Quantum leap

by Thomas Ulrich

For more than a half century, the computer revolution has turned on this simple truth: The shorter the distance between two circuits, the faster the computation speed. Engineers built computers with vacuum tubes, discrete transistors and then integrated circuits — those powerful assemblies of silicon transistors at the heart of modern, high-speed computer design.

But as the performance of the silicon-based integrated circuits approaches its fundamental physical and financial limits, researchers in HP Labs have turned to molecular-scale electronics in a quest to create faster, smaller and less expensive integrated circuits. (Read more about how the team is extending the life of Moore's Law.)

In a quantum leap for integrated circuit design, Senior HP Fellow Stan Williams and his team in the Quantum Science Research group have harnessed a chemical reaction that shuttles electrons back and forth between atoms — permitting molecular-size wire junctions to behave like electronic switches, storing data or performing logical functions. As a result, researchers can fabricate these molecular-scale switches more easily and pack them much more densely than conventional silicon ICs.

Less is more

Packing more devices onto the same-size chip is not the only advantage to building an integrated circuit with molecular junctions. Because individual molecules measure less than 10 nanometers (one nanometer is about the size of five atoms in a row), their electrical properties are governed by quantum physics, instead of the classical mechanics that dominate our world. In their nanoscale world, the physical properties of matter, such as melting point, color and electrical resistance abide by the laws of quantum mechanics.

Since 1995, Williams and his team have searched for ways to improve the performance of electronic devices through quantum physics. The team is a unique one, comprised of scientists and engineers from all over the world and in such varied disciplines as chemistry, physics, computer science, materials science and electrical engineering.

"Our most interesting discoveries come from groups of researchers who combine a deep understanding of several fields to solve a problem that no scientist alone could solve," says Williams.

Instead of a single scientific breakthrough, the Quantum Science Research group has experienced a series of eureka moments.

Defect-tolerant architecture

Phil Kuekes stood at the entrance to Stan Williams' cubicle for the first time in 1996. He had led the team that built Teramac, a defect-tolerant computer that performed a trillion operations per second. A computer architect who has built several generations of world-class machines, he is driven to find new ways to boost computer capacity.

"Everything I knew about computer chips was that they were absolutely perfect, and if they weren't perfect, they were thrown away," Williams says. "Then all of a sudden this computer architect shows up and starts telling me about this crazy machine that he built, and by the way, it has 220,000 broken pieces and it still works perfectly."

"Every chip contains some defects," Kuekes admitted to Williams that day. "But, we can correct manufacturing defects by testing for errors, then downloading software to redirect the flow of data."

Kuekes' discovery convinced Williams that he could create electronic circuits from a world dominated by quantum phenomena. "Phil Kuekes gave us permission to think seriously about building circuits with nanoscale devices, even though the second law of thermodynamics tells us that it is futile to build a perfect machine with molecular-scale components."

Working with atoms

While scientists could visualize the atomic structure of surfaces of molecules some 20 years ago, it was not until Stan Williams and Warren Robinett from the University of North Carolina connected a virtual-reality system with a force-feedback haptic interface to a scanning tunneling microscope that scientists could "feel" individual atoms.

Scientists from HP Labs now routinely use a scanning tunneling microscope and an atomic-force microscope to evaluate their nanoscale materials and devices. They can examine the atomic-scale structure of the surfaces and molecular films that are used in the fabrication of their devices to better understand and optimize their function.

But before engineers could build a complete nano-circuit, Williams and his colleagues had to develop entirely new fabrication techniques.

Less costly fabrication

They developed and adopted an advanced system of manufacturing called nano-imprint lithography -- essentially a printing method that allows inexpensive fabrication of nanoscale features across an entire wafer.

Using this technique, the team was able to fabricate circuits based on HP's patented cross-bar architecture, a checkerboard pattern of horizontal and vertical wires. In 2002, researchers successfully trapped molecules between platinum and titanium wires arranged in an eight-by-eight array within a one square micron area. Each junction performed like an electronic switch — a first step toward creating a working logic or memory device.

Growing nanowires

More recently, scientists "grew" nanoscale wires between two electrodes by exposing a catalytically active particle to gas containing silicon atoms. (Read related paper.)

"It is difficult to make contact between the wire and the electrodes at such a scale," says Ted Kamins, a Distinguished Scientist in the Quantum Science Research group. "So we took the opposite approach: Building the electrodes first and then growing the nanowires between them."

Lateral epitaxial nano-bridges grown across trenches connecting to opposing sidewalls of silicon.

This self-assembling silicon nano-bridge may allow researchers to integrate a variety of sensors into conventional circuitry. Integrating the sensors with the computer makes sense because of the computing power needed to interpret all the information that sophisticated sensors can deliver.

Sensors could, for example, be used to detect toxic gases or to determine the mixture of fuel and air in a carburetor. Another team within the Quantum Science Research group has used a different technique to build a nanowire sensor that can detect complementary fragments of DNA.

The next step is to combine both techniques to create sensors that can detect minute concentrations of biological and chemical materials.

"We have achieved an important goal by looking over the horizon," Williams says. "With these prototypes, designers can begin to integrate molecular scale concepts into products." Although HP will not deliver a nanoscale product immediately, product groups can begin to use the fabrication techniques Williams’ team has developed to manufacture micron-scale components less expensively.

Changing the rules
	Stan Williams and his colleagues at HP Labs are working to extend the life of Moore's Law, a driving force in integrated circuit design first described by Intel co-founder Gordon Moore in 1965. Moore's observation states that the number of transistors fabricated on an integrated circuit doubles every 18 to 24 months. Solid-state microelectronics has followed that performance curve for more than 40 years, but as conventional transistors approach 30 nanometers, they will become extremely expensive to produce. Scientists from the Quantum Science Research group have developed molecular junctions and a process for fabricating them that are as simple and inexpensive as contact printing. Using nano-imprint lithography and their technique for depositing molecular monolayers, scientists from HP Labs hope they will be able to fabricate and demonstrate crossbar circuits with wire-to-wire separation of less than 30 nanometers. That would yield a switch packing density approximately 120 times greater than present-day integrated circuits. "I think we've picked a winner — something that will allow Moore's Law to continue for another half century," Williams says. "I used to think that was impossible. Now I think it is inevitable."

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An 8 by 8 crossbar with 40 nanometer wires fabricated using imprint lithography