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 design-rule dynamic random access memory. ALD-W (A) film was deposited using alternating exposures of and and ALD-W (B) film was treated with for prior to W ALD using and . Finally, ALD-W (C) film was deposited using alternating exposures of and . All the ALD-W films showed excellent step coverage at the contact with an aspect ratio of , but their resistivities were as high as at the thickness of . 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 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 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, , was poor. It was also found that the pretreatment was effective for obtaining the low and stable contact resistance at UHAR contact.