Digital Micromirror Device

Begon Martin Ciapala RichardDeaki Zoltan

Reliability of MEMS: Case studyJanuary 30th 2007

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.

Micromirrors in the back have a residual tilt angle compared to the ones in the front due to the hinge fatigue

Evolution of the bias voltage through the time reported to the number of non functional micromirros

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

• 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

Hinge memory

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