Module 26: Atomic Force Microscopy. Lecture 40: Atomic Force Microscopy 3: Additional Modes of AFM

Transcription

1 Module 26: Atomic Force Microscopy Lecture 40: Atomic Force Microscopy 3: Additional Modes of AFM 1

2 The AFM apart from generating the information about the topography of the sample features can be used to obtain various other information about the nature of samples. Some of the advanced modes of the AFM are: Lateral Force Microscopy (LFM), Phase Contrast Microscopy, Magnetic Force Microscopy (MFM), Electrical Force Microscopy (EFM), Current Sensing AFM (CS- AFM), Scanning Near Field Optical Microscopy (SNOM) etc. In this chapter we discuss some of these modes which are used for obtaining additional information about a surface. We briefly discuss the following modes: Lateral Force Microscopy; Phase Contrast Imaging, Magnetic Force Microscopy (MFM) and Electrical Force Microscopy (EFM). Figure 40.1: Schematic of Lateral Force Microscopy (LFM) 40.1 Lateral Force Microscopy Lateral Force Microscopy (LFM) is an offshoot of Contact mode AFM imaging. During a typical LFM scan, torsional bending of the cantilever occurs in the scan direction and the lateral deflection is measured in addition to its vertical deflection due to surface topography. The lateral deflection is measured by applying a force on the cantilever while it horizontally moves across the sample and strength of the lateral deflection signal is related to the friction force between the sample surface and the tip, the topography of the sample surface and the cantilever lateral spring constant. Also, the angle of torsion is proportional to the magnitude of the lateral force. On 2

3 scanning a flat surface with different friction factor regions, the angle of torsion changes in every region and this allows measurement of the local friction force. For topographically patterned surfaces, the change in cantilever motion is sensed by the detector and a deflection signal is generated which is convolution of surface topography and friction map. To differentiate between friction and topography, two images captured side-by-side utilizing the trace (left-to-right tip motion) of each line in the raster scan and a second pass i.e. retrace (rightto-left tip motion) on the same line. One of these two images is inverted and subtracted from the other to reduce topographic effects in the LFM signal to obtain image of frictional forces. LFM is useful for studying a sample with of inhomogeneous surface compounds like semiconductors, polymer films, data storage devices, surface contaminations etc. Different surface energy domains created by micro contact printing can be identified by LFM Phase Contrast Imaging Phase imaging is carried out in tapping or intermittent contact mode of an AFM for generating a phase contrast image of a sample surface. The AFM records the phase shift signal which is due to delay in the oscillation of the cantilever as it scans a surface along with the topographic data. The time delay in oscillation depends on the local adhesiveness and thus is different for different material phases. Phase imaging is used for differentiating multiple components of composite materials in biological samples or polymer blends. Phase imaging complements LFM and force modulation techniques to map variations in surface properties such as elasticity, adhesion, friction and also in identifying surface contaminants. Sub surface imaging can also be performed by Phase contrast imaging. 3

4 Figure 40.2: Schematic of Magnetic Force Microscopy 40.3 Magnetic Force Microscopy Magnetic Force Microscopy (MFM) is a specific mode of operation which comes under noncontact mode scanning probe microscopy and is unique in identifying domains with different magnetic orientations. In this technique, the force sensing probe is magnetic, which interacts with the magnetic field of the sample when brought in close proximity. Hence, MFM in principle is identical to a simple AFM with a magnetized tip, where the vertical motion of the tip while scanning across the sample is determined by the strength of local magneto-static interactions. MFM produces a 2-D map of the magnetic forces or force gradients from a sample as a function of position, with lateral resolutions of ~ nm. Many kinds of magnetic interactions like magnetic dipole dipole interaction can be mapped using MFM. MFM was developed in 1987 by Y. Martin and H. K. Wickramsinghe shortly after the invention of AFM. It became a powerful technique as it allows investigation of submicron magnetization patterns with high lateral resolution without the need of rigorous sample preparation conditions. The most important part in MFM for high quality images is the magnetic force sensor due to the long-range nature of magnetic forces. Generally, a cantilever and tip assembly made of Si or 4

5 Si 3 N 2 is coated with magnetic thin film (e.g., CoCr). Tips must have a well defined magnetic state in order to obtain quantitative information out of MFM measurements. The spatial resolution in MFM imaging is better if the size of magnetically sensitive region of the tip that is exposed to the sample stray field is as small as possible. Generally, the coated tip has end radii ~10-40 nm. By using appropriate coating materials and techniques, it is possible to tailor the magnetic properties of the tips according to the application. Unknown magnetic state of the tip and its behavior in the magnetic stray field can lead to artifacts and image misinterpretations. The operation of MFM is similar to that of an AFM. Both static and dynamic detection modes can be applied, but mainly the dynamic mode offers better sensitivity. The cantilever-tip assembly is excited to vibrate close to its resonance frequency, with certain amplitude and a phase shift with respect to the drive signal. In the absence of tip-sample forces, the system can be modelled as a damped harmonic oscillator, natural resonant frequency (ω 0 ) and Quality factor (Q) of the cantilever are where k is the spring constant and b is the damping constant. (40.1) In presence of a force gradient on the tip due to the sample stray magnetic field, the cantilever behaves as if it has a modified spring constant,. This results in a shift in the resonance frequency, change of the amplitude and shift in phase of the probe oscillation. All of these are measurable quantities and can be detected using a deflection sensor. The image obtained from a magnetic tip will contain the information of both magnetic and nonmagnetic tip-sample interactions. Critical interpretation of MFM data requires separation of the 5

6 responses due to long range magnetic interactions from short range topographic interactions. The solution to this problem is the method of terrain correction where the tip is allowed to follow the surface height profile using constant distance mode. In the most of the microscopes, the topography is measured in dynamic mode and data is recorded as a separate image during first scan line. This height data is used to lift the tip to a constant distance above the surface during the second (magnetic) scan line, during which the feedback is turned off. Hence, the effect of short range interactions should be eliminated in the second image. Figure 40.3: Schematic of Electrical Force Microscopy 40.4 Electric Force Microscopy Electrostatic Force Microscopy (EFM) is a derivative mode of AFM for qualitative examination of the intrinsic or applied electrostatic properties of a sample like surface potential and surface charge distribution, using a conductive tip. The cantilever deflects due to the electrostatic interaction with the surface allowing the mapping of vertical and near-vertical gradient of the electric field. The magnitude of the deflection is proportional to the charge density. An external voltage can applied between the sample and the biased tip while scanning in non contact mode if the sample surface charge is not large to generate good contrast image. EFM maps the spatial 6

7 variation of the local surface charged domains in a way similar to magnetic domain imaging in MFM. EFM also leads to the measurement of the van der Waals forces the between the tip and the sample surface which are always present in addition to the electrostatic force. Hence, the obtained deflection signal contains information of both surface topography and of surface electrical property. Proper EFM imaging can be done by separating EFM signal from the entire signal. EFM is performed in any of the three modes: amplitude detection, phase detection or frequency modulation. As van der Waals force is inversely proportional to r 6 and electrostatic force is inversely proportional to r 2, van der Waals forces are prominent when tip is closer to the sample while scanning in contact mode. If the tip is lifted up from the sample surface while scanning, van der Waals forces decrease in magnitude and the electrostatic forces becomes prominent. The tip is then biased and scanning is done parallel to the topography line without engaging the feedback. This technique of lift-off is mainly used in all the instruments for performing EFM, to obtain proper topography image and topography free EFM image from the same scan. 7