Beyond Silicon: What's Next?
Silicon still has a lot of life, and will remain the primary platform for integrated electronics well into the future. It will get "tweaked" in many ways in an effort to improve performance and reduce leakage. One of the most important tweaks coming down the road is the integration of III-V materials into the channel regions of transistors, which can greatly enhance mobility.
The second feature in this issue describes work at SEMATECH that addresses three of the most important "grand challenges" in incorporating III-V materials in the channel region of nMOS transistors: improving the high-k/III-V interface, selecting a compatible ultrashallow junction formation technology, and creating a manufacturable process flow.
We surround this story with two related articles: the cover story describes work at Micron into new doping technologies for future device generations, and a third feature describes work at IMEC into high-k metal-gate processing, contrasting gate-first and gate-last options.
In this post-recession period, many are concerned about the escalating costs of R&D and the ever higher costs of semiconductor manufacturing. The cost of next generation lithographic tools alone makes one wonder if only a handful of companies will have the wherewithal to invest in the leading edge technology needed to keep Moore's Law alive.
But wait a minute. There's no rule that says new technologies need to be more expensive. What if some of the new technologies now being developed are less expensive than what's now commonly used? I don't believe that advanced semiconductors will ever be fabricated in the back of a semi-trailer truck, but I believe it's highly likely that lower cost alternatives to some of today's highest priced process technologies are on the horizon.
A great example is shallow junction doping. It's now done in huge and expensive ion implantation machines, but alternatives now in fairly advanced stages of development could provide a much lower cost solution, with better results.
Our cover, for example, depicts a plasma doping (PLAD) tool that not only costs less than conventional technology, but provides a higher throughput and ultimately better device performance. The author does a head-to-head comparison of PLAD and other advanced boron-based ultra-low emerging doping techniques, including beam-line and cluster implants. PLAD is the hands-down winner.
SEMATECH also investigated alternative doping technologies that will likely be required for integrated III-V channel transistors. One of the most promising is mono layer doping (MLD), which offers low-cost, high uniformity and low damage. Also, existing toolsets can be used and, in contrast to ion implantation, MLD does not introduce lattice damage. Another promising approach is selective regrowth. In this technique, either MBE or MOCVD is used to selectively regrow very heavily doped source/drain regions.
Even in the field of lithography, lower cost solutions are being pursued. Although EUV now appears to be the technology of choice, despite its high cost and need for further development of mask and inspection techniques, a group of researchers are working to make multiple-column e-beam (MEB) lithography a reality. Championed by TSMC's Bern Lin and the focus of work at CEA-Leti, and tool maker Mapper Lithography NV (Delft, Netherlands), MEB could be a high throughput, lower cost alternative. It would has a much smaller footprint than EUV, and consumes significantly less power. Nanoimprint lithography is also progressing and is increasingly viewed as a viable candidate, at least for some layers.
One can imagine many other potentially better-performing yet lower cost options moving forward, including nanotechnologies such as molecular self-assembly, where complicated structures are grown from the ground up. Companies are working to incorporate carbon nanotubes into volume manufacturing. Nantero, for example, has developed a method for positioning carbon nanotubes reliably on a large scale by treating them as a fabric that can be deposited using spincoating, and then patterned.
Inexpensive fabrication techniques such as inkjet printing, roll-to-roll web processing and imprint lithography are also quickly advancing for some applications, such as compact discs and signage. As experienced is gained in these areas, those technologies could eventually make their way into more advanced electronic applications.
In short, new technologies are not necessarily more expensive or more prone to variations and yield-limiting defects compared to existing approaches. Just the opposite. Whether or not the historically conservative semiconductor industry can embrace them is another story.