TOPCon has become the mainstream cell technology. The crux of this technology lies in the rear surface engineering, especially the rear passivation scheme. This is achieved through the application of a thin (nano–scale) tunneling oxide layer, topped with a polysilicon layer, which is subsequently doped. This rear passivation stack is applied predominantly either with low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD), while a few also take the physical vapor deposition (PVD) route. TOPCon cells also require aluminum oxide topped with silicon nitride for front surface passivation. In addition, the cell technology also requires thermal processing steps such as boron diffusion.
Jiangsu Leadmicro Nano-Technology Co. Ltd is one of the leading equipment suppliers that is offering all deposition and thermal processing tools required for TOPCon cell production.
In fact, at the recently concluded TaiyangNews annual flagship event High Efficiency Solar Technologies 2023 – Today, Tomorrow And Beyond, Leadmicro's chief scientist Baochen Liao presented the company's latest mass production solutions for TOPCon. In his talk, Liao showcased Leadmicro's latest ALD and PECVD tools for the TOPCon cell process, and its in-house total cell process solutions called as PE-TOPCon technology.
As for individual tool sets, Leadmicro is promoting the ZR series for rear surface passivation. The tool platform consists of 2 variants distinguished by their methods for applying the tunneling oxide – PEALD or PECVD. Both solutions, however, utilize PECVD for polysilicon deposition. The KF series tube type ALD/PEALD tool is designed for the application of aluminum oxide on the front side. The XH series is used for thermal processes such as diffusion, annealing and oxidation.
PECVD gaining traction
Kickstarting its TOPCon research in 2015, Leadmicro shipped its first gigawatt–scale TOPCon turnkey solution in 2022.
However, as mentioned above, the crux of the TOPCon process is rear passivation for which PECVD and LPCVD are the mainstream technologies. While the former is a thermal process, PECVD not only supports lower deposition temperatures, it also seamlessly integrates the doping of the applied polysilicon layer simultaneously (in-situ). This ease of process integration is facilitating the rapid adoption of PECVD, a trend also confirmed by ITRPV's 2021 roadmap, according to Liao. He further substantiates the increasing popularity of PECVD, which conquered a 50% market share, quoting PV InfoLink's August 2023 market share data for various deposition tools.
Going into the details of products, the ZR5000X2 series is an all–PECVD platform, in which all the steps – tunneling oxide as well as polysilicon deposition and in-situ doping – are accomplished using PECVD alone. The tool is increasingly becoming the preferred choice for customers, claims Liao. In fact, the solution being the key, Leadmicro has bagged 15 GW of turnkey solutions in total. The ZR 5000X2 series processes both mainstream wafer formats — M10 and G12. When processing M10, the tool handles a batch size of 768 wafers per each boat, reaching an hourly throughput of 6,200 wafers. The capacity drops to 616 wafers per boat when using G12 wafers, delivering a throughput of 5,000 wafers per hour. Not just high capacity, the ZR series also helps achieve high Voc of over 725 mV.
Liao also shed light on 2 major equipment and process–related improvements for the tube–type PECVD tool. At the equipment level, the continuous deposition of conductive tunneling oxide by passing through high current in the graphite boats create the risk of insulators becoming conductive, which leads to the risk of short-circuits in the parallel graphite boats. The improved design of the ZR series mitigates this risk, according to Liao. On the process front, optimizing hydrogen content, adhesion, and interfacial stress has facilitated getting a grip on the blistering, a common concern among PECVD tube-type reactors.
Passivating front
While not as critical as the rear surface, passivating the emitter side of the TOPCon is also important. The solution is much similar to what is applied on the rear of the PERC cell – aluminum oxide capped with silicon nitride. However, the underlying surface is different – textured and boron doped (active). Leadmicro is promoting its batch-type thermal ALD system called KF 15000S for the deposition of aluminum oxide. The advantage of ALD here is that it inherently meets critical requirement of 100% conformality of the deposited film over the textured surface, leading the superior surface passivation, explains Liao. Liao also presented a comparison chart between the 3 mainstream technologies for the applications – batch ALD, inline ALD and 2-in-1 PECVD. While the efficiency of both the variants of ALD is higher than that of rival PECVD, the batch type ALD wins in terms equipment price, meaning the KF 15000S series reduces the equipment CapEx.
Emitter is also important
Liao also provided a brief overview of the company's thermal processing tools. The XH 10000A series is promoted for all thermal needs of TOPCon process – boron diffusion, annealing and oxidation. The tool platform is tweaked according to application. For boron diffusion, for example, it processes wafers of different dimensions – M6 to G12 – and each of the 6 tubes integrated into the reactor's frame can be loaded with 2,880 of M10 wafers or 2,200 G12 formats. However, the company has not specified the throughput with respect to size.
Discussing the technologies in store for future, Liao emphasized that the company is also working on providing production solutions to next generation of TOPCon technology such as applying the TOPCon structure on both sides of the cells (bifacial-poly), TOPCon–based back contact cell architecture (TBC), and also tools for edge passivation, which can lead to cell efficiencies beyond 26%.
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