Growth and characterization of TiN/SiN(001) superlattice films
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el Flink, Jens Birch, Per O.Å. Persson, Manfred Beckers, and Lars Hultman Thin Film Physics Division, Linköping University, SE-581 83 Linköping, Sweden (Received 5 February 2007; accepted 20 August 2007)
We report the layer structure and composition in recently discovered TiN/SiN(001) superlattices deposited by dual-reactive magnetron sputtering on MgO(001) substrates. High-resolution transmission electron microscopy combined with Z-contrast scanning transmission electron microscopy, x-ray reflection, diffraction, and reciprocal-space mapping shows the formation of high-quality superlattices with coherently strained cubic TiN and SiN layers for SiN thickness below 7–10 Å. For increasing SiN layer thicknesses, a transformation from epitaxial to amorphous SiNx (x 艌 1) occurs during growth. Elastic recoil detection analysis revealed an increase in nitrogen and argon content in SiNx layers during the phase transformation. The oxygen, carbon, and hydrogen contents in the multilayers were around the detection limit (∼0.1 at.%) with no indication of segregation to the layer interfaces. Nanoindentation experiments confirmed superlattice hardening in the films. The highest hardness of 40.4 ± 0.8 GPa was obtained for 20-Å TiN with 5-Å-thick SiN(001) interlayers, compared to monolithic TiN at 20.2 ± 0.9 GPa.
I. INTRODUCTION
Recent studies on TiN/SiNx multilayers have shown that SiN layers can grow epitaxially on TiN layers.1–4 This gives rise to strong interfacial bonding and effective superlattice hardening, with hardness values higher than for both the monolithic constituents [TiN and SiNx (x 艌 1)] and multilayers containing thicker, amorphous SiNx (a-SiNx) layers.1,4 TiN/SiNx multilayers grown on oxidized Si substrates result in polycrystalline films, and for thin (∼3 Å) SiNx layers the typical columnar growth morphology for TiN is maintained.1,2 As the SiNx layer thickness is increased, a-SiNx starts to form, resulting in the growth of polycrystalline TiN layers composed of equiaxed grains. The highest reported hardness values for TiN/SiNx multilayers are observed for SiNx layer thicknesses on the order of 3–7 Å.1,3,5 This thickness is comparable to the thickness of the amorphous tissue phase that has been assumed to surround TiN nanocrystallites within TiN–SiNx nanocomposites at the percolation threshold (10–20 vol% Si3N4).6–8 Here the maximum hardness is achieved at a Si content of 6–10 at.%, corresponding to a SiNx tissue phase thickness of 1–2 mono-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0412 J. Mater. Res., Vol. 22, No. 11, Nov 2007
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layers (∼2–4 Å).8 Outstanding questions for the TiN– SiNx system are for the nature of the SiNx layers in terms of structure and nitrogen content as a function of layer thickness. For the crystalline SiN (c-SiN) tissue phase, the degree of coherency to the TiN phase also needs to be determined. In the present work, epitaxial TiN/SiN(001) superlattices and polycrystalli
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