Chemistry Reference
In-Depth Information
surface, the special effect of certain metals such as copper ions on anisotropy may
indicate that the copper deposition process may be anisotropic instead of silicon dis-
solution in the solution. The etch rate ratio in alkaline solutions, in contrast to
HF solutions, tends to be large and greatly change with solution composition and
etching conditions. Thus, in real practice the HF solutions, due to the near-unity etch
rate ratios, are considered as isotropic etchants whereas those of alkaline echants are
referred to as anisotropic etchants. All anisotropic etchants practically used for etching
silicon are aqueous alkaline solutions.
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Anisotropic etching can be measured by a number of methods: (1) measuring the
in-depth etch rate of samples of different surface orientations,
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(2) using silicon
samples masked by a fan-shaped pattern consisting of radially divergent segments
and measuring the lateral underetch of the segments of different orientations,
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(3)
measuring the depth of etching groove bounded by (111) planes relative to the lateral
etching under the defining mask,
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(4) using spherical, hemispherical, or cylindrical
samples and identifying the orientation of the resulting vertices and faces,
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(5)
analyzing the shape of etched surface profiles of silicon surfaces of different orienta-
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tions.
The orientations most used for anisotropic etching are (100) and (110)
wafers; (111) wafer is rarely used.
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KOH-based
etching
solutions are the most widely used anisotropic
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etchants.
These etchants show the highest etch rate ratio of (100)/(111) and
(110)/(111) planes. Ratios as high as 300-400 for (100)/(111) and 600 for (110)/(111)
can be obtained.
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The etch rate ratio is even higher at room temperature; a ratio as
high as 2500 is achievable for (110)/(111) planes in KOH solution.
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Of the organic
alkaline systems, solutions consisting of ethylenediamine, water, and pyrocatechol
(EDP) are among the most widely used. The etch rate ratio in EDP, although lower than
that of KOH, is rather high compared with other organic etchants as shown in Fig. 7.32.
The (100)
(111) etch rate ratio varies from 12.5 to 50 in the concentration range of
5-40% and temperature range of 60-90 °C.
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Etch rate ratios are almost constant with respect to doping concentration up to
the heavily doped levels where etch rate reduction occurs.
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The degree of
anisotropicity in terms of etch rate ratio of (100)/(111) depends strongly on etching
conditions. For example, in a 20% KOH solution the ratio can change from about
100 near room temperature to about 30 at 100 °C and in EDP solution the etch rate
changes with addition of hydrazine to the solution as shown in Fig. 7.33.
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Figure 7.34
shows that the etch rate of silicon in EDP is independent of the type of doping and
dopant concentration but strongly depends on orientation giving an etch rate ratio of
139:83:8.3Å/s for (100):(110):(111) planes.
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The (111) surface is the slowest etching plane in alkaline etchants in all reported
investigations.
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The fastest etching plane in alkaline solutions, on the other hand,
depends on solution composition. For example, for silicon surfaces of different orien-
tations ranging from 0° to 45° from the (010), the plane of fastest etch rate is (210)
in 35% KOH at 80 °C.
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In the binary and ternary etching systems of hydrazine and
water, and hydrazine, I
/
A, and water, the fastest plane is (211).
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In KOH, (411) was
found to be the fast etching plane in one study
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and (320) in another,
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(331)
in KOH-propanol-water system,
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and (110) in CsOH.
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Table 7.3 shows that (210)
and (310) are the fastest etching planes in TMAH solution whereas (110) and (320) are
P
