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

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

What is a DMD ?

What is a DMD ?

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

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

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!

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

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

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).

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

Hinge memory Efficient heat management added to the previously cited improvements can ensure a lifetime greater than 40000 hours. Hinge memory mean lifetime estimates over testing time

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

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

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

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

Optical properties Optical properties via glass window Enable high quality image

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

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