Scanning Probe Microscopy Measurement and Standards

Summary

This project is tasked with enabling accurate nanomechanical property measurements on materials.  Our goal is to develop standard reference materials and quantitative, reproducible, measurement methods and protocols for atomic force microscopes, to enable accurate dimensional, force, and material property measurements at the nanoscale.

• Atomic Force Microscopes (AFMs) are the most common scanning probe instrument, with over 10,000 AFMs in use in virtually every materials research and development laboratory worldwide.
• Cantilevers used in AFM to interrogate surfaces are very small – the width is less that the width of a human hair.
• The interaction between the cantilever probe tip and a surface is usually measured using a laser beam reflected from the back of the cantilever.  Sub-nanometer cantilever deflections can be measured.

 

Description

Cantilever schematic

The Nanomechanical Properties Group has expertise in a wide variety of SPM techniques, especially atomic force microscopy (AFM); however, the main research thrust for the Group’s SPMM&S project area deals with accurate cantilever stiffness calibration and the use and development of AFM methods for measuring the nanomechanical properties of surfaces. 

Cantilever Stiffness Calibration

• Laser Doppler vibrometry thermal cantilever calibration
• Standard reference cantilevers
• Standards participation (VAMAS)

Nanomechanical Property Measurement

• Contact Resonance AFM (CR-AFM)
• Intermittent Contact Resonance (ICR-AFM)
• Soft Matter AFM (Colloid Probe)

The cantilever stiffness calibration effort is spearheaded by the use and development of the laser Doppler vibrometry cantilever thermal calibration method as a means of accurately determining the stiffness of AFM cantilevers.  This instrument, housed 12 m underground in our Group’s clean room facility is capable of making some of the most sensitive and accurate AFM cantilever stiffness measurements available.  It provides the technical underpinning for complementary Group efforts in AFM standards and method development.

The main focus of the Group’s AFM nanomechanical property measurement capabilities centers around interrogating nanomechanical properties of surfaces by monitoring the resonance frequency shifts of an AFM cantilever as it contacts the surface under controlled static loading.  A recently developed NIST enhancement of that basic technique (Intermittent Contact Resonance or ICR-AFM) has enabled the measurement of near surface nanomechanical properties of materials as a function of depth during imaging.

Impact and Customers:

• A broad spectrum of industries, government agencies, and academic institutions use scanning probe instruments to develop, characterize, and manufacture products from ceramics, metals, polymers, and semiconductors.
• 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; such measurements can be precise, but there is an incomplete understanding of accuracy.
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Major Accomplishments

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

 

Agreement between stiffness measurements and models

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 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 calibration methods

A new "Hammerhead" AFM cantilever has been developed that is capable of providing precise lateral force (e.g. friction) measurements on surfaces using a commercial instrument. The "wings" projecting from the sides of the cantilever are used along with a specially designed calibration chip that allows for precise determination of the optical torsional sensitivity of the photodetector, which is usually very difficult to measure. A production batch of reference material "Hammerhead" cantilevers is being developed and fabricated to make these devices available to the research community.

A new, higher-precision contact-resonance AFM (CR-AFM) method was developed to quantitatively determine material indentation moduli by measuring local mechanical responses.

 

Prototype "Hammerhead" cantilever for precise lateral force measurment in an AFM
 

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. 

Using this CR-AFM technique, select nanostructures have been studied to elucidate their nanomechanical properties. The most recent work included analysis of ZnO nanowires as small as 26 nm in diameter, and AlN nanotubes 200 nm in diameter but with wall thicknesses of only 19 nm.

 

 

 

A split AlN nanotube used for morphology and contact resonance AFM analysis

 

 

Additional Technical Details

This project encompasses the following focus areas:

• Nanoscale Mechanics by Contact Resonance Atomic Force Microscopy—Atomic force microscopy (AFM) provides unique accessibility at the nanoscale. Capabilities in terms of nanoscale mechanical property measurements are added to AFM by contact-resonance AFM (AFM) for a large range of elastic modulus from few GPa to hundred GPa. The nanoscale spatial resolution and robust elastic modulus calibration of CR-AFM are used in quantitative measurements of elastic properties of one-dimensional structures (nanowires, nanotubes, fibers), two-dimensional structures (thin film coatings), nanocomposites etc. Integrative CR-AFM measurements and computation provide a unique mechanical characterization for the next generation of materials and structures used in nanoscale applications and devices.
• Standard Reference Cantilevers for AFM Spring Constant Calibration—AFM is a popular, versatile, research technique that uses a small probe (a cantilever supporting a tip) to interrogate and measure the topography and mechanical properties of surfaces down to the nanoscale. One limitation in interpreting the data and images generated by AFM probes at these very sensitive scales is the lack of accurate determination of the spring constant of the cantilever used to manipulate and control the tip. Manufacturers' specifications for commercial AFM cantilevers provide a stiffness range of a factor of two to four. Techniques are available to narrow the precision of the stiffness but with unknown accuracy as the techniques are not SI traceable.
• Elastic Moduli of ZnO—Zinc oxide (ZnO) nanostructures hold tremendous promise for novel and versatile devices due to their intrinsic semiconducting and piezoelectric properties, as well as the amazing structural diversity that can be achieved by current synthesis techniques. ZnO nanostructures can take the form of nanowires, nanobelts, nanorings, and nanohelices, with demonstrated potential for novel nano-scale piezoelectric devices. The ability to create such devices with predictable and reproducible performance depends on having knowledge of the mechanical properties of ZnO nanostructures.
• Elastic Modulus of Faceted Aluminum—Aluminum nitride has an intriguing combination of physical properties, such as enhanced field emission, large optical band gap, high thermal conductivity, large electrical resistivity, as well as a piezoelectric coefficient comparable to that of quartz. These properties make AlN nanostructures suitable for advanced nanoscale electronic and optoelectronic device applications, and have motivated sustained efforts to synthesize AlN nanostructures in various morphologies: wires, nanoparticles, nanotubes, needles, and platelets. While many applications for AlN nanostructures target their use as field emitters in at panel displays, their superior piezoelectric properties and integration compatibility with silicon substrates make them excellent candidates for sensors, actuators, and nano-electromechanical systems (NEMS).
• Measurement of Elastic Moduli of Nano-Granular Surfaces—Next-generation electronic devices, based on micro- and nanoelectromechanical systems, require accurate knowledge and control of material properties at ultra-small scales. As the size of device elements is reduced further and further, down to the nanoscale, mechanical properties in particular can exhibit significant variations from those of their bulk counterparts due to the increase in the surface-to-volume ratio. The inherent effects of proximate surfaces and interfaces alter the properties of the nanosize entities (e.g., nanowires) or those of reduced-scale constituents (e.g., crystallites in nanostructured materials) and modify the mechanical response of the assembly as a whole. For some particular geometries it is possible to deduce the response of the nanosized constituents by testing an assembly at the macroscale. However, it is obviously more desirable to directly probe the local mechanical properties of a nanostructured material.

Associated Product(s)

Project Summary (PDF)