克服太阳能电池湿化学加工过程中硅堆积的影响

Introduction  

Although the chemical reaction is well known, the anisotropic etching of Si in alkaline  solutions is a complex process. This is particularly true in the solar industry where a large  mass of silicon is typically introduced into the etch bath. The etch by-products (silicates)  affect the balance of the etching specie. If adequate compensation is not made for these byproducts, a significant drop in etch rate and an increase in contamination levels is typically  noticed. Because of this contamination, production lines would suffer from unpredictable  wafer characteristics and hence lower cell performance. 

In order to obtain stable and reproducible manufacturing processes, a reliable and accurate  real-time measurement of the etching constituents becomes necessary. In addition, a  mechanism by which fresh chemicals can be added to the etching bath is also required. The  presence of these etch by-products has been shown to slow down the etch rate even when the  chemical concentration is correct. Simply put, the solubility of etch by-products decreases  once the concentration of silicates increases. And hence, Si mass transport from wafer surface  to the etching solution is impacted. Chemical mixtures involved in solar cell manufacturing  generally include: KOH/IPA, HF/HNO3, HF/HCl, and other compounds. Additives, e.g.  surfactants, are typically used to enhance etch uniformity.

In this study, wafers were processed in a KOH/IPA mixture to produce texturized surfaces as  shown in Figure 1. In-line sensors were installed to monitor the chemicals' concentrations in  real-time. Algorithms were developed to control the chemical concentration by injecting  chemicals and water at a desired time interval to compensate for the loss of chemicals and  water. The system also allows for draining and replenishing the chemicals and water to keep  the silicates under a threshold to maintain consistent etching characteristics under different  bath loading conditions.

Experimental  

Wet chemical processes were conducted on a fully-automated GAMASolar™ wafer etching and  cleaning station. The batch size was 200 wafers per run. All tests were performed with the same  wafer supplier, including wafers from about 100 different ingots. Silicon etching processes were  conducted with the aid of Akrion Systems' patented in-situ chemical concentration control system  (ICE-1™). Measurements of concentrations were taken using inline NIR (near infrared) sensors  installed in the recirculation loop of the process tanks. These sensors measure light absorbance and  transmit it through fiber optics cables to an array of detectors (spectrophotometer).

The light absorbance of given specie in the solution is correlated to its concentration over a wide  range of wavelengths. The signal is then reported to an amplifier that scales the response to a 4-20  mA output. This signal is subsequently fed into an analog module that scales the signal and reports  directly to the system computer which controls the spiking (volumes and frequency) of chemicals to  maintain concentration. A variety of chemicals e.g. HF, HNO3, HAc and applications were also  studied but only the results of KOH/IPA control will be presented here. The goal was to produce  consistent etch rates and texturization patterns similar to those shown in Figure 1 over the entire  bath life and different silicon loading levels.

Results and Discussion: 

This technology provides technical advantages by accurately measuring the concentration of  chemicals to produce the desired process results – in this case the texturization pattern [1-3].  Controlling concentration for uniform, repeatable texturization will help solar cell manufacturers  reduce cost of ownership (COO) and overall cost of manufacturing by extending the usable life of  the chemical bath, which in turn extends the up-time and overall utilization of the tool [4-5].

Conclusions:  

Results show that real-time chemical concentration monitoring and control are critical and  beneficial for advanced solar cell manufacturing. The technology reduces the days required for field  installation by eliminating the time and resources required to dial-in the right chemicals'  concentrations. Utilizing closed loop concentration control, this process will no longer require  many iterations and tedious work until the results are achieved. The technology significantlyreduces rework and wafer miss-processing and hence the manufacturing becomes more robust and  less costly.  

Fig1

Figure 2 shows the calibration curve of the concentration of KOH. Similar calibration is typically  run for IPA. Figure 3 shows the amount of KOH that is required to maintain a stable etch rate over  time. As can be seen from Figure 3, the volume of KOH required for maintaining a constant etch  rate (ER) is linearly related to the mass of Si being etched. The data is in excellent agreement with  stoichiometry and stands good grounds for production applications. The system is also calibrated to  track the etch byproducts (measured as Si) as shown in Figure 4.