Texturing is the heart of wet-chemical processes that are used for wafer conditioning — and accordingly the subject earned an important position among the wet-chemical processed discussed in our recent TaiyangNews Market Survey on PERC Cell Production Equipment 2022. While a previous article on that topic has provided an overview on key trends in texturing stations (see Several Improvements in Wafer Etching), below we summarize major improvements with tool platforms from leading companies.
High Throughput Tools
Increasing throughput has been a subject of focus for equipment makers, and one way of doing it for texturing tools has been to increase the batch size and the trick still maintains its relevance. RENA has launched a very high throughput machine called BatchTex 3 N600, which handles a batch size of 600 to 720 wafers at a time, achieving a throughput of up to 15,000 wafers per hour for M6, and a slightly lower 12,500 for M10/ M12. This is also true for the wet benches from exateq. Its qTex 612 texturing tool processes 612 wafers per batch at a max throughput of 12,000 wafers per hour. Among the others, S.C New Energy's system — capable of processing a batch of 400 wafers at a time — supports a productivity of 8,000 wafers on an hourly basis, and Schmid rates its tool at a maximum of 7,600 wafers per hour while not sharing details about the batch size.
The 2 trends worth highlighting are eliminating the SDE step before texturing and reducing chemical consumption, both helping customers to cut costs, according to a presentation on Latest Developments in Wet-Chemical Solutions for P-type Solar Cells from RENA SVP technology and innovation Holger Kuehnlein during the TaiyangNews Pushing PERC to Its Limits Conference in March 2022. One straightforward way to reduce chemical consumption is to optimize the etching depth. And as for avoiding SDE, exateq's managing director Gerry Knoch said that the step is already subject to deep optimization in terms of the amount of silicon etched from the wafer as well as KOH concentration, while eliminating SDE saves further on costs. "You can save on at least 2 chemical baths, while burdening the texturing process" he added. RENA's Kuehnlein provided further details on the additional care taken as part of texturization: apart from the wafers undergoing a special clean and preconditioning before being subjected to texturing, the process is also optimized by means of additives, all while maintaining process stability and repeatability over the bath lifetime. Keeping the reflectivity intact is yet another non-negotiable item and values about 9% are highly favored, according to Kuehnlein. The other performance indicator of the texturing process is pyramid size, which has been consistently reduced over time, now at 2 μm with +/- 0.5 μm tolerance. However, in addition to meeting these attributes, the texturing process must be strictly aligned to the subsequent passivation step.
Moving to the latest product developments at RENA, the company has introduced a new additive called monoTexH3.7a series that enables cell makers to tailor the angle of the pyramids. While a perfectly shaped pyramid was what the industry had been striving for till the recent past, tailoring the peak pyramid angle to lower the reflectivity is the keyword among the additive suppliers, according to Kuehnlein. The new additive, even without SDE, enables lowering the reflection values by a percentage point from 10% in 2020 to 9%. "We can lower it further, say to 7.5%," said Kuehnlein, but the process stability and compatibility with passivation have to be evaluated very carefully before seeking such low values. Somewhat going against the general sense, this lowered reflectivity achieved with the new-generation additive is at larger pyramid sizes of 2 to 4 μm. Not only it is larger in size, the size distribution is quite wide as well. However, the process is pretty stable with a maximum drift in reflectivity from 9% by just 0.5% over the bath life of 200 runs. In addition, Rena has also improved the ancillary processes such as cleaning steps. For instance, its latest "high ETA clean" involves special additives in the acid-based metal cleaning after texturing.
We have data for 3 models from Schmid, and all of them are based on inline design, including the one that is designed for alkaline texturing. The wet bench has a very generic self-explanatory name, Alkaline Texture + Saw Damage Removal. While alkaline texturing solutions from other vendors are based on batch philosophy, Schmid believes it has a reason to go against the flow. The advantages, according to Schmid's Technology VP Christian Buchner, are low breakage rate, lower chemical consumption, and better process uniformity. "For thin wafers, this is the only process that would work in the future," he said. While wafer thickness reduction has taken the backseat these days, cell technologies such as HJT have the potential to turn this around. The tool runs the wafers in 6 parallel lanes, passing them through one processing bath and 5 cleaning and conditioning baths. It supports a nominal throughput of 7,600 wafers per hour, but the company has not specified the reference wafer size. Schmid is still promoting 2 acidic texturing solutions mainly designed for multicrystalline.
The 2 tools from exateq listed here are each designed to handle SDE and texturing exclusively. Its qTex 612 texturing tool is designed for batch texturing using alkaline media. The tool accepts wafers up to a size of G12. Each process basket accommodates 104 of such wafers, and the loading and unloading of the substrates can be done both automatically as well as manually. The tool consists of 3 baths for active processing and 9 for rinsing and cleaning, processing up to 12,000 wafers per hour with a rated mechanical yield loss of less than 0.1% and a 97% uptime.
qEtch 612 SDE, the company's SDE tool, is also built on the same configuration. Except for the application, it differs very little from its texturing counterpart. This system also uses KOH as the process chemical and hydrogen peroxide, HF and HCL as the cleaning agents. It also has the same number of chemical baths – 3 for process and 9 for cleaning – and rated with the same nominal throughput of 10,000 wafers per hour. The only notable difference according to the listed specifications is its length of 20 m, while the texturing tool measures 26.3 m.
The Text is an excerpt from TaiyangNews recent report on PERC Cell Production Equipment 2022, which can be downloaded for free here.