Concerns about threshold voltage shifts and other performance problems with the gate-first approach to high-k/metal gate creation may cause GlobalFoundries (Sunnyvale, Calif.) and other members of the IBM-led Fishkill Alliance to shift to a gate-last technique, sources said at the International Electron Devices Meeting (IEDM), going on this week in Baltimore.
"My understanding is that the subsequent thermal steps are causing problems with the gate-first approach," said a senior vice president at Qualcomm Corp. (San Diego) who was attending IEDM. "GlobalFoundries seeks a gate-last approach, and if necessary they could drop in a gate-last module independent of IBM," the Qualcomm executive said.
Asked about the potential switch, a senior IBM technology manager said continuance of the gate-first approach after the current 32/28 nm generation is under review. Any shift to a gate-last approach, if it occurs, would come at the 22 nm node or later. "Both of the gate formation approaches have their problems, and there is no doubt that the gate-first approach is significantly simpler," he said, asking not to be identified. "For IBM, gate first will work well at the 32 nm generation, and I would not underestimate the power of incumbency, which could take it to the next (22 nm) generation. After that, we'll have to see what happens."
At IEDM, a knowledgeable source said GlobalFoundries and nearly all the other members of the Fishkill Alliance will force a shift by IBM to the gate-last approach at the 22 nm node. GlobalFoundries is mulling a switch even earlier, at the 28 nm node coming to market in about a year, he added.
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A Toshiba technologist assigned to the Fishkill Alliance said the gate-last approach delivers a lower threshold voltage and higher mobilities, particularly on the PMOS transistor. Channel strain is induced when the dummy gates are removed, providing another significant increase in performance.
At the 45 nm node, when Intel introduced its gate-last process flow, the argument was that the gate-last approach required more restricted design rules (RDRs), the Toshiba manager said. Intel was able to restrict the layout of its poly gate lines to one dimension because of its in-house coordination of the process and the design rules. For foundries, the argument went, too many RDRs would inhibit fabless companies from porting chips from a SiON/poly process to a high-k process.
Intel developed a gate-last approach, announcing it in December 2006 for its 45 nm technology. In that iteration, the hafnium dielectric was deposited by atomic layer deposition (ALD), and a sacrificial polysilicon gate was created. After the high-temperature S-D and silicide annealing cycles, the dummy gate was removed and metal gate electrodes were deposited last.
In the second-generation 32 nm gate-last approach, Intel deposits both the dielectric and the metal electrodes last, further avoiding thermal stress to the gate stack. The Intel approach requires careful control of the etching and CMP steps, among others, but delivers a better work function on the PMOS device in particular. "We have it working, as demonstrated by our 22 nm SRAM announcement a few months ago," said Mark Bohr, a senior fellow at Intel.
Although the gate-first approach more closely resembled the process flow of the pre-high-k era, problems have cropped up, Hoffmann said in a Sunday short course presentation. At various technology conferences this year, researchers have discussed a rolloff of the flatband voltage, shifts in the PMOS threshold voltage, and interface layer regrowth. "When the metal sees a high thermal budget, it has an impact on the work function," Hoffmann said. Importantly, the problems created "fundamental issues for mobility, probably due to remote Coloumb scattering. It takes a fair amount of work to improve the quality of the layers to reduce these changes."
The gate-last approach gains a further performance boost from strain induced when the dummy gate is removed, he added.
Another source said the gate-first approach has yield issues. The capping layer is only ~5 Å. Defects are created from debris generated from the capping layers. Those particles impact yields "and can be the difference between profit and loss for a foundry," he said.