Langmuir 1994,10, 3598-3606
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Scanning Force Microscopy Studies of Enhanced Metal Nucleation: Au Vapor Deposited on Self-Assembled Monolayers of Substituted Silanes David J. Dunaway and Robin L. McCarley* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 Received December 6, 1993. I n Final Form: July 28, 1994@ Scanning force microscopy was used to observe the topography of vapor-deposited Au films on various silane monolayers. Thin (1-10 nm) evaporated Au films deposited onto monolayers of (CH30)3Si(CH&X (X = -SH, -NH2, and -CH3) on SiOz/Si(lll)display morphologies which are dependent on the nature of the Au-X interaction. Macroscopic adhesion tests indicate that the Au is strongly bound to the amine and thiol surfaces. The electrical resistivity of 8 nm thick Au films deposited on the amine-terminated surface was found to be 7 orders of magnitude lower than that for the thiol-terminated surface, indicating that the amine-terminated surface favors an increased number of nucleation sites.
Introduction Of fundamental interest to many chemists workng with thin films on well-defined surfaces is the nature of the interaction between the surface and the thin film. Often this interaction, which is localized over only a few tenths of a nanometer, dictates the properties of materials with macroscopic dimensions. This interaction between the surface and the thin film is of particular importance to the deposition of organothiols on alkanoic acids on metal oxides,9-11 and silanes on Si02 ~ u r f a c e s . ~ JMuch ~-~~ of the research thus far has focused on understanding the structure of these self-assembled monolayers on the given substrate and correlating that information with the macroscopic properties of the thin films.l-l4 Other work has been directed at applications of monolayer films to microelectronics or molecular electronics.2 Recently, supported self-assembled monolayers with pendant tail groups have been investigated as surfaces which could possibly alter the nucleation and growth of vapor-deposited metal film^.'^-^^ Allara has shown1' that the grain size and electrical resistivity of thin Au fdms could be decreased by adsorbing a disulfide monolayer on Al2O3, and Wasser-
* Abstract published in Advance ACS Abstracts, September 15, 1994. (l)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6 , 87 and references therein. (2)Ulman, A. An Introduction to Ultra-ThinFilms From LangmuirBlodgett to Self-Assembly;Academic: San Diego, CA, 1991. (3)Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J . Chem. Phys. 1989,91,4421. (4)Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC.1991, 113,2805. (5)Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J.Am. Chem. Soc. 1989,111,321. (6)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109,3559. (7)Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992,96, 7416. (8)Alves, C. A.;Smith, E. L.; Porter, M. D. J. Am. Chem. SOC.1992, 114,1222. (9)Allara, D. L.; Nuzzo, R. G.;Lungmuir 1985,1,45. (10) Ogawa, H.; Chihera, T.; Taya, K. J.Am. Chem. SOC.1985,107, 1365. (11)Schlotter, N. E.;Porter, M. D.; Bright, T. B.;Allara, D. L. Chem. Phys. Lett. 1986,132,93. (12)Sagiv, J. J . Am. Chem. SOC.1980,102,92. (13)Moaz, R.;Sagiv, J. Langmuir 1987,3,1045. (14)Tillman, N.;Ulman, A.; Penner, T. L. Langmuir 1989,5,101. (15)Goss, C. A.;Charych, D. H.; Majda, M. Anal. Chem. 1991,63, 85. (16) Chaudhury, M. K.; Plueddeman, E. P. J . Adhes. Sci. Technol. 1987,1, 243. (17)Allara, D. L.; Hebard, A. F.; Padden, F. J.;Nuzzo, R. G.; Falcone, D. R. J . Vac. Sci. Technol.A 1983,1, 376.
man et al. demonstrated18 increased adhesion of Au films by using silane monolayers of a thioacetate or thiol on SiOz. We are interested in the fabrication of metal film electrodes with sizes approaching molecular dimensions ( < 5 nm) for studies of unusual transport in fluid electrolytes. In order to predictably produce smooth, electrically continuous metal films, we must understand the quality and nature of the metal interaction with various chemically modified substrates. It is well known that the morphology and electrical properties of thin metal films on insulators are strongly affectedby the nature ofthe insulator,21.22and a few studies have shown that organic modifiers can enhance metal n u c l e a t i ~ n . ~Until ~ - ~ the ~ film attains a thickness which permits the grains to come into contact with each other, the film will be a very poor conductor. If little chemical interaction occurs between substrate and metal (high metal self-diffusion), the metal will be very mobile and will tend to form large crystallites on the surface. Such films are rough and discontinuous at thicknesses as large as 20 nm. The nucleation mechanism associated with these types ofmetal films can be described by the VolmerWeber growth Alternatively, if the metalsubstrate interaction is strong, surface diffusion of the metal will be suppressed, and smoother, more continuous metal films can be formed at lower metal thicknesses. Thus, the chemical interaction between a particular modified surface and a given metal can be evaluated from the metal's morphology and electrical characteristics. Such information is of great importance to those investigating the physical properties of thin metal films. This is evidenced by the number of publications concerning the applications of thin metal films in areas including catalysis,23electronic device f a b r i c a t i ~ nmicrosc0py,~5 ,~~ (18)Wassersman, S.R.; Biebuyck, H.; Whitesides, G. M. J . Mater. Res. l989,4,886. (19)Czanderna,A.W.; King, D. E.; Spaulding,D. J. Vac.Sci. Technol. 1991,A9,2607. (20)Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Lungmuir 1992,8,2707. (21)Bauer, E. 2.Kristallogr. 1958,110,373. (22)Geus, J.W. In Chemisorptionand Reactions on Metallic Films; Anderson, J. R., Ed.; Academic: London, 1971;Chapter 3. (23)Dubois, L. H.; Hansma, P. K.; Somojai, G. A. Appl. Surf. Sci. 1980,6,173. (24)Gupta, D. In Diffusion Phenomena in Thin Films and MicroelectronicMaterials;Gupta, D., Ho, P. s.,Eds.; Noyes: ParkRidge, NJ, 1988. (25)Chidsey, C. E. D.; Loaicano, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988,200,45.
0743-746319412410-3598$04.50/00 1994 American Chemical Society
SFM of Enhanced Metal Nucleation
Langmuir, Vol. 10, No. 10, 1994 3599
Scheme 1. Monolayer Formation Followed by Metal Evaporation on the Molecular Adhesive Condensation Metal Deposition Mepl
electrochemistry,26and chemical sens01-s.~~ Our goal is to control the nucleation and growth of thin metal films using organic modifiers so that thinner, more continuous films can be produced. These films are targeted for use in the fabrication of optically transparent electrodes and nanoband electrodes. Organic layers containing two groups capable of forming chemical bonds with metal and inorganic surfaces have recently been implemented as “molecular adhesives” for vacuum-deposited metal~.l~-~O Allara has demonstrated that 5-10 nm Au films produced with a cystamine adhesive were very smooth (by TEM) and had substantially different electrical properties from Au films deposited on the untreated substrates.17 In contrast, Porter et al. have shown that carboxylic acid-terminated alkanethiol monolayers on Au( 111)do not affect the grain size of vapordeposited Cu overlayers.20 Wasserman et al. demonstrated18that a monolayer of a thiol-tailed silane caused large increases in adhesion in comparison to a methylterminated layer. Goss et al. have shown15 that the bifunctional molecule (CH30)3Si(CH2)sSH can be polymerized onto hydroxylated oxide surfaces such as Si02 through the silane end and can then bind Au on the thiol “surface”. The silane-modified surfaces used by Goss et al. were polymeric, and some problems with reproducible Au film adhesion were experienced. We have adopted a protocol28that produces monolayers of silane anchoring agent on Si02 surfaces, as shown in Scheme 1. This protocol is based on vapor-phasetransport ofthe silane to the oxide surface in a heated environment, so as to promote the condensation reaction but prevent the formation of polymeric species on the oxide surfaces under investigation. This simple procedure for producing surfaces with a given chemical functionality has allowed us to form ultrathin Au films with a range of morphologies and electrical properties. We present here scanning force microscopy29(SFM) images of vapor-deposited Au on monolayers of various substituted silanes deposited on Si02. These images indicate that the grain size of very thin (2 nm) Au films can be systematically controlled by varying the terminal group of the silane. These thin metal films exhibit electrical resistivities which also vary as a function of the silane tail group. Au films deposited on the X = -SH and -NH2 surfaces give rise to Au films which are markedly more conductive than unmodified Si02, but the amineterminated surface gives rise to Au films which are 7 orders of magnitude more conductive (at a Au thickness of 12 nm) than those Au films produced on bare Si02 surfaces. In addition, the adhesion of Au to the amine surface is as strong as that found for Au on Cr-modified Si02, indicating substantial coordination of Au by the amine group. (26) Winograd, N. In Laboratory Techniques in Electroanalytical Chemistry;Kissinger, P. T., Heineman, W. R., Eds.; Dekker: New York, 1984; Chapter 11. (27) Chemical Sensors and Microinstrumentation; Murray, R. W., Dessy, R. E., Heineman, W. R., Janata, J., Seitz, W. R., Eds.; ACS: Washington, DC, 1989. (28) Haller, I. J . Am. Chem. SOC.1978, 100, 8050. (29) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986,56, 93.
Experimental Section Preparationof Silane-ModifiedSiOz/Si Substrates. Appropriate-sized pieces of n-type Si(111) and Si(100) wafers (Virginia Semiconductor, Fredericksburg, VA) with misorientation 10.25” and resistivity of 0.06 Q cm were cleaned in 1:4 30% H202:98% H2SO4 solutions carefully thermostatted at 75 f 5 “C for 20 min (Caution: These solutions are highly oxidizing and should be handled with extreme caution!!).The clean SiOd Si was rinsed with copious amounts of 18MQ cm water and then blown dry with high-purity N2. Samples were modified with the desired alkoxysilane by the method described below. For electrical measurements, the clean SiOdSi samples were oxidized by heating at 1000 “C in an oxygen atmosphere for 2-3 h in order to grow a thick layer (60-100 nm) of insulating SiO2. The oxidized samples were then given the desired silane treatment. We tested several silane deposition protocols and found that a vapor-phase method produced monolayers which were strongly bound to Si02. Other preparations tended to yield either polymeric layers or poorly formed monolayers, as determined by ellipsometry and scanning force microscopy. Some of the failed preparations include the method described by Goss et al.15and a simple dipping of the clean SiOdSi in a millimolar silane solution in to1uene.l7J8 Vapor-phase depositions were performed by suspending the SiOdSi surfaces 2-3 in. above a refluxing -8% (v/v) silane/toluene solution in a quartz sample chamber fitted with a condenser for 4 h.28The quartz chamber and sample rack were always cleaned in a base bath, rinsed with 18MQ cm water, and then dried in a glassware oven immediately before use. Assembly of the chamber was carried out in the laboratory ambient without rigorous exclusion of oxygen or water. The chamber was vented through a toluene bubbler in order to keep the system isolated. After the samples were heated in the silane/ toluene vapor, they were removed, then rinsed with toluene, and dried in a stream of high-purity N2. Instrumentation. Metal evaporations were carried out in a cryogenically pumped vacuum system with a base pressure of 2x Tom(Edwards Auto 306A). Au (Canadian Mint, 99.99%) was thermally evaporated at a rate of 0.1 onto the SiOdSi substrates which had been affixed to a large aluminum holder with Ag paste (SPI Products) or vacuum grease (Dow Corning). The temperature, measured by a thermocouple imbedded in the aluminum holder, did not rise more than 1 “C during the depositions. Thicknesses were monitored with a quartz crystal microbalance. Once evaporations were complete, the chamber was back-filled with high-purity N2 and the samples immediately transferred to the SFM for imaging. Au( 11l)/mica substrates were prepared as previously described.25 SFM images were obtained with a Nanoscope I11 Scanning Probe Microscope (Digital Instruments, Santa Barbara, CAI. Cantilevers with pyramidal Si3N4 tips with a force constant of 0.06-0.58 N m-l were force-calibrated on the sample of interest by measuring the slope of the tip deflection vs tip-sample separation curves. Sample damage was minimized during routine imaging by reducing the tip-sample force to a level which provided stable images for extended periods of time. In the cases where microscopic adhesion testing was performed, the tipsample force was increased until substantial motion of crystallites occurred. Images presented here were low pass filtered and, if necessary, flattened using the manufacturer’s software. All images were obtained in air. The radius of curvature for the tips, Rt,was obtained by imaging atomic step edges (of height H)on Au(lll)/mica surfaces and measuring the lateral distance
Dunaway and McCarley
3600 Langmuir, Vol. 10, No. 10,1994
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traveled by the tip in traversing the step, L. Rt can be found30 from eq 1:
Such analyses gave an Rt of 43 nm, a result which is similiar to that found by others30 using the same type of silicon nitride tip and to that specified by the manufacturer. Ellipsometric measurements were made with a Rudolf 437 ellipsometer equipped with a rotating analyzer detector and 488.0 nm (Ar+laser) light. The angle between the incident beam and surface normal was fixed a t 70". All data were obtained in air and represent the average of five measurements on at least three samples. A film refractive index of 1.45 was used in the calculation of silane film thickness. A freshly cleaned Si wafer with approximately2 nm of native oxide was used as the "blank". We did not observe increases in the Si02 thickness as a function of exposure time in the HzOz/H2S04 cleaning solution a t temperatures of 70-80 "C. Our SFM images indicated some roughening of the native Si02 surface if the temperature of the cleaning solution was above 90 "C. We speculate that this roughening points to oxide growth caused by the H20z/H2S04. Thus, we always carefully controlled the temperature of the cleaning solution a t 75 f 5 "C. Contact angle measurements were obtained with a Ram& Hart Model NRL 100-00 goniometer. The sessile drop method (4pL drop size) employing high-purity water (18 MQ cm) was used. Measurements were performed 1min after the water drop formed on the surface. Five independent measurements on three different samples were used to calculate the contact angles. Electrical resistivity measurements were performed using ? either a two- or four-point-probe technique. A mask with 3 mm wide slots, 3 mm apart, was used to define four contact fingers during deposition of 100 nm of Au on the various surfaces. The mask was removed, and copper wires were attached to the pads at the end of the fingers using silver paint (Ted Pella, Redding, CA). External contact to the copper wires was made using highvacuum electrical feedthroughs. These contact fingers were used to monitor the electrical properties of Au films deposited on top of the finger assembly by placing the finger assembly back in the evaporator and monitoring the overlaying Au film resistance as a function of Au overlayer thickness. We have found virtually identical resistances when the contact fingers were deposited on top of the Au film to be measured, a method which Allara has used in his study of thin Au films.17 Thus with our in situ technique,we feel that shadowing of the Au overlayerfilm by the contact fingers is not a problem. For the four-pointmethod, a PAR 273 PotentiostatlGalvanostat was used as a current source, and the voltage was monitored with a Fluke 8060A digital voltmeter. Current (1-100 nA) was passed through the outer fingers, and the voltage drop across the inner fingers was measured. We have determinedthe contact resistance for our apparatus to be approximately 1-5 Q, using 18 nm Au films. We assumed that this contact resistance was constant throughout the measurement and would only dominate the measured resistance values of fairly thick Au films (dfil, 2 20 nm). Thus, we used a two-point method to measure the resistance of all the films presented here. The two-point method involved using the high-impedance voltmeter to measure the resistance across two of the Au fingers during deposition of the thin Au films and gave similar results in comparison to the fourpoint method. Resistivities were calculated by multiplying the observed resistance times the length of the contact finger (3.0 cm) and the thickness of the Au overlayer (QCM value) and then dividing by the spacing between the two contact fingers (3.0 mm). Reagents. All solventswere HPLC grade or better and were used without further purification. The aminopropylsilane (AI'S), mercaptopropylsilane(MPS),and butylsilane(Aldrich,98%)were vacuum-distilled and stored under Ar a t -20 "C until used. We have noted little difference in silane adhesion with various lots of the silane or with/without silane distillation. All other chemicals were reagent grade or better. (30)Goss, C. A; Brurdield,J. C.; Irene, E. A.; Murray, R. W.Langmuir 1993,9,2986.
B.
:
0 o 7 W 0
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x 1pm constant-force SFM image of bare SiO2/Si(lll). 2-range is 25 nm. (B) 200 nm x 200 nm cross section of image in A.
Figure 1. (A) 1p m
Results and Discussion SiOdSi Substrates. Shown in Figure 1A is a 1pm x 1pm constant-force SFM image of a clean Si(ll1) wafer which has approximately 2 nm of native oxide on the surface (denoted SiOdSi). Mechanical polish marks approximately 50 nm wide were observed on all samples; these are the "lines" running diagonally in Figure 1A. The peroxide/acid cleaning solution had no effect on the surface roughness unless the temperature of the cleaning solution was above 90 "C. Above this temperature, pits and other defects were found with the SFM. If the SiOd Si surfaces were cleaned in the peroxide/acid solution for less than about 5 min, we noted a contamination layer that was easily removed by the SFM tip. There were no noticeable effects of scanning the properly cleaned surface with the SFM tip at forces as high as 180 nN. As can be seen from Figure lA, the surface is extremely flat and devoid of any defects. Similar results were obtained with large scan areas (12 pm x 12 pm). We have obtained a roughness of f0.2nm for these substrates (using 200 x 200 nm scan areas) and have observed only slight variations when wafers from another batch of singlecrystal Si(ll1) are used. The roughness quoted here is a rms value obtained from several 200 x 200 nm scan areas on various wafers. A typical line scan is displayed in Figure 1B. A surface roughness of f l . O nm for similarly prepared Si(100) surfaces was obtained. This increased roughnessis attributed to the cleaning/polishingprocedure used by the manufacturer. Thus, we opted to use the flatter Si(111)for our studies because the expected changes in topography for the Au films should be less than 1nm. We noticed no difference in the electrical properties of thin Au films produced on the oxidized Si(100) and the Si(111)surfaces.
Langmuir, Vol. 10, No. 10, 1994 3601
SFM of Enhanced Metal Nucleation
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Figure 2. (A) 1 pm x 1 pm constant-force SFM image of (CH30)3Si(CH&SH vapor-treated SiOz/Si(111). Z-range is 25 nm. (B) 200 nm x 200 nm cross section of image in A.
Vapor-Phase Silane-ModifiedSiOdSi Substrates. Monolayer films of alkoxysilanes on SiOdSi can be prepared by exposing SiOdSi surfaces to the hot vapors of sufficiently volatile silanes in toluene for a t least 4 h.28 There was no substantial increase in surface roughness for SiOdSi(111)surfaces prepared in this manner (referred to as vapor films) with mercaptopropylsilane (polish marks were still visible). A roughness of f0.4 nm was found for the MPS-treated SiOdSi(ll1) surfaces (200 x 200 nm scans). SFM images did not display any polymeric features (Figure 2A,B). Similar results were obtained with the APS and methyl-terminated surfaces. As expected for such a short-chain alkane monolayer, we did not observe any microstructure attributable to any of the monolayers. Large-scale SFM scans (12 x 12 pm) and optical microscopy (100 x 100 pm) of the various vapor-treated surfaces did not exhibit any particulate matter that could be identified as polymeric siloxane. SFM and optical images of MPS, and especially APS films, on SiOdSi(111) formed by the method of Goss et aZ.,15 consistently displayed 0.1- 1pm particles. These particles could not be dislodged with solvent rinses or sonication. Surfaces with such particles had a hazy appearance. It is well known that the procedure described by Goss et aZ.produces polymeric material. Our vapor-treated surfaces were devoid of such particles, and thus we feel that these surfaces are free of polymeric material. Ellipsometric analysis of these films gave thicknesses indicative of monolayers (0.6-0.8 nm). This assumes a monolayer thickness of 0.7-0.8 nm. The other substituted silanes deposited from the vapor exhibited similar thicknesses. We assumed a refractive index of 1.45 for all of the organic layers. The difference in refractive indices (at the sodium d-line) for the various liquid silanes is less
than 0.03. There is a possibility that the monolayer films made from the different silanes have different densities, and thus the densities of terminal groups on the surface are different. This would not be surprising due to the fact that the deposition of APS is known to be self-catalytic.28 The effects of terminal group density on Au film morphology are discussed below in the section on electrical resistivity measurements. In order to investigate possible artifacts associated with Si02 thickness variations, we used various "blank" native oxide-coated wafers for the ellipsometric background measurement. Wafers cleaned at 75 "C for various lengths of time showed no variation in Si02 thickness. Repetitive cleaning of the same SiOz/Si(lll) sample did not cause any changes in the optical properties of the SiOdSi(ll1) sample. Thus, we feel that the thickness values we obtained for the vapor-treated SiOZ/Si(111)surfaces are attributable to the silane monolayers. Contact angle values for the freshly cleaned SiOdSi(111)surfaces were well below the minimum measurable value of 20", indicating a clean, hydrophilic surface. After the clean SiOdSi(111)surfaces were exposed to refluxing toluene vapors for 4 h, we noted no observable change in wettability. The methyl-terminated surfaces exhibited a contact angle of 74 f 4", which is consistent with a hydrophobic alkane monolayer. As expected, this value is lower than that found for more ordered octadecylsilane monolayers on Si0dSi.l4 We observed no change in the values for the methyl surface over the course of several days exposure to the laboratory ambient. Contact angle values for the MPS surfaces were found to be 51 f 2" and are in agreement with those obtained by Bein31for MPS monolayers on alumina (57"). The contact angle values for the moderately polar thiol surface did not change over the course of several days. Contact angle measurements performed on the APS surfaces yielded a value of 70 f 2", which did not change with time. This value is larger than that found by Bein's group3lfor APS monolayersformed on alumina (52").Our deposition involves the use of refluxing toluenehilane, whereas Bein's group used a solventless, vapor-phase deposition of APS. Our contact angle measurements for the APS surfaces are high for such a polar group (-NH2) but can be explained by either surface contamination or a "burying" of the high-energyamine group into the alkane layer. It is possible that the highly polar amine surface adsorbs organic impurities (either the toluene or the small quantities of impurities present in the HPLC-grade toluene) during the silane/toluene reflux step. We have noted no change in the contact angle after the APS surfaces are soaked in toluene or 18 MQ cm water overnight (70 f2"). No attempts were made at more rigorous cleaning procedures for fear of monolayer loss. It would seem that if substantial surface contamination were present, the adhesion of Au to the amine-terminated surface would be poor. As discussed below, the Au adheres well to the amine surface, indicating a lack of contaminants. If a contamination layer existed on our APS monolayers,it is possible that it could have been removed during evacuation (2 x Torr) prior to Au deposition. The contaminationlayer would of course reform upon removal from the vacuum chamber. Unlike our SFM results for improperly cleaned SiOdSi(11l), we note no removal of material by the tip for any of the monolayer surfaces. Ulman's group has proposed32 that monolayers of hydroxy-terminated silanes on Si02 and hydroxy-terminated alkanethiols on Au reorganize due to the high energy (31)Kurth, D. G.; Bein, T. Langmuir 1993,9, 2965. (32)Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7 , 156.
3602 Langmuir, Vol. 10, No. 10, 1994 A.
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Figure 3. 200 nm x 200 nm constant-force SFM images of 2 nm Au films deposited on SiO2/Si(111)with the followingvaporphase modifiers: (A)none; (B)(CH30)3Si(CH2)3CH3;(C)(CH30)3Si(CH2)3SH;(D) (CH30)3Si(CH2)3NH2. 2-range is 25 nm for all images.
: I 1 , , I d
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of the exposed alcohol group. Contact angles for whydroxyalkanethiol monolayers on Au left in the laboratory ambient were shown to increase with time; Ulman interpreted this as a monolayer reorganization whereby the hydroxyl groupburies itself in the alkane layer, leaving a hydrophobic methylene-like surface. The polarity of the deposition solvent was found to influence the number of exposed hydroxyl groups-nonpolar solvents tended to cause the hydroxyl groups to bury themselves. It is possible that our APS monolayers could undergo a similar surface reorganization during deposition from the toluene/ silane vapor and leave a methylene surface exposed, thus causing high contact angles to be observed. Although the chain length of the APS is somewhat shorter than that of the hydroxyundecanethiol used by Ulman, somewhat similar results might be expected. As noted by Ulman, it is almost impossible to differentiate between surface contamination and surface reorganization. Thus both the surface reorganization and surface contamination models would explain the high APS contact angle values. At this time we are unable to ascertain which of the models is correct. There was no damage to vapor-phase films during SFM imaging, even a t forces as high as 180 nN. Films formed by dipping the SiOdSi in a millimolar silane/toluene solution17J8were easily damaged by SFM imaging at forces above -40 nN, indicating a poorly formed monolayer. In addition, 50 nm Au films deposited on MPS vapor films showed no loss of Au during Scotch Tape adhesion testing. Polymeric layers of MPS on glass have been shown to be effective adhesives for thin Au films;15assuming that the thiol-Au interaction is substantial, we feel confident that the silanes are firmly bound to the Si02 surface. MorphologyofAu on Vapor-PhaseSilane-Modified SiOd Si Substrates. Shown in Figure 3 are constant-force SFM images of 2 nm thick Au films vacuum-depositedon SiOd Si(ll1) with various treatments. There is a noticeable difference in Au crystallite size on the bare SiOdSi(ll1) in comparison to the silane-modified surfaces. We note a lack of atomic steps on the individual Au grains for the various surfaces, indicating a rough polycrystalline film. We routinely find 13-16 nm crystallites of Au deposited on freshly cleaned SiOz/Si(111)surfaces. It is well-known that the difference in surface energies for Au and Si02 is quite high, and adhesion of Au to Si02 is almost nonexist-
Figure 4. Cross sections of 200 nm x 200 nm constant-force SFM images of 2 nm Au films deposited on SiOz/Si(lll) with the following vapor-phase modifiers: (A) (CH30)3Si(CH2)3SH; (B) (CH30)3Si(CH2)3NH2.
ent. Both of these factors lead to fairly rapid Au selfdiffusion, which results in “islanding” of the Au crystallites.22 A similar but larger effect is noted for the (CH30)3Si(CH2)3CH3 (“methyl”) treated surfaces; crystallites of 20-25 nm diameter are observed, indicating little interaction between the Au and methyl surface. Obtaining clear images of thin Au films ((2 nm) on the methyl and bare Si02 surfaces was difficult due to the forces between tip and Au being large in comparison to those between Au and the underlying surface, even a t the lowest imaging forces of 1-2 nN. No attempts were made a t reducing the tip-sample forces by imaging under water, due to loss of the Au film upon exposureto water. Solvents such as propanol, toluene, and dichloromethane also removed the Au films from the methyl and bare SiOdSi surfaces. Both the (CH30)3Si(CH2)3SH(“thiol”)and (CH30)3Si(CH2)3NH2 (“amine”)treated SiOZ/Si(111)samples display smaller grain sizes than those on bare SiOdSi(lll), with the grain sizes on the thiol and amine surfaces being 7-9 and 5-9 nm, respectively. Measured grain heights (obtained from cross sections, Figure 4A,B) were found to be -2-4 nm, but these heights are probably exaggerated due to tip-sample interactions (humidity effects) as discussed by Thundat et a1 .33 and Goss et aL30 Often with surface features like the grains shown here, the cantilever tends to stick and warp upward or downward, depending on scan direction. Such cantilever sticking causes the feature height to be exaggerated. Both grain size and height values are highly reproducible numbers; we have examined 10-20 Au films on each surface. The decreased metal grain size indicates that surface diffusion of Au on the amine and thiol surfaces is substantiallyreduced; we attribute this to strong chemical interactions between Au and these two functionalities. The Au grain size was found to be more homogeneous on the thiol-terminated surface, but the grains appeared to have some void space around them. We call this void (33) Thundat, T.; Warmack, R. J.; Allison, D. P.; Bottomley, L. A.; Lorenco, A. J.; Ferrell, T. L. J. VUC.Sci. Technol. A 1992, 10,630.
Langmuir, Vol. 10,No. 10,1994 3603
SFM of Enhanced Metal Nucleation
space because there are no smaller grains around the larger grains, as evidenced by the SFM images (Figure 3C,D) and cross sections (Figure 4). This void space has been noted (by TEM) for Au deposited on a disulfidetreated surface in a previous study. l7 It appears that some of the Au grains overlap on the amine surface-that is, there seem to be small (5-6 nm) grains clustered around larger (7-9 nm) grains. The presence of smaller grains is more easily seen in the cross section shown in Figure 4B. Note the small grains (shoulders) a t the base of the larger grains. Not all of the small grains are observed in the cross sectional view because of the nature of the line scan and the distribution of the small grains. The larger variation in grain sizes for the Au-amine surface makes it rougher than the Au-thiol surface. We are unsure of how these larger grains form, but it may be that several smaller grains coalesce to form the larger grains. All of the images in Figures 3 and 4 were obtained with the same tip, so the observed differences in grain size or shape were not due to tip differences. Similar results have been observed with a number of different tips. The difficulties of imaging such small objects with a tip with Rt = 43 nm have been discussed by T h ~ n d a and t~~ GOSS.~O We believe that the ability to image the narrow void space between the grains is due to the presence of asparities on the tip, for such small features could not be imaged with such a smooth object -80 nm in diameter. Even with the asparities on the tip, our reported grain heights, grain void space, and grain sizes are likely not the true values due to tip-sample convolution. Although the values we quote are certainly affected by the tip, the differencesin the size, shape, and spacing of the Au grains on the various silane monolayer surfaces are real. Thus, from our observations it appears that the 2 nm Au film on the amine-terminated surface is more continuous in comparison to that formed on the thiol surface. This is confirmed by electrical resistivity measurements discussed later. There is an abundance of literature concerned with adsorption of thiols and disulfides on Organothiols are known to form robust monolayers on Au. Little information regarding the adsorption of amines on Au exists in the literature. Bain has shown that long-chain alkylamines adsorb very poorly on clean Au surfaces.34 Ellipsometric and contact angle measurements indicated that the long-chain amines were not well-packed and were easily displaced by alkanethiols. In addition, various amines have been shown to be readily desorbed from Au electrodes35at potentials near 0 V vs the sodium chloride calomel electrode (SSCE), whereas thiols were not desorbed36until a t least -1.0 V. Such behavior indicates that amines do not interact strongly with Au. A more recent infrared by Xu et al. indicated that longchain alkylamines formed well-defined assemblies on Au surfaces during gas-phase dosing of Au with the alkylamines, but the amine monolayers were readily removed with polar solvent rinses.37 Our results concerning the morphology of Au deposited on amine surfaces, however, point to a fairly strong interaction between Au and the amine. Microscopic and Macroscopic Adhesion of Au on Silane Monolayers. In order to compare microscopic and macroscopic adhesion properties of Au on various substituted (34) Bain, C. D.; Evall, J.;Whitesides, G. M. J.Am. Chem.SOC.1989, 111, 7155. (35) Horanyi, G.; Orlov, S. B. J . Electroanal. Chem. 1991,309,239. (36) Widrig, C. A.; Chung, C.; Porter, M. D. J . Electroanal. Chem. 1991,310,335. (37)Xu., C.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1993,65,2102.
In Vacuo Resistivity Measurements for Evaporated Au Films
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41
2
I
Amine Trial #I
I
Thiol
I
Bare Si02
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M
-.i -6 0
25
50 75 100 125 150 175 200
Au Thickness
(A)
Figure 5. Resistivity plot for various terminated surfaces.
silane surfaces, we imaged 2 nm thick Au films a t various forces and also used Scotch Tape tests on 50 nm Au films to note Au removal. As was discussed earlier, Au films on the methyl-terminated and bare Si02 surfaces could not be imaged a t forces above 2 nN. Au films on these surfaces were completely removed during the tape tests. In contrast, Au films on both the (CH30)3Si(CH2)3SH-and (CH30)3Si(CH&NHa-treated SiOdSi(111)samples were unchanged aRer removal of the tape. Repeated application and removal of tape did not cause any visible damage to the Au. Similar results were obtained when 5 nm of Cr (a commonly used metal adhesion promoter) was used instead of the silanes. We expected that the Au would adhere very strongly to the thiol surface but not to the amine. A previous study using a polymeric amine as an adhesion promoter indicated that there was substantial interaction between vacuum-deposited Au and the polymer, but details concerning the nature of the bonding were not discussed.16 Scanning force microscopy images of 2 nm thick Au films deposited on the amine- and thiol-terminated silane monolayers revealed no difference in microscopic adhesion at high tip-sample loads. We were unable to remove Au deposited on vapor-phase(CH30)3Si(CH2)3SH-or (CH30)3Si(CH2)3NH2-treated SiOZ/Si(111)with forces as high as 180 nN, but we did notice plastic deformation of the Au in the scanned area. Thus with Au films only 2 nm thick, the strength of the Au-thiol and Au-amine interaction is surprisingly large. Electrical Resistivity Studies. Shown in Figure 5 is a plot of log(resistivity) vs Au film thickness for untreated SiOdSi(1l l ) , the oxide treated with (CH30)3Si(CH2)3SH and (CH30)3Si(CH&NH2. (The Au template “fingers” were formed on the clean SiOdSi sample by first coating with 2-5 nm of Cr in order to ensure good adhesion, but the area between the fingers was left as clean SiOz. The resistivity of Au overlayer films on the bare Si02 was unchanged when different thicknesses of Cr were used for the contact fingers.) The untreated surface yielded Au films with a high resistivity a t Au thicknesses less than 10 nm, but then the resistivity of the film decreased fairly rapidly with increased Au thickness until it began to approach the bulk value of 2 x Q cm.38 Such (38)Shackelford, J.F. Introduction to Materials Science for Engineers; MacMillan: New York, 1992; p 543.
3604 Langmuir, Vol. 10, No. 10, 1994 behavior is characteristic of the island-growth mechanism of thin metal films on insulators. Until the grains reach a size which allows contact with other grains, the film will be highly resistive. This is similar to what previous researchers have observed for Au and Ag on bare A1203 surfaces.17 Reduced grain size, as observed by SFM, for the A d thiol-terminated surface was confirmed by the onset of electrical continuity at lower Au film thickness versus those films prepared on bare S i o ~ We . ~ have ~ noted a 2-fold decrease in Au grain size (at Au thickness of 2 nm) for the thiol-terminated surface in comparison to bare SiO2. A similar trend was noted for Au evaporated onto AI203 surfaces treated with 3,3’-dithiodiproprionic acid (a disulfide).17 The Au film thickness a t the onset of electrical continuity for the (CH30)3Si(CHz)~SH-treated SiOz/Si(lll)surfaces was found to be higher than that in the study using adsorbed 3,3’-dithiodiproprionic acid on alumina. We feel that this difference is due to either increased order or a higher surface coverageof the disulfide monolayer in comparison to our MPS monolayers. The proposed structure of the disulfide monolayer is one in which the disulfide bond prevents any “floppiness” of the molecule on the surface.17 Such an increase in the order ofthe film would result in a n increased number of exposed nucleation sites. A similar difference in nucleation sites would occur if the MPS coverage were lower than the 3,3‘dithiodiproprionic acid. A surface with a large number of nucleation sites would cause a deposited metal film to display a lower resistance than a film formed on a surface with fewer nucleation sites. The Au film thickness a t the onset of measurable electrical continuity for Si02/Si(lll)treated with (CH30)3Si(CH&NH2 was observed to be considerably lower than that for bare or thiol-treated surfaces. For Au films 8.4 nm thick, the resistivities on the amine and thiol surfaces were found to be 4.2 f 0.1 x loT552 cm and 1.6 f 0.3 x lo3 52 cm, respectively-a factor of 3 x lo7difference. We show two data sets for different surface modifier and Au film deposition runs in the case of the amine to demonstrate the reproducibility of the procedure. We have repeated this experiment at least 10 times and obtained virtually identical curves. There was no shorting of the Au through the SiOz to the underlying Si, as noted by measurement of the resistance between the “fingers” before the thin Au overlayer was deposited. Resistivity values obtained in vacuo and in the laboratory ambient were found to be within 3-5% of each other. The resistivity values change only 1-3% if the samples are left in vacuum or in the laboratory ambient for up to 3 days. (We did not monitor the values beyond 3 days.) The Au films formed on the amine surface are some of the thinnest, electrically continuous Au films formed a t room temperature that have been reported.17 We note a resistivity of 86 f 2 52 cm for the 3 nm Au on the amine surface, which is smaller than the reported lo4Q cm value for a 3 nm Au film formed on 3,3’-dithiodiproprionic acidtreated a 1 ~ m i n a . lAlthough ~ our resistivity values are smaller than those previously reported for similar syst e m ~variations , ~ ~ in experimental procedures between different laboratories could account for the differences. The novelty o f our work is the unexpected low resistivity values for Au films formed on the amine-terminated monolayer. We are uncertain of the cause of the slight change in slope of the log(resistivity) vs Au film thickness curves (39)Attempts to measure the resistivity for the methyl-terminated surface were met with frustration. Reproducibleelectrical contact with Ag paint could not be obtained due t o delamination problems.
Dunaway and McCarley (near 5 nm Au thickness) for the amine surface. This change is slope is highly reproducible. It is possible that the conductivity of the thin Au film is affected substantially by the chemical interaction of the amine, and the first few layers of Au will exhibit a resistivity lower than that expected in the absence of the amine-Au interaction. The same could be true for the thiol, but the effect would be less if the Au-thiol interaction were less than the Auamine interaction. From the data presented here, it appears that the amine-Au interaction is equivalent to if not larger than the thiol-Au interaction. We are currently investigating the cause of the change in slope of the resistivity plots. From our SFM images, we have observed that the amine interacts strongly with the Au and reduces surface diffusion, thereby decreasing the Au grain size. But, for such a drastic difference in resistivities to occur, we feel that the number of nucleation sites on the amine surface must be substantially larger. A higher density of nucleation sites would allow growing grains to coalesce and form a continuous Au film at an extremely low Au thickness. As can be seen in Figures 3 and 4, there appears to be void space between Au grains for the thiol surface, but there is less for the amine surface at 2 nm Au thickness. We have performed side-by-side SFM experiments for 2 nm Au films on the various surfaces; the void space that we observe is not due to tip artifacts. We note a substantially decreased amount of void space for Au on the thiol surface a t a Au film thickness of approximately 8 nm Au (not shown), which is near the onset (8.4 nm) of measurable electrical resistivity. Thus it appears that the amine-terminated silane is able to cause a decrease in Au surface diffusion and an increase in the number of nucleation sites versus the thiol-terminated surface, which results in a substantially thinner, electrically continuous film. There are several possible explanations for the number of increased nucleation sites on the amine surface. It is possible that the increase is due to increased film order brought about by either a higher silane surface coverage or hydrogen bonding between amine groups on the surface (before metal evaporation). I t is well known that APS deposition is self-catalyzed, so it would not be surprising to have a slightly higher concentration of amine sites in comparison to other silanes deposited for similar amounts of time. The experimental limitations in determining the ellipsometric thicknesses for the amine and thiol monolayers are such that there may be a difference in the film densities. This could lead to a higher density of amine sites. Recent IR studies indicate that hydrogen bonding is present in monolayers of the amine on &03.31 Such hydrogen bonding would cause the amine groups to be exposed and to be structurally more well defined. Our contact angle measurements of the APS monolayer surfaces do not support such a hydrogen-bonding model, unless the high contact angles are due to a volatile organic contaminant which is removed during evacuation prior to the Au deposition. Such a volatile contamination layer could be removed under high vacuum, leaving behind a well-ordered (hydrogen-bonded) amine surface that the Au could nucleate on. We do not have direct evidence that would support arguments for either the increased APS coverage or the hydrogen-bonding theories. At this point we can only conclude that there appear to be more nucleation sites on the APS films in comparison to the MPS films. We were unable to reproducibly image Au films less than 1.5nm thick on the thiol surface, but we could reliably image Au films as thin as 0.3 nm thick on the amineterminated surface; see Figure 6. We attribute this to a
SFM of Enhanced Metal Nucleation
Langmuir, Vol. 10, No. 10, 1994 3605
0.5 nm
1.0 nm
1.5 nm
2.0 nm
x 200 nm constant-force SFM images of Au on (CH30)3Si(CH2)3NH2-treatedSi02. Z-range is 25 nm for all images.
Figure 6. 200 nm
strong interaction of the Au with the amine and the possibly more well-ordered amine surface. In Figure 6A it can be seen that, a t a Au film thickness of 0.5 nm, small crystallites approximately 2-4 nm in diameter are scattered homogeneously on the surface. Many defect regions devoid of Au crystallites were found on samples less than about 2 nm thick. The data in Figure 6 strongly suggest that the amine surface is capable of providing a large number of nucleation sites, and the nuclei formed are tightly held to the surface. Images of Au films less than about 2 nm on the thiol surface were “streaky”,indicating substantial tip-induced damage or loosely bound Au crystallites. We suspect that this inability to obtain stable images of ultrathin Au films on the thiol surface is due to a poorly ordered MPS layer (lower density of silane sites or lack of hydrogen bonding) and weakly bound Au. If the tip-sample interaction is large in comparison to the Au-surface interaction, the tip can push the unconnected Au crystallites around and even remove them. This appears to be the case with the thiol-terminated surface but not the amine surface. In addition, a more ordered organic film (a less soft or more dense film) would prevent tip penetration, and thus tipinduced damage to the Au crystallites would be less evident. We are currently investigating the effects of more ordered, long-chain thiol and amine silanes on nucleation and SFM imaging stability. In addition, we are exploring the synthesis of (CH30)3Si(CH2)3N(CH3)2in order to see how other functionalities influence nucleation and adhesion of Au. Stability of the AulSilane Films. We have performed some preliminary experiments involving exposure of the Au-amine and Au-thiol films to various organic solvents and aqueous solutions in order to judge the stability of the adhesive. Immersion in dichloromethane, acetonitrile, or ethanol overnight did not cause adhesive failure; tape tests of the Au films after exposure indicated no loss of Au. Upon emersion, scraping with Teflon tweezers did not damage the Au film. We noted no loss of Au upon vigorous rinsing with the above organic solvents. Thus, the Adsilane interactions are not disturbed by the organic solvents. When the samples were placed in concentrated aqueous base or acid for more than 30 min, the Au films were completely removed from both the amine and thiol surfaces. This is expected due to cleavage of the siloxane
bonds at these pH values. Soakingthe Au-amine samples in deionized water (C02 saturated) for periods of several days caused complete removal of the Au film during the tape test. Similar results were obtained with ethanol as long as the soak times were in excess of 1day, but only about 50% of the Au was removed during the tape test. The Au-thiol films were unaffected by the protic solvent treatments. Contact angle measurements (70 f 2’) indicated that the aminosilane layer was not removed by the water soakings. Thus, it appears that the protic water (or ethanol) is somehow able to cause displacement of the amine from the Au. This observation was found to be independent of Au film thickness (up to 200 nm), which excludes problems associated with the porosity of the Au film. Xu et al. have reported37that alkylaminemonolayers on Au were easily removed by rinsing with the polar solvent ethanol, so our results are not that surprising. We are unsure of how protic solvents are able to dislodge the Au from the amine surface. Future plans include characterization of the APS surface with ATR-IR before and after Au deposition, and after Au removal by soaking in water. Information gained from such studies will be crucial to the understanding of the Au-amine interaction. The precise nature of the amine-Au interaction observed here is not well understood at this juncture. Several stable Au(1)-amine complexes have been reported, and it is quite plausible that such a complex is formed in the gas phase.40 There is some literature regarding the adsorption of amines on Au, but the observed strength of adsorption was very poor.34,35741 Electron-donating substituents on amines have been shown to increase the interaction of the amine with Au surfaces, but the results are somewhat ambiguous due to possible adventitious surface contaminant^.^^ There have been no previous gasphase measurements of the type of Au-amine interactions described here, and we believe that there may be a reaction between Au atomic vapor and amines which is not found for bulk Au in contact with solutions of alkylamines. Summary This study has demonstrated that the nucleation and growth of thin Au films can be controlled by varying the chemical nature of the surface onto which the Au is deposited. The use of alkoxysilanes terminated with thiol and amine moieties on SiOdSi(ll1) was shown to cause formation of similarly sized Au crystallites during thermal evaporation of Au, but the electrical properties of these films were found to be substantially different at 2 nm Au film thickness. The onset of measurable electrical continuity occurred at 2 nm on the amine surface; this was attributed to a large number of nucleation sites. Although the thiol-terminated surface reduced Au surface diffusion, Au films 8.5 nm thick on the thiol surface were found to be 7 orders of magnitude less conductive than those formed on the amine surface. Both clean SiOz/Si(lll) and the methyl-terminated silane surface displayed larger Au crystallites, indicating substantial surface diffusion of the Au. Thus, we have shown that ultrathin, electrically continuous films of Au can be formed on smooth surfaces with proper control of the surface chemistry of the substrate. Future applications include fabrication of (40) Puddephatt,R.J.In The Chemistry of Gold; Clark,R. J. H., Ed.; Topicsin Inorganic and General Chemistry;Elsevier: Amsterdam, 1978; Monograph 16, pp 253-6. (41) Steiner, U.B.;Neuenschwander,P.; Caseri, W. R.; Suter, U. W. Langmuir 1992,8,90.
3606 Langmuir, Vol. 10, No. 10, 1994 nanoelectrodes, optically transparent electrodes, and molecular devices and assemblies.
Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-92216461, Petroleum Research Fund, and the Louisiana Education
Dunaway and McCarley Quality Support Fund (LEQSF(1993-96)-RD-A-09).We are grateful to Chris Bell for performing the ellipsometric measurements and Larry Curtin for making the contact angle measurements. Helpful discussions with A. W. Maverick and J. T. McDevitt are acknowledged. We thank R. W. Murray for a copy of a preprint (ref 30).
