Etching etchant selection

Microporous silicon is suitable for sacrificial layer applications because of its high etch rate ratio to bulk silicon, because it can be formed selectively, and because of the low temperatures required for oxidation. PS can be formed selectively if the substrate shows differently doped areas, as discussed in Section 4.5, or if a masking layer is used. Noble metal films can be used for masking as well as Si02, Si3N4 and SiC. Oxidation conditions are given in Section 7.6, while the etch rates of an etchant selective to PS are given in Fig. 2.5 b. 

Specific Materials. Because the fundamental physics and chemistry of reactive gas discharges used for etching is not yet fully understood, empirical approaches to the design of etch processes abound. As indicated earlier, etchant selection begins with considerations of product volatility. Virtually any gas or vapor that can dissociate to form etchant species has been considered or studied. An abbreviated list of the most common reactant molecules used to etch films of interest in electronic materials processing is given in Table IV. 

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). 

It is known that HF-HN03-based solutions etch highly doped substrates faster by a factor of about three compared to moderately doped ones. A higher selectivity is reported on addition of chemicals that reduce the HNOz concentration, like H202 or NaN3 [Mul], However, this report suffers from the fact that the etch rate was measured for separate wafers of a homogeneous doping density. For pp+ or nn+ structures, which are not spatially separated, only a low selectivity is observed, because of the autocatalytic behavior of the etchant. 

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... 

Liquid etching has been the preferred method for pattern delineation for thin films for many years (I). Its pervasive use has been due primarily to two considerations. First, although the exact chemistry is often poorly understood, the technology of liquid etching is firmly established. Second, the selectivity (ratio of film etch rate to the etch rate of the underlying film or substrate) can be essentially infinite with the proper choice of etchant solution. 

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