Corresvondence Anal. Chem. 1994,66, 2170-2174
Molecular Imaging Secondary Ion Mass Spectrometry for the Characterization of Patterned Self-Assembled Monolayers on Silver and Gold Greg Glllen,'~tJoe Bennett,t Mlchael J. Tarlov,* and Donald R. F. Burgess, Jr.* Surface and Microanalysis Science Division and Process Measurements Divisionl Chemical Science and Technology Laboratory, National Institute of Standards and Technoiogyl Gaithersburg, Maryland 20899
Self-assembly of allranethiol monolayers on gold and silver substrates is a fast, easy, and convenient method for preparing stable organic films with well-defined physical and chemical properties that can be modified by changing the terminal functional group of the molecule. In this correspondence, we report on the use of secondary ion mass spectrometry (SIMS) for characterizing methods of producing micrometer spatial scale patterns of two chemically distinct monolayers on silver and gold surfaces. Production of these molecular patterns is the crucial first step toward the application of monolayer films for biosensor and microelectronic device fabrication. Notably, we found that SIMS has the necessary sensitivity and selectivity to image the distribution of the intact parent molecular ions from each of the monolayers. T h q SIMS allowed unambiguous confirmation of the correct fabrication of a molecular pattern with micrometer spatial scale resolution. The formation and characterization of self-assembled monolayers (SAMs) of alkanethiol moleculeson metal surfaces (Ag, Au, Cu, Ga, As) is an active area of research.'+ The interest in SAM films stems from their ease of preparation, high quality, and stability. Most importantly, the chemical functionality of the surface of these films can be easily manipulated by variation of the terminal functional group of the thiol molecule. The capability to prepare organic films with tailored surface chemistries is important for fundamental studies of organic surfaces and may also lead to numerous industrial applications. Recently, several groups have demonstrated that it is possible to pattern two or more of these monolayers onto metal substrates using m e c h a n i ~ a l ,pho~?~ tochemi~al,~,~ or electron beam writing technique^.^ The ability to fabricate discrete patterns of molecules on surfaces,
each with specific chemical, biological, or adhesive properties, opens even more possibilities for the technological application of these materials. For example, patterned SAMs could find use as biosensors, corrosion barriers, and resists for semiconductor device manufacture. Our research has focused on developing a new method for UV photopatterning of alkanethiol SAMs on gold and silver surfaces.* We are also experimenting with focused ion and electron bombardment as alternative methods of patterning.1° As research into these new molecular patterning techniques progresses, it has become apparent that new methods of characterizing these materials must also be developed. We have found that one analytical technique, secondary ion mass spectrometry (SIMS), is indispensable for characterizing new patterning procedures. While SIMS is typically used for the characterization of elemental species, there has been growing interest in the application of SIMS to the analysis of organic molecules. In particular, SIMS offers the unique capability of imaging the spatial distribution of molecules on surfa~es.~Jl-l~ In this paper we demonstrate that SIMS is an unambiguous method for characterizing the spatial distribution of two different alkanethiol molecules that have been patterned onto the surfaces of gold and silver substrates. SIMS is one of the few analytical techniques that can give the chemical specificity and monolayer sensitivity required to identify unambiguously molecular species on a micrometer spatial scale.
EXPERIMENTAL SECTION Sample Preparation. The sample preparation procedure is described in detail e l s e ~ h e r e . ~Briefly, J~ an alkanethiol (8) Tarlov, M. J.; Burgess, D. R.F.;Gillcn,G.J. Am. Chem. Soc. 1993,115(12).
5305-5306.
Surface and Microanalysis Science Division.
Proccss Mcasurcmcnts Division. (1) Nuzzo, R.G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidscy, C. E. D. J . Am. Chem. Soc. 1987, 109, 3559-3568. (3) Whitsides, G. M.; Laibininis, P. E. Langmuir 1990, 6, 87-96. (4) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 431-463. (5) Abbott, N. L.;Folkcrs, 3. P.; Whitesidcs, G. M. Science 1992, 257, 1380t
1382.
(6) Ross, C. R.;Sun, L.;Crooks, R. M. Lmgmuir 1993, 9, 632-636. (7) Frisbie, C. D.; Wollman, E. W.; Martin, J. R.;Wrighton, M. S. J . Vac. Sci. Techno/. 1993. I 1 (4), 2368-2372.
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(9) Tiberio, R.C.; Craighcad, H. G.; Lcrcel, M.; Lau, T.; Sheen, C. W.; Allara,
D.L. Appl. Phys. Lett. 1993,62 ( 5 ) . 476-478.
(IO) Gillen, G.; Bennett, J. A.; White, S; Tarlov, M. J. To be. submitted to Appl. Phys. Lett. (11) Briggs, D.; Hcarn, M. J. Surf. Interface AMI. 1988, 13 (4), 181. (12) Gillen, G.; Simons, D. S.; Williams, P. Anal. Chem. 1990, 62, 2122-2130. (13) Schwcitcrs, J.; Cramcr, H. 0 . ; Hcllcr, T.; Jurgcns, U.; Nichuis, E.;
Zchnpfcnning. J. F.; Benninghovcn, A. J. Yac. Sci. Techno/. 1991, A9 (6), 2864-281 1. (14) Todd, P. J.; Short, R.T.; Grimm, C. C.; Holland, W. M.; Markey, S.P. AMI. Chem. 1992,64, 1871. (15) Tarlov, M. J.; Newman, J. G. hngmuir 1992, 8, 1398. 0003-2700l94l03662 170$04.50/0
0 1994 Amerlcan Chemical Society
SAM is prepared on a 200 nm thick silver or gold film that was sputter deposited onto a polished silicon substrate. The monolayer films are formed by immersing the metal film into a le3 M ethanol solution of the thiol of interest for several minutes. For preparationof UV photopatterns, the monolayer samplewas exposed to UV light from a high-pressuremercury lamp at a total spectral power density of 3 W/cm2, through a protective mask (typically a 300- or 400-mesh electron microscopy finder grid). All UV exposures were carried out for 1 h with the sample in air. This UV exposure results in oxidation of the thiol molecules (RS-)in the unprotected regions to their corresponding alkanesulfonates(RS03-).*The UV-exposed samples were then rinsed in ethanol and immersed in a M ethanol solution of a second alkanethiol molecule for 1 min, resulting in the displacement of the weakly bound sulfonate molecules by the second thiol. The result is a complementary pattern of the two alkanethiol monolayers. While electron microscopy finder grids were used as masks in the examples shown here, lithographic masks for creating any type of pattern can be prepared using standard fabrication techniques. SIMS Analysis. Two instrumentswere used for generating molecular ion images. The first, a Cameca IMS-4F ion microscope, was used in a microprobe mode of operation. Monolayer samples were bombarded with a microfocused beam of Cs+ ions at an impact energy of 14.5 keV. The beam was approximately 0.25 pm in diameter. Typical molecular images were acquired in 30 s using a primary ion current of 10pA and a raster size of 500 pm X 500 pm. This corresponds to an accumulated primary ion dose of 1.5 X 10l2ions/cm2. Similar accumulateddoses were received by the sampleduring acquisition of mass spectra. Negative secondary ions were monitored in all cases. A dynamic transfer system was used to enhance transmission of collected secondary ions. The vacuum in the sample region of this cryopumped instrument during analysis was in the 10-1O-Pa range. The upper mass range of this double-focusing instrument, as presently configured, is 310 amu. The second instrument, used for both SIMS imaging and pattern generation, was a Kratos 401LS time-of-flight secondary ion mass spectrometer(TOF-SIMS). The samples were bombarded with 28-keV Ga+ ions from an isotopically enriched Ga liquid metal ion source (> 99% 69Ga). The continuous primary ion beam current for a 1 pm diameter beam was 600 PA. The secondary ion imageswere obtained by pulsing the primary ion beam (pulse duration 4 0 ns; each pulse contains about 150 ions) while maintaining the sample at a constant bias of -3 kV. The repetition rate of the gun was 10 kHz. Secondary ions were extracted into thegrounded, reflectron-typetime-of-flight mass spectrometer(flight length -3 m). The secondary ions were detected by a dual microchannel plate detector with no postacceleration. The signal from the detector was fed to a multistop time-to-digital converter (TDC) with 10-ns time resolution. All secondary ion images were acquired using a total primary ion dose of about 10I2ions/cm2 (static SIMS conditions).
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RESULTS AND DISCUSSION Two requirements must be met when molecular patterns are fabricated using the UV photopatterning procedure. First,
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Flgwe 1. (a, top) Mass spectrum of decanethiol monolayer on gold. (b, bottom) Mass spectrum of decanethiol monolayer on gold after exposure to UV light for 1 h.
UV exposure through a mask in air must selectively oxidize the thiol molecules to sulfonates in the exposed areas16Second, immersion of the exposed sample into a second thiol solution must result in displacement of the weakly bound oxidized species by the second thiol. Our initial SIMS studies focused on the oxidation processes. Figure 1 shows two negative secondaryion mass spectra, acquired with the ion microscope, of a decanethiol monolayer (CHs(CH2)&, m / z 173) deposited on gold. Figure l a shows the mass spectrum of the as-prepared monolayer with the most prominent peaks being the SH- and various gold-sulfur clusters. Only very low intensity (- 100 counts/s) parent molecular ion signals are observed. The low abundance of molecular ions from alkanethiol monolayers on gold has been reported in a previous study in which it was also observed that abundant parentgold cluster ions were formed (not visible in this example due to the limited mass range of the ion microsc~pe).~~ The low parent ion signals and abundant cluster ions were thought to result from thevery strong gold-thiol bond that is characteristic of these monolayer^.^^ Figure 1b shows the samedecanethiol monolayer sample after irradiation with UV light for 1 h. Prominent SO3- and HS04- peaks are now observed as well as the sulfonated parent ion at m / z 221. The fact that we observe little parent ion signal from the unexposed sample but abundant oxidized parent in the UV-exposed sample suggests (16) Huang, J.; Hcmminger, J. C. J . Am. Chem. Soc. 1993, 115, 3342-3343.
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that the oxidized species has a much higher sputter yield and/ or secondary ion yield. X-ray photoelectron spectroscopy (XPS) results indicated that the thiol-to-sulfonate conversion was nearly loo%, although there was some evidence of degradation of the thiol parent by the UV light.8 From the standpoint of molecular patterning, it was necessary to demonstrate that the oxidation had occurred only in the UVexposed regions since some oxidation of these monolayers may occur under normal exposure to atmosphere. Figure 2 shows a molecular ion image of the sulfonated molecular ion ( m / z 221). The field of view is 500 X 500 pm and shows an area of the sample that was exposed to UV light through a largemesh metallic grid. The image shows the expected contrast with the highest signals occurring in the regions not protected by the grid. The second requirement for UV pattern generation is that the more weakly bound oxide species must be replaced by a second thiol molecule, thus creating a pattern of two chemically distinct molecules. For these experiments, monolayers were prepared on silver substrates. Figure 3 shows negative secondary ion mass spectra (generated on the ion microscope) from one representative experiment. Figure 3a shows the as-prepared first monolayer, mercaptoundecanoic acid [HQQC(CH2)&, m / z 2171 on silver. This mass spectrum shows several silver-sulfur cluster ions as well as the prominent peak at m / z 2 17 for the parent thiol molecule. This sample was then exposed to UV light through a 400-mesh electron microscopy grid and subsequently immersed in a second solutioncontainingoctanethiol [CH3(CH2)7S-,m / z 145).The mass spectrum from one of the exposed and then exchanged regions is shown in Figure 3b. A prominent peak at m / z 145 is observed corresponding to the intact thiol parent molecule. No parent ion for the original mercaptoundecanoic acid monolayer is detected in the exchanged region. (Experiments conducted with XPS in which a hexanethiol sample was UV patterned and then reimmersed in the original hexanethiol solution indicated that the sulfonate species produced during the UV exposure are completely removed and that the coveragesof the hexanethiolin the UV-exposedand -unexposed regionswere almost identical.8) To confirm correct fabrication of the pattern, molecular ion images were obtained for each 2172
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Mass (amu) Figure 3. (a, top) Mass spectrum of a mercaptoundecanoic acid monolayer on silver. (b, bottom) Mass spectrum of the same sample after exposure to UV light and exchange with octanethiol.
Figure 4. Molecular ion images of a patterned monolayer of mercaptoundecanoic acid (mlz 217) and octanethiol (mlz 145) on silver. Field-of-view is 350 X 350 pm.
parent ion from the same area of the specimen (Figure 4). These images, with a field of view of 350 X 350 pm, show the spatial distribution of the two thiol molecules on the surface. As predicted, the distributions show the grid pattern with the two molecules in the appropriate, complementary locations. Figure 5 shows images obtained from a new location on the same sample with the field of view decreased to 167 X 167 pm. In this example, the features on the letter "E" are approximately 15 pm in width.
Flgure 6. TOF-SIMS molecular ion image of an ion beam patterned monolayer containing decanethiol (mlz 173) (a, left) and octanethiol (m/z 145) (b, right). Field-of-view is 500 X 500 pm. Figure 5. New area from same sample as shown Figure 4. Fieldof-view is 167 X 167 bm.
The majority of our SIMS imaging experiments have been carried out on alkanethiol monolayers patterned on silver substrates rather than the more commonly studied gold substrates. The decision to use silver was based on our observation that the SIMS parent ion signal (RS-) from an alkanethiol sputtered from a silver surface is typically 1-2 orders of magnitude higher than the parent ion signal for the same alkanethiol sputtered from a gold surface. This significantly higher signal improves our ability to generate the molecular images required to characterize the patterning process. It is possible that this variation in molecular ion signal results from the differences in bonding of alkanethiol monolayers to the two metals. In contrast to gold, alkanethiol adsorption on silver results in a complex reconstruction of the silver surface that may lead to the formation of an ionic surface phase in which silver atoms have been removed from their equilibrium lattice positions. Bonding of alkanethiols to the reconstructed silver surface is presumed to be more ionic than on gold, where covalent interactions are thought to dominate (see ref 17 and references cited therein for a detailed discussion of this effect). While the sputtering and ionization of organic molecules under ion bombardment is not completely understood, it has been empirically observed that the highest ionization efficiencies are obtained from molecules that can be sputtered from the surface as an intact charged ion (preformed ions). These ions are generally formed from systems in which an ionic bond is cleaved in the sputtering process. Examples of such systems would be organic salts or organic compounds associated with a metal or alkali atom in which the bonding is primarily ionic.l* The ionic nature of the silver-alkanethiol bond may be responsible for the molecular signal enhancement we observe. However, other effects such as variations in sputter yield cannot be ruled out. The UV patterning procedure is a very attractive method for molecular pattern generation of alkanethiol molecules because it should be universally applicable regardless of the terminal functional group. However, some potential applications of patterned SAMs, such as resist materials for semiconductor manufacture, may require spatial resolutions (17) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; NUZZO,R. G. J. Am. Chem. Soc. 1991, 113,7152-7167. (18) Colton, R. J.; Kidwell, D. A.; ROSS,M. M. MussSpectrometry in the Anulysis of Large Molecules; McNeal, C. J., Ed.;Wiley: New York, 1986; pp 13-47.
of better than 0.1 pm which is beyond the capability of current photolithographictechniques.19 Therefore, there has also been interest in exploring alternative methods of patterning that would be universally applicable to all types of self-assembled monolayer films and may offer higher ultimate spatial resolution. For these reasons we have begun to explore the possibilities for ion and electron beam bombardment for pattern generation. We have found the Ga+ liquid metal ion gun equipped TOF-SIMS instrument to be an ideal platform for both pattern generation and subsequent molecular imaging characterization. For patterning experiments, the monolayer sample is bombarded with a highly focused,continuousprimary gallium ion beam rastered over a small area of the sample in a rectangular or line pattern. In the example shown in Figure 6, a decanethiol monolayer [CH3(CH2)9S-, m/z 173)] was first prepared on silver. The sample was placed into the TOFSIMS instrument and exposed to the ion beam in a rectangular pattern and in a series of lines. Primary ion doses varied from 5 X 10l2 ions/cm2 for the rectangular pattern to 1 X l O I 4 ions/cm2 for the lines. After ion bombardment, the sample was removed from the TOF-SIMS and immersed for 1 min in a l t 3M solution of octanethiol [CH3(CH2)7S-, m / z 1451. The sample was then returned to the vacuum chamber for molecular imaging SIMS analysis. Figure 6a shows the distribution of the characteristic parent molecular ion for the original decanethiol monolayer. In the ion-bombarded areas, no decanethiol parent ions are observed. Figure 6b shows the distribution of the parent ion of the octanethiol molecule now bound to the ion-bombarded areas. The individual lines are 4-5 pm wide. We have found that as the dose of the Ga ion beam increases, the extent of exchange of the second thiol molecule also increases until a plateau is reached at a primary ion dose of -6 X 1013ions/cm2. To obtain a measure of the efficiency of bonding of the alkanethiols to the ion-bombarded surface, an experiment was conducted in which a decanethiol film was ion beam patterned to a dose of 5 X 1014ions/cm2 (plateau region) and then reimmersed in the original M decanethiol solution. The signal intensity for the decanethiol parent ion was then compared from the ion-bombarded and adjacent unbombarded areas. These data indicate that the coverage in the ion-bombarded regions is approximately 70% that of the unbombarded regions. More specific details of (19) Levenson, M. D. Phys. Today 1993, July, 28-36.
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both ion and electron beam pattern generation in these monolayer films will be reported ebwhere.1° We have used the molecular imaging capability of SIMS extensively to characterize pattern generation in a number of alkanethiol monolayer systems on silver substrates. Other techniques, such as auger electron spectroscopyand scanning electron microscopy, have also been used to image patterned SAMs." The auger method offers high spatial resolution and monolayer sensitivity but requires elemental tags on each molecule, thus limiting its utility to special cases. Scanning electron microscopy has high spatial resolution but lacks chemical specificity.21 A potentially attractive technique for these studies is imaging X-ray photoelectron spectroscopy (XPS). However, at present, imaging XPS systems do not have sufficient spatial resolution or sensitivityto compete with SIMS. These limitations make SIMS, in our estimation, an extremely valuable method for imaging analysis of these patterned monolayers. The main difficulty we have encountered in our SIMS studies is that it is not straightforward to image the distribution of a given monolayer quantitatively because we do not know the relationship between secondary ion signal and- coverage. However, a recent study has demonstrated, at least for mixed monolayer species, that the relative molecular secondary ion SIMS signals were an accurate measure of the coverage of mixed SAM films as determined by contact angle measurementsand ellipsometry.22 This work suggests that with suitable standards it should be possible to quantify the coverage of a SAM film using molecular imaging SIMS. Since we have applied both our magnetic sector ion microscope and TOF-SIMS instruments to the characterization of the patterned monolayers, it is useful to discuss briefly the relative merits of the two instrumental approaches for this specific application. The principal advantage of the TOFSIMS is the parallel mass detection that allows multiple ion images (up to 16 in our instrument) to be obtained simultaneously. The ability to image multiple species simultaneously is very useful for characterizing the often complex chemistry of a patterning process. For quantitative imaging studies of two or more molecules in a pattern, the parallel mass detection also allows for a more straightforward comparison of signal intensities because primary ion beam damage and sputtering influence all of the analytical signals (20) Hickman, J. J.; Ofer, D.; Chaofeng, 2.;Wrighton, M. S.; Laibinis, P. E.; Whitesides,G.M. J. Am. Chem. Soc. 1991,113, 1128-1132. (21) Lopez,G. P.; Biebuyck, H. A.; Whitaridcd, 0 . M. Langmuir 1993,9,1513. (22) Offord, D. A.; John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994, 10, 883-889.
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equally. In the magnetic sector instrument, images are acquired serially, and primary ion beam damage (and sputtering) can result in lower intensitiesfor images acquired later (depending on the primary ion dose). The TOF mass spectrometer also allows for a higher accessible mass range. By Comparison, the magnetic sector ion microscope SIMS instrument has a similar spatial resolution and secondary ion transmission. Because higher primary ion currents can be used, useful molecular images are obtained in a few seconds. For rapid surveys of a process, for example the influence of UV expuretimeon the exchange, the microsoope instrumnt is preferred. Also,when a small number of species are imaged, for example @e two parent molecules in a pattern, ais instrument is also preferable because images can be o W d with higher pixel intensities (Le., good contrast) in a much shorter time. Although not generally used as such, the magnetic sector instrument can be used under static analysis conditions for acquiring fingerprint SIMS spectra when needed.
CONCLUSION Self-assembled monolayer films are important not only as tools to increase our fundamental understanding of organic surfaces but as technologically important materials with many potential applications. The ability to spatially pattern these chemically specificorganic layers has led to many suggestions for interesting and unique new technologies. To characterize molecular patterning techniques, we have applied molecular imaging SIMS. SIMS is unique in having sufficientsensitivity to produce images of intacr parent ions, from a single monolayer of material, with micrometer spatial resolution. This allows us to determine unambiguously the distribution of thiol molecules patterned on the surface. We have also shown that it is possible to use the SIMS instrument itself to generate patterns. ACKNOWLEDGMENT Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the equipment or materials identified are necessarily the best available for the purpose. Recehred for review December 17, 1993. Accepted Afll 1994.' Abstract published in Aduance ACS Absrracrs, May 15, 1994.
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