• Deposition of aluminum oxide for rear passivation is the main step in the PERC process
  • ALD is one of the mainstream technologies employed to deposit aluminum oxide
  • ALD reactors are mainly two types – batch type time based ALD tools and spatial ALD rectors –, while one tool supplier is also offering a hybrid platform

The crux of the PERC process lies in the deposition of aluminum oxide – and, in principle, there are many ways to deposit aluminum oxide for rear passivation purposes. During the inception time of PERC, different deposition technologies were evaluated for this purpose, namely Atomic Layer Deposition (ALD), PECVD, sputtering and APCVD. The method of deposition is responsible for important attributes of the aluminum oxide film. However, only PECVD and ALD have managed to enter the mainstream.

ALD systems were the first to facilitate aluminum oxide deposition for PV. ALD reactors are mainly two types – batch type time based ALD tools and Spatial ALD rectors. Companies like China’s Leadmicro and Korea’s NCD are offering such ALD batch tools tweaked ALD technology to the spatial domain. Dutch companies Levitech and SoLayTec have designed their products based on this spatial ALD principle. Ideal Energy from China has developed a hybrid platform combining the best of all deposition techniques, specifically spatial ALD and inline PECVD.

Leadmicro is currently leading the segment of the aluminum oxide deposition tools for PERC related applications. The company has been a strong advocate of applying aluminum oxide on both sides of the wafer, as a workaround for undesired deposition on the emitter side. Last year, the company introduced a new tool called KF10000, which, as the model name indicates, has a throughput of 10,000 wafers per hour. NCD from Korea is yet another company that is relying on the traditional time based ALD principle. Similar to Leadmicro, it is also promoting deposition of aluminum oxide on both sides of the wafer. This system of NCD supports a maximum throughput of 4,500 wafers per hour.

Ideal Energy’s ALD device, called Ideal ALD, appears similar to inline PECVD from the outside, but operates according to the spatial ALD principle. Unlike other spatial ALD tools, which operate at atmospheric pressure, Ideal Energy’s ALD system accomplishes the process at a vacuum of 5 to 10 bar. The tool now can support a throughput of 7,200 wafers per hour.

Levitech and SoLayTec have designed their products based on this spatial ALD principle that is a single side process. The method allows for an inline operation mode at atmospheric pressure, which saves a lot of time for pumping down and venting the reactor. In SoLayTec’s machines, wafers are moved back and forth under an injector head. The 8-module configuration of the tool has a rated capacity of 4,800 wafers per hour. According to Levitech’s website, the company is now promoting the third generation of its products called Levitrack. This system has horizontal wafer orientation with a throughput of 4,800 wafers per hour. The philosophy is that it transports the wafers unidirectionally from one end of the chamber to the other.

For details on further important aspects of PERC technology, see TaiyangNews Report on High Efficiency Cell Technologies 2019, which can be downloaded for free.