The focus of this research is the fabrication of light modulator nanomirror arrays with on-chip integrated circuits for static and dynamic device characterization. While conventional optical MEMS devices have features on the order of tens of microns, we wish to eventually fabricate light modulators with a reflective area of one sq. µm or less. The idea is to replace unwieldy and expensive masks with spatial light modulator arrays that promise to be more cost-effective and can eliminate the design-to-mask wait period.
Each mirror is modulated with an electrostatic force across a gap of around a few tenths of a micron. Damping is achieved by energy dissipation via a resistor built into the flexure hinge. The schematic diagram of a parallel-plate mirror design is shown in Figure 1. The mirrors tilt when a potential difference is applied between the underlying poly electrode and the mirror. The modulation tilt angle is limited to 1 or 2 degrees from bias position.
We have completed preliminary analysis of heat dissipation and static behavior of the mirror earlier in the project. Fabrication of a parallel-plate structure with sacrificial gap ~ 100 nm and mirror area ranging from 1x1 µm2 to 5x5 µm2 is complete. A recently fabricated mirror device is shown in Figure 2. The current focus of our research is on optical measurement of device properties and IC design for eventual on-chip characterization.
Pure phase modulation (using normal motion of the mirrors) and phase/amplitude modulation (using tilting motion of the mirrors) methods of pattern generation are studied using analytical methods and simulators such as (far-field) SPLAT and (near-field) TEMPEST. The objective is to quantify aerial image in terms of important parameters such as normalized image log slope, contrast, and spots/min feature size, to come up with the most robust method of gray-scaled pattern generation using analog modulation of micromirrors.
Figure 1: Schematic diagram of a parallel-plate micromirror
Figure 2: SEM view of a 5 x 5 µm2 mirror
The primary task of this project is to design, fabricate, and characterize the comb-actuated nanomirror array for EUV maskless lithography to print feature sizes below 100 nm. The technological challenges facing the nanomirror fabrication come from the need to fabricate sub-micron mirrors with sub-50 nm vertical comb gaps, for which a self-aligned process is necessary to build a hidden-hinge and double-comb structure. Moreover, the analog “gray-scaling” printing scheme requires a clear understanding of the difference between the posicast switching and biased switching in an analog scheme. A continuing research effort on this dynamic will follow in the future.
A preliminary process has been designed and tested successfully. Vertical sub-micron comb structures (without a hinge) with released 50 nm comb gaps have been achieved (see figure). We have identified some problems of the process and are currently optimizing the fabrication parameters for further improvement of device structure. We are also investigating the relevant process to form a mirror-compatible amorphous Si thin film with satisfactory photosensitive and resistive properties (for electrical damping).
Figure 1: The SEM pictures of fabricated nanocomb structures with 50-nm comb gaps