- An ability to process larger wafers, improved throughput, reduced texturing time, improving bath lifetime are the generic developments for wet-chemical benches also for HJT
- Including SDE, optimization of the pyramid size and high surface uniformity are the notable optimizations required for HJT wet-benches
- Cleaning, the most important step in wet-chemical station, while can be accomplished with RCA or ozone, the latter is increasingly becoming mainstream
- In this section of the TaiyangNews HJT report, we list 4 wet chemical solutions from 3 leading suppliers.
As with standard cell making, the HJT process also starts with saw damage etching (SDE), followed by texturization and cleaning. All these steps are individually optimized for HJT. However, there are a few generic development areas that are relevant to HJT as well. Regarding cell production tools, a common development across the board, irrespective of the technology, is the ability to process larger wafers, and all the wet benches now have the capability to handle at least the largest commercial wafer size of G12. Attaining higher throughput is also one common improvement but the means are somewhat different. For the texturing tools, the heart of the surface preparation station, reduced texturing time and increasing the batch size are the main routes. As for the process, making the texturing IPA-free by using additives has been an industry-wide standard for a few years already. Improving the bath lifetime was another important development in alkaline texturing.
The deviations from mainstream and specific changes pertaining to HJT starts with SDE. The latest trend among the wet-chemical solutions for PERC is to avoid SDE completely, while SDE or even cleaning before SDE are highly relevant for HJT, according to Holger Kuehnlein, SVP Technology & Innovation at leading wet-chemical technology developer RENA from Germany. The texturing process has to be also aligned with the subsequent passivation step, which is very crucial for HJT. The optimization of the pyramid size is key; the goal here is to strike the right balance between optimum light trapping and high-level surface termination, which collectively helps attain low surface recombination velocity after the PECVD passivation process (see Two-In-One With PECVD).
Maintaining high surface uniformity is also very important to provide a better interface between the crystalline silicon textured surface and the deposited amorphous silicon layers. The primary focus for wet-chemical solutions for HJT is all about process control, which in turn can help reduce the process times, thereby increasing the machine throughput, according to Exateq managing director Gerry Knoch.
Cleaning is perhaps the most important change required for HJT processing. HJT demands a specific cleaning regime involving several surface-cleaning steps to remove organic and metallic impurities. The quality of the deposition – and thus the ultimate passivation – strongly depends on the cleanliness of the wafers, especially since the HJT process is carried out at low temperatures. With standard technologies, any contamination due to small metallic particles is eliminated via gettering, a high temperature process; this is not the case with HJT.
There are two cleaning methods that are largely followed – an ozone-based process and a so-called RCA process using hydrogen peroxide (see TaiyangNews Report on Heterojunction Technology 2020 for details of RCA and ozone-based cleaning steps). While RCA is effective as well as robust, it is also expensive, not only due to the high price of the chemicals of which hydrogen peroxide is at the top, the effluent treatment costs are also high.
The industry has been working on alternative cleaning solutions using ozone. Indeed, ozone-based cleaning has become the standard now, as emphasized by all the wet-chemical solution providers TaiyangNews spoke to. The main challenge with the ozone-based solution is to achieve the required ozone concentration. “Ozone takes time to build up the concentration,” said Exateq’s Knoch, which also affects the uptime of the tool, while the process is rather simple when using hydrogen peroxide.
At the tool level, Exateq is currently promoting a new system called q612. While the “q” represents the product platform, the first digit 6 denotes the number of carriers and 12 indicates the maximum wafer size the tool can process – G12 in this case. As to the ability to process larger wafers, Knoch explained that the earlier q600 tool with the ability to process up to M6 wafer size was already in production since 2017. “We now have 5 years of experience, while others started on this size late, in 2020,” said Knoch. Exateq is also offering one more model, q410, capable of processing 4 baskets and wafer sizes up to M10. While the q612 tool can process 10,000 G12 wafers, q410 supports a throughput of 8,000 wafers per hour, also of G12 size. The wet benches from Exateq are fully integrated; meaning, pumping stations and ozone supply are within the scope of supply.
RENA is also promoting two product platforms for HJT, BatchTex N400 SHJ and BatchTex N600 SHJ, built on its famous BatchN400/600 platform. The tools are based on ozone cleaning and use RENA’s proprietary monoTex additives-based texturing. The company has developed a new additive called monoTexH3.7a that enables cell makers to tailor the angle of the pyramids as explained in detail in TaiyangNews PERC Cell Production Equipment Survey 2022. The additive facilitates a stable process with maximum drift in reflectivity by just 0.5% over 9% throughout the bath life of 200 runs. Both the tools of the company handle baskets with a loading capacity of 100 each, while the number of carriers differs – N400 can process 4 and N600 handles 6. Throughputs vary accordingly as well; the smaller tool supports 8,000 M6 and 7,000 of M10 and G12 wafers per hour, whereas the N600 has a throughput of 12,000 wafers per hour independent of wafer size.
Singulus from Germany is promoting its latest SILEX lll modular wet-chemical system, which – in addition to mainstream PERC – is also suitable for advanced cell architectures such as HJT. The fully automated system is characterized by high throughput for all wafer sizes from M0 to G12 and above, as well as for half cells. The tool is designed for IPA-free alkaline texturing and ozone-based cleaning, which results in a reflectivity of between 11.5% and 12.5%. Singulus claims the SILEX lll supports an annual production capacity of 550 MW and the hourly throughput is given as high as 13,800, 12,000 and 7,200 wafers for M6, M10 and G12 formats, respectively.
YAC and S.C New Energy from Asia are two other important wet-chemical tool suppliers, but they have not responded to our enquiry. YAC from Japan in the past has been an important supplier of wet-chemical solutions from its legacy as a wet-chemical solutions supplier to Panasonic, the pioneer of HJT. For this reason, the company topped the list of wet-chemical tool vendors for HJT, especially during its inception period (about 2 to 3 years ago). However, with the know-how spreading, its prominence is fading out.
The Text is an excerpt from 3rd edition of TaiyangNews’ Heterojunction Technology 2022 report, which provides an overview on the most recent HJT developments as the technology is entering the GW scale production level and can be accessed free of charge here.