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Chemical vapor deposition in high-density low-pressure plasmas: reactor scale-up and performance
Thin Solid Films, 241 (1994) 240 246
240
Chemical vapor deposition in high-density low-pressure plasmas: reactor scale-up and performance J. Pelletier and T. Lagarde Laboratoire de Physique et Chimie des ProcOd~s Plasma, Universitb Joseph Fourier (Grenoble 1), Unitk CNRS, CNET-CNS, France-Telecom, BP 98, 38243 Meylan Cedex (France)
Abstract
In most applications, plasma enhanced chemical vapor deposition (CVD) of high-quality materials requires control of process parameters over a wide range. In particular, high-density low-pressure plasmas capable of producing significant ion fluxes compared to neutral fluxes, and independent substrate biasing which allows accurate adjustment of the ion bombardment energy may lead to high-performance deposited layers. Results in low-temperature Si epitaxy, SiO2, Cu and W deposition are presented which illustrate the capabilities of such plasmas. However, deposition rate and film uniformity remain the key requirements in many industrial fields, including the microelectronics industry with the processing of flat panels and increase in wafer diameters. In fact, the limiting factor of many industrial applications lies in the difficulty of producing large (up to square meters) uniform, dense plasmas. In the conventional cylindrical configuration of plasma reactors, the ability to process large-dimension substrates is dependent on the generation of large volumes of plasma. Unfortunately, due to ion volume recombination, the production of large volumes of uniform dense plasma must be ruled out. Clearly, treatment of a planar substrate with a uniform plasma source is more suitable. The uniform distributed electron cyclotron resonance (UDECR), in which linear microwave applicators can sustain constant amplitude standing waves along a multipolar magnetic structure, can be used to make large planar plasma sources. The performance of such a plasma is presented in terms of uniformity and density, allowing the process optimized in laboratory reactors to be scaled-up to much larger reactors without any alteration in the characteristics.
1. Introduction
Plasma processing of high-performance materials is becoming a critical technology not only in the electronics industry but also in the aerospace, automotive, steel, biomedical, textile, optics and paper industries. Due to the diversity of applications, plasma processing will in the near future have to cover a broad range of geometries, dimensions, chemical systems, electromagnetic designs and plasma-surface interactions. In addition, at the industrial level, plasma processing of materials usually imposes two important requirements: uniformity of the surface treatment and real-time, in situ monitoring for control and analysis. In this situation, the technological challenge lies firstly in developing surface processes able to meet the required specifications, especially in terms of quality, uniformity, processing rate and reproducibility; and secondly in the possibility of transfering processes from one reactor to another, or more generally, to scale processes from a small to a large reactor. The main advantage of plasma processing over many other dry processing techniques lies in the generation, firstly, of active neutral species (reactive atoms, radicals) which allows processing at ambient or even cryogenic temperatures; and secondly of ions which may be used for the energetic bombardment of surfaces. Ion born-
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bardment, which ensures continuous cleaning of the surface during the process, makes it highly reproducible. It can also dramatically influence the properties of the surface layer (morphology, crystallography, compositon, adhesion, surface roughness, stress etc.). Therefore, compared with other conventional dry surface processing techniques, plasma provides, through the plasmasurface interaction, the additional degrees of freedom necessary to reach the strictest process specifications. Therefore, in high-density low-pressure plasmas capable of producing significant ion fluxes with respect to neutral fluxes, independent substrate biasing [1], which allows accurate adjustment of the ion bombardment energy, may lead to high-performance processes: anisotropic and selective etching, low-temperature Si epitaxy and doping, planarizing deposition of SiO2 layers, deposition of high-performance metal and alloy materials, etc. The aim of this paper is to emphasize, using a few selected examples, the necessity of ion bombardment and the desirability of ion energy control in many processes and especially in the preparation of thin films by plasma enhanced CVD (PECVD). In particular, we discuss how to control accurately and independently the various parameters of the plasma-surface interaction. Then, we examine the limiting factors of the production
© 1994
Elsevier Sequoia. All rights reserved
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
of large uniform low-pressure high-density plasmas. We also demonstrate the interest of using uniform planar plasma sources for the treatment of planar substrates. Next, the principle of the UDECR, which can be used to make large planar plasma sources, is described. The performance of such a plasma is presented in terms of density and uniformity, and the future prospects of the technique are discussed.
2. Preparation of thin films by CVD in high-density low-pressure plasmas 2.1. Control of process parameters In most surface treatment applications, the main parameters that characterize the plasma-surface interaction besides surface temperature, are the flux of reactive species and the flux and energy of ion bombardment. The production rates of ions and reactive species increase with power density in the plasma. Since the concentration of reactive species largely depends on ion-molecule reactions occurring in the gas phase, one can selectively control the concentration of reactive species by varying independently the partial pressure of the reactive parent gases. When possible, the ion bombardment energy can also be adjusted independently of plasma generation by biasing the substrate surface with respect to the plasma potential: when the substrate is a conductor, it can be biased by simply applying a d.c. voltage to it; when the substrate is an insulator, it can be biased only by applying a periodic (r.f.) voltage through a low-impedance capacitor [1]. The substrate surface can thus be charged by capacitive coupling. In the following examples, thin films are deposited by PECVD in high-density low-pressure plasmas, allowing the separate control of all the process parameters of the plasma-surface interaction. However, at low pressure, a resonant excitation is generally required to sustain plasmas. Microwave excitation by electron cyclotron resonance (ECR), which can easily produce high-density plasmas at low pressure, can thus deliver ion to neutral flux ratios two or three orders of magnitude higher than conventional r.f. discharges. Finally, at 2.45 GHz, the excitation frequency is much higher than the ion plasma frequency, so that ions cannot be accelerated in the electric field which generates the plasma, thus allowing complete separation between plasma generation and plasma/surface interaction. Hence the interest in ECR excited plasmas for the parametric study of PECVD processes in high-density low-pressure plasmas. 2.2. SiBcon epitaxy and doping Silicon epitaxy in silane plasmas is possible down to a substrate temperature of 300 °C, but epitaxy of ac-
241
ceptable quality requires at least 600 °C, i.e. 200 K lower than in CVD [2]. Hydrogen desorption from the silicon surface by low-energy ion impact appears to be the rate determining step. Introduction of phosphine, diborane or arsenic in the silane plasma produces the intentional doping of the epitaxial silicon. However, doping by arsenic [2] requires the assistance of an ion bombardment energy of a few tens of eV: because of their larger size, arsenic atoms are not easily incorporated into the silicon lattice and tend to remain at the surface during the layer growth. Clearly, ion bombardment forces arsenic atoms to penetrate into the silicon lattice.
2.3. Planarizing Si02 deposition Deposition of silicon dioxide is an important step in the fabrication of semiconductor devices. Present trends in deposited SiO2 films are towards lower deposition temperatures and better control of the quality of the oxides [3-5]. In particular, the characteristics of the oxides deposited without intentional heating from SiH4/O2 mixtures in high-density low-pressure plasmas have been shown to be strongly dependent on ion bombardment energy. For ion energies higher than 50 eV, many of the film characteristics (refractive index, infra-red spectrum, wet etch rate, fixed charge density) are close to those of thermal SiO2 [3-5]. However, in the fabrication of submicron circuits, the challenge is now to deposit SiO2 using a planarizing process, i.e. conforming to the surface topography with void-free filling of high aspect ratio trenches. Such an objective cannot be achieved successfully without simultaneous sputter etching during SiO2 deposition [3-6]. Therefore, without bias, one cannot fill vias with an aspect ratio larger than 0.6. With a 200 V bias voltage, vias with aspect ratios up to 2 can be filled successfully [31. 2.4. Low-resistivity Cu thin films Owing to its low-bulk resistivity (1.7 ktf~cm), copper is one of the most attractive materials for interconnections in sub-halfmicron circuit fabrication. Copper films are usually prepared by evaporation or sputtering. However, CVD and PECVD processes exhibit intrinsic advantages over physical deposition methods, especially in terms of conformal step coverage and fine-grained structure. Copper-containing films have been produced by PECVD in a conventional r.f. discharge from copper (II) hexafluoracetylacetonate with significant incorporation of fluorine in the film [7]. More recently, pure copper films have been prepared at ambient temperature by ECR microwave plasma-enhanced CVD from the copper (II) acetylacetonate-argon-hydrogen system. The bias voltage applied to the substrate and the
242
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
gas phase composition appeared to be critical deposition parameters. Oxygen was never detected in the deposited material owing to the presence of carbon and hydrogen in the copper complex. However, since the spontaneous reaction rate of excess carbon with hydrogen is negligible at room temperature, carbon residues in the films can be removed only by ion induced etching. Thus, with the addition of hydrogen to the plasma (excess hydrogen) and an ion bombardment energy of 50eV, pure copper films with low resistivities (23 ~t~ cm) can be deposited at ambient temperature [8]. 2.5. Low-resistivity W thin films As in the case of copper, PEVD of tungsten films in high-pressure plasmas generally leads to the deposition of the high-resistivity fl-tungsten phase [9]. The presence of this fl-phase, usually observed in low-temperature PECVD [10], is attributed to the incorporation of impurities such as oxygen, fluorine or silicon [9]. In high-density low-pressure plasmas, the intensity of the ion bombardment enhances the desorption of reaction products, i.e. HF and/or SiF4, so that tungsten films with much lox~er resistivities can be deposited. However, without intentional substrate heating, the resistivity of the films strongly depends on plasma conditions [11]. In contrast, at 300 °C, resistivity values between 7 and 9 ~tff~cm are obtained which do not significantly depend on plasma conditions [11]. Clearly, above 300 °C HF desorption is controlled mainly by temperature. 2.6. Remarks The above examples illustrate the different possibilities offered by an intense ion bombardment of the surface and demonstrate the necessity of an accurate and independent control of the ion energy. More generally, the quality of the deposited thin films is largely dependent on all the process parameters defined in Section 2.1. However, all the experiments described above have been carried out in small cylindrical chambers intended to treat substrates not larger than 100 mm in diameter. It is clear that larger substrates will require a scale-up solution for uniform treatment by dense low-pressure plasmas.
3. Plasma homogeneity: theoretical aspects and limitations The aim of this section is to analyze, based on the simplest assumptions, the possibilities of obtaining homogeneous plasmas and, further, to identify the conditions required to achieve a uniform interaction between plasma and surfaces immersed in it. A uniform inter-
action with a plane surface, or with a surface that can be assimilated to a plane surface (curvature radius ~> Debye length) can be obtained under any conditions when the plasma is homogeneous, or when the plasma is unhomogeneous provided that the plasma flux at the sheath edge is uniform. The real question is thus to determine how to produce homogeneous plasmas or uniform fluxes of plasma in a finite-size reactor chamber. 3.1. Homogeneity in bounded plasmas First we consider a discharge with an ion mean free path l~,1 for ion-neutral collisions shorter than the dimensions of the chamber (collisional plasma). Under steady-state conditions and in the absence of ion volume recombination, the plasma can be described by an ambipolar diffusion model with the following equations [12] ~ . nv = s,
(1)
nv = - D a V n
(2)
where n is the plasma density, v~ the plasma directed vleocity, D a the scalar ambipolar diffusion coefficient, and Si the ionization rate. Equations (1) and (2) mean that the plasma produced in a given volume is evacuated outside this volume [eqn. (1)] by diffusion under the influence of density gradients [eqn. (2)]. A homogeneous plasma, defined by X~n = 0
(3)
requires that (eqn. (2)) v~= 0
(4)
This can be achieved (eqn. (1)) only if the ionization rate is negligible in the volume of the plasma, i.e. Si = 0
(5)
This situation can be illustrated by a plasma expanding into a volume that is completely enclosed by a peripheral plasma source of uniform density (Fig. l(a)). However, the introduction of a substrate into such a plasma
@@ n(R)=n0
(a)
n(R)=n0
(b)
Fig. 1. Sketches showing a spherical diffusion plasma generated at the periphery r = R with a density n(R) = no: (a) diffusion in a free volume; (b) case where a concentric spherical object is introduced.
J. Pelletier, T. Lagarde / CVD & high-density low-pressure plasmas
(Fig. l(b)) perturbs the plasma and the homogeneity is lost, except in the two limiting cases of small and large objects compared with the plasma mean free path and to the volume of the plasma respectively [12].
3.2. Plasma homogenity through multipolar magnetic field confinement The fact that plasmas confined by multipolar magnetic fields are homogeneous was qualitatively demonstrated for the first time by Limpaecher and MacKenzie [13]. They also observed a gradual improvement in the homogeneity of an argon plasma as a function of decreasing pressure. More recently, Gauthereau and Matthieussent [14] have measured the radial density profiles in a 1 m diameter, 1 m long multipolar discharge in argon for three gas pressures, namely 5 x 10 - 4 , 10 . 3 and 8 x l 0 - 3 Torr, which correspond to primary electron mean free paths )~pi for ionization of 2.3m, 1.15m and 0.15m respectively [15]. On the other hand, the ion mean free path )~in is less than 10cm throughout the pressure range investigated, so that the plasma can be considered collisional. At the lowest pressure (5 x 10 . 4 torr), i.e. in the collisionless regime for primary electrons, a perfect plasma homogeneity is achieved in the volume of the discharge which is free from magnetic field. In contrast, at the highest pressure (8 x 10 . 3 Torr), i.e. in the collisional regime for primary electrons, the parabolic plasma density profile commonly obtained in non-magnetically confined discharges is recovered. The transition from a homogeneous plasma to a plasma with a parabolic density profile occurs at an argon pressure of the order of 10 .3 Torr, i.e. at the onset of the collisional regime (kpi ~ 2R) for primary electrons. Such experimental results demonstrate the major role played by primary electrons in achieving plasma homogeneity in a multipolar discharge. Thus, bearing in mind that primary electrons produce the plasma and considering that the assumption of a significant ionization rate over the whole plasma volume does not lead to a homogeneous plasma, we must assume the localization of ionization in the peripheral region near the wall of the discharge (see Fig. l(a)). Obviously, a preliminary condition for such a selective peripheral ionization in a discharge confined by multipolar magnetic fields is a mean free path lpi of primary electrons for ionizaton of neutrals larger than the dimensions of the discharge chamber. If this were not the case, the electrons would undergo several inelastic collisions and degrade into plasma electrons before reaching the magnetic sheath and interacting with it. The first mechanism that provides selective peripheral ionization is the reflection of primary electrons in the magnetic sheath: when a flux of electrons reflects on a
243
magnetic mirror, i.e. in a magnetic field gradient, the drift velocity of each electron decreases and then vanishes at the so-called mirror-point [16]. The increase in peripheral ionizaton as compared with volume ionization in the magnetic field free region thus results from the long time spent by the electrons in the magnetic sheath. The second mechanism for peripheral ionization resuits from the irreversible trapping of primary electrons [16] in the multipolar magnetic field, either via elastic collisions of primary electrons with neutrals or via the interaction with electric fields due to plasma instabilities [17]. Then, once trapped, primary electrons oscillate indefinitely between two successive magnetic poles until they suffer inelastic ionizing collisions.
3.3. Limits of plasma confinement In argon plasmas, and more generally in atomic gas plasmas (He, Ne, Hg, Cs, etc.) where ion volume recombination can be assumed negligible [12], the best approach to the problem of plasma homogeneity is to localize plasma generation at the periphery of the reactor. However, in molecular (reactive) gas plasmas, volume recombination of positive ions with electrons (dissociative recombination) or negative ions (mutual neutralization) becomes a significant drawback for homogeneity in high-density plasmas [12]. These two efficient ion recombination mechanisms have rate coefficients ~r of the order of 10 -7 cm 3 s -i. Thus, in order to estimate the importance of the inhomogeneity effects introduced by ion volume recombination, we can calculated the density profile for a plasma assumed to be homogeneous (Si = 0) in the absence of ion recombination. Equation (1) becomes V ' / ~ = --Sr
(6)
where Sr is the volume recombination rate (7)
Sr = ~r n2
Equations (1) and (2) lead to W n = ~r n2
(8)
For the sake of simplicity, we turn now to the one-dimensional case where a plane source (located at z = 0) produces a uniform plasma n(0) = no
(9)
The corresponding spatial density profile as a function of distance z from the plane source is obtained by integration of eqn. (8), and we have , (noO~r"~ '/2 7 -2
n = n o 1-t-/7-A---~/\oua/ z j
(10)
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
244
This equation shows the spatial inhomogeneity introduced by the rate coefficient ~r. Plasma density is decreased by a factor of 4 at a distance ZI]4
= ( 6 D a ~ '/2 \n0~r/
(11)
For example, taking ~r = 1 0 - 7 cm3 S I and D a = 6 × 105 cm 2 s -~, we obtain z~/4= 60cm for no = 10~°cm-3, and zj/4 = 6 cm for no = 1012 c m -3. This result clearly shows that homogeneity of molecular gas plasmas degrades rapidly with increasing plasma density. Hence, homogeneity cannot be ensured when scaling-up highdensity molecular gas plasmas. Since, in actual cylindrical configurations, plasma inhomogeneity increases with diameter, the plasma processing of large-diameter plane substrates requires alternative configurations, such as the plane configuration described below.
3.4. The plane configuration: an ideal scale-up solution As far as achieving a uniform plasma-surface interaction is concerned, the infinite plane configuration where plane substrates are placed parallel to a plane uniform plasma source is undeniably an ideal configuration: each point of the substrate surface with respect to the plane source has the same geometrical factor, i.e. identical solid angle and distance. Therefore, volume recombination cannot affect the uniformity of the plasma-surface interaction. Furthermore, since plasma density decreases as a function of distance from the source, obviously the highest plasma fluxes are obtained at the shortest distance from the source. Therefore, besides providing uniformity, the plane configuration leads to the highest possible substrate to reactor wall surface ratio, minimizing substrate contamination. In addition, the plasma volume to sample surface ratio in a plane configuration, which can be compared with the interelectrode distance in r.f. capacitive discharges, results in the highest efficiency in terms of pumping kinetics. However, as infinite plasma sources are not conceivable, we must consider the more realistic situation of a plane source of finite dimensions. As the spatial influence of the plasma source edge on the uniformity of plasma processing is controlled mainly by the characteristic decay length of the plasma density (eqn. (11)), the edge effects are considerably reduced at the highest plasma densities. Therefore, the real challenge in this domain lies in producing uniform high-density planar plasma sources.
4. DECR plasmas We have seen that, in the absence of ion volume recombination, peripheral excitation favors plasma homogeneity. Thus, using a multipolar magnetic field
confinement, this could be easily achieved by accelerating the electrons trapped in the magnetic sheath. At low gas pressures, due to the necessity to use an efficient plasma excitation, an attractive solution is to transfer HF energy to electrons via electron cyclotron resonance within the multipolar magnetic field. This is the goal achieved in the so-called DECR [18-20]
4.1. Concept of DECR plasmas The development of DECR excitation is based on two ideas: (i) the permanent magnet rows destined for multipolar magnetic confinement are further used to provide the magnetic field intensity Bc needed for ECR coupling Bc = 2z m,.f~
(12)
e
where fc is the frequency of microwaves and e and m,. are the elementary charge and electron mass respectively; (ii) the microwave field required for ECR is provided by linear antennas running along and close to the magnet rows. This scheme results in the generation of the plasma at the periphery of the chamber where the so-called primary electrons (accelerated through ECR) are trapped between the magnetic field cusps (magnetic poles) until they ionize the gas. Figure 2 is a schematic representation of one of the microwave field applicators in a cylindrically shaped DECR discharge chamber. It consists of a linear conductor of cylindrical cross-section, called the antenna, placed a few millimeters above the ground-plane (reactor wall). Permanent magnets are arranged in a similar way as for classical multipolar magnetic field confinement. These magnets provide the required 875 gauss (at an excitation frequency of 2.45 GHz) isomagnetic surface in the vicinity of the antenna along its entire length. The magnetic field is closed by two adjacent magnet rows as in a conventional multipolar field. A magnetic field configuration is thus created whereby microwave field, applied at 2.45 GHz through a coaxial feedthrough, results in ECR coupling along the full
GROUND PLANE PERMANENT MAGNET. _ ~ m = ~ ~
LOBE ANTENNA
:ii,iiiiil;i!i!ii
REGION
Fig. 2. Schematic of DECR excitation.
PLASMA
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
antenna length. Within the pressure range extending from 0.1 to 3 mTorr, the gas is ionized by the primary electrons in the lobes (see Fig. 2) and the plasma produced then diffuses towards the central part of the chamber. In the central magnetic field free part of the chamber, a cold d.c. diffusion plasma is thus obtained, free of high-energy electrons (which remain trapped in the multipolar magnetic field) and whose floating potential is close to the ground potential (reactor wall potential). With argon, such uniform plasmas are currently produced in cylindrical reactors [19], with ion densities exceeding 10II cm -3.
245
constant amplitude standing wave along the linear applicator. To this end, the microwave power sent to the antenna must be large enough for the wave to be reflected at the extremity of the structure, yielding a standing wave pattern. However, with DECR, the conditions of constant amplitude wave pattern can be obtained, for given antenna length and microwave input power, only at a definite gas pressure [20, 21]. None the less, under this unique condition, a linear (up to 2 m) plasma with a uniformity better than a few per cent can be achieved [21].
5. From DECR to uniform DECR plasmas
4.2. Limitations in DECR plasmas A crucial advantage of DECR excitation over all other plasma production techniques is that DECR allows perfect fitting of the plasma source term to the reactor dimensions. However, well-suited for the processing of small substrate diameters, the cylindrical geometry, inadequate for uniform plasma processing over large areas, must be replaced by a planar DECR configuration (see Section 3.4). Such a possible design, which uses a set of parallel linear antennas, is represented in Fig. 3. But the possibility of achieving large areas of uniform plasma assumes that each single linear applicator can generate a uniform plasma along it. In the initial version of the DECR technique (Fig. 2), microwaves propagate mainly in the region of E C R coupling, so that in the presence of plasma, microwaves cannot propagate along the antennas without absorption. Damping of travelling waves along the antennas thus leads to non-uniform plasmas. A possible solution to produce uniform plasmas consists in sustaining a
PLASMA
5.1. Concept of uniform DECR plasmas In order to achieve a constant amplitude wave pattern under any conditions, one possible answer consists in the spatial separation of the propagation region from the absorption region of ECR coupling. In the new DECR design [22], so-called uniform DECR (UDECR), the antennas are set between the magnets of the multipolar magnetic field configuration (Fig. 3), outside the ECR regions but within the regions free from plasma [15] located between the rector wall and specific field lines linking the successive magnet poles. With this UDECR configuration, a standing wave pattern of constant ampliltude is expected. However, this means that the microwave power transferred to electrons varies periodically along the antenna. In fact, since primary electrons responsible for plasma ionization undergo a magnetic drift motion parallel to the magnet rows [16], plasma production along the antennas may remain perfectly uniform, provided the amplitude of the standing wave is constant. Therefore, using UDECR, uniform plasmas are obtained over large pressure and microwave power ranges.
5.2. Performance of UDECR plasmas ECR REGION CUSP MAGNETIC ETIC FIELD UNE
LOBE
N
\
/
Because of the drift velocity along the magnet rows mentioned above, leakage of primary electrons inevitably occurs at the extremities of the magnets, which leads to poor excitation efficiency and to significant contamination due to local wall sputtering. This problem can be circumvented by implementing additional magnets at these extremities in such as way as to close the magnetic structures on themselves, as shown in Fig. 4. This TRACKS
PERMANENT
FORBIDDEN
MAGNETS
REGION
Illllll
GROUND
IIIIII I
ANTENNA PLANE
Fig. 3. Schematic of UDECR excitation.
O~MIB
Fig. 4. Examples of closed magnetic structures.
246
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
improvement leads to a still better unformity and an increase in plasma density (up to 10 ~1 cm-3) as compared with the initial version of DECR (see Section 4.2).
6. Conclusions and perspectives Distribution of plasma sources at the periphery of a closed volume is an attractive concept providing plasma homogeneity in atomic gas plasmas. This has been achieved in cylindrical DECR and UDECR plasmas by combining ECR excitation with a multipolar magnetic field. However, ion volume recombination, responsible for the lack of homogeneity in molecular gas plasmas, imposes limitations on the uniformity of plasma processing. In the case of large plane substrates, this problem can be evaded by adopting a plane configuration for the plasma source. Large plane UDECR plasmas can be obtained by associating parallel linear applicators in a planar configuration. The possibility of producing such uniform plasmas has been proved experimentally. Furthermore, in contr~tst to capacitive discharges, the surface of substrates can be biased independently of plasma generation, allowing the control of the energy of ion bombardment. Clearly, thanks to UDECR, the possibility of scaling processes from a small to a large reactor exists without any alteration in the characteristics. However, the concept of uniform distribution for plasma sources must be extended to gas feeding, pumping, substrate biasing and also substrate heating or cooling, if uniform processing is to be achieved.
References 1 J. Pelletier, B. Petit, Y. Arnal, C. Pomot and M. Pichot, J. Phys., DI9 (1986) 795.
2 R. Burke, J. Pelletier, C. Pomot and L. Vallier, J. Vac. Sci. Technol., A8 (1990) 2931. 3 A. Tissier, J. Khallaayoune, A. G6rodolle and B. Huizing, J. Phys. IV, C2 (1) (1991) 437. 4 F. Plais, B. Agius, F. Abel, J. Siejka, M. Puech, G. Ravel, P. Alnot and N. Proust, J. Electrochem. Soc., 139 (1992) 1489. 5 0 . Joubert, R. Burke, L. Vallier, C. Martinet and R. Devine, Appl. Phys. Lett., 62 (1993) 228. 6 M. J. Cooke and N. Sharrock, Electrochem. Soc. Symp. Proc., 90 (14) (1990) 538. 7 C. Oehr and H. Suhr, Appl., Phys., A45(1988) 151. 8 J. Pelletier, R. Pantel, J. C. Oberlin, Y. Pauleau and P. GouyPailler, J. Appl. Phys., 70 (1991) 3862. 9 W. M. Green, D. W. Hess and W. G. Oldham, J. Appl. Phys., 64 (1988) 4696. 10 Y. T. Kim, S. K. Min, J. S. Hong and C. K. Kim, Jpn. J. Appl. Phys., 30 (1991) 820. 11 A. Belkacem, Y. Arnal, J. Pelletier, E. Andr6 and J. C. Oberlin, Thin Solid Films, 24l (1994) 301. 12 J. Pelletier, Y. Arnal and M. Moisan, in M. Moisan and J. Pelletier (eds.), Microwave Excited Plasmas, Elsevier, Amsterdam, 1992, pp. 249-272. 13 R. Limpaecher and K. R. MacKenzie, Rev. Sci. lnstrum., 44 (1973) 726. 14 C. Gauthereau and G. Matthieussent, Phys. Lett., A 102 (1984) 231. 15 G. Matthieussent and J. Pelletier, in M. Moisan and J. Pelletier (eds.), Microwave Excited Plasmas, Elsevier, Amsterdam, 1992, pp. 303 349. 16 J. Pelletier and G. Matthieussent, in M. Moisan and J. Pelletier, (eds.), Microwave Excited Plasmas, Elsevier, Amsterdam, 1992, pp. 351 384. 17 C. Gauthereau and G. Matthieussent, Phys. Lett., AI21 (1987) 432. 18 M. Pichot, J. Pelletier and Y. Arnal, US Patent 4745 337, 1988. 19 M. Pichot, A. Durandet, J. Pelletier, Y. Arnal and L. Vallier, Rev. Sci. Instrum., 59 (1988) 1072. 20 M. Pichot and J. Pelletier, in M. Moisan and J. Pelletier (eds.) Microwave Excited Plasmas, Elsevier, Amsterdam, 1992, pp. 419434. 21 Y. Arnal, J. Pelletier, L. Pomathiod and L. Vallier, Proc. 8th Int. Colloq. on Plasma Processes (CIP 91), France, Le Vide, Les Couches Minces, Suppl. 256 (1991) 235. 22 J. Pelletier, US Patent 7824210, 1992.
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Chemical vapor deposition in high-density low-pressure plasmas: reactor scale-up and performance J. Pelletier and T. Lagarde Laboratoire de Physique et Chimie des ProcOd~s Plasma, Universitb Joseph Fourier (Grenoble 1), Unitk CNRS, CNET-CNS, France-Telecom, BP 98, 38243 Meylan Cedex (France)
Abstract
In most applications, plasma enhanced chemical vapor deposition (CVD) of high-quality materials requires control of process parameters over a wide range. In particular, high-density low-pressure plasmas capable of producing significant ion fluxes compared to neutral fluxes, and independent substrate biasing which allows accurate adjustment of the ion bombardment energy may lead to high-performance deposited layers. Results in low-temperature Si epitaxy, SiO2, Cu and W deposition are presented which illustrate the capabilities of such plasmas. However, deposition rate and film uniformity remain the key requirements in many industrial fields, including the microelectronics industry with the processing of flat panels and increase in wafer diameters. In fact, the limiting factor of many industrial applications lies in the difficulty of producing large (up to square meters) uniform, dense plasmas. In the conventional cylindrical configuration of plasma reactors, the ability to process large-dimension substrates is dependent on the generation of large volumes of plasma. Unfortunately, due to ion volume recombination, the production of large volumes of uniform dense plasma must be ruled out. Clearly, treatment of a planar substrate with a uniform plasma source is more suitable. The uniform distributed electron cyclotron resonance (UDECR), in which linear microwave applicators can sustain constant amplitude standing waves along a multipolar magnetic structure, can be used to make large planar plasma sources. The performance of such a plasma is presented in terms of uniformity and density, allowing the process optimized in laboratory reactors to be scaled-up to much larger reactors without any alteration in the characteristics.
1. Introduction
Plasma processing of high-performance materials is becoming a critical technology not only in the electronics industry but also in the aerospace, automotive, steel, biomedical, textile, optics and paper industries. Due to the diversity of applications, plasma processing will in the near future have to cover a broad range of geometries, dimensions, chemical systems, electromagnetic designs and plasma-surface interactions. In addition, at the industrial level, plasma processing of materials usually imposes two important requirements: uniformity of the surface treatment and real-time, in situ monitoring for control and analysis. In this situation, the technological challenge lies firstly in developing surface processes able to meet the required specifications, especially in terms of quality, uniformity, processing rate and reproducibility; and secondly in the possibility of transfering processes from one reactor to another, or more generally, to scale processes from a small to a large reactor. The main advantage of plasma processing over many other dry processing techniques lies in the generation, firstly, of active neutral species (reactive atoms, radicals) which allows processing at ambient or even cryogenic temperatures; and secondly of ions which may be used for the energetic bombardment of surfaces. Ion born-
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bardment, which ensures continuous cleaning of the surface during the process, makes it highly reproducible. It can also dramatically influence the properties of the surface layer (morphology, crystallography, compositon, adhesion, surface roughness, stress etc.). Therefore, compared with other conventional dry surface processing techniques, plasma provides, through the plasmasurface interaction, the additional degrees of freedom necessary to reach the strictest process specifications. Therefore, in high-density low-pressure plasmas capable of producing significant ion fluxes with respect to neutral fluxes, independent substrate biasing [1], which allows accurate adjustment of the ion bombardment energy, may lead to high-performance processes: anisotropic and selective etching, low-temperature Si epitaxy and doping, planarizing deposition of SiO2 layers, deposition of high-performance metal and alloy materials, etc. The aim of this paper is to emphasize, using a few selected examples, the necessity of ion bombardment and the desirability of ion energy control in many processes and especially in the preparation of thin films by plasma enhanced CVD (PECVD). In particular, we discuss how to control accurately and independently the various parameters of the plasma-surface interaction. Then, we examine the limiting factors of the production
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Elsevier Sequoia. All rights reserved
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
of large uniform low-pressure high-density plasmas. We also demonstrate the interest of using uniform planar plasma sources for the treatment of planar substrates. Next, the principle of the UDECR, which can be used to make large planar plasma sources, is described. The performance of such a plasma is presented in terms of density and uniformity, and the future prospects of the technique are discussed.
2. Preparation of thin films by CVD in high-density low-pressure plasmas 2.1. Control of process parameters In most surface treatment applications, the main parameters that characterize the plasma-surface interaction besides surface temperature, are the flux of reactive species and the flux and energy of ion bombardment. The production rates of ions and reactive species increase with power density in the plasma. Since the concentration of reactive species largely depends on ion-molecule reactions occurring in the gas phase, one can selectively control the concentration of reactive species by varying independently the partial pressure of the reactive parent gases. When possible, the ion bombardment energy can also be adjusted independently of plasma generation by biasing the substrate surface with respect to the plasma potential: when the substrate is a conductor, it can be biased by simply applying a d.c. voltage to it; when the substrate is an insulator, it can be biased only by applying a periodic (r.f.) voltage through a low-impedance capacitor [1]. The substrate surface can thus be charged by capacitive coupling. In the following examples, thin films are deposited by PECVD in high-density low-pressure plasmas, allowing the separate control of all the process parameters of the plasma-surface interaction. However, at low pressure, a resonant excitation is generally required to sustain plasmas. Microwave excitation by electron cyclotron resonance (ECR), which can easily produce high-density plasmas at low pressure, can thus deliver ion to neutral flux ratios two or three orders of magnitude higher than conventional r.f. discharges. Finally, at 2.45 GHz, the excitation frequency is much higher than the ion plasma frequency, so that ions cannot be accelerated in the electric field which generates the plasma, thus allowing complete separation between plasma generation and plasma/surface interaction. Hence the interest in ECR excited plasmas for the parametric study of PECVD processes in high-density low-pressure plasmas. 2.2. SiBcon epitaxy and doping Silicon epitaxy in silane plasmas is possible down to a substrate temperature of 300 °C, but epitaxy of ac-
241
ceptable quality requires at least 600 °C, i.e. 200 K lower than in CVD [2]. Hydrogen desorption from the silicon surface by low-energy ion impact appears to be the rate determining step. Introduction of phosphine, diborane or arsenic in the silane plasma produces the intentional doping of the epitaxial silicon. However, doping by arsenic [2] requires the assistance of an ion bombardment energy of a few tens of eV: because of their larger size, arsenic atoms are not easily incorporated into the silicon lattice and tend to remain at the surface during the layer growth. Clearly, ion bombardment forces arsenic atoms to penetrate into the silicon lattice.
2.3. Planarizing Si02 deposition Deposition of silicon dioxide is an important step in the fabrication of semiconductor devices. Present trends in deposited SiO2 films are towards lower deposition temperatures and better control of the quality of the oxides [3-5]. In particular, the characteristics of the oxides deposited without intentional heating from SiH4/O2 mixtures in high-density low-pressure plasmas have been shown to be strongly dependent on ion bombardment energy. For ion energies higher than 50 eV, many of the film characteristics (refractive index, infra-red spectrum, wet etch rate, fixed charge density) are close to those of thermal SiO2 [3-5]. However, in the fabrication of submicron circuits, the challenge is now to deposit SiO2 using a planarizing process, i.e. conforming to the surface topography with void-free filling of high aspect ratio trenches. Such an objective cannot be achieved successfully without simultaneous sputter etching during SiO2 deposition [3-6]. Therefore, without bias, one cannot fill vias with an aspect ratio larger than 0.6. With a 200 V bias voltage, vias with aspect ratios up to 2 can be filled successfully [31. 2.4. Low-resistivity Cu thin films Owing to its low-bulk resistivity (1.7 ktf~cm), copper is one of the most attractive materials for interconnections in sub-halfmicron circuit fabrication. Copper films are usually prepared by evaporation or sputtering. However, CVD and PECVD processes exhibit intrinsic advantages over physical deposition methods, especially in terms of conformal step coverage and fine-grained structure. Copper-containing films have been produced by PECVD in a conventional r.f. discharge from copper (II) hexafluoracetylacetonate with significant incorporation of fluorine in the film [7]. More recently, pure copper films have been prepared at ambient temperature by ECR microwave plasma-enhanced CVD from the copper (II) acetylacetonate-argon-hydrogen system. The bias voltage applied to the substrate and the
242
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
gas phase composition appeared to be critical deposition parameters. Oxygen was never detected in the deposited material owing to the presence of carbon and hydrogen in the copper complex. However, since the spontaneous reaction rate of excess carbon with hydrogen is negligible at room temperature, carbon residues in the films can be removed only by ion induced etching. Thus, with the addition of hydrogen to the plasma (excess hydrogen) and an ion bombardment energy of 50eV, pure copper films with low resistivities (23 ~t~ cm) can be deposited at ambient temperature [8]. 2.5. Low-resistivity W thin films As in the case of copper, PEVD of tungsten films in high-pressure plasmas generally leads to the deposition of the high-resistivity fl-tungsten phase [9]. The presence of this fl-phase, usually observed in low-temperature PECVD [10], is attributed to the incorporation of impurities such as oxygen, fluorine or silicon [9]. In high-density low-pressure plasmas, the intensity of the ion bombardment enhances the desorption of reaction products, i.e. HF and/or SiF4, so that tungsten films with much lox~er resistivities can be deposited. However, without intentional substrate heating, the resistivity of the films strongly depends on plasma conditions [11]. In contrast, at 300 °C, resistivity values between 7 and 9 ~tff~cm are obtained which do not significantly depend on plasma conditions [11]. Clearly, above 300 °C HF desorption is controlled mainly by temperature. 2.6. Remarks The above examples illustrate the different possibilities offered by an intense ion bombardment of the surface and demonstrate the necessity of an accurate and independent control of the ion energy. More generally, the quality of the deposited thin films is largely dependent on all the process parameters defined in Section 2.1. However, all the experiments described above have been carried out in small cylindrical chambers intended to treat substrates not larger than 100 mm in diameter. It is clear that larger substrates will require a scale-up solution for uniform treatment by dense low-pressure plasmas.
3. Plasma homogeneity: theoretical aspects and limitations The aim of this section is to analyze, based on the simplest assumptions, the possibilities of obtaining homogeneous plasmas and, further, to identify the conditions required to achieve a uniform interaction between plasma and surfaces immersed in it. A uniform inter-
action with a plane surface, or with a surface that can be assimilated to a plane surface (curvature radius ~> Debye length) can be obtained under any conditions when the plasma is homogeneous, or when the plasma is unhomogeneous provided that the plasma flux at the sheath edge is uniform. The real question is thus to determine how to produce homogeneous plasmas or uniform fluxes of plasma in a finite-size reactor chamber. 3.1. Homogeneity in bounded plasmas First we consider a discharge with an ion mean free path l~,1 for ion-neutral collisions shorter than the dimensions of the chamber (collisional plasma). Under steady-state conditions and in the absence of ion volume recombination, the plasma can be described by an ambipolar diffusion model with the following equations [12] ~ . nv = s,
(1)
nv = - D a V n
(2)
where n is the plasma density, v~ the plasma directed vleocity, D a the scalar ambipolar diffusion coefficient, and Si the ionization rate. Equations (1) and (2) mean that the plasma produced in a given volume is evacuated outside this volume [eqn. (1)] by diffusion under the influence of density gradients [eqn. (2)]. A homogeneous plasma, defined by X~n = 0
(3)
requires that (eqn. (2)) v~= 0
(4)
This can be achieved (eqn. (1)) only if the ionization rate is negligible in the volume of the plasma, i.e. Si = 0
(5)
This situation can be illustrated by a plasma expanding into a volume that is completely enclosed by a peripheral plasma source of uniform density (Fig. l(a)). However, the introduction of a substrate into such a plasma
@@ n(R)=n0
(a)
n(R)=n0
(b)
Fig. 1. Sketches showing a spherical diffusion plasma generated at the periphery r = R with a density n(R) = no: (a) diffusion in a free volume; (b) case where a concentric spherical object is introduced.
J. Pelletier, T. Lagarde / CVD & high-density low-pressure plasmas
(Fig. l(b)) perturbs the plasma and the homogeneity is lost, except in the two limiting cases of small and large objects compared with the plasma mean free path and to the volume of the plasma respectively [12].
3.2. Plasma homogenity through multipolar magnetic field confinement The fact that plasmas confined by multipolar magnetic fields are homogeneous was qualitatively demonstrated for the first time by Limpaecher and MacKenzie [13]. They also observed a gradual improvement in the homogeneity of an argon plasma as a function of decreasing pressure. More recently, Gauthereau and Matthieussent [14] have measured the radial density profiles in a 1 m diameter, 1 m long multipolar discharge in argon for three gas pressures, namely 5 x 10 - 4 , 10 . 3 and 8 x l 0 - 3 Torr, which correspond to primary electron mean free paths )~pi for ionization of 2.3m, 1.15m and 0.15m respectively [15]. On the other hand, the ion mean free path )~in is less than 10cm throughout the pressure range investigated, so that the plasma can be considered collisional. At the lowest pressure (5 x 10 . 4 torr), i.e. in the collisionless regime for primary electrons, a perfect plasma homogeneity is achieved in the volume of the discharge which is free from magnetic field. In contrast, at the highest pressure (8 x 10 . 3 Torr), i.e. in the collisional regime for primary electrons, the parabolic plasma density profile commonly obtained in non-magnetically confined discharges is recovered. The transition from a homogeneous plasma to a plasma with a parabolic density profile occurs at an argon pressure of the order of 10 .3 Torr, i.e. at the onset of the collisional regime (kpi ~ 2R) for primary electrons. Such experimental results demonstrate the major role played by primary electrons in achieving plasma homogeneity in a multipolar discharge. Thus, bearing in mind that primary electrons produce the plasma and considering that the assumption of a significant ionization rate over the whole plasma volume does not lead to a homogeneous plasma, we must assume the localization of ionization in the peripheral region near the wall of the discharge (see Fig. l(a)). Obviously, a preliminary condition for such a selective peripheral ionization in a discharge confined by multipolar magnetic fields is a mean free path lpi of primary electrons for ionizaton of neutrals larger than the dimensions of the discharge chamber. If this were not the case, the electrons would undergo several inelastic collisions and degrade into plasma electrons before reaching the magnetic sheath and interacting with it. The first mechanism that provides selective peripheral ionization is the reflection of primary electrons in the magnetic sheath: when a flux of electrons reflects on a
243
magnetic mirror, i.e. in a magnetic field gradient, the drift velocity of each electron decreases and then vanishes at the so-called mirror-point [16]. The increase in peripheral ionizaton as compared with volume ionization in the magnetic field free region thus results from the long time spent by the electrons in the magnetic sheath. The second mechanism for peripheral ionization resuits from the irreversible trapping of primary electrons [16] in the multipolar magnetic field, either via elastic collisions of primary electrons with neutrals or via the interaction with electric fields due to plasma instabilities [17]. Then, once trapped, primary electrons oscillate indefinitely between two successive magnetic poles until they suffer inelastic ionizing collisions.
3.3. Limits of plasma confinement In argon plasmas, and more generally in atomic gas plasmas (He, Ne, Hg, Cs, etc.) where ion volume recombination can be assumed negligible [12], the best approach to the problem of plasma homogeneity is to localize plasma generation at the periphery of the reactor. However, in molecular (reactive) gas plasmas, volume recombination of positive ions with electrons (dissociative recombination) or negative ions (mutual neutralization) becomes a significant drawback for homogeneity in high-density plasmas [12]. These two efficient ion recombination mechanisms have rate coefficients ~r of the order of 10 -7 cm 3 s -i. Thus, in order to estimate the importance of the inhomogeneity effects introduced by ion volume recombination, we can calculated the density profile for a plasma assumed to be homogeneous (Si = 0) in the absence of ion recombination. Equation (1) becomes V ' / ~ = --Sr
(6)
where Sr is the volume recombination rate (7)
Sr = ~r n2
Equations (1) and (2) lead to W n = ~r n2
(8)
For the sake of simplicity, we turn now to the one-dimensional case where a plane source (located at z = 0) produces a uniform plasma n(0) = no
(9)
The corresponding spatial density profile as a function of distance z from the plane source is obtained by integration of eqn. (8), and we have , (noO~r"~ '/2 7 -2
n = n o 1-t-/7-A---~/\oua/ z j
(10)
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
244
This equation shows the spatial inhomogeneity introduced by the rate coefficient ~r. Plasma density is decreased by a factor of 4 at a distance ZI]4
= ( 6 D a ~ '/2 \n0~r/
(11)
For example, taking ~r = 1 0 - 7 cm3 S I and D a = 6 × 105 cm 2 s -~, we obtain z~/4= 60cm for no = 10~°cm-3, and zj/4 = 6 cm for no = 1012 c m -3. This result clearly shows that homogeneity of molecular gas plasmas degrades rapidly with increasing plasma density. Hence, homogeneity cannot be ensured when scaling-up highdensity molecular gas plasmas. Since, in actual cylindrical configurations, plasma inhomogeneity increases with diameter, the plasma processing of large-diameter plane substrates requires alternative configurations, such as the plane configuration described below.
3.4. The plane configuration: an ideal scale-up solution As far as achieving a uniform plasma-surface interaction is concerned, the infinite plane configuration where plane substrates are placed parallel to a plane uniform plasma source is undeniably an ideal configuration: each point of the substrate surface with respect to the plane source has the same geometrical factor, i.e. identical solid angle and distance. Therefore, volume recombination cannot affect the uniformity of the plasma-surface interaction. Furthermore, since plasma density decreases as a function of distance from the source, obviously the highest plasma fluxes are obtained at the shortest distance from the source. Therefore, besides providing uniformity, the plane configuration leads to the highest possible substrate to reactor wall surface ratio, minimizing substrate contamination. In addition, the plasma volume to sample surface ratio in a plane configuration, which can be compared with the interelectrode distance in r.f. capacitive discharges, results in the highest efficiency in terms of pumping kinetics. However, as infinite plasma sources are not conceivable, we must consider the more realistic situation of a plane source of finite dimensions. As the spatial influence of the plasma source edge on the uniformity of plasma processing is controlled mainly by the characteristic decay length of the plasma density (eqn. (11)), the edge effects are considerably reduced at the highest plasma densities. Therefore, the real challenge in this domain lies in producing uniform high-density planar plasma sources.
4. DECR plasmas We have seen that, in the absence of ion volume recombination, peripheral excitation favors plasma homogeneity. Thus, using a multipolar magnetic field
confinement, this could be easily achieved by accelerating the electrons trapped in the magnetic sheath. At low gas pressures, due to the necessity to use an efficient plasma excitation, an attractive solution is to transfer HF energy to electrons via electron cyclotron resonance within the multipolar magnetic field. This is the goal achieved in the so-called DECR [18-20]
4.1. Concept of DECR plasmas The development of DECR excitation is based on two ideas: (i) the permanent magnet rows destined for multipolar magnetic confinement are further used to provide the magnetic field intensity Bc needed for ECR coupling Bc = 2z m,.f~
(12)
e
where fc is the frequency of microwaves and e and m,. are the elementary charge and electron mass respectively; (ii) the microwave field required for ECR is provided by linear antennas running along and close to the magnet rows. This scheme results in the generation of the plasma at the periphery of the chamber where the so-called primary electrons (accelerated through ECR) are trapped between the magnetic field cusps (magnetic poles) until they ionize the gas. Figure 2 is a schematic representation of one of the microwave field applicators in a cylindrically shaped DECR discharge chamber. It consists of a linear conductor of cylindrical cross-section, called the antenna, placed a few millimeters above the ground-plane (reactor wall). Permanent magnets are arranged in a similar way as for classical multipolar magnetic field confinement. These magnets provide the required 875 gauss (at an excitation frequency of 2.45 GHz) isomagnetic surface in the vicinity of the antenna along its entire length. The magnetic field is closed by two adjacent magnet rows as in a conventional multipolar field. A magnetic field configuration is thus created whereby microwave field, applied at 2.45 GHz through a coaxial feedthrough, results in ECR coupling along the full
GROUND PLANE PERMANENT MAGNET. _ ~ m = ~ ~
LOBE ANTENNA
:ii,iiiiil;i!i!ii
REGION
Fig. 2. Schematic of DECR excitation.
PLASMA
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
antenna length. Within the pressure range extending from 0.1 to 3 mTorr, the gas is ionized by the primary electrons in the lobes (see Fig. 2) and the plasma produced then diffuses towards the central part of the chamber. In the central magnetic field free part of the chamber, a cold d.c. diffusion plasma is thus obtained, free of high-energy electrons (which remain trapped in the multipolar magnetic field) and whose floating potential is close to the ground potential (reactor wall potential). With argon, such uniform plasmas are currently produced in cylindrical reactors [19], with ion densities exceeding 10II cm -3.
245
constant amplitude standing wave along the linear applicator. To this end, the microwave power sent to the antenna must be large enough for the wave to be reflected at the extremity of the structure, yielding a standing wave pattern. However, with DECR, the conditions of constant amplitude wave pattern can be obtained, for given antenna length and microwave input power, only at a definite gas pressure [20, 21]. None the less, under this unique condition, a linear (up to 2 m) plasma with a uniformity better than a few per cent can be achieved [21].
5. From DECR to uniform DECR plasmas
4.2. Limitations in DECR plasmas A crucial advantage of DECR excitation over all other plasma production techniques is that DECR allows perfect fitting of the plasma source term to the reactor dimensions. However, well-suited for the processing of small substrate diameters, the cylindrical geometry, inadequate for uniform plasma processing over large areas, must be replaced by a planar DECR configuration (see Section 3.4). Such a possible design, which uses a set of parallel linear antennas, is represented in Fig. 3. But the possibility of achieving large areas of uniform plasma assumes that each single linear applicator can generate a uniform plasma along it. In the initial version of the DECR technique (Fig. 2), microwaves propagate mainly in the region of E C R coupling, so that in the presence of plasma, microwaves cannot propagate along the antennas without absorption. Damping of travelling waves along the antennas thus leads to non-uniform plasmas. A possible solution to produce uniform plasmas consists in sustaining a
PLASMA
5.1. Concept of uniform DECR plasmas In order to achieve a constant amplitude wave pattern under any conditions, one possible answer consists in the spatial separation of the propagation region from the absorption region of ECR coupling. In the new DECR design [22], so-called uniform DECR (UDECR), the antennas are set between the magnets of the multipolar magnetic field configuration (Fig. 3), outside the ECR regions but within the regions free from plasma [15] located between the rector wall and specific field lines linking the successive magnet poles. With this UDECR configuration, a standing wave pattern of constant ampliltude is expected. However, this means that the microwave power transferred to electrons varies periodically along the antenna. In fact, since primary electrons responsible for plasma ionization undergo a magnetic drift motion parallel to the magnet rows [16], plasma production along the antennas may remain perfectly uniform, provided the amplitude of the standing wave is constant. Therefore, using UDECR, uniform plasmas are obtained over large pressure and microwave power ranges.
5.2. Performance of UDECR plasmas ECR REGION CUSP MAGNETIC ETIC FIELD UNE
LOBE
N
\
/
Because of the drift velocity along the magnet rows mentioned above, leakage of primary electrons inevitably occurs at the extremities of the magnets, which leads to poor excitation efficiency and to significant contamination due to local wall sputtering. This problem can be circumvented by implementing additional magnets at these extremities in such as way as to close the magnetic structures on themselves, as shown in Fig. 4. This TRACKS
PERMANENT
FORBIDDEN
MAGNETS
REGION
Illllll
GROUND
IIIIII I
ANTENNA PLANE
Fig. 3. Schematic of UDECR excitation.
O~MIB
Fig. 4. Examples of closed magnetic structures.
246
J. Pelletier, T. Lagarde / CVD in high-density low-pressure plasmas
improvement leads to a still better unformity and an increase in plasma density (up to 10 ~1 cm-3) as compared with the initial version of DECR (see Section 4.2).
6. Conclusions and perspectives Distribution of plasma sources at the periphery of a closed volume is an attractive concept providing plasma homogeneity in atomic gas plasmas. This has been achieved in cylindrical DECR and UDECR plasmas by combining ECR excitation with a multipolar magnetic field. However, ion volume recombination, responsible for the lack of homogeneity in molecular gas plasmas, imposes limitations on the uniformity of plasma processing. In the case of large plane substrates, this problem can be evaded by adopting a plane configuration for the plasma source. Large plane UDECR plasmas can be obtained by associating parallel linear applicators in a planar configuration. The possibility of producing such uniform plasmas has been proved experimentally. Furthermore, in contr~tst to capacitive discharges, the surface of substrates can be biased independently of plasma generation, allowing the control of the energy of ion bombardment. Clearly, thanks to UDECR, the possibility of scaling processes from a small to a large reactor exists without any alteration in the characteristics. However, the concept of uniform distribution for plasma sources must be extended to gas feeding, pumping, substrate biasing and also substrate heating or cooling, if uniform processing is to be achieved.
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