As the feature size of ULSI (ultralarge-scale integration) circuit devices shrinks, interconnects need to be scaled to maximize the wiring density and wire ability. This increased density leads to smaller contacts or via holes connecting the wires. The reduction in size increases the aspect ratio of these features for the same dielectric thickness. Conventional physical vapor deposition (PVD) techniques such as sputtering are unable to fill high aspect-ratio contacts/vias with aluminum conformally, as shown in Figure 1. This poor conformity is attributed to the shadowing effect by already-deposited nondirectional sputtering atoms and leads to void formation and open failures. Chemical vapor deposition (CVD) is a solution for achieving conformal filling.
This is a prime reason why CVD has been investigated since the 1980s and why a large number of metals have been proposed. Above all, because tungsten as a refractory metal has high resistance to electromigration (EM) failures and tungsten hexafluoride (WF6) as a source gas has some advantages in manufacturing, for example, high vapor pressure (boiling point = 17.4°C) and purity, W-CVD with WF6 based on H2 or SiH4 reduction has been widely studied. Blanket W-CVD has been used for actual via-hole filling for 0.5-μm devices. Figure 2 shows a cross-sectional view of five-level interconnects using W plugs in 0.35-μm devices. Another advantage of W-CVD is that W deposits selectively into via holes if a low flow-rate ratio of reductants to WF6 is used.