Preparation of tungsten-based thin films using a F-free W precursor and tert-butyl hydrazine via 2-and 3-step atomic layer deposition process

Abstract

This article reports the preparation of tungsten metal (W) and other tungsten-based thin films using a F-free W metalorganic precursor tris(3-hexyne) mono-carbonyl tungsten [W(EtC CEt)(3)(CO)] (Et = C2H5), THMCT], tert-butyl hydrazine (TBH), and H-2 plasma via 2- and 3-step atomic layer deposition (ALD) process. Density functional theory (DFT) calculations were performed to simulate the possible mechanisms of W-4(EtC CEt)(4)(CO)(4), as the model of chemisorbed THMCT on the growing surface, with TBH. The experimental findings agree well with the DFT results that the thermal ALD of THMCT and TBH could not deposit the pure W metal films. Instead, THMCT and TBH led to form an amorphous, tungsten oxycarbide, WOxCy (with a minor nitrogen content and a negligible amount of W metal phase), resulting in a very high resistivity of similar to 80,000 mu Omega-cm (at 300 degrees C). Nevertheless, the typical characteristics of ALD, such as the self-limiting growth, and a linear dependency of the film thickness with cycle variation, were obtained with a growth-per-cycle (GPC) of similar to 0.4 angstrom. Post-annealing of the films in the H2 atmosphere resulted in a slight increase in the metallic phase formation with a significant drop in resistivity to similar to 3500 mu Omega-cm. However, the annealing also facilitated the formation of the tungsten oxide phase owing to the oxygen incorporated in the as-deposited films. Introducing an additional H-2 plasma pulse step at the end of each ALD cycle (3-step ALD process) significantly altered the film composition and helped reduce the asdeposited film's resistivity drastically to similar to 650 mu Omega-cm. Also, the process results in the formation of tungsten carbide (WC) as the predominant phase. Finally, the additional high-temperature-post-annealing of these 3-step ALD grown films resulted in extremely low resistivity of similar to 200 mu Omega-cm owing to the formation of body-centered-cubic (BCC) a-W. Thus, the envisaged path in this study to achieve W thin films also provides rooms to grow oxide or carbide dominant phase of W as studied in detail with the help of X-ray photoelectron spectroscopy