MEMS: Manufacturability and Applications to the Aerospace Industry

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6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

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University of Kansas, Fall 2006


MEMS: Manufacturability and Applications to the Aerospace Industry


Ken Thye Lee

Department of Aerospace Engineering, University of Kansas, Lawrence, KS



Micro
-
Electro
-
Mechanical Systems (MEMS) is the integration of mechanical elements,
sensors, actuator
s, and electronics on a common silicon substrate through micro
fabrication technology
1
. These devices generally range in size from a micrometer (a
millionth of a meter) to a millimeter
2
. Common applications of MEMS devices can be
found in inkjet printers,
accelerometers, gyroscopes, pressure sensors, optical switching
and displays. MEMS technology can be implemented by using a number of different
materials and manufacturing techniques. The choice of which material will or can be
used will depend on the devi
ce being created and the market industry in which it has to
operate. Such materials include silicon, polymers and metals. There are several
processes involved in the manufacturing of MEMS devices, which include deposition
process,
photolithography,

etching

processes, wet etching, reactive ion etching (RIE),
deep reactive ion etching (DRIE), and bulk micro machining. Extensive research is
being carried out on alternative breakthrough materials and processes for MEMS
devices in academia and industry. The ongo
ing AEROMEMS project initiated by the
European Union attempts to identify the viability of applying MEMS devices for an
industrial aerodynamic flow separation control application
3
. This paper will attempt to
present a current study of the MEMS technology,
materials and processes available, and
discuss various issues regarding the possible full
-
scale manufacturability and
application of the technology in the aerospace arena for the coming future.



Nomenclature


MEMS = Micro
-
Electro
-
Mechanical Systems

RIE =

Reactive ion etching

DRIE = Deep reactive ion etching

AEROMEMS = Aerospace micro
-
electro
-
mechanical systems

IC = Integrated Circuits

CMOS = Complementary metal oxide semiconductor

SFB = Silicon
-
diffusion bonding

MIT = Massachusetts Institute of Technology

UAV = Unmanned/ uninhibited aerial vehicle



I.

Introduction


MEMS devices first developed in 1970s and commercialized by the 1990s enable all types of systems to be
smaller, faster, less expensive and more cost effective. MEMS are small
-
integrated devices o
r systems that
combine electrical and mechanical components
4
. MEMS devices range in size from the micro to millimeter
level and there
are
millions of MEMS devices in a system. MEMS
technology

have been successfully
demonstrated

and applied to various indus
tries including automotive, aerospace and electronics.
Attention
to the MEMS industry
continues to increase throughout the world. This is mainly attributed to its wide
variety of applications such as inkjet
-
printer cartridges, accelerometers, microengines,

inertial sensors,
microactuators, optical scanners, transducers, fluid pumps, and other sensor applications. These systems
integrate sensors, control actuators and mechanical processes together on a micro scale
to generate
macroscopic effects.



6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

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University of Kansas, Fall 2006


Microeng
ineering or microfabrication development has enabled these devices to be used as new
application to existing technologies are emerging towards miniaturization and integration of lightweight
cost effective materials. In addition, MEMS has also found itself
useful in aerospace applications due to its
small size and manufacturability in large numbers. The microfabrication approach conveys the advantages
of miniaturization and microelectronics to the design and manufacture of electromechanical systems
4
.
Fabrica
tion processes include deposition process,
photolithography,

etching processes, wet etching,
reactive ion etching (RIE), deep reactive ion etching (DRIE), surface micromachining, IC Fabrication,
micromolding and bulk micro machining (wafer bonding). This
paper will briefly discuss the common
processes involved in the manufacturing of MEMS devices.

The emergence of MEMS devices in the last two decades has opened up the possibility for
improvements in aerodynamic performance and research. The potential of ut
ilizing these devices in future
a
erospace technology include
the reactive manipulation of the turbulent boundary layer flows that develop
on aircraft or aerodynamic lifting bodies. Reduction in drag, increase in lift, flow separation control and
buffet sup
pression are among the possible potential benefits of MEMS technology. Aircraft of the future
could benefit from the application of fluid flow control, which not only results in performance
improvements (flying faster, reduced fuel consumption, etc.) but a
lso reduce vehicle size, weight and cost.

The MEMS industry is estimated to have a $10 billion market and a projected annual growth rate
of 10
-
20%
4
. By 2002, the market has reached the $34 billion threshold. Due to significant impacts that
MEMS will have o
n commercial and defense markets, the federal government and industry alike have
shown particular interest in their development
4
.

This paper will address the current technologies and products of MEMS development, the current
technology of microfabrication
processes and the potential large
-
scale application of MEMS devices to the
aerospace industry.



II.

Current Applications of MEMS Devices


The following are some of the examples of MEMS technology and where they are currently being used:

A.

Pressure Transducers
4

The sensing element consists of a flexible diaphragm that deforms in the presence of a pressure
differential across it. The magnitude of diaphragm deformation is converted to an electrical signal, which
appears at the sensor output. The sensor chip consist
s of a thin silicon diaphragm fabricated by bulk
micromachining. Etching the silicon substrate first creates the diaphragm. The sensor die is then bonded to
a glass substrate to create a sealed vacuum cavity under the diaphragm. Finally, the die is mounted

on a
package such that the diaphragm topside is exposed to the environment. The change in ambient pressure
forces the diaphragm to deform downward, resulting in a change of resistance of the piezoresistors. On
-
chip electronics are used to measure the resi
stance change, which causes a corresponding voltage signal to
appear at the output pin of the sensor package
4
.

B.

Accelerometers


Accelerometers measure the magnitude of acceleration of a body or system. MEMS technology
applications to accelerometers are a re
latively new development. One of the recent designs for
accelerometers using MEMS devices is discussed by DeVoe and Pisano (2001)
5
. It is a piezoelectric
accelerometer that is surface micromachined and utilizes a zinc oxide (ZnO) active piezoelectric film.

The
design is a simple cantilever structure, which simultaneously serves as a proof mass and sensing element
4
.
One of the fabrication techniques developed is a sacrificial oxide process based on polysilicon surface
micromachining
4
.





6th Annual KU Aerospace Materials and Processes "Virtual" Conference

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October 17, 2006

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C.

Inertial Sensors


Inertial sensors are a type of accelerometer and are one of the commercial products that utilize
surface micromachining. In automobiles, inertial sensors are used as airbag
-
deployment sensors as well as
tilt or shock sensors. Their use has been limited by

the need of manually aligning and assembling them into
three
-
axis systems, and the resulting alignment tolerances, their lack of in
-
chip analog
-
to
-
digital conversion
circuitry, and their lower limit of sensitivity
4
. A three
-
axis system accelerometer has b
een developed at the
University of California Berkeley
5

to overcome some of these limitations. This type of accelerometer was
designed for the MEMS/CMOS technology and it involves a manufacturing technique where a single
-
level
polysilicon micromachining pr
ocess is combined with a 1.25
-
m
i
cron CMOS.


D.

Microengines


Three
-
level polysilicon micromachining process has enabled current manufacturing technology to
fabricate devices with increased degrees of complexity. The process includes three movable levels of
po
lysilicon, each separated by a sacrificial oxide later, and a stationary le
vel. Operation of the microgears

at
rotational speeds above 300,000 rpm has been demonstrated
4
.


E.

Some other applications


MEMS IC fabrication technologies have also allowed the manu
facture of microtransmissions
using sets of small and large gears interlocking with other sets of gears to transfer power
4
. A recently
developed, micro
-
optoelectromechanical system device is based on MEMS technology. Finally, MEMS
technology has been used
in fabricating vaporization microchambers for vaporizing liquid microthrusters
used for nanosatellites. The chamber is part of a microchannel with a height of 2
-
10 microns, made using
silicon and glass substrates. The nozzle is fabricated in the silicon su
bstrate just above a thin
-
film indium
tin oxide heater deposited on glass
4
.


The fabrication processes that are involved in producing the technologies described
above

will be discussed
in the preceding section.



III.

Fabrication Processes




The most common fa
brication processes used in the manufacture of MEMS devices are briefly addressed
here:

A.

Deposition Process


Deposition process involves

depositing a thin film of material onto a substrate or any deposited
layers. Layer thickness of these films may be withi
n a few tens of nanometers and some allow a layer of
atoms to be deposited at a time. This process mainly used in the manufacture of optics (reflective or non
-
reflective coatings), electronics (insulators, semiconductors and conductors to form ICs) and pac
kaging
(aluminum
-
coated PET film). Deposition techniques can be divided into two categories based on how they
are viewed in terms of chemistry or of physics.





6th Annual KU Aerospace Materials and Processes "Virtual" Conference

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MEMS deposition technology can be classified in two groups
7
:

1.

Depositions that happen because
of a chemical reaction:

o

Chemical Vapor Deposition (CVD)

o

Electrodeposition

o

Epitaxy

o

Thermal oxidation

2.

Depositions that happen because of a physical reaction:

o

Physical Vapor Deposition (PVD)

o

Casting

Reference 7 contains detailed descriptions of the de
position processes mentioned above.

B.

Photolithography

Photolithography also known as optical lithography is a process
that
involves the transfer of a pattern
from a photomask to the surface of a substrate. The substrate used here is often made from crystall
ine
silicon, glass, sapphire and metal. Photolithograpy is also referred to as microlithography or
nanolithography and it involves a combination of:



substrate preparation



photoresist application



soft
-
baking



exposure



developing



hard
-
baking



etching

and vario
us other chemical treatments (thinning agents, edge
-
bead removal etc.) in repeated steps on an
initially flat substrate
8
.

A typical silicon lithography procedure would begin by depositing a layer of conductive metal
several nanometers thick on the substrat
e. A layer of photoresist (a chemical that 'hardens' when exposed to
ultraviolet light) is placed on top of the metal layer. A transparent plate with opaque areas printed on it,
called a photomask or shadowmask, is placed between a source of illumination a
nd the silicon wafer. The
photoresist is then developed and areas of unhardened photoresist undergo a chemical change. After a hard
-
bake, subsequent chemical
treatment etches

away the conductor under the developed photoresist, and then
the hardened photore
sist, leaving the conductor exposed in the pattern of the original photomask.
Photoresist is then applied to the silicon wafer using a spinner
8
.

Lithography is used mainly because it can create patterns over an entire surface simultaneously
and the abilit
y to control its size and shape
8
. However, it requires a substrate to start with, it is not very
effective at creating shapes that are not flat, and it requires extremely clean operating conditions.

C.

Wet Etching


Wet etching is when the material is removed
when immersed in a chemical solution.
Unfortunately, t
he limitation to this process is the
mask that is used to etch the material

that

must be
selected such that it does not dissolve or etches at a s
lower rate than the etched material
. Also, single crystal

materials, such as silicon, possess anisotropic etching (etching in different directions) in certain chemicals.
Anisotropic etching is usually favored in applications where straight sidewalls are required in a material.
Wet etching is also a characterizat
ion of the quality of a wafer. The etch pit
ch

density is a measure for the
amount of dislocations per area
9
. The reader may consult Reference 9 and 10 for more information on wet
etching procedures.


6th Annual KU Aerospace Materials and Processes "Virtual" Conference

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D.

Reactive Ion Etching (RIE)


Reactive ion etching (RIE)
is a technology developed for manufacturing MEMS devices using
plasma to etch the material that is deposited on silicon wafers. Plasma is introduced by RF (radio
frequency) power into the RIE system consisting of a cylindrical vacuum chamber with a wafer p
late
situated at the bottom. The reactive ions are mostly delivered vertically and the process can produce very
anisotropic etch profiles
11
. RIE conditions are dependent on pressure, gas flows and RF power. Other types
of RIE systems include inductively co
upled plasma RIE where high plasma densities can be achieved and
etch profiles tend to be more isotropic
11
.

E.

Deep Reactive Ion Etching (DRIE)


Deep Reactive Ion Etching (DRIE) is a process that is developed for MEMS and is used to create
deep and high aspec
t ratio holes in silicon. This process involves highly anisotropic etch process
es

and is
widely used in applications such as mechanical resonators, microfluidic channels and capacitors. DRIE is a
process that has a higher etch rate than that of RIE. The tw
o main technologies for high rate DRIE are
cryogenic and Bosch (pulsed etching)
11
. DRIE can be combined with diffusion bonding called SFB
-
DRIE
to produce tall structures within crystalline silicon.

F.

Bulk Micromachining
12 & 14

(Wafer Bonding)

Bulk micromac
hining was the most common method used to produce materials in the micrometer
scales. This process produces structures inside the silicon substrate mostly used for bulk micromachining.
Bulk micromachining uses orientation
-
dependent etches on single
-
crystal

silicon. Bulk micromachining
starts with a silicon wafer or other substrates
,

and selectively etches into it, using photolithography to
transfer a pattern from a mask to the surface. Silicon has a crystal structure, which means its atoms are all
arranged
periodically in lines and planes. Certain planes contain weaker bonds, and are more susceptible to
etching. The etch results in pits that have angled walls, with the angle being a function of the crystal
orientation of the substrate
12
. Bulk micromachining
is inexpensive and it is mainly used in early and low
-
budget research.

G.

Surface Micromachining
13


Surface micromachining is useful for producing more complex shapes and polycrystalline
materials that can be machined at different rates in different directio
ns using wet etching. Surface
micromachining is based on the deposition and etching of different structural layers. As the structures are
built on top of the substrate and not inside it, the substrate's properties are not as important as in bulk
micromachi
ning
13
. The expensive silicon wafers can be replaced by cheaper substrates, such as glass or
plastic. The size of the substrates can be much larger than a silicon wafer, and surface micromachining is
used to produce TFTs (thin film transistors) on large a
rea glass substrates for flat panel displays. This
technology can also be used for the manufacture of thin film solar cells, which can be deposited on glass,
but also on PET substrates or other non
-
rigid materials
13
. Modern integrated circuit fabrication u
ses this
technique. Micromachining
technology is still in its infant stage

and usually uses no more than 5 or 6
layers. Surface micromachining is very repeatable for volume production
13
.

H.

Micromolding


In the micromolding process, microstructures are fabric
ated using molds to define the deposition
of the structural layer
4
. One of the most common micromolding processes is the LIGA process. LIGA
(German acronym for x
-
ray lithographie (lithography), galvanoformung (electroplating) and abformung
(molding)) is a
process that was developed by Erwin Willy Becker and Wolfgang Ehrfeld at the Institute

6th Annual KU Aerospace Materials and Processes "Virtual" Conference

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for Nuclear Process Engineering at the Karlruhe Nuclear Research Center. This process is used for the
manufacture of high aspect ratio microstructures. Photsensitve po
lyimides are used for fabricating plating
molds.

There are 3 different types of LIGA with their names depending on the radiation and preform
used
15
:



X
-
Ray LIGA using X
-
Rays produced by a synchrotron.



UV
-
LIGA using Ultraviolet light produced by a UV
-
lamp.



S
ilicon
-
LIGA using DRIE etched silicon as perform.


Reference 14 can be consulted for more detailed information on the LIGA process.

I.

IC Fabrication


IC fabrication technology is the primary technology used to develop MEMS. The major processes that
take plac
e in IC fabrication are:


o

Film growth: A thin film is grown on a silicon wafer used as a substrate.

o

Doping: A low and controllable level of atomic impurity is used introduced into the layer by
thermal diffusion or ion implantation to modulate properties of

the device layer.

o

Lithography: A pattern on a mask is then transferred to the film by photolithography. A typical
mask consists of a glass plate coated with patterned chromium film.

o

Etching: The removal of unwanted regions of a film or substrate for patte
rn dilineation
4
. Wet or
dry etching is used here.

o

Dicing: Finished wafer is sewed or machined into small squares or dice from which electronic
components are made
4
.

o

Packaging: Individual sections are then packaged. This process involves locating, connectin
g and
protecting a device or component.



IV.

Potential Benefits of MEMS Applications in Aerospace


A.

Aerodynamic Flow Control
16


Flow separation control can be achieved by using small MEMS flow separations systems.
Th
e control
of flow separation significant
ly
i
ncreases maximum lift
and

the potential of controlling buffet are
achievable. The key mechanism that delays flow separations
is

the addition of momentum to the boundary
layer using MEMS devices. Conventional systems
are less favored
due to cost and comple
xity of their
installation and maintenance
16
. MEMS technology offers potential to improve efficiency of flow separation
control by minimizing energy input and maximizing the use of the turbulent structures within the boundary
flow itself.
T
hree main concep
ts that are under development by a joint research effort by the European
Union
16
:

1.

The first and simplest is to introduce turbulent flow structures in the near
-
wall region of the
boundary layer that are very similar to those that occur naturally. The syste
m introduces a pair of
counter
-
rotating vortices of similar strength, scale and frequency. The objective of this concept is
to increase turbulent mixing throughout the boundary layer.

2.

The second concept involves

active

detection and enhancement of

the tur
bulent structures that
occur in the flow. This could be achieved by enhancement of the counter
-
rotating stream
-
wise
vortices but the concept require complex integration of sensors, actuators and control systems
16
.

3.

The third concept is based on the idea of
utilizing sub
-
boundary layer vortex generators. Small
passive vortex generators with a height of less than 10 per cent of the boundary layer thickness can
be used to delay flow separation. This concept
utilizes

pulsing jet actuators that introduce large
vo
rtices into the boundary layer to enhance turbulent mixing. The energy requirements are much

6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

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University of Kansas, Fall 2006


higher than the two concepts described above
. For

applications where the boundary layer is very
thin, the
micro
size of MEMS sensors and actuators would be benef
icial.

B.

Micro Jet Engines, Micro Thrusters and Micromachined Air
-
breathing Propulsion Systems and
Actuators


Micromachined actuators are the basic concept that is used to develop micro jet engines, thrusters
and air breathing propulsion systems with potenti
al uses in the c
ivil

aviation and space market of the future.
The continuing development of microengineering using lessons learned from microfabrication techniques
has enabled
the development of
engines and mechanical systems on a micro scale.

Micromachin
ed air breathing propulsors and actuators operates by alternately sucking and blowing fluid
through a small hole
6
. This concept uses the fact that the pump stroke results in a jet and the suction stroke
draws fluid from all directions. Thus, fluid is drawn

in from all directions with low momentum and
expelled at high momentum. This action is similar to that of the locomotion of a squid. Jet velocities of 30
m/s have been reported
6

but it should be noted that the device is not very efficient as a whole.


Mi
cro
-
jet engines are currently being developed by MIT. These devices are comprised of a radial
compressor, combustion chamber and a radial turbine
6
. Experiments with hydrogen as a fuel have shown
that these engines can produce 0.2 N of thrust and a thrust
-
t
o
-
weight ratio comparable to conventionally
size
d engines. RIE has been
used to develop these high aspect ratio devices.




Micronozzles for space applications have also been developed by RIE etching. These nozzles have
been made for supersonic applicatio
ns. Throat diameters from 12 to 30 microns and three
-
dimensional
nozzles have been fabricated using anisotropic etching of single crystal silicon
6
.

C.

Inertial, Pressure, Temperature and Aerodynamics Transducers


Inertial, pressure, temperature, flow rate and

shear stress sensors have been developed recently and
are slowly attracting interest for full scale use on commercial aircraft and spacecraft. Mainly used in UAVs,
these transducers are used mainly for measuring flow parameters and shear stress analysis.
There is large
potential for these MEMS devices as they offer weight and cost saving advantages compared to the
complex measuring instruments found on current aircraft today.

The inertial, pressure, temperature and flow rate sensors can be integrated with

aerodynamic flow
control to enable further technologies involved in introducing more complex turbulent structures and
enhancement of turbulent mixing mechanisms. MEMS shear stress transducers can also be integrated in
aircraft structures to measure accura
te microstrains that will enable more precise structural analysis of
future aerospace materials such as composites.



V.

Conclusions


Future microengineering and microfabrication technologies for the development of existing
MEMS technologies will enable us t
o develop future research in aerodynamics. These devices have the
potential for applications in the aerospace industry where weight and cost are design drivers for aircraft and
spacecraft

technologies
. Microfabrication techniques such as bulk and surface m
icromachining
,

and
micromolding will enable further development of MEMS devices. MEMS transducers can be developed for
a variety of aerospace applications and research. Microactuators such as valves, synthetic jets, boundary
layer control devices, micro je
t engines and microthrusters could enable the development of micro UAVs
and nanosatellites
6
. Boundary layer flow control is essential in improving aircraft performance and to
enable active flow separation control by utilizing flow sensors and actuators. Fu
ture research on alternative
methods of flow separation control using MEMS devices should also be undertaken by academia and
industry. Whether these technologies will materialize will depend on the feasibility and practicality in the
economic sense of thes
e devices for the future aerospace market. These studies have to be carefully
considered by both commercial and defense markets.



6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

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University of Kansas, Fall 2006


VI.

References


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-
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-
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6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

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University of Kansas, Fall 2006


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Anon., online
website, [
http://www.cs.ualberta.ca/~database/MEMS/sma_mems/img/1.jpg
],
University of Kansas, Lawrence 66045, accessed 15 October 2006, 11.00 p.m.

24.

Anon., online website, [
http://www.eur
ophysicsnews.com/full/19/article1/moore_fig1.jpg
],
University of Kansas, Lawrence 66045, accessed 15 October 2006, 11.00 p.m.

25.

Anon., online website, [
http://mmst.kaist.ac.kr/img/researches/SampleLIGAProcess.gif
],
University of Kansas, Lawrence 66045, acces
sed 15 October 2006, 11.00 p.m.



6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

10

University of Kansas, Fall 2006




Figure 1

MEMS devices developed by current technology shown using microphotographs
19
.





Figure 2

Cross section of typical pressure sensors developed by microfabrication techniques and
common pressure transduc
ers in the market
20
.





6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

11

University of Kansas, Fall 2006




Figure 3


Micrograph of a micro accelerometer developed by MEMS fabrication technology
21
.




Figure 4


Inertial sensors that are currently being developed and their applications
22
.





Figure 5


Micrographs of microengines
being developed for nanoapplications
23
.

6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

12

University of Kansas, Fall 2006




Figure 6


Processes involved in bulk and surface micomachining
24
.




Figure 7

Processes involved in LI
GA (micromo
lding) techniques for fabricating MEMS devices
25
.




Figure 8


Flow sensors and pulsed
-
jet actuat
or developed in the AEROMEMS project
16
.



6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

13

University of Kansas, Fall 2006



Figure 9


MEMS hot
-
film flow sensors developed in the AEROMEMS II project
16
.





Figure 10


MEMS microvalve actuator concepts developed in the AEROMEMS II project
16
.

6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

14

University of Kansas, Fall 2006



Figure 11

Schematic of a synthetic jet or

air breathing propulsor
6
.





Figure 12


Schematic of the micro jet engine developed by MIT and
micrographs

of the turbine rotor
and blades fabricated using RIE
6
.








6th Annual KU Aerospace Materials and Processes "Virtual" Conference

"Materials and Process Engineering for Exploration”

October 17, 2006

15

University of Kansas, Fall 2006




Figure 13

Microphotographs of a laser machined converging/diverging nozzle and

supersonic
nozzle for space applications
6
.





Figure 14


MEMS hot wire and thermal shear stress sensor element
17
.




Figure 15


Flexible shear stress array wrapped on a half cylinder mounted on the leading edge of a
UAV for flight tests
17
.