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Tungsten ͑W͒ thin films were deposited using a modified chemical vapor deposition ͑CVD͒ process, called pulsed CVD, and the film properties were characterized as nucleation layers for a W-plug fill process. In this study, the deposition stage is composed of four steps, resulting in one deposition cycle: ͑i͒ reaction of WF 6 with SiH 4 , ͑ii͒ inert gas purge, ͑iii͒ SiH 4 exposure, and ͑iv͒ inert gas purge. The W growth per cycle was extremely linear with a growth rate of ϳ1.32 nm/cycle at 400°C. The growth rate was further enhanced to 1.5-1.9 nm/cycle by increasing the SiH 4 flow rate in the first step and/or by adding H 2 in the first and the third steps. The W film deposited by pulsed CVD showed a much lower roughness ͑ϳ0.7 nm͒ and a better conformality at the contact holes with an aspect ratio of 14, compared to W films deposited by conventional CVD using WF 6 and SiH 4 . The film resistivity was closely related with its phase ͑body-centered cubic ␣-W or primitive cubic ␤-W͒ and microstructure characterized by grain size as well as the film thickness ͑the "size effect"͒. Transmission electron microscopy analysis showed that H 2 addition into the first and third steps increased the grain size from ϳ7 to ϳ13 nm and prevented the film from forming a ␤-W phase with high resistivity, resulting in a lower resistivity of 100 ⍀-cm compared to that of the W film deposited without H 2 addition ͑210 ⍀-cm͒. H 2 addition was also effective in reducing the F and Si impurities in the films. Finally, the film resistivity was discussed on the basis of impurity, roughness, microstructure, and film phase.

Tungsten ͑W͒ thin film as a nucleation layer for a W plug-fill process was deposited using a modified chemical vapor deposition ͑CVD͒ called pulsed CVD, and properties of the films were investigated. Basically, the deposition stage was composed of four separate steps for one deposition cycle: (i) reaction of WF 6 with SiH 4 ; (ii) inert gas purge; (iii) SiH 4 exposure without WF 6 ; and (iv) inert gas purge. A higher deposition rate, ϳ1.5 nm/cycle, was obtained as compared to that of atomic layer deposition ͑ALD͒ (ϳ0.25 nm/cycle). The film deposited by pulsed CVD showed a much lower root mean square ͑rms͒ roughness ͑0.87 nm͒ and better conformality at the contact holes with an aspect ratio of 14 ͑contact height: 3.51 m and top diameter: 240 nm͒ as compared to the layer deposited by conventional CVD using WF 6 and SiH 4 .Chemical vapor deposited ͑CVD͒ tungsten ͑W͒ has been used as a plug material in etched oxide contacts and vias at high aspect ratio in multilevel metallization of dynamic random access memory ͑DRAM͒. 1-5 The conventional W plug-fill process is composed of three steps: 1 (i) silane (SiH 4 ) pretreatment, (ii) W nucleation, and (iii) W plug-fill. Prior to W deposition, Ti/TiN is typically deposited as a glue/barrier layer because of poor adhesion of W on SiO 2 . The first step, a SiH 4 pretreatment induces a homogeneous nucleation of W film on TiN, resulting in a continuous and uniform W film less than 20 nm thick. 5 In the nucleation step, WF 6 is reduced with SiH 4 for fast deposition on TiN with little incubation time while H 2 reduction is used in the plug-fill step with an excellent conformality ͑Fig. 1͒.Generally, the SiH 4 reduction in the W nucleation step produces poor step coverage of W films in deep contacts compared to H 2 reduction. Thus, a thin nucleation layer is favored to minimize the effect on the W plug-fill. However, a thin nucleation layer is subject to be attacked by WF 6 during the plug-fill step, leading to volcano formation. 6 Moreover, as the contact and via sizes continuously shrink with increased aspect ratio, the poor step coverage of the W nucleation layer becomes more problematic. In this respect, a deposition process with better conformality is required. Atomic layer deposition ͑ALD͒ of W is considered as a most promising solution with a superior conformality at the aggressive topographic structures. 7-9 Using Si 2 H 6 , 7 SiH 4 , 8-9 and B 2 H 6 9 as reducing agents of WF 6 , ALD-W films were deposited with very smooth surface and excellent step coverage as compared to those of CVD-W. However, an ideally very low deposition rate of 1 monolayer of W ͑0.25-0.3 nm͒ per cycle as compared to conventional CVD-W process 7,9 may limit its applications in the semiconductor processing.In this article, we report the modified CVD-W nucleation process, called pulsed CVD. We introduced an additional SiH 4 exposure without WF 6 after a short CVD reaction of WF 6 with SiH 4 . And, between the two steps, a purge step using inert gas was carried out to prevent a possible gas-phase rea...