The crux of the TOPCon process lies in the rear surface passivation scheme, and deposition technologies play a very crucial role here. What is required here is forming a thin silicon oxide-based tunneling film, polysilicon topping it and doped subsequently. PECVD and LPCVD are the 2 mainstream technologies for the core of the TOPCon structure.
The latter, which is a thermal process, has the longest track record, as most of the early adopters of TOPCon kickstarted their mass production with LPCVD. However, LPCVD typically had a few shortcomings, such as wraparound, low life of the quartzware, and external doping of polysilicon. PECVD, in addition to supporting lower deposition temperatures, also seamlessly integrates doping of the applied polysilicon layer simultaneously (in situ). The 2 processes have their own set of advantages and limitations.
On the topic of PECVD, Leadmicro, a strong advocate and leading supplier of such tools, offers 2 types of tool platforms. On the all-PECVD platform, all the steps, including tunneling oxide as well as polysilicon deposition and in-situ doping, are accomplished using PECVD alone. The company also offers a PEALD + PECVD tool platform that uses PEALD for tunneling oxide formation and PECVD for the application of polysilicon and subsequent doping. ALD results in very conformal coating, enabling the application of very thin tunneling oxide layers that favor efficiency.
Leadmicro is offering its 5th-generation PECVD all-in-one PECVD tool that supports a throughput of 6,800 wafers per hour. The tool features a no-cooling electrode maintenance solution that enables maintenance of the electrode without the need for the tube to cool down. The platform, which combines PEALD and PECVD in one tool, reaches a throughput of 7,100 wafers per hour.
Laplace is a leading supplier of LPCVD tools. The difference between PECVD and LPCVD starts with the application of tunneling oxide. In LPCVD, the tunneling silicon oxide layer is grown in situ thermally, while the film is deposited in PECVD using oxygen or N2O plasma. According to Laplace, it is well known that thermally grown silicon oxide layers are superior in quality to deposition layers. As for in-situ doping, Laplace highlights that the thermal treatment step is still needed to activate the dopant in the PECVD approach, while in the LPCVD route, the dopant is induced and activated in one step. In order to have some control over the wraparound, LPCVD tools are typically loaded with one wafer per slot. Then, irrespective of the deposition method, the post-deposition chemical treatment is inevitable.
However, Laplace made considerable progress in a few areas quickly thereafter. The company’s improved product platform supports the processing of 2 wafers per slot, with full control over wraparound. This doubles the throughput to 8,600 per hour. With the help of specific recipes and special know-how, Laplace says it has been able to improve the lifespan of the quartz boats to 6 months, and 12 months for quartz tubes as opposed to 4 months in the past.
The thickness of the polysilicon layer has been a topic of interest. Here, equipment makers follow the developments in pastes, which currently support 80 nm. Equipment vendors, paste suppliers and PV manufacturers are all collectively working on reducing this thickness further.
In addition to these, leading PVD tool maker Von Ardenne has also started offering PVD-based solution for TOPCon. VON ARDENNE says its system simplifies the process flow by sequentially depositing the tunnel oxide and doped polysilicon layers without backside wraparound or separate thermal oxidation. It also allows for front and rear SiNx passivation within a single tool. Trials conducted on Fraunhofer ISE’s TOPCon line showed a +0.1% absolute efficiency gain when replacing PECVD SiNx with sputtered PVD SiNx, alongside improvements in open-circuit voltage and fill factor, claims the tool vendor. The PVD process also eliminates the need for hazardous gases like phosphine, silane, and ammonia, significantly reducing permitting complexity, facility costs, and operational expenses – an advantage particularly valuable in emerging manufacturing regions such as the US, India, and MENA. In addition, the PVD-based setup drastically reduces equipment count with its undeniable high production capacity of 1.3 GW with its GigaNova platform. It also lowers cleanroom requirements, leading to overall lower total cost of ownership compared to conventional CVD setups.
As a side note, all the mainstream equipment makers that are supplying production tools for TOPCon are already looking beyond. Companies like Laplace have collected the resources to offer full production line solutions for BC. Leadmicro is taking a step ahead and is already offering deposition tools for tandem technology.
The GigaNova platform from VON ARDENNE was also highlighted as adaptable for future cell architectures. The tool’s chambers, designed for front-side SiNx deposition, could be reconfigured to deposit TCO and ETL/HTL layers, requiring only limited additional investments. In addition to sputtering, this tool platform also supports evaporation, which is required for perovskite technology.
