ELECTROSTATIC WRITING AND IMAGING USING A FORCE MICROSCOPE
F. Saurenbach and B. D.Terris IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099
ABSTRACT. A modified force microscope, the electrostatic force microscope, has been used to study the charging properties of a polymer surface. A polycarbonate surface was charged by placing a small voltage on a tungsten microscope tip and touching the tip to the surface. It was found that the amount of charge transferred depended on the voltage applied to the tip and on the previous contacts to the surface, but that the charge transferred was independent of the number of contacts and the contact time. These results are compared to polymer/metal contact electrification data in the literature.
tungsten wire. The lever is mounted on a piezoelectric bimorph and is oscillated just above its natural resonant frequency. As the tip scans the surface, changes in the tip-to-surface force gradient will shift the resonant frequency of the lever, and thus change the amplitude of oscillation. The lever motion is detected with an optical fiber based interferometer,7 and a feedback loop adjusts the tip height so as to maintain a constant oscillation amplitude. By monitoring the feedback voltage, labeled z-drive signal in figure 1, contours of constant force gradient are measured.
INTRODUCTION When two materials are brought into contact with each other and then separated, there is often a transfer of charge between the two surfaces.' This phenomena of contact, or tribo-, electrification has becn known and studied for years. Except for the case when both materials are clean metal surfaces, there is little consensus on the mechanism of charge transfer. Quite often, the charge transferred is found to be highly variable, and different experimenters rarely obtain the same results. It is not known whether these variations in charge transfer are due to cxperiinental variations, such as in the microscopic composition of the sample surface, contact area, and contact pressure, or if the variation is intrinsic to the charging proccsses.2
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One approach to resolving these issues is to study the process on a microscopic scale, where hopefully the specific charge sites can be identified. To do this, new charge measurement techniques with much greater spatial resolution than those currently available are needed. As described prcviously,3-5 a modified force microscope, the electrostatic force microscope (EFM), offers promise for obtaining such resolutions, arid in addition has the sensitivity to detect the charge from a few clectrons. In the EFM, a weak lever is scanned close to a sample surface. The lever will respond to any changes in the forces acting on it, such as from surface charge, and the lever deflection is meawred. In our EVM, the lever is an etched metal wire, which is bent at approximately 90 degrees near its end to form a sensing tip. I n addition to measuring the sample charge, the end of the tip can be touched to the surface in order to deposit charge.
Signal
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FIG. 1. Schematic of the electrostatic force microscope.
In previous work, a nickel force microscope tip was used to, tribocharge polymethyl methacrylate (PMMA) surfaces and to image the resultant charge.4 The results showed large charge patterns and often contained both positivc and negativc charge. I t was found, however, that the results were sample dependcnt, possibly due to water adsorption by the PMMA. In this work, wc report on the results of using the force microscope tip to charge a polycarbonate surface. In an attempt to produce more repeatable and reliable charge dcpositions, a small voltage ( I to 15 V) was applied between the tip and a n electrode on the back side of the sample during the tip/wrface contact. We found that the surface could be reproducibly charged, and we present our findings on the dependence of the charge transferred on applied voltage, number of contacts, contact timc, and sample history.
The force gradient measured as described above will include all forces felt by the tip, not just electrostatic forces. In addition, the force gradient between the tip and any surface charge depends on the product of the charge and its image charge in the tip, and is thus insensitive to the polarity of the surface charge. In order to distinguish electrostatic forces from other forces, and to determine the sign of any surface charge, an ac voltage is applied between the ti and an electrode located on the back of the insulating sample.415 If there is charge present on the sample surface, the force gradient between the surface charge and the charge due to the ac voltage will cause a tip oscillation at the ac voltage frequency. The total tip motion will therefore consist of an oscillation at the bimorph frequency, with an envelope at the ac voltage frequency. This envelope is detected by a lock-in amplifier, and the phase of the signal is determined by the sign of the charge. This output is labeled "charge signal" in Fig. 1. This technique has been used to image a variety of local charge distributions, including tribocharged samples4 and ferroelectric domain walls,8 with spatial resolution better than 200 nm and sensitivity approaching the charge of a single electron.
INSTRUMENT
RESULTS
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The operation of a force microscope in the noncontact, or attractive, mode has been described in detail previously.67 For theye cxperirnents, the lever/tip is formed by bending an etched
Shown in figure 2 are six charge spots produced by touching the tip to the surface of a 1 mm thick polycarbonate sample. The tip (non-oscillating) was touched while a voltage was applied to the back electrode and the tip held at ground potential. We refer to the negative of the applied electrode voltage as the tip
90/CH 2 9 3 5 - 5 / 9 0 / ~ 5 7 $ 0 1 . 0 0 0 1 9 9 0 EEE
voltage. The tip voltages used were 15, 10,5,0,-5,-10,-15,with -15 being the leftmost spot and 15 the rightmost in Fig. 2. Any charge spot produced with the 0 V tip is below the noise level of our measurement. The figure is a three dimensional rendering of the ”charge signal”. At each x-y scan position, the lock-in output is recorded as the z coordinate, with the phase adjusted so that peaks indicate positive charge and the valleys negative charge. Qualitatively, the height (or depth) of the signal is proportional to the force gradient between the surface charge and the applied ac voltage. As seen in the figure, the sign of the transferred charge is determined by the applied voltage, and the greater thc voltage the more charge is transferred. FIG. 4. The charge image of four charge regions, all written with a tip voltage of -5 V as a function of contact time. From left to right, the contact time was 1, 2, 3, and 5 seconds. The shown scan length, in the long direction, is 10pm.
Finally, the effect of previous tip contacts at a spot was investigated. The sample was first touched with a -5V tip and then contacted again in the same spot with a + 5V tip. The order of the contacts was then reversed on a new spot. The results are shown in figure 5. The left spot in Fig. 5a was produced with a 5V tip and the right spot with a 5V tip followed by a -5V tip. Fig. 5b shows spots produced by a -5V tip (left) and a -5V tip followed by a 5V tip (right). In these data, it appears that the charge transferred is only a function of the last tip contact. However, when the tip voltage for the second contact was much smaller than that for the first contact, this was not the case. Figure 6 shows a spot produced with a -15V tip (left) and a spot produced by a -15V tip followed by a + 5V tip (right). In this case it appears that the charge does depend on previous contacts, and that the + 5V contact reduccd the amount of negative charge left by the -1SVtip.
FIG. 2. The charge image of six charge regions deposited on a polycarbonate surface. The charge was deposited by applying a voltage to a tungsten tip and touching it to the surface. The applied tip voltage used was, from left to right in the figure, -15, -10, -5,5, 10,and 15 volts. The shown scan length, in the long direction, is 20pm. Figure 3 shows the result of multiple contacts of the tip to a single spot on the sample, all with a tip voltage of -5 V. The left most spot was produced by touching the tip to the surface once, the second spot with two touches, the third with 3, and the fourth with 5. It is seen that neither the hcight of thc charge signal nor the size of the charge spot changes significantly with successive contacts of the tip. The amount of charge transferred is independent of number of contacts.
FIG. 3. The charge image of four charge regions, all written with a tip voltage of -5 V as a function of number of tip contacts. From left to right, the tip was touched I , 2, 3, and 5 times to the surface. The shown scan length, in the long direction, is 1Opm.
___ FIG. 5. In (a), the left region was contactcd by a tip with 5V applied to it. The right region was contactcd by a 5V tip, followed immediately with a -5V tip. I n (h), the ordcr of the contacts was reversed. l h e left region was produced by touching with a -5V tip, and the right region with a -SV tip followed by a SV tip. The shown scan length, in the long direction, is 8pm.
The contact time bctwecn thc tip and sample war also sccn to have little eKect on the amount charge transferred. Shown in figure 4 arc five spots produced with contact timcs of I , 2, 3, arid 5 seconds from left to right. All the contacts werc donc with a tip voltage of -5 V. 158
to be included to obtain a realistic model of the dependence of the FWHM on the charge deposit shape and size.
FIG. 6 . The left region was produced by contacting the surface with a -15V tip, and the right region with a -15V tip followed by a 5V tip. The shown scan length, in the long direction, is 15pm.
DISCUSSION Referring to Fig. 2, we see that the charge signal due to the deposited charge increases with the applied tip voltage. It has been shown previously4 that the charge signal is proportional to Qvsin(4 ( c z 2nF.,z 2
I
ac
2 a z
where Q is the surface charge, V sin(wt) is the applied ac bias voltage, and C is the tip-to-electrode capacitance. If the tip scan height (z) is constant, which is valid for small amounts of surface charge, then the charge signal is proportional to the total surface charge sensed by the tip at a given spot. The linear increase in peak height with applied tip voltage suggests that the deposited charge is therefore also linear in tip voltage. The peak widths, as measured by the full width at half maximum (FWHM), are seen to increase slightly as the tip voltage increases. If the charged area were constant as the voltage increased, as might be expected if the contact area is assumed constant, then it would be expected that the FWFIM would also be independent of tip voltage. As an opposing perspective, the charge density could remain constant and the charged area increase with voltage, as might be the case if the charge could spread and the charge density were limited by the coulomb repulsion of the deposited charge. This would lead to an increasing peak width with voltage. Unfortunately, while there is some increase in peak width with voltage, it is not enough, nor over a wide enough range of voltages, to be conclusive. The slight increasc with tip voltage could also be the result of a small increase in contact area due to the expccted increase of contact force with voltage. The data in Fig. 6 support the hypothcsis that the predominate effect is a change in charge density, and not charge area. As discussed above, the left spot was deposited with a -15 V tip, and the right spot with a -15V tip followed by a t 5V tip. If the charge area increased with tip voltage, then it should be possible to deplete the centcr of a large spot by touching with a tip of small, opposite polarity, voltage. Such a process would produce a "doughnut" shaped charge region. This is not seen to happen in Fig. 6, and we have not obscrved such "doughnuts" in any of our experiments. This suggests that the + SV tip reduced the net negative charge density of a small, central, region in which all the charge resides. Note, in fact, that while the peak heights in Fig. 6 are quite different, the FWHM are only slightly different. (The lower half of the right spot is hidden by the plane in the figure). It is difIicult to quantitatively relate the actual size of the charge region to the measured FWI-IM. The instrumental resolution is governed by the tip scan height, which in these experiments is on the order of 100 nm, by the tip radius of curvature, cstimated to be 50-100 nm, and by the long range nature of the electrostatic force, which will cause the irnage charge in the tip to be located a distance from the end of the tip.J,4,8 Thus, even if the off-axis fields from the chargc deposit were calculated, pararncters involving the tip shape would nccd
In addition to the dependence of charge transfer on tip voltage, we have found that the amount of charge transferred is independent of number of contacts (Fig. 3). In general, it has been found that the charge transferred in tribochar ing experiments does increase with multiple contacts,lj with the one notable exception of some liquid-metal/polymer contacts.10 It has been argued that the increase in charge with increasing contacts may be due to different areas being contacted each time, and that the use of liquid metals can avoid this problem. Since we are able to contact small areas with a high degree of accuracy, and thus may truly be contacting the same area, our findings may be viewed to support this conclusion. Our charging, however, was done in the presence of an applied field, which may be significantly different from the referred to work where no voltage was applied. The results of the dependence of charge transfer on contact history are confusing at first. The low voltage results (Fig. 5) suggest that the charge is independent of previous contacts, whereas the higher voltage data (Fig. 6) clearly shows a history dependence. It is possible that to reverse the sign of the deposited charge, the second touch needs to be at an equal or higher tip voltage. Such an effect would arise if the depth (spatial or energetic) of the deposited charge increased with increasing tip voltage. Further experiments would be needed to confirm this. Finally, we find that the charge transferred is independent of contact time. This is not surprising, as the contact times were all quite long (seconds) compared to typical charge transport times. This result is also consistent with previous findings in the literature for materials where charge spreading is not suspected. 11 Note that in both Figs. 3 and 4 there is a slight increase in peak FWHM from left to right. The increase is small compared to the factor of 5 increase in number of contacts or contact time, and may be related to the decay time of the charge spots. The spots were deposited from left to right and then imaged in the same order. (The total scan size is approximately 20pm x 20pm, but only a fraction of it is shown in the figure. The fast scan direction is perpendicular to the line if spots.) The total time from depositing the first spot to imaging the last spot was on the order of 15 minutes, while the half life of the spots was on the order of one hour. This decay effect, however, should enhance any difference between the charge regions, as the spot expected to be largest was deposited last. We therefore conclude that the number of contacts and contact time have, at most, a very minor influence on the charge transferred in these experiments. In the above discussion, we have assumed that the dominate effect of touching the biased tip to the polycarbonate surface is the transfer of real charge. It is also possible that polarization charge is created in the sample due to the large electric field near the tip.12 We would expect, however, that any polarization charge close to the tip would be opposite in sign to the tip voltage. Therefore, the measured charge, being the same sign as the tip voltage, cannot be solely polarization charge and the dominate effect is real charge transfer. It might also be expected that if the technique has high enough spatial resolution, opposite polarity polarization charge, if present, would be seen. This observation of both polarities of charge has, in hct, been reported in tribocharging experiments on PMMA,4 though it is not clear if this is the correct interpretation of that data.
CONCLUSION In conclusion, we have used a force microscope, which has both high lateral resolution and charge sensitivity, to investigate the charging properties of a polycarbonate surface. In voltage assisted charging experiments between a tungsten tip and the surface, we find that the charge transferred depends on the tip
voltage and on the sample charging history, but is independent of number of contacts and contact time. Future experiments are planned to investigate diKerent polymers and to study the charge transfer in the absence of an electric field.
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