Low-thermal-budget electrically active thick polysilicon for CMOS-First MEMS-last integration

Low-thermal-budget,electrically active,and thick polysilicon films are necessary for building a microelectromechanical system(MEMS)on top of a complementary metal oxide semiconductor(CMOS).However,the formation of these polysilicon films is a challenge in this field.Herein,for the first time,the development of in situ phosphorus-doped silicon films deposited under ultrahigh-vacuum conditions(~10^(−9)Torr)using electron-beam evaporation(UHVEE)is reported.This process results in electrically active,fully crystallized,low-stress,smooth,and thick polysilicon films with low thermal budgets.The crystallographic,mechanical,and electrical properties of phosphorus-doped UHVEE polysilicon films are studied.These films are compared with intrinsic and boron-doped UHVEE silicon films.Raman spectroscopy,X-ray diffraction(XRD),transmission electron microscopy(TEM)and atomic force microscopy(AFM)are used for crystallographic and surface morphological investigations.Wafer curvature,cantilever deflection profile and resonance frequency measurements are employed to study the mechanical properties of the specimens.Moreover,resistivity measurements are conducted to investigate the electrical properties of the films.Highly vertical,high-aspectratio micromachining of UHVEE polysilicon has been developed.A comb-drive structure is designed,simulated,fabricated,and characterized as an actuator and inertial sensor comprising 20-μm-thick in situ phosphorus-doped UHVEE films at a temperature less than 500℃.The results demonstrate for the first time that UHVEE polysilicon uniquely allows the realization of mechanically and electrically functional MEMS devices with low thermal budgets.

Single and bundled carbon nanofibers as ultralightweight and flexible piezoresistive sensors

This work demonstrates the application of electrospun single and bundled carbon nanofibers(CNFs)as piezoresistive sensing elements in flexible and ultralightweight sensors.Material,electrical,and nanomechanical characterizations were conducted on the CNFs to understand the effect of the critical synthesis parameter—the pyrolyzation temperature on the morphological,structural,and electrical properties.The mechanism of conductive path change under the influence of external stress was hypothesized to explain the piezoresistive behavior observed in the CNF bundles.Quasi-static tensile strain characterization of the CNF bundle-based flexible strain sensor showed a linear response with an average gauge factor of 11.14(for tensile strains up to 50%).Furthermore,conductive graphitic domain discontinuity model was invoked to explain the piezoresistivity originating in a single isolated electrospun CNF.Finally,a single piezoresistive CNF was utilized as a sensing element in an NEMS flow sensor to demonstrate air flow sensing in the range of 5-35 m/s.

Silicon waveguide cantilever displacement sensor for potential application for on-chip high speed AFM

This paper reviews an initial achievement of our group toward the development of on-chip parallel high-speed atomic force microscopy（HS-AFM）.A novel AFM approach based on silicon waveguide cantilever displacement sensor is proposed.The displacement sensing approach uniquely allows the use of nano-scale wide cantilever that has a high resonance frequency and low spring constant desired for on-chip parallel HS-AFM.The approach consists of low loss silicon waveguide with nano-gap,highly efficient misalignment tolerant coupler,novel high aspect ratio（HAR）sharp nano-tips that can be integrated with nano-scale wide cantilevers and electrostatically driven nano-cantilever actuators.The simulation results show that the displacement sensor with optical power responsivity of 0.31%/nm and AFM cantilever with resonance frequency of 5.4 MHz and spring constant of 0.21 N/m are achievable with the proposed approach.The developed silicon waveguide fabrication method enables silicon waveguide with 6 and 7.5 dB/cm transmission loss for TE and TM modes,respectively,and formation of 13 nm wide nano-gaps between silicon waveguides.The coupler demonstrates misalignment tolerance of ±1.8 μm for 5μm spot size lensed fiber and coupling loss of 2.12 dB/facet for standard cleaved single mode fiber without compromising other performance.The nano-tips with apex radius as small as 2.5 nm and aspect ratio of more than 50 has been enabled by the development of novel HAR nanotip fabrication technique.Integration of the HAR tips onto an array of 460 nm wide cantilever beam has also been demonstrated.