Quantitative Electrical Measurements with Atomic Force Microscopy
Jennifer E. Greene Nanoscience Instruments , 9831 S. 51st Street , C119 , Phoenix , AZ 85044
jgreene@nanoscience.com Introduction
Since the introduction of atomic force microscopy (AFM) from the landmark publication by Binnig, Quate, and Gerber [ 1 ], the fi eld of study has exploded well beyond using interatomic forces to image topography on the nanometer scale. T e ability to measure intermolecular forces and see atoms was scientifi cally tantalizing. Soon aſt er, other material properties could be imaged and measured, including capacitance [ 2 ], magnetic forces [ 3 ], and surface potential [ 4 ].
Over the course of 30 years, AFM instrumentation has evolved and moved from basic research into product development laboratories and manufacturing lines. Capabilities such as resistance and conductance measurements using AFM are now available and are benefi cial for a variety of applications. Two areas of interest that require electrical characterization include solar cells and microelectronics. Examples of these applications are included in this article to illustrate the capabil- ities for quantitative nanoscale electrical characterization using AFM.
Previous limitations of electrical AFM measurements include friction interference, lack of quantitative measurements, diffi culties measuring soſt materials, and limited resistance range. T ese drawbacks are resolved with the ResiScope module for the Nano-Observer AFM (Concept Scientifi c Instruments, France). T e ResiScope has a large dynamic range covering 10 orders of magnitude for resistivity and can provide quanti- tative resistance and current measurements on many diff erent materials. Soſt polymers like those used for organic solar cells can be characterized using a new Soſt ResiScope mode. Available soon, the Soſt ResiScope mode uses a semi-oscillating technique to eliminate friction or damage to the sample. T e Soſt ResiScope mode is currently the only commercially available method for quantitative resistance measurements of compliant samples. Concurrent operation of the Nano-Observer AFM with the ResiScope provides simultaneous imaging and material property measurements.
Materials and Methods AFM imaging modes . Diff erent imaging modes are referred to throughout this article and are briefl y described here. All involve the basic premise of an AFM probe being raster-scanned across a surface. In contact mode, typically the probe is held at a constant force while the cantilever defl ection is monitored through a feedback loop to yield a topographic image. Dynamic force mode, oſt en referred to as either tapping, oscillating, or intermittent-contact, is an imaging technique that oscillates the cantilever so the tip is touching the surface intermittently. Oscillating imaging is benefi cial for soſt or compliant samples. Note that AFM images do not have any
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color information. Color scales are chosen by the user from the instrument soſt ware.
Electric force microscopy (EFM) measures the electrical
fi eld across a sample, usually in a dual-pass mode where the tip is close to the surface to image topography then retracted about 100 nm above the sample to measure the fi eld. A conductive probe is required for this method. Some AFM systems also require additional soſt ware to enable this feature. Magnetic force microscopy (MFM) is a similar technique as it uses a second pass to measure the magnetic fi eld a distance from the surface. MFM requires a probe with a magnetic coating. Kelvin force microscopy (KFM) measures the surface potential of a sample, also requiring a second pass and conductive tip. An option for the Nano-Observer AFM is a high-defi nition mode (HD-KFM) that acquires data in a single pass at the sample surface, yielding higher spatial resolution than the standard KFM mode. HD-KFM is an option that requires additional hardware for the Nano-Observer AFM. ResiScope module . T e ResiScope module works in combination with the Nano-Observer AFM. T e Nano-Observer AFM is capable of advanced modes and integrates the latest technology for high-resolution imaging. A patented fl exure- guided stage with three low-voltage piezoelectric devices is mounted in a massive platform and is combined with a low-noise laser and electronics. Environmental controls for temperature, liquid, and gas are available. For this work, the Nano-Observer AFM modes used included contact, oscillating, and KFM in conjunction with the resistance and current measurements provided by the ResiScope. Conductive probes were also used for these samples. Resistance measurements . T e ResiScope module can provide resistance measurements simultaneously with other dynamic modes like EFM, MFM, or KFM. Resistance data from 10 2 to 10 12 ohms can be obtained. In addition to the resistance data shown in this article, current measurements and current/ voltage spectroscopy data can be acquired over an equivalent range.
T e general principle of the ResiScope is shown in Figure 1 . Resistance measurements are made by applying a DC bias between the sample and a conductive AFM probe, holding the AFM tip at virtual ground. For standard measurements, the tip is scanned in contact mode using the laser defl ection for AFM feedback. As an independent measurement, the ResiScope measures the sample resistance through a fast-response amplifi er (HPA). During the resistance measurement, the digital signal processor (DSP) chooses the best gain in real time to optimize the measurement made by the HPA. T is operating condition aff ords high sensitivity across the entire range of resistivity at a regular scan speed for AFM. Exclusive to the ResiScope, the
doi: 10.1017/S1551929515000991
www.microscopy-today.com • 2015 November
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