An optical memory medium

Abstract: An optical memory medium is described comprising prepits (11, 11') on which a light spot (5) is impinged to obtain information and, if required, guide grooves: each of those (11') among the prepits (11, 11') which are longer than the diameter of the light spot (5) consists of a series of pit portions (11a) and unpitted portions (12); each of the prepits (23) is sandwiched between two discontinuous portions (21a) of guide grooves (21) adjacent to each other; the guide grooves (21) have one or more narrow portions (21b), and each of the prepits (23) is sandwiched between two of the narrow portions (21b) of guide grooves (21) adjacent to each other; or the guide grooves (21) have one or more long portions (21h), one or more short portions (21j), and narrow portions (21i) formed between the long (21h) and short (21j) portions, and each of the prepits (23) is sandwiched between two of the short portions (21j) of guide grooves (21) adjacent to each other.

Description: This invention relates to an optical memory medium with which information can be optically recorded and reproduced or erased. More particularly, it relates to an improved preformat of an optical memory medium. In recent years, with the development of information processing systems, large capacity memories which are inexpensive have been required. To meet this requirement, various optical memory systems have been developed. Such optical memory systems are roughly classified into three categories: read-only type; write once type; and rewritable type. In an optical memory medium which is used in the write once or the rewritable type, information is recorded, reproduced or erased by irradiating a light beam such as a laser beam along tracks (hereinafter, referred to as "recording tracks"). In order to accurately guide a laser spot to an objective position of the optical disk, grooves for guiding the laser spot (hereinafter, referred to as "guide grooves") and pits for prerecorded information are preformed on a glass or plastic substrate. Such pits are called "prepits". The information recorded in the prepits includes generally the track address of a recording track and synchronizing signals for rotating the optical disk at a constant linear velocity. When a recording track is divided into sectors. The prerecorded information also includes the sector address. An example of the preformat portion in a prior art optical memory medium is diagrammatically shown in Fig. 5 see for instance figure 3 of JP-A-62 120 644. In the exemplified configuration, guide grooves 1 formed concentrically or spirally on the surface of a substrate 2. Prepits 3 having a variety of lengths are formed in alignment with the guide grooves 1. Then, a recording layer made of a suitable material such as GdTbFe(Co) is formed by a vacuum deposition, sputtering method or spin coating method. Then, the substrate is covered by a protective substrate, or applied with a hard coating composition is applied, to obtain an optical memory medium. When the guide grooves 1 and prepits 3 are scanned by a laser spot, signals having the waveform as shown in Fig. 5B are obtained from the reflected light beam. The obtained signals are modulated in accordance with the lengths of the prepits 3. The signals will be demodulated and binary coded by setting an appropriate slice level. It is known that the modulated signals are closely related to the physical structure of the prepits 3. As seen from Figs. 6A and 6B, there exists a phase difference between the light beam reflected from an unpitted portion (Fig. 6A) and that reflected from a pitted portion (Fig. 6B). Hence, the strength distribution of the reflected light beam is changed to produce the variation in the light intensity passing through the object lens. Figure 7 shows the relation between the pit widths τ which are normalized by the diameter of the light spot (p x 1/e 2 ; the diameter at which the light strength is 1/e 2 times of the peak strength) and the signal levels obtained from prepits which are longer than the diameter of the light spot, using the depth δ normalized to the wavelength of the light spot as a parameter. In Fig. 7, the signal levels of the reflected beams are plotted in relation to the signal level obtained from unpitted portions. From Fig. 7, it will be seen that the degree of the deflection or the modulation of the reflected light beam is greatest when the width of the prepits is approximately 0.32 times of the diameter of the light spot. Therefore, the prepits are formed so that their width becomes as close to this value as possible. Considering these properties, a substrate for an optical memory medium is produced as follows. A photoresist is applied to a glass substrate, and the substrate is exposed by scanning a laser beam of a short wavelength such as an Ar laser which is modulated in accordance with a coded signal of the information to be prerecorded. The portions exposed to the laser beam are removed by developing, to form prepits or grooves. Using the obtained original glass substrate as a master, the pattern of the original substrate is transferred to a glass or plastic substrate, to obtain a substrate for an optical memory medium. In a section C corresponding to a long prepit, the coded signal has square pulses produced continuously (Fig. 8). In the above-described process of producing the original substrate, it often happens that uneven exposure, developing time and transfer cause a fluctuation in the width of the prepits or grooves of an optical recording medium, resulting in the disturbance of the signal reproduced from the prepits. For example, the waveform obtained from long prepits 3' having a large width (Fig. 9A) is markedly distorted as shown in Fig. 9B. This possibly causes the erroneous detection of the edges of the pulses when binary-coding the data, resulting in misreading of address information or the like. The distorted waveform is likely to occur in the mid portion of a prepit where the width is large, rather than in the end portions where the width is gradually increased or decreased. This is because the diffraction efficiency is lowered in the mid portion, and this appears remarkably when scanning a long prepit. As described above, the control of the width of a prepit which is longer than the diameter of a light spot in producing an original substrate has a great influence on the restraint of the distortion of the reproduced signal in an optical memory medium. Hence, the structure of a conventional optical memory medium requires the fine control of the exposure conditions in producing the original substrate and also a precise control of accuracy in the transfer process, providing an obstacle to the improvement of the yield. Another example of the preformat portion in a prior art optical memory medium is shown in Fig. 10. In the optical memory medium of Fig. 10, guide grooves 1 are formed on a substrate 4, and a record track 2 is formed between guide grooves 1 adjacent to each other. The information such as a track address is prerecorded in the form of prepits 3 formed on the record track 2. The width and depth of the guide grooves 1 have a large effect on the track count signal property for a high speed access operation, and on the track servo property for retaining a light spot on the guide track 2. Consequently, the width of the guide grooves 1 is set within the range from about 0.3 µm to about 0.5 µm. The prerecorded information is read out using the diffraction in the prepits 3. When a light spot moves from a to c as shown in Fig. 11A, the reflected light intensity changes as shown in Fig. 11B. In Fig.11B, I 1 is the reflected light intensity obtained when the light spot is on the prepits 3, and I 2 is the reflected light intensity obtained when the light spot is on a portion other than the prepits 3. The information is read out based on the difference between the light intensities I 1 and I 2 . Therefore, the quality of the reproduced signal depends on the degree of the difference between the light intensities I 1 and I 2 . Because, as described above, the information is prerecorded in the form of prepits 3 formed on the record track 2 disposed between two guide grooves 1, the properties of the information signal which is read out from prepits 3 are greatly affected by the width and/or depth of the guide grooves 1. When the width of the guide grooves 1 is set to be 0.3µm to 0.5µm as stated above, the diffraction efficiency in the prepits 3 is lowered so that the difference between the light intensities becomes small, resulting in that a reproduced signal of high quality cannot be obtained from the prepits 3. This phenomenon may be encountered in a disk such as that disclosed in JP-A-62 110 637 (figure 6). This disk comprises elongate pre-pits on which a light spot is impiged to obtain information (address information). In accordance with the present invention, there is provided an optical memory medium comprising elongate prepits on which a light spot is impinged to obtain information, characterised in that, each of those among said prepits which is longer than the diameter of the light spot comprises a series of circular pit portions and unpitted portions, each of said circular pit portions having substantially the same diameter as each other, said diameter of said circular pit portions and the length of each of said unpitted portions being smaller than the diameter of said light spot. In one embodiment, the length of each of said prepits which are not longer than the diameter of the light spot, is longer than the diameter of each of said circular pit portions. In one aspect of the invention, it is possible to impede a steep change in the diffraction efficiency between the end and mid portions of the prepits even when uneven exposure, uneven developing time or uneven transfer has occurred in the production process of the original substrate. Therefore, the present optical memory medium can prevent the reproduced signal from being distorted, so that a signal of high quality can be reproduced. In other words, the optical memory medium of the invention can surely provide a reproduced signal which is well comparable in quality to that of a conventional medium which has long prepits of an accurately controlled width. Thus, the invention described herein makes possible the objectives of (1) providing an optical memory medium from which undistorted signals of prerecoded information can be produced; (2) providing an optical memory medium which can be produced with a high yield; and (3) providing an optical memory medium the original of which can be easily produced. The invention will be further described by way of example with reference to the accompanying drawings in which:- Figure 1A is a plan view showing the preformat portion of the first embodiment of the invention. Figure 1B is a graph showing the variation of the intensity of the reflected light beam obtained from the optical memory medium of Fig. 1A. Figure 1C is a diagram showing the signal obtained from the reflected light beam of Fig. 1B. Figure 2 is a schematic view of an apparatus for producing an original substrate for the optical memory medium of the invention. Figure 3 is a diagram showing a signal supplied to the apparatus of Fig. 2. Figure 4 is a diagram showing a result of an experiment using a conventional optical memory medium and the optical memory medium of Fig. 1. Figure 5A is a plan view showing the preformat portion of a convnetional optical memory medium. Figure 5B is a graph showing the variation of the intensity of the reflected light beam obtained from the optical memroy medium of Fig. 11A. Figures 6A and 6B illustrate a phse difference between the light beam reflected from an pitted portion and that reflected from a pitted portion. Figure 7 is a graph showing the relation between a pit width and a signal level. Figure 8 is a diagram showing a signal obtained from a conventional optical memory medium. Figure 9A illustrates prepits formed in a conventional optical memory medium. Figure 9B is a diagram showing a signal obtained from the prepits of Fig. 15A. Figure 10 is a perspective view showing another conventional optical memory medium. Figure 11A is a plan view of prepits in the theoptical memory medium of Fig. 16. Figure 11B is a graph showing the variation of the intensity cf the reflected light beam obtained from the prepits of Fig. 11A. Example 1 Figure 1A illustrates diagrammatically an arrangement of prepits in an optical memory medium of the invention. In the optical memory medium of Fig. 1. a row of prepits 11 and 11' is formed to record track address information and/or synchronization information. By scanning a laser spot S along the row, such information can be read out from the medium. Among the prepits 11 and 11', each of the prepits 11' having a length greater than the diameter A of the laser spot S to be scanned consists of a series of pit portions 11a and unpitted portions 12. The length or diameter of the pit portions 11a is smaller than the diameter A of the laser spot S. An apparatus for producing an original substrate for an optical memory medium of the invention is shown in Fig. 2. The apparatus 13 shown in Fig. 2 comprises a laser beam source 14 from which a laser beam is emitted to a modulator 15. The modulator 15 receives the laser beam and a signal to be recorded, and modulates the strength of the laser beam in accordance with the signal. The modulated laser beam is transmitted to an object lens 17 via a mirror 16. The laser beam is converged by the lens 17, and impinged on an original substrate 18, to form a minute focused laser spot on the surface of the substrate. The substrate 18 the surface of which has been applied with a photoresist coating of a predetermined thickness is rotated at a constant linear velocity by a driving motor 20. The apparatus 13 is provided with a feeder 19 which moves the laser spot in the radial direction of the original substrate 18. By the combination of the movement of the laser spot and the rotation of the original substrate 18, prepits are formed concentrically or spirally on the original substrate 18. The signal input into the modulator 15 has been previously processed by a signal processor (not shown) so that the prepits 11' consist of pit portions 11a and unpitted portions 12. More specifically, the signal which corresponds to the signal C in Fig. 14 is converted into a pulse signal train in which each pulse has a width corresponding to the pit portions 11a and the number of the pulse corresponds to that of the pit portions 11a (Fig. 3). After being exposed by the apparatus 13, the original substrate 18 is put into developing and etching processes and the exposed portions are etched away so that prepits are formed. Using the substrate 18 thus formed as a master and conducting processes similar to those of producing so-called video disks or compact disks, an optical memory medium preformatted as shown in Fig. 1A can be produced. When scanned along the prepits 11 and 11', a signal having a waveform which is substantially undistorted can be obtained from the portion corresponding to the prepit 11'. Figure 4 shows a result of an experiment which was conducted to simulate the suppression effect of the medium of this example on the distortion of the reproduced signal. In this experiment, a laser spot having the diameter of 1.3 µm was scanned on a conventional optical memory medium having a long prepit which is 2.8 µm in length, 0.8 µm in width and λ/4 (λ is the wavelength of the laser beam) in depth (shown by a broken line in Fig. 14), and also on an optical memory medium according to the invention which has a prepit 11' comprising three prepit portions 11a having the diameter of 0.8 µm (shown by solid lines in Fig. 4). The prepit portions 11a were arranged in the pitch of 1 µm. In the graph of Fig. 4, the signal level obtained from the conventional medium is shown by a broken line, and that obtained from the present medium by a solid line. As apparent from Fig. 4, the signal obtained from the conventional medium was remarkably distorted in the mid portion of the prepit. In contrast, the signal obtained from the present medium was scarcely distorted. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention specified in the appended claims.

Claims: An optical memory medium comprising elongate prepits (11,11') on which a light spot (S) is impinged to obtain information, characterised in that, each of those (11') among said prepits (11,11') which is longer than the diameter of the light spot (S) comprises a series of circular pit portions (11a) and unpitted portions (12), each of said circular pit portions (11a) having substantially the same diameter as each other, said diameter of said circular pit portions (11a) and the length of each of said unpitted portions (12) being smaller than the diameter of said light spot (S), An optical memory medium as claimed in claim 1, wherein the length of each of said elongate prepits which are not longer than the diameter of the light spot (11) is longer than the diameter of each of said circular pit portions (11a).

Inventors: DEGUCHI TOSHIHISA, INUI TETSUYA, MIEDA MICHINOBU, OHTA KENJI, TAKAHASHI AKIRA, VAN KAZUO, DEGUCHI, TOSHIHISA, INUI, TETSUYA, MIEDA, MICHINOBU, OHTA, KENJI, TAKAHASHI, AKIRA, VAN, KAZUO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B7
IPC codes: G11B7