High-Energy Electron Beam Lithography of Octadecylphosphonic AcidMonolayers on Aluminum
Nikolaj Gadegaard,*,† Xinyong Chen,‡ Frank J. M. Rutten,‡,§ and Morgan R. Alexander‡
Department of Electronics and Electrical Engineering, UniVersity of Glasgow, Glasgow G12 8LT,Laboratory of Biophysics and Surface Analysis, School of Pharmacy, UniVersity of Nottingham,Nottingham NG7 2RD, and School of Pharmacy & iEPSAM, Lennard-Jones Laboratories, Keele
UniVersity, Keele, Staffordshire ST5 5BG, United Kingdom
ReceiVed June 12, 2007. In Final Form: September 20, 2007
Monolayers of octadecylphosphonic acid were self-assembled on silicon substrates sputter coated with aluminum.Patterning of the self-assembled monolayer was achieved by high-energy electron (50 kV) illumination using anelectron beam lithography tool. The change in chemical composition of the exposed monolayer was investigated bytime-of-flight secondary ion mass spectrometry over an area of 100× 100µm2. The electron dose required to fullyexpose the SAM was found to be about 6 mC/cm2. Gratings were exposed with line widths from 10µm to 100 nm.The resulting patterns were imaged using friction force microscopy. It was found that the minimum line width is limitedto ca. 100 nm by the patterning resolution. The pattern resolution achieved, ca. 40 nm, is equal to the grain size ofthe sputter-coated aluminum layer, and the possibility that the grain size limits the pattern resolution is discussed.
Introduction
Creating chemical patterns with lateral dimensions frommicrometers to nanometers is of great interest for variousbiological applications in cell1 or nucleotide2 patterning. Pat-terning is, for example, possible by soft lithography,3-5 photolithography,6 scanning probe microscopy (SPM) manipulation,7,8
or electron exposure.9The contact lithography approach is simplewith few equipment requirements but still requires a mastersubstrate which commonly is produced by electron beamlithography. Patterns with features of less than 100 nm havebeen demonstrated using microcontact printing.10 For directwriting, two SPM approaches have been taken. The dip-penlithography technique uses an atomic force microscope to rasterscan the substrate in a controlled fashion, leaving thiols in apredefined pattern through ink transfer from the tip to thesubstrate.7,8 The smallest surface feature size produced by thedip-pen technique is lines about 15 nm in width. AlkanethiolateSAMs on gold are sensitive to radiation of both UV light andelectrons, though it is worth noting that the exposure mechanismsare different. Leggett et al. have demonstrated the ability to patternthiolate-based SAMs through oxidation of the interfacial head-
group using a scanning optical lithography approach on an SNOMsystem using a UV laser source.11 The area over which patternproduction can be achieved using an SPM approach is generallylimited to the scan size of the instrument, which is typicallyabout 100µm. For biological applications, this size limitationis restrictive since it is similar to the size of a single adhered cell(e.g., fibroblast). Larger areas are accessible through electronexposures of SAMs.
Mostpreviouselectronexposuresof self-assembledmonolayershave been carried out at relatively low energies/accelerationvoltages,<1 kV.12-15 Laibinis et al. used X-rays with an energyof about 1.5 kV. In the substrate the electrons were convertedto photoelectrons, which are responsible for the modification ofthe SAM. Craighead et al. used aminosilanes as SAMs to createchemical patterns on silicon surfaces using<5 kV electrons16
and thiol-based SAMs.17 The electron exposure of monolayersfor chemical conversion requires a relatively large electron dose.The necessary dose required for this can be decreased if theenergy is delivered near the surface. At increasing electron energy,the electrons will penetrate deeper into the material throughoutwhich the energy will consequently be distributed more widely.This means that the incoming electrons pass through themonolayer without great interaction/energy transfer. However,the secondary electrons generated from the substrate willpredominantly be responsible for the actual exposure of themonolayer.18
* To whom correspondence should be addressed. E-mail: [email protected].
† University of Glasgow.‡ University of Nottingham.§ Keele University.(1) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E.
Science1997, 276, 1425-1428.(2) Lipshutz, R. J.; Fodor, S. P. A.; Gingeras, T. R.; Lockhart, D. J.Nat. Genet.
1999, 21, 20-24.(3) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M.Chem. ReV. 1999,
99 (7), 1823-1848.(4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G.
M. Chem. ReV. 2005, 105 (4), 1103-1169.(5) Smith, R. K.; Lewis, P. A.; Weiss, P. S.Prog. Surf. Sci.2004, 75 (1-2),
1-68.(6) Ryan, D.; Parviz, B. A.; Linder, V.; Semetey, V.; Sia, S. K.; Su, J.; Mrksich,
M.; Whitesides, G. M.Langmuir2004, 20 (21), 9080-9088.(7) Hang, S. H.; Mirkin, C. A.Science2000, 288 (5472), 1808-1811.(8) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A.Science1999, 283
Two methods of e-beam lithography of SAMs have beendescribed in the literature. In one, a mask or stencil is used todefine a pattern on the substrate when placed over the substrateand exposed to low-energy electrons (a few hundred electron-volts). A drawback is that not all patterns are possible as a resultof mask restrictions; however, it is a parallel exposure method,which means that the patterning is fast. Another method is to usea scanned electron beam to directly write the pattern on thesubstrate. Converted scanning electron microscopes have beenused to write patterns at low electron energies by raster scanningthe electron beam. However, converted scanning electronmicroscopes are not suitable for large-area patterning, whichrequires precise movement of the sample and optimized electronoptics. Purpose-built electron beam lithography tools can movesubstrates with a very high precision but operate at much higherelectron energies, typically 50-100 kV.
As in conventional fabrication lithography, a number ofdifferent resists can be used to form a topographic pattern.Similarly, electron-induced pattering by chemical conversionhas been carried out on a number of different monolayer materials.Most commonly, thiols have been used as they readily formmonolayers on gold substrates.4 Alternatively, silanes have beenused, which form monolayers on oxide-based materials. Recently,Rundqvist et al. have demonstrated the ability to directly writein the biomolecule fibronectin and use the generated patterns forcell control.19 Here, we have used a phosphonic acid-terminatedmolecule which forms monolayers on aluminum and other oxide-terminated surfaces.20,21Octadecylphosphonic acid (ODPA) hasbeen shown to readily assemble on the oxide surface of aluminummetal with relevance to corrosion prevention of painted andbonded aluminum alloys in the aerospace and automotiveindustries.20-26
Here, we describe the patterning of a new type of self-assembledmonolayer, ODPA SAMs on aluminum, by high-energy electronbeam lithography (50 kV) using a purpose-built tool, an approachthat offers high pattern resolution over large areas. The exposureand patterning were examined by time-of-flight secondary ionmass spectrometry (ToF-SIMS) and friction force microscopy(FFM).
Materials and Methods
Substrates.Virgin silicon (100) substrates (Compart Technology,U.K.) were coated with 100 nm of aluminum by magnetron depositionusing a Plassys magnetron sputtering system.
Substrate Preparation.Aluminum was magnetron sputtered ontosilicon substrates in a membrane-backed turbomolecular-pumpedsystem that obtained a base pressure of minimum 3× 10-8 mbarprior to leaking argon in before a plasma was struck in front of theAl target using a dc magnetron source.
After 1-24 h of covered storage in the ambient atmosphere thealuminum-coated substrates were immersed in a 5 mMethanolicsolution of ODPA for 1-2 h prior to removal, rinsing in 2-propanol,
and drying in a nitrogen gas stream. The ambient storage time hasbeen shown to be critically important in the assembly of someSAMs.27
Electron Beam Lithography. After formation of the self-assembled monolayer the samples were exposed using a LeicaMicrosystems EBPG-5 HR100 high-resolution electron beamlithography tool operating at 50 kV. The electron beam profile ofthe tool is Gaussian. Gratings with 100 nm, 200 nm, 500 nm, 1µm,5 µm, and 10µm widths were exposed. The spot size chosen forthe exposed gratings was 28 nm for 100 nm width and 80 nm forall other widths. The lateral positional resolution of this tool is 5 nm.
ToF-SIMS Analysis.The surface chemistry was established usinga time-of-flight SIMS IV instrument (ION-TOF GmbH, Mu¨nster,Germany) equipped with a single-stage reflectron analyzer and using10 kV postacceleration. Gallium primary ions with an energy of 25kV were used throughout. Large-scale images were acquired bymoving the sample under the pulsed primary ion beam, images witha lateral scale below 500× 500 µm2 by rastering the primary ionbeam. Full raw datasets were saved throughout, allowing retrospectiveconstruction of high mass resolution spectra (m/∆m> 6000 at mass28) from any area within those images.
X-ray Photoelectron Spectroscopy (XPS).XPS was carried outon a Kratos Axis Ultra spectrometer using a monochromatic Al KRX-ray source utilizing electron flood for charge neutralization toachieve optimal spectral resolution. Spectra were acquired usingphotoelectrons collected perpendicular to the surface. Elementalquantification was achieved using relative empirically derivedsensitivity factors provided by the manufacturer.
Atomic Force Microscopy (AFM) Friction Force Imaging.AFM images were acquired in air using a Veeco D3000 AFMinstrument with a NanoScope IIIa controller plus a phase extenderor a Veeco EnviroScope AFM instrument with a NanoScope IIIacontroller plus a Quadrex extender. Silicon nitride DNP-type probes(Veeco) with a nominal spring constant of 0.06 N/m were used inconstant-force mode at 5 Hz scan frequency in the “fast scan” axis,with the scan size ranging from 1µm × 1 µm to 10µm × 10 µm.All friction force images presented have been acquired in the tracedirection.
Results
Dose Analysis: ToF-SIMS Analysis.To determine the effectof the electron dose on the ODPA monolayer on aluminum forhigh-resolution patterning, monolayers were exposed in an 8×8 array of 100× 100 µm squares, each of increasing dose atelectron doses ranging from 0.003 to 12 mC/cm2, Figure 1A.ToF-SIMS analysis was used to provide a molecular analysis ofthe resultant surface pattern.28
Negative secondary ions indicative of the intact SAM werereadily observed, including the dibasic phosphonate molecularion (M) with the addition of one hydrogen,m/z ) 333,CH3(CH2)17P(OH)O2
-, the protonated molecular ion in com-bination with substrate atoms,m/z ) 393, CH3(CH2)17P(OH)-O2AlO2H-, and the dimer in combination with the substrate,m/z ) 691, [CH3(CH2)17PO3]2Al- (Figure 2). These structuressuggest that ODPA forms a bidentate phosphate complex withthe surface, although the ready availability of protons in theSIMS process prevents a conclusive assignment. These ions weresummed and used to produce an image of the SAM distributiondenoted “ODPA” in Figure 1B. To assess the effect of the electronexposure, line scans of the relative intensity of these ions fromthe SAM were carried out, Figure 1C. At a given exposure theintensity of the ODPA molecule reaches a constant valueindicative of complete removal. Only the first six lines of the
(19) Rundqvist, J.; Mendoza, B.; Werbin, J. L.; Heinz, W. F.; Lemmon, C.;Romer, L. H.; Haviland, D. B.; Hoh, J. H.J. Am. Chem. Soc.2007, 129 (1),59-67.
(20) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R.Surf.Interface Anal.2004, 36, 347-354.
(21) Foster, T. T.; Alexander, M. R.; Leggett, G. J.; McAlpine, E.Langmuir2006, 22 (22), 9254-9259.
(22) Allara, D. L.; Nuzzo, R. G.Langmuir1985, 1 (1), 52-66.(23) Maege, I.; Jaehne, E.; Henke, A.; Adler, H. J. P.; Bram, C.; Jung, C.;
Stratmann, M.Macromol. Symp.1998, 126, 7-24.(24) Stratmann, M.AdV. Mater. 1990, 2 (4), 191-195.(25) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R.Langmuir
2007, 23 (3), 995-999.(26) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A.
N.; Nuzzo, R. G.J. Am. Chem. Soc.1991, 113 (19), 7152-7167.
(27) Pertays, K. M.; Thompson, G. E.; Alexander, M. R.Surf. Interface Anal.2004, 36 (10), 1361-1366.
(28) Vickerman, J. C.; Briggs, D.ToF-SIMS: Surface Analysis by MassSpectrometry; IM Publications: Chichester, U.K., 2006.
2058 Langmuir, Vol. 24, No. 5, 2008 Gadegaard et al.
exposure array were analyzed as the signal of the ODPAcomponent had reached the background level at that position.
The ODPA intensity from the irradiated areas plotted versuselectron dose is shown in Figure 3. To generate this figure, thepeak areas were added for the secondary ions associated withthe intact molecular ion plus H and the molecular ion minus OH,CH3(CH2)17P(OH)O2
- and CH3(CH2)16CHdPO2- atm/z) 333
and 315, respectively. To compensate for slightly differentlysized areas selected in the retrospective analysis process, all datahave been normalized against the total counts (which in turn isdirectly related to the number of pixels in each area selected foranalysis). Alternative normalization strategies against the total
ion signal and a number of small (hydro)carbon fragments (notshown) displayed the same behavior.
A number of low-mass negative ions have been correlatedwith the irradiated areas including O-, C2
-, CN-, and C3-. It
was notable that the aluminum oxide species,m/z) 59, AlO2-,
was not seen, indicating that there is a layer obscuring thealuminum oxide surface. In the positive ion spectrum a numberof polyaromatic ions were observed, includingm/z 105, 115,128, 152, 165, and 178 (not shown), similar to observations byHutt and Leggett, who ascribed these ions to polyaromatichydrocarbon species generated under electron irradiation hy-
Figure 1. (A) Schematic representation of the exposure strategy. The highest dose is exposed at the top left corner. The exposure meandersdown as the dose is lowered with the lowest dose in the bottom left corner. (B) SIMS intensity maps for a selection of negative ions withthe total counts (tc) and molecular assignment noted below each image. (C) Retrospective line scans of the integrated OPDA ion intensityof the lines of squares for a summation of ODPA fragment ions with the rows numbered from top to bottom. Only the top six lines wereanalyzed as the exposure was too low to degrade the monolayer for the last two lines.
Electron Beam Lithography of ODPA SAMs on Aluminum Langmuir, Vol. 24, No. 5, 20082059
drocarbon monolayers.29 On the basis of the high-intensity C2-
and C3- fragment ions and the polyaromatic species in the positive
ion spectra, we postulate that this is a cross-linked product ofthe degradation of the ODPA.
Silicon oxide signals, e.g., SiO-, were not observed, indicatingthat the aluminum layer on silicon is not damaged by irradiation.Interestingly, phosphonate fragments (PO2
- and PO3-) are
removed in the irradiated squares at high electron doses, indicatingthat these are lost to the surface with the rest of the moleculeon irradiation.
A series of fluorine-containing ions were correlated with theirradiated areas, including F-, CF-, F2
-, CF3-, COF3
-, C2OF5-,
C3OF7-, C4O2F9
-, and C7O3F15-. These are a clear indication
of a perfluoropolyether at the surface, which is commerciallyknown as Fomblin (Figure 2). SIMS is particularly sensitive tothese compounds due to the stable ions formed in the secondaryion generation process. Fomblin is commonly used as a vacuumpump lubricant for certain systems and in this case possiblyoriginated from low-level contamination of the magnetron sputtersystem used to deposit the aluminum. XPS of the samplesindicated that the elemental composition of the top 10 nm of thesurface was 46 atom % carbon, 24 atom % aluminum, 29 atom% oxygen, and 1.7 atom % phosphorus. No fluorine was detected,presumably because the elemental concentration was below thedetection limit for this element (ca. 0.1 atom % in this experiment).Hence, the Fomblin contamination is concluded to be present at(29) Hutt, D. A.; Leggett, G. J.J. Mater. Chem.1999, 9 (4), 923-928.
Figure 2. Negative ion SIMS spectra from electron-exposed and nonirradiated areas where M) C18H37PO3 and the main peak in the insetis [CH3(CH2)17PO3]2Al-.
Figure 3. Normalized peak intensity of C18 fragments normalized to the total counts as a function of dose for 50 kV electrons.
2060 Langmuir, Vol. 24, No. 5, 2008 Gadegaard et al.
a trace level but is overrepresented in the SIMS spectrum sinceit very readily forms stable negative ions.
Ion beam depth profiling was carried out through the SAMin an area that was not electron-beam-irradiated and the resultanalyzed by SIMS. We display one of the ion images, taken atan ion etch dose sufficient to reveal the alumina signal (AlO-),in Figure 4. The halo of F- and other species around the ion etcharea is from the crater edges. The intensity of the F- signals asa function of ion beam exposure confirms that fluorinated specieswere indeed present below the SAM as a result of introductionin the sample fabrication process. Other interfacial speciesprominent at the crater edge include the PO-, PO2
-, and PO3-
fragments of the ODPA headgroup. In the crater bottom, ionsindicative of the oxide including O- and AlO- were identified.Consistent with the assignment of OH- ions to the SAM, thision intensity was found to be most intense from the unetchedSAM area. Ions seen from the electron-beam-irradiated areas,CN-, C2H-, and C3
-, were observed as a halo around the etchedarea. We suggest that this results from ion-damage-induced cross-linking of the ODPA at the metal oxide surface, in the same wayas electron irradiation induced this effect revealed in the spectrapresented in Figure 2.
Electron-Beam-Exposed Gratings: AFM Analysis.Linesfrom 100 nm to 10µm width (the period is twice that of thewidth) were exposed to 6 mC/cm2, which was chosen on thebasis of the SIMS analysis. The patterns formed were investigatedby FFM since the lateral resolution of SIMS for molecular speciesis not sufficient to enable the lines to be resolved. FFM indicatedthat where the sample was irradiated there was an increase offriction correlated with the increase of friction associated withthe tip-surface interaction in Figure 5. Since the patterns wereimaged in ambient air, this will be caused by both the highersurface energy of the degraded SAM areas and the condensedwater meniscus between the surface and the tip. The periodicityof the line pattern is evident as bright peaks in the spectrum, asindicated by arrows.
To illustrate the successful patterning, horizontal and verticalsections were taken from the friction force micrograph, Figure7. The section has a width of 100 pixels along the line to improve
the signal-to-noise ratio. The electron-exposed line pattern isrunning in the vertical direction of the image, and a section alongthat axis (red line) only illustrates the topographic roughnessfrom the friction force micrograph. A vertical section (blue line),however, shows the patterned 100 nm lines and spaces. The edgeresolution can be estimated from a 90% change of the gray scalevalue (green box) to be about 40 nm. This is similar to the sizeof the grains in the deposited film, Figure 7 inset. Since thetopography can contribute to the friction signal, the FFMresolution estimate may be underestimating the quality/resolutionof the pattern. The topography image shows the facets of thecrystals that intersect the surface, and close inspection of thefriction data suggests that the FFM resolution limit may be thegrain size that can be seen in both the topography and FFMimages.
Discussion
We have used ToF-SIMS to determine the surface chemistryof ODPA monolayers on aluminum substrates exposed to high-energy electrons (50 kV) using a high-resolution beam writer.The absence of alumina ions in the SIMS spectrum of the electron-exposed areas and the increase in intensity of polyaromatic speciessuggest that the ODPA molecules are degraded to form a cross-linked layer. A similar layer was formed under ion bombardmentduring depth profiling. This behavior of alkane SAMs underelectron illumination has previously been reported for alkanethiolson gold.29Trace levels of Fomblin originating from the aluminumdeposition vacuum system were found at this aluminum oxidesurface, detected within this cracked organic surface layer afterelectron irradiation. The presence of this contaminant might beanticipated to reduce ODPA adsorption, although perhaps not toa significant level given its physisorbed state.
The electron dose required to expose ODPA was determinedby ToF-SIMS analysis and used to set the minimal required doseof high-resolution patterns, which was found to be 6 mC/cm2 atan electron energy of 50 kV. From the ToF-SIMS spectra wefound that the exposed areas showed polyaromatic species hadformed. This is in agreement with findings by Hutt and Leggett.29
They used 4 kV electrons to expose alkanethiol SAMs on gold.
Figure 4. ToF-SIMS ion intensity maps of the ion-etched area (central square) in ODPA assembled on an aluminum surface.
Electron Beam Lithography of ODPA SAMs on Aluminum Langmuir, Vol. 24, No. 5, 20082061
From this they proposed that the exposed alkane formed sucharomatic species during exposure to high-energy electrons. Wealso found that after exposure the relevant ToF-SIMS peak forthe aluminum substrate was absent. Thus, it appears that thechemical ODPA functionality is lost during exposure andconverted to aromatic species, but the material is not completelyremoved from the surface. To study the pattern fidelity, we
exposed gratings with dimensions ranging from 10µm to 100nm to an electron dose of 6 mC/cm2. The resolution of the SIMSis insufficient to image the gratings with the smallest dimensions,and thus, the patterns were imaged using FFM. Using FFM, wefound that the edge resolution was about 40 nm, which is similarto the grain size of the sputtered aluminum layer, Figure 7.Comparison with other reports on electron patterning of SAMs
Figure 5. Friction force microscopy of an electron-beam-patterned ODPA SAM with pattern widths of (A) 1000 nm, (B) 500 nm, and (C)100 nm. The Fourier (FFT) transform is derived from the friction force part of the image. The height difference between white and blackin the topography images is 30 nm.
Figure 6. Friction force microscopy of 500 nm wide features. The topography and friction part of the images were overlaid to illustratethe exposed line pattern with respect to the aluminum topography. The height difference between black and white in the topography imageis 30 nm.
2062 Langmuir, Vol. 24, No. 5, 2008 Gadegaard et al.
indicated that much of the work reported in the literature iscarried out at a wide range of electron energies<25 kV. However,there are no reports on using a 50 kV production tool for exposure.Hartnett et al. used two different amine-terminated silanesassembled on silicon substrates.16The monolayers were exposedto low-energy electrons (<5 kV). Line patternes of 1µm wereexposed to a dose of 100µC/cm2. St. John and Craighead usedfluorinated silanes to form monolayer resists.30 The monolayerswere exposed to electrons at 2-25 kV using doses between 370and 3000µC/cm2. Lines of 100-250 nm width were achieved.Hild et al. used organosilanes as monolayer resists on chromiumoxide substrates.31 The monolayer was exposed through a mask(TEM grid) to low-energy electrons, 80-200 V. Dots of 20µmdiameter were exposed to a dose of 350µC/cm2. Golzhauser etal. used a thiol with an aromatic backbone which cross-linkedupon electron exposure.9 Exposures were carried out at electronenergies of 2.5-20 kV using a conventional electron beamlithography tool. At 2.5 kV using a dose of 10 mC/cm2 20-100nm lines were achieved. Rundqvist et al. used PEG thiols ongold substrates and demonstrated a 40 nm line width.32 With theexception of the system used by Go¨lzhauser et al., all the SAM-based resists are degraded during exposure with an achievableresolution of 40-100 nm, which is directly comparable to ourresults. In a single-spot exposure Lercel et al. achieved a 6 nmexposed area.33A direct comparison between the required electrondose for the various SAM systems is very difficult as they havebeen exposed to a range of different electron energies.
The obtainable lateral resolution of the ODPA pattern may belimited by the grain size of the aluminum substrate. Sun andLeggett used UV radiation delivered by a modified scanningnear-field optical microscope to pattern mercaptopropanoic acidSAMs on gold. When backfilled with hexadecanethiol, they foundusing FFM that the grain size determined the achievable patternresolution.11 By comparison with broader patterning resultsachieved on atomically flat gold surfaces, they postulated thatthere is an enhancement in the electric field strength as their UVprobe travels over a grainseffectively an inversion of the
“lightning rod” effect, which results in a grain-size-dependentpattern resolution enabling them to achieve a resolution smallerthan the aperture through which the UV radiation passed.34
Although their system is very different in that they use UV, onemay speculate whether a lightening rod effect for emission ofsecondary electrons from the asperities represented by thepolycrystalline aluminum surface is plausible. In fact, with agrain size similar to that of the aluminum used here, they reacheda similar resolution when using SNOM patterning of thealkanethiols on gold. Since there is an interrelationship of theresolution of the FFM technique and surface topography, thiscould be responsible for the coincidence of this pattern resolutionlimit and the grain size in our data. Further work on aluminumsubstrates of different grain sizes is necessary to confirm thispostulate on this new chemically patterned system.
Conclusions
The vast majority of SAMs used for chemical patterning arebased on thiol or silane functionality to couple the monolayerto the substrate, gold or silicon, respectively. For the first timewe have shown that an alkylphosphonic acid (ODPA) is a suitableSAM for electron-induced patterning using high-resolutionelectron beam lithography on an aluminum substrate. ODPASAMs on aluminum were shown by ToF-SIMS to be degradedby a 50 kV electron beam, with a dose of approximately 6 mC/cm2 or more. Features, down to 100 nm wide lines, were imagedusing friction force microscopy by virtue of the difference insurface chemistry between exposed and unexposed SAM regions.The limit to the patterning resolution was determined fromretrospective line scans of FFM images to be about 40 nm,approximately the same as the aluminum grain size. This edgeresolution is similar to that reported by Rundqvist et al. on PEGthiols on gold.32
Acknowledgment. Weacknowledge technicalassistance fromDr. D. Macintyre and D. McCloy in the Department of Electronicsand Electrical Engineering at the University of Glasgow. N.G.is grateful for a personal fellowship from The Royal Society ofEdinburgh.
LA701733N
(30) StJohn, P. M.; Craighead, H. G.J. Vac. Sci. Technol., B1996, 14 (1),69-74.
(31) Hild, R.; David, C.; Muller, H. U.; Volkel, B.; Kayser, D. R.; Grunze,M. Langmuir1998, 14 (2), 342-346.
(32) Rundqvist, J.; Hoh, J. H.; Haviland, D. B.Langmuir2006,22(11), 5100-5107.
(33) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D.L. Appl. Phys. Lett.1996, 68 (11), 1504-1506. (34) Leggett, G. J.Chem. Soc. ReV. 2006, 35 (11), 1150-1161.
Figure 7. Friction force micrograph of 100 nm wide lines with 100 nm spaces with perpendicular line scans illustrated to the right. Left:sections of the friction force micrograph. The exposed line pattern is running in the vertical direction, and a section along that axis (red line)only illustrates the topographic roughness. A vertical section (blue line), however, shows the 100 nm lines and spaces. The edge resolutioncan be estimated from a 90% change of the gray scale value (green box) to be about 40 nm. Inset: topography image of the surface showingthe grain size. The scale bar is 100 nm.
Electron Beam Lithography of ODPA SAMs on Aluminum Langmuir, Vol. 24, No. 5, 20082063
