Wetting crystal planes

Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

Different etchants used in anisotropic wet etching have specific etch rates for each crystallographic plane in the material being removed. It is the difference in etch rates between planes that produce shapes within the material being etched that appear to follow the planes within the crystalline structure as seen in Fig. 1. For more information on the crystalline structure of silicon and how miller indices are used in designating crystal planes, see The MEMS Handbook [1]. It is commonly accepted that the (111) plane etches the slowest regardless of the etchant used. However, the crystal plane that etches the fastest depends on the etchant composition [2]. Ammonium hydroxide etchants (NH4OH and TMAH) are frequently used to... 

Silicon DRIE is independent of silicon crystal structure, and this enables fabrication of all possible shapes, in contrast to wet chemical anisotropic etching which is limited by silicon crystal planes. In microfluidics this shape freedom has implications for flow profiles, as significantly channel cross sections can be kept... 

Combining DRIE and anisotropic wet etching can produce unique structures. For example, (111) silicon cannot be etched by the standard KOH, TMAH, and EDP etchants, but if an initial trench has been etched by DRIE, fast-etching crystal planes are exposed, and wet etching can proceed. If this is combined with spacer structures to protect sidewalls and another DRIE step, (111) free-standing silicon structures can be made. 

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. 

Calcium and strontium salts of hyaluronic acid, at relative humidities of 66-92%, crystallize in a trigonal unit-cell, with a = b — 2.093 nm and c = 2.83 nm. On drying, the base-plane dimensions reduce to a = b = 1.832 nm, with c = 2.847 nm. Seven water molecules per disaccharide residue exist in the wet form, and two in the dry form. The adjacent chains are antiparallel, and the space group is P3212. The three disaccharide units in the 3(- 0.94) helix are nonequi-... 

Figure 9.7. BF micrographs showing the strrun associated with high-pressure clusters of molecular water in as-grown wet synthetic quartz (crystal W2). (a) Region with a water content corresponding to 2(X)H/10 Si. (b) Region with l,600H/10 Si. No strain is associated with the arrowed clusters because they intersect the foil surface the water escapes, and the stress is relaxed. The strmn field of the circled cluster is characteristic of a lens-shaped inclusion whose plane is normal to the foil surface. (From McLaren et al. 1983.)...

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