New Reliability Assessment Methods for MEMS. Prof. Mervi Paulasto-Kröckel Electronics Integration and Reliability

Transcription

1 New Reliability Assessment Methods for MEMS Prof. Mervi Paulasto-Kröckel Electronics Integration and Reliability

2 Aalto University A merger of leading Finnish universities in 2010: Helsinki School of Economics Helsinki University of Technology University of Art and Design Helsinki

3 Art, Business, Science and Technology School of Art and Design Economics Chemical Technology Electrical Engineering Engineering Science

4 Electronics Integration and Reliability Past and Present Courtesy of VTI Metal ceramic joining Electronics assembly technology, soldering MEMS 3D integration, Bioelectronics & -sensors Compatibility of dissimilar materials and interconnect technologies since 20 years!

5 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

6 Characteristics of reliability assessment of MEMS Reliability evaluation in functional state External stimulus (sensors) Monitoring of output (actuators) Test environment MEMS sensor Known stimulus 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 output output Health monitoring & system control Comparison of output with input Detection of output Control unit MEMS actuator input input

7 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

8 Current status in MEMS reliability assessment Design of Experiment Microelectronics MEMS Modeling Trial & Error Method Functionality Reliabilitytest Compromises typical! Observation Failure analysis Limited system and package level reliability data from environmental tests available Limited physics of failure knowhow

9 Development by Trial & Error Methods - Only isolated areas of a system with functional recipes are known

10 Improvements needed Methods of reliability simulation Modeling Design of Experiment Understanding of materials and specifically materials interactions Methods of reliability evaluation Reliability test Failure analysis Methods of failure analyses for effective identification of root cause

11 Development methodology for reliability 1.0 C TaSi 2 + SiC TaSi 2 + TaC 0.8 Ta 5 Si 3 + Ta 2 C TaC + SiC Ta 2 Si + Ta 2 C 0.6 TaC+C+SiC x(c) 0.4 Ta 2 C TaC TaC+SiC+TaSi 2 SiC Ta 3 Si + Ta 2 C TaC + C 0.2 Ta 2C+TaC+TaSi 2 TaSi 2 + SiC+Si Ta + Ta 2 C Ta 2C+TaSi 2+Ta 5Si 3 Ta+Ta 2C+ C.L. Ta 3Si 0 Ta Ta 3 Si Ta 2 Si Ta 5 Si 3 TaSi x(si) Si 1.0 Ta 2 C TaC

12 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% 2,0 mm thick single Cu layer alumina board for shock impact 1,0 mm thick 8-layer FR4 board for TH 100mm 100mm

13 Test methods 3 axis Gyroscopes Ω x = Ω y 2 = Ω z = Ω axle 1 cos( α) e. g. 360[ dps]cos(54.74 ) 208[ dps] Hollow rotating axle large enough to fit all cables 2. Sample holder jig Placed in an angle of 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

14 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

15 Test methods shock impact Functional evaluation between shock impacts Shock impact tester (up to G) X Y Pneumatic cylinder velocity Z+ Z- Evaluated parameters Offset Sensitivity Noise Four impact orientations: X, Y, Z+, and Z- Health monitoring between the shock impacts Devices were not electrically connected during shock impacts Z- Z+ Rigid strike surface X Y

16 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 / [G] Package failure Electrical failures => Deceleration tolerance has an impact orientation dependency 0 Y X Z+ Z- Package failure Electrical failures Impact Orientation X Y Z+ Z- v

17 Shock impact package failures Displacements Stress distribution Failed device X-orientation v Y-orientation v Z-orientation v

18 Shock impact package failures Y orientation 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%

19 Shock impact electrical failures Electrical failure modes All orientations 2 % 24 % 100 % 80 % All axes failed Z-axis failed X and Y axis failed 60 % 40 % 74 % Z-axis failed X and Y axis failed N = 67 All axes failed 20 % 0 % X Y Z+ Z- Shock Impact Oroentation Transient failures in 22% of the tested gyroscopes X Y Z+ Z- v

20 Shock impact FEM simulation Y-orientation Acceleration (to bring moving structures in contact) G X-orientation G Z-orientation (Deformation enlarged 30 times) G

21 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)

22 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 11 th 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 Cumulative Failure Percentage / [%] Time / [hours] Failure mode 1 F=10 / S=0 Failure mode 2 F=3 / S=14

23 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

24 Test methods Microphone TH 85 ºC / 85 RH: Loudspeaker outside a test chamber Test Chamber Microphones Loudspeaker Heatresistant elastic film 11 cm Multigas corrosion: Loudspeaker inside the test chamber Microphones Loudspeaker Double chamber configuration Protected loudspeaker T operation 90 C waterproof Equipped with internal Humidity sensor Internal microphone for monitoring

25 Mixed gas test method Microphone Power Microphones supplyfixed to a jig Environment chamber Clock signal generator Audio amplifier Gas control unit Monitoring program Humidity Health monitoring and 2 microphones per for Copper coupons for temperature the Loudspeaker board microphones sensor monitoring the atmosphere

26 Health monitoring microphones Magnitude response Difference response Group of Electronics Integration and Reliability, Department of Electronics

27 Chamber setup Microphones in the corrosion chamber for 90 days Volume changes 8 times per hour (28.7 litres/minute) Environment ºC % RH Cl 2 (µg/m 3 ) H 2 S (µg/m 3 ) NO 2 (µg/m 3 ) SO 2 (µg/m 3 ) Test chamber Nordic outdoor ,9 4, Harsh industry

28 Response of the microphones Magnitude Response / [db] Time / [days]