Emitter formation has 2 undesired outcomes – formation of doped glass and an emitter formed on both sides of the wafer along the wafer rims. This makes 2 applications – PSG etch and edge isolation – important post emitter formation. The glass, which is PSG in the case of PERC, is removed with diluted HF solution and doping on the rear side is removed with slight etching on the rear. These 2 steps are accomplished in one go, typically in an inline fashion using HF and HNO3. These wet benches also play an important role in PERC where the rear surface needs to be polished. Typically, PSG etch and edge isolation tools, which etch the rear surface anyway, are tweaked to accomplish this. Equipment suppliers are developing special versions of their tools, making them longer and affording greater flexibility with respect to the degree of rear polishing. While acidic solutions are usually used for this application, using an alkaline chemistry was the trend last year.
The wet-chemical tools for doped glass removal and edge isolation need to be adapted to the TOPCon process, as the doped glass is not PSG but BSG. In addition, this cell architecture requires polysilicon wraparound removal.
RENA is offering 2 different types of solutions – inline and cluster – for polysilicon and doped glass etching and edge isolation. Damian Brunner, senior R&D manager solar & battery at RENA, presented the details of its cluster solution for edge isolation and removal of polysilicon wraparound on day 2 of TaiyangNews Conference on High Efficiency Solar Technologies at the end of 2022, which is still valid. The edge isolation cluster is offered for both PERC and TOPCon. Irrespective of PERC or TOPCon, the wafers go into single-side HF treatment for removal of the glass on the edges and rear. For the singleside HF process, the InEtchSide NIAK3+ product supports a throughput of 6,900 wafers per hour on a footprint of 11 m2. While BSG is harder to etch and usually thicker compared to PSG, RENA has adapted the process, allowing the design to be compact while delivering high throughputs.
With the front glass acting as a mask, the wafers can then be moved into a batch system based on alkaline etching with additive for polishing the rear or to remove the poly wraparound on the front side. In the last part, the surface can be cleaned or even glass can be removed depending on the process flow. The whole process chain is 100% hydrogen peroxide free. Brunner presented the early results with 3 different variations of RENA's process involving BSG removal at different speeds – standard, medium and fast – and benchmarked against the best-known practices in China. The competitor reference process has huge dependency on hydrogen peroxide, while RENA's solution is ozone based. While all the processes resulted in the same efficiency levels, a slight gain in Voc was observed with RENA's solution. All process recipes resulted in good junction isolation, even with the fast and medium HF processes for glass removal, i.e., no negative impact was noticed with fast BSG etch (see Damian Brunner's presentation: Wet Chemical Innovations in SHJ & TOPCon Cell Processing).
In parallel, RENA is also promoting the inline tool platforms for the above-mentioned applications. The company's inline solution for edge isolation is called InOxSide Blue, while its single-side polysilicon etch tool is called InPolySide. Both the products are based on hot alkaline etch. Putting all its best tools and practices into action, in cooperation with Fraunhofer ISE, has resulted in a TOPCon efficiency of close to 24%. RENA's team believes this is a validation of its total wet-chemical solution by an independent and renowned research center for achieving a decent efficiency level.
Another important announcement from RENA is that the company is launching its InEtchSide on the NIAK4 platform. InEtchSide is the company's product for single-side glass etching (PSG and BSG), where the NIAK4 facilitates 10-lane processing for up to G12 wafer sizes and 12 lanes for up to M10. The solution enables single-side etching technology to support close to 1 GW production capacity per single tool.
As to Schmid's edge isolation solutions, it starts with an inline single-side oxide removal with HF, and the rear side polishing is accomplished in a KOH bath at an elevated temperature. While the rear surface is etched, the front surface is protected through a patented water masking technology. This is followed by a rinse and surface cleaning. The subsequent removal of the doped glass depends on the designed process sequence; in any case, the step ends with a rinse and drying.
The same system, very similarly, also works for the removal of polysilicon wraparound and doped glass. It starts again with a HF treatment for preconditioning, followed by etching off wraparound on single side, then a rinse and clean. Depending on the process, the doped glass is then removed with immersion in HF, and finally the wafers are subjected to a rinse and dry. During the rear etching, the front surface is protected with the so-called water masking, Schmid's worldwide patented technology in which the wafer rims are made hydrophobic. In this way, the protective layer of water present on the emitter side is prevented from spilling while protecting the front surface from any chemical exposure. The company's tools are based on a PVDF-PVC design with sophisticated thermal expansion management. They also feature a new transportation system in cleaning modules to ensure clean surface conditions.
Christian Buchner, Vice President of the PV business unit at Schmid underscored that the process requires adaptation of recipes with respect to the method employed for deposition of polysilicon and the doping mechanism. He emphasized that Schmid's tools have been successfully demonstrated to be compatible with films from leading deposition technologies – LPCVD and PECVD. Buchner further notes that the modular nature of the company's tool platforms can also support other advanced technologies beyond PERC and TOPCon (see Christian Buchner's presentation: Innovations in Wet-Chemical Solutions).
As an alternative to the wet-chemical solutions to remove the wraparound, Nines PV is promoting its dry etching tool platform explained in the texturing section. In fact, the benefits of the technology, ultrafast etching rates for example, can be completely reaped in the etching process – be it glass or polysilicon. That is also the reason why the company is mainly promoting its technology for TOPCon. The company's ADE6000-SS-E.ETCH, the largest configuration in its range, supports a max throughput of 18,000 wafers per hour for the single-side etching process. At capacity, just 3 of such tools, needing about 50 m2 of footprint, are sufficient to support a 5 GW production line, according to NinesPV CEO Edward Duffy, and he is optimistic that the feature will help the ADE technology make its way into the market. A half configuration this tool is also available, supporting a throughput of 9,000 wafers per hour.
When it comes to wet benches, exateq has provided data for a glass etching tool that is designed to use HF and reaches very high throughput of 12,000 wafers per hour. RENA has listed 4 products in this section; of these, InOxSide 3+ Fusion and InOxSide 3+ Blue 2 are promoted for single-side etch + PSG etch + cleaning. Both the tools have the same throughput of up to 3,700 wafers per hour on the G12 format. The main difference among them is the chemistry. While both solutions still use HF for glass etching, the former is designed to work with both alkaline and acidic solutions for single-side etching, and InOxSide 3+ Blue 2 relies only a KOH-based alkaline solution. InEtchSide 3+ is only designed for single-side glass removal, thus supporting a high throughput of 5,500 wafers per hour. The InOxSide 3+ Acidic completely relies on acidic chemistry for the single-side etch and doped glass removal that can support a throughput of 3,900 wafers per hour.
Both the products promoted by Schmid in this section, one for PERC and the other for TOPCon, are based on an alkaline chemistry for etching and HF treatment for the glass removal and have the same name – Alkaline Polish / Doped Glass Etch. The TOPCon version is for removing the polysilicon wraparound and doped glass etch. Both the tools, based on 10-lane processing, are rated at a throughput of 10,000 wafers per hour.
S.C New Energy is also offering 2 products for doped glass etching and polishing, and both use HF treatment for glass removal and alkaline KOH chemistry for polishing. Both solutions are based on the cluster philosophy, meaning they use inline single-side process for glass removal and batch-type alkaline process for etching. The main difference is in the throughput; the highest configuration supports 9,600 wafers per hour, while it is 7,000 for the smaller variant.
The article is an excerpt from the TaiyangNews Market Survey on Solar Cell Production Equipment 2023, which also includes detailed product specs and can be downloaded for free here.
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