CA1135850A

CA1135850A - Information-recording element having a dye-containing auxiliary layer

Info

Publication number: CA1135850A
Application number: CA000312268A
Authority: CA
Inventors: Marinus R.J. De Bont; Petrus J. Kivits; Cornelis J. Schoot; Pieter Zalm
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-09-29
Filing date: 1978-09-28
Publication date: 1982-11-16
Also published as: DE2841179C2; IT7828101A0; FR2404888B1; FR2404888A1; IT1099607B; DE2841179A1; AT358295B; AU516911B2; ES473697A1; AU4010778A; GB2005457B; JPS6338773B2; ATA705778A; GB2005457A; NL7809159A; US4230939A; JPS5489605A

Abstract

ABSTRACT:
"Information recording element having a dye-containing auxiliary layer"
The invention provides an information-recor-ding element in which information can be written and read optically. The element is constructed from a transparent substrate, a laser light-absorbing, dye-containing auxi-liary layer provided thereon, as well as a laser light-reflecting recording layer present on the auxiliary layer.
Upon recording information the element is exposed to pul-sated laser light via the substrate, holes being formed in the recording layer. The auxiliary layer stimulates the formation of holes, a saving of laser light energy being obtained. In a favourable embodiment the auxiliary layer has a laser light absorption of from 20 to 80% and a maxi-mum thickness of 2500 .ANG.. In a further favourable embodi-ment the auxiliary layer also comprises an endothermal material, for example, nitrocellulose having a nitrogen content of at least 11 %.

Description

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. 12-9-1978 1.~ . PHN 9226 "Information-recording element having a dye-containing auxiliary layer"

The invention rela-tes to an information re-j ` cording element iIl which information can be written and .i .read optically, which comprises a transparent substrate, an auxiliary layer provided on the substrate, as well as an information-recording layer which i.s provided on the : auxiliary layer and has a reflective po~er with respect to a laser light beam used for writing or reading.
Such an information-recording element is dis-closed in German Auslegeschrift 2,~ ,678. Accordi.ng to ~igure 2B of the Auslegeschrift, the element is exposed to laser light via the substrate, which light is modulated in accordance with t~le information to be written. As a result of the e~posure, substantially circular holes hav-ing a very small diameter are formed in the information-;~ 15 recording layer which will hereinafter be briefly termed recording layer. The holes are formed in that in the ex-posed places the material of the recording layer melts as.
a result of absorption of the amplitude~modulated laser light and the molten material retracts by surface phenomena in an edge-like manner, an aperture or hole being formed.
The recor~i.ng element disclosed in the above-mentioned . Auslegeschrift is constructed from a flexlble transparent polyester substrate, an anti-reflection layer of Bi2S3, ~` As2S3 or Se provided thc-reon, and a metalli.c recording 25 layer of Bi, Ind or Sn provided on the anti-reflection , . . ~ -, ~ .
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2 PHN. 9226 layer. By using an anti reflection layer the percentage of laser light which is absorbed by the recording layer is increased and the overall amount of laser energy which is necessary to form a hole of the desired dimensions in the recording layer is reduced.
If an anti-reflection layer is used it should be borne in mind that not any anti-reflection material is suitable. There is a significant coherence between the properties of anti-reflection layer, recording layer and substrate layer. For example, the above-mentioned Auslege-schrift mentions three discrete anti-reflection materials in combination with three discrete recording materials. In this connection reference may also be made to French Patent Application 2,344,920 - Spong - published on October 14, 1977, which discloses an information-recording element which is constructed from a substrate and a thin layer of a strongly reflecting material provided thereon, for exam- ~
ple, a vapour-deposited aluminium layer, which in turn is ~;
coated with a strongly light-absorbing material, for example, a vapour-deposited organic dye. The element is exposed to pulsatory laser light on the side of the ab-sorption layer, holes being formed in the absorption layer by evaporation of the absorption layer in the exposed ~`
places. Such a thickness of the absorption layer is prefer-ably chosen that the ~hole system is brought in an anti-reflex condition so that the energy loss by re1ection is minimized. On pages 11 and 12 of the above-mentio~ed French Patent Application the coefficient of reflection of such a system is computed on the basis of formulae in which

3~ inter alia the comple~ Fresnells reflection coefficients of the various layers as well as the thicknesses thereof are used as parameters.
The use of an anti-reflection layer according to the above-mentioned Auslegeschrift has the disadvantage that the choice of anti-reflection layer and recording layer is restricted while in addition the anti-reflection ;~
layer must be provided accurately, that is to say with small variations in thickness.
' 3 PHN. 9226.

It is the object of the invention to provide an information-recording element in which the amount of laser light energy required for writing information in the recording layer is reduced by using an auxiliary layer and in which an ample choice of material of both the auxiliary layer and the recording layer is possible. This object is achieved with an information-recording element of the kind mentioned in the preamble which is characterized in that the auxiliary layer is a laser light-absorbing dye-contain-ing layer.
In contrast with the device described in theabove-mentioned French Patent Application 2,344,920,the light-absorbing dye-containing layer in the information-recording element according to the invention is enclosed ~ ' between substrate and recording layer. The light-absorbing layer in the element according to the invention is an auxi-liary layer or activating layer which stimulates the for-mation of holes in the radiation-reflecting recording layer. No holes are formed in the radiation-reflecting ~ ~
20 layer of the known device. This layer may be considered as ;~ -an auxiliary layer for the radiation-absorbing recording -layer. As compared with the device known from the above-mentioned French Patent Application, the information-recording element embodying the invention presents the ad-vantage that the information can be written and read by exposure via the substrate. Herewith it is achieved that the defects, for example, scratches, pits, humps and dust particlesl present on or in the surface of the substrate fall beyond the depth of focus of the objective which 30 focuses the laser light on the recording layer so that the ;~
defects do not adversely influence the quality of the in~
formation to be written and read and in particular do not cause any noise during reading information. The defects have the same or larger dimensions than the holes (image points) formed in the recording layer which have a diameter of ap~
proximately ~.5 /um to l /um.
In connection with the above-men-tioned depth ~`
of focus the transparent substrate has a minimum thickness ?
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of approximately 300/um and preferably a thickJless of approximately 1 mm so as to obtain a ~sufficient mechanical rigidity. The substrate is manufactured from a transparent synthetic resin, for example, polymethylmethacrylate, polycarbonate, polysulphone, PVC or from glass.
Upon writing or recording information a laser light beam which is pulsated in accordance with the infor-mation and has a pulse time of, for example, 5 x 10 7 sec.
is incident via the substrate. The laser light beam passes 10 the substrate~ traverses the auxiliary layer~ in which a part of the laser light is absorbed dependen-t on the com-position of the auxiliary layer, is then reflected against the recording layer~ in which a part of the laser light is absorbed also dependent on the absorption-reflection cha-15 racteristic of the recording layer. The reflected lightagain passes the auxiliary layer~ is partly absorbed and ultimately leaves the element on the side of the substrate.
~s a resul-t of the absorption of energy in the auxiliary Layer and in the recording layer, the tenlperature of said 20 layers will increase in which finally the recording layer will melt in the exposed places and a hole will ~a formed ; in the recording layer as well as in the auxiliary layer as a result of.decomposition and evaporation of the auxi-liary layer in the exposed places.
In a preferred form of the element in accor-dance with the invention an auxiliary layer is used which has a laser light absorption of from 20 to 80 ~ and a maxi-mum thickness of 2500 ~.
A very suitable layer thickness of the auxi-- 30 iary layer is from 300 to 1000 ~.
The element according to the invention per-mits of obtaining a significant saving of laser energry of at least 20 %. With comparatively thick auxili~ry layers the enargy savlng is smaller than when comparatively thin 35 auxiliary layers are used due to the necessary heating of the auxiliary layer by the laser light. The sa~-ing is also smaller wherl auxiliary layers having a comparatively low absorp-t:ion oP laser light are used than when auxiliary ' ' ,, '. .," ,', . ' r~
, 12-9-1978 ~ PIIN 9226 layers are used which show a comparatively high absorptlon of laser light.
The absorption of the auxiliary lay0r can be calculated by means of Lamber-t's Beer's law which, al-though applying to solutions of dyes, permits of neverthe-less obtaining a good approach of the solid layer absorp-tion of the auxiliary layer in the element embodying the invention. According to this law the optical density (OD) ;is equal to the product of the molecular extinction co-10 efficient ( ), -the dye concentration (c) expressed in mol/l and the thickness (d) expressed in cm.
In a further favourable embodiment -the auxi liary layer has a laser ligh-t absorption of from 20 to 50%.
This embodiment is not so much directed to 15 obtaining a maximum saving of laser light, but does present the interesting possibility of reading the element embocly-ing the invelltion via the substra-t~ in reflection. ~s al-ready said9 the element embodying the invention can be read via the substrate In this case ther^- exist two possibili-ties, namely reading in transmission and in reflection.
When reading in transmission, the scanning laser light beam ;which of course is much less energy-rich than the recording laser light beam and, for example, is a factor 10 weaker, - passes the substrate and -the holes presen-t in the auxiliary ;
layer and in the recording layer and is received on the side remote from the substrate and processed in the play-back (reading) apparatus. When reading in reflection, the scanning beam passes the substrate and the parts present of the auxiliary layer~ is reflected against the recording 30 layer~ again passes the auxiliary layer and the substrate and is received on the side of the substrate and ~urther processed.
`The advantage of reading in reflection is that fewer optical elements, for example objectives~ are required in the recording apparatus because the reflected beam tra^-verses for a large part -the same light path but then in the reverse direction as the scanning beam. A second advantage is that errors as a result of vibrations in the optical .

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system~ for example objective vibrations, are partlr compensated for.
In the above-mentioned favourable embodiment, sufficient laser light is reflected for permitting being processed in the optical system. Generally, a-t least 20%
and preferably at least 3O~o of thé amount of light of the scanning beam is required for this purpose. In a pre~erred ~orm the auxiliary layer used in the element embodying the invention comprises an organic dye or a solution or d:is-10 persion of an organic dye in a polymeric binder.
Such an auxiliary layer has a very low thermalconductivi~y so that the thermal energy formed in the auxi-liary layer as a result of light absorption also is op-timally in ~avour of the heating of the layer in the exposed 15 places. The flowing away of thermal energy to the non-exposed places and to the atmosphere is suppressed. In this connection it is to be noted that the anti-reflection layer of inorganic material according to the above-described Ger-man Auslegeschrift 2,514,678 has a considerably higher 20 thermal conductivity.
- The organic dye must preferably show an ab-sorption maximum at the wavelength of the laser light used and must also have a high molar extinc-tiorl coefficient, so that the desired absorption can be realized in a thin layer.
Suitable dyes are, for example, rhodamines such as Thodamin-B and Rhodamine-6G, xanthenes, for exam-ple Pyromine-B and Pyromine-G, acridines, for example Acri~
fIavine, bisphenylmethane dyes, for example, Auramine and ; Michler's Hydrol Blue, triphenylethane dyes, for example, 30 Malachit Green and Crystal Violet, as well as cyanine chromogenic dyes, for example monomethines, trimethines and pentarnethines.
Suitable polymeric binders are, for example, cellulose, polystyrene, polyester9 polether, polyacrylate, 35 polymethylacrylate, polycarbonate, polyamine, polyviny~-; alcoho:l, polyvinylchloride, and copolymers of vinylchloride/
vinylacetate. ~}l auxiliary layer ~Thich consists of dye can simp]y be provided by means of a vapour--deposition process.
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~ n auxiliary layer which contains a solution or dispersion of` the dye in a polymeric binder can be pro-vided on the substrate according to methods which are known se~ For example, a solution of the binder in a suitable solvent, in which solution a dye has also been dissolved or dispersed, can be provided on the substrate surface by m~ans of a pouring, spraying or centrifuging process an~
the solvent inay then be evaporated. A suitable organic solven-t is 3 for example, an alcohol, for example butanol, 10 glycol, methylglycol, an aromatic hydrocarbon, for example 9 benæene, -toluene, a ketone, for example acetone, cyclohexa-none, an ester, for exainple butyl acetate and furthermore tetrahydrofuran and dimethylformamide.
When a dispersion of dye in the polymeric bin-~5 der is used, the particle size of the dye may not be morethan approximately 200 ~. A centrifuging process is to be understood to mean a method in which a quantity of the liquid to be provided is disposed centrally on the substrate after which the liquid is uniformly distributed over the 20 whole surface by rotation of the substrate.
In a further preferred embodiment the auxili-ary ]ayer comprises a polymeric binder which is formed by a radiation-polymerized lacquer on the basis of radiation-sensitive monomers and/or oligomers in which the dye is 25 dissolved or finely diyided.
Very suitable are the UV light-polymerizable thin liquid mixtures of monoes-ters, diesters, triesters or tetraesters of acrylic acid. This applies in particular to .V. light-curable light-sensitive ]acquers containing 30 ~0-90% by weight of a monoacrylate, 5-50% by weight of a diacrylate, triacrylate, or tetraacrylate~ as well as o . 5-3% by weigh-t of a light-sensitive initiator.
A suitable light~sensitive initiator is a ~benzoin derivative, for example, benzoin isobutyl ether, 35 also l~nown commercially as Vicure.
Examples of suitable acrylic acid esters are monoacrylates, for example, alkyl acryla-tes, alkoxy alkyl acrylates, phenoxy alcyl acrylates and phenyl acrylates, ' . ~ '` . ~ `' '.' . : : i.. ,' ` "'. ' ' ' ' `

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8 PHN. 9226, for example, ethyl acrylate, n-butyl acrylate, hexyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, octadecyl acrylate, ethoxyethyl acrylate and phenoxyethyl acrylate;
diacrylates, for example, alcane-diol diacrylates and alkene glycol diacrylates, for example, 1,3-propanediol diacrylate, diethyleneglycol diacrylate and tetraethylene-glycol diacrylate; triacrylates, for example, trimethylol-propane triacrylate and pentaerytritol triacrylate; tetra-acrylates, for example, pentaerytriol tetraacrylate as well as oligomeric acrylic acid esters, for example, polyethyl-ene glycol acrylate and epoxy acrylate.
Also readily useful lacquers are those con-taining a mixture of a radiation-linkable polyene compound which contains per molecule two or several ethylene groups 15 ~-CH=CH-) and a polythiol compound which contains per ;
molecule two or more thiol groups ~-SH).
Polythiol compounds and polyene compounds are sufficiently known from literature and are commercially available. Reference may be made, for example, to the` ~
United States Patent Specifications 3,697,395, 3,697,396, ~`
3,697,397 and 4,008,341. ~`
As regards specific lacquer compositions, re-ference may be made to Canadian Patent 1,111,194 - October 20, 1981 (PHN ~576) and Canadian Patent Application -~
325,476 filed April 12, 1979 (PHN 9103) in the name of Applicants.
The advantage of an auxiliary layer containing radiation-polymerized substances is that the layer can be provided on the substrate surface by means of a matrix so tha~ the substrate-remote surface of the auxiliary layer is particularly readily defined and consequently the recor- ~ `~
ding layer pro~ided on this surface also has a defined structure. It will be obvious that a readily defined "in~
terface" between auxiliary layer and recording layer, which interface falls within the depth of focus of the objective focusing the laser light during writing or reading infor-mation, is of importance to obtain a good information ~uality.
More particularly, the auxiliary layer is . .,~, . , .:i ~,~'^ , ~, , . . . : .

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., , pro~ided by providing a matrix surface with a layer of radiatlon-curable lacquer in whlch a dye is incorporated, providing a substrate on the layer of lacquer, radiation-curing the lacquer layer via the subs-trate or v:ia the matrix, and removing from the matrix the assem~ly of sub-rstra-te and cured dye-containing lacquer layer connected thereto When exposure is carried out via the matrix, the latter should be transparent to the radiation used 9 10 for example UV light, and be manufactured, for example, from glass. A very suitable non-transparent matrix is a nickel matrix, the surface of which has been polished to be very smooth In a par-ticularly interesting embodiment oP
15 the element according to the invention the auxiliary layer also comprises an endothermal material. Upon exposure to laser light, as soon as the decomposition temperature of the endothermal material in the auxiliary layer has been reached, said material will decompose while giving off 2Q energy. The chernical energy of the endothermal material is converted, via the exothermal decomposition reaction9 into calori~ic energy (thermal energy) and potential energy ~pressure build-up). As a result of this, holes will be formed in the recording ~ayer and in the auxiliary layer in the exposed places. The overall energy required for recording~ the information, that i5 to sa~ for creating holes in the recording layer 9 originates only partly from the layer and is provided for the remaining part by the exo-thermal decomposition of the endothermal material present 30 in the auxiliary layer. Briefly, the writing eIlergy is partly provided by chemistry. This may result in an extra saving of laser energy of 50% or more. The required amount of laser energy is small and ;s, for example 1Q to 10 1 Joule per recorded image point, the image point having an 35 area of 1-3/um2.
The small amount of laser energy enables the use of lasers of comparatively low power, for example, solid-state lasers having a power of 1~10 m~. As compared , . ' ' ' . ' ' ' `
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with the usually used gas lasers, said lasers have much smaller'dimensions and a considerably simpler construction.
T'he solid-state lasers can hence be incorporated much more easily and ln addition are considerably cheaper.
The endo-thermal material preferably has an ' energy content released upon decomposition of at least 10 Joule per /um3. Materials having such an energy con-tent are, for example, compounds which show an explosive combustion or deflagration. These include inter alia 10 pyrotechnical compounds and types of powder. Such materials are suf'ficiently described in literature, for example, in "Chemie 1973" R.Meyer Weinheim ~erlag, so that those skilled in the art can easily make a choice on the basis of the desired energy content and, for example, the ignition 15 temperature. It should be borne in mind that when a materi-al having a very low ignition temperature is used, for example lower than 150-200C, the required laser energy becomes smaller, it is true, but on the other hand, due to the larger sensitivity of such a material, extra precautions 20 have to be taken in the processing thereof, which is a cost-price-increasing factor.
According -to Applicants~ good results are achieved if the endothermal material used is nitrocellulose having a nitrogen content of at least 11% or a mixture 25 hereof w:ith nitroglycerine.
A suitable rnix-ture contains f'rom 5 to 25 % by - we::ght of nitroglycerine calculated on the quantity of nitrocell~lose.' - Nitrocellulose or mixtures with nitroglycerine 30 are polymeric substances which ma-y serve as a binder for the dye present in the auxiliary layer. A suitable auxi-liary layer consists, for example, of 80% by weight of nitroce''lulose and 20% by weight od dye, for example, N,N'dioctadecyl rhodamine.
~5 A suitable endothermal material is f~lrthermore a picrate, for example, the pictric acid salt of the dye used in the auxiliary layer.
The image points formed in the recording la~er .
! are arranged in an information track in the form of a spiral or built up from concentric circles. The velocity of writ-ing is high. Per image point an exposure time is ~used which varies from 10 6 to 10 sec., for example, an ex-posure time of 5.10 7 sec.
The usual recording layers having a reflec-ting power with respect to laser light are used in the re-; cording element embodying the invention. For example, layers of metal or metal alloys may be used which general-10 ly have a large reflecting power and can be provided in a simple manner by vapour deposition. Examples hereof are layers of Bi, Te, Ind, Sn, Cu, Al, Pt, Au, Rh, As, Sb9 Ge, - Se, Ga.
Particularly suitable are metals having a high 15 reflection and a low melting point, for example, Cd, Sn, Tl, Ind, Bi and amalgams.
The thickness o r the recording layer is pre-ferably 100-1000 ~. If desired, the layer may be provided with a protective layer of lacquer.
The invention will now be described in greater detail with reference to the following embodiments and the accompanying drawing, in which ~ fig. 1 is a cross-sectionaf view of an infor-matlon-recording element prior -to exposure, and fig. 2 is a cross-sectional view of the ele-ment shown in fig. 1 after information has been recorded.
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Reference numeral 1 in Figure 1 denotes a lmm thick substrate of polymethylmethacrylate (plexiglass~
30 which is provided with a layer 2 of dye (Rhodamine 6G) dissolved in polyether. Layer 2 has been provided by mix-ing 10 ml of a 0.8% solution of the polyether i~ methanol with 50 ml of Rhodamine 6G, shaking -the mixture until all the dye has~been dissolved, and pouring the resulting 35 solu-tion, after filtration. on the substrate plate. The substrate plate 1 having the above-mentioned solution is rotated by m~ans of a centrifuge at a velocity of 1000 rpm., the dye solution being distribu-ted unirormly on the substra-. .

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te surface. After evaporating the solvent, the layer lor -~ilm) 2 is obtained which has a thickness o-f 1300 ~ and an optical density of o,56 a-t a wavelength of 530 n.m.
The dye~containing layer 2 is decolourized over a part of the surface by exposure to a xenon lamp.
The optical density in the decolourixed places is 0.005 at a wavelength of 530 nm.
A 400 ~ thick bismuth film 3 is vapour-depo--sitcd on the layer 2 and herewith the information-recording 10 element according to the invention as shown in Fig. 1 is obtained. It is to be noted that the above-mentioned de-colourization serves exclusively for -taking experimen-ts, as will be described hereinafter. Of course, normally no decolourized spots are present in the element embodying the 15 invention.
The above-described element is exposed to pulsating laser light ( ~ = 530 nm) v~a the substrate (pulse time 500 ns) in the direction denoted by arrows 4.
lt has been found that a laser Iight power of 4.2 mW was 20 necessary in the decolourized places to form a hole having a diameter of 1/um in the bismuth layer 3 and the under-lying dye layer 2. In the non-decolourixed places~ 352 mW
is necessary to make a hole of the same dimensions. An energy saving of 24 o/o is obtained by using a dye-con~ining 25 layer 2.
After the exposure, the situation shown in Fi~. 2 is reached. In F:ig. 2 the same reference numerals ; ~ as in Fig. 1 are used for the same components. Reference numeral 5 denotes the hole having a diameter ~f 1/um 30 formed in the bismuth layer 3 and auxiliary layer 2.
xample 2 ' In a corresponding manner as described in the above example, a 1 mm thick plate 1 (Fig~ 1) of polymethyl-methacrylate was provided on one side with an auxiliary 35 layer 2 containing Rhodamine 6G dissolved in pGlyether. The thic}clless of the auxiliary layer was 600 ~ with an op-tical - den~sity of 0.53. A part of the surface of the au~iliary layer was deco~ourixed by mealls of a xenon lamp. In the de-.

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colourized spots the optical density was 0.002 at a wave-length of 530 nm. By means of a vapour deposition process, a 400 ~ thick bismuth film 3 ~ig. 1) was provided on the auxiliary'layer and the resulting elernent was then exposed to pulsating laser light via the substrate. The pulse time was 500 ns. The ~avelength of the laser light was 530 nm.
It was found that in the decolourized places a laser po~er of 4.0 mW was required to ~orm a hole 5 (~ig. 2) of 1/um diameter in layers 2 and 3. 2.7 mW was necessary ~or the 10 coloured spots so that a savin~ Or 33% was obtained.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An information-recording element, in which in-formation can be written and read optically, which com-prises a transparent substrate, an auxiliary layer provi-ded on the substrate, as well as an information-recording layer which is provided on the auxiliary layer and has a reflecting power with respect to a laser light beam used for writing or reading, characterized in that the auxiliary layer is a laser light-absorbing dye-containing layer.

2. An information-recording element as claimed in Claim 1, characterized in that the auxiliary layer has a laser light absorption of from 20 to 80% and a maximum thickness of 2500 .ANG..

3. An information-recording element as claimed in Claim 2, characterized in that the auxiliary layer has a laser light absorption of from 20 to 50%.

4. An information-recording element as claimed in Claim 1, characterized in that the auxiliary layer com-prises an organic dye or a solution or dispersion of an or-ganic dye in a polymeric binder.

5. An information-recording element as claimed in Claim 4, characterized in that polymeric binder is for-med by a radiation polymerized lacquer on the basis of radiation-sensitive monomers and/or oligomers in which a dye is dissolved or finely divided.

6. An information-recording element as claimed in Claim 19 characterized in that the auxiliary layer also comprises an endothermal material.