GB1590296A

GB1590296A - Information record

Info

Publication number: GB1590296A
Application number: GB52944/77A
Authority: GB
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1977-03-28
Filing date: 1977-12-20
Publication date: 1981-05-28
Also published as: AU514899B2; DE2757737C2; JPS6259372B2; JPH0222453B2; IT1088701B; HK49783A; NL7713230A; FR2385535A1; NL191658C; AU3087677A; JPS53120506A; JPS6352347A; DE2757737A1; FR2385535B1; MY8400273A; NL191658B

Description

(54) INFORMATION RECORD (71) We, RCA CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, of 30 Rockefeller Plaza, City and State of New York, 10020, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates generally to a high-density information storage medium.

More particularly, this invention relates to an information storage medium suitable for use with optical recording and playback methods and apparatus.

In accordance with a feature of the present invention, the efficient coupling of energy from the recording light beam into an absorptive material coating (necessary for the achievement of an adequate recording sensitivity) is achieved by construction of the recording medium in the form of a substrate having a surface which is reflective (at least at the frequency of the light forming the recording beam), with a layer of material transparent at the recording beam light frequency overlying the reflective surface, and a thin layer of material, highly absorptive at the recording beam light frequency, overlying and transparent layer. With both incident light and reflected light (reflected from the substrate surface through the transparent layer) passing through the thin absorptive layer the ability to rapidly elevate the temperature of the absorptive layer material to that required for melting to take place is enhanced.

The absorptive layer may be chosen from a material having high absorptivity and exhibiting long-term stability, such as titanium, platinum and rhodium. Furthermore, the long-term stability of the entire record blank may be enhanced since the reflective surface, being subject to oxidation, is protected from environmental exposure by the light transparent layer.

Pursuant to a further feature of the present invention, the efficiency of energy coupling into the absorptive layer is enhanced by choosing a thickness for the transparent layer (for a given thickness of the absorptive layer) which establishes a so-called anti-reflection condition for the coated substrate at the recording beam light frequency. With energy loss via reflection minimized due to the anti-reflection condition establishment, and with energy loss via transmission into the substrate minimized by the reflective surface presence, a highly efficient heating of the absorptive layer to a melting temperature is realizable.

In accordance with another feature of the present invention the thickness of the absorptive layer is reduced, which significantly decreases the laser power needed for recording thereon. The chosen thickness, however, must still be sufficient to allow the absorption of a maximum fraction of the light incident thereon. When utilizing a metallic absorptive layer, the melting process, resulting from the efficient energy coupling in the absorptive layer which is necessary for information recording, is effective in forming in the thin absorptive layer pits having smooth edges, leading to high definition between the melted pit regions and the undisturbed regions and thus contributing to a high signal-to-noise ratio on read-out.

The absorptive layer preferably has a high melting point, such as a metal, enabling information read-out at relatively high intensity levels, thus reducing the adverse effect of "shot noise" during information read-out.

In accordance with an illustrative embodiment of the present invention, a surface of a substrate a disc form (e.g., glass) is processed so as to form a polished flat surface, which is then coated with a layer of a reflective material (e.g., of aluminum). A layer of material (e.g., a dielectric material, such as a silicon dioxide) which is transparent at the light frequency of a monochromatic light source available for recording use (e.g., an argon laser, providing an output at a wavelength of 4880 angstrom units) is deposited over the reflective layer. Finally, the dielectric layer is coated with a thin layer of material (e.g., an organic dye or a metal) which is absorptive at the light frequency of the recording light source.

In an illustrative recording system (e.g., of the type described in U.S. Patent No. 4 097 895 a record blank of the above-described disc form embodying the priciples of the present invention is subjected to rotation at a constant rotational speed while a beam of light from a light source (e.g., a laser, providing light at a frequency at which the anti-reflection condition is obtained) is focused on the coated surface of the disc. The intensity of the light beam is controlled in accordance with information to be recorded. Illustratively, the control is effected in accordance with carrier waves modulated in frequency by picturerepresentative video signals, with the light beam intensity varying as a result between a high level sufficient to effect melting of the absorptive material and a low level insufficient to effect such melting, the frequency of the level alternations varying as the video signal amplitude changes.

An information track comprising a succession of spaced pits is thus formed in the coated surface of the disc. The pits appearing in those surface regions exposed to the high level beam, due to melting of the absorptive layer material in response to the high level beam exposure with variations in the length and separation of the pits being representative of the recorded information. Where a continuous sequence of pictures is to be recorded, a spiral information track may be formed by providing relative motion, in a radial direction and at a constant rate during the recording between the recording beam and the rotating disc.

Alternatively, in the absence of such relative motion during the recording, a circular information track may be formed, appropriate for "slide" recording purposes.

The result of the above-described recording process is the formation of an information record of a form which facilitates recovery of the recorded information by optical playback processes. The information track of such an information record comprises (1) undisturbed surface regions that exhibit very low reflectance at an appropriate light frequency (due to the anti-reflection thickness choice described previously), alternating with (2) pit regions, formed by the melting process, that exhibit appreciably higher reflectance at the same light frequency (due to complete, or at least partial, removal of the absorptive layer covering of the transparent layer and the reflective surface of the substrate, ensuring departure from the anti-reflection condition). A high ratio between the reflectance of the pit regions and the reflectance of the intervening (undisturbed surface) regions is readily provided.

In playback operations, a light beam is focused upon the information track of a rotating information record of the above-described type embodying the principles of the present invention. The playback beam has a constant intensity at a level insufficient to effect melting of the disc coatings, and is of a frequency substantially corresponding to that at which the undisturbed surface regions exhibit an anti-reflection condition. A photodetector, positioned to receive light reflected from the successive regions of the information track as they pass through the path of the focused light, develops a signal representative of the recorded information. A high readout contrast ratio (due to the large differences in reflectance of the pit regions and the intervening track regions, at the light frequency of the playback beam) is readily obtained, permitting recovery of the recorded video signals with an excellent signal-to-noise ratio.

In the accompanying drawings: Figure 1 provides a cross-sectional view of a portion of a recording medium, exhibiting a construction in accordance with principles of the present invention; Figure 2 illustrates a cross-sectional view of a portion of the information track of an information record formed from a recording medium of the type shown in Figure 1 pursuant to principles of the present invention; and Figure 3 provides a graph of the relationship between dielectric layer thickness and reflectance for an illustrative form of the recording medium of Figures 1 and 2.

In Figure 1, a cross-sectional view of a portion of a record blank 11, formed for use in an optical recording system, shows the construction of a recording medium persuant to an illustrative embodiment of the present invention. The record blank 11 includes a substrate 13, which is illustratively formed in the shape of a disc, a major surface (s) of which is processed to be polished and flat. Desirably, the substrate 13 is formed of a material, such as glass, for example, which may be conveniently processed to achieve such surface.

Overlying the surface "s" of the substrate 13 is a thin layer 15 of a material exhibiting a high reflectivity (over at least a given portion of the light spectrum). Illustratively, the reflecting layer 15 is formed of a metal, such as aluminum, for example, deposited on the surface "s" by an evaporation process.

Overlying the reflecting layer 15 is a layer 17 of material which is light transparent (at least over the aforementioned given portion of the light spectrum). Illustratively, the transparent layer 17 is formed of a dielectric material, such as silicon dioxide, for example, deposited on the reflecting layer 15 by an evaporation process.

Finally, overlying the transparent layer 17 is a thin layer 19 of material which is light absorptive at least over the aformentioned given portion of the light spectrum.

Illustratively, the absorptive layer is formed of a 50 layer of a metal, such as titanium or rhodium, deposited on the transparent layer 17 by an evaporation process.

It will be noted that when depositing titanium in the record blank manufacturing process, a 75 A thick layer of titanium is deposited over the dielectric coating. Upon exposure of the record blank to the atmosphere, the portion of the titanium layer exposed to the atmosphere oxidizes forming an approximately 25 thick layer of titanium dioxide, thus effectively leaving a 50 A thick layer of titanium over the dielectric layer. Titanium dioxide is transparent, at the light frequency of the recording and playback beams, and its effect is inappreciable on the recording and playback processes.

An advantage of the use, in the illustrated construction of the recording medium, of a thin absorptive layer overlying a transparent layer which in turn overlies a reflective surface will be recognized by consideration of the effect achieved when a light beam L (of a frequency in the aforementioned given portion of the spectrum) is directed along an axis (x) normal to the surface "s", and focused at or near the surface of the absorptive layer 19.

Most of that portion of the incident light which reaches the interior boundary of the absorptive layer 19 is not "lost" through transmission into the substrate 13, as would ocur in the absence of the reflecting layer 15, but rather is reflected back through the transparent layer 17 into the absorptive layer 19. The consequence is exposure of the absorptive layer 19 to both incident and reflected light. Where it is desired to effect surface melting in response to recording light exposure, this avoidance of transmission losses into the interior of the recording medium increases the coupling efficiency of energy from the recording light beam into the absorptive material enhancing recording sensitivity. While a similar avoidance of transmission losses into the interior of the recording medium would be achieved by forming a thicker absorptive layer (i.e., increasing the reflectance thereof), the transmission loss avoidance would be offset by high reflection losses.

For optimum efficiency of coupling of energy from the recording light beam L into the absorptive layer 19, reflection losses are desirably reduced to a low level by choosing the thickness (d2) of the transparent layer, 17, with relation to the thickness (dl) of the absorptive layer 19, the thickness (d3) of the reflecting layer 15 and the optical constants of the elements of the system 19-17-15-13, to establish a so-called anti-reflection condition for the system at the recording beam frequency. The achievement of an anti-reflection effect by use of thin films of appropriate thickness and optical properties is well known, per se, and applications of the effect with films of transmissive materials are widespread in optical equipment. Formulae which may be used in arriving at parameter combinations for the absorptive media (19, 17, 15) of the Figure 1 system that result in the desired anti-reflection condition are well known in the art and a presentation thereof may be found in the above mentioned U.S. Patent 4097895.

When the intensity of the focused light beam L is of sufficient magnitude, material of the absorptive layer 19 is elevated to a melting temperature, and melting of the material occurs, forming a pit in the surface of the record blank 11. With suitable modulation of the intensity of the beam L in accordance with a recording signal, as successive regions of the record blank 11 pass through the beam path, an information track may be formed comprising spaced pits in regions of the absorptive layer subject to the high intensity beam exposure, separated by undisturbed regions of the absorptive layer (that were not subject to such high intensity beam exposure).

Figure 2 illustrates a portion of an information record formed when the record blank 11 of Figure 1 is subjected to such controlled beam exposure. As shown in cross-section in Figure 2, the information track comprises a succession of spaced pits p1, P2, P3, P4, separated by regions (ELI, 2, 3, 4) in which the surface of the absorptive layer 19 is undisturbed. For illustrative purposes, the depth of each pit is shown as being equal to the thickness of the absorptive layer 19, whereby the reflective layer 15 is wholly uncovered through the transparent layer 17 in the pit regions. As will be discussed subsequently, while such a depth of melting may desirably result in maximizatin of the readout contrast ratio, it is not essential for good playback results. Thus, in an acceptable alternative to the illustrated form of information record, a residual portion of the absorptive material (with a thickness, of course, less than d1) may overlie the transparent layer 17 in the pit bottoms.

When the light frequency of a playback beam provided by a laser falls in the given spectrum portion for which the disc's layer 19 is highly absorptive and for which the disc's layer 15 is highly reflective, and at or close to the frequency at which the undisturbed regions of the system 19-17-15-13 exhibit the anti-reflection effect, a high readout contrast ratio is realized, permitting video signal recovery with an excellent signal-to-noise ratio.

Illustratively, with use of a deviation range of 7-10 Mhz, color television signals of an NTSC format have been recovered with a video signal-to-noise ratio of 45-50 (peak-to-peak video to rms noise) for a video bandwidth of 5 MHz.

Curve "a" of the graph of Figure 3 shows that the computed minimum reflectance for the illustrative system parameter choices for the system of Figure 1 occurs at a dielectric layer thickness, Dmin, given by the following equation Dmin = 790A + n 1670A where n is zero or a positive integer.

Thus, when a choice for the Dmin thickness of the dielectric layer is associated with the illustrative system parameter choices, the record blank of Figure 1 (and the non-depressed regions of the information record of Figure 2) exhibits an anti-reflection condition for the argon laser output.

It will be appreciated that where the flat substrate is formed of a temperature sensitive material (e.g., polyvinyl chloride), care must be taken to avoid disturbance of the substrate by the heat generated in the recording process. Under those circumstances, the substrate can be effectively insulated from the heat generated by the recording laser by choosing a choice for the Dmin thickness for the dielectric layer which Dmin choice is greater than the thermal diffusion length (1) through the dielectric layer (e.g., a Dmin choice where n > 2).

Curve "b" shows absorption in a soA thick titanium layer, for the illustrative system parameter choices for the system of Figure 1, when computed as a function of dielectric layer thickness.

While the principles of the present invention have been demonstrated with particular regard to the illustrative structures of Figures 1 and 2, it will be recognized that varius departures from such illustrative structures may be undertaken in practice of the invention.

For example, the substrate itself may be formed of material having high reflectivity, eliminating the need for employing a separate reflecting layer in forming a reflective surface underlying the transparent layer. Furthermore, while the invention was particularly described by examples where the absorptive layer was formed of a metallic material, other materials, such as organic dyes (e.g., of a type employed in the aformentioned U.S. Patent 4097895 may be used. The anti-reflection condition for the disc medium obtained by the U.S. Patent is also obtained, through application of the principles of the present invention, with a significant diminution in the thickness of the organic dye due to the above mentioned dielectric layer thickness. For another example, since wideband reflection is not required of the reflecting layer, the metal coating may be supplanted by a multilayer (or even a single layer) dielectric reflector. It should also be appreciated that other forms of optical recording (such as pulsed holographic recording) may use the advantageous record blank structure described herein.

WHAT WE CLAIM IS: 1. An information record, for use in playback apparatus employing a playback beam of light of a given frequency; said record comprising: a substrate having a light reflective surface; a layer of light transparent material overlying said reflective surface; and a layer of light absorptive material overlying said light transparent layer with an information track formed in said absorptive layer; wherein said information track comprises a succession of spaced pits, with variations in the spacing between successive pit edges representative of recorded information, wherein the thickness of said light transparent layer is so chosen, with regard to thickness of said light absorptive layer in all layer regions other than those occupied by said pits, as to establish an anti-reflection condition for all of said layer regions at said given frequency; and wherein the thickness of said light absorptive layer in those layer regions occupied by said pits is less than a thickness required to establish an anti-reflection condition for those layer regions occupied by said pits.

2. An information record in accordance with claim 1 wherein said substrate is in the form of a disc and wherein said light reflective surface comprises a metal layer deposited on a major surface of said disc.

3. An information record in accordance with claim 2 wherein said light absorptive layer is formed by a metallic material.

4. An information record in accordance with claim 1 wherein said pits comprise regions substantially free of said light absorptive material.

5. An information record in accordance with claim 1 wherein said light absorptive layer comprises titanium.

6. A record blank for use in optical recording comprising, in combination: a substrate having a surface exhibiting high reflectivity over at least a given portion of the light

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. regions of the system 19-17-15-13 exhibit the anti-reflection effect, a high readout contrast ratio is realized, permitting video signal recovery with an excellent signal-to-noise ratio. Illustratively, with use of a deviation range of 7-10 Mhz, color television signals of an NTSC format have been recovered with a video signal-to-noise ratio of 45-50 (peak-to-peak video to rms noise) for a video bandwidth of 5 MHz. Curve "a" of the graph of Figure 3 shows that the computed minimum reflectance for the illustrative system parameter choices for the system of Figure 1 occurs at a dielectric layer thickness, Dmin, given by the following equation Dmin = 790A + n 1670A where n is zero or a positive integer. Thus, when a choice for the Dmin thickness of the dielectric layer is associated with the illustrative system parameter choices, the record blank of Figure 1 (and the non-depressed regions of the information record of Figure 2) exhibits an anti-reflection condition for the argon laser output. It will be appreciated that where the flat substrate is formed of a temperature sensitive material (e.g., polyvinyl chloride), care must be taken to avoid disturbance of the substrate by the heat generated in the recording process. Under those circumstances, the substrate can be effectively insulated from the heat generated by the recording laser by choosing a choice for the Dmin thickness for the dielectric layer which Dmin choice is greater than the thermal diffusion length (1) through the dielectric layer (e.g., a Dmin choice where n > 2). Curve "b" shows absorption in a soA thick titanium layer, for the illustrative system parameter choices for the system of Figure 1, when computed as a function of dielectric layer thickness. While the principles of the present invention have been demonstrated with particular regard to the illustrative structures of Figures 1 and 2, it will be recognized that varius departures from such illustrative structures may be undertaken in practice of the invention. For example, the substrate itself may be formed of material having high reflectivity, eliminating the need for employing a separate reflecting layer in forming a reflective surface underlying the transparent layer. Furthermore, while the invention was particularly described by examples where the absorptive layer was formed of a metallic material, other materials, such as organic dyes (e.g., of a type employed in the aformentioned U.S. Patent 4097895 may be used. The anti-reflection condition for the disc medium obtained by the U.S. Patent is also obtained, through application of the principles of the present invention, with a significant diminution in the thickness of the organic dye due to the above mentioned dielectric layer thickness. For another example, since wideband reflection is not required of the reflecting layer, the metal coating may be supplanted by a multilayer (or even a single layer) dielectric reflector. It should also be appreciated that other forms of optical recording (such as pulsed holographic recording) may use the advantageous record blank structure described herein. WHAT WE CLAIM IS:

1. An information record, for use in playback apparatus employing a playback beam of light of a given frequency; said record comprising: a substrate having a light reflective surface; a layer of light transparent material overlying said reflective surface; and a layer of light absorptive material overlying said light transparent layer with an information track formed in said absorptive layer; wherein said information track comprises a succession of spaced pits, with variations in the spacing between successive pit edges representative of recorded information, wherein the thickness of said light transparent layer is so chosen, with regard to thickness of said light absorptive layer in all layer regions other than those occupied by said pits, as to establish an anti-reflection condition for all of said layer regions at said given frequency; and wherein the thickness of said light absorptive layer in those layer regions occupied by said pits is less than a thickness required to establish an anti-reflection condition for those layer regions occupied by said pits.

spectrum; a first coating exhibiting light transparency over at least said given portion of the light spectrum, said first coating overlying said surface; and a second coating exhibiting high absorptivity over at least said given portion of the light spectrum, said second coating overlying said first coating.

7. A record blank in accordance with claim 6 wherein said substrate is in the form of a disc, and wherein said light reflecting surface is formed by a metal coating on a major surface of said disc.

8. A record blank in accordance with claim 7 wherein said light absorptive coating comprises a metallic material.

9. A record blank in accordance with claim 8 wherein said light reflective surface comprises aluminum.

10. A record blank in accordance with claim 9 wherein said light absorptive coating comprises titanium.

11. A record blank in accordance with claim 6 for use with a recording laser providing light of a given frequency wherein said given portion of the light spectrum encompasses said given frequency.

12. A record blank in accordance with claim 7 wherein the thickness of said first coating is so related to the thickness of said second coating, the thickness of said metal coating and the optical constants of said substrate, said metal coating material, and the respective materials of said first coating and said second coating so as to establish an anti-reflection condition for said record blank at said given frequency.

13. A record blank in accordance with claim 6, including a third coating exhibiting transparency over at least said given portion of the light spectrum, said third coating overlying said second coating.

14. A record blank in accordance with claim 13 wherein said second coating comprises titanium; and said third coating comprises titanium dioxide.

15. A record blank or information record, substantially as hereinbefore described with reference respectively to Figure 1 or Figure 2.