Isotropic etchants

Wet etchants fall into two broad categories isotropic etchants and anisotropic etchants. Isotropic etchants attack the material being etched at the same rate in all directions. Anisotropic etchants attack some single-crystal materials (typically silicon) at different etch rates depending on the crystallographic orientation of the substrate, so there is more control of the shapes being produced. 

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. 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

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... 

Figure 1. Cross sections of films etched with liquid or plasma etchants. The isotropic profile represents no overetch, and can be generated with liquid or plasma etch techniques. The anisotropic profile requires plasma...

Most isotropic etchants exhibit a loading effect, wherein a measurable depletion of the active etchant results from consumption in the etch process. In these cases, the overall etch rate depends upon the area of film to be etched. Under extreme circumstances, with carbon-based etch gases, etchant depletion can be so severe that polymer deposition occurs instead of etching. An analysis (85) of the loading effect, which has been extended (86) to include multiple etchant loading and other etchant loss processes, indicates that the etch rate (R) for N wafers each of area A is given by... 

An interesting demonstration of profile control via alteration of the specific chemistry is that of silicon etching in C1F3 mixtures (86). Because a pure chemical (isotropic) etchant (F atoms) is combined with an ion-bombardment-controlled (anisotropic) etchant (Cl atoms), a continuous spectrum of profiles with varying anisotropies is generated by changing the gas composition. 

Several chemical etchants can etch silicon isotropically or anisotropically, be dopant-dependent or not, and have a wide range of selectivity to silicon, which determines the appropriate masking material. Brief but clear descriptions of the silicon chemical etchants and their properties can be found in [2-4, 7]. 

HNA (HF, HN03, CH3COOH, and water [8]) is a complex etch system with highly variable etch rates and etch characteristics dependent on silicon dopant concentrations, the mix ratio of the three acids, the presence or absence of water, and even the degree of etchant agitation. The latter is typical of a diffusion-limited chemical reaction. For the same reason, F1NA etches silicon isotropically. 

When using more aggressive acidic etchants, preferential etching does not occur, and rounded isotropic patterns are created. An example is the isotropic etching of silicon under a patterned mask. The etchant moves down and out from an opening in the oxide mask, undercutting the mask and enlarging the etched... 

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