Papers by Author: Xiao An Fu

Abstract: We report fabrication of lateral, n-channel, depletion-mode, junction-field-effect-transistor (JFET) monolithic analog integrated circuits (ICs) in 6H-SiC. Ti/TaSi2/Pt forms the contact metalization, Ti/Pt the interconnect metal, and the SiO2/Si3N4/SiO2 interlayer dielectric. The threshold voltage and pinch off current indicate that the actual channel doping and thickness is close to the nominal values specified. The wafer yield for good circuits of a single-stage differential amplifier is 54% out of 46 copies.

Abstract: Fully monolithic, transimpedance and differential voltage amplifiers are reported in this paper based on 6H-SiC, n-channel, depletion-mode JFETs. The single-stage transimpedance amplifier has a low-frequency gain of ~222 kΩ at room temperature, with ~2% gain matching for copies on a 6-mm x 6-mm die. The transimpedance gain is set by an integrated resistor and is ~1.1 MΩ at 450oC. The single-stage, differential voltage amplifier has a typical gain-bandwidth of ~2.8 MHz at 600oC and a typical open-loop voltage gain of ~35.8 dB at 25oC, with less than 1-dB gain variation from 25-600oC.

Abstract: This paper reports fabrication and electrical characterization of 6H-SiC n-channel, depletion-mode, junction-field-effect transistors (JFETs) for use in high-temperature analog integrated circuits for sensing and control in propulsion, power systems, and geothermal exploration. Electrical characteristics of the resulting JFET devices have been measured across the wafer as a function of temperature, from room temperature to 450oC. The results indicate that the JFETs are suitable for high-gain amplifiers in high-temperature sensor signal processing circuits.

Abstract: This paper presents silicon carbide sensor interface circuits and techniques for MEMSbased sensors operating in harsh environments. More specifically, differential amplifiers were constructed using integrated, depletion-mode, n-channel, 6H-SiC JFETs and off-chip passive components. A three-stage voltage amplifier has a differential voltage gain of ~50 dB and a gainbandwidth of ~200 kHz at 450oC, as limited by test parasitics. Such an amplifier could be used to amplify the signals produced by a piezoresistive Wheatstone bridge sensor, for example. Design considerations for 6H-SiC JFET transimpedance amplifiers appropriate for capacitance sensing and for frequency readout from a micromechanical resonator are also presented.

Abstract: This paper details the characterization of polycrystalline SiC (poly-SiC) thin films deposited by low pressure chemical vapor deposition. Films were deposited on both Si and SiO2- coated Si substrates using dichlorosilane (SiH2Cl2) and acetylene (C2H2) as precursor gases. Low residual tensile stress films were deposited at 900°C at a pressure of 2 Torr using SiH2Cl2 and C2H2 (5% in H2) flow rates of 35 sccm and 180 sccm, respectively. XRD analysis of these films indicated a (111) 3C-SiC orientation regardless of substrate material. Both resistivity (1.3
-cm) and residual stress gradient (17 MPa/μm) were found to be relatively low and decreased as the film thickness increased. Unintentional nitrogen doping is responsible for the low resistivity measurements and its concentration in the films was about 1.86 x 1016 cm-3. Poly-SiC films exhibiting near-zero residual tensile stress, low stress gradient and relatively low resistivity have favorable properties for design and fabrication of MEMS devices.

Abstract: This paper explores polycrystalline 3C-silicon carbide (poly-SiC) deposited by LPCVD for fabricating flexible ribbon cable interconnects for micromachined neural probes. While doped silicon is used currently, we hypothesized that poly-SiC will provide enhanced mechanical robustness due to SiC’s superior mechanical properties. Paralleling prior work in silicon, forty-two different designs were fabricated from nitrogen-doped poly-SiC films deposited by LPCVD at 900°C using dichlorosilane and acetylene as precursors. The different designs were then tested in bending and twisting modes. Curved beams were found to bend nearly 250% more than straight beams before fracture. Longer beams withstood greater bending and twisting due to greater compliance. Longer and narrower beams generally outperformed shorter beams irrespective of design. Also, doped poly-SiC beams had, on average, breaking angles that were greater than those of identical doped silicon beams by ~50% in bending and ~20% in twisting modes. The paper details the designs studied, describes the fabrication process for the test structures and compares/contrasts the testing and simulation results related to the different designs to identify best design practices.

Abstract: A selective atmospheric pressure chemical vapor deposition (APCVD) process has been developed to deposit porous polycrystalline silicon carbide (poly-SiC) thin films containing a high density of through-pores measuring 50 to 70 nm in diameter. The selective deposition process involves the formation of poly-SiC films on patterned SiO2/polysilicon thin film multilayers using a carbonization-based 3C-SiC growth process. This technique capitalizes on significant differences in the nucleation of poly-SiC on SiO2 and polysilicon surfaces in order to form mechanically-durable, chemically-stable, and well anchored porous structures, thus offering a simple and potentially more versatile alternative to direct electrochemical etching.

Abstract: This paper reports the effect of deposition temperature on the deposition rate, residual stress, and resistivity of in-situ nitrogen-doped (N-doped) polycrystalline 3C-SiC (poly-SiC) films deposited by low pressure chemical vapor deposition (LPCVD). N-doped poly-SiC films were deposited in a high-throughput, resistively-heated, horizontal LPCVD furnace capable of holding up to 150 mm-diameter substrates using SiH2Cl2 (100%) and C2H2 (5% in H2) precursors, with NH3 (5% in H2) as the doping gas. The deposition rate increased, while the residual stress decreased significantly as the deposition temperature increased from 825oC to 900°C. The resistivity of the films decreased significantly from 825°C to 850°C. Above 850°C, although the resistivity still decreased, the change was much smaller than at lower temperatures. XRD patterns indicated a polycrystalline (111) 3C-SiC texture for all films deposited in the temperature range studied. SIMS depth profiles indicated a constant nitrogen atom concentration of 2.6×1020/cm3 in the intentionally doped films deposited at 900°C. The nitrogen concentration of unintentionally doped films (i.e., when NH3 gas flow was zero) deposited at 900°C was on the order of 1017/cm3. The doped films deposited at 900°C exhibited a resistivity of 0.02
-cm and a tensile residual stress of 59 MPa, making them very suitable for use as a mechanical material supporting microelectromechanical systems (MEMS) device development.