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Magnetic test technique could help ensure microelectronics reliability

lp ensure microelectronics reliability Image courtesy of Gary Meek, Georgia Tech.

Taking advantage of the force generated by magnetic repulsion, researchers have developed a new technique for measuring the adhesion strength between thin films of materials used in microelectronic devices, photovoltaic cells and microelectromechanical systems (MEMS).

The fixtureless and noncontact technique, known as the magnetically actuated peel test (MAPT), could help ensure the long term reliability of electronic devices, and assist designers in improving resistance to thermal and mechanical stresses.

"Devices are becoming smaller and smaller, and we are driving them to higher and higher performance," said Suresh Sitaraman, a professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. "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."

Sitaraman and doctoral student Gregory Ostrowicki have so far used their technique to measure the adhesion strength between layers of copper conductor and silicon dioxide insulator. They also plan to use it to study fatigue cycling failure, which occurs over time as the interface between layers is repeatedly placed under stress. The technique may also be used to study adhesion between layers in photovoltaic systems and in MEMS devices.

According to Sitaraman, the team first used standard microelectronic fabrication techniques to grow layers of thin films that they wanted to evaluate on a silicon wafer. At the centre of each sample, they bonded a tiny permanent magnet made of nickel plated neodymium, connected to three ribbons of thin film copper grown atop silicon dioxide on a silicon wafer.

The sample was then placed into a test 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 have been able to provide an accurate measure of the force required to delaminate the sample. The magnetic actuation has the advantage, they claim, 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, Sitaraman says the technique could also be used to generate a large volume of adhesion data in a timely fashion.

So far, the researchers have studied thin film layers about one micron in thickness, but claim their technique will work on layers that are of sub micron thickness. Because their test layers are made using standard microelectronic fabrication techniques in Georgia Tech's clean rooms, Sitaraman believes they accurately represent the conditions of real devices.

"To get meaningful results, you need to have representative processes and representative materials and representative interfaces so that what is measured is what a real application would face," he said. "We mimic the processing conditions and techniques that are used in actual microelectronics fabrication."

As device sizes continue to decline, Sitaraman beleives the interfacial issues will become more important.

He concluded: "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."

The research was supported by the National Science Foundation and has reported in the March 30 issue of the journal Thin Solid Films.

Laura Hopperton

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