OTS (September 2008) -- At the recent FSI Knowledge Services™ Seminar series Stephane Zoll and Bruno Imbert of STMicroelectronics described challenges and solutions of metal stripping for nickel platinum (NiPt) salicide formation using a process based on FSI’s ZETA^® System ViPR™ technology. The process leverages the FSI’s proprietary high temperature capabilities to strip unreacted NiPt with a sulfuric acid/hydrogen peroxide chemistry. This process is able to strip the unreacted Ni and residual Pt while avoiding the parasitic oxidation seen when chlorine based chemistries are used on NiPt silicides annealed at low temperatures. By enabling low temperature annealing, the new method solves a critical problem in the integration of NiPt silicides at the 45 nm technology node.

Silicide processes are a well established means of improving the conductivity of gate polysilicon and source/drain contacts at the transistor level of an integrated circuit. As circuit geometries have continued to shrink, the metal used to generate the silicide has changed from Ti (above 130 nm), to Co (130-90 nm), to nickel (90 nm-65 nm) and now to nickel/platinum (65 nm and below). Each metal brings its own set of challenges.

When NiPt was first introduced at 65 nm the challenge was simply to develop a wet etch chemistry that could remove unreacted Pt. Etchants that remove pure Ni cannot remove Pt, which is typically present at levels of 5% to 10%. The initial solution was found in an aggressive Cl-based chemistry that uses concentrated HCl and H₂O₂ that generates dissolved Cl₂ gas, which subsequently works with HCl to dissolve the Pt.

Another challenge developed at the 45 nm node as junctions in the source/drain region were made more shallow. Due to the shallower junctions, conductive metallic regions (“pipes”) formed by the highly-diffusible Ni penetrated the junction area, causing short circuits in the device. Subsequent investigations revealed that Ni pipe formation can be inhibited by lowering the temperature of the annealing process. However, silicides formed at these lower temperatures include a Ni rich phase that is vulnerable to attack by the Cl chemistry resulting in removal of Ni and oxidation of the remaining silicon which increases contact resistance.

Zoll and Bruno considered both HCl/H₂O₂ and HCl/H₂NO₃ but in both cases the presence of Cl resulted in attack of the Ni rich phase of the silicide. Ultimately the solution was found in a chemistry base on sulfuric acid (H₂SO₄). At elevated temperatures the acid etches unreacted NiPt but the relatively weak oxidant does not attack the Ni-rich phase causing excessive Si oxide formation.

The ZETA^® System ViPR™ technology’s ability to achieve high chemical temperature at the wafer’s surface is ideally suited to the NiPt stripping application. Sulfuric acid based chemistry would not normally be considered for NiPt stripping, however, as Zoll’s presentation clearly demonstrates, at elevated temperatures it not only removes unreacted NiPt, it does so without attacking the Ni-rich silicide. The process shows high selectivity for the unreacted metal versus silicides and dielectrics. The spray technology and flexible, programmable mixing capability of the ZETA^® tool delivers clean, fresh chemicals to the wafer surface, reducing the possibility of cross contamination between wafers and between batches, preserving the flexibility of the cleaning tool to perform multiple processes, and minimizing operating costs for chemical consumption.

The NiPt stripping solution described by Zoll removes a critical roadblock to the use of NiPt silicides at 45 nm. The resulting process reduces temperature restrictions imposed on the anneal step by the stripping chemistry, allowing the use of low temperature anneals that eliminate Ni pipe formation while also avoiding increases in contact resistivity caused by Ni-rich silicide attack and subsequent Si oxidation.

Figure 1. ZETA^® System ViPR™ Technology enables the use of low-temperature silicide annealing to avoid Ni diffusion issues in NiPt silicide formation.

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