Piezoelectric Pressure Sensor
By Lindsey Duffy - EMNT
Department of Electrical Engineering and Electronics,
The University of Liverpool, Brownlow Hill, Liverpool, L69 3GJ, UK
1. Introduction 3. Simulations & Results
Pressure sensors are used to measure the change in pressure in liquids or
3 simulations can be run as a means of analysis of the piezoelectric
gases. The main features of a pressure sensor include, fast response,
pressure sensor model. One is a MemMech simulation, the other a
ruggedness and high stiffness . They can be used in aircrafts, cars,
MemElectro and last but not least is a CoSolveEM.
weather instrumentation and in a lot of other applications that have
pressure functionality implemented.
MemMech: Good when trying to find mechanical, thermomechanical &
Typically in MEMs devices, pressure sensors are made from silicon and piezoelectric effects. Boundary conditions (BC) of pressure loads,
these can be used as car tyre pressure sensors and in disposable blood temperature, etc can be set.
pressure sensors. P = F/A - Pressure (Pascals - Pa) is measured as a means of force over area
applied to an object in a direction perpendicular to a surface. Surface BC of patch fixing & loading can be set. ‘FIX ALL’ fixes the
patches at the top of the pressure sensor. ‘LOADPATCH’ loads the bottom
A piezoelectric pressure sensor (Fig.1) is a device that uses the
patch of the substrate & specifies a pressure .
piezoelectric effect to measure pressure by converting it to an electrical
A pressure of 0.0 to 0.2MPa applied – results showed displacement in the
signal and vice-versa. Piezoelectric sensors are considered to be self-
Z – direction.
generating, as they do not require an external power source to operate
. Piezoelectric materials can be categorised into two groups: crystals For the second model, pressure of 0.0 to 0.0001MPa (1millibar) applied.
and ceramics. The most common piezoelectric material is quartz (SiO2)
but silicon is a commonly used material for building small pressure sensors. Figure 5: Displacement
in the Z, Y, X
Figure 1: An example of a piezoelectric pressure
sensor - Piezoelectric pressure sensors use stacks
of piezoelectric crystal or ceramic elements to
convert the motion of a force-summing device to an
electrical output .
Displacement shows how much a device has
moved . Deeper colour = more displacement in that
area. Red = displacement is at the max in that area
2. Model Stress = force causing deformation/area to which force is applied -
How does a piezoelectric sensor work? For example, when a polarized = F/a.b when applying F electrostatically (Pa or Nm-2).
crystal, such as quartz, is put under pressure, mechanical deformation
takes place, or when the crystals are strained and this is when an More stress in Y
direction = Red.
electric charge is generated. The electric charge can then be
measured using a piezo sensor . This electric charge is usually in the
Rxn Forces stronger
form of voltage or current. on edges of fixed
Figure2: Layout editor – Gnd = faces.
Fabrication of Model: Blue, Membrane = Green &
Beam = Red Figure 6: Stress XY. Figure 7: Mises Stress. Figure 8: Reaction Forces Z.
Process Editor - Layout Editor (Fig.
A „modal analysis‟ can be run to perform a piezoelectric analysis on
2) - Solid Model (Build Mesh) -
the piezo materials – in this case they are Si and ZnO. Piezoelectric results
Analysis = MemMech, MemElectro
include; Displacement, Stress, Electrical Potential and Reaction Force .
But this simulation is too complicated to perform.
Substrate = Silicon_100 Figure 3: Silicon_100 MemElectro: This simulation analyses the electrostatic and
mesh model, illustrating
electroquasistatic effects of a model. BC can be set for a changing
KOH Wet Etching (back etched the pyramid-like
voltage or charge. Applying a voltage to a conductor essentially
substrate) – etches quickly in structure.
identifies it as a conductor to Coventor . E.g. 5V in expect 5V out (or
<100> & <110> but slowly in <111>.
Get a pyramid-like structure (Fig. Figure 4: Silicon_100
model, with the ZnO CoSolveEM: A Coupled Electromechanical Simulation maintains
consistency between the mechanical deformation and the electrostatic
Beam that acts as conductor Voltage Out forcing. Apply a voltage or charge. Capacitance can be measured –
made of ZnO added to substrate C = Q/V (F).
by a „TiedLink‟ (Fig. 4). Fixed faces
5V was set – charge was 2.633295E01pC
(Fig. 9).Therefore giving a Capacitance of
Mapped Bricks mesher settings –
C = Q/V = 2.633295E01/5 = 5.267pF.
name faces fixed or pressure
(were pressure will be applied). Voltage in
Figure 9: Voltage & Charge
Pressure faces CoSolve Results
4. Conclusion & References Figure 10: Piezoelectric disk
deforming when a voltage is
3 simulations were originally planned, but only 2 were done, MemMech & CoSolveEM. The results for MemMech can be seen in
the „Results‟ section.
CoSolveEM simulation was done, but the simulation was still not complete after 19hours, therefore I had to interrupt it. Only results
that were visible after the interruption are shown. The results show for example, when a piezoelectric disk is deformed a voltage is Packaging
Packaging for MEMs pressure sensors must provide a number
generated (Fig. 10).
of factors, which include; mechanical support, environmental
The MemElectro simulation was not performed due to time constraints, but if it had, a voltage of about 5V would have been protection, mechanical stress isolation and a media interface
applied across the beam and I would expect 5V to be the output. If this result was obtained then I would of known that the that is free of fluid leaks. All must be delivered at low cost, have
a low mass and be easy to reproduce .
simulation was successful. Essentially 2 types of packaging can be used for silicon based
A mesh of „Mapped Bricks‟ was used for the model, this is because for a MemMech simulation Hexahedral elements are pressure sensors – Leadless Chip Carrier (LCC) (Fig. 11) or Side
Braze (S/B) (Fig. 12).
recommended over tetrahedral – if geometry permits. You can create a hexahedral mesh region on regions composed of layers
that are topographically equivalent to a cube . Figure 11: LCC
If a MemPZR analysis was to be performed, the mesher settings would have to be changed as bricks cannot be used due to
angle constraints of the model.
Silicon_100 (Silicon crystal) is used as the material for the model. This is because in industry most, if not all piezoelectric pressure Figure 12: S/B
sensors are made of this material. It is because it is readily available, it is cheap and of a high quality material, which makes it References
attractive for use in many MEMs devices. The silicon surface was also back etched using KOH Wet Etching. Etching silicon wafers . http://www.globalspec.com/reference/4396/Introduction-to-
with KOH is a very popular technique, this is because it prevents the silicon becoming decontaminated. It etches quickly in the . Sensorsmag.com, “A Practical Approach to Vibration Detection and
<100> and <110> directions, but slowly in the <111> direction, the result is the pyramid-like structure (Fig.2) of the silicon substrate Measurement” (1999)
. Sensorsmag.com, "Pressure Measurement: Principles and Practice" (2003)
. . Coventor - ANALYZER reference tutorial
The beam is made of Zinc Oxide (ZnO), this is because it is a material that inhibits the piezoelectric effect, so therefore a suitable . “Report on KOH Process Module Etch Characteristics and Design Guide”,
NCSU Nanofabrication Facility. (2005)
piezo material to use here (Fig. 3). . G. Bitko, A. McNeil, “Improving the MEMS Pressure Sensor” Sensors. (2000)