Introduction

Silicon carbide (SiC) is a wide-bandgap semiconductor material (the bandgap width is lying in between 2.36 to 3.3 eV depending on crystal structure) which has found wide application in electronic industry. Wide bandgap allows SiC based devices to be used in a very wide temperature range (up to 1000 °C). Thermal conductivity of SiC at normal conditions is close to copper which opens the opportunity to use SiC in the devices with high current densities. High thermal, radiation and chemical stability is due to the high energy of the bond between Si and C which ensures the stability of SiC-based devices under extreme operating conditions1,2,3,4,5. Thus, due to these properties, silicon carbide is a promising material for power electronics devices6 design as well as various microelectromechanical systems (MEMS) widely used in the automotive industry (hybrid and electric cars), energy industry, oil and gas industry, etc7,8,9,10,11,12,13,14.

One of the most crucial technology operations of the creation of such devices and systems is a precise formation of a profile of a given configuration (grooves, holes, etc.) on/in the silicon carbide substrate. As of today, there is a fairly large variety of techniques for the processing of silicon carbide that allow, to some extent, to solve the following problem: wet etching; etching in solvents, stimulated by femtosecond laser; plasma etching; etching in plasma atmospheric discharge; plasma jet processing, and etcFull size image

As can be seen from the Fig. 1, the reactor of the system consists of discharge and reaction chambers. The reaction chamber (Dinner = 23 cm, H = 33.5 cm) of cylindrical shape is made of stainless steel AISI 321. Plasma in the discharge chamber (Dinner = 23 cm, H = 22.5 cm) was created by applying high-frequency (HF) power to the inductor of specific geometry from the HF generator (f = 6.78 MHz, Wmax = 1000 W) through a resonant matching device. To form the bias potential, a 13.56 MHz HF voltage from a separate HF generator was applied to the substrate holder (electrode).

Since direct measurement and control of the processed substrate surface temperature under plasma etching conditions is itself a technically challenging task, the substrate holder surface temperature was measured in this study. The control was carried out using a type-K thermocouple located in close proximity to the lower plane of the top plate of the holder, to which a sample of silicon carbide was clamped29.

6H-SiC substrates with a thickness of 350 µm were used as etching samples. The standard sample size was 40 × 40 mm. In all experiments, samples were cleaned with acetone, ethyl alcohol and deionized water successively for 10 min each in an ultrasonic bath before the PCE process. In addition, after mounting the SiC substrate in the reaction chamber before the experiment, samples were treated in argon plasma for 10 min to clean up any residual contaminants from the substrate surface. The parameters of the cleaning process in argon were: applied HF power (W = 750 W), pressure in the reaction chamber (P = 0.75 Pa), Ar gas flow (Q = 21.75 sccm), and the distance between the bottom part of the discharge chamber and the surface of the sample (h = 15 cm). A low bias voltage was selected to minimize possible damage to the etching surface by energetic ions during substrate cleaning.

To improve heat transfer between the sample and the substrate holder, the SiC substrate was placed on the substrate holder through the “Ramzai” vacuum lubrication (TU 38.5901248-90). A ceramic O-ring with an outer diameter of 50 mm and an inner hole diameter of 25 mm was placed on top of the sample. Above the ceramic ring a stainless-steel hood (AISI 321) with an opening in the center (25 mm diameter) was placed to press the sample to the table. Thus, the active process region (area of etching) was a circle with a 25 mm diameter.

Sulfur hexafluoride SF6 (GOST TU 6-02-1249-83, purity 99.998%) was used as the etchant main gas. Etching processes were performed in a mixture of SF6 and O2 (high purity, TU 2114-001-05798345-2007). The flow rates of Sulphur Hexafluoride and oxygen were 10.15 sccm and 3 sccm, respectively. The values of HF power (W), bias voltage (Ubias), pressure in the reaction chamber (P), the distance between the bottom part of the discharge chamber and the surface of the sample(h), and etching time (t) in all experiments were fixed and equal to 800 W, -150 V, 0.75 Pa, 15 cm and 30 min respectively. The surface temperature of the substrate holder varied from 15 (temperature of the cooling water) to 300 °C. The etching process was initiated when the substrate holder surface temperature reaches the set point.

The plasma optical emission spectrum (OES) in the chamber during the SiC etching and in the absence of SiC wafer were recorded using the OceanOpticsHR 4000 spectrometer. Spectrum were registered in the wavelength range of 200–1120 nm with a resolution of ~ 0.02 nm and an input slit width of 5 µm. The connection of the spectrometer with the system was carried out by using a fiber optic system for transmission of the plasma emission to the input slit of the spectrometer (see Fig. 1a). The spectra were processed using the SpectraGryph 1.2.14 software30.

After process, the etching depth and roughness of the etched surface was measured. Etching depth was evaluated from microphotographs obtained on the Carl Zeiss Supra 55VP scanning electron microscope with an accuracy of ± 2.5%, as well as direct measurement of the substrate thickness in the etched area with Micron IC 54,793 µm (measuring step is 1 µm, measurement error is 1%). In the second case, measurements were made at five different points of the substrate before and after the etching process. The etching rate (Vetch) was calculated as the ratio of the etching depth to the process time. In addition, microphotographs were used to visually characterize the quality (roughness) of the etch surface. The root mean square roughness (Rms) of the surface was determined by atomic force microscopy (AFM) using the Solver-Pro NT-MDT probe microscope. Each AFM scanning area size was 10 × 10 µm. Each experiment was performed three times to prove the reproducibility of the process. The roughness was determined at five different points within each etching area. One of the measurements was made in the central point of the etching area, and the other four measurement areas were located at two mutually perpendicular diameters (two points each) at equal distances from the center of the etching area. The final roughness value was calculated by averaging the results of these five measurements. The initial Rms of SiC substrates was 9.1 nm.