PROCEEDINGS ARTICLE | February 24, 2009
Proc. SPIE. 7208, MOEMS and Miniaturized Systems VIII
KEYWORDS: Actuators, Mirrors, Ferroelectric materials, Modulation, Silicon, Thermal effects, Aluminum, Laser based displays, Optical modulators, Temperature metrology
Diffractive spatial optical modulators (SOM) with fine pitch pixel array were introduced for the mobile applications of
laser projection display which requires the small volume, low power consumption and high optical efficiency.
Micromechanical designs of piezoelectric (PZT) actuator and mirror ribbon structure were optimized for small volume,
but keeping the same level of the other performance. Even though the same design rule and fabrication equipment were
used for 10 um pitch SOM and 16 um pitch SOM, the optical efficiency of the fine pitch SOM was 78 % for the 0th order
diffraction and is better than that of 16 um pitch SOM (73%). The full on/off contrast ratio has no difference between 10
um pitch and 16 um pitch SOM. All the optical characteristics coincide well with the theoretical estimations. High
displacement of 500nm, which is enough to modulate the three Red, Green and Blue colors were achieved by the control
of the thicknesses and stresses of constituent structural layers. It was found that the stress of Pt/PZT/Pt actuating layer
was the main parameter affecting the initial gap height of the ribbon and also its displacement. For improving the optical
properties of the SOM devices, the required ribbon-flatness could be achieved by applying a stress gradient on the SiN
layer to compensate for the stress unbalance between Al mirror and SiN supprting layer. The temperature sensitive
characteristics of the SOM device, which degrades the image quality, could be minimized by a mechanical compensation
method using a thermal expansion effect of Si substrates. This concept could be applied in most of the bridge type
MEMS structure. The most critical parameter which limit the SOM device lifetime was found to be the ribbon
displacement degradation. By using a temperature accelerating lifetime measurement method based on the displacement
degradation the estimated lifetime was more than 4,000 hrs and is of acceptable level in the mobile application. In short,
the developed fine pitch SOM device, for making small volume of optical module, has sufficient response time and
ribbon displacement for modulating the red, blue and green colors with one SOM chip and is suitable for high quality
embedded laser projection displays. Optical module with VGA is successfully demonstrated for its potential applications
in mobile laser projection display such as a embed projection cellular phone.
