fabrication of sicn ceramic mems using injectable polymer-precursor technique

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  • 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

    Abstract

    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

    1. Introduction

    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 [1] and microcombustors [2] 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 [3], 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 [4]. 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: li.liew@colorado.edu (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 [7]. 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.

    Table 1

    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 [8] 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

    technique.

    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-

    derived ceramics.

    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 t

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