OPTICAL RECORDING ELEMENT AND METHOD OF RECORDING INFORMATION

Abstract: An erasable optical recording element is provided hav­ ing a supporting plate which comprises a recording layer which has a Te-Se-Sb alloy. The ratio of the elements Te, Se and Sb in at.percent lies within a range in the Te-Se-Sb composition diagram which is bounded by a polygon having the corners Te₉₀Se₈Sb₂ - Te₇₈Se₂₀Sb₂ - Te₁₈Se₅₂Sb₃₀ - Te₂₈Se₂Sb₇₀ - Te₉₀Se₂Sb₈₀. The layer thickness of the recording layer is between 80 and 300 nm. During recording the recording layer is converted by exposure to modulated laser light in the exposed places from a more ordered or crystalline state into a less ordered or amorphous state. The locally changed state can be read by means of laser light. A method of recording information is also described.

Description: Optical recording element and method of recording information. The invention relates to an erasable optical recording element having a supporting plate and which comprises a recording layer having a tellurium alloy, in which the recording layer is converted by exposure to modulated laser light in the exposed places from a more ordered or crystalline state into a less ordered or amorphous state and in which the locally changed state can be read by means of laser light. Such a recording element is known, for example, from U.S. Patent Specification 3,530,441. According to this Patent Specification the element and the recording method performed therewith are mainly used for recording images which can be printed visibly. For this purpose the condition is used that the amorphous parts of the recording layer have a stable high electrical (ohmic) resistance and the crystalline parts have a low electrical resistance. After depositing electrical charge upon the recording film, the crystalline or more ordered parts of the film, which represent, for example, the stored information, are not charged and the amorphous parts which have a high resistance are charged. The charge pattern thus obtained can be made visible by causing electrically charged ink particles or pigment particles to adhere to the electrically charged amorphous parts of the recording film. The resulting toner image can be printed, for example, on a sheet of paper. It is stated in column 3, lines 4-8 and 26-30, of the above mentioned U.S. Patent Specification that the stored information can be read in various manners, for example, by determining the electrical resistance, capacity, dielectric constant, refractive index, reflection, light absorption and light transmission. For example, an electron beam may be used which is reflected in accordance with the condition of the various parts of the recording film. These other possibilities are not described in the said U.S. Patent Specification other than in connection with the method shown in Figs. 13-15 in which a recording element is used electrically as a means for light modulation. For this purpose, the element is provided on each side with an electrode to which a voltage pulse can be applied. It is an object of the present invention to provide an erasable optical recording element in which data are recorded in the form of amorphous spots in a polycrystalline matrix by means of pulsated laser light. A further object is to provide a recording element the recording layer of which has a high sensitivity to laser light, which permits of a high information density and which offers the possibility of a high recording rate. By way of example, it is desired to record an amorphous spot of the order of 1/um diameter in, for example, 50 ns. The recording energy then is 0.3 - 0.8 nJ per recorded amorphous spot. Another object is to read the stored information in the form of amorphous spots in a crystalline matrix by means of continuous weak laser light, in particular in reflection, in which a good signal-to-noise ratio is obtained. According to still another object, the recording layer of the recording element together with the information stored therein must be stable and tenable for a long period of time. Moreover, it must be possible to erase the stored information by a simple thermal treatment, the amorphous parts assuming the original crystalline structure. Moreover, it must be possible to record and erase information frequently without permanent deformation or other damage to the recording layer. According to a further object, in a preferred form of the recording element the possibility must be presented to locally erase the amorphous spot by exposure to laser light. Said erasing must furthermore take place preferably in real time. This means that with the same rate of movement of the recording element it must be possible both to write and to erase by using laser light. The Applicants have come to the conclusion that in the realization of these objects the composition of the recording layer is a very important factor. The following problems, which as such constitute an essential element in producing the present invention, play an important role. First of all, the crystallization temperature of the recording material is of importance. At a comparatively high crystallization temperature, the amorphous information areas are stable at room temperature, which is of advantage. On the other hand, the erasing process is then more difficult. The amorphous areas cannot easily be returned to the original crystalline structure. Another aspect is that when reading in reflection the reflection differences between amorphous and crystalline areas must be as large as possible so as to obtain a good signal-to-noise ratio. The layer thickness plays an important role. The disc noise which in crystalline areas is considerably higher than in the amorphous areas is also of importance. Therefore, in the Applicánts' opinion, the crystals in the layer must be as small as possible. Furthermore the thickness of the recording layer is of importance for the recording process. When the layer thickness is too small the possibility exists that upon exposure to pulsated laser light holes will be formed in the recording layer, in addition to or instead of a conversion from crystalline to amorphous. This must be avoided at all costs. When the layer thicknexs is too large, the recording energy is too high and hence the laser light sensitivy, expressed in threshold energy for the information of an amorphous spot, is too low. Another important aspect is the behaviour upon erasing. When recording, the exposed crystalline area must be heated up to the melting point, must then be allowed to melt and then be cooled rapidly to fix the amorphous area. Upon erasing, the amorphous area must be heated up to between the glass transition temperature and the melting temperature and be kept at this temperature until all the amorphous material in the area has crystallized. When the duration of heating for erasure is too short only a partial crystallization takes place as a result of which the information, al-be-it in a weakened condition, still remains. It becomes extra complicated when the requirement is imposed that it must be possible to erase in real time by means of laser light. In this connection the size of the laser light spot used upon erasing, the so-called erasing spot, plays an important role, as well as the rate of movement of the recording element. Moreover, it must be possible to switch the layer many times, for example 100 - 1000 times or even more, without the recording layer being deformed or working loose from the substrate. Switching is to be understood to mean recording plus erasing. The above-mentioned objects are achieved by means of an erasable optical recording element of the type mentioned in the opening paragraph which is characterized according to the invention in that the recording layer comprises a Te-Se-Sb alloy, the ratio of the elements Te, Se and Sb in at.percent lying within a range in the Te-Se-Sb composition diagram which is bounded by a polygon having the corners Te90S e8Sb2 - Te78Se20Tb2 18 Te18Se52Sb30 52 - Te28Se2Sb70 - Te90Se2Sb8 and furthermore that the layer 70 thickness of the recording layer is between 80 and 300 nm. In the above composition range, alloys are found from which recording layers can be manufactured in which amorphous spots can be formed by means of a pulsated laser light beam while using a low recording energy of approximately 0.3 - 0.8 nJ per spot. This enables the use of a laser of a very low power, for example a power of a few milliwatts on the recording layer, and an extremely short pulse time of, for example, 50 ns. The recorded information can be read in reflection, an excellent signal-to-noise ratio being obtained. The recording layer has a good tenability. Moreover, the amorphous spots are very stable at room temperature. The amorphous spots can be erased by a thermal treatment, in particular by heating the recording material for some time at a temperature between the glass transition temperature (devitrification temperature) and the melting temperature of the recording material. In a favourable embodiment of the recording element according to the invention the ratio of the elements Te, Se and Sb in at. percent lies within a range in the Te-Se-Sb diagram which is bounded by a polygon having the corners Te90Se8Sb2 - Te78Se20Sb2-Te39Se21 Sb20 - Te63Se2Sb35 - Te 90Se2Sb8. Recording elements which have particularly advantageous properties can be manufactured by means of the compositions in this preferred range. For example, the amorphous spots can be converted into the original crystalline structure by exposure to laser light in real time. The length of the laser light spot upon erasing, hereinafter termed erasing spot, is, for example, 20 - 100 /um. The exposure time of the amorphous spot is then, for example, 10 - 100 /us. In this case the lasers for erasing and recording may be coupled together so that, #?or example, the erasing spot precedes the recording spot in the recording layer. The width of the erasing spot is 2 /um. Examples of preferred compositions of the recording layer in which the recording information can be erased by means of laser light in real time are Te80Se10Sb10 and Te74Se11Sb15. Another composition, Te50Se30Sb20, has a particularly high stability. Examples of suitable compositions are recorded in the table below. TABLE EMI6.1 <tb> Number <SEP> Composition <SEP> Recording <SEP> Signal-to-noise <tb> <SEP> energy <SEP> (nJ) <SEP> ratio <SEP> (dB/m) <tb> <SEP> 1 <SEP> Te30Se30Sb40 <SEP> <SEP> 1.0 <SEP> 86 <tb> <SEP> 2 <SEP> Te33.3Se33.3Sb33.3 <SEP> 0.6 <SEP> <SEP> 58 <tb> <SEP> 3 <SEP> Te40Se20Sb40 <SEP> <SEP> 1.2 <SEP> 83 <tb> <SEP> 4 <SEP> Te40Se30Sb30 <SEP> <SEP> 0.4 <SEP> 90 <tb> <SEP> 5 <SEP> Te50Se10Sb40 <SEP> <SEP> 0.8 <SEP> 86 <tb> <SEP> 6 <SEP> Te50Se30Sb20 <SEP> <SEP> 0.8 <SEP> 88 <tb> <SEP> 7 <SEP> Te70Se12Sb18 <SEP> 0.5 <SEP> 94 <tb> <SEP> 8 <SEP> Te74Se11Sb15 <SEP> <SEP> 0.5 <SEP> 95 <tb> <SEP> 9 <SEP> Te74Se13Sb13 <SEP> 0.5 <SEP> 94 <SEP> <tb> 10 <SEP> Te80Se10Sb10 <SEP> <SEP> 0.5 <SEP> <SEP> 94 <tb> In a further preferred form the recording layer of the optical recording element accordine to the inventionhas a layer thickness which is approximately equal to wherein > is the wavelength of the irradiating laser EMI6.2 upon reading and na is the refractive index in the amorphous state. The layer thickness in the materials which can be used according to the invention is then generally 80 - 150nm at a # of approximately 800 nm. Extra advantages can be obtained, if, according to a further preferred embodiment a coating layer is provided on the recording layer on the side remote from the substrate. Said coating layer in the first instance serves to protect the recording layer from damage and it also appears that as a result of this the number of switching times increases by a few orders of magnitude. With a suitable choice of the coating layer the contrast upon reading can also be favourably influenced. The coating layer may be, for example, of SiO2. In an embodiment which is suitable for reading by means of reflection differences, the recording layer is present on a transparent substrate. Recording and reading information then takes place on the substrate side ofthe element. Disturbing effects on reading created by dust particles present on the surface of the recording layer remote from the substrate are substantially avoided by said measure. The invention also relates to a method for optically recording information and for erasing the recorded information, in which an optical recording element, which comprises a substrate which is transparent to laser light and has a crystalline recording layer of thickness of 80 - 300 nm provided thereon and comprising a Te-Se-Sb alloy alloy, is rotated at a frequency of 1 - 30 Herz and is exposed from the substrate side to laser light pulsed in accordance with the information and having a pulse energy equal to or smaller than 1 nJ on the layer, as a result of which the crystalline recording layer melts in the exposed places and then cools, amorphous information areas being formed, and in which during erasing the amorphous areas are converted into the original crystalline structure by means of a thermal treatment. A preferred thickness of the recording layer is 80 - 150 nm. A favourable embodiment of this method is characterized in that the amorphous areas are heated to a temperature between the glass transition temperature and the melting point of the recording layer by exposure to a laser light beam, are kept at this temperature for some time until the amorphous part of the recording material has crystallized, and are then cooled so that the amorphous areas are converted in real time into the original crystalline structure. In a further favourable embodiment of the method in accordance with the invention the amorphous information areas are converted into the original crystalline structure by exposure to a laser light beam having a spot length of 1 - 100/umS a spot width of 1.5 - 2.5 fum and an exposure time per amorphous area of 1 - 200 /us. The information stored in the form of amorphous areas in the recording layer of the recording element can be read by means of laser light, for example, by using the difference in reflection between the amorphous areas and the sorrunding crystalline matrix. The invention will now be described in greater detail with reference to the example, the diagram sheet I, and the drawing, in which: Fig.1 is a cross-sectional view of an optical recording element according to the invention,and Fig. 2 shows graphically the reflection of a recording layer as a function of the layer thickness. In the composition diagram shown on the diagram sheet the layer constituents tellurium (Te), selenium (Se) and antimony (Sb) in at.percent are shown at the corners of the triangule. The recording layer used in the recording element in accordance with the invention comprises a Te-Se-Sb alloy the ratio of the elements of which lies within an area ABCDE bounded by a solid line and the corners of which in the given sequence have the composition: A: Te90Se8Sb2, B: Te78Se20Sb2, C: Te18Se52 Sb 30' D: Te28Se2Sb70, E: Te90Se2Sb8. A preferred range as stated in the preamble line within the area ABFGE with the corners F and G being: F: Te39Se21Sb20 and G: Te63Se2Sb35. The compositions stated in the table are recorded in the diagram with a numeral indication corresponding to that in the table. Reference numeral 11 in Fig. 1 denotes a transparent supporting plate. Said plate 11 may be of glass, quartz glass, or of a synthetic resin, for example, polycarbonate or polymethyl methacrylate. On one side the plate 11 comprises a transparent layer 12 of a light-cured lacquer in which a spiral-like follower (guiding) track 13 is provided. The lacquer layer 12 is coated by a 100 nm thick recording layer 14. The recording layer 14 is provided by using a sputtering process. This may also be done by vapour deposition. After the layer has been provided the element is heated until the whole layer 14 has a polycrystalline structure. The recording layer 14 consists, for example, of Te80Se10Sb10. A coating layer 16 of SiO2 is provided on the recording layer 14 by sputtering. When the recording element of Fig.1 is used, the element, for example in the form of a disc having a diameter of 10 cm, is rotated at a frequency of 4 Herz. Via the transparent layer 12 and in the guide track 13, the recording element is exposed to pulsed laser light having an emission wavelength of 800 nm and a pulse time of 60 ns. The laser has a power of 8 mW on the recording layer. In the exposed places of the recording layer 14 an amorphous information spot 15 is formed with a diameter of 1 /us. The data recorded in this manner in the form of amorphous spots in a crystalline matrix are read in reflection by means of a continuous laser light beam having an emission wavelength of 800 nm and a power of 0.4 mW on the plate. For reading, the recording is rotated at 4 Herz. The reading is based on the difference in reflection betweenan amorphous information spot and the polycrystalline surroundings. This difference is illustrated in Fig.2 in which the reflection coefficient (RC) from the substrate side (12) expressed in percentage of reflected light, of a layer having the composition Te80Se1OSblO is plotted graphically against the thickness (t) in nm of the layer. The wavelength of the light used is 800 nm. The broken line 21 in Fig.2 relates to an amorphous layer. The dot-and-dash line 22 relates to a polycrystalline layer. The contrast between the amorphous layer and the crystalline layer is indicated by the line 23. It appears from Fig.2 that the best contrast is obtained with a thickness between 80 and 100 nm.

Claims: 1. An erasable optical recording element having a supporting poate and which comprises a recording layer having a tellurium alloy, in which the recording layer is converted by exposure to modulated laser light in the exposed places from a more ordered or crystalline state into a less ordered or amorphous state and in which the locally changed state can be read by means of laser light, characterized in that the recording layer comprises a Te-Se-Sb alloy, the ratio of the elements Te, Se and Sb in at.percent lying within a range in the Te-Se-Sb composition diagram which is bounded by a polygon having the corners TegOSeg5b2 - Te78Se20Sb2 - Te18Se52Sb30 Te28Se2Sb70 - TegOSe2Sb8 and furthermore that the layer thickness of the recording layer is between 80 and 300 nm. 2. An erasable optical recording element as claimed in Claim 1, characterized in that the ratio of theelements Te, Se and Sb in at.percent is within a range in the Te-Se-Sb diagram which is bounded by a polygon having the corners Tegose8sb2 - Te78Se20Sb2 39 Te39Se21Sb20 21 TE63Se2Sb35 Te90Se2Sb8. 3. An erasable optical recording element as claimed in Claim 2, characterized in that the recording layer has a composition TegOSe1O5b1ow Te14Se11Sb15 or Te50Se30Sb20. 4. An erasable optical recording element as claimed in anyone of the preceeding Claims,characterized in that the layer thickness of the recording layer is from 80 to 150 rim. 5. An erasable optical recording element as claimed in any one of the preceding Claims, characterized in that the recording layer is present on a supporting plate which is transparent to laser light. 6. An erasable optical recording element as claimed in any one of the preceding Claims, characterized in that the recording layer has a coating layer. 7. A method for optically recording information and for erasing the recorded information in which an optical recording element, which comprises a substrate which is transparent to laser light and a crystalline recording layer provided thereon in a thickness of 80 - 300nm which comprises a Te-Se-Sb alloy, is rotated at a frequency of 1 - 30 Herz and is exposed from the substrate side to laser light pulsed in accordance with the information and having a pulse energy equal to or smaller than 1 nJ on the layer, as a result of which the crystalline recording layer melts in the exposed places and is then cooled, amorphous information areas being formed, and in which during erasing the amorphous areas are converted into the original crystalline structure by means of a thermal treatment. 8. A method as claimed in Claim 7, characterized in that the amorphous areas are heated by exposure to a laser light beam to a temperature between the glass transition temperature and the melting point of the recording layer, are kept at this temperature for some time until the amorphous part of the recording layer has crystallized, and are then cooled, the amorphous areas being converted in real time into the original crystalline structure. 9. A method as claimed in Claim 7, characterized in that the amorphous areas are converted into the crystalline structure by exposure to a laser light beam having a spot length of 1 - 100 /um, a spot width of 1.5 - 2.5 /um and an exposure time per amorphous area of 1 - 200 /uS.

Inventors: RIPPENS, WILHELMUS GERTRUDES V. M., VAN DER POEL, CAROLUS JOHANNES
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: G11B7
IPC codes: G11B7, B41M5, G03C1