Anisotropic wet chemical etching

The membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

Figure 16. Schematic diagram illustrating phase-separated columnar-structure-induced anisotropic wet chemical etching. (Reproduced with...

Figure 3.44. Schematic representation of anisotropic wet chemical etching of GcxSei x- Reproduced with permission from reference 151. Copyright 1985 Society of Plastics Engineers.)...

Adhesives for integrated circuits, requirements, 14 AgCl-As2S3, 172 Ag2Se-GejSei., resists development of latent image, 174 anisotropic wet chemical etching, 177/ chemical basis, 174... 

Yoon et al. of Ajou University have successfully fabricated a tungsten filament with an extraction grid and other electrodes formed in nickel on a silicon substrate, as shown in Fig. la [1]. To allow its very high operating temperature to be reached, the filament was suspended over a pit formed by anisotropic wet chemical etching. Emission currents up to 7 nA were obtained with an applied voltage of 35 V. 

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... 

Microfabrication was effected by anisotropic reactive ion etching (RIE) using a plasma [34], Back-side structures were realized by wet-chemical etching. A transparent plate is bonded on top of the microstructures. 

If the material etched away is removed as a liquid, the process is called wet chemical etching and when the material is removed in the gaseous state, it is referred to as chemical etching. Each type can be isotropic, with the same etch rate operating in all directions, or anisotropic, with the etch rates reflecting direction, which may be a spatial direction or crystal orientation. 

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]. 

Wet chemical etching (isotropic and anisotropic) Suspended membranes... 

The most common method of pattern transfer to the substrate is by wet chemical etching. However, all semiconductor wet etching processes exhibit the same basic limitation. This limitation is due to the isotropy of the process, which makes linewidth control difficult for features less than 2 pm when thick substrate layers are used. The need to transfer fine features in thick substrates has led to the development of anisotropic etching techniques such as plasma etching, reactive ion etching (RIE) and sputter etching. 

Silicon has a diamond cubic crystal structure. The Miller indices of the main crystallographic planes of silicon are (100), (110) and (111), respectively. In the wet bulk-micromachining, there are two silicon etching methods isotropic (direction-independent) and anisotropic (direction-dependent) etching. Wet chemical etching solutions are used in this bulk silicon micromachining technology. 

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