Surface etching facets

Since one desires only light reflections from etched facets, the entire surface of the sample should be covered with etch pits. This can be obtained by etching an abraded surface. Fig. 5 shows the results of the action of a preferential etch for germanium on an abraded and on a polished surface. The multitude of pits on a lapped surface raises an interesting question are these pits caused mainly by the multitude of Irregularities produced an the surface by abrasion or by the multitude of mechanically induced dislocations In an attempt to answer this... 

In a study of the dissolution of silicon in NaOH (51), it was found that the ill surfaces have a slower etching rate than the 100 and 110 surfaces. Furthermore, from Fig, 7 it can be seen that within experimental error, the 100 and the llO surfaces etch at the same rate. It is known that the slowest etching planes form the limiting facets on a concave surface, while the fastest etching planes form die limiting facets on a convex surface. To prove that the ill planes had die slow. 

A certain amount of uniform dissolution may occur prior to and during the initiation of pores. For example, as reported in one study, before the formation of the macropores on lowly doped p-Si( 100) in anhydrous HF-MeCN solutions the entire surface is etched forming (111) facets of about l xm. Pores then start to grow at the base of these facets. Once macropores are developed the surface etch rate is greatly reduced, by a factor of 4. 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

According to a second class of model, thermal etching is driven by a need to reduce total surface free energy. According to this theory, faceting will take place even in the absence of any net weight loss. The first to suggest a model of... 

Hondros and Moore (22) also found that heating under vacuum or in nitrogen did not lead to the formation of facets. The latter produced smooth surfaces, and the former led to pit formation. From these data and the studies of earlier workers, the authors concluded that the mechanism of silver etching is the preferential evaporation of silver oxide from certain surface planes. 

Experiments conducted by the same group for the thermal etching of silver in an oxygen atmosphere suggest that evaporation of metal plays little role in the thermal etching process. The thermodynamic model appears to best explain the observations. That is, identical faceted surfaces formed both in the case of suppressed and free evaporation. 

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