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1
HSTSS Silicon Gun Pressure Sensor - Phase IDavid Combes - QinetiQ Malvern22/Jun/2004
2
• Project and aims
• Approach– Modelling, fabrication & packaging
• Packaging issues
• Test results
• Conclusion
• Questions
Introduction
Origins of QinetiQOrigins of QinetiQ
Air Force
Navy
Army
Radar &Electronics
• Air Systems
• Land Systems
• Sea Systems
• Weapons
• Command & Info Systems
• Electronics
• Sensors & Processing
• Structural Materials
• Ranges
• Air Capabilities
• Engine Test
• Chemical & Biological
• Human Sciences
• Chemical & Electronics
• Defence Analysis
RAE
ARE
RARDE
RSRE
CBDELife Sciences
• Sensors & Electronics
• Knowledge & Information Systems
• Future Systems Technology
• Complex Managed Services
• Sensitive areas of research
July 2001
3
• Projectile mounted, reduced cost miniature pressure sensor for HSTSS (Hardened Subminiature Telemetry Sensor System)
– Alternative to piezoelectric, Kistler type gauges
– Acceleration, light and temperature insensitive
– Small, and easy to read out
• To survive gun launch accelerations up to 100,000g
• To survive and measure pressures up to 100,000 psi
– Accurate and repeatable measurement
Overall project aims
• Mainly concerned with prototype sensor element– To survive and measure pressures, and inform further work
required to achieve aims– ~ 1 year programme
• Further phase required for development and testing required to demonstrate fully functional, production ready device
• Output of phase I - 8 packaged sensor elements– Capable of measuring pressures up to 100,000 psi– Capable of surviving gun launch– Minimal form factor
Phase I
4
• Fort Halstead - Gun systems– Gun and ammunition systems research
– Gun and rocket test ranges and instrumentation
• Malvern - Microsystems & Microengineering– MEMS design and modelling
– Electronic sub-system design
– Microstructure fabrication
– Advanced characterisation
– Engineering solutions
Project team
• Bridge configured piezo-resistive silicon sensor
• Provides linear output voltage with change in pressure
• DC response characteristics
• Silicon - robust, cheap, readily processed
• Allows exploitation and application of MEMS technologies, and potential integration of smart functions– Temperature compensation etc
• Suitable for miniaturisation
Approach
5
• Piezoresistive membrane sensor
Approach
Sensor glued intohousing
Interconnect fromsensor to outside
world
Pressure
Blast shield to protectfrom debrisPressure Pressure
A A
membrane
A A
Light Shield
piezo-resistors
Mechanical Modelling
0 100 200 300 400500
1000
1500
2000
2500
Longitudinal @ 0°Transverse @ 0°Longitudinal @ 45°Transverse @ 45°
Distance from membrane centre
Stre
ss (M
Pa)
membrane_edge
• Mechanical finite element analysis– To inform design
– Map stresses in device
– Map strain for piezoresistors
– Package included in analysis
• 100µm fillet radius chosen to give acceptable stresses
6
• Silicon has high theoretical intrinsic strength– Practical strength depends on surface finish and defects– Quoted figures vary widely
• 7GPa - 0.7GPa
• Fabrication process a key component of work– Need smooth etch profile (in all directions)– Use Deep Reactive Ion Etch (DRIE) tool to create initial
circular membrane profile– Post process to smooth further– Etch dice
Fabrication
• Wet etching - wetting problems
• Gas phase etches investigated
• Fluorine based plasma etch-smooth profile, good surface finish
• Wet etching may be used to further smooth
Isotropic etch development
7
• Saw/Cleave– Not highly controllable– Limited geometry (straight)– Forms stress raisers
• Etch dice using DRIE
Etch dicing
Dice etch
Membrane etch
Dice etch
• Steel M12 package, similar to Kistler M6213
• Gasket/attach layer for testing - Epoxy
• Face sealing using compression washer
Packaging
8
Maximum survived pressure
0.0
20.0
40.0
60.0
80.0
100.0
120.0
0 100 200 300 400 500 600
Pre-isotropic etch membrane diameter (µm)
Pres
sure
(kps
i)
t=100µm
Mechanical test results
• Carried out in parallel with mechanical modelling
• Survival test - Budenberg dead weight tested to 100kpsi
• Membranes with no transducer elements tested - available early
• Several membrane designs showed they were suitable for high pressures
• Stress limit of around 2GPa
• P-Type piezoresistors– Allows lower temperature
co-efficient of piezoresistivity
• Simulated using Silvaco’sAthena
• Test devices fabricated to validate models
• Simulated ≈ 680Ω/
• Measured ≈ 710Ω/
Piezoresistors
9
-7
-6
-5
-4
-3
-2
-1
020 30 40 50 60 70 80
Temperature (°C)
% c
hang
e in
pie
zore
sist
ive
coef
ficie
n
<0.1% per °C
Piezoresistors
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 50 100 150 200 250
Stress (MPa)
DR
/R (%
)
Longitudinal
Transverse
at 45°410ppm/MPa
-350ppm/MPa
50ppm/MPa
Temperature sensitivityPiezoelectric coefficients
• Good match with modelling
• Low temperature sensitivity, potential for further gains
Membranes with piezoresistors• Bridge configured sensor
• Two sensitive, two insensitive devices
Piezoresistive Coefficients
Transverse
Longitudinal
Insensitive
Sensitive
Membrane edge
10
• Attachment of suitable bond wires to small die area proved difficult
• Wire-bonder identified which provides solution - Westbond 7440– Some problems obtaining samples -
now solved– Will be testing in next few weeks
• Modifications made to package to allow conventional bonds and soldered connections
Packaging issues
Bond wires
• Uses ceramic header, metal support– Conventional wire bonds to header– Solder connections from header
Modified package
11
• Provides devices with less support– Increases strain in die and membrane
• Die level device failures under reduced pressure
• Analysis of failed devices show modified package not providing sufficient support
• No such problems with unmodified package– Devices in original package type with
bonds made by a Westbond tool (or similar) are the best solution
Modified package issues
-40
-20
0
20
40
60
80
100
120
0 10000 20000 30000 40000 50000 60000
Pressure (psi)
Out
put v
olta
ge (m
V)
1
2
3
Linear (3)
• Devices in modified package tested - Electrical performance as expected– Linear & repeatable
• Epoxy relaxation an issue -alternatives under investigation
• Devices in unmodified package tested with no electrical connections– Most sensitive (weakest)
devices showed they were suitable for full range pressure measurement
Transducer testing
12
• A silicon piezoresistor bridge was identified as capable of meeting requirements
• Models were created to simulate both the mechanical and semiconductor physics of the devices
– Devices were fabricated to validate models
– Good predictive performance
• Processes were developed for fabrication of the mechanical structure and implanted piezo-resistors
• Solutions identified for packaging
Conclusion
• A silicon piezo-resistive pressure sensor has been developed suitable for
– Measuring 100,000psi (high degree of confidence)
– Surviving 100,000g (high degree of confidence)
– Low cost production
– Miniaturisation
– Easy interface
• Phase II funding is being sought to further develop the sensor and packaging to facilitate production of such sensors for ballistic applications
Conclusion
13
Supplemental information
14
• Profitable, growing, high technology P.L.C. formed from majority of DERA
• Approx. 8,500 staff
• Turnover approx. $1.2B – 80% for Ministry of Defence– Commercial work growing rapidly
• One of Europe’s largest science and technology organisations
• Approx. 3200 patents in force, 1400 pending
• 10 spin-out companies
What is QinetiQ?