Design of magnetrons for dc sputtering

Vacuum/volume39/numbers 7/8/pages 717 to 721/1989

0042-207X/89S3.00+.00 Pergamon Press plc

Printed in Great Britain

Design of magnetrons for dc sputtering* J B A l m e i d a , Universidade do Minho, Laborat6rio de Fisica, P-4719 Braga Codex, Portugal

A brief review of the basic physics ofmagnetron sputtering, starting with some physics of a glow discharge and proceeding to the magnetron effect using a single particle explanation of the phenomena is given together with the results of the author's own experience in designing these devices. The design of the magnetic field and the anode, mechanical arrangement of the different components, the vacuum requirements and the choice of an appropriate power supply are all considered.

1. Introduction

Magnetron sputtering has proven to be a very useful deposition and etching technique for several reasons, among which are the low heating effect and minimal damage caused to delicate substrates, the ability to cope with almost any metal or alloy and, indeed, many insulating materials when reactive or radio frequency (rf) sputtering are used, the high rate of coating compared to ordinary diode sputtering, the versatility and adaptability to different shapes and geometries and, last but not the least, the pollution-free nature of the technique. Magnetron sputtering is a technique for industrial applications, rather than for physicists. In fact, the objective is to obtain high etching and deposition rates by means of a large flux of bombarding particles into large areas. There is virtually no concern about the characterization of the bombarding flux in terms of speed distribution ; other methods are more suitable for the investigation of sputtering mechanisms. On the other hand, where the emphasis is on the magnitude of the sputtered flux, this technique has few competitors.

where V is the cathode voltage, j is the ionic current density, m is the ionic mass, e is the electric charge and t0 is the permittivity of vacuum. It follows from this expression that the cathode dark space thickness grows with the cathode voltage and decreases with the pressure, through the ionic current j. The fact that most of the voltage drop appears across the cathode dark space means that virtually only the ions that arrive to this region will be accelerated towards the cathode and used for sputtering; all the others will be lost in the gas. Next to the cathode dark space, in the direction of the anode, there is a region of high ionic density, called the negative glow, where the secondary electrons, generated by collisions of the ions with the cathode and accelerated by the electric field in the cathode dark space, make ionizing collisions with the atoms of the gas. Following towards the anode we now find a region, the positive column, where the electrons have lost most of their kinetic energy and proceed slowly to the anode. The thickness of the cathode dark space is a lower limit for the cathode-anode distance for a stable discharge.

2. Basic operation

2.2. Electron trapping with a magnetic field. The idea of using a

2.1. Glow discharge. This is a method of creating a low pressure plasma by means of an electric field established between a positive electrode (anode) and a negative electrode (cathode). The gas pressure for setting up a glow discharge is usually of the order of 10-2-1 mbar and the degree of ionization is of the order of 1% or less. The discharge is initiated by the collisions of the few free electrons existing in the gas, by virtue of gamma-ray radiation, when they are accelerated by the electric field. After the steady state is reached, which happens in a very short time, the gas begins to glow and a current starts flowing between the two electrodes. The electric field between the anode and cathode is not uniform and, in fact, most of the voltage drop appears adjacent to the cathode in a dark region called the 'cathode dark space' with a thickness d given by (SI units) :

magnetic field for increasing the ionization of the plasma was first suggested and put into practice by Penning in 1940, but its full potential was not realized then and only in the seventies was magnetron sputtering developed as a technique in its own right. The magnetic field adds one degree of freedom to the system, making it a great deal more flexible. The effect of the magnetic field is to increase the distance the electrons have to travel, by bending their trajectories into helices, thus increasing the probabiliy of collisions. With a suitably shaped magnetic field, the trajectories can be so elongated as to break the compromise between pressure and anode-cathode distance. As a consequence, very high degrees of ionization are possible with relatively low pressures, as is required for high efficiency sputtering. The positioning of the anode becomes of little importance and it can generally be placed wherever is suitable. In order to explain the mechanisms of the magnetron we refer to Figure 1, which represents the main features of a planar magnetron. Although the plasma inside the magnetron is known to be rich in collective behaviour, i.e. the particles interact with each other, a single particle picture of the discharge provides some useful understanding of the phenomena taking place. In Figure 1 the cathode is represented by the horizontal surface and

= ~j

* Invited.

2

V 3/2

(1)

717

J B A l m e i d a : Design of magnetrons for dc sputtering

<~'/-//'~\.

...e.<~ ~<"

.>~.

to prevent the electrons that have made no collisions, from being lost, trapping them between the magnetic field and the cathode. In a uniform magnetic field and in the absence of an electric field, the electrons orbit around the field lines in such a way that the magnetic force balances the centrifugal force : o

ev.B

--

meUn

(2)

Fg

Figure 1. Planar magnetron.

where e is the charge of the electron, me is its mass, v, is the component of the velocity at right angles to the field, B is the magnitude of the magnetic field and rg is the radius of the orbit, called gyro or Larmor radius. In the presence of an electric field, the drift velocity of the electrons is the sum of their parallel velocity vv with a velocity vE of magnitude, VE =

the anode (not shown) is located above the cathode. As has been said, the position of the anode is relatively unimportant. The system is completely filled with argon at a pressure around 10 -3 mbar and a voltage of a few hundred volts is applied between anode and cathode. When this voltage is applied, any positive ions within the electric field are accelerated towards the cathode and some will have an energy of 1 eV per volt of the applied voltage. The trajectories of the ions are mostly insensitive to the magnetic field, between 0.02 T and 0.05 T and remain essentially straight. If the ions are energetic enough, they release secondary electrons from the cathode, which contribute to ionize the gas further. Once the secondary electrons are released, there is no real distinction between them and those that are generated within the plasma and are known as primary electrons; they all work together to guarantee the ionization of the plasma. When the steady state is reached, the three main regions of the discharge, 'cathode dark space', 'negative glow' and 'positive column', are established4"6 and the main voltage drop appears across the cathode dark space, while the main ionization takes place within the negative glow. An electron leaving the cathode is accelerated by the strong electric field of the cathode dark space and enters the negative glow; as it crosses the horizontal magnetic field lines, its trajectory is bent back towards the cathode and, unless it makes one collision, it is decelerated by the electric field and ends up on the cathode surface with the same energy as it started with. One such electron has been depicted in Figure 1 labelled as 'electron untrapped' although, as we shall see, it is effectively trapped between the magnetic field and the cathode. If the electron makes one collision on its path, namely one ionizing collision, some of its energy will be lost and the electron will be trapped in a cycloidal orbit. The path to the anode is thus virtually blocked greatly increasing the likelihood of collisions and the degree of ionization. The net effect is the substantial reduction of the cathode dark space thickness, as if the pressure were higher. If the electron has a component of the velocity along the magnetic field lines, it will be lost at the ends if no precautions are taken. In order to avoid this, the magnetic field lines are made to emerge from and pass into the cathode, so that the electron that moves along them will merely be reflected back and forth, as shown in Figure 1. The arches of the magnetic field lines above the cathode surface form a tunnel which must close on itself in order 718

E. B

(3)

parallel to E x B (ref 4). En is the component of the electric field normal to the magnetic field. The magnetron effectively removes most of the electrons kinetic energy, allowing for high degrees of ionization. As a result, the thickness of the cathode dark space is reduced and the following consequences can be observed : (i) the working pressure can be reduced, because there is a higher likelihood of collisions ; (ii) the voltage can be reduced to a few hundred volts, because fewer secondary electrons are needed ; (iii) the sputtered flux is increased by virtue of the reduction in the working pressure and (iv) the electrons don't need to be collected by an anode because they become thermalized and can drift towards any surface at ground potential.

3. Magnetron design 3.1. Magnetic field. The magnetic field design is the key factor governing the operation of the magnetron ; in fact it is responsible for the effective trapping of the electrons and for the uniformity of erosion of the target material. Ideally we would like to erode the target uniformly and this requirement would point to a uniform magnetic field parallel to the cathode. On the other hand, as we saw in section 2.2., the magnetic field lines must be intercepted by the cathode and the tunnel so formed must close on itself; in order to achieve good trapping of the electrons. It follows that a compromise must be reached between trapping efficiency and uniformity of erosion. In practice, however, bad electron trapping usually means unacceptably high pressures and supply voltages and this consideration overrides the uniformity of erosion. One other important point is the fact that it is usually very difficult to avoid some stray magnetic field lines in places where we would like no erosion to take place at all. These must be minimized, but we shall see in section 3.2., some means of dealing with them. The magnetic fields needed for proper operation of the magnetron are relatively small; 0.04 T is about the maximum recommended field strength above the cathode surface because there is virtually no increase in performance above this level. This field strength can easily be achieved with permanent magnets and this is the best way to do it for reasons of simplicity. With the exception of the cases where research on the effects of varying the

J B Almeida :

Design of magnetrons for dc sputtering

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U///////////////Z-/ZT/-/-/-/-/2

[] CATHDI~E

[]

[] MAGNETS

SDFT IRDN

Figure 2. Basic configurationsof the magnetic fieldcircuit.

magnetic field is the objective, we see no point in using magnetic field coils. For a planar magnetron there are three basic configurations of the magnetic circuit (depicted in Figure 2). The second configuration is well adapted to circular geometries, as it can make use of ring magnets of the type used in loudspeakers ; the author has successfully used surplus loudspeaker magnets for this purpose. Some care must be exercised in the design of the centre pole piece in order to avoid saturation. In the first configuration the magnetic field is created by means of the central magnets and it is an easy task to adapt the concept to a rectangular geometry. In spite of the central position of the magnets, it is not easy to avoid some stray magnetic field appearing at the periphery of the device. The third configuration is especially suitable for rectangular geometries and, once more, some care is needed to avoid saturation. None of the basic geometries provides a particularly good magnetic field in terms of uniformily and hence uniformity of erosion. In all cases the material will be preferentially removed from an area some distance away from the centre but not reaching the periphery. When expensive targets are being used and no reprocessing of the wasted material can be envisaged, some alterations can be brought to the basic configurations. These aim at rendering the horizontal component of the field as uniform as possible. 3.2. Anode. As we saw earlier, in section 2.2., a properly designed magnetic field allows all the kinetic energy to be extracted from the electrons, by ionizing collisions, so that eventually all the electrons become thermalized and can drift slowly to whatever surface they find at ground potential. This means that an anode is not needed for the discharge to take place; however most magnetron designs make use of some kind of anode for two main purposes. 3.2.1. Confinement of the discharge. It may be undesirable to have the discharge extend to an unpredictable location somewhere in the chamber. The use of an electrode at ground potential in the place where the discharge is to be stopped effectively puts an end to it there. It is worth noting that the high mobility along the magnetic field lines makes these virtual equipotentials, creating a virtual anode along the lines that touch the real anode.

3.2.2. Preventing unwanted discharge. The fact that it is usually impossible to avoid stray magnetic field frequently leads to undesirable erosion taking place in metal parts at cathode potential. To avoid this it is possible to bring the ground potential to a point situated inside the cathode dark space in the region where the discharge is to be stopped. Alternatively, all metal parts at cathode potential can be electrically insulated, thus shielding the discharge from those places. The latter method, however, can only be used in dc operation.

In Figure 3 we show a typical anode configuration serving both purposes. 3.3. Water cooling. Sputtering is a low temperature process. In fact, heating of the target, even if localized, can lead to evaporation of the material, which is a totally different process and generally unwelcome. In order to keep the target cool, the heat generated by the discharge must be removed by water circulation. The amount of heat that must be removed is : Q = 0.24 V" I

(4)

where V is the applied voltage (V) and I is the current (A). The water flow can be calculated by the following equation : F-

Q cpAT

(5)

where c is the specific heat of water, p is its specific mass and AT is the increase in water temperature. I f F i s to be given in I min ~, Vis in V , / i n A and Tin °C, the previous equation simplifies to: F=

1.44 x 10 2 VI AT

(6)

It must be realized, however, that the target temperature is usually higher than that of the cooling water, because heat is generated on the outer surface of the target and has to be transferred through the target cathode interface to the water. It would be beneficial, from this point of view, to make the water circulate in contact with the inner surface of the target, rather than attaching the latter to a cathode which, in turn, is water cooled. This

~.j~

Anode

Figure 3. Typical anode configuration. 719

J B A l m e i d a : Design of magnetrons for dc sputtering

procedure is not recommended because target materials may not be good structural materials and may not be able to withstand the pressure. Furthermore, as the target becomes eroded its mechanical strength is decreased and it may collapse suddenly, originating a disaster. 3.4. Mechanical considerations. Figure 4 shows details of a magnetron incorporating most of the experience the author has gained in designing several of these devices. The first point to be noted is the limited number of vacuum seals that are used ; this design uses only two standard o-ring seals. This is achieved by building the cathode as part of the vacuum enclosure and leaving the magnets and the water cooling circuit at atmospheric pressure. A lot of thought has been given to the fixing of the magnetron which is now done via a single 40 mm ID hole. The electrical and water supplies are passed through the fixing, at atmospheric pressure. The anode is not shown in the figure because it is attached to the inner surface of the chamber bottom or wall. The cooling of the cathode is done by water that completely fills the cup that forms the magnetic circuit, which is made of chrome-plated soft iron. The water is supplied to the bottom of this cup and is drained from the top, ensuring that it is permanently filled. The permanent magnets are of the bar type, placed vertically at the centre of the cup, immersed in water and in variable number according to the magnetic field that is desired.

from the magnetron and making use of mirrors located internally is worth considering. The provision of some feed-throughs, both mechanical and electrical, is usually unavoidable. These are used namely for : (i) supply of movement for shutters to allow the cleaning of the targets, prior to the deposition ; (ii) supply of movement for the samples, in order to improve film thickness uniformity and/or movement between different magnetron sources ; (iii) passage of electrical signals from monitors: thickness, temperature, etc. and (iv) supply of bias voltage for the sample to improve film adhesion. As movement feed-throughs tend to be a source of vacuum problems, it is advisable to use either the bellows or, better still, the magnetic coupled ones. 4.2. Pumping system. The pumping system must be capable of pumping the chamber to a vacuum of 10 6 mbar in a reasonable time and then maintaining a pressure between 10 3 and 10 : mbar while gas is being supplied to the system. The most c o m m o n systems make use of an oil diffusion pump, backed by a two stage rotary pump and followed by a cold trap ; some systems, though, use more sophisticated pumping, with cryopumps and/or turbomolecular pumps, in order to achieve cleaner background atmospheres or for special sputtering gases.

4. Vacuum requirements 4.1. The vacuum chamber. Virtually any vacuum chamber can be used for magnetron sputtering, if it has been designed for a vacuum of 10 6 mbar or higher. The fact that the magnetron uses a magnetic field for plasma confinement, generally precludes the use of magnetic materials in the vicinity and the author suggests that the whole chamber should be built with nonmagnetic materials; non-magnetic stainless steel is quite good. Some viewing windows, strategically placed, are useful for monitoring the progress of the sputtering process and anomalies of the discharge. These may become covered with sputtered material if they are close to the magnetron, especially if they are directly in front of it. Although this problem is not as severe as in evaporation systems, the alternative of pointing the windows away

5. Power supply It has been shown ~ that the current-voltage characteristic of a magnctron discharge follows a quadratic law : I = / ~ ( V - - V0) 2

(7)

where V0 is a threshold voltage necessary for maintaining the discharge and fl is a parameter strongly dependent on pressure. Table 1 lists some experimentally determined values for these two parameters.

Table 1. Values of/~ and V0 (reprinted from ref 11)

/--M~gne±

/

'

:::::: :

~

~-

720

V0 (V)

[/ (A kV 2)

Magnetron

A1

3.3 5.7 9.5

280 270 245

75 148 200

Planar Planar Planar

AI

1.3 4.5 5.5

400 373 335

39 63 105

RS-gun RS-gun RS-gun

Cd2Sn

2.5 4.0 10.0 14.0

220 180 175 175

26 33 53 78

Planar Planar Planar Planar

CdSe

2.5 5.0 10.0

390 390 390

45 75 120

Planar Planar Planar

Cr

6.0 12.0 19.0

330 320 300

750 1460 2000

Planar Planar Planar

c~hoa~

Hole For e[ectrlcO.[ COr~l:o~C%

Figure 4. Mechanical details of a magnetron.

p (mbar) x 10 3

Target

(see ±ex±)

J B A l m e i d a ." Design

of magnetrons for dc sputtering

The maximum current that any magnetron can stand depends mostly on the efficiency of the cooling system. One rough estimate can be made allowing for a maximum current density of 0.25 A cm 2 on the region of the target where the erosion is higher. Having estimated the maximum allowable current, it is easy to choose a power supply. The power supply must have a current rating equal to the m a x i m u m current and a voltage rating in excess of 600 V. It is important that it can be controlled in constant current m o d e because the discharge can be unstable and because it is the current that relates directly to the rates of erosion and deposition.

6. Gas supply The simplest method to control the gas supply consist of a leak valve directly fed by the gas cylinder. It may be necessary to reduce the pumping speed, in order to achieve the necessary working pressure with a reasonable supply of gas. A more sophisticated approach uses a servo-valve inserted in a control loop to maintain a fixed pressure. If there is the need to supply two gases at the same time, for instance, for reactive sputtering, it is advisable to use a mass flow controller to feed the second gas as a fixed percentage of the main one.

Acknowledgements The author wishes to acknowledge the financial support of the Luso-American F o u n d a t i o n for Development, Lisbon and of C R I O L A B , Porto. Special thanks also to Nelson Almeida of University of Minho for his ingenuity and excellent ideas.

References 1G F Weston, Cold Cathode Glow Discharge Tubes. Iliffe Books Ltd, London (1968). ZL Maissel and R Glang (eds), Handbook of Thin Film Technology. McGraw Hill, New York (1970). 3G K Wehner and G S Anderson, in Handbook of Thin Film Technology (Edited by L Maissel and R Glang), McGraw-Hill, New York (1970). 4j A Thornton, J Vac Sci Technol, 15, 171 and 188 (1978). 5R K Waits, J Vac Sci Technol, 15, 179 (1978). 6m von Engel (ed), Ionized Gases. Oxford, (1965). 7R Behrish (ed), Sputtering by Particle Bombardment. Springer, Berlin (1981). 8p Sigmund, in Sputtering by Particle Bombardment (Edited by R Behrish), p 9. Springer, Berlin (1981). 9j j Bessot, Techniques de L'ingbnieur, M1,657 (1985). 10j B Almeida, M I C Ferreira, M A P Santos and M D Ramos, Nuel Instrum Meth, B18, 651 (1987). ~JW D Westwood, S Maniv and P J Scanlon, J appl Phys, 54, 6841 (1983).

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