Nanofabrication: past, present and future

Editorial

It is a late afternoon in July 2011 and I sit “after work” in a restaurant in Western Massachusetts. It is unbearably hot and humid out, but I love it. I am pondering how to introduce the topic of this special edition of Journal of Materials Chemistry. I am the guest editor of this edition and in this special issue we cover “Materials Chemistry of Nanofabrication”. I find myself thinking about the past of nanoscience & nanofabrication and the future.

In many ways it is amazing how the general concepts of nanotechnology have been accepted by the population at large. In fact, there have been few other major arenas that have so quickly been adopted into the mainstream consciousness; space exploration and nuclear science come to mind. Interestingly, if you were to say the word “nanotechnology” to your average person on the street in London, Tokyo or New York some 20 years ago, you would get a confused blank stare. So I get an idea—I call over several members of the restaurant staff—young people, servers and the barkeep, and I ask them, “What is nanotechnology”? The first response I receive is from a young woman who asks me (while laughing) if I was from the University. I say yes, and then I begin to listen to the other replies. “It is the science of small things”, they all agree on this. “It is the next big industry”; “It is scary, mind control stuff”; “It is the development of small things that will go into the body and fix things”… The conversation continues for a couple of minutes and then they wander off back to their duties. This interchange confirmed my suspicion that the average person has heard of nanotechnology, and that most people associate it with the future, science, and advanced applications. This is something at least.

Nanofabrication is by definition the fabrication of things on the nanoscale. The term is derivative of its precursor, microfabrication. Microfabrication, in turn, finds its roots squarely in the semiconductor industry. In the 1960s, well before most people could comprehend the concept of a handheld calculator or a simple digital watch (much less a smart phone), scientists and engineers were feverishly working on ways to shrink the size of integrated circuits. In fact, the statement by Gordon Moore in 1965 (which was later elevated to legendary status by others as “Moore's Law”) simply described the fact that the transistor counts up until that point had doubled every year. I think it is accurate to say that the continued miniaturization of the integrated circuit is one of the greatest accomplishments in materials science of the 20th century. None of this progress would have been possible without significant developments in materials chemistry. The workhorse of microfabrication is photolithography, the use of light, projection optics and photosensitive polymer layers (photoresists) to produce the tiny patterns that are converted into circuitry. Early photoresists consisted of simple photocurable prepolymers that could be crosslinked into patterns in the millimetre size regime. As critical feature dimensions continued to shrink sub millimetre, new materials were needed. UV sensitive resins based upon Novolak chemistry were developed and could be used to create patterns down to about the micron level. In order to push to submicron patterns, completely new chemistry was needed and in the early 1980's Frechet, Willson and Ito introduced the concept of “chemical amplification”¹ and this technology enabled sub-micron photolithography. In the intervening 30 years, the basic core concepts have remained unchanged and continued incremental developments have pushed the critical dimensions sub-100 nm—a size scale commonly accepted as “nanoscale”. In fact, leading edge semiconductors are currently shipping with the smallest sized critical features at 32 nm, and 22 nm technology is scheduled for production within a couple of years! This will be accomplished with photoresists and UV exposure tools—not any exotic breakthrough technology developed over the last 30 years.

I only mention this abbreviated history for two important reasons: (1) it demonstrates that the largest and most significant implementation of nanofabrication can generally be considered the result of a steady stream of continual incremental improvements of existing technology and (2) these developments have been pursued at high cost and investment solely by one industry, the semiconductor industry. I apologize that in my simplification I have managed to ignore all of the other significant developments that have accompanied the miniaturization of the integrated circuit: e.g. alternative mask lithography techniques, optics, metrology, microscopy and other imaging tools and various new fabrication methodologies.

Let me jump to 2001. I remember noting with great interest when Science magazine announced its selection of Breakthrough of the Year was “Nanocircuits”.² In their explanation for their choice they stated, “The advances have come in several different fields: scanning probe microscopes, technologies for producing carbon nanotubes and nanowires made of various materials, and new organic materials that lend themselves to conducting assemblies. Together, these ways of creating and working with molecular-scale structures have combined to give us the Breakthrough of the Year. However, the Breakthrough is not for the devices themselves, although the work that has produced them deserves high praise. It is for the extraordinary accomplishment of arranging them into circuits that can actually perform logical operations: amplify signals, invert current flows, and even perform simple computing tasks.” I think the best line in their announcement is the realization that “Getting from here to there is apt to be a long and bumpy road, because production scales and economic costs are likely to be formidable. Indeed, we have no idea what a fabrication facility in a post-silicon nanoworld might look like!” Now 10 years later, that last statement could not be truer. We are not in a post-silicon world and in fact, not much closer to realizing that goal. The semiconductor industry has rambled on and continues to break records in miniaturization (and notably continues to break records for fabrication facility cost!).

However, another important realization was made about 10 years ago. A different industry realized that nanofabrication would be crucial to their future products. The magnetic recording industry saw that future products would be hampered unless nanoscale patterning and fabrication were enabled.³ In fact, the critical dimensions needed for magnetic storage for the first time actually exceed the specifications for the IC semiconductor industry. A completely new nanofabrication process was needed and there is general agreement in the industry that patterned magnetic media will be the solution to this problem.⁴

When the initial concepts of nanofabrication were born, and the exploitation of materials properties on the nanoscale were predicted, the general presumption was that these advanced technologies would be developed for, and used primarily for, information technology applications—mainly integrated circuits and computing. However, as developments continue, new applications and needs have emerged. This current issue is a reflection of these trends. In fact, of the 26 contributions for this edition, the majority do not concentrate on technologies tailored specifically for the semiconductor industry. Another interesting development is that whereas the semiconductor industry continues to support fabrication technologies that can tolerate a final cost of > $25 000.00 per square meter of product (conventional silicon wafer fabrication), new nanofabrication techniques are being developed to meet goals closer to $25.00 per square meter (such as roll-to-roll fabrication). How can this be? The realization that top-down patterning and assembly approaches can be replaced or merged with bottom-up self-assembly processes has led to a completely new way of thinking about nanofabrication. Significant challenges to implementation and commercialization exist, but if successful, a completely new paradigm in manufacturing may be realized—one where sustainability, low cost, and high value products can coexist.

I am excited about this issue and the advances that it highlights. Topics include:

• The assembly and manipulation of nanoparticles, nanowires and nano objects

• Advances in nanofabrication techniques and materials

• New routes towards device fabrication

• New device architectures

• High speed, high throughput or low cost nanofabrication

• The use of self-assembled polymeric materials in fabrication

• Modeling and simulation of nanoscale materials

• Characterization and metrology

It will be fun to look back in 10 more years to see where this research has taken us and what the new future challenges will be. I wonder what the impact will be of our work upon our society? It is an exciting time for materials chemistry!

References

1. J. M. J. Frechet, E. Eichler, H. Ito and C. G. Willson, Poly(Para-Tert-Butoxycarbonyloxystyrene) - A Convenient Precursor To Para-Hydroxystyrene Resins, Polymer, 1983, 24(8), 995–1000.
2. D. Kennedy, Breakthrough of the year, Science, 2001, 294(5551), 2429–2429.
3. D. Weller and A. Moser, Thermal effect limits in ultrahigh-density magnetic recording, IEEE Trans. Magn., 1999, 35(6), 4423–4439.
4. C. Ross, Patterned magnetic recording media, Annu. Rev. Mater. Res., 2001, 31, 203–235.

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