Introduction to magnetron sputtering

Introduction to magnetron sputtering

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Jon Tomas Gudmundssona,b , Daniel Lundinc a Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden, b Science Institute, University of Iceland, Reykjavik, Iceland, c Laboratoire de Physique des Gaz et Plasmas LPGP, UMR 8578 CNRS, Université Paris–Sud, Université Paris–Saclay, Orsay Cedex, France

Plasma-based physical vapor deposition (PVD) methods have found widespread use in various industrial applications. In plasma-based PVD processes, the deposition species are either vaporized by thermal evaporation or by sputtering from a source (the cathode target) by ion bombardment. Sputter deposition has been known for decades as a flexible, reliable, and effective coating method. Initially, the dc glow discharge or the dc diode sputtering discharge was used as a sputter source followed by the magnetron sputtering technique, which was developed during the 1960s and 1970s. Magnetron sputtering has been the workhorse of plasma-based sputtering applications for the past four decades. In the planar configuration, the magnetron sputtering discharge is simply a diode sputtering arrangement with the addition of magnets directly behind the cathode target. With the introduction of magnetron sputtering, the disadvantages of diode sputtering, such as poor deposition rate, were overcome as the operating pressure could be reduced while maintaining the energy of the sputtered species, often resulting in improved film properties. Here we discuss the basics of the sputtering process, give an overview of the dc glow discharge, and review the basic physics relevant to the maintenance of the discharge and the sputter processes. Then we discuss the dc glow discharge and its role as a sputter source and how it evolves into the magnetron sputtering discharge. We also discuss various magnetron sputtering configurations in use for a wide range of applications both under laboratory and industrial arrangements. Finally, we introduce pulsed magnetron discharges including high power impulse magnetron sputtering (HiPIMS) discharges.

1.1 Fundamentals of sputtering An important process that takes place in a glow discharge is sputtering, which can occur if the voltage applied to the cathode is sufficiently high. When the ions and fast neutrals from the plasma bombard the cathode target, they not only release secondary electrons, but also atoms of the cathode material. This is referred to as sputtering. When species are sputtered off a cathode target and subsequently used as film forming material, the process belongs to what is referred to as a physical vapor deposition High Power Impulse Magnetron Sputtering. https://doi.org/10.1016/B978-0-12-812454-3.00006-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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(PVD). Sputtering is most easily performed by exposing a cathode target to a gas discharge: either a dc discharge (Kay, 1962) or a magnetron sputtering discharge (Waits, 1978), whereas ion beam sputter deposition is also a well-established PVD technique (Bundesmann and Neumann, 2018). Other PVD techniques include evaporation, pulsed laser deposition, cathodic arc deposition, and ion plating. Sputtering in gas discharges was discovered in the mid-19th century (Grove, 1852). Film formation utilizing sputter deposition, where the cathode target is the source of the film forming material, was first reported by Wright in the 1870s (Wright, 1877a,b). Sputter deposition of thin films had already found commercial application by the 1930s (Fruth, 1932, Hulburt, 1934), but gained significant interest in the late 1950s and early 1960s with improved vacuum technology and the realization that a wide range of materials could be deposited using dc sputtering (Kay, 1962, Westwood, 1976) as well as rf sputtering utilized mainly for dielectrics (Anderson et al., 1962). Here we discuss some of the fundamentals of discharge physics and sputtering. We introduce the dc glow discharge, including its voltage–current characteristics and the various regions observed in its operation, and their properties and role. We discuss some of the fundamentals of plasma physics relevant to sputtering discharges, including electrical breakdown, the relation between the sheath voltage drop and the sheath thickness, and the secondary electron emission, essential for the maintenance of the dc glow discharge. The sputter yield is then discussed in Section 1.1.7, the energy distribution of the sputtered atoms in Section 1.1.8, and collisions within plasma discharges in Section 1.1.9. Finally, we introduce the dc glow sputtering discharge or the dc diode sputtering device in Section 1.1.10. This discussion is intended to give an overview of the fundamental concepts and parameters that are needed to understand the operation of the magnetron sputtering discharge.

1.1.1 DC glow discharge The term gas discharge refers to a flow of electric current through a gaseous medium. For a current to flow, some of the gas atoms and molecules have to be ionized. Furthermore, this current, the discharge current, has to be driven by an electric field. The discharge current, which provides power to the discharge, has to be continuous throughout the length of the discharge. There is a transition in the discharge with regards to which charged species carries the discharge current. In front of the cathode, there is a region, the cathode glow, in which most of the ionization occurs. Outside this region the discharge current is mainly carried by electrons toward the anode and by ions toward the cathode. Energy is needed for the ionization in the cathode glow. In the dc discharge, this is resolved by secondary electron emission from the cathode target. This electron emission is essential for the maintenance of the discharge (see Section 1.1.4). The discharge current is built up by ionization within the cathode sheath, which is due to the secondary electrons that are accelerated by the large electric fields in this region. Thus to describe the current in a dc discharge, the interaction of charged particles with the electrode surfaces has to be taken into account. Let us assume two parallel electrodes separated by a distance L and with applied potential VD . The gap between the electrodes is filled with gas at pressure p, the

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Figure 1.1 The discharge current ID versus the discharge voltage VD for a low-pressure dc discharge. The various operating regimes are noted, with increasing current, Townsend regime, subnormal glow, normal glow, abnormal glow, and arc regime. Reprinted from Gudmundsson and Hecimovic (2017). ©IOP Publishing. Reproduced with permission. All rights reserved.

working gas pressure. The type of discharge that is formed between the two electrodes depends upon the pressure of the working gas, the nature of the working gas, the applied voltage, and the geometry of the discharge. In the following discussion of the dc discharge, we follow the discussion given in a recent review on the foundations of the dc discharge (Gudmundsson and Hecimovic, 2017). The discharge current is shown versus the voltage across a low-pressure dc discharge in Fig. 1.1. A description of the relation between the current and voltage for the dc discharge can be found in review papers such as by Francis (1956) and Ingold (1978) and in a number of textbooks including those of Howatson (1976, Chapter 4), Raizer (1991, Section 8.2), and Roth (1995, Chapter 9) and can be summarized as follows: When a voltage is first applied, the discharge current is very small. This current consists of contributions from various external sources such as cosmic radiation generating free electrons and ions. When the voltage has become large enough to collect all these charged particles, this current remains nearly constant with increased voltage. As the voltage is further increased, the charged particles eventually achieve enough energy to produce more charged particles through collisions with the working gas atoms or by bombardment of the electrodes leading to generation of secondary electrons. As more charged particles are created, the current increases, whereas the voltage is limited by the output impedance of the power supply and remains roughly constant. This region is commonly referred to as the Townsend discharge. The characteristics of the Townsend discharge are very small discharge currents. The Townsend discharge is not luminous since the electron density is low, and therefore the density of excited atoms, which emit visible light, is correspondingly small. Furthermore, it

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is not a self-sustained discharge in the sense that it does not entirely provide its own ionization but requires some external assistance to produce electrons either within the gas itself or from a negatively biased electrode. If the applied voltage is increased further, then the discharge current increases, and eventually this leads to a situation where the plasma density is high enough for the discharge to reorganize the vacuum potential structure and form a cathode sheath, which enables more efficient ionization and therefore a higher current at a given voltage. Then the current increases sharply by several orders of magnitude and becomes independent of the external seed. This is what is referred to as the breakdown point VB (see Fig. 1.1) or subnormal glow and occurs at voltages ranging from two or three hundred volts and upward, depending on the nature of the working gas, the gas pressure, and the separation of the electrodes. Once breakdown has occurred, the discharge becomes self-sustaining and takes the form of a glow, and the gas becomes luminous. As ions bombard the electrode, secondary electrons are emitted. These electrons impact and ionize the atoms of the working gas. Thus more ions are available to bombard the cathode and create more secondary electrons. At this point, the voltage drops, and the discharge current increases abruptly. Electron impact excitation collisions followed by deexcitation with the emission of radiation are responsible for the characteristic glow. This regime is referred to as the normal glow or the dc glow discharge. The ion bombardment of the cathode surface is initially not uniform. The discharge current arranges an optimum current density, and, as the current increases further, more and more of the cathode target surface is subject to ion bombardment. This continues with increased supplied power until a nearly uniform density is achieved covering the entire cathode area. When the whole cathode is covered by ion bombardment, further increase in the power leads to a discharge with a current density at the cathode, which is no longer optimal. Higher currents can therefore only be achieved with higher voltages over the cathode sheath. There is therefore an increase in both voltage and current. This operation regime is referred to as the abnormal glow and is the regime used for sputtering, which is further discussed in Section 1.1.10. The abnormal glow discharge looks much like the normal glow discharge but is more intensely luminous, and sometimes the structures near the cathode merge into one another. As the current density at the cathode becomes large enough for the formation of cathode spots, the discharge makes a transition into the arc regime. The cathode spots can, through a combination of field emission and thermoionic emission, emit electrons more efficiently than the secondary electron emission process, which leads to a second avalanche, increased discharge current, and a drop in the discharge voltage as seen in Fig. 1.1. Eventually a low-voltage high-current arc discharge forms. The dc glow discharge is important historically both for studying the properties of the plasma and for various applications where the dc discharge is used to provide a weakly ionized plasma. The simplicity of the dc glow discharge geometry made it a commonly used plasma generation method for fundamental research in both discharge physics and atomic and molecular physics (Gudmundsson and Hecimovic, 2017). As seen in Fig. 1.1, the dc glow discharge operates in the current range from µA to hun-

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Figure 1.2 A schematic of the dc glow discharge showing several distinct regions that appear between the cathode and the anode. The colors of the various regions assume a neon discharge. The dark spaces are abbreviated as DS.

dreds of mA (current density range 10−5 – 10−3 A/cm2 ), and the working gas pressure is typically in the range 0.5 – 300 Pa. Early studies of the dc glow discharge revealed that it consists of several different regions between the electrodes, which have been illustrated in more or less the same way by several authors (Francis, 1956, Ingold, 1978, Raizer, 1991, Roth, 1995, Nasser, 1971). In Fig. 1.2, we present a schematic of the normal glow discharge in a 0.5 m long tube using neon at 133 Pa as the working gas, which is due to Nasser (1971). The cathode is typically made of an electrically conductive metal. The cathode metal has an influence on the voltage required to maintain the discharge. For a metal that is a good emitter of electrons (see discussion in Section 1.1.4) lower voltages are sufficient. Immediately next to the cathode is a thin dark layer, the Aston dark space. The Aston dark space is followed by the cathode glow, which has a relatively high ion density. The secondary electrons released from the cathode surface are accelerated away from the cathode. These high-energy electrons undergo collisions with neutral working gas atoms at a distance away from the cathode corresponding to the ionization mean free path. In this region the secondary electrons participate in excitation and thereby generate the cathode glow. The cathode glow is followed by the cathode (Crookes or Hittorf) dark space. The regions that extend from the Aston dark space to the cathode dark space together constitute the cathode sheath. Here the electric field is directed toward the cathode, and the space charge is positive and of relatively high density. The cathode dark space is followed by the negative glow (in fact, a region with positive potential), which exhibits a significant light intensity. Most of the ionization occurs here. The boundary toward the cathode dark space is rather abrupt while it is diffuse on the anode side toward the Faraday dark space. The electric field and the energy of the electrons are low in the Faraday dark space. The electron energy available for excitation and ionization is here exhausted. Before entering this dark region, the potential gradient is slightly negative as the space charge reverses. Here the density of electrons has become high enough to carry the entire discharge current and to make the space charge negative. The electron density falls within this dark space region due to recombination and diffusion until the net space charge is zero and the electric field approaches a small constant value and the positive column begins. The positive column is a quasineutral plasma where the electric field is very low. The positive column is simply a long uniform glow, except when striations are formed. The positive column acts as a conducting path between the negative glow region and the anode. The anode

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Table 1.1 The color of selected luminous zones in the dc glow discharge. Gas He Ne Ar H2 N2 Air

Cathode layers red yellow pink red/brown pink pink

Negative glow pink orange dark blue pale blue blue blue

Positive column red/pink red/brown dark red pink red/yellow red/yellow

Based on Francis (1956).

glow is a bright region that appears at the end of the positive column. Often a thin dark space is observed at the end of the positive column (the anode dark space), and a glow close to the surface of the anode (the anode glow). The size, intensity, and color of all the regions described above depend on the nature of the working gas, gas pressure, and applied voltage. Also, some of the features may be absent over particular parameter ranges. The various gases give a discharge of a characteristic color. The colors of the light emitted from the various zones of the dc glow discharge are listed in Table 1.1. If the pressure is reduced, then the cathode dark space expands at the expense of the positive column. This is due the fact that now the electrons have to travel farther (mean free path is longer) to produce efficient ionization. For a secondary electron emission yield in the range 0.05 – 0.1, each secondary electron must initiate an avalanche that produces roughly 10 – 20 ions to maintain the discharge. An electron avalanche is possible within the cathode sheath of glow discharges, because the electron mean free path for ionizing collisions is here smaller than the sheath thickness. As we will see in Section 7.2.3, ionization avalanches within the sheath region are not possible in HiPIMS discharges due to lower pressures and thinner cathode sheaths. How many ions each secondary electron actually produces depends on the ionization mean free path and the distance between the anode and cathode. This relation is qualitatively the statement of Paschen’s law, which relates the breakdown voltage VB to the product of gas pressure and electrode separation and will be discussed in Section 1.1.2. This also shows that the ionization processes in the cathode dark space are essential for the maintenance of the discharge. The potential difference applied between the two electrodes is generally not equally distributed between cathode and anode. The spatial variations of the potential, the electric field, particle densities, space charge, and current densities along the axis of a dc glow discharge are shown in Fig. 1.3. The spatial variation of the plasma parameters shown here was found by particle-in-cell simulation of an argon dc discharge at 50 Pa when a voltage of 400 V is applied across the 5 cm discharge gap (Budtz-Jørgensen, 2001). As the potential profile indicates (Fig. 1.3A), the electric field is large in the vicinity of the electrodes and almost zero within the positive column. Thus almost all the applied voltage drops completely within the first few millimeters in front of the cathode. This region adjacent to the cathode, which is thus characterized by a strong electric field, is the cathode fall, or often referred to as the cathode sheath. We see in

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Figure 1.3 Spatial profiles of (A) the plasma potential, (B) the electric field, (C) ion and electron density, (D) space-charge density, and (E) ion and electron current density. From particle-in-cell simulation of an argon dc discharge at 50 Pa, electrode separation of 5 cm, and −400 V applied to the cathode. Reprinted from Budtz-Jørgensen (2001). ©Budtz-Jørgensen. Reproduced with permission. All rights reserved.

Fig. 1.3C that the sheath region is depleted of electrons, and in Fig. 1.3D, we see that the net space charge is positive in the sheath region in line with our previous discussion of the dark space. The space charge shown in Fig. 1.3D is found by subtraction of the electron density from the ion density. In the plasma bulk, the plasma is quasineutral, and the electron and ion densities are the same. This space charge density leads to the electric field distribution seen in Fig. 1.3B.

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1.1.2 Electrical breakdown Electrical breakdown is an important phenomenon in discharge physics. Here we derive the breakdown voltage as a function of the product pL, where p is the working gas pressure, and L is the distance between the electrodes, which is commonly referred to as the Paschen curve. A similar discussion can be found in various textbooks such as by Lieberman and Lichtenberg (2005, Section 14.3), Raizer (1991, Chapter 7), and Roth (1995, Section 8.6). The electron density and flux grow exponentially as we move axially away from the cathode. Thus the increase in the electron flux is proportional to the flux, or de = α(z)e (z), dz

(1.1)

where e is the electron flux, and α is known as Townsend’s first ionization coefficient and is the inverse of the mean free path for ionization, that is, α(z) ≡ 1/λiz . The electron flux in the direction along the discharge axis (or the direction of the electric field) is  z  e (z) = e (0) exp α(z )dz . (1.2) 0

Due to the continuity of the total charge (creation of equal numbers of electron–ion pairs), we can write 



i (0) − i (L) = e (0) exp

L



α(z )dz





 −1 ,

(1.3)

0

where e (L) from Eq. (1.2) has been inserted. Since the discharge must be selfsustaining, we have e (0) = γsee i (0) and i (L) = 0. Then  exp 0

L

 1 α(z )dz = 1 + γsee

(1.4)

is the condition for self-sustainability. In a vacuum region, the electric field E is a constant, and it follows that the electron drift velocity μe E is also a constant. Hence the electron energy Ee is a constant, allowing us to treat α as a constant in Eq. (1.4). In that case, taking the logarithm of both sides of Eq. (1.4) gives   1 αL = ln 1 + , (1.5) γsee which is the breakdown condition for a dc discharge. The ionization coefficient is usually expressed in the form   Bp α = A exp − , (1.6) p E

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Table 1.2 Constants for the Townsend ionization coefficient. Gas Air H2 Ar N2

A [cm−1 Torr−1 ] 14.6 5.0 13.6 11.8

B [V/(cm Torr)] 365 130 235 325

Range of E/p [V/(cm Torr)] 150 – 600 150 – 400 100 – 600 100 – 600

From Lieberman and Lichtenberg (2005).

where A and B are determined experimentally and found to be roughly constant over a range of pressures and fields for a given gas type. The coefficients A and B for various common gases are listed in Table 1.2. If the minimum voltage at which the discharge initiates, the breakdown voltage, is written as VB = EL, then     BpL 1 ApL exp − = ln 1 + , (1.7) VB γsee which solved for VB gives VB =

BpL

ln(ApL) − ln ln(1 + 1/γsee )

(1.8)

which is a function of the product pL. The product of pressure and distance between the electrodes (pL) is a suitable parameter to characterize the discharge. The curve that shows VB as a function of the product pL is called the Paschen curve. Thus for a fixed discharge length L, there is an optimum gas pressure for plasma breakdown. By differentiating the expression for the breakdown voltage, Eq. (1.8), with respect to pL and setting the derivative equal to zero, we can find the value of pL that corresponds to the minimum breakdown voltage (Raizer, 1991, Chapter 7)   exp(1) 1 (pL)min = ln 1 + , (1.9) A γsee and the minimum voltage is   B 1 , VB,min = exp(1) ln 1 + A γsee

(1.10)

which is referred to as the minimum sparking potential, and is the minimum voltage at which electrical breakdown can occur in a given gas. According to Eq. (1.8), the breakdown voltage is high for low and high pressure and a minimum at pL given by Eq. (1.9). At the lower pressures the ionization process is ineffective due to the low electron-neutral collision probability, whereas at higher pressures elastic collisions prevent the electrons from reaching high enough energy for ionization to occur. The number of gas atoms or molecules in the space between

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the electrodes is proportional to pL. At lower pressure, the distance between cathode and anode has to be longer to create a discharge with properties comparable to those of high pressure with small distance between the electrodes. For low pressure, the electron mean free path is large, and most electrons reach the anode without colliding with gas atoms or molecules. Thus, at low pressure, a higher value of VB is required to generate enough electrons to cause the breakdown of the gas. At higher pressures, the electron mean free path is short. The electrons do not gain enough energy from the electric field to ionize the gas atoms or molecules due to their frequent collisions with the gas molecules. Therefore VB increases as the pressure increases.

1.1.3 The cathode sheath All plasmas are separated from the surrounding walls by a sheath. We have seen in Section 1.1.1, and in particular in Fig. 1.3A, that most of the potential drop over a dc glow discharge appears across the cathode sheath. The relation between the sheath thickness dc , the discharge current density J , and the voltage drop across the cathode sheath Vc was derived by Child (1911), assuming that the initial ion energy is negligible compared to the sheath potential (see also Lieberman and Lichtenberg (2005, Section 6.2)), giving  1/2 3/2 4 Vc 2e J = 0 , 9 Mi dc2

(1.11)

where Mi is the ion mass. Eq. (1.11) is referred to as the Child law or the collisionless Child–Langmuir law. The Child law is valid when the sheath potential is large compared to the average energy of the electrons. A similar relation was derived by Langmuir (1913) for electrons emitted from a hot cathode approaching a cold anode (no thermionic emission). In the collisional regime, where the pressure is high enough that the charged species interact frequently with neutral gas species, we can assume that the ion-neutral mean free path λi is independent of the ion velocity (Lieberman and Lichtenberg, 2005, Section 6.6). This gives the collisional Child law J=

   3/2   3/2 2 5 2eλi 1/2 Vc 0 . 5/2 3 3 πMi dc

(1.12)

Alternatively, assuming that the diffusion of ions is negligible, compared to the drift due to an electric field, and assuming that the ion mobility μi is independent of the ion velocity, we get 9 V2 J = 0 μi c3 , 8 dc

(1.13)

which is referred to as the Mott–Gurney law. It was derived to describe the current at the interface of a semiconductor and insulator (Mott and Gurney, 1948, Chapter V) and later adapted to describe the current through the discharge sheath by Cobine (1958).

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Equation (1.13) is valid only at very high pressures (low drift velocities). The relation between the current density, sheath voltage drop, and the sheath thickness given by Eqs. (1.12) and (1.13) is sometimes referred to as the collisional Child–Langmuir law. We note here that the scalings of the current density with both Vc and dc in Eq. (1.12) are different from Eq. (1.13). It has been demonstrated by experiments that the Mott–Gurney law (Eq. (1.13)) applies to a dc glow discharge in hydrogen (Lisovskiy et al., 2016) and nitrogen (Lisovskiy et al., 2014) for most of the pressure range from 10 to 333 Pa.

1.1.4 Secondary electron emission The emission of secondary electrons as a result of ions or neutrals bombarding a metallic surface plays an important role in discharge physics. The secondary electron emission yield or coefficient γsee is defined as the number of secondary electrons emitted per incident species. The secondary electron emission yield generally depends on the material being bombarded, its surface condition, the type of bombarding species, and the kinetic energy of the bombarding species. The sputter targets are held at high negative potentials, and thus the secondary electrons are accelerated away from the target surface with initial energy equal to the target potential. In many cases, these electrons sustain the discharge by ionization of the neutral working gas. These ions then bombard the cathode target and subsequently release more secondary electrons. As a first approximation, the secondary electron emission yield is independent of the velocity of the bombarding particle while their energy is low, since the electron emission occurs due to transfer of the potential energy of the incoming ion or atom to an electron in the target (Hagstrum, 1954, Abroyan et al., 1967). This constant secondary electron emission yield is attributed to an Auger process and is referred to as potential emission. The energy-dependent portion of the secondary electron emission yield is called kinetic emission. Kinetic emission occurs when a bombarding particle transfers sufficient kinetic energy to an electron in the target. Typically, it starts contributing to the total secondary electron emission yield at a threshold energy of around a few hundred electron volts. This process dominates at higher energies. Both experimental data and theory predict a linear dependence of the secondary electron emission yield on the bombarding particle energy close to the threshold energy, and linear dependence on the bombarding velocity at higher energies (Abroyan et al., 1967, Parilis and Kishinevskii, 1960, Cawthron, 1971, Baragiola et al., 1979, Hasselkamp, 1992). At much higher energies, experimental data show that the electron yield starts decreasing with increasing bombarding velocity. This occurs for a bombarding energy of around 100 keV for H+ (Hasselkamp, 1992). The two different mechanisms are considered to be detachable, so the total electron emission yield is written as γsee = γp + γk ,

(1.14)

where γp and γk are the contributions from potential and kinetic emission to the total yield, respectively. In addition to the energy of the impacting particle, the secondary

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Figure 1.4 Secondary electron emission yields for Ar+ ions and neutral Ar beams incident on various dirty metal surfaces versus particle energy. The solid lines show the secondary electron emission yields for dirty metals, and dashed lines show the secondary electron emission yields for clean metals. After Phelps and Petrovi´c (1999) and Phelps et al. (1999).

electron emission yield depends on the cathode material but rarely exceeds 0.2 for metallic targets bombarded by ions with energy below 1 keV. The condition of the target significantly affects the ion-induced secondary electron emission yield. For clean surfaces, kinetic emission is dominant for projectile energies exceeding about 300 eV/amu (Winter et al., 1991, 2001) and has been observed down to an energy in the range 10 eV/amu (Eder et al., 2000). For Ar+ ion projectiles, this would mean a lower threshold of ∼ 0.4 keV, although kinetic emission would only play a dominant role for impinging ion energies above ∼ 12 keV. However, the threshold for kinetic emission may be reduced in case of dirty or technological surfaces (Phelps and Petrovi´c, 1999). Still, for an energy range of 0.5 – 1.0 keV, relevant in magnetron sputtering discharges, potential emission is expected to dominate. Clean metals, that is, metals free of oxidation, gas adsorbtion, and other contamination generally have a lower kinetic emission yield than contaminated metals (Phelps and Petrovi´c, 1999). The secondary electron emission yields for argon ions γsee,i and argon neutrals γsee,a bombarding “dirty” metal electrodes and “clean” metal electrodes are shown versus the ion or atom kinetic energy of the incident particle in Fig. 1.4. The secondary electron emission yields γsee,i and γsee,a are calculated using the formulas given by Phelps and Petrovi´c (1999) and Phelps et al. (1999). For “clean” surfaces, equations B10 and B12 in Phelps and Petrovi´c (1999) are used, whereas for “dirty” surfaces, equations B15 and B17 in Phelps and Petrovi´c (1999) are used (a correction to B15 is given by Phelps et al. (1999)). For the typical ion bombarding energies expected in magnetron sputtering discharges, including HiPIMS discharges, the secondary electron emission is determined by the potential energy of the arriving ion projectile rather than its kinetic energy. For a potential emission to occur, the potential energy (ionization potential) of the projectile has to exceed twice the work function of the target material. A fit to experimentally

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Table 1.3 The work function φ and the first and second ionization energies for several common elements. Element Ar Al Cu Ti Cr W

φ [eV] N/A 4.08 – 4.28 4.9 4.1 – 4.3 4.5 4.55

Eiz,1 [eV] 15.76 5.99 7.73 6.82 6.77 7.98

Eiz,2 [eV] 27.63 18.83 20.29 13.58 16.50 17.62

From Anders et al. (2007).

determined secondary electron emission yields for various ions on clean surfaces is given as (Baragiola et al., 1979, Baragiola and Riccardi, 2008) γsee = 0.032 (0.78Eiz − 2φ) ,

(1.15)

where Eiz is the ionization energy of the ion, and φ is the work function of the target surface. This process can only occur when the condition 0.78Eiz > 2φ is fulfilled. Another empirical expression is also frequently used (Raizer, 1991): γsee = 0.016(Eiz − φ).

(1.16)

This condition is not fulfilled for most metal ions sputtering a target of the same metal. The work function φ and the first and second ionization energies for several common elements are shown in Table 1.3. Thus, as pointed out by Anders (2008) for a typical metallic magnetron sputtering target, singly charged metal ions cannot perform potential emission. During self-sputtering of a metal target, no secondary electrons are emitted, and the secondary electron emission coefficient is practically zero. We also note looking at Table 1.3 that for some of the common metals like Cr and Ti, the ionization energy to create doubly charged ions is relatively low, compared to the ionization energy of argon. In that case the concentration of the doubly charged ions of the sputtered material is expected to be high. These doubly charged ions can fulfill the condition 0.78Eiz > 2φ and thus create secondary electrons. The presence of a magnetic field close to the cathode surface, such as in the case of magnetron sputtering discharges (see Section 1.2.2), may have a strong influence on the secondary electron emission. Once emitted, the secondary electron motion is governed by the Lorentz equation, which results in helicoidal (arch-shaped) trajectories above the cathode. A fraction of the secondary electrons can thereby return to the cathode surface despite the strong repulsive electric field. Here they are either reflected or (re)captured, which will reduce the secondary electron emission yield. We therefore have to make a distinction between the standard γsee , which is a material-dependent property, and the effective secondary electron emission yield γsee,eff as seen by the discharge. Thornton was one of the first to study this issue in magnetron sputtering discharges (Thornton, 1978). He considered a coaxial cylindrical post magnetron and

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estimated the recapture probability to be r = 0.5 based on a qualitative model for calculating the discharge voltage. Later on Buyle et al. (2004) calculated the recapture probability to be in the range 0.65 – 0.75 for a planar magnetron sputtering discharge configuration. For HiPIMS discharges, the recapture probability has been investigated more recently by Huo et al. (2013) and Gudmundsson et al. (2016), where values of r = 0.50 – 0.86 are reported. In general, the recapture probability is expected to depend on the geometry of the system, the magnetic field configuration, target erosion, and operational parameters, such as working gas pressure (Buyle et al., 2003, Costin et al., 2005). Finally, it should be noted that many of the secondary electrons are thermalized by collisions with the atoms of the working gas, but even at relatively high pressures a substantial number of the electrons retain the full target potential as they impact the anode.

1.1.5 Electron energy distribution function Knowledge of the electron dynamics in technological plasmas helps us to identify which elementary processes are involved in the discharge. The temporal evolution of the electron dynamics is particularly important for pulsed discharges, since it provides information on changes taking place in the plasma when, for example, the cathode voltage is intentionally altered or plasma instabilities occur. Spatial homogeneity of the electron distribution (which also influences other plasma properties) is very important in plasmas where thin films are deposited or surfaces are treated. Electrons constitute the lightest particles in glow discharges, which implies that electrons collide either with particles of the same mass or with particles having a considerably greater mass (neutrals or ions). A distribution function f (t, r, v) gives detailed information on a particle population for a given time t, at a certain position in space, described by the vector r and velocity v, i.e., a six-dimensional phase space of the particle position and velocity. The knowledge of this function allows for calculation of the number of electrons at time t in the volume element d3 r and having a velocity in the range from v to v + dv. For electrons, the electron distribution function (EDF) describes both isotropic and anisotropic electron populations. The electron density ne can be calculated from the electron distribution function as  (1.17) ne (t, r) = fe (t, r, v)dv, v

where fe (t, r, v) is the electron distribution function. The analytical expression of the EDF is often very complicated due to the complexity of low-temperature plasmas. However, from a practical point of view, some experimental techniques provide measurements of the electron energy distribution function (EEDF) ge (t, r, E). This distribution function also describes the electron population, but instead of using the velocity vector, we only gain knowledge of the kinetic energy of the electrons in an interval from E to E + dE. For determination of the EEDF, the electron population must be isotropic in the velocity space, and therefore we lose

Introduction to magnetron sputtering

15

information on the angular distribution. In the case of an isotropic EDF the following expression holds: fe (t, r, v) = fe0√(t, r, v), where we have used the relation between kinetic energy and velocity v = 2E/me , where me is the electron mass. Then the equation for expressing the EEDF as a function of an isotropic EDF is  ge (t, r, E) = 2π

2e me

3/2



E 1/2 fe0 t, r, 2E/me .

(1.18)

When the EEDF is known, the electron density ne can be calculated using Eq. (1.18) or  ∞ ne = ge (E)dE. (1.19) 0

We often use the electron energy probability function (EEPF) gp (t, r, E) = E −1/2 ge (t, r, E)

(1.20)

for visualization of evaluated Langmuir probe characteristics because it allows us to directly determine whether the measured electron distribution is Maxwellian or not (see also Chapter 3, Section 3.1.1 on Langmuir probes).1 When the EEPF is displayed on a semilog plot, it exhibits a straight line if the electron energy distribution is Maxwellian (Godyak and Demidov, 2011). Although the electron energy distribution provides full information on the kinetic properties of an electron gas, it is often convenient to describe the average dynamics of electrons for a given time and location. The effective electron temperature  2 2 1 ∞ Ege (E)dE (1.21) Teff = E = 3 3 ne 0 is often used as a measure of the average electron energy. In the case the EEDF is Maxwellian, we can refer to Te = 23 E as the electron temperature. In the following chapters, we often use the roman typeface symbol T for the voltage equivalent of the temperature such that kB Te [K] = eTe [V].

1.1.6 Electric potentials In Section 1.1.1, we discussed plasma sheaths being established in glow discharge plasmas to maintain plasma quasineutrality. A similar behavior is found in the plasma bulk, which is charged positive to prevent escape of electrons out of the plasma. The potential observed in the plasma bulk or at the plasma boundaries is called the plasma potential Vpl (see Fig. 1.3A), which, in the present context, is an important plasma parameter from the point of view of sheath electron energization (Brenning et al., 2016) 1 Maxwellian electron distributions can be found, for example, in plasmas with high plasma density and

higher pressures, where many collisions between electrons are expected leading to thermalization of the electron population.

16

High Power Impulse Magnetron Sputtering

and for ion acceleration (Hershkowitz, 1994). The plasma potential can theoretically be calculated by solving the Poisson equation, which in the case of no magnetic field and steady-state conditions is written as ∇ 2ϕ = −

e(ni − ne ) , 0

(1.22)

where ni is the ion density, ne is the electron density, e is the elementary charge, and 0 is the permittivity of vacuum. It can easily be shown that the plasma potential is constant when variations in charge neutrality are negligible and the plasma potential is changing significantly only at length scales comparable to or smaller than the Debye length for which charge neutrality is not preserved. Electrically isolated objects inserted into the plasma are bombarded by plasma electrons and ions and will in general take on a slightly negative potential with respect to the plasma potential due to the higher mobility of the electrons. Since neither electrons nor ions can be drained, the charging continues until a sufficiently negative potential balances out the electron and ion fluxes. This potential is known as the floating potential Vfl and gives rise to a zero net current to the inserted object. The magnitude of the floating potential Vfl can be determined by setting the absolute values of the ion and electron fluxes at the electrically isolated object to be identical, that is, |i | = |e |. The floating potential has a repulsing effect on the electron current, which decreases exponentially according to the Boltzmann distribution. Then the equality of ion and electron current densities can be expressed as   Vfl − Vpl 1 i = ns uB = ns v¯e exp , (1.23) 4 Te where uB = (eTe /Mi )1/2 is the Bohm velocity, v¯e = (8eTe /πme )1/2 is the mean electron speed, and Vfl − Vpl is the difference between the floating potential Vfl and the plasma potential Vpl . Solving Eq. (1.23) for Vfl − Vpl , we find Vfl − Vpl = −

  Te Mi ln , 2 2πme

(1.24)

where Mi is the ion mass.

1.1.7 Sputter yield Sputtering is the ejection of atoms due to bombardment of a solid or a liquid surface (the target) by energetic particles, often ions (Ruzic, 1990). The sputter yield Y is defined as the mean number of atoms removed from the target surface for each incident ion. In general the sputter yield depends on the energy of the incoming projectile ion, the ion incident angle (in relation to the target normal), the bombarding ion mass, and the target material. The maximum transferable energy in a collision has to be larger than the surface binding energy or Eth + Esp > Esb / , where Esb is the surface binding

Introduction to magnetron sputtering

17

energy (heat of sublimation) of the target material, Eth is the threshold energy for sputtering, Esp is the binding energy of a projectile to the target surface (Esp = 0 for noble gas ions), = 4Mi Mt /(Mi + Mt )2 is the energy transfer factor in a binary collision, and Mi and Mt are the masses of the projectile and the target atom, respectively (Eckstein, 2007). The minimum ion energy required for sputtering to take place is known as the threshold energy for sputtering and is given by Yamamura and Tawara (1996) as

  i Esb 1 + 5.7 M Mt / if Mi ≤ Mt , Eth = (1.25) if Mi ≥ Mt , Esb × 6.7/

and is typically in the range from some 10 eV to a few hundred eV. For example, if an Ar+ ion bombards a titanium target, then Mi < Mt and Eth ≈ 6.75 × Esb , and if an Ar+ ion bombards an aluminum target, then Mi > Mt and Eth ≈ 9.82 × Esb . There exist in the literature a number of semiempirical models that describe the energy and angular dependence of the sputter yield (see e.g. Sigmund (1969), Bohdansky (1984), Yamamura and Shindo (1984), and Yamamura and Tawara (1996)). Yamamura and Tawara (1996) give various empirical formulas for the sputter yield as a function of ion bombarding energy and data for various combinations of ions and target materials. In the energy range of interest here, 20 – 5000 eV, the sputter yield increases with increasing incident ion energy. The sputter yield is also strongly target material dependent. Sputter yields for Ar+ ions versus bombarding energy, in the energy range 0 – 1000 eV, for various target materials are shown in Fig. 1.5. In this energy range the sputter yield can be approximated by Y (Ei ) = aEib ,

(1.26)

where a, material dependent, and b (roughly 0.5) are fitting parameters that are given for a particular combination of bombarding ion and target materials (Anders, 2010, 2017). For Ar+ bombarding a Cu target, a = 0.1421 and b = 0.468, whereas for the self-sputtering Cu+ bombarding a Cu target, a = 0.0691 and b = 0.556. In addition, the sputter yield increases with increasing angle of incidence, and maximum occurs in the range between 60◦ and 80◦ and falls off rapidly as the angle is increased further (Oechsner, 1975). When the cathode surface is struck by a particle in this energy range, some atoms, referred to as primary knock on atoms, may gain substantial amount of the energy from the incoming ion through the collision. They in turn sputter or strike other atoms transferring momentum yet again. It is of great interest to track these mechanisms and evaluate their combined effects. Computer codes such as TRIM (Transport of Ions in Matter) (Biersack and Haggmark, 1980), SRIM (Stopping and Range of Ions in Matter) (Ziegler et al., 2008, 2010), and TRIDYN (a TRIM simulation code including dynamic composition changes) (Möller and Eckstein, 1984, Möller et al., 1988) are commonly used to calculate the sputter yield. They use a binary collision model and follow the incident particles and all of its cascade atoms until they sputter or their energy is too low to escape the surface potential.

18

High Power Impulse Magnetron Sputtering

Figure 1.5 Sputter yields for Ar+ ions in the energy range 0 – 1000 eV impinging on various target materials. The yields are calculated using a sputter yield calculator from the Surface Physics Group at TU Wien (Surface Physics Group at TU Wien, 2017), which is based on empirical equations for sputter yields at normal incidence by Matsunami et al. (1983).

It should be noted, however, that projectile-target combinations resulting in modifications of the target will lead to a lower accuracy, when using the empirical sputter yield equations discussed. This is particularly relevant for sputtering in reactive gas mixtures, such as N2 or O2 , which may form compounds on the sputter target and thereby altering the material properties. As an example, the sputter yield of Ti when bombarding a pure Ti target and a completely oxidized TiO2 target exhibits a difference in the Ti sputter yield by a factor 15 – 20 between the pure metal and compound targets at 500 eV incident ion energy. Thus target poisoning can have significant influence on the sputter yield as will be discussed further in Section 6.2.3. Using the sputter yield, we can now calculate the sputter rate Rsputter =

Y (Ei )Ji , entarget

(1.27)

where Y (Ei ) is the sputter yield, Ji is the ion current density at the target surface, and ntarget is the atomic density of the target. This can be used to estimate how deep the sputtering process digs into the target surface for a given operation time.

1.1.8 Energy distribution of sputtered atoms The atoms sputtered off the cathode target are considerably more energetic than thermally evaporated atoms (a few eV as compared to about a tenth of an eV). Usually it is desirable to maintain this initial kinetic energy of the sputtered atoms, because of its effect on the film growth (Petrov et al., 1993). Relatively low working gas pressures are typically desired to minimize scattering of the sputtered atoms. The sputter process is, therefore, normally a line-of-sight process where the deposition flux cannot be

Introduction to magnetron sputtering

19

easily controlled, since it consists of neutral atoms. This broad distribution has been measured for sputtered neutrals (Stuart et al., 1969) and is predicted by the Thompson random collision cascade model (Thompson, 1968, 1981). According to the Sigmund– Thompson theory, the energy distribution function can be approximated by fS−T ∝

E , (E + Esb )3−2m

(1.28)

where Esb is the surface binding energy of the target material, and m is the exponent in the interaction potential applied V (r) ∝ r −m (Hofer, 1991). This model predicts an energy spectrum that peaks sharply at 12 Esb , followed by a gradual decrease to higher energies (∝ 1/E 2 ). The energy distribution of atoms ejected from a target is expected to be independent of the nature of the incident ion and the crystal structure of the target. It should be noted that the original Sigmund–Thompson sputter energy distribution function, given in Eq. (1.28), slightly overestimates the probability to sputter-eject energetic atoms. A modified distribution function was later introduced by Stepanova and Dew (2004), where they added a cutoff energy Emax to better reflect experimentally measured profiles     E + Esb n . (1.29) fS−D = fS−T 1 − Emax + E Typical values of the constants are n = 1, m = 0.2 (Stepanova and Dew, 2004), and Emax = 20 eV (Lundin et al., 2013). The angular distribution of the sputtered atoms is often described as a cosine distribution. It means that the relative amount of material sputtered at any particular angle can be compared to the amount sputtered at normal incidence times the cosine of the angle from normal incidence. The overall distribution can thereby be drawn as an ellipse, and in three dimensions, the distribution would appear as an ellipsoid centered on the ion impact point. For a more detailed discussion of the energy and angular distribution of the sputtered material, the reader should consult the reviews given by Hofer (1991) and Gnaser (2007) or the original work by Thompson (1968, 1981) and by Sigmund (1969). If the sputtered material is subsequently ionized, then the ion energy distribution generally shows a narrow low-energy peak, due to thermalized atoms, which are ionized and then accelerated by the plasma potential, and a broad distribution at higher energies, which originates from the sputtered neutrals, which have been ionized by electron impact within the plasma (see e.g. Andersson et al. (2006)). Due to the small mass of the electron, the electron impact ionization does not change the energy of the resulting ion by much. More details on ion energy distributions in HiPIMS discharges are given in Section 4.1.3.

1.1.9 Collisions in gases The glow discharge contains electrons, different types of ions, neutral atoms, and molecules, as well as photons. In principle, we should consider collisions/interaction

20

High Power Impulse Magnetron Sputtering

between all these species, but fortunately some of these collisions are more important than others for the type of low-pressure glow discharges that are used for sputter applications. Before discussing the various collisions, it is necessary to examine the physics involved. The working gas pressure is an important parameter in discharge physics. At low pressure, only a few collisions occur, and the energy transfer between species is inefficient. At high pressure, many collisions occur between the various plasma species, which result in more equal temperatures of the species. The fundamental quantity that characterizes a collision between particles is the cross section σ (vR ), where vR is the relative velocity between the particles before collision. In the definition of the collisional cross section, an element of probability for a certain reaction to occur is implicitly included. The mean free path λ is the average distance traveled by a particle between collisions with other particles and is reciprocal to the product of the gas density (or pressure) and the collisional cross-section σ (vR ): λ=

1 . nσ (vR )

(1.30)

For argon gas at 0.13 Pa (1 mTorr) and room temperature, the average distance traveled by an Ar atom before a collision to another Ar atom is about 8 cm, and most other gases are within a factor of three of this value (provided gas atoms of thermal energy) (Chapman, 1980). When two particles collide, one or both particles may change their momentum or their energy, neutral particles can become ionized, and ionized particles can become neutral. Collision processes can be divided into elastic and inelastic collisions, depending on whether the internal energies of the colliding particles are maintained or not. The total energy, which is the sum of the kinetic and potential (internal) energy, is conserved in a collision. When the internal energies of the colliding particles do not change, then the sum of kinetic energies is conserved, and the collision is said to be elastic. In this case, the kinetic energy is generally exchanged between particles, whereas the total kinetic energy is conserved. Using the energy transfer function 4Mi Mt /(Mi + Mt )2 , it is found that the energy transfer is negligible in electron collisions with heavy particles, such as neutral atoms. For these cases, the electron just changes direction without significantly changing speed. When the sum of kinetic energies is not conserved, the collision is referred to as inelastic. The most important inelastic collisions are listed in Table 1.4. The inelastic collisions often involve excitation or ionization of the colliding particles, so that the sum of kinetic energies after collision is less than that before collision. Inelastic collisions involving electrons are essential to the maintenance of a glow discharge. The most important of these collisions is electron impact ionization. For electron impact ionization of an argon atom, this is written e + Ar −→ Ar+ + 2e, through which an Ar+ ion is formed along with two electrons. Thus a bound electron on the atom is ejected. It is through this multiplication of the electrons and subse-

Table 1.4 The most important inelastic collision types in glow discharges. Chemical symbols are used where appropriate. M refers to any type of metal atom and e refers to an electron. Inelastic reaction Ionization

Reaction example e + Ar −→ Ar+ + 2e

Excitation

e + Ar −→ Ar∗ + e

Recombination

e + Ar+ + body −→ Ar + body

Relaxation Dissociation

Ar∗ −→ Ar + hν e + O2 −→ O + O + e

Dissociative electron attachment Charge transfer Penning ionization

e + O2 −→ O− + O + e Ar + Ar+ −→ Ar+ + Ar Arm + M −→ Ar + M+ + e

Comments A bound electron in the atom is ejected from the atom. Threshold energy for ionization to occur (ionization potential φiz ). For Ar, φiz = 15.76 eV. Excitation of a bound electron to a higher energy level within an atom. Threshold energy for excitation to occur. For Ar, φexc = 11.56 eV. Inverse of ionization. Three body collision (with a wall for example) is typically required. Inverse of excitation. Release of photon with energy hν. Breaking apart a molecule. Requires overcoming the bond strength in the molecule, which is 5.15 eV for oxygen. Often the primary mechanism for negative ion formation in molecular gas. For oxygen, the threshold energy for dissociative attachment is 4.2 eV. Ion-neutral collision. Involves long-lived excited species called metastables, such as Arm with energy levels at 11.56 eV and 11.72 eV.

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High Power Impulse Magnetron Sputtering

Figure 1.6 Low aspect ratio dc glow discharge used for sputtering.

quent acceleration by the electric field that the glow discharge is maintained. This process has a threshold referred to as ionization energy Eiz below which no ionization occurs. The ionization threshold for a few common gases and metals is given in Table 1.3. It is referred to as excitation when a transfer of energy to a bound electron on the atom allows the electron to jump to a higher energy level within the atom with a corresponding quantized absorption of energy. For ions colliding with atoms, the main processes are elastic scattering in which momentum and energy are exchanged. However, also inelastic collisions leading to resonant charge transfer can occur; see Table 1.4. In molecular gases, there are a number of additional important processes that can occur. These may include inelastic collisions, such as vibrational and rotational excitation, dissociation, and dissociative recombination. When negative ions are present, the processes can include attachment, detachment, ion–ion mutual neutralization, and positive–negative ion charge transfer.

1.1.10 DC glow sputter source For decades, the dc glow discharge was used as a sputter source, and there have been a number of reviews written on dc glow discharge sputter deposition (Kay, 1962, Westwood, 1976, Vossen and Cuomo, 1978, Thornton and Greene, 1994). In Fig. 1.6, we show a schematic of such a setup. The upper electrode is the cathode, which serves as a target for ion impact sputtering. In sputter deposition the cathode surface (the target) is the source of the film forming material. The material is then transported through the low-pressure gaseous environment, before it condenses on a substrate to form a film. Such processes can be used to deposit thin films of elemental, alloy, and compound materials as well as some polymeric materials. For sputter applications with the dc glow discharge, the distance between cathode and anode is generally short, so that normally only a short anode zone is present along with the cathode dark space and the negative glow, where the slightly positive plasma potential returns back to zero at the anode. The positive column is commonly absent in these short discharges. The diode sputter sources are often low aspect ratio dc glow discharges (L/R < 1 for a cylindrical configuration) and mainly used for sputtering of metals. Cathode diameters are typically in the range 10 – 30 cm, whereas the spacing between the cathode and anode is 5 to 10 cm. This configuration is referred to as an obstructed dc glow discharge.

Introduction to magnetron sputtering

23

The low pressure dc discharge will adjust the width of the cathode fall region, dc , such that a minimum value of the product dc p = (dp)min is established, referred to as the Paschen minimum. If the length of the dc discharge is less than dc at the Paschen minimum value, then the voltage drop over the cathode fall rises above the Paschen minimum Vc,min . This is desired in some plasma processing applications, where a large voltage drop across the cathode sheath is needed. Almost all the voltage appears across the cathode sheath (dark space or cathode fall). Typical dc glow discharges for sputter deposition require a negatively charged cathode at 2000 – 5000 V and a grounded surface anode. The substrates on which the sputtered atoms are deposited are placed on a substrate holder which is often the lower electrode or the anode. However, the substrate holder may be separate from the anode which then can be grounded, floating, biased, heated, cooled, or some combination of these. A typical dc glow discharge is maintained by secondary electron emission, and the operating pressure must be high enough so that the secondary electrons are not lost to the anode or to the grounded surfaces before performing ionization. These pressures are higher than preferred for optimum transport of the sputtered atoms due to scattering by the working gas atoms. Hence, there is a narrow pressure range around 2 – 4 Pa for dc glow discharge sputtering to be viable. At this pressure, the cathode dark space is about 1 – 2 cm wide. The discharge current density can be as high as 1 mA/cm2 , and the deposition rate is below 10 nm/min at best. The dc sputter source is generally weekly ionized with an ionization fraction of the order of 10−4 . In the dc sputter diode configuration the ions that impinge on the target surface do not have the full cathode fall potential. This is due to the working gas pressure that is high enough to allow charge-exchange collisions and momentum transfer collisions (thermalization) between the accelerating ions and the working gas neutrals. The consequence is that there is a broad energy spectrum of ions and high energy neutrals that impinge on the target surface. The higher the gas pressure, the lower the mean energy of particles that bombard the target. The disadvantages of dc diode sputter deposition include low sputter rate and thus low deposition rate, target poisoning by reactive contaminants, substrate heating due to electrons accelerated away from the cathode target, and that only electrically conductive materials can be used as sputter targets. Also the sputter power efficiency (sputtered atoms/ion-volt) is relatively low in these discharges as they operate at high voltage, and this efficiency decreases with increasing energy.

1.2 Magnetron sputtering To lower the discharge voltage and expand the operational pressure range, the lifetime of the electrons in the target vicinity has to increase. In the 1930s, Penning had proposed the use of magnetic fields in a sputtering system to extend the lifetime of the electrons escaping from the cathode, and trap them in the vicinity of the cathode target (Penning, 1936). Further exploration of the effects of a magnetic field on a dc glow discharge in diode configuration led to the discovery of an additional ionization

24

High Power Impulse Magnetron Sputtering

Figure 1.7 A schematic of the dc planar magnetron discharge used for sputtering.

region in the negative glow in the presence of a magnetic field and increased ion intensity at the cathode target (Kay, 1963). This idea was developed into a cylindrical hollow cathode device in which an axial magnetic field is used to trap electrons, coined as magnetron sputtering (Gill and Kay, 1965). The enhanced ionization due to the addition of a magnetic field leads to acceptable deposition rates and makes operation possible at working gas pressures significantly lower than 4 Pa (Gill and Kay, 1965). These coaxial cylindrical magnetron sputter sources (Thornton, 1978, Thornton and Penfold, 1978) were demonstrated in both hollow cathode (Gill and Kay, 1965, Thornton, 1974) and center cathode configuration (Wasa and Hayakawa, 1967, 1969). This work was followed by the introduction of the planar magnetron sputtering device by Chapin in 1974 (Chapin, 1974, 1979, Waits, 1978). A schematic of a dc planar magnetron sputtering configuration is shown in Fig. 1.7. In the planar magnetron sputtering discharge the cathode target is either a circular or a rectangular plate. In this configuration, the magnetic field can be created by permanent magnets (Waits, 1978), electromagnets (Window and Savvides, 1986a, Wendt and Lieberman, 1990), or a combination of both (Solov’ev et al., 2009, Kozyrev et al., 2011). The center portion of the magnet is of one polarity (north in Fig. 1.7), and the outer periphery is of the other polarity (south in Fig. 1.7). So the assembly consists of an outer annular magnet and an inner magnet of opposite polarity, and the magnetic field lines go out from the center of the cathode and go back into the cathode at the annular. Since the electrons spend more time where the electric field is perpendicular to the magnetic field, the ideal geometry would be to have the magnetic field parallel to the cathode surface. However, in reality the magnetic field produced by the magnet and its associated pole pieces comprise field lines, which extend from the sputter surface and return thereto to form an arch over what is referred to as the erosion region. Within this arch, ionizing electrons and ionized gas are confined forming a dense glow discharge and consequently a high level of sputter activity. We refer to this region as the ionization region. If the cathode plate is circular (can also be rectangular) the magnetic confinement leads to a luminous torus shaped plasma that hovers next to the target. Sometimes a planar magnetron discharge consists of a planar cathode (sputtering source or target) parallel to an anode surface. However, in most cases the anode is

Introduction to magnetron sputtering

25

a grounded shield around the magnetron target (as seen in Fig. 1.7) as well as the walls of the deposition system. Since the electrons move along magnetic field lines with ease, the magnetic field lines closest to the cathode target that go through a grounded structure, for example, the grounded shield, will define a “virtual anode” for the magnetron discharge. The position of the anode, including the virtual anode, is very important for the interaction between the plasma and the substrate. If the anode shields the plasma generated at the cathode from the substrate, then the plasma will be very weak in the substrate vicinity, and the possibility to utilize the plasma to modify the growing film, with, for example, low-energy ion bombardment, will be limited.

1.2.1 DC magnetron sputtering In a conventional dc magnetron sputtering (dcMS) discharge the cathode is kept at a constant negative voltage. Positive ions generated in the plasma are accelerated toward the cathode target generating a vapor of atoms and molecules from the target surface through sputtering (see Section 1.1.7). In the magnetron sputtering discharge the secondary electrons are accelerated by the potential difference between the cathode and the bulk plasma. The main advantage of the planar magnetron sputtering discharge is that the sputtered material flows in the direction normal to the cathode plane. Conventional planar dcMS sources are commonly operated using argon as the working gas in the pressure range 0.1 – 1.5 Pa and an applied cathode voltage in the range of 300 – 700 V, whereas the confining magnetic field strength at the target surface is in the range 20 – 60 mTesla. This leads to current densities of the order of 4 – 60 mA/cm2 and power densities of several tens of W/cm2 (Waits, 1978). This pressure regime and operation parameters define a collision-free sputter deposition process, where the deposition rate is limited by the target power density, and the sputtered atoms almost maintain their energy of a few eV obtained from the sputtering event. The electron density in the substrate vicinity is typically in the range 1015 – 17 10 m−3 (Rossnagel and Kaufman, 1986, Sheridan et al., 1991, Seo et al., 2004, Sigurjonsson and Gudmundsson, 2008). The static deposition rate is in the range 20 – 200 nm/s. The degree of ionization of the sputtered material is generally very low, often of the order of 0.1% or less (Petrov et al., 1994). The majority of the ions bombarding the substrate are ions of the noble working gas. Also, the density of the sputtered particles is much lower than the density of the noble working gas (Naghshara et al., 2011). The primary mechanism for the ionization of the sputtered metal atoms in a dcMS discharge is Penning ionization through impact with the working noble gas atoms that are in the metastable state (Christou and Barber, 2000). The mean free path for the sputtered material with respect to electron impact ionization is over 50 cm (Gudmundsson, 2010).

1.2.2 Addition of magnetic fields The path of the electrons in magnetron sputtering discharges is more complicated due to the presence of both a magnetic field B and an electric field E. As seen in Fig. 1.7,

26

High Power Impulse Magnetron Sputtering

the magnetic field is arched, and the electrons are reflected back into the ionization region (IR) above the race track (discussed in Section 1.2.3) whenever they encounter the cathode sheath edge. They bounce back and forth along the magnetic field lines in cycloid-like trajectories until a collision occurs. The majority of the ionization events occur in this region where the energetic electrons are trapped. That is why it is sometimes referred to as the ionization region. In the absence of an electric field, an electron will gyrate in the magnetic field with the electron cyclotron angular frequency eB , me

ωce =

(1.31)

and the corresponding gyration radius is rce =

ue,⊥ ue,⊥ me = , ωce eB

(1.32)

where ue,⊥ is the electron speed perpendicular to the magnetic field B. Typical values for rce are 1 – 10 mm. This means that electrons in the target vicinity are magnetized, that is, their gyration radius is much smaller than the characteristic size of the confining magnetic field structure. As a comparison, ions have a gyration radius rci of the order of 1 m, which is larger than the characteristic size of the system, and thus the ions are not magnetized by the relatively weak static magnetic field. In the presence of an electric field the electron exhibits a net drift perpendicular to both the B and E field vectors, often referred to as the Hall drift or the E × B-drift (Lieberman and Lichtenberg, 2005, Chen, 2016), which is given by vE =

E×B , B2

(1.33)

and the electrons perform trochoid movements. For a planar magnetron discharge, this drift is in the azimuthal direction, with typical drift velocities around 104 m/s within the ionization region. The resulting azimuthal current is often referred to as the Hall current (Thornton, 1978, Rossnagel and Kaufman, 1987). In addition, there is a drift driven by the electron pressure gradient (or diamagnetic drift) written as v∇p =

∇pe × B . ene B 2

(1.34)

The Hall drift (Eq. (1.33)) and the diamagnetic drift (Eq. (1.34)) result in an azimuthal current flowing above the target race track. Notice that, as pointed out by, for example, Thompson (1964, pp. 160–164), the curved vacuum B field drifts (the drifts of the gyro centers that are proportional to the electron energies E and E⊥ ) give no contribution to the macroscopic current in a homogeneous plasma.

1.2.3 Electron confinement and target utilization Due to the magnetic confinement, a torus-shaped dense plasma hovers in front of the cathode target. Thus in the planar magnetron configurations the sputter-erosion

Introduction to magnetron sputtering

27

is not uniform and leads to the formation of a circular groove-like erosion pattern (the “race track”). This is one of the characteristic features of conventional planar magnetron sputter tools and is a consequence of the charged particle confinement in the magnetic field. For a planar magnetron sputtering discharge, the target utilization is often in the range 26 – 45% (Chapin, 1974, Waits, 1978, Nakano et al., 2017). This race track formation limits the target utilization, resulting in higher operating costs. This also means that the deposition pattern can be non-uniform. Film thickness uniformity must be accomplished by substrate (or target) movement. This issue has been resolved to a large degree by using rotating magnetic assemblies that increase the target utilization and improve the deposited film thickness homogeneity dramatically (Iseki, 2006). Target utilization of up to 77% has been reported using an asymmetric yoke magnet structure (Iseki, 2010). Another option is the rotatable magnetron, which will be discussed in more detail in Section 1.3.2.

1.2.4 Electron heating If the discharge is sustained by secondary electron emission from the cathode and by ion bombardment, then the discharge current at the cathode target consists of electron current Ie and ion current Ii , or ID = Ie + Ii = Ii (1 + γsee ),

(1.35)

where γsee is the secondary electron emission coefficient (see also Section 1.1.4). Note that γsee ∼ 0.05 – 0.2 for most metals (Depla et al., 2009), so at the target the dominating fraction of the discharge current is due to ions. The number of electron–ion pairs created by each secondary electron that is trapped in the target vicinity is then N≈

VD , Ec

(1.36)

where Ec is the energy loss per electron–ion pair created with the flow of secondary electrons into the plasma as the source of energy (Lieberman and Lichtenberg, 2005, Thornton and Penfold, 1978, Depla et al., 2010) and VD is the discharge voltage. As we discussed in Section 1.1.4, not all the secondary electrons are confined in the target vicinity. To account for the electrons that are not trapped, we define the effective secondary electron emission coefficient γsee,eff = me (1 − r)γsee , where e is the fraction of the electron energy that is used for ionization before being lost, m is a factor that accounts for secondary electrons ionizing in the sheath, and r is the recapture probability of secondary electrons. To sustain the discharge, the condition γsee,eff N = 1

(1.37)

28

High Power Impulse Magnetron Sputtering

has to be fulfilled. This defines the minimum voltage needed to sustain the discharge as VD,min =

Ec βγsee,eff

(1.38)

,

where β is the fraction of ions that return to the cathode. Equation (1.38) is often referred to as the Thornton equation. The basic assumption is that acceleration across the sheath is the main source of energy for the electrons (Thornton, 1978). Above breakdown, the parameters m, β, e , and r can vary with the applied discharge voltage, so we can rewrite the Thornton equation for any voltage as (Depla et al., 2009) 1 βme (1 − r) = γsee . VD Ec

(1.39)

Thus a plot of the inverse discharge voltage 1/VD against γsee should then give a straight line through the origin. Depla et al. (2009) measured the discharge dc voltage for a magnetron sputtering discharge with a 5 cm diameter target of 18 different target materials with argon as the working gas while keeping the working gas pressure and discharge current constant. Since all the data are taken in the same magnetron at the same discharge current and working gas pressure, the discharge parameters m, β, e , and Ec are independent of γsee . The authors plotted 1/VD against γsee for working gas pressure of 0.4 and 0.6 Pa and discharge currents 0.4 A and 0.6 A, that is, total of four cases. It was found that for all cases, a straight line indeed results, but that it does not pass through the origin. It has been proposed that the intercept is due to Ohmic heating, that is, the dissipation of locally deposited electric energy J · E to the charged particles that carry the current density J (Brenning et al., 2016). Then the inverse discharge voltage 1/VD can be written in the form of a generalized Thornton equation β H m(1 − r)(1 − δIR ) eC Ie /ID IR δIR 1 = e γ + see VD EcH EcC       a

(1.40)

b

or V1D = aγsee + b, where the intersect is associated with the Ohmic heating process. Here eC is the fraction of the electron energy for the plasma bulk electrons that is used for ionization before being lost from the discharge process, eH is the fraction of the electron energy for the hot secondary electrons that is used for ionization before being lost from the discharge process, and δIR = VIR /VD is the fraction of the total discharge voltage that is dropped across the dense plasma next to the target, the ionization region. Using this formulation, Ohmic heating is thereby the energy gain of an average electron moved across a fraction Ie /ID IR of the potential VIR . It follows that the fraction of the total ionization that is due to Ohmic heating can be obtained directly from the line fit parameters a and b of the measured 1/VD versus γsee . This

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29

Figure 1.8 The relative contributions to the total ionization ιtotal due to Ohmic heating ιOhmic and sheath energization ιsheath . The curves show Eq. (1.40) using a and b from the four combinations of pressure and discharge current in the dc magnetron studied by Depla et al. (2009). They are plotted in the same top-down order as the labels and are drawn solid only in the range of γsee , where they are supported by the measurements of Depla et al. (2009). A blue circle (black in print version) marks the value from the HiPIMS study by Huo et al. (2013). Reprinted from Brenning et al. (2016). ©IOP Publishing. Reproduced with permission. All rights reserved.

ratio can be written as a function of only the secondary electron yield γsee : b ιOhmic = . ιtotal aγsee + b

(1.41)

This relation is plotted in Fig. 1.8 for the four cases studied by Depla et al. (2009). We see that the contribution of Ohmic heating is in the range 30 – 70%, and its contribution decreases with increased secondary electron emission coefficient. Furthermore, Fig. 1.8 also shows a high-power example from an aluminum HiPIMS discharge operated at an argon pressure of 1.8 Pa modeled by Huo et al. (2013), marked by a circle. It is taken at the end of a 400 µs long pulse when the discharge was deep into the selfsputter mode (the current composition of this discharge is discussed in Section 7.1.1). A large fraction of Al+ ions here give an effective γsee close to zero (see also discussion in Section 7.1.1). Note that this HiPIMS case is perfectly consistent with the dcMS cases. The fraction of the discharge voltage that falls over the ionization region δIR =

VIR VD

(1.42)

can be estimated from b=

eC Ie /ID IR δIR . EcC

(1.43)

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High Power Impulse Magnetron Sputtering

Huo et al. (2013, 2017) assume that eC = 0.8, Ie /ID IR ≈ 0.5, and EcC = 53.5 V for Te = 3 V, which gives δIR = 0.15 – 0.19, or 15 – 19% of the applied discharge voltage falls over the ionization region. These results are supported by recent measurements, which have revealed strong electric fields parallel and perpendicular to the target of a dc magnetron sputtering discharge (Panjan and Anders, 2017). The measurements show that the potential can be as high as 30 – 70 V (δIR = 11 – 25%) in the region up to 20 mm over the race track area for a dcMS discharge operated at 270 V and 0.27 Pa. The largest E-fields (and potential drops) result from a double layer structure at the leading edge of an ionization zone. It is suggested that the double layer plays a crucial role in the energization of electrons since electrons can gain several tens of eV when crossing the double layer (Panjan and Anders, 2017). Electrons gain energy when they encounter an electric field, a potential gradient, such as the field in the double layer. The electron heating power Je · E is associated with an acceleration of electrons in the electric field—this electron energization in a double layer is Ohmic heating.

1.3 Magnetron sputtering configurations The planar magnetron sputtering discharge is well established for deposition of thin films of both metallic and dielectric materials. It is widely used in both laboratory settings (using small circular targets) and in industrial applications where the targets are often linear (rectangular and larger). The magnetron sputtering technique can be applied to a large variety of materials and is easily scalable to large areas. The coating uniformity can be in the range of a few percent even for cathodes in the range of meters. Over the past decades, magnetron sputtering has become an extremely important technology for thin film deposition in a wide range of industrial applications. These include metallization in integrated circuits (Hopwood, 1998, Rossnagel, 2008), coatings for wear resistance and corrosion protection (Kelly et al., 1996), large area coating of architectural glass (Nadel et al., 2003), and display applications (KrempelHesse et al., 2009). Depending on the application the applied target voltage can be constant (dc), radio frequency (Nowicki, 1977), or pulsed (Schiller et al., 1993). The configuration can be planar, cylindrical with axial magnetic field, or rotating around a fixed magnetic assembly (discussed in Section 1.3.2). Much of the early exploration of the magnetron sputtering phenomena was made on cylindrical magnetron configurations with axial magnetic field, which were either in the cylindrical post, with the inner cylinder as the cathode target, or the cylindrical-hollow (or inverted magnetron), with the outer cylinder as the cathode target (Thornton, 1978, Thornton and Penfold, 1978). The E × B drift paths go around the cylinder, either on the inside or the outside, depending on the configuration. These configurations are not in much use nowadays except for the hollow cathode magnetron configuration, which is well suited for coating wires or fibers that are allowed to pass through it.

Introduction to magnetron sputtering

31

Figure 1.9 A schematic of the magnetic design commonly used in magnetron sputtering discharges. The three cases, (A) all the field lines originate from the central magnet and pass into the annular magnet (Balanced), (B) all the field lines originate from the central magnet, with some not passing into the annular magnet (Unbalanced type I), and (C) all the field lines originate from the annular magnet, with some not passing into the cylindrical central magnet (Unbalanced type II).

1.3.1 Balanced and unbalanced magnetrons The conventional planar magnetron sputtering discharge is considered to be balanced if the magnetic fluxes through the pole faces of the outer poles and through the pole face of the inner pole are the same, as seen in Fig. 1.9A. If the condition is fulfilled, then the magnetic trap confines the plasma just in front of the cathode target. The substrate thus experiences very little impingement by ions. That is useful when depositing on, for example, heat sensitive substrates. To increase the ion flux to the substrate, an unbalanced magnetron was developed by Window and Savvides (Window and Savvides, 1986a,b, Savvides and Window, 1986). It is based on strengthening or weakening of the magnetic flux through one of the poles, which leads to an unbalance in the magnetic circuit. Window and Savvides (1986a) define two types of unbalancing. In type I, all field lines originate from the central magnet with some not passing into the annular magnet, as seen in Fig. 1.9B. For this case, the unbalanced field lines are directed toward the chamber walls leading to low plasma density in the substrate vicinity. In type II, all field lines originate from the annular magnet with some not passing into the central magnet, as seen in Fig. 1.9C. These unbalanced field lines extend into the substrate vicinity. Some of the secondary electrons can follow these magnetic field lines away from the target toward the substrate. Thus the plasma is not strongly confined to the cathode target region, but is allowed to flow out toward the substrate. This leads to a significant increase in the ion current density in the vicinity of the substrate (Savvides and Window, 1986, Sproul, 1998). As a consequence, the energy of the ions bombarding the substrate during film growth, can be tuned by a substrate bias.

1.3.2 Rotating magnetrons For a few decades, large cylindrical targets have been used for large area coatings. In this configuration the cathode target is a cylindrical tube, and the magnet assembly is installed inside the cylinder, as seen in Fig. 1.10. The rotatable cylindrical mag-

32

High Power Impulse Magnetron Sputtering

Figure 1.10 The rotating magnetron sputtering discharge. The cathode target is a tube that rotates around the fixed magnet assembly with a rotation frequency of roughly 1 Hz. After Wright and Beardow (1986).

netron target was originally proposed to alleviate the problem of low target utilization (Wright and Beardow, 1986). In this configuration the target life increases substantially, and the target utilization can be as high as 90%. The target can rotate during sputtering, so the target erodes uniformly as the target material is continuously exposed to the plasma zone resulting in a uniform erosion around 360◦ of the target surface. These rotatable targets are essential for deposition on large area glass for architectural and automotive applications and for the production of flat panel displays and photovoltaic solar cells (Blondeel et al., 2009). For large area coatings, using a typical inline coater, the substrate is moved relative to the magnetron cathode target in a linear fashion. In particular for reactive sputtering, the active region of the rotatable target is maintained free of dielectric layer build up due to continuous sputtering with rapid rotation speeds (20 rev/min) (Nadel et al., 2003). In the cylindrical configuration the target surface area may be in hundreds and up to tens of thousands of square cm. In some cases the magnet assembly is wobbled for improved layer thickness distribution (Krempel-Hesse et al., 2009).

1.4 Pulsed magnetron discharges Over the years, various modifications have been made to improve the magnetron sputtering technology (see e.g. the reviews by Kelly and Arnell (2000) and Sproul (1998)), in particular, to increase the flux of ions to the substrate and to allow deposition of dielectric films. We will first address the issue of depositing insulating thin films by pulsing the magnetron discharge.

Introduction to magnetron sputtering

33

1.4.1 Definition of pulsed magnetron sputtering discharges The pulsed magnetron sputtering (pMS) arrangement can be either asymmetric bipolar pulsed (Kelly et al., 2000, Sellers, 1998, Scholl, 1998) or a unipolar pulsed (Kouznetsov et al., 1999). The asymmetric bipolar pulsing was developed to address issues in reactive sputtering. These issues will be discussed in detail in Chapter 6. The parameter range is much wider in the pulsed system compared to conventional dc and radio frequency (rf) operation, and the pulsing of the discharge allows for a much greater flexibility due to additional control parameters, such as pulse width, duty cycle (the percentage of the time that the pulse is on), and pulse frequency. In reactive sputtering, the deposited film is a compound and redeposition on the target will therefore result in a different material compared to the composition of the bulk target. If the compound film is an insulator, such as Al2 O3 or Si3 N4 , then a capacitor is formed: The pure metal target (cathode) acts as one conductor, the plasma acts as the other conductor, and in between the insulating film forms the dielectric of the capacitor (Sellers, 1998). The metal cathode target is negatively charged, and a positive charge is collected on the plasma facing side of the insulator film. This is problematic for several reasons: (i) The dc current cannot flow through a capacitor. As we have seen in Section 1.2.4, the discharge current is mainly carried by positive ions impinging on the target. For the part of the target covered by the insulating compound, it means that very few ions will arrive at this zone and sputter the compound. Consequently, this area of the target is poisoned. (ii) The parasitic capacitor may not be able to charge up all the way to the applied discharge voltage. If this is the case, there will be a breakdown of the insulator, which will cause a sudden release of charge carriers. It forces the localized current density to increase into the arc discharge regime, which is not desired (see also Section 1.1.1). (iii) If the entire cathode is covered by the insulating film, then the discharge will be extinguished as soon as the insulator surface voltage drops below the voltage required to sustain the discharge (see also discussion on minimum voltage required in Section 1.2.4). One solution is to use pulsed magnetron sputtering and in particular bipolar pulsed discharges, so that the positive charge accumulated during the negative voltage pulse can be neutralized by electron bombardment during the positive voltage pulse. For this purpose, let us estimate the charging time of an insulator assuming a constant discharge current density J0 = 1 mA/cm2 . The capacitance can thereby be calculated as C = Q/V = (J0 At)/V , and therefore the charging time t = CV /(J0 A), where Q is the charge. Using a typical capacitance per area unit of C/A = 1 pF/cm2 and an applied voltage of V = 1000 V, we find that t = 1 µs. It is thereby possible to generate an almost continuous discharge with a pulse frequency of about 1 MHz. In practice, it is found that the charging time is somewhat longer, since the current is not constant and pulsing frequencies ≥ 100 kHz are typically sufficient (Chapman, 1980, pp. 10–11).

34

High Power Impulse Magnetron Sputtering

Alternatively, it is also possible to use rf magnetron sputtering (usually operated at 13.56 MHz) to deposit dielectric compound films. In particular, when depositing from thick electrically insulating (often compound) target materials, rf power is the only option. Rf sputtering can indeed produce high-quality insulating thin films, such as Al2 O3 from an insulating Al2 O3 target (Voigt and Sokolowski, 2004), but the deposition rates are in most cases considerably lower compared to pulsed magnetron sputtering. Rf power can also be used for reactive sputtering of Al to form aluminum oxide films. However, reactive deposition of aluminum oxide with rf power deposits only at 2 – 3% of the metal deposition rate (Sproul, 1998). Also, rf driven magnetron sputtering systems are complex and difficult to scale up for commercial applications.

1.4.2 Asymmetric bipolar mid-frequency pulsing The asymmetric bipolar dc sputtering discharge was developed to optimize the deposition of insulating films from conductive targets through reactive sputtering (Sellers, 1998). Pulsing the magnetron sputtering discharge in the medium frequency range (10 – 250 kHz) when depositing insulating films can significantly reduce the formation of arcs and, consequently, reduce the number of defects in the resulting film (Schiller et al., 1993). The duty cycle is typically 50 – 90% (Kelly and Bradley, 2009). Asymmetric bipolar pulsing, during which reversed voltage pulses of about +50 to +150 V (or roughly 10 – 20% of the negative voltage amplitude) are added to the normal dc waveform (see Fig. 1.11), are often claimed to be the optimum solution to the target poisoning problem (Sellers, 1998). The reason is that this technique allows for preferential sputtering of the insulating layer that forms on the target surface during reactive sputtering. The working mechanism has been described by Sellers (1998) and can be summarized as follows: First, a typical discharge voltage of VD = −400 V is applied to the cathode target (Fig. 1.11A), which leads to positive ions being accelerated toward the target sputtering predominantly the metal target. The insulating area will not be significantly sputtered, since it collects low-energy ions and the top surface of the dielectric of this parasitic capacitor is thereby charged towards +400 V and thus limits ion acceleration and ultimately reduces the sputter yield. Next, the polarity is rapidly reversed to about +100 V (Fig. 1.11B), which leads to the plasma-facing surface of the insulator to be charged toward −100 V. As the pulse once more reverses and the metal target reaches −400 V, the parasitic capacitor is charged to −100 V (Fig. 1.11C). The effective voltage on the plasma-facing side of the insulator is now −500 V, and consequently the positive ions can sputter this region with an increased energy compared to the pure metal target and thereby reduce the compound fraction of the target. This process requires a high enough frequency to avoid build-up of voltage on the compound covered regions to not cause a breakdown of these parasitic capacitors (Schiller et al., 1993), as described in item (ii) in Section 1.4.1. In practice, this means pulsing frequencies around 100 kHz, which is also in line with our previous frequency estimation concerning sustaining the discharge. Deposition rates during pMS approach those obtained for the deposition of pure metal films. In the case of Al2 O3 , Sproul (1998) reports deposition rates as high as

Introduction to magnetron sputtering

35

Figure 1.11 Preferential sputtering by asymmetric bipolar sputtering. Three points in time are displayed: (A) normal sputter mode, (B) reversal mode, and (C) return to sputter mode. After Sellers (1998).

78% of the metal deposition rate. Note that this technology has also been applied to deposit resistive (not insulating) thin films. In these cases, slightly higher frequencies have been required due to the tendency of the resistive film to self-discharge (Sellers, 1998). Asymmetric bipolar pulsing has rather complex process characteristics due to the steep transients in the voltage waveforms on the µs scale in combination with the spatial variations in the magnetic field and plasma density (Kelly and Bradley, 2009). For ease of comparison with HiPIMS, covered in detail in this book, here we summarize some of the most striking features. From Langmuir probe studies (Glocker, 1993, Bradley et al., 2001) it has been established that the time averaged effective electron temperature Teff is generally greater in pMS compared to dcMS. This is also seen in Fig. 1.12A, where Teff is increased by 33% compared to dcMS values by pulsing at 100 kHz. It is believed that the increased electron heating is due to the pulsed nature of the discharge. The mechanism is likely stochastic heating by the advancing sheathedge during pulse-on, which may then heat the plasma globally through subsequent collisions (Glocker, 1993). In addition, Bradley et al. (2001) show that there is a burst of hot or beam-like electrons shortly after the initiation of the negative voltage pulse, which is probably due to electrons coming directly from the target and accelerated in the cathode sheath. Pulsing the discharge also increases the time-averaged electron density. One example is shown in Fig. 1.12B, where there is an 18% increase at 100 kHz compared to dcMS, although values of up to 300% increase have been reported (Glocker, 1993). However, electron density on the order of ne ∼ 1016 m−3 is too low for the pMS discharge to generate a substantial fraction of ionized sputtered material. As an example, we can estimate the degree of ionization of sputtered Ti by looking at the probability for the sputtered neutral to undergo electron impact ionization when traveling a distance z (see e.g. Lundin et al. (2015) and Gudmundsson (2010) for details on the calculations). By taking the time-averaged values Te = 4.5 eV and ne = 8.4 × 1015 m−3 for the pMS discharge characterized in Fig. 1.12, an estimated velocity of the neutral of 500 m/s, and a typical distance of z = 0.05 cm, we find an ionization probability of

36

High Power Impulse Magnetron Sputtering

Figure 1.12 Langmuir probe measurements at a typical substrate position in pMS. The variation of the electron density ne , and the effective electron temperature Teff during one pMS cycle at 100 kHz pulse frequency. The solid red line (gray in print version) represents the average value from the time-resolved measurements and the dashed blue line (black in print version) represents the value measured in a dcMS equivalent discharge. Data taken from Bradley et al. (2001).

< 1% with a corresponding electron impact ionization mean free path of about 25 cm. It is therefore concluded that although the pMS technique is suitable for reactive sputtering of insulating or poorly conductive thin films, it does not lead to a significant increase of the flux of ions of the sputtered material to the substrate, although some increase of the substrate bias current compared to dcMS has been reported (Kelly and Arnell, 2000).

1.4.3 Magnetron sputtering with a secondary discharge For many applications, a high degree of ionization of the sputtered material is desired as the ion flux to the substrate is known to have a significant influence on the overall quality of the resulting film (Rossnagel and Cuomo, 1988, Colligon, 1995). Furthermore, the energy of the ions bombarding the substrate can be controlled. Over

Introduction to magnetron sputtering

37

the past three decades, there has been significant progress in enhancing the level of ionization in the magnetron sputtering discharge. This was initially achieved by the application of a secondary discharge to a conventional magnetron sputtering discharge (Gudmundsson, 2008), either an inductively coupled plasma assisted magnetron sputtering (ICP-MS) (Yamashita, 1989, Rossnagel and Hopwood, 1994, 1993, Joo, 2000, Hopwood, 2000a) or a microwave amplified magnetron sputtering source (Gorbatkin et al., 1996, Musil et al., 1991, Yoshida, 1992, Xu et al., 2001, Wendt, 2000, Holber, 2000). The secondary discharge typically creates a plasma with an electron density in the range of 1017 – 1018 m−3 and with electron temperatures in the range of 1.5 – 4.5 V (Hopwood and Qian, 1995, Yamashita et al., 1999), which correspond to an electron impact ionization mean free path for the sputtered vapor of a few cm (Hopwood, 1998, Gudmundsson, 2010). The ICP-MS discharge is currently widely used in the semiconductor industry for deposition of metal and compound lines, pads, vias, and contacts (Rossnagel, 2008). The combination of magnetron sputtering and secondary high-density discharges have also been applied to demonstrate a collisionless deposition process at pressures below 0.1 Pa (Yoshida, 1992, Musil, 1998). When the deposition flux consists of more ions than neutrals, that is, M+ > M , then the process is referred to as ionized physical vapor deposition (IPVD) (Hopwood, 2000b). Common to all the IPVD techniques is a very high electron density, typically, ≥ 1018 m−3 . The plasma density increases with increased power density supplied to the target. However, an increased power density leads to overheating and eventually melting of the sputter target. Thus, there is an upper limit to the power that can be delivered through the discharge cathode target.

1.4.4 High power impulse magnetron sputtering More recently, a method using high power pulsed magnetron sputtering discharges in unipolar mode, referred to as high power impulse magnetron sputtering (HiPIMS), or less often as high power pulsed magnetron sputtering (HPPMS), has been proposed as one solution to stay below the power limit for target/magnetron damage, while at the same time achieving a highly ionized flux of the sputtered material (Kouznetsov et al., 1999, Macák et al., 2000). In HiPIMS, this is accomplished using pulsed plasma discharges with a peak power density in the range 0.5 – 10 kW/cm2 (averaged over the target surface) at a low duty cycle in the range of 0.5 – 5%, that is, considerably lower than the 50 – 90% in pMS discussed in Section 1.4.2. The HiPIMS discharge operates by applying square voltage pulses of about 500 – 2000 V, which generates peak current densities of up to 3 – 5 A/cm2 (Helmersson et al., 2006, Gudmundsson et al., 2012). The pulse length is in the range 20 – 500 µs, but typically 30 – 100 µs when depositing thin films, with a repetition frequency of 50 – 5000 Hz. To distinguish this technique from other pulsed magnetron processes, Anders (2011) defines HiPIMS as pulsed magnetron sputtering where the peak power exceeds the time-averaged power by typically two orders of magnitude. Through the use of very high applied instantaneous power densities to the magnetron cathode target, there is a significant increase in the charge carrier density in front of the target during the HiPIMS pulse. In numbers, this means that for the

38

High Power Impulse Magnetron Sputtering

HiPIMS discharge, the electron density in the ionization region close to the target surface is on the order of 1018 – 1019 m−3 (Gudmundsson et al., 2001, 2002), which corresponds to an electron impact ionization mean free path for a sputtered metal atom on the order of 1 cm or less (Gudmundsson, 2010). The characteristics and technical aspects of HiPIMS will be thoroughly dealt with in the following chapters of this book. However, let us first compare this method to the previously discussed dcMS and pMS techniques as well as other HiPIMS-type solutions. First, we already here stress that a few variations of high power pulsed magnetron sputtering discharges exist. For example, superposition of dcMS and HiPIMS discharges (on one magnetron target) have been reported by several authors (Mozgrin, 1994, Samuelsson et al., 2012, Ganciu et al., 2005, Bandorf et al., 2007) and is sometimes referred to as pre-ionized HiPIMS (Va˘sina et al., 2007). Also, decomposition of a single HiPIMS pulse into several individual pulses to produce a pulse sequence (pulse train) has been investigated to some extent (Barker et al., 2013, 2014, Antonin et al., 2015). For this configuration, there is a delay of a few tens of µs between each individual pulse while maintaining typical HiPIMS off-times of milliseconds between each sequence. These and various other hybrid configurations will be described in more detail in Chapter 2.

1.4.5 Modulated pulse power magnetron sputtering Another approach to high power pulsed magnetron sputtering consists of modulating the pulse such that in an initial stage (a few hundred microseconds) the power level is moderate (similar to dcMS levels), followed by a high power pulse (lasting a few hundred microseconds up to a millisecond). Such type of longer pulses are referred to as modulated pulse power magnetron sputtering (MPPMS). This method uses longer pulses, with pulse widths up to 3 ms, at similar repetition rates as found in HiPIMS, that is, in the range between several 10 Hz and a few 100 Hz, leading to duty cycles well above 5% (up to 25%). The main feature of this approach is the superimposition of a macropulse, that is, the previously described longer voltage pulse, with a train of shorter micropulses with frequencies in the range of several 10 kHz. The “on”- and “off”-time of these micropulses, which are typically up to several 10 µs, as well as their frequency can be altered within the macropulses. Using this approach, varying the micropulse frequency and the “on”- and “off”-times arbitrarily, target voltage and discharge current waveforms can be created (Chistyakov and Abraham, 2009, Liebig et al., 2011). Although this technique is not the focus of the book, we will still compare the HiPIMS/MPPMS plasma process characteristics in Chapter 3.

1.4.6 Summary As a summary, we present an overview of the discussed magnetron sputtering discharges in terms of duty cycle versus peak power density (at the target) pt illustrated in Fig. 1.13. We take pt = 0.05 kW/cm2 to be a typical upper limit for a dcMS discharge before target damage sets in. Pulsed magnetron sputtering discharges operated below

Introduction to magnetron sputtering

39

Figure 1.13 An overview of dc and pulsed magnetron sputtering discharges based on the peak power density at the target pt , and the duty cycle. Reprinted with permission from Gudmundsson et al. (2012). Copyright 2012, American Vacuum Society.

this limit are referred to as pulsed dcMS and include the bipolar asymmetrically pulsed discharges, discussed in Section 1.4.2. For all discharges operating above the dcMS limit, the higher peak power must be compensated for by a lower duty cycle. Pulsed magnetron sputtering discharges that are operated at the higher peak power with a low duty cycle are referred to as high power pulsed magnetron sputtering (HPPMS) discharges. For square-shaped pulses, this gives the power density limit line shown in Fig. 1.13. The HiPIMS range in peak power density is defined to lie above a HiPIMS limit of pt > 0.5 kW/cm2 . MPPMS pulses typically begin at a low power level, often in the dcMS range, followed by a stronger pulse of intermediate power density (0.05 < pt < 0.5 kW/cm2 ) or even into the HiPIMS range. These definitions are used throughout the book.

References Abroyan, I.A., Eremeev, M.A., Petrov, N.N., 1967. Excitation of electrons in solids by relatively slow atomic particles. Physics–Uspekhi 10 (3), 332–367. Anders, A., 2008. Self-sputtering runaway in high power impulse magnetron sputtering: the role of secondary electrons and multiply charged metal ions. Applied Physics Letters 92 (20), 201501. Anders, A., 2010. Deposition rates of high power impulse magnetron sputtering: physics and economics. Journal of Vacuum Science and Technology A 28 (4), 783–790. Anders, A., 2011. Discharge physics of high power impulse magnetron sputtering. Surface and Coatings Technology 205, S1–S9.

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Ziegler, J.F., Biersack, J.P., Ziegler, M.D., 2008. SRIM – The Stopping and Range of Ions in Matter. SRIM Co., Chester, Maryland. Ziegler, J.F., Ziegler, M.D., Biersack, J.P., 2010. SRIM – the stopping and range of ions in matter (2010). Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 268 (11–12), 1818–1823.