Langmuir 1995,11, 2143-2150
2143
Growth of Self-Assembled n-Alkyltrichlorosilane Films on Si(100) Investigated by Atomic Force Microscopy K. Bierbaum and M. Grunze” Znstitut fur Angewandte Physikalische Chemie, I m Neuenheimer Feld 253, 69120 Heidelberg, Germany
A. A. Baski,? L. F. Chi,# W. Schrepp, and H. Fuchs# Polymer Research Laboratory, BASF AG, ZKMI 0 5543, 67056 Ludwigshafen, Germany Received November 22, 1994. Zn Final Form: March 13, 1995@ Atomic force microscopy (AFM),contact angle, and ellipsometry measurements are used t o investigate the growth behavior of n-octadecyltrichlorosilane(OTS), n-propyltrichlorosilane (PTS),and n-triacontyltrichlorosilane (TCTS) films on hydroxylated Si(100)substrates. AFM images show that self-assembled monolayers of OTS (CH3(CH2)17SiC13)grow via islands. After an initial nucleation and growth of larger primary OTS islands, smaller secondary islands grow in the areas between the primary islands until the film is complete. The shape and size of the islands and the progress in growth are not well-defined, however, and depend upon the film preparation conditions. In contrast, shorter-chain PTS molecules (CH3(CH2)2SiC13)do not appear to demonstrate this island growth behavior during film formation. Longerchain TCTS molecules (CH3(CH2)29SiC13)show an island-type of growth with different island structures compared to those observed for OTS. We discuss the differences in growth behavior observed under cleanroom and normal laboratory conditions. 1. Introduction
In recent years there has been considerable interest in the properties of thin organic films attached to metal, semiconductor, or insulator surfaces. N-Alkyltrichlorosilanes adsorb spontaneously from solution onto hydroxylated surfaces and form a stable laterally-linked polymer network after reaction with the surface.l These methyl group terminated films and their functionalized derivatives are used in a wide field of applications, e.g., in biosensors for antibody immobilization,2 as adhesion promoters for polymer film^,^,^ for possible applications in molecular electronic^,^ as lubricants,6 and as bonding phases for liquid and gas ~hromatography.~ In addition, functionalized organosilanes can be used in a variety of chemical reactions to design multilayer architectures with defined properties of commercial interest.8 The reaction of OTS with silicon to form self-assembled monolayers was first investigated by Sagiv et al.9and has been widely studied by several other g r o ~ p s . ~ ~ J ~ The mechanism for monolayer formation is not yet fully understood and differing opinions exist in the literature. X-ray reflectivity measurements by Wasserman et al. support a “uniform” model for the growth of OTS mono+ Present address: Naval Research Laboratory, Code 6177, Washington, DC 20375-5432. Present address: Physikalisches Institut, Westfalische Wilhelms-Universitat, 48149 Munster, Germany. * Abstract published in Advance ACS Abstracts, May 15,1995. (1)Maoz, R.; Sagiv, J. J . Colloid Interface Sci. 1984,100,465. (2)Andle, J.; Vetelino, J.; Lec, R.; McAllister, D. Proc. IEEE Ultrasonics Symp. 1989,579. (3)Linde, H.; Gleason, R. T. J . Polym. Sci., Polym. Chem. E d . 1984,
*
_22. _ 3043. ---I
(4)Plueddemann, E. P. J . Adhes. 1970,2,181. (5)Hopfield, J.J.;Onnchic, J. N.; Beratran, D. N. Science 1988,241,
A 1 7.
(6)de Gennes, P.-G. Rev. Mod. Phys. 1985,57,827. (7) Golay, M. J. E. Gas Chromatography;Buttenvorth: Oxford, 1958; p 36. (8) Maoz, R.; Sagiv, J. J . Phys. Chem., submitted. (9)Sagiv, J. J . A m . Chem. SOC.1980,102,92. (10)Tillman, N.;Ulman, A.;Penner, T.L. Langmuir 1989,5,101. (11)Wasserman, S. R.; Tao,Y.-T.;Whitesides, G. M. Langmuir 1989, 5,1074.
layers.12 According to this model, incomplete monolayers consist of OTS molecules which are uniformly distributed over the substrate in a disordered manner. An opposing “island”model is supported by Cohen et al. based on FTIR measurements.13 In this case, incomplete monolayers form partially ordered islands rather than a sparse homogeneous layer. Since FTIR and X-ray reflectivity measurements average information over a large measurement area, characterization by AFM should be more appropriate to determine the local structure and growth behavior of such films. As an example, AFM studies by Schwartz et al.14have shown that OTS film formation on mica occurs by nucleation and growth of self-similar islands. Recently, Fujii et al.15reported a preparation procedure for achieving high-quality hydrophobic OTS monolayers on thermally oxidized silicon substrates and investigated them by AFM and ellipsometry. They determined a refractive index n with n = 1.393 f 0.002 [or the silane layer and a film thickness d with d = 18 f2 A for the OTS layer, in contrast to n = 1.45 given b Wasserman et a1.12 and the theoretical value of 26.2 xl6 F u j i et al. also reported a cross sectional area of 40 A2 for the ostadecyl chains. Wasserman et a1.12report an area of 20 A2which corresponds to the density of surface silanol groups on native silicon dioxide.17J8 It is possible that the different preparation procedure used by Fujii et al. lowers the surface density of OH groups and results in a less densely packed OTS layer. The decreased film density would cause the alkyl chains to tilt and the refractive index to decrease. (12)Wasserman, S.R.; Whitesides, G.M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; h e , J. D. J . Am. Chem. SOC.1989,111, 5852. (13)Cohen, S. R.; Naaman, R.; Sagiv, J. J . Phys. Chem. 1986,90, 3054. (14)Schwartz, D.K.; Steinberg, S.; Israelachvili, J.;Zasadzinski, J. A. N.Phys. Rev. Lett. 1992,69,3354. (15)Fujii, M.; Sugisawa, S.; Fukada, K.; Kato, T.; Shirakawa, T.; Seimiya, T. Langmuir 1994,10,984. (16)Ulman,A. Ultrathin 0rganicFilms;AcademicPress:SanDiego, CA, 1991. (17)Zhuravlev, L. T.Langmuir 1987,3,316. (18)Madeley, J. M.; Richmond, C. R. Z . Anorg. Allg. Chem. 1972, 389,92.
0743-746319512411-2143$09.00/0 0 1995 American Chemical Society
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1pmFigure 1. 5 pm x 5 pm AFM images of the first OTS sample set: (a) afZer 15-s immersion, small irregular-shaped film islands (0.6-0.9 pm diameter); (b) 1-min, secondary nucleation between islands; (c) 5-min, “fillingin” of film material between islands; (d) 35-min, complete monolayer with “scar” outlines from the primary islands. Si substrates were immersed into 1mM OTS solutions immediately after the cleaning procedure while in a cleanroom environment. The dark background is the Si substrate, while the lighter resons represent high& OTS film areas.
In this paper we report recent AFM results that provide evidence for an island-type of growth mechanism for OTS molecules on Si(100). Important aspects concerning the preparation conditions and thus the influence of the state of the silicon substrate are also discussed. In addition, the influence of alkyl chain length on growth behavior and reactivity is investigated for the shorter-chain propyltrichlorosilane (PTS) and longer-chain triacontyltrichlorosilane (TCTS).
2. Experimental Section The silane‘filmswere prepared under cleanroom conditions (class100)or in a normal chemistrylaboratory. As will be shown, the environmental conditions under which films are prepared affect the growth behavior, although the procedures performed were identical. Beforefilm preparation,the Si(100)substrates (Wacker)were cleanedusing a wet chemicaletch for 1hat 80 “Cin a 1:3 solution of hydrogen peroxide (Merck, 30%)and sulfuric acid (Riedel de Haen,95-97%). This etch was followedby an extensiveMillipore water rinse and nitrogen blow dry. For film preparation the
substrates were immersed at room temperature (21“C,45 f 1% air humidity in the cleanroom)for various periods of times in 1 mM solutions of OTS (Merck),PTS (ABCR),or TCTS (ABCR)in bicyclohexyl (Aldrich, 99%). PTS solutions were filtered using 0.5 pm and 0.2 pm syringe-operatedfilters (Millipore)to reduce the amountof polymeric silane aggregates in the solution. TCTS was dissolved in the bicyclohexyl solvent using an ultrasonic bath. After sampleswere withdrawn from the silane solutions, residual nonadsorbed silanes were removed from the substrate by rinsing the samples with Millipore water, stirring them in a chloroform bath for several minutes, and blowing them dry with nitrogen gas. This preparationprocedureappearedto effectively remove excess polymer silane aggregates from the surface. In addition, silicon samples which were immersed for longer than 45 min into OTS or PTS solutions were wiped with a cotton cleanroom cloth after the adsorption to remove excess material. The same procedure was applied to silicon samples which were immersed for longer than 3 h into TCTS solutions. Adsorption experiments which were performed under noncleanroom conditions were carried out in the same manner. The surface topography of the silane films was investigated under ambient conditions with a commercial atomic force
Langmuir, Vol. 11,No. 6,1995 2145
Growth of Films on Si
0.5pm
M
0.1
Figure2. Images of a completeQTSfilm (22.5 h in a 1mM OTS solution): (a)2pm x 2pm; (b)0.5pm values are 8.6 A for (a) and 5.4 A for (b), respectively.
microscope(Park ScientificInstruments). Duringoperation,the AFM detected deflections of the probing cantilever by sensing
the motion of a laser beam focused onto the backside of the cantilever. By use of feedbackto maintain a constant cantilever deflection, constant force images were taken in the repulsive mode with a silicon nitride cantilever having an integrated pyramidal tip. Typical forces were on the order of 10 nN, using a 200 pm long cantilever with a spring constant of 0.06 N/m. No film damage was observed even after multiple scanning. Contact angles were determined on a commerciallyavailable goniometer (Erma, Tokio; Fa. Kruess., Hamburg) at room temperature. Advancing water contact angles were measured by growingsmall sessile water drops on the monolayer surface. The water used for the measurements was distilled, deionized, andfiltered through a Millipore-Q filtrationsystem(specificwater resistivity 18 MQ cm). The contact angle was measured by averaging the results from three independent drops which gave the same angle to within 1.5”. Average film thicknesses were measured using a noncommercial ellipsometer equipped with a 632.8-nm He-Ne laser. The angle of incidence was 70” and the spot size was 1.5 mm. In order to determine the effectivefilm thickness, we assumed that the filmswere isotropic and homogeneous with a refractive index of 1.50.19 This, of course, gives only meaningful results for monolayer coverages.
3. Results and Discussion 3.1. OTS Films. The OTS growth behavior was investigated as a function of immersion time using three different preparation environments: (1)cleanroom with minimal exposure to the ambient; (2) cleanroom prolonged air and water exposure; (3)chemistry laboratory. First, we examine OTS films which were prepared under cleanroom conditions with minimal air and water exposure. Parts a-d of Figure 1show AFM topographs (5pm x 5 pm) of four Si samples immersed for 15 s, 1min, 5 min, and 35 min in the silane solution, respectively. These samples were prepared immediately after the substrate cleaning procedure. After a few seconds of immersion time, OTS islands nucleate on the surface and begin to grow. The extent of this island growth for a 15-s sample is shown in Figure la. The geometrical surface coverage of OTS islands is approximately 18%-20%. The islands are usually somewhat branched with typical diameters of 0.6-0.9 pm. Cross-sections of this0 image indicate a mean island height of h = 25.4 f 2.5 A Fnd a surface root-mean-square (rms) roughness of 8.1 A. (It
+
x
0.5pm. The peak-to-valley
has to be noted that rms values depend weakly on the image size.) Since the theoretical length of a covalently bound OTS molecule is 26.2 A, these islands represent one monolayer of OTS molecules. The fractal appearance of the islands suggests that rather than individual OTS molecules small and mobile precursor clusters attach to the island nuclei and hence lead to growth. The l-min sample (Figure lb) shows a secondary nucleation of smaller dotlike islands (0.06 pm diameter) which occur between the initial primary islands. On the 5-min sample (Figure IC) these secondaryislands continue to grow in number and size with immersion time, filling in the areas between the initial primary islands. The silicon substrate is still visible in the AFM data indicating that a full monolayer coverage is not yet complete. It should be noted that the size of the primary islands for the 5-min sample is reduced (0.3-0.7 pm diameter) with respect to the t Il-min samples, indicating a high mobility of adsorbed OTS molecules and/or OTS clusters. This behavior suggests that the secondary islands are predominantly created by the nucleation of diffusing OTS molecules and/or clusters around OTS nuclei attached to the surface. That indeed the total OTS coverage exceeds the fractional coverage of OTS islands was confirmed by XPS measurements as a function of immersion time.20 These observationsimply the existence of mobile precursor states of OTS molecules and clusters which precede stationary island formation. As the immersion time increases, a complete monolayer is observed in AFM on the 35-min sample (Figure Id), but light contour “scars”of the initial primary islands remain. The complete OTS film is relatively flat with a surface rms roughness of 0.9 A. After even longer immersion times, the scar contours eventually disappear and leave behind only a “mottled”surface texture. Figure 2 shows two AFM images of a long-immersion OTS saomple(22.5 h) with a surface rms roughness of only 0.5 A. To examine the effects of slight variations in the surface preparation on the OTS growth behavior, Si wafers were prepared as described above but were then stored for 4 h in Millipore water and 3 h in air (cleanroom conditions) before immersion into the silane solution. Parts a-d of (19) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, I , 45. (20) Bierbaum, K.; Grunze, M. To be submitted for publication.
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Figure 3. 5 pm x 5 pm images of the second OTS sample set: (a) 10-simmersion, initial nucleation of a few small circular islands (0.05-0.13 pm diameter); (b) 1-min, circular islands up to 0.4 pm in diameter; (c) 5-min, film islands and secondary nucleation between the islands; (d) X-min, incomplete monolayer with progressed secondary nucleation. After the cleaning procedure, the Si substrates were exposed for 4 h to water and for 3 h to air before immersion into 1 mM OTS solutions while in a cleanroom environment.
Figure 3 show AFM images (5 pm x 5 pm) of four Si samples immersed for 10 s, 1min, 5 min, and 15 min in the OTS solution, respectively. The initial growth behavior for t < 5 min for this second sample set is noticeably different from that of the first set. After a 10-simmersion time (Figure 3a), only small circular islands with a diameter of 0.05-0.13 pm are visible, in contrast to the much larger, branched islands observed for a comparable immersion time in the first sample set. After 1min these small islands grow in size to 0.25-0.4pm diameter (Figure 3b), but they still do not resemble the shape and size of the islands in the first sample set. The 5-min samples for both sets, however, have primary and secondary islands which are more similar in size and shape (Figures ICand 3c). Comparing these two sample sets with different surface treatments, the growth process itself remains similar for both sets, but the growth rate and details of the island shape and size depend on the exact surface conditions. It appears that exposure to water and air after the initial cleaning procedure causes large OTS islands to grow more slowly in the initial stages. We
speculate that the longer water exposure creates a larger number of so far unidentified nucleation sites for OTS islands. Hence, a higher number of smaller islands are observed following the latter sample preparation protocol. After the first 5-10 min, however, these differences are no longer as significant. Contact angle and ellipsometry thickness measurements for these two complete sample sets are given in Table 1. Ellipsometry data for the first and second data sets do not differ significantly and generally indicate an increasing film thickness with immersion time. The longer exposed samples of the first data (45 min-22.5 h) are complete films in AFM images and hqve an average ellipsometric sample thickness of 28-30 A, iq reasonable agreement with the theoretical value of 26.2 A. It should be noted that the 35-min sample appears to have an anomalously high sample thickness and contact angle. This anomaly is most likely caused by excess adsorption of polymeric OTS on some parts of the sample, since it was not wiped afier the rinsing procedure. The longer exposed samples show lower film thicknesses and contact
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Growth of Films on Si Table 1. Table of Ellipsometric Film Thicknesses and Water Contact Angles for Given Immersion Times of the OTS Samples from the First, Second, and Third Sample Sets
immersion time 2s
5s 15 s 1 min 5 min 10 min 35 min 45 minu 3 hu 22.5 ha
ellipsometric thickness (A) OTS Sample Set 1 11 11 11 13 18 20 36 28 30 28
water contact angle (deg) 79 77 76 94 99 106 117 109 111 112
OTS Sample Set 2 1s 10 s 1 min 5 min 15 min
1 min 2 min 5 min
8 10 13 18 24
48 64 78 98 118
OTS Sample Set 3 11 13 24
64 72 108
Sampleswere wiped with a cleanroom cloth a h r the rinsing procedure to removeexcessadsorptiondueto longimmersiontimes. angles, because polymeric adsorption was effectively removed by wiping. In contrast to the ellipsometry data, the contact angle measurements are smaller below l-min immersion time for the second data set (longer water and air exposure) versus the first. This result is consistent with AFM data which show smaller initial islands and slower secondarynucleation between such islands for the second set. The measured water contact angles for both data sets reach a maximum value of 117 f lo, a value characteristic of closely packed OTS molecules in a monolayer with exposed methyl groups. This value is slightly higher than literature values of 111-115°.13 An unexpected value in the data is the relatively high contact angle for the 15-minsample in the second data set, which still appears as an incomplete layer in AFM images. Overall, however, contact angle and ellipsometry data monitor the growth of the OTS films. The contact angle data for the preparation procedures also support AFM observations that the OTS films grow differently on substrates exposed to air and water before immersion in the silane solution. To further test the influence of the environment on the growth behavior, samples were prepared with l-min, a-min, and 5-min immersion times in a normal chemistry laboratory. Due to varying temperature, air humidity, and air-borne contaminants in the laboratory as compared to the cleanroom, the size of the observed islands and the progress in growth change. In contrast to the first two sample sets prepared in the cleanroom, a l-min immersion time results in only a few small, circular islands forming on the surface. The number of islands significantly increases after 2 min (Figure 4) and they appear similar in shape to islands of the first sample set. However, these islands are much smaller (0.05-0.2 pm) than those observed for samples prepared in the cleanroom with comparable immersion times (Figyres l b and 3b. The island height is also lower (10- 13A), indicating that the OTS molecules in the islands are less densely packed. A different contamination level may inhibit polymerization of the molecules and cause an increased intermolecular distance in the islands. As a consequence, van der Waals forces between the OTS alkyl chains are not maximized
1pm M Figure 4. 5 pm x 5 pm image of the third OTS sample set. The Si substrate was immersed for 2 min into a 1mM OTS solution immediately after the cleaning procedure while in a normal chemistry environment. A nucleation of small circular islands (0.05-0.2 pm diameter) is visible.
and the chains do not stretch to achieve all-trans conformation, resulting in the observed reduced island height. After a 5-min immersion time, the island development progresses and the OTS film appears almost complete. A water contact angle of 108", however, indicates that a densely-packed, complete OTS monolayer has not been achieved yet. Again, we see that the adsorption kinetics and resulting film growth are influenced by the film preparation conditions. In all of these sample sets an island growth behavior is consistently observed for OTS films on Si. After an initial nucleation and growth of larger primary islands, smaller secondary islands grow in the areas between the primary islands until the film is complete. As described, however, the shape and size of the islands and the rate of island growth can be influenced by the prolonged exposure of the Si substrate to water and air following the cleaning procedure in the cleanroom or exposure to normal laboratory conditions. Possible reasons for this influence involve changesof contaminationlevels, water adsorption, and/or OH binding groups on the surface as a function of exposure time. An AFM study by Barrat et al. also shows that variations in cleaning procedures have an effect on the growth of OTS films.21 They observe hole defects and domain boundaries in complete overlayers which are correlated to the cleaning procedure. 3.2. PTS and TCTS Films. Since island growth is observed for OTS films, a natural question to ask is whether shorter- and longer-chain silanes also grow in this manner. Adsorption experiments with short-chain PTS and long-chain TCTS were performed both under cleanroom conditions and in a normal chemistry laboratory. In the first PTS sample set, Si samples were immersed into PTS solutions for 10 min and 3 h under cleanroom conditions with minimal exposure to air and water. AFM images of both samples exhibit a uniform surface structure with an rms surface roughness of -1.3 (21) Barrat, A.; Silberzan,P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992,20, 633.
Bierbaum et al.
2148 Langmuir, Vol. 11,No. 6,1995
0.5 pm n . Figure 5. 2 pm
x 2 pm images of Si substrates immersed into 1 mM PTS and TCTS solutions immediately after the cleaning
procedure: (a) 10-minimmersion into a 1 mM PTS solution while in a cleanroom environment, a complete film with no detectable islands is visible; (b) 10-min immersion into a TCTS solution while in a cleanroom environment. The remaining samples were prepared in a normal chemistry laboratory for (c) 20-min and (d) 70-min immersion times into TCTS solutions.
A (Figure 5a). Ellipsometric measurements of the 10min and 3-h samples indicate a layer thicknesses of 9 and 10 A, respectively (Table 2). The 3-h value is in good agreement with the film thickness of the sample determined by X-ray photoemission spectroscopy, which was found to be 13 A.22 Give? that the theoretical PTS monolayer thickness is 7.3 A, it appears that a complete film consisting of probably more than one monolayer is present after a 10-minimmersion time. Since PTS reacts so rapidly with the substrate, it is no possible to determine conclusively from these data whether PTS demonstrates island growth. Contact angle measurements (Table 2) of the complete film at 3 h give a value of 97”,which is lower than that expected for a closely-packed monolayer with exposed methyl and/or methylene groups. In the second sample set, Si samples were immersed into PTS solutions for 15 s, 1 min, and 5 min under normal chemistry laboratory conditions. A low water contact angle of 56” indicates that only part of the substrate is covered with (22)Bierbaum, K.; Hiihner, G.; Heid, S.; Kinzler, M.; Woll, Ch.; Effenberger, F.; Grunze, M. Langmuir 1994,II;512.
PTS. However, the ellipsometric film thickness is an unexpectedly high value of 16 in contrast to the values of -10 A for the “complete” films observed for the first sample set. Up to a 5-min immersion time, PTS film growth continues to progress according to contact angle measurements (56-go”), but islands are not observedwith AFM. This absence of island growth under any of our preparation conditions indicates that PTS film growth does not proceed via islands as for the OTS films or that the islands are too small to be imaged in our experiments. Similar to PTS images of 10min and 3 h, TCTS samples prepared under cleanroom conditions show n? island growth. A rough looking surface structure (2.7 A rms) is visible at 10-min (FiguTe 5b) that appears somewhat smoother after 3 h (0.8 A rms). Ellipsometry measurements for the 10-min an$ 3-h samples indicate layer thicknesses of 20 and 53 A, respectively (Tqble 2). The theoretical TCTS monolayerthickness is 42.5 A, indicating that an incomplete layer exists after 10 min and one monolayer with excess adsorption on top of it22is present after 3 h. Contact angle measurements of the complete
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Growth of Films on Si Table 2. Table of Ellipsometric Film Thicknesses and Water Contact Angles for Given Immersion Times of the PTS and TCTS Samples prepared under (1) cleanroom and (2) Normal Chemistry Laboratory Conditions ~
immersion time
ellipsometric thickness (A)
water contact angle (deg)
PTS Sample Set 15 s (2) 1min (2) 5 min (2) 10 min (1) 3 ha (1) 24 ha (1)
16 14 16 9 10 15
56 68 90 90 97 90
TCTS Sample Set 5 min (2) 10 min (1) 20 min (2) 35 min (2) 50 min (2) 70 min (2) 3 ha (1)
19 20 26 28 34 36 53
88 96 95 99 102 106 113
a Samples were wiped with a cleanroom cloth after the rinsing procedure to remove excess adsorption due to long immersion times.
film at 3 h yield a value of 113”(see Table 21, consistent with that for exposed methyl groups in a closely packed layer. Since the 10-min sample shows no islands for an incomplete film according to ellipsometric data, it would suggest that island growth does not occur during film formation for the TCTS film prepared under cleanroom conditions. In a second sample set, Si samples were immersed into TCTS solutions for 5 min, 20 min, 35 min, 50 min, and 70 min under normal chemistry laboratory conditions. Images of the 5-min sample show isolated TCTS islands (0.20.25pm diameter) with heights of 36-42 Arelative to the silicon substrate. A majority of the islands have a mainly cyclic shape with a few being somewhat branched. The number of cyclic islands increases for the 20-min sample (Figure5c),but their size remains approximately constant. A second type of island also can be observed which appears cyclic with holes in the center (0.25-0.3pm diameter). In addition, a secondary nucleation of smaller islands with an apparent lower height occurs between the larger islands. The 70-min sample (Figure 5d) appears nearly complete with a rough-looking structure similarto samples from the cleanroom sample set. Ellipsometric film thicknesses and water contact angles confirm the progressing film growth. Unlike the TCTS samples prepared under cleanroom conditions, the second sample set prepared in the chemistry laboratory demonstrates island growth during film formation, although both data sets appear similar once a complete film is formed. After comparing the three silane films investigated in this study, it appears that PTS is the only molecule which does not demonstrate any detectable island growth for our different preparation conditions. Unlike contact angles for complete OTS and TCTS films which indicate closely packed monolayers with exposed methyl groups, contact angles for complete PTS films are lower and suggest that methylene groups could be exposed instead of methyl groups. Recent NEXAFS data support this phenomenon of disordered PTS films as compared to ordered TCTS and OTS films.22 We suggest that the different growth behavior and difference in final molecular order in the short chain and long chain alkylsilane layers reflect the balance between the chaidchain and headgrouplsubstrate interaction energies. As indicated by the island growth sequence for OTS, adsorbed OTS molecules or small clusters are mobile
on the surface until they nucleate at the perimeter of stable islands or (in the case of the extended watedair cleanroom and chemistry laboratory preparation) are trapped by so far undefined active sites on the surface. It is reasonable to expect that PTS and TCTS mobile precursors also exist on the surface. However, in the case of PTS, the free energy of alkanelalkane interaction is not sufficient to overcompensate the headgrouplsubstrate interaction, hence leading to disordered overlayers as shown in our recent N E W S data.22 In the case of OTS, the alkane/ alkane interaction energy is sufficient to stabilize closepacked alkyl chain arrangements and hence lead to island growth. We note that the NEXAFS results22showed a decreasing average tilt angle of the alkyl chains as a function of growth, indicating that molecules adsorbed on the perimeter of the island are more strongly tilted due to the absence of direct neighbors. Only at saturation coverage was an average molecular orientation close to normal to the surface observed.22For the TCTS molecules no NEXAFS data as a function of exposure are available; only the final adsorption stage was characterized giving a tilt angle close to normal to the surface. The AFM data suggest that the final ordered structure is established similar to the OTS overlayers, although the contact angle measurements indicate that a concentration of gauche conformations persists up to longer exposure times. This may, as discussed below, be associated with the lower overall chemisorption probability of TCTS compared to OTS or PTS. In addition to the growth behavior and structure of these silane films, we can examine their relative reactivities. It is apparent from ellipsometry and AFM data that PTS molecules react much faster with the surface than OTS or TCTS molecules. This high reactivity is also supported by observations of polymer aggregates forming in solution. (Note: During preparation of the silane solutions, PTS solutions pass slowly through 0.5-pm and 0.2-pm syringeoperated filters, whereas TCTS and OTS solutions pass through them without hindrance.) With respect to TCTS versus OTS reactivity, ellipsometry and AFM measurements indicate that TCTS is less reactive than OTS. OTS forms a complete layer after 35 min under cleanroom conditions (-5 min in chemistry laboratory), whereas TCTS films havq only a partial monolayer thickness of 30 A after 1h (36 A after 70 min in chemistry laboratory). As a result, the following sequence in relative reactivity is observed: PTS > OTS > TCTS. This ordering could be due to an increasing degree of gauche conformation of the molecules in solution. As the alkyl chain length increases, it is more likely that reactive sites of the silane molecule become blocked. In addition, the diffusion rate of the silane molecules to the surface is expected to decrease with increasing alkyl chain length, hence limiting the chemisorption rate into the proposed precursor state. The observation that OTS islands act as nucleation sites for ordered growth of the monolayer can be used to induce order in otherwise (in the pure phase) disordered functionalized alkylsilane layers. As will be described in a following this concept has been successfully applied to induce lateral order in o-aminoalkylsilane layers. In conclusion,we have investigated the growth behavior of OTS, PTS, and TCTS films on Si(100)substrates using AFM, ellipsometry, and contact angle techniques. AFM images indicate that self-assembled monolayers of OTS on Si form by an island growth mechanism, in disagreement with previous studies which support a “uniform” (23)Bierbaum, K.; Harder, P.; Heid, S.; W611, Ch.; Grunze, M. In preparation.
2150 Langmuir, Vol. 11,No. 6, 1995 growth model.12 We also find that details of the island growth behavior are influenced by the state of the silicon substrate. This result suggests that changes in binding groups on the substrate affect the surface diffusion of OTS molecules and, therefore, influence the shape and size of developing islands. After the films reach completion, however, they appear quite similar for the various preparation conditions. Longer-chain TCTS molecules also show film growth via islands under certain conditions, but the islands appear different from those observed for OTS films. In contrast, studies with shorter-chain PTS
Bierbaum et al. molecules do not show island growth behavior duringfilm formation under any of the preparation conditions.
Acknowledgment. We thank G. Lupa and H. Kullmann for performing the ellipsometry and contact angle measurements at BASF. Helpful discussions with G. Hahner are also gratefully acknowledged. This study was supported by the Deutsche Forschungsgemeinschaft through Gr 625-13 and as part of the ultrathin organic layer project financed by the German Ministry for Research and Technology. LA940929V
