Etching, wet

In wet etching, the substrate is etched with an etchant solution in a process that involves dipping the patterned resist wafer in the etchant solution and subsequently cleaning and rinsing it with pure deionized water. Wet etch rate is mainly dependent on the composition of the etchant solution and the temperature. Given that 

The choice of etchants is dependent on the nature of the substrate to be etched. Silicon dioxide is etched with a mixture of aqueous solution of hydrogen fluoride (HF) and ammonium fluoride (NH4HF). Paris et al. have reported the following functional expression for the etch rate in angstroms per minute of silicon dioxide  

Chromium is readily etched by aqueous solution of ceric salt, prepared from ceric nitrate, percholoric acid, and water. Chrome masks are now being fabricated with this type of etchant solution.  

Dry etching is the primary etching method currently used to transfer the resist pattern into underlying semiconducting substrates in an IC device. The pattern transfer is accomplished by removing underlying substrate materials that are not 

Parisi, S.E. Haszko, and G.A. Rozgonyi, Tapered windows in Si02 the effect of NH4p HF dilution and etching temperature, J. Electrochem. Soc. 124, 917 921 (1977). 

The etching of a solid material using chemical solution is known as wet etching. During this process, the substrate is either inside the chemical solution or the solution is sprayed on the substrate. 

---

This type of coil was prepared from copper cladded printed circuit board material by applying photolithographic techniques. The p.c. board material is available with difierent copper thicknesses and with either a stiff or a flexible carrier. The flexible material offers the opportunity to adapt the planar coil to a curved three dimensional test object. In our turbine blade application this is a major advantage. The thickness of the copper layer was chosen to be 17 pm The period of the coil was 100 pm The coils were patterned by wet etching, A major advantage of this approach is the parallel processing with narrow tolerances, resulting in many identical Eddy current probes. An example of such a probe is shown in fig. 10. 

A fonn of anisotropic etching that is of some importance is that of orientation-dependent etching, where one particular crystal face is etched at a faster rate than another crystal face. A connnonly used orientation-dependent wet etch for silicon surfaces is a mixture of KOH in water and isopropanol. At approximately 350 K, this etchant has an etch rate of 0.6 pm min for the Si(lOO) plane, 0.1 pm min for the Si(l 10) plane and 0.006 pm miiG for the Si(l 11) plane [24]. These different etch rates can be exploited to yield anisotropically etched surfaces. 

Youtsey C, Adesida I, Romano L and Bulman G, Smooth photoenhanced wet etching of n-type GaN Appl. Phys. Lett 72 560-2... 

Wentworth process Weston cell Wet air oxidation Wet ball mill Wet-end additives Wet etching Wetfastness Wet grinding... 

Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

A powerful feature of wet etching is the abiUty to achieve excellent etch selectivities of one material over another. This can be extremely useful in the fabrication of epitaxial devices with different material layers. Because selective etching allows the removal of specific layers, the final accuracy of the etch can approach that of the epitaxial layers. Etch selectivities of >100 1 have been achieved for citric acid H202 etching of GaAs—AlGaAs and InGaAs—InP stmctures (133). 

Etch Mechanisms. Most wet etches for the compound semiconductors employ oxidation of the semiconductor followed by dissolution of the oxide. For this reason, many wet etches contain the oxidant hydrogen peroxide, although nitric acid can also be used. One advantage of wet etching over dry is the absence of subsurface damage that is common with dry etching. Metal contacts placed on wet-etched surfaces exhibit more ideal characteristics than dry-etched surfaces. 

The distribution of impurities over a flat sihcon surface can be measured by autoradiography or by scanning the surface using any of the methods appropriate for trace impurity detection (see Trace and residue analysis). Depth measurements can be made by combining any of the above measurements with the repeated removal of thin layers of sihcon, either by wet etching, plasma etching, or sputtering. Care must be taken, however, to ensure that the material removal method does not contaminate the sihcon surface. 

Our Electronic Chemicals group is an industry leader in most basic manufacturing of Wet Process Chemistries, from UHP Straights (100 ppt qualities). Custom formulated Wet Etch, Solvents and Solvent Blends, and Cleaning Products. 

A boron-doped diamond electrode was used after wet etching. 

The critical operation is the etching of the sacrificial layer. With a proper wet etching process, the remaining structural layer shows a high quality surface (Fig. 8). The 10 pm thick sacrificial layer has been etched and the layer stays with a perfect plane shape. Stiffness of the structural layer is also visible in Fig. 8b where the substrate have been cleaved near a structure. Attachment points are 500 pm away and there is no bending of the structure. 

Physical vapor deposition Electron-beam evaporation Electroplating Reactive ion etching Wet etching Molecular beam epitajty Chemical-mechanical polishing Rapid thermal processing Vacuum sealing... 

Two microstructured layers of the 2x2 chip were fabricated by photolithography and wet etching in glass (Figure 4.11) [23,24]. These top and bottom layers and a third thinner layer containing holes as conduits were thermally bonded to yield the chip. The way of guiding the micro charmels, as described above, is referred to as two-level crossing. 

The chip is made from glass by photolithography and wet etching followed by thermal bonding. 

The device was realized by deep reactive ion etching (DRIE) using the SU-8 technique, producing vertical side walls [72-74]. This fabrication route was chosen to avoid crystallization, which is known to occur at sharp channel edges. Using DRIE smooth, curved corners can be realized, unlike by conventional silicon wet etching. 

The microfluidic chip system for preparing a miniaturized PMBV/PVA hydrogel consists of a two-chamber chip, an aluminum custom-made chip holder, Teflon capillaries, microtubes, and syringes equipped with a microsyringe pump (Fig. 15). The two-chamber chip was fabricated by a photolithographic wet etching technique. Whereas both channels and chambers (200 pm in depth) were fabricated on the top plate, only chambers (200 pm in depth) were fabricated on the bottom plate. 

Fig. 8.7 Fabrication sequence of a polymer microring resonator (a) prepare a nanoimprint mold (b) spin coat a polymer thin film (c) perform nanoimprinting process (d) separate the sample from the mold (e) dry etch the residual layer (f) create pedestals by wet etch...

---