ALD metal fluoride thin films for UV optics

Tero Pilvi ,  Markku Leskela ,  Mikko Ritala 

Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, Helsinki FIN-00014, Finland

Abstract

Optical components and their coatings are being forced to operate at wavelengths in the deep-ultraviolet (DUV) region because the push for higher density semiconductor devices requires smaller circuit features. These can only be obtained by reducing the wavelength in the photolithographic process used to create the circuits. One of the most anticipated applications is antireflection coating for ArF lithography which is said to be the next main process for the fabrication of semiconductors. Highly damage-resistant dielectric optical coatings are also essential for high output UV lasers. Metal fluorides like CaF2, LaF3 and MgF2 are interesting materials due to their good light transparency in UV range. The problem in depositing fluorides by ALD has been lack of a good fluoride source. The previously used fluoride precursor HF etches silicates and also otherwise is not ideal for ALD.

This paper introduces an Atomic Layer Deposition (ALD) process for preparation of selected fluoride thin films using TiF4 as a fluoride precursor at temperature range of 225–450 °C. Mg(thd)2, Ca(thd)2 and La(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato = C11H19O2) were used as metal precursors. The crystallinity, densities, and roughness of the films were measured by x-ray diffraction and x-ray reflection. Thicknesses and refractive indices of the films were determined from reflection and transmission spectra obtained with a spectrophotometer in the wavelength range of 190–1100 nm. Film morphology was studied with scanning electron microscopy (SEM). Composition and impurity levels of the films were analyzed by time-of-flight elastic recoil detection analysis TOF-ERDA.

The films were polycrystalline and MgF2 grew more columnarly than LaF3 and CaF2 on silicon. Low impurity levels were obtained at 350–400 °C with good stoichiometry. Refractive indices of 1.34–1.42 for MgF2, 1.43 for CaF2 and 1.57–1.61 for LaF3 films were obtained.

 

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Presentation: Oral at E-MRS Fall Meeting 2007, Symposium C, by Tero Pilvi
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-14 15:05
Revised:   2009-06-07 00:44