Microstructural and mechanical properties investigation of electrodeposited and annealed LIGA nickel structures
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I. INTRODUCTION
LIGA, an acronym of the German words “Lithographie, Galvanoformung, Abformung,” is a microfabrication process in which structural material is deposited into a polymethyl-methacrylate (PMMA) mold realized by deep Xray photolithography.[1,2] The LIGA fabricated components range in dimensions from a few microns to several millimeters, with tolerances of about 1 m; small enough for LIGA to be considered a microelectromechanical systems (MEMS) technology. The fabrication process includes six steps: (1) wafer assembly (the pre-exposed wafer typically contains the following layers: adhesion layer, release layer, adhesion layer, and PMMA resist), (2) exposure of a PMMA resist to X-ray synchrotron radiation, (3) chemical development to define a mold, (4) deposition of material into the patterned mold, (5) planarization of overplated deposited features, and (6) final release of the features from the mold and substrate.[3] The LIGA process produces high-precision small-scale components with flat surfaces and vertical sidewalls that can be assembled to create complex microsystems, such as the
THOMAS E. BUCHHEIT, Technical Staff Member, Mechanical and Reliability and Modeling Department, JOSEPH R. MICHAEL, Technical Staff Member, Materials Characterization Department, and TODD R. CHRISTENSON, Technical Staff Member, Photonics and Microfabrication Department, are with Sandia National Laboratories, Albuquerque, NM 87185. DAVID A. LaVAN, Research Affiliate, is with Harvard–MIT Division of Health Sciences and Technology, Cambridge, MA 02139. STEVEN D. LEITH, Technical Staff Member, is with Platform Development, Hewlett-Packard Co., Corvallis, OR 97330. Manuscript submitted January 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
escapement mechanism illustrated in Figure 1.[4] The microstructure and material properties of LIGA microsystems depend primarily on the processing parameters in the electrodeposition step and postfabrication assembly methods. Thus, the focus of this work is on the effect of electroplating, as the commonly employed method of deposition, and annealing, as the essential part of a diffusion-bonding assembly method[5] on the microstructure and material properties of the nickel MEMS components. Most previous mechanical-properties investigations on materials fabricated from the LIGA process have concentrated on electrodeposition of materials that have a significant successful electroplating history, such as nickel and copper.[2,6,7] Electrodeposited nickel as a LIGA structural material is easy to deposit, has good mechanical and corrosion resistant properties, and can be fabricated with low internal stresses.[9,11] As a result, nickel is currently the predominant structural material used in the LIGA process. Also, several binary-nickel alloys and composites have been successfully electroplated,[7–10] providing the basis for a wide range of alloys than can be used to fabricate LIGA parts. However, electrodeposited nickel suffers from problems associated with most electrodeposited materials:
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