1. Hanoi 5-2013
Class: Materials Science Engineering
Doctor : Tr nh Quang Thôngị
Presenter : Hoàng Văn Ti nế

2. Contents
- Introduction
- Experimental method
- Experimental results & discussions
- Conclusions and References

3. Introduction
 Anisotropic etching :
In semiconductor technology ,isotropic etching is
non-directional removal of material from a
substrate via a chemical process using
an etchant substance. The etchant may be a
corrosive liquid or a chemically active ionized gas,
known as a plasma.
 Anisotropic chemical wet etching is one of the key
technologies for fabricating microstructures on a
single-crystal silicon wafer

5. Experimental method
 Purpose : compare the anisotropic etching
properties of KOH and TMAH
(Tetramethylammonium hydroxide)
 Solutions :
 KOH : strong bases
 TMAH : quaternary ammonium hydroxide (QAH)
solutions

6. Specimens
 Using hemispherical
specimens of single crystal
silicon whose surface
exhibited every
crystalographic orientation ,
in order to evaluate the
etching properties as a
function of the orientation.
 The orientation dependence
in the etching rates of the
surface crystals significantly
differed between two
etchants
solid hemispherical specimen of single-
crystal silicon :
- R= 22 nm
- sphericity less than 10 µm
- surface roughness : 0.005-0.007 µm

7. Measuring the profile
 All crystallographic orientations appeared on the
hemispherical surface
 Measuring the profile before and after etching and noting
the change enabled us to calculate the etching rate for any
orientation.
 The profile was measured using a UPMC550-CARAT
(Carl Zeiss , made in Germany ) 3-D measuring machine.
 The surface profile was probed every 2 o
of latitude ranging
from 20 o
to 90 o
and every 2 o
of longitude ranging from 00
to 3600.
 The total number of probe points was 6480

8. Etching conditions

9. Etching conditions
 Etching bath(made of Teflon) was immersed in a
water bath that had heater elements embedded in
the walls and bottom.
 The silicon specimen was first heated to the etching
temperature in a dry container( immersed in the
water bath)
 Transfer the specimen into the etching bath
containing 1 liter of etchant.
 The specimen was held in a Teflon basket so that its
hemispherical surface was at least 5mm away from
the interior of the basket

10. Etching conditions
 Control the etching conditions:
 A silicon chip was thrown into the etchant prior to the
experiments to estimate the time necessary to etch the
hemisphere
 Fresh etchant was used in every subsequent experiment
 A magnetic stirrer was used to equalize the temperature
in the water bath
 The temperature distribution in the etching bath was very
uniform (no variation being detected in the vertical
direction along the center axis of the etching bath)
 The stability of the temperature : ± 0.9 0
C

12.  Comparison of the morphologies at four orientations of the surface etched
in KOH and TMAH solution

13. ORIENTATION-DEPENDENT ETCHING
RATES

14. Etch rate contour map as a
function of orientation
Comparison of
The contour
pattern of the
etching rate in
KOH and
TMAH solutions

15. Comparison of etching rate distribution patterns between KOH and TMAH solutions

16. Effects of concentration and etching
temperature
Dependence of
the etching rates
on the
concentration of
the KOH and
TMAH solutions


17.  Dependence of
the etching
rates on the
temperature of
the KOH and
TMAH solutions


18. Variation in the range of the
measured etching rates
Variation range in
etching rates among
four equivalent (110 )
planes at the same
latitude of 300
on the
same hemisphere
normalized by that of
the (110 ) plane at the
top

19. Variation in the range of
the measured etching rates
The effects of etchant circulation can be
ignored in the case of KOH solutions, but not in
the case of TMAH solutions, especially when
the concentration is 10 wt.%. In the case of
20wt.% TMAH, however, the effects of etchant
circulation were negligible

20. Roughness of etched (100)
surface

21. Summary of the results

22. conclusions
 The orientation dependence was quite different for the ( 111 ) and ( 221)
planes in TMAH and KOH solutions. This suggests that there are
differences in the etching mechanisms of the two etchants in terms of
crystallographic orientation. The etching rate ratio of (111)/(100) in TMAH
was about twice that in KOH .
 The etching rates depended on the concentration of KOH and TMAH
solutions. Many orientation planes had a peak in the etching rates as a
function of the concentration of the solutions. The peak in KOH was at 25
wt.%. The peak in TMAH was at 20 wt.%.
 The activation energies in KOH and TMAH were almost the same for the
(100) , (110) , and (320) planes but not for the (221) and (111) planes.
The activation energies for the (110) and (111) planes at KOH
concentration of 34 wt.% were 0.60 and 0.50 eV, respectively.
 The effects of etchant circulation can be ignored for KOH, but not for
TMAH solutions.
 The roughness of etched (100) surface, which is the smoothest of all the
orientations, was 0.01 m m with KOH and 0.4 m m with TMAH solutions.
The roughness in-creased as the etchant concentration decreased.

23. References
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