Orientation dependent etching

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. 

Anisotropic etching of silicon is routinely used in the fabrication of three-dimensional structures [1,2]. These micro fabrication techniques take advantage of orientation-dependent etch rates where the planes of lowest etch rate, usually the (111) planes, act as etch stops for the dissolution process [3, 4]. In electrolytes such as KOH, the etch rates of the (100) and the (110) planes may be more than two orders of magnitude faster than those of the (111) planes. In buffered NH4F solutions, etch rate enhancements as high as 15 have been reported for the (100) plane in comparison with the (111) surface [5]. 

Wright 3-20 min, well-defined shape and orientation-dependent etch features, smooth background surface, slow etch rate, sharp definition of defects from hot processing, relatively insensitive to dislocation during ciystai growth 388, 389, 433, 652... 

H. Namatsu, K. Kurihara, M. Nagase, and T. Makino, Fabrication of 2-nm-wide silicon quantum wires through a combination of a partially-shifted resist pattern and orientation-dependent etching, Appl. Phys. Lett. 70(5), 619, 1997. 

In the first method, anisotropic or orientation-dependent etching is used to produce a series of parallel micro channels in a silicon wafer. While... 

Sato K, Shikida M, Matsushima Y, Yamashiro T, Asaumi K, Iriye Y, and Yamamoto M 1998 Characterization of orientation-dependent etching properties of single-crystal sihcon effects of KOH concentration. Sens. Actuators, A, 64, pp. 87-93. 

Batterman (31) has attempted to explain the appearance of facets on hillocks in terms of only the measured orientation dependence of the etch rate. Using this, he concluded that the 322 planes are stable hillock facets on germanium etched with an HF + H2O2 + mixture. Irving (32) in a further analysis... 

ELECTROLYTIC ETCHING OF metals produces various results intergranular attack, attack of crystalline surfaces which is orientation dependent formation of etch pits, and anodic oxide films. The behavior of a metal or alloy depends on composition, temperature of the electrolyte, and above all on the electrode potential which varies with the metal. Applications to Al, Fe, stainless steel, Ti, 2r, U, and their alloys will be discussed. 

The (100) surface tends to roughen quicker than the (111) surface and the roughness tends to be permanent on the (100) surface whereas it is transient on the (111) surface. " Such crystal orientation-dependent roughness can also be explained by the anisotropic etching mechanism illustrated in Fig. 7.41. The preferential etching at the (111) steps of the (111) terraces results in the removal of the terraces and reduction of the (111) steps and a reduction of microroughness. 

H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, Anisotropic etching of crystalline silicon in alkaline solutions. I. Orientation dependence and behavior of passivation layer, J. Electrochem. Soc. 137, 3612, 1990. 

The models based on surface passivation suggest that a passive layer, similar to the silicon oxide formed under an anodic potential, exists on (111) silicon at OCP but not on other planes [82, 126, 156]. Instead of oxide, formation of inactive hydration complexes of K+ and OH-has been proposed to block the (111) surface [49, 134]. The fact that the relative etch rates of the different planes vary with the type of solution was attributed to the orientation-dependent adsorption of solvation complexes on the surface. 

Since the etch rate in isotropic etching is the same in all directions, its undercut rate is 1. For orientation-dependent anisotropic wet etching, the undercut rate is basically < 1. If the undercut rate can be controlled in the etching process, the lateral contours along the z direction will be changed, and a special 3D structure can be created with a 2D mask pattern. 

Table 1 lists the etch rates for some crystallographic orientations of silicon in KOH etchant with different concentrations. By understanding and using these orientation-dependent properties, 3D microstructures in a silicon substrate can be well defined. 

Low-temperature ductility is rarely observed in ceramics, which are inherently brittle, but some bulk ceramics show plasticity at ambient temperatures. One example of low-temperature plasticity in MgO is considered here. First, consider a single crystal, where i orientation-dependent properties are of interest. Orientation is one of the factors that influence mechanical properties. It was observed (by etch-pit technique) that the flow in MgO occurs on the 110 (110) slip system. However, it was also found [28] that the 110 (110) slip system contributes to deformation above 600 °C. Details on Plastic deformation in MgO single crystals were presented in Sect. 2.2, Figs. 2.33 and 2.38. Consequently, some information on deformation in polycrystalline ceramics may be of interest. 

Seidel H, Csepregi L, Heubager A, Baumgartel H (1990) An-isotrr c etching of crystalline silicon in alkaline solutions I Orientation dependence and behavior of passivation layers. J Electrochem Soc 137(11) 3612-3626... 

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