Abstract:
A method for preparing a plastic lapping disc overcoated with an abrasive silicon oxide layer. The method comprises the steps of introducing the silicon oxide precursors into an evacuated chamber containing the plastic lapping disc wherein a first major surface of the plastic disc is substantially covered during the glow deposition. The precursors are then subjected to a glow discharge. An abrasive silicon oxide layer is deposited on a second major surface of the disc opposite the first major surface.

Description:
This invention relates to a method for preparing an abrasive lapping disc suitable for lapping video disc styli. More particularly, this invention relates to a method of preparing an abrasive lapping disc by a glow discharge deposition technique. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 3,842,194 of Clemens discloses a capacitive information disc record having a playback system utilizing variable capacitance. In one configuration of the Clemens system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of about 2.6 micrometers and groove depths of about 0.5 micrometer may be used. During playback a pickup stylus of about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick engages the groove as the record is rotating by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information. 
     In systems of the above type, the use of a relatively fine record groove and the record engagement requirements of the pickup stylus result in a stylus tip which is extremely small. 
     In U.S. Pat. No. 4,162,510 of Keizer, a novel keel-tipped pickup stylus structure is disclosed. The keel-tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion, and shoulders interconnecting the body with the constricted terminal portion. The electrode is remote from the keel tip. 
     A second patent of Keizer, U.S. Pat. No. 4,104,832, discloses a pyramidal dielectric support element which is shaped on an abrasive lapping disc having a deep, coarse-pitched groove in order to obtain a keel-tipped stylus. Glow discharge deposited SiO 2  is used by Keizer as an abrasive coating. The coating is prepared by a method which utilizes as starting materials oxygen and an alkoxy-substituted silane of the formula ##STR1## wherein R 1  is selected from the group consisting of H and CH 3 , R 2  and R 3  are independently selected from the group consisting of H, CH 3 , OCH 3 , and OC 2  H 5 , and R 4  is selected from the group consisting of OCH 3  and OC 2  H 5 . 
     Coatings employed in Keizer require periods as long as 30 minutes to shape one diamond stylus. Furthermore, these coatings quickly loose their abrading ability and a second diamond stylus may require two hours to be lapped. 
     Kaganowicz is a copending application entitled, &#34;METHOD FOR PREPARING AN ABRASIVE COATING&#34;, Ser. No. 963,819, filed Nov. 27, 1978, discloses a method for preparing an abrasive silicon oxide (SiO x , 1≦x≦2) coating on the substrate comprising glow discharging precursors comprising silane and a gaseous, oxygen-containing compound selected from the group consisting of N 2  O, H 2  O and CO 2 . 
     In preparing a lapping disc by glow discharge deposition methods a plastic disc is often employed as the substrate. Because a considerable amount of heat is generated during the glow discharge process, the glow discharge deposition must be interrupted to prevent the plastic substrate disc from warping. These interruptions lead to greater time requirements for preparing an abrasive lapping disc. It would therefore be desirable to have a method which allows for the preparation of an abrasive SiO x  coating of suitable thickness without frequent interruption. 
     Wang et al. in a copending application entitled, &#34;METHOD OF DEPOSITING AN ABRASIVE LAYER&#34;, Ser. No. 048,161, filed June 13, 1979, now U.S. Pat. No. 4,260,647 teach a method of depositing an SiO x  layer onto a substrate by depositing a series of thin layers by glow discharge of an organosilane and oxygen. After each interruption of the deposition, a glow discharge is initiated in oxygen prior to a subsequent SiO x  deposition. Because of the vigorous spontaneous interaction between O 2  and SiH 4  the Wang et al. method is not attractive for the present problem. 
     SUMMARY OF THE INVENTION 
     We have found a method for preparing a plastic lapping disc overcoated with an abrasive silicon oxide layer. The method comprises the steps of introducing the silicon oxide precursors into an evacuated chamber containing the plastic lapping disc wherein a first major surface of the plastic disc is substantially covered during the glow discharge deposition. The precursors are then subjected to a glow discharge. An abrasive silicon oxide layer is deposited on a second major surface of the disc opposite the first major surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional view of an apparatus suitable for depositing the abrasive coating. 
     FIG. 2 is a schematic side view of a first assembly which includes a disc and a metal sheet. 
     FIG. 3 is a schematic side view of a second assembly which includes two discs and a metal sheet. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In this invention an abrasive silicon oxide (SiO x , 1≦x≦2) layer may be glow discharge deposited onto a plastic disc substrate for a longer period of time without warping by covering one major surface of the plastic disc with a second plastic disc, a metal sheet or any other convenient thermally conducting or nonconducting material. If a thermally nonconducting material such as a second plastic disc is employed, the nonconducting material serves to increase the thermal mass of the resulting assembly. Also, because only one side of the plastic disc is coated during the glow discharge process, less heat is built-up in the plastic disc. During the glow discharge coating process, the surface of the plastic disc is bombarded with hot particles so the shielding effect can be quite significant. 
     If a thermally conductive material such as a metal sheet is used to cover a major surface of the plastic disc, the thermally conductive material acts as a heat sink which draws off heat from the plastic disc generated during the glow discharge process. In addition, a metal sheet may act as a heat reflector so that some of the radiant heat may be deflected before it reaches the lapping disc. 
     During the glow discharge process the deposition generally continues until a desired thickness is reached. However, if a thick coating is required which causes the plastic disc material to approach its melting or relaxation temperature, then interruption of the glow discharge will still be required, albeit after considerably greater time than for an unprotected plastic disc. 
     The abrasive SiO x  coating is preferably prepared by subjecting gaseous precursors to the glow discharge. Preferably, the precursors are SiH 4  and a gaseous oxygen containing compound such as N 2  O, H 2  O, or CO 2 . More preferably, the oxygen containing compound is N 2  O. 
     The present invention will be further described by means of the Drawing. 
     A glow discharge apparatus suitable for preparing the abrasive material is shown in FIG. 1 generally as 10. The glow discharge apparatus 10 includes a vacuum chamber 12 which is a glass bell jar. In the vacuum chamber 12 are two electrodes 14 and 18 which can be a screen, coil, or plate of a material that is a good electrical conductor and does not readily sputter, for example, aluminum. The electrodes 14 and 18 are connected to a power supply 16 which may be DC or AC. Thus, there will be a voltage potential between the electrodes 14 and 18. The plasma may be enhanced by means of magnets on the electrodes 14 and 18. An outlet 20 into the vacuum chamber 12 allows for evacuation of the system and is connected to a mechanical pump, not shown. First inlet 22 and second inlet 24 are connected to gas bleed systems, not shown, for adding the reactants employed to prepare the abrasive material. 
     In carrying out the process, a substrate 26 to be coated is placed between the electrodes 14 and 18 typically maintained about 5 to 10 centimeters apart. The vacuum chamber 12 is then evacuated through the outlet 20 to a pressure of about 0.5-1×10 -5  torr. A gas which acts as a source of oxygen is added through first inlet 22 to its desired partial pressure. SiH 4  is added through second inlet 24 until the desired partial pressure ratio of SiH 4  to the oxygen source is obtained. 
     In order to begin deposition of an abrasive coating on the substrate 26, a glow discharge is initiated between the electrodes 14 and 18 by energizing the power source 16. For deposition the current should be in the range of 200 to 900 milliamps, preferably 400 to 700 milliamps. Any convenient frequency such as 10 kilohertz may be employed. The potential between electrodes 14 and 18 is generally about 1,000 volts. 
     FIG. 2 is a schematic side view of a first assembly 50. Included in the first assembly 50 is a first plastic disc 52 having a first major surface 54, a second major surface 56 and a first center hole 58. Also included in the first assembly 50 and adjacent to the first plastic disc first major surface 54 is a metal sheet 60 having the same diameter as the first plastic disc 52. A metal sheet first side 62 contacts the first plastic disc first major surface 54. A metal sheet second side 64 is exposed to the glow discharge deposition as is first plastic disc second major surface 56. The metal sheet 60 has a second center hole 66 which is aligned with the first center hole 58 to allow the first assembly to be placed between the two electrodes 14 and 18, respectively, of the glow discharge apparatus 10 as the substrate 26. The first assembly 50 may be held in place as the substrate 26 in the glow discharge apparatus 10 by any convenient means. Preferably, the means allows the first assembly to rotate about its axis and passes through the first center hole 58 and the second center hole 66. Although a metal sheet 60 is shown, the first plastic disc first major surface 54 may be covered by any convenient article such as a second plastic disc. 
     FIG. 3 is a schematic side view of a second assembly 100. In addition to the first plastic disc 52 and the metal sheet 60 shown in FIG. 2, the second assembly 100 includes a second plastic disc 68 having the same radial dimensions as the first plastic disc 52. A second plastic disc first major surface 70 is in contact with the metal sheet second side 64. A second plastic disc second major surface 72 is exposed. The second plastic disc record 68 has a third center hole 74 which is in alignment with the first center hole 58 and second center hole 66. The second assembly 100 is placed between the two electrodes 14 and 18 in the glow discharge apparatus 10 as the substrate 26 and may be held in place by the same means employed for the first assembly 50. In the second assembly 100, one major surface of each of two plastic discs may be coated in a single glow discharge deposition. 
     The plastic disc may be fabricated out of any convenient material such as a homopolymer or copolymer of vinyl chloride or styrene. The metal sheet may be any suitable material such as about 1/16 to 1/8 in. (1.6 to 3.2 millimeters) thick aluminum. The plastic disc major surfaces can be coated with one or more metal layers prior to silicon oxide glow discharge deposition. Metal layers such as copper and Inconel 600 (76.8 atom percent nickel, 13.8 atom percent chromium and 8.5 atom percent iron) are preferred. 
     This invention will be further illustrated by means of the following Examples, but it is to be understood that the invention is not meant to be limited by the details described therein. 
     EXAMPLE 1 
     Two 12-inch (30.5 cm) diameter plastic discs containing a deep, continuous trapazoidal groove in each major surface were placed on each side of a 14-inch (35.6 cm) diameter 0.07-inch (1.78 mm) thick aluminum disc. The plastic disc was compression molded from a molding composition which included 95.25 percent by weight Geon 110×346 (a homopolymer of vinyl chloride available from B. F. Goodrich Co. having a weight averaged molecular weight of 46,200, a number averaged molecular weight of 23,300 and a T g  of 80° C.). The three discs were placed in a 46 cm×76 cm bell jar of a glow discharge apparatus as described in FIG. 1 which was then evacuated to 1×10 -5  torr. 
     N 2  O was added to a partial pressure of 32 micrometers of mercury using a flow rate of 23.6 standard cubic centimeters per minute (sccm). 
     A screw, two washers, and a nut were used to hold the assembly of the three discs in place in the glow discharge apparatus. The assembly was rotated at a rate of 30 revolutions per minute between two 15 cm×15 cm aluminum electrodes. These electrodes covered a strip approximately 6 cm wide on the disc. To create a glow between the electrodes, current was supplied to the electrodes at a rate of 500 milliamps with a potential of about 1,000 volts at 10 kHz. 
     The effect of the glow discharge process on the plastic discs is summarized in Table I. 
     
                       TABLE I______________________________________Time (minutes)        Condition of the plastic discs______________________________________3            no warping6            no warping9            no warping10.5         starting to warp11           warped beyond possible use______________________________________ 
    
     Using the assembly, the glow discharge process could continue for at least 9 minutes before interruption to allow for plastic disc cooling. 
     Control 1 
     A vinyl disc, as described in Example 1, was placed in the glow discharge apparatus and subjected to the same glow discharge conditions of Example 1. The effect of the glow discharge process on the plastic disc is shown in Table II. 
     
                       TABLE II______________________________________Time (minutes)        Condition of the plastic discs______________________________________1            no warping2            warped3            warped beyond use______________________________________ 
    
     These results indicate that in the absence of the metal sheet the glow discharge process must be interrupted about every 1-2 minutes. 
     EXAMPLE 2 
     The same assembly of aluminum discs and two plastic discs was employed as in Example 1, except that the molding composition contained 76 parts by weight of Geon 110×346 and 15 parts by weight of carbon black. The same glow discharge apparatus and conditions of Example 1 were employed. After N 2  O was added, SiH 4  was added at a rate of 3.3 sccm to a partial pressure of 4.3 micrometers of mercury. The effect of the glow discharge deposition on the plastic discs is shown in Table III for plastic discs coated with 50 angstroms of copper followed by 500 angstroms of Inconel 600. 
     
                       TABLE III______________________________________Time (minutes)        Condition of the plastic discs______________________________________5            no warping7            no warping9.5          started to warp10           warped beyond use______________________________________ 
    
     These results indicate that two interruptions would be required to deposit a 1,500 angstrom SiO x  coating at a deposition rate of 75 angstroms/minute. 
     Control 2 
     The conditions and procedures of Example 2 were employed using a single plastic disc of the composition taught in Example 2. The effect of glow discharge deposition on the plastic discs is shown in Table IV. 
     
                       TABLE IV______________________________________Time (minutes)        Condition of the plastic discs______________________________________2            no warping4            started to warp5.5          warped beyond use______________________________________ 
    
     The absence of the metal sheet of Example 2 causes warping to occur in about half the time during glow discharge deposition. 
     EXAMPLE 3 
     The same plastic discs and experimental conditions of Example 2 were employed to prepare SiO x  coatings except as follows. N 2  O was added at a flow rate of 23.6 sccm to a partial pressure of 32 micrometers of mercury. SiH 4  was then added at a flow rate of 3.1 sccm to a partial pressure of 5 micrometers of mercury. The results are shown in Table V. 
     
                       TABLE V______________________________________   Time                 SiO.sub.xTime to the   to Disc              ThicknessOnset   Warping              After    SiO.sub.xof Disc Beyond               Warping  Re-Warping Use       Assembly   Beyond Use                                 fractive(Seconds)   (Seconds) Employed   (Angstroms)                                 Index______________________________________ 60     180       1 Plastic  258      1.379             Disc180     480       2 Plastic  548      1.365             Discs (back-             to-back)600     620       2 Plastic  761      1.298             Discs and             Aluminum             Sheet (as             in Example             2)______________________________________ 
    
     The refractive indeces and thicknesses were determined by ellipsometry. The results indicate that both the time before the onset of warping and warping beyond use were greatest when two plastic discs were placed on either side of an aluminum sheet. However, two plastic discs having a major surface of each disc overlying each other required three times as much exposure to the glow discharge deposition process to warp when compared to a single plastic disc.