Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Redeposition-free of silicon etching by CF 4 microwave plasma in a medium vacuum process regime C. Pakpum a, ⁎ , D. Boonyawan b a Program in Applied Physics, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand b Plasma and Beam Physics Research Facility, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand ARTICLE INFO Keywords: CF 4 microwave plasma Isotropic etching Plasma etching Redeposition-free Silicon ABSTRACT This work presents a redeposition-free process to etch silicon by CF 4 plasma in a modified microwave oven reactor operated in a medium vacuum process regime (25 to 1 × 10 -3 Torr) that only uses a mechanical pump which is introduced at a lower cost compared to the ICP etching system. In order to achieve the capability of the etching process provided by this system, experimental trials were conducted by varying the microwave power in the range of 360–1200 W, gas flow rate 30–90 mL/min and bias voltage at the substrate holder -100 V to -300 V. Plasma species present in the discharge were also monitored by optical emission spectroscopy (OES) for a better understanding of the mechanism of the etching process. The etched sidewalls were observed by scanning electron microscope (SEM), the etched roughness was measured by atomic force microscope (AFM) and the etched depths were measured by a stylus profiling technique. 1. Introduction Fluorine-based plasma etching is considered as an important process step to fabricate many electronic devices in a number of industries. The following are such examples; to etch LiNbO 3 and LiTaO 3 for optical electric field sensors [1–3], to create the structure for a micro fuel cell [4], to form microstructures on a silicon surface for self-cleaning use in solar cells and microfluidic devices [5], to perform plasma nanoscale texturing on carbon thin films for use as counter-electrodes in batteries and dye-sensitized solar cells [6], and to roughen the glass substrate that is used for architectural applications (room lighting and increase efficiency of light out-coupling of emitting diodes) [7]. In order to produce patterning features on a substrate surface, the inductive coupled plasma (ICP) is general widely used to etch silicon by a fluorine-based plasma (the etching process follows this equation: 4F (g) + Si(s) → SiF 4 (g) where F(g) plays a key role in the silicon etching process) in a high vacuum process regime (1 × 10 -3 to 1 × 10 -9 Torr). To achieve such a pressure level involves the use of a diffusion pump or turbo pump, besides using a mechanical rotary va- cuum pump, that leads to high process cost. In addition, when using ICP fluorine-based plasma at an excitation frequency at 13.56 MHz, a re- deposited material can appear as a new layer deposited along the etched sidewall. This has been reported for a variety of etched sub- strates; silicon [8,9], alumina titanium carbide [10] and benzocyclobutene [11]. The redeposited materials are formed by either non-volatile etched by-products sputtered from the etched site and deposited on the sidewall, or formed by plasma species that are able to directly deposit on the etched sidewall. In certain etch conditions the redeposition occurs by both mechanisms. These redeposited materials are undesirable in the etching process due to requiring an additional cleaning process step and after cleaning the residue, an unwanted re- liability issue of the final product may be unavoidable. The applied field frequency (ω/2π) to discharge the plasma is an important factor (among other process parameters such as gas type (inert or reactive), mixed gas ratio, process pressure, substrate tem- perature, input power and bias voltage at the substrate) that influences the plasma species which govern the process and determine whether the deposition or etching process will dominate. Moisan and Wertheimer [12] considered the dependence of applied field at high frequency (radio frequency at 13.56 MHz and microwave frequency at 2.45 GHz) used to ignite the plasma that has an effect on the plasma properties (electron energy distribution function (EEDF), electron density, neutral density) that strongly define the outcome of process, either etching or deposition. The high frequency, in this case, means that applied electric field (ω) is high compared to the ion plasma an- gular frequency (ω pi ), ω > ω pi , which means that ions cannot respond to the applied electric field while only free electrons can absorb energy from that electric field. The energetic free electrons play a key role for https://doi.org/10.1016/j.surfcoat.2020.126018 Received 14 October 2019; Received in revised form 3 April 2020; Accepted 3 June 2020 ⁎ Corresponding author at: 63 M.4 Nong Han, San Sai, Program in Applied Physics, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand. E-mail address: chupong@mju.ac.th (C. Pakpum). Surface & Coatings Technology 397 (2020) 126018 Available online 06 June 2020 0257-8972/ © 2020 Published by Elsevier B.V. T