1. Digital Micromirror Device
Reliability of MEMS: Case study
January 30th 2007
Digital Micromirror Device
Begon Martin
Ciapala Richard
Deaki Zoltan

2. What is a DMD ? Matrix of micromirrors 1024 x 768 mirrors for example
Size of the mirrors: 16 x 16 um

3. What is a DMD ?

4. What is a DMD ?

5. Applications Mainly projection systems (Digital Light Processing)
Other emerging applications such as 3D metrology, confocal microscopy,digital TV

6. Hinge fatigue
Fatigue: slow growth of a crack driven by repeated plastic deformation
Mirror in normal operating mode switches every 200 microseconds
5 years use with 1000 operating hours a year  mirrors switch 90x109 times

7. Hinge fatigue
First approach: anaylsis using bulk properties of the hinge material  showed that fatigue would be a big problem
However… accelerated tests proved that wrong, samples easily exceeding 100x109 switches showing no fatigue.
Explanation: hinge so thin  governed by thin film properties!

8. Hinge memory Most significant mode of failure
Occurs when a mirror operates in the same direction for a long period of time
Main factors are the duty cycle and the operating temperature
Duty cycle: percentage of time a mirror is addressed to one side.(95/5)
Temperature is the dominant factor for hinge memory lifetime

9. Hinge memory
Life test under standard condition of 65°C and 5/95 duty cycle
Bias voltage has to be increased to annihilate residual tilt angle.
Evolution of the bias voltage through the time reported to the number of non functional micromirros
Micromirrors in the back have a residual tilt angle compared to the ones in the front due to the hinge fatigue

10. Hinge memory Cause: Metal creep of the hinge material
Solutions: Selection of a new material with lower degree of metal creep to replace aluminium Improvemed lifetime by a factor of 5 (1000 hours worst-case →not good enough). Implementation of stepped VDD and a “bipolar reset” Allowed mirrors to be efficiently controlled over a wider range of hinge memory. Increased lifetime by a factor of 5 (5000 hours worst-case situation).

11. Thermal management of the DMD
Hinge memory
Thermal management of the DMD
Several sources of heat contribute to hinge memory:
Radiant energy from the light source
Equipment composing the DLP projector and surrounding the DMD
Solution:
Efficient thermal management design required
Heatsinks on the back of most packages to keep the temperature as low as possible
DMD operates at temperatures only 7 to 10 °C above the projector ambient

12. Hinge memory
Efficient heat management added to the previously cited improvements can ensure a lifetime greater than hours.
Hinge memory mean lifetime estimates over testing time

13. Stiction
Induced by an excessive adhesive force between the landing tip and its landing site
Adhesive forces can be induced by:
Surface contamination
Capillary condensation
CMOS defects
Van der Walls forces

14. Stiction
Reliability testing can be done to measure the distribution of surface adhesion across the device to determine the number of operating devices under different switching voltages
Solution to stiction

15. Environment robustness
Based on standard semiconductor tests requirement
Capillarity force
Humidity everywhere
UV light exposure
Thermal testing
Surface contamination
During production process

16. Size vs Robustness Small size enable robustness to mechanical shocks
Lowest resonant frequency in KHz
Test :1500G and 20G in vibration with no mirror breaking
Weaknesses on the package identified and annihilated

17. Optical properties Optical properties via glass window
Enable high quality image

18. Summary of DMD reliability
Hinge memory lifetime
>100’000 hours at normal operating conditions
Random defects
>650’000 hours MTBF (<1500 FIT)
Hinge fatigue lifetime
>3.67 trillion cycle or >250’000 hours
Environmentally robust

19. Conclusion Misleading apparence Concern to reliability
Experience plan must be done to find critical failure modes
Concern to reliability
The reliability of the DMD has been exemplary and should be considered as a reference for development of other MEMS