Magnetic and Electrical AFM Modes
Magnetic and electrical modes, in combination with special cantilevers, enable the measurement of sample properties beyond the topography. Examples are Magnetic Force Microscopy (MFM) and several electrical AFM modes. These modes depend on the detection of magnetic or electrical fields. Key in such measurements is to separate the short-range van der Waals forces from the longer range electrical or magnetic forces. A lifting mechanism enables the probing of longer range electrical and magnetic forces, and deconvoluting them from the short-range van der Waals forces that are present during topographic imaging. The height that the tip is lifted over the sample is often a parameter that needs to be optimized by the user in order to have successful imaging of the magnetic or electrical properties and is typically in the few to hundreds of nanometer range.
The lifting can be done in so called single, interlaced and dual scan line modes. In a single-pass or constant height setup, the slope of the surface is measured from a completed topography image or line before the imaging of the long range interaction is started, and then the tip is scanned at a fixed height above the sample, compensating for the average slope.
Alternative to the single pass are the interlaced and dual pass imaging modes (left and right in schematic below, resp.), providing the topographical information of the surface along with the functional signal. In interlaced mode the forward pass records the topography of a scan line and then the tip is lifted above the sample during the backward pass. In dual pass topography is measured forward and backward in the first pass and the long range signal in the forward and backward movement of the second pass. Dual scan provides the more accurate correlation between topography and long range signal, wheras the interlaced mode is faster.
Since the long range interactions depend on the distance between tip and sample, additional phase contrast is generated when moving at constant slope over a sample with considerable height variation. Contour following, which is an option available for interlaced and dual scanning imaging mode on C3000 and CoreAFM product lines, scans the cantilever above the sample, keeping the separation constant, as depicted schematically below. For very flat samples like hard discs or polished steel contour following is not critical, whereas it may become essential if surface protrusions exceed some tens of nanometers, as shown for an Electrostatic Force Microscopy (EFM) example with 100nm high pedestals.
Magnetic Force Microscopy (MFM)
MFM is a phase imaging mode that uses atomic force microscope cantilevers with a thin magnetic coating in order to probe the magnetic field between a sample and a magnetized tip. This method is commonly used to image any materials with heterogeneous magnetic properties such as magnetic-based hard drives. It can be operated in single, interlaced and dual scan line modes. Any of these modes require optimization of the height above the sample at which the MFM image is collected.
The measurements below show 80 µm topographic (left) and MFM (center) images of steel plus a zoomed in image of the MFM signal (right). The topography shows minor topographical features, whereas the corresponding MFM image reveals 3 distinct areas: the areas showing meandering being the magnetic ferrite phase, in which contrast and frequency depend on the domain orientations.
Electrical Measurement Modes
AFM can probe a wide variety of electrical properties of materials and surfaces. These methods operate either in static mode or dynamic mode, depending on the information being sought. Probing properties such as current, conductance, surface potential, and capacitance are increasingly important in a number of applications including research on semiconductors, solar and battery cells, conductive polymers, and nanoelectronics. These applications have in common that the electrical properties of increasingly miniaturized devices and features need to be measured. Note that all these methods require specialized tips, usually in the form of a conventional silicon AFM cantilever coated with an electrically conductive coating. Probes made of conducting diamond are also suitable for some of these methods. All these modes provide simultaneous topography and electrical property data. A review of some of the main electrical modes and properties are given here.
These electrical modes also often take advantage of interlaced and dual pass scan methods, in which the probe measures topography on the forward or first pass and then the probe is lifted a prescribed distance away from the surface on the backward or second pass, optionally following the topography (contour) of the sample, as shown in the lift modes section.
Conductive AFM (C-AFM)
This is a static mode method where both the current distribution and topography of a surface are mapped simultaneously. It is similar to scanning tunneling microscopy as in both modes a bias voltage is applied between tip and sample, and the tunneling current is measured between the two. However, the advantage of C-AFM, which uses a conductive cantilever as opposed to a sharp metallic wire, is that it provides topography information and current information independently. Single point measurements that measure the current vs. voltage curves (commonly referred to as IV curves) can also be collected in this mode to probe the detailed electrical properties at a position.
Current measurements in air may prove challenging. The contact area between tip and sample is very small (diameter in the 10nm range). Therefore the current density (current per surface area unit) can become very high, rapidly hampering the conductive properties of the cantilever coating. Here a resistor inline with the sample reduces this risk of highly conductive samples.
Surface contamination and a water film between tip and sample commonly present at ambient conditions additionally reduce reliability and repeatability of C-AFM: Only a few nanometers of debris on the tip can block the flow of electrical current. Therefore higher forces are sometimes needed, which require harder tip materials or coatings like conductive diamond-like coatings or platinum silicide.
Such problems are reduced in vacuum systems, like in the example below, recorded in the vacuum of an SEM. The gradual effect of electron beam irradiation on topography and conductivity of a Pt(C) film was measured in situ with AFSEM™. The reduction of height shows the compacting of the film with irradiation dose. At the same time, an increase in local conductivity was observed at higher doses.
3D topography of Pt(C) film irradiated with different electron beam doses
Current through the film of the area inside the blue rectangle in the topography
C-AFM is available for all Nanosurf AFM product lines.
Piezoelectric Force Microscopy (PFM)
This static mode based method is geared towards the study of ferroelectric or piezoelectric materials, which are materials that respond mechanically to the application of an electric field. This mode measures topography simultaneously with mechanical response of the material when an electric voltage is applied with a conductive AFM tip. A sharp conductive AFM tip is brought into contact with the sample and an AC voltage is applied between the tip and sample. The sample will either expand or contract oscillatory due to this applied voltage. The sample motion is then tracked by the cantilever deflection, which is detected with a lock-in amplifier. The amplitude gives information on the piezoelectric tensor of the material and the phase provides information on the polarization direction.
Shown below is a PFM image of a common piezoelectric material, lead zirconate titanate (PZT). The topograph is shown on the left and a 3D view on the right where the coloring represents the piezo response. The phase clearly reveals striped domains inside the grain that were not visible in the topography.
Electrostatic Force Microscopy (EFM)
This mode is the electrical equivalent to MFM and operates in phase imaging mode, but now used for imaging variations in the electric field of the substrate. When scanning the tip lifted above the surface (typically only a few tens of nanometers), a voltage is applied between tip and sample to create a long-range electrostatic force.
EFM images reveal information about surface potential and charge distribution from the phase image: with increasing magnitude of the potential difference between tip and sample, the resonance frequency drops, causing reduction in phase. Thus, a lower phase indicates a larger (absolute) potential difference. This also means that the contrast can be varied with the applied voltage. The example below demonstrates that the phase contrast can be inverted by changing the potential. In this measurement aluminum pedestals on top of a gold substrate were imaged with 3V (top) and -3V (bottom) tip voltage in contour following EFM mode.
Kelvin Probe Force Microscopy (KPFM)
This mode images the surface potential distribution of a sample without direct electrical tip-sample contact. It operates in tapping mode with either a single or dual-pass setup. In the single-pass setup the tip is closer to the sample so there is higher sensitivity and resolution in the Kelvin force measurement, but the topography resolution may suffer somewhat. In the dual pass setup, the tip is further away from the sample, resulting in lower sensitivity and resolution, but the topography can be sharper.
In KPFM an AC plus DC voltage is applied to the cantilever causing an oscillating electrostatic force between tip and sample. The resulting deflection oscillation is detected with a lock-in amplifier and minimized by the DC voltage. The DC voltage used is the local contact potential difference (CPD) between tip and sample. Applications include imaging the Kelvin potential or work function of a surface and measuring applied voltage differences between conductors.
Below is an image illustrating the utility of KPFM. Graphene was deposited onto copper, which was subsequently oxidized. The topography image is shown in the inset revealing the two striated graphene domains and the copper domain in between. The Kelvin probe image shows new features in the surface such as crystal defects within the graphene (circled in white) and differences in coupling to the substrate (circled in grey).
In another example, local charges were placed on an insulating oxide surface layer in a Swiss cross pattern. Imaging the sample with topography (left) did not reveal the pattern, but imaging it in KPFM mode (right) that records the surface potential clearly showed the pattern.