Anisotropic chemical etching

O. Than and S. Buttgenbach, Simulation, of anisotropic chemical etching of crystalline silicon using a cellular automata model, Sensors Actuators A 45, 85, 1994. 

Figure 33 Hybrid multichip module stmcture with five copper conductor levels and six polyimide insulating films deposited by spin coating onto a doped silicon wafer. In this process, the integrated circuits are inserted in cavities that are created by laser drilling or anisotropic chemical etching of the silicon substrate. IC chips are bonded to the substrate by means of a heat-resistant insulating adhesive.

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

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. 

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... 

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

Plasma etching. (PE) Physical-chemical etching with free radicals, supported by ions. Etching profile anisotropic-isotropic, good selectivity. 

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. 

The types of etching process are divided into wet or dry chemical etching and isotropic or anisotropic etching as shown in Figure 2.7. 

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. 

Anisotropic dry chemical etching of silicon (reactive ion etching, RIE). 

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]. 

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.)...

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