Etch hillocks

Since dislocations are linear strain fields, if a crystal is treated in an appropriate atmosphere, impurity ions selectively precipitate along the dislocation. These can be detected by infra-red microscopes, and so the method was used to prove the presence of dislocations during the early period of dislocation studies. If a dislocation is decorated by metallic elements, the dislocations act as a resistance against etching, and only the portion apart from dislocations is etched, and decorated dislocations remain as protrusions. The resulting protrusions are etch hillocks. 

M n (111) photocurrent onset (fig. 1 (n) (b) region I) hole supply, controlled by e — h photogeneration and recombination etch hillocks and ridges... 

When etching n-GaP photoanodically in the onset region I of the photocurrent-potential curve (Table 2, cases M, N, O), etch hillocks and ridges are observed at the surface after removal of only a small amount of material (see, e.g., Fig. 20). In this region of the i- V curve, recombination of the photogenerated charge carriers... 

Photoetched (111)- and (TTT)-faces of GaP in alkaline OBr solutions show a remarkable difference in terms of morphology etch hillocks develop on n-GaP (similar to those formed by photoanodic etching of n-GaP in the onset region of the photocurrent-potential curve), whereas etch pits are formed on p-GaP (similar to those formed by anodic etching of p-GaP in the rising part of the i- V curve) [31]. 

In the case of GaAs photoetching in Cr03-HF solutions, etch hillocks are formed both on n- and p-type crystals [113-114]. Hillocks, however, are also observed in the dark, and their occurrence has been explained on the basis of the complex reaction scheme described by Eqs. (58)-(61). The Cr03-HF etch has been found to be very sensitive to defects at GaAs and even more so at InP crystals [115]. 

It is well known that dislocation etch pits on the surfaces of metals are produced in solutions of salts of other metals as a result of contact displacement reactions (53)(54). The size of pits formed by these solutions depends on the concentration of a salt and the time of etching. However, prolonged etching often leads to the precipitation of mono- or polycrystalline displaced metal at relatively more active sites where dissolution is faster than that at the rest of the surface. Subsequent etching can yield etch hillocks, as observed In the case of etching white tin in acidic solutions of CuSO. Whether etch hillocks or etch pits will be formed at dislocation sites Is determined by the exchange kinetics at the electric double layer and by the diffusion kinetics. 

In addition to pit formation, it is possible for hillocks to be formed on the surface. Hillocks may be formed from humps on the pre-etched surface, protective spots on parts of die surface, or the growing together of etch pits. 

Batterman (31) has attempted to explain the appearance of facets on hillocks in terms of only the measured orientation dependence of the etch rate. Using this, he concluded that the 322 planes are stable hillock facets on germanium etched with an HF + H2O2 + mixture. Irving (32) in a further analysis... 

An attempt was made to show that not all pits or hillocks produced on surfaces by any of the various etching processes are formed at dislocations. Some of the causes of etch pitting were discussed, and techniques for differentiating between the various pits were described. 

Figure 11.22. Schematic sequence of diamond nucleation on an Si hillock (a) initially deposited carbon configuration, and H ion etching, (b) etching of Si surface and formation of tilted diamond nucleus, and (c) growth of the nucleus, and formation of a Si hillock due to H+ ion etching [308],...

Tetramethyl Ammonium Hydroxide (TMAH). TMAH, (CH3)4NOH, etchant has been developed more recently for anisotropic etching of silicon. It is relatively safe to use and presents no special disposal issues. It has a very low oxide etch rate and does not attack aluminum if the soluhon contains a certain amount of silicates. However, hillocks tends to develop in this solution. ... 

In TMAH solutions a smooth etched surface is obtained at concentrations above 22% TMAH. Addition of l-4g/liter pyrazine to 25% TMAH solution increases slightly the etch rate and improves the surface smoothness. Dissolved silicon in the etchant results in higher roughness as shown in Fig. 7.51. In the ternary etching system of hydrazine, IPA, and water,the etched surfaces vary from being highly faceted to very flat. The faceted surfaces are made up of square pyramidal hillocks whereas flat surfaces are marked by a cellular structure. In ethanolamine-based etchants, the roughness of etched surface appears to be associated with the formation of precipitates. ... 

Fig. 7.55. At a microscopic scale, roughness is associated with lattice steps, vacancies, and so only which are determined by the lattice structure of the smface. At a macroscopic level, crystallographic character may be revealed in the topographic features, for example, hillocks formed on the (100) surface. It has been found that the etched surface in 25% KOH has well-defined terraces and step features, whereas it has a nodule type of appearance in 50% KOH solution. Table 7.5 provides a summary of the characteristics of the surface etched features in KOH with respect to the crystal orientation of the surfaces.

In particular, the hillocks formed on the (100) surface are crystallographic structures bounded by four (111) planes resulting from the anisotropic etching and their formation directly contributes to the roughness of the surfaces. Hillocks may, under certain conditions, form in HF-based solutions, " but most commonly in alkaline etchants. ° Because the (111) surface etches very slowly, the formation of hillocks leads to a decreased etch rate. ... 

Roughness developed in a cleaning or etching solution is a result of uneven dissolution across the crystal surface. Many factors in an etching process may cause the uneven distribution of the dissolution rate at both micro and macro scales. One particular example is the roughness associated with the formation of hillocks. In general, any process that causes temporary or permanent surface inhomogeneity will result in preferential dissolution of some areas relative to other areas. 

The surface of the silicon crystal, no matter how it is finished, will have a certain number of lattice defects, which tend to dissolve preferentially resulting in formation of etch pits and other features. Terraces and steps of various sizes are inevitable consequences of anisotropic dissolution of the surfaces misoriented from the (111) surface. Also, a silicon surface, whether initially smooth or not, in HF solutions, has an intrinsic tendency to roughen and form micropores governed by sensitivity of the electrochemical reactions on a semiconductor electrode to surface curvature. Furthermore, the two groups of factors shown in Fig. 7.57 may affect each other. For example, the initial lattice inhomogeneities may provide the sites for deposition whereas localized deposition may enhance the development of etch features such as pits or hillocks. 

F. Shimura, TEM observation of pyramidal hillocks formed on (100) silicon wafers during chemical etching, J. Electrochem. Soc. 127, 551, 1980. 

Formation of pyramidal hillocks bounded by (111) crystal planes on (100) wafer is a problem in NH4OH solutions for all concentrations, temperatures, stirring conditions, and sample preparations. The formation of hillocks significantly reduces the etch rate in the (100) direction [88]. Addition of H2O2 in NH4OH solution has an effect of suppressing the formation of hillocks. 

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