CH634164A5 - READING POINT FOR REPRODUCING A SIGNAL RECORDED ON A RECORDING MEDIUM. - Google Patents

Description

The present invention relates to a read tip for reproducing a signal recorded on a recording medium of the type with variation in electrostatic capacity. The invention relates to a read tip arranged so as to make it possible to reproduce, in the form of variations in electrostatic capacity, a signal recorded, in the form of variations in geometric configuration, on a recording medium, this tip comprising a formed electrode. by ion implantation in the body of the tip.

In general, a signal recording medium of the type with electrostatic variation can have, for example, a structure comprising a recording medium made of synthetic resin, the surface of which has variations in geometric configuration in the form of a plurality cavities, the depth of which varies according to the signal to be recorded, a thin film of a metal which is a good conductor of electricity, formed by evaporation-condensation, on the surface of this support and a thin film of a dielectric material adhering to al metallic film. A read tip for reading and reproducing the signals recorded on such a recording medium of the type with variation of electrostatic capacity is provided with an electrode.

When a stylus provided with an electrode follows the track traced on the surface of a recording medium of the kind just mentioned, the electrostatic capacity between this electrode and the thin metallic film, through the film dielectric or through the dielectric film and an air-filled gap, varies depending on the variation of the geometric configuration of the recording medium. The recorded signal thus detected is reproduced in a known manner in response to this variation in the electrostatic capacity.

In accordance with the prior art, the electrode of a reading tip of this kind consists of an adherent layer obtained by depositing a metal such as tantalum, hafnium or titanium, obtained by the process of evaporation-condensation under vacuum or cathode sputtering, over a well-defined part of the body surface of a sapphire or diamond stylus. Given that the electrode of a known read tip of this kind consists of an adhesive layer formed by evaporation-condensation-vacuum or by cathode sputtering of a metal on the surface of the body of the read tip,

this electrode tends to be easily torn from the surface of the tip during use. This is why the lifespan of such a stylus is too short, its value being of the order of a few hours with a maximum of the order of 100 hours.

In addition, since the values of the resistance to wear of the body of the stylus and of the electrode are clearly different, the wear of the body of the stylus and that of the electrode, as a result of contact between the stylus and the recording medium rotating at high speed, are different from each other. This explains why the known reading peaks do not allow a good reproduction of the signal, in a stable manner, for a long period. It is moreover difficult, during the industrial manufacture of reading tips, to deposit, by evaporation-condensation or sputtering, metal layers all having the same thickness on a large number of reading tip bodies and, in Consequently, there are undesirable differences in the characteristics of the reading tips.

In particular, in the case where the diamond, which is a material having high wear resistance, is used to constitute the body of the reading tip, it is difficult to deposit a metal on this material by evaporation-condensation and, even in the case where an electrode consisting of a metal layer is obtained, it is easily torn off. On the other hand, when using sapphire to form the body of the reading tip, depositing a metallic layer on this material by evaporation-condensation is easier than in the case of diamond, but the duration life of such a stylus is short because sapphire has a lower wear resistance than diamond.

The object of the present invention is to eliminate the drawbacks which have just been mentioned.

To this end, the stylus according to the invention has the characteristics specified in claim 1.

Thus, in the stylus according to the invention, the electrode is not formed by depositing a metal layer by a known process such as evaporation-condensation or sputtering, but this electrode is an integral part of the body of the stylus, formed on a specific part of the surface of the body of the stylus by ion implantation.

Preferably, the ions implanted in said specific part of the surface of the body of the stylus, for

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form the electrode, are ions of a substance chosen from metals, metalloids and semiconductors. Thus, the part of the body of the read tip constituting the electrode cannot be torn off from the surface of this body. In addition, since the wear of this electrode is the same as that of the entire body of the stylus, the latter allows good signal reproduction, stably, during good signal reproduction, stably, for a long period of use. Another advantageous characteristic of the invention lies in the fact that, during the mass production of the read tips, it is possible to give their electrode a uniform thickness corresponding exactly to the same penetration depth, which allows the obtaining economically read tips with uniform characteristics.

The invention will be better understood thanks to the detailed description which follows, given by way of example, of an embodiment of the stylus according to the invention, with reference to the appended drawing in which:

Figure 1 is a bottom view, in perspective, of a read head provided with the read tip according to the invention; and,

Figure 2 is a perspective view, greatly enlarged, showing the end of a read tip according to an embodiment of the read tip according to the invention.

We will first describe, with reference to Figure 1, an example of a read head that can be provided with the read tip according to the invention. The read head 10 comprises a support plate 11 under which is fixed a fixing lug 12 made of a ferromagnetic material. A magnetic plate 13 is placed against a surface of the fixing lug 12 which projects downwards. A cantilever lever 14, in the form of a bipod supports a read tip 15, which will be described below, fixedly mounted at the distal or free end of the lever 14. The two feet of the lever 14 cross, near their base, a transverse bridge 16 to which they are fixed. A magnetic plate 17 is fixed on the rear face of this bridge 16. The lever 14 is stably supported, relative to the plate 11, by the magnetic attraction of the plate 17 with respect to the plate 13. The stylus 15 has an electrode surface on a specific part of its surface. One end of a connection wire 18 is connected to this electrode by means of an electrically conductive adhesive, the other end of the connection wire 18 being connected to a terminal 19 placed under the surface of the support plate 11 .

When using the read head 10, the latter is loaded into an actuating element (not shown) forming part of a rotary cover (also not shown) of a carriage. When the rotary cover is closed, the terminal 19 comes into connection with the conductive core of a coaxial resonator which is in the carriage, which places the latter in a state allowing the reproduction of the signal.

A pickup read head of this kind is described in detail in US Patent No. 4,160,268, entitled "Signal pickup device with tracking control and jitter compensation or a video dise" (signal reading device track control and vibration compensation for video disc).

The reading position of the stylus 15 relative to the recording medium of the type with variation of electrostatic capacity, in the form of a disc (which will be designated hereinafter more simply by the term "disc"), is indicated. in Figure 2. The disc 21 does not have a guide groove but it has a large number of cavities, formed in its surface corresponding to the recorded signal. The cavities 22 are formed in the disc 21 in a manner corresponding to the main signal mentioned above. In FIG. 2, only part of the tracks corresponding to the main signal are shown, these tracks each comprising a succession of cavities 22. These tracks constitute the different turns of the same spiral track and they correspond respectively to the different turns of the disc. 21, the signal recorded on the track consists of the four fields of a video signal. In the embodiment described, the shape of the cavities is such that the line delimiting the edges of each of the adjacent tracks. In other words, the adjacent tracks are contiguous with each other. Consequently, the pitch between the different tracks is practically equal to the width of the track.

Cavities 23 and 24 respectively corresponding to the pilot signals of different frequencies are formed at central positions (coinciding with the lines delimiting the edges of the tracks in the embodiment described) the position of these cavities being approximately intermediate between the central lines delimiting the mutually adjacent tracks.

The disc 21 comprises a base disc 25, made of polyvinylchloride 25 on the surface of which the cavities 22, 23 and 24 are formed, a layer 26 consisting of a thin sheet of aluminum adhering to the upper surface of the base disc 25, and a dielectric film 27 adhering to the exterior surface of the aluminum sheet 26.

The underside 32 of the body 31, constituted by diamond, of the reading tip 15 comprises a part having a maximum width, which is greater than the pitch of the tracks of the disc 21, and a part 33, forming an electrode, having a thickness of 500 to 3000 Å, formed by ion implantation on the rear face of the read tip and on the rear edge of the lower surface 32. When the disc 21 rotates in the direction indicated by the arrow, the read tip 15 moves, following a track, relative to the surface of the disc 21 and the electrostatic capacity between the electrode portion 33 and the thin aluminum layer 26, which constitutes an electrically conductive layer covering the entire surface of the disc, varies correspondingly to the shape of the cavities 22, which allows the reproduction of the main signal recorded in the form of the cavities 22.

Simultaneously, the pilot signals recorded in the form of cavities 23 and 24 are also reproduced by the electrode part 33. In the case where the center of the electrode part 33 deviates from the center line of the track, a level difference in the reproduction of pilot signals. A control signal generated in response to this difference in level is supplied to the pick-up motor elements, which results in an automatic return of the stylus to the track.

We will now describe the electrode portion 33 which constitutes an essential part of the read tip according to the invention.

As the material constituting the body of the stylus 31, one can use sapphire if one wishes to obtain a stylus of a low cost price, but it is preferable to use diamond, whose hardness is higher to that of sapphire, if one wishes to obtain a point of reading having a long lifespan. There are no special conditions regarding the kind of diamond that can be used.

At the time of forming the electrode part 33, the body of the tip is placed, having a prismatic shape, as it appears before polishing, making it possible to give it a particular shape, supported by an appropriate sample holder, in the treatment chamber of an ion implantation device which can be of any type

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known appropriate. In accordance with a particularly advantageous form of implementation of the method of manufacturing the reading tip according to the invention, by ion implantation, hafnium ions (Hf) are implanted in a specific part of the surface of the body of a tip. diamond reading with an ion acceleration voltage of the order of 30 to 100 KeV. By proceeding in this way, the hafnium atoms are entrained in the interstices existing between the carbon atoms of the crystal lattice of the diamond which reduces the electrical resistance of the part of the body of the reading tip in which the ions are implanted at a level allowing the use of this part as an electrode. The depth (i.e. the thickness) of the layer containing implanted ions thus obtained is, for example, 0.3 microns and the electrical resistance of the part (electrode part) formed by this process ion implantation is of the order of 1 KQ / cm (this value corresponds to an example of measurement carried out by means of an electrical resistance measurement device). The electrical resistance of the diamond is, for example, 1016 £ 2 / cm while the electrode part, obtained by ion implantation, has an electrical conductivity.

In accordance with a method of manufacturing the reading tip, it is possible to use, to impart the electrical conductivity to the diamond, not only the implantation of hafnium but also that of ions of metalloids, semiconductors and metals such as aluminum (Al) molybdenum (Mo), titanium (Ti), tantalum (Ta), chromium (Cr), nickel (Ni), copper (Cu), gold (Au), silver ( Ag) and carbon (C).

In addition, the depth of the ion implantation can be adjusted by appropriately adjusting the energy with which this ion implantation is carried out. Although the optimum ion acceleration voltage varies according to the nature of the ions and the implantation depth, the preferred value of this voltage is in the range of 20 to 300 KeV, and it is, for example, order of 100 KeV. Indeed, an ion acceleration voltage less than 5 KeV results in an insufficient implantation depth while an ion acceleration voltage greater than 300 KeV can cause the destruction of the crystal lattice of the diamond.

The quantity of implanted ions is chosen from the range of 1015 to 1025 ions / cm2, preferably with a value of the order of 1 × 1023 ions / cm2. The depth of implantation of the ions is chosen from the range of 0.01 to 0.5 microns, with a preferred value of the order of 0.3 microns.

When the depth of ion implantation, that is to say the thickness of the electrode portion 33 is insufficient, the layer formed by ion implantation cannot function as an electrode. On the other hand, when this depth is too great, it is not possible to obtain a satisfactory reproduction of the signal taking into account the relationship with the dimensions of the cavities 22 in the longitudinal direction of the track. This is why the thickness of the part serving as an electrode is given a value within the limits indicated above.

The depth of ion implantation, the quantity of ions implanted and the other parameters are chosen, depending on the nature of the ions implanted, so as to give the electrical resistance of the part serving as an electrode a value included in the range of 1Q / cm to 104fì / cm.

Then, by abrasion, a specific shape is imparted to the body of the read tip which has just been subjected to the ion implantation operation described above and which therefore comprises a part constituting an electrode formed by ion implantation on a specific part of its surface. A read tip 15 is thus obtained, partially represented in FIG. 2, having a part constituting an electrode 33 and having a shape such that its thickness up to

a certain distance from the end of the tip is less than the transverse width of a cavity 22. It is desirable that the two lateral edges of the electrode part 33 are parallel but a diverging angle of the order of 3 to 8 degrees between these two edges, this angle resulting from the need to facilitate the abrasion process, does not prove to be troublesome in practice.

In the stylus according to one embodiment, the electrode part is formed by ion implantation in the body of a diamond tip and it is therefore not in the form of a layer deposited on the surface of this body. by evaporation-condensation, cathode sputtering, or by any other usual process. This is why the electrode part cannot be torn off from the surface of the body of the stylus even after a long period of reading the disc 21. This is why, in the case where the two edges opposite sides of the electrode part 33 form a divergent angle between them, the end of the life of the read tip is only reached when the electrode part 33 is worn out at the same time as the body 31 of the point and that the width of the electrode at the end of the tip becomes greater than the width of the track. In a known tip of the same kind, the electrode tends to be torn from the body of the tip which results in the fact that the end of the life of this tip is reached after 'about 100 hours. On the contrary, the lifetime of the read tip according to the present invention is very high, since it is of the order of 2000 hours.

Another advantageous characteristic of the method of manufacturing the reading tip, by ion implantation, in comparison with the known methods of manufacturing a reading tip by evaporation-condensation under vacuum or by cathode sputtering, resides in the fact that it makes it possible to manufacture a large number of probe tip bodies in a relatively simple manner, while allowing precise adjustment of the thickness of the electrode part. This process therefore makes it possible to manufacture, with a low cost price, reading tips having a constant quality with little variation from the desired properties.

According to one mode of manufacture, of the reading tip according to the invention, a specific part of the surface of a blank, of prismatic shape, of the body of the reading tip is subjected to an ion implantation process and then gives this blank by abrasion, the shape required for the stylus, without touching the electrode part 33. However, as a variant, ion implantation can be carried out in a specific part of the surface of a body of stylus which has previously been given, by abrasion, a specific form of stylus. In this case, the body of the read tip is oriented so that the surface on which the electrode portion 33 is to be formed is perpendicular to the direction of the ion beam. The lateral parts of the surface of the body of the reading tip opposite the surface on which the part constituting the electrode is formed are inclined relative to the direction of the ion beam and the amount of ion implantation varies considerably depending on the installation angle. This is why there is practically no ion implantation in these lateral parts of the surface and, even in the case where there is a certain ion implantation, this is not sufficient to form a thickness d implantation allowing operation as an electrode. It therefore follows that the part constituting the electrode 3 is formed practically only on the surface shown in FIG. 2.

During the ion implantation process described above, when the operation is continued after obtaining a pro5

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implantation smelter determined by factors such as the nature of the ions and the ion acceleration voltage, atoms corresponding to these ions adhere to the surface of the body of the stylus subjected to the ion implantation process and precipitate on this surface. In this case, the part of the surface on which these atoms adhere and precipitate also acts as an electrode and the thickness of ion implantation mentioned above includes the depth of ion implantation and the thickness of this part made up of atoms. having adhered and precipitated.

In addition, it should be noted that, during the ion implantation process, it is possible not only to adjust the ion acceleration voltage to a constant value but it is also possible to vary this acceleration voltage, if necessary. In this case, when the ion implantation is carried out with a low value of the acceleration voltage, atoms corresponding to the ions adhere to the surface subjected to the ion implantation and precipitate on this surface in a similar manner to that which is described above.

The disc 21 comprises, on the upper surface of the base disc 25, which is made of polyvinyl chloride in the example shown in FIG. 2, a thin sheet of aluminum 26 which functions as a counter electrode for the creation of '' an electrostatic capacity between it and the forming part

electrode 33 of the read tip 15. However, this layer can, as a variant, be constituted by a layer formed by a synthetic resin containing, in mixture, an electrically conductive material such as carbon powder, s in this case, the electrically conductive material mixed in the disc itself functions as an electrode and, moreover, it is suitably covered by the synthetic resin on the surface. This is why, the thin film 26 and the film 27 of the example described above are not necessary, in the latter case.

While, in the example described above, the disc 21 does not have a groove for guiding the stylus, it is clear that this disk could also be provided with a groove for guiding this stylus. In this case, the shape of the end of the tip must of course be such that it corresponds appropriately with that of the guide groove.

It should be noted, moreover, that the main signal recorded in the form of the cavities 22 of the disc 21 can be a 2o television video signal or else a signal of the PCM audio type (pulse code modulation). It is therefore understood that the nature of the signal for the reproduction of which the reading tip according to the invention can be used does not constitute a limitation of the latter.

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Claims (9)

634164 1. Reading tip for reproducing a recorded signal, in the form of variations in geometric configuration, on a recording medium, this tip comprising a body and an electrode formed on a part of the surface of this body and being arranged so as to make it possible to reproduce, in the form of variations in electrostatic capacity corresponding to variations in the geometric configuration of the recording medium, the signal recorded thereon, characterized in that this electrode (33) is constituted by a part of the body of the stylus (31) rendered electrically conductive by ion implantation to a given depth from the surface of said part of the body of the stylus. 2. Reading tip according to claim 1, characterized in that the ions implanted in said specific part of the surface of the body (31) to form the electrode (33) are ions of a substance chosen from metals, metalloids and semiconductors. 2 3. Reading tip according to claim 2, characterized in that said ions are chosen from ions of the following elements: hafnium, aluminum, molybdenum, titanium, tantalum, chromium, nickel, copper, gold, silver and carbon. 4. Reading tip according to claim 2, characterized in that the quantity of ions implanted in said part of the surface of the body of the tip is from 1015 to 1025 ions / cm2. 5. Reading tip according to claim 2, characterized in that the depth of implantation of the ions in said part of the surface of the body of the tip is from 0.01 to 0.5 microns. 6. Reading tip according to claim 2, characterized in that the electrical resistance of said electrode (33) has a value of 1 to 10 "£ 2 / cm. 7. Reading tip according to claim 2, characterized in that said ions are hafnium ions implanted at a depth of up to 0.3 microns below the surface of said part of the surface of the body (31). 8. A method of manufacturing the stylus according to claim 1, characterized in that said ion implantation is carried out with an acceleration voltage of 20 to 300 KeV. 9. Method according to claim 8, characterized in that one carries out said ion implantation while varying the acceleration voltage.