A New Process for Nickel Silicides

With ever-reducing gate dimensions and junction depths, nickel silicides are expected to replace cobalt silicides at poly gate dimensions somewhere below ~40 nm. At this dimension, the sheet resistance of CoSi₂ becomes unacceptably high. The nickel offers the advantages of lower contact resistance, less silicon consumption, lower film stress and lower process temperatures (reducing dopant loss). However, NiSi processes suffer from a problem of excess silicidation, which can cause poly depletion and affect transistor performance. Silicidation also occurs on small diode structures, which can result in high leakage currents.

To address this problem, researchers from Texas Instruments (Dallas) developed a novel process that forms a self-aligned nickel-rich silicide film via a low-temperature in situ anneal, which is followed by high-temperature RTP for controlled silicidation.

The conventional NiSi₂ process involves HF dipping and argon pre-sputter etch prior to nickel PVD. After metal deposition, an optional cap layer can be deposited to prevent ambient effects during silicidation and to reduce silicon diffusion through the top surface.

The researchers proposed that, if the silicidation can be performed at lower process temperatures, with acceptable sheet resistance, excess silicidation could be prevented. They identified two temperature windows where the Rs and nonuniformity was good — at 260-310°C and at 400-550°C. In the lower temperature range, X-ray photoelectron spectroscopy indicated that a nickel-rich silicide forms. The group identified a wet etch solution that would remove unreacted metal atoms without removing the silicided film. The new process uses a low-temperature siicidation after nickel PVD and cap layer deposition, followed by the wet etch and then RTP to convert the nickel-rich silicide to NiSi₂. Improved breakdown characteristics on diodes (breakdown voltage reduced by 50%) indicated that excess silicidation had been minimized.