Alexander E. Braun

Considering Beyond-CMOS Metrology
August 11, 2008

Metrology has become one of the main pillars upon which the semiconductor industry bases its progress. Uncertainty rarely transmutes science into technological progress for the simple reason that if something can’t be measured and quantified, it becomes very difficult to direct or control it. The ITRS has done much to define what the needs will be for each node, alerting academia, industry, and others in the metrology community about what the requirements will be and which are the necessary technologies to be developed.

Now, as the semiconductor industry apparently inevitably progresses toward a post-CMOS future that lies in an increasingly steeper nanoscale regime, metrology must also gird itself in preparation for the beyond-CMOS world, which will include new materials being used now and others as yet unthought of, as well as structures and devices that will require manufacturing processes as beyond ours as ours are to cutting rubylith.

At the last SEMICON West, my attention was caught by a presentation authored by Alain Diebold, Empire Innovation Professor of Nanoscale Science at the College of Nanoscale Science and Engineering at the University at Albany (New York).The presentation was done at the ITRS Public review and is the report of the Metrology Technical Working Group. It dealt with “Beyond-CMOS Metrology,” and during it, he called attention to the fact that if we are to go into the exotic jungles of molecular electronics, ferromagnetic logic, and spin logic devices, we are going to have to be able to image what we are doing.

In the case of molecular electronics, we are—and will be—dealing with soft structures made out of carbon, which can be easily damaged by whatever type of microscope is used. With inorganic materials, of which ferromagnetic and spin logic devices would be composed, this particular problem doesn’t exist, but some form of spin microscopy will have to be developed. Things might get somewhat more complicated, in that some of these future devices could contain graphene, again making them vulnerable to some forms of metrology. Carbon nanofilms are very difficult to do microscopy on, yet some kind of high-resolution microscopy will be essential.



Simulations of TEM imaging stacked layers of graphene are used to understand
optimum experimental conditions.  This figure shows the atomic configuration of
A A A stacking of graphene layers used for image simulation.  Source: Florence
Nelson, College of Nanoscale Science and Engineering.

In addition to just microscopy needs, there are measurement requirements unique to nanoscale materials, such as the need to define the quantum confinement affecting them. Are you trying to make a device based on excitons? Then you will have to measure some aspects and qualities of the excitons. There will also be Berry phase effects, such as those observed during carrier transport measurements of graphene by Philip Kim group at Columbia University (New York). In the area of spin, some breakthrough work has been done by Vince LaBella also of the College of Nanoscale Science and Engineering and others, using beam ballistic emission electron microscopy, to obtain information about this state.

Another factor to consider is that as we dive deeper into nanoscale dimensions, band structure can change optical properties. If one is to do an optical measurement, all of these variables must be understood.

New methodologies, such as a novel use of a single-electron transistor to map out where electrons and holes are in graphene are being explored in Amir Yacoby’s group at Harvard University (Cambridge, Mass.). These form “puddles,” a number of holes where there are electrons—carriers puddling together. This is a spectacular example of something predicted in theory that is being measured using a new technique.

The materials area offers innumerable challenges. For example, we will need to find defects in them. Part of this involves learning exactly what effect geometry can have on these new materials; such as how the edge of a graphene structure impacts its band structure. We will have to discover how to look for contamination in these futuristic materials, as well as understand a nanoscopic measurement when it becomes mesoscopic; that is, going from the very localized to the more wide-ranging. Macroscale devices will require mapping in a wider area than has been necessary when we measure structures in very localized places.

These are the things that keep the metrology community awake at night. We need advances in microscopy and metrology for things like optical measurements, local spin, and new things like electron hole puddles.

We already have very sophisticated systems, like transmission electron microscopy—aberration-corrected TEMs. However, getting that same resolution at very low operating voltages to avoid damaging materials like carbon films is not easy. There will be a growing need to link the modeling and simulation; everything from a theoretical calculation of band structure in a nanowire to optical measurements that will be done on an assembly of nanowires. Both extension and invention will be needed.

Meanwhile, the ongoing collaboration between national labs, industry, and academia must continue, with increased focus on the characteristics and effects of new materials’ applications to future devices. These challenges are not going to be met in just a couple of years; they will require a sustained effort.

Posted by Alexander E. Braun on August 11, 2008 | Comments (0)