GB2148148A - Optical storage medium and method for making same - Google Patents

Abstract

In an optical storage medium comprising a plurality of thin film layers overlying a reflective substrate the thicknesses of the layers are selected to provide an optimum optical performance in response to a plurality of light wavelengths, in particular maximum absorption at the write wavelength frequency, 15% reflectance at the focus-sensing wavelength and low reflectance at the coarse positioning wavelength.

Description

SPECIFICATION Optical Storage Medium and Method for Making Same The present invention relates to optical storage media and more particularly to the design and fabrication of multiple layer digital optical storage recording media.

Over the last few years, dramatic increases have occurred in areal storage density. Tape devices of the early sixties stored only 13 bits per square inch (25.4 mm. square). Today, magnetic disk can store up to 132 million bits per square inch and more with plated magnetic media. In comparison, optical storage units can store up to 600 million bits per square inch, with greater densities within reach.

An optical storage unit typically contains a spinning disk or platter carrying a suitable optical medium.

In a write operation, the input data stream modulates the output of a semiconductor laser diode. The laser optically records the data pattern in the medium by producing marks in the active layer(s) of the spinning platter face. Since the coherent light beam can be focused onto a very small area, individual data marks and tracks can be placed extremely close together. In fact, a typical optical storage unit can store up to forty or more times the data per square inch than can be stored in a comparable magnetic disk. In a typical read operation, light from a gas laser scans the spinning platter. The reflected light is modified by the data marks and detected by a group of photodiodes. A typical data file consists of concentric tracks whose spacing is on the order of 1 micron.The position at which the read or write laser is focused is set according to position information acquired from a reflected beam which is functionally distinct from both the read and write beams.

The spinning platter carries an optical medium whose optical properties vary with the wavelength of impinging light. Special difficulties are encountered when the read, write, and positioning functions are performed at different wavelengths. Optimizing an optical medium for any single function may compromise its response at other wavelengths used for the remaining functions.

In a typical configuration, the concentric user data bands on the storage platter, which can amount to 700 or more, are accessed by means of a coarse access seek mechanism through a first actuator. Once the desired band has been reached, a second actuator positions the laser beams within the band to the desired data track. Fine position information may be acquired from a laser beam that is functionally distinct from all other beams.

Since the spinning platter is made up of a plurality of reflecting, transparent and optically active or absorbing layers, all of varying thicknesses, the process of selecting various layers for optimum performance at the several wavelengths has been substantially a cut and try operation. Since the materials involved in manufacturing such devices can be relatively expensive and the manufacturing process complex, the process of cut and try for varying wavelengths is difficult, expensive and time consuming.

It is therefore a principal object of the present invention to provide a method of designing an optical medium made up of a plurality of selected materials so that characteristics of said optical medium may be optimized for the materials and layer sequence selected.

It is a further object of the present invention to provide means whereby optical characteristics of predetermined materials may be stored and referenced over a range of wavelengths so as to provide desired optical properties for a multiple layer optical medium at specified wavelengths.

It is another object of the present invention to utilize the speed and power of modern computing to enable the designer to design optical media comprised of a plurality of layers of different materials when a number of wavelengths will be applied thereto.

It is a still further object to design and construct an optical medium having a plurality of layers including one or more optically active layers which has optimum optical properties at a plurality of applied wavelengths.

The foregoing objects are effected in an optical medium subjected to a plurality of discrete applied optical wavelengths, the sensitivity of said medium being a function of the maximum energy absorbed by the active layer therein at the write wavelength, the quality of the read back signal being a function of the optical properties at the read wavelength, and performance in both operations being a function of the optical properties at position sensing wavelengths, the invention providing a method of manufacturing said optical medium as a plurality of thin film layers overlying a reflective substrate, and comprising the steps of selecting each of a plurality of layer materials, selecting the thickness of each of said layers, calculating the optical properties of all of said layers together at each of said plurality of wavelengths, repeating said calculation for a plurality of thicknesses to determine the optical properties at each of said wavelengths by said optical media for all of said wavelengths, selecting that combination of thicknesses which result in the optimum optical properties at each said wavelength for all of said plurality of wavelengths and assembling said media with each layer thickness being predetermined in accordance with said selection. In furtherance of the invention the selection and calculation process may be facilitated by the use of a computing facility wherein the characteristics of each of the materials can be entered into an appropriate program established for that purpose and the results computed for each preselected wavelength. The results may be displayed in tabular or graphic modes.

More specifically, the present invention provides for storing, in a computer, data relating to a plurality of materials which may be employed for a multilayer structure of thin films over a reflective layer including the identity of each material and its related optical constants over a range of wavelengths between 200 and 900 nanometers, at 10 nanometer intervals. Linear interpolation is used between the stored values to provide values at each wavelength desired. In operation, the operator enters a series of preselected material names in accordance with a proposed multilayer stack, along with a proposed set of thicknesses for each of said elements. The thickness and the optical constants of each of the layers are entered into a mathematical model which determines the optical properties of the multiple layer medium at the specified wavelength.

The calculations are repeated for each of the multiple wavelengths desired for use in the optical media system contemplated by the present invention. The ultimate result is calculated values for reflectance, phase of the reflected light, and absorption in each layer of the multiple layer stack as a function of applied wavelength. Thus, by entering different sequences of layers, with different thicknesses, an operator may determine an optimized configuration for multiple wavelengths which will result in a response as close as possible to a desired response.

In accordance with the invention, an optical medium consisting of a stack of multiple layer thin films may be fabricated with optical properties optimized for a plurality of different wavelengths of incident radiation.

The invention will be further described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of the cross section of a typical multilayer film stack; Fig. 2 is a flow chart illustrating the sequence of operations employed in the selection processes of the present invention; Fig. 3 is a graphic presentation of data provided in accordance with the present invention; and Fig. 4 is another graphic presentation of data provided in accordance with the present invention.

Fig. 5 illustrates a method of calculating the optical properties of a thin film stack.

With reference to Fig. 1, a typical multilayer arrangement comprising an optical medium which may be utilized in accordance with the present invention is illustrated. As shown, the multiple layer arrangement will include a substrate 10, a first overlying layer 12 such as of aluminum, a second overlying layer 14 such a silica, a third overlying layer 16 which may be an active layer, a fourth overlying layer 18 which may be a dielectric layer and a fifth overlying layer 20 which may be a suitable protective overcoat. A more detailed presentation of the layers and their optical significance is presented in applicants copending U.S.

application 499,666, filed May 31, 1983, entitled "Optical Recording Structure Involving In-situ Chemical Reaction in the Active Structure", and assigned to the present applicant.

Although five layers have been shown in Figure 1, it will be understood that the present invention is theoretically applicable to any number of layers, but as a practical matter from five to twenty layers may be incorporated in the construction of the optical media. For purposes of the present invention, an optimized system will have the following optical properties: Maximum absorption at the recording laser wavelength, approximately 15% reflectance at the focus-sensing laser wavelength, using for example a helium neon laser, and have low reflectance at the wavelength employed to determine laser coarse seek position for location within track bands.

By way of example, three laser wavelengths may be used, 820~850 nanometers for writing, 633 nanometers for reading and focus sensing, and 790 nanometers for coarse seeking. It is understood of course that these parameters and wavelengths may vary, but within the defined characteristics of the invention, may be considered optimized. It is therefore evident that three individual wavelengths differing from one another by several tens of nanometers are applied to the same optical media. It is therefore a necessary requirement that the spectral characteristics be optimized at all three wavelengths.

Certain criteria are employed for purposes of determining the optical properties in a theoretical multilayer thin film structure model. The invention assumes the multilayer thin film structure is built on top of a given substrate, such as aluminum. The invention also assumes that any number of layers may be employed. The parameters that determine the optical properties of the thin film structure are the physical thickness and the optical constants of each layer.

The optical constants defined in the conventional theory of thin film optics are used in the mathematical model employed. Two values are defined and designated as n and k. The parameter n represents the real part of the complex refractive index, whereas k represents the imaginary part of the complex refractive index. The refractive index of a material varies with the wavelength of the incident light. The thickness and optical constants of each layer determine three relevant optical properties of the thin film stack. First is the reflectance of the stack as a function of the wavelength of the applied light. Second is the absorption of the laser light in the active layer as a function of the wavelength of the applied light, and third is the phase of the reflected light relative to the incident light as a function of the wavelength of the applied light.

For selected materials, values of the optical constants may be stored in tables within the model for every 10 nanometers between 200 and 900 nanometers. The program takes the optical constants for a given layer from one of two sources. If numeric values are provided by the operator in specifying a given layer of the stack, those values are used. If, on the other hand, numeric values are not given, the program retrieves the appropriate values from its internal tables. Linear interpolation is employed between stored values.

Using the foregoing principles, it is thus possible to design an optical medium as a plurality of thin film layers overlying a reflective substrate, by selecting each of a plurality of layer materials, selecting the thickness of each of said layers, calculating the optical properties of all of said layers together at each of said plurality of wavelengths, repeating said calculation for a plurality of thicknesses to determine the optical properties at each of said wavelengths by said optical medium for all of said wavelengths, selecting that combination of thicknesses which result in the optimum optical properties at each said wavelength for all of said plurality of wavelengths and fabricating said medium with each layer thickness being predetermined in accordance with said selection.

The sequence of operations employed is illustrated in Figure 2. Thus, as shown, data is input into the computer in an interactive user-friendly sequence which first asks for the wavelength of applied light, the angle of incidence and the state of polarization. Wavelength may be specified in any units desired provided that layer thicknesses are specified in the stack in the same units. The angular incidence is specified in degrees, and the polarization states may be described as either s polarization, p polarization or circular polarization. For each layer of the thin film stack, the identity of the material which makes up the layer, the n value, k value, and thickness are input into the computer. For layers made up of materials whose optical constants have been prestored in the model, an appropriate code such as an asterisk may be specified for n and k.This will indicate to the model that, when computing the optical properties of the thin film stack, it is to retrieve values for that layer from its internal tables. A typical "screen" illustrating the data input phase of the model is illustrated in the following table.

"EXAMPLE" is new.

If you just mis-spelled it, Enter "STACK IS correctname".

Otherwise begin entry of new stack definition: WAVELENGTH ANGLE OF INCIDENCE POLARIZATION (NO DEFAULT) (DEFAULT: 0 DEGREES) (DEFAULT: CIRCULAR) 8350 AMBIENT N (DEFAULT: AIR) (DEFAULT: 1.00029) pmma LAYER DEFINITION & IMPLICIT SYNONYM DECLARATION (NULL TERMINATES) active:=: stc51 30 phase :=:Si02 60 DEFINE THE STACK: MATERIAL THICKNESS N K active phase active phase active silica 800 1.32 aluminum 1000 What's substrate's index ("RETURN" means "don't care")? WAVELENGTH=8350 ANGLE=0 POLARIZATION='CIRCULAR' LAYER (0)='PMMA 'N (0)=* LAYER (1)='STC51 'N(1)=* K(1)=* D(1)=30 LAYER (2)='SI02 'N (2)=* K (2)=* D (2)=60 LAYER (3)='STC51 'N (3)=* K (3)=* D (3)=30 LAYER (4)='Sl02 'N (4)=* K (4)=* D (4)=60 LAYER (5)='STC51 'N (5)=* K (5)=* D (5)=30 LAYER (6)='SILICA 'N (6)=1.32 K (6)=0.0 D (6)=800 LAYER (7)='ALUMINUM 'N(7)=* K(7)=* D (7)=1000 LAYER (8)='SUBSTRATE 'N (8)=1.00; RUN=1 SYNONYM(S):ACTIVE 1 35 PHASE 24 ENTER COMMAND When all the data has been entered, the results are displayed in response to a "RUN" command. The stack reflectance and transmittance as well as reflected phase shift for the specific wavelength is generated and displayed, along with the absorption of each layer of the stack. The resulting screen is illustrated below for a run involving four layers on the substrate.

ENTER COMMAND: run for d(2)=1 0 WAVELENGTH =835 ANGLE=0 POLARIZATI0N='ClRCULAR' LAYER (O)='AIR 'N (0)=1.00029 LAYER (1)='POLYMER17 'N(1)=* K(1)=* D (1)=270 LAYER (2)='STC51 'N (2)=* K (2)=* D (2)=8 LAYER (3)='POLYMER17 'N (3)=* K (3)=* D (3)=100 LAYER (4)='ALUMINUM 'N (4)=* K (4)=* D (4)=100 LAYER (5)='SUBSTRATE 'N (5)=1.00; RUN=88 SYNONYM(S): PHASE 3 COMPUTING...

S P NET STACK REFLECTANCE: 0.009 0.009 0.009 STACK TRANSMITTANCE: 0.000 0.000 0.000 PHASE SHIFT ON REFLECTION: -79.4 -79.4 (DEGREES) TOTAL ABSORPTION BY LAYER: LAYER 1 POLYMER17 0.000000 0.000000 0.000000 LAYER 2 STC51 0.934961 0.934961 0.934961 LAYER 3 POLYMER17 0.000000 0.000000 0.000000 LAYER 4ALUMINUM 0.056437 0.056437 0.056437 ENTER COMMAND: This description illustrates the manner wherein a predetermined number of thin film layers of selected thicknesses and materials built up over a substrate determine optical properties at specified wavelengths for the thin film stack.

The operator may repeat the calculation for various structures until that structure is found which provides a response as close as possible to the desired response at each of wavelengths of interest. It is also possible to command the computer to vary the thickness of a particular layer and display the optical properties as a function of that thickness at the specified wavelength. It is further possible to vary simultaneously the thickness of each of a set of layers and observe the resulting spectral response.

Analysis of the phase of the reflected wave can be plotted as well, and an example thereof is shown in Fig. 4.

The foregoing programs are run under an IBM VM/CMS operating system, although it will be apparent to the skilled practitioner that other comparable computer environments may be employed for the same purpose. The mathematical model or algorithm employed for the calculation of the optical response is shown in the documentation attached hereto as Figure 5, and which is intended to be an example of formulations which may be employed to compute the optical properties of the medium at applicable wavelengths.

Once the appropriate selection of layers and their thicknesses has been made, it remains only for the thin film stack to be fabricated. To this end, onto a suitable substrate material the several layers are sputtered or vapor deposited in sequence until the structure has been completely fabricated. The thicknesses may be accurately controlled by standard techniques evident to those skilled in the art.

As one example of a medium in which the optical properties have been optimized over a plurality of wavelengths, three in this case, reference is again made to Fig. 1 wherein a five layer structure is shown.

The structure is a thin film stack with the materials and dimensions of each layer shown in the following Table I which is intended to be an example of one specific embodiment of a medium created in accordance with the invention.

TABLE I

LAYER MATERIAL THICKNESS (A) LAYER 10 ALUMINUM - LAYER 12 ALUMINUM 1000 LAYER 14 POLYMER 17 400 LAYER 16 STC 51 125 LAYER 18 POLYMER 17 1500 LAYER 20 PMMA Optical performance was optimized in this structure for three wavelengths: 820 nanometers, 632.8 nanometers, and 790 nanometers. The values for each layer in the resultant structure is shown in Table II.

TABLE II

WAVELENGTH REFLECTANCE ABSORPTION 820 nm 10% 65% 632.8 nm 11% 82% 790 nm 8% 70% In the examples, Polymer 17 is a thermoplastic material and STC 51 is a chalcogenide.

Further variations and modifications in accordance with these present inventions will be apparent to those skilled in the art.

Claims (11)

1. A method of manufacturing an optical recording medium subjected to a plurality of discrete applied optical wavelengths, the performance of said medium being a function of its optical properties at each of said plurality of wavelengths, a plurality of thin film layers overlying a reflective substrate, and comprising the steps of selecting each of a plurality of layer materials, selecting the thickness of each of said layers, calculating the optical properties of said layers together at each of said plurality of wavelengths, repeating said calculation for a plurality of thicknesses to determine the absorption of light at each of said wavelengths by said optical medium for all of said wavelengths, selecting that combination of thicknesses which result in the optimum optical properties at each wavelength for all of said plurality of wavelengths and assembling said medium with each layer thickness being predetermined in accordance with said selection.

2. The method of claim 1 wherein each of said materials has a plurality of optical constants associated therewith, and wherein a computer prestores the optical constants for each of a plurality of a selected optical materials, said optical constants being retrieved by said computer upon identification thereto of said materials, and said calculations being based upon said optical constants.

3. The method of claim 1 or 2, wherein said calculation is based upon Fresnel interference principles governing the optical properties of thin film stacks.

4. The method of claim 1,2 or 3, further comprising the steps of employing a computer having a plurality of prestored constants for each of a plurality of different materials over a range of wavelengths varying from 200 nanometers to 900 nanometers, and wherein each of said materials is entered in predetermined sequence in accordance with the build up characteristic of layers of a thin film multiple layer stack over a substrate.

5. A method of tuning a thin film multiple layer stack including light reflecting and light absorbing materials over a reflective substrate, comprising the steps of determining the optical characteristics of each layer in accordance with the thickness of said layer at each of a plurality of wavelength of incident light, adjusting the thickness of each of said layer so as to conform as closely as possible to predetermined spectral characteristics at each of said wavelengths, and assembling a multiple layer thin film stack of predetermined elements with thicknesses of each layer selected so as to provide the closest possible desired optical response at each of the aforesaid multiple wavelengths.

6. In an optical storage medium, the method comprising calculating the optimum thickness of each of a plurality of layers of a thin film multiple layer stack overlying a reflective substrate at each of a plurality of wavelengths, the thicknesses of said individual layers at each of said wavelengths representing the desired optical response at said wavelengths, and assembling a thin film multiple layer stack in accordance with said selected thicknesses.

7. An optical storage medium, comprising a substrate, a first layer overlying said substrate and constituting a phase layer, a second layer overlying said first layer and constituting an active light absorbing layer, and a third layer overlying said second layer and constituting a further phase layer, the thickness and composition of each of said layers being selected so as to render the optical properties of said medium optimum for a plurality of wavelengths.

8. The construction of the medium of claim 7 wherein said optical properties is optimized for three distinct wavelengths.

9. An optical recording medium comprising a substrate having a plurality of coatings thereupon, wherein the thickness of each layer is selected such that the combination of layers has a predetermined response to a plurality of predetermined light wavelengths.

10. A thin film multiple layer stack fabricated according to a method substantially as hereinbefore described with reference to the accompanying drawings.

11. Apparatus for the production of an optical storage medium, comprising a stack of thin layer materials, said apparatus comprising means for storing coefficients indicative of optical response of said materials at predetermined light wavelengths, means for determining the response of a stack of predetermined thin layer thicknesses to a plurality of light wavelengths utilising said predetermined coefficients and means for determining said response for a different set of thin layer thicknesses, and means for determining when said response reaches predetermined minimum requirements.