太阳能电池晶片超声波清洗系统的制作及性能测试

In this study, a midsonic cleaning system for solar-cell wafers with a frequency  of 750 kHz was designed and fabricated. Finite element analysis was used to  design the system. The obtained peak admittance value was 750.0 kHz.  Reflecting the analysis results, the system was fabricated and its admittance  characteristic was measured. The measured data showed 753.1 kHz, a value that  was consistent with the finite element method (FEM) result with 0.4% error. The  acoustic pressure test was performed and the resulting pressures were found to  range from 283% to 328%, with a standard deviations range from 36.8% to  39.2%. Then, the wafer damage test was performed, and no damage was  observed. Finally, the particle-cleaning test was performed; when we applied  1100 W, 99.8% of particles were removed. These results indicate that the  developed midsonic bath has the capability of cleaning effectively without  inflicting wafer breakage.

Ultrasound has been widely applied in a variety of  industries, such as in manufacturing, industrial cleaning and  semiconductor wafer cleaning processes. Regarding  manufacturing fields, Luo et al. and Jang et al. reported  ultrasonic bonding process, and Ng et al. explained ultrasonic  welding application. In addition, it can be used for micro  hole machining, hot embossing, burnishing polishing and  nano-surface reformation process. In our previous works,  ultrasonic waveguides for semiconductor wafer cleaning apparatus are included. Some researches are reported  about megasonic cleaning mechanisms.

In this research, we have used ultrasound for a solar-cell  wafer cleaning system. Recently, solar energy has attracted  attention as a promising clean energy source in the future,  and solar energy generating systems are being widely  developed. The major energy converting photovoltaic parts  are fabricated on solar cell wafers, and as a result the  demands on manufacturing solar cell wafers have been  rapidly increasing.  

The detailed recipes for the cleaning process are as follows.  In the pre-cleaning process, 28 kHz, 40 kHz, and 40 kHz  ultrasonic cleaning processes are involved in a sequence. For  the second and final cleaning, 40 kHz ultrasonic with full  power up to 900 W is used. In this process, due to the wafer  breakage problem, the use of 250 W power is recommended.  The cleaning recipes are illustrated in Fig. 1.

In this article, a midsonic cleaning system with a frequency  of 750 kHz in the midsonic range between 100 kHz and  1 MHz is designed and fabricated. Finite element analysis was  used to design the actuator and the stainless steel plate. Then,  performance tests are processed in the point of acoustic  pressure output and the wafer cleaning efficiency with the  breakage test. Finally, the results are compared with those of  a conventional type, and the performance is discussed.

To design the ultrasonic system, finite element analysis was  performed using commercial FEM tool ANSYS software.  First, in order to design the actuator, it was modelled  two-dimensionally as an axis-symmetric rectangular shape.  The material of the actuator was piezoelectric ceramic, and  the properties were given to the model. For actuation,  electricity should be supplied, the top line and the bottom  line were used as electrodes for the analysis. They were  electrically coupled to each other, and 1 V was applied to  the top while 0 V was applied to the bottom line as a ground.  The calculations were done by changing the frequencies from  600.0 kHz to 900.0 kHz. The obtained admittance graph is  shown in Fig. 3(a), and the peak admittance value was 750.0  kHz. Based on the analysis dimensions, a real piezoelectric  ceramic actuator was fabricated, and the top and bottom  surfaces were coated as electrodes. In order to make it easier  to connect wires, the bottom electrode was drawn to the left  lower corner of the top electrode. Subsequently, the  admittance characteristic of the piezoelectric ceramic actuator was measured and plotted in Fig. 3(b). The peak value was  749.0 kHz, which was consistent with the previous analysis  value. Thus, the value of 750.0 kHz was chosen as the design  frequency of the system. The fabricated piezoelectric ceramic actuator (137 × 39 × 3 mm, a thin rectangular type) is shown  in Fig. 4.

Fig1

After fabricating the ultrasonic bath, the acoustic pressure  distributions were measured over the plate filled with water.  The experimental setup is composed of a hydrophone sensor  (Ø 2.5 × 70.0 mm, Onda corp.) attached on the jig,  three-axis moving columns and a computer analysis system,  as shown in Fig. 7. When measuring the data, the sensor is  moving in a zig-zag path with 0.05 mm steps so that it can  scan the desired area in detail. The measured data is  transported to the computer and the pressure distributions are  displayed in real time. After saving the data file, it is  analyzed by calculating the maximum values and standard  deviation values.