Microstructure and Microtribology of Polymer Surfaces - American

Chapter 11 Probing Adhesive, Mechanical, and Thermal Properties of Polymer Surfaces Using Scanning Probe Microscopy 1

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Gregory F. Meyers , Benjamin M. DeKoven1,3 Michael T. Dineen , Andrew Strandjord1,4, Paul J. O'Connor , Terry Hu , Yi-Hung Chiao , Hubert M. Pollock , and Azzedine Hammiche 1

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Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch011

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The Dow Chemical Company, Corporate R&D, Midland, MI 48667 School of Physics and Chemistry, University of Lancaster, LancasterLA14YB, United Kingdom The use of scanned probe microscopy (SPM) to study adhesion and mechanics of polymer surfaces has been investigated at Dow since 1992 (1,2) and, since 1997, studies of thermal properties at polymer surfaces have begun. Our goal has been to combine the imaging capabilities of SPM with surface property measurements. Initial work used contact mode atomic force microscopy with soft silicon nitride cantilevers to generate force vs. distance (F-d) curves (Figure 1). Operating in an unscanned mode, the sample is pushed into a tip mounted on the end of a soft cantilever, leading to deflection of the cantilever (loading). On retraction of the sample, the lever deflection is also monitored (unloading). During loading, the tip may penetrate into the material, providing information about the mechanical properties of the surface. On unloading, the tip may adhere to the surface, leading to information about tip-surface adhesion. The pull-off portion of the F-d curves is used as a means to provide qualitative information about the chemical nature of the surface, provided there is no plastic deformation resulting from the contact. The loading portion of the curve is used to provide qualitative understanding of the mechanical nature of the contact. The latter illustrates the limitations of beam bending approaches to nanoscale mechanical property measurements using static deflection AFM. The use of resonant AFM techniques with stiffer cantilevers to provide evidence of both chemical adhesion and mechanical deformation was further investigated. Mechanical property measurements of surfaces can be made quantitative using depth sensing nanoindentation, but for soft polymers this approach also has limitations. Finally, scanning thermal microscopy (SThM) was used to identify the presence or absence of thin polymer films on metal surfaces. 3 4

Current address: Intevac, Santa Clara, CA 95054-2704 Current address: IC Interconnects, Colorado Springs, CO

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© 2000 American Chemical Society

In Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


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Distance (depth)

Figure 1. Schematic of deflection vs. distance curve. Deflection can be converted to force using the spring constant. Distance can be converted to depth by removal of the compliance.



192 Experimental F-d curves were made using a "G" head scanner on either a Nanoscope Π (small sample AFM) or Nanoscope ΠΙ (MultiMode AFM) manufactured by Digital Instruments, Inc. The ζ response of the G head scanning piezo as a function of DC offset voltage and frequency (1 or 11 Hz) was measured using interferometry for 50V sweeps. The functional fit of this response was used as a correction to measured displacements. With the Nanoscope Π, silicon nitride cantilevers were used with nominal force constants of 0.58 N/m or 3.1 N/m. Silicon cantilevers were used with the Nanoscope EQ. In this case the spring constant was measured directly from the peak resonance frequency. In both cases the lever deflection was converted to force using deflection from a sapphire single crystal which is assumed to be non-deformable under the specified conditions. The lever deflection calibration was IV = 15 nN for the silicon nitride cantilevers with k= 0.58 N/m. Approximately forty tips were used during the study. Tip radii were measured from scanning electron microscopy images of selected tips. Force curves were collected in air at room temperature by disabling the χ and y scanning, and cycling the ζ piezo a specified distance at a frequency of 12.5 Hz for calibrated loads up to 200 nN. Tapping Mode (trademark of Digital Instruments, Inc.) and phase images were obtained on a Nanoscope ΙΠ (S/N NS3-519) using a Dimension 3000 Large Sample AFM (S/N 3000-1/225, Digital Instruments, Inc., Santa Barbara, CA.) and "G" scanner head (S/N DMLS-288). The system is also equipped with a Phase Extender Module (Model EX-1, S/N 505). In Tapping Mode the lever is oscillated at resonance and the feedback control adjusts for constant tapping amplitude. This is still a contact mode of AFM; however, the normal forces and shear forces are greatly reduced over conventional static deflection contact mode AFM. All images were recorded in constant force mode where the sample ζ position was adjusted during scanning to keep the r.m.s. (root mean squared) lever deflection constant. The tapping lever has a phase component that can change depending on the interaction of the tip with the surface. The phase image plots the phase lag as a function of position and is collected at the same time as the topography. Scanning was carried out in air using commercially available silicon cantilevers and tips with nominal force constants of 48 N/m. Tapping conditions were controlled by zeroing the phase signal (90 degrees) with the tip engaged but not tracking the surface. The setpoint voltage was then reduced and the voltage at which the phase signal was first observed to shift was noted (this is the set-point voltage, AQ). The setpoint was further decreased to improved tracking while noting the direction (sign) of the phase signal until the desired operating setpoint voltage (A ) was achieved. The ratio (r ) of A / A determines the degree of tapping. sp

sp

sp

0

Quantitative nanoindentation was achieved using the Tribometer Micromechanical Testing Instrument (trademark of Hysitron, Inc., Minneapolis, MN) directly coupled


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to a Nanoscope ΙΠ Scanned Probe Microscope (Digital Instruments, Inc., Santa Barbara, CA). The indenter tip was a Berkovich diamond with a nominal radius of 40 nm. The tip was driven electrostatically following a pre-determined loading (25 sec.)- hold (10 sec) - unloading (25 sec.) profile and the displacement of the surface is measured independently using capacitive detection of the actual indenter displacement. Typical loads accessible for this experiment range from 50μΝ to about 5000μΝ. The indenter tip can be used for imaging under the control of the SPM operating in contact mode with a minimum load of about ΙμΝ. The imaging capability is useful for finding a particular feature to indent or for imaging the resulting indent. The calibration and analysis of load-displacement data was based on a published method (3). SThM was carried out in the laboratory of H. Pollock and A. Hammiche in the Physics Department of the University of Lancaster, Lancaster, UK using a modified Topometrix Explorer SPM (Topometrix Corporation, Santa Clara, CA). The microscope uses a small Wollaston wire, bent and etched to form a contact mode AFM tip with a nominal radius of about 200 nm. The tip is used both as a heat source and a heat sensor. A second, reference, tip is held in air in close proximity to the sample for differential measurements. The heat to the tip can be modulated and the material response to the modulated heating can be monitored during imaging via lock-in techniques. For the work described here the microscope was operated in three imaging modes: (1) constant deflection (for topography); (2) constant temperature (DC); and (3) modulated temperature (AC). In an unscanned mode, the tip can be positioned on the surface for local differential thermal analysis (DTA) or local modulated temperature-DTA and local thermomechanical (TMA) measurements (4,22). Several surfaces were used for the current work. For the F-d curves we used polystyrene (monodisperse materials and a commercial Styron 685D grade from Dow Chemical) films spun from toluene onto silicon wafers; commercial low density polyethylene film (LDPE, commercial grades 5301 and 760C from Dow Chemical); thin film silicon nitride as found on the underside of the cantilever substrates; cleaved mica (Electron Microscopy Supplies); sapphire single crystal (Meller Optics); evaporated gold film (0.2 micron); cleaved highly oriented pyrolitic graphite (HOPG, X Y A grade, Union Carbide); polydimethylsiloxane elastomers (PDMS, M. Chaudhury of Dow Corning Corporation); and rubber from a rubber band. The crosslinkable epoxy thermoplastic modified with 5 wt % grafted rubber concentrate (CET-GRC) was prepared according to published procedure (5). The rubber domains are nominally 0.1 micron in diameter and consist of a core-shell structure with a 0.03 micron diameter polystyrene seed. Blocks of the CET-GRC were cryo-polished at -90°C using a diamond knife to provide a smooth face for SPM imaging. Some of these surfaces were post stained in Os0 vapors by suspending the polished blocks in sealed containers over an aqueous solution of 4


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Os04 for several minutes. The stain selectively reacts with unsaturated groups in the polymer surface region, in this case the rubber containing phases. Photodefineable benzocyclobutene (PhotoBCB) is a highly cross-linkable low dielectric polymer made from Diels-Alder addition of vinyl and cyclobutane groups between monomer units consisting of divinyl siloxane bis-benzocyclobutene (6). The partially cross-linked material was spun cast as a 5 micron film onto copper coated silicon wafers. The photoBCB was then patterned using a proximity mask and I-line (365 nm) radiation. The material is negatively defined, i.e., exposed areas are further cross-linked and the unreacted material is then removed during development. The mask leaves circular vias ranging from 5 to 50 microns. Following development and hard thermal cure the vias contain polymer residue ("scum"), which is removed in an oxygen containing plasma step. Results and Discussion Adhesion Using Pull-Off Force In order to make sense of pull-off force data it is important to demonstrate that the measurements are made in the absence of plastic deformation during contact. Comparison of topview contact mode images after contact at low peak loads (15 nN) and high peak loads (150 nN) were used to check for plastic deformation (Figure 2). With the exception of polystyrene, these surfaces showed no evidence of plastic deformation. Polystyrene showed unique patterning during scanning which was found to depend on the polymer molecular weight [1,7]. It is useful to compare typical deflection vs. distance curves for four surfaces which have been subjected to a peak load of 45nN (Figure 3). All exhibit a large hysteresis in unloading due to adhesion, as the force required to break the contact is at least 10X that necessary to form the contact. Note that the pull-off force and pull-off areas are very different for the materials at the same applied load. Also note that slopes of the loading portion of the curves are similar for all surfaces with the exception of the rubber surface which is lower. The LDPE surface exhibited a large pull-off force, but these types of surfaces gave variable results as discussed later. The pull-off force is plotted against applied load for each surface in air (Figure 4). Note that the pull-off forces are independent of load over the range studied. Based on JKR theory (8,9) the pull-off force can be related to surface energy via equation 1: F=3KR /(Y A

tipYsurfac

J

(1)

where R is the radius of the tip (40 nm by scanning electron microscopy), y \ and Ysurface represent the surface energies of the tip and surface, respectively. The surface energy of the tip can be estimated by measuring the pull-off force between the silicon nitride tip and the silicon nitride thin film on the underside of the cantilever substrate and solving for equation 1. This gave a calculated surface energy for silicon nitride of 225 mJ/m . Using this value for the tip allowed for the t p

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Figure 2. Contact mode AFM images of surfaces after contact at 15 nN (top row) and 150 nN (bottom row). Images are at Ιμιη χ Ιμπι and obtained with a silicon nitride cantilever, k = 0.58 N/m.

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200 400 600 800 sample displacement (nm)

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200 400 600 800 sample displacement (nm)

Figure 3. Representative deflection vs. distance curves for polystyrene thin film, sapphire single crystal, rubber and low density polyethylene film obtained with a silicon nitride cantilever (k = 0.58 N/m). The calibration for lever deflection is IV = 15 nN.


196 calculation of surface energies for each material, shown on the right Y axis of Figure 4 and given in Table I. We have used a nominal value of the tip radius (40 nm) in these calculations. Tip-to-tip variations will add additional errors in these estimates. The lowest values were obtained for hydrocarbon surfaces (PS, LDPE, graphite, and rubber); next higher was for metal oxides (silicon nitride, mica, and sapphire), and the highest was for gold metal. The estimated surface energies are consistent with those derived from more macroscopic methods, such as contact angle measurements. Reported values for the surface energies of polystryene and cleaved mica are 33 mJ/m and 300 mJ/m , respectively ( 10).


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Table I. Calculated surface energies from pull-off data over the applied load range of 20 nN to 75 nN (silicon nitride cantilever, k = 0.58 N/m, and tip with radius R = 40 nm). Material

surface energy (mJ/m ) 12 ± 1 29 ± 2 53 ± 8 71 ± 5 109 ± 5 226 ± 11 244 ± 2 7 287 ± 4 2

polystyrene graphite polyethylene rubber sapphire silicon nitride mica gold

The reproducibility and accuracy of the pull-off measurements depended on many factors, such as surface and tip cleanliness, the relative curvature of surface features to the curvature of the tip (i.e. roughness and contact area), and variations in tip shape. Examples are shown for sapphire and LDPE surfaces (Figure 5). The pulloff force from a sapphire surface left open in the laboratory for several months gave an initially high pull-off force at low applied load and then low pull-off forces for subsequently higher loads. Contact mode AFM images revealed a surface layer 3-4 nm thick that could be altered during scanning. If the sapphire surface was cleaned for several minutes in a UV-ozone chamber, the surface gave consistent pull-off values at all loads. The high initial pull-off force on the contaminated surface was probably due to necking while the lower pull-off forces at higher loads probably resulted from pull-off between the lower surface energy contaminant on both the tip and surface. Variations in pull-off forces were also noted for LDPE films. In this case variations were found to be related to surface roughness and, therefore, contact area. Measurements were obtained on two films with local roughness that was either less than or greater than the tip radius (Figure 5). When the roughness is less than the tip radius in the vicinity of contact, the pull-off forces are consistently lower.



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Load(nN)

Figure 4. Pull-off force vs. maximum applied load during contact for each surface using a silicon nitride cantilever (k = 0.58 N/m). The surface energy scaling is based on JKR theory as described in the text.

Load(nN)

Figure 5. Pull-off force vs. maximum applied load during contact demonstrating the effect of surface contamination (sapphire) and roughness (LDPE) on measurement accuracy.


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This summarized our understanding of pull-off forces circa 1992 (2). Despite these problems we were encouraged by the potential for SPM to provide measures of local surface energy on polymers. Other groups have made much progress in this area since 1992 and these measurements can be made quantitatively for specific systems (e.g. using chemically functionalized tips and surfaces) and under specific conditions(e.g. in liquids) (11). Further understanding of the nature of tip-surface contacts on polymers is necessary before the technique can be made for general use. Qualitative information regarding relative differences in surface adhesion can now be obtained routinely while imaging using, for example, phase detection in Tapping Mode AFM (TMAFM) (12). In this technique the cantilever is driven to resonance and the phase lag of the lever interacting with the surface is measured as a function of position during scanning. Adhesive interactions can dominate phase image contrast when the lever is operated in a low amplitude and light tapping regime, where sample deformation is minimized, as described later. Mechanical Properties Based on Lever Deflection of Soft Cantilevers The lever deflection is plotted against sample displacement for all surfaces (Figure 6). The slopes of these curves during loading were similar despite the fact that the nominal moduli of the materials varied by three orders of magnitude. We were surprised to find, for example, that LDPE and sapphire had similar mechanical behavior under experimental conditions. Only at the highest applied loads (ca 200 nN) do the slopes begin to change and distinguish between materials. Surface displacements for loads up to 150 nN are less than 2 nm with the exception of elastomers. Using soft cantilevers (k = 0.58 N/m) our experiments could only distinguish rubbers or polydimethysiloxane (PDMS) elastomers from other polymers, metals, and metal oxide surfaces. This observation is readily explained by the relative differences in stiffness between the cantilevers used and the stiffness of the surfaces under investigation. Using Hertzian contact mechanics, Radmacher et al. (13) derived equations for calculating surface stiffness (k ) and tip penetration depths (d) during contact, as shown below: s

2

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1/3

2

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d = [(97t /16)[F /E* R]]

(2)

k = ÔF/ôd = [(6/7C )(E* RF )] s

l/3

0

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Ε* = (πΕ)/(1-ν )

(3) (4)

Equation 2 gives the penetration depth, equation 3 gives the surface stiffness and equation 4 is the calculated effective modulus based on bulk modulus, E, and Poisson's ratio, v. We have applied this analysis to our materials to calculate the surface penetration and surface stiffness using the measured radius of our tip and the known bulk mechanical constants of the materials at an applied load of 50 nN (Table II).



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sample displacement (nm) Figure 6. Lever deflection vs. sample displacement for all surfaces (IV deflection = 15 nN force, silicon nitride cantilever k=0.58 N/m). Measured surface penetration is less than 2 nm for all surfaces up to loads of 150 nN with the exception of elastomers.


200 Table II. Calculated surface stiffness and tip penetration depths for several materials using a silicon nitride cantilever (k=0.58N/m, tip radius=40 nm) and a SOnN contact force. material penetration V MN/m) E(N/m ) depth (nm) PDMS 0.28 10 270 0.5 polyethylene 26 10 0.4 2.9 polystyrene 3x 10 0.33 1.4 54 sapphire 4x 10 0.06 1300 0.2 2

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!l

Only for the PDMS elastomer is the stiffness of the surface on the order of the cantilever stiffness used to probe the mechanical properties. All other surfaces show negligible penetration. The sensitivity to measure mechanical properties of polymer surfaces using AFM under static deflection depends on the relative stiffness of the cantilever and surface (14). The soft (k

Microstructure and Microtribology of Polymer Surfaces - American

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