Dual-metal reflective layer recordable/erasable optical media

Recordable/erasable optical storage media are disclosed. The medium of the present invention generally comprises a rigid substrate, an expansion layer, a retention layer, and a dual-metal reflective layer. A single, active layer may be substituted for the expansion and retention layers. A protective layer may or may not be present. The dual-metal reflective layer comprises a first metal sublayer and a second metal sublayer with a graded first metal/second metal alloy at the interface of the first metal sublayer and the second metal sublayer. Methods for applying the dual-metal reflective layer to a polymer-coated substrate and substrates produced by such methods are also provided.

BACKGROUND OF THE INVENTION 
The present invention relates in general to the field of recording media; 
in particular, to a recordable/erasable optical storage medium with a 
dual-metal layer for reflection and methods for producing this layer as 
well as media produced by these methods. 
Optical data storage media in the form of compact disks are well known as 
an alternative to long-playing records and magnetic tape cassettes. The 
disks with which consumers are familiar are optical read-only disks, and 
the common disk player is designed specifically for this type of disk. 
These disks have a reflective surface containing pits that represent data 
in binary form. A description of these pits and how they function is 
provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13 
(incorporated herein by reference). 
Compact disks are currently produced by a process similar to the process 
used to produce conventional long-playing records. The process, referred 
to herein as the mastering process, starts by first polishing a plain 
glass optical disk. This disk has an outside diameter from 200 to 240 mm, 
a thickness of 6 mm and undergoes various cleaning and washing steps. The 
disk is then coated with a thin chrome film or coupling agent, a step 
taken to produce adhesion between the glass disk and a layer of 
photo-resist, which is a photosensitive material. Data on a compact disk 
master tape are then transferred to the glass disk by a laser beam cutting 
method. 
The glass disk is still completely flat after it is written on by the laser 
beam because pits are not formed until the glass is photographically 
developed. The disk surface is first made electrically conductive and then 
subjected to a nickel evaporation process. The disk, now known as the 
glass master, then undergoes nickel electrocasting, a process that is 
similar to that used in making analog phono records. A series of metal 
replications follow, resulting in a disk called a stamper. The stamper is 
equivalent to a photographic negative in the sense that it is a reverse of 
the final compact disk; that is, there are now bumps where there should be 
pits. This stamper is then used to make a pressing on a transparent 
polymer such as polyvinyl chloride, poly(ethyl-methacrylate), and 
polycarbonate. The stamped surface is then plated with a reflective film, 
such as aluminum or other metal, and finally a plastic coating is applied 
over the film to form a rigid structure. 
The player operates by focusing the laser beam on the reflective metal 
through the substrate and then detecting reflected light. The optical 
properties of the substrate, such as its thickness and index of 
refraction, are thus critical to the player's detection systems and 
standard players are designed specifically with these parameters in mind. 
The pits increase the optical path of the laser beam by an amount 
equivalent to a half wavelength, thereby producing destructive 
interference when combined with other (non-shifted) reflected beams. The 
presence of data takes the form of a drop in intensity of the reflected 
light. The detection system on a standard player is designed to require 
greater than 70% reflection when no destructive interference occurs and a 
modulation amplitude greater than 30% when data are present. These 
intensity limits, combined with the focusing parameters, set the criteria 
for the compact disks and other optical data storage media that can be 
read or played on such players. 
Media on which data can be recorded directly, and read directly from, have 
a different configuration and operate under a somewhat different 
principle. One example is described in U.S. Pat. No. 4,879,709 (Clark) 
(incorporated herein by reference), see also U.S. Pat. No. 4,719,615 
(Feyrer, et al.) (incorporated herein by reference). 
The medium described in Clark includes a lower expansion layer of a rubbery 
material that expands when heated. The expansion layer is coupled to an 
upper retention layer that is glassy at ambient temperature and becomes 
rubbery when heated. Both layers are supported on a rigid substrate. The 
expansion and retention layers each contain dyes for absorption of light 
at different wavelengths. Data are recorded by heating the expansion layer 
by absorption of light from a laser beam at a "record" wavelength to cause 
the expansion layer to expand away from the substrate and form a 
protrusion or "bump" extending into the retention layer. While this is 
occurring, the retention layer rises in temperature above its glass 
transition temperature so that it can deform to accommodate the bump. The 
beam is then turned off and the retention layer cools quickly to its 
glassy state before the bump levels out, thereby fixing the bump. 
Reading or playback of the data is then achieved by a low intensity "read" 
beam that is focused on the partially reflecting interface between the 
retention layer and air. When the read beam encounters the bump, some of 
the reflected light is scattered, while other portions of the reflected 
light destructively interfere with reflected light from non-bump areas. 
The resulting drop in intensity is registered by the detector. Removal of 
the bump to erase the data is achieved by a second laser beam at an 
"erase" wavelength that is absorbed by the retention layer and not by the 
expansion layer. This beam heats the retention layer alone to a rubbery 
state where its viscoelastic forces and those of the expansion layer 
return it to its original flat configuration. The write, read, and erase 
beams all enter the medium on the retention layer side, passing through 
retention layer before reaching the expansion layer. 
In U.S. copending application Ser. No. 416,082 now U.S. Pat. No. 5,001,699 
(assigned to the assignee of the present invention and incorporated herein 
by reference) an optical storage medium includes a triple layer recording 
structure having an expansion layer, a retention layer, and a very thin 
reflective layer interposed between the retention and expansion layers. 
See also U.S. Pat. No. 4,896,314 (Skiens et al.). 
In one embodiment, copending U.S. application Ser. No. 07/516,509 (assigned 
to the assignee of the present application and incorporated hereby 
reference) describes a liquid reflective layer that is provided adjacent 
the retention layer opposite the expansion layer. Additionally, improved 
expansion and retention layers are also described therein. Copending U.S. 
application Ser. No. 07/414,041 (assigned to the assignee of the present 
application and incorporated herein by reference) describes a polymer 
"active" layer that is provided adjacent to the substrate and that serves 
to combine functions of the expansion and retention layers into a single 
layer. Copending U.S. application Ser. No. 07/414,044 (assigned to the 
assignee of the present application and incorporated herein by reference) 
describes a single, dual-purpose, retention/reflective layer that is 
adjacent to the expansion layer. 
SUMMARY OF THE INVENTION 
The present invention provides recordable/erasable optical storage media. 
More particularly, it provides optical storage media with a dual-metal 
layer for reflection and methods for producing this layer as well as media 
produced by these methods. The dual-metal layer is advantageous in that it 
provides improved stability for the reflective layer. 
In addition to the dual-metal reflective layer, the media of the present 
invention generally include a rigid substrate and an expansion layer 
adjacent to the substrate. Usually adjacent to the expansion layer is a 
retention layer. Alternatively, the functions of the expansion and 
retention layers can be combined into a single, active layer. The 
dual-metal reflective layer is usually adjacent to the retention layer or 
the active layer if the latter is substituted for separate expansion and 
retention layers. The dual-metal reflective layer can also be sandwiched 
between the expansion layer and the retention layer. A protective layer is 
optionally present and is usually adjacent to the dual-metal reflective 
layer. 
The media of the present invention are susceptible to expansion and 
relaxation, to writing data thermally, to erasing data thermally, and to 
reading data optically.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The present invention provides recordable/erasable optical storage media 
wherein a dual-metal reflective layer is present. In one embodiment, as 
shown in FIGS. 1 and 2, the medium comprises a substrate 4 onto which an 
expansion layer 6 is provided. A retention layer 8 is adjacent to the 
expansion layer. A dual-metal reflective layer 10 is adjacent to the 
retention layer. A protective layer 12 may also be present, although it is 
not necessary. 
The substrate 4 is formed from a rigid transparent material that permits 
substantially full transmission of light for recordation, reading, and 
erasure. The substrate is sufficiently thick and rigid to provide 
structural integrity to the optical medium, and it does not deform in 
response to pressure caused by expansive forces in the adjacent active 
layer. Recordation bumps in the expansion or active layer, caused by the 
layer's thermal expansion upon absorption of the write beam's light 
energy, protrude away from the substrate because of its rigidity. With 
this layer arrangement, the bumps protrude into the reflective layer, as 
described below. 
The substrate may be constructed from a wide variety of readily available 
materials. Merely by way of example, the substrate can be fabricated from 
glass, polymers, or amorphous polymers. In a preferred embodiment, the 
substrate is made of polycarbonate. In many embodiments, the substrate 
will be the thickest layer, with a thickness of about 1 mm or more, 
preferably about 1.2 mm. The thickness of the substrate must be such that 
it imparts rigidity to the optical medium. 
In the embodiment shown in FIGS. 1 and 2, an expansion region or layer 6 is 
adjacent to the substrate. The expansion layer is usually formed of a 
material that absorbs a percentage of light energy passing through it; 
displays a high coefficient of thermal expansion, particularly when 
compared to the other layers of the medium; and displays a high 
coefficient of elasticity to the extent that it will expand readily when 
heated at the temperatures encountered during a recordation process 
without exceeding its upper expansive limit. The expansion layer must also 
contract to its original relaxed condition upon cooling. 
When at room temperature, the expansion layer material should be near or 
above its softening temperature, which is preferably below 30.degree. C. 
and more preferably at or below 20.degree. C. By softening temperature, it 
is meant the temperature at which the modulus of elasticity of the 
material of the expansion layer has dropped to 25% to 50% of its maximum 
modulus. A coefficient of thermal expansion above about 1.times.10.sup.-4 
/.degree. C. is preferred, with those greater than about 3.times.10.sup.-4 
/.degree. C. more preferred, and those greater than about 
5.times.10.sup.-4 /.degree. C. most preferred. The degree of single pass 
absorptivity of light energy should be between 20% and 40% in the 
wavelength range from 850 nm to 650 nm such that the expansion layer may 
be heated with a laser beam at a write wavelength. To maintain the ability 
to read data recorded on the optical media on standard detection 
mechanisms, such as those found on conventional compact disk players, a 
maximum double pass absorption at the standard compact disk read 
wavelength (780 nm) of 10% is most preferred. 
Accordingly, the expansion layer material may comprise a base resin 
selected from a group including, but not limited to, rubbers, such as 
silicone rubbers, styrene-butadiene rubbers, and natural rubbers such as 
butyl rubbers; epoxies; polyurethane; polymers; amorphous polymers; 
cellulose acetate-butyrate; poly(vinyl butyryl); polyamides; acrylic 
polymers; polyvinyl acetate; silicone resins; styrene-butadiene 
copolymers; vinyl chloride-vinyl acetate copolymers; and mixtures thereof. 
Materials with high elasticity, such as elastomers and polymers with 
elongations greater than 15%, are preferred for construction of the 
expansion layer. 
In a preferred embodiment, the expansion layer is an epoxy resin with a 
softening temperature below 50.degree. C. and preferably at 30.degree. C. 
or below. In another preferred embodiment, a base resin or mixture of 
resins is mixed with appropriate curing agents to form the expansion 
layer. For example, a bisphenol A/-epichlorohydrin epoxy resin (Shell 828, 
manufactured by Shell Chemical) and an epichlorohydrin-dimer fatty 
acid-based epoxy resin (Shell 871, manufactured by Shell Chemical) may be 
mixed in approximately equal amounts with a nonstoichiometric (e.g., 2.6x) 
amount of a curing agent, such as Versamid V150 (a polyamide resin that is 
an adduct of a polyamine with a dibasic fatty acid, manufactured by 
Henkel). Additionally, Shell 828 may be mixed with a nonstoichiometric 
(e.g., 1.5x) amount of a curing agent such as Dow DEH52 (an aliphatic 
polyamine-epoxy adduct, manufactured by Dow Chemical). 
The thickness of the expansion layer is selected in accordance with the 
optics of the system. For example, in order to maintain the minimum bump 
size during data recordation with the greatest write sensitivity during 
recording, a laser beam should be maintained as small as possible as it 
passes through the expansion layer. Accordingly, most of the expansion 
layer should be within the focal depth of the write beam. For recording 
systems having optical parameters similar to those found in standard 
compact disk players, the write beam is diffraction limited and has a 
focal depth of approximately 1.0-2.0 microns. In such cases, best results 
can be obtained with an expansion layer having a thickness of 
approximately 0.5 to 3.0 microns, preferably about 1.0 microns. 
The expansion layer is bonded to the substrate and, in the embodiment 
depicted in FIGS. 1 and 2, to the retention region or layer 8. This is 
achieved by methods known in the art. For example, coating of the 
expansion layer onto the substrate may be accomplished by a wet chemical 
process, such as spin coating or web coating. The retention layer is then 
deposited onto the expansion layer, utilizing, for example, vacuum 
deposition, sputtering, or chemical vapor deposition. 
The retention layer is usually formed from a material that absorbs a 
percentage of light energy passing through it; displays a glass transition 
temperature that is above room temperature, preferably about that of the 
expansion layer; is rubbery, when above its glass transition temperature, 
with sufficient elasticity to permit it to conform to the contour of the 
distortion formed in it by the heated expansion layer; and displays 
sufficient rigidity and strength below its glass transition temperature 
such that it will hold the expansion layer in an expanded condition, even 
when the expansion layer is cooled to ambient temperature. 
In preferred embodiments, the retention layer is formed of material or 
combinations of materials that display at least some light absorption at 
the wavelength of an erase beam. The wavelength of the erase beam may be 
chosen from a wide spectrum of available light wavelengths. The degree of 
absorptivity may vary from wavelength to wavelength, and from retention 
material to retention material, but may be, for example, about 30% to 45% 
at wavelengths between about 650 nm and 860 nm. 
Accordingly, the retention layer material includes, but is not limited to, 
rubbers, such as silicone rubbers, styrene-butadiene rubbers, and natural 
rubbers such as butyl rubbers; epoxies; polyurethane; polymers; amorphous 
polymers; cellulose acetate-butyrate; poly(vinyl butyryl); polyamides; 
acrylic polymers; polyvinyl acetate; silicone resins; styrene-butadiene 
copolymers; vinyl chloride-vinyl acetate copolymers; and mixtures thereof. 
In a preferred embodiment, the retention layer is an epoxy with a glass 
transition temperature of about 80.degree. to 120.degree. C. and may be, 
for example, about 105.degree. C. The retention layer will typically have 
a Young's modulus of at least about 400,000 psi. The thickness of the 
retention layer is approximately 0.5 to 1.5 microns, and a preferred range 
is from about 0.5 to about 1.0 microns. 
In another embodiment, as shown in FIG. 3, a single polymer layer, termed 
the active region or layer 7, is adjacent to the substrate and alleviates 
the need for separate expansion and retention layers. Both recording and 
erasure can be accomplished with a single laser beam, and there is 
necessarily one less separate coating operation involved in the 
manufacture of a medium with an active layer. 
The active layer is comprised of any polymer whose softening temperature is 
above room temperature (approximately 20.degree. C.) and that has a 
relatively large coefficient of expansion above its softening temperature. 
By softening temperature, it is meant the temperature at which the modulus 
of elasticity of the material of the active layer has dropped to 25% to 
50% of its room temperature or maximum modulus. A softening temperature 
range of approximately 30.degree. C. to 175.degree. C. is preferred, with 
a more preferred range of approximately 90.degree. C. to 130.degree. C. 
The coefficient of expansion should be greater than 200.times.10.sup.-6 
/.degree. C., and is preferably greater than 250.times.10.sup.-6 /.degree. 
C., and more preferably greater than 300.times.10.sup.-6 /.degree. C. 
The active layer material may comprise a base resin selected from a group 
including, but not limited to, epoxies, polyurethane, polymers, amorphous 
polymers, cellulose acetate, cellulose acetate-butyrate, polystyrene, 
polysulfonamide, polycarbonate, cellulose nitrate, 
poly(ethyl-methacrylate), poly(vinyl butyryl), aromatic polyesters, 
polyamides, acrylic polymers, polyvinyl acetate, silicone resins, alkyd 
resins, styrene-butadiene copolymers, vinyl chloride-vinyl acetate 
copolymers, nitrocellulose, ethylcellulose, and mixtures thereof. In a 
preferred embodiment, the active layer is an epoxy with a softening 
temperature of about 80.degree. C. to 120.degree. C. and may be, for 
example, about 105.degree. C. 
In more preferred embodiments, a base resin or mixture of resins may be 
mixed with appropriate curing agents to form the active layer or region. 
In particular, a bisphenol A/-epichlorohydrin epoxy resin (Shell 828, 
manufactured by Shell Chemical) and an epichlorohydrin-dimer fatty 
acid-based epoxy resin (Shell 871, manufactured by Shell Chemical) may be 
mixed in approximately equal amounts with a nonstoichiometric (e.g., 2.6x) 
amount of a curing agent, such as Versamid V150 (a polyamide resin that is 
an adduct of a polyamine with a dibasic fatty acid, manufactured by 
Henkel). Additionally, Shell 828 may be mixed with a nonstoichiometric 
(e.g., 1.5x) amount of a curing agent such as Dow DEH52 (an aliphatic 
polyamine-epoxy adduct, manufactured by Dow Chemical). 
The thickness of the active layer is approximately 0.5 to 3.0 microns. A 
preferred range is 1.0 to 2.0 microns. The active layer is bonded to the 
substrate by methods known in the art. For example, coating of the active 
layer onto the substrate may be accomplished by a wet chemical process, 
such as spin coating or web coating. 
The dual-metal reflective region or layer 10 is adjacent to either the 
retention layer or to the active layer, depending upon the selected 
embodiment. Alternatively, the dual-metal reflective layer can be 
sandwiched between the expansion and retention layers, as shown in FIG. 4. 
As shown in FIG. 7, the dual-metal reflective layer of the present 
invention is heterogeneous and comprised of a first metal subregion or 
sublayer 20 and a second metal subregion or sublayer 22 with a graded 
first metal/second metal alloy 24 at the interface of the first and second 
metal subregions or sublayers. The first metal comprises any highly 
reflective metal that forms low melting alloys with gallium in the range 
from about 0.degree. C. to about 25.degree. C., preferably in the range 
from about 20.degree. C. to about 25.degree. C. Preferred metals are those 
selected from the group consisting of indium, silver, bismuth, lead, zinc, 
and alloys thereof. The second metal is comprised of gallium or alloys 
thereof. 
The heterogeneous dual-metal reflective layer of the present invention is 
advantageous in that it adheres like a solid metal film yet performs like 
a passive liquid metal. The first metal forms a solid polymer-metal 
interface with the retention layer, as in the embodiment shown in FIG. 1; 
or with the active layer, as in the embodiment illustrated in FIG. 3; or 
with the expansion and retention layers, as in the embodiment shown in 
FIG. 4. The second metal then wets well to the mushy solid or liquid/solid 
mixture of the first metal and partially diffuses into it to form a graded 
alloy that behaves as a passive liquid metal. The metals of the first and 
second layer are both very soft, highly reflective, and very soluble in 
each other. As such, the dual-metal reflective layer offers little or no 
resistance to recordation bump formation during the laser write process, 
and the texture of the dual-metal layer is such that the layer remains 
stable during the erasure process. 
Specifically, the dual-metal reflective layer can vary in thickness from 
about 2,000.ANG. to about 10,000.ANG., preferably from about 4,500.ANG. to 
about 6,000.ANG., and more preferably from about 4,500.ANG. to about 
5,200.ANG.. The first metal sublayer can have a thickness in the range 
from about 500.ANG. to about 1,500.ANG., preferably from about 800.ANG. to 
about 1,200.ANG. and more preferably from about 1,000.ANG. to about 
1,200.ANG.. The second metal sublayer can vary in thickness, depending 
upon the selected thickness and content of the first metal sublayer. In 
general, a preferred thickness range is from about 3,000.ANG. to about 
6,000.ANG., preferably from about 3,500.ANG. to about 4,000.ANG.. 
Additionally, in a preferred embodiment, the second metal sublayer is 
comprised of separately sputtered regions or layers and, more preferably, 
five separately sputtered layers, each layer having a thickness from about 
700.ANG. to about 800.ANG.. In this latter more preferred embodiment, the 
second metal sublayer has a thickness from about 3,500.ANG. to about 
4,000.ANG. and the first metal sublayer has a thickness from about 
1,000.ANG. to about 1,200.ANG., resulting in an overall dual-metal 
reflective layer thickness from about 4,500.ANG. to about 5,200.ANG.. 
If the overall reflective layer is relatively thick, for example from about 
5,000.ANG. to about 10,000.ANG., then recordation bumps deform and are 
encompassed by the reflective layer but do not protrude into the 
protective layer 12, if such a layer is present. This is shown in FIG. 5, 
with an active layer present. If, however, the reflective layer is 
relatively thin, for example, less than about 5,000.ANG., then the 
recordation bumps deform the reflective layer and protrude into the 
protective layer, if present. This is shown in FIG. 6, with an active 
layer present. 
One or more protective regions or layers 12 may be present, which serve to 
protect the recordation bumps from damage due to contact with external 
objects. Characteristic of this layer is that it is sufficiently 
compliant, when the dual-metal reflective layer is thin, to allow the 
recordation bumps to easily protrude into it and to offer little 
resistance to their formation. In addition, the protective layer 
preferably is relatively thick when compared to the other layers so that 
the bumps are not transmitted through the reflective layer, into the 
protective layer, and subsequently through the protective layer to its 
outer surface. It is also preferred, although not necessary, that the 
protective layer have a high thermal conductivity to enable it to function 
as a heat sink for purposes of rapid cooling of the active layer, if 
present, immediately following formation of the bumps. A thermal 
conductivity of at least 5.times.10.sup.-4 cal/((cm.sup.2 /.degree. 
C.)(sec/cm)) will provide adequate results. Suitable materials for use as 
a protective layer include silicone and acrylate. 
The protective layer may or may not be required depending on the 
functionality, storage, and handling of the optical disk. Thus, in an 
application where the disk is stored and operated in a protective case or 
cartridge, a protective layer may not be required. A protective layer may 
also not be required where the dual-metal reflective layer is of 
sufficient thickness so that the recordation bumps are not transmitted 
through the reflective layer to its outer surface. PG,14 
In one embodiment, as shown in FIG. 1, the protective layer will enclose 
and contain the dual-metal reflective layer, this being accomplished by 
the protective layer being provided with an inner ring 14 and an outer 
ring 16 that extend to and contact the retention layer or the active 
layer, if the latter is present instead of a retention layer. The 
thickness of the inner and outer rings is selected in accordance with the 
desired thickness of the reflective layer. In a preferred embodiment, the 
width of the rings is between about 1 mm and 4 mm. Alternatively, as shown 
in FIGS. 3 and 4, the dual-metal reflective layer may extend to the edges 
of the disk so that the protective layer does not contain it. The 
thickness of the optional protective layer is on the order of tens of 
microns, as it must be thick enough to protect the recordation bumps from 
external abuse. Preferably, the protective layer is about 2 microns thick. 
The present invention also includes methods for applying the dual-metal 
reflective layer to a polymer-coated substrate, such as is present in 
optical storage media. These processes require a minimum of treating and 
also do not require substrate cooling. The present processes comprise the 
steps of: sputtering a first metal onto a polymer-coated substrate and 
then sputtering a second metal onto the sputtered first metal. The first 
metal comprises any highly reflective metal that forms low melting alloys 
with gallium in the range from about 0.degree. C. to about 25.degree. C., 
preferably in the range from about 20.degree. C. to about 25.degree. C. 
Preferred metals are those selected from the group consisting of indium, 
silver, bismuth, lead, zinc, and alloys thereof. The second metal is 
comprised of gallium or alloys thereof. 
Preferred conditions for sputtering the first metal include performing the 
process in a vacuum chamber, preferably having an initial chamber pressure 
of about 5.times.10.sup.6 torr or less. The first metal, preferably 
indium, is sputtered to a thickness from about 500.ANG. to about 
1,500.ANG., preferably from about 800.ANG. to about 1,200.ANG., and more 
preferably from about 1,000.ANG. to about 1,200.ANG., at a rate from about 
80.ANG./sec to about 120.ANG./sec, preferably at about 120.ANG./sec. 
Preferred conditions for sputtering the second metal onto the sputtered 
first metal include performing the second step immediately without 
breaking the vacuum whereby alloying of the second metal sublayer with the 
first metal sublayer is improved. If the second metal sublayer is applied 
in the same vacuum chamber immediately after applying the first metal 
sublayer, the formation of an oxide layer is avoided. 
The second metal is sputtered to a preferred total thickness from about 
3,000.ANG. to about 6,000.ANG., preferably from about 3,500.ANG. to about 
4,000.ANG., at a rate from about 10.ANG./sec to about 15.ANG./sec, 
preferably at about 12.ANG./sec. 
In a preferred embodiment, the second metal sublayer is sputtered more than 
one time. Herein, an initial layer having a thickness from about 700.ANG. 
to about 800.ANG. is applied. After pausing, preferably for about 1 to 
about 2 minutes and more preferably about 2 minutes, a second layer is 
deposited, and this process can be repeated until a preferred amount of 
about five separate layers have been deposited. Other numbers of separate 
layers can be employed, although heating and cooling of the previously 
sputtered layer is a critical factor. Pausing between sputtering separate 
layers of the second metal allows the target layer to cool. If the target 
layer is allowed to cool too much, moisture condensation will be present. 
If, however, the target layer is heated too much, then the metal will melt 
and become unwieldy to handle. As the melting point of gallium is about 
29.8.degree. C., the second metal sublayer should not be allowed to heat 
to this level. 
The present invention also includes dual-metal reflective layer, 
polymer-coated substrates produced by the above-discussed methods, 
preferably those substrates produced by the methods wherein the second 
metal is sputtered more than one time and, more preferably, about five 
times. 
FIGS. 5 and 6 illustrate the invention during/after the writing (either 
during the initial recording or during subsequent recordings) of an 
optically detectable recordation bump 18; however, for purposes of 
illustration, the recordation bump is not necessarily in proportion to the 
respective layers. To write, a laser beam (indicated as h.nu.) enters the 
substrate and passes into the active layer, or into the expansion layer in 
an alternative embodiment, where it is absorbed at a particular 
wavelength, known as the write wavelength. The absorptive characteristics 
of the layer may be imparted thereto using methods that will be apparent 
to those of skill in the art, such as by the addition of light-absorptive 
dyes or pigments. Since the medium of the present invention need not be 
wavelength-specific, a broad range of dyes or pigments is available for 
this purpose. 
In addition, except for the ability to pass a portion of the wavelength 
energy that is employed for the purpose of reading the recorded data, 
these dyes or pigments need not be wavelength-specific and may therefore 
absorb light energy over a broad spectrum of wavelengths. Thus, during 
recordation, the laser beam is absorbed by a dye or a pigment contained 
within the active layer, or the expansion layer, that will absorb light 
from the laser beam at the write wavelength to cause the active layer, or 
the expansion layer, to expand away from the substrate and form 
recordation bumps extending into the reflective layer. Dyes or pigments 
that can be used singly or in combination are nigrosin blue, aniline blue, 
Calco Oil Blue, ultramarine blue, methylene blue chloride, savinal blue, 
Monastral Blue, Malachite Green Oxalate, Sudan Black BM, Tricon blue, 
Macrolex green G, DDCI-4, and IR26. Preferred among these are savinal 
blue, Tricon Blue, and Macrolex green G. 
Because the absorption of the beam by the dye or pigment occurs 
progressively throughout the complete thickness of the active layer, if 
present, a thermal gradient is created within the layer. This gradient 
depends on the amount of energy that is absorbed at a given depth within 
the layer. The incident surface of the active layer is necessarily heated 
to a higher temperature than its opposite surface, which results in a 
progressive absorption through the layer, and a thermal gradient is formed 
between the two surfaces. The heated spot of the active layer is confined 
by a surrounding low temperature area, and expansion can take place only 
away from the substrate. The dual-metal reflective layer acts as a passive 
component so that when a recordation bump is formed, the bump will 
protrude into the reflective layer and it will conform around the bump. In 
addition to improving reflection, the dual-metal reflective layer acts as 
a heat sink during recordation and causes the active layer to cool more 
rapidly at its opposite surface. When the laser beam is turned off, the 
opposite surface of the active layer will cool much more rapidly than the 
incident surface, resulting in a temperature below the softening 
temperature of the polymer for the opposite surface. The opposite surface 
thus becomes rigid and locks the recordation bump into place while the 
hotter incident surface area is still expanded. 
If an expansion layer is used, the dye(s) or pigment(s) in the expansion 
layer absorb a high proportion of the energy at the wavelength of the 
writing laser to form a heated area in the layer. The heated spot of the 
polymer expansion layer is confined by a surrounding low temperature area, 
and expansion can take place only away from the substrate and into the 
reflective layer. 
After the optical medium has been recorded, as described above, erasure can 
be achieved by methods known in the art. For example, this may be 
accomplished by "spot" erasure, where either a different laser with a 
larger focused point, or the same laser used to record on the medium but 
defocused to a slightly larger spot, can be used to focus a light beam 
through the substrate and, in one embodiment, the active layer. Erasure 
therein occurs when a recorded area of the active layer is heated 
relatively slowly to the softening temperature of the active layer and is 
then cooled slowly so that the active layer relaxes to its original 
unwritten state. By heating the active layer slowly, a steep temperature 
gradient is not formed through the thickness of the layer such as was 
formed during recordation. The cooling rate of the active layer is now 
slower than its viscoelastic restoring forces so that the polymeric 
material of the layer returns to its original spatial arrangement. 
If, instead, an expansion layer is present, erasure occurs when a recorded 
area of the expansion layer is heated relatively slowly to above the 
melting point temperature of the retention layer to a temperature at which 
the polymer expansion layer softens. The layers are then cooled slowly so 
that both the expansion layer and the retention layer return to their 
original spatial arrangements. By heating and then cooling the expansion 
layer slowly, the cooling rate of the layer is now slower than its 
viscoelastic restoring forces so that the polymeric material of the layer 
relaxes to its original unwritten state. The retention layer then 
resolidifies over the expansion after the relaxation of the expansion 
layer. Suitable dyes for incorporation into the retention layer include 
those dyes described above for the expansion layer. 
Unlike much of the prior art, there is no requirement herein that the write 
or recordation wavelength be different from the erasure wavelength. The 
write wavelength chosen can be the same wavelength as used for erasure, 
and this is preferred. The previous need for two lasers, to record and to 
erase, with different wavelengths corresponding to the absorption 
frequencies of different dyes in separate expansion and retention layers, 
is eliminated by the present invention. 
Reading of the recorded data (recordation bumps) from the optical disk is 
achieved by focusing a light beam, chosen from a wide spectrum of 
available light wavelengths, through the substrate and through the active 
layer or the expansion layer. Playback or reading can be accomplished, for 
example, on standard compact disk systems. The media of the present 
invention are compatible with such standard systems: the active layer or 
the expansion layer is transparent at the read wavelength of 780 nm; the 
reflectance of the reflective layer is 70% or greater at the read 
wavelength; the recordation bumps generate interference with the reflected 
light beam; and the interference is then detected by the read or playback 
system. 
It is to be understood that the above description is intended to be 
illustrative and not restrictive. Many embodiments will be apparent to 
those ordinary of skill in the art upon reviewing the above description. 
By way of example, although the invention has been illustrated with 
reference to the use of lasers as the radiant energy source, other sources 
can be used and will be readily apparent to one of ordinary skill. The 
feature of a dual-metal reflective layer can be combined with other 
arrangements of the substrate, expansion and retention layers (or 
substituted active layer), and protective layer. The scope of the 
invention, therefore, should be determined with reference to the appended 
claims, along with the full scope of equivalents to which such claims are 
entitled, and should not be limited to the above description.