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 SiH ₄, and ALD-W (B) film was treated with B ₂H ₆ for 5 s prior to W ALD using WF ₆ and SiH ₄. Finally, ALD-W (C) film was deposited using alternating exposures of WF ₆ and B ₂H ₆. All the ALD-W films showed excellent step coverage at the contact with an aspect ratio of ∼14, but their resistivities were as high as 125-145 μΩ 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 α-W or primitive cubic β-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°C annealing. This is clearly different from ALD-W (A) and ALD-W (B) with polycrystalline grains of α-W and β-W, and β-W was transformed to α-W after 800°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 B ₂H ₆ pretreatment was effective for obtaining the low and stable contact resistance at UHAR contact.