1. Device Characterization: Testing :
Photolithography
- Apply negative photoresist. SU-8;
- Mask and exposure to UV light during 40-50
sec (intensity of UV light: 2.6mW/cm^2)
- Post bake on hotplate for 60s at 120 oC;
- Remove developed photoresist with TMAH
solution for 55 seconds;
- Rinse twice with DI water for 30s each time
Oxidation
- Grow SiO2 layer during 24hrs at 1100 oC
MEMS Simulation:
FEA simulation:
Ansys is used to simulate the deflection of
a diaphragm with characteristic length of
4000µm
, with entire surface fixed (except
membrane area), under 14.5 psi vacuum.
Deformation result is shown below. The
maximum deformation is 0.4µm, at the
center.
Figure: Total deformation of MEMS diaphram
Analytical calculation:
Motivation:
Automotive market has become a key driving market
for Micro-Electro-Mechanical Systems (MEMS) in
recent years. Various MEMS sensors have been
included to prevent accidents, provide better user
experience and reduce pollution. Among others,
MEMS pressure sensors, play an important role in
many applications. Small size, cost-effective, long
lasting, and low energy consuming, appropriate
piezoresistive automotive pressure sensors are widely
employed in engine, tire, as well as passenger
compartment, to reduce emissions, enhance safety and
improve users’ comfort. This rises our interest in
designing, fabricating and testing a simple membrane
that can be included in a piezoresistive pressure
sensor.
PIEZO-ELECTRIC DIAPHRAM BULK MANUFACURING
ME169 MEMS Design and Fabirication Spring 2016
Design and Manufacturing Team: Shahriar Shariat, Kevin Sayo, Yue Guo
Faculty Mentor: Professor Green, PhD
Fabrication method:
Materials selection
- 4x 10cm <100> silicon wafer, polished both
sides;
- 4x 10cm <100> silicon wafer, polished single
side
Background Research:
In MEMS fabrication, reliable etching of silicon is the
most important factor. In this project, TMAH is
chosen as etchant for the anisotropic etching, as 1) it
has high selectivity towards silicon dioxide film, 2)
non toxic and 3) CMOS compatible.
Relation between etching rate and parameters such as
TMAH concentration and process temperature is
investigated in order to achieve highest performance
possible.
Figure: Anisotropic etch rate of <100> oriented Silicon as a function of
temperature and TMAH solution concentration
As a result, 25% TMAH and 95oC are chosen for the
process.
Boron etch stop
- 4x Boron etch stop: B-155 spun on one side of
each wafer;
- Low temp prebake (180-187C, 10 mins);
- Drive in furnace at temperature of 1100 oC
for 1hr 15min;
- Remove Boron with HF solution during 25
minutes
Mask design
- Designed mask to allow creation of 15 sizes of
diaphragm
Bulk micromachining
- Native oxide is stripped before each etching
session
(since TMAH selectivity of Oxide vs. Silicon
can prevent any etching at all);
- Etch and check progress regularly (since etch
rate varies significantly during process thus
causes very unpredictable depths
(Etch rate varies from ~40µm/hr to ~75µm/hr
with 25% TMAH, at 95oC)
Strip exposed silicon oxide to expose bare
silicon
- Opened Diaphragm holes (BOE to strip oxide)
~20 minutes until water sheets off
(hydrophobic);
- Remove photoresist
Figure: Final product
Analysis of Results:
Most wafers broken during etching or
rinsing process where grill was designed.
This undesired result is caused by silicon
lattice structure. Research shows that when
a <100> oriented silicon wafer is patterned
with an etch mask, the etch front is
bounded by the <111> planes. The final
shape of the etched pit is a V-groove or an
inverted pyramid. The sides are aligned to
the <110> planes independently of the
initial shape of the etch mask.
Measures under profilometer show
significant deflection on a 4000µm*4000µm
membrane with a thickness of 250µm,
when under vacuum conditions (up to
14.5psi). However, measurement was
biased by the deformation of gasket. True
deflection should be close to 1µm range,
according to simulation.
Design Analysis and Calculations:
The angle between side walls and the <100> plane
(54.7 degrees) is taken into account when designing
circular openings on the mask.
Figure: Side profile after anisotropic etch
Acknowledgements:
We would like to thank Dr. Green, Professor Lee
and Neil Peters for his knowledge and support
throughout this project.
Conclusion :
Overall the project was carried out with
success: final product met design
characteristics, functionality is tested and
result was as expected.
Selected references :
[1] Colin. (2011, Febrary 21). MEMS Library. Retrieved
from MEMS Library: http://memslibrary.com/
[2] Barakat, Nael; Plotkowski, Alexander; and Jiao,
Heidi, "Design and Computational analysis of
Diaphragm Based Piezoresistive Pressure Sensors for
Integration into Undergraduate Curriculum" (2011).
Faculty Scholarly Dissemination Grants. Paper 207.
Figure: Test settings Figure: Measurement settings
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𝒑𝒃 𝟒
𝑬𝒉 𝟑
= −𝟎. 𝟎𝟏𝟑𝟖
𝟗𝟗𝟗𝟗𝟕𝟒( 𝟎. 𝟎𝟎𝟒 𝟒
𝟏𝟗𝟎𝟎𝟎𝟎 ∗ 𝟏𝟎 𝟔 𝟎. 𝟎𝟎𝟎𝟐𝟓 𝟑
= 𝟏. 𝟏𝟗𝝁𝒎
-5 inMg
0 inMg
-15 inMg
-10 inMg