Current-sensing atomic force microscopy (CSAFM) is an extended SPM mode used for simultaneously probing the conductivity and topography of a sample. By combining the advantages of scanning tunneling microscopy and force microscopy, CSAFM is capable of measuring and mapping the localized electric properties of resistive samples.
CSAFM utilizes electrically conductive AFM cantilevers and operates in standard contact mode. A current generated by applying a voltage bias between the substrate and the conducting cantilever can be used to construct a spatially resolved conductivity image. It also enables local ‘current vs. voltage’ measurements (I/V) with purely topographic feedback.
CSAFM is a measurement mode that is useful in a wide variety of material characterization applications, including thin dielectric films, ferroelectric films, nanotubes, conducting polymers, and semiconductor devices. CSAFM has also been applied in the study of electron-transfer processes in single molecules and transport processes in ion-conducting membranes.
The KLA Tencor7500 AFM/SPM microscope is a high-performance scientific instrument that delivers high-resolution imaging with integrated environmental control functions. The standard KLA Tencor7500 includes contact mode, acoustic AC mode, and phase imaging with one universal scanner that can operate in both open-loop mode and closed-loop mode.
Switching imaging modes with the 7500 is quick and convenient, owing to the scanner’s interchangeable, easy-to-load nose cones. All 7500 microscopes come with the lowest-noise closed-loop position detectors available so as to provide the ultimate convenience and performance in imaging — without sacrificing resolution or image quality.
With the addition of a preamp, the standard nose cone of the KLA Tencor7500 microscope permits the use of CSAFM mode. Preamps for the 7500 are available in three different sensitivity settings: 0.1 nA/V, 1 nA/V, and 10 nA/V, yielding current ranges of ±1 nA, ±10 nA, and ±100 nA, respectively.
Each of these preamp modules comes as a plug-and-play board (see Figure 1) that inserts into the interface slot on the scanner. Simply switching between different plug-and-play boards changes the 7500’s current sensitivity, greatly increasing the flexibility of the microscope as well as reducing the complexity of its use.
Figure 1. Universal scanner
Example 1: Resistivity Mapping of Titanium-Tungsten (TiW) Films
TiW is a common material employed for multilevel metallization in VLSI technologies. TiW provides a good barrier to prevent Si diffusion into Al, the commonly used metal for interconnects in semiconductor devices. It also adheres strongly to both SiO2 and Si3N4 films and provides a clean and uniform nucleating surface for Al.
TiW is an alloy of titanium and tungsten, usually containing 10 wt% of Ti and 90 wt% of W in the film. Such film typically consists of columnar grains of W, while the Ti forms a solid solution with some W atoms at the grain boundaries. The distribution of Ti is an important factor in the behavior of TiW film. Resistivity of TiW film is reported to be in the range of 50–80 μΩ·cm.
Figure 2 shows CSAFM topography and current images obtained simultaneously for a TiW film deposited on Si. The topography image reveals that the TiW film has a surface roughness of about 6 nm rms.
The current images display higher conductivity at the areas between the granular domains, corresponding to the Ti/W liquid solution formed at the grain boundaries between columnar W phases. The change in polarity between current images (B) and (C) is a result of the opposite DC bias applied to the sample during imaging.
Figure 2. CSAFM imaging of a TiW thin film: (A) topography, (B) current at –0.05 V, and (C) current at +0.10 V. Scan size: 1.5 μm x 1.5 μm.
Example 2: Conductivity Measurement on Semiconductor Devices
A particularly important CSAFM application is in the study of physical failure analysis of integrated semiconductor devices. CSAFM has been successfully utilized to localize the failure site on marginally failing devices and to obtain full electrical data on single bit failures. It is also a viable alternative tool to both voltage contrast imaging and in-FIB pico-probing.
CSAFM simultaneously provides topography information with nanometer-scale resolution as well as electrical characteristics of the surface. It can perform current mapping of large areas and provide current-voltage (I-V) characteristics of specific contacts, vias, and even foreign particles in a nondestructive manner. The current image of the ‘region of interest’ yields an easy-to-interpret map of the possible defective contacts or vias.
CSAFM is more sensitive than passive voltage contrast (PVC) methods, as the contrast mechanism is the direct current measurement of the surface. For bulk silicon devices, CSAFM is easily performed by biasing the silicon substrate while scanning with the conductive tip to measure the current passing through each point of contact.
Figure 3 presents the CSAFM topography and current images of a piece of SRAM de-processed to the bare silicon level, exposing the PMOS and NMOS structures. The current image (B) clearly reveals the different conductivity of differently doped regions.
Figure 3. CSAFM topography (top) and current (bottom) images of SRAM.
With the help of constant-force-feedback control, current-sensing AFM delivers nanometer-scale current measurement, high resolution, and spatially resolved conductivity imaging for a wide variety of material characterization applications, including thin dielectric films, ferroelectric films, nanotubes, conductive polymers, IC devices, and many more. CSAFM is also useful for studying electron-transfer processes in single molecules and transport processes in ion-conducting membranes.