Investigating electrical charged samples by scanning probe microscopy: the influence to magnetic force microscopy and atomic force microscopy phase images. Carlos A. R. Costa, 1 Evandro M. Lanzoni, 1 Maria H. O. Piazzetta, 2 Fernando Galembeck 3 and Christoph Deneke. 1 1 (LCS), Laboratório Nacional de Nanotecnologia (LNNano), Centro de Pesquisa em Energia e Materiais (CNPEM), Campinas, São Paulo, Brazil. 2 Laboratório de Microfabricação (LMF), Laboratório Nacional de Nanotecnologia (LNNano), Centro de Pesquisa em Energia e Materiais (CNPEM), Campinas, São Paulo, Brazil. 3 Laboratório Nacional de Nanotecnologia (LNNano), Centro de Pesquisa em Energia e Materiais (CNPEM), Campinas, São Paulo, Brazil. This technical memorandum discusses artifacts in magnetic force microscopy images due to the influence of electric field gradients above sample surface. We find that the technique is very sensitive to the influence of electrical charges, an effect not yet discussed in detail in literature. Our results show that electrical charges give rise to a signature in the magnetic force microscopy, which is indistinguishable from a magnetic signature. Furthermore, we find a strong correlation between electrical artifacts and Kelvin force microscopy images. Finally, we investigated the influence of electrical charges on the atomic force microscopy phase contrast normally ascribed to different materials. Again, a clear influence of the electrical field is observed. Our results indicate that great care has to be taken in the interpretation of magnetic or phase contrast images, when electrical fields are involved, as in many polymer samples. Introduction Scanning probe microscopy (SPM) is a well established technique to study the magnetic structures of thin films or nanostructures like recording media, nanoparticles, and nanowires [1-3]. Hereby, the magnetic nanometer sized structure is studied by a modified atomic force microscopy (AFM), where an interaction between a magnetic tip and the magnetic domains of the sample is used to achieve lateral resolution in the sub 10 nm regime. As the technique is derived from AFM and carried out with commercial AFM microscopes, it is normally called magnetic force microscopy (MFM). For non-magnetic tips, the AFM contrast arises from changes of the topography of the sample height, which ismeasured moving the tip up and down keeping the amplitude of the cantilever vibration constant. The technique allows as well the monitoring of the phase of the AFM cantilever vibration in comparison to the 1
modulator of the tip. Any change in this phase relation is ascribed to changes in material tip interaction resulting in a viscoelastic contrast. For MFM measurements a Co-coated Si-tip is magnetized and then scanned at a defined distance above the sample following the AFM deduced topography. The microscope is operated in intermittent contact mode, i.e. the tip is vibrated in z direction (vertically to the sample) during the scanning and the amplitude change as well as the phase change compared to the free vibration resonance of the tip is monitored. The contrast in the MFM images is ascribed to the magnetic interaction between tip and sample, giving rise to a force in z direction. This force results in a change of the effective spring constant of the tip. For high magnetic force gradients the tip exhibits an increased effective spring constant, whereas for low magnetic force gradients the effective spring constant of the tip decreases. This change induces a shift of the vibration resonance resulting in an amplitude and phase change of the vibrating cantilever. Normally, any change in phase and contrast is attributed solely to magnetic forces present above the sample, neglecting any influence of possible electrical charges presents in the sample [4]. In this technical memorandum, we investigate in detail and systematically the influence an electric field gradient above sample to MFM images acquired with a commercial AFM using a lithographical patterned Kelvin force microscopy (KFM) calibration sample. We find that the commonly used metalized MFM cantilever is sensitive to the electrical charge present resulting in an apparent MFM contrast for the electrical test structure. Furthermore, we carry out AFM of this test sample to investigate the influence of the electrical field to the AFM phase contrast. We observe that also the AFM phase contrast is sensitive to the electrical fields present resulting in a set of artifacts exhibited in the AFM phase change. Experimental setup The sample used was a non-magnetic glass surface with interdigitated Al stripes, prepared using microlithography techniques in the microelectronics facility in the Laboratório de Microfabricação, LMF/LNNano/CNPEM as demonstrated in Figure 1. The stripes are then contacted by wires to allow the application of an electrical field. To obtain an electrical field gradient on the sample, we connect the fingers to a variable voltage source. Figure 1: Illustration of non-magnetic glass surface with interdigitated Al stripes sample, connected with wires to a voltage source. AFM experiments were carried out using a commercial AFM instrument (NanoSurf FlexAFM). The scan head was mounted within an environmental chamber, which allows full control of the relative humidity and temperature (30%UR and 25 C). Following standard MFM techniques [5], the cantilever resonance frequency was 2
determined. For measurements, the cantilever is excited slightly of resonate towards lower frequencies and topographic images are taken in intermittent contact mode (AFM). The magnetic signature is deduced by scanning the tip a second time above the sample and registering the phase shift of the tip vibration. To introduce a pseudo magnetic signal, we apply different voltages between the interdigitated Al fingers. To qualify the electrical potential of the sample, KFM was carried out. During intermittent contact mode measurement, the mechanical oscillation of the tip is tracked by the photodetector and analyzed by two feedback loops. The first loop is used in the conventional way to control the distance between tip and sample surface, while scanning the sample at constant oscillation amplitude. The second loop is used to minimize the electric field between tip and sample by adjusting the tip bias voltage to minimize electrostatic forces between tip and sample. A lock-in amplifier applied a 15 khz electrical AC signal to the tip and measures the tip vibration at this frequency while scanning, adding a DC bias to the tip to recover the undisturbed 15 khz mechanical oscillation. The KFM image is built using the tip s bias voltage fed to the tip, at every pixel, thus detecting electric potential gradients throughout the scanned area. Results and discussion Figure 2 shows AFM topography [Fig 2(a)] as well as MFM image [Fig. 2(b)] on the same area without any voltage applied between the Al stripes. AFM was acquired in intermittent contact and MFM was acquired in phase contrast 250 nm above the surface. As the Al stripes have been fabricated by lithography followed by evaporation of Al, they are higher than the flat glass substrate. This is clearly seen in the topographical contrast of Fig. 2(a), where the Al stripes can easily be identified running from the top to the bottom of the image. The AFM the stripes show a 500 nm width of with a ca. 2.5 µm spacing. The height of the stripe is determined to ca. 200 nm. Figure 2(b) shows the MFM contrast. As Al is nonmagnetic, we expect no signature from the strips. Figure 2: SPM image of interdigitated Al stripes (a) topography (AFM) showing a clear height contrast, which allows the finger to be identified. (b) MFM image of the same area without any voltage applied between the fingers. Indeed, only a weak signal in the phase image (Fig. 2(b)) is observed, which is ascribed to a slight difference in the interaction of tip with Al and the glass substrate even though the tip is 250 nm above the sample. This material-based contrast is well known in AFM and used to identify different materials. It is interesting to point out that this interaction is still visible for this large distance between tip and sample. 3
Things change drastically, when a voltage is applied to the sample. Figure 3(a) shows the AFM topography of our sample with 5V applied between the fingers. The same area is depicted in Fig. 3(b), exhibiting a strong MFM contrast between the biased and grounded Al fingers. The obtained AFM topography, observable in Fig. 3(a), is similar to the obtained contrast with no voltage applied. In contrast to Fig 2(b), the MFM image acquired from the sample and depicted in Fig 3(b) depends largely on the applied voltage. Hence, the MFM signal exhibits a strong phase variation but this arises from the electrical field gradient above the sample. KFM and MFM images. Hence, the observed contrast in the MFM and the KFM could be ascribed to a magnetic field or to an electric potential giving rise to a shift in amplitude and phase of the SPM probe, when scanned over the sample surface. These results clearly show that commercial MFM tips are sensitive to any electric potential gradient present on the sample surface. Figure 4: Images of interdigitated Al stripe with a 5V bias applied: (a) topography (b) KFM image. Figure 3: (a) AFM topography of the sample (b) MFM phase shift 5 V applied between the Al stripes. To further illustrate the influence of the electrical voltage, KFM imaging was carried out to deduce the electrical potential of the surface. The results are shown in Figure 4, Fig. 4(a) depicts the AFM topography and Fig 4(b) the KFM image of the same area using the Co-coated tip. During image acquisition, a 5 V bias was applied to the figures to introduce a polarization to the sample. This seems to be a predictable result, as commercial MFM tips are similar in their design to commercial KFM tips. Usually Cocoated Si-tip are used for magnetic experiments whereas Pt/Ir-coated Si-tip are used for electric experiments. Both exhibit similar mechanical characteristics: a resonance frequency of 75 khz and a force constant 2.8 N/m. Finally, verified the influence of an applied voltage to the AFM phase signal in intermediate contact mode when using a metal-coated tip. Again, the AFM topography, as seen in Fig. 4(a), is similar to the topography images depicted in Fig. 2(a) and 3(a). Figure 4(b) shows the deduced potential map of the sample by KFM. A simple visual comparison indicates a strong correlation between the 4
References [1] Binnig, G.; Quate, C. F.; Gerber, C.; Atomic Force Microscope, Physical Review Letters, 56, 930-933 (1986). Figure 5: AFM phase contrast images: (a) without applied voltage (b) 5 V bias applied to the stripes. Figure 5 (a) shows the AFM phase contrast signal obtained with our test sample without applied voltage. Due to the difference between materials of the stripes (Al) and the substrate (glass), a phase shift is observed when the tip is above the Al stripe. Figure 5 (b) depicts the same area with 5 V applied to the Al fingers. A clear change in the phase contrast image is visible compared to the first condition indicating that the AFM phase signal is additionally influenced by the applied voltage over the sample. [2] Gouveia, R. F.; Costa, C. A. R.; Galembeck, F.; Water Vapor Adsorption Effect on Silica Surface Electrostatic Patterning, Journal of Physical Chemistry C, 112, 17193-17199 (2008). [3] Atomic Force Microscopy, Oxford, NY, 2010. [4] Baytekin, H. T. et. al., Control of Surface Charges by Radicals as a Principle of Antistatic Polymers Protecting Electronic Circuitry, Science, 341, 1368-1371 (2013). [5] Scanning Probe Microscopy and Spectroscopy, Method and Applications, Cambridge University Press, NY, 1994. Conclusion The presented results show a strong influence of an electric field gradient on any kind of conductive AFM cantilever probe. The standard MFM imaging technique cannot distinguish magnetic and electric forces above the surface as well as AFM phase contrast cannot distinguish viscoelasticity properties from electric forces in intermediate contact mode using a metal coated tip. To have greater confidence in assigning observed MFM images to actual magnetic sample features, we suggest that contrast should be compared in micrographs acquired before and after tip magnetization, to avoid or at least to decrease the effect of electric sample features on the observed MFM contrast. 5