Method for Making a Stylus Having a Graphite Electrode

The present invention relates to a method for making a stylus for use in recording signals on a lacquer disc or reproducing signals from the record and to a stylus manufactured by the method, and in particular to a method for forming an electrode on a surface of a diamond body.

In video recording and reproducing systems of the grooveless capacitance detection type, the cutting stylus is formed of a diamond body for cutting the surface of a lacquer disc to form minute pits along sprial or concentric tracks. The diamond body is provided with an electrode to release electrostatic charges which build up at the diamond tip as the result of the cutting. The reproducing stylus is also formed of a diamond body which rests over several tracks and the electrode reads just one information signal from each track of a disc record by sensing its goemetric surface variations. It is required that the stylus be maintained in a stable contact relation with the surface of the disc which spins at a high speed. The electrode is conventionally formed by vacuum deposition, sputtering, or ion plating of a conductive material such as halfnium or titanium. Since the conductive material that forms the electrode is chemically different from the underlying body, the electrode, formed as a layer, is physically attached to the underlying body. This results in the electrode having the tendency toward separating from the underlying diamond body when subject to heat or friction which are likely to encounter during stylus manufacture or actual use. Another conventional method for making the electrode involves ion injection. Because of the strong bonding between carbon atoms that form the diamond body, it is difficult to inject enough ions to provide a highly conductive electrode.

According to the present invention, a block of diamond is heated within an environment of either vacuum or inert gas each containing a small amount of oxygen at an elevated temperature for purposes of removing noxious gas from the reaction chamber, a process known as "degassing". After the degassing process, the diamond body is heated at a temperature lower than the degassing temperature, preferably in a range from 1100.degree. C. to 1500.degree. C. under the pressure of 3.times.10.sup.-2 Torr. to 200.times.10.sup.-2 Torr. for a period ranging from 30 minutes to 9 hours. This heating process allows the carbon atoms of the surface areas of the diamond body to form a thin layer of graphite having a sheet resistance of as low as 600 ohms. The presence of oxygen is essential to the conversion of surface layer to graphite. The amount of oxygen contained in the reaction chamber is given by partial pressure which is preferably 1/5 of the total pressure of the reaction chamber. The diamond body is then subject to a lapping process whereby the conductive layer is shaped into the form of a stylus electrode having a width of about 1 micrometer. Being integrally formed with the underlying body, the graphite electrode is not separable from the diamond body when subject to heat or friction which might occur during use.

A 3-mm cube of diamond was placed into an oven. The oven was evacuated to as low as 10 Torr. and the diamond body was heated at an elevated temperature of from 1200 C. to 1600 C. for a period from 5 minutes to 1 hour at a pressure of 2.times.10.sup.-2 Torr. to 1.times.10.sup.-2 Torr. for the purpose of removing noxious gas from the oven. After the degassing process the temperature was lowered to a level in range between 1100.degree. C. and 1500.degree. C. can the absolute pressure of the oven was increased to a level in a range between 3.times.10.sup.-2 Torr. and 200.times.10.sup.-2 Torr. The reaction chamber contained a small amount of oxygen whose partial pressure ranges from (0.6).times.10.sup.-2 Torr. to 40.times.10.sup.-2 Torr. Under these conditions the heat treatment process was continued for a period from 30 minutes to 9 hours. With this heat treatment, the carbon atoms of the surface areas were converted into a black layer of graphite. The graphite layer was found to have a conductivity which depends on the period of the heat treatment. It was also found that the presence of oxygen is essential to the conversion of surface layer to graphite. Tests were conducted to ascertain the relationship between the period of the heat treatment and the sheet resistance of the conductive layer. It was found that the sheet resistance is inversely variable as a function of the period of the heat treatment. More specifically, the sheet resistance was 20 killohms, 2 killohms, 700 ohms and 600 ohms, respectively, for periods of 30 minutes, 2 hours, 4 hours and 9 hours.

Scoring tests indicate that the conductive layer is not separable from the underlying body. It is considered that there is a smooth transition from the diamond body to the graphite layer which accounts for the strong bonding with which they are tied together.

The same results were obtained when the diamond body was heat-treated in an environment of an inert gas containing a small amount of oxygen.

After the heat treatment processes, the diamond body was subject to a lapping process to shape the conductive layer into the form of a stylus electrode having a width smaller than the width of a recording track.