Surface Micromachining

In bulk micromachining processes the materials are removed by different etching techniques (described in the previous chapter) while in surface micromachining processes the materials are added through thin film deposition techniques in order to form micromechanical structures. Thin films are deposited using low pressure chemical vapor deposition (LPCVD) of polycrystalline silicon, silicon nitride and silicon dioxides. The films can be deposited sequentially and removed as necessary in order to build microdevices. Surface micromachining is a relatively simple method as compared to bulk micromachining. It can produce a very thin and higher precision structure than bulk micromachining, and the process can be controlled more easily. There are many advantages in surface micromachining as compared to bulk micromachining processes. A comparison between surface and bulk micromachining is presented in the following section.

In the history of literature, surface micromachining fabrication process began in the 1960's, but it has been rapidly expanded over the past few decades. The application of surface micromachining to batch fabrication in MEMS devices was put into practice in recent years. The first example of a surface micromachined device was a resonant gate transistor (Nathanson and Wickstrom 1965). The height for this kind of device was limited to less than 10 micrometers. It was shaped like an x-y plane surface and hence the process has been named the surface micromachining process. A survey indicates that Gabriel et al. presented the first possible application of polysilicon surface micromachining in 1989 (Gabriel et al. 1989). Subsequently, microscale movable parts like pin joints, gears, springs and many other mechanical and optical components have been developed (Editor 1997; Editor 1991). The first commercial product based on surface micromachining, namely, the ADXL-50, a 50-g accelerometer for activating air-bag deployment was developed by Analog Devices (Editor 1991). Thereafter, Digital Micromirror Device™ was developed by Texas Instruments (Editor 1977). Recently, there are many commercial devices available which are manufactured using surface micromachining processes. Four major issues are dealt while developing the surface micromachining conformant devices.

• The understanding and control of the material properties of micro structure films (e.g., Polycrystalline silicon, silicon oxide and silicon nitrite)

• Design constraints such as size and layer positioning

• Adoption of precision releasing methods which consider the film stress and sticking properties of the materials

• The packaging methods

A brief overview in this regard is presented in this chapter. The following section summarises the difference between the bulk and surface micromachining processes. The properties of microstructures, bonding and packaging processes are also discussed.

6.3.1 Bulk versus Surface Micromachining

Surface micromachining is mainly active in the x-y plane and it is relatively difficult to process on the z-axis. By definition, bulk micromachining removes materials from the substrate in either direction, while surface micromachining normally adds a thin film. Bulk and surface micromachining have many sub-processes in common. Both the methods employ photolithography, oxidation, diffusion and ion implantation techniques. CVD (Chemical Vapour Deposition) method is adopted for oxide, nitride and oxy-nitride formation. Plasma etching is used in both processes. They also use many common materials like polysilicon, Al, Au, Ti, Pt, Cr and Ni. However, the processes differ in many ways like in the use of anisotropic etchants, anodic and fusion bonding, use of Si <100> as a starting material, different types of etch stops, double-sided processing and electrochemical etching in bulk micromachining, and the use of dry etching in patterning and isotropic etchants in release steps in surface micromachining. The use of polysilicon in surface micromachining avoids the fabrication difficulties associated with bulk micromachining. Polysilicon increases the freedom of the integration of complicated microdevice features. The use of a sacrificial layer in surface micromachining has advantages in the assembly of thin and tiny mechanical structures. Bulk micromachining uses larger die, waste material, and involves high production costs as compared to its counterpart. There is a V2t effect in the bulk micromachining structure of thickness t, and there is no such effect on surface micromachining. Bulk micromachining does not induce any stress but this is the biggest limitation and disadvantage of surface micromachining. A brief dimensional comparison is drawn in Table 6.1.

Table 6.1. Comparison between bulk and surface micromachining (Source - J. Micromechanics and Microengineering 8 (1998))

Property

Bulk

Surface

Processing complexity

0

0>+

Lateral dimensions

3-5 mm

100-500 ¡.t m

Vertical dimensions

100—SO 0 it m

0.5-2 /¿m

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