Four-mask process based on spacer technology for scaled-down lateral NEM electrostatic actuators

Multi-spacer technique for low-voltage, high-aspect-ratio lateral electrostatic actuators

“…To overcome this limitation, sidewall stringer and spacer approaches were developed [3][4][5][6][7]. The processes are self-aligned, thereby preserving the symmetry of bi-directional lateral electrostatic actuators.…”

“…Unlike the sidewall stringer process, where the stringer itself is used as the structural layer, for the spacer process, the spacer is then transferred to an underlying planar structural layer by a reactive-ion-etch (RIE). Furthermore, sublithographic beam width and gap size between fixed and movable electrodes are accomplished by using closely spaced islands for the mold or hardmask [6,7]. To enable stiff anchors, the mold or hardmask is selectively removed with a photoresist mask using an additional photolithographic step.…”

“…In the previously demonstrated four-mask spacer process using a polysilicon structural layer [7], the set of nitride spacers, LTO hardmask, and 6:1 BOE etchant was chosen for simplicity of choice. This combination is, however, not ideal since nitride has over 4X lower etch selectivity than oxide in RIE.…”

“…With these choices, we present the fabrication process and mask design schemes for two types of scaled lateral electrostatic actuators: coupled and decoupled designs of springs and movable electrodes. Figure 1 shows cross-sections of the masks that are transferred to an underlying structural layer in the two masking schemes: the previous work (1a) [7] and the present work, the "multi-spacer process" (1b). For the previous masking scheme, without the buffer layer, the isotropic etchant should be chosen not to etch the structural layer as well whereas for the present masking scheme, this is not required.…”

“…Comparison of two masking schemes showing cross-sections of the masks that are transferred to an underlying structural layer: (a) the previous technique without a buffer layer[7], (b1) the multi-spacer technique in this work before anisotropic etch of the buffer layer and (b2) after anisotropic etch of the buffer layer. The buffer and spacer are chosen to be the same material for simplicity of the process.…”