Imaging Local Material Properties
The AFM's capabilities go beyond imaging topography, however. The same force-sensing concepts can be used to quantify near-surface physical properties on the nanoscale. For many applications, such measurements provide valuable information that imaging morphology alone cannot.
For example, functional properties such as electrical, magnetic, and electromechanical response impact applications ranging from photovoltaics to nonvolatile memory and data storage. To interrogate functional behavior on the nanoscale, a number of AFM modes4 have been developed based on electrostatic, capacitive, magnetic, and related tip-sample interactions.
AFM modes that probe electrical properties include conductive AFM (CAFM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KFPM). The example in Figure 4 shows CAFM evaluation of a photoactive film. The nanoscale information provided by these techniques is often complementary to that obtained by probe station methods, which test a whole device. AFM electrical techniques can also be used to assess uniformity, identify defects, and otherwise assure quality.
Other capabilities for functional characterization are provided by piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM). PFM characterizes static and dynamic electromechanical response of piezoelectric, ferroelectric, and multiferroic materials. In contrast, MFM uses a magnetized tip to assess the magnetic behavior of ferromagnetic and multiferroic materials.
In other applications, mechanical and tribological properties such as modulus, adhesion, and friction are critical for performance and reliability. The AFM's sensitivity to low forces enables mechanical measurements with much higher vertical and lateral resolution than possible otherwise. As Figure 5 shows, today's AFMs provide other nanomechanical techniques5 besides the classic force curve method. These newer, faster imaging techniques can also measure viscoelastic response, of particular importance for polymers and biomaterials.
This article has briefly reviewed the capabilities of today's AFMs for nanoscale surface characterization. Recent instrumentation advances such as higher spatial resolution, faster imaging rates, and enhanced measurements of physical properties make AFMs more valuable than ever before. Future refinements that extend these capabilities even further will help AFMs keep pace with technology's continuing demands for better device control on smaller length scales.