Nucleation of CVD-TiN on tungsten
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Using Chemical Vapor Deposition (CVD), TiN was deposited on sharp tungsten needles. The reactant gases were TiCl4, N 2 , and H 2 . A Transmission Electron Microscopy (TEM) investigation revealed that the first nuclei of the CVD-TiN coating on tungsten did not consist of 8 -TiN, but were a mixture of a - T i N and e-TiN. These results were also verified with x-ray measurements. From these experimental results a possible mechanism for the initial growth of TiN on tungsten is suggested. It may be that the change in relative concentrations of the different titanium nitrides suggested as mechanism of the initial growth of CVD-TiN can be applied in general for all TiCl 4 /H 2 /N 2 /metal systems where the original substrate surface material partly or completely consists of a metal with catalytic properties.
I. INTRODUCTION
deposited on tungsten needles, having tip radii of less than 50 nm. This technique of coating needles to study the coating was originally suggested by Norden et al. for use in the field ion microscope (FIM).5'6 After deposition in our CVD-reactor, the needles are then directly mounted in the TEM without any further sample preparation. A drawback with this technique is that only coatings having a thickness less than 500 nm can be investigated, in order to allow imaging electrons to penetrate the areas of interest; consequently, deposition has to be carried out for very short times. To ensure that the filling and evacuation of the CVD reactor after deposition do not influence the deposits, these operations have to be done within seconds. A CVD system was, therefore, designed especially for preparation of needles suitable for TEM analysis. Besides improving control of the deposition atmosphere, the reactor was designed to operate at 400-600 Pa. To confirm that the used deposition processes yield the same results as a process
Titanium nitride can exist in three different phases, the hexagonal a-phase, the tetragonal e-phase, and the cubic S -phase.1 In Fig. 1, the phase diagram for the T i - N system is shown. The 5-phase of titanium nitride has during the last two decades been used for many purposes including tribological and decorative coatings.2 Although the kinetics of CVD-TiN growth on TiN is well explained,3'4 little is known about the nucleation and the initial growth. Transmission electron microscopy (TEM) was used here to analyze the nucleation as the phase information with spatial resolution on a nm scale can be obtained with this technique. The normal way to investigate thin films of coated material with TEM is to prepare cross sections, which, however, is a laborious technique. Furthermore, the original microstructure of the sample could be altered during thinning. Thus, to avoid the preparation steps of the samples prior to investigation by the microscope, titanium nitride was 3000 T[°C]
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FIG. 1. Phase diagram of the Ti-N system (from Ref.
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