Method of forming stable images in electron beam writable glass compositions

A silicate glass article comprising an amount of SiO.sub.2 effective as a network former and, PA0 at least one surface of said article having a substantially continuous silver and hydration content over its area, effective to render said surface darkenable upon exposure to electron beam radiation but substantially not thermoplastic, PA0 said article having been substantially darkened by electron beam irradiation over at least a portion of at least one surface thereof, PA0 and said article having geometric characteristics suitable for functioning as a photomask reticle, optical disk or similar archival storage medium.

BACKGROUND OF THE INVENTION 
This invention relates to new glass articles having energy-sensitive 
portions which can be written by high-energy beams to record stable 
images. 
Various devices are known for archival storage media. These include 
magnetic materials, etched chrome masks, and many others. Attempts have 
also been made to use glass for such applications. However, further 
improvements are still desired. 
SUMMARY OF THE INVENTION 
This invention is based on the surprising discovery that silicate glass 
compositions, hydrated and containing silver, can be effectively written 
with high-energy beams, e.g., electron beams, to produce 
high-optical-density images which are stable. These advantageous 
properties are achieved, surprisingly, with essentially no content in such 
compositions of transition metals having 1-4 d-electrons in the atomic 
state. 
Accordingly, this invention relates to a glass article comprising the 
following composition in mole %: 
______________________________________ 
SiO.sub.2 
30-95 
P.sub.2 O.sub.5 
0-20 
B.sub.2 O.sub.3 
0-30 
Al.sub.2 O.sub.3 
0-40 
RO 0-40 
R.sub.2 O 
1-35 
halide 
0-10 
______________________________________ 
and essentially no transition metals having 1-4 d-electrons in the atomic 
state, 
wherein RO is MgO, CaO, SrO, BaO, ZnO and/or PbO, 
R.sub.2 O is Li.sub.2 O, Na.sub.2 O, K.sub.2 O, Rb.sub.2 O and/or Cs.sub.2 
O, 
halide is Cl, F, Br and/or I, 
at least one surface of said article having a substantially continuous 
silver and hydration content over its area, effective to render said 
surface darkenable upon exposure to electron beam radiation but 
substantially not thermoplastic, 
said article having been substantially darkened by electron beam 
irradiation over at least a portion of at least one surface thereof, 
and said article having geometric characteristics suitable for functioning 
as a photomask reticle, optical disk or similar archival storage medium. 
In another aspect, this invention relates to a glass article comprising the 
following composition in mole %: 
______________________________________ 
SiO.sub.2 30-95 
P.sub.2 O.sub.5 
0-20 
B.sub.2 O.sub.3 
0-30 
Al.sub.2 O.sub.3 
0-40 
RO 0-40 
R.sub.2 O 1-35 
halide 0-10 
______________________________________ 
and essentially no transition metals having 1-4 d-electrons in the atomic 
state, 
wherein RO is MgO, CaO, SrO, BaO, ZnO and/or PbO, 
R.sub.2 O is Li.sub.2 O, Na.sub.2 O, K.sub.2 O, Rb.sub.2 O and/or Cs.sub.2 
O, 
halide is Cl, F, Br and/or I, 
at least one surface of said article having a substantially continuous 
silver and hydration content over its area, effective to render said 
surface darkenable upon exposure to electron beam radiation but 
substantially not thermoplastic, 
said article substantially not exhibiting alterable photo-anisotropic 
effects induced by actinic UV radiation, 
and said article having geometric characteristics suitable for functioning 
as a photomask reticle, optical disk or similar archival storage medium. 
In a further aspect, this invention relates to a silicate glass article 
comprising an amount of SiO.sub.2 effective as a network former and, 
at least one surface of said article having a substantially continuous 
silver and hydration content over its area, effective to render said 
surface darkenable upon exposure to electron beam radiation but 
substantially not thermoplastic, 
said article having been substantially darkened by electron beam 
irradiation over at least a portion of at least one surface thereof, 
and said article having geometric characteristics suitable for functioning 
as a photomask reticle, optical disk or similar archival storage medium. 
In yet a further aspect, this invention relates to a silicate glass article 
comprising an amount of SiO.sub.2 effective as a network former and, 
at least one surface of said article having a substantially continuous 
silver and hydration content over its area, effective to render said 
surface darkenable upon exposure to electron beam radiation but 
substantially not thermoplastic, 
said article substantially not exhibiting alterable photo-anisotropic 
effects induced by actinic UV radiation, 
and said article having geometric characteristics suitable for functioning 
as a photomask reticle, optical disk or similar archival storage medium. 
In other aspects, this invention relates to methods of forming an image 
readable by actinic radiation, e.g., UV or visible radiation, comprising 
writing such an image in a glass article of this invention by 
electron-beam irradiation thereof. The invention also relates to methods 
of erasing such images by heating of the glass articles containing the 
images. 
This invention also relates to various other aspects and features as 
described below. 
A wide range of silicate glass compositions is applicable for this 
invention. In essence, these need be adequate to provide a silicate glass 
having the necessary characteristics for a given application and contain 
therein at least one surface which has a content of silver and is 
hydrated, typically substantially continuously over the entire area of the 
surface, such that the resultant hydrated silver composition is sensitive 
to high-energy radiation such as electron beams to darken under its 
influence. 
The compositions also are to be essentially devoid of all transition metals 
having 1-4 d-electrons in the atomic state. These include Sc, Ti, V, Y, 
Zr, Nb, La, Hf, Ta and W. Preferably, the glasses of this invention are 
also essentially devoid of all metals having 1-4 d-electrons in the atomic 
state, such as, for example, Gd, Lu, Th, Pa, U, Np, Cm, etc. 
Within the context of this invention, the term "substantially (or 
essentially) free of" a component means that the component is not 
intentionally added to the batch composition for preparation of a glass 
but, rather, is unavoidably present therein as an impurity in other batch 
components and, thus, will be present in the final glass composition in 
trace amounts. Similarly, other impurities can be derived from processing 
of the glass melt. Nevertheless, it is highly preferred that the glass 
compositions used in this invention be entirely devoid of transition 
metals having 1-4 d-electrons in the atomic state, and also all such 
metals. Where the presence of such undesired components, e.g., such 
metals, is practically unavoidable, the resultant trace amounts should be 
preferably maintained less than 0.1 mole % if possible; of course, higher 
amounts can be included where the consequent adverse effects can be 
tolerated in the end-use. 
By the expression "substantially darkened by electron-beam radiation" is 
meant a degree of darkening which is significantly greater than any 
inadvertent darkening which might occur, for example, in SEM measurements 
or analyses of a glass composition. Typically, the degree of darkening 
will be that which is sufficient for use of any resultant image based on 
the darkened portions as an archival storage medium as described above. 
The precise optical density involved will depend on the end-use 
requirements and can be, for example, equal to or greater than 0.1, 0.5, 
1, 1.5, 2, 2.5, 3, and higher, etc. 
Thus, the silver-containing surfaces of this invention will typically have 
silver more or less uniformly and homogeneously distributed continuously 
over a surface, as opposed to discretely, e.g., in a pattern. 
Furthermore, the silver-containing surfaces of this invention preferably do 
not exhibit any substantial alterable photo-anisotropic effects induced or 
inducible by actinic UV radiation such as the effects described in U.S. 
Pat. Nos. 4,191,547, 4,296,479 and 4,297,417, each of which is entirely 
incorporated by reference herein. Thus, the glass articles of this 
invention will not be exposed to sufficiently high doses of UV radiation 
whereby such alterable photo-anisotropic properties could be induced where 
applicable. Thus, in normal use as an archival storage medium, the glass 
articles of this invention would be exposed to low-level UV radiation 
effective for reading an image in the medium. Typically, many months, on 
the order of many years, will pass before a sufficient dose of UV 
radiation has been received by a glass article of this invention to 
produce any significant alterable photo-anisotropic effects, if ever, for 
any glass articles of this invention which might somehow inherently be 
capable of producing such effects. Preferably, the glass articles of this 
invention exhibit essentially no alterable photo-anisotropic effects 
induced by any kind of radiation, e.g., e-beam, X-ray, etc. 
A preferred composition for use in conjunction with this invention has the 
following composition in mole %: 
______________________________________ 
SiO.sub.2 40-90 
P.sub.2 O.sub.5 0-5 
B.sub.2 O.sub.3 0-30 
Al.sub.2 O.sub.3 0-15 
RO 5-30 
.SIGMA.Li.sub.2 O, Na.sub.2 O, K.sub.2 O 
5-25 
halide 0-6. 
______________________________________ 
A most preferred composition for use in this invention has the following 
composition in mole %. 
______________________________________ 
SiO.sub.2 50-85 
P.sub.2 O.sub.5 0-2 
B.sub.2 O.sub.3 0-20 
Al.sub.2 O.sub.3 0-5 
RO 5-20 
.SIGMA.Li.sub.2 O, Na.sub.2 O, K.sub.2 O 
5-25 
Cl 0.1-6. 
______________________________________ 
Silica is used in the glasses of this invention as the principal glass 
former. Levels of silica too low generally result in glasses which possess 
thermomechanical properties, particularly thermal expansion, which are 
undesirable in applications of the materials of this invention to 
situations requiring the recording, reading, or transferring of 
microscopic optical density patterns generated in the material by 
electron-beam irradiation. High levels of silica impart lower thermal 
expansion values but decrease the meltability of such compositions. 
The addition of Al.sub.2 O.sub.3 and RO (R=Mg, Ca, Sr, Ba, Zn, Pb) impart 
chemical durability to the produced glass compositions and also impart 
during manufacturing of the glass articles an increased stability and 
reduced tendency towards crystallization. Zn is preferred, as well as Ca. 
The presence of R.sub.2 O (R=Li, Na, K, Rb, Cs) is of particular importance 
when the materials are to be subjected to an ion-exchange treatment of 
silver for alkali ions. Levels of alkali ions too low give insufficient 
ion-exchangeability, whereas levels too high reduce the chemical 
durability of the produced glasses resulting in the potential chemical 
attack of glass samples during the ion-exchange treatment. Li, Na and K 
are preferred. 
The presence of halides in the produced glass articles, preferably Cl, 
significantly enhances the sensitivity of such glass articles to electron 
beams. 
It is essential that glass compositions of the present invention be free 
from transition metal oxides having from 1 to 4 d-electrons in the atomic 
state as these constituents impart to electron beam induced images in the 
glass an instability with time and temperature which is undesirable for 
application of these materials as archival storage media. 
The presence of polyvalent species, such as As.sub.2 O.sub.3, Sb.sub.2 
O.sub.3, SnO.sub.2, Bi.sub.2 O.sub.3 and GeO.sub.2, can act as electron 
donors to silver ions entering the glass structure during ion exchange. 
Thus, in the glass compositions, they will increase the extent to which 
silver ions are reduced to silver metal precipitates during ion exchange 
Correspondingly, the glasses of this invention preferably should be 
substantially devoid of these components also. 
It is also possible for the glass compositions of this invention to contain 
other components achieving desirable properties for the glass per se, 
e.g., small sensitizing amounts of Cu. 
The glass articles of this invention can comprise a single composition 
overall having a silicate base composition, essentially homogeneously and 
uniformly containing a silver and hydration content. Alternatively, the 
glass articles can comprise a bulk portion consisting essentially of the 
base silicate composition. This bulk portion can have a surface layer 
consisting of essentially the same (or even a different) silicate glass 
composition having the mentioned silver and hydration content. The layer 
can be coated onto the bulk portion or can be monolithic therewith, e.g., 
prepared by suitable treatment of a surface of the bulk composition-based 
glass article per se. 
The glass articles of this invention, of course, will have other 
characteristics necessary or desirable for the intended end-use. Thus, 
they will be substantially non-thermoplastic so that they provide surfaces 
which have sufficient rigidity and integrity for the intended 
applications, such as use in archival storage, e.g., photomasks, optical 
disks, optical elements such as diffraction gratings, gunsights, etc. For 
example, the glass articles of this invention will have surfaces, 
especially writable surfaces, which are not "thermoplastic" as defined in 
U.S. Pat. No. 4,160,654 which is entirely incorporated by reference 
herein. Thus, the articles of this invention will not be capable of 
thermoplastic molding, pressing or extrusion. They will be 
non-thermoplastic in the sense that highly sensitive measurements of the 
nature of the surface before hydration (e.g., interferometrically with a 
sensitivity/accuracy of at least 5 um) will show no significant or 
substantial difference in surface quality after hydration treatment. The 
surfaces will be substantially nondeformable under conditions 
conventionally employed to deform glass surfaces defined as being 
"thermoplastic." 
Similarly, the glass articles of this invention will have geometric 
characteristics which are necessary or desirable for the intended end-use. 
Thus, where the article is to be employed as a photomask or optical disk 
or similar archival storage medium, it will generally have a high degree 
of flatness on the written surface and typically be in the form of a 
rectangular or disk-shaped plate. These characteristics will be 
satisfactory to meet relevant industry specifications. Consequently, prior 
art silver-containing silicate surfaces of optical waveguides, optical 
fibers and similar structures are not included in the scope of this 
invention. 
Essentially any known method for achieving hydration of silicate glasses 
and/or for achieving a silver content in silicate glasses can be employed 
to prepare the glass articles of this invention. These include acid 
treatment for hydration (see, e.g., Roger F. Bartholomew, "Water in 
Glass," Treatise on Materials Science and Technology, Vol. 22, Glass III, 
eds. Minoru, Tomozawa et al., 75-128 (1982), Academic Press; standard salt 
bath ion exchangers (see, e.g., U.S. Pat. No. 3,528,847), the well known 
combination hydration and silver ion-exchange treatments (see, e.g., U.S. 
Pat. Nos. 4,160,654, 4,191,547 and 4,297,417, incorporated by reference 
above), CVD techniques (Thin Film Science and Technology, "Coatings on 
Glass," H. K. Tulker, Elsevier, 1984), thin-film coating techniques 
(Tulker, supra), silver-bismuth pool electric migration Electro-Float, 
Silver-bismuth, Pool Treatment, Vol. II, Glass Science and Technology, 
"Color Generation and Control in Glass," C. R. Bansford, Elsevier (1927)), 
ion-beam implantation (Tulker, supra), sol-gel techniques (Tulker, supra), 
etc. 
The preferred method is the silver ion exchange which simultaneously 
hydrates a silicate glass. 
The latter can be achieved by conventional acidic silver salt bath 
treatment. The well known technique is straightforwardly applicable to 
this invention in conjunction with routine optimization. Typically, 
suitable temperatures will be up to 374.degree. C., as mentioned in the 
cited U.S. patents, e.g., 200.degree.-370.degree. C., preferably, 
300.degree.-370.degree. C. The pH of the silver salt acidic solution is 
generally less than 4 and preferably less than 2. Typically, the treatment 
will be conducted in a conventional autoclave using a pressure of at least 
200 psig, more typically greater than 400 psig. The time of reaction 
(including time to heat up and cool down), as is well known, will depend 
on the desired depth of ion exchange and will typically be up to 16 hours, 
but more commonly approximately 2-3 hours. 
Typical ion-exchange baths will include sufficient acid to achieve the 
necessary pH, preferably nitric acid, but also other strong mineral acids 
such as boric acid, hydrochloric acid, sulfuric acid, etc. Similarly, the 
precise silver salt utilized is not critical. Where halides are desired in 
the silver layer, silver halides, of course, can be employed. However, 
preferably the salt will be silver nitrate in accordance with the prior 
art. The concentration of the silver salt will typically be 90-95% of its 
solubility limit in the bath in order to enhance silver exchange but 
provide adequate safety against undesired precipitation. However, of 
course, much lower concentrations of silver are also applicable with 
correspondingly higher treatment times in order to achieve a given layer 
thickness. 
The ion-exchange bath also classically contains salts of other cations 
which correspond to oxides contained in the base glass composition being 
treated in order to provide controllability of exchange. Most preferably, 
these other components will include alkali metal salts such as chlorides, 
but preferably the nitrates, e.g., preferably lithium nitrate since the 
alkali metal cations are quite mobile. Other employable salts include 
those of boron, alkaline earth metals, etc. Typically, these salts are 
included in the amounts discussed in the prior art, for example, 50-350 
g/l. Also, typically included in these baths is silica gel in amounts 
equal to or in excess of saturation and also halides, e.g., as a salt, a 
liquid or even as a gaseous additive. 
Ion-exchange layer thicknesses achieved can be conventionally varied and 
are routinely selectable, e.g., by appropriate modification of bath 
composition and treatment time for a given composition. For the 
applications of this invention where high resolution images are desired, 
typically, relatively small layer thicknesses will be desired, for 
example, less than 10 um, preferably less than 5 um, more preferably less 
than 2 um, and even lower. Also for considerations related to resolution, 
the ion-exchange layer thickness will preferably be selected to be smaller 
than the depth of penetration of the writing beam, such as an electron 
beam. For example, 20 keV electron beams will typically penetrate silicate 
glasses to a depth of 3-5 um. The ion-exchanger layer thickness will thus 
be chosen to be less than this, e.g., typically about 75% less. However, 
of course, the precise thicknesses will not be critical in accordance with 
this invention where resolution is not as important, e.g., where the 
archival storage media are being used to write image features 
significantly above the 1-10 um level, e.g., for images used in 
conjunction with printed circuit boards or higher wavelength optical 
devices, gunsights, etc. 
By "hydration content" herein is simply meant the compositional variations 
achieved in silicate compositions in accordance with the particular 
conventional hydration treatment utilized as discussed above. Typically, 
such treatments introduce into the silicate glass composition, H.sup.+, 
H.sub.3 O*, H.sub.2 O, silanol groups, etc. 
The high-energy sensitive glass articles of this invention are preferably 
written by electron-beam irradiation, typically of 5 keV to 100 keV 
strength, the precise value not being critical and lower and higher values 
being applicable. Typically, the glass articles of this invention will 
have a sensitivity such that they can be stably darkened to provide an 
image having an optical density greater than 1.5 or 2 by only 2-6 scan 
counts of a 20 keV electron beam common in the industry, e.g., provided by 
a MEBES III system of Perkin Elmer. However, the media of this invention 
can also be written via other high-energy beams, such as X-ray radiation, 
or, where desired, with very high-energy UV radiation. The resultant 
images will be very accurately and stably readable using radiation to 
which the materials of this invention are not darkenable, e.g., UV 
radiation of a wavelength greater than about 360 nm or via visible 
radiation. The reading wavelength will not be critical as long as it 
satisfies the properties of the intended end-use and does not cause 
further significant "writing" of the glass article containing the written 
image. 
The images achieved in accordance with this invention are erasable, simply 
by heating the written glass articles to a temperature effective for 
erasure. Suitable temperatures for a given glass can be routinely 
determined by a few routine experiments. The images are also erasable by 
instantaneous localized heating via a focused or directed heat supplying 
medium such as a laser or other energy source. 
A major advantage of this invention is that the images having adequate 
optical density for the intended end-use are also highly stable against 
all relevant ambient conditions encountered, e.g., in storage, shipping, 
use, etc. 
In another application of the glass articles of this invention, they can be 
used for generation of relief images in their surfaces by conventional 
chemical and/or thermal treatments. A written image will render surfaces 
of the articles of this invention more or less susceptible to such 
treatments whereby selective etching can be achieved, e.g., by exposure of 
the entire bulk of the material to such treatments or of the entire 
surface of the material to such treatments or of only local sections of 
the material to the treatment, e.g., by reaction with chemical beams, ion 
beams, lasers, other focused or directed energy beams, etc. See, e.g., EP 
0074157. 
The preparation of the glasses of this invention can be achieved as 
follows: the required chemicals are selected and blended into a batch 
which is then melted in a fused silica, refractory or platinum crucible 
from 1100.degree. C. to as high as 1500.degree. C. depending on the chosen 
composition. The glass is then refined at temperatures exceeding 
1300.degree. C. for periods typically from 2 to 4 hours, again depending 
on the glass composition and melt viscosity, with varied intervals of 
stirring. The glass is then typically cast into a steel mold and annealed 
at the transformation temperature plus about 20.degree. to 30.degree. C. 
for two hours, followed by cooling at 30.degree. C./hour to room 
temperature. The resultant casting of glass can be cut and fabricated into 
thin plates for treatment by an ion-exchange procedure to activate the 
surface layer to electron beams.

EXAMPLES 
The following examples illustrate the application of glass materials 
prepared in conformance with the present invention. The selected glass 
composition is first melted without the positive inclusion of any 
constituents which can lead to later spontaneous reduction of silver ions 
during the ion-exchange process to sensitize the active layer to electron 
beams. The preparation of these glasses proceeds in accordance with the 
above-described melting and forming procedures. 
Polished glass plates of the examples are surface treated by silver ion 
exchange to activate the surface layer to electron beam irradiation. The 
samples are electron beam irradiated on a JEOL 35c scanning electron 
microscope operating at 20 keV, with a beam current of 23.6 nA, focused 
onto a 100 micron spot with a 2-second exposure time. A typical resultant 
optical density is shown in FIG. 1, upper curves. Following a short term 
increase in optical density, which reaches completion in less than 24 
hours following electron beam exposure, there is no drop in optical 
density. 
Polished glass plates of the examples are ion exchange surface treated at a 
higher temperature than those above and then electron beam irradiated 
under the same conditions. A typical resultant optical density is shown in 
FIG. 2. There is no evidence, within the measurement reproducibly of 0.2 
OD in the UMSP-80 microdensitometer, for the short, less than 24 hour, 
relaxation of optical density to higher values exhibited in FIG. 1. In 
addition, no decrease was recorded for periods of time up to 10 days after 
electron beam exposure. The overall lower optical density exhibited by 
this sample is a result of an ion-exchange penetration depth, measured by 
analyzing interference in intensity from light reflected from the front 
face and the layer, of more than 5 microns. The penetration depth of the 
electron beam used for the irradiations is only 4.5 microns. 
COMATIVE EXAMPLES 
A composition is also melted and surface treated as described above but 
contains a transition metal oxide containing 1 to 4 d-electrons in the 
atomic state, i.e., TiO.sub.2. It exhibits a time instability of electron 
beam induced darkening in the material as shown in the lower curves of 
FIG. 1. Similar behavior of electron beam induced optical density 
instability with time occurs for samples containing other transition metal 
oxides which contain from 1 to 4 d-electrons in the atomic state. Surface 
treatment of these compositions at the higher temperature mentioned above 
fails to stabilize optical density. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions. 
______________________________________ 
Table in Mole % Oxide* 
Examples 
1 2 3 4 5 6 7 8 
______________________________________ 
SiO.sub.2 
74 70 76 74 74 74 74 72 
B.sub.2 O.sub.3 
2 2 2 2 2 2 1 2 
Al.sub.2 O.sub.3 
1 1 1 1 1 1 1 1 
P.sub.2 O.sub.5 1 
MgO 
CaO 7 2 
SrO 
BaO 
ZnO 7 7 7 7 7 7 7 
PbO 
Li.sub.2 O 
4 5 3.5 6 4 4 4 4 
Na.sub.2 O 
8 10 7 8 8 8 8 8 
K.sub.2 O 
4 5 3.5 2 4 4 4 4 
Cl 3 3 3 3 0 3 3 3 
______________________________________ 
*Exception is Cl in mole % element. 
RO's (R = Mg, Sr, Ba) are similarly included in total amounts up to 7 mol 
% as replacements for ZnO and/or SiO.sub.2. 
RO's (R = Pb) are similarly included in total amounts up to 2 mole % as 
replacements for ZnO and/or SiO.sub.2. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever.