Unipolar Pulse Mode Research Articles

High Rate Deposition of Piezoelectric AlScN Films by Reactive Magnetron Sputtering from AlSc Alloy Targets on Large Area.

This paper reports on the deposition and characterization of piezoelectric AlXSc1-XN (further: AlScN) films on Si substrates using AlSc alloy targets with 30 at.% Sc. Films were deposited on a Ø200 mm area with deposition rates of 200 nm/min using a reactive magnetron sputtering process with a unipolar–bipolar hybrid pulse mode of FEP. The homogeneity of film composition, structural properties and piezoelectric properties were investigated depending on process parameters, especially the pulse mode of powering in unipolar–bipolar hybrid pulse mode operation. Characterization methods include energy-dispersive spectrometry of X-ray (EDS), X-ray diffraction (XRD), piezoresponse force microscopy (PFM) and double-beam laser interferometry (DBLI). The film composition was Al0.695Sc0.295N. The films showed good homogeneity of film structure with full width at half maximum (FWHM) of AlScN(002) rocking curves at 2.2 ± 0.1° over the whole coating area when deposited with higher share of unipolar pulse mode during film growth. For a higher share of bipolar pulse mode, the films showed a much larger c-lattice parameter in the center of the coating area, indicating high in-plane compressive stress in the films. Rocking curve FWHM also showed similar values of 1.5° at the center to 3° at outer edge. The piezoelectric characterization method revealed homogenous d33,f of 11–12 pm/V for films deposited at a high share of unipolar pulse mode and distribution of 7–10 pm/V for a lower share of unipolar pulse mode. The films exhibited ferroelectric switching behavior with coercive fields of around 3–3.5 MV/cm and polarization of 80–120 µC/cm².

Deflection of high-intensity pulsed ion beam in focusing magnetically insulated ion diode with a passive anode

The focused high-intensity pulsed ion beam (HIPIB) of 100 ns order pulse is generated with respect to its spatial stability in two types of magnetically insulated ion diodes (MIDs) with geometrical focusing configuration using the passive anode, i.e., insulation of electrons with an external magnetic-field and a self-magnetic field, respectively. Anode plasma formation for the ion beam generation is based on different processes in the two types of MIDs, as the surface breakdown on the polymer-coated anode operated in the unipolar pulse mode for the external-magnetic field MID and the explosive electron emission on the graphite anode in the bipolar-pulse mode for the self-magnetic field MID. Typical energy density per pulse is in the range of 3–6 J/cm2, at an accelerating voltage of 200–300 kV with a pulse duration of 120–150 ns. The spatial deviations of the HIPIB is evaluated by measuring the energy density distribution by using an infrared diagnostic method considering neutralizing during the ion beam propagation to the focal plane with a spatial resolution of 1 mm. The ion beam deviation is about ±1.5 mm for the external-magnetic field MID and ±2.5 mm for the self-magnetic field MID, leading to a fluctuation in the energy density of 1%–12%, and 9%–27% within a 10 mm range at the focal point, respectively. It is revealed that the displacement of different parts of a beam spot occurs nonsynchronously, mainly attributable to the intrinsic diode processes of plasma generation and expansion, and ion beam extraction from the anode–cathode gap, while the influence of magnetic field in the transportation region is negligible. The ion beam spatial deviation has a major influence on the shot-to-shot stability of ion beam, and it is suggested that the stability can be enhanced via diode process improvement.

Stabilization of ion beam generation in a diode with self-magnetic insulation in double-pulse mode

AbstractThe paper presents the results of statistical studies of ion beam generation in different geometry diodes with explosive emission cathode in a mode of self-magnetic insulation. The experiments were carried out using the TEMP-4M pulsed ion beam accelerator during its operation in both unipolar pulse mode (100 ns, 250–300 kV) and bipolar-pulse mode with the first negative (300–600 ns, 100–150 kV) followed by a second positive (120 ns, 250–300 kV) pulse. It is found that the standard deviation of the total energy and energy density of the beam does not exceed 10–11%, while the same shot-to-shot variation in ion current density was found to be 20–30%. The mechanism of the energy density stabilization from pulse to pulse may be associated with the charge exchange between accelerated ions and stationary molecules and formation of accelerated neutrals. We observed a high correlation between the energy density (or total beam energy) and the duration of the first voltage pulse. We performed analysis of Blumlein statistical performance when the Blumlein was terminated with a resistive load and with a self-magnetically insulated ion diode. A characteristic feature of Blumlein operation in the double-pulse mode is an excellent reproducibility of breakdown of the preliminary spark gap, the variation in breakdown voltage is <2%. At the same time, the shot-to-shot variation in the breakdown voltage of the main spark gap in both bipolar- and unipolar pulse mode is 3–4 times higher than that for the preliminary spark gap. To improve the statistical performance of the main spark gap we used the first voltage pulse at the output of Blumlein to trigger the main spark gap. The new trigatron-type regime of the main spark gap operation showed a better reproducibility of the first pulse duration, with the time jitter not exceeding 10 ns in a set of 50 pulses.

Improvement in the statistical operation of a Blumlein pulse forming line in bipolar pulse mode.

The paper presents the results of studies on shot-to-shot performance of a water Blumlein pulse forming line of 1-1.2 kJ of stored energy. The experiments were carried using the TEMP-4M pulsed ion beam accelerator during its operation in both unipolar pulse mode (150 ns, 250-300 kV) and bipolar-pulse mode with the first negative (300-600 ns, 100-150 kV) followed by a second positive (120 ns, 250-300 kV) pulse. The analysis was carried out for two cases when the Blumlein was terminated with a resistive load and with a self-magnetically insulated ion diode. It was found that in bipolar pulse mode the shot-to-shot variation in breakdown voltage of a preliminary spark gap is small, the standard deviation (1σ) does not exceed 2%. At the same time, the shot-to-shot variation in the breakdown voltage of the main spark gap in both bipolar-pulse and unipolar pulse mode is 3-4 times higher than that for the preliminary spark gap. To improve the statistical performance of the main spark gap we changed the regime of its operation from a self-triggered mode to an externally triggered mode. In the new arrangement the first voltage pulse at the output of Blumlein was used to trigger the main spark gap. The new trigatron-type regime of the main spark gap operation showed a good stability of breakdown voltage and thus allowed to stabilize the duration of the first pulse. The standard deviation of the breakdown voltage and duration of the first pulse did not exceed 2% for a set of 50 pulses. The externally triggered mode of the main gap operation also allowed for a decrease in the charging voltage of the Blumlein to a 0.9-0.95 of self-breakdown voltage of the main spark gap while the energy stored in Marx generator was decreased from 4 kJ to 2.5 kJ. At the same time the energy stored in Blumlein remained the same.

High-rate deposition of Ta-doped SnO2 films by reactive magnetron sputtering using a Sn–Ta metal-sintered target

Ta-doped SnO2 films were deposited on glass substrate (either unheated or heated at 200°C) by reactive magnetron sputtering with a Sn–Ta metal-sintered target using a plasma control unit (PCU) and mid-frequency (mf, 50kHz) unipolar pulsing. The PCU feedback system precisely controlled the flow of the reactive and sputtering gases (O2 and Ar, respectively) by monitoring either discharge impedance or the plasma emission of the atomic O* line at 777nm. The planar target was connected to the switching unit, which was operated in unipolar pulse mode. Power density on the target was maintained at 4.4Wcm−2 during deposition. The lowest obtained resistivity for the films deposited on heated substrate was 6.4×10−3Ωcm, where the deposition rate was 250nmmin−1.

High-rate deposition of high-quality Sn-doped In2O3 films by reactive magnetron sputtering using alloy targets

Sn-doped In2O3 (ITO) films were deposited on heated (200°C) fused silica glass substrates by reactive DC sputtering with mid-frequency pulsing (50kHz) and a plasma control unit combined with a feedback system of the optical emission intensity for the atomic O* line at 777nm. A planar In–Sn alloy target was connected to the switching unit, which was operated in the unipolar pulse mode. The power density on the target was maintained at 4.4Wcm−2 during deposition. The feedback system precisely controlled the oxidation of the target surface in “the transition region.” The ITO film with lowest resistivity (3.1×10−4Ω cm) was obtained with a deposition rate of 310nmmin−1 and transmittance in the visible region of approximately 80%. The deposition rate was about 6 times higher than that of ITO films deposited by conventional sputtering using an oxide target.

High Rate Reactive Sputter Deposition of TiO2 Films for Photocatalyst and Dye-Sensitized Solar Cells

We fabricated dye-sensitized solar cells (DSCs) using TiO2 films deposited on various substrates by reactive magnetron sputtering with midfrequency pulsing and process control units, and evaluated their performance. Two pulse modes, i.e., unipolar pulse and pulse packet modes, were utilized to deposit TiO2. The highest conversion efficiency achieved was 3.7% when 10-µm-thick TiO2 films were deposited on glass substrates coated with fluorine-doped tin oxide (FTO) using the unipolar pulse mode in the oxide mode and postannealing in air. On the other hand, the conversion efficiency achieved was 2.7% for cells with 10-µm-thick TiO2 films deposited on glass substrates coated with FTO using the pulse packet mode without postannealing, and it dropped to 1.3% when the unipolar pulse mode was used. The relationships between the photocatalytic decomposition activity and the DSC characteristics with regard to the TiO2 films were investigated in detail. The reduction in the density of defects in the TiO2 films led to an improvement in both the photocatalytic activity and the DSC characteristics.

High rate reactive magnetron sputter deposition of Al-doped ZnO with unipolar pulsing and impedance control system

Al-doped ZnO (AZO) films were deposited on quartz glass substrates, unheated and heated to 200 °C, using reactive sputtering with a special feedback system of discharge impedance combined with midfrequency pulsing. A planar Zn–Al alloy target was connected to the switching unit, which was operated in a unipolar pulse mode. The oxidation of the target surface was precisely controlled by a feedback system for the entire O2 flow ratio including “the transition region.” The deposition rate was about 10–20 times higher than that for films deposited by conventional sputtering using an oxide target. A deposition rate of AZO films of 390 nm/min with a resistivity of 3.8×10−4 Ω cm and a transmittance in the visible region of 85% was obtained when the films were deposited on glass substrates heated to 200 °C with a discharge power of 4 kW.

Local arc discharge mechanism and requirements of power supply in micro-arc oxidation of magnesium alloy

To study the requirements of the power supply in micro-arc oxidation (MAO) of magnesium alloy, many experiments were performed under the DC, unipolar, and ambipolar pulse output modes. Based on the experimental results and electric arc theory, the separate local arc discharge mechanism was put forward. It is considered that magnesium MAO process consists of three stages including anodic oxidation, micro-arc oxidation, and large-arc discharge in turn with increasing source voltage. The MAO film is composed of metal oxides that resulted from numerous discrete local arc discharges, which accumulate the non-equilibrium structure after undergoing sudden heating and cooling cycles. Separate local arc discharge is caused by the process in which the oxygen-based gas is ionized in the conduct channel bearing electric field intensity, changed from insulator to conductor that presents sharp negative resistance effect, and produced partially high temperature to ignite locally metal oxidation. The local arc discharge model is described as four courses: gas created from electrolysis, arc discharge, metals oxidization, and cooling and shrinking of oxides. The purpose of pulse supply is to inhibit the large-arc discharge by intervening proper cooling time, which cannot be actualized by a unipolar pulse mode because of the strong capacitive load characteristics but can be reached by an ambipolar pulse supply because the negative pulse period acts as cooling time. Using a discharge loop to remove the influence of load capacitive, a new type of pulse power supply for MAO is developed, so that the large-arc problem is resolved effectively, the film-forming efficiency is improved, and the pollution of the film and electrolyte caused by negative voltage is avoided.

Pulse mode effects on crystallization temperature of titanium dioxide films in pulse magnetron sputtering

Pulse mode effects on the crystallization temperature and photocatalytic properties of TiO 2 films were investigated. TiO 2 films were deposited on silica glass substrates using a pulse magnetron sputtering apparatus in the unipolar and the bipolar pulse modes. X-Ray diffraction showed that in the bipolar pulse mode the rutile phase TiO 2 film grew at lower substrate temperature compared with the unipolar pulse mode. The photocatalytic activities obtained from the photoreduction of Ag ions at the surface of TiO 2 films indicated that the anatase phase TiO 2 films grown in the bipolar pulse mode had higher photocatalytic activity compared with those grown in the unipolar pulse mode. This suggests that the bipolar pulse mode is an effective technique to achieve higher photocatalytic activity of TiO 2 film.

A versatile coating tool for reactive in-line sputtering in different pulse modes

Recently, FEP developed a new family of flange mounted magnetron sources together with pulsed powering and process control. This system can be applied either as single magnetrons powered in unipolar pulse mode, e.g. with a pulsed d.c. voltage, or can be applied as a pair of magnetrons powered in bipolar mode, e.g. with an alternating pulsed voltage between both magnetrons that act alternately as the anode and cathode of the discharge. The pulse unit can be switched between the two pulse modes. This allows to investigate directly the influence of the pulse mode and pulse parameters using the same magnetron configuration and the same type of pulse power supply. Experimental results of the reactive deposition of SiO 2, Si 3N 4 and TiO 2 demonstrate the specifics of both process modes. There are effects of the pulse mode observed for all this materials that are independent on the target material. Furthermore, we observed effects of the duty cycle, i.e. the ratio of the pulse-on-time to the cycle time of the rectangular current or voltage pulses. The deposition rate and thermal substrate load exhibit a pronounced dependence on the pulse parameters. The values of film properties like refractive index and internal stress will be discussed for the materials SiO 2, Si 3N 4 and TiO 2. The flexibility of the pulse sputtering system with free choice of pulse mode allows the universal use either for basic research or in production. The investigations made with this system demonstrate the new possibility to use the pulse parameters to influence film properties.