Silicon, isotropic etching

Silicon Micromachining, Fig.1 Schematic of cross-sectional view of isotropically etched silicon microchannels using wet chemical etchants of NHA... 

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. 

C.I. Silicon and Poly silicon. The isotropic etching of silicon (Si) and polycrystalline silicon (Poly-Si) by atomic fluorine (F) is probably the most completely understood of all etch processes, particularly for the cases in which F atoms are produced in discharges of F2 (36) and CF4/O2 (42). Fluorine atoms etch (100) Si at a rate (A/min) given by (36) ... 

Aluminum, copper, diamond, gold, silicon Anisotropic etching, isotropic etching... 

Isotropic etching of silicon is done with an aqueous mixture of HF+HN03 + CH3COOH (hydrofluoric, nitric add and acetic add). Often the acetic add is omitted. The etch rates are very high at room temperature 940 pm/min with 20-46% HN03 (69% HNO3, 31% H20) complemented by HF (49% HF, 51% H20). The quality of the silicon surface produced will depend on the solution used. Smoother surfaces are achieved with a higher proportion of nitric acid and a lower proportion of acetic acid. 

Silicon can be isotropically etched in Xep2 according to the overall reaction ... 

NOJ/NO. HNO3 is a commonly used oxidant in HF solutions for isotropic etching of silicon. Figure 6.18, reported by Kooij et shows the etch rate and... 

There are three major etching solution systems with respect to the uniformity of the etched surface, that is, relative etching rates on silicon surfaces of different crystallographic characters (1) isotropic etching system represented by HF-HNO3, (2) anisotropic etching system represented by alkaline solutions, and (3) defect etching system represented by HF-CrOs solutions (Fig. 7.1). 

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]. 

Fig. 5.1.1 Isotropic etching of silicon wafers a) top view b) cross section along AA with agitation c) cross section along AA without agitation...

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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