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J -| 1 I I I I I L
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DUAL LAYER OPTICAL MEDIUM HAVING
PARTIALLY REFLECTING THIN FILM
FIELD OF THE INVENTION 5
The present invention relates generally to the field of optical media, and more specifically to the area of optical media which employ two or more information storage layers. 10
BACKGROUND OF THE INVENTION
There is a seemingly never-ending demand in the field of data storage for media having increased storage capacity and )5 performance. In the field of pre-recorded optical discs, such as compact discs and video discs, increased storage capacity is usually achieved by increasing the storage density per unit area of the disc. However, the maximum data storage density achievable in an optical recording system is limited by the 2Q smallest feature that the optical system can resolve. For conventional far-field imaging systems, the smallest resolvable feature size is limited by diffraction effects to approximately the wavelength of the available light source, usually a solid state laser diode. Thus, one method of increasing disc 25 storage capacity is to decrease the wavelength of the laser diode. However, while the wavelengths available from laser diodes have been steadily decreasing, the decreases have not been dramatic due to limitations in solid state technology and materials. ^
A number of other techniques for increasing storage capacity of optical recording systems have been proposed. These include: (1) higher efficiency data coding schemes, e.g., pulse-width modulation; (2) optical and/or magnetic super-resolution; (3) zoned recording at constant angular 35 velocity; (4) advanced data channel detection methods, such as partial response/maximum likelihood detection, and (5) recording on both the grooves and land areas of the disc.
While the preceding methods for increasing storage capacity all rely upon increasing the storage density per unit 40 area of the disc, an alternative method for increasing the capacity of an optical disc is to employ additional storage layers on the disc which can be independently recorded or reproduced. Thus, the approach in this case is to increase the addressable area of the disc. This approach is attractive 45 because it has the potential to substantially increase media storage capacity with only a modest increase in media and recording system complexity.
If multiple storage layers, e.g., 2, are to be reproduced by optical beam(s) provided on one side of the disc, then one of 50 the storage layers of the disc must be reflective enough so that it may be reproduced by the optical beam(s), but transparent enough so that the beam(s) may penetrate the first storage layer and pass on to a second storage layer. However, such a disc has proved to be difficult to construct, 55 especially, where only a single laser is employed.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an optical 60 disc having a partially reflecting layer and a transparent spacer layer that allows a single reproducing optical beam to focus on either of two different planes within the disc. The disc includes a transparent substrate having a pattern of pits in one of its sides. A partially reflective layer adjacent the pit 65 pattern has an index of refraction having a real component (n) between 2.6 and 3.2 and an imaginary component (K)
less than 0.4, measured at any wavelength within the range of from 500 to 850 nm. A transparent polymer spacer layer is provided over the partially reflective layer, and a highly reflective layer is provided over the spacer layer.
In one embodiment of the present invention, the substrate comprises polycarbonate and the spacer layer comprises a photopolymer. A second pattern of pits may be provided in the side of the spacer layer adjacent the highly reflective layer. The internal surface reflectivity of the partially reflective layer preferably varies by less than ±0.03 over variations in thickness in the partially reflective layer of +10%. The spacer layer has a thickness of from about 5 to 100 um.
In another embodiment of the present invention, the partially reflective layer includes silicon carbide. One preferred ratio of the silicon to the carbon in the partially reflective layer is 1:1.3. In yet another embodiment, the partially reflective layer includes silicon carbide containing from about 5 to 15 atomic % oxygen. The partially reflective layer is preferably 30 to 80 nm thick.
The present invention also includes optical storage systems which include the media described above. The systems further include a focused laser beam positioned to enter the medium through the substrate, means for adjusting the focal position of the laser beam on either the partially reflective or highly reflective layer, and a photodetector positioned to detect the reflected laser beam exiting the medium.
As used herein, the terms "silicon carbide" or "SIC" mean mixtures of silicon and carbon ranging in composition from 30-50 atomic % silicon, 35-60 atomic % carbon, and 0-20 atomic % oxygen, as measured by x-ray photoelectron spectroscopy, and having silicon-carbon stoichiometrics ranging from SiC0 9 to SiC14.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows an optical data storage system according to the present invention.
FIG. 2 is a computer-generated graph of internal interface reflectivity at 650 nm as a function of thickness for various materials.
FIG. 3 is a computer-generated graph of internal surface reflectivity at 650 nm as a function of thickness for silicon carbide according to the present invention.
FIG. 4 is a computer-generated graph of apparent reflectivity at 780 nm as a function of thickness for silicon carbide according to the present invention.
FIG. 5 is a graph of the real component of the index of refraction (n) as a function of wavelength for various materials according to the present invention.
FIG. 6 is a graph of the imaginary component of the index of refraction (K) as a function of wavelength for various materials according to the present invention.
FIGS. 7A-7C show photomicrographs of various layers of the optical recording medium constructed according to Example 1.
An optical data storage system 10 according to the present invention is shown in FIG. 1. Optical storage medium 12 comprises a transparent substrate 14, a partially reflective thin film layer 16 on a first data pit pattern 15, a transparent spacer layer 18, and a highly reflective thin film layer 20 on a second data pit pattern 19. An optical laser 30 emits an optical beam toward medium 12, as shown in FIG. 1. Light from the optical beam which is reflected by either thin film
layer 16 or 20 is sensed by detector 32, which senses modulations in light intensity based on the presence or absence of a pit in a particular spot on the thin film layers.
Although patterns 15 and 19 are referred to as "data pit patterns," pit patterns 15 and 19 may be any pattern of pits 5 or grooves that is capable of storing information, be it data, servo or tracking information, format information, etc.
The capability for independently reading either the first or second pit pattern 15 or 19 is based on the comparatively limited focal depth characteristic of typical optical disc 10 readout systems. The lenses employed in typical optical recorders/players to form a diffraction limited laser radiation spot on the media storage layer have moderately large (0.4 to 0.6) numerical apertures to improve resolution and increase storage density. Such lenses exhibit focal depths 15 (i.e., the range of focus variation over which the focused spot size remains approximately diffraction limited) of about 2 um; for large focus variations the size of the illuminated spot grows rapidly. Consequently, if partially reflective thin film layer 16 exhibits adequate transmission and the distance 20 separating the two data pit patterns 15 and 19 is large relative to the optical system focal depth, it is possible to focus the laser 30 on either data pit pattern with acceptably low "cross-talk" from the other data pit pattern. Thus, although the light from laser 30 will be reflected back toward detector 25 32 by both layers 16 and 20, only the layer upon which the laser is focused will strongly modulate the reflected light intensity, thereby enabling data readout.
The data pit patterns 15 and 19 on medium 10 can be reproduced most easily by first focusing on one of the 30 reflective layers 16 or 20, and then reproducing the data on that entire layer before switching focal position to focus on the other reflective layer. In the alternative, it may be desirable to switch focus position one or more times before completely reproducing the data contained in one of data pit 35 patterns 15 and 19. In either case, use of two data pit patterns separated by transparent layer 18 effectively doubles the data storage capacity of optical recording medium 10.
Transparent substrate 14 may be a polymeric material suitable for optical disc substrates which supports molding 40 of data pit pattern 15 with sufficient fidelity, such as polycarbonate or amorphous polyolefin. Alternatively, it is possible to use a flat substrate of, for example, glass or polymethylmethacrylate, and form data pit pattern 15 by means of photopolymer replication, as will be described for the 45 formation of data pit pattern 19.
Transparent spacer layer 18 may be a polymer, such as a photocurable polymer, which has a complex refractive index with a real component, n, ranging from about 1.45 to 1.6 and 5Q an imaginary component, K, of less than 10 4 and more preferably less than 10"5. Transparent spacer layer 18 should be thick enough to allow laser 30 to focus on either of data pit patterns 15 and 19 with a minimum of cross-talk. This translates into a thickness that is preferably within the range 55 of from about 5 to 100 um, and more preferably from about 30 to 50 um.
Highly reflective layer 20 may be a metallic layer which exhibits high reflectivity at the laser wavelength used to reproduce the data. Currently available laser diode sources ^ radiate at wavelengths ranging from about 600 to 850 run. Aluminum, gold, silver, copper and their alloys can exhibit suitably high reflectivity in this wavelength range. Highly reflective layer 20 preferably has a reflectance of at least 70%, and more preferably at least 80%. 65
In order to minimize the complexity and cost of optical data storage system 10, it is desirable that the average
readout signal levels from each of the data pit patterns 15 and 19 be approximately equal. Thus, the apparent reflectivities from layers 16 and 20, as seen by detector 32, should also be approximately equal.
As used herein, the term "apparent reflectivity" refers to the fraction of optical power incident upon transparent substrate 14 which, when focused to a spot on a flat region of either layer 16 or 20, could, in principle, be sensed by a photodetector in an optical readout device. It is assumed that the readout device comprises a laser, an appropriately designed optical path, and a photodetector. It is further assumed that the optical element in the optical path which is in closest proximity to transparent substrate 14 is a high (>0.4) numerical aperture objective lens. As used herein, the terms "internal surface reflectivity" or "internal interface reflectivity" refer to the fraction of optical power incident upon an interface within the media structure (e.g., the interface between transparent substrate 14 and partially reflecting layer 16 or the interface between spacer layer 18 and highly reflecting layer 20) which is reflected.
In order to estimate the necessary reflectivity from partially reflective layer 16, we assume that highly reflective layer 20 consists of aluminum, which reflects about 80 to 85% of the light incident on the internal interface between spacer layer 18 and highly reflective layer 20. It is further assumed that the refractive index real component, n, of spacer layer 18 is 1.5, that substrate 14 is polycarbonate with a refractive index real component, n, of 1.57, and that reflections at the air-substrate interface do not contribute to the readout signal. If we further assume that partially reflecting layer 16 is an ideal material which exhibits no absorption, it can be shown that a reflectivity of about 0.35, as observed at the internal interface between substrate 14 and the partially reflecting layer will yield a balance in the apparent reflectivities from layers 16 and 20. While a partially reflecting layer 16 which exhibits no absorption is ideal, real partially reflecting layer materials are likely to exhibit some absorption. If we choose a hypothetical partially reflective layer which absorbs 25% of the light it does not reflect and define this as an upper limit for acceptable absorption, we find that an internal surface reflectivity of about 0.25 is required to balance the reflectivity of layers 16 and 20. In this case, the apparent reflectivities from both layers is about 30% less than for the case of a partially reflecting layer which exhibits no absorption. Thus, the preceding examples define a range for the internal surface reflectivity at the interface between the substrate 14 and layer 16 of from about 0.25 to 0.35. Taking into account the attenuation due to reflections at the substrate-air interface, the above range corresponds to an apparent reflectivity seen by an optical readout device of about 0.24 to 0.33.
Candidate materials for partially reflecting layer 16 include metals, semiconductors and dielectrics. Metals, however, are generally strongly absorbing and may be expected to cause excessive signal attenuation. Furthermore, the reflectivity of metallic films typically is a very strong function of film thickness. FIG. 2 is a computer-generated graph based on optical modeling showing internal surface reflectivity for incident light of wavelength 650 nm as a function of thickness calculated for films of gold (Au), aluminum (Al), and silicon (Si) films sandwiched between a 1.2 mm thick polycarbonate substrate and a slab of n=1.5, K=0 material, which approximates the effect of transparent spacer layer 18.
Examination of FIG. 2 reveals that the reflectivity of an Al or Au partially reflecting layer changes very rapidly with thickness, making control of film thickness and uniformity