Thermolithographic Patterning of SolGel Metal Oxides on Micro Hot

sable and thermally isolated from each other. Thin films of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane. (TFS) or hexyltrichlorosilane (HFS...
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Anal. Chem. 2003, 75, 4360-4367

Articles

Thermolithographic Patterning of Sol-Gel Metal Oxides on Micro Hot Plate Sensing Arrays Using Organosilanes Nancy Ortins Savage,† Sonya Roberson,‡ Greg Gillen,‡ Michael J. Tarlov,† and Steve Semancik*,†

National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899

Sol-gel-derived SnO2 and Fe2O3 were selectively deposited on elements of micro hot plate (µHP) arrays. The silicon micromachined µHP arrays contain heating elements (100 µm × 100 µm) that are electronically addressable and thermally isolated from each other. Thin films of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (TFS) or hexyltrichlorosilane (HFS) assembled on surfaces of the arrays served as thermally sensitive resists whereby heating of specific µHPs resulted in removal of organosilane films only in heated areas. TFS-masked surfaces were characterized with condensation figures and secondary ion mass spectrometry (SIMS) imaging. TFS was removed from regions heated above 400 °C to expose hydrophilic surfaces, while TFS films in unheated areas were unaffected and remained hydrophobic. Sol-gel tin oxide spin-coated on the thermally patterned arrays adhered only to the hydrophilic regions and was repelled from the hydrophobic areas masked by the TFS films. By using HFS films, it was possible to selectively deposit two sol-gel materials, SnO2 and Fe2O3, on different µHPs in the same array as confirmed by SIMS imaging. Both materials showed varying degrees of electrical response to hydrogen and methanol in gas-sensing measurements. The development of array devices for both liquid- and gasphase chemical sensing has been actively pursued over the past several decades. Gas sensor arrays composed of multiple polymer or metal oxide sensors can discriminate between different odors, identify pure and binary mixtures of organic solvent vapors in air,1-4 and classify more complex mixtures, such as different types of coffee.5-7 The advantage of array devices is that multiple gas* Corresponding author. Fax: 301-975-2643. E-mail: [email protected]. † Process Measurements Division. ‡ Surface and Microanalysis Division. (1) Szczurek, A.; Szecowka, P. M.; Licznerski, B. W. Sens. Actuators, B 1999, 58, 427-432. (2) Zee, F.; Judy, J. W. Sens. Actuators, B 2001, 72, 120-128. (3) Patel, S. V.; Jenkins, M. W.; Hughes, R. C.; Yelton, W. G.; Ricco, A. J. Anal. Chem. 2000, 72, 1532-1542. (4) Meijerink, M. G. H.; Strike, D. J.; de Rooij, N. F.; Koudelka-Hep, M. Sens. Actuat., B. 2000, 68, 331-334. (5) Albert, K. J.; Walt, D. R.; Gill, D. S.; Pearce, T. C. Anal. Chem. 2001, 73, 2501-2508.

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sensitive, but not necessarily gas-selective, materials can be combined to generate gas-specific patterns. Thus, instead of having multiple sensors with each dedicated to a specific analyte, one may use a small subset of less specific sensors combined with computational methods to decipher the contents of a complex mixture both qualitatively and quantitatively. Previously, our group has demonstrated the utility of MEMS micro hot plate (µHP) arrays for gas sensing by temperatureprogrammed methods and the selective screening of potential oxide sensor materials. The arrays have 4, 16, 36, or 48 individually addressable elements, and each can be probed simultaneously for resistance changes in response to gases through the use of metal contacts on the surface of the hot plates. A challenge of working with such microarray devices is the selective deposition of sensing films on the small area of each µHP (∼100 µm × 100 µm). At present, our approach has been to deposit materials on µHPs through chemical vapor deposition whereby material is only deposited on the localized hot plate areas held at elevated temperatures.8-12 In this report, we demonstrate selected area deposition of aqueous-phase sol-gel materials through control of the surface-wetting properties of the µHPs. In the approach, organosilanes are formed on µHP arrays and used as heat-sensitive resists. Heat supplied by the µHPs themselves is used to desorb the organosilane films and change the wetting properties of selected µHPs from hydrophobic to hydrophilic. Spin-coated solgels are found to adhere only to the hydrophilic µHP surfaces. Organosilanes spontaneously react with hydroxylated surfaces to form films of specific chemical functionality for a variety of applications. These films and monolayers have been used to (6) Pardo, M.; Niederjaufner, G.; Benussi, G.; Comini, E.; Faglia, G.; Sberveglieri, G.; Holmberg, M.; Lundstrom, I. Sens. Actuators, B 2000, 69, 397-403. (7) Capone, S.; Siciliano, P.; Quaranta, F.; Rella, R.; Epifani, M.; Vasanelli, L. Sens. Actuators, B 2000, 69, 230-235. (8) Semancik, S.; Cavicchi, R. Acc. Chem. Res. 1998, 31, 279-287. (9) Cavicchi, R. E.; Suehle, J. S.; Kreider, K. G.; Shomaker, B. L.; Small, J. A.; Gaitan, M.; Chaparala, P. Appl. Phys. Lett. 1995, 66, 812-814. (10) Semancik, S.; Cavicchi, R. E.; Wheeler, M. C.; Tiffany, J. E.; Poirier, G. E.; Walton, R. M.; Suehle, J.; Panchapakesan, B.; Devoe, D. L. Sens. Actuators, B 2001, 77, 579-591. (11) DiMeo, F., Jr.; Cavicchi, R. E.; Semancik, S.; Suehle, J. S.; Tea, N. H.; Small, J.; Armstrong, J. T.; Kelliher, J. T. J. Vac. Sci. Technol., A 1998, 16, 131138. (12) Semancik, S.; Cavicchi, R. E.; Kreider, K. G.; Suehle, J. S.; Chaparala, P. Sens. Actuators, B 1996, 34, 209-212. 10.1021/ac0301797 Not subject to U.S. Copyright. Publ. 2003 Am. Chem. Soc.

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enhance properties related to sensing, biocompatility, adhesion, corrosion inhibition, and water repellency.13-19 In addition, lithographic patterning of organosilanes is possible through methods such as UV, excimer laser, or electron beam exposure,20-23 as well as through maskless techniques such as “dip-pen” lithography.24 In this work, we describe a thermal lithographic technique where the creation of hydrophobic and hydrophilic regions is controlled for the selective deposition of sol-gel materials on µHPs. EXPERIMENTAL SECTION Micro hot plate arrays are silicon-based devices made through complementary metal oxide semiconductor processing. Each array consists of multiple µHPs, which are individually addressable for temperature control and electronic probing of deposited films. Figure 1 shows a top view scanning electron microscopy (SEM) micrograph of a four-element µHP array as well as a schematic of the multilayer stack of the µHP. Each multilayer stack consists of a serpentine polysilicon heater for resistive heating (30-550 °C), an aluminum plate layer for heat distribution, and a patterned titanium-tungsten surface metal layer that is used for making electrical contact with deposited films for sensing applications. The electrically conductive layers in the structure are isolated vertically from one another with SiO2. During the wafer-processing steps, regions around the µHPs are left open down to the silicon wafer so that a silicon etchant (tetramethylammonium hydroxide) can be used to release the µHP. The suspended microbridge structure created thermally isolates each µHP from the remainder of the device. Etched µHPs are mounted in 40 pin dual-in-line chip packages and wire bonded with aluminum leads. The µHPs are typically 100 µm on a side and weigh less than 0.2 µg. Their low thermal mass allows rapid heating and cooling with thermal rise and fall times between 2 and 5 ms.8 The µHPs were designed at NIST and fabricated at MIT-Lincoln Laboratories. For this work, four-element µHP arrays with interdigitated metal contacts, such as that shown in Figure 1A, were used for sensing measurements and 16-element µHP arrays with two parallel contacts were used for some of the characterization studies. The strategy developed for the selected area deposition of solgel metal oxides on µHPs is depicted in Figure 2. The µHP surface, which is composed primarily of SiO2 with additional surface metal contacts, is treated with a solution of an organosilane to render the surface hydrophobic. By heating specific µHPs, the silane film can be locally removed to expose the underlying SiO2, (13) Zybill, C. E.; Ang, H. G.; Lan, L.; Choy, W. Y.; Meng, E. F. K. J. Organomet. Chem. 1997, 547, 167-172. (14) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 17491753. (15) Tada, H.; Nagayama, H. Langmuir 1995, 11, 136-142. (16) Tada, H.; Nagayama, H. Langmuir 1994, 10, 1472-1476. (17) Nakagawa, T.; Soga, M. J. Non-Cryst. Sol. 1999, 260, 167-174. (18) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752-3758. (19) Shin, H.; Collins, R. J.; De Guire, M. R.; Heuer, A. H.; Sukenik, C. N. J. Mater. Res. 1995, 10, 699-703. (20) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829-5839. (21) St. John, P. M.; Craighead, H. G. J. Vac. Sci. Technol., B. 1996, 14, 69-74. (22) Masuda, Y.; Seo, W. S.; Koumoto, K. Jpn. J. Appl. Phys. 2000, 39, 45964600. (23) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 1236-1241. (24) Ivanisevic, A.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7887-7889.

Figure 1. (A) SEM micrograph showing a top view of a four-element micro hot plate with interdigitated electrodes. (B) Schematic of the multilayer that makes up one micro hot plate. Layers of SiO2 exist between each multilayer for electrical isolation but have been removed from the figure for clarity.

which is hydrophilic. Aqueous sol-gel solutions can then be spun over the entire array, wetting only the exposed µHPs. Organosilane films were deposited on µHP arrays as well as 5-mm2 SiO2/Si substrates used for some characterization experiments. The SiO2/Si substrates were prepared by immersing diced Si wafer pieces in a 1:1 solution of methanol/HCl for 30 min, rinsing with water, soaking in concentrated sulfuric acid for 15 min, and then treating with boiling Nanopure water for 1 h before rinsing with acetone. The surface metals of the µHPs are susceptible to attack by acids, so these microsubstrates could not be cleaned in the same manner as the larger substrates. Both the µHPs and the SiO2/Si substrates were treated in a UVOCS (Montgomery, PA.).25 UV/ozone cleaner for 15 min immediately prior to organosilane deposition. This procedure has been shown previously to both remove surface organic contamination and produce hydroxylated surfaces appropriate for organosilane deposition.26 Analytical Chemistry, Vol. 75, No. 17, September 1, 2003

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Figure 2. Schematic diagram outlining the strategy for thermal lithography of silanes on micro hot plates.

(Tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (TFS; Gelest Inc., Tullytown, PA.)25 and hexyltrichlorosilane (HFS; Aldrich).25 were both used as received. Silane solutions (0.02 mM) were prepared with dry hexane in a N2-flushed drybox just prior to film deposition. To deposit films, cleaned substrates or µHPs were soaked in the desired silane solution for 10 min and rinsed with copious amounts of hexane. Tin oxide sol-gels were prepared from the hydrolysis of SnCl4.27,28 Ammonium hydroxide was added dropwise to a solution of tin chloride pentahydrate in water until a precipitate formed. The precipitate was then rinsed three times with water and peptized (resuspended in solution) with ammonium hydroxide until a transparent sol was formed. Fe2O3 sol-gels were prepared similarly from FeCl3, except that acetic acid was used for peptization.29 Sol-gels were spin-coated on the µHP arrays at 2000 rpm for 120 s until the film was dry. The µHPs were heated by applying appropriate voltages across the serpentine heater. Each µHP was calibrated prior to use, with the specific voltage(s) needed to reach the desired temperature(s) determined from both the resistance of the serpentine heater and the thermal resistivity of the polysilicon. A voltage of 10-11 V was typically used to heat the µHPs to 400 °C. Sensing measurements were performed in a home-built stainless steel sample cell where the µHPs were exposed to controlled, known gas mixtures of either methanol or hydrogen in air at a measurement temperature of 400 °C. The total flow rate of gases over the sample was 1 L/min, and the concentration of methanol or hydrogen in air ranged from 25 to 75 µmol/mol (25-75 ppm). (25) Certain commercial equipment or materials are identified in this report to specify adequately the experimental procedure. Such identification does no imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment are necessarily the best available for the purpose. (26) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600-7604. (27) Mulvaney, P.; Greiser, F.; Meisel, D. Langmuir 1990, 6, 567-572. (28) Jin, Z.; Zhou, H.-J.; Jin, Z.-L.; Savinell, R. F.; Liu, C.-C. Sens. Actuators, B 1998, 52, 188-194. (29) Orel, B.; Macek, M.; Svegl, F.; Kalcher, K. Thin Solid Films 1994, 246, 131-142.

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Conductance changes of the sample were acquired by a specifically designed computer program. Contact angle measurements of water were made with a Rame Hart Inc. (Mountain Lakes, NJ.)25 NRL contact angle goiniometer. Scanning electron micrographs were obtained with a Hitachi (Pleasanton, CA.) FE-SEM S-4000.25 Optical Micrographs were taken with a Nikon (Melville, NY.) Labophot-2A microscope25 outfitted with a Digital Instruments Inc. (Sterling Heights, MI.) Spot DI-SP100 digital camera.25 Secondary ion mass spectrometry was performed with an Ion-TOF IV25 time-of-flight instrument using a 25-keV Ga+ primary ion beam and operated to detect either negative or positive secondary ions. A low-dose electron flood gun was used for charge compensation. All experiments were conducted under static analysis conditions with primary ion doses less than 1012 ions/cm2. RESULTS AND DISCUSSION Characterization of thin films formed on the ∼100 µm × 100 µm surfaces of the µHPs is difficult. Therefore, contact angle measurements were first made on TFS films prepared on the macroscopic SiO2/Si substrates to ascertain the assembly protocol and the film quality. Cleaned, UV ozone-treated substrates had contact angles of 27° ( 2° (average of 12 measurements). The contact angle increased to 103° ( 2° (average of 6 measurements) after 10 min of exposure to the TFS solution. These contact angle measurements agree with reported values of organosilane films with similar R groups. For films prepared from solution, contact angle values in the literature range from a Θa (advancing contact angle) of 94° ( 2° (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-methyldichlorosilane on silicon30 to 103° for TFS on native oxide Si(100) wafers.31 A contact angle of 106° has also been reported for gas-phase dosed silane monolayers of the trimethoxysilane with the same R group.26 (30) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielson, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435-8442. (31) Geer, R. E.; Stenger, D. A.; Chen, M. S.; Calvert, J. M.; Shashidhar, R.; Jeong, Y. H.; Pershan, P. S. Langmuir 1994, 10, 1171-1176.

Figure 3. Optical images of condensation figures for TFS-treated micro hot plate surfaces where M1, M2, and M3 have been heated to 350, 400, and 450 °C, respectively. (A) Array is cooled with dry ice and moisture freezes on the surface. (B) Array at room temperature after being cooled.

While direct observation of the contact angles for films on the µHPs was not possible, changes in the surface hydrophobicity could be ascertained using condensation figures. This technique correlates the pattern of moisture that condenses on a cold surface with surface composition.32-34 For these experiments, a fourelement µHP array was first modified with TFS. Next, three of the µHPs were heated to 350 (M1), 400 (M2), and 450 °C (M3), respectively, for 60 s. The entire µHP array was then cooled by placing pieces of dry ice on the surrounding electronics package while the array was observed with an optical microscope. Initially, moisture condensed on the surface of the µHPs, preferentially wetting the three µHPs that had been heated. The water droplets on the surface then froze and increased in concentration. Next, the dry ice was removed and the sample allowed to return to room temperature. As shown in Figure 3, upon warming to room temperature, water formed continuous sheets on the surfaces of µHPs M1, M2, and M3, while beads formed on the remainder of the array. The difference in wetting behavior between µHPs M1, M2, and M3 and the other regions of the array suggests that heating renders the µHP surfaces hydrophilic, while the remainder of the array, including µHP M4, remains hydrophobic. Chemical analysis with secondary ion mass spectrometry (SIMS) was used to characterize the TFS films and verify their selective desorption from heated µHPs. Figure 4 shows the negative ion spectra of two SiO2/Si substrates, one blank and one that has been modified with TFS. No parent ion for TFS was detected, though several characteristic fragments of fluorinated (32) Lopez, G. P. B.; Hans, A.; Frisbie, D.; Whitesides, G. M. Science 1993, 394, 647-649. (33) Aizenberg, J. B.; Andrew, J.; Whitesides, G. M. Nature 1998, 394, 868871. (34) Hofer, R. T.; Spencer, M. N. D. Langmuir 2001, 17, 4123-4125.

Figure 4. SIMS spectra of SiO2/Si substrates. (A) Clean, untreated substrate. (B) TFS-treated substrate.

organosilanes were observed in the spectrum.35 None of these fragments were observed in the spectrum of the blank SiO2/Si substrate. From the spectrum of the TFS-derivatized substrate, one can observe that the most abundant species is F- (m/z ) 19) and it was therefore chosen for the imaging studies described below. Table 1 summarizes the SIMS data for the 20 most intense peaks. For SIMS imaging studies, the organosilane film was formed on a 16-element µHP array. Each µHP was heated to a specified temperature (350-450 °C) and length of time (one or more 1-s pulses) or not heated at all. SIMS images of the array elements (35) Munster High Mass Resolution Static SIMS Library; Ion-TOF GmbH, Germany, 1998-1999.

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Figure 5. SIMS images of TFS-treated micro hot plates showing fluorine secondary ion signal. (A) No heating. (B) 400 °C, one 1-s heating pulse. (C) 400 °C, five 1-s heating pulses. (D) 450 °C, five 1-s heating pulses.

Table 1 mass 1.0079 11.998 13.0079 15.9954 17.0035 18.9996 24.0025 36.0071 38.0030 39.0135 43.0075 48.0114 55.0124 59.9811 62.9853 69.0141 78.9872 93.0250 100.9988

ion

intensity (%)

CCHOOHFC2C3F2HF2C2F- + SiCH3C4C3FC5OSiFCF3C5FC3F3Si2CHO2-

6 0.339 0.366 5.39 0.92 100 0.5 .084 1.28 0.259 0.417 0.076 0.0799 0.8279 0.290 0.6417 0.3638 0.1245 0.1209

H-

were then obtained to determine which temperatures and times were effective in removing the silane film. Figure 5 shows the results for µHPs that were kept at room temperature as well as those that were heated to 400 and 450 °C for either one or five 4364

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1-s pulses. Figure 5A shows that, without heating the device, the surface of the µHP and the surrounding area is coated with the fluorinated organosilane film. Differences appear to be rather small between portions of the films covering SiO2 and those over the metal electrodes. Heating the film with just one 1-s temperature pulse at 400 °C reduces the observed fluorine secondary ion signal by 31% (Figure 5B) and increasing the thermal exposure to five 1-s pulses reduces the fluorine signal an additional 42% (Figure 5C). Finally, increasing the temperature to 450 °C for five 1-s pulses highly reduces the observed fluorine signal an additional 10%. The observed decreases in the fluorine secondary ion signal on the surface of the µHPs with temperature is strong evidence that the organosilane film has been largely removed as a result of heating. (Preliminary XPS measurements made before and after heating the organosilane films on µHPs indicate an original organosilane film thickness of ∼5 nm or less.) Our condensation figure results indicate that brief heating to 400 °C (5 s, cumulative) or to 450 °C (1 s) is sufficient to largely remove the TFS film from the surface of µHPs. Longer times (60 s) are required at lower temperatures (350 °C) to change the hydrophobicity of the µHP surface. It has been reported that heating organosilane films results in decomposition with formation of various gaseous species (dependent on the silane, i.e., ethanol

Figure 6. (A) SIMS images of TFS-treated micro hot plates showing fluorine secondary ion signal after S2 and S3 were heated. (B) SIMS image showing the tin secondary ion signal after spin-coating the sol-gel tin oxide onto the array of (A).

Figure 7. Results of sensing measurements for TFS-treated, SnO2-coated micro hot plate array. The conductivities of S2 and S3 change with the varied gas composition. S1 and S4, which was protected by the TFS film during SnO2 deposition, show no response to either different concentrations of methanol (left) or hydrogen (right). All four micro hot plates were held at 400 °C during the experiment.

and ethylene from the decomposition of tetraethyl orthosilicate).36,37 The temperatures used for treating the modified µHPs is thought to be sufficient for film decompostion based on results from other studies. For example, for vinyltriethoxysilane and (36) Gamble, L.; Henderson, M. A.; Campbell, C. T. J. Phys. Chem. B 1998, 102, 4536-4543. (37) Gamble, L.; Jung, L. S.; Campbell, C. T. Langmuir 1995, 11, 4505-4514.

diethyldiethoxysilane films, B-hydride elimination reactions lead to the cleavage of the Si-C bond and formation of ethylene, which has been detected at 377 °C, and acetylene (VTES only), detected at 447-527 °C.37 (3,3,3-Trifluoropropyl)trimethoxylsilane films have been observed to give off trifluoroproylene as a decomposition product between 277 and 427 °C as well as ethylene around 387 °C.36 Analytical Chemistry, Vol. 75, No. 17, September 1, 2003

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Figure 8. SIMS positive ion images of (A) Sn (on S1 and S3) and (B) Fe on (on S2 and S4) that shows localized deposition of sol-gel tin oxide and iron oxide on array elements achieved using HFS masking.

While the changes in hydrophobicity of the µHP surface as well as the reduction in fluorine secondary ion signal with temperature suggest the elimination of the silane from the surface, it is likely that some residue from the silane decomposition remains on heated µHP surfaces. In our experiments, it is reasonable to expect SiO2 as a byproduct as well, with deposition on both the existing SiO2 surface of the µHP and the electrodes. Both XPS and temperature-programmed static SIMS studies of organosilane desorption from TiO2(110) surfaces indicate decomposition of the silane involves cleavage of the Si-O(-C) and Si-C bonds, leaving SiO2 on the surface as well as fluorine in the case of (3,3,3-trifluoropropyl)trimethoxylsilane.36,37 As indicated below, this SiO2 must be at very low concentrations, as there is no noticeable interference with the electrical contact between the electrodes and deposited sensing films. The utility of these thermally generated organosilane masks was first demonstrated through selected area deposition of solgel SnO2. Four µHPs were derivatized with the TFS, and two of the four µHPs were then heated to 400 °C for 60 s to fully remove the silane film. Sol-gel SnO2 was next spin-coated over the entire array 4 times. Between each sol-gel spin coat, the three previously heated µHPs were held at 400 °C for 15 min. Due to the hydrophilicity of the exposed µHPs compared to the hydrophobicity of the remaining array, it was expected that the waterbased sol-gel suspension would adhere only to the exposed hydrophilic regions. The selective deposition of the tin oxide sol-gel on the TFSmasked µHP array was assessed with SIMS. Images of a fourelement µHP, monitored for both the fluorine secondary ion signal (negative ion image) and the complementary tin secondary ion (m/z ) 120, positive ion image), are shown in Figure 6. Micro hot plates S2 and S3 had been heated to remove the TFS film, while µHPs S1 and S4 remained hydrophobic. The image shows 4366

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that the tin oxide sol-gel film only deposited on the heated, and therefore hydrophilic, µHPs. Thicknesses of films deposited on µHPs are very difficult to obtain nondestructively, and no direct measurements were attempted in these studies. The lack of significant charging during SIMS analyses, however, suggest that tin oxide film thicknesses were less than 100 nm. For the gas-sensing measurements, all four µHPs were held at 400 °C (subsequently removing the organosilane film from the S1 and S4 µHPs) and then all were exposed to both methanol and hydrogen test gases in air. Figure 7 shows the conductivity changes of the four µHPs during testing as the gas composition was varied between 100% air and 25, 50, and 75 µmol/mol (25, 50, and 75 ppm) methanol or hydrogen in air. Two of the µHPs (the two coated with SnO2, S2 and S3) show similar conductance responses to the presence of methanol and hydrogen gases. The other two µHPs (S1 and S4) exhibit no response, confirming that the organosilane mask adequately protected these µHPs from SnO2 deposition during spin-coating. The experiments discussed above demonstrate the utility of organosilane films for selectively depositing sol-gel oxides. We now demonstrate that the thermal lithographic technique can be used to pattern multiple sol-gel materials on µHP sensor arrays. Following the same protocol as with the TFS film, SnO2 was spin-coated on an HFS-masked array resulting in the deposition of SnO2 on two µHPs, S1 and S3. The array was then exposed to a fresh HFS solution to deposit the silane film on top of the SnO2 surface. Subsequent heating of µHPs S2 and S4 then removed the HFS from these elements and exposed the underlying hydrophilic silica. Fe2O3 was then spin-coated on the array, selectively depositing on µHPs S2 and S4. Postitive ion images of Sn and Fe secondary ion signals shown in Figure 8 indicate that both oxides have been successfully deposited on the specified µHPs using the HFS mask. We again estimate the thicknesses of

Figure 9. Results of sensing measurements for HFS-treated, SnO2-coated and Fe2O3-coated micro hot plates in the four-element array shown in Figure 8 for the same testing protocol as described for Figure 7, with different concentrations of methanol (left) and hydrogen (right).

the deposited SnO2 and Fe2O3 films to be less than 100 nm. Figure 9 displays sensing measurements from this array, where the SnO2 sensor shows greater gas sensitivity than the Fe2O3 to both of the gases tested. For these experiments, it was necessary to change the mask material from TFS, which was useful in the characterization of the µHP array surfaces, to HFS, a nonfluorinated silane. In early experiments with Fe2O3 sol-gel deposition, TFS was found to be ineffective as a mask for processing this oxide. HFS, however, worked well as a mask for both the Fe2O3 and SnO2 sols. Both differences in the organosilane films and differences between the two sols contribute to the failure of the Fe2O3/TFS system. First, the two sols have different pHs; the SnO2 is basic while the Fe2O3 is acidic. In addition, there may be differences in the surface polarity between the fluorinated and nonfluorinated organosilane films. Studies have shown that the surfaces of fluorinated organosilane films have a significant surface polarity, with the surface of the film being negatively charged.38 The positively charged Fe2O3 particles in the acidic sol might be attracted to the fluorinated organosilane surface rather than repelled from it, as would be expected by hydrophobic/hydrophilic interactions, making TFS a poor mask for the acidic sol.

CONCLUSION The thermal lithographic method described above is a new approach to patterning surfaces. Using this technique, one takes advantage of the integrated heating capabilities of the µHP arrays, eliminating the need for many of the steps involved in photolithographic approaches. In particular, the alignment of a shadow mask with specific features on the array surface (i.e., one or more micro hot plate sensors) that are targeted for the removal of a silane film is a requirement with photolithographic methods. This is unnecessary with thermal lithography, however, as the micro hot plates themselves define the area of exposure. The approach has been employed to produce arrays with multiple gas-sensitive materials that can be used for recognizing and quantifying analtyes in gas mixtures. ACKNOWLEDGMENT The authors acknowledge Dr. Richard Cavicchi for useful conversations regarding this work, and Christopher Montgomery for preparation of the µHP devices. N.O.S. acknowledges the National Research Council/National Institute of Standards and Technology Postdoctoral Research Program for financial support.

Received for review May 5, 2003. Accepted May 13, 2003. (38) Robinson, G. N.; Kebabian, P. L.; Freedman, A.; DePalma, V. Thin Solid Films 1997, 310, 24-28.

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