Alkaline etching

The morphology of alkaline-etched (100) and (110) silicon surfaces varies from rough surfaces that exhibit micron-sized pyramids or ridges [Sc5] to smooth orange peel-like surfaces, depending on the etchant composition and substrate doping density. Mirror-like surfaces can be obtained on (111) crystal planes. 

During the alkaline etching process the silicon surface is covered with Si-OH and Si-H. The ratio of this surface species depends on pH and temperature [Pi3, A13],... 

Acidic silicon etchants are mainly used for two purposes for the delineation of crystal defects, as discussed in Section 2.5, or to remove silicon in an isotropic manner. Isotropic etching adds another degree of freedom to the design of micromechanical structures, because all alkaline etches are anisotropic. Most isotropic etchants for silicon were developed in the early days of silicon crystal technology and exhaustive reviews on this topic are available [Tu3, Rul]. A brief summary is given below. 

Fig. 9.9 SEM micrograph of an n-type silicon electrode with an etched macropore array (5 2 cm, (100), 3 V, 350 min, 2.5% HF). Pore growth was induced by a square pattern of pits produced by standard lithography and subsequent alkaline etching (inset upper right). In order to measure the depth dependence of the growth rate, the current density was periodically kept at 5 mA cm 2 for 45 min and then reduced to 3.3 mA crrf2 for 5 min. This results in a periodic decrease in the pore diameter, as indicated by the white labels on the left-hand side. After [Le9].

In order to circumvent these shortcomings, a fabrication process based on macro PS as a sacrificial layer has been proposed [Le30]. The process sequence is shown in Fig. 10.25. First etch pits in the desired pore pattern are formed on the n-type silicon wafer surface by photolithography and subsequent alkaline etching. Then deep macropores are formed by electrochemical etching according to the... 

Cleaning can be a chemical treatment that includes acid or alkaline etching of the adherend surface. The etching process, especially, removes stubborn oxides (such as on aluminum or iron) and roughens the surface on a microscopic scale. 

A proprietary alkaline cleaner, Prebond 700, appears to be satisfactory for a number of metal adherends including titanium and is recommended as a versatile one-step surface preparation process.48 A proprietary alkaline etch solution, Turco 5578, is available from... 

Polyethylene terephthalate cannot be solvent-cemented or heat-welded. Adhesives are the prime way of joining PET to itself and to other substrates. Only solvent cleaning of PET surfaces is recommended as a surface treatment. The linear film of polyethylene terephthalate (Mylar) provides a surface that can be pretreated by alkaline etching or plasma for maximum adhesion, but often a special treatment such as this is not necessary. An adhesive for linear polyester has been developed from a partially amidized acid from a secondary amine, reacted at less than stoichiometric with a DGEB A epoxy resin, and cured with a dihydrazide.72... 

Etch rate reduction is not observed for highly doped silicon in HF-based solu-tions. It is observed on highly boron doped silicon for (100), (110), and (111) orientations in all major alkaline etching solutions.Figure 7.31 shows the dependence... 

Y. Nemirovsky and A. El-Bahar, The non equilibrium band model of silicon in TMAH and in anisotropic electrochemical alkaline etching solutions. Sensors Actuators 75, 205, 1999. 

Etching Surface modification by removal of near-surface layers by means of acids (acid etching) or alkali solutions (alkaline etching). 

Acid or alkaline etch 4. Acid or alkaline etch... 

Chemical Acid etching, e.g. chromic, nitric or mixed acids Alkaline etching, usually proprietary solutions based on sodium hydroxide. Conversion coatings... 

Chemical Air oxidation Add etching, e.g. nitric, hydrochloric, hydrofluoric or mixed adds Alkaline etching, usually proprietary solutions, e.g. TURCO 5578 sodium hydroxide or hydrogen peroxide-based solutions Conversion coating, e.g. in phosphate-fluoride solutions... 

Glasses can also be ablated with alkaline solutions, but the alkaline etching process is much less effective. 

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