U.S. patent application number 11/156305 was filed with the patent office on 2005-12-15 for low-temperature fabrication of glass optical components.
Invention is credited to Aitken, Bruce G., Chatterjee, Dilip K., Raguin, Daniel H..
Application Number | 20050274145 11/156305 |
Document ID | / |
Family ID | 32107517 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050274145 |
Kind Code |
A1 |
Aitken, Bruce G. ; et
al. |
December 15, 2005 |
Low-temperature fabrication of glass optical components
Abstract
In one aspect, a method is provided for molding from glass
complex optical components such as lenses, microlens, arrays of
microlenses, and gratings or surface-relief diffusers having fine
or hyperfine microstructures suitable for optical or
electro-optical applications. In another aspect, mold masters or
patterns, which define the profile of the optical components, made
on metal alloys, particularly titanium or nickel alloys, or
refractory compositions, with or without a non-reactive coating are
provided. Given that molding optical components from oxide glasses
has numerous drawbacks, it has been discovered in accordance with
the invention that non-oxide glasses substantially eliminates these
drawbacks. The non-oxide glasses, such as chalcogenide,
chalcohalide, and halide glasses, may be used in the mold either in
bulk, planar, or power forms. In the mold, the glass is heated to
about 10-110.degree. C., preferably about 50.degree. C., above its
transition temperature (Tg), at which temperature the glass has a
viscosity that permits it to flow and conform exactly to the
pattern of the mold.
Inventors: |
Aitken, Bruce G.; (Corning,
NY) ; Chatterjee, Dilip K.; (Rochester, NY) ;
Raguin, Daniel H.; (Acton, MA) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
32107517 |
Appl. No.: |
11/156305 |
Filed: |
June 16, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11156305 |
Jun 16, 2005 |
|
|
|
10283402 |
Oct 29, 2002 |
|
|
|
Current U.S.
Class: |
65/64 |
Current CPC
Class: |
C03B 2215/412 20130101;
G02B 3/005 20130101; C03C 3/32 20130101; G02B 3/0031 20130101; C03B
2215/60 20130101; Y02P 40/57 20151101; C03B 19/06 20130101; C03B
11/082 20130101; G02B 3/06 20130101; C03B 2215/414 20130101; C03B
2215/16 20130101; C03B 2215/41 20130101; C03B 2201/86 20130101;
C03C 17/22 20130101; G02B 5/045 20130101; C03B 2215/17 20130101;
C03B 2201/82 20130101; C03B 2201/80 20130101; G02B 3/0056 20130101;
C03C 2217/77 20130101; C03B 2201/88 20130101; C03B 11/086
20130101 |
Class at
Publication: |
065/064 |
International
Class: |
C03B 023/00 |
Claims
1-30. (canceled)
31. A precision optical element made according to a method
comprising the steps of: a) providing a non-oxide glass with a
glass transition temperature (Tg) of up to about 550.degree. C.:
(b) providing a mold having an active surface that has an optical
finish, wherein said active surface if made of a titanium alloy
with a composition, in terms of weight percent, comprising about
98-80% Ti. 1-10% Al, and 1-10% V; (c) placing said glass in said
mold, (d) heating said mold and glass to an operational temperature
from about 10.degree. C. to about 110.degree. C. above the Tg: and
(e) pressing the mold when the viscosity of the glass reaches about
10.sup.6-10.sup.12 poise; wherein, optionally, said titanium active
surface is coated with a protective coating, said coating being one
selected from the group consisting of: (A) a release agent: and (B)
a material having a crystallization temperature higher than at
least an operational temperature, said material being further
coated with a release agent.
32.-55. (canceled)
56. A precision optical element formed from a non-oxide glass by a
molding or embossing method, wherein the method comprises:
providing a glass having a glass transition temperature (Tg) up to
550.degree. C. as granular, planar, or bulk-solid items; providing
a two part mold having an active surface with an optical finish,
which may be used with or without a protective coating, wherein
said active surface is either optionally: (A) coated with a layer
of non-reactive material, or (B) made from either a titanium alloy
or a nickel alloy, or (C) both made from either a titanium alloy or
a nickel alloy and coated with said non-reactive material; charging
said mold with said glass, heating said mold to an operational
temperature of at least 10.degree. C. above said Tg; and
hot-pressing said glass.
57. The precision optical element according to claim 56, wherein
said temperature is at least about 50.degree. C. above said Tg.
58. The precision optical element according to claim 47, wherein
the method further comprises inserting blocks into said mold, at
least one of said blocks presents a section that faces said wafer
or powder.
59. The precision optical element according to claim 47, the method
further comprises placing on a surface of said blocks a layer of
non-reactive material that is non-reactive with said glass at said
operational temperature.
60. The precision optical element according to claim 47, wherein
said non-reactive material is boron nitride.
61. The precision optical element according to claim 31, wherein
said non-oxide glass is a chalcogenide glass.
62. The precision optical element according to claim 31, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic sulfide, germanium sulfide and
germanium-arsenic-sulfide glasses.
63. The precision optical element according to claim 62, wherein,
in atomic/element percent, germanium is in the range of 0-35%,
arsenic is in the range of 0-55% and sulfur is in the range of
30-85%.
64. The precision optical element according to claim 31, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic selenide, germanium selenide and
germanium-arsenic-selenide glasses.
65. The precision optical element according to claim 31, wherein,
in atomic/element percent, germanium is in the range of 0-35%,
arsenic is in the range of 0-55% and sulfur is in the range of
30-85%.
66. The precision optical element according to claim 31, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic telluride, germanium telluride and
germanium-arsenic-telluride glasses.
67. The precision optical element according to claim 31, wherein,
in atomic/element percent, germanium is in the range of 0-45%,
arsenic is in the range of 0-60% and selenium in the range of 25%
to about 100%.
68. The precision optical element according to claim 61, wherein to
modify the optical, thermal and/or mechanical properties of said
optical element's chalcogenide glass, said glass further comprises
one or more elements selected from the group consisting of
phosphorus, gallium, selenium, tin, antimony, thallium, chlorine,
bromine, iodine, a rare earth element, lithium, sodium and
potassium.
69. The non-oxide glass according to claim 31, wherein said glass
is a chalco-halide glass.
70. The non-oxide glass according to claim 31, wherein said glass
is a halide glass.
71. The precision optical element according to claim 56, wherein
said non-oxide glass is a chalcogenide glass.
72. The precision optical element according to claim 56, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic sulfide, germanium sulfide and
germanium-arsenic-sulfide glasses.
73. The precision optical element according to claim 72, wherein,
in atomic/element percent, germanium is in the range of 0-35%,
arsenic is in the range of 0-55% and sulfur is in the range of
30-85%.
74. The precision optical element according to claim 56, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic selenide, germanium selenide and
germanium-arsenic-selenide glasses.
75. The precision optical element according to claim 56,wherein, in
atomic/element percent, germanium is in the range of 0-35%, arsenic
is in the range of 0-55% and sulfur is in the range of 30-85%.
76. The precision optical element according to claim 56, wherein
said non-oxide glass is a chalcogenide glass is selected from the
group consisting of arsenic telluride, germanium telluride and
germanium-arsenic-telluride glasses.
77. The precision optical element according to claim 56, wherein,
in atomic/element percent, germanium is in the range of 0-45%,
arsenic is in the range of 0-60% and selenium in the range of 25%
to about 100%.
78. The precision optical element according to claim 71, wherein to
modify the optical, thermal and/or mechanical properties of said
optical element's chalcogenide glass, said glass further comprises
one or more elements selected from the group consisting of
phosphorus, gallium, selenium, tin, antimony, thallium, chlorine,
bromine, iodine, a rare earth element, lithium, sodium and
potassium.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to optical components and
their manufacture in glass. In particular, the invention relates to
methods and apparatus for molding or embossing non-oxide glasses
(e.g., chalcogenide glasses) with low glass-transition temperatures
(T.sub.g) into devices of varied geometry having complex or fine
surface features.
BACKGROUND
[0002] Optical elements have had various uses in many diverse
technologies, including sensors, image projectors, displays (e.g.,
liquid crystal display (LCDs), plasma display, and
electro-luminescence display), as well as opto-electronic devices
for telecommunications. As the telecommunications industry
develops, the need to develop precision optical elements that
incorporate microstructures increases. In telecommunication
devices, optical elements may be used, for instance, in fiber and
laser couplers, optical switches, or as diffraction gratings for
WDM applications, and densely packed microlens arrays (MLAs) or
networks for wavelength management modules or collimator
applications. Precision optical elements require highly polished
surfaces or exacting surface figures and qualities. The surfaces
demand fabrication in proper geometric relationship to each other;
and, where the elements are to be used in transmission
applications, they will be prepared from a material of controlled,
uniform, and isotropic refractive index.
[0003] Numerous methods and materials may be used to fabricate
complex, precision optical elements. Because a great majority of
conventional machining processes for the manufacture of optical
components are unsuited for producing very small features,
components having surface features or dimensions of 500 microns or
smaller typically can be fabricated only through a few methods of
limited applicability. Fabrication of microstructured surfaces
using polymers have leveraged off of processes developed by the
semiconductor industry for making integrated circuits. Using
photolithography and ion etching techniques, some have created
submillimeter surface features. These methods, however, are not
conducive to large-scale manufacturing. The process time needed to
etch a microstructure is proportionally dependent on the required
total depth of the microstructure. Moreover, the methods are not
only expensive, but can produce only a limited range of feature
types. Also, etching processes can create rough surfaces. A smooth
concave or convex profile or true prismatic profiles cannot be
readily achieved using either of the two aforementioned
techniques.
[0004] Molding or hot embossing of plastics or glass materials, on
the other hand, can form submillimeter-sized features. Plastics can
conform to molds and reproduce faithfully intricate designs or fine
microstructures. Unfortunately for many telecommunication
applications, plastic materials are not ideal since they suffer
from several shortcomings. Plastic materials are often not
sufficiently robust to withstand, over time, environmental
degradation. First, they exhibit large coefficients of thermal
expansion, and limited mechanical properties. Plastic optical
devices often cannot long withstand humidity or high temperatures.
Both the volume and refractive indices of plastics can vary
substantially with changes in temperature, thereby limiting the
temperature range over which they may be useful. Plastics cannot
transmit high-power light, due to internal heating of the material.
Thus, well before a plastic component actually melts, its surface
features will degrade and its index of refraction may change.
Either change is unacceptable in an optical context. Furthermore,
since plastics for optical applications are available in a limited
range of dispersion and refractive index, plastics can provide only
a restricted transmission range. Hence, their usefulness even
within the restricted bandwidth is limited by the tendency to
accumulate internal stresses, a condition that results in
distortion of transmitted light during use. In addition, many
plastics can scratch easily and are prone to yellowing or
developing haze and birefringence. Application of
abrasive-resistant and anti-reflective coatings, unfortunately,
still has not fully solved these flaws. Finally, many chemical and
environmental agents degrade plastics, which makes them difficult
to clean effectively.
[0005] In comparison, glass possesses properties that make it a
better class of optical material over plastics. Glass normally does
not suffer from the material shortcomings of plastics, and it can
better withstand detrimental environmental or operational
conditions. Hence, glass is a more preferred material. Glass
optical components represent a different class of devices than
those made from plastics and the molding processes used are more
stringent.
[0006] Precision optical elements of glass are customarily produced
by one of two complex, multi-step processes. In the first, a glass
batch is melted at high temperatures and the melt is formed into a
glass body or gob having a controlled and homogeneous refractive
index. Thereafter, the glass body may be reformed using repressing
techniques to yield a shape approximating the desired final
article. The surface quality and finish of the body at this stage
of production, however, are not adequate for image forming optics.
The rough article is fine annealed to develop the proper refractive
index and the surface features improved by conventional grinding
and polishing practices. In the second method, the glass melt is
formed into a bulk body, which is immediately fine annealed, cut
and ground into articles of the desired configuration.
[0007] Both of these methods have their limitations. On one hand,
grinding and polishing are restricted to producing relatively
simple shapes, such, such as flats, spheres, and parabolas. Other
shapes and general aspheric surfaces are difficult to grind and
complicated to polish. On another hand, conventional techniques for
hot pressing of glass do not provide the exacting surface features
and qualities, which are required for clear image forming or
transmission applications. The presence of chill wrinkles in the
surface and surface figure deviations constitute chronic
afflictions.
[0008] The molding of glass traditionally has presented a number of
other problems. Generally, to mold glass one must use high
temperatures, typically greater than about 700.degree. C. or
800.degree. C., so as to make the glass conform or flow into a
requisite profile as defined by a mold. First, at such relatively
high temperatures, glass becomes highly chemically reactive. Due to
this reactivity of glass, highly refractory molds with inert
contact surfaces are required. Some materials used to fabricate
molds include silicon carbide, silicon nitride or other ceramic
materials, or intermetallic materials, such as iron aluminides, or
hard materials, such as tungsten. In many cases, however, such
materials do not present sufficient surface smoothness or optical
quality for making satisfactory optical surface finishes. Precision
optical elements require highly polished surfaces of exacting
microstructure and quality. Metal molds can deform and
re-crystallize at high temperatures, which can adversely affect the
surface and optical qualities of the article being molded. This
means additional costs to repair and maintain the molds and higher
defects in the product. Second, also due to the reactivity of the
glass at high temperatures, often the molding need to be done in an
inert atmosphere, which complicates the process. Third, the
potential for air or gas bubbles to be entrapped in the molded
articles is another drawback of high-temperature molding. If
captured within the glass, gas bubbles tend to degrade the optical
properties of the article. The bubbles distort images and generally
disrupt optical transmission. Fourth, even at high temperatures,
hot-glass molding cannot create efficiently on the surface
intricate, high-frequency, submillimeter microstructures, such as
those required for diffraction gratings.
[0009] In the past, workers in the field of molding technology have
endeavored to develop several techniques for the manufacture of
optical elements. These techniques, however, have yet to
satisfactorily overcome the deficiencies of glass molding. Hence, a
new method or an improvement of existing technology is needed to
for the manufacture of precision optical elements with deep or fine
microstructures, such for diffraction gratings or microlenses. The
method should be cost-efficient, expedient and enable high-volume,
mass production of fine-figured microstructures in multiple,
identical glass optical elements. The present invention can satisfy
these needs.
SUMMARY OF THE INVENTION
[0010] The present invention pertains, in part, to a cost-effective
method of making a precision optical element with fine optical
microstructures by means of molding or embossing. It has been
discovered in accordance with the present invention that the
drawbacks associated with glass molding can be substantially
eliminated through the use of non-oxide based glasses as the
material to be molded. Suitable glass compositions include
chalcogenide, chalco-halide, and halide glasses, which typically
all have low glass transition temperatures (T.sub.g). An example of
a halide glass may be a fluoro-zirconate glass (e.g., ZBLAN). Of
the three kinds of glass, a sulfide glass is preferred, or more
particularly, a germanium-arsenic-sulfide glass. The advantages of
a non-oxide or chalcogenide glass includes high refractive index,
lower molding temperature, excellent thermal stability and good
environmental durability. The high refractive indices of these
glasses are particularly beneficial, since they reduce the extent
of sag required for making a lens of a given focal length. The low
molding temperatures of these glasses are attractive because they
obviate the need for expensive molds or masters, such as chemically
vapor deposited silicon carbide or silicon nitrides, required for
molding higher-temperature oxide glasses.
[0011] In brief, the method of manufacture comprises several steps.
First, provide a non-oxide glass with a Tg of up to about
550.degree. C. Second, provide a mold having at least a first
portion and a second portion. At least an active, molding surface
is formed of a material selected from: titanium alloys; nickel
alloys; silicon carbide; silicon nitride; or refractory ceramic
composite of silicon carbide and silicon nitride, or a refractory
metal such as tungsten and its alloys. The mold components have an
active surface that has an optical finish and can be used with or
without a protective coating a protective coating. Place the glass
in the mold. Then, heat the glass, the mold, or both to an
operational temperature from about 10.degree. C. to about
110.degree. C. above the T.sub.g. Press the mold when the viscosity
of the glass reaches about 10.sup.6-10.sup.12 poise. At room
temperature, the glass may take the form of granular, planar,
bulk-solid items (e.g., respectively, a powder frit, a wafer or
planar (disk) body, a bulk-solid ingot or monolith of any practical
three-dimensional shape), or a combination thereof. When the glass
is in the form of a wafer or powder, the method further comprising
inserting blocks into the mold. When blocks are used, the method
may further comprise applying or placing a layer of material, which
is non-reactive with the glass at the operational temperature, on a
surface of the blocks that is in contact with the glass material.
This release coating, such as a boron nitride, may be spray coated
or sputtered on the surfaces of the mold defining the profile or
the master defining the pattern. The method further comprises the
steps of hardening the glass in the mold either through natural or
forced cooling, then removing the glass. Further processing of the
resultant embossed or molded glass article may be included, such as
fine annealing or polishing. The present method permits the glass
to be molded in an atmosphere that contains oxygen, such as ambient
air, as well as enclosed in conventional inert gas atmospheres.
[0012] Once heated to the operational temperature, under pressure,
the glass sags into the mold to conform to a master design, whereby
the surface-relief structure of the master is transferred into the
glass. When the starting materials are granular in form, such as
glass frit, the molding process can sinter the individual glass
particles into a solid article without trapping air pockets or
other occlusions, which may mar the final product. Unlike certain
methods indicated before such as, grinding, polishing, reactive ion
etching, which are based on precise material removal processes, in
the present fabrication process, fabrication times are not
dependent on, nor directly determined by the depth of the
microstructure.
[0013] In another aspect, the present invention relates to a mold
assembly comprising a first or upper component and a second or
lower component. The mold may be made using a variety of materials,
which may include for example silicon carbide; silicon nitride; a
refractory ceramic or composite of the two or more metals, alloys,
ceramics and glass. A preferred material is a titanium alloy of a
nominal composition, in terms of weight percent, consisting
essentially of about: 80-98% Ti (titanium); 1-10% Al (aluminum);
and 1-10% V (vanadium). Titanium alloys of such compositions have
been used in military aircraft compressors and bio-implants but not
employed as mold materials for glass moldings. Surface treatment of
Ti-6Al-4V alloys, such as nitriding, can improve surface wear
properties of the material.
[0014] Additional features and advantages of the present
molding/embossing of glasses will be explained in the following
detailed description. It is understood that both the foregoing
general description and the following detailed description and
examples are merely representative of the invention, and are
intended to provide an overview for understanding the invention as
claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The foregoing and other objects, features and advantages of
the invention will become more apparent from a reading of the
following description in connection with the accompanying drawings
in which:
[0016] FIG. 1 is an isometric view of a cylindrical lens array;
[0017] FIG. 2 is an isometric view of a prismatic lens array;
[0018] FIG. 3 is an isometric view of an array of hyperfine lenses
having a high-density factor;
[0019] FIG. 4 is an isometric view of an array of hyperfine lenses
having a low-density factor;
[0020] FIG. 5 is an isometric view of a blazed-type hyperfine
grating;
[0021] FIG. 6 is an isometric view of a sinusoidal-profile
hyperfine grating;
[0022] FIG. 7 is a front view of a fixture of the mold assembly in
which the molding/embossing method provided by the invention may be
carried out; and
[0023] FIGS. 8A, B, and C are sectional views of pairs of mold
blocks, which may be used in the apparatus shown in FIG. 7.
[0024] FIG. 9 depicts a cross-sectional schematic of surface-relief
microlens.
[0025] FIG. 10 is a graph plotting sag of a microlens as a function
of .DELTA.n for different numerical apertures and a focal length of
1000 micron meters (.mu.m), see Eq. (3). Note that the sag of the
microlens decreases as the index of refraction difference
increases.
[0026] FIG. 11 depicts a schematic of a blazed grating.
[0027] FIG. 12 shows the profilometer trace of the mechanical
surface of a microlens-type profile molded from a chalcogenide
glass. The master was a titanium alloy substrate machined with a
mill to create a 513 micron meters (.mu.m) deep divot.
[0028] FIG. 13 shows profilometry data for a chalcogenide-molded
diffractive lens having a 9 .mu.m zone spacing. The fidelity of the
replicated zones are excellent and the measured width of the zone
transition is essentially limited by the lateral resolution of the
imaging apparatus.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention provides, in part, a one-step process
of fabricating complex optical components, such as gratings, lenses
of short focal length, or high-density microlens arrays (MLAs). The
process overcomes the molding limitations of other glass molding
methods. Optical lenses have had various uses in many diverse
technologies or applications, such as in television or computer
displays (e.g., LCDs).
[0030] To be technically precise, molding refers to a process of
shaping a ductile or fluid starting material to a final object and
the embossing refers particularly to a process of imprinting a
design on the final object. Molding and embossing, however, are
synonymous for the purpose of this invention. As used herein, the
term `molds` and `masters` are also synonymous: the mold is
normally used for shaping process and the master is used for
imprinting designs.
[0031] As used herein, the term "fine microstructure" refers to a
single lens, a lenslet or an array of lenses having smooth, curved
features less than or equal to about 500 microns along at least one
dimension. The individual lens elements may be concave or convex;
spherical, aspherical or fresnel; cylindrical (as illustrated in
FIG. 1) or prismatic (FIG. 2); and disposed on either a planar
substrate or one a curved substrate. Arrayed lenses may be produced
in various densities. In a "high-density factor" array, as shown in
FIG. 3, the lens elements abut or lie close to one another; an a
"low-density factor" array, as shown in FIG. 4, the lens elements
are spaced wider apart. In all four illustrations, the dimension
"a" is less than or equal to about 500 microns.
[0032] As used herein, the term "hyperfine microstructure" refers
to a single lens, a lenslet or an array of lenses or microlenses
having smooth, curved features less than or equal to about 100
microns along at least one dimension, preferably less than or equal
to about 10 microns.
[0033] As used herein, the term "fine grating" refers to a
blazed-type grating, as illustrated in FIG. 5, or surface-relief
diffuser, with groove spacing "a" less than or equal to about 500
microns; or a curved-profile grating, as illustrated in FIG. 6,
with groove spacing "a" less than or equal to about 500 microns.
The depth of the grooves can be up to about 100 microns deep. The
groove spacing may be variable or fixed. The grooves themselves may
be disposed on a planar or surfaced surface.
[0034] As used herein, the term "hyperfine grating" refers to a
blazed-type grating, as illustrated in FIG. 5, or surface-relief
diffuser, with groove spacing "a" less than or equal to about 100
microns, preferably less than or equal to about 10 microns.
[0035] Depending on the particular material, hyperfine gratings and
lenses may reflect or transmit optical radiation. Generally, the
molding, isothermal pressing and/or embossing of oxide glasses are
high-temperature processes, involving temperatures in the range of
about 700-1200.degree. C. or greater. Oxide-based glasses,
particularly glasses containing silicates and lead oxides are
highly reactive in their softened state. To prevent chemical
reaction between the mold material and the softened glass,
expensive mold materials, such as chemical vapor-deposited (CVD)
silicon carbide, reaction bonded silicon nitrides and hard
refractory metals and alloys, are required along with release
coatings on the molding surfaces.
[0036] As an alternative, researchers have turned to sol-gel
processes to produce highly transparent glass, relatively opaque
ceramics, or hyperfine-featured surfaces, such as described in U.S.
Pat. No. 5,076,980, or PCT Application Publication No. 93/21120,
which are incorporated herein by reference. Workers in the field
believed that to mold hyperfine features in glasses would be rather
difficult since vitreous materials retain significant viscosity at
practical working temperatures, which would prevent the molten
glass from accurately and reliably conforming to the
hyperfine-featured mold.
[0037] We have found, however, that non-oxide glasses, in
particular chalcogenide glasses, function very well for molding
hyperfine featured surfaces in contrast to common optical or
non-optical glasses that contain some type of oxide, such as oxides
of silicon, aluminum, boron, lead and the like. For many
specialized applications in optics, opto-electronics, and optical
telecommunications, more particularly, development and application
of non-conventional glasses may be the only practical material for
engineers to use. It is believed that the ability to produce
consistently by a molding process optical elements, such as
microlenses or diffractive gratings, having sharp transition angles
with features on the order of microns or submicrons, is not obvious
from previous glass molding technologies using either oxide or
non-oxide glasses.
[0038] Chalcogenide glasses are distinguished in their material
compositions from conventional optical glass families, in that they
contain in their glass-forming matrix a chalcogen element instead
of oxygen. A chalcogen element may be one or more elements of the
sulfur group (e.g., S, Se, or Te) in the periodic table, and may be
combined with arsenic, antimony, germanium, phosphorous, gallium,
indium, etc. Additionally, chalcogen elements may be mixed with a
halide (fluorine, chlorine, bromine, iodine) to create
chalco-halide glasses. Since sulfides generally react with ambient
oxygen at high temperatures, it was thought that the reactivity of
such glasses would be a major obstacle for development of a molding
process. The chalcogenide glasses, however, were unexpectedly
resilient to chemical reaction in ambient atmosphere, and has
little potential for undesired deformation or contamination during
the manufacturing process.
[0039] Moreover, chalcogenide glasses have very interesting
properties, which further distinguish them from conventional
oxide-based glasses. Chalcogenides exhibit excellent optical
transparency in the near and far infrared (IR) spectral region
(>700 nano-meters (nm)). This is an important attribute of
chalcogenide glasses for fabricating optical lenses, since optical
telecommunication uses transmittance in the infrared spectrum.
Silicates, by comparison, tend to absorb or are opaque in the
mid-IR. Moreover, chalcogenides may be used in heat sensing
applications, such as for forward looking infrared (FLIR) systems,
or guidance in the nose of a missile. Certain chalcogenide glasses
have potential applications as infrared transmitting materials and
as switching devices in computer memories, because their
conductivity changes abruptly when particular threshold values are
exceeded. Moreover, chalcogenide glasses can function as
semiconductors, not insulators as are most common oxide glasses,
and are better thermal conductors; thereby, having the potential
for better thermal management when packaged in modules for
telecommunication uses.
[0040] Other advantages of chalcogenide glasses include high
refractive indices. One does not necessarily need to have an
aspheric lens when lenses can be made from a non-oxide glass with a
higher index of refraction. Generally, chalcogenide glasses exhibit
higher refractive indices in the range of about 1.8 to greater than
3, preferably .gtoreq.2.0, which affords much flexibility in design
parameters, such as sag of a lens or period of a grating. Using
smaller sag, one can produce lenses with reduced radii of
curvature, and optical refractive lenses with less distortion in
the optical pathway than oxide glasses with lower refractive
indices of .ltoreq.1.5. Thus, a spherical lens is not a handicap in
this situation. The distance of the vertex to the plane of
substrate is less. Hence, the sag required is smaller and
shallower. Chalcogenide glasses also exhibit third-order,
non-linear refractive index of about 80 to 1000 times higher than
that for silica; and, their phonon level energy is very low
(.about.300 cm.sup.-1), which makes them an excellent host for
optical amplifiers or lasers doped with rare earth element ions
(e.g., erbium (Er), neodymium (Nd), praseodymium (Pr), thulium
(Tm), ytterbium (Yb), etc.).
[0041] In contrast to oxide-based glasses, chalcogenide glasses
exhibit lower softening temperatures and low glass transition
temperatures, Tg (.about.10.sup.13.4 poises). This feature makes
chalcogenide glasses attractive candidates for molding or
embossing. As used herein, the term "low-Tg" refers to a glass that
has a Tg.ltoreq.about 500.degree. C. Oxide-based glasses such as
silicates typically have a high Tg over about 600.degree. C. and
are prone to chemically react with the material of the mold.
Although phosphate glasses have T.sub.g.about.300-320.degree. C.,
and can be molded at about 400-450.degree. C., they have refractive
indices that are considerably lower than chalcogenide glasses.
Hence, phosphates require greater sag to produce comparable lens or
other optical elements.
[0042] Chalcogenide glasses can be molded at temperatures of about
200-600.degree. C. or less, typically about 250-350.degree. C.,
depending on composition, since they characteristically have glass
transition temperatures (T.sub.g) of less than about 500.degree. C.
Although some chalcogenide glasses have Tg in about 350-480.degree.
C., more commonly, the Tg is less than or equal to about
300.degree. C. (e.g., .about.130.degree. C. or .about.190.degree.
C. to .about.200.degree. C. or .about.250.degree. C.). Hence, one
can take advantage of the low temperature properties of
chalcogenides to achieve adequate fluidity to permit `hyperfine
structure` molding.
[0043] Chalcogenide glasses generally have coefficients of thermal
expansion, on the order of about 10-50 ppm/.degree. C., or for the
examples in Table 1, of about 20-40 ppm/.degree. C. The thermal
coefficients of the mold material selected may be in the order of
about 2 to 40 ppm/.degree. C., preferably about 5-30 ppm/.degree.
C. Considering the field of Ge--As-sulphide glasses, the
compositional range of moldable glasses, which would satisfy the
parameters of the mold, includes about 0-35% Ge, about 0-55% As,
about 30-85% S. To modify the optical, thermal, and/or mechanical
properties of these glasses, phosphorus (P), gallium (Ga), indium
(In), selenium (Se), tin (Sn), antimony (Sb), tellurium (Te),
thallium (Tl) chlorine (Cl), bromine (Br), and iodine (I) may be
added as optical constituents. Other elements, such as the rare
earths, or fluxes (e.g., Li, Na, K) may be also included.
[0044] In Table 1, we compare the material and optical properties
of a few select chalcogenide and oxide-based glasses. For two
chalcogenide glasses, Examples 1 and 2, Table 1 gives their
properties and respective concentrations in atomic percent of
germanium, arsenide, and sulfur. Example 3 is a low-Tg
(.about.330.degree. C.) oxide-based glass having a fluorine-free
composition, as disclosed in U.S. Pat. No. 5,021,366, incorporated
herein by reference. The glass of Example 3 is currently being
manufactured and distributed to companies that mold optical
elements (e.g., Geltech, Orlando, Fla.; Eastman Kodak, Rochester,
N.Y.). Example 4 is an oxide-based glass by Schott Glass
Technologies, Inc., and Example 5 is fused silica (HPFS.RTM.) by
Corning, Inc. One notes from Table 1 that the Tg of each of the
chalcogenide glasses is lower than that of the other oxide-based
glasses, including that of Example 3.
1TABLE 1 Material & Optical Properties of Select Chalcogenide
& Oxide-based Glasses Ex. 4 Ex. 1 Ex. 2 (Oxide glass, Ex. 5
(8.75% Ge, (12.5% Ge, Ex. 3 Schott Glass Fused Silica 17.5% As, 25%
As, (Phosphate glass, Technologies, (HPFS from Property 73.75% S)
62.5% S) Corning, Inc.) Inc.) Corning, Inc.) Index of .about.2.37
.about.2.48 1.60 1.51 1.46 refraction (n.sub.D) at the sodium D
line, 589 nm Index of 2.186 2.284 refraction (n.sub.1549) at 1549
nm dn/dT (ppm/.degree. K) NA NA -11.0 2.5 10 (in Nitrogen)
Wavelength 530 ->2400 560 ->2400 400-1600 350-2000 200-2000
transmission range (nm) CTE (ppm/.degree. C.) 40 20 15 7.1 0.52
Density (g/cm.sup.3) 2.72 3.03 3.80 2.51 2.20 Tg (.degree. C.)
.about.150-160 .about.245 330 559 1585 Ts (.degree. C.) .about.261
.about.348 Softening Point
[0045] In general, chalcogenide glasses will be more likely than
conventional oxide-based glasses to oxidize or otherwise react
chemically in air when heated. Despite this, however, we have
discovered surprisingly that chalcogenide glasses (Ge--As sulfides)
may be molded in air without undergoing any detectable oxidization.
Example glasses of Ge.sub.25As.sub.50S.sub.60 and
Ge.sub.8.76As.sub.17.5S.sub.73.75--respect- ively have Tg of
245.degree. C. and 150.degree. C, and CTE of 20 and 35 ppm/.degree.
C. Thus, when employing chalcogenides glasses according to the
present invention, the need to use an inert atmosphere during the
processing of the glass is either abated or eliminated, which is a
great advantage.
[0046] A number of other of suitable chalcogenide glasses will
include, for example, sulfide glasses that have a composition
comprising, in atomic/element percent, about 25-90% S, 0-50% As,
0-45% Ge; selenide glasses comprising, in atomic/element percent,
about 25-100% Se, 0-60% As, 0-45% Ge; or, telluride glasses
comprising, in atomic/element percent, about 25-90% Te, 0-50% As,
0-45% Ge. A specific type of germanium-arsenic-sulfide is described
in U.S. Pat. No. 6,277,775, incorporated herein, which contains a
source of phosphorus ion as a co-dopant to effect dispersion of a
rare earth metal ion dopant in the glass. Co-assigned U.S. patent
application Ser. No. 09/894,587, incorporated herein, describes
another kind of chalcogenide glass, which contains molecular
clusters. Another specific chalcogenide example includes a Ge--As
selenide glass having a composition in atomic/element percent of
about: 12.5% Ge, 25% As, 62.5% Se, with a Tg of about 219.degree.
C.
[0047] Alternative kinds of glasses may include chalco-halide
glasses. Chalco-halide glasses are similar in composition to the
sample chalcogenides except for the addition of Cl, Br, and I. A
typical system would be glasses encompassed by the member
components As--S--I, where Tg can range from below room temperature
for very I-rich species to about 250.degree. C. for I-poor
compositions. Similar glasses exist in the systems: As--S, Se--Cl,
Br; Ge--S, Se--Cl, Br, I and Ge--As--S, Se--Cl, Br, I, as given in
the review paper by J. S. Sanghera et al., J. Non-Cryst. Solids,
103 (1988), 155-178, incorporated herein by reference.
[0048] Another major class of chalco-halide glasses are the
so-called TeX or TeXAs glasses, containing Te and a halogen X with
or without a crosslinking element such as As. For thermally stable
lenses, the TeXAs glasses would be more preferred over the TeX
glasses. Typical examples of these and other chalco-halides are
presented by J. Lucas and X-H. Zhang, J. Non-Cryst. Solids 125
(1990), 1-16, and H-L. Ma et al., J. Solid State Chem. 96, 181-191
(1992), incorporated herein by reference.
[0049] Halide glasses also may be employed for applications
according to the present invention. Particular glass examples may
be drawn from the wide family of fluorozirconate glasses of which a
typical example, referred to as ZBLAN, has a composition in terms
of mole percent of about: 53% ZrF.sub.4, 20% BaF.sub.2, 4%
LaF.sub.3, 3% AlF.sub.3, 20% NaF, with a Tg of about
257-262.degree. C. Other possible halide glasses include the Cd
halides of which the following is a typical example: 17% CdF.sub.2,
33% CdCl.sub.2, 13% BaF.sub.2, 34% NaF, and 3% KF, with a Tg of
about 125.degree. C. Broad compositional ranges for these kinds of
halide glasses are given in U.S. Pat. No. 5,346,865, incorporated
herein, which include: 42-55% CdF2 and/or CdCl2, 30-40% NaF and/or
NaCl, 2-20% total of BaF2 and/or BaCl2+KF and/or KCl, with optional
halides as listed.
[0050] These two illustrative halide glass families are not
necessarily fully inclusive of all halides as there are also
fluorindate and fluorogallate glasses in which the major
constituents are typically alkaline earth fluorides, (e.g.,
ZnF.sub.2, CdF.sub.2 and InF.sub.3 and/or GaF.sub.3). Having Tgs
similar to that of ZBLAN, Tgs for these glasses can range from
about 260-300.degree. C. These glasses. A representative example
is: 19% SrF.sub.2, 16% BaF.sub.2, 25% ZnF.sub.2, 5% CdF.sub.2, 35%
InF.sub.3, with a Tg of 285.degree. C. When molding halide glasses
according to the present invention, it is preferred that a
non-reactive coating be used with the mold material to prevent the
halide species from reacting with air.
[0051] The method of the present invention, in part, is adapted
from a proprietary process developed by J. Mareshal and R.
Maschmeyer at Corning Inc., which is described in U.S. Pat. Nos.
4,481,023, 4,734,118, 4,854,958, and 4,969,944, the contents of
which are incorporated herein by reference. According to the
Mareshal-Maschmeyer process, a glass preform having an overall
geometry closely approximating that of the desired final product is
placed into a mold, the mold and preform are brought to a
temperature at which the glass exhibits a viscosity between
10.sup.8-10.sup.12 poises, a load is applied to shape the glass
into conformity with the mold, and the thereafter the glass shape
is removed from the mold at a temperature above the
transformational range of the glass and annealed.
[0052] In contrast, the present inventive molding process does not
require that one use either molten or solidified glass gobs, or
that preforms be in a near final shape. The glass material to be
molded or embossed may be in the form of regularly or
irregularly-shaped bulk-solids, such as ingots or a disc or wafer.
For example, one can place a glass wafer, from 0.25 to 2 mm in
thickness and 50 to 300 mm in diameter, in between two halves of
the mold. Alternatively, fine glass frit powders (e.g., less than
0.1 mm in diameter particle size) may be used. When glass frit is
used, the powder contains particles of sufficiently small,
irregular-sized glass particles to enable them to consolidate in
the heated mold when pressure is applied. The powder consolidates
initially to form a preform (such as a wafer, a gob or a
rough-shaped lens or grating), which surprisingly contained little
if any occlusions. This ability to use glass materials of virtually
any shape can reduce fabrication costs and simplify the molding
process.
[0053] A wide variety of temperatures and molding pressures may be
employed successfully to form glass articles of high precision,
provided that certain minimum criteria are met:
[0054] First, the molding operation will be conducted at
temperatures at which the glass has a much higher viscosity when
compared with customary glass pressing procedures. Thus, the glass
will be molded at viscosities of about 10.sup.6-10.sup.12 poises,
with a preferred range being about 10.sup.7-5.times.10.sup.11
poises, and a more preferred range of about 10.sup.8-10.sup.10
poises. Any non-oxide glass composition may be deemed a suitable
candidate for the inventive molding process, provided a suitable
mold material is available, which is capable of being fashioned
into a good surface finish, is sufficiently refractory to withstand
the pressing temperature and pressure, and is not substantially
attacked by the glass composition at molding temperatures.
[0055] Second, the inventive molding operation will involve an
ostensibly isothermal condition during the period wherein the final
figure of the shaped article is being formed. As employed herein,
the term isothermal means that the temperature of the mold and that
of the glass preform, at least in the vicinity of the mold, are
approximately identical. The temperature differences permitted are
dependent upon the overall size and specific design of the final
glass shape, but the difference will, preferably, be less than
20.degree. C. and, most desirably, less than 10.degree. C. This
isothermal condition will be maintained for a period of sufficient
length to allow the pressure on the molds to force the glass to
flow into conformity with the surface of the mold.
[0056] Normally, the glass products molded in accordance with the
inventive process contain too much thermal stress to be suitable
for use in optical applications and, therefore, a fine annealing
step is demanded after molding. Because of the isothermal
environment utilized in the pressing procedure, however, and the
fact that the molded articles essentially totally conform to the
mold surfaces, the articles shrink isotropically, thereby
permitting them to be fine annealed without any significant
distortion of the relative surface figure. Moreover, this annealing
without distortion can be achieved outside of the mold with no
elaborate physical support for the molded shape. This practice
leads to much shorter mold cycle times and precludes the need for
recycling the molds. In summary, there is no need to cool the mold
under load with the glass shape retained therewithin to a
temperature below the transformation range or transition
temperature of the glass. That is, the molds can be held at
temperatures where the glass is at a viscosity of about 10.sup.13
poises (the minimum temperature at which the pressed articles are
removed from the molds), rather than cooling the molds below the
transformation range, perhaps even to room temperature, and then
reheating. Such cycling consumes much energy and adversely affects
the life of the mold.
[0057] An important detail in the development of a glass molding or
embossing process is the choice of material from which the mold or
master is formed. The mold material should be chemically stable at
the operational molding temperature, and it should not react
chemically with the glass during molding. In other words, the
material of choice for the mold, which defines the profile or
pattern to be formed on glass, preferably should possesses a
similar thermal expansion to that of the glass and have a
recrystalization temperature that is substantially higher than the
temperature to which the mold is heated. Usually, it is preferred
that the mold has a coefficient of thermal expansion (CTE) that is
substantially compatible with the CTE of the glass material. A
material with a CTE that satisfies this and the other criteria,
however, has been difficult to find. Design parameters need to take
account of the large contraction of the glass that occurs when the
mold cools when there is a large disparity between CTE of the mold
and that of the glass. (It should be noted that, at times,
selecting mold/master materials and glasses of differing CTEs may
be necessary to take advantage of the differential in expansion
and/or contraction to help with the release of the molded/embossed
parts from the molds/masters.) Although matching the CTE of the
mold material and the glass is important, more important is that
the mold material should have a recrystallization temperature of
that is substantially higher than the operational temperature of
the mold.
[0058] Although silicon carbide (SiC), silicon nitride
(Si.sub.3N.sub.4), refractory ceramic composite of the two could be
used to form the mold, ceramic materials, however, can be expensive
and require coatings. Refractory metals (such as tungsten) and
alloys may also be used as molds, but some materials such as
titanium alloys are preferred in practicing the present invention
Normally, a large difference in the coefficient of thermal
expansion exists between a metal alloy surface compared to that of
a glass. A titanium alloy, such as Ti-6Al-4V alloy, however, was
unexpectedly discovered to be very suitable for embossing and
molding of chalcogenide (sulfide) glasses, particularly glasses
with a coefficient of thermal expansions in the range of about 20
to 40 ppm/.degree. C. The titanium alloy has a coefficient of
thermal expansion of within the range of approximately 8 to 12
ppm/.degree. C., and a recrystallization temperature in the range
of about 700 to 800.degree. C.
[0059] Molds made from titanium alloys can be used to process
chalcogenide glass blanks up to a temperature of about
500-550.degree. C., which should be suitable for molding such
glasses having a Tg of up to about 450.degree. C. or 500.degree. C.
The titanium alloy mold is made preferably of a nominal
composition, in terms of weight percent, of about: 80-98% Ti
(titanium); 10-1% Al (aluminum); 10-1% V (vanadium), preferably
consisting essentially, in weight percent, of about 90% Ti, 6% Al,
4% V. Commercially available Ti-6Al-4V alloys are normally used in
structural applications for chemical industries and also in
military aircrafts, but not for manufacturing glass molds.
According to the present invention, the mold blocks can be made of
ceramics and their composites, metals or alloys, preferably of
titanium or nickel alloys, particularly Ti-6Al-4V alloy, where the
glass to be processed into optical components is a non-oxide glass,
more particularly a chalcogenide type of glass.
[0060] The specific structure of the molding apparatus is not
critical to the operation of the inventive process. The press
should contain some mechanism for moving the molds against the
glass preform and some constraints against the motion of the molds.
Such constraints are demanded to achieve the geometrical
relationships required among the optical surfaces. It will be
appreciated that such constraints may be constructed in a variety
of ways. An apparatus developed in the laboratory for molding
lenses are illustrated in FIG. 7, which is exemplary only and not
limiting. Hence, for example, the addition of mechanisms for
automatic loading and unloading of the glass, alternative sources
of heating, cooling and press motion, and assignment of the
essential functions to separate or different mechanical elements
are considered to be within the technical competence or ingenuity
of a worker of ordinary skill in the art.
[0061] FIG. 7 shows a face-on view of a fixture of a mold assembly
8 used. The assembly 8 accommodates a base plate 10 on which a
stationary mold half is disposed, standing on insulator standoffs
14. A moveable mold half 16 is guided by guideposts 18. A mechanism
(not shown) connected to a guidance device 20 for the mold assembly
on the mold half 16 is connected to an actuator for applying
pressure via the mold halves against glass material to be molded or
embossed. This material is supported between two mold blocks 22 and
24. The blocks are captured in cavities in the mold halves 12 and
16. The mold halves are heated by electrical heater elements 26
located in each mold half 12 and 16. The mold halves may be split
to capture the heater elements 26.
[0062] FIGS. 8A, B, and C, depict a sectional view of three
alternate pairs of mold blocks used in the apparatus of FIG. 7. The
mold masters of FIG. 8B may have various forms, such as spherical
and aspheric, single plano-convex, plano-concave, array of such
lenses, and lens arrays. The mold blocks may be cylindrical in
shape. The opposite mold surfaces 28 and 30 of the mold blocks 22
and 24 define the profile of the optical component or element to be
molded. Profiled cavities on the mold assembly will form various
types of lenses on one side of a substrate of the glass material
being molded. The profile of the molded object may be of either a
convex or concave spherical lenses defined by a single concave
semi-spherical mold cavity 32 (cavity for concave lens profile is
not shown) in the mold surface, as shown in FIG. 8A. Alternatively,
the mold cavity described can be of aspheric shaped (not shown) to
produce aspheric glass lenses. For a lens array, such as depicted
in FIG. 8B, of convex (FIGS. 3 or 4) or concave (not shown) lenses
there may be multiple cavities 32 of semi-spherical or aspheric
(not shown) shapes. Other profiles, such as double convex or double
concave may be formed even on opposite sides of the glass substrate
which is formed in the mold, as for example with the mold blocks 22
and 24 shown in FIG. 8C.
[0063] In the fabrication of surface-relief microlenses (41) on a
substrate (42), one critical parameter is the sag of the lens, s.
As defined in FIG. 9, this parameter designates the height that the
vertex (45) of the microlens extends above the substrate surface
(43). For a microlens having a spherical shape of radius R, the sag
is given by Eq. (1):
s=R-{square root}{square root over (R.sup.2-(D/2).sup.2)}, (1)
[0064] where D is the diameter of the microlens. The radius of
curvature R is calculated from the indices of refraction of the
incident medium n.sub.1 and that of the substrate n.sub.2 and the
desired focal length f according to Eq. (2):
R=(n.sub.2-n.sub.1)f (2)
[0065] Substituting Eq. (2) into Eq. (1), one obtains
s=f.DELTA.n[1-{square root}{square root over
(1-(NA/.DELTA.n).sup.2)}], (3)
[0066] where .DELTA.n=n.sub.2-n.sub.1 and NA is the numerical
aperture of the lens defined by 1/(2F/#), where the f-number (F/#)
of the lens is f/D. Graphing the sag of the microlens s as a
function of .DELTA.n for a fixed f-1000 .mu.m, but various
numerical apertures one obtains the curves represented in FIG. 10.
One notes that as .DELTA.n increases, the sag of the microlens
decreases. By reducing the sag of the lens, one increases the
manufacturability of the lens (e.g., in a molding process, it is
less likely that air will be trapped in the mold if the sag of the
microlens is less). One of the most important reasons for using
high-index glasses, however, is for reducing optical
aberrations.
[0067] Spherical surfaces are not ideal surfaces from an imaging
perspective. To image a given object, one ideally requires an
aspheric surface in order to reduce aberrations. Aspheric surfaces,
however, are more complicated to fabricate and to test than
spherical surfaces. For low numerical apertures (<0. 1) the
error between the desire aspheric surface and the manufacturable
spherical surfaces is generally small. For larger numerical
apertures, however, this deviation increases, thereby degrading the
image quality of the optical system. By using materials of higher
indices of refraction (e.g., chalcogenide glasses with
n.sub.2>2.0), one can reduce the spherical aberrations of lens
since the required radii of curvatures for a set focal length is
reduced.
[0068] For a blazed grating, a substrate (51) is patterned with a
series of blazed facets (52) across one of its surfaces, see FIG.
11. The desired depths of these facets is given by 1 d = n ,
[0069] where .lambda. is the wavelength of operation and
.DELTA.n=n.sub.2-n.sub.1. The depth of the grating therefore
decreases linearly with .DELTA.n. By decreasing the depth of the
grating, one decreases the width of the shadow region (53) of the
grating. The shadow region is the region in which from a
geometrical optical analysis there is no light due to the shadowing
caused by the sidewalls of the grating facets. The shadowing
reduces the efficiency of the grating in the order of interest by
introducing unwanted diffraction effects. By reducing the depth of
the grating (e.g., by increasing .DELTA.n), one increases the
theoretical diffraction efficiency attained by the grating.
[0070] According to an embodiment for carrying out the present
molding method, the glass material, either in the ingot, wafer or
powder form, is placed between the opposing mold surfaces 28 and 30
of the mold blocks 22 and 24. Both upper and lower halves of the
mold blocks or masters 22 and 24 are heated simultaneously. Heating
and cooling rates and dwell times are determined based on working
conditions and particular glass species, and may be controlled
precisely using a digital temperature controller. In general, by
increasing power to heating elements 26, the mold or master is
heated at a predetermined rate to an operational temperature that
is at least about 10.degree. C. above the Tg of the specific glass
material in the mold. Typically, the operational temperature is
within a range of about 10-110.degree. C. above Tg. In other words,
the glass has a viscosity of about 10.sup.6 to about 10.sup.12
poises. Preferably, the temperature is about 20.degree. C. to about
90.degree. C. above Tg. More preferably, the temperature is about
30-70.degree. C. above Tg. For certain chalcogenide glass samples
that have a Tg in the range of approximately 100.degree.
C.-350.degree. C., or about 130.degree. C.-250.degree. C., the
operational temperature of the blocks 22 and 24, preferably are
about 50.degree. C. above Tg, or in the range of 150.degree. C. to
400.degree. C.
[0071] Concurrently with heating of the precursor glass material, a
predetermined pressure is applied mechanically to the upper half of
the mold against the lower half to form the optical components into
the desired shapes and with hyperfine microstructures (i.e., lens
or microlens array or diffraction grating, diffractive optical
pattern or combinations of such lenses and patterns). A mechanical
driver, for example a screw drive, connected to the guidance
devices 20 on the upper mold half 16 may be employed to actuate the
pressing. Alternatively, the mold assembly 8 can be put between the
platens of a hydraulic press or electrically driven press, such as
a machine to apply pressure on the mold blocks 22 and 24. As with
heating, the exact amount of pressure depends on various factors,
including the Tg of the glasses or the complexity of the features
in the profile to be molded. The pressure (force) of the mold
translates to about 10 to 5000 pounds per square inch (psi) of the
mold surface area. For example, for glass species mentioned as
suitable for molding according to the present process, the pressure
can be in the range from 1 to 100 psi. Press the mold when the
viscosity of the glass reaches about 10.sup.7-10.sup.11 poises,
preferably about 10.sup.10 poises. After reaching the pre-selected
peak operational temperature, the pressure is held on the mold
block assembly for a dwell time, suitably from about 0.1 to 10
minutes to ensure the completion of flow of glass material within
the mold profiles or the designed patterns. For a certain
composition of a specific chalcogenide glass, a dwell time, 10
sec., at the optimum temperature, 50.degree. C. above Tg, and
pressure, 20 psi, was utilized for completion of movement and
filling of the viscous glass within the mold cavity. Then the
pressure was gradually released. The mold is allowed to cool at a
predetermined rate, 20.degree. C./min., to 50 C or poises, and the
optical component having the desired profile is extracted from the
mold assembly 8. For industrial fabrications, it is preferred that
the temperature of the mold is not cooled all the way to room
temperature and also use of cooling using forced air or nitrogen.
Additional process steps for removing the molded product are either
described by Mareschal and Maschmeyer or familiar to those in the
art.
[0072] For certain glass compositions, it may be desired to apply a
release coating on the opposing mold surfaces 28 and 30 of the mold
halves 22 and 24. The release coatings may include: graphite carbon
coating, molybdenum-di-silicide, fluorocarbon (CF.sub.x), boron
nitride, noble metals and alloys, and some commercially available
release coatings. Boron nitride (BN) was determined to be the best
release coating for molding/embossing of sulfide glasses. The
release coating material can be effectively spray coated or sputter
deposited on the mold surfaces. It will be noted that the molding
and embossing processes in this investigation are carried out in
ambient air. An airtight enclosure having an inert atmosphere or
vacuum was not required for such molding and embossing processes.
In the course of working with these materials, however, it was
noticed that born nitride coated mold surfaces had longer service
lives (i.e. more cycles of molding/embossing before repolishing the
mold surfaces) compared to non-coated mold surfaces.
[0073] The foregoing general description of the method for
fabricating optical components in glass and the apparatus for
executing the method should be taken as illustrative and not
limiting of possible variations or modifications. The examples in
the following section further illustrate and describe the
advantages and qualities of the present invention.
EXAMPLES
[0074] In a series of studies, it was endeavored to mold and/or
emboss optical components having fine, complex microstructures in
compact shapes and sizes, using non-oxide glasses from either
bulk-solid or powder forms (a wafer, a cube, a irregular shaped
agglomerate or powder) in the mold. Optical components, such as
MLAs, required a variety of complex shapes for densely packed
individual lenses, aspheric lenses, or diffraction gratings, which
may be used in optical switches, optical displays and the like. In
particular, chalcogenide glasses, in which oxide species are
absent, impart unexpectedly favorable molding characteristics. For
the experimental examples described below, all moldings were
carried out in air. No inert atmospheres such as nitrogen or argon
was used during molding. In some specific cases, forced air or
nitrogen gas was used to facilitate cooling of the molded parts and
their removal.
[0075] The glass transition temperatures (Tg) of the chalcogenide
glass samples chosen for experiments ranged from about 160.degree.
C. to about 245.degree. C. and indices of refraction were in the
range of about 2.3 to about 2.5. The precursor glass material was
in the form of a wafer having thickness in the range of 0.25 to 2
mm and 20 to 300 mm in diameter. Alternatively, fine glass powders
(less than 0.1 mm in diameter), and cube or irregular shaped solid
block of glass were also used as precursor material. Note this is
in contrast with the preforms required in the molding processes
developed by Kodak (EP 1 069082 A2), Corning (U.S. Pat. No.
4,481,023), and possibly, by Geltech. We placed the glass, either
in wafer or in powder form, between the opposing mold surfaces of
the mold blocks. The temperature of the mold blocks was increased
from room temperature to about 50.degree. C. above the Tg of the
specific glass material in the mold by increasing the power input
to the heater elements. The temperature of the blocks was in the
range of 220 to 300.degree. C. in the case of the chalcogenide
glasses used (Table 1, Examples 1 & 2). Concurrently with
heating of the glass precursor material, we applied mechanical
pressure in the range of 1 to 100 psi to the upper mold portion.
The amount of pressure applied depended on factors such as the
spatial frequency and depths of the features in the profile to be
molded as well as the Tg of the glasses. In order to apply pressure
on the mold blocks, we placed the mold assembly between the platens
of a hydraulic or electrically driven press, such as an Instron
machine. After reaching the pre-selected peak temperature, the
pressure was applied and was held on the mold block assembly for a
dwell time, typically from about 5 to 60 seconds, to ensure the
complete flow of glass material within the mold. After this dwell
time, we gradually released pressure and reduced the temperature of
the mold block back to room temperature, as programmed in the
temperature controller.
[0076] After investigating several mold materials (including
titanium and its alloys, aluminum and its alloys, and steels,
particularly series 440 stainless steels), a specific titanium
alloy, Ti-6Al-4V alloy, and/or electroless high phosphorous nickel
alloy were chosen as suitable mold materials, specifically for
their stability at the molding temperature used for molding these
sulfide glasses. These materials have coefficients of thermal
expansion (CTE) of approximately 10 to 20ppm/.degree. C. and a
recrystallization temperature in the range of 700 to 900.degree. C.
Oxidation of some of the mold materials, particularly aluminum and
iron-based alloys, prevented the use of those materials.
[0077] We molded precision microlenses, such as those required to
collimate fibers, to measure the change in surface profile (e.g.,
radius of curvature) between that of the master and that of the
molded glasses. The mold cavity was machined using a carbide tool
and then diamond-polished using of 10 .mu.m grit. The higher
precision masters we molded were fabricated utilizing single point
diamond turning. But the mold material was not subject to any other
special treatment, except to anneal for relief of stresses due to
machining.
[0078] The present invention can replicate microstructures as deep
as 500 .mu.m in chalcogenide glasses in a fraction of an hour. Such
a process can convey commercial advantage in the manufacturing of
cost-effective, precision, optical microstructures for optical
surface-relief elements. In contrast, reactive ion etching
techniques can take as long as 12 to 24 hours to etch 50 to 100
.mu.m deep microlenses into fused silica. The capital expense of
reactive ion etching and the associated support equipment and human
resources is significant and is typically measured in millions of
dollars.
[0079] FIG. 12 illustrates the result of a microlens-profile molded
using the glass of designated Example 1, in Table 1 (8.75% Ge,
17.5% As, and 73.75% S), as the fine precursor frit (.about.10
.mu.m). For this experiment, the titanium mold or master had a
concave surface-relief structure of diameter of 3 mm clear aperture
and a sag of 513 .mu.m. In order to mold this type of structure,
the mold surface was coated with BN by aerosol assisted spraying.
After the frit powder was deposited in the mold cavity at room
temperature, the mold was raised to 300.degree. C. at a rate of
20.degree. C./min. The material was held at 300.degree. C. for
about 5 minutes before a force of 20 psi was applied. The force was
released after about a minute and the mold was cooled down to room
temperature at a rate of 1.degree. C./min. One notes from the
profile trace in FIG. 12 that the molding process had no difficulty
replicating a 513 .mu.m deep structure.
[0080] The molding of microstructures as fine as 5 .mu.m can be
resolved also using the glass of Example 1. At an operational
molding temperature and pressure of only 245.degree. C. and 50 psi,
respectively, we were able to fabricate a 1.3-mm diameter,
0.94-.mu.m deep, diffractive lens having 5 .mu.m as its smallest
grating period, using a single-point diamond-turned,
high-phosphorous electroless nickel substrate as the mold master.
Table 2 summarizes the experimental conditions. For this
experiment, the wafer of Example1 was inserted into the mold only
after it had already reached 245.degree. C. Based upon the
sharpness of the diffractive zones achieved, it believed that the
resolution of the Example 1 glass is significantly less than 5
.mu.m. Potentially, the resolution limit of the present molding
process and chalcogenide glasses can be refined and used to
replicate wavelength dispersion gratings with grating periods of
approximately 1 .mu.m or finer, such as for wavelength division
multiplexer (WDM) modules.
2TABLE 2 Parameters for Molding Experiment Parameter Value Glass
Example 1 in wafer form (5 mm diameter, 2 mm thickness) Master
High-phosphorous electroless nickel Release coating None Max.
temperature 245.degree. C. Pressure 50 psi Molding time 30 min.
(heating, pressing, cooling)
[0081] Afterwards, the molded-glass, diffractive, optical
microstructures were examined using a Zygo NewView 100 for surface
roughness and grating feature fidelity. The surface finish of the
molded structure was found to average 55 .ANG. rms, while the
master had a surface finish of 40 .ANG. rms. FIG. 13 illustrates
the 2-dimensional and 3-dimensional profiles of diffractive
structures replicated according to the present invention. The
fidelity with which the structures (grating grooves of .about.9
.mu.m or larger) were reproduced was excellent. To the lateral
resolution of the NewView (1 .mu.m), we observed no signs of
degradation of the sharp zone transitions when comparing those of
the master to those of the replica. Gratings with a 2-D
cross-section of about 8 .mu.m or about 5 .mu.m wide zone periodity
can be made for diffractive lens. Certain precautions may be
advisable, however, such as applying release coatings to overcome
adhesion issues. Although adhesion issues can degraded the quality
of sub-8 .mu.m period zones, the viscosity of the glass of Example
1 was sufficiently low enough to replicated with excellent fidelity
the sharp zone transitions of the nickel diffractive master. Hence,
it is believed that the viscosity-limited resolution of the molding
process using Example 1 is significantly less than 5 .mu.m.
[0082] In addition to investigating the resolution limits of the
process, we also investigated different release coatings. A number
of release coatings, such as graphite carbon,
molybdenum-di-silicide, fluorocarbon, boron nitride, some
commercially available release coatings (e,g, Zinc Stearate Mold
release, Thermoset release, Dry Film Mold release, Rocket Release
etc. manufactured by Stoner Incorporated, Quarryville, Pa.) were
evaluated from the criteria of their effectiveness in improving the
surface smoothness of the molded optical components and the life
cycle of the mold surfaces. For each release coating, we evaluated
their effectiveness in maintaining the surface quality of the mold
cavity and the lifetime of the release-coated mold surfaces. From a
number of experiments, it was determined that boron nitride (BN)
was the most effective release coating material of the group for
the molding. Boron nitride coated mold surfaces, particularly for
the titanium alloy molds, had longer service lives (i.e., more
cycles of molding/embossing in between cleaning or repolishing of
the mold surfaces) compared to non-coated mold surfaces.
[0083] In general, the coatings for releasing surface-sensitive
components from the molds are best applied either by physical or
chemical vapor deposition techniques. In the absence of thin-film
coating facilities, the release coating material may be effectively
spray coated on the mold surfaces. Spray coating on polished mold
surfaces, however, tends to increase the roughness of the mold
surfaces because of the particulate nature of the spray-coating
material. As a result, the molded lens surfaces replicated the
roughness of the mold surfaces caused by spray-coatings.
Sputter-deposited films/coatings are more preferred for optical
surface release due to the smoothness of the resulting release
coating surfaces. For chalcogenide, in particular sulfide glasses,
and chalco-halide glasses, surprisingly, a release coating was not
required when titanium alloy and high phosphorous electroless
nickel molds were used. Other nickel alloy surfaces also do not
necessarily require a release coating.
[0084] The present invention has been described generally and in
detail by way of examples and the figures in detail and by way of
examples of preferred embodiments. Persons skilled in the art,
however, can appreciate that the invention is not limited
necessarily to the embodiments specifically disclosed, but that
substitutions, modifications, and variations may be made to the
present invention and its uses without departing from the scope of
the invention. Therefore, changes should be construed as included
herein unless they otherwise depart from the scope of the invention
as defined by the appended claims and their equivalents.
* * * * *