By Debra Vogler, Senior Technical Editor
Thermal budgets are becoming ever tighter, as memory manufacturers move beyond the 70nm node. Some manufacturers are also moving from batch furnaces to single-wafer tools to bypass the longer processing times arising from the larger thermal masses in conventional furnaces. With the introduction of its Atmos dual-chamber, single-wafer 300mm tool (based on its Helios platform), Mattson Technology Inc. says users will be able to address thermal budget constraints as they extend the well-known process of selective oxidation to more advanced nodes — where material changes that accompany the smaller device geometries and thinner films mean structures are less forgiving with respect to over-processing.
Memory manufacturers are tackling the replacement of the conventional gate contact — replacing doped polysilicon (p-Si) and tungsten silicide (WSix) with tungsten/tungsten nitride (W/WN), see image, below — because it allows lower sheet resistances in combination with small aspect ratios . The “old” gate stack used a nonselective RTO process, but because of tungsten’s low oxidation resistance, the “new” stack requires a selective oxidation process, one that oxidizes only the lower poly-silicon sidewall while leaving the W/WN sidewall unoxidized.
While selective oxidation is a well-known process with a long history, the new technology, done at atmospheric conditions, will encompass a wider range of oxidation applications, some because of the tool’s high steam concentration (>50%), explained Andreas Toennis, VP and GM of Mattson’s thermal products group. Examples of processes enabled include: selective oxidation of gate stacks; various oxidations at lower thermal budget compared to dry oxidation and in situ steam generation; and improved oxide quality and corner rounding behavior for shallow trench isolation (STI) liners, flash tunnel oxides, and sidewall oxides for STI.
Toennis noted that historically, the oxidation process used pure oxygen, making the process much harsher and difficult to control. “The issue is not necessarily processing at high steam concentrations, but the ability to be able to do it at all,” he told WaferNEWS. “With thermal budget reduction becoming a necessity, steam oxidation offers an ability to smoothly oxidize certain films, so the ability to use steam in the oxidation regime at all different types of concentrations (<1% and all the way up to almost 100%) would be ideal."
The company says it has had an inquiry from one logic customer for the application of selective oxidation to metal gates. “This selective oxidation that is being used so extensively now in DRAM, may have some potential application in logic as well and it’s being explored,” said Sing-Pin Tay, director, product technology, Mattson’s thermal products group.
Toennis added that the most likely scenario for using metal in the gate structure for logic devices is 32nm. From a timing standpoint, he noted that 45nm is in process development at leading IDMs and will finish at the end of this year or mid-2007. Technology for 32nm is in R&D stage and progressing into process development probably by mid-next year or end of next year — the time frame in which Mattson’s technology would be explored for logic gate structure applications. “On the memory side, we see it being applied at earlier nodes and it’s now being introduced at 70nm production,” said Toennis. — D.V.
 G. Roters, R. Hayn, W. Kegel, O. Storbeck, S. Frigge, G. Feldmeyer, et al., “Selective Oxidation of Tungsten-Gate Stacks in High Volume DRAM Production,” Proc. ECS, 2003.