Microwave plasma is widely used in the fields of material processing, surface treatment, and thin film deposition due to its advantages of electrodeless discharge, high plasma density, and strong group activity. The application of magnetic field is one of the effective means to regulate and optimize the plasma properties. By applying magnetic field, the trajectory of electrons in the plasma can be influenced, which in turn affects the density, temperature distribution, and energy distribution of the plasma. Consequently, this can control, to some extent, the chemical reaction rate and pathway of the plasma. In addition, magnetic fields can be used to improve the stability of the plasma and to increase the interaction region between the plasma and matter, thus expanding its application prospects in scientific research and industrial applications. For instance, the regulation of microwave electron cyclotron resonance (ECR) plasma by magnetic fields can obtain high-density, wide-range low-temperature plasma. Electron cyclotron resonance-microwave plasma chemical vapor deposition (ECR-MPCVD) in order to make the free range of electrons is long enough to ensure the electron cyclotron resonance, and the working air pressure is generally controlled at 10^-3~1 Pa. In the process of conventional MPCVD preparation of diamond and other thin-film materials, the lower working pressure poses certain challenges and difficulties in controlling the growth rate and temperature of the thin films. Under higher pressure, microwave plasma tends to congregate, significantly reducing the uniformity of radical spatial distribution. Therefore, it is necessary to continuously investigate and explore the strategies, patterns, and mechanisms of the magnetic field regulation of microwave plasma at different working pressures. Currently, there are few reports on the regulation of microwave plasma by magnetic fields at hectopascal levels (≥100 Pa).

In order to regulate the microwave plasma, two sets of coaxial magnetic field coils were installed on a reactor of waveguide coupled microwave plasma chemical vapor deposition (MPCVD) device with the dual-substrate set-up. By providing adjustable current (0-300 A) to the coils through the adjustable constant current source, a stable and uniform magnetic field can be generated in the direction of the resonator’s axis, with a magnetic field strength of approximately 0.105 T. Under the conditions of microwave power of 550 W and a pressure of 400 Pa, optical emission spectroscopy(OES) was employed to collect emission spectra from the plasma region along the horizontal direction on the surface and in the middle of the substrate holder, both with and without the uniform magnetic field, at substrate holder spacings of 20 mm and 30 mm. The distance between adjacent acquisition points was 5 mm. The electron temperature of the plasma was diagnosed using the hydrogen atom Balmer series H_α and H_β lines. The effects of the uniform magnetic field on the plasma shape, spatial distribution of radicals, and electron temperature at different substrate stage gap were investigated.

The experimental results show that at an operating pressure of 400 Pa, the presence of a uniform magnetic field causes the plasma sphere to transition from a spherical shape to an ellipsoidal shape, with the plasma at the center being compressed and the plasma on the surface of the substrate table being stretched (Figure 4). This change results in a more uniform distribution of plasma on the surface of the substrate table. The spectroscopic diagnostic results show that when the substrate spacing is 30 mm, the introduction of a uniform magnetic field significantly reduces the intensity of hydrogen plasma radicals on the surface of the substrate table, with the intensity of H_α and H_β changing by more than 75%, and the distribution uniformity on the surface of the substrate table is significantly improved. When the substrate spacing is 20 mm, the intensity of H_α and H_β radicals on the surface of the substrate table increases by 10% at the center 0 mm under the action of a uniform magnetic field, but the intensity of radicals decreases sharply by 25%. The intensity of H_α and H_β radicals in the middle of the substrate table shows varying degrees of decrease under the action of a uniform magnetic field, but the change amplitude is small when the substrate spacing is 20 mm (Figure 6). Moreover, a uniform magnetic field improves the distribution uniformity of plasma electron temperature along a direction parallel to the substrate table (Figure 7).

The effects of uniform magnetic field on the shape of microwave hydrogen plasma and the spatial distributions of H_α and H_β groups, as well as the electron temperature of the plasma in the resonance cavity of a dual-substrate waveguide-coupled MPCVD device with different substrate spacings at a working pressure of 400 Pa were investigated by optical emission spectroscopy. The analysis results show that at the pressure of 400 Pa, the presence of a uniform magnetic field causes the plasma to expand along the direction perpendicular to the magnetic field and to be compressed in the direction parallel to the magnetic field, which leads to the transformation of the plasma from a spherical shape to an ellipsoidal shape. When the gap between substrates is 30 mm, the uniformity of the radial distribution of H_α and H_β groups in the plasma is significantly improved under the magnetic field. Meanwhile, the introduction of the magnetic field results in a more uniform distribution of plasma electron temperature along the radial direction of the substrate.