Design and fabrication of hidden hinge monocrystalline silicon micromirrors for maskless lithography

Doctoral thesis, 2008

Micromirror-based maskless lithography has recently received attention as an attractive candidate to tackle challenges associated with the continued de-vice scaling in the semiconductor industry. The micromirrors work by diffrac-tion and the requirements on planarity are very tough, e.g. peak-to-valley dif-ferences of a few nanometers. Increasing the resolution in future systems by decreasing the wavelength further increases the planarity requirements. This thesis deals with a novel micromirror structure intended to meet the planarity requirements in future optical maskless lithography systems. The mirror consists of standing flexure hinges hidden underneath the mir-ror surface. Comprehensive mechanical analysis and simulations of the struc-ture is presented and it is shown that the mirror can be designed to meet future planarity requirements. Even though the rewards are clear the realization poses several challenges which are addressed in this thesis. The proposed fabrication process relies on aligned low temperature trans-fer bonding where the bond alignment accuracy is a critical parameter. A method to achieve submicron post bond alignment accuracy of non-transparent substrates is presented. Successful fabrication requires bonding of μm2-sized areas. A novel method for high spatial resolution mechanical characterization of such small bond interfaces is presented. The method achieves more than two orders of magnitude higher resolution than has been reported previously. Another important challenge is the formation of the hidden monocrystal-line silicon hinges. Simulations show that hinge widths down to tens of nano-meters are required for future mirrors. A method based on local oxidation of silicon has been used to achieve hinge widths down to 20 nm. This method also tends to increase the uniformity across hinges due to stress dependent oxida-tion. Arrays of monocrystalline silicon hidden hinge mirrors have been fabri-cated using a low temperature transfer process. Down to 16×16 μm2 sized mir-rors have been realized and actuation characteristics of fabricated mirrors agree with theory and simulations.