U8005235587A
`5,235,587
`[11] Patent Number:
`United States Patent
`Bearden et al.
`[45] Date of Patent:
`Aug. 10, 1993
`
`
`[19]
`
`[54] OPTICAL DATA STORAGE APPARATUS
`AND METHOD
`
`4,927,263
`
`5/1990 de Groot eta]. ....................... 365/5
`OTHER PUBLICATIONS
`
`[75]
`
`Inventors: Alan J. Bearden, Berkeley; Michael
`P. O’Neill, Orinda, both of Calif.
`
`[73] Assignee:
`
`The Regents of the University of
`California, Oakland, Calif.
`21 A l. N .: 676 263
`]
`[
`.pp
`0
`’
`122] Filed:
`M113 279 1991
`
`[63]
`
`[56]
`
`Sarid, D. et a1.,
`25(8):l968—1972.
`Hansma, P. K. et
`242:109—215.
`Rugar, D. et al., Rev. Sci. Inst. (1988) 59:2337—234-0.
`Wang, C. P., Lasers & Optronics, (Sep. 1987) pp. 69—71-
`Acket, G. A. et 211., IEEE J. Quantum Elec., (1984)
`QE-20(10):1163—1169.
`Potter, 1. c., J. App1. Physics, (1969) 40(12):4770—4776.
`Deferrari. H. A.,
`J. Acoust. Soc. Am-
`(1967)
`42(5):982—990.
`$396??? 91:6 A' e‘ “1" 1' A°°“S" 50° Am' (1966)
`( )-
`‘
`-
`Primary Examiner—Paul M. Dzierzynski
`Assistant Examiner—Kiet T. Nguyen
`Attorney, Agent, or Firm—Peter J. Dehlinger
`'[57]
`ABSTRACT
`An optical data storage apparatus and disc in which
`information is stored in the form of multiple submicmn
`depths at information-storage sites on the disc surface.
`The depth information is read by directing a focused
`laser beam onto the storage sites, back reflecting a por-
`tion of the reflected beam into the laser cavity, and
`converting power fluctuations in the laser beam to sub-
`micro“ dismnce measurements-
`
`148
`
`IEEE J. Quantum E1ec.,
`
`(1989)
`
`1988)
`
`14,
`
`a1., Science (Oct.
`
`11 Claims, 9 Drawing Sheets
`
`150
`
`Related US. Application Data
`Continuation-impart ofSer. No. 414,897, Sep. 29, 1989,
`Pat. No. 5,029,023.
`Int. (:1.5 ..............................................-0113 7/00
`[51]
`[52] us. c1. .................................... 369/106; 369/112;
`359/116; 369/275-3; 369/275-4
`[58] Field of Search ................... 369/106, 54, 55, 112,
`369/116, 121, 275.1, 275.3, 275.4, 275.5
`References Cited
`U.S- PATENT DOCUMENTS
`4,161,752 7/1979 Basilico .......................... .. 369/275.4
`4,361,402 11/1982 Costa .......... ..
`.... .. 356/73,]
`4,375,088
`2/1983 de Han et al.
`369/275.3
`4,441,179 4/1984 Slaten ......... ..
`369/275-5
`4,443,873 4/ 1984 Anthon ....... ..
`""" 369/110
`4,451,914 5/1984 LaBudde et a1.
`""“
`4,554,836 11/1985 Rudd
`4,556,967 12/1985 Braat
`369/2754
`4,so7,214 2/1989 Getreuer
`369/275.3
`4,399,327
`2/1990 Bates et a1. ........................ .. 369/106
`Photo-
`dotocco:
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`.152
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`US. Patent
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`Aug. 10,1993
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`Sheet 1 of9
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`5,235,587
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`Photo-
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`dctoctor
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`1
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`52
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`154
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`LASER
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`150
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`122
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`126
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`44
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`156
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`158
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`159
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`140 ‘
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`7
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`120
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`135
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`134
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`US. Patent
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`Aug. 10, 1993
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`Sheet 2 of 9
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`Sheet 3 of 9
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`Aug. 10,1993
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`US. Patent
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`Aug. 10, 1993
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`Sheet 4 of 9
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`3d
`196
`202
`'
`III-V.” Flg°5A
`¢ Develop, plasma etch,
`
`192
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`194
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`strip
`
`164
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`192
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`194
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`164
`
`¢
`
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`
`204
`
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`205
`
`¢ Recoat.
`expose
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`
`
`Fig.5c
`
`203
`
`192
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`194
`
`164
`
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`
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`
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`¢ Repeat cycles
`
`192
`
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`
`.
`
`190
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`V Fig. SE
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`192
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`194
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`US. Patent
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`Aug. 10,1993
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`Sheet 5 of 9
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`US. Patent
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`Aug. 10, 1993
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`Sheet 6 of 9
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`10
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`2
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`4
`5
`8 10 12
`15
`LOG DOSE (microcoullcmz)
`
`20
`
`Fig.
`
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`
`100
`
`1.200
`
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`
`FREQUENCY (Hz)
`
`Fig.
`
`do999cP.
`
`REMAININGAFTERDEVELOPMENT(mlcrons)
`ammqmuo
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`THICKNESS
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`Sheet 8 of 9
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`
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`10
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`100
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`
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`5,235,587
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`
`
`1
`
`OPTICAL DATA STORAGE APPARATUS AND
`METHOD
`
`This application is a continuation-in-part of copend-
`ing application Ser. No. 07/414,897, filed Sep. 29, 1989,
`now US. Pat. No. 5,029,023.
`'
`
`FIELD OF THE INVENTION
`
`The present invention relates to a high-density optical
`data storage apparatus and storage disc.
`
`BACKGROUND OF THE INVENTION
`
`5
`
`10
`
`15
`
`Optical data discs are widely used for digital informa-
`tion storage. In a typical optical storage disc, informa-
`tion is encoded at data-storage positions on the disc in
`the form of two-state optical “spots” which are read as
`either “0” or “1”. The disc is read by directing a laser
`beam onto the disc, at selected data-storage positions,
`and determining from the reflected light beam, which of 20
`the two states was seen at each location.
`It will be appreciated that the density of stored infor-
`mation in this type of storage disc depends directly on
`the density of data-storage sites which can be achieved
`on the disc surface. The site density, in turn, is limited
`by the ability of the focused laser beam to resolve adja-
`cent data-storage sites. Since a focused laser beam has a
`spot size of at least about 1-2 microns, the data-storage
`sites on a disc surface must be spaced by at least 1-2
`microns.
`
`25
`
`30
`
`It would be desirable to provide an optical data stor-
`age disc in which the density of information on the disc
`can be increased severalfold over current discs.
`
`SUMMARY OF THE INVENTION
`
`It is a general object of the invention to provide a
`high-density digital data storage apparatus, and storage
`disc in the apparatus.
`The optical data storage disc of the invention has a
`substrate which defines a plurality of data-storage posi-
`tions. Multiple-bit information is stored at each data-
`storage position by a structure adapted to reflect a fo-
`cused coherent light beam directed against
`the disc
`surface at one of 2N selected submicron displacement
`distances above or below a defined disc surface, corre-
`sponding to one of a selected 2N bits of information,
`where N>2.
`In one embodiment, the substrate carries one and
`preferably multiple transparent layers, each having a
`thickness between about 1-10 microns. Each layer de-
`fines 2N submicron depths at which reflecting structure
`can be located, for each data-storage position, for re-
`flecting focused coherent light from that depth at that
`position.
`The data storage disc is used in an information stor-
`age apparatus constructed according to another aspect
`of the invention. The device includes a laser for produc-
`ing a coherent output light beam, a lens for focusing the
`output beam onto the surface of the data-storage disc,
`and structure for moving the beam to selected data-stor-
`age positions on the disc at successive, known times.
`A photodetector in the device is used to measure the
`power output of the laser, and the time-dependent
`changes in the measured power of the output beam are
`converted to position-dependent displacement distances
`at the selected data-storage sites, for determining the
`selected one of the 2N bits stored at each such site.
`
`35
`
`45
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`50
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`65
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`2
`Also disclosed is a method of retrieving digital infor-
`mation. In the method, a focused laser beam is directed
`onto the surface of an optical data disc of the type de-
`scribed above. As the beam is moved over the disc to
`selected data-storage positions, a portion of light re-
`flected from each position is back reflected into the
`laser cavity, causing a change in laser beam power re-
`lated to the depth of the reflecting site. The laser power
`variations are converted to digital information related
`to the 2N depths at each storage position.
`These and other objects and features of the invention
`will become more fully apparent when the following
`detailed description of the invention is read in conjunc-
`tion with the accompanying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`focused beam preferably has a beam diameter of be-
`
`FIG. 1 is a simplified schematic view of a data-stor-
`age device constructed according to the invention;
`FIG. 2 is an enlarged, fragmentary portion of a data-
`storage disc constructed in accordance with the inven-
`tion;
`FIG. 3 is a sectional view of the FIG.-2 disc, taken
`generally along line 3—3 in FIG. 2;
`FIGS. 4A4F illustrate steps for producing a data-
`storage disc constructed according to one embodiment
`of the invention;
`FIGS. 5A—5E illustrate steps for producing a data—
`storage disc like the one shown in FIG. 4F, according
`to a different construction method;
`FIGS. 6A-6D illustrate steps for producing a data-
`storage disc according to another embodiment of the
`invention;
`FIG. 7 is a plot showing the thickness in a photoresist
`layer, after development, as a function of log dose irra~
`diation applied to the layer;
`FIGS. 8A—8C illustrate steps for producing a data-
`storage disc according to another embodiment of the
`invention;
`FIG. 9 illustrates a multi-layer optical data-storage
`disc formed in accordance with the invention.
`FIG. 10 shows spectra of laser power variations pro-
`duced by target surface vibrations, as measured at sev-
`eral vibration amplitudes, indicated at the left of the
`spectra in nanometers, and over vibrational frequencies
`between 200—2200 Hz;
`FIG. 11 shows a plot of peak power fluctuation at a
`fixed frequency, as a function of increasing peak ampli-
`tudes of vibration;
`FIG. 12 shows a theory parameter diagram of an
`experimental configuration used for measuring laser
`power output as a function of displacement of a piezo-
`electric transducer; and
`FIG. 13 is a plot of laser power amplitude as a func-
`tion of oscillation amplitude of a target (light line) and
`calculated from theory (dark line).
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`A. Data-Storage Device
`
`FIG. 1 is a schematic view of a portion of an optical
`data-storage apparatus or device 120 constructed in
`accordance with the invention. The apparatus includes
`a stable-resonator laser 122 and an adjustable-focus
`objective lens 124 for focusing the laser output beam,
`indicated at 126, onto the surface of a data-storage disc
`128 also formed in accordance with the invention. The
`
`

`

`3
`tween about 0.5-2 microns at the focal point of the lens,
`and the depth of focus of the beam is preferably be-
`tween about 1—2 microns.
`
`5,235,587
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`10
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`15
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`One preferred type of laser is a low-power He-Ne gas
`laser operating in two longitudinal modes, each of 5
`which is linearly polarized. An internal linear polarizer
`blocks the passage of one of the two modes, so that only
`a single linearly polarized low-power beam is available
`externally. One laser of this type which is suitable com-
`mercially available is a Uniphase Model llOlP He-Ne
`single-mode gas laser (Uniphase, San Jose, Calif).
`Where it is desired to perform target mapping at target
`vibrations in the 10-100 MHz range, as described be-
`low, a semiconductor laser, such as are commercially
`available, may be used.
`The construction and characteristics of the disc will
`be described below. For present purposes, it is noted
`that the surface of the disc defines a plurality of data-
`storage positions, such as indicated at 130a, 1301;, and
`130C which are located within concentric tracks 132a,
`132b, 132c, respectively, on the disc surface. At each
`data-storage position, information is stored in the form
`of a reflective region having a selected displacement
`distance above or below a defined surface plane in the
`disc. The number of different displacement distances is
`2N, where N is greater than 2 and preferably 3 to 8 or
`more. That is, the disc stores 2” bits of information at
`each data-storage position.
`The disc is mounted on rotary motor 134 in the de-
`vice for rotation, at a selected speed about the disc axis,
`indicated at 136, according to conventional disc drive
`construction.
`
`20
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`25
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`30
`
`An optical assembly 138 in the device is designed for
`shifting the position of the focused beam to different
`selected tracks on the data disc. The assembly includes
`lens 124 and pair of mirrors 140, 142 interposed between
`the laser and lens for directing the beam along a select-
`ed-length horizontal path in the figure. Mirror 142 and
`lens 124 are carried on a lens carriage 144. The carriage
`is shiftable, along a horizontal axis 134 in the figure,
`corresponding to a radial line 135 on the surface of the
`disc, to position the focused beam at selected tracks on
`the disc.
`
`Shifting in the optical assembly is performed by a
`motor 148 which is operany connected to the carriage,
`and designed to shift the beam in selected increments,
`e.g., 0.005 to 0.2 mm, corresponding to the radial spac-
`ing between adjacent tracks on the disc. The increment
`of shifting is also adjusted to preserve the phase rela-
`tionship between the output beam and the beam re-
`flected from the disc back into the laser. This is
`achieved by making the increments of shifting, and
`therefore the spacing between adjacent tracks on the
`disc, equal to an integral number of wavelengths of the
`coherent laser beam. The position of the optical assem-
`bly is under the control of a controller 150 which in-
`structs the motor to selected track positions on the disc,
`according to well-known disc drive construction.
`The power of the laser output is measured by a pho-
`todetector 152 placed behind the optical cavity of the
`laser, for receiving light from the cavity through the
`rear mirror of the cavity and an opening 154 formed in
`the rear of the housing. The photodetector includes a
`silicon photodiode (shown) which is designed to mea-
`sure light intensity. One suitable type of photodiode is
`an EG&G SGD lOO-A silicon photodiode. The photo-
`diode is connected to an operational amplifier 156 in the
`photodetector which outputs a DC component voltage
`
`35
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`4
`which is linearly proportional to the power of the light
`beam detected by the photodiode. The operational am-
`plifier characteristics are such that the voltage signal
`output which is typically obtained is in the microvolt-
`/millivolt range.
`The amplifier is connected to a signal converter 158
`which operates to convert time-dependent amplitude
`changes in the voltage signal received from the photo-
`detector to binary data values at each of the selected
`disc locations which was “read.” The construction and
`operation of the converter will be apparent to those
`skilled in the art. The data information from the con-
`verter is supplied to a data-handling device 159, such as
`a microprocessor, which in turn can provide signals to
`controller 150 for accessing information from the disc.
`In operation, the laser beam in the device is focused
`onto the surface of a data disc, at a selected track in the
`disc. As the disc rotates, each data-storage position is
`seen by the laser beam as one of 2” surface displace-
`ments above or below a defined reference plane. These
`surface displacements, in turn, produce a change in the
`phase of the light reflected back into the laser, causing
`a proportional change in the power output of the laser,
`as discussed below. The time-dependent changes in
`power are converted by converter 158 to time-depend-
`ent displacements read on the disc, and these are assooi-
`ated with given addresses on the disc according to
`known methods.
`
`It will be appreciated that each data-storage position
`represents a selected one of 2”bits of information. Since
`the density of data-storage positions on the disc can be
`made substantially as high as in conventional discs
`(where 2 bits of information only are stored at any stor-
`age position), the density of information on the disc, and
`the speed with which information can be accessed from
`the disc is enhanced by a factor of up to 21"'1.
`
`B. Data-Storage Disc
`
`FIG. 2 shows an enlarged fragmentary plan view of
`the surface of a data storage disc, such as disc 128,
`constructed in accordance with the present invention.
`Tracks, such as tracks 1320, 132b, and 132C in the disc
`are indicated by solid lines, and data-storage positions,
`such as positions 1300, 130b, and 130C, are indicated by
`dotted lines. The data-storage positions on each track
`are spaced from one another by a distance preferably
`between about 5 to 200 microns, and adjacent tracks
`have‘ a spacing between about 0.4 and 5 microns. The
`disc may be further encoded with radial and disc-angle
`information, for determining disc position, and with
`track-position information, to insure proper beam place-
`ment with respect to any selected track, according to a
`conventional optical data disc construction.
`FIG. 3 shows an enlarged fragmentary cross-sec-
`tional view of disc 128, taken generally along line 3—3
`in FIG. 2, i.e., along track 132c. The disc includes a
`substrate 160 which may be any suitable, preferably
`rigid disc material, such as aluminum, glass, or the like.
`The substrate supports a layer 162 whose outer surface
`defines the data-storage locations on the disc. The thick-
`ness of the layer defines the 2” different displacement
`distances, or thicknesses, corresponding to a selected
`one of 2” bits of information which can be stored at
`each data-storage location.
`For purposes of simplicity, the discs illustrated herein
`have 8 (23) defined displacement distances, which may
`include the surface plane of layer 162 and seven increas-
`ing depths below the surface, or eight subsurface
`
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`5,235,587
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`depths. The different displacement distances are indi-
`cated by depth markings, such as markings 164, shown
`at the left in FIG. 12 and related figures which follow.
`Thus, for'example, the data stored at position l30c cor-
`responds to a depth 5xd, and the data stored at position
`130c’, to a depth 7xd.
`FIGS. 4A—4F shows one method of forming a disc
`170 (FIG. 4F) of the type described. Initially a substrate
`172 is coated with an etchable layer 174, such as a sili-
`con layer, using known coating methods. Layer 174,
`which has a preferred thickness of at least 2 microns, is
`coated with a photoresist coat 176, for example, by spin
`coating to a desired thickness, for example, 1-5 microns.
`Depth markings in the layer, such as markings 164, are
`indicated at 164 in the figures.
`The coated disc is covered with a mask 178 whose
`openings, such as openings 180, correspond to the posi-
`tions of the data-storage positions which are to have the
`greatest depth, Nd, where N=2N. The masked photore-
`sist coat is exposed to UV light, producing exposed
`regions, such as regions 182 in the resist coat (FIG. 4A),
`and these regions are removed by development (FIG.
`48), according to known photolithographic methods
`(e.g., Introduction to Microlilhography, ACS Symposium
`Series, Thompson, L. F., et al., eds., ACS, 1983). The
`disc surface is then plasma-etched for a selected period
`sufficient to etch the uncovered regions of layer 174 to
`a depth (1, corresponding to the distance between the
`outer surface of layer 174 and the first depth marking.
`The disc is then covered with a second mask 184
`whose openings, such as openings 186, correspond to
`the positions of the data-storage positions which are to
`have the next-greatest depth (N— 1)d (FIG. 4D). The
`masked photoresist coat is exposed to UV light, these
`regions are removed by development, and the coated
`substrate is plasma etched under the previously selected
`conditions. This second plasma etch step is effective to
`etch the just-uncovered regions of layer 174 to a se
`lected depth d, and the already-uncovered regions to a
`depth approximately equal to 2d (FIG. 4B).
`With each repeated cycle, new regions of the layer
`are uncovered and etched to a depth d, and previously
`uncovered regions are etched an additional increment d
`in depth. After 21" cycles, e.g., 8 cycles, all of the data-
`storage positions have been etched to a selected depth,
`and the photoresist layer is removed by stripping. As
`seen, the resulting disc 170 is composed of an underly-
`ing substrate and an outer layer defining plural data-
`storage positions, each with a selected one of 2N depths
`below the surface of the layer.
`FIGS. SA—SE illustrate another method for forming a
`data-storage disc 190 (FIG. 5B) of the type described
`above. Here a substrate 192 having an etchable layer
`194 is coated with a suitable photoresist layer 196, as
`above. The resist layer is exposed to UV light through
`a mask 198 whose openings, such as openings 200, cor-
`respond to the data-storage positions which are to have
`the greatest depth, Nd. The light-exposed regions, such
`as regions 202, are removed by development (FIG. 5A),
`as above, and the disc is chemical-etched or plasma-
`etched until the uncovered regions of layer are etched
`to a selected depth Nd (FIG. 5B).
`The substrate is then stripped, coated with a second
`coat 203, and this coat is exposed to UV, using a second
`mask 204 whose openings, such as openings 206, corre-
`spond to the data-storage positions which are to have
`the next-greatest,
`(N— l)d depth. The light-exposed
`regions, such as regions 208, are removed by develop-
`
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`ment and the disc is chemical-etched or plasma-etched
`until the uncovered regions of layer are etched to a
`selected depth of (N— l)d (FIG. 5D).
`With each step, a new pattern of data-storage posi-
`tions with progressively shallower etch regions is cre-
`ated, until after 2” steps, a desired disc 190 composed of
`an underlying substrate and an outer layer defining
`plural data-storage positions, each with a selected one
`of 2N depths below the surface of the layer, is formed
`(FIG. 5E).
`FIGS. 6A—6D illustrate a method of forming a disc
`210 (FIG. 6D), based on the ability to control the depth
`of removal of material from a photoresist coat, accord-
`ing to the dose of radiation applied to the coat. FIG. 7
`shows a plot of coat thickness remaining after develop-
`ment in a positive-tone electron-beam resist coat, as a
`function of total e—beam dose (in microcoul/cmz) ap-
`plied to the coat (plot from Thompson, supra, p. 105).
`As seen, the thickness remaining in the coat after devel-
`Opment is a well-defmed linear function of log dose, up
`to a saturation dose of about 1012 microcoul/cmZ.
`In preparing the disc, a substrate 112 is coated with a
`positive-tone e-beam resist coat 115 (Thompson, supra)
`having a preferred thickness of 2 or more microns. The
`resist layer is then c0vered with a mask 214 whose
`openings, such as openings 216, correspond to the data-
`storage positions which are to have the greatest depth,
`and the coat is irradiated with an electron beam (3-
`beam) for a period corresponding to a desired depth of
`coat removal,
`i.e., corresponding to a selected dose.
`The effective irradiation depth is indicated in coat 215
`by shaded regions, such as regions 218, in FIG. 6A.
`The first mask is then replaced with second mask 220
`whose openings, such as openings 222, correspond to
`the data-storage positions which are to have the next-
`greatest depth. The coat is now irradiated with an e-
`beam for a period corresponding to the new selected
`depth of coat removal, as indicated by regions 224 in
`FIG. 6B.
`
`This procedure is repeated for data-storage positions
`at each of the selected 2N depths, as shown in FIG. 6C.
`After all 2N irradiation steps, the irradiated coat is de-
`veloped to remove cavities in the coat corresponding in
`depth to the selected irradiation doses, as shown in FIG.
`6C. After baking, to remove developing solvent from
`the resist coat, the coat may be covered with a transpar-
`ent protective coat.
`FIGS. 8A-8C illustrate a method of forming a data
`storage disc 226 in which the regions of reflection in a
`disc surface are provided by changes in the index of
`refraction at selected data-storage positions and se-
`lected depths within a uniform-thickness layer. The disc
`shown in the figures is composed of a substrate 228
`coated with a layer 230, preferably about 2 microns
`thick. Layer 230 is formed of a transparent polymer,
`such as polyethylene, whose index of refraction can be
`selectively varied,
`in localized regions of the layer,
`according to the degree of polymerization at the local-
`ized regions.
`Layer 230 is initially covered with a mask 234 whose
`openings, such as openings 236, correspond to the data-
`storage positions which are to have the greatest depth
`Nd, and the coat is irradiated with an e-beam whose
`energy is calibrated to penetrate the layer to a depth
`Nd, producing a localized change in index of refraction
`at that depth. The area of localized change in index of
`refraction is shown by solid line, such as line 238, at the
`bottom of an irradiated region, such as region 240.
`
`
`
`

`

`
`
`7
`The first mask is then replaced by a second mask 242
`whose openings correspond to the data-storage posi-
`tions where index of refraction changes are to have the
`next-greatest depth (N—l)d. The layer is now irradi-
`ated with an e-beam whose energy is calibrated to pene-
`trate the layer to the next-up selected depth, producing
`index of refraction changes at that depth at the mask
`open positions, as shown in FIG. 8B. These steps are
`repeated until index of refraction changes at each of the
`2N depths is achieved (FIG. 8C).
`
`C. Multiple Layer Disc
`
`The data storage disc of the invention can be con-
`structed to include two or more transparent layers in a
`stacked configuration, as illustrated in FIG. 9. Here a
`stacked-disc device 246 is composed of a substrate 248
`and a series of stacked layers, such as layers 250, 252,
`and 254, each constructed according to one of the sin-
`gle-layer embodiments described above. That is, each
`layer, such as layer 254, has a surface region 256 which
`defines a plurality of data-storage positions, such as
`positions 260, and data is stored at each region in the
`form of a depth (or height) which is a selected one of
`2N displacement distances below (or above) the surface
`plane of the layer. Each layer has a preferred thickness
`of between about 10-50 microns, and the disc may con-
`tain up to several hundred layers.
`In a data-storage device which uses a multi-layer disc
`of this type, the plane of focus of the coherent light
`beam is shifted for “reading” a selected layer by suitable
`lens shifting means.
`As an example of the increased data-storage density
`which can be achieved in the stacked disc, the adjust-
`able position objective lens will be assumed to be a
`100>< microscope scope objective lens having a NA
`value of 0.6. This lens has a focal plane depth of lOu.
`Such an objective will distinguish layers of holes spaced
`20p apart. A medium having an index of refraction of
`0.01 would provide 0.1% retroreflected light intensity.
`If 2 mm of useful depth is available (this will depend on
`the actual material used), 1000 separate layers could be
`formed.
`
`D. Surface Depth Resolution
`
`FIG. 10 illustrates the resolution of a laser system
`with back reflection, such as employed in the apparatus
`of the invention, to detect surface displacements in the
`direction normal to the surface, e.g., surface displace-
`ments due to variations in the depth of surface displace-
`ments on the optical data storage disc of the invention.
`The target here is attached to a commercially avail-
`able piezoelectric transducer (PZT) having a response
`of about 4.4 nm displacement/V. A stiff paper card was
`attached to the surface of the transducer to serve as the
`vibrating target surface. Voltages from a sine-wave
`oscillator (10 Hz to 20 kHz) or the synchronized sweep
`frequency output from an audio spectrum analyzer
`(Hewlett-Packard 3580A) were used to drive the trans-
`ducer directly or through an amplifier for the larger
`motional amplitude ranges.
`The PZT vibration amplitude is shown at the left axis
`in FIG. 10, and the corresponding power output in
`voltage units, along the right axis. Foreach voltage
`applied to the transducer the frequency was swept or
`varied from 200 Hz to 2.2 kHz. The flatness of the traces
`indicates the flat response of the piezoelectric over the
`range of driving frequency.
`
`5,235,587
`
`8
`It can be appreciated from the spectra shown in FIG.
`10 that the amplitude of the measured signals is linearly
`related to the amplitude of the transducer vibrations.
`For example, with reference to the two spectra at the
`top in FIG. 10, a tenfold increase in vibration amplitude
`(from 10 to 100 nm) corresponds to an approximately
`tenefold increase in detected laser output measured as a
`voltage; similarly, a lOO-fold increase in vibration am-
`plitude yields an approximately lOO-fold increase in
`measured voltage. The noise in the spectra for vibration
`amplitudes below 1 nm is due to background electronic
`noise and random laser light beam output variations.
`Similar results were obtained when the target surface
`was a transducer covered with brushed steel, plastic,
`and mylar, indicating that the data disc surface can be
`formed of a variety of materials. In particular, it has
`been discovered that a distance-dependent laser modua-
`tion effect produced by retroreflection from a surface is
`observed even when the reflector is a diffuse, non-
`specular surface. In fact, the paper card reflector used in
`the studies described above has
`readily apparent-
`“fuzzy” surface irregularities which are in the several
`micron size range, as judged by light microscopy. Yet
`the reflected beam from this surface is able to produce
`linear distance-modulated laser power effects in the
`distance range between l-lO nm, as discussed below
`with respect to FIG. 11.
`The signal-to-noise ratio as monitored by the output
`of the operational amplifier is due to laser-light intensity
`fluctuations and electronic noise in the photodetector.
`The exact noise voltage at the output of the current-to-
`voltage electronics depends on the specific laser’s am-
`plitude fluctuations, the detector and its associate cir-
`cuitry, and on the bandwidth of the measurement. For
`the bandwidth of the circuit used, the noise voltage was
`2 microvolts, giving a signal-to-noise ratio of at least 40
`dB.
`
`In the method described above, displacement mea-
`surements down to the 10 picometer range have been
`made. The limiting noise in the apparatus is due pre-
`dominantly to laser amplitude fluctuations, which in
`theory can be reduced by up to two orders of magni-
`tude. It is also noted that detector noise levels down to
`10—2 pm/(Hz)*i have been reported (D. Rugar et al.,
`Rev. Sci. Instru., 59:2337, 1988). Thus, assuming that
`mechanical vibrations in the microscope can be reduced
`sufficiently, the data storage device should be able to
`detect position-dependent surface displacements down
`to the 01—1 picometer range, particularly by employing
`phase-lock or time-averaging signal-to-noise improve-
`ment
`techniques. Even within the range of demon-
`strated linearity between 1-10 nm (FIG. 11 below), a
`disc surface layer having a thickness of 10 microns
`would be able to accommodate between 1,000 and
`10,000 (about 210 to 213) bits of information at each
`data-storage position. Typically, the disc is designed to
`encode 23, 24, or 25 bit words at each location.
`The range of linearity with respect to amplitude can
`be determined by keeping the frequency constant as the
`amplitude is varied and plotting the power output (volt-
`age) versus amplitude as in FIG. 11. The linear relation-
`ship holds over a range of vibration amplitudes extend-
`ing below about one quarter of the incident
`light’s
`wavelength. For vibration amplitudes greater than this
`upper limit, the relationship becomes non-linear. How-
`ever, as seen below, the relationship between vibration
`amplitude and measured laser power output in the high~
`amplitude range is still predictable, and therefore useful
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`4O
`
`45
`
`50
`
`55
`
`65
`
`
`
`

`

`5,235,587
`
`10
`beam, producing a relatively constant frequency varia-
`tion in depth. The resulting response amplitude of the
`laser contains a baseline or reference voltage level cor-
`responding to the planar regions of the disc between
`adjacent data-storage positions, and peaks of different
`heights corresponding to one of the 21V different surface
`depths at a data-storage position.
`E. Surface Reflectance Effects
`
`
`
`9
`in determining large—amplitude displacements from the
`measured power output fluctuations of the laser. For a
`He—Ne laser,
`the wavelength of emitted light is 632
`nanometers; thus, the range of linearity (taking back-
`ground noise into consideration) extends up to about
`150 nm.
`
`FIG. 12 is a theory parameter diagram of an experi-
`mental configuration used for measuring laser power
`output as a function of displacement of a piezoelectric
`transducer (PZT) 88. Here Le represents the length of
`the laser cavity 90, defined by two mirrors 92 and 94,
`and L represents the distance between mirror 94 and a
`reflecting surface 96 carried on the PZT. Experimen-
`tally, the PZT was placed 20 cm from the laser exit
`port, and was driven by a DC signal from a PZT high-
`voltage amplifier, or a 40 Hz AC sine wave signal cou-
`pled to the amplifier, A silicon photodetector (not
`shown) was operated in the photovoltaic mode so that
`its voltage output was linearly proportional to the light
`intensity input. The detector was positioned at the rear
`of th