Silicon wafer, etching rates

Figure S-A2. Silicon wafer etched using the microplasma etcher for 1 min. VHF (very-high-frequency) power, substrate bias power, and Ar-SFe flow rates are 150 W, 4 W, 200 seem, and 100 seem, respectively.

Electrochemical etching is one way of controlling the etch rate and determine a clear etch stop layer when bulk micromachining Silicon. In this case, the wafer is used as anode in an HF-Electrolyte. Sufficiently high currents lead to oxidation of the silicon. The resulting oxide which is dissolved by the HF-solution. Since lowly doped silicon material is not exhibiting a notable etch rate, it can be used as an etch stop. 

All standard cleaning processes for silicon wafers are performed in water-based solutions, with the exception of acetone or (isopropyl alcohol, IPA) treatments, which are mainly used to remove resist or other organic contaminants. The most common cleaning procedure for silicon wafers in electronic device manufacturing is the deionized (DI) water rinse. This and other common cleaning solutions for silicon, such as the SCI, the SC2 [Kel], the SPM [Ko7] and the HF dip do remove silicon from the wafer surface, but at very low rates. The etch rate of a cleaning solution is usually well below 1 nm min-1. 

In order to guarantee a constant etch rate across the wafer independent of feature size the etching is restricted to narrow gaps around the desired patterns. This allows to remove narrow as well as extended areas of the silicon. The areas to be removed are not bonded to the bottom glass substrate. The released areas are removed afterwards just by turning the sandwich upside down. 

In anisotropic crystallographic wet chemical etching of silicon, the dependency of the etch rate on crystal orientation is exploited. Even along the main levels of the crystal, for example along the (111) and (110) levels, the etch rate can vary by a factor of 100. Aqueous solutions of alkaline hydroxides such as KOH and NaOH are anisotropic etch solutions for silicon. The etching speed in individual directions will depend on the temperature and the etch solution used. The relation of the etch rate to the crystal s direction is shown in Figure 2.8 for the widely produced wafers made of (10 0)-type silicon [3]. 

The most characteristic feature of etching in KOH solution is its anisotropic nature, i.e., the etch rate varies with the crystal orientation of the silicon wafer. The etch rate for the major crystal planes follows 110 > 100 > 111. The exact difference between the etch rates of these planes depends on concentration, temperature, and measurement procedure. For example, in 20% KOH at 100 °C the etch rates for (110) (100) (111) are 930 560 19A/s corresponding to a ratio of 50 30 1. The difference between the etch rates of (110) and the (100) planes and that of the (111) plane is larger at room temperature, giving a ratio of 160 100 1 in 20% KOH. Addition of... 

FIGURE 7.17. Etch rate of a silicon wafer as a function of the pH of an aqueous solution of NH, and NaOH at 70°C. After van den Meerakker and van den Straaten. (Reproduced by permission of The Electrochemical Society, Inc.)... 

The most characteristic feature of etching in KOH solution is its anisotropic nature, that is, etch rate varies with the crystal orientation of the silicon wafer. The etch rates for the major crystal planes are 110 > 100 > 111 [82], The exact difference between the etch rates of these planes depends on concentration, temperature, and measurement procedure. 

A planar substrate, such as silicon wafer, could be micromachined by a sequence of deposition and etching processes. This results in three-dimensional microstructures which can be implemented in cavities, grooves, holes, diaphragms, cantilever beams etc. The process referred to as silicon micromachining often employs anisotropic etchants such as potassium hydroxide and ethylene diamine pyrocatechol. The crystallographic orientation is important as the above-mentioned etchants show an etch-rate anisotropy. The ratio for the (100)-, (110)- and (111)- planes is typically 100 16 1. The technique of electrochemical etch stop could be applied for control of the microstructural dimensions. An alternative... 

An oxide layer on a silicon wafer Is most commonly removed with HP or NH4P-HP (BOB) solutions. The reaction results In formation of silicon hexafluoride which is water soluble (10). The buffered HP Is used when a constant oxide etch rate Is required. Complication due... 

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