fabrication of sicn ceramic mems using injectable polymer-precursor technique
Post on 04-Jul-2016
Embed Size (px)
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