Novel Chemistry for Deposition of MgF2 Thin Films

NASA’s Jet Propulsion Laboratory, Pasadena, California

Magnesium fluoride (MgF2) thin films are useful for many different optics applications. In particular, they are useful for ultraviolet anti-reflective and protective coatings. However, in the far UV, one needs a very small, controllable amount of material to get the best optical performance. That is difficult to achieve with conventional methods. Atomic layer deposition (ALD) is an ideal UV-compatible thin-film deposition technique due to its ability to deposit uniform, pin-hole free films with angstrom-level thickness control. Therefore, it is an ideal technique to use to deposit protective thin films in the 2-nm thickness range. However, conventional ALD-MgF2 reactions are very unpredictable due to the low reactivity and volatility of the precursors.

A new chemistry was developed to deposit these films based on Mg(EtCp)2 and HF. This reaction proceeds readily, as the two constituents are very well behaved during the deposition process.

The compound Mg(EtCp)2 is evaporated by heating in a stainless steel bubbler. The material is then carried into the deposition chamber by pushing with an inert carrier gas (in this case, argon). Once a saturated coverage is obtained, the Mg(EtCp)2 is purged from the gas phase in the system and a puff of HF is introduced. The fluorine in the HF reacts with the Mg to make MgF2, and the hydrogen reacts to form 2 HEtCp molecules. The reaction is complete when all the Mg has been converted to MgF2. This series of exposures is repeated until an MgF2film of the desired thickness has been grown.

This novel reaction is the first demonstration of an MgF2 ALD deposition method that is compatible with deposition on ALD aluminum metal with simple and volatile precursors. The use of HF also ensures a very clean thin film.

This work was done by Harold F. Greer, Shouleh Nikzad, Michael C. Lee, and Wesley A. Traub of Caltech; and Steven George and Matthew Beasley of the University of Colorado for NASA’s Jet Propulsion Laboratory. NPO-48678

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