Brown-developed sensor bares faults in smallest electrical components

The magnetic-sensing microscope allows Brown researchers to watch electricity flow through the world’s tiniest components. They are using the device to find defects in integrated circuits and micromachinery. The design opens the door to wider application of magnetic-sensing technology for imaging electrical current flow.

by Scott J. Turner

Two Brown physicists have created a microscope that allows them to witness electricity flow through the world’s tiniest components.

The new device uses a magnetic sensor to uncover defects in the smallest and most complex integrated circuits at a resolution 1,000 times greater than current technology. It removes a barrier to further shrinking of integrated circuits: As chips get smaller, non-visual defects become harder to find.

“This microscope will allow manufacturers to find defects in each embedded wire in ever-tinier circuits,” said Professor Gang Xiao. He developed the instrument’s hardware and software with Ben Schrag, who will receive his Ph.D. this month.

A current-density image, taken with a Circuit Scan 1000 high-resolution magnetic microscope, reveals a tiny flaw in one of two 0.25-micron metal wires in an integrated circuit chip. Further magnification (detail) shows the right-hand wire has a "mouse bite" along one of its edges, where the electrical current shows up as a tiny hot spot.

Although magnetic sensing is used extensively, it is not applied widely for imaging electrical current flow, said Schrag. The only method that uses magnetic imaging to see current flow is restricted to extremely low temperatures, employing cryogenic aids such as liquid nitrogen.

The Brown device scans chips at room temperature, which opens the way to greater use of magnetic sensing technology. The researchers envision a small, non-invasive form of remote detection, such as a “pass-over and detect” pen tipped with a magnetic sensor, for use in finding internal cracks within aircraft, sensing biological agents in the environment or body, or recognizing counterfeit bills or other objects.

Until now, when an integrated circuit failed, a manufacturer had to rip the chip open or study a test model to find out what went wrong. The microscope scans chips intact.

“Where this is so valuable is that it is a non-destructive method,” said Jim R. Lloyd, a research staff member at IBM and a specialist in electron-induced atomic migration in semiconductor integrated circuits. “This technology also provides the opportunity to see how damage formed in real time in real chips.”

Previously, little or no technology existed for actually “watching” electrical current flow, said Schrag. Whenever current runs through wires, such as those embedded within the semiconducting material of an integrated circuit, it creates a magnetic field. By measuring spatial changes in that magnetic field, the microscope visualizes electrical current, even within wires buried under layers of advanced materials, he said.

The microscope features some of the same magnetic-scanning technology found in computer hard drives. A scanner does not touch what it reads. Instead, a magnetic sensor the size of a petite pea moves quickly back and forth over a circuit through which current flows. The sensor collects information, which is converted by algorithms into a color picture of electron flow. Color changes in the image reflect the intensity of electron flow as well as the presence of defects.

“The device allows us to see the evolution of hot spots on each wire in a circuit and how each defect moves down the wire in the form of electrons moving atoms,” said Xiao. “To see a collection of atoms moving as a function of time is a capability that did not exist until now. We are witnessing the flow of electricity. It appears similar to an image of human blood flowing.”

Xiao and Schrag have filed patents on several aspects of the technology, which has been transferred to Micro Magnetics, a Fall River, Mass., company. The firm makes scanning devices for manufacturers of integrated circuits (computer chips).

Integrated circuits are tiny electronic devices made out of semiconductor material. They are used in all sorts of gadgets such as microprocessors, video and audio equipment, cars and other items. Some integrated circuits may contain more than one million electronic components.

The National Science Foundation funded this work.