Plasma Power Effect on the Surfaces of a Quartz Crystal During Etching using Tetrafluoroethane Gas

The performance of a quartz crystal microbalance (QCM) biosensor can be enhanced by patterning the surface of the SiO2 substrate. In this study, the patterning was realized by a plasma etching process. The etching of the SiO2 was carried out using a tetrafluoroethane (CH2FCF3) plasma. The plasma was generated by applying power from a generator. The generator used in this research was a low frequency 40 kHz plasma generator. The generator was equipped with automatic matching circuits, which ensured the stability of plasma power during the experiments. The specimens were produced with a power ranging from 40 watts to 120 watts for 1 hour. The pressure of the chamber was fixed at 1 Torr. The processing gas for this study was a commercial CH2FCF3 gas. The flow rate of the gas was 20 ml/min. The purpose of this research was to study the effect of plasma power on the etching rate and the anisotropy of the etched SiO¬2 surface. The etching rate and the anisotropy strongly correlate with the quality of patterning. Measurement and observation of the etched SiO2 surface were carried out using an optical microscope and a TMS-1200 (Topography Measurement System). The optical microscope was used to determine the etched area from the unetched one, while the TMS was utilized to obtain the thickness and the surface profile. The results show the highest etching rate, i.e., 17.90 nm/min, was obtained by applying a plasma power of 100 watts. The rate demonstrated a relatively slow etching process due to a complex mixture of fluorine (F) and the CH2FCF2 compound. This slow etching rate is preferable for controlling nano-profiles of the pattern. Furthermore, the applied power also had an effect on the anisotropy of the etched profile, and the results of this research show that the best anisotropic coefficient, i.e., 4.8×10-2, occurred in the process with an optimized 110-watt power. The anisotropy was defined as the ratio of the vertical etching rate and the horizontal etching rate. This ratio is important in determining the quality of the profile of the patterned QCM.

Anisotropic coefficient; Etching rate; Low-frequency generator; Tetrafluoroethane gas

The results of this study demonstrate effective etching of SiO2 by means of a CH2FCF3 plasma. The depth of etching, etching rate and anisotropy coefficient were observed as a function of power. The optimum power for this process can be found in the range of 100 to 110 watts, where the maximum etching rate and maximum anisotropy coefficient were observed. Power higher than 110 watts resulted in a screening effect which reduced both the etching rate and the anisotropy coefficient.

The results of this study demonstrate effective etching of SiO2 by means of a CH2FCF3 plasma. The depth of etching, etching rate and anisotropy coefficient were observed as a function of power. The optimum power for this process can be found in the range of 100 to 110 watts, where the maximum etching rate and maximum anisotropy coefficient were observed. Power higher than 110 watts resulted in a screening effect which reduced both the etching rate and the anisotropy coefficient.

This work was financially supported by The Ministry of Research, Technology and Higher Education (RISTEKDIKTI) Republic of Indonesia through LPPM Brawijaya University.

Fridman, A., 2008. Plasma Chemistry.  Cambridge University Press

Goyal, A., 2006. Ultrasensitive Quartz Crystal Microbalance Integrated with Carbon Nanotube. Thesis Doctor of Philosophy. Graduate School Department of Electrical Engineering, Pennyslvania State University

Li, X., Hua, X., Ling, L., Oehrlein, G.S., Barela, M., Anderson, H.M., 2002. Fluorocarbon-based Plasma Etching of SiO2: Comparison of C4F5/Ar and C4F8/Ar discharges. J. Vac. Sci. Technol. A, Volume 20(6) pp. 2052–2061

Lieberman, M.A., Lichtenberg, A.J., 2005. Principles of Plasma Discharges and Material Processing. Wiley-Interscience, John Wiley & Sons, Inc., Publication Hoboken, New Jersey, USA

Masruroh, Djoko, D.J.D.H., Rahayu, S., Sakti, S.P., 2016. Viscoelastic and Morphological Behavior of Stearic Acid Layer on Top of Polystyrene as Immobilisation Matrix for QCM Sensor. Materials Science Forums, Volume 848 pp. 757–762