GB2191787A - Process and arrangement for sputtering a material by means of high frequency - Google Patents

Abstract

In the sputtering of a material by means of high frequency discharge between two electrodes (5, 6), as illustrated in Fig. 1, the problem arises that material is sputtered off both electrode surfaces if the electrode surface which is actually not to be subjected to sputtering is not at least ten times as large as the surface of the electrode which carries or consists of the material, the sputtering of which is desired. Accordingly in order to prevent undesired accompanying sputtering even in relation to a smaller surface area of the electrode (6, 11) which is not to be sputtered, arranged in the region of said electrode are magnets (9) so that the force lines (10) at least partially pass out of the surface which is not to be sputtered, and back into same again, and a so-called electron trap is formed over the electrode surface of the electrode which is not to be sputtered, thereby preventing the emission of sputtered material therefrom. <IMAGE>

Description

SPECIFICATION Process and arrangement for sputtering a material by means of high frequency The present invention relates two a process and an arrangementforsputtering a material by means of high frequency comprising at leasttwo electrodes of which at least one carries the material to be sputtered and at least one other forms the opposite electrode.

In order to increase the plasma density in the sputtering space or chamber in sputtering arrangements, magnetic fields have already long been in use. Initially such arrangements were developed for direct current sputtering wherein the direction of the sputtering effect is clearly defined by the polarity ofthe electrical field.

It is always the negatively charged cathode which is sputtered and forthat reason reference is frequently also made to cathodic sputtering. By means of magnetic fields in the discharge chamber, it is possible to increase the length ofthe electron paths and thus increase the number of ions produced per electron. US patent No.4 166018 (Chapin) describedforthefirsttime an arrangement having flat electrodes, a so-called planar magnetron arrangement, in which the magnetic field lines pass out of the cathode surface and back into same and therefore form a kind oftunnel over the surface of the cathode.The electrons of the plasma are so-to-speakenclosed in the tunnel and reflected by the negatively charged cathode. If magneticfield tunnel is also still closed in itself at the plane of the cathode, a large part of the electrons can be kept within the tunnel, on an endless path. Such an arrangement represents a highly economical way of cathodic sputtering by means of dcvoltage.

It is also known to produce a sputtering effect by means of high frequency. In that case, in spite of using a high frequency voltage which continuously alternates in respect of polarity, there is a preferential direction in regard to the sputtering effect, more specifically, experience has shown that it is always the side with the smaller electrode surface area from which material is preferentially sputtered off(lBM J.Res.Deveiop. March 1970, page 168). The sputtering-off effect is of course not strictly limited to the side with the smallerelectrode, but at the same time there is a sputtering-off effect also occurring from the larger electrode surface area if the difference between the plasma potential and the potential of the larger electrode exceeds the sputtering threshold.The lowerthe ratio in respect of surface area between the larger and the smaller electrode surfaces, the greaterthe amount of material that is also sputtered off the larger electrode surface at the same time, until, at a ratio of 1:1, neither of the two surfaces is any longer preferred in regard to the direction of sputtering-off. In the case of arrangements which are supported by a magnetic field, ofthe previous type (magneticfield atthe smallerelectrodeto be sputtered), sputtering of the larger electrode can still occureven with ratios in respect ofsurface area of 10:1.

In order to achieve such a ratio in respect of surface area between the two electrodes in high frequency sputtering in a vacuum chamber, that sputtering-off is practically ensured only from one side, hitherto the wall of the vacuum chamber itselfwasfrequently used as the opposite electrode, the surface area of which, in relation to the electrode carrying the material to be sputtered, was substantially largerthan 10:1,so that sputtering ofthe chamberwall was something that no longer had to be feared.

Another option which is described in US patent specification No. 3 661 761 provided that that electrode in the vacuum chamber, from which material was not to be sputtered off, should be provided with protuberance portions in order suitably to increase the surface area thereof in relation to the electrode carrying the material to be sputtered.

Magneticfields have also already been used in relation to high frequency sputtering in order to increase the plasma density, thereby to achieve a higher sputtering rate. Such an arrangement is illustrated for example in US patent specification No. 3 369 991. In the arrangement illustrated therein in Figure 1, disposed opposite the electrode 22 carrying the material Tto be sputtered, as a counter-electrode, is a holding means 80 for substrates which are to be coated, wherein there are provided magnets 90 whose field lines pass practically perpendicularly through both the substrate and also the target surface Tfrom which material isto be sputtered off.In order to guarantee sputtering only ofthe latter, once again those surfaces which are atthe same potential as the holding means are larger in that, besides other earthed components, at leastthe metallic floor of the vacuum chamber additionally acts as an electrode surface. It can be estimated from Figure 1 of the above-indicated US patent specification that the total of the earthed electrode surface areas which are notto be sputtered, to the total of the electrode surface area T carrying the material to be sputtered, is in a ratio which is certainly substantially higherthan 10:1 so that material is only sputtered off the latter.

As stated therefore there is no longer any fear of substantial accompanying sputtering of the larger electrode surface in high frequency sputtering with a ratio in respect of electrode surface areas of higherthan 10:1.An interface ortroublesomeeffectmayoccurinthe range of 10:1 to about 5:1, particularlywhen operating with high sputtering voltages. However, in the semiconductor art and often also when producing thin films for optical purposes, the sputtering of material from both electrodes is already no longer acceptable with ratios in respect of surface areas of higherthan 5:1 and even more so with a ratio below such a figure, because of the danger of contamination of the surfaces to be processed or the films or layers to be applied thereto.Sputtering of material from both electrodes also results in a substantially worse level of efficiency because a part of the energy of the gas discharge is used for the undesired sputtering ofthe opposite electrode.

In a high frequency sputtering installation, British patent specification No. 1 358411 discloses maintaining a weak axial magnetic field (which promotes the gas discharge at low pressures) between the two electrodes of which one carries a material to be sputtered and the other carries a substrate which isto be coated, and, in orderto protect the substrate or the layer growing thereon from electron bombardment, generating in the direct vicinity in front of the working table carrying the substrate a comparatively strong additional magnetic field whose force lines form arcs overthe substrate surface. The first-mentioned magnetic field is typically of a strength of only 100 Gauss (0.01 T) while the second magnetic field has a magnetic flux density of 1000 Gauss (0.1 T).Due to the co-operation of the two fields, the electrons formed in the gas discharge are deflected from the central region ofthetable (where the substrates lie) and electron bombardment ofthe substrates is thereby suppressed. However, if the surface area ratio ofthe electrodes falls below a value of 10:1, it is no longer possible with that known arrangement to suppress the accompanying sputtering effect, in respect ofthe larger ofthe two electrodes.

In comparison therewith the underlying object of the present invention is that of providing a process and an arrangementforsputtering a material by means of high frequency, wherein accompanying sputtering of the larger electrode surface is very substantially suppressed, even with disadvantageous surface area ratios.

That object of the invention can be achieved bythe process and an arrangement as set forth in the claims.

The effect ofthe magnetic field arrangement according to the invention, for suppressing sputtering ofthe opposite electrode which is of larger surface area, is based on the consideration that an electron trap is formed by the magnetic field lines which pass into and out ofthe opposite electrode and which form an arc thereover, in a similar manner as over the cathode in previous do voltage sputtering arrangements.That achieves a substantial increase in the charge carrier density in front of the electrode which is not to be sputtered and surprisingly that does not resultfor example in increased sputtering off of material from said electrode (as would have been expected having regard to the known effect, of substantially increasing the sputtering rate, of a magnetic electron trap in front of the cathode in the case of planar magnetron arrangements), but rather it results in substantial suppression ofthe sputtering-off phenomenon, as is confirmed by experience with the apparatus according to the invention. That is possibly explained bythefact that the increased charge carrier density in front ofthe surface which is not to be sputtered gives rise to a substantially lower voltage drop than would occurwithoutthe presence of that magnetic field. That means thatthe sputtering effect can also be practically completely suppressed at that electrode side. Sputtering then no longer occurs even with a ratio in respect of surface area of the largerto the smaller electrode, of below 10:1.It is only if the ratio ofthe electrode surface area which is notto be sputtered to the electrode surface area which is to sputtered falls below a figure of 1:1 that high frequency sputtering ofthe electrode which is nowsmaller can also no longer be prevented by a magnetic field. It is precisely that latter situation thatoccurs in the known planar magnetron arrangements where the surface to be sputtered is substantially smallerthan the insidewall of the sputtering chamber, which wall serves as the counter-electrode. While therefore in dc voltage cathode sputtering the electron trap in front ofthe cathode serves to increase the sputtering rate, the electron trap according to the invention provides for a reduction in the sputtering effect at the larger electrode. That prevention relies on a modification of the voltage distribution in the discharge space.

With suitable dimensioning (Figure 1) ofthe magnetic field configuration and the geometry ofthe discharge space,the magnet arrangement according to the invention makes it possible to attain the known advantages of a magnetisfield-supported HF-discharge, even with disadvantageous surface area ratios. That relates in particuiarto higher rates on the surface to be sputtered, at lower discharge voltages and low gas pressures. It is also possible to provideforvery homogenous material removal profiles at the surface to be sputtered, which hitherto was only possible with magnetic fields which were moved relative thereto.

Sputtering ofthe electrode which is larger in terms ofsurface area can also be reliably prevented by means ofthe invention in known arrangements (magnetic field in the region of the surface to be sputtered). For optimum operation of the process according to the invention it is recommended that the magneticflux density should be so set in the region ofthe opposite electrode that the plasma density in the electron trap which is produced under the action ofthe magneticfield is at least one third greater than the plasma density which would occuratthe same location under otherwise identical conditions but without the action ofthe magnetic field.By measuring the plasma density on the one hand with a magnetic field and on a second occasion without a magnetic field, under otherwise identical conditions, it is therefore possible to ascertain beforehand whether the adjustment of the magnetic field is correct. Another parameter which is very useful for adjustment purposes is the devoltage potential difference between the counter-electrode and the plasma in the discharge space. The magnetic field should be of such a value that the do voltage potential difference assumes a value of less than 100 V.

Aservice-proven arrangement according to the invention for sputtering a material by means of high frequency is characterized in thatthe ratio 4)1/4) ofthe sum 4)1 of the absolute amounts of the magnetic fluxes passing through the opposite electrode surface, to the sum 4 ofthe magnetic fluxes which pass through the opposite electrode surface and which are taken into account with theirsign is greaterthan 2:1.

The invention will be described, by way of example, in greater detail hereinafter by means of embodiments with reference to the accompanying diagrammatic drawings. In the drawings: Figure 1 shows an etching installation in which the surface to be sputtered are at earth potential and the counter-electrode which is acted upon bytunnel-shaped magnetic field lines is connected to the high frequencyvoltage, Figure2 is a diagrammatic view of an arrangement in which the high frequency feed is at the side to be sputtered whereas the opposite electrode which is not to be sputtered is formed by the earthed wall ofthe vacuum chamber, and Figure3 is a diagrammatic view of an installation wherein the active surface of the electrodeto be sputtered is equal in size to the surface ofthe opposite electrode.

Figure 1 shows a vacuum chamber 1 which comprises an upper portion 2 which is substantially in the shape of a downwardly open box having a flange resting on a bottom 3 in the form of a plate firmly fixed to the flange and provided with a gas inlet 21.A sealing 22 is provided between the flange and the bottom 3. The chamber 1 can be evacuated byway of a suction connection 4to which a vacuum pump (not shown) is connected. On the side towards the vacuum space, the bottom plate 3 is formed as an electrodeS which carries the substrate 5' which is to be sputtered, that is to say in this case etched, the electrode 5 being disposed opposite a hood-shaped electrode 6 constituting the opposite electrode which is notto be sputtered.In operation the latter electrode 6 is connected to an external high frequency source, with the interposition of a capacitor 8, byway of high frequency voltage supply means 7 which is passed vacuum-tightlythrough the chamberwall. As can further be seen from Figure 1, the vacuum chamber 1 is enclosed by an electromagnetic or a permanent magnet 9 whose field lines, extending into the chamber 1, form tunnel-like arcs in accordance with the invention on the inward side of the cylindrical portion 11 ofthe electrode 6, in that at least a substantial part of the force lines 10 pass out ofthe surface 11 and backinto same.

The high frequency voltage supply means 7 has a voltage-carrying conductor 12 which is carried by an insulating plate 13which in turn is vacuum-tightly screwed to an opening 16 in the wall ofthe chamber 1 by means of a flange ring 14 and using a sealing ring 15. The conductor 12 projects through the opening 16 into the chamber 1 where it is formed as a carrier 17 for the electrode 6. An insulator 18 and a metal nut 19 serve for pressing a sealing ring 23 againsttheconductor 12 and the insulating plate 13.

The above-described arrangement can be used for the sequential processing of individual wafers in a semiconductor production installation. In that arrangement, some 10 nm of the oxide is to be etched away from the surface of the wafer before the actual coating operation is carried out in the same vacuum chamber.

Etching rates of over 30 nm/minute for SiO2 on an Si-wafer were achieved with the arrangement shown in Figure 1.A crucial consideration in that respect is that, on a 6" (152.4 mm) wafer, a degree of uniformity of better than +/- 5% is achieved and with a power density of around 1.2W/cm2 in a 13.56MHz argon discharge of 0.5 Pa pressure, a self-bias voltage of only 600 VDe (!) occurs.

Hitherto, approximately the same etching rates of about30 nm/minute could only be achieved by means of extremely high voltages of about 2 kV and pressures around 1 Pa. The use of such high voltages and high pressures hitherto represented a problem in semiconductor manufacture, which now seems to be solved by the invention.

With the described arrangement, for example a ratio of 14)1/4) of 3.5 was measured for the opposite electrode which is notto be sputtered, wherein I 4 1was of value of 7 x 10-4V.s and was ofavalue of 2 x 1 V.s. Those magnetic fluxes were determined by point-wise measuring of the entire electrode surface by means of a Hall probe (for example a Bell Gaussmeter, model 620).

The following table shows that and two further examples, in tabularform: Example Ibl IPI/ No.1 7 2 3.5 No.2 7.5 1.5 5 No.3 8.7 0.3 29 (All data of the magneticfluxes in 1 0-4 voltseconds 10-4weber; 1 Wb = 1 Vs).

As a measurement in respect of the effect which is achieved by the magnetic field arrangement according to the invention, it is possibleto measure the potential difference between the plasma in the discharge space and the electrode which is not to be sputtered; in all three examples, that potential difference was below 100 volts.

Figure 1 shows as a further possible configuration that a magnet20 may also be arranged behind (beneath) the electrode5 carrying the substrates to be sputtered, in order in that way additionally to achieve a similar effect of increasing the sputtering rate, as in the known magneticfield-supported do voltage cathodic sputtering apparatuses. In that case, tunnel-like magnetic field lines are also produced over the surfaces from which material isto be sputtered, the field lines there resulting in an increased level of charge carrier concentration and thus as stated an increased sputtering rate.As the electrode surface 5 orthe surface area of the substrateto be sputtered is substantially smaller than the surface area of the opposite electrode, sputtering occurs there at any case, which cannot be suppressed by the field of the magnet 20. However possible sputtering ofthe opposite electrode 6 is prevented in this arrangement in accordance with the invention by the magnetic field 10.

In Figures2 and 3 parts which are attributed the same function in regard to the invention are denoted bythe same reference numerals as follows: reference numeral 25 denotes the electrode carrying the surfaces to be sputtered, 26 denotes the opposite electrode which is not to be sputtered and in the vicinity of which are arranged respective magnets 27 whose field lines 28, in accordance with the claim, at least in part pass out of said electrode 26 and back into same again and therefore form a tunnel over the electrode surface. The chamber wall is denoted by reference numeral 29, the evacuation connection by reference numeral 30, the high frequency supply means by reference numeral 31 and the high frequency generator by reference numeral 32.In Figure 2, the lower portion of the chamber wall represents the opposite electrode which is not to be sputtered. On the other hand, in Figure 3 the lower portion ofthe chamber wall is the electrode to be sputtered orthe carrier of the material to be sputtered. In comparison with the arrangement shown in Figure 2, the arrangement of Figure 3 is also distinguished in that the active electrode surface to be sputtered and the counter-electrode are equal in size in terms of surface area; nonetheless, by virtue of the arrangement according to the invention of a magnet behind the counter-electrode, sputtering thereof is substantially prevented, (while in the previous state ofthe art, with electrode surfaces ofthe same size, when using high frequency, both were subjected to sputtering in likefashion).

In the present description the reference to active electrodes means those electrode portions through which the discharge current flows. It is desirableforthe back of the electrodes, which is remote from the sputtering space, to be provided with screening or shield means as indicated for example at 33 in Figure 2, whereby discharge is suppressed on the back of the electrodes (in perse known manner). Parts ofthe chamberwall may also be used for such so-called dark space screening means, as illustrated in Figures 1 and 3.

Claims (9)

1. A process for sputtering a material by means of high frequency discharge between at leasttwo electrodes, of which at least one electrode ca rriesthe material to be sputtered and anotherformsthe opposite electrode which is not to be sputtered, wherein a magnetic field is produced in the region ofthe opposite electrodethefield lines of which, for a substantial part forming an arc over the opposite electrode, pass out ofthe opposite electrode and return backthereinto, wherein the magnetic flux density is so adjusted in a region of the opposite electrode that an electron trap is formed in front of said opposite electrode.

2. A process according to Claim 1 wherein the magnetic flux density is so adjusted in the region ofthe opposite electrode that the plasma density in the electron trap which is produced underthe effect ofthe magnetic field is at least one third greater than the plasma density at the same location which would occur under otherwise identical conditions but without the action of the magnetic field.

3. A process according to Claim 1 or 2 wherein the magneticfield is so adjusted that the voltage difference between the opposite electrode and the plasma in the discharge space assumes a value of less than 100V.

4. An arrangementforsputtering a material by means of high frequency comprising at leasttwo electrodes of which at least one carries the material to be sputtered and another forms the opposite electrode which is not to be sputtered, and magnets for producing a magneticfield in the region of the opposite electrode, which are so arranged that a substantial part of the magnetic field lines passes out ofthe opposite electrode and backthereinto, forming an arc over the surface ofthe opposite electrode, wherein the ratio F/F' of the whole of the opposite electrode surface F actively participating in the discharge to the whole F' ofthe electrode surfaces carrying the material to be sputtered is at most 10:1.

5. An arrangement according to Claim 4 wherein the ratio I | of the sum l + l 14)1 ) of the absolute amounts ofthe magnetic fluxes passing through the opposite electrode surface, to the sum + of the magneticfluxes which pass through the opposite electrode surface and which are taken into account with their sign is greater than2:1.

6. An arrangement according to Claim 4 or 5 wherein the electrodes practically completely enclose the discharge space except four apertures serving for evacuation and for mutual electrical insulation.

7. An arrangement according to Claim 4,5 or 6wherein the electrode carrying the material to be sputtered also has associated therewith a magnetic field whose force lines at least in part pass out of said electrode and return back into same.

8. A process according to Claim 1 substantially as herein described with reference to the accompanying drawings.

9. An arrangement according to Claim 4 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawings.