Dielectric Films Of TiO₂ And AlN By Pulsed Bias Arc Ion Plating

Dielectric films are the most widely-used functional films, which play increasing roles in the machinery, electronics, information, aerospace and other fields. But their poor conductivity brings difficulty to synthesis in some PVD processes. Take ion plating as an example, micro-arcs caused by charge accumulation not only severely degrade film quality, but also damages electrical components of the control system. Thus, ion plating, including widely-used arc ion plating, has always been considered unsuitable to fabricate dielectric films. Pulsed bias arc ion plating (PBAIP) is an advanced thin film deposition technology emerging in recent years. It inherits the advantages of arc ion plating such as high ionization rate and deposition rate and brings in new features such as low deposition temperature and residual stress, grain refinement and droplet reduction. These favorable features make PBAIP a potential technique to fabricate fine functional films. However, whether PBAIP can be used to fabricate dielectric films has not been extensively attempted. The clarification of this issue is of great significance to the development of arc ion plating technique and new film materials.Micro-arcs due to charge accumulation have been effectively suppressed by properly matching pulse frequency, duty cycle and bias voltage. Dense and smooth dielectric films such as TiO₂, TiO_2-xN)x and AlN have been successfully synthesized by using pulsed bias arc ion plating technique on glass, Si(100) and 316L stainless steel substrates, which represent respectively insulator, semiconductor and conductor. It has been sufficiently proven by our experiments that PBAIP can be used to synthesize dielectric films. The influence of pulsed bias on growth morphology, microstructure and properties of TiO₂, TiO_2-xN_x and AlN films has been investigated by varying the bias voltage from 0V to -900V. The results show that on whatever substrates, pulsed bias exhibits good regulation and control ability on film color, deposition rate, hardness and elastic modulus, adhesion force, microstructure, surface roughness and optical properties, which has further proven the feasibility of using PBAIP to fabricate dielectric films. The experiment results have been rationally interpreted by using a plasma sheath model and numerical simulations. Detailed results are listed as follows.(1) Synthesis of TiO₂ filmsUniform and transparent TiO₂ films have been deposited on three kinds of substrates with pulsed bias voltage varying in a fairly wide range from 0V to -900V. Droplets on film surface are few and small, film surface quality is good. As-deposited TiO₂ films on glass deposited at -300V are atomically smooth with RMS roughness of 0.113nm, which results in a high refractive index 2.51 at 550nm, closed to the maximum reported in the literatures. Deposited at the same bias, -300V, and TiO₂ films on 316L stainless steel substrates have a high adhesion force of 82N.Most of the as-deposited TiO₂ films on glass are in amorphous state. The film colors are uniform and different with the substrate bias. XPS results show that stoichiometric proportion of Ti and O is 1:2 and high bias voltage helps bonding between Ti and O. With the increase of bias voltage, absorption edge of the TiO₂ films red-shifts first, and then blue-shifts. Meanwhile deposition rate and film hardness increase first and then decrease. The hardnesses of the films deposited at -100V and -300V are at above 11GPa. Optical band gap is nearly constant, 3.27eV.Most of the as-deposited TiO₂ films on Si substrates consist of the Rutile phase, and the films deposited at -900V show a preferred orientation along the (220) direction. Film phase changes with the bias voltages after annealing at 600℃for 1h in air. AFM results show that the films deposited at 0V are smooth, surface islands are small and their density is high; The surface of the films at -900 V is wavy, surface islands are large and their density is low. FT-IR and Raman spectra also show that pulsed bias has apparent effects on film microstructure and bonding.Only the TiO₂ films deposited at -500V on stainless steel substrates are composed of Anatase phase, those deposited at other biases are amorphous. Deposition rate of the films on stainless steel first increases, and then decreases with the increase of bias voltage. Film hardness is higher than that of the substrates, while elastic modulus is similar to that of the substrates, which results in higher adhesion force.(2) Synthesis of TiO_2-xN_x filmsN substitutional doping has been realized by using PBAIP in a mixed atmosphere. N ions substitute some O ions in TiO₂, forming an N-Ti-O structure. The wavelength threshold for optical absorption of the films blue-shifts to 400nm after N doping. Droplets on the surface of TiO_2-xN_x films deposited at -300V on glass substrates are scarce, resulting in a low friction coefficient of 0.13. With the bias voltage increasing from 0 to -300V, adhesion force of TiO_2-xN_x films on 316L stainless steel substrates increases by 40%, from 45N to 65N.(3) Synthesis of AlN filmsAlN films have also been successfully synthesized on glass, Si(100) and 316L stainless steel substrates by PBAIP with bias voltage varying from 0 to -500V. Many "needle-like" droplets appear on the film surface due to the low melting point of Al. AlN films deposited at 0V on Si substrates show no preferred orientation. AlN films at -50V show a preferred orientation along the hexagonal A1N (110) direction. At -100V and -300V, the preferred orientation changes to fcc-AlN (200). As the bias voltage further increases to -500V, the preferred orientation changes back to hexagonal AlN (110). AlN films on Si substrates deposited at 0V, -100V and -300V are composed of the fcc and hexagonal phases, and the films at -50V and -500 V consist of the hexagonal phase, resulting in a higher film hardness of 33GPa. Hardness of A1N films on 316L stainless steel substrates at -100V and -300V is higher, above 30GPa, due to thr formation of the hexagonal phase.Finally, the experimental results have been discussed within the framework of pulse plasma sheath. The fundamental reasons for the suppression of micro-arcs lie in two aspects. One is that plasma sheath thickness shows a dynamical oscillation in step with the pulsed bias. The accumulated positive charge is neutralized instantly. Accumulated charge quantity cannot meet the intensity for micro-arc breakdown by matching pulse frequency, duty cycle and bias voltage. More importantly, pulsed bias is always effective in accelerating the deposition ion species and influences the film growth. Thus pulsed bias can indirectly regulate and control film microstructure and properties. This is the unique feature in the synthesis of dielectric functional films using pulsed bias arc ion plating technique.