OPTICAL STORAGE STRUCTURE

Abstract: In an optical storage device having an active layer including a Chalcogenide alloy, a metal, preferably nickel, is codeposited with the Chalcogenide, in order to inhibit darkening of the storage structure.

Description: -I- OPTICAL STORAGE STRUCTURE This invention relates to optical storage devices of the type comprising a rotatable disk having an active structure enabling variation of optical properties by means of focused write radiation, such as a laser beam, and reading data stored thereon by means of focused read radiation beam. The invention is more particularly directed to an optical recording medium of the above type wherein the active layers of the structure are stabilized. An optical structure- of the above type is disclosed, for example, in U.S. application No. 499,666, Muchnik, filed May 31, 1983, and copending application (STO-109), both assigned to the present assignee. In accordance with the above disclosure, a preferably disk shaped substrate, for example, of aluminum or plastic, is provided with a planarizing layer, for example a thin coat acrylic material, to provide an optically smooth surface. A reflective layer for the structure is deposited on the planarizing layer, and thereupon the three layer structure disclosed in application S.N. 499,666 is deposited. The phase layer, for example of a fluorocarbon, is tuned, i.e., it has a thickness such that destructive interference occurs between radiation reflected from the active layer and radiation transmitted by the active layer and reflected from the reflective layer and transmitted by the active layer. This destructive interference phenomenon occurs for both read and write beams. It enhances the write sensitivity by increasing the coupling of the write beam energy to the active layer. It also enhances the read signal by increasing the reflectivity contrast between unwritten marks and unwritten surrounding SUBSTITUTE SHEE" regions . In order to eliminate the effect of any dust or small particles falling on the structure, a dust defocusing layer is provided on the structure, being adhered thereto by an adhesion layer deposited on the matrix layer. The phase and matrix layers, and adhesion and dust defocussing layers are transparent to radiation of the read and write frequencies. Tuned structures of the above type have a good signal to noise ratio, good sensitivity, and otherwise good performance. ; It has been found, however, that the recording medium may produce unstable reflectance upon exposure to various sources of heat. Such sources may include over- exposure, the heat of an ultraviolet lamp employed to cure the defocusing layer, and repeated exposure to the read beam, which may occur for exa ple when the read beam dwells on a single point of the structure for an extended period. The instability of the structure has been found to occur even when the read beam has fairly low power, i.e., even less that that deemed sufficient for reliable operation. The insta¬ bility results in darkening of the medium, resulting in some cases in loss of control of the focusing serro-mechanis . In addition, the effect results in an increase in the noise floor spectrum. When the structure is subject to higher exposure levels, such as for example the write beam, write marks are created that are typically surrounded by a dark ring. In this regard the term "dark" refers to a lower reflectance than the surrounding areas. Such dark regions tend to grow with repeated beam exposure, thereby resulting in SUBSTITUTE SH! a unstable mark length. This effect is un¬ desirable since information may be recorded in the length of the mark. Briefly stated, in accordance with the invention, this effect has been found to be overcome by the alloying of a metal, preferably nickel, with the Chalcogenide alloy. In order that the invention may be more clearly understood it will now be disclosed in greater detail with reference to the accompanying drawings, wherein: Figure 1. is a cross-sectional view of an optical recording structure in accordance with the invention; and Figure 2 is an electron-beam photograph of a portion of the active layer of the recording structure. One form of optical storage device, in SUBSTITUTE SHEET accordance with the invention, as illustrated in the cross-sectional view of figure 1, is comprised of a substrate 10 of, for example, aluminum or a plastic material. The substrate 10 may have a thickness of about 0.075 inches and be in the form of a disk of 14 inches diameter. These parameters are of course not limiting to the present invention. The surface of the substrate disk 10 conventionally has small imperfections, micro irregularities, tooling marks, polishing streaks, etc., which are undesirable for the optical properties of the recording media, and in order to remove these imperfections, a planarizing layer 11 is preferably provided on at least one surface of the substrate. The planarizing layer may have a thickness of for example 2 to 25 micrometers and may comprise an acrylic layer solvent coated by spin coating. The surface of this layer should have a micro roughness less than 5 nanometers, and may be aluminized for this purpose. The planar active layer also serves to prevent corrosion of the substrate, as well as to provide a chemical barrier between residual substrate contamination and the three layer structure of the phase layer, active layer and matrix layer described in the following paragraphs. As one example, the planarizing layer has been comprised of Rohm and Haas Acryloid A-10. This material is a solvent base methyl- methacrylate thermoplastic resin in a Cellosolve acetate having a viscosity of 800 to 1200 cps (Brookfield 25 degrees C), 30% plus or minus 1% percent solid, a density of 8.6 lbs. per gallon and a glass transition temperature of the polymer -8- of 105 degrees C. The Acryloid A-10 resin was dissolved in a solution of Cellosolve acetate and butyl acetate with a final solvent ratio of 9:1, Cellosolve acetate to butyl acetate. The Cello¬ solve acetate was Urethane grade (boiling point of 156.2 degrees C), and the butyl acetate was spectral grade (boiling point of 126.5 degrees C). The butyl acetate may be substituted by Cellosolve acetate. The solution has a solid content of 22%, and a viscosity of 133 cps (Brookfield at 21 degrees C), filtered to 0.2 micrometers. The planarizing layer provides a base for the reflecting layer 12. The reflecting layer is preferably of aluminum, although copper or silver may be alternatively employed. A thickness of about 100 nanometers is preferred, although this dimension is not critical. It must be highly reflective at the read, write and coarse seek wavelengths employed, for example 633, 830 and 780 nanometers reflectively. The reflectivity should be equal to or greater than 0.85 in air, at these wavelengths. The reflective layer 12 is preferably formed by sputtering onto the planarizing layer, for example employing a Leybold-Heraeus in-line vacuum deposition system. It will of course be apparent that the invention herein is not limited to the above structure wherein the reflective layer is formed on a planarizing layer, and other suitable tech¬ niques for forming a reflective surface of the required planarity, supported by the substrate, may be employed. The next three layers, defining a three- layer structure are comprised of a phase layer 13 on the reflective layer, an active layer 14 on the phase layer and a matrix layer 15 on the active layer. The phase layer and matrix layer may be of a plasma polymerized fluorocarbon with a fluorine to carbon atomic ratio of (for example only) 1.8. The active layer is preferably STC-68 tellurium alloy (Te 65 Se 2Q As.. Ni, Q ). In response to a write beam (for example a laser beam) the optical energy of the beam is dissipated as thermal energy in the active layer, whereby the active layer agglomerates within the fluorocarbon phase and matrix layer. This agglomeration affects the optical transmittance of the three layer structure in accordance with the signal modulation of the write beam. At the read wavelength and coarse seek wavelengths the active layer absorbs energy to a different extent in the written and unwritten areas, to develop a reflective contrast. The phase layer optically adjusts the absorption and reflectivity of the three layer structure at the read, write and coarse seek wavelengths, the phase layer thereby having a tuned thickness to effect destructive interference at the active layer for beams of the read and coarse seek wavelengths, as a result as reflection of these beams at the reflective layer 12. The phase layer 13 also similarly isolates the active layer from the heat sinking effect of the highly conductive reflecting layer, thereby enabling the energy of the write beam to be effectively dissi¬ pated in the active layer. In addition, as discussed above the phase layer provides a matrix into which the active layer can be dispersed. The phase layer may have a thickness, for example, of 80 nanometers with an index of refraction of about 1.38. The active layer is a thin layer having discrete island-like globules. The layer therefore has irregular or discontinuous upper and lower surface characteristics defined by the globular surfaces. The mass equivalent average thickness of the active layer is thus about 7 to 8 nanometers. The globules denote discrete particles of dimension averaging within the range of 1 to 8 nanometers. It must be stable chemically, optically and in atomic structure. It has an amorphous lattice structure, with a glass transition temperature greater than 80 degrees C. The agglomeration of the globules in response to the heat generated by the write beam is illustrated in the electron beam photograph of figure 2, wherein it is seen that the material of the active layer has agglomerated to form enlarged globules 30 interspersed by large transparent areas, in the generally circular region 31 that has been exposed to the write beam, the surrounding region 32 of the active layer remaining substantially reflective. (The globules are of course invisible in light due to their small size, radiation of smaller wavelength being required to analyze them.) In this photograph the diameter of the exposed area 31 of the active layer was about one micron. The matrix " layer 15 may have a thickness of, for example, 270 nanometers. The fluorocarbon phase layer and fluorocarbon matrix layer are preferably formed by plasma polymerization, and the active layer formed by sputtering, for example employing a Leybold- Heraeus in-line vacuum deposition system. An adhesion layer 16 is provided on the matrix layer. The adhesion layer, in addition to providing the proper surface energy for the application of the outer defocusing layer, also provides adhesive coupling between the matrix layer and the defocusing layer. While the defocusing layer 17 may be applied directly to the matrix layer 15, it has been found that adequate bonding by such direct application is not achieved, for example, when the defocusing layer is of a material such as acrylic polymer. The defocusing layer must be adhered firmly to the storage structure, such that it will not loosen by the variable forces acting thereon, such as the centrifical force cause by rotation of the disk, and similarly induced forces that may effect a gradual deterioration of adhering forces. The adhesion layer 16 thus serves to inhibit the eventual separation of the defocusing layer 17 from the matrix layer 15, in use. This adhesion layer is preferably comprised of a layer of aluminum from 1 to 10 nanometers thick, preferably about 2 nanometers thick. The adhesion layer 16 may be formed by sputtering aluminum, for example employing a Leybold-Heraeus vacuum deposition system. The outer layer 17 of the structure is a defocussing layer, which serves to optically defocus dirt and dust particles and the like which have come to rest thereon. The defocusing effect prevents interference with the optical structures formed in the active layer, in writing and reading data, and in the optical seeking operations. The critical properties of the defocussing layer are that it be sufficiently thick to defocus dust particles that lay on the surface of the disk. In this sense, it is desirable that the layer be set to have, for example, a working thickness of about one millimeter, or one thousand nanometers. This ■ - - *"" * - selection must be balanced however in view of factors that suggest the desirability of a thinner defocussing layer, such as the difficulty of depositing an extremely thick layer with uniformity of thickness and optical integrity, and the rendering of the disk more vulnerable to film stress induced warping from thicker films. . In one embodiment of the invention, the defocussing layer was an acrylic polymer with a thickness of about 178 nanometers, composed of an acrylic polymer having a viscosity of 18 plus or minus 3 cps (Brookfield, UL, 12 rpm, 25 degrees C). Its surface tension was 27 plus or minus 3 dynecm 1. The refractive index as a liquid was 1.455 plus or minus 0.005, and as a solid 1.494 plus or minus 0.005. The glass transition temperature was 56 degrees C and the density was 1.06 plus or minus 0.001 g-cc (25 degrees C). The shrinkage upon curing was 12%, and the water pickup was 0.5%. The material was prefiltered to 0.2 microns before use. The dust defocusing layer may be applied by rotating the disk in a horizontal plane, at a speed of, for example, 20 rpm. The acrylic polymer is preferably applied to the surface of the adhesion layer by means of a nozzle controlled to move from a predetermined inner diameter position of the disk to a predetermined outer diameter position, for example, between an inner diameter position of about 7.6 inches and the outer diameter position of 13.945 inches on a disk of about 14 inches diameter. The rotation of the disk during the application of the acrylic polymer achieves a thickness uniformity of plus or minus 0.005 inches in the active area of the disk, for example, between diameters 8.66 inches and 13.84 inches. The micro roughness of this surface is no greater than 100 angstrom units rms, and surface undulations having spatial wavelengths from 5 millimeters to 50 millimeters is less than 1,000 angstrom units PP. No defect is permissible greater than 200 nanometers in size. Following deposition of acrylic polymer, the layer is cured in ultraviolet light for a time less than 60 seconds, the curing being effective before removal of the coated disk from the deposition apparatus. The uniformity of exposure of the layer to ultraviolet curing light must be better than 90 percent, since uniformity is needed not just for an even cure, but also so that any change induced in the media is uniform. In the above example, the intensity of the curing light at the disk surface must be 25 milliwatts per centimeter or greater, preferably with the spectral intensity concentrated around 360 millimeters. The intensity of infrared radiation during curing must be low, for example, less than 22 milliwatts per centimeter, since the dust defocusing layer may be damage by infrared radiation prior to curing. An optical storage structure as above described in the form of a disk of about 14 inches diameter, is adapted to be rotated at a rate of for example, about 1300 rp . Writing of data on the disk is effected by a laser beam, at the write frequency, with a diameter of 0.5 plus or minus 0.05 nanometers, the beam having a write power equal to or less then 16 milliwatts. The reading photodetectors are adapted to read spot sizes of about 0.75 nanometers. in copending application serial no. 499,666, Muchnik et al. , filed May 31, 1983, assigned to the assignee of the present application, illustrations are provided for showing, at the submicroscopic level, the interface between the matrix layer, active layer and phase layer, showing that the discrete globules of the active layer are encapsulated between the fluorocarbon material of the matrix layer and phase layer. Said prior application points outs that the mode by which the optical properties of the three layer are varied in response to heat from a laser beam is not known. It is believed at the present, however, that the change of optical properties is effected by agglomeration of the materials rather than chemical reaction. As discussed above, it has been found that the active layers comprised of a tellurium, selenium, arsenic alloy are subject to a darkening effect that results in unstable reflectance. Such darkening may be associated with an amorphous-to- crystaline phase transition in the active layer alloy. Such transitions in similar alloys have been studied previously, and have been proposed for use in reversable optical recording. (AW Smith Applied Optics 13(4) 79, (1974) and AE Bell and SW Spong, Applied Physics Letters 38, Number 11, June 1981). For purposes of an archival recording medium adapted to be written only once, however, such darkening is undesirable and may result in a complete loss of signal in time. In accordance with the invention, the effect of darkening has been overcome by alloying the chalcogenide alloy, i.e., the alloy of tellurium and selenium (with or without arsenic), with a metal such as nickel. For example, it has been found that employing an alloy of Te g5 Se 2 n As 5 N iιn results in a dramatic improvement in the disk performance. Mark lengths remain stable even though subject to a 40-hour test on a single track with the read beam intensified to be 2-1/2 times normal read power. In addition, the noise floor spectrum, signal to noise ratio, and mark lengths have been found essentially unchanged. It has further been found that, in the formation of the active layer, it is necessary to ■ simultaneously deposit the nickel or other metal with the remainder of the component of the alloy. Sequential depositions of the metal and chalcogenide no not provide as great an improvement. In production, in accordance with a preferred embodiment, a planar magnetron sputtering target was employed of the desired composition. Various compositions that have been found to be satisfactory are given in Table I. From this table it is apparent that satisfactory results have been obtained employing weight percentages of nickel of from about 2% to about 6% in the Chalcogenide alloy, or, from about 5 to 10 atomic percent. While the best results have been obtained using nickel, especially Alloy STC-68 as shown in Table I, improved results have also been observed using the aluminum alloys shown in Table II. TABLE I Alloy Atomic Percent Weight Percent STC66 Te75 Sel5 As5 Ni5 Te83.7 Sel0.4 As3.3 Ni2. STC67 Te80 Sel5 Ni5 Te87.4 SelO.l Ni2.5 STC68 Te65 Se20 As5 NilO Te76 Sel5 As3.5 Ni5.5 STC69 Te70 Se20 As2 NilO Te80 Sel4 Asl.5 Ni4.5 TABLE II In one manner of deposition, a strip of nickel was overlayed on a larger sputtering target of the composition Te65, Se20, As5, so that the nickel and tellurium alloy were cosputtered by an argon ion beam by a secondary ion arrangement. In another manner, a single target can be made as an intermixture or conglomerate of aluminum along with Te, Se and As, in a relatively homogeneous structure. While the invention has been disclosed and described with reference to a single embodiment, it will be apparent that variations and modifications may be made therein, and it is therefore intended in the following claims to cover each such variation and modification as falls within the true spirit and scope of the in¬ vention.

Claims: WHAT IS CLAIMED IS: 1. In an optical recording structure comprised of an active layer of a Chalcogenide alloy embedded between phase and matrix layers of a material transparent to writing and reading beams, the improvement wherein said active layer further comprises a metal. 2. The optical recording structure of claim 1 herein said Chalcogenide alloy is comprised of at least one material of the group, tellurium, selenium and arsenic. 3. The optical recording structure of claim 1 wherein said metal comprises nickel. 4. The optical recording structure of claim 3 wherein said active layer is comprised of from 2 to 6% of weight by nickel. 5. The optical recording structure of claim 1 wherein said active layer is comprised of from 5 to 10 atomic percent of nickel. 6. The optical recording structure of claim 1 wherein said Chalcogenide alloy is comprised of tellurium, selenium and arsenic, and said metal is nickel of from 2 to 6 weight percent. 7. The optical recording structure of claim 1 further comprising a reflective layer on said phase layer. 8. In the method of forming an optical . recording structure comprising depositing a layer of material transparent to a radiation beam on a reflective layer, depositing an active layer of a Chalcogenide alloy on the phase layer, and depositing a matrix layer of a material transparent to said radiation beam on said active layer; the improvement wherein said step of depositing said Chalcogenide alloy further comprises codepositing therewith a metal. 9. The method of claim 8 wherein said step of codepositing a metal comprises codepositing nickel with a weight percent of 2 to 6% with said Chalcogenide alloy. 10. The method of claim 9 wherein said step of depositing said Chalcogenide alloy comprises depositing tellurium, selenium and arsenic. £2

Inventors: MUCHNIK BORIS J, REVAY ROBERT E, SPONG FRED W, MUCHNIK, BORIS, J., REVAY, ROBERT, E., SPONG, FRED, W.
Applicants: STORAGE TECHNOLOGY CORP, STORAGE TECHNOLOGY CORPORATION
Application year: 1985
Main classification: G11B7
IPC codes: G11B7