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Langmuir 1993,9, 1617-1620
Scanning Electron Microscopy for Imaging Photopatterned Self-Assembled Monolayers on Gold Eric W. Wollman, C. Daniel Frisbie, and Mark S. Wrighton' Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received September 4,1992. In Final Form: March 22,1993 Scanning electron microscopy (SEM) can be used to detect images resulting from irradiation of selfassembled monolayersof bis(undecanyl4-azidobenzoate)disulfide, I, on Au. The resulting surface-confined aryl azide has previously been shown to be photosensitive and yields primary photoproducts which are derivativesof 3H-azepineand hydrazinewhen irradiated with UV light in the presence of amines. Irradiation of the monolayer, Au-I, through a Cr-on-glass mask and a thin f i i of N-(2,2,2-trifluoroethyl)-N-(2ferrocenylethyl)amine,11, results in the surface attachment of 3 X 10-10 mollcm2, or approximately one monolayer,of ferrocenylcentersas demonstratedby cyclicvoltammetry. Maps of F and molecular fragments of ferrocene obtained by secondary ion mass spectrometry (SIMS) confirm that I1 is selectively attached to the surface only where irradiated. Contrast observed in the SEM images is a function of surface composition and length of exposure to energetic electrons. The critical finding is that SEM reveals the same image that is established by the SIMS technique. SEM, therefore, can be used as a routine and convenient method to obtain high lateral resolution images of photochemically patterned monolayers. Introduction We report here that scanning electron microscopy (SEMI1"' can detect patterns resulting from nonuniform irradiation of a photosensitive self-assembled monolayer (SAM)on Au. This result is perhaps best understood in light of two well-established facts about SEM. First, materials contrast in SEM occurs because different materials have different secondaryelectron yields (6)under primary electron bombardment. For example, Au (6 = 1.5) appears brighter than Al(6 = 0.8) if the two materials are in the same field of view in a scanning electron microscope, since Au emits more secondaryelectronsunder primary electron bombardment than Al. Second,surface adsorbates are known to change the magnitude of the secondary electron yield from many materials2*mby changing the surface potential of the materia12t7n8or by scattering the secondary electrons once they are emitted from the substrate.9 The relative influence of various adsorbed species on 6 can therefore also be the basis for contrast in SEM images. However, there have been only a few reports involving the use of SEM to image the lateral distribution of surface adsorbates.lG12 Our interest in surface chemistry and in mapping the distribution of surface-confiied molecules has led us to investigate the use of SEM in conjunction with other high lateral Author to whom correspondence should be addressed. (1) Goldatein, J. I., Yakowitz, H., Ede. P ~ c t i c a Scanning l Electron Microscopy; Plenum Prw: New York, 1976. (2) sei^, H. J. Appl. P h p . 1983,64, R1-R18. (3) Newbury, D.E. Scanning Electron Microscopy; IIT Research htitute: Chicago, IL,1977; Vol. I, pp 663-567. (4) Amelinckx, S.,Gevers, R., Rsmaut,. G., Van Landuyt, J., Eds. Modem Diffiaction and Imaging Technrques in Materio1 Science; American Elnevier Publishing Co., Inc.: New York, 1970. (6) McKay, K. G. Advances in Electronics a d Electron Physics; Academic Preae: New York, 1948; Vol. 1. (6) Bruining, H. Physic8 and Applicatione of Secondary Electron Emission; Pergamon Prw: London, 1964. (7) Palmberg, P. W. J. Appl. Phys. 1967,38,2137. (8)Padarrmee, H.; J d , A. J. Appl. Phys. 1976,60,1112. (9) Ono, S.;Knaya, K. J. Phys. D Appl. Phys. 1979,12,619. (10) Venablee, J. A.;GrWitb, B. W.; Harland, C. J.; Ecker, K. H. Rev. Phys. Appl. 1974,9,419. (11) Itoh, M.;Yokota, T.;Fufiehima, A.; Honda, K. Thin Solid Films 1986,144, L116. (12) Kanisawa, K.; O W , J.; Hirono, S.;Inoue, N. Appl. Phys. Lett. 1991, MI, 2363.
0743-746319312409-1617$04.00/0
resolution surface analytical techniques, namely Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS),in the analysis of photochemically patterned SAMs. The preceding report describes independent discovery of SEM imaging of SAMs patterned by means other than photo~hemistry.~~ The fact that SEM is sensitive to chemical changes in a monolayer of material is fundamentally intriguing. At the very least, the use of SEM opens the possibility of imaging patterned monolayers more conveniently and at higher lateral resolution than is presently possible by AES or SIMS. In this work, S A I V ~ Shave ~ ~ Jbeen ~ prepared by reaction of Au surfaces with a disuNde, I, which contains a photosensitivearyl azide moiety, SchemeI. Our laboratory has already reported that near-W irradiation of Au-I through a film of pure amine, 11,results in the irreversible attachment of 3 X 10-lO mol/cm2, or approximately one monolayer, of ferrocenyl centers to the surface, as shown by cyclic voltammetry.16 The two photoproducts shown in Scheme I, the azepine, Au-IIa, and the hydrazine, AuIIb, are the primary products, though it is known that the azepine undergoes further photoreaction to yield bicyclic products.17 Irradiation (A > 320 nm) of a photosensitive Au-I SAM through a Cr-on-glass mask and a thin film of pure I1 selectively produces photoproducts Au-I1 only where the Au-I is irradiated. With this methodology, arbitrary functional groups may be written to the Au surface in any desired pattern by choosing the appropriate amine and photomask. Our key finding is that the Au-I1 photopattern is observable by SEM. Experimental Section Chemicals. Methylcyclohexanesolvent for derivatiziig Au
slides with I was wed as purchased from Aldrich. Hexane and ~~
~~~~
~~
(13)Lopez, G. P.; Biebuyk, H. A.; Whitaiden, G. M. Longmuir, preceding paper in this h u e . (14) For a general overview of SAMe e m Whiteidea, 0 . M.; Laibiinia, P. E.Longmuir 1990,687. (?S) Hickman,J. J.; Ofer, D.;Zou, C.;Wrighton,M. S.;Laibinb, P. E.; Whtadee, G. M.J. Am. Chem. SOC.1991,113,1128. (16) Wollman, E.W.;Lorkwic, I. M.; Wrighton,M. 5.Submittad to J. Am. Chem. SOC. (17) Odum, R. A.; Schmall, B. J. Chem. SOC.,Chem. Commun. 1969, 1299.
0 1993 American Chemical Society
1518 Langmuir, Vol. 9, No.6, 1993
Wollman et al.
Scheme I. Preparation of Photopatterned SAMs on Au
I
*
I s
\\\
II hv
e
h
0.
Au
AU-II
Au-I
acetone for rinsing the derivatized Au substrateswere purchased from Mallinckrodt and used without further purification. Ethanol was obtained from Pharmco. Dibutyl- and diethanolamines were obtained from Aldrich. Synthesisof N-(2,2,2-trifluoroethyl)N-(Zferrocenylethyl)amine, speciesII,N-2-(2,6dichlorophenyl)ethyl-N-propylamine,and N-decyl-N-pentadecafuorooctylamine will be published elsewhere.16 Formation of Au-I SAMs. Polycrystalline Au films were prepared by electron beam evaporation of 50 A of Ti onto a SisN, coated Si wafer (100) as an adhesion layer followed by lo00 A of Au. The Au-coated wafers were fractured into 1.5 X 0.75 cm2 slides and these were immersed in a 1 mM methylcyclohexane solution of the disulfide, I. The Au slides were allowed to soak a t least 5 h before use. Mask Fabrication. The Cr-on-glass mask was fabricated at MIT and was originally designed for use in the fabrication of microelectrodesby standard lithographictechniques. The feature sizes on the mask range from 2 to 100 pm. Photopatterningof Au-I SAMs with Amine. A 1.5 X 0.75 cm2 Au substrate was removed from the methylcyclohexane solution of I and rinsed copiously with ethanol, acetone, hexane, and ethanol again, then blown dry with N2. A drop (-0.5 mL) of the appropriate amine was sandwiched between the Cr side of the Cr-on-glass mask and the Au-I surface, and this assembly was irradiated (A > 320 nm, 10 min) through the mask with light from a high-pressure Hg lamp. After irradiation, the mask was separated from the Au substrate, and the Au substrate was washed sequentially in ethanol, acetone, and hexane and then allowed to soak for an additional 5 min in ethanol before being blown dry with N2. The final soaking procedure ensured complete removal of unreacted amine from the surface. Three different amines could be patterned onto an Au-I SAM (Figure 4) by sequentially irradiating the surface through a thin film of the desired amine sandwiched between the substrate and a piece of Pt foil with a 125-pm slit cut into it. The slit in the Pt foilwaspositioned differently for eachof the three irradiations. The rinsing procedure was the same as already described. Scanning Electron Microscopy. SEM images of the photopatterns were obtained using a Perkin-Elmer 660 scanning Auger spectrometer. The Auger instrument is capable of performing simply as a scanning electron microscope. Images were acquired using a 3-keV electron beam operating a t 2 nA, except when noted otherwise in the figure caption. Acquisition time for each imagewas 70 s so that typical doses were 1electron/ nm2 (16 pC/cm2). Secondary Ion Mass Spectrometry. Detailed procedures have been described elsewhere.'* The F-map of the photopattemed substrate was acquired using a Fisons 1x70s magnetic sector SIMS instrument, using a 28-keV Ga+ beam operating a t (18) (a) Friebie, C. D.;Martin, J. R;Duf'f, R R,Jr.; Wrighton, M. S. J. Am. Chem. Soc. 1992,224,7142. (b) Friabie, C. D.;Wollman, E.W.; Martin, J. R,Wrighton, M.S.J. Vac. Sei. Technol., in preas.
A.
'm
F'Map m/z = I9
B.
C.
SEM High Magnification
U
IOpm
Figure 1. (A) 'BF- element map obtained by secondary ion mass spectrometry (SIMS) of a photopatterned Au-I monolayer film using chemistry shown in Scheme I. The map was acquired in 26 s using a 50-pA, 16-keV Ga+ primary ion beam. Total dose to the sample was 2 X 10l2ions/cm2or 0.02 ions/nm2. (B)Scanning electron micrograph of the same Au-I monolayer film. The dark areas correspond to regions where F was mapped in A. The micrograph was obtained in 70 s using a 3-keV, 2-nA primary electron beam, resulting in a total dose of 1 primary electron/ nm2. (C)Scanning electron micrograph showing eight 2.0 pm wide lines located a t the center of the micrograph in B. The image was acquired using a dose of 100 primary electrons/nm2. 25 nA/cm2. The map was acquired in 26 s, yielding a total d m to the surface of 2 X 10l2ions/cm2, or 0.02 ion/nm*.
Results and Discussion We have determined at high lateral resolution the distribution of Au-I1 on photopatterned substrates by
Langmuir, Vol. 9, No. 6, 1993 1519
SEM for Imaging SAMs on Gold A.
SEM Photopatterned AU-I
SEM Photopatterned AU-I
H 300pm
B.
Optical Micrograph of Mask
m,
Figure 3. SEM showing the effect of exposure of a photopatterned Au-I surface to a dose of primary electrons greatly in excess of that necessary to obtain the photomicrograph. The light rectangle a t the center of the photograph, marked A, was exposed to 61 primary electrons/nm2. The field of view was enlarged by reducing the magnification, and the micrograph acquired. The area outside A, marked B, was exposed to a dose of only 3 primary electrons/nm2.
Figure 2. (A) SEM of an Au-I film irradiated through a mask, shown in B, and a thin film of pure HN(CIOH~I)(CSH~FI& The light areas of the micrograph correspondto the location of surfaceattached fluorinated amine. Total dose to the surface during the image acquisition was 10primary electrons/nm2. (B) Optical micrograph of the Cr-on-glassmask used to photopattern Au-I monolayers.
mapping the location of F, a substituent of 11,by SIMS.18 Figure 1A shows a SIMS map for 19F-ion from analysis of a Au-I sample irradiated to form patterned Au-11. The map for F reproduces the image of the mask, with F mapped to the irradiated regions. The selectivity is at least 40:l as determined by a pixel analysis of the light and dark areas of the map. This confirms that the chemistry according to Scheme I can be used to prepare patterned molecular monolayers.16 Our significant finding is that the patterned Au-I SAM can be imaged by SEM. Figure 1B shows an SEM micrograph of the same sample shown in Figure 1A. The dark areas correspondto regions where F is present. Parts A and B of Figure 1confirm that the SEM image is due to contrast differences which result from the different chemicalcomposition in the irradiated and nonirradiated regions. Near-UV irradiation of Au-I through a mask in the absence of amine also results in an observable SEM image, presumably due to decomposition of the aryl azide. Importantly, no image is detected by SEM after patterned irradiation of a SAM of alkanethiol (C18H37SH) on Au, confirmingthat the patterning is due to photoreaction of the aryl azide. Irradiation of Au-I in the presence of amines other than I1also yields high contrast SEM images as illustrated for Au-I surfaces irradiated in the presence of H N ( C ~ O H ~ ~ ) ( CFigure ~ H ~ F2A, ~ ~or) in , the presence of N-2-(2,6-dichlorophenyl)ethyl-N-propylamine, Figure 3. Figure 2B shows an optical micrograph of the mask used to prepare the patterned monolayers in our work. Figure 1C shows an SEM micrograph of the small features located at the center of Figure 1B at low magnification. Each of eight photopatterned lines shown in Figure 1C is 2.0 pm wide and separated from its neighborsby 2.0pm, demonstrating that SEM can be used to image patterned monolayers with at least 1pm lateral resolution. In principle, monolayer patterns with features as small as 50 A (0.005 pm) could be resolved by SEM? although we are not certain of the ability to obtain this resolution when imaging monolayers owing to the associated electron beam damage. SIMS is presently capable of 100nm resolution, and while in principle AES can map features as small as 100 nm, in practice the extremely low signal resulting from a monolayer of organic material
125pm
H Figure 4. SEM of an Au-I SAM (A) patterned sequentially with dibutylamine (B), diethanolamine (C), and HN(CloH21)(CSH2F15) (D). The image was acquired using a 3-keV, 2-nA electron beam.
prevents such high lateral resolution imaging. The point is that our data show that SEM provides lateral resolution equal to, or better than, AES or SIMS and is an easier and more rapid technique for detecting images of patterned monolayers. We find that the contrast observed in our SEM images is affected by exposure of the surface to the primary electron beam. Figure 3 shows an SEM image of a Au-I film photopatterned with N-2-(2,6-dichlorophenyl)ethylN-propylamine. Prior to the acquisitionof the SEM image, the area correspondingto the bright rectangle at the center of the micrograph, marked A, was exposed to a 6.6-nA primary electron beam for 4.5 min, resulting in a dose of 61 electrons/nm2. The SEM image of the entire field of view, areas A and B, was then acquired using a dose of 3 primary electrons/nm2. Area A therefore received 20times more primary electrons/nm2 than area B. Regions in A which consisted of the native Au-I surface exhibit a dramatic increase in brightness as the direct result of electron bombardment. As aryl azide has previously been shown to be sensitive to electron bombardment,lg we conclude that contrast changes are due to changes in surface chemistry as a result of exposure to energetic (19) Cai, S.X.; Nabity, J. C.; Wybourne, M. N.; Keana, J. F. W. Chem. Muter. 1990, 2,631.
1520 Langmuir, Vol. 9, No. 6, 1993
electrons. Interestingly, the image becomes brighter initially in the region exposed to electrons (area A), not darker as is common in SEM. The preceding paper by L6pez et al.l*also discusses complex changes in contrast that occur during exposure of patterned monolayers to primary electrons. They conclude that electron-stimulated desorption of adventitious adsorbates as well as electronbeam-induced graphitization are important factors governing time dependenceof contrast. These processes may also be observed in our SEM studies, although efficient (61primary electrons/nm2,or 9.8 C/m2)electron-induced decomposition of the aryl azide in Au-I is the major mechanism of initial (brighter) contrast change in our images. We also note that the SEM images of the photopatterned monolayers are observablefor remarkably long times, on the order of hours, if low current densities (-lo-' A/cm2)are used. Despite decomposition of Au-I, we are able to acquire high-quality images of the monolayer photopatterns as shown in Figures 1, 2, and 3. We are pursuing further studies aimed at elucidating the sources of contrast in the secondary electron images of photopatterned SAMs. For example, we are able to pattern adjacent portions of a Au-I monolayer with several different amines. Figure 4 shows an SEM image of such a multiply patterned surface. Differences in contrast are
Wollman et al. observed for each amine-labeled photoproduct in the field of view, demonstrating that SEM is sensitive to a continuous "gray-scale" of subtle chemical changes in the patterned photoproducts. Lopez et a1.13provide excellent examples of differentiation of different monolayers in the same field of view by SEM and correlate molecular functionality to contrast. We have also begun to explore the affect of different substrates on the observed images. Preliminary work with aryl azides attached to Si surfaces via silosy groups shows that it is also possible to image photopatterned monolayers on Si by SEM. Indeed, we have already found SEM to be of great use in assessing the quality of image transfer in surface photochemistry on Au, e.g. Figure 2A and Figure 2B.
Acknowledgment. This work was supported partially by the Officeof Naval Research and the Defense Advanced Research Projects Agency through the Department of Defense University Research Initiative. We also acknowledge partial support from the National Science Foundation for this research. The authors thank John R. Martin and Elisabeth Shaw of the MIT Surface Analysis Facility for their assistance as well as Professor John B. VanderSande for useful discussions.
