Origins of Roughness

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. 

FIGURE 7.57. Schematic illustrations of the possible origins of surface roughness. 

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. 

a typical process flow for advanced ICs consists of 300 to 500 steps, 30% of which are wafer cleaning steps. Many process steps during IC fabrication may introduce contamination, which must be cleaned before the next process step. For example, in processes such as steam oxidation, resist etching, and ion implantation, metallic contamination typically introduces a surface concentration of 10 to lO Vcm. The need for wafer cleaning can be separated into three areas (1) preparation of the wafer surfaces for oxidation, diffusion, deposition, and metallization (2) preparation for the application of photoresist and (3) removal of photoresist after the etching process.  

FIGURE 7.58. Schematic illustrations of the physical location of contaminant on the surface of Si (a) on bare surface, (b) on surface covered with an oxide, and (c) within the surface oxide. 

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