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Do the two consecutive layers of the same material deposited by PVD behave as one

My query is related to optical Thinfilm deposition by PVD in vacuum. If we deposit same material film, two times, one over the other does it behave as single film having the overall thickness as sum of the two films or it behaves as two individual films. Say for example for MgF2 film. We deposit a lambda/4 layer once and take the film out of the chamber and again deposit a lambda/4 layer. Will it behave as lambda/2 layer or as two films of lambda/4 thickness.

D.K.Kohli
Centre for Advanced Technology - Indore, India

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Generally speaking, it's not a good idea to expose the substrates to air then coat again. It depends on the degree of oxidation of the surface between coating steps. If the surface oxidizes, it will most likely form two layers, with poorer adhesion of the second layer unless you do a good sputter cleaning job between layers. If you have no oxidation, the second layer may grow on the lower layer in such a way to form a single thicker layer. But again, you may not get good adhesion.

Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover
San Diego, California
 

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Optically, the two thinfilms will act as one layer unless, as Jim mentioned, there is some kind of contamination that gets on the first layer when you vent it up to atmosphere. There almost always will be something that gets on the surface when you vent to atmosphere, the question is how much can your optical system tolerate. If you are making high end thinfilm optics then you will probably see a degradation in performance. If you are making low end optics you probably won't notice.

John Davis
- Berthoud, Colorado, USA
 

 Ed. note: The grammatical error of eliding the space in 'thin film' to the single word 'thinfilm' on this and other letters is the deliberate error of finishing.com, to allow more efficient targeting of searches for this topic.