Ed’s Threads 070615
Musings by Ed Korczynski on June 15, 2007

IBM HK+MG gate-first processing
At the VLSI Symposium on June 14th, and after months of a mainstream press hype-war with Intel, IBM finally unveiled some of the details of its new high-k/metal-gate (HK+MG) transistor technology. Mukesh Khare, IBM project manager for high-k/metal-gate development, presented integration details of the new transistors while keeping specifics of materials and processing confidential. The key information is that their HK+MG “gate first” approach keeps the same processing sequence used by traditional SiON gates, allowing for both technologies to be run on the same line and minimizing integration costs.

“We did a lot of work to look at gate-first and gate-last, and both approaches have challenges,” explained Khare, in an exclusive interview with SST and WaferNEWS. “We picked the approach that is simple, scalable, and also migrate-able.”

Gate-first is simple in terms of changes to existing processes, and looks scalable to smaller device geometries. “Migrate-able” means making it easy to port designs from SiON transistors. Indeed, gate-first processing seems to be the best overall approach -- if you can find a material that can withstand the high temperatures used in device annealing. Keeping most of the existing process flow intact, 45nm will still use tungsten plugs for contacts.

Transistor formation typically requires ~1000°C annealing to allow atoms to settle into proper places after ion-implantation, which inherently damages silicon crystals. Any gate materials in place during annealing must withstand such temperatures without losing their properties. In particular, the high-k dielectric material must maintain a certain composition and material phase to ensure that the transistors do not leak current.

All IBM will officially say to date is that its gate-first high-k material is hafnium-based, which is the currently known default standard, but they will not yet specify anything else. The material is likely to be a blend of hafnium, silicon, oxygen, and nitrogen, which can be seen as just adding the hafnium to the SiON currently used. Hafnium atoms have a relatively higher oxygen coordination number and are simply larger (atomic number 72, compared to silicon at number 14, and oxygen and nitrogen at 8 and 7, respectively), so adding them to the SiON currently used increases the dielectric constant of the layer based on density functional theory. The thickness of the inversion layer under the gate (Tinv) with conventional oxynitride is typically, at best, 18-19 Å -- IBM’s HK+MG transistors reportedly demonstrate Tinv ~12Å, something achieved, by working for over 10 years on fundamental materials engineering.

Though not needing any fundamentally new metrology techniques, every film will require control. For example, compositional changes with nitrogen depth have already been used with nitrided-oxide gates (SiO:N), so one possibility is a nitrided-hafnium silicate (HfSiO:N). Nearly all the recent HK dielectrics that have been shown for CMOS transistors have been stacks of layers with atomic-level engineering of the interfaces. The specific composition and gradients within the layers are officially secret, but it is highly likely that there is at least one atomic layer of SiO at the bottom.

HK+MG transistors at nanometer-scale nodes are constrained by the same trade-offs between speed and leakage (for HP or LSTP circuits, respectively) as with SiON+poly transistors. Engineering the dielectric stack to be either fastest/leaky or fast/tight for a target HP or LSTP, there’s a single HK gradient-stack and one metal used for both NFET and PFET gates. Poly-silicon tops the metal gates. “After more than three years on the 300mm pilot line, there’s been a lot of learning and we’re on track,” Khare noted.

For planar devices, there are more options in terms of ALD, CVD, or PVD, explained Khare. He claims that the cost to use HK+MG is similar to that needed for any new technique like using a dual-stress liner, and so it adds minimal additional cost to the final wafer, but not all designs will need the performance improvement so some chips at 45nm and 32nm will still use SiON+poly. “It depends on the product needs. It is a very powerful technology. It’s very simple,” stated Khare. “The materials challenge was very high k, and that’s one thing we focused on.”

—E.K.

Labels: CMOS, high-k, HK+MG, IBM, metal gate, transistor, VLSI

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070615: IBM HK+MG gate-first processing

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