AVS 51st International Symposium
    Thin Films Tuesday Sessions
       Session TF2-TuM

Paper TF2-TuM10
High Reflectivity X-Ray Mirrors from W/Al@sub 2@O@sub 3@ Nanolaminates Fabricated by Atomic Layer Deposition

Tuesday, November 16, 2004, 11:20 am, Room 303C

Session: ALD and Applications
Presenter: F.H. Fabreguette, University of Colorado
Authors: F.H. Fabreguette, University of Colorado
Z.A. Sechrist, University of Colorado
R.A. Wind, University of Colorado
S.M. George, University of Colorado
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Nanolaminates can display novel optical, thermal, electrical or mechanical properties. In this study, atomic layer deposition (ALD) was used to fabricate W/Al@sub 2@O@sub 3@ nanolaminates for x-ray mirrors. High x-ray reflectivity depends on precise thickness control, minimal interfacial roughness and high density contrast between the absorber (W) and spacer (Al@sub 2@O@sub 3@) layers in the superlattice that defines the Bragg mirror. The sequential, self-limiting ALD surface reactions allow for atomic control of each layer thickness. Al@sub 2@O@sub 3@ ALD films are amorphous, whereas W ALD films are polycrystalline. To minimize the surface roughness, the W ALD growth temperature was decreased from 177°C to 125°C. This temperature reduction lowered the surface roughness and also changed the tungsten crystalline phase from the @alpha@-phase with some @beta@-phase to the pure @alpha@-phase. This transformation is desirable because the @alpha@-phase has a higher density than the @beta@-phase. Quartz crystal microbalance (QCM) was used as an in-situ probe to optimize the nucleation of Al@sub 2@O@sub 3@ ALD on W and W ALD on Al@sub 2@O@sub 3@ at 125°C. Atomic force microscopy (AFM) studies revealed that smoother films resulted from nanolaminates that had shorter nucleation periods. The optimized W/Al@sub 2@O@sub 3@ nanolaminate growth parameters were used to grow W/Al@sub 2@O@sub 3@ x-ray mirrors. These Bragg mirrors were fabricated with variable bilayer thickness, D, spacer-to-absorber ratio, @gamma@, and number of bilayers, N. The measured x-ray reflectivities were compared with the predicted x-ray reflectivities from the Fresnel equations. Reflectivities as high as 70-80% were obtained using the best set of D, @gamma@ and N parameters. These results demonstrate that the W/Al@sub 2@O@sub 3@ nanolaminates fabricated by ALD are competitive with commercial x-ray mirrors prepared using sputtering techniques.