Nanoscale Elastic-Property Mapping with Contact-Resonance-Frequency AFM*
- PDF / 718,745 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 87 Downloads / 179 Views
O8.2.1
Nanoscale Elastic-Property Mapping with Contact-Resonance-Frequency AFM∗ D. C. Hurley, A. B. Kos, and P. Rice National Institute of Standards & Technology Boulder, CO 80302-3328, U.S.A. ABSTRACT We describe a dynamic atomic force microscopy (AFM) method to map the nanoscale elastic properties of surfaces, thin films, and nanostructures. Our approach is based on atomic force acoustic microscopy (AFAM) techniques previously used for quantitative measurements of elastic properties at a fixed sample position. AFAM measurements determine the resonant frequencies of an AFM cantilever in contact mode to calculate the tip-sample contact stiffness k∗ . Local values for elastic properties such as the indentation modulus M can be determined from k∗ with the appropriate contact-mechanics models. To enable imaging at practical rates, we have developed a frequency-tracking circuit based on digital signal processor architecture to rapidly locate the contact-resonance frequencies at each image position. We present contact-resonance frequency images obtained using both flexural and torsional cantilever images as well as the corresponding vertical contact-stiffness (k∗ ) image calculated from flexural frequency images. Methods to obtain elastic-modulus images of M from vertical contact-stiffness images are also discussed. ∗ Contribution of NIST, an agency of the US government; not subject to copyright. INTRODUCTION As critical dimensions shrink below 1 µm, new tools are required to investigate material properties on commensurate scales. In particular, nanomechanical information—knowledge on nanometer length scales of mechanical properties such as elastic modulus, strength, adhesion, and friction—is needed in many emerging applications. The need is driven by the increasing integration of multiple materials on micrometer and nanometer scales. The complexity of such systems increases the demand for accurate property values for predictive modeling. Furthermore, because localized variations in properties are often the cause of failure, it is increasingly important to assess not just the “average” properties from a single sample position, but to visualize spatial variations in properties. One approach to meet this objective combines nanoindentation techniques with force modulation and scanning [1]. This promising method is limited in lateral spatial resolution by the radius (a few hundred nanometers) of the Berkovich diamond indenter used. Therefore, methods that exploit the increased spatial resolution of atmomic force microscopy (AFM) are also being developed. Those methods that promise quantitative image information are typically dynamic approaches in which the AFM cantilever is vibrated at or near the frequencies of its resonant modes. Two of these approaches are contact methods called ultrasonic AFM [2,3] and atomic force acoustic microscopy (AFAM) [4,5]. In this paper, we describe our progress towards quantitative imaging of nanoscale elastic properties using AFAM. Using new signal acqusition and processing techniques, we hav
Data Loading...
