Scanned Probe Microscopy Measurement and Standards

Our goal is to develop standard reference materials and quantitative, reproducible, measurement methods and protocols for scanned probe microscopes, to enable accurate dimensional, force, and material property measurements at the nanoscale. For example, our approach will allow force measurements in atomic force microscopes to be both precise and accurate.

A NIST Standard Reference Material (SRM 3461) will be produced that will enable accurate calibration of the flexural stiffness of AFM test cantilevers. An array of extremely precise rectangular cantilevers will be microfabricated from silicon-on-insulator wafers. The uniformity of the cantilever population on each wafer will be verified with resonance frequency measurements using Laser Doppler Velocimetry and Euler-Bernoulli modeling. Calibrations traceable to the International System of Units (SI) will be performed on a statistical subset of the population using the electrostatic force balance developed by the Manufacturing Engineering Laboratory at NIST.

Further, AFM methods will be developed to enable mechanical property measurements at the nanoscale. Contact resonance methods will be used to determine elastic properties. AFM adhesion measurements under different environmental conditions will elucidate the important parameters that control nanoscale surface interactions.

Impact and Customers:

• A broad spectrum of industries, government agencies, and academic institutions use scanned probe instruments to develop, characterize, and manufacture products from ceramics, metals, polymers, and semiconductors.

• Atomic Force Microscopes (AFMs) are the most common scanned probe instrument, with an estimated 10,000 AFMs in use in virtually every materials research and development laboratory worldwide.

• Calibrating delicate measurement tools such as AFMs is difficult: at small length scales, the forces that affect probe-material interactions, and their relative magnitudes, are often unknown, but are certainly dominated by surface effects.

• Currently, the lack of SI-traceable stiffness calibration standards hampers progress towards making AFM force measurements quantitative; suchmeasurements can be precise, but there is an incomplete understanding of accuracy.

A series of extremely uniform prototype reference cantilever arrays were created that can be used to calibrate the spring constants of atomic force microscopy cantilevers and other micromechanical structures. Nominal spring constants were estimated to be in the range of 0.02 Nm-1 to 0.2 Nm-1. Resonance frequency measurements were used to assess the uniformity of devices from different portions of a silicon-on-insulator wafer, and from different processing batches. Variations of less than 1% (relative standard deviation) in resonance frequency attested to the high degree of uniformity achieved. Independent calibration of cantilevers in an array using an electrostatic force balance indicated that the actual spring constants ranged from 0.0260 ± 0.0005 Nm-1 (±1.9%) to 0.2099 ± 0.0009 Nm-1 (±0.43%). The results confirmed the feasibility of creating uniform reference cantilevers and calibrating them using an SI-traceable 

 
Agreement between Euler-Bernoulli beam theory and measurements performed on the cantilever array using an electronic force balance

technique, making these devices excellent candidates for force calibration standards for AFM. An SRM production batch is currently in process.

A method for calibrating the stiffness of AFM cantilevers was developed using the prototype reference cantilever array. A series of force-displacement curves was obtained using a commercial AFM test cantilever on the reference cantilever array, and the data were analyzed using an implied Euler-Bernoulli model to extract the test cantilever spring constant from linear regression fitting.The method offers improvements in precision over the reference cantilever method (factor of five) and the added mass calibration method (factor of three) that are currently used for AFM calibration.

Comparison of precision of reference array calibration method versus reference cantilever method

 
A new, higher-precision contact-resonance AFM method was developed to quantitatively determine material indentation moduli by measuring local mechanical responses. A dual reference method has been shown to be capable of extracting the modulus of a material within 3% of the calculated expected value without any assumptions of the probe's mechanical properties.

First contact resonance frequencies obtained on three different surface materials

Finally, adhesion forces between a gold sphere and flat gold substrate were studied using AFM in different humidity environments. The pull-off force measured in a vacuum is found to be a small fraction of that in ambient air or nitrogen atmosphere. Calculations of capillary condensation forces, including the effects of elastic deformation of the contacting bodies and adsorption layers, revealed that the meniscus force is the dominant source of the observed difference in pull-off forces. The experimental data showed that nitrogen purging did not eliminate the meniscus contribution to the pull-off force.