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This section focuses on two processes that can produce high aspect ratio (> 100:1), batch-fabricated components that can be integrated with IC-based wafer processing: HEXSIL and LIGA (Lithographie, Galvanoformung, Abformung).
The HEXSIL method of producing high-aspect-ratio parts (mentioned in the taxonomy section of Chapter 2) involves a combination of DRIE and surface micromachining techniques. HEXSIL combines HEXagonal honeycomb geometries for making rigid structures with thin-films and SILicon for surface micromachining and CMOS electronics. The trenches serve as reusable molds that can be sequentially filled with polysilicon and sacrificial layers of oxide. After patterning and the removal of sacrificial layers, structural members with large lateral dimensions (ranging up to centimeters) can be formed from arrays of polysilicon honeycombs. Thus, through the HEXSIL process, batch processing of thin-film layers can be used to produce elements that form a transition between the millimeter and micrometer worlds.
An example of HEXSIL is a pair of tweezers that can pick up particles ranging roughly from 1 to 25 µm and place them on platforms (also made of HEXSIL) under operator control (Figure 3-1; Keller and Howe, 1997). This basic process has also been combined with nickel plating to produce highly conducting regions on the HEXSIL plates for contacts and conducting patterns. Thermal expansion of resistively heated HEXSIL regions has been used to actuate HEXSIL structures, such as the tweezers. Using interconnected levers, the very tiny expansion in polysilicon beams can be multiplied to produce multiple millimeters of motion (Keller and Howe, 1995).
The HEXSIL process is an interesting example of the way high-cost machines and processes (e.g., DRIE) can support a major leap forward in MEMS. The development costs for trench etchers were paid by IC producers who saw ways to increase the density of semiconductor-memory arrays by adding a third dimension on the chip. For MEMS, DRIE etchers promise nearer-term, silicon-compatible processing of high-aspect-ratio structures. As this promise becomes a reality, MEMS-specific DRIE machines can be expected to evolve. By the same reasoning, fine-structured, nonplanar metal-film plating apparatus and techniques for reliable deep trench film coating and etching will also be mastered. Because HEXSIL currently uses IC-based technologies, compatibility with these technologies is not an issue. An important area of research required to make HEXSIL a designable and versatile MEMS process, however, is the establishment of the basic mechanical properties (e.g., internal stress, Young's modulus, fatigue strength) of polysilicon so that it can be qualified for new applications.
Small, precision-metal components have historically been produced by serial methods, such as computer numerical controlled (CNC) milling or micro-electron discharge machining (micro-EDM). Although serial methods are capable of producing high-precision parts in a variety of metals, they can result in high per-piece costs or part-to-part variations. To drive down the cost of high-precision parts to a level supportable by general systems use, batch-fabrication methods need to be developed.
LIGA (Becker et al., 1986) was developed at the German nuclear research center, the Kernforschungszentrum Karlsruhe, for the production of high-precision, high-aspect-ratio parts in a batch-processing environment. LIGA utilizes x-ray exposure of a resist film, typically polymethylmethacrylate (PMMA), followed by electroplating into the template produced by the exposure to yield a primary metal part. This metal part can be either the final device or, if multiple plastic or metal copies are desired, the master for an injection mold. Figure 3-2 illustrates a basic LIGA process. Figure 3-3 shows metal and plastic parts produced using LIGA.
The LIGA method generally uses nickel or permalloy (NiFe) as the electro deposited material. Subsequent injection molding usually uses plastics. Multilevel LIGA enables fabrication of components from more than one material or material type for bimorphic applications or friction and wear reduction.
Using multilevel exposure techniques in LIGA processing, which was demonstrated recently (Guckel et al., 1996b), provides for the formation of complex structures, the integration of multiple material layers, and some degree of batch assembly. Multilevel LIGA has a number of added processing requirements, including planarization of the sequentially electroplated layers, adhesion of PMMA to the planarized levels, alignment of the layers, and electroplating of multiple layers.
Compatibility and Manufacturing Constraints of LIGA
The compatibility of LIGA and silicon processing (IC and MEMS) was demonstrated by Guckel et al. (1989), who produced photodiodes in the silicon substrate as part of a motor-position sensing system, and by the HI-MEMS Alliance in the development of a hydrostat that directly integrates LIGA with bulk micromachining (Egert and Felde, 1995).
Researchers have successfully demonstrated the utility of LIGA for the production of prototype metal and plastic precision parts, but the transition of LIGA into manufacturing level processes has been slow. A number of issues and challenges still face LIGA before it can be accepted and integrated into a manufacturing environment. Although some