Thursday, October 25, 2018

Patterned films by atomic layer deposition using Parafilm as a mask

Recently I have worked a lot with etch people. These guys are definitely a completely different breed from us ALD people. and I have gotten a bit annoyed of their obsession in using Capton Tape all the time. It is rather annoying to cut perfect pieces and then try to separate the protective film on the double sided sticky tape and then it is alos very tedious to clean up after you exposed it to a plasma with nasty gases. Now there is an alternative worth testing also for etch process development!  And for sure - ALD guys are behind it once again saving the etchers and Litho people like we did for multiple patterning.
Here is a very practical and low cost method for all of you who have the need to depositing patterned films by ALD or protect the substrate or certain areas from ALD film growth like the back side or edge. Zhang et al for University of Helsinki has investigated using Parafilm as a mask to block the film growth on selected areas of the substrate surface. Interestingly it works also well above the melting temperature of Parafilm (60 °C) since parfin in this form is very viscous up to the range 275 to 300 °C, below which quite many ALD processes produce high quality films.

A box of parafilm (From Wikimedia Commons, the free media repository)
On an interesting side note, the researchers from Helsinki has actually an Ir-ALD based device on board of the Mercury space ship BepiColombo heading towards the planet as we speak. According to Prof. Ritala [LINK] it is a ALD-Ir coated microchannel plate for x-ray focusing. However if this device had any Parafilm patterning involved in the making I do not know. Anyway in about 7 years we all look forward to see some results from the Satellite using high class Ir-ALD from Helsinki.
The study concludes:
  • The use of Parafilm as a blocking mask is a simple, clean and effective method for ALD film patterning.
  • The blocking ability was tested for ALD of Al2O3, TiO2 and Ir.
  • The Parafilm method works up to 275 °C but above 300°C the melted film will flow on vertical samples
  • Outgasing from the Parafilm at elevated temperatures may hinder growth around the edge of the Parafilm (observed for TiO2 and Ir ALD).
  • Potential use of the method besides R&D is seen for protecting contact areas in IC and MEMS

Patterned films by atomic layer deposition using Parafilm as a mask

Zhang, Chao ; Kalliomäki, Jesse ; Leskelä, Markku ; Ritala, Mikko
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, January 2018, Vol.36(1)

This paper presents a new method for depositing patterned films by atomic layer deposition (ALD) using Parafilm as a mask to block the film growth on selected areas of the substrate surface. This offers an easy and efficient method for large area patterning from the millimeter to even meter scale as needed, for example, for protecting contact areas in integrated circuits and microelectromechanical system devices and in preventing film growth on the backside of substrates. It is shown here that Parafilm can protect the substrate against Al2O3, TiO2, and Ir film growth effectively. However, outgassing from the Parafilm affects the film growth on the unmasked areas differently for the three materials. For Al2O3, there are no significant effects on the growth rate and film quality on the nonmasked areas and the thickness profiles next to the Parafilm masked areas are narrow. For TiO2 and Ir, by contrast, the thickness profiles are wider and outgassing also slows down the Ir growth. Energy dispersive x-ray analysis confirms that Ir does not grow on Parafilm. Al2O3 and TiO2 films do grow on Parafilm, but after the ALD process, the Parafilm can be peeled off easily with tweezers and the areas under the Parafilm mask are without any deposition.

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