MFP NanoIndenter - First AFM-Based NanoIndenter for Quantitative Materials Characterization from Asylum Research

Topics Covered

Background
Introduction
Innovative, Robust Design
Monolithic Design Eliminates Drift and Errors in Depth Measurements
Nanopositioning for Accuracy and Precision
Diffraction-limited Optics Provide High Resolution Viewing of Tip and Sample
Easy-to-use Pre-calibrated Setup and Calibration Verification
Push and Turn Adjustments Maintain Calibration
Direct Measurement for Tip Characterization and Accurate Results
Applications

Background

Asylum's MFP NanoIndenter is a true instrumented indenter and is the first AFM-based indenter that does not use cantilevers as part of the indenting mechanism. These characteristics and the use of state-of-the-art AFM sensors provide substantial advantages in accuracy, precision and sensitivity over other nanoindenting systems.

Introduction

Unlike cantilever indenters, the MFP NanoIndenter (Figure 1) moves the indenting tip perpendicular to the surface. This vertical motion avoids the lateral movement and errors that are inherent in cantilever-based systems. Compared to conventional commerciallyavailable instrumented nanoindenters, the MFP NanoIndenter provides lower detection limits and higher resolution measurements of force and indentation depth with the superior precision of AFM sensing technology.

Figure 1. The MFP NanoIndenter intregrates the quantitative capabilities of instrumented nanonindenters with the resolution of AFM/SPM to provide advanced materials characterization with enhanced accuracy, precision and sensitivity.

The indenter is completely integrated with the AFM, providing the unique ability to quantify contact areas by performing AFM metrology of both the indenting tip and the resulting indentation (Figure 2). These direct measurements enable analysis of material properties with unprecedented accuracy relative to indirect calculation methods. The design uses passive actuation through a monolithic flexure, minimizing drift and other errors in depth measurement.

Figure 2. Indenting on dentin (left of crack), and enamel (right) on a human tooth sample. The indentations in each row (one row is circled) were all created with the same maximum force. The smaller indents on the enamel demonstrate that it is harder than the dentin, 70µm scan. Sample courtesy D. Wagner and S. Cohen, Weizmann Institute of Science.

The positioning accuracy in the sample plane is subnanometer using the MFP's closed loop nanopositioning sensors. The NanoIndenter Head utilizes advanced diffraction-limited optics coupled with CCD image capture for precision navigation of the tip to areas of interest on the sample.

The integrated software provides a full complement of experimental control and analysis functions, including standard analysis method templates. The system kit includes a set of nanoindenting tips, three different sample mounts, two calibration standards for sensitivity and spring constant verification, as well as the tools and accessories necessary to perform indenting experiments on a full range of materials. This highly quantitative tool, combined with high-end AFM capabilities, breaks new ground in the characterization of diverse materials including thin films, coatings, polymers, biomaterials, and many others. The MFP NanoIndenter Module is available in standard (spring constant typ. 4,000N/m) and low force (spring constant typ. 800N/m) versions exclusively for the Asylum MFP-3D™ AFM.

Innovative, Robust Design

At the heart of the NanoIndenter is our exclusive sensored closed loop head, designed with a robust flexured transducer for quantitative measurements.

Monolithic Design Eliminates Drift and Errors in Depth Measurements

With conventional nanoindenters, electrical actuation typically causes small parts to heat up, resulting in drift and, consequently, measurement errors. The monolithic design (Figure 3) of the flexured and sensored Z axis of the MFP NanoIndenter eliminates these drift problems and provides for quantifiable results.

Figure 3. The NanoIndenter transducer is a flexured, sensored design for unprecedented precision and accuracy.

Nanopositioning for Accuracy and Precision

Displacement of the MFP's indenting flexure is performed with a piezo actuator and measured with our patented low-noise, sensored Nanopositioning System (NPS™). The force is computed digitally as the product of the spring constant and the measured indenter flexure displacement. This measurement is generated by converting the optical signal (measured at the MFP photodetector) into the displacement of the vertical indenting flexure. The indenter provides unprecedented resolution because the two quantities of interest - depth and force - are computed based on displacements measured with state-of-the-art AFM sensors. Unlike conventional instrumented nanoindenters that cannot quantitatively measure the force in real time, the optical lever detection enables high bandwidth, true force feedback. This allows repeatable imaging, quantitative feature measurement, quantitative force curves, and accurate positioning for manipulation and lithography.

Diffraction-limited Optics Provide High Resolution Viewing of Tip and Sample

The NanoIndenter optics and camera assembly provides viewing of the indenter tip and sample at an angle of 20 degrees from horizontal (Figure 4). The indenter tip can be positioned with an accuracy of 20 micrometers with a 5x objective on a reflective surface.

Figure 4 Optical view of 10um calibration grating and cube corner tip. Reflection of the tip (bottom) can be seen on the sample.

Different regions of the sample can be viewed with the optic's independent translation stage. The design allows for easy exchange of objectives to accommodate different sample requirements. A built-in iris diaphragm provides adjustable depth of field and the camera allows for adjustment of exposure time, gain, frame rate, saturation, and gamma.

Easy-to-use Pre-calibrated Setup and Calibration Verification

The spring constant is calibrated with three independent methods to minimize errors in your measurements - the added-mass, reference spring, and micro-balance methods. Calibration of the indenter flexure assembly is performed at the factory; hardware and software are provided for calibration checks by users, ensuring precision and accuracy over the life of the instrument.

Push and Turn Adjustments Maintain Calibration

The advanced design of the NanoIndenter head includes push-and-turn adjustments that maintain the pre-set calibration parameters and protect against accidental changes.

Direct Measurement for Tip Characterization and Accurate Results

Tip characterization is extremely important for quantitative analysis in nanoindenting applications. Conventional nanoindenters must use indirect methods to evaluate the effect of the indenter tip geometry on the indentation results, such as indenting on a standard sample (fused silica) with application of theoretical and experimental assumptions. In contrast, the MFP NanoIndenter allows direct tip metrology using standard AFM techniques (Figures 5). This method specifically avoids the theoretical assumptions and associated experimental errors inherent in conventional methods (e.g. Oliver-Pharr). Similarly, AFM metrology of resulting indentations (Figure 6) provides additional experimental data to improve upon the accuracy of theories for data analysis. In addition, damaged or worn tips can be identified through AFM imaging and discarded before invalid data are collected.

Figure 5. Direct measurement of tip (Berkovich) area function shows the relationship between depth (AFM Z-sensor data, top) and contact area (numerical integration of the data, bottom), 5µm scan.

Figure 6. Granular materials have contact areas that are discontinuous and need to be measured for a correct estimation of material properties. Indium tin oxide, 800nm scan.

Applications

The NanoIndenter is ideal for a variety of nanoindenting applications including:

  • Elastic behavior of metals, ceramics, polymers, etc.
  • Dislocation phenomena in metals
  • Fractures in ceramics
  • Mechanical behavior of thin films, bone, biomaterials
  • Residual stresses
  • Time-dependent mechanical characteristics of soft metals and polymers
  • Combined nanoindenting with current-voltage measurements (IV)
  • Combined nanoindenting and Piezoresponse Force Microscopy

The Asylum MFP-3D™ AFM platform allows accurate estimation of elastic rebound, pile-up and sink-in material volumes. AFM imaging is key to identification of cracks, displacement, and failure zones in indented samples, as well as imaging of features revealing physical phenomena.

Source: "The MFP Instrumented NanoIndenter for Quantitative Materials Characterization" from Asylum Research

For more information on this source please visit Asylum Research

Date Added: Jul 31, 2009 | Updated: Jun 11, 2013
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