Dc Magnetron Sputtering Discharge Research Articles

Structure of DC magnetron sputtering discharge at various gas pressures: a two-dimensional particle-in-cell Monte Carlo collision study

We report the results of a particle-in-cell Monte-Carlo collision simulation of an axially symmetric DC magnetron discharge with a 5 cm diameter flat cathode in argon at pressures from 1 to 10 mTorr at a constant discharge current of about 0.5 A. Calculations show that the cathode region, where almost the entire discharge voltage drops, consists of a cathode sheath 0.1–0.2 mm wide and a presheath about 2 cm wide, where most of the ionizations occur, separated by a region 0.25–0.35 mm wide, where the plasma potential remains almost unchanged and the plasma density reaches its maximum value. Most of the discharge voltage drops in the presheath at low gas pressure, and in cathode sheath at high pressure. The ratio of sheath to presheath voltages increases linearly with pressure. The distribution of the ionization rate has two maxima: near the cathode sheath and in the presheath. The fraction of ionizations near the cathode sheath increases with pressure. The electron energy distribution function (EEDF) is generally a two-temperature function. At low pressures at a distance of less than 1 cm from the cathode, the EEDF becomes one-temperature. A high-energy tail is observed on the EEDF near the cathode; the fraction of electrons in the tail (in the order of tenths of a percent at 10 mTorr) and their energy, determined by the sheath voltage, increase with pressure. The electron temperature decreases with pressure due to a decrease of the electric field in the presheath, which leads to a decrease of energetically accessible regions of collisionless electron motion and to a corresponding decrease in the energy that electrons can obtain in these regions. The dependence of the discharge voltage on the gas pressure has a minimum at about 3 mTorr, which occurs due to the competition of two processes on pressure increase: a decrease in the electron temperature and a decrease in the fraction of electrons returning back to the cathode. Plasma density waves are observed in the presheath region at pressures of 1–3 mTorr.

Spatial distribution of plasma parameters by a dual thermal-electrostatic probe in RF and DC magnetron sputtering discharges during deposition of aluminum doped zinc oxide thin films

Aluminum doped zinc oxide thin films deposited by magnetron plasma sputtering are essential for various optoelectronic applications. So far, the oxygen negative ions and the atomic oxygen are regarded as responsible for the poor spatial uniformity of thin film resistivity. While various methods are available for thin film characterization, understanding the growth mechanism requires spatial-resolved measurements of plasma parameters. This work uses a dual thermal-electrostatic probe that is able to reveal the spatial distribution of plasma density, electron temperature and plasma potential. The results exhibit a parabolic profile for plasma density and flat profiles for electron temperature and plasma potential, with no correlation with the strong distribution of thin film resistivity that mirrors the erosion track on the target surface.

Comparison of 1D and 2D particle-in-cell simulations for DC magnetron sputtering discharges

One-dimensional and two-dimensional particle-in-cell (PIC)/Monte Carlo collision simulations are performed for a balanced direct current magnetron sputtering (DCMS) discharge. The plasma parameters obtained by both simulations above the target racetrack share similar features and magnitudes under similar discharge intensities, i.e., similar local discharge current densities. The commonly observed high-energy tail of electron energy distribution in DCMS discharges is reproduced in both simulations. The accuracy and applicability of using one-dimensional simulations to qualitatively investigate the characteristics of balanced DCMS discharge are confirmed. In terms of capturing the key physical mechanisms in DCMS, one-dimensional PIC simulation is an efficient method, which could largely alleviate the computational expense and preserve the physical fidelity of modeling results.

Spectroscopic investigation on the near-substrate plasma characteristics of chromium HiPIMS in low density discharge mode

High power impulse magnetron sputtering (HiPIMS) discharge promises high ionization fraction and energetic ions in comparison with dc magnetron sputtering discharge. But acknowledge on the characteristics of HiPIMS plasma in the near-substrate region (substrate vicinity), which is of great importance for film deposition, is still limited. Here, optical emission spectroscopy (OES) combined with the collisional-radiative modelling are developed and used to determine the electron temperature and the number density of neutral sputtered atom for the chromium HiPIMS plasma in substrate vicinity. The OES analysis demonstrated the HiPIMS discharge of Cr sputtering process in low density mode was dominated by the electron impact ionization of argon atoms and excitation of chromium atoms. As the HiPIMS plasma in the substrate vicinity is far from the local thermal equilibrium state, the relative intensities of transition lines to ArI 4p states was used to calculate the electron temperature. Subsequently, the neutral chromium atoms density about 1017 m−3 was reported in the near-substrate region. Our findings have important implications for species generation in low density HiPIMS discharge, with applications in synthesis of dense chromium coatings.

Structural, morphological and electrical properties of nickel oxide thin films deposited by reactive sputtering

This paper is devoted to the study of the influence of oxygen content in the nickel oxide films on the film structural, morphological and electrical properties. Nickel oxide films have been synthesized by reactive DC magnetron sputtering discharge by varying the oxygen flow rate (1.9<Q(O2)<3.6sccm) for various deposition time. XRD analyses revealed the polycrystalline nature of the as-deposited films and also a phase transition from nickel oxide (111) to nickel oxide (200) associated with nickel non-stoichiometry in the NiO structure. The polycrystalline films presented an average crystallite size of 15–30nm and a surface roughness of 1–10nm. In-plane stress measurements have established the correlation between crystallite size and intrinsic compressive stress and also the ion-penning effect of negative oxygen ions during the film growth. A maximum stress of 10GPa was found for lower film thickness (10nm). By adjusting the oxygen concentration, conductive AFM (C-AFM) and resistivity measurements by the four point method have revealed at room temperature an electrical transition from insulating to conductive state. C-AFM and four point measurements showed respectively an increase in the collected current and an abrupt decrease of the mean resistivity from 107 to 10Ωcm when the stoichiometry varies from NiO0.96 to NiO1.14. This transition is related to the non-stoichiometry attributed to nickel vacancies. Finally, low-temperature (290–100K) electrical conduction measurements confirmed the weak dependence of Ni-deficient nickel oxide films with film thickness and showed that charge carrier conduction is a thermal-activated process.

Experimental evidence of warm electron populations in magnetron sputtering plasmas

This work report on the results obtained using the Langmuir probe (LP) measurements in high-power dc magnetron sputtering discharges. Data show clear evidence of two electron components, such as warm and bulk electrons, in the sputtering plasma in a magnetic trap. We have also used optical emission spectroscopy diagnostic method along with LP to investigate the plasma production. Data show that there is a presence of low-frequency oscillations in the 2–3 MHz range, which are expected to be generated by high-frequency waves. Analysis also suggests that the warm electrons, in the plasmas, can be formed due to the collisionless Landau damping of the bulk electrons.

Development of a compact combined plasma sensor for plasma surface engineering processes

A combined sensor for the investigation of plasma-based surface engineering processes has been developed, which basically consists of a quartz crystal microbalance that is simultaneously used as a heat flux sensor and a planar Langmuir probe in one active element. The sensor can thus measure deposition flux, heat flux, and charged particle flux laterally resolved at the same time and position. The setup and working principle of the sensor are shown, and the suitability for process investigations is demonstrated exemplarily for a dc magnetron sputtering discharge for Ti thin film deposition.

Measuring the energy flux at the substrate position during magnetron sputter deposition processes

In this work, the energetic conditions at the substrate were investigated in dc magnetron sputtering (DCMS), pulsed dc magnetron sputtering (pDCMS), and high power impulse magnetron sputtering (HiPIMS) discharges by means of an energy flux diagnostic based on a thermopile sensor, the probe being set at the substrate position. Measurements were performed in front of a titanium target for a highly unbalanced magnetic field configuration. The average power was always kept to 400 W and the probe was at the floating potential. Variation of the energy flux against the pulse peak power in HiPIMS was first investigated. It was demonstrated that the energy per deposited titanium atom is the highest for short pulses (5 μs) high pulse peak power (39 kW), as in this case, the ion production is efficient and the deposition rate is reduced by self-sputtering. As the argon pressure is increased, the energy deposition is reduced as the probability of scattering in the gas phase is increased. In the case of the HiPIMS discharge run at moderate peak power density (10 kW), the energy per deposited atom was found to be lower than the one measured for DCMS and pDCMS discharges. In these conditions, the HiPIMS discharge could be characterized as soft and close to a pulsed DCMS discharge run at very low duty cycle. For the sake of comparison, measurements were also carried out in DCMS mode with a balanced magnetron cathode, in the same working conditions of pressure and power. The energy flux at the substrate is significantly increased as the discharge is generated in an unbalanced field.

Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. II. Positive ions

Spectra of the ion mass and energy distributions of positive ions in reactive (Ar/O2) and nonreactive (Ar) dc magnetron sputtering discharges have been investigated by energy-resolved mass spectrometry. The results of three sputter target materials, i.e., Cu, In, and W are compared to each other. Besides the main gas constituents, mass spectra reveal a variety of molecular ions which are dependent on the target material. In reactive mode, ArO+ is always observed in Ar/O2 but molecules containing Ar and the metal were exclusively found for the Cu target. The occurrence of the different ions is explained in the context of their bond strengths obtained from density functional theory calculations. The energy spectra generally contain the known low-energy peak corresponding to the plasma potential. Differently extended high-energy tails due to sputtered material were observed for the different targets. Besides these, high-energetic ions were detected with up to several 100 eV. Their energies are significantly different for Ar+ and O+ with Ar+ strongly depending on the target material. The spectra are discussed together with results from transport of ions in matter (TRIM) calculation to elucidate the origin of these energetic ions.

Study on the Effect of Base Pressure on Magnetron Sputtering Discharge Plasma by Optical Emission Spectroscopy

Argon plasma was generated by DC magnetron sputtering discharge at a constant input power and sputtering pressure. The emission spectra of the argon plasma, in a region of 300 nm to 800 nm, were recorded with three different base pressures (4×10−4 Pa, 13×10−4 Pa and 26×10−4 Pa). From the spectra, the Boltzmann temperature (TB) was calculated by using the atomic Boltzmann plot method. The intensity ratio of N and N2 lines provided the information on the changes in the degree of dissociation of nitrogen molecules. The obtained results showed that with the increase in base pressure the dissociation of N2 increased and the Boltzmann temperature (TB) decreased.

Growth and structural properties of Mg:C thin films prepared by magnetron sputtering

We investigate the growth and structure properties of Mg:C thin films. The films are prepared using a dc magnetron sputtering discharge where the electrical resistance over the films is monitored during growth in-situ with a four point probe setup. The structural properties of the films are investigated using X-ray diffraction measurements and the elemental composition and binding in the films is determined using elastic recoil detection analysis and X-ray photoelectron spectroscopy. The results show that during co-sputtering the carbon flux influences the initial stages of the film growth. The films are made of polycrystalline magnesium grains embedded in a carbon network, the size of which depends on the carbon content, but amorphous phases cannot be excluded. The XPS measurements show the presence of carbidic carbon whereas X-ray measurements find no Mg:C phases. The overall stability of the films is found to depend on the carbon content, where stable films capped with a 14 nm Pd layer cannot be obtained with carbon content above 18%.

Electron energy distributions and plasma parameters in high-power pulsed magnetron sputtering discharges

We report on the results obtained using time-resolved Langmuir probe measurements in high-power pulsed dc magnetron sputtering discharges. Time evolutions of the electron energy distribution and the local plasma parameters were investigated at a substrate position of 100 mm from a planar target of 100 mm diameter during a high-rate deposition of copper films. The average target power density in a pulse was 500 W cm−2 at a repetition frequency of 1 kHz, a voltage pulse duration of 200 µs and an argon pressure of 1 Pa. The electron energy distributions with two energy groups and sharply truncated high-energy tails were observed during a pulse. After a fast rise in a 50 µs initial stage of the pulse, the kinetic temperature of electrons, defined using the mean electron energy, remained in the range from 10 500 to 12 200 K till the pulse termination. The temperature of weakly populated hot electrons decreased rapidly in the initial stage of the pulse approaching the kinetic temperature approximately 100 µs after a pulse initiation. High plasma densities, being in the range 1 × 1012–2 × 1012 cm−3 for 100 µs, were achieved at the substrate position with a 50 µs delay after establishing the 125 µs steady-state discharge regime with the target power density of 650–680 W cm−2 during a pulse. The plasma potential slowly increased from 0.5 to 3.5 V during the pulse and 25 µs after its termination.

Plasma parameters in a planar dc magnetron sputtering discharge of argon and krypton

The electron energy distribution function (EEDF) and plasma parameters in a planar dc magnetron sputtering discharge in argon and krypton were determined using a Langmuir probe. Two groups of electrons are observed in the discharge. The electron temperature of the cold electrons is roughly independent of the discharge pressure, while the electron temperature of the hot electrons decreases with increased discharge pressure. The electron density increases with increased pressure and is roughly a factor of 2–3 higher for a krypton discharge than for an argon discharge.

Preparation and characterization of magnetron sputtered, ultra-thin Cr0.63Mo0.37 films on MgO

We report on the preparation of lattice matched heteroepitaxial films in a dc magnetron sputtering discharge. CrxMo1−x thin films were grown on MgO(1 0 0) substrate, choosing the composition of the binary metal alloy to be x=0.63, so that the film is lattice matched to the substrate when the 〈100〉 orientation of the film is parallel to the 〈110〉 orientation of the substrate. Ex situ x-ray diffraction and low angle x-ray reflectivity measurements were performed to determine the film structure, film thickness, as well as the surface and interface roughness. We determine the optimum growth temperature to be 200 °C. Using this temperature 0.4–15 nm thick films were prepared. The resistivity of the films was measured ex situ using a four-point-probe. The resistivity dropped rapidly with increasing thickness. The resistivity versus thickness data was found to be well described with Namba’s model, which includes film roughness as a parameter.

Spatially-resolved spectroscopic optical emission of dc-magnetron sputtering discharges in argon-nitrogen gas mixtures

The spatially-resolved spectroscopic optical emission of various lines of a dc-magnetron sputtering discharge (with cylindrical symmetry) is studied. The discharge works with an aluminium-based alloy cathode. Ar-N2 gas mixtures are used. Emission lines corresponding to the cathode (Al) and the gas discharge species (Ar, N2, N2+, N) are considered. The three-dimensional emission profiles, I (r, z), of the lines are measured, where r is the distance from the centre of the cathode and z is the height from the cathode. I (r, z) profiles are studied as a function of the total pressure, the gas composition and the electrical power. In order to compare the results, I (r, z) profiles are fitted with an empirical analytical function. It turns out that the three-dimensional emission profiles of the various lines behave differently. The results are understood by taking into account the motion of electrons in our measured magnetic field distribution and the role of electrons in populating the upper levels of the optical transitions.

Electron energy distribution function in a dc magnetron sputtering discharge

The electron energy distribution function (EEDF) of a dc magnetron sputtering discharge ignited in either pure argon gas or a mixture of Ar and 4.7% methane is measured in the substrate vicinity (7 cm from the cathode) using ac differentiation of a cylindrical Langmuir probe characteristic. The results show the presence of two energy groups at a small discharge current l = 0.1 A for two values of argon gas pressure p = 0.4 and 1.07 Pa. The high-energy electron group is more pronounced at the lower pressure p of the sputtering gas and disappears with increasing l. The small amount of methane gas in the argon does not affect significantly EEDF at the location of measurement, if compared with that obtained in pure argon. Fitting the measured EEDF to the equation f(E) = a·E 0.5 exp (−b·E x) we find values of x in the range 1.43–1.70, which correspondsto a distribution between those of the Maxwellian (x = 1) and Druyvesteyn (x = 2) ones. The mean electron energies 〈 E〉 M calculated from the measured EEDF are in the range of 4.8–7.1 eV, depending on the discharge conditions. For our conditions of operation, the comparison of 〈 E〉 M values with those obtained by means of the standard Langmuir technique [〈E〉 L = 1.5( kT e )] shows a reasonable agreement (〈E〉 L = 4.4–6.0 eV).