CA1159307A

CA1159307A - Thermo-optical data writing process and data medium for performing this process

Info

Publication number: CA1159307A
Application number: CA000369013A
Authority: CA
Inventors: Jean Cornet
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1980-01-23
Filing date: 1981-01-21
Publication date: 1983-12-27
Also published as: EP0033430B1; FR2474222A1; JPS56127937A; DE3065566D1; EP0033430A1; FR2474222B1; US4577291A; US4404656A

Abstract

ABSTRACT OF THE DISCLOSURE
THERMO-OPTICAL DATA WRITING PROCESS AND
DATA MEDIUM FOR PERFORMING THIS PROCESS
The invention relates to thermo-optical inscription or writing on a data support comprising an organic intermdiate layer (2) covered by a metal layer (3).
The invention relates to a writing process consisting of creating in the metal layer 3 a plastic deformation as a result of the gaseous pressure exerted due to the local thermal degradation of intermediate layer 2. Inscription produces a bulge-like impression without perforating the metal layer (3).
The invention more particularly applies to the thermo-optical inscription and storage of data on an optically readable support by phase or amplitude contrast.
Figure 4.

Description

` ~5930'7 BACKGROUND OF THE INVENTION
The present invention relates to the thermo-optical recording of data which are to be optically read. It more particularly aims at a thermo-optical writing or inscription process using a low power laser and a thermosensitive support enabling on the one hand the reading of the data immediately following their recording without any intermediate development stage and on the other hand the copying of the data by an overall, rapid method without deterioration of the original etching or any modification of the etching character-istics during the copying process.
The structures proposed for writing data with low power lasers (typically below 15 mW) generally operate by thermal ablation. This is, for example, the case with layers of tellurium, bismuth, or vitreous chalcogenide alloy and even certain metal layers deposited on thermally degradable organic intermediate layers. The data are stored in the form of holes in said structures and during reading said holes create an amplitude contrast making it possible to detect the data. ~Iowever, the data cannot be copied in an overall, simple and fast manner, for example by means of a polym-erization process which forces a liquid to enter the cavity of the relief, followed by the hardening thereof by an appropriate means (thermal or photographic). This duplication method transforms the amplitude contrast of the original into phase contrast, i.e. the copies cannot be read with the same reader as that used for reading the original. ~ photographic or holographic method must bq used to ensure that the copy retains an amplitude c~ntrast. ~n this case the photographic mekhod has serious disadvantages in that it requires an excellent contact between the emu~sion and the recorded sur~ace of the original, which is generally di~icult -to achieve : ' ' ' . ' , :
'- : .

~S~31307 over the entire area of the supports to be brought into contact due to dust. It is also necessary to work in a darkroom with top quality optical instruments, whilst using emulsions with a very high resolution. However, the holographic method requires cumbersome equipment.
Other structures have been developed by the Applicant having the special feature of providing better impression characteristics, particularly from the writing sensitivity standpoint and the reading signal to noise ratio than the structures operating by thermal ablation. These structures are constituted by a ductile metal layer covering a highly expansible organic layer, The light absorption by the metal layer during impact of the laser beam leads to the heating of the organic layer which reacts to this heating by a powerful local thermal expansion without change of state. In turn this ex-pansion creates an unreversible stretching in the metal layer, exceeding the elastic limit, but remaining this side of the elongation at break. The main disadvantage of this type of structure is that the considerable thermal expansion of the organic layer is generally obtained by adding a large quantity of plasticizer to the polymer. However, the plasticizer is a liquid which behaves like a solvent and consequently leads to a drop in the mechanical properties of the polymer, particu-larly its hardness and its softening point. As a result particular care must be taken during the subsequent deposition of the metal layer to ensure that no mechani-cal stress is introduced into the organic layer which, on relaxincJ, may im~air the morphology of the structure, both bq~ore and af-ter ~torage o the data. ThereEore khq metallic layqr must be deposited at a very low speed, bq perEectly stainless, have a limited hardness, a limited mechanical rigidity and limited adhesion to the organi.c layer~ As a rqsult the data medium is .. , , ~ , ...
.. . . ~ ."..

.
, ., .:::

31~7 difficult to manipulate without special precautions. In addition, the deposition of a random protective layer ~thermosetting or photo-hardenable resin, varnish, etc.) is not possible, because it systematically leads to stresses which may denature the recorded impression.
The only acceptable protection system is the use of a cap leaving a free space abo~e the sensitive layer.
However, this cap virtually doubles the cost of the data medium. Finally there is no possibility of using direct duplication by polymerizable liquid or contact photogra-phy for producing copies ~rom such a fragile structure.
BRIEF SUMMARY OF THE IN~ENTION
_ .... _ _ ...
The invention relates to a thermo-optical da-ta writing process consisting of the surface heating by means of an intensity-modulated, focused radiation beam of a thermosensitive data medium having at least one metal layer covering an organic intermediate layer, wherein the thermal energy given off in the metal layer by the impact of the beam causes gas evolution by local degradation of the organic intermediate layer, this gas evolution and the associated loss of adhesion lead to stretching in the metal layer beyond the elastic limit and on this side of the elongation at break of the ductile material constituting said metal layer~
The invention als-o relates to a data medium incorporating an organic intermediate layer covered by at least one metal layer wherein the localized heating of the metal layer in the impact area of a focused radi-ation beam produces a gas evolution by local degradation o~ the organic intermedia-te layer, this gas evolution and -the resul-ting loss Oe adhesion bring about a plas-tic s-trqtching o~ the metal layer which is beyond the elastic limi-t and on this side of ~he elongation at break oE the ductile material conskituting the metal layer.

~ ~1 5~3a7 _RIEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereina~ter relative to non-limitative embodiments and the attached drawings, wherein show: Fig. 1 a thermosensitive data medium and a focused, optical writing beam.
Fig. 2 a deformation of the medium of Fig. 1 during the writing phase.
Fig. 3 the relief obtained on the sur~ace of the medium of Fig. l in the case of an inscription made without perforation or change of state.
Fig. 4 the relief obtained on the surface of the medium of Fig. l in the case of an inscription performed according to the invention.
Figs. 5 and 6 - explanatory diagrams.
Fig. 7 a first variant of the data medium according to the invention.
Fig. 8 a data medium reinforced by an electrolytic deposit according to the invention. 0 Fig. 9 the aspect of the metal protection of Fig. 8 after renewal of the elements o~ the radiation-transparent medium.
Fig. 10 the duplication process applicable to the recorded medium according to the invention. 5 Fig. ll the protection by means of a cap of the data medium according to the invention.
Fig. 12 another variant of the data medium according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
3~ Fig. l i~ a sectional view o~ a data medium whlch can be ~hermo~optically inscribed by means of the radia-ted energy containecl in an inscription or writing beam ~ ~ocused by a lens 5. The da~,a to be wri-t~en is generally in thq ~orm o~ an angularly modulated electri-aal signal. ~his signal modulates th~ intensi-ty o~ beam '`', : ','" '`' ~ ' , ", :'' .,~ ':`, ,' ~5~307 4, for example by means of an electrically controlled optical modulator not shown in Fig. 1. The radiated energy is produced by a coherent light source in order to obtain a very small spot by focusing on the surface of the data medium. When the spot moves on the surface of the medium in a direction perpendicular to the plane of Fig. 1 a relief impression is formed and has the con-figuration of a succession of protuberances of non- `
uniform length and spacing. This relief materializes a track, whose phase contrast can be optically read.
There can be an amplitude contrast when the relief obtained leads to a thickness reduction of the structure of the medium, because this mechanical effect resulting from a stretching of the thin layer leads to a variation in the transmittance or optical reflectance.
The thermosensitive medium of Fig. 1 comprises a substrate 1 covered by an intermediate layer 2 of organic material, which is in turn covered by a metal layer 3.
Under the impact of the writing beam 4, the metal layer 3 is locally heated due to the absorption of the radiated energy and transfers this heating to the intermediate layer 2. Fig. 2 shows the appearance of the data medium surEace as a result of the heatiny of the intermediate layer 2. When this heating causes no change of state either in layer 3 or in intermediate layer 2, it is possible to obtain a permanent impression after the passage o~ the writing beam 4 on the basis of an expansion of the bulge or swelling 7 of intermediate layer 2, which causes a stretching ~ in layer 3 which is beyond the elastic limit. The residual deEormation ob~ained is illu~krated in Fig. 3. The relief im-pression has in its centre a bulge 7 and the latter is linked wi~h the non de~ormed portions of layer 3 by a small annular deprq5si.0n resulting from the rebalancin~

` ' ~lS5~3()7 of stresses. It can be seen in Fig. 3 that the organic material of the intermediate layer 2 remains in contact with layer 3. The deformation without change of state descrihed hereinbefore is compatible with a low writing beam power and makes it possible to obtain an impression which can be easily read when using a highly plasticized ~`
polymer in intermediate layer 2. To increase the perma-nent deformation, bearing in mind the limited adhesion of layer 3 to intermediate layer 2, the metal used has a limited hardness. As a result the data medium produced in this way is completely devoid of mechanical strength.
Moreover, the deposition of layer 3 on intermediate layer 2 must be performed at a slow speed so as to prevent any stress liable to cause folds. The structure using a soft layer deposited on a highly plasticized polymer intermediate layer undergoes perforation on reaching the power threshold ~ at which the intermedi-ate layer is degraded. The perforation of layer 3 leads to the appearance of an edge with difficulty reproduc-ible contours, which reduces the signal to noise ratioof the read-back signal of the impression.
According to the invention for obtaining a relief impression without perforation of layer 3 and which has better mechanical strength, a metal is chosen which has a better adhesion to intermediate layer 2 and an unplasticized or only slightly plasticized polymer material is adopted for the latter. Stretching beyond the elastic limit of layer 3 is largely due to the for-mation oE a gas bubhle ob~ained by local degradation of ~hq intermedia~e layqr 2. The impression pxod~ced in this way is shown in Pig~ ~, where it is possibl~ to see that layer 3 has disengaged from inte~mediat~ layer 2 due ~o the local degradati4n o~ layer ~ with the Eor-ma-tion o~ ara~er 6.
Th~ diagram of Fi~ 5 illustrates the response ~ ~ 5Sa 307 characteristics of the two types of data medium referred to hereinbefore. They differ by their sensitivities to the inscription radiation and by their mechanical strength values. The writing beam power E is plotted on the abscissa and the signal to noise ratio which can be obtained by readiny the impression on the ordinate. In the case of a data medium constituted by a highly plas-ticized polymer intermediate layer 2 covered by a rela-tively soft, moderately adhering, precious metal layer 3, the response characteristic 8 is obtained. On this side of the power threshold EV leading to a degradation of intermediate layer 2, i.e. to the left of dotted line 9, it is clear that an easily readable impression can be obtained with a limited inscription power. Beyond the degradation threshold EV the characteristic 8 is bent, because the degradation of the intermediate layer 2 leads to the inevitable perforation of metal layer 3.
The response characteristic 10 relates to a data medium according to the invention having a reduced sensitivity to the inscription, which has a better mechanical strength. This support has an intermediate layer 2 which is unplasticized or only slightly plasti-cized and a metal layer 3, which is harder and more adhesive due to the lncorporation of an appropriate element. The part of characteristic 10 to the left of line 9 indicates a low reading signal to noise ratio, because the relief of the impression is much more marked than in the case of the data medium to which character-istic 8 refers. This results from the lower expansion coefficient oE the polymer and the raising of the qlastic limit of thQ mckal layer 3, Fig. 6 makes it possible to compare the elongation-tqnsion curves o~ metals consti-tuting layer 3 o~ the two types oE daka medium having respectively -the \- ~
~L5g3137 responsecharacteristics 8 and 10 of Fig. 5. The pro-portional elongation is plotted on the abscissa of Fig.
6, whilst the mechanical tension ~ is plotted on the ordinate.
Curve 11 relates to a soft metal layer pro-duced from a precious alloy without adjuvant. A sig-nificant stretching beyond the,elastic limit can be obtained with a mechanical tension ~A. At A the dis- -~
charge line 13 encounters the zero tenslon axis, so that a significant impression relief is ensured because the permanent elongation OA is considerable. Such a stretch-ing justifies the good signa] to noise ratio obtained, despite the lack of degradation of intermediate layer 2 in the case of characteristic 8 of Fig. 5.
Curve 12 relates to a much harder metal layer 3. The same mechanical tension ~A only just exceeds the higher elastic limit and the discharge line 14 shows that plastic stretching has given rise to a limited permanent elongation OB. This situation corresponds to the part of characteristic 10 in Fig. 5 to the left of line 9. According to the invention use is made of that part of characteristic 10 to the right of line 9. The degradation of the intermediate layer 2 leads to the local disengagement of the metal layer 3 and the volume of gas given off leads to a supplementary plastic stretchi,ng, which may remain on this side of the elon-gation at break OD of the metal layer. Thus, along the discharge line 15 there is a permanent elongation OC
guaranteeing an excellent reading signal to noise ratio, as illustra-ted by -the part of ~he characteristic 10 to the right oE line 9.
~ h~ diagrams oE E'igs~ 5 and 6 show that by utiliæing a plastic stretching without perforakion of the metal layer 3, the proaed~re is different level with 35 intermediate layer 2 because the latter c~uses s~retch- ;

. . : , .
, ..

~S930~
g ing either by expansion without change of state, or by local thermal degradation supplying most of the defor-mation work of the metal layer.
According to the writing process according to the invention the impression of Fig. 4 is obtained !
whose significant relief is produced due to the thermal degradation of the organic intermediate layer 2 and its maintainance corresponds to plastic stretching without perforation o~ the metal layer 3.
The data medium suitable Eor performing the writing process has an intermediate layer 2 of a ther-mally degradable organic material with a thickness of approximately 10 to 300 nm. Layer 2 is covered with a thin metal layer 3 with a thickness of 5 to 1~ nm. Sub-strate 1 can be of a random nature, i.e metallic, vitreous or organic, opaque or transparent, f~exible or rigid and of a random thickness. As a non-limitative example substrate 1 is transparent and rigid and has a thickness between 1 and 1.5 nm. The organic intermedi-ate layer 2 is preferably chosen with a low degradationtemperature, a high hardness and a limited compressi-bility in such a way that the deformation resulting from the degradation of the polymer remains confined to the laser beam impact area. The metal layer 3 fulfils several functions It serves to absorb the incident radiatlon for converting it into thermal energy. Under the impact oE the laser it must also confine the bubb:Le created by the degradation of the polymer, which implies khat it is completely continuous (without cracks, crakers, eta.) and musk adhere ~irmly to the or~anic intexmcdiate layer, Fina~ly it must have high ductility and hardness, so that i;t undergoes a siyni~iaan-t de~or~
mation under the pressure created by the ya~ bubble, without there being any typ~ o~ ~raature or ~erforation.
In view o~ ~he Eact tha~ the de:Eorma-tion can bc oE a .. . .

. . . .... . . . ~ . :
,, ~

~5~3~7 very considerable level (up to approxi~ately 300 nm) without the formation of any hole or bulge, a very good signal to noise ratio is obtained during read-out. The writing process by degradation can be used on almost all thermoplastic polymers which can be degraded at low temperature.
However, when making the choice account must be taken of the ease of thin layer or semi-thick layer deposition on large surfaces, the microscopic quality of the layer, i.e. its continuity, absence of graining or other defects, as well as its transparency and adherence to the substrate. As a non-limitative example it is possible to use for the organic intermediate layer 2 polymethyl methacrylate, polystyrene, polycarbonates, :
polysulphone or cellulose derivatives ~nitrocellulose, cellulose acetate, ethyl cellulose, celluse aceto- `
butyrate). In certain cases these substances are com-mercially obtainable in the form of thin or thick sheets, so that the actual substrate 1 acts as the thermally degradable layer, In all cases these substances can also be deposited on a substrate 1 by drawing or cen-trifuging from solutions in simple solvents (acetone~
cyclohexanone, butyl acetate or the mixture of solvents known under the trade name "AZ Thinner" of the SHIP~E~
Company). Other deposition methods can also be used, such as vacuum sublimation or cathodic sputtering.
Moreover, it may be advantageous to add to the polymer an adjuvant able to reduce the temperature at which gaseous emission occurs, i.e. improving the sensitivity.
3n ~his is the case with certain plasticiæers such as adipate~, azelakes and ~ebacates Oe isodecyl, butoxy-ethyl or e~hylhexyl or isodecyl, cyclohexyl or ethyl-hexyl phthala-tes.
For p~oducing the metal layer 3, the m~tals 3~ a.re chosen as a Eunction of their duc-tility, their ;, ~ ' ' ;

:
~L5930~

deformation shock resistance, their hardness, their limited oxidizability and their possibility of being used in the production of perfectly continuous thin layers which will adhere to the polymer. The ductility and shock resistance are generally obtained with gold, silver or platinum-based alloys in the homogeneous phase. The hardness and adhesion are obtained by choos-ing as the addition element one of the following ele-ments Cu, Cr, Ni, Al, Sb, Ta, Ti, Mn, Si, Zr, Co, Pd provided that the precious metal content exceeds or is equal to 60% by weight, so that the alloy retains a good oxidation resistance and remains in the high ductility homogeneous phase. It is therefore possible to use the dental alloys Ag - Pd - Cu. Moreover, other addition ;~
elements can be used without passing beyond the scope of the invention. It is also possible to use non-precious alloys such as aluminium bronzes (e.g. Cu/0, Fe7 5, A122 5) or tin bronzes (e.g. Cu70, Sn5, Pb25) and Cu-Ni, Cu-Pd and Ni-Pd alloys, which are known to have a high oxidation resistance, as well as interesting mechanical properties for the purpose of the present invention. As a non-limitative example the Applicant has obtained excellent results with the alloy Cr20Au80 (by weight).
All these metals make it possible to produce thin layers by vacuum evaporation and these layers have a known absorptlon power of 50% for incident radiation when the thickness of the layers is between 4 and 10 nm. More-over, annealing in an oxidizing atmosphere (e.g. at 55C
~or 48 hours) makes it possible in certain cases-, e.g.
that of Cu-~u alloys, to considerably incxease the hardness and adhesion o~ the metal layers without sig-niEicantly reduing their ductility.
One o~ the advantacJes of the invention is that the adhesion o~ the metal layer to the polymer and the q~istence oE a signi~iaant rad:ial thermal gradient tends ,~, , - , , .
,: ;

~59307 to confine the gas bubble to the centre of the writing or inscription spot, i.e. gives a much smaller defor-mation than that of the said spot. In fact it can be as small as 0.3 ~m, whilst leading to a signal to noise ratio above 50 db. This leads to a transfer function with a cut-off frequency rejected at high frequency. In addition, the size of the inscription and the signal to noise ratio rapidly vary as a function of the power of the inscription beam, as is shown by characteristic 10 of Fig. 5. This provides the possibility of storing the data at several levels by regulating the power (the only limitation being the available power, because holes are never formed).
In addition, the effect of reducing the thick-ness of the metal layer as a result of its plastic deformation can be utilized for reading the data con-tained in the impression by local reflectance reduction and by a correlative transmission increase. Thus, there is an amplitude contrast which completes the phase contrast associated with the relief of the impression.
Although the thickness of the metal layer is limited, its hardness and the mechanical rigidity of the polymer make it easy to manipulate the latter medium by using only elementary precautions, particularly for providing protection against dust and ~ingermarks. Thus, the structure has a long service li~e. However, it may be advantageous to more completely protect the data con-tained in the sensitive layer.
Fig. 11 shows a conventional construction o e ~0 ~hi~ pro-kection u~ing a cap 21 giving an ernpty space above the metal layer 3. ~he inscripkion can be made by m~ans o a ligh-t beam 4 traversing the ~ubstrate, here in the ~orm o~ the in~rm~diate layer 2 and ~ocused on the metal layer 3 by a lens 5 oE op~ical axis XX. The protective cap 21 can be transparent t~ the inscription .. ..... . . .

. - . . . . ~ .

radiation and the substrate can be opaque if read-out is by reflection. For read-out by transmission, the two elements giving access to thP metal layer 3 must be transparent to the read-out radiation.
However, simpler and less onerous protective methods can be used. As shown in Fig. 7 the sensitive layer 3 can be protected without being degraded in any way by means of a thick layer 16 of photo-polymerizable or thermo-polymerizable resin, a varnish or a polymer deposited by the wet method (drawing, centrifuging, etc.) or by the dry method (sublimation, cathodic sputtering). This layer 16 can be deposited both before and after inscription to the extent that its hardness is not so high that it opposes the growth of the deEor-mation under the impact of the laser beam. As a non-limitative example layer 16 is of thermosetting silicone of type Sylgard 184 (trade mark) produced by DOW CORNING
Company, a photopolymerizable, acrylic laquer or a nitro-cellulose coating deposited by centrifuging from a so-lution o~ 100 g/l of AZ Thinner of the SHIPLEY Company.It should be noted that the thick layer pro-tection de-scribed here is effective not only in protecting the sensitive layer 3 from possible mechanical damage, but also protects it against corrosion if 3 is not com-pletely stainless, which is the case with Cu-Au alloys with a very high copper content.
Fig. 8 shows another protective method con-sisting of depositing a metal layer 7 on the recorded data medium. This deposit can be produced on metal layer 3 by the electrolysis of a metal with a high mechanical and chemical strength, e~g. nickel. Unlike str~lctures etched by ablation~ th~ struature according to the inven~.lon can be covered with a thick metal layer 17 with~ut the relief disappearing, because i-t is still legible through substrate 1. The con~iguration defined . . ' ' ' ' , .

: `~ ' ~ ' '' ' ~` ~
~593~)7 :~ .

by the data medium protected by a thick metal layer 17 according to the invention is a structure which is par-ticularly satisfactory when a ~ong-term storage is desired. It should also be noted that a degradation of substrate 1 is not prejudioial to the reading of the data because the damaged substrate and the intermediate layer 2 can be removed by dissolving in a suitable solvent and can be~replaced by a new protective layer 18, e.g. a photopolymerizable resin, as illustrated in ~ ~ `
Fig. 9.
During this operation the etching on the thick metal layer 17 is not damaged. Prior to storage in ~ , accordance with the process described hereinbefore, the relie~ can be copied once or severaL times by a contact process.
Fig. lO lllustrates the general copying process by et~hing by means of a photopolymerizable or thermopolymerizable liquid ;19. The copy can be produced~
on a rigid or ~flexible~pla;stic support 20. In the -;~
copying operation the~data medium is coated with liquid 19, then the~support~20;of~ the COpy; 15 engaged with~the liquid coating l9 by~means o~ a device making it possi~
ble to obtain a continuous liquid ilm without gas bubbles or other de~ects~ which also has a uni~orm thickness. Following irradiation with an ultraviolet lamp in either dir~ection or following thermal treatment,~
the polymerizable resin ~19 adheres in preferred manner~
to the support 20~. This permits ~a complete~separation of copy 10, 20 ~rom~original~1-2-3 without any damage~
occurring to the copy or to the original and the latter can be u~ed again eith~r ~or~complementary inscripti~n or ~ a n~w CQpy~
A ~ypical data ~edium aca~rdin~ t~ the in~ ~
v~n~ion can, ~r ~xample, h~v~ the ~:llowin~ ch~racte~- ;
~5 istics. ~ubstrste 1 1~ ~ormed by a p~lymethyl metli- j ; :

l ,. ,. . , : :

- 15 - ~5~30~ ~

acrylate disk of diameter 356 mm and thickness 1.5 mm.
Thls substrate 1 is covered with a nitrocellulose inter-mediate layer 2 of thickness 500 nm deposited by cen-trifuging at a speed of 6 revolutions s 1 from a so-lution containing 8 g of nitrocellulose per litre of AZThinner. After evaporating the solvent the organic intermediate layer 2 is covered with a layer 3 of an alloy Cr20Au80 (by weight) with a thickness of approxi-mately 8 nm by vacuum evaporation at a speed of 0.3 nm s 1. The inscription of the recording medium is carried out through the substrate 1 with a beam from a helium-neon laser (~ = 633 nm) modulated at the frequency of 2 MHz. The projecting lens used has a numerical aperture of 0.45. The disk is rotated at a speed of 25 revo-lutions s 1 and the inscription is made over a radius of130 mm, leading to a succession of impressions 0.4 ~m wide and 2.5 ~m long, with a high relief of 150 nm when the incident power is 8 mW. Using such a recorded medium the read-out signal measured with the spectrum analyzer and related to the optical noise is at a level of 64 db for a 30 kHz band. The relief can be copied by photopolymerization of an acrylic liquid on a rigid polymethyl methacrylate support 20 by means of a 100 W
UV lamp located at a distance of 18 cm. The radiation time is 12 seconds. Read-out on a conventional machine revealed no degradation of the read out signal, either for the original or for the copy.
Fig. 12 shows a variant of the data medium according to the invention. Substrate 1 carries a plas-ticized organic intermediate layer 2 successivelycovered by a metal in-termedia-te layer 30, e.g. of go:ld, which is in turn covered by a harder metal layer 3. The lntcrmediate layer 30 makes ik possible to obtain low stresses during deposition on a relatively highly plas-ticized organic intermediate layer 2~ ~'hus, rnetal layer - 16 - ~ ~30~

3 can be deposited whilst taking fewer precautionsO
Such a structure makes it possible to envisage a two-phase etching process. In the first phase a continuous relief impression materializing a pretrack is produced by rneans of an inscription beam 4 which does not degrade the intermediate layer 2. In the seconcl phase the relief 7 of this pretrack is inscribed with a beam 40 modulated by the data, which amplifies it due to the degradation of intermediate layer 2. Thus, a succession of protuberances is obtained having a more marked relief 70 than that of the pretrack. The production of a pre-track by degradation of the intermediate layer 2 is also compatible with the subsequent inscription of the data.
The use of a pretrack can be conceived in two different ways. It is possible to record the data on a track at the side of the pretrack or it can be recorded above the pretrack. The pretrack may involve a slight deterioration of the organic intermediate layer or an expansion without any change of state of the said inter-mediate layer.
Although in the foregoing description and inthe subsequent drawings the essential characteristics of the present invention have been described and represent-ed in connection with pre~erred embodiments thereof, it is obvious that all appropriate modifications can be made thereto without passing beyond the scope of the invention. In particular the deposition of a very thin layer of a few nm of chrome, nickel, platinum or copper on the sensitive layer 3 before or after etching with a view to increasing the hardness of layer 3 and conse-quently the number of copies obtainable Erom the same ori~inal does not pass beyond the saope of the in~ention.
In addition, the metal layer 3 can advantageously be Eorme~ by one oE the ~ollowing alloy5:

- 17 ~3~)7 Elemen ts Example Cd - Sb cdl8sb82 Mg - Sb M~17Sb83 Al - Sb 33 67 Cd - Cr Cdl7Cr83 Mg - Cr Mgl7Cr83 Al - Cr A133Cr67 :
., .

' ., ,

Claims

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

1. A thermo-optical data writing process con-sisting of the surface heating by means of an intensity-modulated, focused radiation beam of a thermosensitive data medium having at least one metal layer covering an organic intermediate layer, wherein the thermal energy given off in the metal layer by the impact of the beam causes gas evolution by local degradation of the organic intermediate layer, this gas evolution and the associ-ated loss of adhesion lead to stretching in the metal layer beyond the elastic limit and on this side of the elongation at break of the ductile material constituting said metal layer.

2. A writing process according to Claim 1, wherein the organic intermediate layer is formed from a thermoplastic polymer serving as the substrate.

3. A writing process according to Claims 1 or 2, wherein two writing phases follow one another with a view to producing a first permanent relief and, whilst being guided by the latter, establishing a second perma-nent relief.

4. A writing process according to Claim 1, wherein the metal layer is reinforced by a deposit following inscription.

5. A writing process according to Claim 4, wherein this deposit is an electrolytic deposit of adequate thickness to serve as a definitive support for the metal layer.

6. A writing process according to Claim 5, wherein the intermediate layer and its original support are removed and replaced by a protective layer.

7. A writing process according to Claim 1, wherein after inscription the metal layer serves for the transfer of an impression, which is carried out by means of a fluid photopolymerizable material layer.

8. A writing process according to Claim 1, wherein the impression inscribed in the metal layer is protected by a cap.

9. A data medium for performing the writing process according to Claim 1, comprising an organic intermediate layer covered by at least one metal layer, wherein the localized heating of the metal layer in the impact area of a focused radiation beam produces a gas evolution by local degradation of the organic intermedi-ate layer, this gas evolution and the resulting loss of adhesion bring about a plastic stretching of the metal layer which is beyond the elastic limit and on this side of the elongation at break of the ductile material con-stituting the metal layer.

10. A data medium according to Claim 9, wherein the organic layer is constituted by at least one polymer, the metal layer being formed by an alloy incorporating precious metals from the group gold, silver, platinum and an adjuvant for increasing the adhesion thereof with respect to the organic layer and the deformation shock resistance, said adjuvant being chosen from among the following elements: copper, chrome, nickel, aluminium, antimony, tantalum, titanium, zirconium, cobalt, palladium, manganese, silicon.

11. A data medium according to Claim 9, wherein the organic layer is constituted by at least one polymer, the metal layer being constituted by one of the following alloys: aluminium bronze, tin bronze, copper-nickel, copper-palladium, nickel-palladium, cadmium-antimony, magnesium-antimony, aluminium-antimony, cadmium-chrome, magnesium-chrome and aluminium-chrome alloys.

12. A data medium according to Claim 10, wherein the polymer belongs to the group including poly-methacrylates, polycarbonates, polystyrene, cellulose derivatives and polysulphone.

13. A data medium according to Claim 12, wherein the polymer is plasticized so as to reduce its degradation temperature.

14. A data support according to Claim 13, wherein the plasticizer incorporated into the polymer is an adipate, azelate or sebacate of isodecyl, butoxyethyl or ethylhexyl or an isodecyl, cyclohexyl or ethylhexyl phthalate.

15. A data medium according to Claims 13 or 14, wherein the metal layer is separated from the organic intermediate layer by an intermediate metal layer.

16. A data medium according to Claim 9, wherein protective means are provided for the metal layer.

17. A data medium according to Claim 16, wherein these means comprise a thick electrolytic deposit of metal made after writing the data.

18. A data medium according to Claim 9, wherein the metal layer is covered with a metal deposit which cooperates with the latter with a view to forming a resistant impression.