U.S. patent application number 11/657930 was filed with the patent office on 2007-06-21 for fabrication of ceramic microstructures.
This patent application is currently assigned to President and Fellows of Harvard College. Invention is credited to Pascal Deschatelets, George M. Whitesides, Hong Yang.
Application Number | 20070142202 11/657930 |
Document ID | / |
Family ID | 22876105 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070142202 |
Kind Code |
A1 |
Yang; Hong ; et al. |
June 21, 2007 |
Fabrication of ceramic microstructures
Abstract
The invention provides ceramic molded solid articles and methods
for making these articles on the micron scale. Articles are molded
from ceramic precursors, optionally using molds including at least
one portion that is elastomeric.
Inventors: |
Yang; Hong; (Rochester,
NY) ; Deschatelets; Pascal; (Louisville, KY) ;
Whitesides; George M.; (Newton, MA) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, P.C.
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
President and Fellows of Harvard
College
Cambridge
MA
|
Family ID: |
22876105 |
Appl. No.: |
11/657930 |
Filed: |
January 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09940072 |
Aug 27, 2001 |
7198747 |
|
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11657930 |
Jan 24, 2007 |
|
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60233156 |
Sep 18, 2000 |
|
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Current U.S.
Class: |
501/77 ;
264/317 |
Current CPC
Class: |
C04B 35/571 20130101;
B81C 99/0085 20130101; B81C 99/008 20130101; B82Y 30/00 20130101;
B28B 13/021 20130101; Y10T 428/26 20150115; B81C 2201/034 20130101;
C04B 35/589 20130101; C04B 35/622 20130101; B82Y 10/00 20130101;
B28B 7/342 20130101 |
Class at
Publication: |
501/077 ;
264/317 |
International
Class: |
B29C 33/76 20060101
B29C033/76; C03C 3/064 20060101 C03C003/064 |
Goverment Interests
GOVERNMENT FUNDING
[0002] This invention was sponsored by the pace and Naval Warfare
Systems Grant No. N66001-98-1-8915. The government has certain
rights in the invention.
Claims
1. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
filling the mold with a ceramic precursor; heating the ceramic
precursor under a moisture-free atmosphere to produce a structure
comprising a ceramic, the structure having a Young's modulus that
does not change more than 10% upon heating to 1400.degree. C. in an
inert atmosphere; and removing the mold from a product formed from
the ceramic precursor.
2. The method of claim 1, wherein the ceramic precursor comprises
at least two different elements.
3. The method of claim 2, wherein the at least two different
elements are selected from a group consisting of carbon, nitrogen,
boron, silicon, phosphorus, aluminum and hydrogen.
4. The method of claim 1, wherein the ceramic precursor comprises
at least three different elements.
5. The method of claim 1, wherein each element of the ceramic
structure is derived from the ceramic precursor.
6. The method of claim 1, wherein the step of heating is performed
under an inert atmosphere.
7. The method of claim 1, wherein prior to the step of filling, the
ceramic precursor is prepared to have sufficient viscosity to
completely fill the mold.
8. The method of claim 7, wherein the viscosity of the ceramic
precursor is adjusted to have a value less than about 500
cm.sup.2/s.
9. The method of claim 1, wherein prior to the step of filling, the
mold is treated such that it is inert with respect to reaction with
the ceramic precursor and any subsequent products resulting from
the ceramic precursor.
10. The method of claim 9, wherein the step of treating the mold
comprises reacting the mold with an agent selected from the group
consisting of alkylating, silylating, fluoroalkylating, or
alkylsilylating agent.
11. The method of claim 1, wherein the step of filling comprises
positioning a surface of the mold against a surface of a substrate
to create a cavity which the ceramic precursor fills.
12. The method of claim 11, wherein the substrate is selected from
the group consisting of silicon, silicon dioxide, silicon nitride,
and any substrate with a smooth metallic surface.
13. The method of claim 11, further comprising treating the
substrate surface to render the substrate inert with respect to
reaction with the ceramic precursor and any subsequent products
resulting from the ceramic precursor.
14. The method of claim 13, wherein the step of treating comprises
silanization.
15. The method of claim 1, wherein the step of filling comprises
allowing the ceramic precursor to enter a volume of lower
pressure.
16. The method of claim 1, wherein the step of filling comprises
allowing the ceramic precursor to enter a volume by means of
capillary action.
17. The method of claim 1, further comprising the step of curing
the ceramic precursor in the mold.
18. The method of claim 17, wherein the ceramic precursor is cured
chemically.
19. The method of claim 17, wherein the ceramic precursor is cured
thermally.
20. The method of claim 17, wherein the ceramic precursor is cured
in the mold at a temperature of at least 100.degree. C.
21. The method of claim 17, wherein the ceramic precursor is cured
in the mold under an inert atmosphere.
22. The method of claim 17, wherein the precursor is cured in the
mold under a moisture-free atmosphere.
23. The method of claim 1, wherein the solution contains
tetrabutylammonium fluoride.
24. The method of claim 1, wherein the product comprises a cured
ceramic precursor and after removing the mold, the method further
comprises heating the cured ceramic precursor to a temperature of
at least 1000.degree. C. to produce a ceramic.
25. The method of claim 1, further comprising transferring the
product to a substrate selected from the group consisting of
silicon, silicon dioxide, silicon nitride, and metal.
26. The method of claim 1, wherein the ceramic precursor is a
single precursor.
27. The method of claim 1, wherein the ceramic precursor comprises
a polymer.
28. The method of claim 1, wherein the ceramic precursor comprises
an oligomer.
29. The method of claim 1, wherein the mold exhibits elastomeric
properties.
30. The method of claim 29, wherein the mold comprises
polydialkylsiloxane material.
31. The method of claim 1, wherein the step of filling the mold is
performed under an inert atmosphere.
32. The method of claim 1, wherein the step of filling the mold is
performed under a moisture-free atmosphere.
33. A method comprising: providing a silanized mold having at least
one component with at least one dimension less than 100
micrometers; providing a ceramic precursor having sufficient
viscosity to completely fill the mold, wherein the viscosity of the
ceramic precursor is adjusted to have a value of less than about
500 cm.sup.2/s; thereafter, filling the mold with the ceramic
precursor; and dissolving the filled mold.
34. The method of claim 33, further comprising heating the ceramic
precursor in the mold to produce a structure comprising a
ceramic.
35. The method of claim 33, further comprising the step of curing
the ceramic precursor in the mold.
36. The method of claim 35, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
37. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
reacting the mold with an agent selected from the group consisting
of an alkylating, silylating, fluoroalkylating, or alkylsilylating
agent, such that the mold is inert with respect to reaction with a
ceramic precursor and any subsequent products resulting from the
ceramic precursor; thereafter, filling the mold with the ceramic
precursor; and dissolving the filled mold.
38. The method of claim 37, further comprising heating the ceramic
precursor in the mold to produce a structure comprising a
ceramic.
39. The method of claim 37, further comprising the step of curing
the ceramic precursor in the mold.
40. The method of claim 39, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
41. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
positioning a surface of the mold against a surface of a substrate
to create a cavity which a ceramic precursor fills; treating the
substrate surface to render the substrate inert with respect to
reaction with the ceramic precursor and any subsequent products
resulting from the ceramic precursor; and dissolving the mold
containing the ceramic precursor.
42. The method of claim 41, further comprising heating the ceramic
precursor to produce a structure comprising a ceramic.
43. The method of claim 41, further comprising the step of curing
the ceramic precursor in the mold.
44. The method of claim 43, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
45. A method comprising: providing a silanized mold having at least
one component with at least one dimension less than 100
micrometers; allowing a ceramic precursor to enter a volume of
lower pressure in the mold; and dissolving the mold containing the
ceramic precursor.
46. The method of claim 45, further comprising heating the ceramic
precursor to produce a structure comprising a ceramic.
47. The method of claim 45, further comprising the step of curing
the ceramic precursor in the mold.
48. The method of claim 47, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
49. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
filling the mold with a ceramic precursor; curing the ceramic
precursor in the mold under a moisture-free atmosphere; and
dissolving the mold containing the ceramic precursor.
50. The method of claim 49, further comprising heating the ceramic
precursor to produce a structure comprising a ceramic.
51. The method of claim 49, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
52. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
filling the mold with a ceramic precursor; and dissolving the
filled mold.
53. The method of claim 52, further comprising heating the ceramic
precursor to produce a structure comprising a ceramic.
54. The method of claim 52, further comprising the step of curing
the ceramic precursor in the mold.
55. The method of claim 54, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
56. A method comprising: providing an elastomeric mold comprising
polydialkylsiloxane material having at least one component with at
least one dimension less than 100 micrometers; and filling the mold
with a ceramic precursor; heating the ceramic precursor in the mold
to produce a structure comprising of a ceramic, the structure
having a Young's modulus that does not change more than 10% upon
heating to 1400.degree. C. in an inert atmosphere; and dissolving
the mold containing the heated ceramic precursor.
57. The method of claim 56, further comprising the step of curing
the ceramic precursor in the mold.
58. The method of claim 57, wherein the ceramic precursor is cured
in the mold under a moisture-free atmosphere.
59. A method comprising: providing a mold; silanizing the mold; and
filling the mold with a ceramic precursor.
60. The method of claim 59, further comprising heating the ceramic
precursor to produce a structure comprising a ceramic.
61. The method of claim 59, further comprising the step of curing
the ceramic precursor in the mold.
62. The method of claim 61, wherein the ceramic precursor is cured
in the mold under an inert and/or a moisture-free atmosphere.
63. A method comprising: providing a mold having at least one
component with at least one dimension less than 100 micrometers;
and filling the mold with a ceramic precursor.
64. The method of claim 63, wherein the ceramic precursor comprises
at least two different atom types.
65. The method of claim 64, wherein the at least two different atom
types are selected from a group consisting of carbon, nitrogen,
boron, silicon, phosphorus, aluminum and hydrogen.
66. The method of claim 63, wherein the ceramic precursor comprises
at least three different atom types.
67. The method of claim 63, further comprising heating the ceramic
precursor to produce a ceramic structure.
68. The method of claim 67, wherein each atom type of the ceramic
structure is derived from the ceramic precursor.
69. The method of claim 67, wherein the step of heating is
performed under an inert atmosphere.
70. The method of claim 67, wherein the step of heating is
performed under a moisture-free atmosphere.
71. The method of claim 63, wherein prior to the step of filling,
the ceramic precursor is prepared to have sufficient viscosity to
completely fill the mold.
72. The method of claim 71, wherein the viscosity of the ceramic
precursor is adjusted to have a value less than about 500
cm.sup.2/s.
73. The method of claim 63, wherein prior to the step of filling,
the mold is treated such that it is inert with respect to reaction
with the ceramic precursor and any subsequent products resulting
from the ceramic precursor.
74. The method of claim 73, wherein the step of treating the mold
comprises reacting the mold with an agent selected from the group
consisting of alkylating, silylating, fluoroalkylating, or
alkylsilylating agent.
75. The method of claim 63, wherein the step of filling comprises
positioning a surface of the mold against a surface of a substrate
to create a cavity which the ceramic precursor fills.
76. The method of claim 75, wherein the substrate is selected from
the group consisting of silicon, silicon dioxide, silicon nitride,
and any substrate with a smooth metallic surface.
77. The method of claim 75, further comprising treating the
substrate surface to render the substrate inert with respect to
reaction with the ceramic precursor and any subsequent products
resulting from the ceramic precursor
78. The method of claim 77, wherein the step of treating comprises
silanization.
79. The method of claim 63, wherein the step of filling comprises
allowing the ceramic precursor to enter a volume of lower
pressure.
80. The method of claim 63, wherein the step of filling comprises
allowing the ceramic precursor to enter a volume by means of
capillary action.
81. The method of claim 63, further comprising the step of curing
the ceramic precursor in the mold.
82. The method of claim 81, wherein the ceramic precursor is cured
chemically.
83. The method of claim 81, wherein the ceramic precursor is cured
thermally.
84. The method of claim 81, wherein the ceramic precursor is cured
in the mold at a temperature of at least 100.degree. C.
85. The method of claim 81, wherein the ceramic precursor is cured
in the mold under an inert atmosphere.
86. The method of claim 81, wherein the precursor is cured in the
mold under a moisture-free atmosphere.
87. The method of claim 63, further comprising removing the mold
from a product formed from the ceramic precursor.
88. The method of claim 87, wherein the step of removing the mold
comprises physically removing the mold.
89. The method of claim 87, wherein the step of removing the mold
comprises dissolving the mold.
90. The method of claim 89, wherein the step of dissolving
comprises dissolving the mold in a solution containing fluoride
anions.
91. The method of claim 90, wherein the solution contains
tetrabutylammonium fluoride.
92. The method of claim 87, wherein the product comprises a cured
ceramic precursor and after removing the mold, the method further
comprises heating the cured ceramic precursor to a temperature of
at least 1000.degree. C. to produce a ceramic.
93. The method of claim 87, further comprising transferring the
product to a substrate selected from the group consisting of
silicon, silicon dioxide, silicon nitride, and metal.
94. The method of claim 63, wherein the ceramic precursor is a
single precursor.
95. The method of claim 63, wherein the ceramic precursor comprises
a polymer.
96. The method of claim 63, wherein the ceramic precursor comprises
an oligomer.
97. The method of claim 63, wherein the mold exhibits elastomeric
properties.
98. The method of claim 97, wherein the mold comprises
polydialkylsiloxane material.
99. The method of claim 63, wherein the step of filling the mold is
performed under an inert atmosphere.
100. The method of claim 63, wherein the step of filling the mold
is performed under a moisture-free atmosphere.
101. An article comprising a free standing ceramic structure with
at least one component with a dimension less than 50 micrometers,
the at least one component being integral with the article.
102. The article of claim 101, wherein the article is a molded
article.
103. The article of claim 101, wherein the ceramic comprises a
formula Si.sub.wB.sub.xN.sub.yC.sub.z.
104. The article of claim 101, wherein the ceramic has an oxide
content of less than about 30% by atomic composition.
105. The article of claim 101, wherein the ceramic is substantially
free of structural degradation upon exposure to air at a
temperature of greater than 1000.degree. C. for at least 2
hours.
106. The article of claim 101, wherein the at least one component
has an aspect ratio of at least about 2:1.
107. The article of claim 101, wherein the at least one component
has an aspect ratio of at least about 4:1.
108. The article of claim 101, wherein the article is capable of
withstanding temperatures greater than 2000.degree. C.
109. The article of claim 101, wherein the article can withstand
temperatures less than 1500.degree. C. in the presence of air for
at least about 80 hours resulting in a change of less than a 10% in
Young's Modulus of the article.
110. The article of claim 101, wherein the at least one component
has a dimension less than 25 micrometers, the at least one
component being integral with the article.
111. The article of claim 101, wherein the at least one component
has a dimension less than 15 micrometers, the at least one
component being integral with the article.
112. A method comprising: providing a mold; filling the mold with a
ceramic precursor; and dissolving the mold.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/940,072, filed on Aug. 27, 2001, which
claims priority to U.S. Provisional Patent Application Ser. No.
60/233,156 filed Sep. 18, 2000, the entire contents each of which
are incorporated by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to ceramic solid
articles, and more particularly to a method for making these
articles on the micron scale by molding them from ceramic
precursors.
BACKGROUND OF THE INVENTION
[0004] In the fields of chemistry, biology, materials science,
microelectronics, optics, and medicine the development of devices
which are small relative to the state of the art and which are
conveniently and relatively inexpensively produced is
important.
[0005] A well-known method of production of devices, especially in
the area of microelectronics, is photolithography. According to
this technique, a negative or positive resist (photoresist) is
coated onto an exposed surface of an article. The resist then is
irradiated in a predetermined pattern, and portions of the resist
that are irradiated (positive resist) or nonirradiated (negative
resist) are removed from the surface to produce a predetermined
pattern of resist on the surface. This is followed by one or more
procedures. According to one, the resist serves as a mask in an
etching process in which areas of the material not covered by the
resist are chemically removed, followed by removal of resist to
expose a predetermined pattern of a conducting, insulating, or
semiconducting material. According to another, the patterned
surface is exposed to a plating medium or to metal deposition (for
example under vacuum) followed by removal of resist, resulting in a
predetermined plated pattern on the surface of the material. In
addition to photolithography, x-ray and electron-beam lithography
can be used in an analogous fashion. Lithography techniques such as
those mentioned above typically require relatively expensive
apparatus, and are relatively labor intensive. The techniques
require the design and fabrication of chrome masks, access to clean
rooms, and other requirements commonly known to those skilled in
the art.
[0006] Microelectromechanical systems are an area of relatively
intensive research. These systems involve the fabrication of
microscale structures prepared from silicon, or occasionally from
other material such as gallium arsenide, silicon carbide, silicon
nitride, metals, glasses, or plastics, by typical integrated
circuit industry microfabrication techniques such as
photolithography or additive/subtractive processes such as
deposition and etching. While interesting systems have been
developed, simplification and increased versatility would be
advantageous.
[0007] Ceramic structures such as borosilicon carbonitride have
numerous applications. Ceramics have found extensive use in
connection with products to be used in harsh operating conditions
such as when exposed to high temperatures, highly oxidative
environments, and when exposed to aggressive chemical conditions.
Ceramics are also known for their high strength, hardness, low
thermal conductivity, and low electrical conductivity.
[0008] U.S. Pat. No. 5,698,485 (Breck) describes a process for
producing ceramic microstructures from mold inserts structured by
using the technique lithographic, galvanoformung, abformung (LIGA),
a lithographic structuring method requiring the use of high energy
radiation. LIGA requires a high energy radiation sources such as UV
radiation, X-rays, or ion beams. Bruck also describes structuring
simple geometrically shaped PTFE, PC or PMMA mold inserts through
machining.
[0009] PCT international application number PCT/US98/02573
(Schueller) describes the fabrication of carbon
microstructures.
[0010] Mat. Res. Innovat. 1999,2,200 (Jungermann, et al.) describes
the synthesis of amorphous Si.sub.2B.sub.2N.sub.5C.sub.4. Nature
1996, 382, 796 (Riedel, et al.) discloses the synthesis of
amorphous silicoboron carbonitride stable up to 2000.degree. C.
[0011] These and other techniques for the use of ceramics,
including the production of small-scale devices, are useful in some
circumstances. However, these techniques typically involve more
than a desirable number of fabrication steps, and in many cases it
would be advantageous to reduce the cost, and increase versatility,
associated with these techniques. Additionally, micromachining is
an expensive technique requiring specialty equipment.
[0012] Accordingly, it is an object of the invention to provide a
technique for forming ceramic solid structures on the micron scale
conveniently, inexpensively, and reproducibly.
SUMMARY OF THE INVENTION
[0013] The present invention provides techniques for forming
ceramic solid structures. In one aspect, the invention provides a
method which involves filling an elastomeric mold with a precursor
of the ceramic solid structure. The ceramic precursor, in one
embodiment, is characterized as a viscous fluid where the viscosity
can be attained by setting the precursor and allowing it to
maintain its shape. The method can also involve removing the mold
from a product of the ceramic precursor. In one embodiment the mold
is removed by physically separating the mold from the ceramic
precursor. In an alternate embodiment the mold is removed by
dissolving the mold. In yet another embodiment, the mold is treated
to aid removal of the mold from the ceramic precursor. In a further
embodiment, treatment can involve silanizing a surface of the mold.
The filling step comprises, according to one embodiment, filling
the ceramic precursor between the surface of the mold and a
substrate. The mold then is removed from the product of the ceramic
precursor and the ceramic precursor remains on the substrate. In a
further embodiment, the ceramic precursor is removed from the
substrate and a free-standing ceramic solid structure is formed.
The free-standing structure is rigid enough to maintain its shape
without support along all surfaces of the structure. For example, a
small portion of the structure can be held with a support such as a
clamp or sharp tweezers.
[0014] In another embodiment, the invention provides a method that
includes filling the mold with the precursor of the ceramic solid
structure, and setting the ceramic precursor. A solid structure, in
a shape of the mold, is thereby formed. This method can be carried
out using the technique described above, namely, forming the solid
structure against the surface of the mold, and can involve other
described steps. In one embodiment, the ceramic precursor is set
thermally. In another embodiment, the ceramic precursor is set by
curing chemically.
[0015] In a further embodiment, a method of the invention involves
filling, simultaneously, at least two ceramic solid structures from
ceramic precursors of the structures. This can involve forming the
ceramic precursors against at least two indentations in a surface
of a mold, and allowing the ceramic precursors to solidify against
the at least two indentations. The precursors then can be heated to
form the at least two ceramic solid structures.
[0016] In the above methods, one or more ceramic precursors can be
placed against one or more surfaces of the mold (or one or more
indentations in a surface that define two or more molds), setting
the ceramic precursor, removing the mold, and thermally setting by
heating the precursor to form the ceramic solid structure. The
method can involve heating the ceramic precursor to form the
ceramic solid structure having a dimension of less than 100
.mu.m.
[0017] In another embodiment, a method of the invention involves
applying the ceramic precursor of the ceramic solid structure to an
indentation pattern in the surface of an elastomeric mold, applying
the elastomeric mold to a surface of a substrate to encapsulate the
ceramic precursor between the substrate surface and the indentation
pattern, and curing the ceramic precursor. Then, the mold is
removed from the substrate and from the ceramic precursor. The
ceramic precursor then is heated to form a free-standing ceramic
solid structure having a dimension of less than 100 .mu.m.
[0018] According to another aspect, articles are provided in
accordance with the invention. In one embodiment, the invention
provides a free-standing, ceramic solid structure having a
dimension of less than 100 .mu.m. In another embodiment, the
dimension is less than 50 .mu.m, and in yet another embodiment less
than 25 .mu.m. In other embodiments the structure has a dimension
of less than 15 .mu.m, less than 10 .mu.m, less than 5 .mu.m, or
less than 1 .mu.m.
[0019] The invention provides, according to another embodiment, an
article comprising a hexagonal grid.
[0020] According to another aspect, a microgear is provided in
accordance with the invention.
[0021] In yet another aspect, the invention provides a method that
involves forming a ceramic solid structure that is a replica of a
template structure. The method involves forming the ceramic
precursor of the ceramic structure against the surface of a mold
cast from the template, and allowing the precursor to take the form
characterized by the mold.
[0022] Other advantages, novel features, and objects of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings, which are schematic and which are not
intended to be drawn to scale. In the figures, each identical or
nearly identical component that is illustrated in various figures
is represented by a single numeral. For purposes of clarity, not
every component is labeled in every figure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1A illustrates schematically a technique for forming a
ceramic solid structure against a surface using a mold, from a
ceramic precursor;
[0024] FIG. 1B shows another scheme for forming a ceramic solid
structure against a surface using a mold, from a ceramic
precursor;
[0025] FIG. 2 is a schematic illustration of a technique for
transferring a ceramic precursor of a ceramic solid structure from
indentations in a mold to a substrate surface, forming ceramic
solid structures at the surface, and removing free-standing
articles from the surface;
[0026] FIG. 3 is a photocopy of a scanning electron micrograph
(SEM) image of a free-standing ceramic structure;
[0027] FIG. 4 is a photocopy of a SEM image of a free-standing,
ceramic structure;
[0028] FIG. 5 is a photocopy of a SEM image of a free-standing,
ceramic structure; and
[0029] FIG. 6 is a photocopy of three SEM image of a free-standing,
ceramic solid structure, the structure being a micron scale tooth
of a gear with an aspect ratio of approximately 3-4 to 1.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention provides techniques for forming
ceramic solid structures from ceramic precursors of these
structures, using elastomeric molds, and structures produced
according to the techniques. A variety of structures useful in
microanalytical, microelectronic, micromechanical, chemical,
medical and biomedical fields can be produced according to the
techniques. Furthermore, the present invention provides techniques
for easily reproducing intricate microscale ceramic solid
structures consistently.
[0031] The ceramic solid structures of the invention, for example
borosilicon carbonitride, can be used in applications where light
weight, high chemical resistance, high oxidative resistance, and/or
high temperature thermal stability are required properties. There
are a small number of materials such as silicon carbide, silicon
nitride, and metal alloys which can maintain their mechanical
strengths at high temperatures, typically exceeding 1300.degree. C.
Silicon carbide (SiC) and silicon nitride (Si.sub.3N.sub.4) are the
most commonly explored materials for high temperature mechanical
applications. However, silicon carbide, silicon nitride, and metal
alloys suffer several drawbacks at high temperatures. Silicon
carbide and silicon nitride are difficult to fabricate, undergo
phase transformations, and degrade mechanically at temperatures
above 1400.degree. C. Metal alloys, such as nickel aluminum-based
materials, used for high temperature mechanical applications are
also not effective at high temperatures. Metal alloys usually fail
structurally or melt at temperatures above 1395.degree. C.
[0032] On the other hand, ceramics, such as borosilicon
carbonitride, can maintain their structural integrity at
temperatures exceeding 2000.degree. C. in an inert atmosphere. In
air, ceramics have excellent oxidative stability and are able to
withstand temperatures greater than 1050.degree. C. for longer than
two hours. It has also been shown that ceramics can sustain their
oxidative stability in air at temperatures up to 1500.degree. C.
Even under these extreme conditions, ceramics maintain their
density. For example, borosilicon carbonitride, maintains its low
density of approximately 1.8 grams per cubic centimeter at high
temperatures. In addition, when ceramics are exposed to harsh
environments, they are able to maintain their structural integrity.
One indicator of a ceramic's ability to maintain its structural
integrity is its Young's modulus. The Young's modulus is a measure
of strain, expressed by the ratio of a stress on a given unit of a
substance to an accompanying distortion. It is a feature of the
invention that the Young's modulus does not change significantly
when a ceramic solid structure of the invention is exposed to harsh
environments.
[0033] Another test to determine the oxidative stability of a
ceramic solid structure is to expose the ceramic solid structure to
an oxidative environment at high temperature. The ceramic article
is then cooled down to 25.degree. C. Next, a scanning electron
micrograph (SEM) is taken of the ceramic solid structure. The
absence of structural defects appearing on the ceramic solid
structure as seen on the SEM image is a good indication of the
thermal and oxidative structural integrity of the ceramic solid
structure.
[0034] The invention utilizes an elastomeric mold comprising an
indentation pattern that can be used to transfer a ceramic
precursor of a ceramic structure from the mold to a substrate
surface or that can serve as a mold that when, positioned proximate
to a surface of the substrate, can define a region in which the
ceramic precursor is positioned. As used herein, "elastomer"
describes a compound which exhibits rubbery properties. In other
words, an elastomeric compound will recover most of its original
dimensions after extension or compression.
[0035] The substrate can be any surface known in the art and can
even be another mold.
[0036] In one embodiment, the ceramic precursor is comprised of
three or more different atom types and preferably at least four
different atom types such that a quarternary ceramic results. An
illustrative example of atoms types which can be found in the
ceramic precursor are: carbon, nitrogen, boron, silicon,
phosphorus, aluminum, and/or hydrogen. The ceramic precursor can be
monomeric, oligomeric or polymeric.
[0037] FIG. 1 illustrates schematically a technique for forming the
ceramic structure at the substrate surface. An article 20, which
serves generally as the elastomeric mold, includes an application
surface 22 including a plurality of indentations 24 that together
define a linear, patterned array of indentations contiguous with a
contact surface 26. In one embodiment, after forming article 20,
article 20 is treated, for example by silanization with an agent
that is not reactive with the ceramic precursor. This allows
removal of the mold from a resulting solid without causing
destruction of the solid. For example silanization prevents the
ceramic structure 38 from adhering to article 20. This treatment
can be accomplished with a variety of agents, such as alkylating,
silylating, fluoroalkylating, or alkylsilylating agents. Article
20, according to one embodiment, is a mold or an applicator used to
transfer the precursor of the ceramic structure, in a linear
pattern, to a region or regions proximate the substrate surface.
Article 20 also can define a forming article or micromold placed
proximate the substrate surface and used to guide the ceramic
precursor of the ceramic structure so as to position the ceramic
precursor in a pattern at a predetermined region or regions
proximate the substrate surface. As used herein, the term
"proximate" is meant to define at the substrate surface, that is,
in contact with the substrate surface, or at a position near the
substrate surface and fixed relative to the substrate surface.
[0038] When article 20 is placed proximate a surface 28 of a
substrate 30, contact surface 26 of the article seals portions of
the surface 28 that it contacts, thereby forming channels 32
defined by indentations 24 and portions 34 of the substrate surface
28 not contacted by the contact surface 26. In this manner a
micromold is created, which is defined by the article 20 and the
substrate surface 28. A ceramic precursor 36 of a ceramic structure
is placed adjacent one or more openings of the channels 32 and
introduced into the channels 32 and allowed to flow adjacent
portions 34 of the substrate surface 28 in register with the
indentations 24. The ceramic precursor 36 can be urged to flow via,
for example, pressure applied to the ceramic precursor as it is
positioned so as to enter the channels, or vacuum created within
the channels by, for example, connection of the outlets of the
channels to a source of vacuum. Alternatively, according to one
aspect of the invention, the ceramic precursor can be allowed to
flow into the mold via capillary action. Capillary filling of the
mold is especially useful when the mold is of very small dimension
(in particular in the micro scale) and is defined herein to mean
that when a ceramic precursor is positioned adjacent an opening or
the channel 32 formed by a portion 34 of the substrate surface and
the indentation 24 of article 20, the ceramic precursor will flow
into at least a portion of the channel spontaneously. In alternate
embodiments, the filling step is performed under an inert
atmosphere and/or moisture-free atmosphere. As used herein, "inert"
means almost entirely or entirely unreactive. In addition, as used
herein "moisture free" means an environment which has less than
0.1% water or even up to less than 1% water.
[0039] Subsequent to introduction of the ceramic precursor 36 into
channel 32, the ceramic precursor 36 can be solidified, or cured,
before removal of article 20 from the substrate surface 28. The
curing step is generally carried out to partially or fully
cross-link the ceramic precursor, or polymerize a precursor that
can include monomeric or oligomeric species. In one embodiment, the
thermal setting or curing temperature is greater than 100.degree.
C. In another embodiment, the ceramic precursor is cured
chemically. When cured chemically, the curing step can be performed
under an inert atmosphere and/or under a moisture free
environment.
[0040] The article 20 can be removed by peeling article 20 off of
the cured ceramic precursor 36. In an alternate embodiment, article
20 is removed from the cured ceramic precursor 36 by dissolving
article 20. In a further embodiment, article 20 is dissolved using
a solution containing a fluoride anion provided by compounds such
as tetrabutylammonium fluoride. The cured ceramic precursor 36 can
then be heated usually at a temperature greater than the thermal
curing temperature. In an alternate embodiment, the cured ceramic
precursor 36 can by heated without removing the article 20. It is
during the heating step that two changes can occur to the cured
ceramic precursor. First, the ceramic precursor undergoes a
polymerization reaction. Second, the ceramic precursor will be
pyrolyzed. As used herein, "pyrolyzation" means treatment of a
ceramic precursor to reaction conditions, such as high temperature.
Under these reaction conditions, a material which undergoes
pyrolysis may have any one or a combination of any of solvent,
gases, heteroatoms, and/or functional groups removed. In addition,
ceramics which undergo pyrolysis may undergo a reorganization of
the atomic matrix. After heating, the article 20 is removed from
the heated ceramic precursor 36.
[0041] The pattern of parallel indentations 24 formed in the
surface 22 of the article 20 is for illustrative purposes only. Any
pattern, for example a pattern defined by a single indentation or
many indentations, one or more of the indentations defining a
non-linear pathway of uniform or non-uniform depth, is intended to
fall within the scope of the invention. Various patterns are
illustrated in subsequent figures. The indentation pattern can be
of a variety of dimensions and, according to one aspect of the
invention, includes a region having a dimension of less than 100
.mu.m. The dimension can be a lateral dimension or vertical
dimension. "Lateral dimension" is meant to define a dimension
parallel to application surface 22. "Vertical dimension" is meant
to define a dimension perpendicular to the surface. A lateral or
vertical dimension can be a dimension of a feature that is a
portion of an overall structure. That is, an overall structure
could have a width of 5 millimeters parallel to the surface and a
height of 5 millimeters relative to the surface, but include a
ridge on the top of the structure of 75 .mu.m height and 75 .mu.m
width. That sub-feature would, by definition, have a lateral
dimension of 75 .mu.m and a vertical dimension of 75 .mu.m.
According to preferred embodiments, the indentation pattern
includes a portion having a dimension of less than about 100 .mu.m,
less than about 50 .mu.m, less than about 30 .mu.m, less than about
20, 15, 10, 5 or 1 .mu.m. The dimension of the indentations can be
altered by deforming the article 20.
[0042] Where the article 20 is placed adjacent to the substrate
surface 28 and the ceramic precursor fills channels 32, ceramic
structure or structures 38 resulting from the technique can have
lateral dimensional features that correspond to the lateral
dimensional features of indentations 32 of the article. For some
structures, it is not necessary that the ceramic precursor
completely fill the channels 32 and some minute flaws may be
tolerated. In other embodiments in which channels 32 may not be
completely filled, the lateral dimension of ceramic article 38
formed from the ceramic precursor is to be minimized. According to
this embodiment, ceramic precursor 36 is introduced into the
channels 32 in an amount small enough that the ceramic precursor
wets only the corners of the channels. When the ceramic precursor,
substrate, and article are selected such that the ceramic precursor
will wet the micromold efficiently via capillary action, when a
small amount of ceramic precursor is supplied to the article
channel or channels, the ceramic precursor will selectively wet
portions of the channels having a low interior angle relative to
the rest of the channel (such as corners 39 defined by the abutment
of contact surface 26 against the substrate surface 28 at the edge
of region 34 of the substrate surface). When the ceramic precursor
wets the corners selectively and the ceramic precursor is heated to
form the ceramic structure, the resulting structure can define a
pattern having a dimension smaller than that of the lateral
dimension of indentation 24. According to this embodiment the
lateral dimension of structure 38, at its narrowest, is narrower
than the narrowest lateral dimension of channel 24 of the article,
and can have a height significantly less than the height of the
channel. The lateral dimension of the ceramic structure 38
according to this embodiment can be on the order of less than or
equal to about 100 .mu.m or 50 .mu.m, or preferably less than about
20 or 10 .mu.m, more preferably less than about 5 .mu.m or 1
.mu.m.
[0043] Any suitable material can define the substrate 30 of the
invention. The substrate surface 28 can be of the same material as
the bulk material of substrate 30, or a different material.
Substrates exposing a variety of functional surfaces such as
hydrophobic, hydrophilic, and biologically compatible or
non-compatible surfaces are known, and are suitable for use with
the invention. In other embodiments, free-standing ceramic
structures are formed. In particular, a silicon wafer carrying a
fluorosilane film is particularly useful. Other substrate materials
include silicon, silicon dioxide, silicon nitride, polymers and
metals such as gold and chromium. In a further embodiment, the
substrate is treated so that it is inert. Article 20 similarly can
be formed of a variety of materials. According to one embodiment,
substrate surface 28 and/or contact surface 26 of article 20 is an
elastomer or other conformable material. Preferably, contact
surface 26 and more preferably, for ease of fabrication, the entire
article 20, is formed of an elastomer, most preferably polydimethyl
siloxane (PDMS). When the elastomer defines substrate surface 28 or
contact surface 26, or preferably article 20, an optimal seal is
created between contact surface 26 and portions of the substrate
surface 28 adjacent and contiguous with portions 34 that with the
indentations 24 define the channels 32. This results in optimal
confinement of the ceramic precursor 36 to the channels 32.
According to the invention pressure can be applied to the article
20 against the substrate 30 during micromolding, but according to
embodiments in which an elastomer is used as described, pressure
need not be applied as the elastomer conforms well to the surface
against which it mates thus sealing the channels 32. The article 20
can be fabricated of an elastomer in a manner analogous to the
fabrication of a stamp from an elastomer as described in
co-pending, commonly-owned U.S. application Ser. No. 08/131,841 by
Kumar, et al, entitled "Formation of Microstamped Patterns on
Surfaces and Derivative Articles", filed 4 Oct. 1993, and
incorporated herein by reference.
[0044] FIG. 2 illustrates another embodiment of the invention in
which, rather than forming ceramic structures via micromolding, a
microtransfer molding technique is used to transfer a set of
patterns, in parallel, to a substrate surface 28 (parallel molding
of a variety of similar patterns via micromolding as illustrated in
FIG. 1 is embraced by the invention as well). In FIG. 2 a transfer
article 40, which can be made of the same material from which
article 20 is made, includes a plurality of indentation patterns 42
that can be identical. Application of the ceramic precursor 36 to
the plurality of indentation patterns 42, followed by positioning
the indentation patterns of the applicator adjacent the substrate
surface 28, results in transfer of the ceramic precursor, in the
pattern, to the substrate surface 28. Heating can take place prior
to or after removal of the article 40 from the substrate surface
28, preferably after removal of the article following brief, mild
heating. Specifically, ceramic precursor 36 is applied to the
indentation patterns 42 preferably by coating the surface 41 with
ceramic precursor 36, spreading the ceramic precursor back and
forth across the surface 41 with an applicator to assure filling of
each of the indentations 42, and removal of excess ceramic
precursor until a small drop of the ceramic precursor is left
essentially in the center of the indentation pattern. Then, the
surface 41 of the applicator is brought into contact with the
substrate surface 28 to transfer the ceramic precursor, in the
pattern, to the substrate surface 28. At this point, the substrate
and applicator are, together, gently thermally cured to, for
example, 150.degree. C. by gradually raising the temperature for
one hour, and then the applicator 40 removed, leaving a plurality
of pre-heated ceramic precursors 44, arranged in patterns
corresponding to the indentation patterns 42 of applicator 40, on
the substrate surface 28. Following heating under an inert
atmosphere, ceramic microstructures 46 can be allowed to remain on
the substrate surface 28, or can be removed from the substrate
surface 28 to define free-standing articles.
[0045] According to the embodiment illustrated in FIG. 2, the
ceramic precursor 36 of the ceramic articles 46 is transferred
essentially instantaneously to the substrate surface 28, in the
indentation pattern 42.
[0046] As used herein, "ceramic solid structure" includes a class
of materials comprising inorganic, nonmetallic solids which can
withstand high temperatures during manufacture and use. An example
of a ceramic solid structure would be borosilicon carbonitride
(SiBNC).
[0047] Borosilicon carbonitride (SiBNC) can be synthesized from
several preceramic polymers. Borosilicon carbonitride ceramics have
several beneficial characteristics which make them useful for
applications under high temperature conditions. Borosilicon
carbonitride materials are light weight and stable at temperatures
exceeding 2000.degree. C. in an inert atmosphere. They also show
excellent resistance to oxidation at temperatures up to
1500.degree. C. in air.
[0048] In one embodiment, the ceramic article demonstrates
structural integrity when exposed to harsh oxidative and thermal
conditions. In another embodiment of the invention, the ceramic
article is structurally stable when exposed to harsh oxidative and
thermal conditions as demonstrated by an SEM analysis of the
ceramic article. In a further embodiment, the ceramic article
demonstrates its structural integrity upon being exposed to air for
two hours at 1050.degree. C. as demonstrated by SEM analysis of the
ceramic article.
[0049] In a preferred embodiment, the ceramic article's Young's
modulus will not change more than 10% upon heating to 1400.degree.
C. in an inert atmosphere. In another preferred embodiment, the
ceramic article's Young's modulus will not change more than 10%
upon heating to 1400.degree. C. in air. In a further preferred
embodiment, the Young's modulus at 25.degree. C. will not change by
more than 10% upon heating the ceramic article to 1400.degree. C.
followed by cooling the ceramic article down to 25.degree. C.
[0050] A simple screening test for determining useful precursors of
ceramic materials of the invention involves providing a known
precursor and testing its viscosity to determine whether it can
fill a mold having dimensions on the order of dimensions desired in
a final ceramic structure. If the material is too viscous, it can
be determined whether viscosity can be lowered by dilution with a
suitable solvent, preferably a low-boiling solvent that can be
driven off via gentle heating prior to pyrolyzation and
condensation. In another embodiment, viscosity of the ceramic
precursor is controlled by a reaction with ammonia. In an alternate
embodiment, a ceramic precursor viscosity of less than 500
cm.sup.2/s is sufficient.
[0051] FIG. 3 is a photocopy of two SEM images of the same
free-standing, ceramic solid structure 46, fabricated in accordance
with the technique of FIG. 1. By use of the term "free-standing",
it is meant that the structure need not be supported by a
substrate, and is not an integral part of a substrate. The
structure has a variety of different feature sizes. Structure 46 is
a hexagonal grid with a vertical dimension of approximately 50
.mu.m and a lateral dimension of approximately 20 .mu.m.
[0052] FIG. 4 is a photocopy of three SEM images of the same
free-standing, ceramic solid structure 66. However, the ceramic
structure of FIG. 4 has been heated in air to 1050.degree. C. for
two hours. FIG. 4 demonstrates the high oxidative stability of
ceramics at high temperatures as no obvious oxidative defects are
seen in the SEM images.
[0053] FIG. 5 is a photocopy of two SEM image of another
free-standing, ceramic solid structure 66. Structure 66 is a
hexagonal grid with a lateral dimension of approximately 70
.mu.m.
[0054] FIG. 6 is a photocopy of three SEM image of the same
free-standing, ceramic solid structure 76. Structure 76 is a micron
scale tooth of a gear with an aspect ratio of approximately 3-4 to
1.
[0055] The technique of the invention can be carried out on
nonplanar, for example, curved surfaces when a flexible or
elastomeric applicator (20/40) is used. Where a nonplanar substrate
28 exists, a flexible or elastomeric applicator 20 or 40 can be
bent to conform to the nonplanar substrate surface 28 and the
technique carried out as described. Where the ceramic precursor 36
of a ceramic solid structure is viscous enough, the nonplanar
substrate, such a cylindrical article (capillary or the like) can
be rolled across the surface 41 or 22 of applicator 40 or 20,
respectively, the pattern transferred to the nonplanar article, and
the ceramic structure heated on the surface of the article.
[0056] Example ceramics that can be made in accordance with the
invention include SiCN, SiC, SiN, BN and SiAlCN.
[0057] The function and advantage of these and other embodiments of
the present invention will be more fully understood from the
examples below. The following examples are intended to illustrate
the benefits of the present invention, but do not exemplify the
full scope of the invention.
EXAMPLE 1
Fabrication of Ceramic Structure Molds
[0058] The photomasks used for the fabrication of masters were
transparency films having opaque patterns of features printed by a
high-resolution printer (.gtoreq.3386 dpi). The masters were
fabricated using SU-8 photoresist (MicroChem Corp., Newton, Mass.)
on silicon wafers (Test grade, Silicon Sense, Inc., Nashua, N.H.)
by conventional photolithographic techniques. The bas-relief
masters of SU-8 were silanized in a dessicator that was connected
to a vacuum, and contained a vial with small amount of
(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane. The
silanization was used to prevent adhesion of PDMS to the masters.
Casting prepolymer, Sylgard 184 (Dow Corning, Midland) on these
silanized masters and curing at 70.degree. C. formed the PDMS molds
with bas-relief structures as negative replicas of the structures
in the SU-8. The PDMS molds were heated at 160.degree. C. in an
oven for .about.2 h to complete the cure. The surface of the PDMS
mold was oxidized with an oxygen or air plasma in a plasma cleaner
(SPI Plasma PREP II.TM.; www.2spi.com) at a pressure of .about.1 mm
Hg for 2 min. These molds were then exposed to vapors of
(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane for
.about.12 h to obtain hydrophobic surfaces. Polished silicon wafers
were used as substrates and treated with oxygen plasma and
silanized by the aforementioned reagent prior to use.
EXAMPLE 2
Synthesis of Polymeric Ceramic precursor
[0059] All syntheses were performed under an argon atmosphere in
Schlenk-type glassware. The polymeric precursor was synthesized
from 1-(trichlorosilyl)-1-(dichloroboryl)ethane (TSDE). The
duration of the polymerization was adjusted under an ammonia
atmosphere in order to obtain a preceramic polymer with a suitable
viscosity for microtransfer molding (.mu.TM) and vacuum assisted
micromolding in capillaries (MIMIC) at temperatures of
120-140.degree. C. The product was a colorless transparent
glass-like solid at room temperature. At 120.degree. C., the
preceramic polymer has a viscosity similar to that of thick
honey.
EXAMPLE 3
Synthesis of 1-(Triclororosilyl)-1-(dichloroboryl)ethane (TSDE)
[0060] Boron trichloride (99.9%, Aldrich, 0.1 mol, .about.9 mL at
-78.degree. C.) was condensed at -65.degree. C. in a round-bottom
flask equipped with an addition funnel. Trichlorovinylsilane (97%,
Aldrich, 0.1 mol, .about.13 mL) was pre-mixed with triethylsilane
(99%, Aldrich, 0.1 mol, .about.16 mL), and added dropwise to
BCl.sub.3 over 15 min. The mixture was stirred for one hour and
then allowed to warm to room temperature. The reaction mixture was
distilled using a Vigreux column at .about.40.degree. C./12 mbar to
obtain TSDE. The product is a colorless liquid at room temperature
and extremely sensitive to moisture and air. (Caution: BCl.sub.3 is
highly corrosive and should be handled with care.)
EXAMPLE 4
Synthesis of the Preceramic Polymer
[0061] TSDE (.about.0.06 mol) was added dropwise to a solution of
.about.60 mL of methylamine (anhydrous, 98+%, Aldrich) in
.about.180 mL of dry hexane (100.0%, A.C.S. reagent, J. T. Baker,
Phillipsburg, N.J.) at -65.degree. C. under argon. The mixture was
allowed to react overnight under vigorous stirring. After reaction
was complete, the mixture was warmed to room temperature, and a
white solid was removed by filtration using house vacuum. Hexane
was removed by distillation from the filtrate using a rotary
evaporator (Model: R110, Buchi) under moderate vacuum (.about.20 mm
Hg). This polymer was heated for 1-2 h at 100.degree. C. under an
atmosphere of ammonia, followed by degassing under vacuum (.about.3
mm Hg) for 30 min. Upon cooling to room temperature, a colorless
glassy solid was obtained and stored under argon.
EXAMPLE 5
Fabrication of Microstructures of SiBNC Ceramics
[0062] The manipulations were conducted in a glove bag under argon.
The fabrication of preceramic microstructures using .mu.TM involves
the following steps: (i) Silanized PDMS molds were placed on a hot
plate with their structured surface facing upward. The temperature
of the hotplate was set at 120.degree. C.; (ii) A flask containing
preceramic polymer 1 was heated on the hotplate. After this polymer
melted into a viscous liquid, it was transferred onto the top of
the molds using a spatula; (iii) After the polymer filled the voids
of the PDMS mold, a silanized silicon wafer was placed on the top.
A pressure of .about.5 psi (.about.3.times.10.sup.4 Pascals) was
applied on the wafer using a weight; (iv) The temperature of the
hotplate was then raised to 200.degree. C. at a rate of 2.degree.
C./min. The polymer was cured at this temperature for 1 h and
cooled to room temperature on the hotplate. An alternative process
used vacuum assisted MIMIC. In this method, a PDMS mold was placed
directly against the surface of a silanized silicon substrate on a
hotplate. The preceramic polymer was melted at 120.degree. C. and
transferred to the entrance of the PDMS mold using a spatula. The
channels were filled with the polymer by applying vacuum in the
channels and by capillary flow. The hotplate was heated to
200.degree.C., held at that temperature for 1 h, and cooled to room
temperature.
[0063] Once the hotplate was cooled down, we first tried to peel
off the PDMS mold from the silanzied substrate and from the
preceramic microstructure. The mold could sometimes be removed from
the substrate, leaving the preceramic microstructures still
attached to the silicon substrate. In these cases, the preceramic
structures along with the substrate were transferred into a quartz
tube furnace for pyrolysis and condensation. When the PDMS molds
could not be peeled off from the polymeric microstructures, we used
a 1.0 M TBAF solution in THF (Aldrich) to dissolve the PDMS mold.
In this case, the microstructure and the PDMS mold were normally
released from the silanized substrate first. The preceramic
microstructure became freestanding after the dissolution of the
PDMS mold by the TBAF solution. This freestanding preceramic
structure was placed on top of a silicon substrate and transferred
into the quartz tube furnace for pyrolysis and condensation.
[0064] The conversion of polymeric microstructures into ceramics
started with the purge of the samples positioned in the center of a
quartz tube with argon for .about.15 min. The temperature of the
furnace (model: TF55035A, Linderberg/Blue) was raised to
1050.degree. C. at a rate of 1.degree. C./min and held for 2 h at
that temperature. After the heating, the furnace was cooled to room
temperature at a rate of 2.degree. C./min. If the preceramic
structures were freestanding, the final ceramic structures could be
easily removed from the silicon substrate. Some microstructures
that were initially attached to the silanized silicon substrate
released spontaneously. The test of thermal stability of the
ceramic microstructure was performed using a freestanding
microstructure of hexagonal honeycomb grid following a similar
heating procedure but in air.
[0065] Those skilled in the art would readily appreciate that all
parameters listed herein are meant to be exemplary and that actual
parameters will depend upon the specific application for which the
methods and apparatus of the present invention are used. It is,
therefore, to be understood that the foregoing embodiments are
presented by way of example only and that, within the scope of the
appended claims and equivalents thereto, the invention may be
practiced otherwise than as specifically described.
* * * * *
References