## 1 Introduction  
In the past decades, carbon nanostructures such as carbon nanotubes and diamond films have attracted a lot of attention due to their extraordinary mechanical, thermal, and electric properties [1; 2; 3; 4; 5; 6; 7]. Diamond films can be characterized by their remarkable field emission properties, wide energy gap, low heat conductivity, great optical penetration, high hardness, and chemical inertness. In addition, it comes with high biocompatibility, which can be used in medical applications. In recent years, micro- and nano-scale diamond films have been used in different industrial, engineering, and medical applications due to their distinctive physical and chemical properties [4; 5; 6; 7]. However, numerous limitations still exist in practical applications. Various factors such as film growth rate, surface roughness, area uniformity, adhesion, stress, operating pressure, and temperature must be further investigated to fully explore the potential of diamond films. There are several ways to fabricate a diamond film, including microwave plasma enhanced chemical vapor deposition (MPECVD), hot filament chemical vapor deposition, and microwave plasma jet chemical vapor deposition (MPJCVD) [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. Compared to vacuum-based plasma processing, microwave plasma processing at atmospheric pressure does have some good advantages. First, the equipment cost is comparatively low, and it eliminates the limitations associated with vacuum compatibility. Second, the low-temperature and high-pressure plasma processes greatly enhance the formation of active chemical species, for example, through high chemical selectivity and low ion densities. In addition, the damage to the film surface is also relatively low, while the adhesion between material interfaces is extremely good. For these reasons, it is very attractive to employ microwave plasma to fabricate diamond films. For the past few decades, diamond film coating has been widely used in industries in cutting tools as surface protective films due to its excellent mechanical properties. The traditional vacuum cathodic arc deposition method had low deposition rates of several tens of nm min\({}^{-1}\). Using an MPJCVD and applying a high substrate bias voltage, the deposition rate was increased to several thousand nm min\({}^{-1}\)[27]. Hence, MPJCVD has been proven to substantially enhance the deposition rate of diamond films. Furthermore, by applying a multistage magnetic field during deposition to the MPJCVD, a uniform thickness polycrystalline diamond film with a large diameter of up to 4 to 7 inches could be produced [28], demonstrating that the improvement of the deposition temperature and thickness uniformity of an MPJCVD for large-scale diamond film can be achieved with some tailored designs or modifications. Here, the MPJCVD fabrication method was chosen to fabricate the diamond films on a variety of substrates as it might serve as a better surface treatment. Plasma is a group of charged gas molecules in which the total numbers of positive charges (mostly positive ions) and negative charges (mostly electrons) are approximately equal. Thus, plasma in general is considered electrically neutral. In addition to the two charged particles, there are often a number of excited neutral gas molecules. Therefore, a normal plasma reactor contains neutral gas molecules, ions, electrons, and maneuverable neutral gas molecules. There are different ways to generate plasma, such as by applying a strong electric field to the gas or bombarding it with an electron beam or laser, which can excite the gas into a plasma state [8]. Plasma technology has become prominent in the technological advances of various industries. In semiconductor processes, film growth and etching for different materials are generally treated by plasma.