Comparison of Chemical Lithography Using Alkanethiolate Self

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Comparison of Chemical Lithography Using Alkanethiolate Self-Assembled Monolayers on GaAs (001) and Au Chuanzhen Zhou,† Aaron Trionfi,‡ Jason C. Jones,‡ Julia W. P. Hsu,‡ and Amy V. Walker*,† †

Department of Chemistry and Center for Materials Innovation, Washington University in St. Louis, Campus Box 1134, One Brookings Drive, St. Louis, Missouri 63130 and ‡Center for Integrated Nanotechnologies, Sandia National Laboratories, P.O. Box 5800 MS-1415, Albuquerque, New Mexico 87185-1415 Received September 2, 2009. Revised Manuscript Received November 22, 2009

We have investigated the efficiency of bifunctional pattern formation in alkanethiolate self-assembled monolayers (SAMs) adsorbed on GaAs (001) and Au, using time-of-flight secondary ion mass spectrometry. Two patterning techniques were employed: electron beam lithography and UV photopatterning. Previous work has always assumed that complete degradation of the SAM was necessary for the formation of well-defined multifunctional patterned surfaces, requiring large electron doses or long UV irradiation times. We demonstrate that well-defined multifunctional patterned surfaces can be produced on GaAs (001) with only partial degradation of the SAM, allowing greatly reduced electron beam doses and UV irradiation times to be used. Using electron beam lithography we observe that sharp well-defined patterns can form after an electron dose as low as 450 μC cm-2. We also demonstrate that only 50% of the monolayer must be photooxidized in UV photopatterning, reducing the exposure time needed by a factor of 3. In contrast, patterning of alkanethiolate SAMs adsorbed on Au requires much higher electron doses (g1250 μC cm-2) and photooxidation times (2 h). The substantial differences observed on these two substrates appear to arise from differences in the SAM structure on GaAs and Au. These results suggest that alkanethiolate SAM resists may be a suitable technology for nanometer scale lithography of GaAs and possibly other semiconductors.

1. Introduction In chemical lithography a surface is patterned using a combination of lithographic and chemical preparation techniques to incorporate multiple chemical functionalities whose reactivity controls the subsequent deposition of metals,1-3 semiconductors,4 minerals,5 nanoparticles,6,7 biomolecules,7,8 and polymers.9-11 Self-assembled monolayers (SAMs) are attractive resist materials for chemical lithography because they form high-coverage, durable thin films (thickness: 1-2 nm) on metals and semiconductors12,13 which allow, in principle, pattern resolutions of e1 nm.14 Further, the properties of SAMs can easily be tuned by selective *Corresponding author. Present address: Department of Materials Science and Engineering, University of Texas at Dallas, 800 W. Campbell Rd, RL 10, Richardson TX 75080. E-mail: [email protected]. Telephone: 972 883 5780. Fax: 972 883 5725. (1) Sondag-Huethorst, J. A. M.; van Helleputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285–287. (2) Zhou, C.; Nagy, G.; Walker, A. V. J. Am. Chem. Soc. 2005, 127, 12160– 12161. (3) Lu, P.; Demirkan, K.; Opila, R. L.; Walker, A. V. J. Phys. Chem. C 2007, 112, 2091–2098. (4) Lu, P.; Walker, A. V. ACS Nano 2009, 3, 370–378. (5) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehing, M. C. Langmuir 1994, 10, 619–622. (6) Prabhakaran, K.; G€otzinger, S.; Shafi, K. V. P. M.; Mazzei, A.; Schietinger, S.; Benson, O. Nanotechnology 2006, 17, 3802–3805. (7) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466–2468. (8) Salaita, K.; Wang, Y.; Mirkin, C. A. Nat. Nanotechnol. 2007, 2, 145–155. (9) He, Q.; Kller, A.; Grunze, M.; Li, J. Langmuir 2007, 23, 3981–3987. (10) Steenackers, M.; K€uller, A.; Ballav, N.; Zharnikov, M.; Grunze, M.; Jordan, R. Small 2007, 3, 1764–1773. (11) Schmelmer, U.; Jordan, R.; Geyer, W.; Eck, W.; G€olzh€auser, A.; Grunze, M.; Ulman, A. Angew. Chem., Int. Ed. Engl. 2003, 42, 559–563. (12) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (13) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (14) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793–1807.

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modification of functional groups while leaving the rest of the molecule unchanged.12,13 A number of different lithographic techniques have been developed to pattern SAMs, including UV photopatterning,15-21 electron beam lithography,7,14,22-26 nanoimprinting,27 microcontact printing27,28 and dip pen nanolithography.8,27,29 Electron beam lithography and UV photopatterning are attractive because they produce chemically well-defined, contamination-free SAM patterns on the nanometer scale.7,14,17-19,22-26 Electron beam patterned SAMs can act as either positive (soluble after electron beam exposure) or negative (insoluble after electron beam exposure) resists, depending on their chemistry and the etching procedure. Upon electron beam irradiation aliphatic SAMs (15) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305–5306. (16) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024–1032. (17) Sun, S.; Leggett, G. J. Nano Lett. 2002, 2, 1223–1227. (18) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414–2415. (19) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381–1384. (20) Zhou, C.; Walker, A. V. Langmuir 2006, 22, 11420–11425. (21) Zhou, C.; Walker, A. V. Langmuir 2007, 23, 8876–8881. (22) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476–478. (23) Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1993, 11, 2823–2828. (24) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139–1143. (25) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663–3667. (26) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol. A 1996, 14, 1844–1849. (27) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (28) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (29) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661–663.

Published on Web 12/10/2009

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disorder, fragment and decompose forming a graphitic layer, and so act as positive resists for wet etches22,24,25,30 but negative resists for reactive ion etching (RIE) (a dry etch process).24 Aromatic SAMs, however, undergo electron-induced quasi-polymerization, making them negative resists.31 One of the simplest ways to produce a surface with multiple functionalities is to first pattern a SAM (SAM#1) and then displace the photoreacted or electron beam-damaged areas by adsorption of a second SAM (SAM#2).16,20,21 By using appropriately chosen terminal groups for the two SAMs, one can control the deposition of metals,1-3 biomolecules7,32 and other compounds with high spatial resolution.4,5,10,11 The major disadvantage of chemical lithography using SAM resists adsorbed on metal surfaces is that high electron doses (normally .1 mC cm-2,14,30,33 and in some cases as high as 20100 mC cm-2 ,1,11,34) or long exposures to UV light (>1 h.)20,21 are required to produce well-defined patterns, whereas current resist technologies require much lower electron doses or UV exposure times. For example, the critical electron dose for poly(methyl methacrylate) (PMMA), a common positive electron beam resist, is 350 μC cm-2 at 50 kV.35 Likewise, in photolithography exposure times of a few seconds to a few minutes are employed, depending on the wavelength and power of the UV lamp used and the properties of the photoresist layer.36 In this paper we investigate the efficiency of formation of bifunctional patterned SAM surfaces on GaAs (001) and Au employing both electron beam lithography and UV photopatterning. To date, it has always been assumed that the formation of well-defined patterned bifunctional SAMs on any substrate requires that SAM#1 must be fully degraded. However, we show that patterned SAM surfaces on GaAs(001) can be produced without complete degradation of SAM#1. This allows for use of greatly reduced electron doses and UV exposure times compared to patterning of SAMs on Au, where SAM#1 must be completely degraded before well-defined patterns form. The electron doses required to form well-defined patterned SAMs are similar to the critical electron doses for currently employed electron beam resists.35 We estimate on GaAs (001) that only ∼24% of SAM#1 must be degraded using electron beam lithography in order to form sharp, well-defined patterns. Similary, in UV photopatterning only ∼50% of SAM#1 must be photooxidized in order to produce sharp patterns, leading to a 3-fold reduction in the exposure time required.

2. Experimental Section 2.1. Materials. Undecanethiol (UDT) (99%), hexadecanethiol (HDT) (99%), 16-mercaptohexadecanol (MHL) (99%) and 16-mercaptahexadecanoic acid (MHA) (99%) were obtained from Asemblon (Redmond, WA). Tetradecanethiol (TDT) (98%) and octadecanethiol (ODT) (98%) were purchased from Fluka (Milwaukee, WI) and Aldrich (St. Louis, MO), (30) Tanii, T.; Hosaka, T.; Miyake, T.; Ohdomari, I. Jpn. J. Appl. Phys., Part 1 2004, 43, 4396–4397. (31) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; G€olzh€auser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401–2403. (32) Zhou, C.; Qi, K.; Wooley, K. L.; Walker, A. V. Colloids Surf. B 2008, 65, 85–91. (33) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772–7782. (34) G€olzh€auser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806–809. (35) McCord, M. A.; Rooks, M. J. In SPIE Handbook of Microlithography, Micromachining and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Publications: Bellingham, WA, 1997; Vol. 1, Microlithography, pp 139-250. (36) Levinson, H. J.; Arnold, W. H. In SPIE Handbook of Microlithography, Micromachining and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Publications: Bellingham, WA, 1997; Vol. 1: Microlithography, pp 13-138.

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respectively. Eicosanethiol (ECT) was supplied by D. L. Allara, The Pennsylvania State University. All alkanethiols were used without any further purification. Single-side polished n-type GaAs (001) wafers (dopant: Si, (1.1-1.9)  1018/cm3) (American Xtal Technologies, Fremont, CA) were used for all studies. Native oxide Si Æ111æ wafers were obtained from Addison Technologies, Inc. (Pottstown, PA) and were cleaned using piranha etch before use. Chromium and gold were purchased from Goodfellow (Oakdale, PA) and Alfa Aesar (Ward Hill, MA), respectively, and were of greater than 99.99% purity. 30% ammonium hydroxide in water was purchased from JT Baker (CMOS grade) (Phillipsburg, NJ) and anhydrous ethanol (ACS/USP grade) was obtained from Aaper Alcohol (Shelbyville, KY). The 70% nitric acid and 37% hydrochloric acid were obtained from Aldrich (St. Louis, MO). 2.2. SAM Preparation. SAMs of alkanethiolates with different alkyl chain lengths and terminal groups on GaAs(001) were prepared using the procedure first reported by McGuiness et al.37 Briefly, GaAs substrates were immersed in ammonium hydroxide solution for 5 min to remove the native oxide layer, rinsed with degassed ethanol and dried using nitrogen gas. The GaAs substrates were then immersed in degassed 3 mM ethanolic solutions of the corresponding alkanethiol (MHA, HDT, ODT and ECT) and ∼10 mM ammonium hydroxide prepared in a nitrogenpurged glovebox. The substrates were immersed in the solution for approximately 20 h at ambient temperature (21 ( 2 C). After removal from solution, the samples were rinsed with copious amounts of degassed ethanol and dried using nitrogen gas. The preparation and characterization of SAMs on Au have been described in detail previously.38-41 Briefly, first Cr (∼50 A˚) and then Au (∼1000 A˚) were thermally deposited onto clean native oxide Si wafers. Self-assembly of well-ordered monolayers was achieved by immersing the resulting Au substrate into a 1 mM degassed ethanolic solution containing the corresponding alkanethiol (MHA, ODT, or HDT) for 24 h at ambient temperature (21 ( 2 C). After removal from solution, the samples were rinsed with ethanol and dried using nitrogen gas. To ensure that the prepared SAMs were free from significant chemical contamination prior to UV photopatterning and electronbeam lithography, for each batch a sample (∼1  1 cm2) was taken and characterized using single wavelength ellipsometry and time-of-flight secondary ion mass spectrometry (TOF SIMS). 2.3. Electron Beam Patterning. After the formation of the SAM (SAM#1), each sample was prepared for electron beam lithography by electron-beam evaporating 30 nm thick silver alignment marks onto it using a simple shadow mask procedure. The alignment marks were needed to locate the areas in SAM#1 exposed to electrons for the TOF SIMS experiments. The SAM#1 samples were then patterned in a LEO 440 electron beam microscope modified with a commercial nanometer patterning generation system (NPGS; J. C. Nabity, Bozeman MT). The patterning was performed at an accelerating voltage of 40 kV and a beam current of 2 nA. Arrays of squares, either 200  200 μm2 or 100  100 μm2, were patterned in SAM#1 with electron doses ranging from 450 to 4000 μC cm-2. For SAMs adsorbed on GaAs(001), the patterned SAM#1 was then immersed in 30% ammonium hydroxide solution for 5 min. Upon removal from the ammonium hydroxide solution, the sample was rinsed with degassed ethanol and then immersed in a freshly made ethanolic solution containing 3 mM of a second alkanethiol (SAM#2) and ∼10 mM ammonium hydroxide for 20 h at ambient temperature, 21 ( 2 C, in a nitrogen-purged glovebox. For Au samples, the electron beam patterned SAM#1 (37) McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zharnikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231–5243. (38) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483. (39) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682–691. (40) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. Z. J. Am. Chem. Soc. 1989, 111, 321–335. (41) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569.

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was immersed into a freshly made 1 mM ethanolic solution of a second alkanethiol (SAM#2) for 24 h at ambient temperature, 21 ( 2 C. After removal from the SAM#2 solution, the samples were rinsed with copious amounts of ethanol and dried using nitrogen gas. In the areas exposed to the electron beam SAM#2 displaces the electron beam degraded SAM#1, leading to a patterned SAM#1/ SAM#2 surface. Two different patterned SAMs were prepared on the Au and GaAs(001) substrates: MHA/HDT and ODT/HDT. The samples were immediately transferred to the TOF SIMS for analysis. Two samples of each patterned SAM were prepared and two spots on each surface were analyzed. 2.4. UV Photopatterning. To UV photopattern the SAM (SAM#1) surface, a mask (copper TEM grid, Electron Microscopy Inc.) was placed on top of the SAM surface and the assembly placed approximately 50 mm away from a 500 W Hg arc lamp equipped with a dichroic mirror and a narrow band-pass UV filter (280 to 400 nm) (Thermal Oriel, Spectra Physics Inc.). The SAM surface was exposed to the UV light for different periods of time ranging from 5 to 90 min. To measure the extent of photooxidation of SAM#1, a second unmasked sample of SAM#1 was placed simultaneously under the filtered UV lamp, so that the ion intensity of the photoproducts could be determined. To replace the photooxidized SAM#1 with SAM#2, the patterned SAM#1 on GaAs(001) was immersed in 30% ammonium hydroxide for 5 min. It was then immersed into a freshly made ethanolic solution containing 3 mM of a second alkanethiol (SAM#2) and ∼10 mM ammonium hydroxide for 20 h at ambient temperature, 21 ( 2 C, in a nitrogen-purged glovebox. For Au samples, the patterned SAM#1 was then immersed into a freshly made 1 mM ethanolic solution of a second alkanethiol (SAM#2) for 24 h at ambient temperature, 21 ( 2 C. After removal from solution, the samples were rinsed with ethanol and dried using nitrogen gas. Samples were immediately transferred into the TOF SIMS instrument for analysis. In the areas exposed to UV light, SAM#2 displaces the photoreacted SAM#1 resulting in a patterned SAM#1/SAM#2 surface. Three different patterned SAMs were prepared on GaAs(001): MHA/ HDT, ECT/HDT and ODT/HDT. Two different patterned surfaces were prepared on Au: HDT/UDT and MHA/HDT. Two samples of each type of patterned SAM were prepared and two spots on each patterned surface were analyzed.

Figure 1. Electron beam patterned ODT/HDT SAM adsorbed on GaAs(001). Negative ion TOF SIMS images centered at mass m/z 285 (ODT-, where ODT = S(CH2)17CH3-) and 257 (HDT-, where HDT = S(CH2)15CH3-). The ODT SAM was exposed to electron beam doses of 450, 550, 750, and 950 μC cm-2. The images are shown using a heat scale where the scale bar displays the maximum and minimum number of counts per pixel. Also, shown is the percentage decrease in the ion intensity, ΔI, of the molecular cluster ion, 69Ga2ODTþ, upon electron beam exposure. The error in ΔI is (15%. Area of analysis: 500  500 μm2, 128  128 pixels2. Note: The low ion intensity of m/z 257 observed in the ODT areas is due to a fragment of ion ODT, S(CH2)15CH3-.

2.5. Time-of-Flight Secondary Ion Mass Spectrometry. TOF SIMS analyses were conducted using a TOF SIMS IV (ION TOF Inc., Chestnut Ridge, NY) instrument equipped with a Binmþ (n = 1-7, m = 1, 2) liquid metal ion gun. The instrument consists of an airlock, a preparation chamber, and an analysis chamber, separated by gate valves. The preparation and analysis chambers were maintained at less than 5  10-9 mbar to avoid sample contamination. The primary Biþ ions had a kinetic energy of 25 keV and were contained within a ∼100 nm diameter probe beam. The total accumulated primary ion dose was less than 1  1010 ions/cm2, which is within the static SIMS regime.42 The secondary ions were extracted into a time-of-flight mass spectrometer and reaccelerated to 10 keV before reaching the detector. Peak intensities were reproducible to within (10% from sample to sample and (6% from scan to scan. The images shown are 500  500 μm2 divided into 128  128 pixels2. For edge resolution measurements images were obtained using an analysis area of 100  100 μm2 divided into 256  256 pixels2. The edge resolution was obtained by determining the lateral distance between 84% and 16% of the maximum ion intensities for each sample and spot, and the results averaged. We note that the pixel size is 391  391 nm2 and the errors reported are typically (1 pixel.

3. Results and Discussion For brevity, we will only present the results for electron beam lithography and UV photopatterning of methyl-terminated SAMs (42) Vickerman, J. C.; Briggs, D. TOF-SIMS: Surface Analysis by Mass Spectrometry; IM Publications/Surface Spectra: Chichester, U.K., 2001.

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Figure 2. Electron beam patterned ODT/HDT SAM adsorbed on Au. Negative ion TOF SIMS images centered at m/z 679 (Au2ODT-, where ODT = S(CH2)17CH3)) and 651 (Au2HDT-, where HDT = S(CH2)15CH3)). The ODT SAM was exposed to electron doses from 500 to 4000 μC cm-2. As in Figure 1, the images are shown using a heat scale, and the percentage decrease in the ion intensity of the molecular ion Au2(ODT)- is also displayed. Area of analysis: 500  500 μm2, 128  128 pixels2.

(SAM#1). In the first set of experiments, ODT was used as SAM#1 and HDT as SAM#2. The results for a carboxylic acid terminated SAM, MHA, are similar (see Supporting Information). After an electron dose of 500 μC cm-2, patterns are clearly observed on both GaAs(001) and Au in the molecular ion intensities of ODT, ODT-, and Au2(ODT)- (Figures 1 and 2, respectively). However, the adsorption of HDT in the electron beam-damaged areas is only observed on GaAs(001) after an electron dose of at least 450 μC cm-2 (∼6 electrons per SAM molecule) (Figure 1), while no HDT adsorption is observed on Au after an electron dose of 4000 μC cm-2 (∼54 electrons per SAM molecule), the highest electron dose used (Figure 2). To quantify the quality of the pattern on GaAs, the edge resolution was determined for the ODT and HDT areas, respectively. DOI: 10.1021/la9033029

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We observe that the best edge resolutions, 1.3 ( 0.4 μm, were obtained for electron doses g750 μC cm-2 (∼10 electrons per SAM molecule). Similarly, bifunctional patterned SAM surfaces on GaAs(001) can be formed by electron beam lithography of acid-terminated SAM resists, such as MHA. In this case after an electron dose of 450 μC cm-2 (∼6 electrons per SAM molecule) sharp, welldefined SAM patterns are observed (see Supporting Information). This behavior is in agreement with recent studies showed that acid-terminated SAMs are more susceptible to electron-induced damage than methyl-terminated SAMs.43 The data in Figure 1 clearly show that well-defined bifunctional patterned SAMs on GaAs(001) can be fabricated without the complete degradation of the SAM resist. We observed a ∼21% decrease in the intensity of the molecular ion of ODT on GaAs(001), Ga2Mþ, after an electron dose of 750 μC cm-2 suggesting that ∼21% of the monolayer has been degraded. We note that the use of ion intensities to quantify changes in samples is somewhat controversial. There are two approaches to quantify SIMS ion intensities: relative sensitivity factors44 and use of an internal standard.45 In this case we have used the relative sensitivity factor method to quantify the changes in the SIMS ion intensities and have assumed that the ionization cross-section for the Ga2Mþ ion does not significantly change during this experiment. Thus, the relative sensitivity factor for the ion remains constant. This seems reasonable since for other ions in the mass spectra, such as hydrocarbon fragment and S-containing ions, the intensities remain approximately constant throughout the electron exposures. The concentration of the undamaged SAMs, Cundamaged,d at a given electron dose d in μC cm-2 is given by C undamaged, d ¼ RSF  I M, d C meth chain where RSF is the relative sensitivity factor, IM,d is the intensity of the molecular ion after the SAM has been exposed to an electron dose d in μC cm-2 and Cmeth chain is the concentration of the SAM methylene chains. The percentage decrease in the molecular ion intensity, ΔI, is given by RSF  I M, d ¼0 C meth chain -RSF  I M, d C meth chain ΔI % ¼ RSF  I M, d ¼0 C meth chain ! I M, d ¼0 -I M, d  100% ¼ I M , d ¼0

!

where IM,d = 0 is the intensity of the molecular ion before electron exposure. The error in ΔI is typically (17%. However, on Au no adsorption of HDT is observed in the electron irradiated ODT even after an electron dose of 4000 μC cm-2 (Figure 2); in this case ∼28% of the monolayer has been damaged, suggesting that on Au a much larger amount of electron beam-induced damage is required for the adsorption of a second SAM. To investigate the amount of SAM degradation required to form well-defined patterns on Au we studied MHA SAMs because these are more susceptible to electron-induced damage.43 Sharp MHA/HDT patterns formed after an electron dose of 2000 μC cm-2 (∼27 electrons per SAM molecule). From the decrease in the McLaffery ion intensity (m/z 60, CH2C(OH)OHþ), which is characteristic of the acid terminal group, we estimate that ∼77% of the SAM layer has been degraded suggesting that most (43) Zhou, C.; Jones, J. C.; Trionfi, A.; Hsu, J. W. P.; Walker, A. V. J. Phys. Chem. C, DOI:10.1021/jp905612p. (44) Andersen, C. A.; Hinthorne, J. R. Anal. Chem. 1973, 45, 1421–1438. (45) Cornelio, P. A.; Gardella, J. A., Jr. ACS Symp. Ser. 1990, 440, 379–393.

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Figure 3. UV photopatterned ODT/HDT SAM adsorbed on GaAs(001). Negative ion TOF SIMS images centered at mass m/z 285 (ODT- where ODT = S(CH2)17CH3-) and 257 (HDTwhere HDT = S(CH2)15CH3-). The ODT SAM was exposed to UV light for times ranging from 5 to 45 min. The images are shown using a heat scale, as in Figure 1. Also shown is the extent of photooxidation of ODT, χ, as measured for each sample. Area of analysis: 500  500 μm2; 128  128 pixels2.

of the SAM1 must be damaged in order to see pattern formation upon exposure to SAM#2. We next consider UV photopatterning. To investigate whether well-defined patterned surfaces on GaAs(001) can also be formed without the complete degradation of the SAM, we photopatterned an ODT SAM on GaAs and then exposed the photopatterned ODT to HDT. TOF SIMS images of the resulting constructs are shown in Figure 3. Upon exposure to UV light alkanethiolate SAMs adsorbed on GaAs(001) photooxidize to form sulfonates and sulfates, indicated by the formation of MSO3- and MSO4- ions, respectively. The extent of photooxidation, χ, can be measured using the relative ion intensity method,46,47 which has been successfully employed previously to study the photooxidation of SAMs adsorbed on metals and GaAs(001).16,48-53 It is given by χ ¼

½MSO3 -  ½MSO3 -  þ ½MS - 

where [MSO3-] and [MS-] are the ion intensities of MSO3- and MS-, respectively. For ODT MS- is CH3(CH2)17S-, the molecular ion of the unoxidized SAM, and MSO3- is CH3(CH2)17SO3-. χ = 1 indicates that the reaction is complete. In Figure 3 the pattern can be discerned after only 5 min UV exposure of ODT (SAM1), corresponding to an extent of photooxidation of 0.27. The edge resolution for the pattern 5.3 ( 0.5 μm and 8 ( 1 μm as measured using the ODT- and HDT- ion intensities. After 15 min UV exposure (χ = 0.49) a well-defined pattern is observed with an edge resolution of 5.3 ( 0.5 μm, as measured using both the ODT- and HDT- ion intensities. No further improvement is obtained with additional ODT UV exposure. Thus, only 50% of the ODT SAM must be photooxidized in order to form a well-defined, sharp ODT/HDT pattern on GaAs(001). To investigate this further, we performed two similar experiments in which MHA and ECT were photopatterned, followed by adsorption of HDT (see Supporting Information). As in the case of ODT, only 50% of the SAM must be photooxidized for a well-defined pattern to form. (46) Bordoli, R. S.; Vickerman, J. C. Surf. Sci. 1979, 85, 244–262. (47) Brown, A.; Vickerman, J. C. Vacuum 1981, 31, 429–433. (48) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089–4090. (49) Brewer, N. J.; Janusz, S.; Critchley, K.; Evans, S. D.; Leggett, G. J. J. Phys. Chem. B 2005, 109, 11247–11256. (50) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657–6662. (51) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795–4801. (52) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174–184. (53) Zhou, C.; Walker, A. V. J. Phys. Chem. C 2008, 112, 797–805.

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Figure 4. UV photopatterned HDT/UDT SAM adsorbed on Au. Negative ion TOF SIMS images centered at m/z 711 (Au(HDT)2-, HDT = S(CH2)15CH3) and 571 (Au(UDT)2-, UDT = S(CH2)10CH3). The HDT SAM was UV photopatterned for between 10 and 120 min (40-120 min shown). Images are shown using a heat scale, as in Figure 1. Also shown is the extent of photooxidation of HDT, χ. Area of analysis: 500  500 μm2, 128  128 pixels2.

SAMs adsorbed on Au, however, must be nearly fully photooxidized before a sharp pattern can be formed. This is similar to electron beam lithography that the SAM must be almost fully degraded before a well-defined bifunctional SAM pattern is formed. For example, Figure 4 displays the formation of a patterned SAM after photopatterning an HDT SAM followed by backfilling of the photooxidized areas with UDT. The extent of photooxidation of HDT, χ, is: χ ¼

½MSO3 -  ½MSO3 -  þ ½AuM2 - 

where [MSO3-] and [AuM2-] are the ion intensities of MSO3and AuM2-, respectively. For HDT, AuM2- is Au(S(CH2)15CH3)2- (Au(HDT)2-), a molecular cluster ion of unoxidized HDT, and MSO3- is CH3(CH2)15SO3-. We observe HDT must be almost fully photooxidized (χ = 0.98) before an HDT/UDT pattern is observed. At χ = 0.85 a pattern is observed in the HDT molecular ion intensity, but no pattern is observed in the UDT molecular cluster ion intensity (Au(UDT)2-). At χ = 0.98 the measured edge resolution is 8 ( 1 μm using Au(UDT)2- and Au(HDT)2-. There are two possible reasons why SAMs adsorbed on GaAs(001) and Au require different amounts of degradation in order to form well-defined and sharp patterns. First, SAMs on Au and on GaAs(001) are prepared in different ways. On GaAs(001) an etchant, ammonium hydroxide, is employed to remove any oxidized GaAs substrate prior to adsorption of the second alkanethiol, whereas on Au no etchant is used. The etching step may reduce the time required to form a well-defined SAM pattern because the etch removes the topmost layer of the GaAs substrate and along with it both damaged and undamaged SAM molecules, prior adsorption of SAM#2. Second, SAMs adsorbed on GaAs(001) and Au have very different structures. SAMs with more than 9 methylene units in the alkanethiolate backbone adsorbed on Au are well-ordered,12,13 with large domain sizes (∼250 nm2).54 In contrast, SAMs adsorbed on GaAs(001) are well-ordered only if there are at least 15 methylene units in the backbone, and have small domain sizes.55 For example ODT on GaAs(001) has an average domain correlation length of 70 A˚, corresponding to a domain size of ∼49 nm2.55 (54) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667–668. (55) McGuiness, C. L.; Blasini, D.; Masejewski, J. P.; Uppili, S.; Cabarcos, O. M.; Smilgies, D.; Allara, D. L. ACS Nano 2007, 1, 30–49.

Langmuir 2010, 26(6), 4523–4528

Figure 5. UV photopatterned HDT/UDT SAM adsorbed on Au. Prior to immersion in the 1 mM ethanolic solution of UDT, the photooxidized HDT was immersed in dilute aqua regia solution for 5 min. Negative ion TOF SIMS images centered at m/z 711 (Au(HDT)2-, where HDT = S(CH2)15CH3) and 571 (Au(UDT)2-, where UDT = S(CH2)10CH3). The HDT SAM was UV photopatterned for between 10 and 120 min (40-120 min shown). The images are shown using a heat scale, as in Figure 1. Also shown is the extent of photooxidation of HDT, χ. Area of analysis: 500  500 μm2; 128  128 pixels2.

Such small domain sizes suggest that the SAM contains a large number of domain boundaries. These domain boundaries may account for the faster displacement of degraded SAM molecules on GaAs than on Au by providing an easy route for the second alkanethiol to diffuse to, and adsorb in, the photooxidized or electron beam-damaged areas. To test whether the etching step affects the patterning process, the following experiment was performed. HDT SAMs on Au were photopatterned for various lengths of time. Each patterned HDT (SAM#1) was then immersed into a dilute aqua regia solution (HNO3:HCl:H2O, 1:1:50) for 5 min. Dilute aqua regia is a wellknown etchant for gold and its oxides.56 After the etch, each sample was immersed into a 1 mM ethanolic solution of UDT (SAM#2) to form a patterned HDT/UDT SAM surface. In Figure 5 we observe that a pattern becomes visible in the HDT molecular ion intensity, Au(HDT)2-, at χ = 0.85, corresponding to an edge resolution of 8 ( 1 μm. A pattern can be discerned in the UDT molecular cluster ion intensity (Au(UDT)2-). However, the contrast in Au(UDT)2- intensity is not sufficient to determine the edge resolution at this χ. The sharpest HDT/UDT patterns are formed when the HDT has been almost fully photooxidized (χ = 0.98) prior to adsorption of UDT. At χ = 0.98 the edge resolution in the etched sample is the same, 8 ( 1 μm (measured using both the Au(HDT)2- and Au(UDT)2- ion intensities), as that in the “unetched” HDT/UDT sample (made using the normal preparation procedure for SAMs on Au). Since the pattern becomes visible at the same extent of photooxidation (χ=0.85) and continues to improve until the HDT has completely photooxidized in both the “etched” (Figure 5) and “unetched” (Figure 4) cases, we conclude that the use of an etchant is not an important factor in the formation of multifunctional patterned SAMs. Rather, it appears that the monolayer structure is a critical parameter for forming well-defined patterned SAMs.

4. Conclusions These results clearly demonstrate that SAMs adsorbed on GaAs(001) do not need to be fully degraded for successful chemical lithography. We observe that only 50% of the SAM needs to be photooxidized when using UV photopatterning in (56) CRC Handbook of Metal Etchants; Walker, P.; Tarn, W. H., Eds.; CRC Press: Boca Raton, FL, 1991.

DOI: 10.1021/la9033029

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order to form sharp patterned SAM surfaces. This leads to a 3-fold decrease in the time required for photopatterning. In electron beam lithography of methyl-terminated SAMs on GaAs, we estimate that only ∼20% of the SAM must be degraded in order to form similarly sharp patterns to form on GaAs(001), corresponding to an electron dose of 750 μC cm-2 for methyl-terminated SAMs. For MHA an even lower electron dose is required 450 μC cm-2. In contrast, on Au the SAM must be almost fully degraded for sharp patterns to form. In UV photopatterning at least 85% of the SAM must be photooxidized to form well-defined patterns. No pattern formation is observed in electron lithography of methylterminated SAMs, even at the highest electron doses employed (4000 μC cm-2). Sharp patterned SAM surfaces can produced using MHA, which is more susceptible to electron beam-induced damage, at electron doses g1250 μC cm-2. In this case, we estimate that ∼77% of the SAM has been damaged prior to adsorption of the second SAM. The electron beam doses required for SAMs adsorbed on GaAs(001) (e450 μC cm-2 for MHA and 750 μC cm-2 for methyl-terminated SAMs) are similar to the critical dose for poly(methyl methacrylate) (PMMA), 350 μC cm-2 at 50 kV,35 a positive resist commonly used in electron beam lithography. Further, the UV exposure times required for SAMs adsorbed on GaAs(001) are only slightly longer than those needed to process a conventional photoresist (including prebake, exposure and postbake steps).36 Since SAMs are resistant to chemical attack from

4528 DOI: 10.1021/la9033029

Zhou et al.

acids25,30 and bases,21,22,24,25 and are compatible with reactive ion etching processes,24 these results suggest the alkanethiolate SAM resists may be a suitable technology for high-resolution, nanometer scale lithography of GaAs and possibly other semiconductor substrates. Acknowledgment. A.V.W. acknowledges the financial support of the National Science Foundation. This work was performed in part at the US Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos and Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the US Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. Supporting Information Available: Figures showing negative ion TOF SIMS images and edge resolution profiles for electron beam patterned MHA/TDT adsorbed on GaAs(001); negative ion TOF SIMS images of electron beam patterned MHA/HDT adsorbed on Au; negative ion TOF SIMS images of UV photopatterned ECT/ODT adsorbed on GaAs(001); negative ion TOF SIMS images of UV photopatterned MHA/HDT adsorbed on Au. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(6), 4523–4528