Note: Descriptions are shown in the official language in which they were submitted.
~03739
Hoe 87/K 093
MAGNETO-OPTIC RECORDS
Speclficatlon
BACKGROUND OF THE INVENTION
Field of the Invention
The present inventlon relates to the fleld of
erasable magneto-optic recordlng and more particularly
to the fleld of record structureæ that allow a higher
_ signal-to-noise ratio of signals output in response to
reading of the record.
Descrlptlon of the Prlor Art
`10Magneto-optic (MO) recording is sometimes re-
ferred to as thermomagnetic recording, or thermo-mag-
neto-optlc recordlng. These terms refer to technology
that is being actively developed in many laboratories
around the world because it promlses to comblne the
capaclty and packing density of optical recording to-
gether with the erasability of conventional magnetic
recordlng. The basic prlnciples of MO recording are
set forth in U.S. Patent #3,949,387 issued April 6,
1976 for ~EAM ADDRESSABLE FILM USING AMORP~OUS MAG-
NETIC MATERIAL to Chaudhari, et al. Distinct fromconventional optlcal recording in which an optical
coating is ablated, for example, to produce reflec-
tance variations ln a laser beam that reads the re-
cord, MO recording is accomplished by inducing and
detecting relatively small rotations of the plane of
polarization of a linearly polarized read beam re-
flected off the record. This is sometimes referred to
as Kerr MO recording, and such rotatlon of the plane
of polarization as the "polar Kerr magneto-optlc ef-
fect", or, in context, as ~ust the "Kerr effect." Theamount of such polarization rotation of the read beam
is most desirably dependent only on the magnetization
of the MO thin film recording structure at the parti-
KD-1001 -1-
~:~03739
cular locatlon on which the read beam ls lncident on
the record.
As understood, the normal practlce at the present
state of the art of MO recordlng is to address the MO
record for reading, erasing, and wrlting by means of a
focussed laser beam that is dlrected through a trans-
parent disk of glass or plastic which serves as a sub-
strate for the thin film MO recording structure. Such
addressing for wrlting or erasing ls done in the pre-
sence of a magnetic fleld. The substrate of the MO
) record, typlcally about 1.2 mm thick, also serves to
provlde defocussing of dust that may be on the surface
of the substrate, and may be provlded wlth grooves or
other features to provide informatlon for tracklng,
etc. One problem with this MO record ls caused by thebirefringence of such substrate.
Birefringence refers to the phenomenon in which
an optical materlal, ln a state of stress, for exam-
ple, becomes non-homogeneous and non-isotropic with
respect to the index of refraction. That is, light
havlng different states of polarization sees sllghtly
different lndices of refraction in such optical mater-
ial. Since readout of recorded lnformation by means
- of the Kerr effect is based on detecting relatively
small rotations of the plane of polarization (of the
order of one degree), and since the read beam of light
makes a double pass through such substrate, the opti-
cal retardatlon caused by rotatlon of the plane of
polarlzatlon due to blrefrlngence is a very serious
problem for magneto-optic recording. In particular,
for state of the art MO records, a tolerance of 30 na-
nometers for double pass optlcal retardatlon and a
further tolerance on the gradient of the retardation
of about 30 nanometers/cm may be requlred to assure
satisfactory performance of such substrate with re-
spect to birefringence.
KD-1001 -2-
1303739
It has proven qulte dlfflcult to achleve thls low
level of blrefrlngence ln preferred optlcal plastlcs,
such as polycarbonate (PC) t used for such substrates.
Thus, in the current llterature lt ls recognlzed that
from most polnts of vlew, bisphenol-a-polycarbonate ls
a good candldate for such MO disk substrates. See,
for example, Alan B. Marchant, "Retardation Effects in
Magneto-optic Readout", SPIE Proc. of Optical Mass
Data Stora~e II, (San Diego, CA, August, 1986) 695.
Marchant noted that despite years of research which
have resulted in such substrates wlth a very low op-
tical retardation for normally-incldent light, the
problem ls substantlal with lncreases in the angle of
lncldence relatlve to normal. Marchant also noted
that even vertlcal blrefrlngence of such sub6trates ln
a dlfferentlal detectlon system ls a serlous problem.
~ ased on read-only dlgltal audlo disk technology,
for example, the most economical means of providing a
grooved transparent substr~te is in~ection moldlng of
a suitable optical plastic. Thls is the method by
whlch substrates are lmpressed wlth grooves and digl-
tal audlo lnformatlon. The commerclal success of such
read-only dlgltal audlo dlsks has lead to attempts to
ln~ectlon mold such optlcal plastlcs as polymethyl-
methacrylate (PMMA) and PC as substrates for use assubstrates ln MO records. However, the stress result-
lng from such moldlng leads to blrefrlngence problems
when PC ls the materlal used for such MO record sub-
strates. Although adequate performance wlth respect
to substrate blrefrlngence has been achleved wlth
PMMA, PMMA has undesirable permeablllty to water vapor
and expands ln a humld envlronment. Thls dlmensional
lnstability causes cracklng and peellng of the thln
fllm MO recordlng structure.
Desplte the problems of blrefrlngence noted ln
the Marchant article, there have been continued at-
KD-1001 -3-
1;~03739
tempts to use such conventlonal substrates both to
support the thin film MO recording structure and to
transmlt the record and read beams to and from the
thln film MO recordlng structure. This ls lndlcated
by an artlcle publlshed in August, 19~, by Treves and
Bloomberg, entitled, "Effect of Birefringence on Opti-
cal Memory Systems", in SPIE Proc. of Optlcal Mass Da-
ta Stora~e II, (San Diego, CA). They reported on such
conventional substrates and concluded that optical bi-
refringence must be controlled since substantial re-
duction in the signal-to-noise ratio is attributed to
the birefringence of the substrate. As recently as
March 13, 1987, Toda, et al., in an article entitled
"Analysis of Signal to Noise Ratio in Magneto-Optical
Disk Using a Polarlzation Slmulator", Topical Meetin~
on OPtical Data Stora~e, OPtical Societ~ of America
Tech. Di~est Series, Vol. 10, (Statellne, Nevada,
3/11-13/87) pp. 34-37, also reported on such conven-
tional substrates. They concluded that the reta~da-
tion of a PC substrate showed complicated dependenceon the lncident angle of the laser beam. Since MO
recording, llke all optical recording methods, uses a
focussed beam of light for writing and reading, and
since focussed beams include rays of all angles of in-
cidence out to a limiting angle set by the numericalaperture of the recording obJective lens, readout of
MO signals through PC substrates is vulnerable to
these effects.
Such continued attempts to use such conventlonal
substrates to perform the above-noted multiple func-
tions of support (for the thin film MO recording
structure) and beam transmission have apparently ig-
nored disclosures relating to types of optical record-
ing other than MO recordlng. In those other types of
3~ optical recordlng physical changes such as melting or
evaporation, or chemical changes such as decomposi-
KD-1001 -4-
. . .
130373~
tion, effect the recording function. In one form of
such recording by physical changes, uslng ablative
write-once media, mark formatlon is created either by
e~ecting material from the viclnity of the mark, or by
causlng material to withdraw from the mark region lnto
a surroundlng rim, or both. In elther case, a free
surface unconstralned by any overlayer i6 required, or
is said to be beneflcial. For example, in U.S. Patent
#4,074,282 lssued February 14, 1978 for RADIATION-
SENSITIVE RECORD WITH PROTECTED SENSITIVE SURFACE to
- ! Balas, Jr., et al., llght is lncident upon an ablative
recording medium deposited on the inner surface of a
substrate dlsk, which ls one of two disks that have an
air gap between them. The light is incident on and is
transmitted through the other disk, which is a window
disk, across the air gap and directly onto the abla-
tive recordlng medium. However, such disclosure rec-
ognizes that in a number of situations, such as when
the ablated material would coat the window, the inci-
dent light should be transmitted through the substratedisk and not through the window disk. Since the
above-referenced conventional use of substrates in MO
recording conforms to this latter teaching of the
4,074,282 Patent, it appears that such conventional MO
substrate practices and the earlier non-MO recording
practices (e.g. Patent 4,074,282) both teach the use
of the substrate for both the support and beam trans-
mission functlons. In view of the above-referenced
articles, however, when such teachings are followed
there are substantial, unsolved problems with bire-
fringence that detract from the overall performance of
such MO records.
SUMMARY OF THE INVENTION
In contrast to the prior art in which an incident
laser beam has been transmitted through a substrate
KD-1001 -5-
.. .... .. ..
1303'739
that supports a thln fllm M0 recordlng structure, mak-
ing lt necessary to contlnue efforts to reduce the bl-
refrlngence of such substrate fabricated from materi-
als havlng the undesirable blrefrlngence, the M0 re-
cord of the present lnventlon provldes a high quallty,durable, economlcal, erasable optlcal record structure
havlng mlnimal blrefrlngence problems, such that the
slgnal-to-nolse ratlo ls enhanced relative to that of
the conventional structures for erasable magneto-optlc
records.
A record for sensing radlatlon lntenslty modula-
tlon in the presence of a magnetlc fleld accordlng to
the prlnclples of the present inventlon separates the
functlon of supportlng the thin fllm M0 recordlng
structure of the substrate from the substrate's optl-
cal transmlsslon functlon so that the ldeal substrate
can be selected wlthout regard to the transmlsslve
optlcal properties thereof.
An M0 record deslgned ln accordance wlth the
prlnciples of the present lnventlon has a substrate
that may have very poor blrefrlngence propertles, yet
the record provides lmproved slgnal-to-nolse ratlos ln
read beams slnce neither the record beam nor the read
beam ls transmltted through the substrate.
In an M0 record of the present invention, the
substrate ls effectlvely the bottom of the record,
wlth a thln film M0 recording structure on the top
surface of the substrate, where such top surface sup-
ports a wlndow through whlch recordlng energy ls
transmitted ln the presence of a magnetic blas fleld
for M0 recording.
Wlth these and other obJects in mind, a flrst
preferred embodlment of an M0 record for recordlng ln
response to radlatlon lntensity modulatlon in the pre-
sence of a magnetlc fleld accordlng to the present ln-
vention is responslve to radlatlon lncldent on a given
KD-1001 -6-
1303739
side thereof. The record lncludes erasable material
for M0 recording in response to such inten~ity modu-
lated radlatlon and to the magnetlc fleld. A sub-
strate is provlded for supporting the erasable mag-
neto-optic material on the side of the material away
from the glven slde of the record. The 6ubstrate is
fabricated from a material havlng preferred support
characterlstics that are selected without regard for
the transmisslve optlcal characterlstic6 thereof. A
support structure overlles at least a portlon of the
\ substrate for provldlng at least one support surface
spaced from the erasable magneto-optlc material. A
wlndow ls mounted on the support surface for formlng
an alr gap on the slde of the erasable magneto-optlc
material toward the given slde of the record. The
window has flat surfaces and ls fabricated from a ma-
terial that is optically transparent and that has mln-
imal birefrlngence so that the radlatlon incldent on
the glven side of the record is transmitted through
the wlndow with mlnimal optical retardation and is not
transmitted through the substrate before being lncl-
dent on the erasable materlal for M0 recording ln the
presence of the magnetlc field.
A second preferred embodiment of the present ln-
ventlon relates to a record for recordlng ln response
to radiation intenslty modulation in the presence of a
magnetic field, where the radiation is incident on a
given side of the record. The record includes a sub-
strate and a wlndow. The window forms the glven side
of the record. The wlndow is flat and fabricated from
a material that is opticall~ transparent and has mini-
mal birefringence so that the incident radiation is
first transmitted therethrough with minimal optical
retardation. A transparent adhesive (such as optlcal
cement) is provided between the surface of the window
opposlte to such given side of the record and a thin
KD-1001 -7-
~303'739
film M0 recording structure deposlted on the sub-
strate. The thln film M0 recordlng structure lncludes
a magneto-optic layer sandwiched between a transparent
layer and a protectlve layer. The protectlve layer
lles between the substrate and the magneto-optlc lay-
er, and the transparent layer overlies the magneto-
optic layer and therefore is adJacent to both the mag-
neto-optic layer and the transparent adheslve. The
optically transparent layer and the protectlve layer
are formed from hermetlc materlal and the magneto-
! optic layer records ln response to such radlatlon
lntenslty modulatlon ln the presence of a magnetlc
field. The substrate ls provlded for supportlng the
protectlve layer and thus the remainder of the record.
The substrate ls fabricated from a material having lm-
proved film support and moisture impermeability char-
acteristics, eg. dlmenslonal stability, without regard
for the transmlssive optlcal characterlstlcs thereof
since the intenslty modulated radiatlon ls not trans-
mitted through the substrate.
A thlrd preferred embodiment of the present ~n-
ventlon provldes for the lncorporation of the erasable
magneto-optic layer into a tuned multl-layer interfer-
ence structure to further enhance and optlmize the
~5 difference of the effect~ve Kerr angle of polarization
rotatlon corresponding to different magnetlzations (up
or down) of the magneto-optic recordlng layer. The
tuned multi-layer interference structure is a four
layer structure comprised of a reflective layer over-
lying the substrate, a transparent dielectrlc spacer
protective layer overlying the reflective layer, a
magneto-optic recording layer overlylng the spacer
protective layer, and a protective overlayer of trans-
parent dlelectric overlying the magneto-optic layer.
A fourth preferred embodiment of the present in-
vention provides two of the records of the first, se-
KD-1001 -8-
, ~ _ ... . . ... ...... . .. ... .
`` 1303739
cond, or third embodlments, wlth the substrates back-
to-back or sharlng a common substrate to form a double
sided MO record.
BRIEF DESCRIPTION OF THE DRAWINGS
Other ob~ects, features and advantages of the
present lnvention will be apparent from an examination
of the following detailed descriptions which include
the attached drawings in which:
FIG. lA is a vertlcal cross-sectlonal vlew of a
flrst preferred embodiment of a record for recordlng
in response to radiatlon lntenslty modulatlon in the
presence of a magnetlc fleld accordlng to the present
lnventlon, showlng a magneto-optic recordlng layer
between a substrate and a wlndow;
FIG. lB is an enlarged vlew of a portion of the
record shown ln FIG. lA, lllustratlng the magneto-
optic layer between protectlve layers;
FIG. 2A is a vertical cross-sectional vlew of a
portlon of a second preferred embodiment of a record
for recordlng in response to radiation lntensity modu-
latlon ln the presence of a magnetic fleld accordlng
to the present invention, where a spacer for separat-
ing a window from a substrate is in the form of trans-
parent adhesive;
FIG. 2B ls an enlarged vlew of a portion of the
record shown ln FIG. 2A, lllustrating a thin fllm mag-
neto-optic recording structure between the adhesive
and the substrate;
FIG. 3A is a vertical cross-sectional view of a
fourth preferred embodlment showing a two-sided mag-
neto-optlc record for recording in response to radia-
tlon intensity modulation in the presence of a mag-
netic field according to the present lnventlon;
FIG. 3B is an enlarged vlew of a portlon of the
two-sided record shown ln FIG. 3A; snd
KD-1001 -9-
............................... .. .. .
~303739
FIG. 4 is an enlarged vlew of a third embodlment
of a magneto-optlc record showing a tuned multi-layer
lnterference structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Summar.V of the Preferred Embodiments
The first preferred embodlment of the present ln-
ventlon ls shown ln FIG. lA as a record 10 for recor-
! dlng ln the presence of a magnetlc fleld (represented
by arrows 11) and ln response to lntenslty modulatlon
of energy such as radlatlon ln the form of a laser
beam lndicated by arrow 12. The record 10 according
to the present invention records in response to such
radiation 12 incident on a given side 13 thereof. The
record 10 lncludes a thin film magneto-optic (MO) re-
cording structure 14 that includes a layer 15 of eras-
able material for MO recordlng. A substrate 16 ls
provlded for supportlng the thln fllm MO recordlng
structure 14 on a slde 17 thereof opposlte to the gl-
ven side 13 of the record 10. The substrate 16 is
fabricated from materlal havlng preferred support
characterlstlcs that are selected wlthout regard for
the transmissive optlcal characteristics thereof. A
support structure 18 in the form of concentric rlngs
19-19 overlles at least a portlon of the substrate 16
for providing at least one support surface 20 spaced
from the erasable magneto-optic layer 15. A window 21
ls mounted on the support surfaces 20 for formlng an
alr gap 22 on a side 23 of the erasable magneto-optic
layer 15 toward the glven slde 13 of the record 10.
The wlndow 21 has flat surfaces 24-24 and is fabri-
cated from a material that ls optlcally transparent
and that has mlnlmal blrefringence so that the light
radiation 12 incident on the given side 13 of the re-
KD-1001 -10-
1;~03'739
cord 10 is transmltted through the wlndow 21 wlth
mlnlmal optical retardatlon and ls not transmitted
through the substrate 16 before belng incldent on the
erasable layer 15 for M0 recordlng.
A second preferred embodlment of the present in-
vention ls shown ln FIGs. 2A and 2B as a record 25 for
recordlng ln response to lntenslty modulatlon of the
radlation 12 in the presence of the magnetlc fleld 11,
where the radiatlon 12 ls lncldent on a given side 13'
of the record 25. The record 25 includes a substrate
26 and a window 27. The wlndow 27 forms the glven
slde 13' of the record 25. The wlndow 27 ls flat and
fabrlcated from a materlal that ls optlcally trans-
parent and has minimal blrefringence so that the in-
cldent radlatlon 12 ls flrst transmltted therethrough
wlth mlnimal optlcal retardatlon. A transparent ad-
hesive 28 (such as optical cement) is provlded between
a surface 29 of the window 27 opposite to such given
side 13l of the record 25 and a thin fllm M0 recordlng
- 20 structure 30 deposited on the substrate 26. The thin
fllm M0 recording structure 30 lncludes a layer 31 of
magneto-optlc material sandwlched between a transpar-
ent layer 32 and a protectlve layer 33. The protec-
tive layer ~3 lies between the substrate 26 and the
magneto-optlc la~er 31, and the transparent layer 32
overlles the magneto-optlc layer 31 and therefore ls
ad~acent to both the magneto-optic layer 31 and the
transparent adheslve 2~. The optlcally transparent
layer 32 and the protectlve layer 33 are formed from
hermetic materlal and the magneto-optic layer 31 is
responslve to such radlatlon 12 ln the presence of the
magnetic field 11 for recording the intensity modula-
tion of the radiatlon 12. The substrate 2~ is provl-
ded for supporting the protective layer 33 and thus
the remainder of the record 25. The substrate 2~ ls
fabricated from a material havlng excellent fllm sup-
KD-1001 -11-
.... .
13~3739
port and moisture impermeablllty characterlstlcs with-
out regard for the transmissive optlcal characteris-
tics thereof slnce the radlation ls not transmitted
through the substrate 26.
A thlrd preferred embodlment of the present ln-
vention is shown in FIG. 4 as provldlng a tuned multl-
layer lnterference structure 34 instead of the thln
film recordlng structures 14 and 30 of the respectlve
records 10 and 25 and lnstead of the comparable thln
film recording structure of the fourth embodiment
shown in FIGs. 3A and 3B. In place of such struc-
tures, the structure 34 lncludes a reflectlve layer 35
directly on the substrate 16 of the first embodiment,
for example, for reflecting the lncldent radlatlon 12.
A transparent dlelectrlc spacer protective layer 36
overlies the reflective layer 35. An erasable mag-
neto-optic layer 37 similar to that of the first em-
bodiment is provided on the spacer protectlve layer
36. A protectlve overlayer 38 of transparent dielec-
tric material overlies the magneto-optic recording
layer 37. The thicknesses of the layers 35, 36, 37
and 38 of the interference structure 34 are selected
so that the interference structure 34 is tuned to
further enhance and optimlze the difference of the
effectlve Kerr angle of polarlzatlon rotatlon corres-
ponding to different dlrectlons of magnetization (up
or down) of the magneto-optic recording layer 37. If
the interference structure 34 ls provlded on the sub-
strate 16 of the flrst embodlment (of the record 10),
slnce the lntenslty modulated radlation 12 ls not
transmitted through the substrate 16, the substrate
material is selected without regard for the trans-
missive optlcal characterlstics thereof.
A fourth preferred embodlment of the record of
the present invention is shown in FIGs. 3A and 3B as a
double-sided optical record 40 including dual thin
KD-1001 -12-
1;~03739
fllm magneto-optic recording structures 41-41. Each
dual structure 41-41 includes a layer 42-42 of eras-
able magneto-optic materlal for recording ln the pre-
sence of the magnetic field 11 in response to inten-
sity modulation of the radiation 12 incident on respe-
ctive first and second sides 43 and 44 of the record
40.
The record 40 also includes a substrate 45 for
supporting the dual thin film magneto-optic recordlng
structures 41-41 on opposite sides 46 and 47 thereof
! so that the M0 layer 42 shown as the upper layer 48
(FIG. 3B) is toward the first side 43 of the record 40
and the M0 l~yer 42 shown as the lower layer 49 (FIG.
3B) is toward the second side 44 of the record 40. The
substrate 45 ls fabrlcated from a materlal having sup-
port characterlstics selected wlthout regard for the
transmlsslve optlcal properties thereof. Spacers such
as rings 50-50 overlie at least a portion 51 (FIG. 3A)
of each such opposlte slde 46 and 47 of the substrate
45 for providing support surfaces 52-52 (FIG. 3A)
spaced from each erasable magneto-optlc layer 42-42.
Wlndows 53-53 are mounted on the support surfaces 52-
52 for forming an alr gap 54 on a slde 55 ~FIG.3B) of
3 the upper layer 48 toward the flrst slde 43 of the
record 40 and for formlng the alr gap 54 on a slde 56
of the lower layer 49 toward the second slde 44 of the
record 40. The wlndows 53-53 have flat surfaces 57
and are fabrlcated from a material that ls optlcally
transparent and has minlmal blrefringence so that the
incident radlation 12 ls transmltted therethrough wlth
mlnlmal optlcal retardatlon. The thin film magneto-
optic recording structures 41-41 can be ln the form of
the lnterference structure 34 of the thlrd embodiment.
Also, the air gaps 54-54 can be replaced b~ the trans-
parent adheslve 28 of the second embodlment depending
on what operational characteristics are desired for
KD-10~1 -13-
1303739
the dual record 40.
Detailed Description of Record 10
Referrlng agaln to FIGs. lA and lB, the record 10
is shown in detail as including the substrate 16. The
substrate 16 is formed in the shape of a rigid dlsk
from material that has thermal and humidity coeffi-
cients of expansion that are compatlble wlth the thin
film magneto-optic recording structure 14. Signifl-
cantly, the rigid structural and e~pansion character-
lstics of the materlal of the substrate 16 mag be se-
lected wlthout regard to the transmissive optical
characteristics thereof since the incident radiation
12 is not transmitted through the substrate 16. In
addition, the material from which the substrate 16 is
formed is selected for having characteristics suitable
for having grooves 58 or other features for tracking,
etc. molded therein. In a preferred embodiment of the
record 10, the substrate 16 ls fabricated from po,ly-
carbonate materlal that has been in~ection molded to
ZO form the grooves 58. Such substrates 16 are avail-
able, for example, from Idemitsu Petrochemical Co.,
_ Ltd., New .York, New York. The substrate 16 of the
preferred embodiment has a diameter of 130 mm., a
pltch of 1.6 microns of the grooves 58 and a thickness
of 1.2 mm. As shown in FIGs. lA and lB, the grooves
58 are provided on the side 59 (FIG. lB) of the sub-
strate 16 that ls toward the given slde 13 of the
record 10 on whlch the radlation 12 is incldent.
Optionally, a layer 60 of hermetic sealing mater-
lal may be provlded on a lower surface 61 of the sub-
strate 16. The lower surface 61 is opposite tG the
glven side 13 on which the radiation 12 is incident,
thus the hermetic sealing material may be selected
without regard to the transmissive optical properties
thereof. Thus, the material of the hermetic sealing
KD-1001 -14-
1303739
layer 60 may be metal, and i6 prefer~bly alumlnum.
As shown ln detall ln FIG. lB, on the grooved
surface 59 of the substrate 16, the thln fllm magneto-
optlc recordlng structure 14 ls shown. The structure
14 includes a lower protectlve layer 62 that ls ad~a-
cent the surface 59 of the substrate 16. The protec-
tlve layer 62 ls formed from a materlal that ls selec-
ted for lts hermetlc seallng characterlstlcs and rel-
atlvely strong adheslon to the materlal of the sub-
strate 16. However, since the radiatlon 12 is not
) transmltted through the protectlve layer ~2, such se-
lectlon of the material for the protectlve lower layer
62 ls wlthout regard to the transmlsslve opt~cal pro-
pertles thereof. In a preferred embodlment of the re-
cord 10, the protectlve lower layer 62 ls formed from
slllcon nltride havlng a thlckness of about 90 nano-
meters.
The erasable magneto-optic layer 15 ls deposited
over the protectlve layer 62. For recordlng ln re-
sponse to the radlation 12 using the Kerr effect, the
magneto-optlc layer 15 is formed from an alloy of rare
earth elements and transltlon metals. E~amples of
_~ such materlals are dlsclosed ln U.S. Patent #3,949,387
for BEAM ADDRESSABLE FILM USING AMORPHOUS MAGNETIC
MATERIAL, lssued on Aprll 6, 1976 to Chaudharl, et al.
In partlcular, such alloy may be selected from the
groups conslstlng of the elements Tb, Fe, and Co; Gd,
Tb and Fe; Gd, Tb, Fe and Co; and Tb and Co. Such
alloy may also be formed from such groups, whereln
each such group further lncludes one or more elements
selected from the group conslstlng of Pt, Ti and Cr.
In a preferred embodlment of the record 10, the alloy
of rare earth elements and trans~tion metals forming
the magneto-optlc layer 15 ls formed from 22~ Tb, 70
Fe and 8~ of Co, wlth 2~ Pt [(Tb22 Fe70 C8)98 Pt2]
The thickness of the preferred alloy is 90 nanometers
KD-1001 -15-
.. . .
1303739
10~. The alloy may be deposlted onto the protective
lower layer 62 by sputterlng ln a well known manner.
Still referrlng to FIG. lB, the thln fllm struc-
ture 14 ls shown lncludlng an optically transparent
layer 63 provlded on the magneto-optlc layer 15. The
transparent layer 63 ls selected for havlng charac-
terlstics of optlcal transparency, hlgh inde~ of re-
fraction and hermetic seallng properties. In a pre-
ferred embodlment of the record 10, the optically
transparent layer 63 may be formed from sillcon ni-
i tride havlng a thlckness of 90 nanometers + 10~.
Referring agaln to FIG. lA, the support structure18 ls shown formed from the rings 19-19 whlch are an-
nular and concentric. The rlngs 19-19 overlle annular
portlons 64 (FIG. lA) of the substrate 16. The rlngs
19-19 are provlded wlth the support surfaces 20 that
support and malntaln the wlndow 21 spaced from a sur-
face 65 (FIG. lB) of the optlcally transparent layer
63 to form the air gap 22. Preferably the rings 19-19
are bonded directly to the annular portions 64 of the
substrate 16. The corresponding portions of the M0
layer 15 and the layers 62 and 63 are masked off dur-
_ ing depositlon for thls purpose. The rings 19-19 may
i be formed from material having a uniform but non-cri-
tical thickness of .05 to .5 mm in the dlrectlon in
which the radlation 12 is lncident on the side 13 of
the record 10. However, lf the materials from which
the window 21 and the substrate 16 are made differ
substantially with respect to expansion, eg. thermal
expanslon, -the spacer rings 19-19, partlcularly the
outer ring 19, should be made of a compllant, elas-
tomeric material.
The air gap 22 between the upper surface 65 of
the optically transparent layer 63 and the bottom
flat surface 24 of the window 21 thus h~s a thickness
of about .05 to .5 mm. An advantage of providlng the
KD-1001 -16-
... ...
~303739
alr gap 22 ln magneto-optlc recordlng relates to the
larger Fresnel reflectlon coef~iclent associated with
the air/transparent layer 63 interface, as compared
with that of a plastlc substrate/protectlve layer
interface. The Fresnel coefficlent for interfaces
between non-absorblng dlelectrlcs ls glven by
(n0-nl)/(n0+nl) where n0 and nl are the lndlces of re-
fractlon on either side of the interface. Slnce the
lndex of air ls l.O and is much lower than the index
of plastics (typically of the order of 1.5) the mag-
i nitude of the Fresnel coefflcient ls much larger at an
alr/dlelectrlc interface than at a plastic/dielectric
interface. Thls results ln a better anti-reflectlon
condition, and therefore a better "bloomlng" enhance-
ment of the Kerr effect and playback slgnal for alr
incldence than for substrate incidence of the light.
See, for example, the discussion of blooming at page
67 of the text "Magnetlc Domalns and Technlques For
Their Observation" by R. Carey and E.D. Isaac, Aca-
demic Press, 1963.
Further, "optically smooth" surfaces of sub-
strates have some degree of micro-roughness. Such
_ micro-roughness is usually expressed in terms of the
"root mean square roughness" of the surface; i.e., the
r.m.s. devlation of the actual surface proflle from an
ideal perfectly smooth flat surface. This micro-
roughness can be measured by various means, a conven-
ient one being a measurement of "total integrated
scatter" (TIS) of llght reflected from the surface.
See for e~ample the article, "Comparlson of Techniques
for Measuring the Roughness of Optical Surfaces" by
Jean M. Bennett, OPtical En~ineerin~, Vol. 24, No. 3,
(May-June 1985), pp. 380-385 and references cited
therein.
This micro-roughness induced scatter is one
source of noise in a slgnal derived from an optical
KD-lOOl -17-
i303739
record. For a glven roughness of the substrate sur-
face, the nolse ln the slgnal due to thls roughness
will be conslderably less for alr lncldence of the
radlation 12 onto the thln film structure 14 than for
substrate incldence onto ~uch structure 14 (as in the
prlor art). In partlcular, accordlng to the above
~ennett reference, the total lntegrated scatter
(TIS) _ (4~ )2 where ~ ls the r.m.s. roughness, ~
is the wavelength of the llght, and TIS ls the total
scattered fractlon of the reflected llght. In a
medlum of lndex of refractlon, n, the optical thlck-
ness of a given scattering surface displacement, ~,
becomes n~ whereas the wavelength becomes ~/n. There-
fore, TIS in a medlum of lndex n becomes:
(TIS)n ~ ¦4~n~ /n)]2 ~ n4(4~ )2 b n4(TIS) i
Thus, in terms of llght scattering, a given surface ls
n4 times worse from the substrate-lncident side as
compared to the air-incident side. Therefore, to the
extent that micro roughness is a factor in determining
the signal-to-noise ratlo of a slgnal derlved from an
M0 optical record, it ls better for the optical-beam
to address the recordlng structure from an alr amblent
rather than from the amblent of the substrate. Slnce
the alr gap 22 is ad~acent the upper surface 65 of the
transparent layer 63, the effects of microroughness
are reduced in the M0 records 10 and 40.
As shown in FIG. lA, the window 21 is mounted on
the support surfaces 20 of the rings 19-19. The win-
dow 21 has optimum transmissive optical characteris-
tics including the flat surfaces 24. Further, thewindow 21 is fabricated from a material that is opti-
cally transparent and that has mlnlmal blrefringence
so that the incldent radiatlon 12 ls transmitted
through the wlndow 21 wlth mlnimal optlcal retarda-
KD-1001 -18-
~303739
tlon. Such characterlstlcs are provided by polymeth-
ylmethacrylate, polymethylpentene and polyolefln. In
a preferred embodlment of the record 10, the wlndow 21
ls formed from chemlcally tempered sheet glass havlng
a diameter of 130 mm and a thlckness of 1.2 mm. Such
wlndows are available from Glaverbel, Brussels, Bel-
gium. As a result of such chemical tempering, the
flat surfaces 24 are ln a state of compressive stress
yet retain the desired minlmal birefringence.
10Prior to deposition of the thin films that form
i the hermetlc seallng layer 60 on the lower surface 61
of the substrate 16 and the thln fllm structure 14 on
the grooved side 59 of the substrate 16, the surfaces
59 and 61 of the substrate 16 are treated by ion beam
15 bombardment to promote good adhesion of such films
thereto.
Detailed Disclosure -
Fourth Embodiment - Record 40
Referring now to FIGs. 3A and 3B, it may be
20 understood that the record 40 includes the substrate
45 that ls provlded with grooves 66 that are slmilar
to the grooves 58 formed ln the substrate 16. The
J substrate 45 may be formed by a single in~ection mol-
ded member (as shown) or by two members 45' (FIG. 3A
25 ~olned along a llne 45ll, each member 45~ being slmi-
lar to the substrate 16 of the record 10. The record
40 is symmetrical with respect to a plane 67 (FIG. 3B)
that ls parallel to the flrst and second sides 43 and
44. Thus, the sides 43 and 44 of the record 40 are
30 constructed and function the same. Therefore, the de-
scription of one of the sides 43 and 44 applies to the
other side.
Each dual thin film magneto-optic recording
structure 41-41 formed on the substrate 45 is simllar
35 to the thin film magneto-optic recordlng structure 14
KD-1001 -19-
1303739
of the record 10, and lncludes the protectlve lower
layer 62 and the optlcally transparent layer 63 provl-
ded on the record 10. The M0 layer 42 of the struc-
ture 41-41 ls the same as the MO layer 15. Further,
the spacers 50-50 shown ln FIG. 3A are simllar to the
rings 19-19 provlded in the record 10. The spacers
50-50 thus form the air gap 54 whlch has the same ad-
vantages as the air gap Z2 of the record 10. The
spacers 50-50 also support the windows 53-53 in op-
posite confronting relationship with respect to the
`~ upper surface 65 of the transparent layer 63 in a man-
ner similar to the rings 19-19 of the record 10. The
rings 50-50 are slmilarly sealed to the substrate 45
to form the air gap 54 as an annularly-shaped space.
Simllarly, each of the wlndows 53-53 ls formed from
materlal selected for desired transmissive optical
characterlstlcs, as in the manner of the window 21 of
the record 10. It may be understood then that the re-
cord 25 provides the ability to record on both sides
43 and 44, which increases the amount of data that can
be stored on a single record 40.
The record 40 shown ln FIGs. 3A and 3B may alter-
natively be provided with the tuned multi-layer inter-
.J ference structure 34 shown in FIG. 4 lnstead of the
thin film magneto-optic recording structure 41-41.
The advantages of using such structure 34 are dis-
cussed below.
Detailed DescrlPtion - Record 25
Referring ln detail to FIGs. 2A and 2B, it may be
understood that the substrate 26 may be designed with
the same criteria in mind as used for the substrate 16
of the record 10, for example. Similarly, the respec-
tive transparent layer 32, the magneto-optic layer 31
and the protective layer 33 are selected in a manner
3~ slmilar to that for the respectlve protective layer
KD-1001 -20-
1303739
63, the magneto-optlc layer 15 and the optlcally
transparent layer 62 of the record 10. The trans-
parent adhesive layer 28 may be an ultraviolet light
cured optlcal photopolymer. Thls has characterlstlcs
5 as a cement and is also optlcally transparent. The
thlckness of the transparent adheslve layer 28 ls not
crltlcal, but ls about 20 mlcrometers ln a preferred
embodlment of the record 25. The wlndow 27 adheres
dlrectly to the transparent adheslve layer 28 and ls
10 selected wlth the same crlterla ln mlnd as the wlndows
21 and 53-53 of the records 10 and 40.
Referrlng to FIGs. 2A and 2B, lt may be observed
that the record 25 embodles certain of the advantages
of the records 10 and 40. In particular, the sub-
15 strate 26 of the record 25 may be designed wlth the
same obJectlves ln mlnd as used ln the design of the
substrate 16 and the substrate 45 of the respectlve
records 10 and 40. In partlcular, the functlons of
the substrate 26 may be separated from that of the
20 functlon of the window 27, such that the ldeal charac-
teristics of the substrate 26 may be obtained without
regard to the transmlssive optlcal characterlstlcs
thereof. Similarly, because of such separation of
r functlons, the characteristics of the window 27 can be
25 selected from the standpoint of optlcal transmlsslon
wlthout blrefringence. This advantage is believed to
more than off-set the results of elimlnatlng the air
gap from the record 25 and the resultant loss of the
"blooming" and surface roughness effect.
.
30Detailed Di~closure - Structure 34
The third preferred embodiment of the present
inventlon ls shown in FIG. 4 as provlding the tuned
multi-layer interference structure 34 lnstead of the
thln fllm recording structures 14, 30 and 41-41 of the
35respective records 10, 25 and 40, and instead of the
KD-1001 -21-
1303739
thln film recordlng structure ~1-41 of the record 40
shown ln FIGs. 3A and 3B. The structure 34 lncludes
the reflectlve layer 35 that is deposited dlrectly on
the substrate 16 of the record 10 or the record 40,
for example, for reflectlng the lncident radlatlon 12.
The reflectlve layer 35 ls preferably alumlnum. The
transparent dielectrlc spacer protectlve layer 36
overlles the reflectlve layer 35. The spacer layer 3~
ls preferably silicon nl~rlde. The erasable magneto-
optic layer 37 may be slmilar to that of the record 10and ls provided on the spacer protective layer 36.
For use in the interference structure 34, the magneto-
optic recordlng layer 37 is preferably an alloy com-
posed of [(Tb22 Fe70 Cog)g~ Pt2]. The protective15 overlayer 38 of transparent dlelectric material is
preferably also silicon nitride and overlies the mag-
neto-optlc recordlng layer 37. The thicknesses of the
layers 35, 36, 37 and 38 of the lnterference structure
34 are designed using well known techniques which
allow for some variatlon in thicknesses while still
achieving the desired tuning to enhance and optimize
the difference of the effective Kerr angle of polari-
zation rotation corresponding to different directions
of magnetization (up or down) of the magneto-optic re-
cording layer 37. Such enhancement is in addition tothat resulting from the use of the alr incidence
structure of the records 10 and 40. In a preferred
embodiment of the interference structure 34, the
thl- es~e re as follows:
KD-1001 -22-
.
1303739
Layer Thlckness
Thlckness
La~er(nanometers)
Reflectlve 35100
Spacer 36 50
magneto-optic 37 24
Overlayer 38140
With the lnterference structure 34 provided on the
substrate 1~ of the record 10, since the lntenslty
~~ 10 modulated radlatlon 12 is not transmitted through the
substrate 16~ the substrate materlal may be selected
without regard for the transmlssive optlcal charac-
terlstlcs thereof.
Charac_eristics of Records 10, 25, and 40
Referrlng to FIGs. lAj 2A, and 3A, it may be un-
derstood that the records 10, 25, and 40 are provided
with a window, such as the wlndow 21 of the record 10,
that is on the side of the records 10, 25, or 40 on
which the radiation 12 is lncldent. As shown ln FIGs.
1~, 2B and 3B, the preferred angle of lncldence of a
central ray 6g of the radiation 12 onto the respective
'. windows 21, 27 and 53 is 90. The typical radiation
12 is a focussed beam of light having a numerical
aperture ln the range of .5 to .65. The magnetic
25 field 11 is produced by a coil 69 (FIGs. 1 and 2), a
similar coll 69 belng used with the record 40. The
magnetlc fleld 11 is perpendicular to the plane formed
by the flat surfaces of the records 10, 25 and 40,
such as the flat surfaces 24-24 (FIG. lA). The radia-
tion 12 is transmitted through the window 21 of the
record 10, through the window 27 of the record 25 and
through the windows 53-53 of the record 40. The radi-
ation 12 is then transmltted through the air gaps 22
and 54-54 respectiYely, or in the case OI the record
KD-1001 -23-
i303739
25, through the transparent adheslve layer 28, and ls
then transmitted through the optlcally transparent
layers 63 of the records 10, 25 and 40. When the ln-
tenslty of the radlation 12 is of a relatively hlgh
value, the radiation 12 ls referred to as a "record
beam" slnce the record 10 records in the presence of
the magnetic fleld 11 when the lntenslty ls hlgh. In
particular, when the modulatlon of the intensity of
the radiatlon 12 results ln the hlgh intenslty record
beam, the materlal of the respective magneto-optlc
layers 15, 31 or 42-42 ls locally heated by the fo-
cussed radiatlon 12, causing the coerclvity of such
material to fall below the value of the magnetic field
11, thus-enabling the direction of magnetization of
the material of the respective magneto-optic layers
15, 31 and 42-42 to be locally reversed from their
original unrecorded conditlon by becoming aligned with
the magnetic field 11. During recording, the magnetic
field 11 ls oriented in a dlrection opposite to the
direction of magnetization of the material of the un-
recorded magneto-optlc layers 15, 31 and 42-42. For
e~ample, if the dlrection of magnetization of the mag-
neto-optic layers 15, 31 and 42-42 is initially up in
FIGs. lA, 2A and 3A, then at the particular location
on whlch the record beam of the radiation 12 is inci-
dent on the magneto-optic layers 15, 31 or 42-42, the
direction of the magnetlzatlon wlll be reversed and
will be downward. Thus, although the magneto-optic
layers 15, 31 and 42-42 do not record in response to
only the record beam, the record beam of radiation 12
and the magnetic field 11 result in recording of the
radiation intensity modulation in the form of the re-
versal of the magnetization at the locatlons at which
the M0 layers 15, 31 or 42-42 are heated as described.
In the case of the magneto-optic recording layer 37 of
the lnterference structure 34, there is greater heat-
KD-1001 -24-
, . _ .. . ... . .
1~03739
ing of the magneto-optlc recordlng layer 37 due to the
lnterference structure.
To read the record thus produced by the radlatlon
12 in the form of the record beam, radlation ln the
form of a plane polarlzed read beam of focussed llght
energy 12 having a substantially reduced level of in-
tensity is dlrected perpendicularly to the respective
sides 13, 13' and 43 of the records 10, 25 and 40.
The read beam 12 is transmitted through the respective
windows 21, 27 and 5~-53 and across the respective alr
gaps 22 and 54-54 or through the adheslve layer 2~ and
ls transmitted through the optlcally transparent lay-
ers 63 and onto the respective magneto-optic layers
15, 31 and 42-42. At locations on whlch the record
beam of radlation 12 was dlrected onto the magneto-
optic layers 15, 31 or 42-42, the resulting reversal
of the magnetization at such locations will, according
to the polar Kerr magneto-optic effect, rotate the
plane of polarization of the read beam 12 in a direc-
tion opposite to that of non-reversed locations. The
read beam 12 is reflected off the magneto-optic layers
15, 31 and 42-42 as indicated by the arrows 70 (FIGs.
--, lA, 2A and 3A) and the rotations of the plane of
polarization are detected. In the case of the mag-
neto-optic recordlng layer 37 of the lnterference
structure 34, the read beam of radlation 12 is reflec-
ted ln part off both the recordlng layer 37 and the
reflectlve layer 35, such parts belng approximately
out of phase by vlrtue of proper cholce of thlckness
of the transparent dlelectrlc spacer protectlve layer
36.
The advantages of the structure of ths records 10
and 40, both of which include alr incldence of the
read optical radlation 12 onto the thin film magneto-
optic recording structures 14 and 41-41, respectively.
whether ln the form shown in respective FIGs. lA or 3A
KD-1001 -25-
1303739
or uslng the lnterference structure 34 shown ln FIG.
4, lnclude better anti-reflectlon "blooming" enhance-
ment of the Kerr slgnal. Further, such records 10 and
40 avoid the necesslty of uslng an optlcal quallty
grooved substrate having stringent tolerances on blre-
frlngence and blrefrlngence gradlent. Instead, the
substrates 16 and 45, respectlvely, may be used wlth-
out regard for transmlsslve optlcal characterlstlcs.
The effect of micro-roughness of the substrates 16 and
45 is conslderably reduced for alr lncldence as com-
pared to substrate lncldence of the read beam of radl-
ation 12. Slnce the substrates 16 and 45 need not be
transparent, non-transparent materials such as metals
can be used on elther slde of the substrates 16 and
45, elther to seal out molsture from the bulk of the
substrates, or to lmprove the adhesion or barrler pro-
perties of the protectlve layer 62. The substrate
surfaces 46 and 47 (FIG. 3B) and 59 (FIG~ 1~) may be
vigorously treated by plasma or by ion bombardment to
cross llnk and harden such surfaces, to activate such
surface for improved adhesive bondlng of the thin film
recording structures 14 and 41-41 thereto (as to the
respective records 10 and 40) or for improved adhesive
bonding of the reflective layer 35 to such substrate
surfaces, and to seal low molecular welght fragments
of polymer, or plastlclzer awa~ from such surfaces,
without regard to transmlsslve optlcal propertles.
Dust defocusslng ls provlded by the transparent
windows 21, 27 and 53-53 having the flat surfaces,
~0 such as the surfaces 24 and 57-57 of the respectlve
records 10 and 40.
E~perimentally, it has been observed that inter-
ference effects assoclated wlth the alr gaps 22 and
54-54 are not a serlous problem; thls ls most llkely
due to the compensatlng effect of reflectance and Kerr
angle factors contrlbuting to the Kerr slgnal.
KD-1001 -2~-
1303739
It has also been experlmentally observed that the
carrler-to-noise ratlo of a test slgnal observed
through the respective wlndows 21 and 53-53 and across
the air gaps 22 and 54-54 ls conslstently better than
that observed when the read beam radlatlon 12 is
transmltted through a substrate as in the above-de-
scrlbed prlor art M0 records. The lmprovement has
been about 3.7 dB ln carrler-to-nolse ratio.
Whlle the preferred embodlments have been de-
scribed ln order to lllustrate the fundamental rela-
`~ tlonships of the present lnventlon, lt should be
understood that numerous variations and modlflcatlons
may be made to these embodiments without departing
from the teachings and concepts of the present lnven-
tion. Accordlngly, lt should be clearly understood
that the form of the present lnventlon descrlbed above
and shown in the accompanylng drawlngs ls lllustratlve
only and ls not lntended to limit the scope of the
invention to less than that descrlbed ln the following
claims.
KD-1001 -27-
