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 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 [4]. 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.

    5. Demolding

    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

    structures.

    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

    s r2ht

    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

    microstructures.

    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

    above.

    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

    bonding interface.

    8. Conclusion

    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-

    temperature applications.

    Acknowledgements

    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

    technique.

    References

    [1] D.M. Burns, V.M. Bright, Optical power induced damage to

    microelectromechanical mirrors, Sens. Actuators A 70 (1999) 614.

    [2] C. Rossi, D. Esteve, C. Mingues, Pyrotechnic actuator: a new

    generation of Si integrated actuator, Sens. Actuators A 74 (1999)

    211215.

    [3] C. Jacob, S. Nishino, P. Pirouz, C.A. Zorman, A.J. Fleischman, M.

    Mehregany, Structural and electrical characterization of single-crystal

    4-inch CVD grown 3C-SiC films, Silicon Carbide Related Mater. 142

    (1996) 429432.

    [4] R. Riedel, G. Passing, H. Schonfelder, R.J. Brook, Synthesis of dense

    silicon-based ceramics at low temperature, Nature 355 (1992) 714

    716.

    [5] L. An, R. Riedel, C. Konetachny, H.J. Kleebe, R. Raj, Newtonian

    viscosity of amorphous silicon carbonitride at high temperature, J.

    Am. Ceram. Soc. 81 (1998) 13491352.

    Fig. 9. Illustration of process steps for polymer-based bonding of SiCN

    parts: (a) two solid polymer parts; (b) bonding parts together with a layer

    of liquid polymer; (c) crosslinked structure, and (d) ceramic multi-layer

    component.

    L.-A. Liew et al. / Sensors and Actuators A 89 (2001) 6470 69

  • [6] R. Riedel, L.M. Ruwisch, L. An, R. Raj, Amorphous silicoboron

    carbonitride ceramics with anomalously high resistance to creep, J.

    Am. Ceram. Soc. 81 (1998) 33413344.

    [7] R. Raj, L. An, S. Shah, R. Riedel, Oxidation kinetics of amorphous

    silicon carbonitride ceramics, J. Am. Ceram. Soc., accepted for

    publication, November 2000.

    [8] H. Lorenz, M. Despont, N. Fahri, J. Brugger, P. Vettiger, P. Renaud,

    High-aspect-ratio, ultrathick, negative-tone-near-UV-photoresist and

    its applications for MEMS, Sens. Actuators A 64 (1998) 3339.

    Biographies

    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-

    temperature environments.

    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-

    tronics packaging.

    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

    Sub-Division.

    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

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