Bob Michaels

April 16, 2012

3 Min Read
Magnetic Test Could Help Improve Reliability of Medical Electronic Devices

The magnetically actuated peel test developed by Georgia Tech researchers tests the stresses to which microelectronic chips are subjected. (Image courtesy of Greg Ostrowicki and Suresh Sitaraman)

Engineers at the Georgia Institute of Technology (Georgia Tech; Atlanta) are exploiting the force generated by magnetic repulsion in an effort to develop a new technique for measuring the adhesion strength between thin films of materials used in microelectronic devices, photovoltaic cells, and microelectromechanical systems (MEMS). Known as the magnetically actuated peel test (MAPT), the researchers' fixtureless and noncontact technique could eventually help ensure the long-term reliability of electronic devices and also assist designers in developing electronic devices with improved resistance to thermal and mechanical stresses.

"Devices are becoming smaller and smaller, and we are driving them to higher and higher performance," remarks Suresh Sitaraman, a professor at Georgia Tech's George W. Woodruff School of Mechanical Engineering. "This technique would help manufacturers know that their products will meet reliability requirements and provide designers with the information they need to choose the right materials to meet future design specifications over the lifetimes of devices."

Fabricated from insulator and conductor layers, microelectronic chips are susceptible to thermal stress that can be created when heat generated by electronic devices causes the materials of adjacent layers to expand. This stress can cause the layers to delaminate, leading to microelectronics failure.

Together with doctoral student Greg Ostrowicki, Sitaraman first used standard microelectronic fabrication techniques to grow layers of thin films on a silicon wafer. At the center of each sample, they bonded a tiny permanent magnet made of nickel-plated neodymium connected to three ribbons of thin-film copper grown on top of silicon dioxide. The sample was then tested in a station consisting of an electromagnet below the sample and an optical profiler above it. Voltage supplied to the electromagnet was increased over time, creating a repulsive force between the like magnetic poles. Pulled upward by the repulsive force on the permanent magnet, the copper ribbons stretched until they finally delaminated.

With data from the optical profiler and knowledge of the magnetic field strength, the researchers can provide an accurate measure of the force required to delaminate the sample. The magnetic actuation has the advantage of providing easily controlled force consistently perpendicular to the silicon wafer.

Because many samples can be made at the same time on the same wafer, the technique can be used to generate a large volume of adhesion data in a timely fashion. However, because device failure often occurs gradually over time as the layers are subjected to repeated heating and cooling cycles, the Georgia Tech researchers plan to cycle the electromagnet's voltage on and off. "A lot of times, layers do not delaminate in one shot," Sitaraman notes. "We can test the interface over hundreds or thousands of cycles to see how long it will take to delaminate and for that delamination damage to grow."

Thus far, Sitaraman and his team have studied thin film layers about 1 µm in thickness, but they say that their technique will work on submicron-thick layers as well. Because their test layers are made using standard microelectronic fabrication techniques in Georgia Tech's cleanrooms, Sitaraman believes that they accurately represent the conditions of real devices.

As device sizes continue to shrink, Sitaraman says the interfacial issues will grow more important. "As we continue to scale down the transistor sizes in microelectronics, the layers will get thinner and thinner. Getting to the nitty-gritty detail of adhesion strength for these layers is where the challenge is. This technique opens up new avenues."

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