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REACTIVE PLANAR MAGNETRON SPUTTERING OF SiO2 FIELD OF THE INVENTION
This invention relates to a method for producing a transparent film of silicon dioxide on a substrate.
DESCRIPTION OF PRIOR ART
Silicon dioxide -films, have a variety of known uses in the microelectronics industry, and as well can be used for anti-glare coatings for CRT displays, automobile windshields, etc., due to the low refractive index and stability of silicon dioxide. A convenient way of depositing metal oxide films generally, on a substrate, is by sputtering, in particular by planar magnetron sputtering. Sputtering of some other rnetal oxides generally3 has been accomplished in the past, by sputter-ing directly from a target of the required metal oxide. However, sputtering from an oxide target is known to be considerably slower than sputtering from a metallic target. A krlown technique for forming films of the oxides oE some metals, is reactive sputtering. In this technique, the target is of the metal desired, while oxygen is introduced between the target and the substrate. The introduced oxygen then reacts with sputtered metal to form the corresponding oxide, which is then deposited on the substrate. Provided the oxygen flow rate is carefully controlled, the target will remain metallic with a resulting high sputtering rate being possible, in comparison to sputtering from an oxide target. For example, it has been known to produce oxides of Cd2Sn alloy, as well as oxide fiIms of In and In/Sn alloy, by reactive sputtering from metallic targets.
It is also known in sputtering, that there are many oxides which at a sufficient voltage, violently loose electrons through an arc or spark-like discharge to ground or to the anode. The foregoing problem is compounded at high current operation, typically above about 6 amps9 as sufficient curren~ is then available to sustain an arc once it is initiated. In an apparatus to produce films on a commercial scale, currents greater than 20 amps, and as large as 150 amps, are required to produce a sufficient throughput. The usual method to extinguish such an arc, is to momentarily turn off the sputtering power supply, which is obviously undesirable.
Silicon dioxide is an example of a material with the above problem. That is, silicon dioxide deposits formed on a cathode during sputtering, can violently lose electrons to cause an arc to the anode or ground, which arc is sustained in a commercial apparatus due to the high cathode current. Such deposits can occur on the cathode, in particular on the clamps usually arranged to hold the target on the cathode. These deposits occur even when the target is of pure silicon, since in conventional systems, some silicon sputtered from a pure silicon target will react with the oxygen present and inevitably be deposited on the remainder of the cathode, in particular on the target clamps thereof. The foregoing problem is so severe with regard to silicon dioxide, that very little literature can be found on d.c. reactive sputtering of silicon, despite the fact that a high rate, reactive sputtering technique which could produce silicon dioxide, is of great interest in the areas previously mentioned.
~ '7 It is desirable then, to provide a relatively high rate reactive sputtering method to produce silicon dioxide Eilms, which method is not severely affected by the above mentioned violent discharge from silicon dioxide deposits on the cathode, and resulting sustained arc from the anode or ground to the cathode.
SUMMARY OF THE INVENTION
A method of producing a transparent film of si]icon dioxide on a substrate is provided. The method requires sputtering silicon toward the substrate from a silicon target which forms part of a cathode. The target is clamped along edges of it to the remainder of the cathode, by a clamp set of at least one clamp an outer surface of each of which extends upwardly away from the target surface to a position thereabove. The target is further covered by a baffle with a flux receiving strip, disposed between the entire target and the substrate. Oxygen is admitted between the substrate and baffle at a rate sufficient so that at least a portion of the silicon sputtered toward the substrate, is oxidized, while the target remains metallic. An rf discharge is applied to the substrate at a power density o-f substantially .08 to .16 watts cm~2. The substrate is dimensioned and spaced from the cathode so that the rf discharge at the substrate, generates a self-bias voltage on the substrate between substantially -70 to -100 volts. All of the foregoing steps are executed simultaneously.
Preferably, in the above method, each clamp of the clamp set which initially extends at the target surface, substantially perpendicularly upward from the target surface.
Furthermore, during the method, the silicon is preferably covered by a baffle with a flux receiving strip which has a total open area of no more than 50% of the target surface area (the "target surface area" referring to the surface area of the target received on the cathode (and hence forming a part thereof), from which sputtering can take place).
The method additionally preferably comprises maintaining the total gas pressure at less than substantially .5 Pa. Usefully, the silicon is sputtered from a preferably elongated silicon target having two opposed side edges, and which target is clamped along substantially the entire length of its two side edges by two elongated clamps of the clamp set, each clamp as described.
DRAWINGS
Embodiments of the invention will now be described, with reference to the Figure which is a transverse cross-section of an apparatus constructed to execute the method of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Referring now to the drawing, a planar magnetron sputtering cathode 2 is shown, which cathode is elongated, and similar in construction to that disclosed in U.S. Patent No.
4,486,289, entitled PLANAR MAGNETRON SPUTTERING DEVICE, issued December 4, 1984,by Parsons et al. It will thus be appreciated that the entire assembly shown in the Figure is elongated in a manner as shown in that patent. Cathode 2 has body 4 with a oo]ing cavity 6 disposed therein, and two recessed target re-ceptacles 5 formed in opposite sides thereof. A row of permanent magnets 8 is disposed within cooling cavity 6 in a manner again similar to that shown in the foregoing patent to Parsons et al.
Cathode 2 also includes a clamp set consisting of two elongated clamps 10 having outer ends 12 which are removably fastened by screws or the like (not shown) to cathode body 4. Clamps 10 also have curved inner ends 14 such that when clamps 10 are clamping targets 28 of substantially pure silicon in target receiving surfaces 5 of cathode 2 in the manner shown in the Fig~lre, an outer surface 11 of the clamps extends gradually away, initiaLly perpendicularly, from respectlve target surfaces to positions thereabove; that is, to positions to the left and right respectively of silicon targets 28 as shown in the Figure.
The apparatus shown also includes a frame formed of lower and upper frame members, 15 and 16 respectively, to which ground shields 21 are mechanically and electrically connected.
Cathode 2 is connected to frame members 15 and 16, but electric-ally insulated therefrom, by virtue of elongated lnsulators 18.
The lower (from the direction as viewed in the Figure) of such insulators 18 has a cooling fluid inlet passage 19 extending therethrough, while the upper of such insulators 18 has a cooling fluid outlet passage 20 extending therethrough. Passages 19 and 20 communicate with cavity 6 in cathode 2 by means of elongated passages 21, substantially equal in length to cavity 6. Frame members 15 and 16 are also provided with elongated fluid cooling lines 17 extending therethrough, to cool the members 15, l6.
Two metallic substrate electrodes 26 are disposed apart from respective target receiving surfaces 5, and hence respective received targets 28, as shown in the Figure. Electrodes 26 are of such a size, and spaced an appropriate distance from target receiving surfaces 5 and received targets 28, such that when an rf discharge is applied to them of a power density of substantially between .08 to .16 watts cm 2, each substrate electrode 26 generates a self-bias voltage of between substantially -70 to -100 volts (that is + 10 volts). The actual spacings of all of the elements of the apparatus shown, and their dlmensions, are listed below in Table 1. Substrate electrodes 26 are arranged to receive adjacent to them, a continuous, timed feed of an elongated substrate sheet 30, the Eeed being from either the upward or downward direction as viewed in the Figure.
Substrate sheet 30 may for example, be a transparent, plastic sheet which is continuously fed from a roll.
Two baffles 23, are disposed between respective targets 28, so as to cover respective targets 28. Baffles 24 each have an elongated flux receiving strip 24, which is disposed to be in the path of most of the sputtered silicon from the corresponding target 28, during operation of the apparatus. Each strip 24 has a plurality of baffle openings 25 therethrough, evenly spaced thereon. A sufficient number of openings 25 are provided, and with an adequate cross-sectional area, such that the total open area (i.e. the total cross-sectional area of openings 25 above exposed surfaces of targets 28) of each strip 24, is no greater than substantially 50% of the surface of each target 28 (i.e. the exposed surface of the received target 28).
Operation of the above apparatus in accordance with the presenc method will now be described. First, the entire assembly ~25i~
Item Dimension (inches) a b 5/8 c 1 7/8 d 2 e 2 f 3 g 6 h 7 3/4 ~f~
shown in Figure 1 is disposed within a housing, which is evacuated. Frame members 15 and 16 are connected to the usually metallic housing, and the positive side of a suitable clc power supply connected to such housing, and hence to frame members 15 and 16 and target shields 22, which will act as anodes. The negative sicde of the power supply is of course connected to cathode 2 by connection of the negative lead to metallic cooling lines 19 and 20. An rf power source is connected to substrate electrodes 26, the power source producing a power density on each of the substrate electrodes 26 of between about .08 to .16 watts crn . As previously mentioned, due to the relative dimensions of substrate electrodes 26 and cathode 2, and the distance the foregoing are spaced apart, substrate electrodes 26 will, as a result of the foregoing rf power supply discharge, generate a self-bias voltage of between about -70 to -100 volts. A suitable cooling fluid, such as water, is then supplied through cooling lines 17, and also into inlet cooling line 19 and hence into cavity 6 and then out of outlet cooling line 20. Argon and oxygen are admitted into the evacuated housing, between baffles 23 and substrate electrodes 26. The flow rates are maintained so as to maintain a total pressure of between about .3 to 2.0 Pa, and preferably below .5 Pa in order to minimize scattering of sputtered silicon, and thereby maximize the deposition rate of silicon dioxide. Provided the oxygen is admitted between baffles 23 and electrodes 26, the exact location of its admission, and the direction in which it was admitted, does not appear to have any effect on the deposited silicon dioxide films. When the apparatus of the Figure has the dimensions given in Table 1, and the cathode sputtering is about 10 kw, it has been found by ~ 25'~
routirle experimentation, that a suitable oxygen flow rate is about 40 cc/minute.
As a result of the above, sputtering of silicon targets 28, will take place. It has been found that of course much of the sputtered silicon will be deposited on baEfles 24, in particular on flux receiving strips 24 thereof, as well as on clamps 10. A portion of the silicon will of course pass -through openings 25 in strips 24, and react with oxygen being introduced between baffles 24 and electrodes 26, to be deposited as a film of silicon dioxlde on those portions 29 of substrate sheets 30 disposed immediately adjacent front faces 27 of respective substrate elect-rodes 26. Such portions 29 at any time act as the actual substrates at that time. Baffles 23, prevent sufficient oxygen from reaching targets 2~, as to cause a problem with target oxidation. It should be noted that oxygen flow rates must of course be kept reasonable, as some oxygen will pass through openings 25 in strips 24, and if the oxygen flow rate is too high, silicon targets 28 may become oxidized, with consequent severe reduction in sputtering rate. However, as long as the oxygen flow rate is not excessive and for the particular dimensions specified in Table 1, and the total open area of strips 24 is no more than 50% as described, target oxidation is not a problem. Most, but not all, oxygen which does pass baffles 24, will be gettered on baffles 24 by sputtered silicon flux.
The above apparatus was operated as described, with no cathode to anode arcing problems generally associated with sputtering of silicon, as a result of violent discharges producecl from silcion dioxide deposits as previously described. It has been found however that if clamps 10 are replaced with target ~ ~ 5 ~
clamps which are flush with the outer surfaces of targets 28, as has often been the construction of target c]amps in the past, persistent arcing from such flush clamps would start, and then quickly destroy process control, after about 20 minutes of operation regardless of the sputtering power and operating pressure. To correct such problem, either the power to cathode 2 had to be turned off, or the oxygen flow rate had to momentarily be reduced to a low level. In such case, stable operation would again then continue Eor about 20 minutes, beEore the arcing problem resurfaced. Such monitoring of the apparatus for arcing problems, and the consequent shut off of power to cathode 2, or reduction in oxygen flow rates, is obviously undesirable in a large scale operation, and will affect control of the silicon dioxlde films deposited on the substrates. By using clamps 10 with outer surfaces 11 of a shape which extends above the surfaces of targets 28 as described, in conjunction with baffles 24 described, the foregoing arcing problem was completely eliminated. Apparently, this is as a result of a high deposition of silicon on inner ends 14 of clamps 10, which inner ends 14 are closest to targets 28. In prior arrangements where the target clamps were flush with the target surfaces, such a high rate of deposit of metallic silicon could not occur. The high rate of metallic silicon on inner ends 14 of clamps 10, apparently ensures that the outer surfaces 11 of clamps 10 at inner ends 14, remain conductive. As a result, apparently no charge can build up on any silicon dioxide deposits which may form on inner ends 14, which charge might otherwise cause an arc to an anode. Thus, the fact that the outer surface 11 of each clamp 10 extends upwardly away from the corresponding target 28 when received in a target seat 5, to a position above such target 28, is particuarly important in preventing the arcing problem previously associated with deposition oE silicon dioxide -films by sputtering.
With the particular dimensions shown in Table 1, the total open area of each of flux receiving strips 24, must be no greater than 50% of the surface area of each corresponding target 28. If the open area of strips 24 is larger than 50%, stable operation of the silicon sputtering target near the metal to oxide mode transition point, is very difficult or impossible to maintain, due to the arcing problem associated with silicon dioxide build up. The foregoing 50% maximum open area of strips 24, was again obtained for the geometry shown in Figure 1, and may vary somewhat depending upon the exact configuration of the apparatus used in any particular case. Such maximum open area, can be determined in different apparatus configurations, by no more than routine experimentation, the open area belng increased to the point where arcing or target oxidation becomes a problem.
It should be noted that in any event, periodic cleaning of strips 24 is necessary during continuous operation of the apparatus, otherwise openings 25 will become filled with deposits.
The relative dimensions of the substrate electrodes 26 and the cathode 2, as well as their spacing, are important from the point of vlew of generating the required negative bias on the substrate electrodes 26. As mentioned, this should be between substantially -70 to -100 volts. Apparently equally good silicon dioxide films were obtained on the substrates, with the negative bias at any voltage within the foregoing range. However, when the negative bias was down to substantially -50 volts, the ~52~
silicon dioxide film had a yellow tinge. In addition, when the negative bias was larger than -100 volts (i.e. the absolute value of the negative bias was greater than 100 volts), substantial resputtering attended to occur on the substrates 30. Such resputtering tends to damage heat sensitive substrates in particular, such as plastic films.
As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice oE this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.