A scanning conduction microscopic method for probing abrasion of insulating thin films

Tribology Letters 1 (1995) 159-175 159

A scanning conduction microscopic method for probing abrasion of insulating thin films

J.T. Dickinson a, K.H. Siek a and K.W. Hipps b

~ Department of Physics and b Department of Chemistry, Washington State University, Pullman, WA 99164-2814, USA

The use of scanning force microscopy (SFM) to probe wear processes at interfaces is of considerable interest. We present here a simple modification of the SFM which allows us to make highly spatially resolved measurements of conductivity changes produced by abrasion of thin insulating films on metal substrates. The technique is demonstrated on fluorocarbon polymer thin films deposited on stainless steel substrates.

Keywords: scanning force microscopy; atomic force microscopy; spatial probe of conductivity; tribology; polymer thin fdms; abrasion; wear

1. In troduct ion

The scanning force microscope (SFM) is a useful tool for examining the conse- quences of tribological wear, particularly on inherently fiat surfaces such as single crystals. However, 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 conducting substrate. 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 S T M / S F M imaging [1], and spatially resolved potent iometry used recently to image potentials of metallic structures on integrated circuits [2]. Point-by-point measurements of semiconductor conductivity have also been made using an SFM with conducting tips in efforts to probe semiconductor doping profiles [3] as well as spatially controlled potentiometry for thin-film structures [4]~

�9 J.C. Baltzer AG, Science Publishers

160 J.T. Dickinson et al. / Scanning conduction microscopy of thin films

2. Experimental

Film topography was characterized with a Digital Instruments Nanoscope III scanning force microscope (SFM) operated in the contact mode. SFM scans were acquired at scan sizes ranging from 200 • 200 nm 2 to 100 • 100 ~tm 2 and at tip velocities from 0.2 to 200 ~tm/s. A simple modification of the SFM allowed simultaneous, spatially resolved measurements of film conductivity. A schematic of the experimental arrangement is shown in fig. 1. Scanning conduction microscopy (SCM) measurements were made using commercial triangular Si3N4 SFM tips (115 ~tm 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 nm, the pressure applied by the tip is about 20 MPa, well below the yield stress of most materials. Forces were determined from the measured displace- ments 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 completed by a 30 ~tm, stainless steel wire spot-welded to the conducting substrate.

Current measurements were performed with a Keithley 620 electrometer whose output was amplified with a Tektronix 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 topographical image acquired simultaneously. For improved time resolution, current measurements were occasionally acquired at higher sampling rates during individual scans with a LeCroy 6810 digi-

Meta l Coated

Rlimiting ~ electrometernA - ktA

Insulating thin film on Metal Substrate

Double Stick Tape (isolates substrate from ground)

D C - 2 k H z Differential

2 0 m V - 5 V to digitizer

Fig. 1. Schematic diagram of an experimental arrangement for measurement of the spatial distribu- tion of conductivity of a thin film on conductive substrate. One of the most important components is

R l i m i t i n g .

3". T. Dickinson et aL / Scanning conduction microscopy of thin films 161

tizer. A bias voltage (typically, 1 V) was applied to the sample through a limiting resistor (Rlimit ing , typically 1-10 Mf~) to prevent current-induced damage to the metal coating of the SFM tip when metal-to-metal contact was made. When the SFM tip is over a region where the polymer film is thick, the effective tip-to-substrate resistance is high (> 1012 f~) and little current flows through the circuit. When the tip crosses a bare region of the substrate, the tip-to-substrate resistance falls to low values (< 105 ~) and the resistance of the circuit is approximately equal to that of the limiting resistor. Regions of intermediate conductivity are also evident. Thus, a typical SCM image is a three-dimensional plot of the magnitude of the current vs. x-y .

The useful lifetimes of the conducting tips were limited by wear of the metal coating and ranged from 10 to 30 scans. High currents ( > gA) associated with low limiting resistances or high bias voltages, however, dramatically shortened tip lifetimes. Both positive and negative bias voltages produced nearly equal currents with no apparent nonlinearities, suggesting ohmic conduction. Little effort was made to electrically 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.

Samples of polytetrafluoroethylene (PTFE) coated stainless 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 nm (KrF) through a quartz window onto a PTFE substrate (Goodfellow chemicals) mounted in vacuum at a pressure of 10 -4 Pa. The laser was operated at a pulse repetition rate of 10 Hz and a fluence of 4 J /cm 2 per pulse. The ablation products were collected on a stainless steel substrate held at 340~ TEM and XRD analysis confirmed that the resulting 300 nm thick films were semi- crystalline PTFE. A few specimens of plasma-deposited, ~ 130 nm thick, perfluor- oalkoxy (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 2. Although not well controlled, the wear produced by successive linear strokes was indeed cumulative and served as a simple means to demonstrate the technique.

3. Results and discussion

Fig. 2 shows (a) a 10 x 10 gm 2 SFM topographical scan and (b) the SCM image for a PTFE film on stainless steel prior to abrasion. The film was scanned at a tip velocity of 20 gm/s with a 100 mV bias voltage applied through a 10 Mf~ limiting resistance; a full-scale reading on the electrometer corresponds to 10 nA. As shown in fig. 2b, the unabraded films are basically insulating under the tip; sampling sev-


Fig. 2. (a) An SFM topographical scan and (b) conduction scan of a nonabraded PTFE coated stainless steel sample.

eral regions of the film, we found one submicron-sized region of enhanced conductivity (similar to the small region in the foreground shown in fig. 2b) in approximately 20% of the scans. These features were reproducible upon repeated scans. The large area of no current indicates that there is no evidence of noise due to dis- charges that might be generated by contact electrification [9-11]. Spatial resolution, as determined by the smallest patches of conductivity observed on otherwise nonconduct ing 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 conduc-

s T. Dickinson et aL / Scanning conduction microscopy ofthinfilms 163

tivity) and the morphology of the conducting coating at the tip. One would expect that small modulations in conductivity would result in much lower resolution.

Limited abrasion (five strokes of the swab) was then carried out and several (typically 5-10) scans were carried out on different locations. Fig. 3 shows a typical SFM topographic scan and the SCM scan, again at a scan rate of 20 I~m/s with a 100 mV bias on the tip (full scale equals 10 nA). Some conduction through the film is evident in the SCM scan. All of the conduction zones shown have conductivities

Fig. 3. (a) An SFM topographical scan and (b) conduction scan of the same PTFE film (fig. 2) after the light abrasion (five strokes).

164 J.T. Dickinson et aL / Scanning conduction microscopy of thin films

well below 10 nA, i.e., well below the value for tip-to-bare metal contact. The conduction zones lay in a flat region between two large protrusions. 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. 4. The surface relief of the film has dropped slightly, perhaps due to removal of particulates (a common problem with thin films grown by pulsed

Fig. 4. (a) An SFM topographical scan and (b) conduction scan of the same PTFE film (fig. 2) after a total of eight strokes.

3". T. Dickinson et aL / Scanning conduction microscopy of thin films 165

laser deposition) and the number of conduction regions has increased markedly. The total area of conduction zones is now 2% of the scanned area. These regions appear to be distributed randomly across the surface, and are not strongly corre- lated with surface relief: both the highest and lowest regions of the surface display conduction zones. A few of the conduction zones display full scale current readings (10 nA) but most do not. All of the conduction zones in fig. 4 are a micron across or less, with the majority being much smaller.

Additional abrasion (a total of 16 strokes) further increases the fraction of the scanned area covered by conduction zones, as shown in fig. 5; the conduction zones

Fig. 5. (a) An SFM topographical scan and (b) conduction scan of the same PTFE film (fig. 2) after a total of 16 strokes.

166 aT. T. Dickinson et al. / Scanning conduction microscopy of thin films

now cover approximately 6% of the surface. Again, most of the conduction zones display less than full scale current readings. Again, no obvious correlation between the topographic features and the conduction zones is observed.

Wiping the surface with the cotton swab 50 times produces large conduction zones, as shown in fig. 6b. Much of the surface now displays full-scale current readings. About 30% of a random sample of 10 x 10 gm 2 scans on this highly abraded

Fig. 6. (a) An SFM topographical scan and (b) conduction scan of the same P T F E film (fig. 2) after a total of S0 strokes.

Jr. T. Dickinson et al. / Scanning conduction microscopy of thin fitms t67

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. 6b. The highly conducting zone in fig. 6b is in a region (fig. 6a) which tends to be elevated.

Because our abrasion was not well controlled, quantitative analysis of the data is not particularly meaningful. Nevertheless, the fraction of the scanned area occu- pied by conduction zones is a clean, monotonically increasing function of the "extent of abrasion" (number of strokes). The conducting area fraction increased rapidly over the first several strokes, and subsequently displayed a slower increase proportional to the square of the number of strokes. An improved abrasion appara- tus is under construction that will allow for controlled wear under more technologi- cally interesting conditions.

Fig. 7 shows the influence of the bias voltage on conduction in the area imaged topographically in fig. 7a. The tip velocity was 20 gm/s. The SCM image of fig. 7b was acquired with a tip bias of 100 mV, where a full scale current reading corresponds to 10 nA; in the scan of fig. 7c, the tip bias was increased to 1 V and the full scale current reading corresponds to 100 nA. The scaled current values vs. position for any particular point on the surface scale are almost identical over the two scans. This invariance of the image indicates that over a limited range of applied voltages (a factor of 10), the measured current is a linear function of tip bias.

Fig. 8 shows a 500 x 500 nm 2 SCM image of a slightly abraded PTFE film that includes a high current zone (center left) and an intermediate current zone (center right). In the high current region on the left, the current equals the 10 nA limit (/limit) imposed by the limiting resistor, and thus corresponds to exposed metal where the PTFE has been abraded off. In the intermediate current zone, the current is approximately a third of/limit o v e r the entire region. This would imply an effective resistance of 20 Mf~ (V = 0.1 V, i = 3 hA, Rtotal = Rtip-film -1- Rlimit). Regions where the current is less than/limit a r e observed throughout all of the SCM scans. These regions tend to be isolated, although they are often adjacent to high current patches, and are very reproducible from scan to scan.

Fig. 9a shows a 200 x 200 nm 2 topographical image of an abraded PTFE film. The SCM image (fig. 9b) was acquired with a 275 mV tiP bias; a full-scale electrometer reading corresponds to 100 nA (only one direction of the scan is shown). At a scan rate of 5 Hz and acquiring 256 digitized points/line, current measurements are made at 0.4 ms intervals, at points separated by 0.8 nm. A portion of a single SCM trace (the endpoints are marked with the arrows in fig. 9b) is displayed vs. time in fig. 9c showing various levels of conductivity including full-scale current readings. The first highly conducting plateau (6-14 ms) is fairly flat, whereas the other regions show more modest conduction and "spiky" features. By digitizing at higher rates, we can expand any region we desire. The i(t) plot of fig. 9d was acquired in the same region (actually during the same scan) marked by the vertical arrow in fig. 9c at a rate of 1 gs/channel. During the I ms time interval displayed, the tip has moved only 2 nm. Thus, the current fluctuations in this interval are


Fig. 7. (a) An SFM topographical scan and conduction scan for a PTFE film on stainless with (b) 100 mV and (c) 1 V bias voltages applied between the tip and the metal substrate.

J. T. Dickinson et al. / Scanning conduction microscopy of thin films 169

Fig. 8. A 500 x 500 nm 2 conduction scan showing regions of both high and intermediate conduction on an abraded PTFE film on stainless steel.

due primarily to temporal fluctuations in the current, with perhaps a small contri- bution due to spatial variations in conductivity sampled by the tip motion.

The current at a fixed position can be examined as the tip approaches and leaves the surface. Fig. 10 shows simultaneous current and force measurements as the tip approaches a region of intermediate conductivity on an abraded PTFE film (275 mV tip bias; full scale current corresponds to 100 hA). Note that (1) the onset of measurable current (labeled A) occurs after contact when the force has reached (~ 15 nN; (2) the current continues to rise as the force increases (A to B); but then (3) the current stops rising at B, at a force of ,-~ 50 nN, even though the force continues to increase. Current and force measurements made as the tip is withdrawn are shown in fig. 10c. 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 (D). Note that adhesion produces an attractive force between the tip and surface as 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 connection could be due to direct polymer-tip adhesion, perhaps dominated by electrostatic forces [11].

On high current patches ("metal-to-metal"), slightly higher forces are required to produce measurable currents, as shown in fig. 11. During approach, the onset of the current (A) is ~ 20 nN, and maximum current is reached at a force of ~-, 65 nN (B). During retraction of the tip the current begins to drop at ~ 60 nN (C) and reaches zero at ~ 55 nN. Note again that in fig. 1 lc, an adhesive force (nega-

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tive - towards the surface) is produced in the final stages of tip withdrawal, imply- ing continued physical contact without measurable current. The higher adhesion seen here (in comparison with fig. 10c) and long duration at constant force (which corresponds to ~., 450 nm of displacement) is troublesome but may be due to an intervening water layer (which would have poor conductivity), consistent with the hydrophilic nature of an oxidized metal surface.

With slight differences in the hysteresis of conduction during make vs. break, the force dependence of the current is nearly the same for these two regions which display different levels of conduction. Note, however, that the fluctuations in the currents, comparing (fig. 10c and 10d vs. figs. 11 c and 11 d) are noticeably higher for the intermediate currents. We suggest that this is due to the conduction through thinned polymer, generally involving a hopping mechanism, which would tend to be "noisier" [12]. The conduction from the tip to the stainless steel may also be com- plex but definitely has lower amplitude fluctuations. We would also expect any minute vibrations of the tip to contribute to the fluctuations.

During scanning, increasing the applied force significantly affects the current measurements. Fig. 12 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, Rlirniting = 1 MI), and a scan rate of 0.5 Hz. Figs. 12a and 12b show 4 x 4 gm a topographic 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 extensive, as shown in fig. 12c, 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 El, and Poisson's ratio Ul, and a plane of Young's modulus E2, and Poisson's ratio ua, the contact area is given by

(3P(kl + k2)R) 2/3 A = ~ 4 ' (1)

where

2 1--ui k i - - - -

Ei

where P is the applied load. For a low modulus polymer, A can increase consider- ably 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 decrease the effective contact resistance and thus account for the higher currents observed at higher contact forces. In general, we found that larger patches of intermediate conductivity 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 elastically deforms the soft thin film; eventually, the steel bin-


Fig. 12. (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.


ders further deformation (which dramatically decreases k2) and therefore inhibits further increases in the contact area.

4. Conclusion

We have shown that sputter deposited metal coatings on commercial Si3N4 tips offer sufficient conductivity to allow SCM images to be acquired which provide a spatial probe of wear on these polymer coated metal samples. Undamaged polymer films normally showed no detectable conductivity with applied forces up to

100 nN; repeated scans over several areas revealed small patches of conductivity. Thinned regions produced by wear exhibited intermediate conduction, and exposure of"bare" metal with high conductivity were readily detected. The accu- mulation of damage with increasing tribological loading is readily apparent in the evolution of these features. In some cases, useful correlations can be made between the simultaneously acquired AFM topography and SCM scans. The high spatial resolution observed in the conduction scans is probably due to the nature of the damaged surface (e.g., exposure of very small regions of conduction to the metal substrate) and perhaps only limited portions of the tip participating in the current path. The magnitude of the conducting current showed a threshold in the applied compressive force of about 20 nN for both the low and high conductivity regions. A plateau in conduction current occurred at applied compressive force of about 60 nN. Upon retraction of the tip from the surface, measurable hysteresis in the current is observed, probably due to contact area being maintained by adhesive forces. Scanning at higher compressive forces and/or applied bias produced measurable current in larger areas without any obvious change in topography.

Numerous efforts are currently being made to produce stable conducting cantilevers which would facilitate SCM studies, e.g., important work by Thomson and Moreland [13]. Coatings employing higher cohesive energy 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 diamond tips, if made sufficiently conducting, could be very robust and allow a variety of material sys- tems 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.

Acknowledgement

We would like to thank Les Jensen and X.D. Wang for his assistance during early stages of this experiment. This work was supported by grants from the National Science Foundation under the Surface Engineering and Tribology Program CMS- 9414405, the Division of Materials Research DMR-9201767, and an Instrumenta- tion Grant DMR-9205197.

J. 71. Dickinson et aI. / Scanning conduction microscopy of thin films 175

R e f e r e n c e s

[1] L.A. Wenzler, T. Han, R.S. Bryner andT.P. Beebe, Rev. Sci. Instr. 65 (1994) 85. [2] M. Anders, M. Miick and C. Heiden, J. Vac. Sci. Technol. A 8 (1990) 394. [3] J. Snauwaert, L. Hellemans, I. Czech, T. Clarysse, W. Vandervorst and M. Pawlik, J. Vac. Sci.

Technol. B 12 (1994) 304. [4] M. Anders, M. Mueck and C. Heiden, J. Vac. Sci. Technol. A 8 (1990) 394. [5] Digital Instruments, Incl, Santa Barbara, CA, USA. [6] G.B. Blanchet and S.I. Shah, Appl. Phys. Lett. 62 (1993) 1026. [7] G.B. Blanchet, C.R. Fincher Jr., C.L. Jackson, S.I. Shah and K.H. Gardner, Science 262

(1993) 719. [8] W. Jiang, M.G. Norton, L. Tsung and J.T. Dickinson, J. Mater. Res., to be published. [9] S. Lee, L.C. Jensen, S.C. Langford and J.T. Dickinson, Electrical transients generated by the

peel of a pressure sensitive adhesive from a copper substrate. Part I. Initial observations, J. Adhes. Sci. Technol., to be published.

[10] L. Scudiero, J.T. Dickinson, L.C. Jensen and S.C. Langford: Electrical transients generated by the peel of a pressure sensitive adhesive from a copper substrate. Part II. Analysis of fluctuations, J. Adhes. Sci. Technol., to be published.

[11] R.G. Horn and D.T. Smith, Science 256 (1992) 362. [12] J.T. Dickinson, S.C. Langford and L.C. Jensen, J. Mater. Res. 8 (1993) 2921. [13] R.E. Thomson and J. Moreland, National Institute of Standards and Technology, Boulder,

Development of highly conductive cantilevers for atomic force microscopy point contact measurements, to be submitted.

Documents

A scanning conduction microscopic method for probing abrasion of insulating thin films