Friday, January 11, 2008

"MEMS: Measurement scheme could also boost chip yields"

Most instruments used to measure properties in microelectromechanical systems (MEMS) are far too big to determine standard physical properties such as stiffness. To remedy this, the National Institute of Standards and Technology (NIST) has developed a set of testing procedures for MEMS-sized structures using optical non-contact instrumentation. Besides MEMS makers, NIST claims that CMOS semiconductor makers could also take advantage of its measurement regime to increase wafer yields by reducing the frequency of failures. NIST has encapsulated much of its non-contact optically based measurement expertise in a Web-based MEMS Calculator. The approach enables engineers to plug in measured values made with readily available optical interferometers to determine standard mechanical properties. NIST engineers also contributed to a mechanical version of the regime based on the American Society for Testing and Material (ASTM) standard E 2245. The entire testing regime enables measurement the residual stress in thin films using SEMI Standard MS4-1107. These results, NIST claims, can aid in design strategies, fabrication techniques and post-processing methods to increase yields by reducing failures from electromigration, stress migration and delamination. Stress is measured using a mechanical engineering standard called Young's modulus, which determines a material's elasticity and stiffness. For macroscopic objects, Young's modulus can be determined by pushing on both ends of a beam, then measuring how far the beam deflects. Since chip measurements don't allow physical contact, the NIST technique instead uses an optical vibrometer that measures the vibration frequency of a thin film. Young's modulus can then be used to calculate the thin film's resonant frequency, thereby allowing the chip maker to predict how the thin film will react under various loads. Standard MS4-1107 is available from the Semiconductor Equipment and Materials International.