| We study the design and scaling issues of parallel-plate and vertical-comb micromirrors for their potential applications in both DUV and EUV maskless lithography. The transient and resonant behavior of various types of electro-mechanically coupled structures with different degrees of freedom is investigated using the perturbation method, linear control theory, and numerical simulation. Non-dimensional control parameters are identified and transient optimization is carried out to minimize the systems' settling time. We examine the dynamic characteristics such as stability, damping, and the settling time of damped parallel-plate and vertical-comb microactuators using different switching schemes. It is found that vertical double-comb tilting micromirrors and doubly clamped double-flexure piston micromirrors have superior stability. We also show that for both micromirror types, the transient response can tolerate the probable fabrication variations in resistance and spring properties.; We propose using the mirror hinge as a built-in resistor to introduce optimal electrical damping in EUV micromirrors operating in vacuum. We have developed a low-temperature (<430°C) IC compatible SiGe process, in which SiGe can be doped at different levels to function as a structural (conductive) and damping (resistive) material. It is demonstrated that SiGe resistivity requirement (10--100 Ohm-cm) can be met by varying the dopant gas (B 2H6) flow rate. However, a repeatability problem owing to dopant injector clogging is discovered and a non-clogging dopant gas such as BCl3 is proposed for future study. Self-aligned processes using "spacer nanolithography" to define ultrathin nano-scale actuation gaps for low-voltage operation (e.g., <2 V) have been developed to fabricate both tilting and piston micromirrors. We have successfully constructed double-comb tilting micromirrors with 300-nm fingers and 40-nm finger gaps, and double-flexure piston micromirrors with 80-nm thick flexures and 80-nm actuation gaps. The mirror sizes are in the range of 10 to 0.5 mum. |
