We develop metrology for rapid, nondestructive mapping of mechanical properties with nanoscale spatial resolution, by use of AFM contact-resonance spectroscopy methods. These methods are applied to specific material systems to better understand the nanomechanical behavior of surfaces, thin films, and devices. Objectives for this year include: (i) Develop methods for accurate quantitative measurements of compliant materials such as polymers (M < 10 GPa); use methods to evaluate nanoscale size effects in polymeric systems such as nanoimprint lithography structures. (ii) Use newly-gained knowledge of cantilever forces to assess potential for quantitative depth assessment (of buried interfaces, defects, etc.). (iii) Determine physical limits on the ultimate spatial resolution of these methods. 

Background

“In order to be widely used, future nanodevices will require nanomechanical measurements that are rapid, accurate, predictive, well-understood, and representative of a device or system’s environment in real time” (NNI Interagency Workshop on Instrumentation and Metrology for Nanotechnology, 2004). This vision describes the need for new measurement tools for emerging nanotechnology applications, a field estimated to create a $340 billion market for materials and processing within the next 15 to 20 years. The vision also specifically emphasizes the need for nanomechanical information—knowledge on nanometer length scales of mechanical properties such as elastic modulus, strength, adhesion, and friction. Existing methods to obtain mechanical-property data include instrumented (“nano-“) indentation, scanning acoustic microscopy, and microtensile testing. However, such methods often have drawbacks: they are destructive, lack the desired spatial resolution, or are restricted to specialized test specimens.

Another issue is the growing need to visualize the spatial distribution in mechanical properties rather than make do with an “average” value. Increasingly, applications involve several disparate materials integrated on the micro- or nanoscale (e.g., electronic interconnect, composites). The complexity of fabricating such systems dictates the use of predictive modeling to save time and money. Yet modeling can correctly predict system performance only if the property data used as input are accurate at the relevant length scales. Furthermore, in such heterogeneous systems it is frequently the localized variation or divergence in properties that causes failure (void formation, fracture, etc.). Engineering these complex systems thus requires quantitative nanomechanical imaging to enable better prediction of nanoscale reliability and performance.

Approach

Atomic force microscopy (AFM) methods promise to meet these needs. The small radius of the AFM cantilever tip (~5 nm to 50 nm) enables in situ imaging with nanoscale spatial resolution. Many techniques have been demonstrated to exploit the AFM’s capabilities, with several designed to provide information about mechanical properties. However, most of these methods do not provide the quantitative data needed. A subset of emerging AFM methods that does provide quantitative mechanical-property information is contact-resonance spectroscopy. Approaches such as atomic force acoustic microscopy (AFAM) involve vibrating the AFM cantilever while its tip is in contact with a test material. In this way, the resonant modes of the cantilever — the “contact resonances” — are excited. From measurements of the contact-resonance frequencies, information is obtained about the interaction forces between the tip and sample (e.g., contact stiffness). Models for the tip-sample contact mechanics are then used to relate the contact stiffness to mechanical properties of the sample such as elastic modulus.

So far in this project, we have developed contact-resonance spectroscopy methods for quantitative modulus measurements. Issues affecting measurement precision and accuracy such as tip shape have been investigated. Most recently, we have created tools for nanomechanical mapping, and have demonstrated their utility in assessing elastic modulus and thin-film adhesion.