NEMS sensor improves AFM

June 3, 2011 — The US National Institute of Standards and Technology’s (NIST) Center for Nanoscale Science and Technology (CNST) has improved atomic force microscopy (AFM) by replacing the microscope’s optical instrumentation with a nanomechanical cantilever probe and nanophotonic interferometer on a chip.

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Figure. Scanning electron micrograph (SEM) of the cantilever-microdisksystem. A calculated z-component of the magnetic field is overlaid on the structure. SOURCE: NIST.

The AFM maps local tip-surface interactions by scanning a flexible cantilever probe over a surface. CNST researchers replaced the AFM’s bulky optical sensing instrumentation, which limits the tool’s sensitivity, stability, and accuracy.

The CNST researchers nano-fabricated an integrated sensor combining a nanomechanical cantilever probe with a high-sensitivity nanophotonic interferometer as a monolithic unit on a single silicon chip. The package is chip-scale, self-aligned, and stable. Fiber optic waveguides couple light with the sensor, enabling interfaces with standard optical sources and detectors.

The cantilevers are orders of magnitude smaller than those used in conventional laser-based AFMs. The detection bandwidth is increased significantly, because each smaller structures has an effective mass less than a picogram. System response time is a few hundred nanoseconds.

The probe was fabricated adjacent (>100nm gap) to a microdisk optical cavity. Light circulating within the cavity is influenced by probe tip motion. This readout technique is based on cavity optomechanics. The cavity’s high optical quality factor (Q) means that the light makes tens of thousands of trips around the inside of the cavity before leaking out of it. During this circulation, information is gathered about probe position. Small probe-cavity separation and high Q gives the device sensitivity to probe motion at less than 1 fm/√Hz, while the cavity is able to sense changes in probe position with high bandwidth.

The probe is 25µm long, 260nm thick, and 65nm wide. Probe stiffness is comparable to conventional microcantilevers, maintaining high mechanical gain. Simple probe-tip geometry changes can significantly vary the mechanics of the probe tip, which can be used to "tune" combinations of mechanical gain and bandwidth for various AFM applications.

Results were reported in Nano Letters, "Optomechanical transduction of an integrated silicon cantilever probe using a microdisk resonator," K. Srinivasan, H. Miao, M.T. Rakher, M. Davanco, and V. Aksyuk, Nano Letters 11, 791-797 (2011). Access the article here:

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