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Organic and Inorganic Contamination on Commercial AFM Cantilevers Yu-Shiu Lo,† Neil D. Huefner,† Winter S. Chan,† Paul Dryden,† Birgit Hagenhoff,‡ and Thomas P. Beebe, Jr.*,† Department of Chemistry & Center for Biopolymers at Interfaces, University of Utah, Salt Lake City, Utah 84112, and Tascon GmbH, Mendelstrasse 11, 48149 Mu¨ nster, Germany Received March 30, 1999 It has been found that a common shipping and packaging material for commercial AFM cantilever tips, poly(dimethylsiloxane) (PDMS), causes a thin layer of silicone oil contamination on AFM cantilever tips. Due to the similarity of elemental compositions between silicone oils and AFM cantilevers (both contain silicon and oxygen), it is difficult to detect such contaminants with the widely used surface characterization technique, X-ray photoelectron spectroscopy (XPS), since XPS provides mainly elemental and short-range chemical information. However, by using static time-of-flight secondary-ion mass spectrometry (TOFSIMS), a technique that is extremely surface-sensitive, silicone oils on AFM cantilevers can easily be identified by their molecular fragments. A simple dip cleaning procedure using a mixture of concentrated sulfuric acid and hydrogen peroxide (piranha solution) was found to be an easy and effective way to remove organic contamination, including silicone oils, from AFM cantilever tips. It has also been shown, in both XPS and TOF-SIMS spectra, that a small amount of Au is present on the tip side of AFM cantilevers. This is most likely due to thermal diffusion of Au during the deposition of Au on the back side of the cantilevers, placed there to enhance laser reflectivity in the detection system of AFM instruments. No simple dipping approach was found to remove Au contamination on the tip side without also damaging the required Au coating on the back side of the cantilevers.
Introduction Since its invention in 1986,1 the atomic force microscope (AFM) has gained much attention in various fields and numerous advances have been made in the past decade.2-4 AFM monitors the repulsive or attractive interactions between the tip and the sample in a relatively small contact area by detecting the deflection of a flexible cantilever on which the tip is mounted. With high force and displacement sensitivities, and the capability of operating in various liquid environments, AFM has been used not only for imaging surfaces with excellent lateral spatial resolution but also for probing local chemical and mechanical interactions between AFM tips and samples with comparable spatial resolution and single-molecule force resolution. A wide variety of AFM cantilevers with different specifications and mechanical properties that are suitable for different kinds of systems are now provided by several manufacturers.5 To achieve an added degree of chemical specificity for various applications such as mapping of surface chemical functionalities6-10 and investigation of † ‡
University of Utah. Tascon GmbH.
(1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (2) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298-1328. (3) Louder, D. R.; Parkinson, B. A. Anal. Chem. 1995, 67, 297A303A. (4) Bottomley, L. A. Anal. Chem. 1998, 70, 425R-475R. (5) The major AFM cantilever manufacturers include Park Scientific Instruments, TopoMetrix, Digital Instruments, Olympus, and others. (It is our understanding that Park Scientific Instruments and TopoMetrix have merged into ThermoMicroscopes.) (6) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071-2074. (7) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M J. Am. Chem. Soc. 1995, 117, 7943-7951. (8) Noy, A.; Sanders, C. H.; Vezenov, D. V.; Wong, S. S.; Lieber, C. M. Langmuir 1998, 14, 1508-1511.
specific chemical11-16 and biological interactions,17-27 surface modification of the AFM tips is necessary. This can be done by a self-assembly technique,6-9,12-16,22 by functionalized microsphere attachment,19,23 and by chemical21,24-26 or physical17,18,20,27 immobilization of biological species on the AFM tips. To produce reliable and reproducible results in topographic or “friction-force” imaging, and in various kinds (9) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960-10965. (10) Nakagawa, T.; Soga, M. Jpn. J. Appl. Phys. 1997, 36, 52265232. (11) Williams, J. M.; Taejoon, H.; Beebe, T. P., Jr. Langmuir 1996, 12, 1291-1295. (12) Han, T.; Williams, J. M.; Beebe, T. P., Jr. Anal. Chim. Acta 1995, 307, 365-76. (13) Wenzler, L. A.; Moyes, G. L.; Raiker, G. N.; Hansen, R. L.; Harris, J. M.; Beebe, T. P., Jr. Langmuir 1997, 13, 3761-3768. (14) Wenzler, L. A.; Moyes, G. L.; Harris, J. M.; Beebe, T. P., Jr. Anal. Chem. 1997, 69, 2855-2861. (15) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357-4368. (16) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446-455. (17) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415-17. (18) Moy, V. T.; Florin, E.-L.; Gaub, H. E. Science 1994, 266, 257259. (19) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 0, 354-357. (20) Chilkoti, A.; Boland, T.; Ratner, B. D.; Stayton, P. S. Biophys. J. 1995, 69, 2125-2130. (21) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 7713. (22) Boland, T.; Ratner, B. D. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 5297-301. (23) Stuart, J. K.; Hlady, V. Langmuir 1995, 11, 1368-1374. (24) Hinterdorfer, P.; Baumgartner, W.; Gruber, H. J.; Schilcher, K.; Schindler, H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3477-3481. (25) Dammer, U.; Hegner, M.; Anselmetti, D.; Wagner, P.; Dreier, M.; Huber, W.; Guntherodt, H. J. Biophys. J. 1996, 70, 2437-2441. (26) Allen, S.; Chen, X.; Davies, J.; Davies, M. C.; Dawkes, A. C.; Edwards, J. C.; Roberts, C. J.; Sefton, J.; Tendler, S. J. B.; Williams, P. M. Biochemistry 1997, 36, 7457-7463. (27) Lo, Y.-S.; Huefner, N. D.; Chan, W. S.; Stevens, F.; Harris, J. M.; Beebe, T. P. Jr. Langmuir 1999, 15, 1373-1382.
10.1021/la990371x CCC: $18.00 © 1999 American Chemical Society Published on Web 07/09/1999
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of force probing, a clean, or at least a known, AFM tip chemistry is necessary. Cleanliness is also important for the quality and effectiveness of further surface modifications on the AFM tip. Thin layers of contaminants may change the reactivity or adsorptivity of surfaces so that the desired modifications may not really be carried out on surfaces. Since most researchers do not independently verify or characterize putative surface modifications with surface-sensitive spectroscopies, the contamination problem is an especially insidious one. Many procedures for cleaning the AFM tip-cantilever assembly have been developed. Such procedures include ultraviolet ozone treatment,10 aggressive acid-based baths,16,24,27 and plasma etching.19,23,26,28 Other groups use as-received cantilevers with only simple cleaning methods such as rinsing with organic solvents or no precleaning at all. Because the AFM tip is so small, characterization of its surface chemistry is often difficult. One indirect approach is to characterize reference surfaces that are made of the same materials as the AFM tips and treated with the same process as the tips. Unless this reference surface was subjected to the same storage conditions, it might not be representative of the actual cantilever and tip surface chemistry, and therefore, the experimental design employing a reference surface is potentially flawed. Surfacesensitive methods to directly analyze the contact area of the tip are becoming more widely available, and a reasonable alternative is to perform a surface microarea analysis on the cantilever legs in the vicinity of the pyramid tip instead. In a recent analysis of as-received cantilevers from Park Scientific Instruments (now known as ThermoMicroscopes) by time-of-flight secondary-ion mass spectrometry (TOF-SIMS), we found that silicone oils (low-molecularweight polysiloxanes) are present as a contamination layer on the cantilevers and chips. Other cantilever manufacturers use similar packaging and shipping methods. In at least some cases, the shipping and packaging materials (PDMS, poly(dimethylsiloxane)) seem to be responsible for this contamination. Evidence of the release of silicone oils from AFM storage containers could also be seen from the change of contact angle of water on glass substrates stored in these same polymeric containers for various lengths of time. Such contaminants cannot be unambiguously identified by XPS or Auger spectroscopy because these techniques do not provide the necessary long-range molecular information to make such an assignment. In addition to silicone oils, both XPS and TOF-SIMS spectra show significant amounts of Au on the tip side of the cantilevers. This gold can also be considered a contaminant in many cases. Immersion of the contaminated cantilever chips in hexane overnight was found to remove only some of the silicone oils, while immersion in a mixture of concentrated sulfuric acid and hydrogen peroxide (piranha solution) was found to effectively remove the silicone oils from the AFM cantilever tips. The purpose of this note is to point out this apparently little-known fact about the contamination of as-received AFM cantilevers and to suggest a proper cleaning procedure when using these as-received AFM cantilevers, particularly when subsequent surface chemical modifications will be attempted. Experimental Section TOF-SIMS Characterization. Static TOF-SIMS was performed with a TOF-SIMS IV instrument (Cameca/ION-TOF, (28) Senden, T. J.; Drummond, C. J. Colloids Surf. A 1995, 94, 2951.
Langmuir, Vol. 15, No. 19, 1999 6523 Paris/Mu¨nster). For the as-received and hexane-cleaned AFM cantilever chips, spectra were acquired using a 25-keV, 2.3 pA pulsed primary Ar+ ion gun with a pulse width of ∼800 ps. The irradiated area was approximately 50 µm × 50 µm on the wafer part of the underside of the AFM cantilever chip. For the piranhacleaned AFM cantilevers, a 30 keV, 1.2 nA Ga+ primary ion beam pulse was used with a pulse width of 15 ns, and the irradiated area was about 120 µm × 120 µm on the cantilever part of the underside of the AFM cantilever chip. To obtain spatially resolved ion images, Ga+ was used as the primary ion with kinetic energy of 25 keV, a current of 3 pA, and a pulse width of 30 ns. The image sizes are given in the figures. In all cases, charge compensation was achieved by applying low-energy electrons (∼30 eV) from a pulsed flood gun. All spectra were acquired with a primary ion dose below 1012 cm-2. XPS Characterization. X-ray photoelectron spectroscopy was performed using an ESCALAB 220i-XL (formerly Fisons, now VG, East Grinstead, U.K.). Spectra were collected in the binding energy regions of Si 2p (103.4 eV), C 1s (284.6 eV), O 1s (531.6 eV), and N 1s (397.9 eV). High-resolution multiplex spectra of the individual elements were acquired from 200-µm-diameter areas, using a 20-eV pass energy and signal averaging for 50 scans, requiring ∼100 s per spectral region. Due to sample charging on insulator samples, a low-energy electron flood gun was used for charge compensation, and peak positions were assigned by referencing the methylene component of the C 1s peak to a binding energy of 284.6 eV and linearly shifting all other peaks by an equal amount, as is customary. Contact-Angle Measurements. A standard contact-angle goniometer (model A-100, Rame-Hart) was used to obtain contactangle measurements. Water (18 MΩ‚cm) was used as the sessile drop for all samples. Advancing contact angle measurements were made visually within 30 s after applying the drop to the sample. Each value was reported as an average with one standard deviation, from at least 12 measurements. Cleaning Procedures. Two different procedures were used to clean the AFM cantilever tips. In the first method, cantilever chips were immersed in reagent-grade hexane (Fisher) overnight, rinsed with hexane, and then dried in air. In the second method, cantilever chips were cleaned in piranha solution, H2SO4/H2O2, 70:30 (v/v), for 30 min, rinsed with copious amounts of deionized water, and then dried on a hot plate at 150 °C for 1 h. Caution! The piranha solution has very strong oxidizing power and is extremely dangerous to handle in the laboratory; gloves, goggles, and face shields are needed for protection.29 It is strongly recommended to work with small volumes for safety and disposal reasons. All cleaned cantilever chips were stored in a similarly cleaned glass Petri dish in air prior to characterization. Storage in various polymeric containers (including disposable Petri dishes) resulted in the reaccumulation of poly(dimethylsiloxane) on the samples, perhaps due to the presence of silicone-based polymer release agents. Glass microscope slides (Fisher Scientific) were cleaned with the same piranha cleaning procedure and were stored in methanol before use.
Results and Discussion TOF-SIMS Characterization. In the course of a TOFSIMS analysis on as-received AFM cantilever chips, several unexpected peaks appeared in the spectrum. These peaks were subsequently identified as fragments of silicone oils. TOF-SIMS has especially high sensitivity for the detection of silicone oils. Molecular fragments of silicone oils appear in the positive-ion spectrum at m/z ratios of 43, 73, 147, 177, 191, and 207 amu, as well as at some higher masses.30 Figure 1 shows static TOF-SIMS positiveion spectra acquired from an as-received AFM cantilever sample. Several siloxane-type molecular fragments are identified. The ubiquitous and uniform distribution of the silicone oils on the as-received cantilevers can be easily (29) Kern, W. Handbook of Semiconductor Wafer Cleaning Technology; Kern, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993. (30) Hohlt, T. A. Static SIMS Handbook of Polymer Analysis; Hohlt, T. A., Ed.; Perkin-Elmer: Eden Prairie, MN, 1991; pp 164-165.
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Figure 3. Plot of contact angle of deionized water droplets vs storage time of glass test substrates on the silicone mats in the commercial AFM cantilever container. The curve starts to level off at about storage times of 200 h (∼8 days) and indicates a saturated adsorption or a full coverage of silicone oils on the substrates. One-sigma error bars were calculated from at least 12 replicate measurements.
Figure 1. TOF-SIMS positive-ion spectrum of an as-received AFM cantilever. Several peaks of characteristic molecular fragments from silicone oils (poly(dimethylsiloxane), PDMS) are labeled.
Figure 2. Spatially-resolved TOF-SIMS ion images of an asreceived AFM cantilever. Image A depicts the ion map of Si+, which could come from both the silicone oils and silicon nitride cantilever itself. Images B-D are ion maps of three characteristic silicone oil fragments CH3Si+, C3H9Si+, and C5H15OSi2+ at m/z ratio of 43, 73, and 147, respectively. The distribution of the silicone oil contamination over the whole cantilever can be easily seen from these images.
seen from the TOF-SIMS ion images of some characteristic silicone oil (poly(dimethylsiloxane), PDMS) fragments. The ion maps of Si+, CH3Si+, C3H9Si+, and C5H15OSi2+ (at m/z ratio of 28, 43, 73, and 147, respectively) taken on an as-received AFM cantilever are shown in Figure 2. As we attempted to locate the source of the silicone oil contamination, it became evident that it was most likely a result of the method of packaging and storing of the
cantilevers. A common and convenient method of shipping AFM cantilevers is to place them on a cured silicone mat situated in the bottom of an outer container/lid package. Individual cantilever chips and wafers of several cantilevers naturally adhere to, and can be easily removed from, these silicone mats. Present in these silicone materials are lower-molecular-weight forms of the silicone polymers, sometimes referred to as silicone oils, possessing a finite vapor pressure at room temperature. After prolonged exposure of the AFM cantilever-chip assemblies to the silicone mats, a significant contamination layer can build up. A TOF-SIMS analysis was also performed on a piece of the silicone mat from the AFM cantilever storage container. The presence of strong silicone oil signals in the spectra (similar to the spectra shown in Figure 1, data not shown) provides direct evidence that the silicone mat used as packing material was the cause of the silicone oil contamination detected on as-received AFM cantilever chips. It is possible that this kind of contamination may also be observed when the cantilevers are not shipped or stored with silicone mats but are instead merely stored in various plastic containers without silicone. Polysiloxanes, a major source of silicone oils, are frequently used as chemical release agents in many molded plastic containers.31,32 Contact-Angle Measurements. To further study how soon the silicone oil contamination builds up, several clean glass substrates were stored in the plastic AFM cantilever container on the silicone mat for various lengths of time. Contact angles of deionized water droplets were measured on the sides which were not in direct contact with the silicone mats. The release of silicone oils from silicone mats and its time-dependent accumulation can be observed through the change in contact angles. The contact-angle measurement versus storage time on the silicone mat is plotted in Figure 3. These clean glass substrates yielded contact angles of less than 10° prior to exposure to the contaminants. There is a drastic increase in contact angles up to the 1-week exposure time (168 h). After 1 week, the (31) Owen, M. J. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; John Wiley & Sons: New York, 1985; Vol. 14, pp 411-420. (32) Owen, M. J.; Jones, J. D. Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 10, pp 7688-7694.
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Figure 4. XPS survey spectrum of an as-received AFM cantilever with Au coated on the back side. Due to the fact that the cantilever’s Si3N4 bulk material typically exists as a silicon oxynitride near the surface, the detection of Si, O, and N would not normally lead one to suspect a polymeric contamination like silicone oils. Arrows shown indicate the presence of approximately 1.6% Au on the tip side of the cantilever legs, also unexpected.
contact angle starts to level off, indicating a saturated adsorption or a complete coverage of hydrophobic silicone oils on the hydrophilic clean glass substrates. XPS Characterization. The XPS spectra of the asreceived AFM cantilevers are shown in Figure 4. Although XPS has routinely detected Si and O on the cantilever legs, without molecular fragment information it was not possible to attribute these signals to a contamination source such as siloxanes. The cantilever’s Si3N4 bulk material exists as a silicon oxynitride near the surface,11,28,33 and therefore the detection of Si, O, and N would not normally lead one to suspect a polymeric contaminant like siloxanes, unless a careful analysis of chemical shifts was conducted. Small-spot XPS analysis of regions on the cantilever legs, taken on the tip side, clearly indicate the presence of Au. This analysis yields the same results regardless of proximity to the tip, but an analysis of the tip side of the chip (not on the legs) does not show the presence of gold, surprisingly. A standard elemental quantification based on literature sensitivity factors34 indicates approximately 1.6% Au on the tip side of the AFM cantilevers. Apparently gold diffusion from the intentionally coated back side of the cantilever assembly to the tip side, during subsequent processing steps, is the source of this gold contamination. A shorter edge-diffusion distance on the cantilever legs (a few micrometers) than on the chip region (∼480 µm) results in a significant amount of gold on the tip side of the former structures but not on the latter. A spot XPS analysis of only the chip region of the cantilever-chip assembly would normally miss this contamination problem. Spatially localized TOF-SIMS spectra (not shown here) also reveal the presence of Au on the tip side of the cantilever legs but not on the chip. Cleaning Procedures. In an attempt to thoroughly remove the silicone oil contamination from AFM cantilever tips, two different cleaning procedures were employed. The first method consisted of soaking the contaminated (33) Bousse, L.; Mostarshed, S. J. Electroanal. Chem. 1991, 302, 269274. (34) The equation Cx ) (Ix/Sx)/(∑Ii/Si) was used for the calculation of the atomic concentration ratio of the species on surfaces. C, I, and S represent the atomic concentration, XPS intensity, and XPS sensitivity factor, respectively. S values used with the ESCALAB 220i-XL are as follows: C 1s, 1.00; N 1s, 1.80; O 1s, 2.93; Si 2s, 0.95; Au 4f, 17.12.
Figure 5. TOF-SIMS positive-ion spectrum of the tip side of an AFM cantilever leg with Au coating on the back side, after it was cleaned with piranha solution. Compared to Figure 1, the removal of silicone oils by piranha solution is clearly shown from the absence of the silicone oil fragment peaks in this spectrum. A zoom-in spectrum at m/z 39 is given in the inset as an example to show how the elemental K+ signal was discriminated from the C3H3+ hydrocarbon fragment, both at nominal mass 39 amu, due to the high mass resolution of TOFSIMS. The presence of a Au peak (197 amu) indicates that the gold contamination on the tip side of the cantilever was not completely eliminated by the piranha cleaning procedure.
cantilever chips in hexane overnight. Both XPS and TOFSIMS were performed following this cleaning procedure. The spectra were not significantly different before and after this mild cleaning procedure and are not shown. It was found from the TOF-SIMS results that the amount of silicone oils was reduced; however, some silicone oils remained. In addition to the still-present silicone oil signals, some new hydrocarbon fragments that probably came from residual hexane or ambient air were also detected. The second, and more aggressive, cleaning procedure consisted of incubating the cantilevers in a mixture of concentrated sulfuric acid and hydrogen peroxide, also called piranha solution, for 30 min. Both sulfuric acid and hydrogen peroxide are powerful oxidizing agents that react rapidly with almost any kind of organic compound. One of the products of this type of reaction is hydrogen gas. Immediately after the immersion of the as-received AFM cantilever chips into piranha solution, visible gaseous bubbles evolved from the cantilevers, indicating the presence of organic contaminants on the surfaces. In some cases the gold coating on the back side of the chip was peeled away in the course of incubation. However, observations made under an optical microscope indicated that typically the gold coating on the back of the cantilevers remained intact, and subsequent use in the AFM indicated a sufficient amount of gold for the laser reflection. The effective removal of silicone oils from the surface of AFM cantilever tips by piranha solution was shown clearly from TOF-SIMS analysis. The positive-ion static SIMS spectra taken from the tip side of the cantilever leg after being cleaned with piranha solution are shown in
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Figure 5. A comparison with Figure 1 (note different intensity scales) indicated that essentially all silicone oils were successfully removed. Several alkali and alkaline earth metal ions (Na+, K+, Mg+, Ca+) and trace amounts of other metal ions (Al+, Fe+, Cu+, etc.) were detected. They were most likely the residues of the piranha solution left on the cantilever chips or are intrinsic bulk impurities in the Si3N4 cantilever material. The presence of Ga+ was due to the gallium primary ion gun used in this analysis. Some contaminant hydrocarbon fragments were also observed. These fragments were probably introduced to the system from ambient air. Au on the cantilevers was not completely removed by piranha solution as can be seen by the presence of a Au+ peak in the spectrum on a much-expanded scale. It is expected that completely eliminating gold contamination present on the tip side will not be possible using a simple dipping method, without damaging the necessary gold coating on the back side of the cantilevers. Conclusion From a TOF-SIMS analysis, silicone oils (low-molecularweight polysiloxanes) were found to be present on asreceived AFM cantilever chips that were shipped and
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stored on silicone mats by a major manufacturer. On the basis of water contact angle studies, silicone oils released from this kind of packing material formed a contamination layer on substrates stored on these mats in just a few days. Such contaminants will likely exist in other commercial AFM cantilever chips, including those that are not stored or shipped on silicone mats, since silicones are commonly used as chemical release agents in many molded plastic containers. Two cleaning procedures were attempted. On the basis of XPS and TOF-SIMS results, we can conclude that soaking in hexane overnight is not enough to remove the silicone oil contamination on as-received cantilevers. A more aggressive procedure with piranha solution was shown to completely remove polysiloxane contaminants from the AFM cantilever tips. Acknowledgment. This work was supported by grants from the National Science Foundation (CHE9357188), from the Center for Biopolymers at Interfaces, and by the UROP program at the University of Utah (N.D.H. and W.S.C.). LA990371X
