mechanical Characterization of Thin Films by Micromechanical Techniques
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supporting structures, like microbridges or membranes, fabricated in detached films. The other is to study films adhered to supporting, microsized structures made of some other material, usually silicon, and to deposit the film either before or after micromachining the support material. Both approaches have advantages and drawbacks. Using free-standing, single-layer structures of film material, the properties of the thin film itself (in detached condition) are measured. Many film materials are not easily removed from their substrates, however, and thin coatings often interact strongly with their substrates.7 Hence their mechanical behavior in a filmsubstrate composite can be quite different from what they display in the form of freely suspended single-layer structures. Measurements on such structures might be misleading, consequently, if one is interested in the fundamental material parameters of a well-adhered thin coating. Micromechanical processing facilitates the manufacturing of microsized singleor double-layer specimens. It is possible to design microscale experiments for evaluating the fundamental material parameters of the thin film in a manner resembling macroscopic experiments. This makes analysis of the experimental results easier and more straightforward, and to some extent, previous experience from the corresponding macroscopic experiments can be used. This article gives a very condensed description of the micromachining processes used in specimen preparation, and a more comprehensive discussion of problems
connected with the models for analytical evaluation. Special attention is paid to the influence of various sources of error. The article reviews numerous reports on micromechanical measurements of elasticity, internal stress, fracture, adhesion, plasticity, thermomechanics, and microtribology. It gives both theoretical background and experimental problems for different types of measurements.
Micromachining Since the field of micromechanics has evolved from microelectronics, it is natural that the materials and processes used in the two fields are similar. Although alternative microfabrication processes exist, such as beam-induced etching/deposition8 or the LIGA process for metals, polymers and ceramics,9 conventional semiconductor processing based on planar lithography and etching still is predominant within micromechanics. In the present context two types of micromachining are of particular interest: bulk micromachining for fabricating structures directly in the bulk material (usually a single crystalline wafer), and surface micromachining which involves structures cut from layers (single or multiple) deposited on a substrate. Some micromechanical devices combine the two techniques, for instance when a surfacemicromachined structure is "freely" suspended over a cavity or hole in the bulk substrate. The lateral geometry of the desired structure is defined in the same manner in the two processes. The body to be machined is coated with a thin layer of a mask material (such as SiO2 on a silicon w
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