poly-SiC Films Research Articles

Doped polycrystalline 3C-SiC films with low stress for MEMS: part II. Characterization using micromachined structures

Surface micromachined resonators, stress pointers and cantilevers were used to characterize nitrogen doped polycrystalline 3C-SiC (poly-SiC) films for their suitability as structural materials for MEMS applications. The films were deposited on 100 mm diameter silicon (Si) and silicon dioxide (SiO2) wafers using dichlorosilane (SiH2Cl2) and acetylene (C2H2) as precursors and ammonia (NH3) as the dopant source gas in a high throughput, low-pressure chemical vapor deposition furnace. The average Young's modulus was found to be 330 GPa for the films with the best combination of resistivity and residual stress. The device level tensile stress and stress gradient measured by stress pointers and cantilevers indicated that the residual tensile stresses were significantly affected by doping gas flow rate and deposition pressure. The best films deposited on SiO2 exhibited an average tensile stress of 74.2 MPa and an average stress gradient of 222 MPa µm−1.

Characterization of Thermoelectric Properties of Heavily Doped n-Type Polycrystalline Silicon Carbide Thin Films

The thermoelectric properties of heavily doped n-type 3C polycrystalline silicon carbide (poly-SiC) films are investigated for microelectromechanical systems (MEMS) applications in harsh environments. Two MEMS structures are designed and fabricated to measure the Seebeck coefficient and the lateral thermal conductivity of poly-SiC thin films. The van der Pauw structure is used to determine electrical resistivity. The obtained Seebeck coefficient is -10μV/K at room temperature, increasing in magnitude to -20 μV/K at 300 °C. The power factor is in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> W·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> within the tested temperature range. The measured lateral thermal conductivity of poly-SiC thin film is 64 W·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , significantly lower than that of undoped single-crystalline SiC, due to increased phonon-grain-boundary and phonon-impurity scatterings. The decrease in the thermal conductivity of the heavily doped poly-SiC film benefits its thermoelectric figure of merit, which is 4.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> .

A polycrystalline SiC-on-Si architecture for capacitive pressure sensing applications beyond 400 °C: Process development and device performance

Abstract

The Poly-SiC MSM Photodiode on Ceramic SiC Substrate for Low-Cost Ultraviolet Light Sensing Applications

Pseudo 6H poly-SiC (P6H-SiC) films are deposited on the pressureless-sintering ceramic SiC substrate by rapid thermal chemical vapor deposition technology to develop metal-semiconductor-metal (MSM) photodetectors (PD) for low-cost and high-performance ultraviolet (UV) detecting applications. The sensing performances of the Au/P6H-SiC/ceramic-SiC MSM PD were examined by measurement of photo and dark currents with and without the irradiation of 366 and 254 nm UV lights. At room temperature/250°C, the photo/dark current ratio of the P6H-SiC MSM PD is ~ 20.66/8.98, respectively, under irradiation of UV light, which are higher than that of ~ 4.88/0.22 for the polycrystalline β-SiC MSM PD on silicon substrate.

Low-energy ion bombardment to tailor the interfacial and mechanical properties of polycrystalline 3C-silicon carbide

Low-energy Ar+ ion bombardment of polycrystalline 3C-silicon carbide (poly-SiC) films is found to be a promising surface modification method to tailor the mechanical and interfacial properties of poly-SiC. The film average stress decreases as the ion energy and the bombardment time increase. Furthermore, this treatment is found to change the strain gradient of the films from positive to negative values. The observed changes in stress and strain gradient are explained by ion peening and thermal spikes models. In addition, the poly-SiC films show a significant enhancement in corrosion resistance by this treatment, which is attributed to a reduction in surface energy and to an increase in the compressive stress in the near-surface region.

Very Thin Poly-SiC Films for Micro/Nano Devices

We report characterization of nitrogen-doped, very thin, low-stress polycrystalline silicon carbide (poly-SiC) films suitable for fabricating micro/nano devices. The poly-SiC films are deposited on 100 mm-diameter (100) silicon wafers in a large-scale, hot-wall, horizontal LPCVD furnace using SiH2Cl2 and C2H2 as precursors and NH, as doping gas. The deposition temperature and pressure are fixed at 900 degrees C and 4 Torr, respectively. The deposition rate increases substantially in the first 50 minutes, transitioning to a limiting value thereafter. The deposited films exhibit (111)-orientated polycrystalline 3C-SiC texture. HR-TEM indicates a 1 nm to 4 nm amorphous SiC layer at the SiC/silicon interface. The residual stress and the resistivity of the films are found to be thickness dependent in the range of 100 nm to 1 microm. Films with thickness less than 100 nm suffer from voids or pinholes. Films thicker than 100 nm are shown to be suitable for fabricating micro/nano devices.

Very Thin Poly-SiC Films for Micro/Nano Devices

We report characterization of nitrogen-doped, very thin, low-stress polycrystalline silicon carbide (poly-SiC) films suitable for fabricating micro/nano devices. The poly-SiC films are deposited on 100 mm-diameter (100) silicon wafers in a large-scale, hot-wall, horizontal LPCVD furnace using SiH2Cl2 and C2H2 as precursors and NH3 as doping gas. The deposition temperature and pressure are fixed at 900 °C and 4 Torr, respectively. The deposition rate increases substantially in the first 50 minutes, transitioning to a limiting value thereafter. The deposited films exhibit (111)-orientated polycrystalline 3C-SiC texture. HR-TEM indicates a 1 nm to 4 nm amorphous SiC layer at the SiC/silicon interface. The residual stress and the resistivity of the films are found to be thickness dependent in the range of 100 nm to 1 μm. Films with thickness less than 100 nm suffer from voids or pinholes. Films thicker than 100 nm are shown to be suitable for fabricating micro/nano devices.

Electrical, mechanical and metal contact properties of polycrystalline 3C-SiC films for MEMS in harsh environments

Polycrystalline 3C-SiC (poly-SiC) is a promising structural material for microelectromechanical systems (MEMS) used in harsh environments. In order to realize poly-SiC based MEMS devices, the electrical, mechanical and metal contact properties of poly-SiC have to be optimized. The poly-SiC films, reviewed here, are deposited by low pressure chemical vapor deposition using 1,3-disilabutane (DSB) as the single precursor, and are in-situ doped using NH 3 (n-type) and trimethylaluminum (p-type). The effects of doping on film structure, resistivity, residual stress and strain gradient have been examined. A bi-layer deposition scheme is developed to minimize the strain gradient without compromising the electrical resistivity of film. The temperature coefficient of resistivity (TCR) is also characterized and is found to be negative over 304–638 K. Ohmic nickel and platinum contacts to n-type doped poly-SiC are obtained and characterized. A stable metal contact up to 300 °C in air is demonstrated.

Mechanical properties of a 3C-SiC film between room temperature and 600 °C

Silicon carbide (SiC) is widely recognized as the leading candidate to replace silicon in micro-electro-mechanical systems devices operating in harsh environments. In this work, cantilevers and bridges in SiC are designed, fabricated and evaluated between room temperature (RT) and 600 °C. The active material is a cubic poly-SiC film deposited on a poly-Si layer which is separated from the Si substrate by a thermal oxide. From surface profiling and optical observations, it is deduced that an average residual strain of +5 × 10−4 is present in the 2.7 µm thick film, with a gradient of 2.5 × 10−4 µm−1. The structures are excited either mechanically or electrostatically. Their resonance frequency is measured by laser Doppler velocimetry and used to derive the Young's modulus and residual stress in the heteroepitaxial layer (330± 45 GPa and 200± 20 MPa, respectively). The temperature coefficient of Young's modulus is found to be −53± 2 ppm K−1 in the range RT to ∼ 300 °C, while an analytical expression is given for the temperature dependence of the Young's modulus between RT and 500 °C. The residual tensile stress is found to depend on temperature in a complex manner.

A study of electrical properties and microstructure of nitrogen-doped poly-SiC films deposited by LPCVD

The electrical properties and microstructure of polycrystalline 3C-SiC (poly-SiC) films were studied for different nitrogen doping concentrations. Nitrogen-doped poly-SiC films were deposited by LPCVD (low pressure chemical vapor deposition) at 900 °C and 2 Torr using 100% SiH 2Cl 2 (35 sccm) and 5% C 2H 2 (180 sccm) as the Si and C precursor gases, and 1% NH 3 (20–100 sccm) as the dopant source gas. The resistivity of the poly-SiC films at 30 °C decreased from 1.466 Ω cm to 0.036 Ω cm as the flow of NH 3 dopant source gas increased from 20 sccm to 100 sccm. The surface roughness and crystalline structure of the poly-SiC films did not depend on the dopant concentration. The average surface roughness was 19–21 nm, and the average surface grain size was 165 nm. In XRD spectra, poly-SiC is so highly oriented along the (1 1 1) plane at 2 θ = 35.7° that other peaks corresponding to SiC orientations are not presented. Resistance and temperature coefficient of resistance (TCR) measurements were carried out in the 30–450 °C temperature range. While the resistance decreased with increasing measurement temperature, the magnitude of the resistance change was much larger in the films with lower doping concentration. On the other hand, the linearity of resistance variation was better in the films with higher doping concentration. In case of poly-SiC films deposited with 20 sccm and 100 sccm 1% NH 3, the average TCR is −2050.3 ppm/°C and −1957.0 ppm/°C, respectively.

Novel Polycrystalline SiC Films Containing Nanoscale Through-Pores by Selective APCVD

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.

Nitrogen-Doping of Polycrystalline 3C-SiC Films Deposited by Low Pressure Chemical Vapor Deposition

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.

Mechanical Testing of Flexible Silicon Carbide Interconnect Ribbons

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.

Characterization of Low Stress, Undoped LPCVD Polycrystalline SiC Films for MEMS Applications

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.

Deposition and properties of polycrystalline β -SiC films using LPCVD with different dopant amount

The physical and electrical properties of polycrystalline β-SiC were studied according to different nitrogen doping concentration. The crystalline peaks SiC(111), SiC(220), SiC(311) and SiC(222) appeared in XRD analysis of poly-SiC films deposited on Si substrates covered with thermally-grown SiO2. Resistance of nitrogen-doped SiC films decreased with increasing temperature. The size of resistance ratio was much bigger in films with low doping. On the other hand, the linearity of resistance variation was better in films with high doping.

Fabrication and testing of bulk micromachined silicon carbide piezoresistive pressure sensors for high temperature applications

This paper explores the development of high-temperature pressure sensors based on polycrystalline and single-crystalline 3C-SiC piezoresistors and fabricated by bulk micromachining the underlying 100-mm diameter (100) silicon substrate. In one embodiment, phosphorus-doped APCVD polycrystalline 3C-SiC (poly-SiC) was used for the piezoresistors and sensor diaphragm, with LPCVD silicon nitride employed to electrically isolate the piezoresistor from the diaphragm. These piezoresistors fabricated from poly-SiC films deposited at different temperatures and doping levels were characterized, showing -2.1 as the best gauge factor and exhibited a sensitivities up to 20.9-mV/V*psi at room temperature. In a second embodiment, epitaxially-grown unintentionally nitrogen-doped single-crystalline 3C-SiC piezoresistors were fabricated on silicon diaphragms, with thermally grown silicon dioxide employed for the piezoresistor electrical isolation from the diaphragm. The associated 3C-SiC/SiO/sub 2//Si substrate was fabricated by bonding a (100) silicon wafer carrying the 3C-SiC onto a silicon wafer with thermal oxide covering its surface. The 3C-SiC handle wafer was then etched away in KOH. The diaphragm was fabricated by time etching the silicon substrate. The sensors were tested at temperatures up to 400/spl deg/C and exhibited a sensitivity of 177.6-mV/V*psi at room temperature and 63.1-mV/V*psi at 400/spl deg/C. The estimated longitudinal gauge factor of 3C-SiC piezoresistors along the [100] direction was estimated at about -18 at room temperature and -7 at 400/spl deg/C.

Polycrystalline silicon carbide as a substrate material for reducing adhesion in MEMS

The in-use adhesion characteristics of polycrystalline cubic silicon carbide (poly-SiC) films when used as a substrate material in MEMS applications are investigated using micromachined polycrystalline Si (poly-Si) cantilever beam arrays. The detachment lengths greater than 1500 μm are obtained, corresponding to an apparent work of adhesion of less than 0.006 mJ/m2. This is to be compared to the detachment lengths of less than 200 μm when poly-Si substrate is used, corresponding to the apparent work of adhesion of greater than 20 mJ/m2. To help understand the mechanism leading to the significant reduction in in-use adhesion, the poly-SiC surfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurement. Based on the data, it is suggested that the topography as well as the slower oxidation rate of poly-SiC films may be responsible for the observed adhesion reduction.

The mechanical properties of polycrystalline 3C-SiC films grown on polysilicon substrates by atmospheric pressure chemical-vapor deposition

This paper presents the results of a study to determine Young’s modulus, residual stress, and burst strength of polycrystalline 3C silicon carbide (poly-SiC) films grown on as-deposited and annealed polysilicon substrate layers. The biaxial modulus and residual stress were determined for bulk micromachined poly-SiC diaphragms using an interferometric load-deflection measurement apparatus. The load-deflection data were analyzed using a least-squares fitting technique to extract the biaxial modulus and residual stress values, and Young’s modulus was calculated assuming a Poisson ratio of 0.15. Poly-SiC films comprised of equiaxed grains exhibited Young’s modulus values ranging from 452 to 494GPa, while columnar films with a high degree of (110) texture exhibited Young’s modulus values between 340 and 357GPa. The residual stress for these films did not exhibit a discernable relationship with microstructure; however, the values exhibited a general dependence on growth temperature. Poly-SiC films grown at 1280 °C had residual stress values ranging from 401 to 486MPa, while a film grown at 1160 °C had a residual stress value of 113MPa. Burst strength was determined using a combination of finite element analysis and burst pressure measurements of the suspended diaphragms. Poly-SiC films grown at 1280 °C exhibited an average burst strength value of 1718MPa, while the poly-SiC film grown at 1160 °C had an average burst strength value of 1321MPa.

Measurement of residual stress and elastic modulus of polycrystalline 3C-SiC films deposited by low-pressure chemical vapor deposition

Residual stress and in-plane biaxial modulus of polycrystalline 3C-silicon carbide (poly-SiC) films are studied using bulk and surface micromachined microstructures. The poly-SiC films are deposited on 100-mm-diameter (100) silicon wafers in a high-throughput, low-pressure chemical vapor deposition furnace at 1173 K using dichlorosilane (SiH 2Cl 2) and acetylene (C 2H 2) precursors at deposition pressures from 61 to 666 Pa. The resulting 0.5 to 2.7 μm films are highly textured, (111) oriented, polycrystalline 3C-SiC. Suspended diaphragms of these films are fabricated by standard wet chemical bulk micromachining of the silicon substrate from the back side. The load-deflection technique is used to extract the residual stress and in-plane biaxial modulus. For the center slot position in the boat, the films' residual stress changes from high (e.g., 726 MPa) to a relatively lower (e.g., 265 MPa) tensile stress as the deposition pressure increases from 61 to 333 Pa at fixed precursor flow rates. The average biaxial modulus of the films is 482 GPa; accordingly, the average Young's modulus is 401 GPa, assuming a Poisson's ratio of 0.168. However, the poly-SiC diaphragms fabricated from films deposited in the 380 to 666 Pa pressure range buckle due to compressive residual stress. Therefore, micro strain gauges are also fabricated and used to measure compressive residual strains in these films, which are − 0.032% at 500 Pa and − 0.024% at 666 Pa. Using 401 GPa as Young's modulus, these correspond to residual stresses of − 129 MPa and − 98 MPa, respectively.

Polycrystalline silicon-carbide surface-micromachined vertical resonators-part II: electrical testing and material property extraction

This manuscript is the second of a two part series describing the fabrication and testing of megahertz frequency, polycrystalline silicon-carbide (poly-SiC) micromechanical resonators made from films deposited by atmospheric pressure chemical vapor deposition. In Part I, the development of deposition and patterning techniques suitable for the fabrication of vertically actuated, clamped-clamped beam resonant structures was detailed (see Wiser, Chung, Mehregan, and Zorman, Polycrystalline Silicon-Carbide Surface-Micromachined Vertical Resonators-Part I: Growth Study and Device Fabrication, J. Microelectromech. Syst., vol. 14, no. 3, Jun. 2005). This paper describes the testing procedures used to determine the nominal resonant frequencies and quality factors for these resonators, as well as the methods used to calculate the Young's modulus and residual stress of the poly-SiC films from the resonant frequency data. Poly-SiC devices with a resonant frequency ranging from 1 to 4 MHz and quality factors of around 2500 were successfully tested. The quality factors were lower than expected for devices of this design. Modeling and experimental results indicate that the low values are likely due to insufficient doping in the poly-SiC films. Both the Young's modulus and residual stress values calculated for the poly-SiC films (436 GPa and 121 MPa, respectively) compare favorably with values reported in the literature.