Abstract:
An improved apparatus for preparing a coating by subjecting gaseous reactant precursors to a glow discharge is disclosed. The improvement comprises an electrode which is larger in all dimensions than the substrate which is to be coated. Electrically insulated magnets are arranged such that the area of the magnets is proportionally distributed with respect to the area of the substrate to be coated, whereby enhanced uniformity of quality and thickness of the deposited coating are provided. Unexpectedly the coating deposition rate increases substantially without increasing the power required or precursor consumption, and further, the temperature does not markedly increase during deposition.

Description:
This invention relates to an apparatus for depositing a coating onto a substrate by glow discharge. More particularly, the invention relates to an improved electrode used in such an apparatus. 
     BACKGROUND OF THE INVENTION 
     Kaganowicz, in U.S. Pat. No. 4,328,646, disclosed a method for preparing an abrasive silicon oxide (SiO x ) coating on a plastic substrate, such as a patterned lapping disc, which includes the steps of subjecting silane and a gaseous, oxygen-containing compound to a glow discharge. The resulting abrasive SiO x  coatings deposited on the substrate are suitable for lapping a hard material, such as diamond. 
     Abrasive coated discs of the type mentioned above have been successfully employed to micromachine diamond articles of micron and sub-micron dimensions, such as styli for high density information discs. In order to meet the stringent specifications of a high density information disc playback system, the diamond styli must be machined to a high degree of accuracy. It has been found that uniformity of quality and thickness of the SiO x  coating on the stylus lapping disc is important to the successful manufacture of video disc styli. 
     Many commercially available glow discharge systems suitable for depositing SiO x  coatings onto lapping disc substrates employ electrodes which are smaller in area than a typical stylus lapping disc (30.5 cm in diameter). To adapt to such a system, the lapping disc substrate is mounted parallel to the electrodes so that the disc can be rotated through the electrode area in order to coat the entire substrate surface with the product deposited during the glow discharge. Because it is known that coatings of different quality and thickness are deposited in different areas of a glow discharge system, magnets have been added to, but electrically insulated from, the electrodes. The magnets keep the glow in the electrode area, thereby providing somewhat better uniformity of the deposited coatings. However, the variations across a substrate with respect to uniformity of thickness and quality of SiO x  coatings deposited on the lapping disc in this manner, although improved, are still less than desirable for the highly precise manufacture of the above described styli. Also, when using an electrode smaller than the substrate, if a higher power is sought to be used to promote a more rapid deposition, heat builds up in the system which warps the plastic substrates. Merely enlarging state of the art electrodes to be comparable in size to the lapping disc substrate does not enhance the uniformity of the deposit. 
     It should be stressed that the desired improvements in the uniformity of quality and thickness of a coating across the substrate should not be obtained at the expense of established standards for the production of stylus lapping discs, if possible. Factors such as power usage, SiO x  precursor consumption and deposition rate are important economic considerations. Further, the temperature in the system should be minimized as a practical matter to avoid damaging the heat-sensitive plastic substrates used in the production of lapping discs. 
     SUMMARY OF THE INVENTION 
     An improved apparatus for preparing a coating by subjecting gaseous precursors to a glow discharge is disclosed. The improvement comprises an electrode which is larger in all dimensions than the substrate which is to be coated. Electrically insulated magnets are arranged such that the area of the magnets is proportionally distributed with respect to the area of the substrate to be coated, whereby enhanced uniformity of quality and thickness of the deposited coating are provided. Unexpectedly, the coating deposition rate increases substantially without increasing the power required or precursor consumption, and further, the temperature does not markedly increase during deposition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an electrode used in the prior art glow discharge systems. 
     FIG. 2 is a lateral view illustrating the size of an electrode and arrangement of magnets with respect to the substrate to be coated in a prior art glow discharge apparatus. 
     FIG. 3 is a cross-sectional view of an apparatus suitable for depositing a glow discharge coating according to the present invention. 
     FIG. 4 is a lateral view illustrating the size of an electrode and an arrangement of magnets with respect to a substrate to be coated according to the present invention. 
     FIG. 5 is a cross-sectional view of an electrode used in the glow discharge apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross-sectional view of a prior art electrode assembly 10 and a substrate 44 which is to be coated. The prior art electrode assembly 10 comprises an electrode 12 separated from a magnet compartment 18 containing magnets 16 by insulators 14. The prior art electrode assembly 10 measures about 12 cm wide by 15 cm high by 6 cm deep. 
     FIG. 2 is a lateral view of a prior art electrode assembly 10 showing the placement of the assembly 10 and the arrangement of the magnets 16 in relation to the substrate 44 to be coated. Although the substrate 44 is rotated, at any given instant approximately 25% of the inside track is within the electrode area compared to only about 10% of the outside track. This causes coatings of SiO x  deposited along the inside track to be as much as 40% thicker than coatings deposited on other areas of the substrate 44. 
     A glow discharge apparatus of the present invention is shown in FIG. 3 generally as 20. The glow discharge apparatus 20 includes a vacuum chamber 22, which can be a glass bell jar. In the vacuum chamber 22 are two electrodes 26 and 32 which can be a screen, coil or plate of a material that is a good electrical conductor and does not readily sputter, such as aluminum. The electrodes 26 and 32 are connected to a power supply 36, which may be DC or AC, to produce a voltage potential. Behind the electrodes 26 and 32 are magnets 28 and 34. These magnets 28 and 34 are typically electrically insulated from the electrodes 26 and 32. The magnets 28 and 34 are typically behind the electrodes 26 and 32 but can be placed in any location where they would keep the glow in the vicinity of the electrodes 26 and 32 without interfering with deposition. The electrodes 26 and 32 and the magnets 28 and 34 make up the electrode assemblies 24 and 30. An outlet 38 from the vacuum chamber 22 allows for evacuation of the system and is connected to a pumping station, not shown. A first inlet 40 and a second inlet 42 are connected to gas bleed systems, not shown, for adding the reactants employed to prepare the glow discharge deposited coatings. 
     In order to begin deposition of a coating on the substrate 44, a glow discharge is initiated between the electrodes 26 and 32 by energizing the power source 36. For deposition the current should be in the range of about 200 to 900 milliamps, preferably about 400 to 700 milliamps. Any convenient frequency may be employed. The potential between electrodes 26 and 32 is generally about 1,000 volts. The magnets 28 and 34, along with the energized electrodes 26 and 32, create an electromagnetic field which keeps the glow in the vicinity of the electrodes 26 and 32 for a more efficient reaction of the precursors. As the precursors react in the glow, the coating is deposited onto the substrate. The power for the glow is maintained until the desired thickness has been deposited. 
     FIG. 4, in accordance with the present invention, shows the spacial and dimensional relationship of an improved electrode assembly 30 to a superimposed substrate 44 (shown with dotted lines). FIG. 4 also illustrates an arrangement of magnets wherein the area of the magnets 34 is distributed in a substantially proportional manner with respect to the area of the substrate 44. The substantially proportional arrangement of magnets 34 in the electrode assembly 30 of the present invention can be effectively accomplished by positioning the magnets 34 along the circumference of the electrode assembly 30. As shown in FIG. 4, the magnets 34 extend by varying lengths toward the center of the electrode assembly 30. A glow created between the electrode assembly 30 and the substrate 44 will be more evenly distributed over the substrate 44 by the magnets 34. Uniformity of the coating is further improved by rotating the substrate when coating a round substrate 44 using a round electrode assembly 30. 
     It should be stressed that although the embodiment in FIG. 4 contemplates a round substrate 44 and a round electrode assembly 30, the concept of the present invention is readily adaptable to other shapes. For example, to coat a square substrate, one should use a square electrode assembly larger than the substrate. Also, magnets should be arranged around the perimeter of the square electrode extending towards the center. The lengths of the magnets should vary so as to provide a substantially proportional distribution of magnet area with respect to the area of the substrate to be coated. 
     In utilizing the improved electrode assembly 30 as shown in FIG. 4, the uniformity of quality (as measured by refractive index) and thickness of deposited coatings across the substrate are substantially improved. With electrode assembly 30 being significantly larger than prior art electrodes, it was expected that the power and amount of precursors would have to be increased substantially beyond that used in prior art methods, in order to initiate a glow. Surprisingly, however, a glow was initiated using the present apparatus with no increase in power or the amount of precursors, and the deposition rate of the coating substantially increased. Further, the temperature of the substrate during deposition was considerably lower than in prior art depositions, offering more protection to heat-sensitive substrates and providing more flexibility of the power and time variables during deposition. 
     FIG. 5 is a cross-sectional view of a typical embodiment of an electrode assembly 30 of the present invention and its relationship to a substrate 44. The electrode 32, a non-sputtering, good electrical conductor, such as aluminum, can be connected to the power supply (not shown) by any convenient means. The electrode 32 is electrically insulated from the magnets 34 by an insulator 46, which can be of any suitable electrically insulating material such as glass or quartz. As a convenient means of holding the arrangement of magnets 34 in place, support plates 48 are used to &#34;sandwich&#34; the magnets 34. The support plates 48 can be of any material which does not readily sputter, such as aluminum. A suitable electrode assembly 30 is about 35 cm in diameter and only about 1.5 cm in thickness. 
     The invention will be further illustrated by the following Example but it should be understood that the invention is not meant to be limited to the details described therein. 
     EXAMPLE 1 
     A grooved polyvinyl chloride disc substrate, 30.5 cm in diameter, was charged to a glow discharge system parallel to, and about 5 cm from, an electrode assembly consisting of a 0.32 cm thick by 35 cm diameter aluminum electrode with a magnet arrangement sandwiched between two 0.32 cm×35 cm aluminum support plates behind the electrode. The sandwiched magnets were separated from the electrode by a 0.32 cm×30 cm glass insulator as shown in FIG. 5. The assembly was held together with nylon screws. The magnet arrangement consisted of six 15 cm long magnets, four 12 cm long magnets, and sixteen 4 cm long magnets equally spaced around the circumference of the electrode assembly, extending towards its center, as shown in FIG. 4. The system was evacuated to 10 -5  Torr. N 2  O was introduced at a flow rate of 170 sccm, and SiH 4  was introduced at a flow rate of 25 sccm, to a pressure of 45 microns. A glow was initiated by applying 300 watts of RF power (at 13.56 MHz) to the electrode. The glow was maintained for 100 seconds. Thickness and refractive index of the deposited coating were measured at 20 points along a line spaced 5 cm from the outer edge of the substrate. The results are summarized in TABLE 1. 
     CONTROL 
     Using a prior art electrode of the type illustrated in FIGS. 1 and 2, a grooved polyvinyl chloride disc was processed as in Example 1, except that after 100 seconds the glow was shut off to allow the substrate to cool off. A second 100 seconds of glow was required to get the desired 1000 Angstrom thickness of SiO x . Thickness and refractive index were measured at 20 points across the disc as in Example 1. The results are summarized in TABLE 1. 
     
                                           TABLE 1__________________________________________________________________________Average       Average             ElectrodeCoating       Refractive                Deposition                      Deposition                             TemperatureThickness     Index  Time  Rate   After Deposition__________________________________________________________________________Example 1 1135Å (±50Å)         1.421 (±.003)                100 sec.                      681Å/minute                             ˜25° C.Control 980Å (±200Å )         1.396 (±.009)                200 sec.                      294Å /minute                             ˜95° C.__________________________________________________________________________ 
    
     While the present invention has been described with respect to silicon oxide deposition by glow discharge onto a disc-shaped substrate using a round electrode assembly, it it not appropriate to so limit the scope of the concepts described therein. Uniformity of any product deposited by glow discharge can be enhanced by using the improved glow discharge electrode assembly of the present invention. Further, variously shaped substrates could be more uniformly coated by using the present electrode assembly.