Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 349

a relatively nonporous and clean back side of the finished wafer. 471 It typically removes
0.5 to 1 mil of material from each side of the wafer. It has been reported that for the
etching to remove the damaged layer, KOH solutions offer better results in terms of
yield, flatness of back side, and metal contamination compared with solu-
tions. 490 solution has been used to remove the damaged
layer on mechanically polished surfaces. 302
Chemomechanical polishing, which combines mechanical abrasion and chemical
etching, is a basic processing technology for the production of flat, defect-free reflec-
tive surfaces. A widely used chemomechanical polishing technique for silicon uses an
aqueous colloidal silica suspension. 213 The polishing proceeds by a combination of the
reactions of silicon with the solution and the mechanical removal of the reaction prod-
ucts by the polishing pad and the abrasive fluid. Electropolishing has been proposed as
an alternative technique to mechanical polishing for wafer polishing. 33,303,306

Selective Material Removal. Selective etching can be realized by using
masking, anisotropicity, pn junction, focused illumination, heavy boron doping, and
potential control. All of these methods can be used for lateral selective etching, but only
the methods that provide a boundary with a clear difference in the etch rates of the
materials making the boundary, such as a pn junction or a junction between boron
heavily doped and moderately doped materials, can be used for in-depth selective
etching. Etching over a confined area on (100) substrate in an anisotropic solution
results in the formation of a cavity as illustrated in Fig. 7.62(2). In the case of an illu-
minated pn junction as shown in Fig. 7.62(4), the photogenerated holes in the n region
are flown to the p region resulting in dissolution of the silicon in the p region. When
the illumination is confined in an area, the photoenhanced dissolution occurs either
within the illuminated area or outside of the illuminated area depending on the mater-
422,600,758

ial type. For n -type material, illumination generates a selective etching which

leads to the formation of a cavity at the illuminated area as shown in Fig. 7.62(6). On
the other hand, for p -type material, illumination results in the formation of a column
as shown in Fig. 7.62(8).
In-depth selective etching requires that the material to be removed have a much
larger etch rate than the material beneath it so that etch stop occurs at the end of the
etching. For etching alkaline solutions, heavily boron doped silicon can be used as the
material for etch stop. This is based on the difference of several orders of magnitude
in the etch rates between heavily doped and lowly doped materials. Etch stop can also
be realized using the difference in the dissolution rates of p- and n -type silicon at anodic
potentials in HF solutions as shown in Fig. 7.62(7). For example, in-depth selective
etching of a p -type silicon can be realized by anodic polarization in HF solutions using
n -type silicon for etch stop.

In-depth selective etching of silicon in alkaline solutions can also utilize the dif-
ferent passivation potentials between p- and n -type materials in alkaline solutions such
152,511,536 EDP, 112 NH 4 OH, 521,1004 hydrazine, 205,462448 and TMAH. 516 In this method,
as shown in Fig. 7.62(9), an anodic voltage sufficient to cause passivation of n -Si is
applied via an ohmic contact. Due to the potential drop in the reversely biased pn junc-
tion, the p-

as KOH,

is maintained at a potential negative to the passivation potential and is
etched. On complete removal of the p -Si, the junction disappears and the etch stops
because the n -Si is passivated. A current peak, corresponding to the formation of the

Si