When using multi-arc ion coating equipment to deposit TiN thin film on the surface of polished high-speed steel, with other parameters unchanged, the influence of the bias voltage on the deposition rate of the thin film was investigated. The experimental results showed that with the increasing of the negative bias, the deposition rate increases continuously, but after the negative bias reaching a certain value, the deposition rate decreases with the increase of the bias voltage.
Due to its high hardness, low friction coefficient, good chemical inertness, unique color and good biocompatibility, TiN films are widely used in the mechanical, plastics, textiles, medical industry, microelectronics and other industries. It has become one of the most widely used thin film materials in industrial research and application now. There are many methods for preparing TiN films, among which multi-arc ion plating is one of the most widely used technologies in the industry today. This technology originated in the 60s and has been rapidly developed since then. The multi-arc ion-deposited thin film has many advantages such as strong film base adhesion, high density of the film layer, wide range of materials that can be plated, good coating around, and low deposition temperature. However, there are many factors that affect the quality of the film during the coating process. Domestic and international studies have shown that the main process parameters affecting multi-arc ion plating are the cathode target current, reaction gas pressure, substrate negative bias, nitrogen partial pressure, and substrate deposition temperature.
In this paper, we mainly study the influence of negative bias on the deposition rate. When the substrate is negatively biased, ions in the plasma will be accelerated by the negative bias electric field to the substrate. When reaching the surface of the substrate, the ions bombard the substrate and transfer the energy obtained from the electric field to the substrate, which causing the temperature of the substrate to rise. Therefore, the substrate negative bias voltage has big influence on deposition rate, the internal residual stress, binding force of the film and the substrate, the frictional properties of the membrane/base in coating process. Changing the negative bias of the substrate can adjust the energy of the deposited ions and the temperature of the substrate surface to control the coating quality. The influence of negative bias on the structure and properties of multi-arc ion plating TiN has been studied and reported. However, the effect of negative bias on the deposition rate of thin films is rarely reported. This article intends to study the influence of negative bias on the film deposition rate.
1. Experimental Method
Polished high-speed steel is used as the base material. The sample was ultrasonically washed with absolute ethanol for 20 min, and then the surface of the substrate was wiped with absolute ethanol and acetone solution, then dried, and repeatedly placed it on the base frame of the multi-arc ion coating system, the distance between target and substrate is 250 mm. When the vacuum chamber was pumped to a background vacuum of 2.6×10 −3 Pa, Argon gas was charged to 5 Pa~10 Pa, and a negative bias voltage of 500 V was applied to the workpiece. After maintaining for 2 min to 3 min, the voltage was raised to 900 V. Argon gas forms a lavender plasma glow under low-voltage discharge, and under the action of an electric field, argon ions bombard the workpiece. After the glow wash, the argon gas is reduced to about 2 Pa, a negative bias voltage of 900 V is applied to the workpiece, and the Ti target is ignited, then the substrate is bombarded with high-energy metal ions. After cleaning, the negative bias voltages were adjusted to 0 V, -50 V, -100 V, -150 V, -200 V, and -250 V respectively. And TiN films were deposited. During the coating process, the arc voltage U = 20 V, the arc current I = 65 A, and the deposition time was 30 min. X-ray diffraction was used to analyze the phase structure of the film. The microstructure of the coating was analyzed by scanning electron microscopy. Film thickness was measured using an XP-2 profiler. The deposition rate was then calculated based on the measured thickness and deposition time.
2. Results and Analysis
2.1. The phase structure of the film under different biases
Figure.1 shows the X-ray diffraction pattern of the film. The analysis shows that the phase composition of the film is TiN phase. When no bias is applied, diffraction peaks corresponding to TiN(200) and (220) crystal planes can be observed, but (111) diffraction peak is almost zero. The strongest peak in this line is from the base Fe(111), which indicates that the film thickness is small and the X-rays have penetrated the substrate. With the increasing of the bias voltage, the (111) crystal orientation begins to appear, and the (200) preferred orientation is relatively weakened. When the bias voltage reaches 200 V, the TiN film shows a strong (111) preference. We note that the Fe(111) peak gradually weakens as the bias voltage increasing, which indicates that the film thickness gradually becomes larger.
Fig. 1 XRD diffraction pattern of TiN films obtained under different bias voltages
2.2. Coating Surface Morphology
Fig. 2 Surface morphology of TiN coating
In the multi-arc ion plating coating, there are dispersed particles on the surface. It is generally believed that the target is melted into tiny droplets under the high temperature of the local arc and ejected, and then adhered to the coating surface in the form of solid particles. The hardness of these domains is lower than that of the TiN film. These soft spots are detrimental to the performance of the coated tool and also reduce the surface finish of the tool. It can be observed by scanning electron microscopy that the TiN coating surface particles are generally in the range of 1 μm to 2 μm, and the number of particles up to 5 μm is small.