The Crux Of PERC

ALD Based Aluminium Oxide System In PERC Processing

The Crux Of PERC

Low cost, high throughput passivation: Leadmicro is a pioneer in batch type ALD tools, which have played a major role in PERC’s entry into high volume production with their low cost and high throughput. (Source: Leadmicro)

  • Rear passivation is the key for the PERC cell architecture
  • While rear passivation requires a stack of alumnus oxide and silicon nitride; ALD is one of the mainstream method to apply aluminium oxide
  • A batch type ALD from Lead micro and a hybrid platform combining the inline PECVD and special ALD from Ideal Deposition are the two popular solutions

Passivation is a process which deactivates bulk defects and reduces the surface recombination of charge carriers, safeguarding cell efficiency (Please check our TaiyangNews Report on TOPCon Solar Technology 2021 for more details on the basics of passivation).

Rear passivation is what differentiates the PERC cell architecture from the standard BSF structure, in which silicon nitride deposited with PECVD are the employed materiel and method of deposition for front side passivation.  The material of choice for rear passivation is aluminum oxide hands down. As for the deposition of this material, ALD and PECVD tools used for applying aluminium oxide and capping the SiNx layer on the rear. In fact, the crux of the PERC process lies in passivating the rear surface. And during the early days of PERC development, aluminum oxide deposition systems were the key ingredients of the PERC upgrade package.

ALD systems were the first to facilitate aluminum oxide deposition for PV. The fundamentals of ALD can be found in our earlier PERC reports, e.g. PERC Solar Cell Technology 2017 Edition.. We received ALD system data from 2 companies for this survey – Leadmicro, a batch ALD developer, and Ideal Deposition that has a hybrid platform on offer. Leadmicro is also marketing an ALD enabled smart PV turnkey solution for high efficiency solar cells, having already provided such a line for TOPCon to Suntech, which its CTO Wei- Min Li highlights as the world’s first. Li spoke about Smart ALD Enabled PERC at the TaiyangNews Pushing PERC to Its Limits Event.

Leadmicro has played a pivotal role in facilitating PERC’s entry into high-volume production. Its ALD solution is both low cost and high throughput. One of the main obstacles for batch ALD was wraparound on the front, similar to LPCVD in TOPCon. However, depositing ALD on top of the front SiNx layer has alleviated any risk of forming inversion layers. Leadmicro has switched to ozone, replacing water as oxidant, making the process more effective as a result. The industry has received the technology very well. According to Li, the company has delivered over 100 GW worth of batch ALD tools within a short 5-year span since 2016. For this survey, we have received data for Leadmicro’s KF20000S, a 2021 model. The tool is a tube reactor and 6 tubes are integrated into the reactor shell. The company, however, has not provided several details of its system, including the wafer size at which it delivers its throughput of 20,000 wafers per hour.Not only throughput, Leadmicro’s batch reactors score high even in performance, according to Li.

At the given thickness of 3-5 nm, the passivation properties of the deposited films are superior in terms of interface defect density, higher negative fixed charge density, high thickness uniformity (deviation of 3% or less) and conformality, claims Li. As already mentioned, while ALD solutions do require a PECVD tool for applying a capping layer, Li claims that ALD still provides the best cost of ownership and better yields based on an analysis of 5 years of production data. Speaking of the other benefits, Li said that ALD results in dense films that also have less hydrogen content and, hence, less prone to LeTID. Leadmicro is also working on a special solution to address PID.

Ideal Deposition has developed a hybrid platform that combines the best of all deposition techniques, specifically spatial ALD and inline PECVD. The system closely resembles the remote plasma inline PECVD tools. Unlike the erstwhile spatial ALD tools, which operate at atmospheric pressure, Ideal Energy’s ALD system works in vacuum. While vacuum pumps make the tool relatively more expensive, this low pressure deposition not only improves the reliability in mass production, but also eases the integration. The wafers are processed in a carrier that moves through different reaction zones to achieve the deposition on a single side, similar to spatial ALD systems. According to the company, this hybrid platform combines the high efficiency potential of ALD with PECVD’s operational benefits. The approach of placing wafers in carriers reduces possible breakage. Considering all these benefits, the concept did sell well; the company has shipped 170 sets of these reactors of which 30 were installed overseas, and developing 4 generations of tools in the process so far, said Zhang Dan, VP of sales, Ideal Deposition, in his presentation Deposition Equipment High Efficiency PERC Cell Production at the TaiyangNews conference. When compared to batch tools, which have a better price proposition and smaller footprint, the technology department of Ideal Deposition claims that its hybrid platform results in better thickness uniformity and quality of the aluminum oxide film. As a result, both the yield and efficiency are better, and the deposition philosophy ensures that these aspects are safeguarded when moving to larger wafer formats, the team emphasizes. As with inline PECVD tools, the Ideal ALD is also flexible in terms of the ability to add further process chambers depending on the required throughput.

In fact, up to 3 chambers can be opted for with the iALD-IV, the lone model the company has provided data for in this current survey. It can process wafers with edge lengths from a lowly 156 mm up to 230 mm. It is quite easy for the inline batch systems like Ideal Deposition’s to process different wafer sizes as this mainly involves changing the carrier. The carrier of the tool has 80 pockets to accommodate G12 wafers, while the slots increase to 108 with M10. The ALD tool has a rated throughput of 10,500 wafers/h for the G12 format, while it increases to 14,500 with M10. These throughput figures are assuming a film thickness of up to 10 nm, which Dan emphasizes is quite a big number for tube reactors, especially when the throughput has to be maintained.

The other highlighted advantages are the ability to process thin wafers and low wafer breakages. The tool can easily support the processing of half wafers in its present state, a highly discussed topic these days. Against the usual practice of cutting a fully processed cell into half, this approach involves processing already half cut wafers, and this at a time when the cutting is becoming inevitable with larger sizes. This process helps reduce edge losses significantly, especially in the case of high efficiency cell technologies such as HJT. However, its relevance to PERC remains unclear. Semco is the latest company to join the stream of ALD tools suppliers for aluminum oxide. However, the report has been finalized by the time the company send the details of its product.

The Text is an excerpt from TaiyangNews recent report on PERC Cell Production Equipment 2022, which can be downloaded for free here.

About The Author

Shravan Chunduri

Shravan Chunduri is Head of Technology at TaiyangNews. Shravan caught the solar bug vey early in this career, starting 20 years ago in research, followed by solar manufacturing, then writing and consulting. His responsibility spans from writing technology articles and reports.

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