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Ultrafast ALD compared to other technologies

SoLayTec, Manager Marketing and Sales, Roger Görtzen
In the 1980s, when solar cell production was not as substantial as it is today, titanium dioxide (TiOx) was the anti-reflection coating (ARC), a layer that could easily be deposited by atmospheric pressure chemical vapour deposition (APCVD). This type of deposition was simple and cheap, but it did not provide surface passivation…
Then came the introduction of SiNx by plasma enhanced chemical vapour deposition (PECVD), which has since been the solar industry standard for the front of the solar cell. Apart from ARC, it is also excellent in its surface passivation for the front of the solar cell.

Passivation with Al2O3
We all know the current necessity to boost efficiency and reduce the cost of the price per Wp. When focusing on high-efficiency solar cells, the effect of rear surface recombination becomes an important design parameter. At the Department of Applied Physics, Eindhoven University of Technology, Dr. Hoex (currently working at SERIS, Singapore) published his work in 2008 entitled: “On the c-Si surface passivation mechanism by the negative-charge-dielectric Al2O3”. In a nutshell he revealed that this is an excellent passivation material for p- and n-type c-Si from 5 to 10nm Al2O3. It creates a high negative charge density. However, the key remark of his work in which he used thermal and plasma atomic layer deposition (ALD ) was that these tools were not really suitable for mass production.

End-user criteria
This was the signal for equipment vendors to begin development. Before going into any detail, let us briefly consider the needs of the end users. The Cost of Ownership of the Al2O3 layer per wafer/cell has to be as low as possible; the most important cost drivers for this are material costs, the downtime for cleaning, and the tool price plus its throughput. Furthermore, the tool has to be screened for its process performance; like the passivation quality, uniformity of the layer and layer deposit on just one side (i.e., no other side deposition OSD).

Ultrafast ALD
Ultrafast ALD from SoLayTec uses spatial ALD for Al2O3 deposition. This technology is based on the spatial separation of precursor gases instead of time-based separation. The different gases are confined in specific process areas as the wafers pass by. Every point on the wafer is sequentially in contact with the wafers. This creates the (real) ALD effect and reaction. In the SoLayTec machines the process places in the core deposition units where a wafer is moved upstream and downstream through an injector head twice per second resulting in eight layers per second (deposition rate of 1nm/sec).


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