Page 231 - Sami Franssila Introduction to Microfabrication

210 Introduction to Microfabrication

(111) slope formation (110) slope formation (100) slope formation (311) slope formation at (311) slope growth
under the etching mask the intersection between
(100) and (111) planes
(100)
(110) (100) (100) (100) (100)
(110)
A C (311)
B
B (111) C (111)
(111) (111)
(111) (111) (110) (111) (110) (111)
(100) (100) (111) (311)
A (110)
Etching mask
(100) (110)
(100) (100) (100) (100)
(100)
(311)
(100)
A-A cross-section A-A cross-section A-A cross-section A-A cross-section A-A cross-section
(110) (100) (100)
(311) (311) (311) (311)
(111) (111) (111) (111)
(111) (111)
(111) (111) (111) (111)
B-B cross-section B-B cross-section C-C cross-section C-C cross-section C-C cross-section
(a) (b) (c) (d) (e)
Figure 21.8 Convex corner undercutting time evolution. Reproduced from Shikida (2001), by permission of Springer

Figure 21.9 The effect of mask polarity on shape: top row; initial mask opening; bottom row and etched shape (oxide
mask shown grey)

Time evolution of various structures, with convex and Condition 21.1 leads to vertical walls that are (100)
concave corners, are shown in Figures 21.8, 21.9 and planes, and Condition 21.2 leads to 45 walls that are
◦
21.10. (110) walls. This is shown in Figures 21.11 and 21.12.
If the structures are aligned along the [100] direction KOH etchant, 25 to 50%, fulfils Condition 21.1, and
(45 relative to wafer flat) instead of the usual flat KOH–IPA solution is an example of Condition 21.2.
◦
direction [110], new possibilities arise. For instance, When the rate condition is close to limit values, as is
◦
45 walls suitable for fibre coupling mirrors and 90 ◦ the case with <25% TMAH, inadequate stirring or some
sidewall mesas can be made. These structures depend on other disturbance can lead to unexpected changes in
relative etch rates of (100) and (110) planes according final shapes.
to Conditions 21.1 and 21.2: If double-sided lithography and etching is done
(to be discussed in more detail in Chapter 28), more
√ elaborate shapes appear, for example, vertical sidewalls
◦
rate{100}/rate{110} < 1/ 2 90 walls (21.1)
√ and inward slanted (111) planes. This is illustrated in
◦
rate{100}/rate{110} > 2 45 walls (21.2) Figure 21.13.