A comparative study of the atomic-layer-deposited tungsten thin films as nucleation layers for W-plug deposition

The properties of three different kinds of atomic-layer-deposited (ALD) W thin films were comparatively characterized and investigated as nucleation layers for the W-plug process of 70 nm design-rule dynamic random access memory. ALD-W (A) film was deposited using alternating exposures of WF6 and SiH4 and ALD-W (B) film was treated with B2H6 for 5 s prior to W ALD using WF6 and SiH4. Finally, ALD-W (C) film was deposited using alternating exposures of WF6 and B2H6. All the ALD-W films showed excellent step coverage at the contact with an aspect ratio of similar to 14, but their resistivities were as high as 125-145 mu Omega cm at the thickness of 20 nm. High resistivities of ALD-W films are discussed on the basis of impurities cooperation such as Si and B, phase (body-centered-cubic alpha-W or primitive cubic beta-W), crystallinity (crystalline or amorphous), and grain size. It was found that ALD-W (C) film formed an amorphous phase, which was stable until 900 S C annealing. This is clearly different from ALD-W (A) and ALD-W (B) with polycrystalline grains of alpha-W and beta-W, and beta-W was transformed to beta-W after 800 degrees C annealing. The formation of amorphous W resulted in the formation of large-size grains of chemical-vapor-deposited W film deposited on ALD-W (C) and the reduction in the resistivity of W-plug stack. The integration results showed that the reduced resistivity of W-plug stack with ALD-W (C) provided a significantly lower resistance at the W bit line contact. Another advantage of the integration scheme with ALD-W (C) was its stable contact resistance at the ultrahigh aspect ratio (UHAR) contact even though the step coverage of the underlayer, TiN, was poor. It was also found that the B2H6 pretreatment was effective for obtaining the low and stable contact resistance at UHAR contact. (c) 2006 The Electrochemical Society.