MS-3-P-1855 APT versus STEM EDX 3D mapping of transition metal nitride thin films

Me-N thin films exhibiting a nanocomposite (NC) structure are well known to achieve extremely high values up to 60 MPa [1-2]. According to literature, the nanocomposite structure consists of crystalline MeN (Me = Ti, Cr, Al etc.) nanograins with sizes from 5-20 nm surrounded by an amorphous SiNx matrix. Two aspects of the nanocomposite structure are important: 1. the unambiguous observation by TEM of the two differently structured phases due to the sample thickness superior to the grain size; 2. the structure on the atomic level of interfaces between the crystalline, facetted grains and the amorphous matrix. From the literature review, it is not clear, to which extent the atomic structure of the interfaces between the crystalline grains and the amorphous matrix influences the hardness of the NC coating. There are two contradictory theories. The first claims, that the matrix surrounding the crystalline Me-N grains should be amorphous, beginning from the first layer [3,4]. Conversely, the second theory explains the high hardness by the growth around the crystalline grains of an epitaxial, crystalline layer of the matrix having a thickness of 1-2 monolayers. The aim of this study was to image in 3D the distribution of chemically different phases of the NC structure. For this purpose Atom Probe Tomography (APT) and HAADF Tomography combined with EDX mapping were applied. The APT technique permits reconstructing in three dimensions the chemical composition of a sample with a nearly atomic resolution. The HAADF imagining allows for tomography with a certain chemical contrast, however simultaneous collection of EDX maps will provide more chemical information for reconstruction. This is possible on Tecnai Osiris TEM operating at 200kV equipped with an ultrafast Super EDX system. The sample, grown on a Si substrate, was a TiSiN coating having a fine-grained structure with three amorphous SiN layers having thicknesses of 5 and 10nm. First a pillar with a length of 1200nm and a diameter of 200nm was cut by FIB, Fig. 1(a). The next step was to shape a 800nm long needle and diameter below 100nm transparent to electrons. HAADF and BF STEM were used to evaluate the needle cut form the TiSiN coating for tomography. The chemical layout of the sample was verified by EDX mapping, Fig.2. STEM tomography was compared with the results obtained by APT, exhibiting much higher resolution. Thus 3D chemical imaging may shed some light into the origin of the high hardness in nanocomposites.

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