Etchants anisotropic 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. 

Tetramethyl Ammonium Hydroxide (TMAH). TMAH, (CH3)4NOH, etchant has been developed more recently for anisotropic etching of silicon. It is relatively safe to use and presents no special disposal issues. It has a very low oxide etch rate and does not attack aluminum if the soluhon contains a certain amount of silicates. However, hillocks tends to develop in this solution. ... 

In particular, the hillocks formed on the (100) surface are crystallographic structures bounded by four (111) planes resulting from the anisotropic etching and their formation directly contributes to the roughness of the surfaces. Hillocks may, under certain conditions, form in HF-based solutions, " but most commonly in alkaline etchants. ° Because the (111) surface etches very slowly, the formation of hillocks leads to a decreased etch rate. ... 

Barycka and 1. Zubel, Silicon anisotropic etching in KOH-isopropanol etchant, Sensors Actuators A 48, 229, 1995. 

TMAH, (CH3)4NOH, etchant has been developed more recently for anisotropic etching of silicon [59, 109, 141]. It is relatively safe to use and presents no special... 

KOH, is rather high compared to other organic etchants as shown in Fig. 26. Etch rate ratios are almost constant with respect to doping concentration up to the heavily doped levels where etch rate reduction occurs [45, 80, 92]. A highly defective surface, such as a saw-damaged surface, may not exhibit anisotropic etching rates [152],... 

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]. 

Combination of DRIE and isotropic plasma etching can be used to create channels inside silicon. After the DRIE step, a sidewall passivation step is performed, and etching is then continued with an isotropic etchant. Anisotropic SFg/02 DRIE recipe can quite easily be tailored to be isotropic by, for example, raising the pressure and eliminating the oxygen. Similarly, isotropic SFg plasma release etch can be used to fabricate free-standing silicon structures. 

Silicon Micromachining, Fig. 2 Schematic of cross-sectional view of anisotropically etched microchannels (a) in (100) silicon and (b) in (110) and (c) of 3-D aerial view of anisotropically etched microchannels in (100) silicon using wet chemical etchants of KOH, EDP, or TMAH... 

Isotropic versus anisotropic etch Glass is always etched isotropically because the etchant etches all exposed surfaces at the same rate. This is independent of the etchant used and arises because glass does not have different planes. Depending on etchant, sihcon is either etched isotropically or anisotropically. Anisotropic etching occurs where material on one plane is removed at a greater rate than material on a different plane. The difference is most apparent in terms of channel shape and aspect ratio. Isotropic... 

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