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After more than 40 years of development, a large complement of IC engineers have been trained. These engineers provide a very important resource that directly contributes to the continued development of ICs. By taking advantage of the freedoms provided by the IC design procedures, engineers have come up with new designs and ideas that have extended the IC process far beyond what was first envisioned.
Clearly, the characteristics of the IC process just described should be applied to the production of MEMS as much as possible. Focusing on ways to leverage the multibillion-dollar investment in the IC infrastructure will be effort well spent.
Many of the processes that have been refined in IC technology to produce electronic devices can be adapted to make the mechanical structures needed in MEMS. These processes include those that support photolithography, plasma etching, wet etching, diffusion, implantation, chemical-vapor deposition, sputtering, and vacuum deposition. The most sophisticated IC production uses very high performance equipment (to control submicron line widths, for example). Such fine dimensional control is not required in typical MEMS applications, which therefore might be able to use earlier generation equipment. Thus, in some cases, MEMS fabrication facilities can make use of older IC processing lines, thereby reducing startup costs (for new industrial ventures) or making it feasible to open MEMS-capable fabrication facilities in government laboratories or universities.
USING EXISTING INTEGRATED CIRCUIT-BASED PROCESSES
This section enumerates several IC-based fabrication processes that have been used to produce MEMS. Opportunities and technical challenges for each fabrication process are highlighted, and recommendations are given to address the technical challenges of IC-based MEMS processing technologies.
Existing IC-based technologies that have been used to produce MEMS are generally described by the terms bulk micromachining or surface micromachining. In bulk micromachining, the mechanical device is composed of the substrate material (e.g., single-crystal silicon), whereas in surface micromachining, the mechanical device is made from material deposited as part of the fabrication process. In a few cases, this distinction does not apply because sequential steps produce a composite device, but the dominance of either surface or bulk micromachining in the process is usually apparent. Compatible processing with ICs has been demonstrated using either technique, but the complexity of the process, the sizes and possible shapes of the mechanical elements, the sizes of the chips, the minimum sizes of the features, the costs, and the yields are all strongly influenced by the chosen process and the level of system integration in the MEMS.
Bulk Micromachining Processes
Bulk micromachining was first demonstrated decades ago. In its original form, it produced structures by using anisotropic wet etching of the single-crystal substrate. By combining the constraints of directionally dependent and impurity dependent etching with photolithographic patterning, a number of useful three-dimensional configurations (Figure 2-1), notably cantilevers, diaphragms, and orifices, can be produced. The rates of the anisotropic etches are greatly reduced by heavy boron doping, and either this effect or the presence of a pn-junction is often employed to control etch depths. The original bulk-micromachining process is widely used today, especially for the production of pressure sensors. Newer techniques have also been introduced to add features to bulk micromachining.
Two techniques rely on wet-chemical etching or RIE (reactive-ion etching) to form structures from bulk material. Released structures are formed by etching through the bulk material or by undercutting the bottom structures to be released with a selective wet or plasma-etch step and a masking material. Released structures can also be formed using a substrate with two or more layers: the micromachined device is formed from the silicon remaining in the upper layer after the lower (buried) layer is dissolved, releasing the structures selectively.
Other techniques used to micromachine bulk material include scanned, focused-ion-beam or laser ablation to remove materials; masked ion-beam etching or ion milling; and mechanical removal of the unwanted silicon. These technologies are serial rather than batch processes and do not usually provide the economies of scale offered by most IC manufacturing techniques. Serial scanning tools are useful for cross-sectioning or calibrating suspended MEMS, however, by selective material removal or selective material deposition.
A bulk-micromachined accelerometer (Figure 1-4) highlights the characteristics of the wet-chemical etching of single-crystal silicon for MEMS. The process involves lithographic patterning of the device onto a silicon dioxide mask layer. This step is followed by a pattern-transfer step that exposes areas for subsequent wet-chemical etching using potassium hydroxide (KOH) or other suitable wet etch. The KOH etch is anisotropic and faster on different crystallographic planes. The crystal orientation of the surface is normally the plane so the silicon etches much slower in the normal direction than in the direction lateral to the surface. The shape of the finished structure has sloped sidewalls and facets on corners or curved patterns. Etched square patterns become inverted pyramids. The etching times may be minutes or hours.
Two advantages of wet-chemical micromachining are that large structures can be micromachined from silicon in a short time and that the chemical-etch equipment is simple and inexpensive. Disadvantages of wet-chemical processing are