The document describes the design of a MEMS sensor to measure fluid flow parameters such as flow rate, velocity, direction, and pressure using hotwire principles. It involves designing resistive elements on a silicon carbide substrate that can produce heat when current is passed through and allow changes in temperature to be detected. Fabrication of low-cost silicon carbide is also discussed using porous silicon as a template in a chemical vapor deposition process. The sensor aims to provide accurate readings of flow and pressure in applications such as aerospace, medical devices, automotive, and semiconductor manufacturing.

1. MEMS-BASED
MEMS BASED FLUID
FLOW SENSOR
by Jean P. Cortes
P

2. MEMS?
MEMS: Micro Electro Mechanical Systems
MEGR Mathematics
MEMS Biology
ECGR
Physics
y Chemistry
Micro motor.

3. MEMS?
A)
B) C)
A) Micro gears, B) Micro pump, C) micro
wind blades.

4. Objectives
Design and evaluate a MEMS sensor with
capabilities in measuring parameters,
such as:
• Fluid Flow rates
• Velocity/direction
• Pressure
based on thermal convection
convection.

5. Heat transfer
Conduction
within the same material
Convection
Through a medium (depends on the
interaction with the medium)
Radiation
Every material radiates energy.
On the device:
less conduction
dependence on convection
high thermal conductivity fast response

6. Flow measurement
Thermal fl
Th l flow sensors b
based on H t i
d Hotwire
principles:
1) heat is produced (Joule’s effect)
2) heat is transferred by convection
3) ΔT is α to the flow rate.
I2 Rw = h Aw (Tw – Tf)

7. Hotwire principle
A) B)
Flow
HEATER
A) Hotwire: flow rate depends on the loss of heat at the heater
B) Hotwire anemometry: flow rates and direction can be
measured.
d
For flow measurement one or three elements can be used.

8. Example of MAF in a car
Schematic representation of the Hotwire anemometry.

9. Analytical analysis
Thin film
Thi fil resistor powered by a constant
it db tt
current source is introduced in fluid medium.
In thermal equilibrium with it ambient, th
I th l ilib i ith its bi t then
I2 Rw = h Aw (Tw – Tf)
The film resistance is:
Rw= Rref [1 + TCR (Tw-Tref)]
* for optimal operations TCR must be high.
Rate: the heat transfer coefficient h is a function of
fluid velocity vf according to King’s law

10. Analytical analysis
* a, b, and c are calibration constants to be found empirically…
Direction: the heat is removed form the heater and
transfer to the detectors This calculations are
detectors.
going to be obtained empirically.

11. Pressure measurement
Pressure sensors based on Hotwires
1) heat is produced ( p = Iw2 Rw )
2) heat is kept constant
3) h
) heat transfer due to I
fd Impingement rate:
i
4) The Tw increases as P goes down

12. Pressure measurement
Vacuum
Chamber
Heater
Gas
Molecules Pumping
Schematic representation of the application to measure pressure.
For pressure measurement one element is needed.

13. Design
100 μm
Top view 100 μm
100 μm
μ
Heater (W)
2 detectors (Ni/Pt)
TCR = 3900 ppm/oC
500 μm
SiC support
Metal
0.5 μm
10 μm
SiC
500 μm 300 μm
500 μm
Design of the three resistor for flow measurement.
Side view Si

14. Design
Top view 100 μm
150 μm
100 μm
μ
50 μm
Heater (W)
2 detectors (Ni/Pt)
TCR = 3900 ppm/oC
500 μm
SiC support
Metal
Mtl
0.5 μm
10 μm
SiC
500 μm 300 μm
500 μm
Design of the one resistor for vacuum measurement.
Side view Si

15. Why SiC?
Can withstand harsh environments (chemical
inert, very high temperatures T > 1500 oC)
High thermal conductivity (3.2- 4.9 W/cm K)
Hardness and wear resistance high Young’s
resistance, Young s
modulus, very low thermal coefficient of
expansion
High resistance (102 to104 Ω-cm).
The Challenge:
• SiC is expensive
• Develop a way to fabricate SiC at low cost

16. Fabrication of SiC
Equipment:
Power source
Solution
Teflon cell (2 beakers) cathode
Container V
Two electrodes (Pt)
anode
• Si is inexpensive
• Can be used for electronic
circuits
• Porous Si can
accommodate the large
strain between Si and SiC

17. Fabrication of SiC
Low pressure chemical vapor deposition chamber.
Inside
Top view

18. Project Plan – Fall 08

19. Project Plan – Spring 09

20. Cost
Total Cost for making HOT-MEMS
Total Tooling Total Labor Manufacturing
Operation Cost
cost Cost/Wafer
cost
Cleaning $21.00 $2.50 $23.50 $0.94
Oxidation $23.00 $2.50 $25.50 $0.51
Metal Deposition $260.00 $2.50 $262.50 $17.50
Mask creation $25.00 $50.00 $75.00 $3.74
Photolithography $25.80 $5.00 $30.80 $30.80
Metal Etch $4.20 $2.50 $6.70 $6.70
Dicing $10.50 $5.00 $15.60 $15.50
Total $369.50 $70.00 $439.60 *$75.69
18000 devices…….so the final price is ~ $ 0.00456
•6 in wafer

21. Applications - flow
Aerospace, velocity measurements at various
A l it tt i
ranges of interest
Medical, respirators for pre-matured born, and
diagnostics of asthmatic attacks
Automotive industry,
Education, educational training of students on
fluid related experiments,
Semiconductor industry, for almost every
process where vacuum is involved

22. Summary
o W have designed a sensor suitable f
We h di d it bl for
both: flow and vacuum measurement.
o We have started the process of
fabricating SiC using porous silicon as
template.
o Low cost process promises mass
ow ost pro ess
production of the device.
o High thermal conductivity of silicon
carbide improves the detector’s
response.
response

23. Acknowledgments
Dr. Mohamed-Ali H
D Mh d Ali Hasan ( d i
(advisor)
)
Dr. Asghar Hashmi
Dr. R. Gary Wilson
Dr. Stuart Smith
Payam Shoghi
Namit Singh
Ayman Zhobi
Tanya Dias
Dr. Horacio Estrada