Inductively-coupled plasma-enhanced chemical vapour deposition of hydrogenated amorphous silicon carbide thin films for MEMS☆
Introduction
Silicon carbide (SiC) with its outstanding physical and mechanical properties, as well as its chemical inertness and high temperature stability is an interesting, but technologically challenging material with great opportunities for MEMS applications in harsh environments [1], [2]. Young’s modulus values of crystalline silicon carbide (e.g. 3C-SiC or 6H-SiC) in the range of 400–500 GPa [3], [4], [5] and hardness of 30–50 GPa [5], [6] are reported, which enables the realisation of extremely stiff and robust membranes or cantilevers. However, state-of-the-art deposition of crystalline SiC thin films requires chemical vapour deposition (CVD) processes at very high temperatures of 1050 °C and above, which strongly limits the available materials and process steps during MEMS fabrication [7]. To overcome these limitations, low temperature deposited hydrogenated amorphous SiC (a-SiC:H) can be applied as highly chemically inert and up to 650 °C thermally stable layers to improve device properties [8].
In the past years, several papers have been published that report on a-SiC:H thin film properties under varying deposition parameters using conventional capacitively-coupled plasma-enhanced CVD (CCP-CVD) technique, often simply referred to as plasma-enhanced CVD (PECVD) [9], [10], [11], [12], [13], [14]. However, ICP-CVD has several outstanding advantages compared to CCP-CVD. It provides higher plasma densities and a reduced ion bombardment of the growing film surface. Additionally, ICP-CVD deposited thin films have a very good spatial homogeneity due to the uniform plasma distribution in radial and axial direction [15], [16]. The ICP process is shown to be capable of depositing nanocrystalline SiC thin films even at 500 °C [17] and is for example used for the realisation of Ni/SiC Schottky diodes [18] or for Si quantum dots synthesis in an a-SiC:H matrix [19]. Despite these very interesting features, to the best of the authors‘ knowledge, a-SiC:H thin films deposited by ICP technique have not been investigated so thoroughly. Therefore, it is the aim of this study to analyse the impact of various parameters naming reactive gas flow ratio (rCH4), ICP-power (PICP), substrate temperature (Ts) and deposition back pressure (pb) on the deposition rate as well as on important MEMS-related thin film properties, so to demonstrate their promising potential for the realisation of robust MEMS. This includes the impact on the refractive index (n) and mechanical film parameters like residual stress (σ), mass density, hardness (Γ) and Young’s modulus (E). Furthermore, the quality of the films and their chemical composition is validated using Fourier transform infrared (FT-IR) spectroscopy, indicating the strong impact of deposition conditions on the chemical bonds, and thus demonstrating the clear correlation between mechanical film properties and SiC bond density.
Section snippets
Experimental details
The thin films used for this study were deposited on 350 μm thick, double side polished 4″ Si (100) wafers. The native oxide layer was removed by a 20 s buffered hydrofluoric acid wet etch step and the wafer surface was cleaned with an EVG EV 300 megasonic wafer cleaner prior to deposition. The thin films synthesis was carried out using an OXFORD INSTRUMENTS Plasmalab System 100 at various deposition parameters. Argon (Ar), methane (CH4) and silane (SiH4) are used as precursor gases, with rCH4
Results
CVD deposition of thin films is a complex process and changing one parameter slightly can have a huge impact on properties and quality of the resulting layer. Therefore, a careful selection of the process parameters has to be done to prevent a possible misinterpretation of the results. In the following section, the impact of rCH4, PICP, Ts and pb on the deposition process is shown and the resulting optical and mechanical properties, as well as the chemical composition of as-deposited a-SiC:H
Discussion
As presented above, variation of the deposition parameters has a huge impact on the resulting chemical and mechanical properties of a-SiC:H thin films. Obviously, the chemical and mechanical thin film characteristics are highly correlated. Especially the amount of SiC bonds seems to reflect the resulting mechanical thin film parameters very well. To discuss these correlations, the areal density of SiC bonds δSiC obtained from FT-IR is plotted versus the residual stress σ and the mass density ρ
Conclusions
This study has shown the possibility to tailor key thin film properties of ICP-CVD deposited a-SiC:H thin films to a great extent which provides huge opportunities for the design and realisation of MEMS devices. Refractive index values at λ = 632.8 nm for example can be modified from 1.68 for highly carbonated a-SiC:H up to 3.49 for a methane-free deposition process by variation of the reactive gas flow ratio. This variation also has a major impact on the resulting mechanical thin film properties.
Acknowledgements
This work has been (partially) supported by the Linz Center of Mechatronics (LCM) in the framework of the Austrian COMET-K2 programme ACCM.
T. Frischmuth was born in Nuremberg, Germany, in 1984. During his diploma thesis he worked on droplet microfluidics and its possible application for cell analysis using Raman spectroscopy at TU Wien, where he received his MSc. degree in biomedical engineering in 2012. Since then, he is working at the Institute of Sensors and Actuator Systems at TU Wien where he received his PhD in 2016. His research mainly focuses on the material properties and synthesis of hydrogenated amorphous silicon
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T. Frischmuth was born in Nuremberg, Germany, in 1984. During his diploma thesis he worked on droplet microfluidics and its possible application for cell analysis using Raman spectroscopy at TU Wien, where he received his MSc. degree in biomedical engineering in 2012. Since then, he is working at the Institute of Sensors and Actuator Systems at TU Wien where he received his PhD in 2016. His research mainly focuses on the material properties and synthesis of hydrogenated amorphous silicon carbide thin films and the implementation thereof into microelectromechnical systems.
M. Schneider was born in Ottweiler (Saar), Germany, in 1983. He graduated in the field of physics in 2009 with his diploma work on Lorentz angle measurements in highly irradiated silicon strip detectors for high energy collider applications such as the large hadron collider at CERN. The same year, he started his PhD thesis at the department of Microsystems Technology at Vienna University of Technology. He finished his PhD in 2014 on the study of material properties of aluminium nitride thin films and is currently working as a Postdoc in the same department. His current research focus is on silicon carbide.
D. Maurer studied Equipment Engineering at the University of Applied Sciences in Villach and received his BSc. degree in 2007. After finishing his first study he worked as CAD construction engineer at Mechatronic Systemtechnik GmbH in Villach. Between 2009 and 2011 he studied Systems Design at the University of Applied Sciences in Villach. He received his MSc. degree after finishing the diploma thesis at Infineon Technologies Austria AG where he has been employed in 2010. Since October 2011 he has been working as process integration engineer for MEMS silicon microphones in the semiconductor production at Infineon Technologies Austria AG.
T. Grille was born in 1964 in Marburg/Lahn, Germany. After an apprenticeship as a chemical-technical assistant at the NTA Prof. Dr. Grübler gGmbH he became a state-certified process and environmental engineer. Since 1987 he worked on various fields in industry and science, e.g. on GaAs semiconductors and on high-efficiency solar cells at the Fraunhofer Institute, on DRAM in SIMEC Dresden and on power semiconductors in the IGBT Wafer FAB Start-Up in Lenzburg. Since 2000 he is employed at Infineon Technologies Austria AG as a product matrix-leader in the MEMS Group for Si-Microphones where he received multiple innovation and high performance awards.
U. Schmid started studying physics and mathematics at the University of Kassel in 1992. He performed his diploma work at the research laboratories of the Daimler-Benz AG on the electrical characterization of silicon carbide (6H-SiC) microelectronic devices. In 1999, he joined the research laboratories of DaimlerChrysler AG (now Airbus Group) in Ottobrunn/Munich, Germany. In 2003 he received his Ph.D. degree from the Technische Universität München. From 2003–2008, he was post-doc at the Chair of Micromechanics, Microfluidics/Microactuators at Saarland University. Since October 2008, he is full professor for Microsystems Technology at the Vienna University of Technology.
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Selected paper from EUROSENSORS 2015 conference, September 6-9, 2015, Freiburg, Germany.