Silicon etch characteristics

Electrical impedance measurements for silicon etching in HF have been used to analyze the potential distribution at the interface [72, 73, 75, 79, 80]. Analysis of these data has resulted in insight into the kineties of the dissolution process and the potential distribution at the silicon/HF interface. The characteristic features of the... 

Silicon etching in KOH solutions have been extensively investigated, resulting in a body of information that shapes the current understanding of the etching behavior of silicon in alkaline solutions. The major characteristics and the principal reaction processes involved in all alkaline solutions appear to be similar to that in the KOH system although the detailed characteristics vary from system to system. Most notably,... 

Anisotropic etching, that is, different dissolution rates on different crystal planes, is a characteristic feature of silicon etching in alkaline solutions. Strictly speaking, the etch rate of silicon always depends, to a various extent, on crystal orientation in all etching solutions, acidic or alkaline. However, the etch rate difference among different planes is small in acidic HF solutions compared to those in alkaline solutions. Figure 26 shows the etch rate ratios of(100)/(lll) and (110)/(111) planes in various solutions. 

The many etching characteristics of silicon and the numerous etching systems provide a large range of variation in the etch rate. This range of etch rates, in combination with various etching techniques, provides many methods in selective removal of materials on silicon as illustrated in Fig. 36, allowing the fabrication of diverse structures on silicon. 

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. 

Yamamura K, Mitani T (2008) Etching characteristics of local wet etching of silicon in HF/HNO3 mixtures. Surf Interface Anal 40 1011-1013... 

Colloidal nanolithography, deep silicon etching and nanomolding are the techniques used to achieve fibrillar polymer sfructures which mimic the gecko foot hairs these nanofibrils are densely packed, perpendicular and strongly adhesive to a synthetic surface, and due to these characteristics are promising materials for integration in flexible membranes and exploitation of new adhesives [169]. 

There are other, nonhydrogel, new materials for chromatographic and electrophoretic separations [7,8,103,164,199,214,377,407], Eor example, Volkmuth and Austin [407] proposed electrophoretic studies in microlithographic arrays of posts and channels etched into sihcon wafers. This material may be useful for studying fundamental transport characteristics of macromolecules in defined media, and many recent studies have been conducted to develop chromatography and electrophoresis on silicon wafers with micron-scale channels... 

Flockhart, S. M., Dhariwal, R. S., Experimental and numerical investigation into the flow characteristics of channels etched in (100) silicon, J. Fluids Eng. 120 (1998) 291-295. 

Etch rate and homogeneity and anisotropic characteristics are the predominant factors in determining the resulting micro system device properties. Temperature and concentration of the KOH solution as well as the doping concentration of the silicon material have the largest impact on these properties and have to be thoroughly controlled. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

The electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

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