Isotropic and anisotropic etching

Fig. 2.72a-c Construction of buried micro-channels by combined anisotropic and isotropic etching. Reprinted from Kandlikar and Grande (2002) with permission... 

Kim JS, Lee MY, Kang MS, Yoo KP, Kwon KH, Singh VR, Min NK (2010) Fabrication of high-speed polyimide-based humidity sensor using anisotropic and isotropic etching with ICP. Thin Solid Films, Procedia Engineering 5 264—267... 

Figure C2.13.5. Schematic illustrations of isotropic etching by a neutral gas and anisotropic plasma etching.

Eig. 8. Schematic of etching directionaHty showing (a) isotropic etch, and (b) anisotropic etch. 

The high selectivity of wet etchants for different materials, e.g. Al, Si, SiOz and Si3N4, is indispensable in semiconductor manufacturing today. The combination of photolithographic patterning and anisotropic as well as isotropic etching of silicon led to a multitude of applications in the fabrication of microelectromechanical systems (MEMS). 

Acidic silicon etchants are mainly used for two purposes for the delineation of crystal defects, as discussed in Section 2.5, or to remove silicon in an isotropic manner. Isotropic etching adds another degree of freedom to the design of micromechanical structures, because all alkaline etches are anisotropic. Most isotropic etchants for silicon were developed in the early days of silicon crystal technology and exhaustive reviews on this topic are available [Tu3, Rul]. A brief summary is given below. 

Once the resist has been patterned, the selected regions of material not protected by photoresist are removed by specially designed etchants, creating the resonator pattern in the wafer. Several isotropic (etching occurs in all directions at the same etch rate) and anisotropic (directional) etching processes are available. Wet chemical etching is an isotropic process that... 

Fig.1 The scheme of anisotropic surface etching and isotropic deposition of Au—TMTU complexes at step edges of the Au(ni) surface in TMTU-containing electrolytes [23].

The simplest and perhaps the most useful measure of anisotropy is the ratio of lateral or horizontal undercut distance (dh) to the vertical etch distance (dy). This ratio is inversely related to the quality of replication. In anisotropic etching, djdv = 0, and exact dimensional transfer is achieved. A low-quality transfer is obtained with isotropic etching, in which djdv = 1. Anisotropic etching is imperative for high-density-device fabrication. 

Figure 10.12 A microstructure after anisotropic, non-selective etching (left), anisotropic, selective etching (middle), and isotropic, selective etching (right).

The types of etching process are divided into wet or dry chemical etching and isotropic or anisotropic etching as shown in Figure 2.7. 

If the material etched away is removed as a liquid, the process is called wet chemical etching and when the material is removed in the gaseous state, it is referred to as chemical etching. Each type can be isotropic, with the same etch rate operating in all directions, or anisotropic, with the etch rates reflecting direction, which may be a spatial direction or crystal orientation. 

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