Terahertz Chips could make Portable Medical Scanners

Microphotos of high-frequency CMOS chips developed in Ehsan AFshari’s lab: above, an oscillator circuit; below, a frequency doubler. Large white areas in the images are contact points where external wires are attached to the chip. At radio frequencies, the length of wires is critical A new mathematical analysis enables researchers to design circuit elements to get the best possible performance. Courtesy of Afshari Lab

Terahertz radiation, used in airport body scanners, promises a wide range of applications in science and medicine, from detecting cancer and tooth decay to inspecting food through its packaging. Its range of wavelengths — between microwaves and infrared light — penetrate cloth, paper and leather and a very short distance into the skin — all without the damaging effects of X-rays. Terahertz devices also can detect unique signatures of explosives.

Such applications require a portable, low-power radiation source, but most terahertz sources are still bulky and expensive, usually involving lasers and vacuum tubes. Cornell researchers have demonstrated new ways to generate signals in the lower end of the terahertz range on a microchip at 10,000 times more power than previously possible, with the inexpensive CMOS chip technology used in many everyday electronic devices.

Solid-state terahertz devices could range from hand-held medical scanners to portable weapons scanners for the military, said Ehsan Afshari, assistant professor of electrical and computer engineering, who reported on new approaches to generating high-frequency signals at the 2011 IEEE International Solid-State Circuits Conference February 22, 2011, in San Francisco. A paper on related work appears in the March 2011 issue of the IEEE Journal of Solid-State Circuits.

The maximum frequency at which a chip can operate and the power it can put out are limited by the physical characteristics of the material. Oscillator circuits seldom reach the maximum possible frequency or power, said Afshari.

The best previous effort on a CMOS chip generated a signal at 410 GHz with an output power of 20 nanowatts (billionths of a watt). Using new techniques, Afshari has built CMOS oscillators operating at up to 480 GHz with an output of 0.2 milliwatts (thousandths of a watt) — 10,000 times higher power. These are still very low-power signals, roughly comparable to Bluetooth devices, but enough for medical instruments that might be held close to the skin.

“We broke the record, but it’s more important than that,” Afshari said. “Nobody can break our record because we have a method that can look at any given process and come up with a topology that can guarantee the maximum power and frequency.”

At radio frequencies, the length and shape of wires and other components are critical. Afshari and graduate student Omeed Momeni developed a mathematical analysis to calculate the characteristics of these components that would achieve the highest possible frequency and power on a given chip material.

The next step, Afshari said, will be to work with Cornell researchers who are familiar with gallium nitride, a material capable of operating at much higher frequencies and with power levels up to 2,000 times more than can be handled by silicon. Cornell is considered a world leader in gallium nitride research, he noted. Computer simulations, Afshari said, indicate that a gallium nitride device could generate frequencies up to 1 terahertz with enough power to scan a 1-meter-square area 10 meters away, with resolution down to 1 square centimeter — more than adequate for a soldier or police officer to scan an approaching stranger for weapons.

The research was supported by the Semiconductor Research Corporation through the Center for Circuit & System Solutions and by the National Science Foundation. Chips were manufactured through the Taiwan Semiconductor Manufacturing Company University Shuttle Program.