EP0888612A1 - Apparatus and methods for forming and use with variable pit depth optical recording media - Google Patents
Apparatus and methods for forming and use with variable pit depth optical recording mediaInfo
- Publication number
- EP0888612A1 EP0888612A1 EP97917035A EP97917035A EP0888612A1 EP 0888612 A1 EP0888612 A1 EP 0888612A1 EP 97917035 A EP97917035 A EP 97917035A EP 97917035 A EP97917035 A EP 97917035A EP 0888612 A1 EP0888612 A1 EP 0888612A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- transfer function
- intersymbol
- recited
- photoresist
- nonlinear
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/10009—Improvement or modification of read or write signals
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/14—Digital recording or reproducing using self-clocking codes
- G11B20/1403—Digital recording or reproducing using self-clocking codes characterised by the use of two levels
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/14—Digital recording or reproducing using self-clocking codes
- G11B20/1496—Digital recording or reproducing using self-clocking codes characterised by the use of more than three levels
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B27/00—Editing; Indexing; Addressing; Timing or synchronising; Monitoring; Measuring tape travel
- G11B27/10—Indexing; Addressing; Timing or synchronising; Measuring tape travel
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B27/00—Editing; Indexing; Addressing; Timing or synchronising; Monitoring; Measuring tape travel
- G11B27/10—Indexing; Addressing; Timing or synchronising; Measuring tape travel
- G11B27/19—Indexing; Addressing; Timing or synchronising; Measuring tape travel by using information detectable on the record carrier
- G11B27/28—Indexing; Addressing; Timing or synchronising; Measuring tape travel by using information detectable on the record carrier by using information signals recorded by the same method as the main recording
- G11B27/30—Indexing; Addressing; Timing or synchronising; Measuring tape travel by using information detectable on the record carrier by using information signals recorded by the same method as the main recording on the same track as the main recording
- G11B27/3027—Indexing; Addressing; Timing or synchronising; Measuring tape travel by using information detectable on the record carrier by using information signals recorded by the same method as the main recording on the same track as the main recording used signal is digitally coded
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B27/00—Editing; Indexing; Addressing; Timing or synchronising; Monitoring; Measuring tape travel
- G11B27/36—Monitoring, i.e. supervising the progress of recording or reproducing
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/004—Recording, reproducing or erasing methods; Read, write or erase circuits therefor
- G11B7/005—Reproducing
- G11B7/0052—Reproducing involving reflectivity, absorption or colour changes
Definitions
- the present invention relates generally to a method and apparatus for mastering a compact disc with multiple level pits. Specifically, a method and apparatus are disclosed for creating a layer of photoresist of a depth that is precisely controlled so that more than two levels of pits varying within approximately one quarter wavelength of the reading laser light are created. In one embodiment, a method and apparatus for intersymbol interference compensation that compensates for intersymbol interference between different depth pits is further disclosed.
- a compact disc In a compact disc (CD and CD-ROM), information is stored on the surface of a disc as pits which are read by a laser. The pits are transferred to the plastic surface by a metal stamper which is produced, through a series of intermediates, from a master. The information is transferred to the master during the mastering process.
- the mastering process involves using a focused laser beam to record the information onto a thin layer, generally about 150 nm thick, of photosensitive material deposited onto a glass substrate.
- the photosensitive material can be positive photoresist or a thermally ablative medium (so-called "Direct Mastering"). Photoresist is, however, currently the most common medium used for mastering.
- stamper 1 14 Each metal mother may then be used to form a stamper 1 14.
- the stamper is then used to stamp a polycarbonate plastic substance 115 into the form of a disc 116 in a molding process.
- stamper 114 is removed from disc 116, disc 116 is coated with a reflective metal coating 118 and then a protective plastic layer 120 is deposited over the surface of disc 116.
- Protective plastic layer 120 functions to protect the pits from being damaged.
- the master disc is used to create a negative disc, i.e., a disc which has bumps in locations which correspond to pits in the compact disc being produced.
- the father disc creates a series of mother discs which have pits corresponding to the master and the final compact disc which is being produced.
- the mother creates a stamper which is a negative of the master and finally, the stamper creates the compact disc which has pits that correspond to the pits created in the master disc.
- SUBSTITUTE SHEET (RULE 28) series of logical 0's, and the number of zeroes in the series is proportional to the length of the flat region between edges.
- More information could be stored on the same amount of CD surface area if, instead of merely detecting edge transitions between pit and land, a CD reader could detect multiple levels of pits. For example, if eight different levels of pits could be distinguished, then three bits of information could be stored in the area of a minimum-length pit (16 different levels would encode four bits, 32 levels would encode five bits, and so on). To further increase the information capacity of the CD, rather than following a variable-depth pit by a land region, another variable-depth pit could immediately follow the preceding one, and so on, so that each variable-depth pit immediately abuts each adjacent variable-depth pit.
- An optical disc that stores more than one bit of information at each pit or symbol location by modulating the depth of the pits is referred to as a pit depth modulated (PDM) disc.
- PDM pit depth modulated
- a method of mastering pits of variable depth would be necessary in order to allow the density and data-rate speed increases afforded by a variable depth pit scheme such as mentioned above.
- a multistep mastering method for variable depth pits is described in U.S. Patent No. 5,235,587, issued August 10, 1993, to Bearden, et al. This method suffers from the need to do multiple mastering steps, one step for each depth of pit desired, which is undesirable. It would be advantageous if mastering of variable-depth pits could be done in a single mastering step and if mastering variable-depth pits could be accomplished using the same photoresist and mastering benches used commonly today in mastering CD's. It would furthermore be desirable if such pits could be read by currently available optical readers with minimal modification.
- U.S. Patent No. 4,150,398, issued to Kojima, et al. describes a single step mastering process suitable for use in storing analog signals on a disc using photoresist.
- Kojima, et al. describes a photo-sensitive recording medium which is photo -reacted to a degree varying substantially linearly in correspondence with the intensity of the light incident on the medium over a range of light intensities.
- the intensity of the light beam and the degree of modulation of the light beam by the signals are selected to maintain the maximum and minimum intensities of the modulated light beam within a predetermined range so as to form simultaneously on the record medium a tracking path portion and recorded signal portion.
- the range of depth of the reacted photoresist described in Kojima, et al. is between 200 nm and 800 nm.
- SUBSTITUTE SHFET (RULE 28) Discs mastered according to the teaching of Kojima, et al. are not readable by optical disc readers.
- Current optical readers detect an intensity difference which results from the interference of light reflected from the bottom of each pit with light reflected from the surrounding disc surface. When the depth of the pits is one quarter of the wavelength of the light used to read the disc, then light reflected from the bottom of the pit interferes destructively with light reflected from the disc surface so that the decrease in the intensity of the reflected light may be detected.
- the wavelength of the laser used by most detectors is 780 nm. Since the index of refraction of the plastic used in most compact discs is approximately 1.55, the wavelength of light in the plastic is approximately 500 nm.
- One quarter of a wavelength is therefore a distance of about 125 nm.
- Kojima' s minimum displacement distance of 200 nm to reach the linear region of the photoresist characteristic curve is therefore greater than one quarter of the wavelength of the laser used to read the optical disc.
- the overall dynamic range of depth change taught by Kojima, et al. of between 200 nm and 800 nm is also greater than one quarter wavelength.
- Kojima, et al. does not address the problem of creating multiple level pits less than one quarter of a wavelength deep in a repeatable manner.
- Kojima, et al. teaches using a photoresist with a steeply varying linear characteristic curve over a predetermined dynamic range.
- Kojima, et al. does not teach how the coating and development process may be engineered to enable the repeatable production of discrete level pits.
- Current optical disc mastering using photoresist processing likewise does not produce precisely controlled multiple pit levels.
- Current optical discs are produced from masters which are created from photoresist coated discs.
- a single desirable pit depth is chosen which is less than one quarter of a wavelength.
- the photoresist is exposed by a laser beam intensity that is modulated by cycling from a fixed maximum power to off, basically an "on or off mechanism.
- a pit depth of exactly one quarter of a wavelength would create complete destructive interference for reflected light and therefore optimum contrast at the reader with neighboring lands, a somewhat smaller pit depth level is generally selected to provide a better tracking signal.
- the depth of pits formed is slightly less than the actual depth of the pits on the master.
- the thickness of the photoresist layer is typically approximately 150 nm and essentially all of the photoresist is removed to create a pit or none of the photoresist is removed to leave a land. Other thicknesses of photoresist are possible. No attempt is made to precisely remove intermediate amounts of
- Intersymbol interference occurs on both conventional CD's as well as PDM discs.
- Light reflected from one pit also tends to interfere with light reflected from another pit, especially when the pits are smaller than the reading laser spot of the optic stylus, resulting in intersymbol interference.
- the depth and location of one pit or symbol therefore, tends to influence or interfere with the signal that is read from neighboring signals.
- the effect of the interference is greater when the symbols are closer together.
- a "modulation transfer function" (MTF) describes the transformation of the detected signal that results from the diffraction of light from neighboring pits.
- Optical data disc readers currently use analog filtering of the detector signal to equalize the frequency response of the system.
- the equalization is an attempt to compensate for the MTF, which predicts how much contrast an optical imaging system will generate when scanning different spatial frequencies.
- Current art uses a simple frequency equalization as discussed in chapter 2 of Principles of Optical Disc Systems (Bouwhuis, Braat, Huijser, Pasman, van Rosmalen, and Immink, 1985, Adam Hilger Ltd., Boston, MA).
- the magnitude of the MTF decreases monotonically with increasing spatial frequency, reaching zero at a limit called the optical cutoff frequency.
- the peak- to-peak signal from a series of 0.83 ⁇ m pits and lands on a CD is approximately 40% that from 1.6 ⁇ m marks.
- the cutoff frequency, which for CD corresponds to 0.43 ⁇ m pits and lands, a CD reader would detect no contrast at all. Since the shorter marks correspond to higher temporal frequencies in the detector signal, one can "equalize" the contrasts of long and short marks by increasing the high-frequency gain in the electronics.
- CD's and DVD's are constant linear velocity (CLV) systems.
- CLV is inconvenient for data storage because the spindle speed must be adjusted each time the drive seeks data at a different disc radius. When the reader head seeks information at a different radius, the drive must wait for the spindle to change rotational speed to maintain the linear velocity.
- Certain magneto-optical drives use analog equalization filters and operate at CAV. This is accomplished by dividing the disc radially into several zones, each with its own data rate and equalization filter. The zones must be narrow enough so that the change in linear velocity is small enough over a single zone so that one filter can operate over an entire zone even though the linear velocity is changing.
- the depth of photoresist removed during the development process could be precisely varied to achieve different discrete pit levels within a range of less than 125 nm, or a range corresponding to one quarter of the wavelength of the light which is to be used to read the disc.
- equalization filters could be developed which could adapt to different linear speeds so that nonconstant linear velocity disc drive systems could be used.
- the pit depth modulation of a PDM disc could be designed to precompensate for all or part of intersymbol interference.
- the present invention provides apparatuses and methods for fabricating an optical disc master, for equalization, and for precompensation to correct for intersymbol interference.
- the data levels to be stored on the disc are determined and a substrate is coated with a layer of photoresist material.
- a variable power laser beam irradiates the photoresist in a way which causes the photoresist to be partially exposed to a depth corresponding to the data level being represented. Controlled development of the photoresist allows repeatable pit levels to be formed.
- a method of compensating for intersymbol interference on an optical disc includes measuring an intersymbol linear transfer function.
- the intersymbol linear transfer function substantially describes a linear portion of the effect of intersymbol interference on an optically detected read signal from an optical disc.
- the intersymbol linear transfer function is convolved with a write signal to produce a linearly transformed portion of the optically detected read signal.
- An inverse linear transfer function of the intersymbol linear transfer function is determined.
- the inverse linear transfer function has the property of canceling the effect of convolving the intersymbol linear transfer function with the write signal.
- the optically detected read signal is convolved with the inverse linear transfer function.
- a method of compensating for intersymbol interference includes measuring an intersymbol linear transfer function that substantially describes the linear portion of the effect of intersymbol interference on an optically detected read signal from an optical disc.
- the intersymbol linear transfer function is convolved with a write signal to produce a linearly transformed portion of the optically detected read signal.
- An inverse linear transfer function of the intersymbol linear transfer function is determined.
- the inverse linear transfer function has the property of canceling the effect of convolving the intersymbol linear transfer function with the write signal.
- the inverse linear transfer function is divided into a short portion and a long portion.
- the write signal is convolved with a precompensation transfer function and the precompensation transfer function is substantially the inverse of the long portion of the inverse linear transfer function.
- SUBSTITUTE SHEET (RULE 26 ⁇ read signal is convolved with the short portion of the inverse linear transfer function so that the linear portion of the effect of intersymbol interference on an optically detected read signal from an optical disc is canceled.
- a method of compensating for intersymbol interference on an optical disc includes measuring an intersymbol nonlinear transfer function.
- the intersymbol nonlinear transfer function substantially describes the nonlinear portion of the effect of intersymbol interference on an optically detected read signal from an optical disc.
- the intersymbol nonlinear transfer function is nonlinearly convolved with a write signal to produce a nonlinearly transformed portion of the optically detected read signal.
- An inverse nonlinear transfer function of the intersymbol nonlinear transfer function is determined.
- the inverse nonlinear transfer function has the property of canceling the effect of nonlinearly convolving the intersymbol nonlinear transfer function with the write signal.
- the write signal is nonlinearly convolved with a precompensation transfer function and the precompensation transfer function is substantially the same as the inverse nonlinear transfer function so that the nonlinear portion of the effect of intersymbol interference on an optically detected read signal from an optical disc is canceled.
- FIGURES IA through IL are schematic diagrams illustrating how polycarbonate optical discs are produced.
- FIGURE 2 is a schematic diagram of a variable exposure system used to create multilevel pits on an optical disc.
- FIGURE 3 is a schematic diagram illustrating the exposure of a photoresist layer.
- FIGURE 4 is a graph illustrating a laser power modulation curve.
- FIGURE 5A is a graph illustrating residual photoresist after development plotted against exposure time for different development times.
- FIGURE 5B is a graph illustrating residual photoresist after development plotted against exposure time when undiluted developer was used.
- FIGURE 5C is a graph illustrating residual photoresist after development plotted against exposure time when developer that was diluted 50% (1 : 1) was used.
- FIGURE 6 is a graph illustrating exposure energy versus pit depth for one embodiment.
- FIGURE 7 is a graph illustrating a plot of bump height (in nm) verses cutting laser power (in mW) taken from two different runs of AFM analysis of stampers produced by the procedure outlined in the embodiment below.
- FIGURE 8 is a block diagram which illustrates an optical data storage and data recovery system.
- FIGURE 9 is a graph which illustrates Fraunhofer diffraction results for 0.5 ⁇ m length pits on a 0.5 ⁇ m wide track.
- the present invention provides an apparatus and method for mastering variable-depth pits in photoresist media by using a laser beam to expose the photoresist by scanning across the disc as the disc rotates and by controlling the development of the photoresist on the disc.
- the present invention further provides an apparatus and method for providing equalization and for compensating for intersymbol interference when pits on an optical disc are read.
- Variable-depth pits are produced in photoresist media by providing a different type of modulation to the laser beam than is used currently to master compact discs.
- the method used in a preferred embodiment involves modulating the power of the laser to intermediate power levels between a maximum power and no power. Pits of different depth are defined by the photoresist in areas that are exposed to the intermediate power levels of the laser. By careful control of the power level and the development procedure, the different depth pits are defined in a reproducible manner.
- the modulation of the power level is further controlled to provide precompensation for intersymbol interference between variable depth pits.
- FIGURE 2 is a schematic diagram of a variable exposure system used to create multilevel pits on an optical disc.
- a laser 200 provides a laser beam 212 that exposes the photoresist.
- An acousto-optical modulator 210 modulates the strength of the laser beam so that the beam variably exposes the surface of the disc.
- An electro-optical modulator is also used in certain embodiments.
- An optical system 220 focuses the laser beam onto the surface of disc
- Modulation source 240 computes the desired pit depth and outputs a control signal to acousto-optical modulator 210 so that the appropriate intensity of light irradiates disc 230 at each pit location.
- modulation source 240 determines a pit depth that also compensates for intersymbol interference. In order to properly control the light intensity, the intensity of laser beam 212 is measured at some point after the
- SUBSTITUTE SHEET (RULE 2B) beam travels through modulator 210.
- a detector 242 is used to measure the intensity just before the beam enters optical system 220.
- pit levels are determined based on the amount of information to be stored in each pit and the characteristics of the reading device which is reading the data. In one embodiment, 3 bits of information are to be stored in each pit, and 8 different data levels are determined. In another embodiment 2 bits of information are stored in each pit and 4 different data levels are determined. In general, greater than two bits of information are stored at each pit. It should be noted that numbers of data levels such as 10 which represent a fractional number of bits are allowed. Those data levels are then mapped to output levels of the reading device. Pit depth levels are determined which will produce the output levels. An amount of laser energy which will expose the photoresist a sufficient amount to produce the desired pit depth level is determined.
- the laser energy which exposes the photoresist is varied by changing the laser power.
- Other embodiments change the amount of time that a single power laser exposes photoresist. This is accomplished by varying the rotation rate of the disc to vary the amount of time that disc portions are exposed to the laser beam.
- the laser beam is pulsed at a variable high frequency so that different disc portions receive different quantities of laser energy.
- FIGURE 3 is a schematic diagram illustrating the exposure of a photoresist layer.
- a laser beam, 310 is allowed to expose portions of the photoresist layer, 320. This results, after controlled development, in variable depth pits 330.
- the photoresist layer, 320 is applied to a glass or plastic substrate 300 by traditional coating methods such as spin or dip coating.
- the thickness of layer 2 may be, but need not necessarily be the same as used traditionally in CD mastering, i.e. 120-170 nm. The thickness must be at least as thick as the deepest pit (approximately 170 nm in the preferred embodiment) required but may be thicker by 1 or more orders of magnitude.
- the type of photoresist could be either negative or positive.
- the photoresist is a positive photoresist such as that used currently in CD mastering, Shipley S1800 for example, but a photoresist such as Olin Ciba Geigy OCG 825, commonly used for IC manufacturing may also be used. It will be apparent to one of ordinary skill in the art that other photoresists may be used as well.
- the laser beam is allowed to expose the photoresist, and the amount of the photoresist that is exposed varies according to the power of the laser.
- FIGURE 4 is a graph illustrating a laser power modulation curve. A laser power modulation curve 410 used in one embodiment of the invention is shown.
- the development conditions of the exposed photoresist are controlled so that the desired pit levels may be precisely defined by the modulated laser power. Both the developer concentration and the development time are precisely controlled, as well as other parameters such as temperature, developer age and water purity. In one preferred embodiment, the temperature is controlled at 23 degrees C.
- FIGURE 5 A is a graph illustrating the residual amount of photoresist remaining after development versus the exposure time of the laser energy for three different development times.
- Curve 510 corresponds to a 30-second development time and has a relatively flat slope.
- Curve 520 corresponds to a 60-second development time and has a steeper slope.
- Curve 530 corresponds to a 120-second development time and has a much steeper slope than curve 510 or 520. This shows that if a shorter development time is used, it is possible to better control the variation in pit depth for a given variation in laser power.
- the concentration of the developer is also important . Typically concentration ranges from 0.1 to 0.3 Molar are acceptable. Concentration of 0.15 Molar is typically most preferred.
- OCG 825 positive photoresist is spin coated onto a blank glass master at a thickness of approximately 1 ⁇ m.
- the prepared photoresist substrate is allowed to be exposed by an He-Cd laser operating at a wavelength of 442 nm.
- the laser is modulated by sending the laser beam through an
- SUBSTITUTE SHEET (RULE 28) Acousto-Optic Modulator (AOM) which causes the exiting beam's intensity to be modulated based on the voltage applied to the AOM.
- AOM Acousto-Optic Modulator
- Other methods of modulating the laser will be apparent to one of ordinary skill in the art.
- a lower applied voltage causes less light to be transmitted through and a higher voltage allows more light to be transmitted through the particular AOM system used.
- the laser is passed through an optical train to focus the beam to a small spot on the photoresist, about 5 ⁇ m in diameter in the present embodiment although a much smaller spot limited only by diffraction limits, can also be achieved.
- the laser exposes the photoresist As the laser exposes the photoresist, a higher voltage is fed to the AOM when a deeper pit is desired and a lower voltage is applied when a shallower pit is desired.
- the photoresist After the photoresist is exposed, there is an optional soft-baking step in which the photoresist is heated at a temperature of around 100 degrees C for a desired period of time. In the process that yielded the FIGURE 6 results, the softbake step was skipped.
- the exposed photoresist was developed in an appropriate developer solution. An OCG 934 developer solution at a concentration of 2 parts developer to one part water was used and the photoresist was developed for 3 minutes and subsequently washed with distilled water for another 1 minute. The development process may be monitored spectroscopically for completion such as is done currently in the compact disc and IC industries.
- the developed photoresist was then hard-baked at 120 degrees C for 30 minutes, but no hard bake step is necessary.
- the exposure energy is determined by the linear speed of the scanning laser relative to the disc, the width, and the intensity of the beam.
- a mastering speed of 2.82 m/s may be used to master a disc in one hour.
- Slower mastering speeds use a lower power laser and provides sharper boundaries between pits, but require more time.
- a linear relationship such as the one obtained is desirable, but not required.
- variable depth pits may be created by selecting the energy level that will produce the correct pit depth. It is therefore possible to vary the laser power to produce reproducible pit depths.
- the developer concentration, development time, type of development temperature, beam characteristics, depth of focus of beam, and scanning speed may all be varied in accordance with the present invention.
- the lands are not etched and the pits are exposed and etched to multiple levels between 0 and a distance approximately 1/4 of a wavelength of the laser that will read the disc. (If negative mastering were used, the pits and lands would be reversed.)
- all of the land regions could be exposed and etched to a certain level and the pit regions could be exposed and etched to vary between a distance of 0 and approximately 1/4 of a wavelength of the reading laser.
- FIGURE 7 is a graph illustrating a plot of bump height (in nm) verses cutting laser power (in mW) taken from two different runs of Atomic Force Microscopy (AFM) analysis of stampers produced by the procedure outlined below for one preferred embodiment.
- a glass master was prepared by applying a photoresist coating, in this case Shipley Microposit 1400-5, to a thickness of 165 nm.
- the photoresist-coated glass was inserted into a commercial laser beam recorder (LBR), in this case one developed by Disc Manufacturing Inc. (DMI) which uses an Argon ion laser operating at 457.9 nm.
- LBR commercial laser beam recorder
- DMI Disc Manufacturing Inc.
- FIGURE 7 shows a plot of the data generated from these measurements.
- the heights of the bumps are plotted as a function of the laser power used to cut them.
- This slope is approximately 169 nm/mW in one case and approximately 1 10 nm/mW in the second. It is estimated that at least 8 discrete pit levels could be defined within the depth range shown.
- SUBSTITUTE SHEET (RULE 2 ⁇ )
- a master created using the above method may be used to create a stamper for PDM CD's having multiple data levels.
- the development parameters, laser power range, linear speed and beam characteristics are chosen in accordance with the present invention.
- a layer of photoresist is exposed to a variable amount of laser energy. The laser exposure energy is varied by varying the laser power or time during which the photoresist is exposed.
- Intersymbol interference occurs on both conventional CD's as well as PDM discs. Intersymbol interference is especially problematic for PDM discs which are detecting smaller signal changes than conventional CD's.
- the present invention attacks the problem of reversing the effects of interference in two ways.
- DSP digital signal processor
- a DSP filter works by sampling and digitizing the signal at some rate and then mathematically convolving the signal data with a set of fixed coefficients. By choosing the coefficients, one can produce a filter with any desired frequency response, up to the Nyquist limit of 1/2 the sampling frequency.
- a nonlinear mathematical algorithm is used to cancel out nonlinear intersymbol interference effects.
- the storage capacity of an optical disc system is improved in two ways.
- the speed of data recovery can also be improved since the DSP filter may be adjusted to work with a nonconstant velocity optical disc driver, eliminating the need to adjust the linear speed of the disc when different areas are accessed. Data recovery is further improved by removing nonlinear inter ⁇ symbol interference effects from the reader signal.
- the nonlinear techniques which work in conjunction with DSP, are applied either as an extra signal processing step or as precompensation before the data is written. It is also possible, using precompensation that compensates for nonlinear and some linear effects, to simplify the DSP filter or to use an analog filter for the remaining linear effects.
- SUBSTITUTE SHEET (RULE 28) Data is encoded as a groove of varying depth on the surface of a reflective disc. The data is first expressed as a series of multi-level symbols xi. Then, within the limits of the mastering and replication process, each symbol is converted into a segment of a groove with constant depth. These segments, called “pits", all have the same length.
- pit is used throughout this specification to refer to an area on the disc which represents a data symbol it should be noted that this invention is specifically not restricted to this particular surface mo ⁇ hology.
- a "pit” can be considered to be any type of multi-level mark, for example an area whose reflectivity has been modified by some sort of writing process or a mark with different possible widths.
- FIG. 8 is a block diagram which illustrates an optical data storage and data recovery system.
- a data signal 802 includes a sequence of symbols x t received from the modulation encoder. The value of each ⁇ : is one of M possible levels, so that a level is a real number denoting the value of a data symbol.
- Data signal 802 is fed to a precompensator 804 which precompensates the signal to help remove nonlinear intersymbol interference effects, and possibly some linear effects.
- the output of precompensator 804 is a signal 806 which represents the sequence of levels to be written w
- the levels to be written w represent the precompen sated levels which are to be written to the disc. These levels are not necessarily discrete.
- Signal 806 is input to an Optical Disc Data Channel (ODDC) 808.
- ODDC 808 represents all steps in the process from disc mastering to reading which affect the data sequence that is recovered.
- signal 806 is first transformed by an S-curve calibrator 810 which compensates for the responsiveness of the writeable disc medium to laser intensity.
- S-curve calibrator 810 which compensates for the responsiveness of the writeable disc medium to laser intensity.
- a one-to-one mapping converts each level w t into the appropriate modulation signal for the mastering laser, taking into account the nonlinear response of all elements in the ODDC.
- the output of S-curve calibrator 810 is sent to a laser modulator 812 which controls a laser writing system 814.
- An optical reader head 820 (for example a CD or DVD reader head) reads the disc and outputs a sampled reader signal 822, which is
- SUBSTITUTE SHEET (RULE 28 ⁇ denoted r, .
- Reader signal 822 is input to a DSP deconvolver 824 which compensates for the MTF.
- the recovered levels 826 denoted as y, are obtained.
- Recovered levels 826 are the convolution of s, the transfer function of the deconvolver, and r t . Once the deconvolution is completed, except for noise and imperfections in the signal processing, y, should equal the original data x These values are passed on to the modulation decoder.
- precompensator 804 and DSP deconvolver 824 cancel out the Intersymbol Interference (ISI) from the Optical Disc Data Channel (ODDC), enabling successful recovery of the data.
- Precompensator 804 compensates for the nonlinear ISI effects and DSP deconvolver 824 compensates for the linear effects.
- Nonlinear effects are removed in the current preferred embodiment by precompensation to avoid the processing requirements of removing nonlinear ISI effects from the sampled reader signal, r,. In other embodiments, nonlinear effects are removed after r,. is read.
- the design of precompensator 804 and DSP deconvolver 824 is based on a simulation of how the ODDC affects a sequence of symbols.
- the process of reading a disc was simulated using a model based on Fraunhofer diffraction theory. Such a simulation shows the inherent nonlinearity in the ODDC.
- the data was encoded as a simple rectangular groove having a depth which changes at the start of new pits.
- the depths were chosen so as to produce evenly spaced signals when "read” by a simulated disc player.
- the pits were shortened to as small as 0.5 ⁇ m the simulated reader signal exhibited ISI as expected.
- Figure 9 is a graph which illustrates Fraunhofer diffraction results for 0.5 ⁇ m length pits on a 0.5 ⁇ m wide track.
- 1.0 represents the signal level from a flat area on the disc, and 0.0 represents zero reflected intensity.
- a curve 902 represents the track depth profile on the disc.
- a curve 904 represents a Fraunhofer simulation of the signal read by the optical disc reader.
- a set of samples 905 represent samples of curve 904 taken at various pit locations.
- a curve 906 represents the signal that would be expected from a linear system.
- the spatial frequency maps to a temporal frequency in the read signal.
- a filter with an increased gain at higher frequencies could therefore compensate for the effect described above.
- the linear speed of the reader head varies relative to the disc surface, as it would if a constant angular velocity (CAV) drive were used, then the spatial frequency would not map to the temporal frequency of the read signal.
- the present invention provides a digital filter which compensates for changing linear speeds to effectively filter the signal and remove linear ISI effects.
- the frequency response of a DSP filter is referenced to the data sampling clock, which by design is marking off specific distances on the disc. Such a filter could therefore remove ISI equally well at any linear velocity. It would therefore be possible to read a disc at CAV, reducing seek times.
- the ISI has an unusual character.
- a deep mark amongst shallow neighbors produces a signal with a different shape than that from a shallow mark amongst deep neighbors.
- the sequence a can now be inte ⁇ reted as a set of (linear) convolution coefficients.
- the MTF and a are completely equivalent descriptions of the linear impulse response of the ODDC.
- the second term is the nonlinear ISI and arises not from destructive interference between pit and land, but between pit and adjacent or nearby neighboring pit. It acts to lower the reader signal, consistent with Figure 2.
- the third term represents the remainder of the expansion of the sin 2 term in the equation above.
- both real data and Fraunhofer diffraction simulations can be better modeled by an expression which is more general than Eq. (I):
- the linear part of Eq. (3) is invertible. As long as the ISI is not too severe, one can calculate a sequence of coefficients # such that a ⁇ J - w g mce a can ⁇ e physically measured or measured using a computer simulation, a can be derived from the physical measurements of a. In one embodiment, the reader is preprogrammed with the coefficients of a. In other embodiments, the coefficients of a are encoded on the disc. Likewise, B can also be physically measured. In the fitting process, the B* form can not produce effects similar to a*W; thus they can be fit separately. B, however, is more difficult to invert. The following discussion shows the nonlinear term B is dealt with in one embodiment using an iterative data recovery method.
- a precompensation method is used so that the iteration may be performed once, before the data is sent through the ODDC, simplifying the design of the disc player.
- Eq. (4) is instead inte ⁇ reted as the precompensation step. If it is specified what the reader signal r should be, then W n) is the sequence that must be fed to the ODDC in order to achieve that output. If r is constrained to be equal to the original data sequence x, then Eq. (4) represents an algorithm which completely precompensates the data for all ISI. If precompensation is performed according to this constraint, then the sequence r can be passed on to the modulation decoder with no further processing.
- noise is filtered by the ODDC in the same way as w, it turns out to be unfavorable to amplify some frequency components of x over others, as a '1 does. Doing so only forces one to confine the original discrete levels to a smaller dynamic range to ensure that w never exceeds the dynamic range of the ODDC.
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- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Optical Recording Or Reproduction (AREA)
- Manufacturing Optical Record Carriers (AREA)
Abstract
Description
Claims
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US62019696A | 1996-03-22 | 1996-03-22 | |
US620196 | 1996-03-22 | ||
PCT/US1997/004941 WO1997035304A1 (en) | 1996-03-22 | 1997-03-24 | Apparatus and methods for forming and use with variable pit depth optical recording media |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0888612A1 true EP0888612A1 (en) | 1999-01-07 |
EP0888612A4 EP0888612A4 (en) | 1999-12-29 |
Family
ID=24484980
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP97917035A Withdrawn EP0888612A4 (en) | 1996-03-22 | 1997-03-24 | Apparatus and methods for forming and use with variable pit depth optical recording media |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0888612A4 (en) |
AU (1) | AU707392B2 (en) |
CA (1) | CA2249403C (en) |
WO (1) | WO1997035304A1 (en) |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2002540660A (en) | 1999-03-23 | 2002-11-26 | コーニンクレッカ フィリップス エレクトロニクス エヌ ヴィ | Method for decoding a stream of channel bits of a signal related to a binary channel signal into a stream of source bits of a signal related to a binary source signal |
KR100425659B1 (en) * | 2000-03-06 | 2004-04-03 | 샤프 가부시키가이샤 | Optical disc provided with pits of different depths, optical disc reproducing apparatus for reproducing the optical disc, and method for tracking the optical disc |
KR100694036B1 (en) * | 2000-06-01 | 2007-03-12 | 삼성전자주식회사 | Disc with grooves and pits of different depths and method for manufacturing thereof |
JP2006252743A (en) | 2005-03-08 | 2006-09-21 | Qinghua Univ | Multilevel read-only optical disk and its manufacturing method |
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JPH03141031A (en) * | 1989-10-26 | 1991-06-17 | Brother Ind Ltd | Optical recording medium and its reader |
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1997
- 1997-03-24 CA CA002249403A patent/CA2249403C/en not_active Expired - Fee Related
- 1997-03-24 AU AU25493/97A patent/AU707392B2/en not_active Ceased
- 1997-03-24 EP EP97917035A patent/EP0888612A4/en not_active Withdrawn
- 1997-03-24 WO PCT/US1997/004941 patent/WO1997035304A1/en not_active Application Discontinuation
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US4455632A (en) * | 1980-04-25 | 1984-06-19 | U.S. Philips Corporation | Apparatus for optically reading a record carrier with two types of information areas |
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Also Published As
Publication number | Publication date |
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AU2549397A (en) | 1997-10-10 |
CA2249403A1 (en) | 1997-09-25 |
AU707392B2 (en) | 1999-07-08 |
CA2249403C (en) | 2001-01-09 |
WO1997035304A1 (en) | 1997-09-25 |
EP0888612A4 (en) | 1999-12-29 |
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