New Reliability Assessment Methods for MEMS
Prof. Mervi Paulasto-Kröckel
Electronics Integration and Reliability
Aalto University
Helsinki School of Economics
Helsinki University of Technology
University of Art and Design
Helsinki
A merger of leading Finnish universities in 2010:
ChemicalTechnology
Electrical Engineering
Economics ScienceArt andDesign
Engineering
Art, Business,Science and Technology
School of…
Electronics Integration and Reliability –Past and Present
Metal – ceramic joining
Electronics assembly technology, soldering
1990 2000 2010
Compatibility of dissimilar materials and interconnect technologies since 20 years !
MEMS 3D integration, Bioelectronics & -sensors
Courtesy of VTI
Outline
• Reliability challenges in MEMS – status of the industry• What kind of changes are needed?• Reliability characterization of gyro
– Development of methods– Results
• TH and shock impact tests• FA
• Reliability characterization of microphone– Reliability assessment methods– Results
• Summary
Characteristics of reliability assessment of MEMS
• Reliability evaluation in functional state– External stimulus (sensors)
– Monitoring of output (actuators)
• Definition of failures– Based on functional characteristics
– More than one criterion for failure
– Requires real-time monitoring system
• Methods for health monitoring are device specific and non-standardized
Note: Standardized methods to produce loading still apply!
• Thermal cycling, mechanical shock, vibration, temperature / humidity (e.g. 85/85), corrosion
MEMSsensor
Test environment
Known stimulus
Control unit
Comparison of output with input
Health monitoring &
system control
MEMSactuator
Detection of output
outp
utou
tput
inpu
tin
put
Reliability challenges in MEMS• Use environment
• Mechanical shock impacts and vibrations (specifically moving parts w and w/o impacting surfaces!)
– Mobile devices vulnerable to high-G shocks
• Rapid changes of temperature
• Moisture (specifically open structure devices!)
• Defects and contaminations from processing
• Typical failure modes• Unwanted interactions at contacting surfaces – friction, adhesion,
stiction and wear• Fracture• Corrosion, delamination
• Package reliability• Maintain hermeticity, package induced stress
Current status in MEMS reliabilityassessment
Design of Experiment
Modeling Reliability test
Failure analysisCompromisestypical !
Trial & ErrorMethod
Functionality test
Observation
MEMS
Microelectronics
• Limited system and package level reliability data from environmental tests available
• Limited physics of failure knowhow
Development by Trial & Error Methods
- Only isolated areas of a system with functional recipes are known
Improvements needed
Design of Experiment
Modeling Reliability test
Failure analysis
Understanding of materials and specifically materials interactions
Methods of reliability evaluation
Methods of reliability simulation
Methods of failure analyses for effective identification of root cause
Development methodology for reliabilityTaSi2 + SiC
TaSi2 + TaC
Ta5Si3 + Ta2C
Ta2Si + Ta2C
Ta3Si + Ta2C
Ta + Ta2C
Ta2C TaC
TaC + C
TaC + SiC
TaSi2 + SiC
TaSi2 + TaC
Ta5Si3 + Ta2C
Ta2Si + Ta2C
Ta3Si + Ta2C
Ta + Ta2C
Ta2C TaC
TaC + C
TaC + SiC
C
Ta Si
x(Si)
x(C)
0 0.2 0.4 0.6 0.8
0.8
1.0
1.0
0
0.2
0.4
0.6
TaC
Ta2C
SiC
Ta3Si Ta2Si Ta5Si3TaSi2
TaC+C+SiC
TaC+SiC+TaSi2
TaSi2+SiC+Si
Ta+Ta2C+ Ta3Si
Ta2C+TaC+TaSi
2Ta2C+TaSi
2+Ta5Si3
C.L.
MEMS Gyroscope reliability
• Device: a 3-axis MEMS Gyroscope– CoC assembly of ASIC and MEMS– Dimensions: 3.1 mm x 4.2 mm x 0.8 mm
• Reliability characterization:– FEM simulations and shock impacts in all
three orthogonal axes and various shock levels 1,500G – 15,000G
– Non-functional and functional tests– Temperature/humidity test 85°C / 90 RH%
100m
m
100mm
1,0 mm thick 8-layer FR4 board for TH
2,0 mm thick single Cu layer alumina board for shock impact
1. Hollow rotating axle – large enough to fit all cables
2. Sample holder jig – Placed in an angle of 54.74°to excite all 3
axes of the gyroscopes
3. Servo motor
4. Clutch coupling
5. Servo drive to control the motor
6. PC software – Control of the angular velocity
– Acquisition of angular velocity data
7. Wireless communication unit on the rotating axle
8. Wireless communication unit of the PC
9. Slip rings – Power to the wireless communication unit
and the gyroscopes
Test methods – 3 axis Gyroscopes
][208)74.54cos(][360..)cos( dpsdpsgeaxlezyx ≈°⇒⋅Ω=Ω=Ω=Ω α
1
2
7
4 3
8
5
9
6
Test methods – healt monitoring
• Failure criterion: predefined change in the – Offset– Sensitivity– Noise
in the output of any of the three axis
• Repeated at different angular velocities: – 0, ± 450, ± 1350, and ± 1800 degrees per second
• The health monitoring procedure was repeated once per hour
Test methods – shock impact
XY
Z+ Z-
Y
Z-
Z+
Shock impact tester(up to 100 000 G)Functional evaluation between shock impacts
velocity
X
• Evaluated parameters– Offset– Sensitivity– Noise
• Four impact orientations: X, Y, Z+, and Z-
Rigid strike surface
Pne
umat
ic
cylin
der
• Health monitoring between the shock impacts
• Devices were not electrically connected during shock impacts
Shock impact results
• Decelerations to produce package failures is about two times that of electrical device failures
• Differences in deceleration tolerance were analyzed statistically
1. Package level failures• Z+ differs statistically significantly from
others• Z- differs from X statistically significantly
2. Electrical device failures• Y differs statistically significantly from Z+
=> Deceleration tolerance has an impact orientation dependency
X Y Z+ Z-v
Y X Z+ Z-
Package failure 8800 10288 14975 8388
Electrical failures 3919 4525 5189 4319
0
2 000
4 000
6 000
8 000
10 000
12 000
14 000
16 000
18 000
De
cele
rati
on
/ [
G]
Impact Orientation
Package failure
Electrical failures
X-o
rient
atio
nY-
orie
ntat
ion
Z-o
rient
atio
nDisplacements Stress distribution Failed device
v
v
v
Shock impact – package failures
Shock impact – package failures
Borosilicate glass– Fracture paths (averages):
• In the borosilicate glass: 70 %• In the silicon: 16 % • Along the fusion bonded interface: 14 %• Along the the anodic bonding interface: 0%
• Y orientation
24 %
74 %
2 %
All orientations
Z-axis failed All axes failed
X and Y axis failed
N = 67
• Transient failures in 22% of the tested gyroscopes X Y Z+ Z-
v
• Electrical failure modes
Shock impact – electrical failures
0 %
20 %
40 %
60 %
80 %
100 %
X Y Z+ Z-
Shock Impact Oroentation
All axes failed Z-axis failed X and Y axis failed
Shock impact – FEM simulationY-
orie
ntat
ion
X-o
rient
atio
nZ
-orie
ntat
ion
Acceleration (to bring moving
structures in contact)
≈≈≈≈ 4 500 G
≈≈≈≈ 1 500 G
(Deformation enlarged 30 times)
≈≈≈≈ 1 800 G
Shock impact – active element failures
• Failure analyses of internal failure sites:– The “cap” of the device is thinned down by DRIE etching– Observation windows are cut in the thinned-down caps by FIB– Observation by the SEM or optical microscopy
• Examples of internal failure modes:– Fractured comb arms(A)
– Fractured comb fingers(B)
– Stuck MEMS elements(C)
– Chipped edges (D)
(A) (B) (C) (D)
Temperature / humidity test (85°C / 90 %RH)
• Failures detected in 13 out of 27 gyroscopes after 180 days of exposure
– 10 failures before 50 days – 11th failure after 148 days
=> At least two different failure mechanisms
• Early failures:– Operation recovered after the devices were
removed from the test environment– Mass decrease of 6% was measured during 7
days at room temperature (devices removed from the substrate by shearing)
=> Failures are most likely due to short circuits by absorbed moisture
– No delamination or voiding of the underfill or the RDL polymer detected
10 1 0001001
5
10
50
90
99
Time / [hours]
Cum
ulat
ive
Fai
lure
Per
cent
age
/ [%
]
Failure mode 1
F=10 / S=0
Failure mode 2
F=3 / S=14
MEMS Microphone reliability
• Device: Multi-chip module composed of– MEMS chip: acoustic sensor
– ASIC
• Reliability characterization:– TH 85°C/85% RH test
– Multigas corrosion
– Shock impact test
Test methods – Microphone
• TH 85 ºC / 85 RH: Loudspeaker outside a test chamber
• Multigas corrosion : Loudspeaker inside the test chamber
Test Chamber
LoudspeakerMicrophones
Heat-resistant
elastic film
11 cm
Loudspeaker
Double chamber configuration
MicrophonesProtected loudspeaker• Toperation ≤ 90°C• “waterproof”• Equipped with internal
• Humidity sensor• Internal microphone
for monitoring
Loudspeaker
Microphones fixed to a jig
Copper coupons for monitoring the atmosphere
Environment chamber Gas control unit
Health monitoring for the microphones
Mixed gas test method – Microphone
2 microphones per board
Power supply
Clock signalgenerator
Audio amplifier
Humidity and temperature sensor
Monitoringprogram
Health monitoring – microphones
Group of ElectronicsIntegration and Reliability,Department of Electronics
Magnitude response
Difference response
Chamber setup
• Microphones in the corrosion chamber for 90 days
• Volume changes 8 times per hour (28.7 litres/minute)
EnvironmentºC % RH
Cl2 (µg/m 3)
H2S(µg/m 3)
NO2(µg/m 3)
SO2(µg/m 3)
Test chamber 30 70 19 262 188 136
Nordic outdoor 5 78 0,9 4,6 28 30
Harsh industry 25 49 15 135 23 550
Response of the microphones
Time / [days]
Mag
nitu
deR
espo
nse
/ [dB
]