Transcript Design and Simulation of a MEMS Piezoelectric - Asee

Design and Simulation of a MEMS Piezoelectric Micropump
Alarbi Elhashmi, Salah Al-Zghoul, Advisor: Prof. Xingguo Xiong
Department of Biomedical Engineering, University of Bridgeport, Bridgeport, CT 06604
Introduction
Micropumps are MEMS (Microelectromechanical Systems) actuators used to drive
microfluid to flow toward certain destination. Micropumps usually refer to the pumps
that are fabricated on the scale of microns (1μm=10-6m). Various micropumps based
on different actuation techniques have been reported, such as electrostatic
micropumps, piezoelectric micropumps, thermodynamic micropumps, smart polymer
micropump, magnetic micropumps, etc. Among them, piezoelectric micropumps are
attractive designs due to their easy operation and large pumping force. In a
piezoelectric micropump, a pumping chamber is connected to inlet and outlet ports,
with one-way microvalves to regulate the pumping flow direction. The pumping
chamber is sealed by top diaphragm, with a piezoelectric actuator deposited on top of
it. When AC driving voltage is applied, piezoelectric actuator will expand and shrink
periodically, causing the diaphragm to bend up and down. Hence the microfluid is
sucked into the chamber from inlet, and pumped out from outlet periodically. Due to
their small size, low cost, low power consumption, and high efficiency, micropumps
have been widely used in many applications in the field of Biomedical Engineering
such as Micro Total Analysis System (μTAS), Lab-on-a-chip, and micro drug delivery
systems.
Values
2000
Length of valve (μm)
210
Thickness of pump wall (μm)
400
Width of pump (μm)
4000
Length of pump (μm)
4000
Thickness of PZT (μm)
8
Length of PZT (μm)
800
Width of PZT (μm)
800
Height of pump (μm)
610
Driving voltage (v)
50
Operating frequency (Hz)
60
Flow rate (µL/min)
149.4
1
1200um
Figure 4. Dimensions of the top view
Piezoelectric Micropump
Figure 5. First vibration mode of
micropump (f = 0.634kHz)
Figure 6. Second vibration mode of
micropump (f = 0.641kHz)
Figure 7. Third vibration mode of
micropump (f = 0.817kHz)
Figure 8. Fourth vibration mode of
micropump (f = 0.826kHz)
Piezoelectric Micropump Design
In this poster, a bulk-micromachined piezoelectric micropump is proposed. The
proposed micropump consists of two parts: top and bottom silicon structures with
piezoelectric material located in the top diaphragm. Both top and bottom structures are
shown in Figure 1. In the top silicon structure, it is a square shape that has a gap in
the middle. The square shape also has two inlet and outlet valves. In the bottom
silicon structure, it has a gap in the middle with two openings. Both top and bottom
structures are bonded together via silicon direct bonding technique. When the
piezoelectric material bends the diaphragm down, the outlet valve will open to allow
fluid to exit the system while the inlet valve will be closed and vice versa.
800um
400um
Width of valve (μm)
800um
Measurement Variables
600um
1200um
MEMS micropumps are important components for many bioMEMS devices such as
Micro Total Analysis System (μTAS), Lab-on-a-chip, and microdrug delivery systems.
Many activation techniques can be used in pumping the microfluid in MEMS pumps;
among them, piezoelectric activation is an attractive one due to its low cost and simple
fabrication. In this poster, the design and simulation of a MEMS piezoelectric
micropump is proposed. The device consists of top and bottom silicon structures that
are bonded together. Two valves are embedded in the pump on the both side: one
inlet, and one outlet. In addition, the piezoelectric material is bonded on the diaphragm
on the top part. When voltage is applied to the piezoelectric actuator, the membrane
bends down and up, which makes the inlet and outlet valves open and close, with both
valves moving in the same direction. Hence, fluid is pumped into the chamber from the
inlet valve and pumped out from the outlet valve. A theoretical model is used to
analyze the working principle of the micropump. Based on analysis, a set of optimized
design parameters are suggested. ANSYS simulation has been used to verify the
function of the device. A fabrication flow has been suggested for the fabrication of the
micropump. The proposed micropump can be used for various bioMEMS applications.
2000um
Table 1. The optimized design parameters
of the MEMS Piezoelectric Micropump
4000um
Abstract
Figure 9. Fifth mode of piezoelectric
micropump (pumping mode, f = 0.862kHz)
1 - Diaphragm
2
2 - Piezoelectric Actuator
3
3 - Top cover
4 - Valve
4
5 - Chamber
5
6 - Bottom Structure
Figure 10. Sensitivity analysis when
experiencing pressure of 10 MPa
6
Figure 1. 3D view of the structure of a MEMS Piezoelectric Micropump
Theoretical Analysis
Assume the diameter, thickness, and operation voltage of the PZT material are d, h and u
separately, the frequency of operation is f (~60 Hz). The resulted flow rate of the
micropump can be derived by calculating the stroke volume. Since the PZT material in
design has a square shape, we can approximate the diameter by calculating the volume
of a cube and comparing it with the volume of a cylinder. In our micropump design, the
diameter is 902.7 µm.
Vstroke = 8 x 10-11 x (d / h) x u
Vstroke = 4.15 x 10-12 µL
Flow rate = Vstroke × f
= 4.15 x 10-2 X 60
= 2.49 µL/sec
= 149.9 µL/sec
Figure 11. Stress contour plot when
experiencing pressure of 10 MPa
Device Fabrication
The device is fabricated with bulk-micromachining process, as shown in Figure 12.
The top and bottom silicon structures are fabricated with DRIE (Deep Reactive Ion
Etching) etching separately. Aluminum is used as etching mask for Si DRIE etching.
After that, the top and bottom structures are aligned and bonded together with Silicon
Direct Bonding (SDB) technique. The steps are listed as follow: (1) deposit Al layer on
Si wafer; (2) photolithography to define the chamber and valves; (3) etching a small
area to define the chambers of the pump; (4) Remove Al layer and re-deposit Al layer
as etching mask; (5) Use photolithography to define the valves (6) mask-less etching
until the valves structure are formed (7) the bottom structure will follow the same
steps. (8) top and bottom structures are aligned and bonded together with silicon
fusing bonding, device is fabricated. For each selective silicon etching, it has seven
sub-steps: (1) Oxidation; (2) Photoresist spinning; (3) Photoresist patterning; (4) Al
etching; (5) Photoresist removal; (6) Silicon DRIE etching; (7) Al removal. During the
bonding process, precise alignment is required to ensure the proper function of the
device.
Fabrication
process of top
structure
Design Optimization and Simulation
Based on theoretical analysis, we derived a set of optimized design parameters of the
piezoelectric micropump, as shown in Table 1. According to the design parameters,
the piezoelectric micropump has the intrinsic vibration frequency of 60 Hz. ANSYS
FEM simulation is used to extract the vibration modes as well as the stress distribution
of the device. The bending shape of the diaphragm structure under given voltages
can also be observed. The ANSYS simulation results are in good agreement with
theoretical prediction. The first five vibration modes and the corresponding resonant
frequencies of the micropump are shown in Figures 5 – 9.
3. DRIE etching of Si
4. DRIE etching of Si
2. Photolithography
5. Photolithography
1. Deposit Al layer
Fabrication
process of bottom
structure
4. Deposit Al layer
Bonding of top and
bottom structures
Figure 12. Fabrication flow of the micropum (cross-sectional view)
Conclusions and Future Work
Figure 2. Undeformed shape of the
micropump (top structure)
Figure 3. Undeformed shape of the
micropump (bottom structure)
In this project, a bulk-micromachined piezoelectric micropump design is proposed. The
working principle of the micropump is analyzed in details. The bending of diaphragm
and pumping rate of the micropump are derived. Based on the analysis, an optimized
design is suggested. ANSYS simulation is used to verify the performance of the
device. The fabrication flow of the device is also proposed. The fabrication is based on
Si DRIE etching and silicon direct bonding (SDB) techniques. In the future work, we
will further improve the device’s performance, such as increasing the pumping flow
rate and reducing the driving voltage of the micropump.