Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique
Post on 04-Jul-2016
Fabrication of SiCN ceramic MEMS using injectablepolymer-precursor technique
Li-Anne Liew*, Wenge Zhang, Victor M. Bright, Linan An,Martin L. Dunn, Rishi Raj
Department of Mechanical Engineering, NSF Center for Advanced Manufacturing and Packaging of Microwave,
Optical, and Digital Electronics (CAMPMODE), University of Colorado, Boulder, CO 80309-427, USA
In this paper, a novel and cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived
ceramics is described. Micro-molds are fabricated out of SU-8 photoresist using standard UV-photolithographic processes. Liquid-phase
polymers are then cast into the molds and converted into monolithic, fully-dense ceramics by thermal decomposition. The resultant ceramic,
based on the amorphous alloys of silicon, carbon and nitrogen, possess excellent mechanical and physical properties for high-temperature
applications. This capability for micro-casting is demonstrated in the fabrication of simple single-layered, high aspect ratio SiCN
microstructures. A polymer-based bonding technique for creating more complex three-dimensional structures is also presented.
# 2001 Published by Elsevier Science B.V.
Keywords: MEMS; High-temperature; Polymer; Ceramic
MEMS for extreme temperature environments have
attracted much attention due to their many potential appli-
cations, such as optical MEMS for high power laser applica-
tions  and microcombustors  for MEMS power
sources. However, the high temperature environments
remain a significant challenge to current MEMS technology.
The fabrication of MEMS for high-temperature applications
is a two-fold problem: selecting suitable refractory materials
and developing appropriate microfabrication techniques.
Traditional microfabrication processes, such as surface
micromachining and LIGA, rely on polysilicon and plas-
tic/nickel as structural materials, respectively. These mate-
rials cannot operate at high temperatures for extended
periods of time. At the same time, state-of-the-art ceramics
that are designed for high-temperature environments cannot
be easily processed using existing microfabrication techni-
ques. Moreover, CVD of SiC , a technique currently
under development for high-temperature MEMS, is time-
consuming and expensive. Also, the planar nature of
CVD prevents the fabrication of complex three-dimensional
structures. Therefore, the development of new materials and
appropriate microfabrication techniques for high-tempera-
ture MEMS is of both scientific and practical interest to the
MEMS community. Fig. 1 compares the aspect ratios
achieved and maximum operating temperature of tradition-
ally-microfabricated MEMS with that of the polymer-
derived SiCN MEMS that are described in this paper.
2. Injectable polymer derived ceramics
A novel fabrication technique for high-temperature
MEMS using an injectable polymer-derived SiCN ceramic
has been developed. This new technology is based on the
recently developed polymer-derived ceramics, which are
bulk ceramics fabricated by the thermal decomposition of
compacted crosslinked polymer powders . The polymer-
derived ceramics are amorphous alloys of silicon, carbon,
and nitrogen (SiCN) which remain thermally stable up to
15008C. The compositions of the new ceramics can bevaried through the use of different polymer precursors,
and can be tailored to produce SiCN with excellent thermal
and mechanical properties. Table 1 compares the physical
properties of SiCN with those of Si and SiC. Youngs
modulus, Poissons ratio, and density are in the same range
as those of SiC and Si. The creep resistance of SiCN is
Sensors and Actuators A 89 (2001) 6470
* Corresponding author. Tel.: 1-303-492-3842;fax: 1-303-492-3498; URL: http://mems.colorado.edu.E-mail address: email@example.com (L.-A. Liew).
0924-4247/01/$ see front matter # 2001 Published by Elsevier Science B.V.PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 5 4 5 - 8
comparable to that of SiC and Si3N4 [5,6], while its oxida-
tion resistance exceeds that of the same materials . In
addition, the thermal shock resistance of SiCN appears very
promising for high-temperature applications.
SiCN may be obtained from liquid- or powder-based
polymer precursors. However, the SiCN obtained from
the powder-route shows relatively low strength and hardness
due to the high porosity of powder-derived ceramics in
general (typically 10 vol.% porosity). In addition, pow-der-processing cannot be easily integrated into existing
microfabrication techniques. Therefore, liquid polymer pre-
cursors were used to develop a novel micro-casting techni-
que for the fabrication of SiCN MEMS structures. Fig. 2
compares the microstructure, strength, and hardness of SiCN
samples obtained through powder- and casting-routes.
Fig. 1. Diagram of microstructure aspect ratio against maximum use
temperature for different MEMS materials and processes.
Comparison of physical properties of SiCN, Si, and SiC
SiCN Si SiC
Density (g/cm3) 2.20 2.33 3.17
E modulus (GPa) 158 163 405
Poissons ratio 0.18 0.22 0.14
CTE 106 (K) 0.5 2.5 3.8Hardness (GPa) 15 11.2 30
Strength (MPa) 250 175 418
Toughness (MPa m1/2) 3.5 0.9 46
Fig. 2. (a) Scanning electron micrographs showing the microstructure of polymer-derived SiCN from powder-route and (b) from casting-route. This figure
shows that the material resulting from casting is fully dense in that no defects can be seen despite a higher magnification than that in (a). (c) Comparison of
strength and hardness (which are not directly related to each other) of samples obtained from powder- and casting-routes.
L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470 65
3. Microfabrication of SiCN MEMS
Fig. 3 outlines the process flow in the fabrication of SiCN
MEMS. First, a mold is fabricated using standard photo-
lithographic techniques. The liquid polymer precursor is
then cast into the mold, and the mold and polymer precursor
are then heated or thermal-set at 2508C to solidify thepolymer. After thermal-setting, the polymer becomes a
transparent solid, and may be separated from the mold if
suitable techniques are used. After thermal-setting, the
polymer part is crosslinked by heating to 4008C underisostatic pressure. After crosslinking, the polymer becomes
infusible, remaining transparent. In the final stage (pyroly-
sis), the crosslinked polymer part is heat-treated at10008Cto convert it to a monolithic ceramic part.
The commercially-available Ceraset (CerasetTM, Lanxide
Company, USA) was used as the starting polymer. A catalyst
(Dicumyl peroxide, Aldrich Chemical, Milwaukee WI,
USA) is added to lower the thermosetting temperature to
1408C, so that it is compatible with photoresist processing.The mold was made using SU-8 photoresist  from Micro-
Chem Corp. SU-8 is a well-known negative, epoxy-type
photoresist based on EPON SU-8 resin. Its key features are
its ability to produce high-aspect ratio structures and low
optical absorption in the near-UV range, which leads to
straight vertical sidewalls. Other advantages of using SU-8
as the mold are: (1) standard UV-photolithography can
produce high-aspect ratio structures; (2) batch fabrication;
(3) the SU-8 photoresist decomposes during pyrolysis, enabling the release of the structures using the lost mold
Fig. 4 outlines the micro-casting technique in more detail.
First, the photoresist is spun onto a substrate (a). Then, the
photoresist is patterned using standard UV-lithography and
developed, producing cavities of desired shapes (b). The
liquid precursor (Ceraset) is then cast into the cavities by
spinning, and the wafer is thermal-set in an oven for 20 min.
At the end of this step, the Ceraset is solid and there is a thin
layer of it covering the entire wafer (c). This thin top layer of
Ceraset is polished-off (d). The wafer is then crosslinked
under isostatic pressure (e), and during pyrolysis the SU-8
decomposes and the SiCN part no longer adheres to the
substrate (f). The final result is a free-standing high aspect-
ratio microstructure (g).
Of course, this schematic shows only one structure being
fabricated; in reality, multiple structures are fabricated at
once on a wafer, which is one of the attractions to micro-
casting using photoresist molds.
Fig. 5(a) shows an SU-8 mold for a micro-gear. Fig. 5(b)
shows the same mold when filled with the polymer pre-
cursor. For small cavities (less than 20 mm wide), often theliquid does not fully fill the cavity due to the surface tension
of the liquid and/or air bubbles in the mold cavity. This can
be solved by placing the filled mold in vacuum to remove
most of the air bubbles, and then pressing a thin film of
Teflon over the wafer to force the liquid into the cavities.
This film is left on the mold during thermosetting, and alsoFig. 3. General processing steps in the fabrication of injectable polymer-
Fig. 4. Fabrication process for SiCN MEMS.
66 L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470
serves to minimize the thickness of the extra top layer of
Ceraset that needs to be polished-off.
4. Crosslinking and pyrolysis
The key to the success of the casting-route is the applica-
tion of isostatic pressure during crosslinking. The heat-
treatment during crosslinking generates gaseous by-pro-
ducts. This out-gassing may cause the formation of micro-
cracks which disintegrate the sample . In the casting
approach, the applied isostatic pressure provides an obstacle
to the nucleation of bubbles and microcracks. The gaseous
by-products would need to build up sufficient pressure to
overcome the applied pressure before bubbles and micro-
cracks can nucleate. Due to the open structure of the
polymer, the gases will diffuse out of the sample before
they can build up a pressure that is high enough to overcome
the applied pressure. Thus, transparent and defect-free cross-
linked polymers can be obtained by the application of
isostatic pressure during crosslinking. The fact that no
defects can be seen in the final ceramics indicates that
the infusible polymer network has enough strength to with-
stand the further heat-treatment during pyrolysis.
The crosslinked samples are then pyrolyzed at 10008C for8 h to convert the polymer into a ceramic. Very low heating/
cooling rates are used: the heating rate is about 18C/min to4008C and 258C/h to 10008C; the cooling rate is 18C/min to
room temperature. Higher heating rates result in the produc-
tion of most of the gaseous by-products at a narrow tem-
perature range (H2 at 6007008C, and CH4 at 6008C), whichcould generate defects in the pyrolyzed sample and degrade
the mechanical strength of the structures. Examples of
microstructures fabricated from this casting process are
shown in Fig. 6.
Initially during pyrolysis, the SU-8 photoresist mostly
decomposed but left a thick, hard residue on the walls of the
samples, as can be seen in Fig. 6 and also in Fig. 7(a). These
SU-8 by-products, being chemically resistant to etchants and
strongly attached to the SiCN, could not be easily removed
through either chemical or physical means. Attempts to
burn-off the residue at 10008C in an oxygen atmospherewere also not successful. The by-products are the result of
reactions between the liquid Ceraset and the polymer mold
at the interface, producing unknown compounds. Many
attempts were made to de-mold the parts before pyrolysis
in order to prevent the reactions.
One drawback of using SU-8 is that it is not easily
removed, even when using the solvent provided by the
manufacturer. It was found that the SU-8 could be stripped
if the solvent was heated to about 908C. However, beforepyrolysis, the Ceraset is highly reactive and chemically
Fig. 5. (a) Photoresist mold (200 mm diameter and 75 mm thick); (b) mold filled with Ceraset.
Fig. 6. (a) Gear (200 mm diameter and 45 mm thick) made from the mold shown in Fig. 8; (b) cantilever beam (137 mm thick); (c) tensile test sample (142 mmthick).
L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470 67
reacts with many polymers. Various photoresists that could
be easily stripped were experimented with as alternatives to
SU-8, but the highly reactive nature of Ceraset still proved to
be the limiting factor. This problem was finally solved
through a combination of an extended bake and extended
UV-exposure of the SU-8, following the mold fabrication
but prior to casting. This ensured that the SU-8, being a
branched polymer, was completely crosslinked before the
Ceraset was cast into it. Results were encouraging. SU-8
thus remained the material of choice for mold fabrication,
and the lost mold technique was pursued following this
post-fabrication treatment of the mold prior to casting.
Fig. 7(b) shows the marked improvement in surface quality
following this treatment.
Despite the success of the post-fabrication treatment of
the mold in producing SiCN parts that were clean and
residue-free, it is still advantageous to de-mold prior to
pyrolysis, especially if polymer-based bonding (decribed
later in this paper) is to be used to fabricate multi-layer
6. Size limitations
We have studied the size range of samples that can be
successfully cast. It has been found that the samples will
crack at the crosslinking and pyrolysis stages if the length
exceeds a certain critical value, which is a function of the
sample thickness, being larger for thicker samples. Ideally,
there should not be any limit on the sample size, which is
proven by casting in macro-sized Teflon molds that have
been conventionally machined. In this case, the thermally-
set samples are released from the Teflon mold and cross-
linked and pyrolyzed as free-standing samples. The diameter
of these samples can be up to 10 mm (which is limited by the
size of our furnace). The reason for the observed scale limit
for the micro-cast samples is explained with the aid of Fig. 8.
The Ceraset experiences volume shrinkages of 5 and 25%
during crosslinking and pyrolysis, respectively. However, at
the same time the Ceraset adhesion to the silicon substrate
results in a tensile stress, s, in Ceraset structure. This tensilestress in turn induces a shear stress, t, at the silicon/Ceraset
Fig. 7. (a) SiCN cantilever beam encased in SU-8 residue; (b) SiCN gear tooth (75 m thick), obtained from SU-8 mold that had undergone post-fabricationtreatment to prevent residue formation.
Fig. 8. Induced stresses during heat-treatment of Ceraset on silicon.
68 L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470
interface. Assuming a disk-shaped sample, the tensile and
shear stresses are related by
where s increases with the progress of heat-treatment. Let tcand sc be the critical values for which the sample will peel-off from the substrate and fracture, respectively. Then, if the
ratio r/h is too large (which corresponds to thin samples),
then sc will be reached first and sample will crack. Ifthe sample is thick, then tc will be reached first andsample will peel-off from the substrate without cracking.
The problem can be prevented by using crosslinked Ceraset
as the substrate, because it will shrink together with the
7. Polymer-based bonding
Another advantage of using a liquid precursor is the
ability to use it as an adhesive layer to bond two or more
SiCN parts together. This polymer-based bonding tech-
nique thus may be used as a means of fabricating monolithic
multi-layer structures, whose final thicknesses might not
otherwise be achievable due to the size limitations described
The basic bonding process is shown schematically in
Fig. 9. First, two thermal-set or crosslinked solid parts
are fabricated by casting (a). The same liquid polymer is
then spread on the desired location in the same way that glue
is spread on components to be attached. The liquid nature of
the polymer precursor allows atomic level contact at the
interfaces between the adhesive layer and the solid polymer
parts. The thermal-set or crosslinked components are then
aligned and held together (b). The assembled structure is
then thermal-set again to solidify the adhesive layer, and
then crosslinked under isostatic pressure (c) to establish
chemical bonds between the polymer in the adhesive layer
and the polymer in the original components. Therefore, after
crosslinking, the whole structure becomes a solid piece of
polymer without a noticeable interface. This bonded struc-
ture is identical to a single cast piece. The sample is then
pyrolyzed (d). Further examination of parts produced by this
method indicated that there are no observable defects at the
A novel micro-casting process based on injectable poly-
mer-derived SiCN has been developed. The resulting cera-
mic structures exhibit excellent mechanical and thermal
properties, making this process very promising for fabricat-
ing MEMS for high-temperature applications. SU-8 photo-
resist molds enable the use of low-cost microfabrication
facilities. A polymer-based bonding technique can be used
whereby SiCN parts are bonded together to produce com-
plex, three-dimensional monolithic ceramic components.
Compared to the existing MEMS materials and processes,
the new injectable polymer-precursor technique substan-
tially enhances the manufacturability of MEMS for high-
This work is supported by the Defense Advanced
Research Projects Agency (DARPA) and U.S. Air Force
under contract # F30602-99-2-0543. Also, thanks to Mr.
Tsali Cross for developing the polymer-based bonding
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Li-Anne Liew received her BS and MS degrees in Mechanical Engineering
from the University of Colorado at Boulder in 1998 and 2000, respectively.
She is currently a doctoral research assistant in the Department of
Mechanical Engineering at the University of Colorado, Center for
Advanced Manufacturing and Packaging of Microwave, Optical and
Digital Electronics (CAMPMODE). Her research interests are in the
design and packaging of MEMS sensors, MEMS for biomedical
applications, and the design and fabrication of MEMS for high-
Wenge Zhang received his BSME degree from Dalian University of
Technology in Dalian, China, in 1982 and his MS degree in Mechanical
Engineering from the University of Colorado, Boulder, in 1995. He joined
the University in 1991 and is currently a research associate for the Center
for Advanced Manufacturing and Packaging for Microwave, Optical and
Digital Electronics, at the University of Colorado. His research interests
include low-cost prototyping and thermal management of MCMs,
thermosonic flip-chip bonding and optoelectronics packaging. He has 14
years of research experience in mechanical design, computer control
systems and logic circuit design, and 5 years of research experience in
thermosonic flip-chip bonding, MCM substrate fabrication, and optoelec-
Dr. Victor M. Bright is an Associate Professor of Mechanical Engineering
and the Director of the MEMS R&D Laboratory, University of Colorado at
Boulder. Prior to joining the University, he was an Associate Professor and
the Director of the Microelectronics Research Laboratory in the
Department of Electrical and Computer Engineering, Air Force Institute
of Technology, Wright-Patterson Air Force Base, Ohio (June 1992
December 1997). Prof. Brights research includes MEMS, silicon
micromachining, microsensors, microactuators, MEMS self-assembly,
MEMS packaging, opto-electronics, and semiconductor device physics.
Dr. Bright received the following awards in the area of MEMS: Best
paper of the MCM98 International Conference and Exhibition on
Multichip Modules and High Density Packaging, 1998; R.F. Bunshah Best
Paper Award at the 1996 International Conference on Metallurgical
Coatings and Thin Films. Dr. Bright has authored and co-authored more
than 70 papers in the areas of MEMS. He is a member of IEEE, ASME,
and SPIE. He serves on the Executive Committee for ASME MEMS
Dr. Linan An obtained his PhD degree from Lehigh University in the field
of materials science and engineering. His research includes processing of
ceramic materials, microstructural design of ceramics, high temperature
behavior, oxidation and corrosion of ceramic materials, and mechanisms of
fracture in brittle solids.
Dr. Martin L. Dunn is an Associate Professor of Mechanical Engineering.
He received the PhD in Mechanical Engineering in 1992 from the
University of Washington. Prior to joining the faculty at the University of
Colorado, he was a postdoctoral appointee in a solid mechanics group at
Sanida National Laboratories, and a design engineer in a transducers group
at the Boeing commercial Airplane Company. Professor Dunns research
interests include (i) the micromechanical behavior (fracture and deforma-
tion) of materials and structures for microelectromechanical systems
applications, (ii) the micromechanics and physics of heterogeneous media,
including defects, polycrystals, and composites, with emphasis on
piezoelectric solids, and (iii) the acoustic characterization of material
microstructure. His research is based on a strong coupling between
theoretical and experimental efforts. Professor Dunn has published over 60
articles in refereed journals on these subjects and has been the principal or
co-principal investigator on grants and contracts in these areas from NSF,
DOE, ONR, NIST, DARPA, and Sandia National Laboratories.
Dr. Rishi Raj prior to joining C.U. Boulder in 1996, had served on the
Faculty of Materials Science and Engineering at Cornell University for 21
years. He received the PhD in Applied Physics from Harvard University in
1970. His research is in processing of high-temperature metals,
composites, and ceramics. His most significant contributions have been
in understanding how interfaces control high-temperature processing and
mechanical behavior. He has over 200 publications of which 150 are in
refereed journals, and include 14 U.S. Patents. He has been a Guggenheim
Fellow, an Alenxander von Humboldt Senior Scientist Awardee, and a
Matthias Scholar at Los Alamos National Laboratory.
70 L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470