By Ed Korczynski, Sr. Technical Editor
In another example of the old one-liner that “all that is old is new again,” the old technique of solid-source doping is being used by Intel for a critical process step in so-called “14nm node” finFET manufacturing. In the 7th presentation in the 3rd session of this year’s IEDM, a late news paper written by 52 co-authors from Intel titled “A 14nm Logic Technology Featuring 2nd-Generation FinFET Transistors, Air-Gapped Interconnects, Self-Aligned Double Patterning and a 0.0588m2 SRAM Cell Size” disclosed that solid source doping was used under the fins.
As reported by Dick James of Chipworks in his blog coverage of IEDM this year, the fins have a more vertical profile compared to the prior “22nm node” and are merely 8nm wide (Fig. 1). Since Intel is still using bulk silicon wafers instead of silicon-on-insulator (SOI), to prevent leakage through the substrate these 8nm fins required a new process to make punch-through stopper junctions, and the new sub-fin doping technique uses solid glass sources. Idsat is claimed to improve by 15% for NMOS and 41% for PMOS over the prior node, and Idlin by 30% for NMOS and 38% for PMOS.
Solid glass sources of boron (B) and phosphorous (P) dopants have been used for decades in the industry. In a typical application, a lithographically defined silicon-nitride hard-mask protects areas from a blanket deposition in a tube furnace of an amorphous layer containing the desired dopant. Additional annealing before stripping off the dopant layer allows for an additional degree of freedom in activating dopants and forming junctions.
In recent years, On Semiconductor published how solid-source doping on the sidewalls of Vertical DMOS transistors enable a highly phosphorous doped path for the drain current to be brought back to the silicon surface. The company shows that phosphorous-oxy-chloride (POCl) and phospho-silicate glass (PSG) sources can both be used to form heavily doped junctions 1-2 microns deep.
The challenge for solid-source doping of 8nm wide silicon fins is how to scale processes that were developed for 1-2 microns to be able to form repeatable junctions 1-2 nm in scale. Self-aligned lithographic techniques could be used to mask the tops of fins, and various glass sources could be used. It is likely that ultra-fast annealing is needed to form stable ultra-shallow junctions.
Intel is notoriously protective of process Intellectual Property (IP) and so has almost certainly ensured that any equipment and materials suppliers who work on the solid-source doping process sign Non-Disclosure Agreements (NDA) with amendments that forbid acknowledging signing the NDA itself, so it is pointless to directly ask for any further details at this point. However, slides from John Borland’s recent presentation at the NCCAVS Junction Technology Users Group meeting provide a great overview of the publicly available information on finFET junction formation, and include the following:
…higher dopant activation can be realized at low temperatures if the junction is amorphous and recrystalized by using SPE (solid phase epitaxy) recrystalization of the junction as also shown in the data by Intel.
Also seen at IEDM this year in the 7th presentation of the Advanced Process Modules section, Taiwanese researchers—National Nano Device Laboratories, National Chiao Tung University, and National Cheng Kung University—joined with Californian consultants—Current Scientific, Evans Analytical Group—to show “A Novel Junctionless FinFET Structure with Sub-5nm Shell Doping Profile by Molecular Monolayer Doping and Microwave Annealing.” They claim an ideal subthreshold swing (~60 mV/dec) at a high doping level. Poly-Si n & p JLFinFETs (W/L=10/20 nm) with SDP experimentally exhibit superior gate control (Ion/Ioff >10E6) and improved device variation.