Abstract
In this paper, the working principle of the SPIK2000A pulsed DC magnetron sputtering power supply controller is thoroughly analyzed. Based on the technical requirements of magnetron sputtering for selective absorbing coatings, stability tests, arc suppression performance evaluation, and deposition rate measurements under various operating conditions are conducted. The experimental results demonstrate that the SPIK2000A controller significantly enhances the deposition rate by increasing the sputtering voltage and power. It also exhibits superior arc suppression capabilities, making it a suitable solution for improving the efficiency and quality of coating processes in all-glass vacuum solar collectors.
0. Introduction
Magnetron sputtering coating systems play a crucial role in the fabrication of selective absorbing coatings for all-glass vacuum solar collector tubes. To enhance the performance of these coatings, it is essential to achieve a sufficient thickness of the dielectric layer, which helps reduce reflectance and increase absorption efficiency. Currently, AlN is commonly used as the dielectric material in Al-N/Al and Cu-Al/SS selective absorbing coatings. However, the deposition rate of AlN using conventional magnetron sputtering techniques is typically around 1.5 nm/min. For high-quality coatings, a dielectric layer thickness of 60–80 nm is required, leading to process times of up to 40–60 minutes. This long duration reduces production efficiency. To address this issue, a pulsed magnetron sputtering approach was introduced to improve the deposition rate of the AlN layer.
1. Working Principle of the SPIK2000A Pulsed DC Magnetron Sputtering Power Controller
1.1 Control Mechanism
The control system architecture is illustrated in Figure 1. Plasma energy is supplied via a large capacitor within the SPIK unit. The controller delivers a fixed voltage with high instantaneous current, ensuring a strong energy reaction during operation. The DC power continuously charges the SPIK capacitor, filtering out ripples to provide stable DC output. The controller supports a maximum programmable frequency of 50 kHz, with pulse timing parameters such as T+on, T+off, T-on, and T-off adjustable. It also features fast arc detection with suppression times less than 2 μs, and supports multiple operation modes including DC+, DC-, UP+, UP-, and BP.
Figure 2 shows the symmetric (single-DC) and asymmetric (double-DC) pulse output waveforms. These waveforms provide high instantaneous power, enabling the generation of high-density plasma and allowing arbitrary waveform pulses to be programmed.
1.2 Experimental Setup and Connection Methods
The SPIK2000A pulsed DC magnetron sputtering power controller was provided by Shin Chang Motor Co., Ltd. The coating system used was an all-glass vacuum solar collector tube coater with an inner cavity diameter of φ750 mm, featuring a cylindrical aluminum target and capable of coating φ37 full glass vacuum tubes. An OS-5020 oscilloscope from Korea EZ.DIGITAL was used for signal monitoring. The controller was connected in series between the original power output and the cathode/anode of the coater, with its own separate control power supply.
2. Experimental Procedures and Results
2.1 Stability Test
2.1.1 Operating Modes
Figure 3 presents the waveform under the negative pulse mode, with T+on = 20 μs and T+off = 10 μs. The red curve represents the output of the DC pulse controller, showing a square wave with a working time of 20 μs and a non-working time of 10 μs. The voltage remains relatively stable, fluctuating within ±10 V. The blue curve indicates the input voltage to the controller, varying between 264 V and 304 V, reflecting a larger voltage fluctuation from the DC power supply.
Figure 4 illustrates the bipolar pulse mode, with T+on = 10 μs, T+off = 10 μs, T-on = 40 μs, and T-off = 10 μs. The waveform is a typical square wave with some fluctuations, particularly during the positive pulse phase. In reactive sputtering, only the negative potential state allows effective sputtering, while the positive state can help reduce charge accumulation and arc formation.
2.1.2 Discharge Curve Test
As shown in Figure 5, when the argon flow rate was set to 57 SCCM, background vacuum at 1.4 × 10â»Â³ Pa, and current at 20 A, the target voltage and power were measured under different nitrogen flow rates. Using the pulse control mode, both the target voltage and power increased significantly compared to the standard mode. The inflection point of the discharge curve shifted depending on the operating mode.
However, under the double-pulse mode, when the nitrogen flow was high, occasional overvoltage exceeding 800 V occurred, triggering system protection. Adjusting the parameters reduced the overvoltage but did not eliminate occasional voltage or current fluctuations.
2.1.3 Arc Detection Test
The SPIK2000A controller includes arc detection and counting functions, though it does not display cumulative counts. During testing, different arc thresholds were set, including ±200A, ±150A, ±100A, etc. When the arc threshold was above ±150A, few arcs were detected. At ±100A, the arc count increased as the range narrowed. At ±50A, thousands of arcs were recorded, and the count rose with higher nitrogen flow. Setting the threshold to ±40A or lower caused frequent arcs, often exceeding the controller’s limit of 10,000, leading to arc suppression and shutdown. At ±30A, the power supply failed, and the target surface did not glow.
This indicates that arc phenomena are common during reactive sputtering for Al-N/Al coatings. Most arcs occur around ±75A, with fewer at ±100A and almost none above ±150A. While the SPIK controller offers some arc suppression, it cannot fully prevent arcs. Without it, more arcs would likely occur.
Although bipolar pulses do not significantly improve the sputtering rate and waste power, they may offer benefits in optical or semiconductor coatings where microstructure quality is critical.
2.2 Deposition Rate Test
Figure 6 shows the single-layer deposition rate under optimized operating modes. Using the DC pulse controller, the deposition rate increased significantly compared to the standard mode. For example, the rate increased from 1.45 nm/min to 3.93 nm/min and 4.93 nm/min, representing a 2.7 and 3.4-fold improvement, respectively. Similar improvements were observed for the absorber and anti-reflective layers.
Among the three operating modes, the positive and negative pulse mode yielded the highest deposition rate, followed by the negative pulse mode, with the DC mode being the lowest. However, when considering power efficiency, the positive and negative pulse modes showed a 1.5-fold improvement over the others, though the absolute value did not change much. Overvoltage could occur in these cases.
Overall, the use of negative and bipolar pulses increases the sputtering voltage and power, enhancing the deposition rate. While power efficiency does not improve significantly, there are clear advantages in terms of production efficiency, process time reduction, and overall energy savings.
3. Conclusion
1. Under the same working conditions, the SPIK control mode significantly increases the sputtering target voltage and power, leading to a substantial improvement in deposition rate. However, the power efficiency does not show a significant increase.
2. The SPIK control mode effectively detects and suppresses large arcs during the sputtering process, contributing to the preparation of high-quality coatings.
3. Implementing the SPIK control mode offers clear advantages in improving production efficiency, reducing process time, enhancing product performance, and lowering overall energy consumption.
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