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Spectroscopy Measures Nanoscale Material Strain

LAFAYETTE, Ind., Aug. 28, 2014 — A new spectroscopy technique could lead to microelectronics that perform better under stress and heat.

A team from Purdue University developed the technique, called nanomechanical Raman spectroscopy.

Raman spectroscopy can provide information about the chemical makeup of a material. “But we have not been able to incorporate in-situ stress or deformation into those chemical signatures,” said Dr. Vikas Tomar, an associate professor in Purdue’s School of Aeronautics and Astronautics. “Now we have combined nanomechanical measurements into Raman spectroscopy.”

A new research platform uses a laser to measure the “nanomechanical” properties of tiny structures undergoing stress and heating, an approach that could help improve designs for microelectronics and batteries. Courtesy of Ming Gan/Purdue University.

The new technique was used to look at silicon cantilever beams about 7 µm thick and 225 µm long. These beams were heated and stressed simultaneously.

The applied stress was controlled by a nanomechanical loading system, and micro-Raman spectroscopy was used to measure the surface stress as the samples deformed. The surface stress was found to be lower than the applied stress at all temperatures, according to the study.

Heating the cantilever to 100 °C while also applying stress to the structure, which influences mechanical properties, was found to cause a dramatic increase in deformation. Specifically, heating reduced bonding forces between atoms on the surface of the structures.

“Now we can understand as the material is deforming how the interface stresses are developing, and this will allow us to predict how to modify them,” Tomar said.

The findings are significant as silicon structures measured on the microscale and nanoscale form essential components of semiconductor processors, sensors, batteries and microelectromechanical systems (MEMS).

“The functioning of such devices has been found to be highly affected by their operating temperature,” Tomar said. “Such densely packaged devices generate considerable heat during operation. However, until now we have not been able to measure how heating and surface stress contribute to mechanical properties.”

Purdue has filed a patent application for the new platform. The work was funded by the National Science Foundation.

The research was published in Applied Physics (doi: 10.1063/1.4892623).

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