MEMS Young’s Modulus and Step Height Standards Validated
Two more MEMS standard test methods have recently been validated with round robin results. They were published in October 2009 and become available on CD-ROM in November 2009. One of the test methods is for measuring Young’s modulus (SEMI MS4) and entitled, “Test Method for Young’s Modulus Measurements of Thin, Reflecting Films Based on the Frequency of Beams in Resonance.” The other is for measuring step heights (SEMI MS2) and entitled, “Test Method for Step Height Measurements of Thin Films.” Written by the National Institute of Standards and Technology (NIST) staff member Janet Marshall in the Semiconductor Electronics Division of the Electronics and Electrical Engineering Laboratory, the original unvalidated test methods were published in 2007.
To validate the standards, a MEMS Young’s Modulus and Step Height Round Robin Experiment was held with eight participants where measurements were taken (following the steps in the original standards) on test chips designed and fabricated specifically for this work. The results were analyzed and incorporated into the standards, after which they were successfully re-balloted in July 2009. The technical basis for the two test methods and the data from the round robin experiment will be provided in an upcoming NIST publication entitled, “MEMS Young’s Modulus and Step Height Measurements with Round Robin Results.”
The Young’s modulus standard applies to thin films, such as found in MEMS materials, which can be imaged using a non-contact optical vibrometer, stroboscopic interferometer, or comparable instrument. It is based on the average resonance frequency of a single-layered cantilever oscillating out-of-plane. Young’s modulus measurements are an aid in the design and fabrication of MEMS devices and ICs. For example, knowledge of Young’s modulus values can lead to calculations of residual stress, which can, in turn, contribute to circuit design strategies and fabrication and post-processing methods that help increase fabrication yield by reducing the frequency of failures from electromigration, stress migration, and delamination for the $100 billion semiconductor industry.
The step height standard applies to thin films, such as those found in MEMS materials, which can be accurately imaged using an optical interferometer or comparable instrument with the capability of obtaining topographical 2-D data traces. Step height measurements can be used to determine thin film thickness values that are an aid in the design and fabrication of MEMS devices and can be used to obtain thin film material parameters, such as Young’s modulus, the material parameter mentioned above that has been eluding the MEMS community for many years. SEMI MS2 for step height measurements is instrumental in determining the cantilever beam thickness required for the SEMI MS4 Young’s modulus calculations.
In support of these new validated standards, quick and easy calculations can be performed on the NIST Semiconductor Electronics Division Web site (http://www.eeel.nist.gov/812/test-structures/MEMSCalculator.htm) which also includes the resources and data calculation pages necessary for four previously published MEMS test methods for measuring in-plane lengths (ASTM E 2244), residual strain (ASTM E 2245), strain gradient (ASTM E 2246), and wafer bond strength (SEMI MS5).
The two recently published standard test methods are under the jurisdiction of the SEMI North American MEMS Committee and are the direct responsibility of the MEMS Materials Characterization Task Force.
In addition, work is starting on developing a technology roadmap for MEMS, a challenging task given the widely varied MEMS industry. An initial template draft can be found at www.semi.org/memsroadmap. SEMI has published six other MEMS standards and is looking forward to contributions from professionals like you.
Visit the MEMS Standards Committee homepage at www.semi.org/MEMSCommittee, where the SEMI Standards MS2-1109 and MS4-1109 are available for purchase. For more information on MEMS Committee activities, please contact Susan Turner at email@example.com.
About the Author
Janet C. Marshall received a B.S. Degree in physics from Muhlenberg College, Allentown, PA in 1978, and an M.S. degree in electrical engineering from Lehigh University, Bethlehem, PA in 1981. She worked at Bell Laboratories, Allentown, PA from 1978 to 1981 and has been with NIST since then. Her current interests are with MEMS, specializing in the area of standardization.