No abstract

Integrating complex microcomponents, let alone microsystems, as monoliths often is not feasible. Microcomponents with parts made of different materials or by different processes can be assembled, however. In this way, closed chambers can be made which contain the movable parts. Assembly and interconnection techniques thus play a significant role in microsystems technology and have major impacts on the quality, reliability, and economic viability of microsystems. Adhesive bonding techniques have been developed and successfully applied in making complex microcomponents on a laboratory scale as well as in a small series production of micropumps. These bonding techniques will be described in this paper, and the great potential of adhesive bonding as a microassembly technique in microsystems technology will be demonstrated by three examples.

Silicon wafers have been anodically bonded to sputtered lithium borosilicate glass layers (Itb 1060) at temperatures as low as 150-180°C and to sputtered Corning 7740 glass layers at 400°C. Dependent on the thickness of the glass layer and the sputtering rate, the sputtered glass layers incorporate compressive stresses which cause the wafer to bow. As a result of this bowing, no anodic bond can be established especially along the edges of the silicon wafer. Successful anodic bonding not only requires plane surfaces, but also is determined very much by the alkali concentration in the glass layer. The concentration of alkali ions as measured by EDX and SNMS depends on both the sputtering rate and the oxygen fraction in the argon process gas. In Itb 1060 layers produced at a sputtering rate of 0.2 nm/s, and in Corning 7740 layers produced at sputtering rates of 0.03 and 0.5 nm/s, respectively, the concentration of alkali ions in the glass layers was sufficiently high, at oxygen partial pressures below 10\4 Pa, to achieve anodic bonding. High-frequency ultrasonic microanalysis allowed the bonding area to be examined non-destructively. Tensile strengths between 4 and 14 MPa were measured in subsequent destructive tensile tests of single-bonded specimens.

Capillary adhesive bonding is used successfully to integrate microsystems. To ensure high reliability and quality of the interconnection technique, it is imperative that the propagation of adhesives in the joints be controlled. Two adhesives frequently used in capillary adhesive bonding were examined: a one-component, UV-curing methacrylate adhesive (Dymax 191-M), and a two-component epoxy resin bonding adhesive curing at room temperature (Epo-tek 302-3M). The propagation of these adhesives in joints with different gap heights of 2-20 m between two PMMA surfaces and between one PMMA and one PI surface was measured and compared with the theoretical adhesive propagation in accordance with the Hagen-Poiseuille equation for a gap flow, with the capillary pressure taken into account.Once the dynamic viscosity, the wetting angle and the surface tension of the adhesive have been determined as a function of the measuring time, the measured propagation of the Epo-tek 302-3M and Dymax 191-M adhesives can be described in good agreement with the theory for all the gap heights under study.

Micromechanical structures must be assembled where monolithic integration is not feasible because of incompatible processes, complex geometries or different materials involved. In this way, a system can be built up of products from various suppliers, which may promote the development of a market for microsystems. In micromechanics, assembly must establish mechanical and fluidic contacts in addition to electrical connections. Assembling single components is relatively expensive. This problem may be overcome if batch processes are conducted. A batch process for LIGA structures was developed which has been used to make microfluidic components by transferring a diaphragm to microstructures.