Ed’s Threads 070824
Musings by Ed Korczynski on August 24, 2007

Intel finds signs of heterogeneous life after silicon
With CMOS circuits using silicon channels running into performance limitations, doping with germanium and straining the lattice have already been used to push the limits. The limits of silicon remain, though, and so compound semiconductors are again under consideration as building blocks for mainstream ICs—this time integrated on top of 300mm silicon wafers.

The first solid-state transistors were built with germanium (Ge), but Ge crystals do not grow a nice surface-passivating oxide as do silicon crystals, and the oxide is vital to prevent surface defects that would otherwise kill performance. Grown silicon oxide has been the heart of cost-effective high-volume IC production for over 40 years. Even high-k (HK) gate dielectrics based on hafnium-oxides will typically be used on top of a grown and partially etched-back silicon oxide layer.

What comes after high-k and metal-gates (HK+MG)? Heterogeneous integration of compound semiconductors will completely replace the silicon channel. Silicon wafers would still be used, but only as physical supports for different NMOS and PMOS transistor materials. The new channel materials will be deposited and/or grown using technologies such as metal-organic CVD (MOCVD), solid-phase epitaxy (SPE), and spike/flash anneals.

Though HK+MG has been spun as the biggest change in 40 years, when compound semiconductors are integrated with silicon it will be a far greater integration challenge for it will require an even greater number of new materials. Now Intel researchers have reported passing a milestone along the road many travel to heterogeneous integration: integrated superior device performance.

This summer, Intel researchers achieved success at fabricating high performance devices using both indium antimony (InSb) and indium gallium arsenide (InGaAs) channels, and in each case they perform as well as their counterparts on GaAs wafers. On their internal blog, they show curves for the performance of both types of devices, with individual transistors both higher performing and consuming less power than equivalently sized silicon devices.

Mike Mayberry, Intel’s director of components research, has articulated five general areas of research and develop needed to create a competitive commercial heterogeneous semiconductor fab technology:
1) put compound semiconductors on silicon wafers,
2) find different high-k dielectrics for the gates,
3) develop PMOS materials to go with known NMOS,
4) develop enhancement-mode devices instead of depletion-mode, and
5) “Make them small enough to compete with silicon transistor densities.”

The last is the tricky part. These circuits are conceived of as being possible at or past the 22nm node, so that means high-yielding dense 32nm and likely 22nm CMOS circuits will already be in production using strained SiGe. Heterogeneous circuits will have to compete with Si/SiGe chips on price, so transistor packing density will be crucial.

By the time InSb and InGaAs could be integrated on silicon wafers, finFETs should also be options along with planar transistor structures. In an email exchange with me after his initial blog posting, Mayberry answered some questions relating to finFETs as possible structures for heterogeneous integration. “It is possible that in the timeframe we’re discussing that trigate structures could be a strong candidate and thus, that builds pressure to find a vertical solution for III-V,” he confirmed, but he admitted that, “we do not as yet have a solution to that problem.”

If finFETs are used, new metal alloys will need to be developed to be able to form minimal resistance electrical contacts to these new materials. Even if planar structures are used, the contacts will almost certainly be to the channel material instead of silicon to avoid the need for a >5nm transition zone from material to material that would otherwise limit packing density. Due to the need for this transition layer in planar devices with silicon contacts, and the performance advantages to a reduced number of boundary layers, it seems likely that metal contacts will be direct to the new compound semiconductor materials.

In my email exchange with Mayberry, he confirmed only that “we have not decided how to make electrical contact for those future devices.” One thing he knows—and will let us know—is that for Intel’s current work, SOI would just get in the way and everything is grown on bulk silicon wafers.

This is all starting to get into some serious materials engineering. Mayberry’s blog pre-announces that they will show enhancement-mode device results using these materials at IEDM this December. Since these newest devices will be “normally off” they should result in minimal leakage and reduced power-consumption for circuits. We can almost certainly expect unexpected subtle second-order integration challenges with all of these new materials, with the semiconductor industry now moving into “interesting times.”

–E.K.

Labels: compound semiconductor, heterogeneous, IC, integration, Intel

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070824: Intel finds signs of heterogeneous life after silicon

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