FSI Adds Steam to Photoresist Clean Step
FSI International has developed an all-wet cleaning process that preserves ultrashallow junctions while cleaning carbonized photoresists. Speaking at the Sematech Surface Preparation and Cleaning Conference, FSI chief technologist Jeff Butterbaugh said the photoresist stripping technique injects steam into a sulfuric peroxide mixture.
David Lammers, News Editor -- Semiconductor International, 3/26/2009
A wet clean method from FSI International (Chaska, Minn.) that preserves ultrashallow junctions (USJs) while cleaning carbonized photoresist was presented yesterday at the Sematech Surface Preparation and Cleaning Conference (SPCC, Austin, Texas).
Working with Varian Semiconductor Equipment Associates (Gloucester, Mass.), the companies studied the use of plasma immersion ion implantation doping (PLAD), millisecond anneals, and an all-wet photoresist stripping technique that removes very little of the doped silicon from the junction regions. The cleaning process involves injecting steam into a sulfuric peroxide mixture (SPM), which shortens the time required to clean a wafer of implanted photoresist to ~five minutes.
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| Jeff Butterbaugh, chief technologist, FSI International |
Jeff Butterbaugh, chief technologist at FSI, told an SPCC audience of about 100 people that by next year, advanced logic devices are expected to have junctions with &100 Å dopant depths. The International Technology Roadmap for Semiconductors (ITRS) also projects that starting next year, material loss from each cleaning step must be kept to 0.3 Å, down from an accepted 0.4 Å thus far.
The biggest cleaning challenge comes from removing implanted photoresist which becomes carbonized. The amorphous carbon “is the most difficult challenge,” Butterbaugh said, adding that “anyone developing an implanted PR removal process must be sure to get rid of that photoresist, particularly at the edge. You have to remove all residues.”
Dealing with the carbonized photoresist set FSI’s chemists to the task of improving the reactivity of SPM. To make it more reactive, FSI has focused on creating more radicals, which Butterbaugh said are instrumental in the removal of carbonized photoresist.
At higher temperatures and higher concentrations the radicals have more oxidation power. Rather than add hot water which would dilute the SPM, the FSI approach involves adding steam in the process chamber to raise the temperature of the mixture to just below ~260°C, near the boiling point. At the surface of the wafer, the temperature is in the 200-220°C range.
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| Adding steam to the process chamber shortens the time required to remove implanted photoresist. (Source: FSI International) |
“We see a significant improvement to the photoresist stripping capability by adding steam to the process,” he said.
Butterbaugh, who is a co-chair of the Front End Processing committee of the ITRS, said plasma doping appears to be one solution to achieving USJs. Results thus far are encouraging: after implant and the photoresist stripping steps, the dopants appear to remain where they are supposed to be, and the sheet resistances are close to optimum, he said. Imaging of a 10 Å square area showed that silicon loss was kept to &2 Å.
