The use of scanning conduction microscopy to probe abrasion of insulating thinfilmsJ. T. Dickinson, L. C. Jensen, K. H. Siek, and K. W. Hipps Citation: Review of Scientific Instruments 66, 3802 (1995); doi: 10.1063/1.1145441 View online: http://dx.doi.org/10.1063/1.1145441 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/66/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electromechanical response of amorphous LaAlO3 thin film probed by scanning probe microscopies Appl. Phys. Lett. 105, 012906 (2014); 10.1063/1.4889853 Contrast in scanning probe microscopy images of ultrathin insulator films Appl. Phys. Lett. 88, 063107 (2006); 10.1063/1.2172397 Nanostructural conductivity and surface-potential study of low-field-emission carbon films with conductivescanning probe microscopy Appl. Phys. Lett. 75, 3527 (1999); 10.1063/1.125377 Surface potential of ferroelectric thin films investigated by scanning probe microscopy J. Vac. Sci. Technol. B 17, 1930 (1999); 10.1116/1.590851 Scanning probe microscopy of thermal conductivity and subsurface properties Appl. Phys. Lett. 61, 168 (1992); 10.1063/1.108207

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The use’of 3canning conduction microscopy to probe abrasion of insulating thin films

J. T. Dickinson, L. C. Jensen, and K. H. Siek Department of Physics, Washington State University, Pullman, Washington 99164

K. W. Hipps Department of Chemistry, Washington State University, Pullman, Washington 99I64

(Received 31 January 1995; accepted for publication 29 March 1995)

The use of scanning force microscopy (SFM) to probe wear processes at interfaces is of considerable interest. A simple modification of the SFM which allows one to make highly spatially resolved measurements of conductivity changes produced by abrasion of thin insulating films on metal substrates is presented here. The technique is demonstrated on fluorocarbon polymer thin films deposited on stainless-steel substrates. 0 1995 American Institute of Physics.

I. INTRODUCTION

The scanning force microscope (SFM) is a useful tool for examining the consequences of tribological wear, particu- larly on inherently flat surfaces such as single crystals. How- ever, on moderately rough surfaces, changes in topography due to tribological loading are often difficult to interpret in terms of wear processes. We have examined the wear of an important class of coatings, namely fluorocarbon thin films deposited on stainless steel, using a simple modification of standard SFM techniques. Metal-coated silicon nitride ‘tips are used to probe current flow between the tip and the con- ducting substrates. Simultaneous topography and conduction images are acquired. During wear (performed outside of the SFM), localized thinning of the film and exposure of bare metal are easily and unambiguously detected by this method on size scales less than 50 nm. We describe the technique and present results on two types of fluorocarbon thin films. This technique is similar in spirit to the use of combined STM/SFM imaging,’ and spatially resolved potentiometry used recently to image potentials of metallic structures on integrated circuits2 Point-by-point measurements of semi- conductor conductivity have also been made using an SFM with conducting tips in efforts to probe semiconductor dop- ing profiles3 as well as spatially controlled potentiometry for thin-film structures.”

II. EXPERIMENT

Film topography was characterized with a Digital Instru- ments Nanoscope III SFM operated in the contact mode. SFM scans were acquired at scan sizes ranging from 200 X300 nm2 to 100X 100 pm2 and at tip velocities from 0.2 to 200 pm/s. A simple modification of the SFM allowed simul- taneous spatially resolved measurements of film conductiv- ity. A schematic of the experimental arrangement is shown in Fig. 1. Scanning conduction microscopy (SCM) measure- ments were made using commercial triangular Si,N, SFM tips (115 pm from tip to base)5 that were sputter coated with 300 A of Au or Ag at room temperature. Both sides of the cantilever were coated to minimize cantilever deflection due to stresses in the metal coating. The SFM was operated in the contact mode, using a nearly constant compressive force,

typically 30 nN. Assuming a tip radius of 50 run, the pressure applied by the tip is about 20 MPa, well below the yield stress of most materials. Forces were determined from the measured displacements and quoted force constants for the commercial cantilever beams used in this experiment. The sample was mounted on double stick tape. to insulate it from ground. Electrical contact to the conductive cantilever was made through the cantilever clamp. The circuit was com- pleted by a 30 pm, stainless-steel wire spot welded to the conducting substrate.

Current measurements were performed with a Keithley 602 electrometer whose output was amplified with a Tek- tronix 3A9 differential amplifier. The amplifier output was digitized in phase with the acquisition of the SFM image, providing a SCM image that corresponded to the topographi- cal image acquired simultaneously. For improved time reso- lution, current measurements were occasionally acquired at higher sampling rates during individual scans with a LeCroy 6810 digitizer. A bias voltage (typically) 1 V was applied to the sample through a limiting resistor @limitins, typically l-10 Ma) to prevent current-induced damage to the metal coating of the SFM tip when metal-to-metal contact was

Scanning Conduction Probe

Metal Coated

Double Stick Tape tes substrate from ground)

FIG. 1. Schematic diagram of an experimental arrangement for measure- ment of the spatial distribution of conductivity of a thin Urn on a conductive substrate. One of the most important components is rGiting.

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FIG. 2. (a) Scanning conduction microscope scan of a nonabraded PTFE coated stainless-steel sample; (b) SCM scan of the same PTFE film after the light abrasion (five strokes); (c) SCM scan after a total of eight strokes; (d) SCM scan after a total of 16 strokes; (e) SCM scan after a total of 50 strokes; (f) SFM topographic scan corresponding to (e).

made. When the SFM tip is over a region where the polymer limiting resistor. Regions of intermediate conductivity are film is thick, the effective tip-to-substrate resistance is high also evident. Thus a typical SCM image is a three- (>lOr’ a) and littl e current flows through the circuit. When dimensional plot of the magnitude of the current versus n-y. the tip crosses a bare region of the substrate, the tip-to- The useful lifetimes of the conducting tips were limited substrate resistance falls to low value (<lo5 a) and the re- by wear of the metal coating and ranged from 10 to 30 scans. sistance of the circuit is approximately equal to that of the High currents (> ,xA) associated with low limiting resis-

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SFM Force and Current vs Time for PTFE Film on Stainless Steel During: Contact and Detachment o Intermediate Current Region

Time (msec) Time (msec)

HG. 3. (a) The force and (b) conduction current vs time during approach and contact for an intermediate conduction region on an abraded PTFE,ktainless - ._ sample. (6) The force and (d) conduction current vs time during retraction.

tances or high bias voltages, however, dramatically shortened tip lifetimes. Both positive and negative bias voltages pro- duced nearly equal currents with no apparent nonlinearities, suggesting ohmic conduction. Little effort was made to elec- trically shield the sample; pickup or perhaps ground loops generated small but detectable modulations in the current signal which are visible in the images presented here.

mulative and served as a simple means to demonstrate the technique.

HI. RESULTS AND DlSCUSSlON

Samples of polytetrafluoroethylene (PTFE) coated stain- less steel were prepared in our laboratory by laser ablation deposition.6-8 PTFE films, -200 nm thick, were produced by focusing the output of a Lambda Physik EMG 203 MSC laser operating at 248 rmr (KrF) through a quartz window onto a PTFE substrate (Goodfellow chemicals) mounted in vacuum at a pressure of 1O-4 Pa. The laser was operated at a pulse repetition rate of 10 Hz and a fluence of 4 J/cm2 per pulse. The ablation products were collected on a stainless- steel substrate held at 340 “C. TEM and XRD analysis con- firmed that the resulting 300 run thick films were semicrys- talline PTFE. A few specimens of plasma-deposited, -130 nm thick, perfluoroalkoxy (PFA) films on stainless steel were also examined. In both cases, stainless steel exposed by wear processes would be oxide coated.

Film abrasion was performed by manually drawing a cotton swab dampened with acetone across the surface. The applied normal force was approximately 2 N over a nominal contact area of 3 mm’. Although not well controlled, the wear produced by successive linear strokes was indeed cu-

Figures 2(a)-2(e) shows 10X10 m2 SCM images for a PTFE film on stainless steel prior to and following progres- sively increasing extent of abrasion. The scans were acquired at a tip velocity of 20 &s with a 100 mV bias voltage applied through a 10 MQ limiting resistance; a full-scale reading on the electrometer corresponds to 10 nA. As shown in Fig. 2(a), the unabraded films are basically insulating un- der the tip; sampling several regions of the film, we typically observed one or two submicrometer-sized regions of en- hanced conductivity [similar to the small region in the fore- ground shown in Fig. 2(a)] in approximately 20% of the scans. These features were reproducible upon repeated scans. The large area of no current indicates that there is no evi- dence of noise due to discharges that might be generated by contact electrification?-‘1 Spatial resolution, as determined by the smallest patches of conductivity observed on other- wise nonconducting films, is as small as 20 nm. We suspect that this resolution is significantly influenced by the structure of the conducting patch (steep changes in conductivity) and the morphology of the conducting coating at the tip (i.e., perhaps only a limited portion of the tip is participating).

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r Pm (cl

FIG. 4. (a) An SFM topographical scan and conduction probe scan of an abraded PFA film on stainless steel acquired at a compressive force of 50 nN and (b) the accompanying conduction scan. (c) A conduction scan taken over the same region at a force of 100 nN.

One would expect that small modulations in conductivity on the surface would result in much lower resolution.

Figure 2(b) shows a typical SCM scan after limited abra- sion (five strokes of the swab). Some conduction through the film is evident in the SCM scan. All of the conduction zones shown have conductivities well below 10 nA, i.e., well be- low the value for tip-to-bare metal contact. The total area of the conduction zones (exhibiting current flow above the

noise level) corresponds to 0.6% of the scanned area. Further abrasion {a total of eight strokes) increases the

area of the conduction zones, as shown in Fig. 2(c). The total area of conduction zones is now 2% of the scanned area. A few of the conduction zones display full-scale current read- ings (10 ~4) but most are at intermediate values. All of the conduction zones in Fig. 2(c) are a micrometer across or less, with the majority being much smaller.

Additional abrasion (a total of 16 strokes) further in- creases the fraction of the scanned area covered by conduo tion zones, as shown in Fig. 2(d); the conduction zones now cover approximately 6% of the surface. Again, most of the conduction zones display less than full-scale current read- ings.

Wiping the surface with the cotton swab 50 times pro- duces large conduction zones, as shown in Fig. 2(e). Much of the surface now displays full-scale current readings. About 30% of a random sample of 10X10 ,u,m’ scans on this highly abraded surface showed large, highly conducting patches. The remainder of the scans displayed large numbers of small, highly conducting regions over most of the scan, similar to the upper third of Fig. 2(e).

SFM topographs taken simultaneously with these par- ticular SCM scans generally showed little correlation with the regions of conduction. The surface relief actually dropped progressively with abrasion, perhaps due to the re- moval of particulates (a common problem with thin films grown by pulsed laser deposition). In Fig. 2(f) we show the SFM topograph corresponding to the SCM scan of Fig. 2(e) where the highly conducting zone appears to be in a region which tends to be evaluated.

Because our abrasion was not well controlled, quantita- tive analysis of the data is not particularly meaningful. Nev- ertheless, the fraction of the scanned area occupied by con- duction zones is a clean, monotonically increasing function of the “extent of abrasion” (number of strokes). The con- ducting area fraction increased rapidly over the first several strokes, and subsequently displayed a slower increase pro- portional to the square of the number of strokes. An im- proved abrasion apparatus is under construction that will al- low for controlled wear under more technologically interesting conditions.

The influence of the bias voltage on conduction was ex- amined over a range of 10 mV-1 V. Basically, images re- mained invariant when current scales were normalized. Over this limited range of bias, the measured current is a linear function of tip bias. Expanded scale images showed that higher biases allowed intermediate conducting regions to be detected that were in the noise at lower bias voltages. Typical resistances of the tip and film in these intermediate conduc- tivity regions were lo-30 Ma. Conduction occurred with both plus and minus bias voltages.

The current at a fixed position can be examined as the tip approaches and leaves the surface. Figure 3 shows simulta- neous (a) force and (b) current measurements as the tip ap- proaches a region of intermediate conductivity on an abraded PTFE film (275 mV tip bias, full-scale current corresponds to 100 ~4). Note that (1) the onset of measurable current (la- beled A) occurs after contact when the force has reached

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-15 nN; (2) the current continues to rise as the force in- creases (A to B), but then (3) the current stops rising at B, at a force of -50 nN, even though the force continues to in- crease. (cj Force and (d) current measurements made as the tip is withdrawn are shown in Fig. 3(c) and 3(d). The current does not start to drop until C, when the force has dropped to -25 nN, and decreases continuously until the force is very nearly zero CD). Note that adhesion produces an attractive force between the tip and surface as the tip is withdrawn further; nevertheless, no current is observed after D even though there is obviously a connection between the tip and surface up to the instant of detachment. This physical con- nection could be due to direct polymer-tip adhesion, perhaps dominated by electrostatic f0rces.l’

On high current patches (“metal-to-metal”), slightly higher forces were often required to produce measurable cur- rents, with onsets at -20 nN, and maximum currents reached at a force of -65 nN. During retraction of the tip the current begins to drop at -60 nN and reaches zero at -55 nN. Even higher adhesive forces were observed in the final stages of tip withdrawal from the high conductivity regions, again im- plying continued physical contact without measurable cur- rent. This higher adhesion may be due to an intervening wa- ter layer (which would have poor conductivityj, consistent with the hydrophilic nature of an oxidized metal surface.

With slight differences in the hysteresis of conduction during make versus break, the force dependence of the cur- rent is nearly the same for these two regions which display different levels of conduction. However, the fluctuations in the currents were noticeably higher for the intermediate cur- rents. We suggest that this is due to the conduction through thinned polymer, generally involving a hopping mechanism, which would tend to be “noisier.“” The conduction from the tip to the stainless steel may also be complex but definitely has lower amplitude fluctuations.

During scanning, increasing the applied force signifi- cantly affects the current measurements. Figure 4 shows the influence of changing the applied force on an abraded sample of PFA/stainless (which has similar mechanical properties as PTFE) with the tip bias set at 1 V, Rlimitins=l M0, and a scan rate of 0.5 Hz. Figure 4(a) and 4(b) shows 4X4 pm2 topo- graphic and SCM scans acquired with an applied force of -30 nN. Enhanced conduction is observed around the edge of an elevated region of the sample. When the applied force is doubled to 60 nN the conduction zones are more exten- sive, as shown in Fig. 4(c), occupying a significant fraction of the elevated region. Increasing the force applied by the tip increases the contact area A. Assuming elastic deformation only, the contact area between a sphere of radius R, Young’s modulus E 1 and Poisson’s ratio u,, and a plane of Young’s modulus E2 and Poisson’s ratio y, the contact area is given by

A=7T 1

3P(kl +kz)R 2’3 1 4 ’ where ki=y (1)

t

and P is the applied load. For a low modulus polymer, A can increase considerably as the applied load is increased. For rough surfaces (including the tip), A would increase even more dramatically with P. This increase in area would de-

crease the effective contact resistance and thus account for the higher currents observed at higher contact forces. In gen- eral, we found that larger patches of intermediate conductiv- ity were observed at higher applied loads and/or higher bias voltages. The saturation in current as the force is increased is attributed to the effect of the steel substrate as the tip elasti- cally deforms the soft thin iilm; eventually, the steel hinders further deformation (which dramatically decreases k,) and therefore inhibits further increases in the contact area.

It should be noted that numerous efforts are currently being made to produce stable conducting cantilevers which would facilitate SCM studies, e.g., important work by Thom- son and Moreland.13 Coatings employing higher cohesive en- ergy metals such as W and Rh may provide more durable tips, with greater tolerance for higher contact forces and higher currents, and therefore longer lifetimes. Doped dia- mond tips, if made sufficiently conducting, could be very robust and allow a variety of material systems to be studied. In principle, these methods would also be of considerable utility for studying on a nanometer scale the wear of very hard coatings on metals-and semiconductors.

ACKNOWLEDGMENTS

We would like to thank X. D. Wang for his assistance during early stages of this experiment. This work was sup- ported by grants from the National Science Foundation under the Surface Engineering and Tribology Program (Grant No. CMS-9414405), the Division of Materials Research (Grant No. DMR-9201767), and an Instrumentation Grant (No. DMR-9205197).

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