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On the UV-assisted rapid thermal diffusion of phosphorus in silicon

There are a number of publications that have been claiming for years that the contribution of UV light enhances diffusion mechanisms in Si. Without precise explanations about the claimed photoelectrical interactions explaining this mechanism significantly enhanced (and therefore faster) diffusion processes are claimed for identical processing temperatures.

In order to understand if UV radiation could effectively enhance diffusion, the FLASH consortium decided that it would be worth to have a systematic view to this topic. Jipelec offered the possibility to execute experiments in a prototype RTP reactor that is equipped with additional excimer lamps and that allows to switch on these UV lamps independently from the conventional lamps.

After careful and systematic planning of experiments between FhG-ISE, CEA Genec and RWE Schott Solar an experimental matrix on etch- polished, mirror-polished and double-side polished Cz-Si wafers was prepared applying the following three diffusion sources: spin-on diffusion from liquid dopants, diffusion from APCVD deposited PSG (phosphor silicate glass) layers with an undoped SiOx capping layer and diffusion from pre-diffused (POCl3) wafers.

The difficult topic of temperature measurement during RTP processing was addressed first to make sure that any comparison between samples that were processed with additional UV light and samples that were processed with the normal spectrum of the RTP lamps would be reproducibly executed at the same temperature. It turned out that open loop processing was mandatory in this experiment to achieve unambiguous results.

The fundamental investigation about the influence of UV radiation on the diffusion process has been executed in a prototype reactor of Jipelec as shown in the schematic below.

The excimer UV lamps emitting at a wavelength of 222 nm are located above the wafer. The power density of UV lamps impinging on the top wafer surface is in the range of 60 mW/cm2. This is in 2 orders in magnitude lower than the power coming from the THLs. However, according to publications of authors that reported about UV-enhanced diffusion mechanisms, this intensity would definitely suffice to observe UV-enhanced diffusion. This is not the case. UV-enhanced diffusion does not occur, and can definitely be dropped.

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Fraunhofer Institut Solare Energiesysteme
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