NEMS Measurement Science

Nanoelectromechanical systems (NEMS) offer many new capabilities in sensing and electronics that are expected to provide significant advances in consumer products, medical diagnostics, and homeland security and defense systems. In order to realize the promise of NEMS technology, the manufacturing of these systems must be scaled up and the resulting cost, quality, and reliability of these mechanisms must be improved. One major limiting factor in achieving this transition to high-throughput manufacturing is the lack of measurement science related to the inspection and dynamic characterization of NEMS. As a result, this project focuses on the development of optical methods for NEMS displacement measurements and dynamic characterization approaches based on these optical methods that can be used to quantify the quality of manufactured NEMS, detect defective devices before packaging, and provide feedback for manufacturing process control, all at the wafer level for high-throughput nanomanufacturing.

To develop the measurement science for enabling high-throughput nanomanufacturing of nanoelectromechanical systems (NEMS) with exceptional yield and low cost and to accelerate the introduction of NEMS into U.S. sensor and electronics markets by 2014.

The field of NEMS has developed over the last decade through a merging of microelectromechanical systems (MEMS) and nanotechnology.Research on nanoscale sensors represents approximately one third of all nanotechnology R&D^[1] and has been identified by the National Nanotechnology Initiative (NNI) as a "tremendous opportunity to realize revolutionary advances in application areas ranging from medicine and health to national security." ^[2] NEMS sensors are capable of unprecedented sensitivity (e.g., zeptogram-resolution mass measurement^[3]) due to the unique properties of nanomaterials and other favorable scaling effects. NEMS also offer many new capabilities in radio frequency (RF) electronics, including low-loss mechanical switching and high quality factor resonators and oscillators. Although there has been significant research on device development, particularly in academia, research on the nanomanufacturing issues for NEMS has been limited to date, thereby delaying the commercialization of this technology. This research will focus on the measurement science needed for the successful transition of NEMS to the marketplace through high-throughput nanomanufacturing, which has largely been ignored.

The 2011 International Electronics Manufacturing Initiative (iNEMI) Roadmap has identified dynamic wafer-level testing as one particularly critical component that is currently missing for MEMS/NEMS, noting that, "It is critical to measure the dynamic characteristics of the MEMS product early at wafer level in the product development phase as well as final manufacturing. This dynamic test data can be used to optimize the design and process, predict final product performance at final test, and improve overall product quality and yields."^[4] We will address this need by developing advanced optical measurement methods for the dynamic characterization of NEMS that can be used for inspection, system testing, and process measurements to increase production yield and reliability. These optical methods are geared towards dynamic displacement measurements and will provide exceptional measurement speed and resolution, making them well suited to the high bandwidth and small motion of NEMS and the short test times required for wafer-level characterization.

Dynamic testing of NEMS requires the measurement of either the system's mechanical or electrical response to known inputs. The mechanical response is a function of the displacement of the mechanism, which is generally fast and small in amplitude due to the size of the mechanism. Therefore, displacement measurements with high-resolution and bandwidth are necessary to characterize the mechanical response. Optical methods for displacement measurement have a number advantages over other methods, such as capacitive detection, including high measurement speed, exceptional displacement resolution, and minimal measurement loading effects. Accordingly, we have developed two optical instruments for NEMS displacement measurements, the nanoscale motion microscope (NMM) and a custom near-field scanning optical microscope (NSOM). The NMM combines homodyne interferometry for out-of-plane displacement and laser reflection for in-plane displacement. The NSOM is also designed to measure out-of-plane displacement but with better spatial resolution than the NMM due to the nanoscale aperture used in this instrument. We will use these instruments to develop new measurement methods for characterizing the dynamic electromechanical response of NEMS that can be used for wafer-level testing.

This research will have three components: 1) upgrading the instrumentation to improve displacement resolution further, 2) the development of NEMS test methods based on displacement measurements, and 3) the application of these test methods to identifying geometric variations and defects in fabricated mechanisms. Instrument upgrades will include a differential interferometry capability to minimize the effect of drift in the microscope, spatial filtering for confocal measurements with better rejection of stray light, and calibration procedures that will improve the measurement accuracy. The test methods will focus on measuring the dynamics of NEMS and determining the parameters of interest from these measurements, such as resonant frequencies, quality factor, and stiffness, to determine whether a mechanism is performing as designed. The test methods will include experimental modal analysis at the nanoscale and time-domain analysis for step response and ring down measurements. An immense amount of information is encoded in the dynamics of a mechanism and changes in these dynamics provide insight into the quality of fabricated devices. As a result, the test methods can be used to measure variations in device properties and detect defects, which can be used to reject defective components before packaging and adjust process parameters using statistical process control. The sensitivity of the test methods to geometric tolerances, material properties, and defects will be explored and demonstrations of various conditions found during wafer-level dynamic testing will be performed. A set of NEMS test structures will be designed and fabricated in support of these experiments. Although NEMS are the focus of this work, all of these results will be directly applicable to other micro- and nanosystems including nanostructures, MEMS, and biological structures.

[1] http://www.whitehouse.gov/sites/default/files/microsites/ostp/pcast-nni-report.pdf , pp. 18.

[2] National Nanotechnology Initiative, Nanotechnology-Enabled Sensing, 2010. http://www.nano.gov/NNI-Nanosensors-stdres.pdf, pp. 34.

[3] Y.T. Yang et al., Nano Letters, 6, pp. 583, 2006.

[4] 2011 iNEMI Roadmap, MEMS/Sensors Chapter, p. 76.

• Outcome: Developed a custom metrology-grade optical microscope that is capable of measuring out-of-plane NEMS motion using homodyne interferometry and in-plane NEMS motion using laser reflection.

• Outcome: Developed a custom near-field scanning optical microscope (NSOM) for the measurement of out-of-plane NEMS motion with spatial resolution that is several times better than a far-field optical microscope.

• Output: Demonstrated the measurement of in-plane NEMS oscillations with amplitudes below 0.1 nm and 200 MHz bandwidth. 

• Output: Demonstrated shear-force measurements using the NSOM, which will be used to detect the presence of a NEMS during the NSOM probe approach.

• Impact: Provided displacement measurements of RF MEMS for a major U.S. manufacturer of RF components to determine the relationship between mechanical and electrical modes.  

There has yet to be a demand for standards related to the measurement of NEMS since NEMS is a new technology and companies are almost solely focused on the development and protection of their intellectual property. However, as mentioned above, our research also applies to MEMS, where the industry is more mature and has reached a point that it is open to the development and use of standard test methods. Laser Doppler vibrometers and white light interferometers are commonly used in the MEMS industry to measure the motion of MEMS. Therefore, we will engage industry in the first year in planning to develop documentary standards and standard reference materials for optical displacement measurements for MEMS based on our proposed measurement methods. No such standards exist today so these efforts will aim to lay the groundwork for future standards by establishing a MEMS standard test methods committee and by publishing a draft standard for in-plane displacement measurements using laser reflection. These activities will likely take place through SEMI, which is responsible for most of the MEMS standards currently used by the industry.