Research On The Discharge Characteristics Of Twin Aluminum Targets Reactive High Power Impulse Magnetron Sputtering And Deposition Process Of Alumina Films

Owing to their excellent properties,such as high hardness and wear resistance,high transparency,good thermal conductivity as well as excellent chemical and thermal stability,Al₂O₃ film has received increasing interest in different industrial applications from optoelectronic devices,machining coatings to aerospace applications.Compared with its amorphous form,many of the desirable properties are tied in with the crystalline phases of Al₂O₃ films.However,growth of crystalline Al₂O₃ films by traditional film deposition methods at low temperature is usually difficult.To resolve this problem,twin targets reactive high power impulse magnetron sputtering（TTR-HiPIMS）was proposed,and the plasma and ion flux characteristics has been investigated by means of time-resolved Langmuir probe,energy-resolving quadrupole mass spectrometry.2D PIC-MCC（Particle in Cell-Monte Carlo collision）model was also developed,and the physical mechanism of the closed-field TTR-HiPIMS discharge asymmetry was explained.Al₂O₃ films were prepared at low substrate temperature,and the relationship between the resulting film quality and the plasma parameters were also discussed.The influence of magnetic field configuration（mirror-field and closed-field）on the discharge characteristics of TTR-HiPIMS has been studied.Time-resolved Langmuir probe diagnostics revealed similar trend（strong modulation during the pulse period）,but higher values of plasma potential,floating potential and effective electron temperature for mirror-field discharge in comparison with the data obtained at closed-field configuration.However,the electron density achieved in closed-field discharge is obviously higher than the corresponding mirror-field,with its peak value about 1.3 times higher than the corresponding mirror-field.Time-averaged mass spectrometer results showed that the change of the magnet configuration does not seem to significantly influence the gas composition of the discharges.The ion energy distribution functions（IEDFs）recorded at both discharges consists of three components: a main narrow peak at low energy followed by a broad shoulder and then a high energy tail.The mirror-field discharge exhibit higher peak energies but lower fluxes compared to closed-field discharge.Investigation of the ions evolution has revealed that for both discharges the ions intensity evolution corresponds well with that of the electron density.The signal for the mirror-field discharge peaked exactly at the end of the pulse,whereas fluxes of the ion species in the closed-field reached the highest value 10 μs after the voltage pulse termination.To better understand the discharge characteristics of closed-field TTR-HiPIMS,the plasma parameters（Vp,Vf,Teff and ne）at symmetry positions（y=-40 mm and y=40 mm）with respect to the plane of geometric symmetry of the magnetron（the plane crosses both axes of the magnetrons）are studied and compared.Fine temporal resolution of the closed-field TTR-HiPIMS probe characteristics revealed a rapidly climbing and sinking followed by a gradual increasing again in the plasma potential,floating potential and effective electron temperature during the pulse-on period.In the pulse-off time,two-fold decay,the initial fast drift decay and the subsequent slower diffusive decay,was observed for these parameters.While for the electron density,it demonstrated a steep increase up to 60 μs after the initial of voltage pulse,followed also by a two-stage exponential-like decrease.The initial fast decay of ne is caused by the rapid loss of the high-energy electrons,which is determined by the drift motion.It is also clear that the plasma density between pulses remains at a relatively high level(1×10¹⁶m^-3),which is helpful for the reignition of the next active plasma pulse and thus in favour of the low pressure operation of TTR-HiPIMS discharge.We have compared the recorded plasma potential and floating potential values for the two symmetric positions.The results show that there are very small variations in the recorded plasma potential.However,the values for the floating potential at y = 40 mm position is generally lower than y =-40 mm position during the pulse-on time.However,the values for the recorder effective electron temperature and electron density at y = 40 mm position is generally higher than y =-40 mm position,this means that an asymmetry behavior of plasma parameters was observed.In order to find out the physical mechanism that governs this asymmetry behavior,a two-dimensional PIC-MCC model was developed to simulation the short-pulse closed-field TTR-HiPIMS discharge.The discharge potential has a c1ear1 y expressed radial dependence,which follows the pattern of the magnetic field.The gradient of the potential is steepest near the cathode at the race track.It is also clearly visible that the high density plasma（e^-,Ar⁺ and O₂⁺）region is centered on the race-track and spatially limited by the magnetic field configuration of the magnetron cathode.The density and energy of the plasma species increases as the discharge develops in time.It is found that the plasma particles motion in TTR-HiPIMS discharge is influenced by different types of guiding center drifts,but the formation of denser plasma structures in the +y part was mainly explained by the presence of the gradient B drift.Al₂O₃ films were synthesized on silicon substrates by TTR-HiPIMS technique,and effects of the magnetic field configuration,oxygen flow rate,substrate temperatures and bias voltage on microstructure,surface morphology,chemical composition and optical transmittance of the deposited films were investigated.It is confirmed that the magnet configuration appeared to have little influence on the deposition rate.However,the extended ion bombardment energies in the mirror-field discharge results in a higher film roughness and lower atomic O/Al ratio in the as-deposited film（1.43<1.51）.As the oxygen flow rate was increased from 8 to 16 sccm,the deposition rate of Al₂O₃ films decreased from 110 to 85 nm/h,the surface defect density also increased with the oxygen flow rate.Grazing incidence X-ray diffraction results indicated that all the as-deposited films were polycrystalline γ-Al₂O₃ Scanning electron microscopy showed that all the films have a smooth surface,which indicated that the arc events were almost completely suppressed in the sputtering process.The films prepared at substrate temperatures ranging from 300 to 500 °C contained fine nanocrystals of sizes in the range 15-25 nm,with the corresponding root-mean-square roughness increased from 2.06 to 4.24 nm.When the substrate bias reached-40 V,fully crystallined Al₂O₃ film can be deposited at temperatures as low as 230 °C,and the film deposited on ITO substrate had high optical transmittance（≈80%）in the wavelength range of 200–800 nm.This is mainly due to the high ratio of ion to metal fluxes（Ji/JMe>10）and high IEDFs presented in TTR-HiPIMS discharge,which can supply an additional kinetic energy to the surface and near-surface region of the growing films,and compensate for the crystallization activation energy.