CA1137345A - Process for making a reflective data storage medium - Google Patents

Abstract

ABSTRACT

A method for making a reflective data storage medium by creating a volume concentration gradient of silver precipitating nuclei on one surface of a silver halide emulsion coated photoplate. This volume concentration is then built up by a silver diffusion transfer nega-tive development using primarily solution physical development until the surface becomes reflective.
Lastly, a thermal annealing step is used to increase reflectivity, recording sensitivity and produce a more uniform contrast ratio.

Description

" ~i37345 PROCESS FOR MAKING A
REFLECTIVE DATA STORAGE MEDIUM

The invention relates to recording media, and more par-ticularly to a reflective silver data recording and storage medium useful for reading laser recordings im-mediately after laser writing.

Previously, many types of optical recording media have been developed for laser writing. Some 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. Pres-ently 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 ab-lated at a spot, and dielectric materials whose refrac-tive-index may be changed at a point, causing a scatter-ing of light when scanned with a read laser.

The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several ad-vantages: First, they can be produced for research pur-poses in small quantities with commercially available sputtering equipment. Second, they can be read either by reflection or by transmission. Third, films of tel-lurium and bismuth have relatively high recording sensi-tivities.

Fortunately, for all of these reasons, metal films have ~3734S

enabled a large amount of research to be conducted andprogress to be made in the design of optical data storage systems. To date, tellurium and amorphous mix-tures thereof have evolved as the most widely used of the metal films. However, tellurium must be manufac-tured by a relatively expensive, batch-type, vacuum sputtering technique; it does not form a tenacious coating; and it introduces manufacturing and environmen-talcomplications because of its toxicity and since it rapidly oxidizes in air it must be encapsulated in an airtight system in order for it to achieve an acceptable archival life. It has been reported in the literature that by forming amorphous tellurium mixtures with arsenic and selenium the rate of oxidation is reduced.

What is particularly desirable about tellurium is that it has a low melting temperature for a metal, 450C, and also a very low thermal conductivity of 2.4 watts per meter per degree Kelvin at 573K. In comparison, silver metal has a melting temperature of 960C and a thermal conductivity of 407 watts per meter per degree Kelvin at the same elevated temperature. When these two metals are considered for laser recording with short pulses of laser power, the tellurium is far superior from a re-cording sensitivity standpoint since the low thermal conductivity keeps the heat generated by the laser beam confined to a small area and the lower melting tempera-ture facilitates the melting of the hole. Conversely, silver metal, because of its high thermal conductivity, about 170 times that of tellurium, would not normally be considered suitable for laser recording.

1~37345 A surface can reflect large percentages of the incident light without being electrically conductive. It is known that if very small, electrically conductive metal spheres or spherical particles are distributed through a dielectric medium, the effective dielectric constant or refractive index will rise owing to the added induced dipoles of the metal particles.

- When photographic gelatin is heated above 245C, it gives up all retained water, exhibits a pyrolysis which frees some carbon, and is transformed ppysically from long helices to a shorter, random coil configuration typical of polymer materials.

Although it is possible to produce reflective metallic coatings of many types of substrates by vacuum sputter-ing or evaporation, silver is relatively unique in thatit can also be produced and patterned by photographic techniques. Previously, a reflective silver laser re-cording medium was the subject of a prior U.S.A. patent application wherein a processed black silver emulsion was converted to a reflective recording medium by heat-ing at a temperature in the range of 250C to 330C in an oxygen containing atmosphere until a shiny reflective appearance was achieved. The heating process appeared to break up the black filamentary silver grains into tiny grain segments of a few hundred angstroms. Over a period of minutes the heat and oxygen combined to create the surface reflective component of silver which is more concentrated at the surface and decreases monotonically into the body of the converted emulsion without creating a clearly defined layer of silver. It is believed that ` li3734S

the conversion process includes the creation and then decomposition of silver oxide. High contrast digital-data recordings with reflective contrast variations of +20% can be accomplished with a 5 milliwatt laser beam 0.8 microns in diameter and with a pulse length of 100 nanoseconds.

A silver diffusion transfer negative reflective photo-graphic process leading to a reflective data storage medium without the use of a thermal processsing step was the subject of a second prior U.S.A. patent applica-tion. The reflective electrically non-conducting data storage and laser recording medium was made from a com-mercially available photosensitive silver-halide emul-sion by a silver diffusion trarsfer negative process and relies on the high refractive index of the silver-gelatin composite at the emulsion surface to create the reflectivity. In that application a well defined layer of reflective silver gelatin was created at a surface of the silver-halide emulsion by a latent image formation followed by a special monobath development treatment in-volving a small amount of chemical development and chem-ical diffusion transfer of the silver complexes and so-lution physical development of the latent image. High contrast digital-data recordings with reflective con-trast variations of +40~ were accomplished with a 13milliwatt laser beam 0.8 microns in diameter and with a pulse length of 100 nanoseconds. These reflective con-trast variations appeared to be associated with local variations in reflectivity owing to local variations of silver density within the gelatin layer.

li3734S

Silver diffusion transfer negative and reversal processes have been described in the patent literature. In U.S.
Patent 3,464,822 Blake discloses a silver diffusion trans-fer reversal process for creating electrically conducting silver images for the fabrication of printed circuit boards. That invention, in turn, is based upon silver diffusion transfer process inventions of the reversal type, leading to black non-reflective and non-conductive images, one example being U.S. patent 2,500,421 by E. H.
Land. The silver diffusion transfer reversal process forms the basis of direct positives by the Polaroid Land process of Polaroid Corporation and the Gevacopy and Copyrapid processes of Agfa-Gevaert. These reversal processes should be distinguished from the silver dif-fusion negative process. One such process leading toblack non-reflecting and non-conducting images, is de-scribed in U.S. patent 3,179,517 by Tregillus.

The present invention provides a method of making a re-flective electrically non-conducting data storage medium comprising, forming an areawise, latent image exposure in a photo-sensitive silver-halide emulsion layer disposed on a sub-strate, thereby defining an area for data storage, said exposure creating an areawise layer of silver precipita-ting nuclei having a volume concentration gradientthrough the depth of the emulsion, said emulsion having unexposed photosensitive silver-halide remaining therein in concentrations inversely related to said nuclei con-centration, 113734~

contacting said unexposed photosensitive silver-halide emulsion layer with an aqueous monobath comprising a weak silver-halide developing agent for developing said latent image and a rapid-active silver-halide solvent for reacting with unexposed and undeveloped silver-halide to form soluble silver ion complexes which are trans-ported by chemical diffusion transfer to said silver precipitating nuclei of said latent image where silver of said silver ion complexes is precipitated and absorbed on said nuclei in the presense of said developer acting as a reducing agent, thereby forming a reflective elec-trically non-conducting silver negative of said latent image in said area for data storage, and heating said reflective electrically non-conducting data storage medium at least to 250 until the reflectivity of the surface increases by at least 5% of the initial reflectivity percentage.

Among the advantages of the preferred embodiments of the invention are the fact that it may be manufactured with-out the use of a vacuum system and on a continuous basis and which may be used to record low-reflective spots in a reflective field with relatively low energy laser pulses. Control indicia and certain data base data may be pre-recorded by photographic means to facilitate the use of discs or plates in both the recording apparatus and the playback apparatus. Replication of optically recorded media is permitted by photographic contact printing, readable in reflection or transmission. A
further significant advantage is that a high sensitivity laser recording and data storage medium can be fabricated from commercially advailable photoplates. Disclosed here-in is a method of achieving a higher recording sensi-tivity and reduced variations in reflective contrast -than achieved in prior art recording media while re-taining the valuable attributes of the laser recording materials disclosed in those applications.

It has been discovered that use of a silver-halide emul-sion photoplate and a negative silver diffusion transfer photographic process leads to a well defined silver layer and that if this is followed by a step of thermal annealing, the result will be a reflective data storage medium of high recording sensitivity and redueed varia-tion in reflective contrast. High contrast digital-data reeordings have been achieved with reflective contrast variations of only +10% with a 3 milliwatt laser beam 0.8 mierons in diameter and with a pulse length of 100 nanoseeonds. The reflective surface may be patterned or formatted prior to laser recording by photographie exposure through a photomask.

The processing steps proceed as follows: First a volume concentration gradient of silver precipitating nuclei is created at one surface of the emu]sion by actinie radiation or other methods, with the gradient decreasing in concentration in the depthwise direction. This is followed by a single step monobath silver diffusion transfer negative development process that is primarily a solution physical development process which is used to build up the volume concentration of silver at the sur-face containing the precipitating nuclei until the sur-face becomes reflective.

The final thermal annealing step results in a greater silver concentration at the surface and a slight pyro-lization of the gelatin which leads to an increase in laser recording sensitivity, a more uniform reflective contrast ratio and a higher surface reflectivity. Al-though this thermal annealing step may be carried out in an inert atmosphere, the silver diffusion to the sur-face appears to be faster and more complete if carried out in an oxygen containing atmosphere.

The completed ref]ective surface layer is typically less than one micron thick; has a reflectivity of 20~ to 50%;
is electrically a non-conductor and thermally a poor conductor since the matrix is typically gelatin, which holds the high concentration of tiny particles and ag-glomerates of silver particles which are separated andisolated from each other by the gelatin matrix. Although the layer reflects light like a metal, it melts easily with a focussed laser beam, with the result that its recording sensitivity is almost twice as high as laser recording media using tellurium or the electrically non~conducting silver layer of the prior art.

A principal step in the process is an exposure or sur-face activation of the area to be used for data record-ing or alternatively non-data recording, which affects mainly the si]ver-halide grains close to one of the sur-faces of the emulsion. Such an exposure or activation creates a surface latent image having a depthwise ex-posure gradient, with a concentration of exposed silver-halide which is greatest at the one surface and least in the interior of the emulsion. The surface of great-est concentration may be either the surface distal to ` ` 1~37345 9 _ the substrate or proximate thereto, depending on where laser writing will initially impinge on the medium. For example, if laser writing is to be done on the upper surface, the emulsion surface distal to the substrate would have the greatest concentration of exposed silver-halide.

The surface latent image may include images in the photo-graphic recording sense or may cover the entire surface, but is always located primarily at a surface of a photo-graphic emulsion, which also contains some unexposedsilver halide, in the interior of the emulsion. Such a surface latent image may be made by light itself, i.e., by intentionally exposing one surface or the other of the photosensitive emulsion to light where data record-ing will occur, the remaining area being masked. Alter-natively the surface treatment may be made by a surface activating chemical, namely a fogging agent, such as hydrazine or borohydride salt such as potassium boro-hydride, which performs a surface latent image activa-tion on silver-halide emulsions similar to a light ex-posure. Alternatively, during the original manufacture of the silver-halide photographic plate or film a very thin gelatin layer containing silver-precipitating nuclei would be included at the surface distal to or the sur-face proximate to the substrate, which would be the basisfor creating a reflective surface at either of these two surfaces.

The second principal step of the process involves con-tacting the exposed or activated and unexposed silver-halide with a monobath containing a silver-halide de-veloping agent for developing the surface latent image created in the exposure or activation step. Simultane-ously a silver-halide solvent in the monobath, prefer-ably a solubl;e thiocyanate or ammonium hydroxide, reacts rapidly with unexposed and undeveloped silver-halide to form soluble,complexed silver ions which are transported by diffusion transfer to nuclei of the developing latent image or in the alternative case to the layer containing nuclei, where the silver in the complexed silver ions is precipitated in the presence of the silver halide de-veloping agent. This process forms a reflective silver image which is a negative of the light exposed or surface activated latent image.

The third principal step is a thermal annealing process typically for a few minutes at about 300C preferably in an o~ygen containing atmosphere. This thermal an-nealing step apparently causes diffusion of silver par-ticles to the reflective surface, slight pyrolizes the gelatin thereby freeing some carbon, removes the water contained by the gelatin and transforms the physical structure of the gelatin from long helices to a shorter, random coil configuration typical of polymer materials.
The thickness of the gelatin layer also shrinks in this process. The heating step leads to an increase in sur-face reflectivity apparently due to the increase in silver vo]ume concentration at the surface. This step also increases the laser recording sensitivity, apparent-ly because the carbon coated, light brown gelatin is ~ 1137345 more absorptive of the laser beam energy than is clear gelatin. This step also results in a significantly more -uniform reflective contrast ratio of the recorded spots, which may be caused by a more uniform reflective surface or more uniform gelatin absorptivity or both. Although this annealing step may be carried out in an inert atmo-sphere, the silver diffusion to the surface appears to be faster and more complete if carried out in an atmo-sphere containing oxygen.

Recording is accomplished by puncturing through the re-flective surface with a laser beam so as to create a hole in the reflective component which may later be de-tected by a variety of means such as reduced reflection of the hole; scattering of light from the hole; in-creased light transmission through the hole.
!
An advantage of the above method for making a reflective recording medium is that it allows a low-cost manufac-turihg process to create a formatted reflective silver layer on the medium which can be used for very low power laser recording. Several embodiments of the present method may be carried out by continuous manufacturing operations, as opposed to batch operations, but batch procedures may also be used.

In the drawings:

Figure 1 is a top plan view of the recording medium of the present invention.

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~137345 Figure 2 is a side sectional view of the recording medium of Figure 1, taken along lines 2-2.

Figures 3-8 are detail views of the recording medium of Figure 1 showing the results of different combinations of photographic processing steps for making the finished recording medium.

Figure 9-11 are side sectional views of three versions of the recording medium of Figure 1 showing methods of lase'r reading or writing.

Figure 12 is a plot of relative reflective contrast ratio versus laser beam power at the surface for a thermally annealed monobath developed photoplate com-pared to two similarly developed photoplates which had not been thermally annealed.

Figure 13 is a plot of the relative reflective contrast ratio versus laser beam power at the surface for a ther-mally annealed photoplate which had been monobath de-veloped compared to two thermally converted photoplates of the same original type which had been chemically black prior to thermal conversion.

The reflective laser recording medium of the present in-vention is made in ihree principal steps which can be carried out in a continuous flow process: the first step involving formation of a surface latent image, the sec-ond step involving chemical diffusion transfer of silverion complexes, the third step being thermal annealing which appears to involve a thermal diffusion transfer of silver metal to the reflective surface, a slight pyrolysis of gelatin that frees carbon and a shrinkage of the gelatin thickness.

I. Surface Latent Image Formation Surface latent image formation for a laser recording medium is achieved by exposing a region of unexposed photographic emulsion to light or to a fogging agent over the area where laser writing is to be done. Alter-natively during the original manufacture of the silver-halide photographic plate or film a very thin gelatin layer containing silver-precipitating nuclei would be included at the surface distal to or the surface proxi-mate to the substrate, which would be the basis for creating a reflective surface at either of these two surfaces. To record control indicia on the medium, part of the emulsion may be masked or alternatively may have been exposed and chemically developed prior to this sur-face latent image formation step. Typically such a medium is a disk, as illustrated in Figure l; however, it could be a square or rectangular plate.

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 rota-tion. While the recording medium of the present inven-tion is described as a disc, a disc configuration is not - il3734S

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 hon-rotating rectangular plate. However, rotating discs are pre-ferred for fast random access to medium amounts of data and non-rotating rectangular plates in stacks are pre-ferred 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 sepa-rate different recording fields, to carry control in-formation, 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 con-trol signal recording thereon. The recording field 19 is shown to have a plurality of concentric, circumfer-entially-spaced servo guides 23 thereon. Such servo guides 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 3-8.

Figure 2 shows a side sectional view of the recording medium of Figure 1. The medium consists of a substrate 27 which is a sheet-like layer which may be transparent or translucent, preferably a dimensionally stable mater-ial, like glass or other materials such as ceramics orplastics which can withstand the temperatures used in the thermal annealing step. Opaque, light-absorptive materials will work in those applications of the present invention where light transmission through the substrate is not desired. 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 ab-sorbed 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 preferably both.

For th~ case where the substrate is transparent, record-ing and reflective reading of the data can be done through the substrate as shown in Figures 10 and 11, or from the side distal to the substrate as shown in Figure 9. For transmissive read, the configurations of Fig-ures 10 and 11 may be used. If the substrate is absorp-tive then reflective read is the only possibility and the configuratibn of Figure 9 would be used.

The thickness of the substrate is not critical when the laser beam is directed onto the surface as shown in Figure 9, but it should have sufficient thickness to provide strength for resistance against breakage. If ` ` 1137345 the laser beam is directed through a transparent sub-strate, as in Figures 10 and 11, then in order to main-tain focus of the beam the thickness of the transparent subs~rate 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 1/8 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 by surface latent image formation and silver diffusion transfer into components 32 and 33 in Figures 9, 10 and 11. This process for creating the reflective layer 32 does not require any chemical constituent within the emulsion other than a conventional silver halide held in a suitable colloid carrier, preferably gelatin. They may also contain optical and chemical sensitizers, anti-fogging agents, stabilizing compounds, emulsion harden-ers and wetting agents. However, when commercial photo-plates or films are used, they may contain certain phys-ical characteristics or added chemical ingredients which could lead to favorable or unfavorable results. For example, most photographic films have a gelatin overcoat over the silver-halide emulsion that might have a thick-ness of 1 micron. Since layer 32 is not electrically conducting but reflects owing to its high dielectric constant, any moderately thick, high dielectric constant coating over it will reduce its reflectivity, particu-larly if the gelatin is heated to a high temperature and becomes absorptive to light owing to a partial pyrolysis of the gelatin to carbon. Thus, if a photographic plate with an overcoat is to be used the overcoat must be re-moved, such as with an enzyme or hot water. For high quality surfaces it would be preferable to start with a medium without an overcoat.

One of the advantages of the gelatin is that it has a relatively low melting temperature, less than 400C, which aids laser recording. Such relatively low melting temperature carriers are preferred in the present in-vention, but they must be able to withstand the tempera-tures used in the thermal annealing step of up to about 330C.

If a screening dye is used within the emulsion to create an exposure gradient in conjunction with actinic radia-tion exposure, the dye should be selected so that it is not trapped within layer 32 so as to cause a streaked surface of non-uniform reflectivity. However such streaking may be reduced in non-uniformity by the anneal-ing processing step.

Emulsion thicknesses of 3 to 6 microns are adequate tocontain sufficient silver-halide emulsion to build up the reflective layer by the complexing and diffusion transfer steps. If thicker commercial emulsions are used along with long processing times, the reflective layer may become too thick or too thermally conducting to permit recording with low-power lasers. The thicker coating requires a higher laser beam power to penetrate it and a higher thermal conductivity leads to faster heat flow away from the spot being recorded, also lead-ing to higher recording powers.

If a hardened emuision is desired it may be preferable to harden or cross link the gelatin after forming re-flective layer 32. If the emulsion is hardened initial-ly, then it will swell to a reduced extent during mono-bath processing thereby reducing the rate at which the silver halide is dissolved and complexed, thus extending the process time.

Small silver-halide grains typically found in commercial-ly available high resolution or high definition photo-plates used in photomask making, holography and high-resolution recording are excellent for producing reflec-tive laser-recording materials. These emulsions typical-ly have mean grain sizes of .05 micron and a spread of about .007 micron. One type, the Agfa-Gevaert Millimask HD photoplate, has a mean grain size of .035 micron and a spread of .0063 micron. The finer grains appear to result in minimizing the micro variations or granularity in reflectivity and thickness of the reflective component and thereby permit recording and reading of smaller holes than for coarse grain emulsions. The finer grain emul-sions also dissolve faster in the monobath owing to their greater surface-to-volume ratio which leads to a shorter process time.

High resolution emulsion coated glass plates having these characteristics are commercially available and are known :' -`~ ` 1137345 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-Gevaert of Belgium, Konishiroku Photo Industries Co., Ltd. of Japan and Eastman Kodak Company.

The shiny reflective component 32 in Figures 9, 10 and 11 result from the photographic monobath processing de-scribed herein but the silver is present initially as silver halide and reflectivity does not initially exist in the emulsion. Thus at the inception the silver of reflective component 32 is found in the photographic emulsion 29, which is uniform in its composition. An insert subbing layer, not shown, is usually used to attach the substrate 27 to the emulsion 29. Following the photographic monobath processing of the present in-vention the emulsion 29 of Figure 2 produces a reflec-tive component 32 at the emulsion surface shown in Figure 9, with a low-reflective underlayer 33 beneath it. The reflective layer 32 is more sharply defined in thickness when nuclei are included during manufacture or when a fogging agent is used for surface activation.
Thus, although Figures 9, 10 and 11 depict a sharp boundary for reflective component 32, if light exposure is used such is not the case but actually the concentra-tion falls off and continues into underlayer 33. The fall-off is more rapid if the emulsion had contained a screening dye.

Thus when light exposure is used underlayer 33, while not completely depleted of silver, contains much less silver than reflective component 32. Optically, under-layer 33 is either clear or reddish in color and is transmissive to red light having wavelengths of 630 nanometers and longer. Underlayer 33 tends to be clear or slightly yellow if the silver-halide therein is not subject to latent image formation. Underlayer 33 tends to be amber or red if latent image formation occurs in the underlayer. As described hereinafter, better def-inition of the reflective component occurs where a fogging agent is ued for surface latent image formation.
Since the depth of penetration of the fogging agent can be controlled, for example by the length of time of emulsion dipping into the fogging agent, the unfogged silver-halide below this penetration depth forms under-layer 33. Since the silver in the unfogged silver-halide region subsequently goes into solution as a silver complex, some of which is deposited on silver nuclei in reflective component 32, the underlayer 33 becomes substantially clear and is essentially gelatin.

On the other hand, if surface latent image treatment is achieved by means of exposure to light, the depth of treatment is more difficult to control, but is made easier with screening dyes. The purpose of the screen-ing dye is to attenuate actinic radiation through the depth of the emulsion so that there is surface latent image formation through only a fraction of the depth of the emulsion. Screening dyes are usually a narrow band-width to absorb either blue or green light, but not both.

~ , Thus if this type of dye is used the actinic radiation must also be narrow band or filtered accordingly, otherwise unwanted actinic radiation will penetrate the emulsion. Thus, in general, actinic radiation ex-posure does not leave a clear boundary between regionsof surface latent image formation and regions of no surface latent image formation. Rather, there is a gradient with good surface latent image formation closest to the light source where there is strongest exposure and weak latent image formation further away where there is weakest exposure. In this case the monobath develops the weak latent image in the under-layer 33 which thereby forms a nuclei base for further silver deposits from the silver complex with the result that the underlayer has a red or amber color.

Either method of surface latent image treatment creates an exposure gradient with a greater concentration of exposed silver-halide near the surface of the emulsion where the exposure is greatest. Portions of the ex- -posed and partially developed silver-halide grains be-come silver nuclei where silver will be reduced from silver ion complexes during diffusion transfer. When the densest concentration of exposed silver-halide grains is desired at the emulsion surface distal to the substrate, either method of surface latent image treat-ment may be used. However, when the surface having the highest exposed silver-halide concentration is de-sired proximate to the substrate, then either nuclei are included in manufacturing or actinic radiation ex-posure through the transparent substrate is necessaryto create the surface latent image. An emulsion heavily --``` 1137345 dyed with a screening dye is necessary in this case to create a surface latent image conce~tration proxi-mate to the substrate. A short chemical photographic development cycle before monobath development may be used to help create the required silver precipitating nuclei prior to the creation of the silver complex and thus enhance diffusion transfer and reflectivity proxi-mate to the substrate. Owing to the dielectric con-stant of the glass a much higher volume concentration of silver is necessary to give the same reflectivity as compared to an emulsion side reflective layer. The required layer of high concentration silver precipita-ting nuclei at the substrate or distal to the substrate can also be incorporated during the photoplate manu-lS facturing process.

Once craters are created penetrating reflective compo-nent 32, the data contained in the craters may be read by changes in reflectivity of the shiny reflective com-ponent throughout the visible spectrum and into the near infrared where it i5 ultimately limited in its usability as reflective component 32 becomes more and more trans-parent and therefore less reflective. The craters also - may be detected by transmission of red light, 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.

It should be noted that both the recording areas 17, 19 and the non-recording guard band 21 of Figure 1 initially ---` 113734S

havè silver-halide emulsion covering a substrate. Thus, the designation of ~ecording and non-recording areas is arbitrary and the entire surface could be used for re-cording if desired. However, as a matter of convenience, it is preferable to designate areas as non-recording areas. The boundaries between recording and non-re-cording areas may be defined by concentric lines, just as the servo guides 23 of Figure 1, which have been greatly enlarged in the Figure, may be defined by lines.
Typically, servo guides are closely spaced concentric circles or adjacent lines of a spiral, with data being written on or between the lines. Such servo guide lines, as well as line boundaries for non-recording areas, may be photographically recorded on the record-ing 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 photograph-ically 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 pre-recorded on the re-cording medium of the present invention may be applied by photographic techniques since the starting material for the recording medium is an unexposed commercially available photoplate used in the manufacture of semi-conductor integrated circuits or film-based materials of similar quality. A principal characteristic of silver-halide emulsion photosensitive materials for use in the present invention is fine grain size so that the ` 113734S

reflectivity granularity is minimised and very small holes can exhibit measurable changes in reflectivity.
Large grain sizes would lead to greater granularity which would tend to mask changes in reflection created by small holes. Pre-recording of information may be achieved by masking off areas as described herein.
After photographic processing, this pre-recorded infor-mation may be read in reflection since the pre-recording areas will consist of either highly reflective white silver areas or low reflective black silver areas or low reflectivity clear gelatin areas.

The photographic techniques which may be used to pre-record data base and control information are closely related to the fabrication of emulsion photomasks in the semiconductor industry. Lines having a thickness of one micron may be made using these photomask manu-facturing techniques. Some procedures for creating a pre-recorded line pattern are illustrated in Figures 3-8.

With reference to Figure 3, fine grain silver-halide emulsion medium 11 is exposed to actinic radiation in the areas for data recording but the line pattern con-sisting of the circular lines 23a, 23b aad 23c is masked from the radiation. This procedure creates a surface latent image formation in the data recording areas. The masked areas are then unmasked and the emulsion is subjected to the monobath processing de-scribed herein which creates the reflective surface for laser recording on 11 in Figure 4. If the recording 11373~S

areas are to be activated by actinic radiation, it is preferable that the emulsion contain a screening dye which is absorptive to the actinic radiation so that the latent image of the silver nuclei is concentrated on the surface. Although a screening dye is preferred, it is not essential to creating a reflective surface.
Without a screening dye the silver concentration gradi-ent will not fall off as rapidly from the surface into the body and a higher power laser beam may be required for recording, There are two principal reasons that the silver can be concentrated at the surface distal to the substrate wihtout use of a screening dye. Firstly, the photons irradiating the surface are absorbed by the silver halide as they create silver atoms; thus, there is a greater exposure at the emulsion surface than at the body. Secondly, when the emulsion is dipped into the monobath the surface nuclei begin to grow by chemical development mbre rapidly than the inner silver nuclei since they contact the developer first. Thus, when the solution physical development part of the monobath development begins, more of the complexed silver ions will precipitate on the surface where the silver nuclei will be larger and more numerous. Also it is known that it requires four silver atoms per silver-halide grain for the grain to participate in chemical develop-ment. Thus, any absorption by the silver halide will result in a higher probability of silver-halide grains on the surface having the four atoms of reduced silver than for internal grains. Commercially available photo-plates containing screening dyes include Eastman Kodak's High Resolution Plate - Type II, and three Agfa-Gevaert photoplates: Millimask Negative, Millimask Reversal, and Millimask Precision Flat HD. Denser screening dyes than these are necessary to create the desirable reflec-tivity at the surface proximate to the substrate.

The circular lines 23a, 23b, and 23c which were masked represent low reflectivity servo guides which would pro-vide information as to whether the recording laser is recording on the data track or has moved off the edge of the data track. To provide additional information to the servo system, the servo guides could contain a reflective and non-reflective pattern shown in Figure 5, which would provide information as to whether the correction requires a movement to the right or left.
Notç that the right and left servo guides would provide different frequency signals to the playback system.
The dashed pattern shown could be created in the master by means of a photomask or by interrupting a laser photographic recording beam.

For the servo guides or any other indicia markings to be in the form of low reflective black silver, as op-posed to clear gelatin markings discussed above, the servo guides themselves could be exposed through a mask or by means of a continuous or interrupted laser beam.
Figure 6 illustrates the making of such indicia where actinic radiation is used first to expose servo guides 43a, 43b, 43c and the remaining area 41 would be shield-ed. Then a normal chemical or direct development would be used to create a black low reflectivity pattern as ~ ` li37345 shown in Figure 7 which would be bleached out, or patterned as in Figure 8. No fixing would be used since the silver halide in region 41 would be used in the subsequent monobath processing to create reflective recording areas. Also note that the lines 43a, 43b, and 43c could have been broken into a pattern such as those shown in Figure 7. With the track guides and possibly other indicia recorded in black silver, the next step would be to expose the surface latent image in the remaining areas for laser recording. After the final step of thermal annealing the black and clear patterned servo guides would be converted to a pattern - of high and low reflectivity.

Surface latent image formation is done in the recording area 41 of Figure 8, as well as recording area 11 of Figure 4 previously mentioned, in either of three ways:
first, by exposure of the unexposed silver-halide emul-sion data recording area tw actinic radiation such as by mercury arc lamp, incandescent lamp, xenon flash lamp onto an emulsion containing a screening dye for the entire bandwidth of the actinic radiation or second-ly, by means of a surface activation of a fogging agent such as hydrazine in aqueous solution or in gaseous state, or for example, potassium borohydride in aqueous solution, or thirdly, by including a silver-precipita-ting nuclei layer near the emulsion surface where the surface latent image is desired. Surface latent image formation would be followed by processing as described below.

1~37345 When the surface latent images are created by a fogging agent, it is of no consequence that the screening dye may have been washed out in the earlier development process. The surface activation of the emulsion could take place either by a few-second dip in a fogging agent, such as an aqueous carrier containing hydrazine or by exposure to hydrazine gas for a period of minutes.
Penetration of the fogging agent to the interior of the emulsion can be minimized by starting with a dry emulsion. After monobath development, the finished laser recording medium would have the appearance shown in Figures 5 or 8. Note that the patterned black con-trol indicia 43 of Figure 8 would be low reflective black compared to the shiny silver recording areas of 41 prior to the thermal annealing step.

Use of a fogging agent creates nucle where silver in silver ion complexes may be reduced and adsorbed. As an alternative to use of a fogging agent, preformed silver-precipitating nuclei may be disposed in the un-exposed silver-halide emulsion, for example in the manufacturing process. The commercially available instant photographic films of the Polaroid-Land photo-graphic system have such nuclei layers in contact with the silver-halide emulsion. Note that the use of sil-ver-precipitating nuclei layers incorporated in the emulsion does not preclude the possivility of pre-recorded control indicia. The non-data recording areas may be exposed first in a pattern and chemically de-veloped to low reflectivity black silver and not fixed.

-` ` 1137345 After bleaching out the black silver the entire plate is then given a monobath development to create reflec-tive data recording areas and in the non-data recording areas to create low reflective regions where the black silver had been bleached out and also reflective regions adjacent to the bleached out black regions.

An alternate method of surface latent image formation is by means of actinic radiation exposure of the data recording area. It is desirable for the medium to con-tain a screening dye to limit the exposure primarily tothe surface, but this dye may be washed out if the medium was previously processed as for example in pro-ducing black silver control indicia. This problem can be overcome by a dyeing process after the chemical de-velopment process is completed or by utilization of apermanent, non-soluble screening dye in the initial manufacture of the emulsion, which does not cause non-uniform reflectivity. The monobath processing may be carried out in the sam~e manner as was described in the case of the fogging agent activation. Also, as describ, ed, the black silver areas created by the initial ex-posure and chemical development could be bleached out before monobath processing.

The surface latent image formation methods create a depthwise exposure gradient, with a concentration of exposed silver-halide which is greatest at one emulsion surface where exposure was greatest. That concentration falls off in the depthwise direction, rather abruptly in the case of fogging agents, such that the concentra-tion of latent image formation falls continually from 1~373g5- 30 -the exposed surface and is lowest at or near the oppo-site emulsion surface. The unexposed silver-halide exists in concentrations inversely related to the ex-posure concentration. After monobath processing, the volume substrate will exceed the lowest concentration in the body of the emulsion by a ratio typically ex-ceeding 5:1.

The reflective component 32 of Figures 9-ll is thus derived from the silver in the silver-halide emulsion.
While this reflective silver component may appear at either of the two emulsion surfaces and is concentrated there, the thickness of the reflective component is not well defined when created by actinic radiation ex-posure because some radiation penetrates below the surface of the emulsion and a silver latent image is created. An advantage of using a fogging agent for surface latent image formation as compared to actinic radiation exposure is that it creates a better defined reflective layer and a lower silver concentration with-in the body of the emulsion. With both of these pro-cesses, silver halide in a commercially available photo-graphic emulsion is the starting material for creating the laser-recording medium in the present invention, and the finished product may be considered to be silver particles in a gelatin dielectric matrix, the halide being removed in the monobath processing.

To use the laser recording medium of the present in-vention, laser light is focused on a spot on the re-flective component either from the side distal to the substrate or through a transparent suhstrate. For laser 3'7;~5 recording as opposed to data storage applications the reflectivity of the reflective layer preferably ranges between 15% and S0~; thus, the remaining percentage of incident radiation of 85~ to 50~ is either absorbed by the reflective component or partly passes through it.
The absorved power distorts or melts the gelatin sup-porting the reflective component so as to reduce the reflectivity at the spot and create an adequate con-trast in reflective reading of the recorded data. For data storage applications, i.e., laser reading but not recording the reflectivity may be as high as possible and the thickness of the reflective layer is not criti-cal. The reflective component 32 is located on the underlayer as shown in Figure 9 and Figure 11 and ad-jacent to the substrate as shown in Figure 10. In allthree cases a reflective read procedure can be used -for example, as described in U.S. Patent 3,657,707. In the cases shown, the recording laser beam need only affect the reflective component, and further penetration into component 33 is not needed.

In Figure 9, the substrate could be either transmissive or opaque if reflective read is used, but must be trans-missive to the read laser beam if transmissive read is used. The component 33 would consist of a red or amber silver gelatin complex if a soluble screening dye and actinic exposure were used to create component 32, but would be essentially clear gelatin if fogging agent surface activation were used or if the emulsion had been manufactured with a silver precipitating nuclei layer included. The color of component 33 would have little effect on reflective read methods but would affect transmissive read methods. If component 33 is red in color, transmissive reading can be accomplished to a limited extent by use of a red or near infrared laser beam provided that the opacity of the undisturbed re-flective coating blocks about 90% of the light and the recorded craters permit transmission of at least about 50% of the light. If component 33 is essentially clear gelatin it would permit transmissive reading with a green or blue laser as well; and since the reflective component is more opaque at these wavelengths, a higher contrast would be achieved than in the case of a red or infrared laser being used for transmissive read.

Figure 10 illustrates a configuration which could have been produced by photographic exposure using narrow band blue or green actinic radiation through a transparent substrate 27 onto an emulsion heavily dyed to attenuate the selected narrow band actinic radiation. Commercial-ly available soluble screening dyes with adequate ab-sorption properties can accomplish the task. Dyes con-tained in commercial photoplates are not adequate to achieve the desired reflectivity. After final proces-sing the component 33 would be red or amber in color.
Recording and reflective reading would be achieved through the substrate. Transmissive read could be ac-complished to a limited extent by use of a red or near infrared ]aser beam such that the opacity of the reflec-tive coating blocks 90% of the read-beam radiation and the recorded craters permit transmission of at least 50% of the light. If this eonfiguration were produeed by use of an emulsion whieh had been manufaetured with a silver precipitating nuclei layer included, component 33 would be essentially clear gelatin and transmissive read also could be accomplished in blue and green as deseribed in the previous paragraph.

Figure 11 illustrates a eonfiguration where both the substrate and the underlayer are transmissive to vis-ible and near infrared radiation. It has the advantagethat layer 32 ean be eoated with a non-optieal flat proteetive layer whieh would serve to eneapsulate layer 32. This type of proteetive layer could not be used in the configuration of Figure 9 because it would be in the optical path. The essentially clear gelatin component 33 would be created by fogging agent surface aetivation or aetinie radiation exposure distal to the substrate of an emulsion heavily dyed with a sereening dye so that almost no silver latent images in the body of the emulsion are reduced during monobath development~
This configuration can also be produced by use of an emulsion which had been manufactured with a silver pre-cipitating nuclei layer included at the location of layer 32. In this case in addition to reflective read at visible wavelengths and near infrared, the component 33 also permits transmissive read at these wavelengths by laser light traversing substrate 27 for transmission through the essentially clear gelatin component 33 and through crater 30 in component 32.

1~37345 Figures 9, 10 and 11 show emulsion coating 29 on sub-strate 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 about 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 let-ter R. As mentioned previously, these recording areasmay also contain pre-recorded data base data and con-trol 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 writter 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 in-dicia 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 need be no data storage distinction between the photographically pre-recorded non-reflective spots and non-reflective spots made by laser writing. In the recording mode the pre-recorded control information is used to determine the location of the data craters being recorded.

~1373K

II. Silver Diffusion Transfer We have found that a very thin, highly reflective, silver surface may be formed by the diffusion transfer of appropriate complexed silver ions to a layer of silver precipitating nuclei. This reflective layer is electrically non-conducting and has low thermal con-ductivity and may be patterned photographically, these latter two properties being highly desirable for laser recording media. The complexed silver ions are created by reaction of an appropriate chemical and the silver halide used in conventional silver-halide emulsions.
A developing or reducing agent must be included in this solution to permit the complexed silver ions to be precipitated on the nuclei layer. This combination of developing agent and silver complexing solvent in one solution is called a monobath solution. Preferred monobath formulations for highly reflective surfaces include a developing agent which may be characterized as having low activity. The specific type of develop-ing agent selected appears to be less critical thanthe activity level as determined by developer concen-tration and pH.

The developing agent should have a redox potential sufficient for causing silver ion reduction and ad-sorption or agglomeration on silver nuclei. The con-centration of the developing agent and the pH of the monobath should not cause filamentary silver growth which gives a black low reflectivity appearance.

- il37345 The developed silver particles should have a geometric shape, such as a spherical or hexagonal shape which when concentrated form a surface of high reflectivity.

Developing agents having the preferred characteristics are well known in the art and almost any photographic developing agent can be made to work by selection of concentration, pH and silver complexing agent, such that there is no chemical reaction between the develop-ing agent and complexing agent. It is well known that photographic developing agents require an antioxidant to preserve them. The following developing agent/anti-oxidant combinations produced the typical indicated reflectivities for exposed and monobath developed Agfa-Gevaert Millimask HD photoplates.

For Monobaths Using Na(SCN) As a Solvent And Silver Complexing Agent Approximate Developing Agent Antioxidant Maximum Reflectivity p-methylaminophenol Ascorbic Acid 46%
20 p-methylaminophenol Sulfite 37%
Ascorbic Acid 10%
p-Phenylenediamine Ascorbic Acid 24%
Hydroquinone Sulfite 10%
Catechol Sulfite 60%

For Monobaths Using NH40H As a Solvent And Silver Complexing Agent Developing Agent Antioxidant Typical Reflectivity Hydroquinone Sulfite 25%
Catechol Sulfite 30 The preferred solvents/silver compleXing agents, which must be compatible with the developing agent, are mixed therewith, in proportions promoting the complete diffu-sion transfer process within reasonably short times, such as a few minutes. Such silver complexing agents in practical volume concentrations should be able to dissolve essentially all of the silver halide of a fine grain emulsion in just a few minutes. The solvent should not react with the developing silver grains to dissolve them or to form silver sulfide since this tends to create non-reflective silver. The solvent should be such that the specific rate of reduction of its silver complex at the silver nuclei layer is ade-quately fast even in the presence of developers of low activity, which are preferred to avoid the creation of low-reflectivity black filamentary silver in the initial development of the surface latent image.

The following chemicals act as silver-halide solvents and silver complexing agents in solution physical de-velopment. They are grouped approximately according totheir rate of solution physical development; that is, the amount of silver deposited per unit time on pre-cipitating nuclei, when used with a p-methylaminophenol-ascorbic acid developing agent.

~i37345 Most Active Thiocyanates (ammonium, potassium, sodium, etc.) Thiosulphates (ammonium, potassium,sodium,etc.) Ammonium hydroxide Moderately Active ~ picolinium - ~ phenylethyl bromide Ethylenediamine

2-Aminophenol furane n-Butylamine 2-Aminophenol thiophene Isopropylamine Much Less Active Hydroxylamine sulfate Potassium chloride Potassium bromide Triethylamine Sodium sulfite From the above it can be seen that the thiocyanates and ammonium hydroxide are amongst the most active solvents/complexing agents. While almost all developers suitable for solution physical development can be made -~ il373~5 to work in the silver diffusion transfer process of the present invention with the proper concentration and pH, not all solvents/complexing agents will work within the desired short development time or in the proper manner.
For example, the thiosulfate salts, the most common silver-halide solvent used in photography`and in Pola-roid-Land black and white instant photography 's dif-fusion transfer process, does not work in this process for two reasons. Its complexed silver ions are so stable that it requires a strong reducing agent to pre-cipitate the silver on the nuclei, and this strong re-ducing or developing agent would have the undesirable effect of developing low reflective black filamentary silver. It has another undesirable effect, also ex-hibited by the solvent thiourea; namely, that it formsblack, low reflecting silver sulfide with the develop-ing silver grains. On the other hand in the black and white Polaroid-Land process black silver is a desirable result. Sodium cyanide is not recommended, even though it is an excellent silver-halide solvent, because it is also an excellent solvent of metallic silver and would thus etch away the forming image. Is is also about 50 times as toxic as sodium thiocyanate, which is a common photographic reagent.

The process chemicals can be applied by a variety of methods, such as by immersion, doctor blades, capillary applicators, spin-spray processors and the like. The amount of processing chemicals and temperature thereof should be controlled to control reflectivity. Prefer-ably, the molar weight of the developing agent is less ~13734S

than 7% of the molar weight of the solvent since higher concentrations of developing agent can lead to low re-flective filamentary silver growth, exceptions to this ratio being found among p-phenylenediamine and its N, N-dialkyl derivatives having a half-wave potential be-tween 170 mv and 240 mv at a pH of 11.0, which have lower development rates and require higher concentra-tions, i.e., up to 15 grams per liter and minimum of about 2 grams per liter. These derivatives and their half-wave potentials are listed in Table 13.4 of the book entitled The Theory of the Photographic Process, 3rd ed., Macmillan Company (1966). The concentration of the solvent in the form of a soluble thiocyanate or ammonium hydroxide should be more than 3 grams per liter but less than 75 grams per liter. If the concentration is too low the solvent would not be able to convert the halide to a silver complex within a short process time and if the solvent concentration is too great the latent image will not be adequately developed into the necessary silver precipitating nuclei causing much of the silver complex to stay in solution rather than be precipitated. The process by which the silver complex is reduced at the silver precipitating nuclei and builds up the size of the nuclei is called solution physical development.

It is important to note that in solution physical de-velopment, as used herein, the silver particles do not grow as filamentary silver as in direct or chemical development, but instead grow roughly equally in all ~13734S

directions, resulting in a developed image composed of compact, rounded particles. As the particles grow, a transition to a hexagonal form is often observed. If the emulsion being developed has an extremely high den-sity of silver nuclei to build upon and there is suf-ficient silver-halide material to be dissolved, then eventually the spheres will grow until some contact other spheres forming aggregates of several spheres or hexagons. As this process takes place the light trans-mitted through this medium initially takes on a yellow-ish appearance when the grains are very small. This changes to a red appearance as the particles build up in size and eventually will take on a metallic-like reflectivity as the aggregates form.

The first two processing steps of the present invention may be achieved by physical phenomenon, chemical treat-ments or manufacturing techniques but when these steps are linked together in the proper processing sequence, the result is a reflective data storage medium which may also be used for laser recording even without thermal annealing. Table 1 presents 14 experimental examples to illustrate some of the variations of the individual steps that may be used and to present an overview of the two steps necessary to create a reflec-tive data storage medium. Such a medium can also beused for laser recording but its recording sensitivity and relative contrast ratio may be improved by thermal annealing as described in the next section.

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III. Thermal Annealing The first two process steps described, namely surface latent image formation and silver diffusion transfer, will result in the creation of a reflective data storage medium which can also be used for laser recording.
These process procedures and the nature of the resulting reflective data storage media are the subjects of U.S.
Patent No. 4,278,756 issued July 14, 1981 by E. ~l.
Bouldin and J. Drexler. In accord with the present in-vention the laser recording sensitivity is increased, the variations in reflective contrast ratio are reduced and the surface reflectivity percentage is raised by a thermal annealing step typically at about 300C, prefer-ably in an oxygen containing atmosphere and typically for three to five minutes.

Although the exact method of enhancement of these per-formance characteristics by thermal processing is not fully understood, the observed results give some clues to the phenomena involved. The reflectivity is always observed to rise upon heating to about 300C for several minutes, even in an inert atmosphere such as nitrogen, but the rise in reflectivity percentage may be two-to-four times as great when a pure oxygen atmosphere is used as illustrated in Example 1. Thus, it appears that the silver volume concentration at the reflective surface increases with heat and even more so with heat and oxygen. In order for the silver volume concentra-tion at the surface to increase, one might conclude that ~137345 the process involves a thermal diffusion transfer of silver metal to the surface. It is possible that during the process, silver oxide is formed and reduced to sil-ver again since at the temperatures involved the very unstable silver oxide can form. Since the thickness of the gelatin shrinks substantially during the annealing process, this effect should increase the silver volume concentration.

Also during the thermal annealing step the gelatin matrix pyrolizes slightly, thereby freeing some carbon and increasing the absorptivity of the gelatin. This effect is presented in Example 4 where percentage ab-sorptivities are given for five process temperatures and three wavelengths of visible light and one wave-length of infrared liqht. It is believed that an in-crease in absorptivity of the light brown gelatin leads to increased absorption of the incident laser power during the recording process which in turn results in increased laser recording sensitivity.

The thermal annealing step involves the heating of the reflective emulsion coating after the first two previ-ously mentioned process steps to a temperature of ap-proximately 280C to 360C in air and 250C to 360C in oxygen or 310C to 360C in an inert atmosphere, with an oxygen containing atmosphere being preferred. The presence of oxygen reduces the required temperature or process time and appears to yield a more complete processing. Long process times above 340C may lead to excessive pyrolizing of the gelatin. Electrical re-sistance measurements on the shiny layer 32 in Figure8 indicate no measurable electrical conductivity.

~137345 Heating methods include an air convection oven, a con-tacting hot source or radiant heating. All three methods may be used for the thermal annealing.

An air convection oven manufactured by GCA Corporation under the name Precision Scientific Model 605, capable of 2500 watt input, has been used for this purpose.
Although it works for experimental purposes, there are two drawbacks. First, it can only be used in a batch process and, second, the glass substrate is subject to significant thermal shock when the plates are inserted or removed from the convection oven.

An improvement over the air convection oven is a con-tacting hot source made up of a pre-heat section, ther-mal annealing section and cooling section. A pusher bar is used to move the work piece from one section to the next. The cooling section and pre-heat section are de-signed to hold three work pieces. Thus, a batch of three plates can be converted. The contacting plate is aluminum, and thermo couples are inserted to determine the proper temperature settings. This arrangement mini-mizes the thermal shock, but it is still a batch process.

Radiant heating is preferred since it can be a continu-ous-flow process and involves a minimal thermal shock.
That is, if a conveyor belt is used such that plates may be moved under radiant heating sources, the intensity of the heating sources or their closeness to one another can be adjusted to create the desired heating profile.

A radiant heater was designed to anneal one plate at a time. The radiant heater for plates up to 4" x 4" in size consisted of a planar array of five type 500T3 (Westinghouse) quartz-envelope infrared heaters rated at 500 watts each. The tubes were moun~ed on 3/4"
centers and spaced 1-3/4" from the surface of the plate to be annealed. A 5" x 5" x 1/8" quartz plate was interposed between the source and the workpiece to serve the function of sealing the annealing chamber while admitting the infrared radiation. The plate to be annealed was enclosed in the sealed chamber which contained the desired atmosphere. A cylindrically shaped chromium-plated reflector having a radius of 3"
was mounted above the heaters, with the array positioned approximately on the diameter of the cylinder.

Example 1 A photoplate coated with a commercial Konishiroku Photo Industries ST emulsion 3 microns thick containing no screening dye is immersed in a 1% water solution of sodium hydroxide to remove the anti-halation backing and after washing is exposed to room light for 10 min-utes. It is then immersed in a monobath developer solu-tion consisting of p-phenyenediamine, 5.4 grams, Q -ascorbic acid, 5 grams; NaOH, 2 grams; NaSCN, 5 grams;
with water added to bring volume up to 1 liter. Develop-ment time was 5 minutes. The plates were washed and dried. The reflectivity of the surface was 12.3%.
After heating a sample in nitrogen at 318C for 5 min-utes the reflectivity rose to 15.9~, that is, 29~ above the initial reflectivity percentage. After heating a second sample at 328%C in nitrogen for 5 minutes the reflectivity rose to 19.5%. Annealing in an oxygen atmosphere rather than nitrogen raised the reflectivity further. For example, when heating was at 318C for 5 minutes in oxygen the resulting reflectivity was 27.7 and when heating was at 328C, other conditions being the same, the resulting reflectivity was 31.8%.

Example 2 A photoplate coated with a commercial Agfa-Gevaert Millimask HD emulsion 4.5 microns thick and containing a screening dye was exposed to sunlight for several minutes and then immersed for five minutes at 23C in a monobath which contained the following formulation:
p-phenylenediamine, 5.4 grams; Q-ascorbic acid, 5.0 grams; KBr, 0.5 gram; and NaSCN, 10.0 grams; with water added to bring volume up to 1 liter; and with a pH = 11 achieved by adding NaOH. After drying, samples "A" and "B" (representing the prior art) exhibited a range of reflectivities of 20% to 24~ at 633 nanometers. Sample "C" was annealed for five minutes in oxygen at 320C
exhibited a reflectivity of 36% and represents an exam-ple of the process of this invention.

Laser recording was then accomplished with an argon laser using the green line at 514 nanometers. The beam diameter was approximately 0.8 micron at the media sur-face, and pulse lengths of 100 nanoseconds were used.

Tests were conducted to determine how the reflective contrast ratio varied with laser-beam power. Measure-ments were made starting at beam powers of 28 milliwatts and down to under 3 milliwatts. The results of those tests for three samples are shown as curves "A", "Bl' and "C" in Figure 12. The ratio of reflected power from the unrecorded surface to that of the hole at 24 milliwatts was in the range of 7:1 for all three samples. At each measured power level, the contrast was measured at 32 points and averaged. At their rated laser recording power level of 13 milliwatts at the above beam diameter and pulse length, samples "A" and "B" exhibited reflec-tive contrast ratio variations of +48% and +36% at the 1 sigma distribution points derived from the 32 measured contrast ratios at each power level. Sample "C" ex-hibited reflective contrast ratio variations of only +16~ at the 7 milliwatt level. Neither sample "A" or "B" could be used effectively for recording below 5.7 milliwatts since at that level the reflective contrast variation reached +50%. In comparison sample "C" ex-hibits a reflective contrast variation of less than 40%
at the 3.6 milliwatt level.

Example 3 A photoplate coated with a commercial Konishiroku Photo Industries ST emulsion 3 microns thick containing no screening dye is immersed in a 1~ water solution of sodium hydroxide to remove the anti-halation backing and after washing is exposed to room light for ten minutes. It is then immersed in a monobath developer solution consisting of p-phenylenediamine, 5.4 grams;
Q -ascorbic acid, 5 grams; NaOH, 2 grams; NaSCN, 5 grams; and KBr, 0,5 grams; with water added to bring volume up to 1 liter. Development time was five minutes before the plates were washed and dried. After themal annealing for five minutes in oxygen at 320C the sam-ple exhibited a reflectivity of 25.5%, and is desig-nated sample F and represents an example of the process of this invention.

Laser recording was then accomplished with an argon laser using the green line at 514 nanometers. The beam diameter was approximately 0.8 micron at the media sur-face, and pulse lengths of 100 nanoseconds were used.
Tests were conducted to determine how the reflective contrast ratio varied with laser-beam power. Measure-ments were made starting at beam powers of 28 milli-watts and down to 1.3 milliwatts. The results of this series of tests are shown in Figure 13 as the curve representing sample "F'l. Curves 'ID'' and "E" represent tests performed on the reflective laser recording medium of the prior art (represented by patent applica-tion serial number 012, 235) and using the same Konis-hiroku Photo Industries ST emulsion as the starting material. The ratio of reflected power from the un-recorded surface to that of the hole at 24 milliwattswas in the range of 7:1 for samples "D" and "F" and slightly lower for "E". At each measured power level, the contrast was measured at 32 points and averaged.

At their rated laser recording power level of 4.6 milliwatts at the above mentioned beam diameter and pulse length, samples "D" and "E" exhibited variations in reflective contrast ratios of +16% and +20% at the 1 sigma distribution points derived from the 32 meas-ured contrast ratios at each power level. At the 2.8 milliwatt level they exhibit variations in reflective contrast of +22~ and +29% respectively. In contrast Sample "F" exhibited variations in reflective contrast ratios of only +8.5~ at 4.6 milliwatt laser beam power and only +10~ at 2.8 milliwatts.

Example 4 A group of glass plates coated with a 3-micron thick clear gelatin produced from a commercial Konishiroku Photo Industries ST photoplate is heated in air for five minutes at various temperatures to illustrate how the pyrolizing of the gelatin at different temperatures leads to different degrees of light absorption at dif-ferent wavelengths. The percentage a~sorptions are as follows:

Percentage Liqht Absorption of 3 Microns of Gelatin As a Function of Process Temperature And Light Wavelength Wavelengths of Light 25 Processing Blue Green Red Infrared Temperature(488 nm)(514 nm)(633 nm) (830 nm) 276C 65% -- 16% --296C 78% -- 25% --316C 90% 77% 45~ 18%
339C 99% 89% 62% 25%
360C -- 93% 75% 30%

- 1~3734S

In summary, the combination of creating latent images, processing by the chemical silver diffusion transfer negative process and thermal annealing creates a more sensitive reflective DRAW laser recording material than any described in the prior art. Further, this three-step process leads to the lowest variations in reflec-tive contrast obs,erved amongst the various silver-gelatin data storage media of the prior art. Example 1 illustrates how the state of annealing, as measured by reflectivity, increases with temperature and with the presence of oxygen in the surrounding atmosphere. Ex-ample 2 and its associated Figure 12 illustrate how the thermal annealing increases the reflective contrast ratio and lowers the usable threshold recording level from 5.7 milliwatts to 3.6 milliwatts and significantly reduces the variations in the reflective contrast ratio over a wide range of laser power levels. In this ex-ample Agfa-Gevaert HD emulsion was used as the starting material in both the process of this invention and the prior art process. Example 3 and its associated Figure 13 illustrate how the three-step process of this inven-tion results in a reflective laser recording material that performs better at lower power levels; that is, has a higher recording sensitivity than that exhibited by the three-step process described in the prior art under patent application serial number 012, 235. In this example the same Konishiroku Photo Industries ST
emulsion was used as the starting material for the process of this invention and the prior art process.
Example 4 illustrates how the thermal annealing step increases the absorptivity of the gelatin matrix layer to the recording laser beam which may explain the in-crease in laser recording sensitivity after annealing.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a reflective electrically non-conducting data storage medium comprising, forming an area-wise, latent image exposure in a photosensitive silver-halide emulsion layer disposed on a substrate, thereby defining an area for data storage, said exposure creating an areawise layer of silver precipitating nuclei having a volume concentration gradient through the depth of the emulsion, said emulsion having unexposed photosensitive silver-halide remaining therein in concentrations inversely related to said nuclei concentration, contacting said unex-posed photosensitive silver-halide emulsion layer with an aqueous monobath comprising a weak silver-halide developing agent for developing said latent image and a rapid-acting silver-halide solvent for reacting with unexposed and un-developed silver halide to form soluble silver ion com-plexes which are transported by chemical diffusion transfer to said silver-precipitating nuclei of said latent image where silver of said silver ion complexes is precipitated and adsorbed on said nuclei in the presence of said developer acting as a reducing agent, thereby forming a reflective electrically non-conducting silver negative of said latent image in said area for data storage, and heat-ing said reflective electrically non-conducting data storage medium at least to 250°C until the reflectivity of the surface increases by at least 5% of the initial re-flectivity percentage.

2. The method of claim 1 wherein said medium is heated until the absorptivity of the emulsion rises to at least 20% at the wavelength of a laser to be used for recording.

3. The method of claim 1 or 2 wherein said heating is carried out in air.

4. The method of claim 1 or 2 wherein heating is carried out in an oxygen containing atmosphere.

5. The method of claim 1 or 2 wherein said heating is carried out in an oxygen atmosphere.

6. The method of claim 1 or 2 wherein said heating is carried out at a temperature of at least 280°C.

7. The method of claim 1 or 2 wherein said heating is by means of radiant heating.

8. The method of claim 1 or 2 wherein said heating is carried out for a time of between one-half minute and 20 minutes.

9. The method of claim 1 or 2 wherein said heating is carried out for a time of between one and five minutes.
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