Micromechanical properties of silicon-carbide thin films deposited using single-source chemical-vapor deposition

C. R. Stoldt, M. C. Fritz, C. Carraro, R. Maboudian
2001 Applied Physics Letters  
1,3-Disilabutane is used as a single-source precursor to deposit conformal silicon-carbide films on silicon atomic-force-microscopy cantilevers. By measuring the resonance frequency of the cantilever as a function of silicon-carbide film thickness and developing an appropriate model, the value of the film's elastic modulus is determined. This value is in good agreement with those reported for silicon-carbide films deposited using conventional dual-source chemical-vapor deposition. Additionally,
more » ... tion. Additionally, we comment on the feasibility of integrating this process into the fabrication technology for microelectromechanical systems. Silicon carbide ͑SiC͒ is an attractive material for demanding mechanical and high-temperature applications, as well as for use in abrasive, erosive, and corrosive media. It is tough, possesses low-friction characteristics, and is second only to diamond in wear resistance. SiC is also a semiconductor of great interest in high-power, high-temperature, and high-radiation applications. 1,2 Furthermore, SiC is one of the foremost biocompatible materials, 3 making it a potential candidate for biological applications. The superior mechanical, chemical, and electrical capabilities make SiC an exceptionally attractive material in microelectromechanical systems ͑MEMS͒. 4 -6 As it stands, SiC technology remains technically demanding and a nonstandard process in the Si-based integrated circuit ͑IC͒ fabrication laboratories. As a consequence, the development of a simple method for integrating thin, conformal SiC coatings as well as structural layers into Si-based micromechanical systems is of immediate interest. Conventional SiC chemical-vapordeposition ͑CVD͒ processes generally utilize multiple precursors such as silane and acetylene, and require elevated substrate temperatures in excess of 1000°C. High temperatures cause significant deformation of released polycrystalline-Si microstructures, 7 making postprocess SiC coating challenging if not impossible. This indicates that low-temperature alternatives to conventional SiC CVD methods must be considered. Ideally, a simple CVD method should utilize a single precursor that forms high-quality SiC films at temperatures low enough to enable SiC deposition postprocessing. Lee and co-workers 8 have demonstrated that epitaxial cubic-SiC films can be formed on single-crystalline Si at temperatures of 900-1000°C utilizing 1,3-disilabutane ͑DSB͒ as a single precursor. Polycrystalline-Sic films at temperatures as low as 650°C have been achieved, although temperatures near 800°C yield a higher growth rate and improved crystallinity. 9 These temperatures make postprocessing of Si-based MEMS tangible. At room temperature, DSB is a liquid with a vapor pressure of 27 Torr, making the handling aspects much more simplified when compared to conventional dual-source CVD. Also, the use of a single precursor insures stoichiometry, hence, it eliminates the need for an elaborate gas handling system. These features make this method of deposition ideal for integrating SiC into MEMS technology. However, the micromechanical properties of SiC films produced using DSB have not been investigated. In this letter, we report a method for determining the elastic modulus of polycrystalline-SiC films using Si cantilevers commonly employed in atomic-force microscopy ͑AFM͒. Through this study, the feasibility of using a single CVD precursor for coating Si-based micromechanical structures is also demonstrated. The SiC deposition proceeds as follows. Before deposition, the DSB ͑Gelest Inc., Ͼ95% purity͒ is further purified by freeze-pump-thaw cycles using liquid nitrogen, after which it is introduced into the vacuum chamber via a variable leak valve and nozzle. The base pressure of the CVD chamber is below 2ϫ10 Ϫ9 Torr. Prior to introduction into the CVD chamber, each Si AFM cantilever is dipped in concentrated HF to remove the native oxide, then rinsed with acetone and dried under N 2 . This procedure is confirmed not to change the previously determined cantilever resonance frequency. In the CVD chamber, the cantilever is positioned 3 cm below the exit of the nozzle. The cantilever is radiatively heated from below by a spiral tungsten filament, and its temperature is calibrated and monitored by a thermocouple attached to the heating stage. Deposition begins by pressurizing the CVD chamber with DSB to 5ϫ10 Ϫ5 Torr with the cantilever temperature held at 100°C. The cantilever temperature is then increased at a rate of 50°C/min to the growth temperature of 780°C at constant DSB pressure, where it is then held for the duration of the deposition ͑from 0 to 180 min͒. The growth rate under these conditions is 3.5 nm/min. Control experiments performed on uncoated cantilevers show that prolonged annealing at 780°C in the absence of DSB does not change the resonance frequency. a͒ Electronic
doi:10.1063/1.1383277 fatcat:hmluqbpwyrhp5hbat6khedgovu