SANTA CLARA, Calif. – IBM sketched out its visions of the fab future at the recent Common Platform Technology Forum, the chip alliance of IBM, Globalfoundries and Samsung. Specifically, an IBM scientist spotlighted double patterning tricks with immersion lithography. Big Blue also showed advances in fully depleted silicon-on-insulator and plans for silicon photonics, nanowires and other new twists ahead on the semiconductor road map. The following pages provide a few excerpts from the talks, starting with lithography.
“Since the 45 nm node we have been on a steady downward path, but [extreme ultraviolet lithography] can bring us back,” said Gary Patton, chief technologist in IBM’s semiconductor research group said, referring to the foil below. “EUV represents “the biggest change in the history of lithography [because] EUV light is extremely hard to work with—it’s absorbed by any material, including reflective lenses and masks,” he added.
Nevertheless, “I believe CMOS scaling will continue, [but] it will require disruptive technologies such as carbon nanotubes and silicon photonics,” he said.
The Rayleigh resolution limit (k1=0.61) of ASML's EUV tool (NA=0.33) is 25 nm. So by the time they use for 7 nm, it would need the same OPC and enhancement tricks used for 193 nm immersion today. But EUV was originally justified as a way to avoid these tricks. In that sense, it has already failed its promise.
There are two competitive semiconductor technologies today: FD-FinFETs by Intel and FDSOI by IBM. Major difference is scalability. Based on the semiconductor device physics theory, the FDSOI channel thickness required to suppress transistor leakage current is 7nm for 22nm-node, 4nm for 14nm-node, 3nm for 10nm-node, and 2nm for 7nm-node. Meanwhile, for FD-FinFETs the fin width (equivalent to channel thickness) required is 22nm for 22nm-node, 14nm for 14nm-node, 10nm for 10nm-node, and 7nm for 7nm-node or fin width = gate length, Lg. What a large difference favoring FinFETs! For the 22nm-node FDSOI the channel thickness of 7nm is required while for 22nm FinFETs the fin width as large as 22nm is required to suppress transistor leakage current. That is why Intel’s 22nm FinFETs are in high Volume manufacturing for almost two years, and 14nm will be manufactured at 2014, but FDSOI at 22nm and below will not be manufacturable because Soitec can’t deliver such thin 7nm, 4nm, 3nm and 2nm FDSOI. What Soitec can deliver today is high volume manufacturing of 28nm SOI wafers with minimum 12nm SOI and 25nm buried oxide. FinFETs are not dependent on Soitec wafers.
IBM Patton predicts the next big thing after FinFETs will be carbon nanotubes. But he doesn’t say at what technology node FinFETs will end? Intel Mark Bohr said FinFETs can be extended to the end of scaling. I disagree with Mark. In my opinion it is plausible to manufacture the fin width equal to 7nm, but not below because of the quantum confinement induced device variability. The manufacturability of the 7nm carbon nanotube with possibly 3nm or less nanotube diameter has not been demonstrated yet. The other critical issues are self-heating, source/drain resistance and quantum confinement effects. Skim
These subwavelength optical confinement activities are related to plasmons. The surface plasmon polariton modes in metamaterials is the source of negative refraction. But plasmons have their own well-known limits.
I think that is the reason for the interest in silicon photonics. A group at Northwestern has made a bow-tie shaped 3D metamaterial nanocavity with a negative index of refraction and demonstrated a laser that defies the diffraction limit of light, emitting coherent IR wavelength light from a cavity structure that is much smaller than the wavelength. This shouldn't be possible either, but it is.
Sometimes we think we know, based on the best available science, only to find out later that the science wasn't quite right. Nano-photonics is at this stage, just now finding oddities in optical principles that have been considered sacrosanct since the 19th century.
If photonic chips can be built with the same CMOS process, and it looks like they can, then I think we will be using photons instead of electrons before 2 nm is reached. Yet there remains the possibility that the as of yet untested predictions are not quite accurate, so we won't know for certain where the quantum limit is until we can actually reach it and the theoretical physics versus experimental physics debates are settled.
David Patterson, known for his pioneering research that led to RAID, clusters and more, is part of a team at UC Berkeley that recently made its RISC-V processor architecture an open source hardware offering. We talk with Patterson and one of his colleagues behind the effort about the opportunities they see, what new kinds of designs they hope to enable and what it means for today’s commercial processor giants such as Intel, ARM and Imagination Technologies.