Ultra-deep LIGA process

The ultra-deep (UD) LIGA strategy for die fabrication that has features up to two millimeters in thickness and 100 microns in linewidth has been developed at SRRC, Taiwan. Here, a successive exposure strategy has been demonstrated to overcome the shortage of hard x-rays generated by a medium energy light source such as the 1.5 GeV Taiwan Light Source. Furthermore, this successive exposure process accumulates much more irradiation dosage in the photoresist that leads to a reduction of the developing time and thus produces a UD microstructure with very high aspect ratio. The present process makes use of a conformal mask technology that permits the sidewalls of the microstructure to be perfectly aligned after multiple exposures and developments. Since the total reflection of x-rays inhibits further dosage deposition on the sidewall after the first exposure, the successive exposure process improves the precision of the microstructure. The main concerns of fabricating a UD microstructure include the low diffusion speed involved in developing a high-aspect-ratio microstructure and the high residual stress between photoresist and substrate. A deeper microstructure can be achieved by choosing proper photoresist material, increasing the diffusion speed of development and designing a suitable structure to balance residual stress. A low temperature process is essential to keep the thermal stress from destroying UD microstructures.