Anisotropic etching

Figure 2.9 shows the shapes obtained when silicon is etched anisotropically. The four (111) levels cut the surface of a disk of (100) material along the (11 0) directions within it. The etchant, in turn, attacks each layer of the (100) levels and removes... 

In contrast to isotropic etching, anisotropic etching is a fabrication technique that removes material in specific directions allowing for the production of geometric characteristics such as sharp corners, flat surfaces, and deep cavities. Relative to microfabrication, selectivity can be defined as the ratio of the etch rate of the target material to the etch rate of other materials. The definition can also be applied more specifically to single crystalline materials such as silicon where it would be considered the ratio of the etch rate in the target direction to the etch rate in other directions. 

A fonn of anisotropic etching that is of some importance is that of orientation-dependent etching, where one particular crystal face is etched at a faster rate than another crystal face. A connnonly used orientation-dependent wet etch for silicon surfaces is a mixture of KOH in water and isopropanol. At approximately 350 K, this etchant has an etch rate of 0.6 pm min for the Si(lOO) plane, 0.1 pm min for the Si(l 10) plane and 0.006 pm miiG for the Si(l 11) plane [24]. These different etch rates can be exploited to yield anisotropically etched surfaces. 

Dry etching is a commonly used teclmique for creating highly anisotropic, patterned surfaces. The interaction of gas phase etchants with surfaces is of fundamental interest to understanding such phenomena as undercutting and the dependence of etch rate on surface structure. Many surface science studies aim to understand these interactions at an atomic level, and the next section will explore what is known about the etching of silicon surfaces. 

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

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

The flux of F radicals to tire wafer is nearly isotropic. Anisotropic etching is due to ions tliat are incident on tire wafer essentially perjDendicular to tire surface (see above). 

In order to design and optimize anisotropic dry etching processes, severai issues must be understood ... 

Youtsey C, Adesida I and Bulman G 1997 Highly anisotropic photoenhanced etching of n-type GaN Appl. Phys. Lett. 71 linebreak 2151-4... 

Veprek S and Sarott F A 1982 Electron-impact-induced anisotropic etching of silicon by hydrogen Plasma Chem. Plasma Proc. 2 233-46... 

Giiiis H P, Choutov D A, Martin K P, Bremser M D and Davis R F 1997 Highiy anisotropic, uitra-smooth patterning of GaN/SiC by iow energy eiectron enhanced etching in DC piasma J. Electron. Mater. 26 301-5... 

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

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