Plasma-Enhanced Atomic Layer Deposition of Ta-N Thin Films

Abstract

The plasma-enhanced atomic layer deposition ͑PEALD͒ of tantalum nitrides ͑TaN͒ thin films has been performed using terbutylimidotris͑diethylamido͒tantalum and hydrogen radicals at a temperature of 260°C. The film thickness per cycle is also self-limited at 0.8 Å/cycle, which is thinner than that of the conventional atomic layer deposition ͑ALD͒, 1.1 Å/cycle. X-ray diffraction analysis indicates that the as-deposited films are not amorphous but polycrystalline mixed with cubic TaN and TaC. The film crystallinity as well as the film density increases with the pulse time and the electrical power of the hydrogen plasma used. By using the hydrogen radical as a reducing agent instead of NH 3 , which is a typical reactant gas used in ALDs and metallorganic chemical vapor depositions of TaN, the films show a much lower electrical resistivity and show no aging effects under exposure to air, owing to the increased film density and crystallinity, and the presence of TaC bonding. In addition, it has been shown that films, which are formed by the PEALD, retain perfect step coverage on the submicrometer holes with an aspect ratio of 10:1.As increasing demand for smaller, faster electronic devices in ultralarge scale integrated devices ͑ULSI͒, copper ͑Cu͒ with a lower resistivity and an enhanced electromigration resistance represents a promising material for providing better performance as an interconnect metal than aluminum ͑Al͒ alloys which are currently in use. 1 However, Cu forms Cu-Si compounds and an easily drift through the oxide at a low temperature. 2 Therefore, a diffusion barrier between Cu and its underlying layers is thought to be a prerequisite for Cu to be used in applications for silicon integrated circuits. For this purpose, transition metal nitrides represent attractive alternatives for use in Cu diffusion barriers. Among these, tantalum nitride has received the most interest because of its high thermal stability and resistance to forming compounds with copper. 3,4 Until now, TaN films have been formed using a collimated and ionized physical vapor deposition ͑PVD͒ technique, since the aspect ratio of contact and via holes continuously increases, this technique will eventually meet application limits in forming conformal TaN films on the holes due to its line-of-sight type approach. In order to overcome this technological barrier, metallorganic chemical vapor deposition ͑MOCVD͒ of TaN films has been studied over the past 10 years, but conformal deposition without generating particles and an improvement in electrical resistivity remain primary issues. 5 In this respect, atomic layer deposition ͑ALD͒ represents an attractive candidate for forming TaN thin films on contact and via holes, because of its inherent abilities to grow conformal thin films almost perfectly and to prevent particle generation caused by intermixing between the reactant gases used, and to control the film thickness digitally on an atomic scale. [6][7][8] In practice, the upper deposition temperature in ALD is determined from the self-decompo...

Synthesis and in situ characterization of low-resistivity TaNx films by remote plasma atomic layer deposition

“…Using 30 s H 2 plasma in the ALD cycle resulted in a resistivity as low as 380 ⍀ cm which is similar to the best values reported in the literature ͑350-400 ⍀ cm͒. 4,9 Park et al reported that the decrease in resistivity relates to an increase in mass density and degree of crystallization of the TaN x films. 9 We note that one data point ͑3 s H 2 plasma͒ does not follow the general trend and the corresponding film was specifically investigated for its low resistivity ͑FPP: 48 ⍀ cm͒.…”

“…% reported by Kim et al 4 and the C content of 20 at. % reported by Park et al 9 The C content in the films originates from the PDMAT ligands and could not effectively be reduced by prolonged plasma exposure time as indicated by our experiments. Showing a different trend than the N and O content with H 2 plasma exposure time, the constant C content could be an indication of a redeposition process involving ligand surface reaction products dissociated by the plasma after being released from the surface.…”

“…38 In a plasma-assisted ALD process using TBTDET precursor and a direct H 2 -Ar plasma, Park et al reported a growth rate of 0.06 nm/ cycle. 9 Ritala et al reported a growth rate of 0.025 nm/ cycle for conductive TaN x films deposited in a thermal ALD process from TaCl 5 precursor and NH 3 using Zn as an additional reducing agent. 10 …”

“…4,9 Park et al reported that the decrease in resistivity relates to an increase in mass density and degree of crystallization of the TaN x films. 9 We note that one data point ͑3 s H 2 plasma͒ does not follow the general trend and the corresponding film was specifically investigated for its low resistivity ͑FPP: 48 ⍀ cm͒. The resistivity of this film was stable and not affected by storage of the sample under ambient conditions, as observed from repetitive FPP measurements over a longer time span.…”

“…Due to its layer-by-layer growth, atomic layer deposition ͑ALD͒ is believed to be the method of choice for deposition in demanding 3D features. 4,[8][9][10] Sequentially employing two self-limiting surface reactions, a submonolayer of material is deposited per ALD cycle and the process is proven to yield excellent uniformity and conformality. [11][12][13] Since different TaN x crystal phases exist including low-resistivity cubic TaN and very-high-resistivity Ta 3 N 5 , 14 successful integration of TaN x films synthesized by ALD requires control over film stoichiometry and composition.…”