MEMS Fabrication1

16 MEMS Fabrication1 Marc J. Madou 16.1 Wet Bulk Micromachining: Introduction Nanogen, Inc. 16.2 Historical Note 16.3 Silicon Crystallography Miller Indices • Crystal Structure of Silicon • Geometric Relationships between Some Important Planes in the Silicon Lattice 16.4 Silicon as a Substrate and Structural Material Silicon as Substrate • Silicon as a Structural Element in Mechanical Sensors 16.5 Wet Isotropic and Anisotropic Etching Wet Isotropic and Anisotropic: Empirical Observations • Chemical Etching Models 16.6 Etching With Bias and/or Illumination of the Semiconductor Electropolishing and Microporous Silicon 16.7 Etch-Stop Techniques Introduction • Boron Etch Stop • Electrochemical Etch-Stop Technique • Photo-Assisted Electrochemical Etch Stop (for n-Type Silicon) • Photo-Induced Preferential Anodization, PIPA (for p-Type Silicon) • Etch Stop at Thin Films-Silicon on Insulator 16.8 Problems with Wet Bulk Micromachining Introduction • Extensive Real Estate Consumption • Corner Compensation 16.9 Wet Bulk Micromachining Examples 16.10 Surface Micromachining: Introduction 16.11 Historical Note 16.12 Mechanical Properties of Thin Films Introduction • Adhesion • Stress in Thin Films • Strength of Thin Films 16.13 Surface Micromachining Processes Basic Process Sequence • Fabrication Step Details • Control of Film Stress • Dimensional Uncertainties • Sealing Processes in Surface Micromachining • IC Compatibility 16.14 Poly-Si Surface Micromachining Modifications Porous Poly-Si • Hinged Polysilicon • Thick Polysilicon • Milli-Scale Molded Polysilicon Structures 1This chapter was originally published in Madou, M.J. (1997) “Wet Bulk Micromachining” and “Surface Micro- machining” (chaps. 4 and 5), in Fundamentals of Microfabrication, CRC Press, Boca Raton, FL. © 2002 by CRC Press LLC 16.15 Non-Poly-Si Surface Micromachining Modifications Silicon on Insulator Surface Micromachining 16.16 Resists as Structural Elements and Molds in Surface Micromachining Introduction • Polyimide Surface Structures • UV Depth Lithography • Comparison of Bulk Micromachining with Surface Micromachining 16.17 Materials Case Studies Introduction • Polysilicon Deposition and Material Structure • Amorphous and Hydrogenated Amorphous Silicon • Silicon Nitride • CVD Silicon Dioxides 16.18 Polysilicon Surface Micromachining Examples 16.1 Wet Bulk Micromachining: Introduction In wet bulk micromachining, features are sculpted in the bulk of materials such as silicon, quartz, SiC, GaAs, InP, Ge and glass by orientation-dependent (anisotropic) and/or by orientation-independent (isotropic) wet etchants. The technology employs pools as tools [Harris, 1976], instead of the plasmas studied in Madou (1997, chap. 2). A vast majority of wet bulk micromachining work is based on single crystal silicon. There has been some work on quartz, some on crystalline Ge and GaAs, and a minor amount on GaP, InP and SiC. Micromachining has grown into a large discipline, comprising several tool sets for fashioning microstructures from a variety of materials. These tools are used to fabricate microstructures either in parallel or serial processes. Madou (1997, Table 7.7) summarizes these tools. It is important to evaluate all the presented micromanufacturing methods before deciding on one specific machining method optimal for the application at hand—in other words, to zero-base the technological approach [Block, B., private communication]. The principle commercial Si micromachining tools used today are the well-established wet bulk micromachining and the more recently introduced surface micromachining (see Section 16.10). A typical structure fashioned in a bulk micromachining process is shown in Figure 16.1. This type of piezoresistive membrane structure, a likely base for a pressure sensor or an accelerometer, demonstrated that batch fabrication of miniature components does not need to be limited to integrated circuits (ICs). Despite all the emerging new micromachining options, Si wet bulk micromachining, being the best characterized micromachining tool, remains FIGURE 16.1 A wet bulk micromachining process is used to craft a membrane with piezoresistive elements. Silicon micromachining selectively thins the silicon wafer from a starting thickness of about 400 µm. A diaphragm having a typical thickness of 20 µm or less with precise lateral dimensions and vertical thickness control results. © 2002 by CRC Press LLC most popular in industry. An emphasis in this chapter is on the wet etching process itself. Other machining steps typically used in conjunction with wet bulk micromachining, such as additive processes and bonding processes, are covered in Madou (1997, chaps. 3 and 8, respectively). Wet bulk micromachining had its genesis in the Si IC industry, but further development will require the adaptation of many different processes and materials. To emphasize the need for micromachinists to look beyond Si as the ultimate substrate and/or building material, we have presented many examples of non-Si micromachinery throughout the book. The need to incorporate new materials and processes is especially urgent for progress in chemical sensors and micro-instrumentation which rely on non-IC materials and often are relatively large. The merging of bulk micromachining with other new fabrication tools such as surface micromachining and electroplating and the adaptation of new materials such as Ni and polyimides has fostered a powerful new, nontraditional precision engineering method. A truly multidisciplinary engineering education will be required to design miniature systems with the most appropriate building philosophy. After a short historical note on wet bulk micromachining we begin this chapter with an introduction to the crystallography of single crystal Si and a listing of its properties, clarifying why Si is such an important sensor material. Some empirical data on wet etching are reviewed and different models for anisotropic and isotropic etching behavior follow. Then, etch stop techniques, which catapulted micromachining into an industrial manufacturing technique, are discussed. Subsequently, a discussion of problems associated with bulk micromachining such as IC incompatibility, extensive real-estate usage, and issues involving corner compensation is presented. Finally, examples of applications of wet bulk micromachining in mechanical and chemical sensors are given. 16.2 Historical Note The earliest use of wet etching of a substrate, using a mask (wax) and etchants (acid-base), appears to be in the fifteenth century for decorating armor [Harris, 1976]. Engraving hand tools were not hard enough to work the armor and more powerful acid-base processes took over. By the early seventeenth century, etching to decorate arms and armor was a completely established process. Some pieces stemming from that period have been found where the chemical milling was accurate to within 0.5 mm. The masking in this traditional chemical milling was accomplished by cutting the maskant with a scribing tool and peeling the maskant off where etching was wanted. Harris (1976) describes in detail all of the improve- ments that, by the mid-1960s, made this type of chemical milling a valuable and reliable method of manufacturing. It is especially popular in the aerospace world. The method enables many parts to be produced more easily and cheaply than by other means and in many cases provides a means to design and produce parts and configurations not previously possible. Through the introduction of photosensitive masks by Niépce in 1822 [Madou, 1997, chap. 1], chemical milling in combination with lithography became a reality and a new level of tolerances came within reach. The more recent major applications of lithography-based chemical milling are the manufacture of printed circuit boards, started during the Second World War, and, by 1961, the fabrication of Si-integrated circuitry. Photochemical machining is also used for such precision parts as color television shadow masks, integrated circuit lead frames, light chopper and encoder discs, and decorative objects such as costume jewelry [Allen, 1986]. The geometry of a ‘cut’ produced when etching silicon integrated circuits is similar to the chemical-milling cut of the aerospace industry, but the many orders of magnitude difference in size and depth of the cut account for a major difference in achievable accuracy. Accordingly, the tolerances for fashioning integrated circuitry are many orders of magnitude smaller than in the chemical milling industry. In this book we are concerned with lithography and chemical machining used in the IC industry and in microfabrication. A major difference between these two fields is in the aspect ratio (height-to-width ratio) of the features crafted. In the IC industry one deals with mostly very small, flat structures with aspect ratios of 1 to 2. In the microfabrication field, structures typically are somewhat larger and aspect ratios might be as high as 400. © 2002 by CRC Press LLC Decorating armor. (From Harris, T. W., Chemical Milling, Clarendon Press, Oxford, 1976. With permission.) Isotropic etching has been used in silicon semiconductor processing since its beginning in the early 1950s. Representative work from that period is the impressive series of papers by Robbins and Schwartz (1959, 1960) and Schwartz and Robbins (1961, 1976) on chemical isotropic etching, and Uhlir’s paper on electrochemical isotropic etching [Uhlir, 1956]. The usual chemical isotropic etchant used for silicon was HF in combination with HNO3 with or

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