MEMS Mega-pixel microthruster
array

AFOSR TechSat-21 # F49620-99-C-0012
– Honeywell: Dan Youngner, Son Thai Lu
– Princeton: Edgar Choueiri
– $~160K for ~18 months
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p1
Satellites: The traditional
vision



Big
Expensive
Single,
connected
unit
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p2
Not all parts of huge satellites are
necessary for all missions

Antennas: Only need elements along periphery,
other key locations
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p3
Micro-satellites: The new
vision






Cluster / network
that behaves as one
“Virtual satellite”
Effective size =
100’s m x 100’s m
Smaller
Cheaper
Robust
» redundant parts
» if one dies, another wakes up to
take its place
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
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What are “small” satellites?
< $10K
<1 Mont h
<1, 000
<1 W
<0. 5 MB
Nano- Sat
1- 10
< $100K
<6 Mont hs
<10, 000
<10 W
<5MB
Mi c r o- Sat
10- 100
< $1M
<18 Mos
<100, 000
<100 W
<500 MB
Mi ni - Sat
100- 500
< $10M
< 3 Yr s
<1, 000, 000
<1 KW
<2 GB
Medium*
500- 1000
< $100M
< 5 Yr s
<10, 000, 000
<10 KW
v ar i abl e
Large*
>1000
>$100M
> 5 Yr s
>10, 000, 000
>10KW
v ar i abl e
Satellite Launches in the '90's
160
140
120
100
80
60
40
20
0
LARGE
MEDIUM
MINI
Year of launch
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
MICRO
1992

c al
Rat e
Dat / S
)
<1
1991

Ty pi
Dat a
( Dat a/
at
Pi c o- Sat
1990

Football-size
~10kg
Shorter
missions
They are the
future
Number of Satellites

Sat el l i t e
Cl as s
Appr ox .
Mas s Range
Ty pi c al
Mi s s i on
Ty pi c al
( Kg)
Rec ur i ng Cos t Li f e ( Yr s ) Si z e ( c m^ 3)
Appr ox .
Tot al
Sat el l i t e
Power
( Wat t s )
Source: NASA
p5
Micro-satellite relative motion

Issue : Satellites in cluster don’t stay put
with respect to one another
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
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Hill Orbitals: Manageable
chaos
(if only it were that simple)

Ideal world: microsatellites orbit one
another in perfect
“Hill orbitals”
» complicated, but manageable


Real world: They don’t
Continual/ periodic
station-keeping
required
» to adjust position
» to adjust orientation
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p7
Micro-thrusters on Microsatellites

Conceptual
Honeywell
Satellite
(H-Sat 101)
targeted for
2003
deployment
MEMS Microthrusters
Pulsed
Plasma
Thruster
(3 - 6and/or
places)
MEMS Micro-thrusters
(3 places)
Extendable S-band
communication antenna
MEMS Gyro, GPS, DGPS,
Earth, Sun and Star Sensor Package
Thin Film inflatable
solar arrays (1 KW)
Three axis momentum
system in one integrated
package
Li-Ion or Ni-Cd
Battery Pack and
Regulated Power Supply
Rad-hard PowerPc
Processor
Plug&Play Payload
Interface Station
Multi-functional structure
housing with micro heat pumps,
electrical conduits data transmission
conduits.
(Jack Jacobs)
Pre-processors for
Payload Requirements
MEMS Mega-pixel Microthruster Array.
Variable Payloads
(Radar payload shown)
Honeywell / Princeton. F49620-99-C-0012
Data storage (1 GFLOP)
module
p8
The Vision:
A million separate one-shot engines on a
chip




Co-axial stack of 4
silicon MEMS wafers
Wafer #1: array of 1
million pixel igniters
Wafers #2 & #3:
arrays of 1 million coaxial hollow
fuel/oxidizer filled
cavities
Wafer #4: Egress
channels
MEMS Mega-pixel Microthruster Array.
800Å Silicon nitride
diaphragm (Option
B only)
50
00µ
m
-2
Wafer #3:
Cavities will be either left
empty (Option A) or filled
with oxidizer (Option B).
>˜500µ
600Å Silicon nitride
diaphragm
solder seals
laser joined
at cryogenic
temperatures
Wafer #2:
Cavities filled with fuel.
400Å Silicon nitride
diaphragm
Emitter
Resistor
SiO2 wall
surrounds
each pixel
Au on wall
wets
to solder
Silicon Nitride
Pixel
Signal
Voltage
Pixel
Address
Line
V
ss
Vdd
V
ss
Vdd
G
Signal
Line
Wafer #1:
Infrared emitter array
Pixel
Address
Line
Rad-Hard CMOS Pixel
Transistor Circuitry
Honeywell / Princeton. F49620-99-C-0012
p9
Thruster array: scale =~100:1
wafer #4
egress

Wafer thickness =
wafer #3
fuel
cavity length =
250 mm

Cavity width =
40
wafer #2
~40 - 135 mm
Gallium
Au/Cr/Au
oxidizer
10
wafer #1
Methanol
igniter
wafer #2
(oxidizer)
Na in BP
Gallium
Au/Cr/Au
Thermal
Emitter
IPA
Silicon
Single thruster
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
Array of thrusters
p 10
The igniter arrays



Mature Honeywell
product: IR scene
projector arrays
(10 yr, $10M)
Rad-hard RICMOS
underlayers for
address / drive
512 x 512 arrays:
»

up to 2 mW/pixel
Temps to ˜900°C in
˜10 msec (in std
vacuum package)

TM
Other arrays:
»
up to 100 mW/pixel
4-inch wafer containing
four 512 x 512 pixel arrays,
eight 128 x 128 pixel arrays, and
several emitter and FET test
structures.
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
SEM photo of pixels. Each pixel
is 51µ x 51µ (center to center)
p 11
IR Scene Projector Display

Synthetic
infra-red
scene of
rocket
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 12
Wall Rupture Strength: FEM


Finite element modeling
Assumes 100 atmosphere explosion in chamber
Figure 4a: Radial stress in Pixel sidewalls when the internal pressure in one cavity is 100 atmospheres.
MEMS Mega-pixel Microthruster Array.
Figure 4c: Deflections with a 100 atmosphere explosion in a cavity, magnified 1000x. The maximum
deflection at the center of a cell wall is 0.6E-6 cm, or about 600 angstroms.
Honeywell / Princeton. F49620-99-C-0012
p 13
Wall Rupture Strength:
Summary
Max Stress vs. Corner Radius and Wall
Thickness for 100 micron pitch cell with 100
Atm Load
Wall
strength:
» function of
wall thickness
» function of
radius of
curvature
» function of
wall
roughness
» function of
silicon yield
strength
2500
h=5
h=10
h=15
h=20
2000
Stress (MPa)

risky region
1500
1000
safe region
500
0
0
5
10
15
20
25
Corner Radius
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 14
SiN Diaphragm Rupture
Strength
Function of thickness, radius, material properties
SiN diaphragm rupture strength.
Film Stress = 300 M pa. Thickness = 100 - 3,000 angstroms
Young's modulus = 4E12 dynes/cm2. E/(1- ) = 2E12 dynes/cm2
100A
Rupture pressure
(atmospheres)

200A
200
300A
150
400A
100
600A
50
800A
1000A
0
0
20
40
60
Diaphragm Radius (microns)
80
1500A
2000A
3000A
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 15
Cavity shapes & dimensions
Chip

Optimal design
unknown

Diaphragm radius
When in doubt,
build them all
CHIP1
CHIP2
CHIP3
CHIP4
CHIP5a
CHIP5b
Test Cell Even
Test Cell Odd
20 microns
20 microns
60 microns
60 microns
45 microns
25 microns
20 microns
60 microns
Diaphragm 1
Diaphragm 2
Diaphragm 3
(t = 200 angstroms)
(t = 600 angstroms)
(t = 1500 angstroms)
burst pressure
18 atm
18 atm
5 atm
5 atm
8 atm
15 atm
18 atm
5 atm
burst pressure
55 atm
55 atm
18 atm
18 atm
24 atm
42 atm
55 atm
18 atm
burst pressure
140 atm
140 atm
45 atm
45 atm
63 atm
110 atm
140 atm
45 atm
= cavity, diaphragm
= filament
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 16
Fuel-filled cavity arrays



RIE holes through
silicon wafers
Etch-stop on thin SiN
diaphragm
Fill cavities with fuel,
oxidizers
»
»
»
»


Iso Propyl alcohol
sodium azide (NaN3)
Na in Benzophenone
Phosphorus in Carbon
Disulfide
Wafer-to-wafer bond
Many variations of
cavity size, shape,
space
Diaphragms fracturing during wafer dicing
SEM photos of cavities etched into 250µm Si wafer.
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 17
Filling cavities with fluids

Procedure for filling
cavities:
» pump chamber to <1
torr base pressure
» cool die to ~77ºK
» open valve,
disbursing fluid
» back-fill chamber
with dry N2, keeping
puddle on die
» warm die to room
temp
fluid to be
disbursed
microthruster
die
valve
Vacuum
chamber
cooling
stage
To vacuum pump
LN2 in
LN2 out
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 18
Low Temp Bonding of Fuel / Oxidizer
Wafers

Requirements:
» low temp
– don’t ignite fuels
– don’t rupture diaphragms
»
bonds when wet
– with methanol
– with other fuels/oxidizers

» inert wrt fuels / oxidizers
Approach:
» Gallium solder (melts at 29.8C)
– evaporated onto -55C substrate
(Jeff Ridley)

»
Gold as seal (eutectic with Ga)
»
Excellent mutual soluability
Status:
» Under development, promising
–
–
–
–
eutectic forms
Ga/Au seal is strong
alignment is difficult
diffusion barriers / adhesion
layers still an issue
MEMS Mega-pixel Microthruster Array.
Looking through Pyrex wafer at Ga-Au seal on Gold wafer
Cavities are empty; Region around the cavities are filled with water
Honeywell / Princeton. F49620-99-C-0012
p 19
Low temperature ignition
Brite IR Emitter Pixels (128 x 128 HDR array)
250


<160°C max
ignition temp
Strategy:
Hypergolic
fuel/oxidizer
combination
choice is still
under
evaluation
200
Pi xel Temperature (C)

150
100
50
0
0
50
100
150
200
250
300
350
400
Pixel Current (uA)
Option # -->
Material in
contact with
igniter
Material in 1st
cavity wafer
#1
Methanol
#2
Methanol
Na in
Benzophenone
(BP)
Phosphorus
in Carbon
Disulfide
(P in CS2)
Material in
2nd cavity
wafer
Methanol
Methanol
MEMS Mega-pixel Microthruster Array.
#3
Sodium
Azide
(NaN3)
Sodium
Azide in
solvent
Sodium
Azide in
solvent
Honeywell / Princeton. F49620-99-C-0012
#4
Sodium Azide
(NaN3)
#5
Sodium Azide
(NaN3)
Sodium Azide,
dried to a
powder under
conditions where
it coats the walls
Sodium Azide,
dried to a
powder under
conditions where
it coats the walls
Na in BP
or
P in CS2
Methanol
p 20
Interim Strategy: Laser
Ignition

Issues with electrical ignition:
» BRITE wafers are rare, high-priced
– (1 @ $30K)
» Available power from BRITE is low (~1.6 mW)
– too low to initiate some reactions

Solution: Pyrex wafers + YAG laser
» Pyrex is abundant, optically transparent
» YAG laser: ~100kW for ~6nsec (0.6mJ)
– more than enough to initiate all conceivable
reactions
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 21
Expected Performance of Pixel
Array
Attribute
MEMS
Thruster
Array*
Mass of Fuel per pixel
Mass of support structure per pixel
Number of pixels
Total mass of thruster, including fuel, controllers
Isp
Impulse per pixel
Total impulse
Minimum power required to ignite
1.6 µg
0.8 µg
˜1E6
2.4 gm
200 s
3 µN s
3 N sec
10 mW
Macro
Ion
Thruster
**
NA
NA
NA
13.8 kg
2500 s
NA
158 kN s
1.2 kW
* Estimated performance based on standard chemical explosion characteristics.
Subject to further modeling and verification.
** Data from “Electric Propulsion for Low Earth Orbit Communication Satellites,”
by Steven R. Oleson, NASA Lewis Research Center, 1997.
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 22
Testing





To be performed by
Professor Choueiri,
Princeton.
Electric Propulsion and
Plasma Dynamics Lab
(EPPDyL)
Large (8 ft dia x 24 ft
long vacuum tank
@ 1E-5 torr
Laser interferometric
measurements
more information at: http://cougarxp.princeton.edu:2112/eppdyl/personnel/eyc.html
MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 23
Similar Technologies


Carol Rossi & Kris Pister:
BSAC & LAAS, France
» thermally ignite explosive
trapped in pixel
» patented, demonstrated
» designed primarily for
drug dispensing
» issues: low density (3mm
x 3mm)
David H. Lewis: TRW
» MCNC MUMPS fab
» 19 thrusters on 6mm x
6mm square
» thermally boil liquid.
» issues: Low density, Low
Isp, thermal fratricide
MEMS Mega-pixel Microthruster Array.
http://bsac.berkeley.edu/microrockets/microrockets.html
http://design.caltech.edu/micropropulsion
Honeywell / Princeton. F49620-99-C-0012
p 24
MEMS Mega-pixel Microthruster
Array Status
Anticipate 1st testable structure by
March, 2000.
 … stay tuned.

MEMS Mega-pixel Microthruster Array.
Honeywell / Princeton. F49620-99-C-0012
p 25