New Magnetic Testing Technique Boosts Reliability

With electronic devices being pushed toward ever-higher levels of performance, it's little wonder that reliability has emerged as a major concern. Modern microelectronic chips are fabricated from multiple layers of different materials (insulators and conductors). Thermal stress can occur when heat generated during the operation of the devices causes the materials of adjacent layers to expand at different rates, causing the layers to separate. This process, known as delamination (or de-bonding), is a major cause of microelectronic device failure.

But thanks to a research team from the Georgia Institute of Technology (Atlanta, GA), a new safeguard may be on the horizon. Suresh Sitaraman, a professor at Georgia Tech's George W. Woodruff School of Mechanical Engineering, along with doctoral student Gregory Ostrowicki, 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) devices (Fig. 1).

The fixtureless and noncontact techniqueknown as the magnetically actuated peel test (MAPT)could help ensure the long-term reliability of electronic devices, assisting designers in improving durability under thermal and mechanical stress (Fig. 2 and Fig. 3). First reported in the March 30, 2012 issue of the journal Thin Solid Films, Sitaraman and Ostrowicki's work has received support from the National Science Foundation.

The Georgia Tech researchers used their technique to measure the adhesion strength between layers of copper conductor and silicon dioxide (SiO2) insulator. Using standard microelectronic fabrication techniques, they grew layers of thin films to evaluate on a silicon wafer. At the center of each sample, they bonded a tiny permanent magnet made of nickel-plated neodymium (NdFeB), connected to three ribbons of thin-film copper grown atop SiO2 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 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 and on the same wafer, the technique can be used to generate a large volume of adhesion data in a timely fashion. The test station is small enough to fit into an environmental chamber, allowing the researchers to evaluate the effects of high temperature and/or high humidity on the strength of the thin-film adhesion. This is particularly useful for electronics that are intended for harsh conditions, including automobile engine control systems and aircraft avionics.

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