Chemistry Reference
In-Depth Information
7
Etching of Silicon
7.1. INTRODUCTION
Etching, in the context of this topic, refers to the dissolution processes that uniformly
or preferentially remove material from silicon crystals immersed in a solution. Etching
of silicon has been extensively explored due to its useful applications in the fabrica-
tion of electronic devices. Since the 1950s, when etching started to be used in device
fabrication processes, numerous investigations have been carried out to develop
and characterize the etching systems for micromachining, delineation of defects,
surface polishing, and so on. The amount of information generated from these
investigations is enormous considering the diversity of circumstances under which
etching is performed, and the infinite possible combinations of solutions, materials,
processing procedures, and structure/geometry factors that may be involved in a given
circumstance.
The information considered in this chapter is limited to the etching aspects that
are directly relevant to the electrochemical properties of the silicon material, whereas
those related to etching procedures and structure/geometry are not dealt with in detail.
Emphasis is put on the planar etch rate, which is the most important quantitative para-
meter in an etching solution. The spatially preferential, or nonplanar, etch rates are dis-
cussed in relation to the planar etch rates. The preferential etchings caused by the effect
of doping and crystal orientation, which are widely utilized in device fabrications, are
dealt with in detail particularly the mechanisms in relation to the large amount of elec-
trochemical information presented in the other chapters. Also, the surface roughness
resulting from etching, which essentially affects all aspects of silicon technology, is dis-
cussed in a separate section. Finally, etching as a process used in the various applica-
tions involving the fabrication of devices is briefly reviewed.
7.2. GENERAL
The two principal etching solution systems for silicon are HF solutions and alka-
line solutions. This is because silicon is inert in aqueous solutions due to the formation
of an insoluble surface oxide except for HF solutions or alkaline solutions in which the
oxide is soluble. Various chemical agents can be added to these two solutions so as to
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