charge writing and detection by electric scanning probe techniques

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Nikolaus Knorr, Sony Deutschland GmbH, Materials Science Laboratory, Stuttgart, Germany

Stefan Vinzelberg, Asylum Research, Mannheim, Germany

Introduction

Understanding the generation mechanisms and dissipation behavior of electric charge on the surface and in the bulk of

insulating materials is of importance to the study of a variety of phenomena. These include triboelectrification and electrostatic

discharge (ESD), as well as to several current and future applications, such as cable insulation, laser printer and photocopier

xerography, charge based data storage, plastic material recycling, and electret field-effect transistors, microphones, and dust

filters. Remarkably, the charging and discharging mechanisms of insulators are often not understood in detail, and even today

the identity of the charge carriers remains obscure in many cases.1,2,3 A convenient tool for investigating these mechanisms is

the atomic force microscope (AFM) because the charging can be realized with the same probe used for the analysis, and electric

field sensitive AFM modes such as Electrostatic Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KPFM), also known

as Scanning Kelvin Probe Microscopy (SKPM), provide highly resolved spatial information of the charge distribution with

extremely high sensitivity. Accordingly, these methods have been used since their invention for the study of charging and

discharging phenomena on insulators.4,5

Charge Writing and Detection by Electric Scanning Probe Techniques

Biased AFM-probe-induced charge writing has found widespread interest, often in conjunction with data storage or nanoxerography. Charge pattern and xerographically assembled nanoparticles with feature sizes below 100 nm have been generated.6 Triboelectric (frictional) and contact charging phenomons are also well suited for investigation by EFM and KPFM because both the morphology and the charge distribution of the contacted surface are detected simultaneously in high resolution. Triboelectricity is a common phenomenon experienced any time two materials come into contact. Tribocharging affects us frequently in our daily lives due to the widespread use of highly insulating synthetic fibers and plastics with low surface and volume charge dissipation. Research on triboelectric charging mechanisms has recently found renewed interest,3,7,8 and AFM studies have helped in revealing new insights such as prevalent bipolar charging characteristics on micrometer length scales and the link of morphological features to the charge distribution.9 In this application note, biased-probe induced and triboelectric charging of thin insulating films is covered, with the focus on practical aspects of the AFM techniques. An Asylum Research MFP-3D™ AFM was used for all work described.

EFM and KPFM Modes for Charge DetectionEFM and KPFM investigations are useful for high resolution studies of any system where electric fields are present, such as biased electrodes and devices, or for contact potential differences of materials. Here, we are interested in the electric fields generated by uncompensated charge carriers deposited on, or injected into, insulators. The physical system under

study is that of a thin insulating sheet or film of thickness d, with its free face charged by the biased AFM probe, by rubbing, or by evaporation of water, and its opposing face in good mechanical and electrical contact to a grounded back electrode. For both EFM and KPFM scans, the so-called dual-pass method is applied, with a first scan in AC mode recording the topography and a second scan for electric field recording along the same scan path in the non-contact mode but with an adjusted lift height h added to the previously recorded height information.

Typically, bare or metal covered (Au, Pt, or PtIr) AFM probes made of strongly doped Si with medium-range force constants are used for EFM and KPFM. Here, Pt-covered AFM probes with relatively stiff cantilevers (40 N/m force constant) have been used to reduce the distortion of the topographic scan by the attraction between probe and charged surface, and to increase the resolution and sensitivity of the EFM scans by small EFM amplitude scanning (typically h = -10 nm, measured with respect to the height of contact in the topographic scan).

EFMVertical force gradients generated by the electric fields emanating from the charged sample and interacting with the probe modify the harmonic potential of the cantilever spring (Figure 1). Attractive force gradients weaken the restoring cantilever force while repulsive force gradients cause the

AFMAsylum Research

opposite effect (Figure 2A). This results in a negative or positive shift of the resonance frequency of the cantilever (Figure 2B). In EFM mode, these resonance shifts are detected by measuring the phase or the amplitude change of the cantilever oscillation which is mechanically excited by a piezo element at or near its resonance frequency. For small resonance shifts, the phase signal is proportional to the resonance frequency shift. Larger frequency shifts can be tracked by using a phase lock loop-type feedback that adjusts the drive frequency such that the phase remains at 90°. In this work, the phase signal (φ) was evaluated.

Attractive force gradients make the cantilever effectively “softer,” reducing the cantilever resonant frequency and increasing the phase. Conversely, repulsive force gradients make the cantilever effectively “stiffer,” increasing the resonant frequency and reducing the phase. A quantitative interpretation of the EFM signal is difficult because it depends in a complicated fashion upon the geometries of the probe, sample, and charge pattern due to the long range nature of the Coulomb force. For example, a point charge q on the surface of the insulting film will induce mirror charges in the probe apex (qp) and the back electrode, which will all be mirrored again ad infinitum at the probe, dielectric, and electrode surfaces. For proximate scanning of the charge, the dominant force on the probe will be generated by the interaction of q and qp, yielding an attractive force which is proportional to q2 (Figure 3). For strong or extended charge patterns, the fields reaching the cone of the probe and the cantilever will also considerably affect the cantilever oscillation. To obtain stronger and better defined charge signals, usually a constant DC bias (Vp ) is applied to the probe during scanning of the second pass. In this case, for a point charge q, often only three forces need to be considered: the forces between the charge in the biased probe (Qp ) and its mirror charges in the dielectric and the electrode (Qm), between Qp and q, and between q and qp (Fig. 3).10 For Vp well above the surface potential of q at the probe, Δφ is approximately proportional to q.

KPFMJust as for EFM, there are several different KPFM modes. In our experiments, the cantilever is no longer driven mechanically by the piezo in the second pass, but an AC voltage VAC sin(ωt) is applied to the AFM chip with ω /2π at the resonance of the cantilever. Any electric fields reaching the probe will exert a force on the charges induced at the probe by the applied AC voltages and excite a mechanical oscillation of the cantilever at the resonance frequency. The oscillation is however nullified by a feedback applying an additional DC voltage (VDC) to the AFM chip, annihilating the external fields at the probe. In the case of charge deposited on an insulating film,VDC is equal to the surface potential (VSP) generated by the charge at the probe with respect to the grounded back electrode. Mapping VDC (x,y) thus reflects the distribution of the surface potential on the sample surface. The origin of the force on the probe can


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Figure 2: (A) The harmonic potential of the cantilever spring is modified by an attractive or repulsive force gradient. (B) This modification leads to a shift of the cantilever resonance frequency and an associated phase shift which is detected by the AFM control electronics.

Figure 1: In Electric Force Microscopy (EFM), the electric field generated by charge on the sample surface interacts with the AFM probe at the end of the vibrating cantilever beam (distortion of the electric field by electrostatic induction at the probe and cantilever are not considered).


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be derived by considering that the energy of the capacitor (C ) formed by the probe and the electrode is given by:

E = ½ CV2 Eq. 1

The z component of the electrostatic force is F = ½ dC / dz V 2. Because V = (VDC – VSP ) + VAC sin(ωt), the vertical electrostatic force is the sum of three terms: a static term, one term oscillating with a frequency of ω, and one with 2ω. The ω-term is:

Fω = dC / dz ( VDC – VSP ) VAC sin(ωt) Eq. 2

which is nullified by a feedback loop for VDC = VSP. For a proper operation of the feedback loop, the insulating film has to be thin enough (d < 50 µm for the usual type of AFM probes), so that the probe-electrode capacity generates sufficient forces on the probe for the regulation mechanism to work efficiently.

Uncharged insulators have a surface potential VSP close to zero. However, measured VSP values can easily exhibit offsets of several volts caused by fields emanating from electrostatically charged objects close to the cantilever substrate. For example, the plastic cap of the AFM probe holder can get charged easily. To reduce the offsets, the probe holder may be discharged, for example, with an antistatic gun or by wiping with a wet cloth.

Similar to the difficulties in interpreting EFM signals quantitatively, surface charge densities are generally difficult to

derive from the KPFM-determined surface potential values at the probe apex due to the long range nature of the Coulomb field, especially for strong and concentrated charge patterns. KPFM scans are, however, somewhat less difficult to interpret as compared to EFM scans, because of the reduced parameter set (no Vp ) and the fact that the measured electric field distribution remains almost unaffected by the presence of the probe (the concept of mirror charges in the probe is not applicable because the electric fields are nullified by the applied voltage). In a working approximation, charge densities which are constant on an area large enough to be resolved by KPFM are proportional to the measured surface potential.11 Under these assumptions, the surface density σ of charges that are located on the surface of the insulating film can be approximated by the capacitor equation:

σ (x,y) = εε0 VSP (x,y) d -1 Eq. 3

where ε is the dielectric constant of the insulting film and ε0 the electric permittivity of free space.12 Eq. 3 follows from the fact that the charge induced in the probe/back-electrode capacitor by the applied voltage has to match the charge at the insulator surface in order to nullify the electric fields between the charge and the probe. For a uniform space charge density ρ in the insulating film, it follows from the Poisson equation that:

ρ = 2εε0 VSP d-2.13

The lateral resolution of KPFM and EFM probes can not be determined easily. It is, however, generally advisable to use long and slender probes for higher resolution.14 In addition, the use of rather thin insulating films is beneficial because the range of the electric fields of the charges at the surface is reduced by the compensating fields of the mirror charges in the proximate back electrode. It should, however, be noted that, for biased-probe charge writing on very thin films (d < 50 nm), electric break down can easily damage the probe.15

Biased-Probe Induced Charge WritingMany different types of insulating material are suitable for AFM charging experiments. Insulating sheets can be either glued to an electrode substrate, or a metal film can be evaporated onto the backside of the sheet. Alternatively, insulating films can be deposited directly on an electrode sample, for example, by thermal evaporation or by spin- or dip-casting from solution. The latter methods have the advantage of typically providing discharged and well-attached films of low surface roughness. The quality of the contact of the film to the sample is crucial because for loose contacts, charge will also accumulate on the side of the sheet facing the electrode. Frequently used systems are oxide layers grown on doped Si wafers and thin (d < 100 nm) spin-coated films of poly(methyl methacrylate) (PMMA) which can be strongly and persistently charged by comparatively low voltages (<50 V) under ambient conditions. Charging of insulators has been found to be highly variable in different laboratories and for different experimenters due to, amongst

Figure 3: Center linecuts of EFM phase-scans of a positively charged spot at x = 0 on a 40 nm PMMA film. For Vp = 0, no force acts on the probe (φ ≈ 90°) for | x | > 150nm. Closer to the charge, φ is increased due to attraction of q and qp. For Vp = -5V, φ is increased (φ ≈ 100° for |x | > 250 nm), due to the attraction between Qp and Qm. Close to the charge, φ is further increased due to the attraction of Qp and q and q and qp. For Vp = +5V, φ is increased symmetrically (φ ≈ 98° for |x | > 250 nm), also due to the attraction between Qp and Qm, which have however interchanged signs. Close to the charge, φ is reduced due to the repulsion of Qp and q, which is however somewhat attenuated by the attraction of q and qp. For smaller positive Vp , these opposing forces result in camel-shaped phase scans.

other things, its strong dependence on the humidity. Therefore, for quantitative charging experiments it is advisable to work under a well-defined atmosphere by performing the AFM experiments in a glove box (Figure 4) or in a humidity cell. A low humidity will also reduce the charge dissipation rate.16

AFM charge writing can be accomplished by applying a constant bias or voltage pulses to the continuously scanning probe, or point-wise by applying a voltage pulse to the probe at a fixed location. In Figure 5, the surface of a 160 nm PMMA film has been charged continuously while scanning in contact mode by application of +50 V bias voltages using MicroAngelo,™ the built-in tool in the MFP software. To avoid damage to the AFM electronics, bias voltages above ±10 V were applied to the back electrode, electrically connected by a metal clamp which touches the electrode at a point where the insulating film has been removed. In this case the cantilever substrate was held at ground. For ±10 V, the authors verified that charge writing is symmetric for the two alternatives of bias application (cantilever substrate versus back electrode). In AFM charge writing of insulators, a probe which is positively biased relative to the back electrode usually yields positive charging and surface potential values at the sample surface and vice versa. To avoid confusion, voltages stated in the following text always refer to potentials applied to the cantilever substrate relative to the back electrode.

Charge Spot WritingCharge spots were written in Figure 6 by square voltage pulses of up to 80 V on a 12 µm thick float glass slide and on a 1.4 µm thick spin-cast film of poly(1-trimethylsilyl-1-propyne) (PTMSP), which is a high-free-volume glassy polymer of intrinsic porosity (inner surface: 550 m2/g).17 First, the probe was brought into contact with the polymer film by using contact mode AFM feedback with a controlled, low load force (typically 25 nN). Next, a square voltage write pulse was applied by triggering an external function generator connected to an external high voltage amplifier, and then the tip was retracted before moving to the next spot location. Usually, no modification in the topography at the contact position due to indentation of the tip is observed, because of the low load forces and relatively low voltages (<100 V) used.

Figure 7 summarizes the performance of some functions of AFM based charge writing which are relevant for data storage. All charge spots shown have been written and scanned in ambient air. Figure 7A indicates the high reproducibility of the method over a 90 x 90 µm2 scan area, 7B demonstrates the targeted reversing in polarity (erasing) of single bits, 7C their rewriteability, 7D illustrates the insensitivity of charging to the applied load and that writing from a lift height is possible, and 7E demonstrates high resolution scanning of charge spots which are as small as 30nm (full width half max).

Charge Spot Writing Charging MechanismThe surface potential peak heights and widths of the charge spots are not correlated to the applied pulse voltages in a simple manner, one reason being that the size of the charged surface is often too small to be resolved by the probe, yielding average values of the surface-potentials above the charge pattern and the neighbouring uncharged areas.16 It has however been shown that the KPFM peak volumes, which are proportional to the total


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Figure 4: MFP-3D AFM operated in a glovebox, which is in turn located inside of an acoustic isolation chamber.

Figure 5: Surface potential scan of a charge pattern written on PMMA in ambient air in contact mode using the Asylum Research MicroAngelo built-in tool software. Scan size: 8 µm x 8 µm. Color scale: 1 V.

amount of surface-near charge, approximately follow a square law in the applied pulse voltage, offset by a threshold voltage Vth. The pulse width dependencies of KPFM peak volumes, heights, and widths can be approximated by power laws of positive exponents smaller than one.15 Figure 8 shows results for glass and PTMSP (including data from Figure 6), and for charge written on a 1.0 µm thick polyvinyl acetate (PVAc) spin-cast film.

Vth is a key parameter shedding light on the charging mechanism. Vth is increased for more hydrophobic materials of low water permeability and water uptake capability. Based on this and other observations, such as the comparably weak effect of the probe load on the amount of injected charge (Figure 7), an electrochemical charging mechanism of ionic charge carriers mediated by the field-adsorbed water meniscus at the biased probe has been proposed.15 Supposedly, either water ions are injected from the water meniscus into the insulating film (Figure 9) or, for materials with easily dissociable surface groups, ions from the insulating material are removed to the water phase driven by the high electrochemical potential of the biased water meniscus. According to the former mechanism, Vth will be reduced for polymers with a high degree of free volume or high water swell-ability, because the penetrating ions may keep a larger fraction of their hydration shell when entering the polymer. The validity of the proposed charge injection mechanism is supported by the fact that charging is strong and achieved at low applied voltages for dried amorphous materials with a comparably high degree of water permeability such as SiO2, Al2O3, PMMA, PVAc, polycarbonate (PC), PVAc, polyacrylamide (PAAm), poly(acrylic acid), and PTMSP.

IV. Triboelectric ChargingPronounced triboelectric bipolar charge patterns can be generated easily by rubbing an insulating film with almost any other material. Strong VSP signals in the few voltage range can be achieved by using comparatively thick films (d = 1 µm) according to Eq. 3. It is advisable to either use pointed rubbers,


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Figure 6: Surface potential scans of charge spots written in 2% RH on (A) glass, and (B) PTMSP, by 20 V, 40 V, 60 V, 80 V 0.5s in length pulses. Scan size: 10 µm x 5 µm. Color scale: 5 V.

Figure 7: KPFM study on different aspects of charge spot writing for data storage. (A) 25 x 25 array of spots written on a large scale by (5 V / 0.5s) pulses. (B) 9 x 9 array of charge spots (-7 V / 0.5s) with a 3 x 3 array erased by (7 V / 0.5 s) pulses. (C) Spots rewritten by 1, 102, and 104 (-30 V / +30 V, 104 Hz) square wave cycles, from bottom to top. For higher cycling numbers, peak widths are increased. This spot broadening can be reduced by the use of asymmetric pulses of shifted bias or pulse width. (D) From bottom to top: charge spots written in contact with load forces of 1 µN and 10 nN (+30 V / 5 ms), and with lift heights of h = 100 nm, 150 nm, and 200 nm by (+40 V / 10s) pulses. (E) Charge spots written (+15 V, +16 V, +18 V, +20 V / 1s, from bottom to top) and scanned with a high aspect ratio (~7:1) AFM probe. (A,B) on 20 nm PMMA; (C,D) on 40 nm PMMA; (E) on 30 nm COC.


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Figure 8: Peak volumes, heights and widths of the KPFM charge peaks, as a function of applied voltage pulse height and width. Charge spots written on PTMSP ( g ), glass (5), and PVAc (•), all in 2% RH. Pulse widths in (A) are 0.05s for PVAc and 0.5s for glass and PTMSP. Pulse heights in (B) are +80 V for glass; +80 V and -60 V for PTMSP; and ±50 V for PVAc (closed symbols for negative, open symbols for positive voltages). Lines indicate power law fits in (B) and square law fits in (A) with Vth = -3 V for glass; ±19 V for PTMSP; and -10 V and +24 for PVAc.

so that the entire width of rubbed track can be scanned (the maximum scan range of the MFP-3D scanner is 90 µm x 90 µm), or to scan the edge of broader rubbed tracks with the cantilever facing the untouched surface, in order to have a pristine surface area on the scan which provides a surface potential reference. In several literature studies, the AFM probe itself has been used for the rubbing, which is pointed indeed and offers high control of the applied load.5,18 Also, the scanning can be performed easily after the charging without the need for relocating the rubbed surface area. Relocation can be tedious if the rubbed track is small and if the rubbing did not affect the morphology of the film to such an extent that the scratched can be resolved with the camera. Tracks which are extended in one dimension can be located by single line EFM or KPFM scanning perpendicular to the rubbing direction and subsequent translation of the sample with the micrometer screws in the same direction by the distance of the scanning range. Alternatively, the rubbed surface can be

marked by a scratch or ink close by. For the examples shown here, as well as in Ref [9], rubbing was performed by hand-held pointed hard cotton swabs (HUBY-340 BB013, curvature radius ∼100 μm), which were covered with polymer films before use by soaking in viscous polymer solutions and subsequent drying. The use of low-cost swabs covered with freshly prepared polymer films guarantees that the polymer surfaces on the swab are discharged before use and allows for single-use experiments with pristine surfaces.

Important external parameters for tribocharging are the ambient pressure, the temperature and the relative humidity (RH). Most charging phenomena can conveniently be observed at room temperature and atmospheric pressure. The effect of RH on the charging behavior is multifaceted because adsorbed water can be the source of the triboelectric charge (for example by dissociation of the water molecules into positive and negative water ions), but it can also act as a sink of charge, because


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ionic charge carriers have much higher mobilities on the surface and in the volume of most materials at increased RH.16 At low RH (<10%), strong charge patterns can be generated when at least one rubbing partner is made of a somewhat hydrophilic material and the charge patterns are generally fairly durable over many days. Examples are given in Figures 10 and 11 for PAAm, PVAc, and PMMA films rubbed by cyclic olefin copolymer (COC), PMMA, and PAAm. Some interesting observations are: rubbed tracks are generally charged bipolarly, at onsets of tracks material from the rubber is deposited and surfaces are strongly charged, nanoscopic strongly charged particles are frequently observed along the rubbed track (Figure 11), and stick-slip friction is reflected in the charge pattern.

At higher RH, however, the charge patterns are often rapidly dissipated, especially for materials with a high degree of water uptake capability or water permeability. Rubbing combinations of two hydrophobic materials, such as Teflon, polystyrene, polydimethylsiloxane (PDMS), cyclo olefin polymers (COP), polyvinylchloride (PVC), silicone rubber, polyethylene (PE), polypropylene (PP) will generate much weaker charge pattern at low humidity, but stronger ones at higher humidity (RH > 50%), and charge patterns on such materials are stable even in higher humidity.

Triboelectric Charging MechanismThe mechanism of triboelectric charging is under debate. To explain the complexity of the observations, assumedly several different mechanisms contribute to the charging, their prevalence depending on the type of materials involved and the conditions under which the triboelectrification is generated. In Ref. 9, an insulator/insulator triboelectric charging mechanism by separation of dissociated water ion pairs stemming from adsorbed water has been proposed: the positive water ions, i.e. hydrated protons, are preferentially dragged along the surface together with an excess of adsorbed water in front of a conformal rubbing contact due to their lower adhesion to

and increased separation from the surface, leaving behind the relatively stronger bound and near-surface negatively charged water ions. This explanation finds support in the observation that water droplets evaporating on hydrophobic surfaces generate a pronounced bipolar charge pattern of a negatively charged dewetted circumferential boundary and a strongly positively charged core (Figure 12). Presumably, negative water ions are preferentially left behind on the surface by the receding three-phase line due to their higher affinity than the positive water ions to the polymer/water interface, leading to an accumulation of excess positive ions in the shrinking evaporating droplet.

SummaryThe EFM and KPFM methods have been discussed, including interpretation of the measured variables with regard to uncompensated charge deposited on insulating dielectric films. Practical hints have been given for the controlled generation of charge patterns by triboelectrification and biased-probe induced charging, as well as methods for quantification of the measured signals for the latter. Possible charging mechanisms have been discussed, which may, interestingly, be caused by dissociation and ion separation of adsorbed water in both cases.

Figure 9: Possible charging mechanism for a positively biased probe: I) Ambient water is field-adsorbed. II) Water is dissociated (6H2O → O2 + 4H3O+ + 4e–). III) Positive water ions are injected (for example, H3O+).

Figure 10: Gallery of various surface potential AFM images illustrating the charge patterns on polymer films generated by rubbing polymer covered cotton swabs across the surface (top row: PAAm rubbed by COC). The rubbing direction is from top to bottom. Image size is 90 µm x 90 µm; the lower right shows a complete trace of PVAc rubbed by PAAm (110 µm x 162 µm composed of two separate scans). The color scale varies slightly from image to image but in all images, the potential signal covers a range of a few volts.

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The foregoing application note is copyrighted by Oxford Instruments Asylum Research, Inc. Oxford Instruments Asylum Research, Inc. does not intend the application note or any part thereof to form part of any order or contract or regarded as a representation relating to the products or service concerned, but it may, with acknowledgement to Oxford Instruments Asylum Research, Inc., be used, applied or reproduced for any purpose. Oxford Instruments Asylum Research, Inc. reserves the right to alter, without notice the specification, design or conditions of supply of any product or service. Application Note 23 – 6/2014.

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Figure 11: (A) PMMA rubbed by COC at 2% RH. The surface potential data have been overlaid as color on the 3D topography derived from the height data.

Figure 12: Surface potential scan of bipolar charge pattern generated by evaporated droplets of deionized water on a 1 µm thick COC film. Scan size: 90 µm x 90 µm. Color scale: -0.5 V to 0.2 V. At the positively charged spots VSP reaches up to 5 V.

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charge writing and detection by electric scanning probe techniques

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