CA1134508A - Broadband reflective laser recording and data storage medium with absorptive underlayer - Google Patents

Abstract

ABSTRACT

A broadband laser recording and data storage medium for direct read after writing, formed from a photosensitive silver-halide emulsion in four steps. First, a non-saturating actinic radiation exposure is used to create latent images. A normal photographic development is used to produce a medium of gray neutral density. The surface of the remaining silver halide is fogged in a water or alcohol based solution to create a very thin layer of silver precipitating nuclei on the surface.
Finally, a single-step, negative silver diffusion trans-fer process is used to dissolve the unexposed and un-developed silver halide, forming silver ion complexes.
These complexes are transported by diffusion transfer to the sites of the silver precipitating nuclei and the filamentary silver on the surface, where the silver com-plexes are reduced to metallic silver on both the nuclei and the filamentary silver to form a high concentration of non-filamentary silver particles at the surface of a low melting temperature colloid matrix which is highly reflective of light and electrically non-conducting.

Description

A BROADBAND REFI,ECTIVE_LASER RECORDING
AND
DATA STORA&E MEDIUM WITH ABSORPTIVE UNDERLAYER

The invention relates to laser recording media, and more particularly to a broadband reflective silver data re-cording and storage medium.

Previously, many types of optical recording media have been developed for laser writing. Some of these media require pos-t write processing before they can be read, and some can be read immediately aEter 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 evapor-ation, thin films of dyes or other coatings which can be ablated at a spot and dielectric materials whose re-fractive-index may be changed at a point, causing a scattering of light when scanned with a read laser.

The most common D~AW media are thin metal films, usually on a glass substrate. Thin metal films have several ad-vantages: First, they can be produced for research purposes in small quantities with commercially available sputtering equipment~ Second, they can be read either by reflection or by transmission. Third, films of tell-urium and bismuth have relat-vely high recording sensi-tivities.

These thin metal films have enabled a large amount of research to be conducted and progress to be made in the .

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design of optical data storage systems. To date, tellurium and amorphous mixtures -thereof have 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 atenacious coating; and it introduces manufacturing and environmental complications because of its toxicity and since it rapidly oxidiz~s in air it must be encapsula~ed in an airtight system in order for it to achieve an ac-ceptable archival li~e.

What is particularly desirable about tellurium is thatit has a low meltin~ temperature for a métal, 450C, and also a very low thermal conductivity of 2.4 watts per meter per degree Kelvin at 573K. For example, in com parison, 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 thin, electrically conductive films of these two metals are compared for laser recording with short pulses of 20 laser power, the tellurium is far superior from a record- -ing sensitivity standpoint since the low thermal con-ductivity keeps th~ heat generated by the laser beam confined to a small area and the lower melting tempera-ture facilitates the melting of the hole.

The use of an electrically conducting, con-tinuous metal film of silver as a reflective laser recording medium would be impractical for precisely the same reasons that tellurium has been adopted. This is, si~ver melts at more than twice the temperature and has a thermal con-ductivity about 170 times higher. Desolte these apparent ~, ` :
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disadvanta~es, non-filamentary silver can be used quite eEfectively if it is dispersed throughout a dielectric such as gelatin with a sufficiently high vol~e concen--tration to create a reflective surface but low ènough 5~ in volume concentration that the silve!r layer is not continuous. Under these special circums-tances the làser beam need only melt the dielectric to record data on the reflective surface, not the silver itself~

A reflective silver laser recording medium of this gen-eral type was the subject of a prior U.S~A. patent ap-plication wherein a processed black filamentary silver emulsion was converted to a reflective non-electrically conductive reflective recording medium by heating at a temperature in the range of 250 to 330C in an oxygen containing atmosphere until the surface developed a shiny reflective appearance. This laser recording ma-terial worked effectively with laser beams of visible wavelength but its recording sensitivity~fell by about a ~actor of three for semiconductor lasers, which~gener-ate light beams in the near infrared at about 830 nm.Also the high temperature heating process precluded the possibility of using plastic film substrates c~mmonly used for photographic films and other plastics.
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A second prior U.S.A. patent application described a reflective data storate medium derived ~rom silver-halide emulsion and using a silver diffusion transfer process. No heating was required to create the reflec-tive surface; reflectivities up to 24.4% to green light were achieved. However, the recording sensitivity of this material was less than that of the process described in the first U.S~A. patent application which yielded rèflectivities up to 17.2~.
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In a third U.S.A. patent application a medium was dis-closed in which the recording sensitivity was greatly improved over that described in the second U.S nA~ appli-cation. However it was necessary to add an annealing step at a temperature at 250C and above to achieve the desired results. Also, although the recording sensi-tivity was very good with a green laser beam at 514 nanometers and with a red laser beam at 633 nanometersr it fell off by about a factor of three when the laser wavelength was increased to 830 nanometers. This effect was similar to that observed with materials produced by the method of the ~irst U.S.A. patent application. By the method of the third U.S.A. patent application the best sensitivities were achieved with media having re-flectivities in the green of 25.5% although reflectivi-ties up to 36.6% were observed from less sensitive samplesO

These last three referenced U.S.A. patent applications described a reflective data storage or laser recordi~ng media produced from silver-halide emulsions so as to create the desired reflective but non-electrically con-ducting surface desired for efficient laser recording.
These photographic materials have the added advantage that photographic-type techniques can be used for either replicating master discs or pre-recording data or con-trol markings on the reflective surface. However, these media were limited,in recording sensitivity in the long wavelengths near 830 nanometers, and in achieving high reflectivity along with high sensitivity and in requir-ing a relatively high temperature process to be used forhigh sensitivity DRAW recording applications, which limited the selection of plastics that could be used as substrates.

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345~3 The present invention provides a method of making a reflective optical data storage medium comprising exposing at least a portion of a silver~halide emulsion photosensitive medium to a non-saturating leve} of ac-tinic radiation, developing said emulsion to a gray color having an op-tical density to red light of between 0.05 and 2.0, forming an areawise layer of silver precipitating nuclei on the surEace of said developed emulsion at sites of undeveloped silver halide, contacting said developed and nucleated emu:Lsion with a monobath comprising a silver-halide solvent and a silver reducing agent, whereby unexposed and undeveloped silver halide forms soluble silver complexes and is transported by diffusion transfer to said precipitating nuclei where said silver complexes are reducea to metallic silver.~

Among the advantages of the preferred embodiments of the inventlon are the fact that the laser recording and data storage mediu may be manufactured without the use of a vacuum system and on a continuous basis and which may be used to record low-reflectivity spots in a reflective field wi-th relatively low energy laser pulses. Another advantage is that at the long red wavelength and near-infrared wavelengths, high recording sensitivities may be achieved. The manufacturing process disclosed herein also permits the use of a wider variety of plastic ~ . , . ;. - . .
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substrates and achieves higher reflective surfaces with-ou-t lowering the recording sensitivity so as to increase the reflective light to facilitate automatic focusing and increase the signal level when data is read.

S It hàs been discovered that the silver halide in a photo-sensitive emulsion of a photoplate or film may be e~-posed and developed into two o~tically contrasting layers by a series of steps so as to create a laser recording medium that is absorptive to laser beams both in the visible and near infrared. A partially transmissive mirror-like reflective upper layer is formed atop an absorptive underlayer, both of which absorb light energy in the ultraviolet, visible and infrared spectra.

This two-layer medium absorbs and dissipates the heat of laser beams impinging either in the visible wavelength or near infrared, making this laser recording medium very broadband to the wavelength of the recording laser.
The reading of the data on~the reflective surface is~al-so broadband. Further, the reflective surface may con-tain pre-recorded data by patterning with lower or high-er reflectivity markings by means of a photographic ex-posure through a photomask~ This pre-recorded data lS
also broadband in that it can be read with a visible or near-infrared laser. Laser recording is done by melt ing spots in the gelatin matrix which contains the re-flective surface which are later read as spots of low reflectivity.

The mirror-like reflective layer consists mainly of a relatlvely~high volume concentration of non-filamentary .

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silver particles and a lower concentration of fila-mentary silver particles supported in a matrix of gela-tin. The dark underlayer consists mainly of a moderate volume concentration of filamentary silver particles supported in a matrix of gelatin. Other colloid matrices could be used to support the silver particles~

In brief, the two-layer medium is made as follows: The surface of a fine grained silver-halicle emulsion photo-sensitive medium is exposed briefly to a low-to-moderate level of actinic radiation. This exposed silver-hallde is then developed to an optical density typically 0.05 to 2.0, as measured with red light of a photograph1c densitbmeter. This gelatin layer containing filamentary silver particles exhibits an optical density of typically 0.05 to 0.8 for a 3 micron emulsion and 0.1 to 1.5 for a 6 micron emulsion. After this initial processing step, the emulsion layer is gray in appearance, but a large amount of the silver halide in the emulsion remains un-altered. A very thin layer of unexposed~silver halide at the surface of thls partially developed emulsion layer is then chemically fogged to form a very dense ~;~ layer of sllver precipitating nuclei at that surface.
The fogged medium is finally subjected to a negative silver diffusion transfer step wherein the silver halide in the emulsion is solvated to form soluble silver com-plexes. These silver complexes are precipitated on the silver precipitating nuclei to form a reflective layer comprising non-filamentary silver particles which ag-gregate with the filamentary silver~ The degree of refle~tivity of the surface may be adjusted over a range , ~ ~ .

3~8 of values depending upon the ratios of the -two types of silver. This same mechanism also causes some of the silver ion complex to precipitate on the filamentary silver in the absorptive underlayer, increasing the op-tical density to red light of this already developedunderlayer typically by at least a factor of two in-crease in light absorption.

The final result of these two exposure/development se-quences is a superior reflective laser recording medium which is comprised o~ a very thin layer of reflective but non-electrically conducting reduced non-filamentary silver and a much smaller amount of filamentary silver, under which lies a highly absorptive layer consisting primarily of filamentary silver in a gelatin matrix.
This absorptive underlayer typically has a fine optical density to red light of between 0.2 and 3Ø The orig-inal silver-halide emulsion photosensitive medium which eventually results in the above described reflective laser recording medium is usuaIly coated;on either~a~
plastic or glass substrate. The reflective surface~ has~
a reflectivity to green light of 44% for a typical ;
sample. ~ -Laser recording on this double-layer medium can be made very efficient. The absorptive filamentary silver par-ticles in the reflective layer can be increased until -~ the medium acceptable reflectivity is reached. These filamentary particles are absorptive over a very wide spectrum range from ultraviolet to near infrared, per- -mitting a wide variety of lasers to be used for record-ing. Also, the light energy that is not absorbed by : .

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the reflective layer is almos-t entirely absorbed by the underlayer which causes a rise o~ temperature at the interface oE the reflective layer and lmderlayer, there-by facilitating the melting oE the reflective la~er.
Recording is accomplished by use of a laser beam to melt the gelatin at a spot in the reflective layer, thereby reducing the reflectivity at the spot. Before record-ing, the reflectivity of the reflective layer is specu-lar; in other words incident light perpendicular to the surface will be reflected back towards its origin in a parallel line. After recording, perpendicular incident light will be diffusely reflected because the light re-turning towards origin will be scattered as opposed to parallel. This latter effect and the increased absorp-tivity at the spot lead to a lowered reflectivity. Theabsorptive underlayer would be only slightly penetrated by the recording process. None of the silver in either layer is melted during the recording process.
-Several distinct advantages result from this method of making reflective laser recording media. First, it is a very sensitive laser recording medium. Second, slnce the surface reflective layer retains its laser light ab-sorptive and reflective properties over the visible and near-infrared wavelengths, it is a very broadband laser recording medium. Third, since different reflectivites of the surface may be achieved, pre-recordings may be produced using appropriate photomasks to create the de-sired exposure patterns. Fourth, the high recording sensitivity is achieved without resorting to a high-temperature heating process, thereby permitting the useof certain commercially available plastics as substrat~s.

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Fifth, the silver-halide emulsion of this invention is inexpensive owing to the thinness of the emulsion.

In the drawings:

Figure 1 is a plan view illustrating a laser recording and data storage medium prepared in accordance with the method of the present invention;

Figure 2 is a side seotional view illustrating a non-saturation exposure and gray de~elopment step for a photosensitive mediunt used in making a laser recording and data storage medium in accord with the present in-vention;

Figure 3 is a side sectional view of the surface chemi-cal fogging step to create nuclei on the photosensitive medium of Figure 2 in accord with the method of the : 15 present invention;

Figure 4 is a side sectional view of the negati~e dif-fusion transfer step from the emulsion layer~to the :
: nuclei on the photosensitive medium of Figure 3 i.n accord with the method of the present invention;
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Figure 5 is a side sectional view illustrating a masking and exposure s-tep of a photosensitive medi~ in an alter-nate method for making a laser recording and:data stor-age mediunt of the present invention;
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Figure 6 is a side sectional view illustrating the sec-ond non-saturating exposure step and the gray develop-~ment step in the alternate method first illus-trated in Figure 5;

3~38 Figure 7 is a side sectional view illustrating the surface chemical fogging step to create nuclei in tha alternate method for the photosensitiva medium Qf Fig-ures 5 and 6;

Figure 8 is a side sectional view of the negative dif~
fusion transfer step from the emulsion :Layer to the nuclei in the alterna-te method for the photosensitive medium of Figure 7; and , .
Figure 9 is a side sectional view of the record1ng med-ium of Figure 4 illustrating the method of las~r record-ing.

A broadband reflective laser recording and data storage medium is made in four principal steps: an initial ex~
posure to actinic radiation, photographic development, 15 surface chemical fogging or nucleation, and silver dif- -fusion transfer.~ The~finished medium may be a dlsk,~ as shown in Figure 1, or may have~other shapes.
- ~ ' Initial Exposure - :
The first step in the process of making the present in~
vention is the exposure of a silver~halide photosensi~
-tive medium to actinic radiation~ This initial exposure is non~saturating, leaving as much as two~thirds or more of the photosensitive silver halide unactivated. ~With reference to Figure 2, the silver-halide photosensitive medium is preferably a fine grain silver halide photo-plate, such as thoso used to produ- .emicondu-tor , -" :
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: ` , , ' 1~ 15~ 3 photomasks. The initial exposure may be obtained from a weak source or from a very brief exposure -to a moder-ate source of actinic xadiation. Actinic radiation is the generic term which describes any exposure which creates a latent image. Latent image is the term which describes activation of unexposed silver halide.

The initial exposure results in the formation of a la-tent image which when photographically developed pro-duces a medium of gray color having an optical density of 0.05 to 2Ø This gray medium plays in important role in the performance of the broadband reflective laser recording medium of this invention. Clear gelatin absorbs very little light energy and thus a laser re-cording material using it will not be very sensitive.
The present invention relates to thè discovery that black filamentary silver particles may be formed to create a light absorptive medium while leaving a large portion of silver halide unactivated. This remaining silver halide then is used to create the reflective surface of the present invention.

Exposure oE the silver-halide photosensitive medium may be of uniform intensity over the surface of the medium, as illustrated in Figure 2. This would yield a uniform density of the latent images within the photosensitivity medium, which when photographically developed results in a uniform gray tone or optical density throughout the me~ium.

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5~8 An alternative to a uniform exposure and thus a uniform density of laten-t images is a patterned exposure of variable intensity, as illustra-ted in Figure 5. For example, the exposure of the silver-halide photosensi-tive medi~ may be composed of alternating concentricbands of high and low intensity actinic radiation over the surface of the photosensitive medi~. By changing the intensity oE the exposure in an alternating fashion, by means of a shielding mask 14 having t~o degrees of transmissivity to the actinic radiation, the density of latent images within the photosensitive medium will differ in proportion to the intensity of the exposure levels. By patterning this differential exposure to form concentric or spiral bands of higher and lower density latent images, it is possible to create a light absorptive layer which may then be used by further pro-cessing to create a servo track pattern of two different reflectivities.

This initial gray layer becomes an absorptive under~
layer in the final product, whic~h is covered by a very thin layer of reflective reduced silver. The combina-tion of a reflective silver coating over a light ab-sorptive underlayer is in turn supported by a supporting substrate 19. This supporting substrate may be either glass or some plastic or ceramic material. It is not necessary that this supporting substrate be transparent to either the exposing actinic radiation or to the radi-ation produced hy the optical reading device.

It is clear also that the combination of reflective silver coating over absorptive underlayer may be placed .
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on both sides of such a supporting substrate. For cx-ample, it is possible and practical to use a photopla-te which has disposed on its opposite sides silver-halide photosensitive mate~ial. The fact that the photosensi-tive material which finally results in the silver coat-ing over an absorptive substrate covers opposite sides of the supporting substrate has no detrimental effect on the utility of the final product and in fact provides twice the data storage capacity.

The silver-halide photosentivie medium hereinabove dis-cussed may be a commercially available black and white photoplate or black and white film product such as a strip film with or without a gelatin overcoat. Photo-plates used for semiconductor photomasks or holographic recordings have no overcoat. The smaller the grain sizes of the silver-halide emulsion the higher the reso-lution of xecording of the final product which results from the application of this invention. The emulsion grain size should be less than 5% to 10~ of the record-ing hole size for best results. As lS shown in the~ex-amples which follow, commercially available hig~ resolu-tion silver-halide emulsion photoplates used in making semiconductor integrated circuits are particularly use-ful in the practice of this invention. These photoplates have grain sizes primarily under .05 micron and will yield non-filamentary si]ver particles for the high reso-lution reflective layer produced in~the final process ~-~
step. The silver halide in such plates is held in~a colloid matrix, normally gelatin. But the invention is by no means limited to these photoplates nor indeed is it limited to using only commercially available silver-halide photosensitive materia`s. Ary photos~nsitive :
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silver-halide emulsion with grain sizes primarily under - .05 micron may be used in the practice of the present invention for high resolution laser recording~ For lower resolution recordings the silver halide grain ~ 5 sizes may be larger than .05 microns~

; For purposes of this patent applicativn, the term "sil-ver halide emulsion" means a silver-halide emulsion without a gelatin overcoat, unless an overcoat is specified.

It is clear that there are many difEerent initial ex-posures which may be used in accordance~with this ln-vention. The two set forth herein, a uniform exposure and exposure through a mask having different transmissi- -vities to actinic radiation are but two broad examples.
Examples will suggest several different variations in this initial exposure step.

B. Photographic Development The second step of the present~invention is concerned with the photographic development of the latent images formed in the initial expo~sure. This produces a gelatin layer contai~ing filamentary silver particles exhibit-ing a gray optical density as measured with red light of a photographic densitometer typically between 0.05 to 008 for a 3 micron thick emulsion and 0.1 to 1.5 for a 6 micron thick emulsion. This development creates a gray layer, on the surface~of which will be formed a reflective layer. The gray layer becomes the absorptive :~ ~

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113A5~8 underlayer, which in the recording/playback mode looks like a neutral density Eilter when the initial ~ray layer is at the higher range of optical densities and has a ~ -reddish hue when the initial gray layers are of lower optical density. The starting gray layer has an initial optical density typically of 0.05 to 2.0, as measured with red light, although for some applications a range of 0.1 to 0.8 may be preferred.

Referring to Figures 2, 3, 6 and 7, the black dots in photosensitive medium 11 illustrate the formation of filamentary silver as a result o~ development, The volume concentration of activated silver halide deter-mines the volwme concentration of filamentary silver.
The volume concentration of filamentary silver in turn determines the optical density of the absorptive under-layer. The extent of development also affects the op-tical density.

When non-filamentary silver particles are distributed throughout the gelatin, light transmitted through the gelatin glves the appearance of a reddish color owlng to the scattering effect of the tiny non-filamentary sllver particles. When filamentary silver particles are distributed throughout the gelatin, light passing through the gelatin gives the appearance of a gray color, as in a neutral d~ensity filter. When both non-filament-ary and filamentary silver particles are distributed throughout the gelatin, the color of the light passing through the gelatin can range from a reddish color to a neutral gray to a reddish gray, depending upon the con-cen-trations of the two types of silver. The absorption ' ~ ' '.., ', , ~ ' , : ' "
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L5~8 created by the gray-appearing filamentary silver is very broadband from the visible through the near-infrared.
Added to this absorptivity is tha-t of the non-fi:lamentary silver which adds to the absorptivity in the green and blue wavelengths but has a lesser effect at the red and infrared wavelengths.

Photographic development is the process of converting the activated silver halide which comprises the latent image into filamentary silver. This conversion is really a reduction of the activated silver to form a black fiIamentary silver. The development process is well known in the photographic industry. Gene~ally, commercially available silver-halide photosensitive materials which may be used in accord with this inven-tion have manufacturer suggested developing agents.

In addition to the relatively simple interplay betweenexposure time and development, we must consider the desired optical density of the absorptive~underlayer formed by this development step. ~It must be remembered that the product produced by the disclosed invention is a broadband optical laser recording and data storage medium. As such, there are many possible applications of the product of this invention. The specifications of one user may not be the same as others, requiring flexibility in the sensitivity of the medium.

The principal role of the initial actinic exposure step is to create a latent image throughout the silver-halide emulsion layer which may be photographlcally developed into a gray layer created by filamentary silver parti-cles in the gelatin matrix. No fixing step is used :: :

~1345~3 after the development since the remaining silver halide is used to create the desired reflective surface, The stronger the initial exposure and the stronger the de-velopment, the darker the gray color of the media and the less silver halide is left for creating the reflec-tive layer. Thus, if a light shielding ~ask 14 is used to create a pattern of latent images and -then if this is followed by a second moderate exposure~ the developed plate will have areas o different gray densities. When such a media is processed to create a reflective surface, the reflectivity will vary in inverse relationship to ~ -the gray density of the media. Thus, ~he actinic ex-posure step can perform the valuable role of pre-pattern-ing or pre-recording the reflective laser recording media as well as creating the absorptive underlayer needed for efficient laser recording. The development step is designed to translate the patterned exposure into a patterned gray layer having areas of different gray densities.

The preferred initial optical density falls within a wide range for a number of reasons. For e~xample, if a high surface reflectivity of 60`% is required then the initial density would be under 0.5 since the higher the nitial density the lower the final reflectlvity. This high reflectivity may be required to achieve a high signal-to-noise ratio or ease of automatic focusing.
Along these same lines, if two or more reflectivities are required for pre-recording servo-tracks or such, the high reflectivities would require lower initial gray denslties.

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As a second example, if a high recording sensitivi-ty is desired, then -the optical densi-ty oE -the underlayer should be higher than the minimum amount.

The third example involving the optica;L density is that it is not critical from a recording sensitiv:lty stand-point. From a theoretical standpoint, an optical density of 1 indicates a 90% absorption of the laser beam and an optical density of 2 implies a 99% absorption. The exposure or processing difference to achieve these two optical densities would be great, but the effect on sensitivity would be small; that is, only about 10%.

The two purposes of the absorptive underlayer will be discussed below in Section E.

It is important to keep the unexposed and undeveloped silver halide in that condition between the photographic development step and the next step. For this reason, the development step and the surface chemlcal fogging step are performed in the absence of light or using a~
safe light, so as to keep the level of actinic radiation to a minimum. In addition, it is~necessary to limit the exposure of the medium to actinic radiation in the transition from ~he initial exposure to the photographic development. This is obvious because a carefuIly con-trolled initial exposure would be ruined by exposure in transitlon to stray actinic radiation.

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~ ~ ' , -- ~ 1 3 45~ t 3 C. Surface Chemical Fogging With reference to Figure 3 and 7, after the silver-halide photosensitive medium has been exposed and developed to an optical density of 0.05 to 2.0, the surface of the photosensitive medium is then fogged. Fogging, or nucleation, is the process of creating silver precipi-tating nuclei. These nuclei form a thin layer where the silver in silver ion complexes may be redu~ed to metallic silver and absorbed. Essentially, all fogging does is to create an area where transported silver ion complexes may be aggregated and reduced to reflective silver.
The nuclei formed by fogging axe indicated by the *
signs in Figures 3 and 7.

It is clear from the figures that the photosensitive medium ll must be penetrated by the fogging agent to create a surface layer of nuclei. Generally, silver-halide photosensitive material utilize gelatin as a sus-pensive colloidal medium for the silver-halide emulsion.
Thus, to form a layer of silver precipitating nuclé;at the surface of the medium it is necessary to slightly penetrate the silver-halide emulsion. It is well known that when silver-halide emuIsion absorbs water, the emulsion swells. This swelling results in a rapid and deep penetration of the fogging agent in any water based solution into the emulsion. This is not desirable since this would create a thick layer of nuclei which would result in a thicker, less reflective surface.

When a photoplate or other photosensitive medium without a gelatin overcoat is used, we find that the use of a .

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water miscible alcochol, an example of which is methanol, almost entirely eliminates the swelling and thus mini-mizes the peneiration of the fogging agent. This small penetration yields a thin and highly dense nuclei layer which, after the fogging step, becomes the desired thin highly reflective data recording mediumO When the photo-sensitive medium contains` a gelatin overcoatl also commonly called a supercoating, a water or alcohol based fogging solution is preferable in order for the fogging solution to penetrate the overcoat and create nuclei at the surface of the silver-halide emulsion.

Again, it must be kept in mind, that the purpose of this invention is to devise a method for making a:re-flective laser recording medium. The medium which~re~
sults from this invention has a very thin reflective silver surface layer, preferably a small fraction oE a micron, which covers a much thicker dark underlayer having an optical density of 0~2 to 3Ø This reflec-tive silver surface is created by transporting silver ion complexes from the silver halide in:the underlayer to the silver precipitating nuclei in the surface layer~
and then reducing the silver complexes fo.rmed at the nuclei to reflective non-filamentary silver particles It is clear that the most efficacious location for the ~ 25 sllver precipitating nuclei is at the surface of the ~ silve.r-halide photosensitive medium.

It is desirable to limit the penetration of the Eogging agent to as close to the surface of the photosensitive medium as possible and practical. In this regard, methanol or any other water miscible alcohol is especial-: ly useful when the photosensitive medium used is a photo-plate vithout an overcoat. Uowever, any aqueous soleio~ ..

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of a reducing agent will penetrate -the gelatln of most commercially available photosensitive materials, thus creating a deptllwise decreasing concentratlon of silver precipitating nuclei~ Generally, fogging agents are used in a bath and penetra~e the entire medium. However, in the method of the present inVentiQn selected solvents and time control the depth of penetration. It is impor-tant that the emulsion be uniform in d:ryness prior to emersion in the fogging agent in order to prevent vary-ing degrees of penetration of the fogging agent.

The objective is to create a very thin, dense layer ofnuclei. When~there is no gelatin overcoat on top of;the photographic emulsion, methanol is a very useful carrier of the fogging agent since it penetrates the gelatin much more slowly than water-and, therefore, its pene-tration can be limited and controlled. When a gelatin overcoat is present, the fogging agent must be able to penetrate through the overcoat to reach the photo-sensitive silver halide without penetrating deeply into ~;
the silver-halide emulsion. Water~or alcohol can be used for this purpose~since they are effective in pene-trating the overcoat. -: ~
A fogging agent is a vèry active reducing agent. Anyone of the hundreds of photographic developers are re-ducing agents which could theoretically be made activeenough to fog silver halide with the correct adiustments o concentration and p~I. All would have some solubility in methanol, but it would be questionable if one could dissolve enough developer to be active while simultan- ~
30 eously dissolving enough antioxidant -to protect the ~ ;

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developer from air oxidation. Of -the simpler compounds that woul.d be ac-tive.reducing agents ancl soluble in metha.nol., two examples are h~drazi.ne and hydroxylamine.
Both are fogging agents at high pH. However, both are silver-halide solvents that could part:ially dissolve the surface of the silver halide that is being fogged, which would be an undesirable effect. Borohydride is an example of a practical compound for the method of this invention, as it is very active in reducing silver halide, is not oxidized by air, and has no silver-halide solvent properties. Borohydrides of lithium r sodium, potassium, cesium and rubidium would be useful.

Consonant with the above limitation, depthwise penetra-tion of the fogging agent is kept slight, typically less than 10~ of the depth of the photosensitive medium. The competing factors of penetration and the duration of exposure to the fogging agent combine to determine the factors in the fogging step. By limiting depthwise penetration of the nucleation agent to typically five or ten percent of the depth of the emulsion, or one micron or less, the final reflective silver layer will occupy typically approximately the top five or ten per-: cent of the medium and the gray underlayer will occupythe remaining ninety percent. Usually the photosensi-tive medium is less than 15 microns thick.

The chemical surface fogging or nucleation step may beeliminated by incorporating a thin layer of nucleating agent for silver precipitation. This is a common prac-tice in silver diffusion transfer processesO In Chapter 16, "Diffusion Transfer and Monobaths," of The Theory of the Photographic Process, Fourth Edition, T. H. James, ~ ,, . .

~39~g38 :

- 2~ -a numher of types of eEfective nucleating materials are mentioned which have been incorporated into a ~ilver precipitating layer including copper, silver, silver sulide, selenium, cadmium sulfide, lead sulfide, and mercuric sulfide. When a reflective surface is the ob-jective rather than a black surfacer it is important that round shaped crystals of siIver are grown - not the filamentary type which leads to a black surface. Pure silver particles of a round-like shape would be prefer-red for this nucleating layer since sulfides tend togrow filamentary silver, which leads to a low reflec-tivity surface.

D. Silver Dlffusion Transfer to Nuclei After having formed a thin layer of silver precipitating nuclei on the sur~ace of the silver-halide photosensi-tive medium, the final step of the method of the present invention entails transporting the silver in the re-maining silver halide to the silver precipitating nuclei~;
and by means o~ silver complexes there reducing the ~
silver. This proc2dure is usually accomplished by plac-ing the nucleated photosensitive medium in a monobath.
This monobath contains bo~h a silver-hdlide solvent and a silver reducing agent. This step is also done in the dark or using a safe light until silver diffusion trans-Fer is complete.

The two elements of this monobath, a silver-halide sol-vènt and a silver reducing agent, comprise a silver diffusion transport and reduction system. The silver-halide solvent acts on the silver halide in the photo-sensitive medium to produce mobi~e silver ion complexes.

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~3~8 These free silver complexes are transported wi-thin the photosensitive medium to and through the surface of -the medium. These silver complexes are then subjected to reduction, producing metallic silver on the silver nuclei and on the filamentary silver at the surface. This is represented by the clustered dots in the reflective layer 17 of Figures 4 and 8.

The reflective layer formed in this step lS e1ectrical1y non-conducting and has low thermal conduc-tivity and may be patterned photographically, these latter two prop-erties being highly desirable for laser recording media.
The complexed silver ions are created by reaction of an appropriate silver solvent and the silver halide left undisturbed in the emulsion~ 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 mo~obath formulations for highly reflective surfaces include a developing agent which may be characterized as having low activi~ty. The specific type of developing agent selected appears to be less critical than the activity level as dete~mined by de veloper concentration and pH.
:
The developing agent should have a redox potential suf-ficient for causing silver ion reduction and absorption or agglomeration on silver nuclei. The concentration of the developing agent and the pH of the monobath should not cause filamentary silver growth which gives .

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~3~L5~8 a black low reflectivity appearance. The developed silver particles should have a geometric shape, such as a spherical or hexagonal shape which when concentrated form a good reflectivity surface.

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 developing agent and complexing agent. It is well known ~hat photographic developing agents require an antioxidant to preserve them. The following are typical developing agentlantioxidant combinations which may be used in con-junction with a sodium thiocyanate (NaSCN) solvent) complexing agent.

For Monobaths Using (Na(SCN) As_a Solvent And Silver Complexin~ Agent : . .
DeveIoping Agent ~ Antioxidant p-methyaminophenol Ascorbic Acid 20 p-methylaminophenol Sulfite Ascorbic Acid p-Phenylenediamine Ascorbic Acid Hydroquinone Sulfite Catechol Sulfite The preferred solvents/silver complexing agents, wh~ch must be compatible with the developing a~ent, are mixed therewith r in proportions promoting the complete dif-fusion 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 oE a fine grain emulsion in just a few minutes. The solvent . ~ .
...

:~ ' , : . `

'~ . ' ' , _ 3 ~3~8 should not react wi.th the developing silver grains to dissolve them or to foxm silver sulfide since this tcnds to crea-te non-reflec-tive silver. The solvent shoul~ be such that the specific rate of reduc-tion of its silver complex at the silver nuclei layer is adequately fast even in the presence of developers of :Low activity, which are pr~erred to avoid the creation of low~reflec-tivity black filamentary silver in the initial develop-ment 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 to ~ :
their rate of sclution 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.

Most Actlve ~ ~
Thiocyanates lammonium, potassium, sodium, etc.) : :
Thiosulphates (ammonium, potassiumrsodium, etc~) Ammonium hydroxide ::
Moderately Active (Y picolinium - ~ phenylethyl bromide Ethylenediamine l-Aminophenol furane n-Butylamine 2-Aminophenol thiophene Isopropylamine : :

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Much Less ~ctive IIydroxylamine sulfate Potassium chloride Potassium bromide Triethylamine Sodium sulfite From the above it can be seen that -the thiocyana-tes and ammonium hydroxide are amongst the most active solvents/
complexing agents. While almost all developers suitable for solution physical development can be made -to work in the silver diffuslon transfer process of the presen-t in-vention with -the proper concentration of pH,~not all solvents/complexing agents will wor]c within the desired short developmen-t time or in the proper manner. For example, the thiosulfate salts, the most common silver-halide solvent used in photography and in Polaroid-Land blac~ and white instant photography's diffusion trans-fer process, does not work in this process for two reasons. Its complexed silver lons are~so stable;Lhat ;
it requires a strong reducing agent to precipitate~the silver on the nuclei,~and this strong reducing or de-veloping agent would have the undesirable effect of de-veloping low reflective black filamentary silver. It has another undesirable effect, also exhibited by the solvent thiourea; namely, that it forms black, low re-flecting silver sulfide with the developing silver grains. On the other hand in the black and white Pola-roid-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~oE metallic silver and would thus etch away the forming image. It is also about 50 tlmes~

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as toxic as sodium thiocyanate, which is a common photo-graphic reagent.

In addition, iE the solvent concentration is -too low the solvent would not be able to convert -the silver halide to a silver complex within a short process time and if the reducing agent is too weak the unexposed, undeveloped silver halide will bypass the silver precipitating nuclei causing much of the silver complex to go into 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 nucle 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 ~ 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 density of silver nuclei to build upon and there is sufficient silver-halide material to be dissolved, then eventually the spheres will grow until some contact other spheres forming aggregates of several spheres or hexagons.

During the initial exposure and development steps, a gray layer is formed. The black filamentary silver which comprises the layer is present throughout the photosensitive medium. Thus the reflective la,yer forme~

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in the final step, diffusion transEer, con-tains some filamentary silver. The filamentary silver is important in determining the absorptivity of the reElective layer.
Typically, the vol~ne concentration of silver in the reflective layer exceeds the average concentration in the underlayer by a ratio of at least 3 to 1. Usually, the volume concentration of silver particles in the re-flective layer is a minimum of 20~ and a maximum of 50%.
Filamentary silver in the reflective layer may be as little as 1% and as much as 50% of the total silver in the reflective layer.

Also, in producing the gray layer as -the second step in creating this broadband reflective recording media, re-call that the initial optical density of this layer could fall anywhere within the range oE 0.05 and 2Ø
The optical density of the gray Iayer is raised by the silver diffusion transfer step which precipitates more silver on the filamentary si~ver of this layer, as~
shown by the dashe~ present in Figures 4, 8 and 9,~
raising its density to between perhaps 0.2 and 3.0 in the finished product. For most applications, however, the initial density would still fall within the range of about 0.1 to 0.8 optical density. All oE the above optical densities are measured with red light.
. ~
In the second set of figures, numbers 5 through 8, a mask is used in the initial exposure, thereby creating areas of greater and lesser optical~density upon de-velopment. In the final step of this invention, a :

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reflective layer of differing reflectivities, 17a and 17b, is formed by negative diffusion trans~er.

In Figure 5, the photosensit.ive medi~l 11 is exposed through mask 14 to a non-saturating level of actinic radiation from source 13~ Mask 14 has a transmissive area 14b and an absorptive area 14a which creates a pattern at the surface of the photosensitive medium of two different intensities of actinic radia-tion. Next, in Figure 6, the mask has been removed and the same photosensitive medium 11 is exposed to a uniform level of actinic radiation. The cumulative effect of these successive exposures is to form a latent image of at least two differing densities while activating less than half of the photosensitive silver halide present in the photosensitive medium. In this example, the densi.ty of filamentary silver formed during development in area lla is less than that in area llb. Figure 7 shows the for-mation by surface chemical fogging of a thin layer of silver precipitating;nuclei. In Figure 8 a reflective layer is formed by negative diffusion transfer as dis-cussed immediately above.

The reflective layer ln Figure 8 has two different re-flectivities, shown by 17a and 17b. The reflecti.vity of 17a is grea*er than that of 17b for two distinct rea-sons. F.irst, area lla contains a larger concentrationof unexposed undeveloped silver halide than does llb.
Thus there are more immediately availa~le silver ion complexes in~Ila when the photosensitive medium is ex-posed to the monobath. Since the concentration of silver precipitating nuclei is areawise constant/ the only relevant factor in formation of reflective sllver ' ', . . :
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is the availabili-ty of silver ion complexes. Thus since more complexes are available in lla than in llb, more silver will aggregate and subsequently be reduced in 17a than in 17b.

Second, as will be recalled from the section on photo-graphic development, filamentary silver is present throughout the developed photosensitive medium. In Figures 6, 7 and 8 the density of fllamentary silver in llb is greater than in lla. The concentration of fila-mentary silver particles is present in the reflectivelayer in essentially the same concentration or density as it is in the rest of the photosensitive medium im-mediately below the reflective layer. Thus the amount of filamentary silver present in reflective layer 17a is less than that in 17b and as you would expect the resulting reflectivity in 17b is reduced in comparison to 17a, owing to the higher absorptivity of the fila- ;
mentary particles in 17b. It should be remembered that the quantity of reflective silver present in the reflec-tive layer is substantially greater than the amount of absorptive filamentary silver, but where the density of filamentary silver varies in an areawise fashion its incorporation into the reflective layer does have an impact on local reflectivity. The result of the silver diffusion transport and reduction step is a very thin - layer of reflective silver particles over an absorptive underlayer. This very thin reflective layer is easily distorted, melted or punctured by a laser and thus is suitable for laser recording. Reflectivity between 10%
30 to 75% can be attaln-d rhe normal`y I igh thermal and ~ , :
-5~8 electrical conductivity of silver is not present sincethe silver particles are not in contact. Both layers are non-conductive electrically. Another disadvantage of the use of silver is its cost. In this invention, however, very little silver is used both to form the reflective silver coating on the opaque substrate or underlayer and there is very little silver to begin with in the silver-halide photosensitive medium. Thus this invention has the advantages of being inexpensive and using commercially available materials as well as being amenable to well known photochemistry.

E. Laser Recording Refe,rring,now to Figure 9, there is an illustration of recordiny on the broadband reflective laser recording medium of this invention. Reflective layer 17 has been pitted by a laser beam forming a shallow crater 23 which is a hole in the reflective layer. It is important to notice that the penetration of the laser beam lS very shallow, barely penetrating the reflective layer. The reflective gelatin matrix which incorporates the ref~lec-tive silver is melted, callsing a loss of reflectivity where the melting occurs.
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Before recording, the refIectivity of the reflective layer is specular; in other words, incident light per- `
2S pendicular to the surface~will be reflected back towards its origin in a parallel line. After recording~ light which is perpendlcularly incident on a crater or pit will be diffusely reflected because the light returning towards the source will be scattered as opposed to pàral-lel. This later effect and the increased absorptivityof the spot lead to a lowered reflectivity.
' ~: .
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' - ` -~3~08 Crater 23 may be 1 micron in diameter or less, thou~h for some purposes a larger crater may be desired. For example, in analog recording ovals of more than one mic~on length are sometimes used.

When laser recording is being done the higher the absorp tivity of the surface the easier it is to melt craters which appear to be holes in the surface. However if the reflected light is being used for reading the record-ed data or for automatic focusing then a high reflectiv~
ity is desired. The incorporation of various amounts of filamentary silver with the~non-filamentary silver of the reflective layers offers an important means;of ad-justing the reflectivity of the desired level. Also the filamentary silver particles are absorptive even at lS near infrared wavelengths which results in these media being a broadband laser recording media.

The absorptive underlayer, 21, which is the resul~ of the exposure and development steps, serves two purposes in laser recording. First, the filamentary-silver ab-sorbs the light energy of the recording laser beam,converting it into heat. The reflective layer of this invention is partially -transmissive, especlally in the near infrared where semiconductor lasers operate. Thus transmitted light energy through the reflective layer is absorbed by the absorptive underlayer immediately be-neath the reflective surface it strikes. The light energy is converted into heat, causing a temperature rise in the underlayer and the reflective layer in con-tact with it~ Th~s in turn makes it easier to me t the `

~':

34S~8 reflective layer by raising its temperature. The con version of light energy into heat by the filamerltary silver has a synergistic effect in the recording mode, because as the temperature of the reflective layer in-creases the temperature rise necessary to reach themelting point of the reflective gelatin matrix decreases.
A similar effect may be achieved by adding to the emul-sion materials absorptive at the wavelength of the re-cording laser, for example, by dyeing the gelatin.

Second, gelatin is a good thermal insulator. Thin metal layers such as tellurium used for laser recording have a higher degree of therm21 conductivity than does the gelatin layer containing either the reflective or fila-mentary silver. Thus heat dissipated in the ref:l.ective làyer does not flow rapidly into the substrate thus heat energy is conserved and the recording process is effi-cient. The broadband laser recording and data storage medium of this invention has the advantage of metal-like reflectivity while avoiding the disadvantages of hlgh~ :
thermal conductivity and high melting temperature con-comitant with the use of a metal film. Also, gelatin melts at a temperature of about 350C compared to 450C
for tellurium, the most common laser recording material used today.

Example 1 This example illustrates how the level of the initial light exposure is related to the final reflectivity of .:
the surface layer. If the first exposure is intense, then the emulsion photoplate will develop dark gray to black, leaving no silver halide to produce the reflective .

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layer. For the same reasons, a slight ini-tial exposure will result in a final highly reflectiv~ surface. To illustrate this point, a commercial Agfa-Gevaert Milli-maslc HD emulsion, 4.5 microns thick, containing a screen-ing dye, was exposed on a Mark VII sensitome-ter manu-factured by ~dgerton, Germeshausen & Grier, Inc~ The plate was placed emulsion-side down and exposed with the instrument's tungsten light source through a stepped wedge stepped in optical density units of 0.1. The photoplate was exposed for 10 2 sec. through -the stepped wedge. The photoplate was developed for 4 minutes in a developer with the following formula-tion: sodium sulfite, 36.9 ~rams; hydro~uinone~ 7.9 grams; phenidone, 0.52 grams; potassium hydroxide 7.4 grams; potassium bromide, 2.7 grams; benzotriazole, 0.07 gram; with water added to bring volume up to 1 liter.

After development, washing, and drying, the resulting neutral optical density was measured with red light on a Macbeth densitometer Model TR527. The optlcal densl-ties were as follows:

Initial Optical Density Step Number (with Red Light) 6 5.80 7 4.26 8 2.69 9 1.40 0.65 11 ~.31 30No initial 0.0 exposure ":

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A simllar plate was processed as described previously and was i~nersed in a surface fogging solution of 0.5 grams per liter KBH~ and 0.5 grams per liter NaOH in methanol ~or 15 seconds to create silver nuclei on the surface. This nucleated plate was then washed for 1 minute and placed in a monobath developer solution con-taining: sodium sulfite, 10 grams; ascorbic acid, 2.5 grams; Elon (Kodak developer~, 0.25 grams, sodiurn hydrox-ide, 2.0 grams; sodium thiocyanate, 125 grams; with water added to bring volume up to I liter. After im~
mersion for 2 minutes in the monobath, the photoplate was analyzed for reflectivity and optical density. The resulting optical densities were measured with a Macbeth densitometer Model TR 527, and the reflectivities were measured with a system comprised of a He-Ne laser and an International Light, Inc. Model IL 710A research photometer. The 633-nanometer light reflected from the ~sample was compared to an aluminum mirror of 92% ref~lec-tance. The results were as follows~

Final Optical Density 5tep Number(with Red Ligh-t)Reflectivity 6 5.85 9.8%
7 5.84 10.3%
8 4.97 10.2 9 3.15 18.0%
1.97 35.0%
11 1.32 38.5%
No initial 0.88 45.8%
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This example shows that by changing the initial exposure level as d~ne above by use of a stepped wedge as a mask, it is possible to make an optical data storage medium with areas of reduced reflectivity where higher actinic exposure occurred and areas of higher reflectivity where lower intensity actinic exposure occurred.

Example 2 The first part of this example illustrates that only a low initial actinic exposure is necessary to create the desired amount of filamentary-type silver in-the~surface layer and underlayer. The volume concentration of the filamentary silver in the finished med:ia is determined by two effects. The first is the initial exposure~ and development, and the second is the immersion of the photoplate in the monobath. The first effect creates a given number of filamentary silver particles, while the second effect results in an intensification of the build-up of the filaments by the metallic deposit of silver from,the silver complex created when the monobath inter~
acts with~the silver halide. ~
.
A photoplate coated with a commercial Konishiroku emul-sionj 3 microns thick, with an anti halation backlng and containing no screening dye, was exposed on a Mark VI r sensitometer manufactured by Edgerton, Germeshausen &
Grier, Inc. The photoplate was placed emulsion-side down and exposed with the instrument's tungsten light source through a stepped wedge stepped in optical denslty units of 0.1~ Actinic radiation exposure was 2 x 10 seconds. The exposed photoplate was then developed for ~: , ~ ' ' '-' S~3 4 minut~s and not fixed. ~h~ anti-halation backing is removed in this process. The developer formulation was as follows: sodium sulfite, 36.9 grams; hydroquinone, 7.9 grams; phenidone, 0.52 gram; potassium hydroxide, 7.4 grams; potassium bromide, 2.7 grams; benzotriazole, 0.07 gram; with water added to bring volume ~Ip to 1 liter. -After washing and drying, the resulting neutral optical densities were measured in red light on a Macbeth densi tometer, Model TR 527. Each step o~ the stepped wed~e is defined by a number. The step identification and associated optical density is presented in the ~wo col-umns on the left:
":
Optical Density Optical Density (with Red Light) (with Red Light) Following Initia~
After Exposure, Develop-Initial Exposure ment, and Immersion Step Numberand Development ~ in Monobath~
4 1.05 ~ 3.03 .03 3.00 6 .77 2.91 7 .66 2.75 8 .5~ 2.53 9 39 2.13 -q;: ~ :
.20 1.45 .10 1.15 12 .~' 1.30 ~ : , , , ::
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S1~8 This exposed and developed stepped wedg~ was then im-mersed for 3 minutes in a monobath solution comprising:
sodium sulfite~ 10 grams; ascorbic acid, 2.5 grams;
Elon tKodak developer), 0.25 gram; sodium hydroxide, 2.0 grams; sodium thiocyanate, 60 grams; with water added to bring vol~e up to 1 liter. Note the sub-stantial rise in optical density compared to the initial optical density.

The second part of this example illustrates how the in-itial gray density is related to the final reflectivity for the Xonishiroku photoplate coated with an ST emul-sion 3 microns thick. A second sample was expose~ and developed by the identical procedure described above.
However, before inserting the sample into the monobath solution for 3 minutes, it was thoroughly dried, then immersed for 15 seconds in a fogging solution compris-ing~ KBH4l 0.15 gram; and NaOCH3, 0.6 gram with metha-nol added to bring volume up to one liter. The photo-plate was then washed thoroughly before and after im-20 mersion in the monobath. ~ ~ ;

The reflectivitles were then measured for the more re-flective step numbers at 633 nanometers using a DR2J
microflectometer system manufactured by Gamma Scientific Inc. The results were as follows:

Reflectivity Measured Step Number at 633 Nanometers 8 7.8%
9 21.8~
48.4%
11 63.1%
12 66%

.. . . ..
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~3~ 8 Thus, we see tha-t step lO had an initial absorptivity of .20 and a final reflectivity of 48~ at 633 nanometers.
For higher initial absorptivity, the final reflectivity was lower and vice versa.

S ~

This example illustrates that the reflective laser re-cording made by the method of this invention permits recording at lower laser powers than any of the previous media described by the same inventors in earlier patent applications, and also that this high sensitivity is achieved in conjunction with a reflectivity of at least 44~ which is considerably higher than that obtained earlier. Also, the reflectivity remains at a relatively ;-high level into the near infrared. `~

A 3-inch square photoplate coated with a commercial Konishiroku ST emulsion 3 microns thick and containing no screening dye, was exposed for 1.0 seconds at 3580 nanoamps/cm2 on an ~ltratech contact printer, Model CP210. This~is approxlmatély equivalent to 10 lumens/
ft.2 for 1.0 second. It was then developed in a devel-oper solution for 5 minutes and not fixed. The devel-oper was comprised of: soaium sulfite, 36.9 grams;
hydroquinone, 7-.9 grams; potassium hydroxide, 7.4 grams;
potassium bromide, 2.7 grams; benzotriazoIe, 0.7 gram;
witil water added to bring volume up to 1 li-ter~ The photoplate was then washed in water for 10 minutes.
Next, the photoplate was oven dried at 40C for 20 minutes. The photoplate was then dipped ~or 15 seconds in a solution of KBH4, 0.15 gram; and NaOCH3, 0.6 gram;
with methanol alded to bring volume up to one l.ter.

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The photoplate was then washed in water Eor 10 minu-tes and no-t dried. To achieve silver diffusion transfer, the photoplate was then immersed for 3 minutes in a monobath developer solution comprising: sodium s~lfite, 10 grams;
ascorbic acid, 2.5 grams; Elon (Kodak developer), 0.25 gram; sodium hydroxide, 2.0 grams, sodium thiocyanate, 60 grams; with water added to bring volume up to 1 liter.
The final wash in water was for 10 minutes, followed by an air oven drying for 20 minutes at 40C.

The resultiny mirror-like coating on the glass substrate is a broadband reflective laser recording and data stor-age medium. Laser recording was accomplished with an argon laser using the green line at 514 nanometers. The laser beam diameter was approximately 0.8 micron at the surface of the medium, and pulse lengths of lO0 nano-seconds were used. Tests were conducted to record low reflectivity spots with the laser beam by melting the reflective surface of the gelatin. Then the reflectivity of the hole was compared to the reflectivity of an ad-jacent area. A comparison of these reflectivitles leadsto a relative contrast ratio measurement. This pro-cedure~was repeated for 60 holes at a given laser power level. (The samples of the prior art were~tested;with 32 holes.~ The relative contrast ratio was determined by averaging results from the 60 holes. Also calculated was the statistical distribution of the rela-tive con-trast ratio for the 60 holes and a ~ 1 sigma distribu-tion was calculated. These resulting data are presented as Sample 4 on Table 1. For comparison with perfo~nance of prior art medi, the performances of samples l, 2 and .

:

3~5~3 3, also shown on Table 1, are prescnted as miles-tones of the prior art. Note that for all media, -the rela-tive contrast ratio decli.nes with the reduction of laser power. It should be understood that as the laser 5 power declines near the lower levels, the recorded hole :
gets smaller and smaller. Thus, when the 0.8-micron : .
laser beam is directed at, say, a 0.6-micron hole, some ~
light is reflected back from the undisturbed reflective . :
area around the hole. This is important to understand in evaluating the data contained in Table 1.

Also, acceptable recordings may be defined as those ~ :
which give the smallest ~ 1 sigma distribution as a per- ~.
centage of the average relative contrast ratio. Thus we see that for prior art sample 2, the material is un-usable at 2.8 milliwatts since the distribution of 834 i5 actually larger than ~he relative contrast of 640. If we were to set an arbitrary recording sensi-tivity limit at the power level where, say, the sigma distribution is no more than 20~ of the relative;con-trast ratio, we could arrive at a means of comparinglaser recording sensitivities of the three prior art samples 1, 2 and 3 to that of sample 4. By this def~i-nition, sample 1 would have a minimum power requirement of about 2.2 milliwatts; sample 2 would have a minimum power requirement of more than 15.4 milliwatts; sample 3 would have a minimum power requirement of 1.7 milli-watts; and sample 4, using the method of the present invention, would have a minimum power requirement of 1 mllliwatt th--, it is the most sensiti.ve ~E the g-oup.

, ~ -~3~c~

In addition, sampIe 4 has a reflectivity of at Ieast 44~, which compares to 17%, 21% and 25.5~, respectively, for samples 1, 2 and 3. This is importan-t whenevex the reflected signal is used as in automatic :Eocusing and in maximizing the signal-to noise ratio. The re1ec-tivity of sample 4, produced by the method of the pres-ent invention, maintains adequate reflectivity over a significant range. The reflectivities of this sample were measured with a DR2J microreflectometer system manufactured by Gamma Scientific Inc.

Reflectivity Data: Sample 4 ,' .
Wavelength Reflectivi-ty Light Source , 514 nm 50% Spectral re~lectometer 514 nm 44% 0.8-micron laser beam 633 nm 60% ~ spectral reflectometer 633 nm ~2% HeNe laser 830 nm 36% Spectral reflectometer : ~ :
The light absorptivity of several samples were compared to determine whether sample 4 is more broadband in ab-sorptivity. Since sample 2 has a much lower recording sensitivity than the others, it was left out of the comparison. For visible measurements, a Macbeth densi-tometer, Model~TR 527 was used; and for the near in-frared absorptivity measurements, a Beckman DR-2 spectro-photometer was used. The results were as follows:
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Optical Densities Wav~length Pr or Art _ _ Present Invention or color~ le 1 Sample 3 Same~e 4 Blue 4.65 4.8 3.44 Green 3.89 4.25 2.77 Red i.36 2.83 1.92 780 nm 1.04 1.11 1.77 830 nm i.o4 1.05 1.51 900 nm 1.03 1.04 1.51 ~
:
The absorptivity characteristics of sample 4 are clearly more broadband; that is, more uniform over the spectral area of interest. Over the spectrum shown, the optical densities of samples l and 3 vary by about a factor of

4 compared to a fac-tor of 2 for sample 4. Also, the optical density in the near infrared is considerably higher for sample 4 than samples 1 and 3, makin~ it more suitable for recording with a near in~rared semiconduc- ;
tor diode laser, which typical~ly operates at 780~nano-meters and 830 nanometers.

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Claims (23)

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

1. A negative photographic process for making a reflec-tive optical data storage medium comprising areawise exposing a silver-halide emulsion photosensitive medium to a uniform non-saturating level of actinic radiation, de-veloping said emulsion to a gray color having an optical density to red light of between 0.05 and 2.0, the optical density being directly related to the exposure while leaving both exposed, developed silver and unexposed, un-developed silver in place, forming an areawise layer of silver precipitating nuclei on the surface of said devel-oped emulsion at sites of undeveloped silver halide, con-tacting said developed and nucleated emulsion with a monobath comprising a silver-halide solvent and a silver reducing agent, whereby unexposed and undeveloped silver halide forms soluble silver complexes and is transported by diffusion transfer to said precipitating nuclei where said silver complexes are reduced to metallic silver leaving exposed developed silver in place, thereby pro-ducing a layer of reflective metallic primarily non-filamentary silver over a grey layer of primarily filamen-tary silver.

2. The method of claim 1 wherein said exposure of said photosensitive medium is carried out by directing uniform intensity actinic radiation over the surface of said photosensitive medium.

3. The method of claim 1 wherein said exposure of said photosensitive medium comprises creating a pattern of two or more levels of non-saturating actinic radiation over the surface of said photosensitive medium.

4. The method of claim 1, wherein said forming of an areawise layer of silver precipitating nuclei is carried out by contacting a surface of said photosensitive silver-halide emulsion with a fogging agent.

5. The method of claim 1 wherein said fogging agent comprises a solution of borohydride anion.

6. The method of claim 1, wherein said developing of said gray color is to an optical density between 0.1 and 0.8.

7. The method of claim 1, wherein said areawise layer of silver precipitating nuclei is formed by providing a silver precipitating layer at the air-emulsion interface of said silver-halide emulsion.

8. A negative photographic process for making a reflec-tive optical data storage medium comprising areawise ex-posing a silver-halide emulsion photosensitive medium to a uniform non-saturating level of actinic radiation, devel-oping the exposed silver hallde of said exposed medium to low-reflective filamentary gray silver, while leaving both exposed, developed grey silver and unexposed, undeveloped silver halide in place, forming a layer of silver precipi-tating nuclei at sites of undeveloped silver-halide at the surface of the gray medium, dissolving unexposed silver halide in said medium to. form soluble silver ion complexes while leaving developed grey silver in place, transporting by sil,ver diffusion transfer soluble silver ion complexes to said precipitating nuclei and said developed filamen-tary silver and reducing and precipitating said soluble silver complexes to reflective metallic silver at said nuclei and said developed filamentary silver in a reflec-tive metallic layer of primarily non-filamentary silver less than one micron thick over a low-reflective grey layer of primarily filamentary silver.

9. The method of claim 8 wherein said exposure of said photosensitive medium is carried out by. directing uniform intensity actinic radiation over the surface of said photosensitive medium.

10. The method of claim 8 wherein said exposure of said photosensitive medium comprises creating alternating pat-terns of higher and lower non-saturating levels of actinic radiation over the surface of said photosensitive medium.

11. The method of any one of claims 8-10, wherein said silver precipitating nuclei is formed by creating a depth-wise decreasing concentration in said reduced medium.

12. The method of any one of claims 8-10, wherein said silver precipitating nuclei are formed by contacting said reduced medium with a solution of a borohydride anion.

13. A negative photographic process for making a reflec-tive optical data storage medium comprising areawise exposing a silver-halide emulsion photosensitive medium to a moderate exposure of actinic radiation through a mask having at least two levels of optical density to the ac-tinic radiation, where said mask is disposed between the source of said actinic radiation and said photosensitive medium, exposing the entire silver-halide photosensitive emulsion to a weak to moderate exposure of actinic radia-tion, developing said exposed emulsion to create low-reflective, grey filamentary silver while leaving both exposed, developed grey silver and unexposed undeveloped silver halide in place, nucleating a surface layer of un-exposed silver halide in said developed medium whereby silver precipitating nuclei are created in said surface layer, and contacting said nucleated emulsion with a mono-bath comprising a silver-halide solvent and a silver reducing agent leaving developed grey silver in place, whereby unexposed and undeveloped silver halide forms soluble silver ion complexes and is transported by diffu-sion transfer to said nuclei where silver is reduced to reflective metallic silver on said nuclei and to. said filamentary silver where silver is reduced to metallic silver on said filamentary silver thereby forming a reflective metallic layer of primarily non-filamentary silver over a low-reflective grey layer of primarily filamentary silver.

14. The method of claim 13 wherein said nucleating step comprises forming silver precipitating nuclei at the surface of said. developed medium.

15. The method of claim 13 or 14, wherein said nucleating step comprises. contacting said developed medium with a solution of a borohydride anion.

16. A reflective data recording medium comprising, a colloid matrix underlayer disposed on a substrate, said underlayer having primarily filamentary silver particles therein having an optical density to red light between 0.2 and 3.0, and absorptive of visible and infrared light, and a reflective surface layer primarily of non-filamen-tary individual silver particles in said colloid matrix, said reflective surface layer. disposed atop said under-layer, having maximum sliver particle dimensions pri-marily under .05 microns, some of which are aggregated with similar particles, and having a volume concentration of silver particles greater in said surface layer than in said colloid matrix underlayer, said reflective surface layer having at least one area having substantially uni-form reflectivity said uniform reflectivity being between 10% and 75%.

17. The data recording medium of claim 16 wherein the volume concentration of silver in said reflective surface layer exceeds the lowest silver volume concentration within said colloid matrix underlayer by a ratio of at least 3:1.

18. The data recording medium of claim 16 wherein said reflective surface layer is less than one micron thick.

19. The data recording medium of claim 16 wherein said volume concentration of silver particles in said reflec-tive layer is a minimum of 20% and a maximum of 50%.

20. The data recording medium of claim 16, wherein said colloid matrix layer is photographic gelatin of the type used in the manufacture of silver-halide emulsions.

21. The data recording medium of claim 16, wherein said reflective surface layer comprises primarily non-filamen-tary silver particles and at least 1% filamentary silver particles of the total silver volume concentration in said reflective layer.

22. The data recording medium of claim 16, wherein the thickness of the colloid matrix layer is less than 15 microns.

23. A data recording medium of claim 16, wherein said reflective surface layer is electrically non-conductive.