GB2042200A - Laser pyrographic reflective recording medium - Google Patents

Abstract

A recording medium for laser writing and a method of making same wherein a silver-halide emulsion coating (29) disposed on a substrate (27) is strongly exposed to actinic radiation and then developed, or otherwise processed for maximum blackness. The black opaque emulsion is preferably converted to a reflective recording medium by heating at least to 250 DEG C in an oxygen containing environment until the emulsion coating assumes a reflective appearance (32). Prior to developing, patterns may be photographically imposed on the medium to provide control indicia for the recording system or data base information to a playback system or to provide a means of replicating master recordings. <IMAGE>

Description

SPECIFICATION Laser pyrographic reflective recording medium This invention relates to a laser pyrographic reflective recording medium and more particularly to such a storage medium useful for directly reading laser writing immediately after laser writing.

Previously, many types of optical recording media have been developed for laser writing. For example, an article in Optical Engineering, Vol. 15, No. 2, March-April, 1976, p. 99 discusses properties of a large number of media. Such of these media require post write processing before they can be read, and some can be read immediately after laser writing. The media of interest herein are for "direct read after write" capability, commonly known as "DRAW" media. Presently known laser DRAW media are thin metal films in which holes may be melted, composite shiny films whose reflectivity at a spot may be reduced by evaporation, thin films of dyes or other coatings which can be ablated at a spot, and dielectric materials whose refractive-index may be changed at a point, causing a scattering of light when scanned with a read laser.

Today, these media are generally manufactured by means of vacuum deposition on a batch basis rather than continuous-flow basis and are therefore expensive and it is difficult to achieve a uniformity of quality for large production quantities of the product since many batches would be involved. Refractive-index-change materials which have been proposed for future manufacture on a continuous basis have the disadvantage that in order for these media to be read by reflection, a metal undercoating must be applied, thereby introducing a batch type production process with the above mentioned disadvantages.

The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several advantages: First, they can be produced easily in small quantities with commercially available sputtering equipment. Second, they can be read either by reflection or by transmission. Third, films of tellurium and bismuth have relatively high recording sensitivities.

Fortunately, for all of these reasons, metal films have enabled a large amount of research to be conducted and progress to be made in the design of optical data storage systems. To date, tellurium has evolved as the most widely used of the metal films. However, tellurium must be manufactured by an expensive, batch-type, vacuum sputtering technique; it does not form a tenacious coating; and it introduces manufacturing and environmental complications because of its toxicity.

In U.S. patent 3,911,444 Lou, Watson, and Willens disclose a vacuum deposited metal film recording media for laser writing incorporating a separately deposited plastic film undercoat between the metal film and a flexible transparent substrate in order to require less energy to write with a laser and to prevent impurity transfer between the substrate and the radiation absorbing film.

In Example I of U.S. patent 3,567,447 Chand discloses that upon heating a processed silver-halide emulsion coated photoplate, the non-image areas, clear gelatin, darkened to a transparent reddish colour and the image areas assumed a reflective metallic sheen. Chand used the reddish colour to delineate nonimage areas to be removed chemically, thereby leaving hardened opaque image areas, with clear non-image areas.

In U.S. patent 3,893,129 Endo discloses exposed and partially developed film for recroding laser writing by means of heating the film to cause local deformation which scatters light.

In U.S. patent 3,689,894 Laura and Eng disclose exposed and developed microfilm to record data by optically writing transparent bits of data with black areas of the microfilm by burning holes through the black silver-halide emulsion.

Many other patents reveal light absorptive media for recording laser writing. An object of the present invention is to provide a moderately reflective DRAW laser recording material which may be manufactured without the use of a vacuum system and on a continuous basis and which may be used to record non-reflective spots in a reflective field with relatively low energy laser pulses.

Accordingly the invention provides an information storage medium for pyrographically recording laser writing, of the type having a sheet substrate, the improvement characterized by a shiny reflective surface coating disposed on the sheet substrate, the shiny coating comprising reflective silver particles distributed within a gelatin matrix.

The invention also provides a method of making a laser pyrographic information and storage medium comprising: photographically processing a silver-halide emulsion coating on a substrate to blackness; and converting the black photographically processed silver-halide emulsion coating at least until a shiny reflective component appears on the emulsion coating surface.

The black photographically processed silver-halide emulsion coating can be converted by heating at least to 2500C in an atmosphere having a substantial percentage of oxygen.

Temperatures between 280 C and 340 C are preferred to produce the reflective coating during a time cycle on the order of one-half minute to 20 minutes. Heating methods can include the use of a convection oven, a contacting hot source, or radiant heating. A resulting shiny reflective emulsion coated transparent or absorptive substrate is a DRAW laser pyrographic reflective recording medium.

An advantage of the recording medium of the present invention is that it may be made of a continuous flow basis. The emulsion coating is tenacious with respect to its substrate and is not toxic when laser writing ablates portions of the coating. Another advantage is that the emulsion coating may be previously exposed to desired patterns by normal photographic means to provide control indicia for the recording apparatus and/or playback apparatus and for replication of master disc recordings.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein :- Figure 1 is a top plan view of a preferred recording medium of the present invention; Figure 2 is a side sectional view of the recording medium of Figure 1; Figures 3 to 7 are detail views of the recording medium of Figure 1 showing sequential processing steps for making the finished recording medium; and Figure 8 is a side sectional view of the recording medium of Figure 1 showing laser writing.

Figure 1 shows a disc 11 having an inner periphery 13 and an outer periphery 15. The interior of the inner periphery 13 is void so that a centering collar may be used to hold disc 11 on a spindle for high speed rotation. While the recording medium of the present invention is described as a disc, a disc configuration is not essential for operating of the recording medium. For example, the recording medium may be a flat sheet-like material which could be square and with a central hub rather than a hole. It could also be a non-rotating rectangular plate. However, rotating discs are preferred for fast random access to medium amounts of data and non-rotating rectangular plates in stacks are preferred to provide intermediate speed random access to large amounts of data by mechanically selecting a plate and scanning it by mechanical and electro-optical means.

The disc of Figure 1 is photographically partitioned into recording and non-recording areas. For example, a first annular recording zone 17 could be spaced from a second annular recording zone 19 by an annular guard zone 21. The function of the guard zone may be to separate different recording fields, to carry control information, such as timing signals and to provide space for data read-write transducers to reside when not over recording areas. While such guard bands are preferable, they are not essential to the operation of the present invention. It should be noted that the recording fields are for data and control signal recording, while the guard band is not for data recording, but may have control signal recording thereon. The recording field 19 is shown to have a plurality of concentric, circumferentially-spaced servo tracks 23 thereon.Such servo tracks are thin lines which define the spaces between circular paths wherein data are written. The pattern for such lines is applied photographically as explained below with reference to Figures 3 to 7.

Figure 2 shows a side sectional view of the recording medium of Figure 1. The medium consisting of a substrate 27 which is a sheet-like layer which is transparent, translucent, or opaque; preferably a high temperature, dimensionally stable material, like glass, ceramic, and thermoset and thermoplastic polymide plastics. One of the requirements of the substrate material is that it should withstand temperatures to at least 2800C and most likely 320 C but up to 340 C, without thermal deformation. Transparency or absorptivity of the substrate is desired so that when the light beam of the reflective playback apparatus impinges upon a recorded spot, it either passes through the substrate or is absorbed by it with minimum reflection.If the substrate is absorptive, it may be absorptive at the wavelengths of the recording beam or the reading beam, or both. Thus, not all common photographic substrates may be used. For example, the most common photographic film bases are polyester polyterephalate, polycarbonate, or cellulose triacetate - which normally have maximum continuous operating tempteratures of 145"C, 132"C, and 205 C, respectively.

Further, in order to coat a photographic emulsion on a substrate, it must be etchable, dimensionally stable, non-destructible by sunlight, have good mechanical strength, and should also be relatively inexpensive and currently available in adequate quantities. Further, when data recordings are to be read only in reflection rather than transmission, the substrate may be a non-reflecting, opaque material. There is one plastic-polymide that meets all of these conditions. It is available as a thermo-plastic and a thermoset plastic, can operate at 3200C for minutes, and is etchable with alkalines.

For the case where the substrate is transparent, the recording medium may be read in a transmissive mode, for example as in U.S. patent application serial No. 845,332, entitled Error Checking Method and Apparatus for Digital Data in Operational Recording Systems, assigned to the assignee of the present invention.

The thickness of the substrate is not critical when the laser beam is directed onto the surface as shown in Figure 8, but it should have sufficient thickness to provide strength for resistance against breakage. If the laser beam is directed through a transparent substrate, then in order to maintain focus of the beam the thickness of the transparent substrate would have to be very uniform (for example, as obtainable from float glass or selected high quality drawn glass). Also, the thickness of the substrate may depend on the overall size of the recording medium being used. For a 12-inch disc, a thickness of inch may be suitable.

The purpose of substrate 27 is to support a silver-halide emulsion coating 29, which is uniformly applied to the substrate in a conventional manner and which is converted subsequently to layers 32 and 33 in Figure 8.

Currently available silver-halide emulsions from 3 to 6 microns thick are adequate, provided they are characterized by very fine silver-halide grain size, i.e. onlu a small percentage of the grains are larger than 0.07 microns. This grain size appears to be important because when grain sizes become larger than approximately 0.06 microns the conversion from black to reddish colouration, which subsequently becomes metallic, appears to be less complete. Emulsion coated glass plates having these characteristics are commercially available and are known as photoplates which are used to make photomasks for the manufacture of semiconductor integrated circuits. For example, emulsion coated photoplates suitable for use herein are manufactured by Agfa-Gaevaert of Belgium, Konishiroky Photo Industries of Japan and the Eastman Kodak Company.

The shiny reflective component 32 in Figure 8 results from the thermal processing described herein and reflectiveity does not initially exist in the emulsion. At the inception the material of reflective component 32 is mostly, excepting oxygen, all found in the photographic emulsion 29, which is uniform in its composition.

A subbing layer, not shown, is usually used to attach the substrate 27 to the emulsion 29. Following the thermal conversion of the present invention the emulsion 29 of Figure 2 produces a reflective component 32 at the emulsion surface shown in Figure 8, with a non-reflective under-layer 33 beneath it. The reflective component 32 is not well defined in thickness, but exhibits a silver concentration gradient, with most of the silver near the surface, and less extending downwardly. Thus, although Figure 8 depicts a sharp boundary for reflective component 32, actually such is not the case and is only pictured in this manner to explain the contrast which exists between the surface of the material and the underlying emulsion. The presence of more silver particles at the surface and less below after heating is surprising and not completely understood.

It is believed that heat causes breakup of filamentary silver particles into much more mobile, smaller particles which, in the presence of oxygen at the surface concentrate and become reflective.

Underlayer 33, while not completely depleted of silver, contains less silver than reflective component 32.

Optically, underlayer 33 is partially transmissive to red light having wavelengths of 630 nanometers and longer, so that once craters are created penetrating reflective component 32, the craters may be detected by transmission of red light through the underlayer 33, provided that the opacity of the reflective layer is sufficiently great at the selected wavelength to permit detection of the craters through differences in light transmission. The data contained in the craters may also be read by changes in reflectivity of the shiny reflective component through the visible spectrum and into the near infrared where it is ultimately limited in its usability as it becomes more and more transparent and therefore less reflective in the non-data areas.

It should be noted that both the recording areas 17, 19 and the non-recording guard band 21 of Figure 1 have silver-halide emulsion covering a glass substrate. Thus, the designation of recording and nonrecording areas is arbitrary and the entire surface could be used for recording if desired. However, as a matter of convenience, it is preferable to designate areas as non-recording areas. The boundaries between recording and non-recording areas may be defined by concentric lines, just as the servo tracks 23 of Figure 1, which have been greatly enlarged in the Figure, may be defined by lines. Typically, servo tracks are closely spaced concentric circles or adjacent lines of a spiral, with data being written on or between the lines.Such servo track lines, as well as line boundaries for non-recording areas, may be photographically recorded on the recording medium prior to any data recording. Moreover, other alphanumeric information or data base information which is to be a permanent part of the recording medium also may be applied to the recording medium at an early time in the processing cycle since it becomes a permanent part of the recording medium.

One of the advantages of the present invention is that the permanent information to be recorded on the recording medium of the present invention may be applied by photographic techniques since the starting material for the recording medium can be an unexposed commercially available photoplate used in the manufacture of semiconductor integrated circuits. After thermal conversion this information may be read in reflection since the black image areas will become highly reflective and the clear non-image areas will be only slightly reflective.

The photographic techniques which may be used to pre-record data base and control information are well known in the semi-conductor industry. Lines having a thickness of one micron thickness may be made. The typical procedure for creating a line pattern is illustrated in Figures 3 to 6. With reference to Figure 3, the medium 11 is exposed to a line pattern consisting of the circular lines 23a, 23b and 23c. The line pattern exists as a latent image in the silver-halide emulsion, the remainder of which is unexposed to light.

Figure 4 illustrates the processing of the plate 11 through a well known commercially available developer which causes the image pattern of lines 23a, 23b, 23c to become black, characteristic of black silver, the remainder of the material being transparent.

In Figure 5, the developed silver areas 23a, 23b, 23c are bleached out so that they are clear. The bleach does not affect the unexposed zones of the recording material 11.

In Figure 6, the entire recording medium is not strongly exposed to actinic radiation, such as by a mercury arc lamp, incandescent lamp or xenon flash lamp, and developed for maximum blackness (fixing is optional). The object of this step is to provide black zones for conversion into the reflective data recording fields. Of course, the guard zone 21 in Figure 1 between recording fields will also have the same black character, only its use is different. This exposure, development and fixing step would be the only processing procedure taken, prior to heating, if there were no information to be pre-recorded, such as the servo tracks 23a, 23b, 23c and there were no other pre-recorded alphanumeric or control indicia. In fact if no images are to be photographically recorded a fogging developer could be used in some cases to avoid the need for the exposure step. Fogging developers are known for use in reversal processing where it is desired to develop all remaining silver in a silver-halide emulsion. See "The Theory of the Photographic Process", 4th Ed., McMillan (1977) p.422. The minimum extent of blackness opacity must be such that the reflective component 32 subsequently produced by thermal conversion is adequately reflective to the wavelength of light used in reflective playback. A thin reflective component with adequate reflectivity, less than 0.5 micron thick is preferred in order to minimize energy needed to punch through the coating for laser pyrographic writing.

As an alternative to wet chemical development for creating black emulsion, dry thermal development may also be used. In thermal development, a latent image is developed by heating an exposed photosensitive thermographic material. Various types of materials exist, but in each case very mild heating causes development. Heating to about 11 50C for five seconds causes development in typical thermographic materials. One type of material which may be thermally developed contains a developer composition together with silver-halide grains. Heating causes the developer to become activated, sometimes using moisture derived from the emulsion carrier. In either instance, whether a chemical developer or a dry developer is used, the photographic emulsion is processed for maximum blackness.

Once the recording medium has achieved the described level of blackness, with or without pre-recorded indicia thereon, thermal conversion into a reflective medium may commence by heating the emulsion coating to a temperature of approximately 280 C to 3400C in air, or 2500C to 3400C in oxygen, until a shiny reflective component 32 in Figure 8 appears. The coating initially is first converted to a dark cherry red transmissive medium. This conversion, indicated in Figure 7, begins to occur at temperatures as low as 200 C. At higher temperatures, specifically at about 3000C the coating starts to become reflective in less than a minute.After further heating, reflectivity is clearly evident at the upper surface and the material 11 has a characteristic gold colour. Electrical resistance measurements on the shiny component 32 in Figure 8 indicate no measurable conductivity. Heating methods include the use of a convection oven, contacting hot source or radiant heating. Radiant heating is preferred because it heats the emulsion fast and uniformly and can be programmed easily to minimize thermal shock to the substrate.

The shiny component 32 also has low thermal conductivity. It is believed that silver grains which form the shiny component 32 are individually separated from each other by gelatin. In other words, the mild temperatures used in thermally converting the emulsion coating into a shiny component 32 are low enought to preserve the insulative properties of the gelatin. Higher temperatures would char or burn the gelatin, perhaps removing it by ablation. It appears that the mild temperatures used herein are adequate to stimulate silver grain breakup, and cause a dispersion of the grains which appears to be necessary for the creation of the reflective layer.

The oxygen component of the heating atmosphere is preferably maximized because it lowers both the temperature and the time required for processing. Although heating in air will work, a pure oxygen environment is better. A minimum percentage of oxygen, at least a few percent and preferably much more, such as the substantial percent of oxygen found in air, is necessary to create the shiny silver component 32 by thermal conversion.

At a minimum, the shiny reflective component must be visible on the surface of the material to be useful for reading the data by differential reflectivity. However, in some instances it may be desirable to have a thicker coating of the shiny material; in this instance longer heating would be required. For example, in the instance of thermally converting the entire thickness of the coating, heating for above 20 minutes is needed.

Conversion may occur at temperatures between 2800C and 340 C. The hig her the conversion temperature, the faster the reaction and the more complete conversion; however, 3200C is selected as a preferred maximum temperature so as to minimize the charring of the gelatin in the emulsion coating and to minimize possible thermal damage to the substrate. Charring in gelatin is noted by an amber colour in the material.

Note that the shiny component 32 only occurs where black silver previously existed. The shiny component is thus derived from the silver in the developed silver-halide emulsion. While the silver appears at the surface and is concentrated there, the thickness of the shiny component is not well defined because of a silver concentration gradient diminishing toward the direction away from the exposed surface. It was shown above that certain black silver areas could be removed by bleaching in order to leave control indicia or line boundaries. Clear indicia markings of simple types can also be introduced by a negative processing procedure in which a mask or an intermittent beam is used to create the black images which outline the clear indicia.

While a silver-halide emulsion is the straining material for the recording medium of the present invention, the finished product is considered to be a silver gelatin complex, the halides being substantially removed in the exposure and development process. The finished product is characterized by a reflective silver component at the surface thereof having a silver gradient with more silver at the surface and less below, but with some silver throughout the gelatin.

To use the recording medium ofthe present invention laser light is focused on a spotatthe surface ofthe coating of the recording medium. Enough laser energy is delivered to the stop to remove the shiny reflective material. The shiny material is primarily at the surface and since a reflective read procedure is used, for example as described in U.S. patent 3,657,707, the recording laser beam need only penetrate and remove the shiny coating - not the full depth of the emulsion coating. Transmissive type reading can be accomplished to a limited extent if a red or very near infrared laser beam is used such that the capacity of the coating blocks 90% of the light and the recorded craters permit transmission of at least 50% of the light.

Figure 8 shows emulsion coating 29 on substrate 27 covered by shiny component 32 having a crater 30 damaging the shiny component created by means of laser light indicated by the rays 31. The size of the craters is kept at a minimum, preferably slightly under one micron in diameter but no larger than a few microns in diameter to achieve high data densities. Data written by means of laser light are recorded in the recording areas 17, 19 shown in Figure 1, designated by the letter R. As mentioned previously, these recording areas may also contain prerecorded data base data and control indicia which may be disposed over essentially the entire area of the medium. No data is recorded in the guard band 21, designated by the letter G, although this region may have control indicia written therein.Control indicia in either of the areas may be written by means of photographic techniques or by pyrographic methods such as laser writing.

Thus, the recording medium of the present invention may contain a mix of pre-recorded data and control indicia which has been applied to the recording medium by photographic techniques, as well as subsequently written data applied to the recording medium by laser pyrographic writing. There is no data storage distinction between pre-recorded non-reflective spots and non-reflective spots made by laser writing. In the read mode, data base data and control information are accessible on the same disc as data, while in the record mode the control information is used.

The table below lists the relative contrast measurements obtained from laser writing and reading as shown in Figure 8 on a sample of this laser pyrographic reflective recording medium on a glass substrate.

Measurements were made by recording and reading 32 spots at each of 16 power levels, or a total of 512 spots, with an argon laser generating the green 514 nanometer line. Electron micrographs and optical microscope photographs were taken on the holes created. These photos confirmed that the depth of the holes at rated power levels is considerably less than the 0.8 micron hole diameter itself; also, that the lower power levels the holes are reduced in size. The table illustrates that the contrast is almost unchanged from power levels of 28 milliwatts down to 6.9 milliwatts, indicating that as long as the power is above the required level, the material performs well. There is no apparent "overpower" effect. Note that there is little further degradation in contrast down to 4.6 milliwatts.Finally note that the usable contrast (when the median contrast is much larger than the 1 Sigma distribution value) is as low as 2.2 milliwatts. From these data it can be concluded that the medium could be rated at 5 milliwatts for 0.8 micron beam recording with 100 nanosecond pulses. This is slightly less than that required to record on a thin layer of vacuum-deposited tellurium - a metal which is one of the more popular laser recording materials.

The above described reflective recording medium requires much less laser energy and creates considerably less recording debris than prior media in which holes were burned through an entire 6 microns thick black silver-halide emulsion.

The method herein described results in melting and removal of less than 0.5 micron of the thermally converted emulsion. Thus, the laser energy required and the debris generated are reduced by more than one order of magnitude.

Note that the thermally converted processed silver-halide emulsion under the reflective coating, 33, plays a thermal insulating role in improving recording sensitivity similar to what is accomplished by the plastic film undercoat of the prior art. However, the present coating method does not require the added step of a batch-type vacuum deposition procedure. A continuous flow manufacturing process may be used to produce the reflective coating.

Laser writing and reading on pyrographic reflective recording medium Laser Beam: 514 Nanometers Wavelength 0.8 Micron Diameter 100 Nanosecond Pulses Pulsed Power Relative Contrast Statistical Distribution at Surface of Ratio Averaged of Contrast Ratios Recording Over 32 Spots of the 32 Spots Medium (in (+ 1 Sigma) Milliwatts) 28 2417 + 276 24.3 2361 + 442 21 2518 1267 18 2700 + 296 15.4 2690 1314 12.8 2804 + 267 10.4 2634 + 270 8.7 2651 1 325 6.9 2498 + 336 5.7 2221 1459 4.6 2156 1432 3.6 1860 + 624 2.8 1725 + 380 2.2 1217 1250 1.7 654 1188 1.3 279 1145

Claims (12)

1. An information storage medium for pyrographically recording laser writing, of the type having a sheet substrate, the improvement characterized by a shiny reflective surface coating disposed on the sheet substrate, the shiny coating comprising reflective silver particles distributed within a gelatin matrix.

2. A medium as claimed in claim 1 wherein the shiny reflective surface coating has a silver concentration gradient whereby the silver concentration is greatest at the surface boundary away from the substrate and diminishes in concentration toward the substrate.

3. A medium as claimed in claim 1 to 2, wherein the shiny reflective surface coating has a reflective silver component extending throughout said coating.

4. An information storage medium for pyrographically recording laser writing, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

5. A method of making a laser pyrographic information and storage medium comprising: photographic- ally processing a silver-halide emulsion coating on a substrate to blackness; and converting the black photographically processed silver-halide emulsion coating at least until a shiny reflective component appears on the emulsion coating surface.

6. A method as claimed in claim 6, wherein the converting is effected by heating the material to at least 2500C in an atmosphere having a substantial percentage of oxygen.

7. A method as claimed in claim 5 or 6 wherein the step of photographically processing the silver-halide emulsion comprises the steps of: exposing said silver-halide emulsion coating on a substrate to actinic radiation; and developing the silver-halide emulsion coating.

8. A method as claimed in claim 5, 6 or 7, wherein the step of photographic processing includes photographically defining in the emulsion areas for recording data by laser pyrographic means and areas set aside for no laser pyrographic recording.

9. A method as claimed in any of claims 5 to 8, wherein the step of photographic processing includes photographically defining in the emulsion lines defining servo tracks.

10. A method as claimed in any of claims 5 to 9 wherein the step of photographic processing includes photographically defining in the emulsion lines defining control indicia.

11. A method as claimed in claim 8, wherein the step of photographic processing includes recording data base data in a photographically defined area set aside for no laser pyrographic recording.

12. A method of making a laser pyrographic information and storage medium, substantially as hereinbefore described with reference to the accompanying drawings.