Abstract 

Starting already 60 years ago silicon (Si) has been the material of choice for micro electromechanical systems (MEMS), thanks to its well-established technology base. However, its limitations in terms of electrical, thermal and mechanical properties, such as its relatively low band gap, restrict its performance in high-temperature or harsh environments applications. To overcome these challenges, silicon carbide (SiC) has emerged as a promising alternative. Single crystal SiC exhibit superior properties, including a wide band gap, high breakdown electric field strength and thermal stability, making it ideal for high-power, high-temperature and chemically aggressive applications. This work focuses on polycrystalline 3C-SiC featuring high hardness, thermal stability, wear resistance and biocompatibility, making it a perfect substitute for Si for thermo-mechanically loaded components, such as cantilevers and membranes. The controlled deposition of 3C-SiC on Si or SiO2 coated Si substrates offers the potential for tailored thin film properties, enabling the creation of polycrystalline 3C-SiC-on-insulator (SiCOI-like) structures. This advancement in material properties can significantly improve the performance of 3C-SiC-based MEMS devices, surpassing the limitations of Si and paving the way for next-generation, high-performance applications in extreme environments. This work investigates the deposition, characterization and application potential of polycrystalline 3C-SiC thin films grown on both Si and SiO2 covered Si substrates. An alternating approach is used to feed the precursor gases of silane and propane into the reaction chamber. The study primarily focuses on the influence of process gas flow rates on thin film growth and properties. A higher growth per cycle was consistently observed with a lower carrier flow rate, attributed to hydrogen inhibition and surface passivation effects. X-ray photoelectron spectroscopy (XPS) depth profiling revealed an increased oxygen content of 7.5 % in thin films grown on SiO2, causing substantial differences when compared to those grown on Si. Higher rates in growth per cycle were measured for Si substrates compared to SiO2. The carbonization layer thickness, however, was consistent for each substrate, featuring a 20 nm amorphous carbon layer at the 3C-SiC/Si interface, while a 10 nm graphite layer was identified at the interface on SiO2. Furthermore, smoother surfaces of 3C-SiC thin films grown on SiO2 compared to Si were measured with a minimum RMS roughness of 2.6 and 3.5 nm, respectively. X-ray diffraction (XRD) analysis confirmed the polycrystalline nature of the films, with the <111> crystallographic orientation being most prominent. Higher crystal quality was confirmed for SiO2 covered Si substrates, indicated by lower full-width at half maximum (FWHM) values of XRD rocking curves of 1.112 ° for Si and 0.446 ° for SiO2. Next, the coefficient of thermal expansion (CTE) of 3C-SiC thin films was investigated, showing significant variation with process gas flow rates. The CTE values ranged from 4.37 to 13.96 ppm/K at 900 °C, resulting in a minimal CTE-mismatch between 3C-SiC and Si of 3.8 %. The impact of the CTE-mismatch is demonstrated on device level with micro hotplate (μHP) structures featuring thermal excitation with patterned platinum (Pt) structures. As expected, significant reduction of thermally-induced deflections was demonstrated when a lower mismatch of the individual CTE values was achieved. Electrical characterization of nominally undoped 3C-SiC films showed that a carefully selected process gas flow ratio resulted in films with low impurity levels, indicated by high film resistivities of 2.3 Ω·cm. For n-doped 3C-SiC thin films, using the alternating feed approach, a minimum film resistivity of 0.02 Ω·cm was measured at room temperature, confirming the effectiveness of the newly designed in-situ doping scheme. Further analysis showed that excessive nitrogen incorporation leads to the formation of an amorphous insulating SiCxNy layer due to the presence of high ammonia flow rates. On the device level, 3C-SiC-based μHP structures exhibited reduced thermally-induced deflections compared to those with Pt-based structures, confirming the potential of integrated 3C-SiC heating structures as minimal deflections resulted. In conclusion, this work highlights the significant impact of deposition parameters on the growth, structural and electrical properties of polycrystalline 3C-SiC thin films, with promising applications in MEMS and high-temperature device applications.

Publications

PhD thesis

Journal articles

  1. Moll, Philipp, et al. "Ultra-low CTE-mismatch of 3C-SiC-on-Si thin films for high temperature MEMS applications." Sensors and Actuators A: Physical (2025): 116262, https://doi.org/10.1016/j.sna.2025.116262
  2. Moll, Philipp, et al. "Polycrystalline LPCVD 3C-SiC Thin Films on SiO2 Using Alternating Supply Deposition." Journal of Microelectromechanical Systems (2024), https://doi.org/10.1109/JMEMS.2024.3472286
  3. Moll, Philipp, et al. "Impact of alternating precursor supply and gas flow on the LPCVD growth behavior of polycrystalline 3C-SiC thin films on Si." Sensors and Actuators A: Physical 372 (2024): 115376, https://doi.org/10.1016/j.sna.2024.115376

Conference contributions

  1. Moll, Philipp, Georg Pfusterschmied, and Ulrich Schmid. "Impact of Excess Carbon at the 3C-SiC/SiO 2 Interface Using LPCVD-Based Alternating Supply Deposition." 2024 IEEE 37th International Conference on Micro Electro Mechanical Systems (MEMS). IEEE, 2024, https://doi.org/10.1109/MEMS58180.2024.10439413
  2. Moll, Philipp, Georg Pfusterschmied, and Ulrich Schmid. "Robust Polycrystalline 3C-Sic-on-Si Heterostructures with Low CTE Mismatch up to 900° C for MEMS." 2023 IEEE 36th International Conference on Micro Electro Mechanical Systems (MEMS). IEEE, 2023, https://doi.org/10.1109/MEMS49605.2023.10052144 

Funding

  • TU Wien