Online proceedings - EDA Publishing Association

11-13 May 2011, Aix-en-Provence, France
seismic mass and the length of the supported beams in Figure
3 and Figure 4(a) are listed in (3) to (4).
W = 2H (1)
L =3. 2H (2)
m
> 23 HW
(3)
W3 > 21.1
H
(4)
Some other comer compensations suggested a oriented
band and fan-like [110]-oriented side beams [6] and a
modified band design [7] to reduce the required groove
width, W 3 for the compensation pattern. However, if the edge
width for the corner compensation is limited, such as a small
seismic mass with large etching depth, compensation designs
fail due to the overlap of neighboring patterns as shown in
Figure 4(b). If overlap of compensation occurs, the etched
mesa dimensional will be incorrect and convex corners will
undercut.
Wn = 2*H - 0.0007C 2 - 0.02C (5)
Ln= 3.2*H + 0.0101C 2 - 3.66C (6)
B = 1.91C (7)
where C is the overlap of adjacent conventional -oriented
bands as shown in Figure 4(b).
(a)
Figure 4 Overlap of adjacent -oriented bands due to limited width of
square
This study presents a corner compensation for a mesa with
high aspect ratio using oriented compensating bands
augmented with a mandatory separation and a -oriented
beam to the truncated bands due to the overlap of
adjacent compensation patterns. An empirical equation is
presented from the simulation of anisotropic etching using
Intellisuite.
II. METHODOLOGY
In the wet etching of seismic mass, conventional oriented
bands of corner compensation is good if the ratio
between the etching depth and the width of the mesa top is
smaller than 0.35. If the aspect ratio (H /W m ) is larger than 0.35,
the adjacent compensation patterns overlap. Here a modified
design is presented as shown in Figure 5. The overlap bands are
truncated with a separation of 20 (μm). A [110]-oriented beam
is introduced to the truncated pattern. The width and the length
of the band and beam are related to the etching depth (H). An
empirical formula for KOH etching is presented from various
simulations using Intellisuite and least squared curve fitting as
shown in (5)~(7).
(b)
Figure 5 A modified -oriented bands for limited width of square.
If the aspect ratio of mesa is smaller than 0.35, a
conventional oriented band compensation will perfect
convex corners. If a small mesa is fabricated using deep bulk
wet etching, the proposed design provides a satisfactory convex
corner for the seismic mass with aspect ratio between 0.35 and
0.625. Several compensation design based on the proposed
method are shown in Figure 6 to Figure 8. With the increase of
aspect ratio, minor corner undercut presents. If the aspect ratio
is larger than 0.625, corner undercut will still aggravate.
The verification result for a mesa with width of 400 (μm)
and etching depth 250 (μm) is shown in Figure 9. The etchant
is 30% 80°C KOH. The corresponding aspect ratio of the mesa
is 0.675. Only minor undercut can be observed in convex
corners using the proposed compensation design, while typical
compensation using simple -oriented bands fail
completely for this case.
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