Papers by Author: Christian A. Zorman

Abstract: We report on experimental explorations of using focused ion beam (FIB) nanomachining of different types of silicon carbide (SiC) thin membranes, for making robust, high-quality stencil masks for new emerging options of nanoscale patterning. Using thin films and membranes in polycrystalline SiC (poly-SiC), 3C-SiC, and amorphous SiC (a-SiC) with thicknesses in the range of t~250nm−1.6μm, we have prototyped a series of stencil masks, with nanoscale features routinely down to ~100nm.

Abstract: This paper details the development of amorphous hydrogenated silicon carbide (a-SiC:H) films as structural material that is resistant to biofouling. The a-SiC:H films were deposited by PECVD and evaluated for their mechanical and anti-biofouling properties. It was found that the as-deposited films exhibited compressive residual stresses that could be converted to moderate tensile stresses upon a post deposition anneal. The amorphous films exhibited a much lower Young’s modulus but similar burst stress when compared to polycrystalline 3C-SiC films of like thickness. The as-deposited a-SiC:H films were more resistant to biofouling than silicon and silicon dioxide surfaces. Coating the a-SiC:H films with polyethylene glycol (PEG) significantly improved the anti-fouling characteristics for extended periods.

Abstract: We report an initial experimental exploration of engineering very thin, suspended amorphous silicon carbide (a-SiC) membranes into vibrating micromechanical devices. We show that micromachined a-SiC thin square membranes can make interesting multiple-mode flexural resonators, with frequency spectra exhibiting many measurable resonant modes over a wide frequency range (100kHz–10MHz) in the low radio frequency (RF) bands. Initial demonstration and preliminary data suggest interesting and rich dynamical, nonlinear, and dissipative properties in these micromechanical resonances. Specifically, for instance, at room temperature (T≈300K) and in moderate vacuum (e.g., ~20mTorr), resonant modes of an a-SiC square membrane (thickness: t≈1.5µm, size: 1mm×1mm) are observed in the ~100kHz–5MHz range, with measured quality factors (Q’s) in the range of ~2,500–9,000.

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: 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.