 |
Description  |
|
|
FIELD OF THE INVENTION
The present invention relates to optical recording elements, including
recordable optical compact disks.
BACKGROUND OF THE INVENTION
There are many known types of optical recording elements. One of the
currently popular elements is the compact disk (CD). Digital information
is stored in the form of high optical density marks or pits on an
otherwise reflective background. In this format, the optical information
is most often in the form of read only memory or ROM. Optical information
is not usually recorded in real time but rather is produced by press
molding. In a typical process, the optical recording substrate is first
press molded with a master containing the digital information to be
reproduced. The thus formed information is then overcoated with a
reflective layer and then with an optional protective layer. In those
areas having the deformations or pits, the optical density is higher than
in those areas not having the deformations.
It is desirable to produce recordable optical recording elements which,
when recorded in real time, produce a record that mimics the conventional
CD on read out. Read out is at about 780 nm. In this manner, information
can be added to the CD and the CD can be used on a conventional CD player.
One element of this type is the so called "Photo CD". In the system in
which this element is used, conventional photographic film is first
processed in a conventional manner. Then, the images from the film are
digitized and the digital information is recorded in a CD readable form on
a CD compatible optical recording element (Photo CD). Images can then be
played back on a CD-type player into a conventional television. Since the
element has a capacity for a number of digitized images that is greater
than the number of images on a typical roll of consumer film, it is
anticipated that the user will want to add images to a preexisting CD.
These recordable optical recording elements consists of a polycarbonate
substrate containing a continuous tracking groove. An organic dye layer
(recording layer) is solvent coated onto the grooved substrate.
Fabrication is completed by coating a reflector layer, a protective layer,
and a label in fashion similar to a conventional CD digital audio disc or
CD-ROM disc. In the recordable element the digital information is written
into the dye layer with the focused beam of a diode laser operating in the
near infrared region of the spectrum.
Commercially useful recordable optical recording elements have stringent
requirements. The recording layer must have the required reflectivity, and
must also be able to couple with incident laser irradiation to provide
features having adequate optical contrast. The layer must also have good
stability towards light, heat and humidity for acceptable shelf life
(incubation stability). Since the Photo CD is a consumer product, it must
be capable of withstanding extreme environments. Between the time the
original images are recorded on the Photo CD element and the time
subsequent images are recorded, the element might be placed in strong
sunlight.
In addition the optical properties of the recording layer (its' refractive
indices) must be finely tuned especially as regards `k` (the imaginary
component of the index of refraction) which controls the absorption of
light at the writing laser wavelength. K must be finite but low to insure
a balance between coupling and reflectivity.
Experimentally, it is difficult to find a single dye with the appropriate
value of k. It is common in the art to combine symmetrical cyanine dyes
(usually at least two dyes, one with a high k and one with a low k) to
meet the k requirements. See U.S. Pat. No. 5,391,413. Indodicarbocyanine
dyes have been used frequently. However, these dyes often have less than
the desired light stability and will in fact fade to an unusable state
after only a few days of intense sunlight exposure.
Optical recording layers containing metallized formazan dyes are disclosed
in U.S. Pat. No. 5,294,471. These recording layers do not have sufficient
capability to couple with incident laser irradiation to form the necessary
contrast and sensitivity.
U.S. Pat. No. 5,547,728 discloses optical recording elements having
recording layers comprising a mixture of metallized formazan dye and a
cyanine dye. Such elements are useful but need improvements in recording
sensitivity, incubation stability and light stability. Most of the
expressly disclosed metallized formazans therein have a strong electron
withdrawing group (typically NO.sub.2). This results in a relatively large
absorption at 780 nm. As a result, those formazans have, as part of their
complex refractive index high k values (about 0.15). This relatively large
k value has a negative impact on reflectivity when used with other
recording layer dyes also having high k values.
Thus, there is a continuing need for optical recording materials that have
the necessary optical characteristics such that they are CD compatible,
can couple with incident laser irradiation to form features with
sufficient contrast and yet are light and dark (incubation) stable.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a metallized
formazan dye having an value of 0.03 to 0.07 for the imaginary part of the
refractiv index k. For convenience the metallized formazan dyes used in
this invention are sometimes referred herein as "low k" formazans.
This invention provides optical recording elements having improved
incubation stability and light stability compared to prior art elements
containing similar components. The use of low k metallized formazans
defined above allows the use of greater concentrations of metallized
formazans without adversely affecting the targeted k value for the
recording layer.
DETAILED DESCRIPTION OF THE INVENTION
Any cyanine dyes should be useful in the recording layer dye mixture as
long as the resulting mixture has the refractive index attributes
mentioned above. A particularly useful group of such dyes are
indodicarbocyanine dyes. Typical indodicarbocyanine dyes include:
##STR1##
The preparation of the low k formazan used herein are prepared according to
the methods described in U.S. Pat. No. 5,294,471. Low k metallized
formazan fall within the scope of structure (I):
##STR2##
wherein R.sub.1 represents alkyl of 1-12 carbons, or alkyl substituted
with one or more groups selected from the group consisting of hydroxy,
acyloxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl,
arylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl, aryloxycarbonyl,
acetyl, aroyl, alkyl-aminocarbonyl, arylaminocarbonyl,
alkylaminocarbonyloxy, arylaminocarbonyloxy, acylamino, amino, alkylamino,
arylamino, carboxy, trihalomethyl, alkyl, heteroaryl, alkylureido,
arylureido, succinimido and phthalimido; or aryl, or alkyl substituted
aryl or alkoxy substituted aryl;
R.sub.2 represents sulfonyl morpholine, sulfonamide, halogen, cyano,
alkoxycarbonyl or alkyl sulfonyl.
In the definitions of structure I alkyl means 1-12 carbons (including
cycloalkyl of 5-7 carbons), aryl means 6-10 carbon atoms, alkoxy means
1-12 carbons and hetaryl means 5-10 membered unsaturated rings with at
least one heteroatom.
In addition, each of the alkyl, cycloalkyl, aryl, hetaryl and alkoxy groups
may be further substituted with one or more groups chosen from hydroxy,
acyloxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl,
arylsulfonyl, thiocyano, cyano, nitro, halogen, alkoxycarbonyl,
aryloxycarbonyl, acetyl, aroyl, alkylaminocarbonyl, arylaminocarbonyl,
alkylaminocarbonyloxy, arylaminocarbonyloxy, acylamino, amino, alkylamino,
arylamino, carboxy, trihalomethyl, alkyl, aryl, hetaryl, alkylureido,
arylureido, succinimido, phthalimido and the like.
Metallized low k formazan dye of claim within structure I include those
presented in table 1.
TABLE 1
______________________________________
1 #STR3##
2 #STR4##
3 #STR5##
4 #STR6##
5 #STR7##
6 #STR8##
7 #STR9##
______________________________________
OPTICAL RECORDING ELEMENTS
The optical elements of the invention comprise a light transmitting,
typically pregrooved substrate, the dye recording layer overlaying the
substrate, a light reflective layer overlaying the light absorptive layer
and a protective layer overlaying the light reflective layer. The
recording process will produce marks of lower reflectivity than the
unmarked areas of the disk when written and read with a diode laser
emitting between 770 and 800 nm. The substituents on the dye molecules are
selected such that the real part of the complex refractive index (n) of
the unwritten recording layer measured with a 780 nm light source is
greater than 1.8 and the imaginary part (k) is less than 0.15.
The substrate may be any transparent material that satisfies the mechanical
and optical requirements. The substrates are generally pregrooved with
groove depths from 20 nm to 250 nm, groove widths 0.2 to 1 .mu.m and a
pitch 0.5 to 2 .mu.m. The preferred material is polycarbonate; other
materials are glass, polymethylmethacrylate and other suitable polymeric
materials.
The preparation of the optical recording element of the invention is
achieved by spin coating of the dye mixture, with or without addenda, from
a suitable solvent onto a transparent substrate. For coating, the dye
mixture, with or without addenda, is dissolved in a suitable solvent such
that the dye is 20 or less parts by weight to 100 parts of solvent by
volume. The dye recording layer of the element is then overcoated with a
metal reflective layer under reduced pressure by resistive heating or a
sputter method and finally overcoated with a protective resin.
Coating solvents for the dye recording layer are selected to minimize their
effect on the substrate. Useful solvents include alcohols, hydrocarbon
halides, cellosolves, ketones. Examples of solvents are
2,2,3,3-tetrafluoropropanol, tetrachloroethane, dichloromethane, methyl
cellosolve, ethyl cellosolve, 1-methoxy-2-propanol,
4-hydroxy-4-methyl-2-pentanone. Preferred solvents are alcohols since they
have the least effect on the preferred polycarbonate substrates. Mixtures
containing these solvents can also be used.
Useful addenda for the recording layer include stabilizers, surfactants,
binders and diluents.
The reflective layer can be any of the metals conventionally used for
optical recording materials. Useful metals can be vacuum evaporated or
sputtered and include gold, silver, aluminum, copper and alloys thereof.
The protective layer over the reflective layer is similarly conventional
for this art. Useful materials include UV curable acrylates.
One preferred protective layer is disclosed in U.S. Pat. No. 5,312,663.
This patent discloses a two layer structure in which the layer adjacent to
the reflective layer is spin coated and the second layer is screen
printed.
An intermediate layer, to protect the metal layer from oxidation, can also
be present.
The element of the invention can have prerecorded ROM areas as described in
U.S. Pat. No. 4,940,618. The surface of the substrate can have a separate
heat deformable layer as described in U.S. Pat. No. 4,990,388. Other
patents relating to recordable CD type elements are U.S. Pat. Nos.
5,009,818; 5,080,946; 5,090,009; 4,577,291; 5,075,147; and 5,079,135.
Examples of representative metallized formazans from Table 1 were combined
with a cyanine dye to form optical recording layers and optical recording
elements. For each of the recording layer mixtures studied the cyanine dye
to formazan ratio was varied so as to form an element capable of achieving
the required refractive indices, the aim being to meet the 70% minimum
reflectivity for optical data discs published in International Standard
ISOIEC 10149. The combined formazan cyanine dye mixture was dissolved in
an appropriate solvent (typically 2,2,3,3-tetrafluoro-1-propanol (TFP))
and spun coated onto silicon wafers. Spectroscopic ellipsometric analysis
of the coated wafers provided the refractive index. The film thickness
corresponding to the first reflective maximum and the reflectivity was
calculated. The ratio of components was varied until the required
reflectivity at 780 nm was achieved.
Measurement of a single component was made by coating the single component,
such as a dye, as described above. K was determined by spectroscopic
ellipsometric analysis also as described above.
The following examples demonstrate the light and incubation stability of
the optical recording layer used in the elements of this invention.
For each of the dye mixtures studied, an optical recording element was
prepared by first dissolving the dye mixture in an appropriate solvent.
The solvent mixture was MP/TCE ›1-methoxy-2-propanol and
2,2,2-trichloroethanol (97:3)! unless otherwise stipulated. The solution
was spin coated onto a grooved polycarbonate substrate using methods well
known to those skilled in the spin coating art. A complete recording
element was prepared by applying 70 to 100 nm of gold to the dye film by
sputtering and the gold layer was overcoated with 5 microns of a
photocrosslinkable lacquer (Daicure SD-17). The element was tested by
recording an EFM pattern using a commercially available media tester
(MT-16 from Philips NV) and the pattern read back on a CD-CATS reader
(commercially available from Audio Development Informationsteknik AB
Sweden).
The light stability of the coated dye mixtures in optical recording
elements was evaluated in some examples by preparing appropriate mixtures
of low k formazans with a cyanine dye. The mixture was dissolved in
2,2,3,3-tetrafluoro-1-propanol at 2% solids and spun coated on 2"X 2"
pieces of polycarbonate at 600rpm. After drying, the spectra of the
resulting recording layers was measured and the absorbance at the
wavelength corresponding to that of maximum absorbance of CD-1 (714 nm)
was recorded. The recording layers were then exposed to a 50 Klux high
intensity daylight source for periods extending to 14 days, the absorbance
loss (at 714 nm) noted after each increment of exposure. In control
compositions containing only CD-1 (no formazan dyes) the loss was 100
percent after three days exposure.
The criteria for incubational stability, 2 weeks at 80.degree. C./85% RH
and light stability, 5 days of 50 Klux light exposure are that contrast
(I11R) not drop by more than 0.05 after 2 weeks incubation, that
reflectivity not drop below 65%, that symmetry not increase or decrease by
more than 10 units, and that BLER not rise above 50. BLER is error rate.
COMPARATIVE EXAMPLE
A dye mixture containing 4 parts of CD-1 and 1 part of a metallized
formazan according to structure 1 (wherein R.sub.1 =3-C.sub.7 H.sub.15 and
R.sub.2 =NO.sub.2) in 1-methoxy-2-propanol was coated as a recording layer
on a 45 nm deep grooved polycarbonate substrate to a thickness
corresponding to the first reflectance maximum. The formazan used in this
example is typical of prior art metallized formazans having strong
electronegative groups and high k values of about 0.15. An optical
recording element was completed and tested as described above both fresh
and after 1 week incubation at 80.degree. C./85% RH.
The element, which was disclosed to afford good light stability in U.S.
Pat. No. 5,547,728, was found to be incubationally unstable.
Examples 1-5
Optical recording elements were formed from dye mixtures containing 7 parts
of CD-1 and 3 parts of various low k formazans (LF-1, LF-2, LF-3, LF-4 and
LF-5) dissolved in a mixture of 1-methoxy-2-propanol and
4-hydroxy-4-methyl-2-pentanone and coated on a 80 nm deep grooved
polycarbonate substrate. The elements were tested as described above and
passed all incubation and light stability criteria.
Example 6
The dye LF-6 was tested as in example 1 and found to pass the incubation
criteria.
Example 7
The dye LF-3 was tested as in example 1 with CD-1 except that the ratio was
6:4 and that the substrate was 200 nm The resulting element passed the
incubation and light stability criteria.
Example 8-9
The dyes LF-5 and LF-7 were tested as in example 7 except that the
substrate was 50 nm. The element containing LF-5 was tested only for
incubation stability. It passed the incubation stability criteria.
The element containing LF-7 passed both the incubation and light stability
criteria.
This invention has been described with particular reference to preferred
embodiments thereof but it will be understood that modifications can be
made within the spirit and scope of the invention.
* * * * *
|
|
 |
|
 |
Description |
|
