0Copyright 1995 American Chemical Society
APRIL 1995 VOLUME 11, NUMBER 4
Letters A Room Temperature Method for the Preparation of Ultrathin SiO, Films from Langmuir-Blodgett Layers C . L. Mirley and J. T. Koberstein" Department of Chemical Engineering, Institute of Materials Science, U-136, University of Connecticut, Storrs, Connecticut, 06269-3136 Received December 9, 1994. In Final Form: January 27, 1995@ Langmuir-Blodgett films prepared from diacid-terminated poly(dimethylsi1oxane)can be transformed to SiO, ultrathin films (t = 14A) by exposure to W-ozone at room temperature and atmospheric pressure. The kinetics of the reaction is followed by grazing-incidenceinfrared spectroscopyand the film composition is determined by angle-resolved X-ray photoelectron spectroscopy. The results show that UV-ozone treatment oxidizes 90% of the methyl side groups to hydroxyl and bridging oxygen species. This technique represents a novel low temperature route for the preparation of ultrathin SiO, films from polysiloxane starting materials.
Introduction Currently, there is much interest in developing low temperature techniques for preparing silicon-containing ceramics.l Silicon oxide materials are a particularly important class of ceramics since most silicon semiconductor technology is based on the ability to form thin Si02 layers.2 Conventional methods for preparing Si02 layers on silicon wafer surfaces usually involve heating the silicon in air to temperatures in excess of 1000 OC. Techniques for producing Si02 layers on substrates other than silicon include sol-gel methods where solutions of tetrahydroxysiland2-isopropanol are spin-coated then baked a t tem~ peratures up to 600 "C to give a hard ~ o a t i n g .Thermal chemical vapor deposition* and photochemical vapor deposition6 methods give high-quality thin silicon oxide films a t somewhat lower temperatures, i.e., 110-500 "C. Siloxane polymers have also been used as starting materials to produce stoichiometric Si02 films by avariety Abstract published inAduance ACSAbstracts, March 15,1995. (1)Seyferth, D. Adu. Chem. Ser. 19w); No. 224,565. (2)Deal, B.E.In The Physics and Chemistry ofSiOz and the Si-Si02 Interface; Helms, C. R., Deal B. E., Eds.; Plenum Press: New York, 1988,p 5 and p 26. (3) Bhushan, B.; Gupta, B. Handbook of Tribology; McGraw-Hill, Inc.: New York, 1991;p 14.48. (4)Nyman, M.; Desu, S.; Peng, C. Chem. Mater. 1993,5,1636. (5)Maruyama, T.; Tago, T. Thin Solid Films 1993,232,201. @
of techniques. Pyrolytic degradation of high molecular weight poly(dimethylsi1oxane)(PDMS) has been found to produce silicon oxide fi1ms.l. However, this procedure requires high temperatures ('800 "C) and the ceramic yield is usually low unless the PDMS is highly cross-linked. Photo-induced reactions of polysiloxane films using W excimer laser irradiations or a combination of Wthermal treatment7also gives amorphous Si02 networks. The latter technique reduces the processing time but still requires processing temperatures of 400 OC to initiate the condensation reactions leading to Si02 film formation. Recently, Kalachev et a1.8have shown that LangmuirBlodgett films of poly(diethylsi1oxane)could be converted to quartz-like organoceramic ultrathin films near room temperature conditions by exposure to a low pressure (0.04 mbar) and low temperature (35 OC) oxygen plasma. Potential applications for ultrathin SiO, films include metal-oxide-semiconductor devices such as insulated-gate field-effect transistors (MOSFET) where the SiO, thick(6)Joubert, 0.; Hollinger, G.; Fiori, C.; Devine, R.; Paniez, P.; Pantel,
R J.Appl. Phys. 1991,69 (9),6647. (7)Klummp, A.;Sigmund, H. Appl. Su$. Sei. 1989,43,301. (8) Kalachev, A.A.; Mathauer, K.; Hohne, U.; Mohwald, H.; Wegner, G. Thin Solid Films 1993,228,301.
0743-746319512411-1049$09.00/0 0 1995 American Chemical Society
1050 Langmuir, VoE. 11, No. 4, 1995
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ness needs to be on the order of 30 A to give an insulating layer of electrical tunneling dimension^.^ In our laboratory, we have found that ultrathin organoceramic films can also be prepared by exposing deposited polysiloxane LB films to ultraviolet light (W)and ozone. W light emissions in the 185-254 nm range produce ozone from atmospheric oxygen and simultaneously excite organic molecules. Ozone, being the second strongest oxidizing agent next to fluorine, then reacts with the organic portion of the molecules to produce noncondensing, volatile species such as carbon dioxide and water.1° The advantages to this treatment are that it is fast, requires only standard equipment, and is carried out at room temperature and atmospheric pressure. In this paper, we describe some of the properties of organoceramic films prepared using this method.
Experimental Section Materials. Carboxylic acid-terminated poly(dimethy1siloxane) (diacPDMS)was prepared by Dr. I. Yilgor of Goldschmidt Chemical Corp.ll Vapor phase osmometry and gel permeation chromatography confirmed that the M, = 2100 and the polydispersity was 1.93. Endgroup titration showed that the acid functionality was exactly two. Water from a Millipore Super-Q water purification system was used for preparation of the LB films. Substrates for LB film deposition were composed of gold or silver vacuum evaporated onto glass slides. Equipment. The LB trough used to prepare the LB films was a constant perimeter type that was built in-house. A detailed description ofthis apparatus is describedelsewhere.12 AUVOCS
TlOXlO/OES UV-ozone cleaner was used for treating the deposited LB films. The apparatus contains a low pressure mercury-quartz lamp, generatingUV emissions in the 185 and 254 nm range (10 mW/cm2). The thickness ofdeposited LB films was measured using a variable-anglespectroscopic ellipsometer (J.A. Woollam Co.) capable of measuring the refractive index over a wavelength range of 2500-10000 A. Composition and thickness of the LB films were measured using a Perkin-Elmer (9)Roberts, G.Langmuir-Blodgett Films; Plenum Press: New York, 1990;Chapter 7,p 379. (10)Vig, J. In Treatise on Clean Surface Technology;Mittal, K, Ed.; Plenum Press: New York, 1987. (11)Yilgor, 1.;McGrath, J. E. Adu. Polym. Sci. 1988,86, 9. (12)Mirlev, C. L.: Lewis. M. G.: Koberstein. J. T.: Lee. D. H. T. Langmuir 1994,10, '2370.
Physical ElectronicsPHI 5300X-ray photoelectronspectrometer WS),equippedwitha monochromaticAI KaX-ray source(146.6 eV) and hemispherical analyzer. Grazing-incidence infrared (GIR)spectra were measured using a Matson Cygnus 100FTIR having an MCT detector with 4-cm-l resolution. LB Film Preparation. A diacPDMS solution was prepared in chloroform (4mg/mL). The solution (1OOpL)was spread onto a water subphase containing CdClz (2 x 10-4) and KHCO3 (2.4 x M); pH = 7.65, T = 19 "C. The floating LB film was compressed at 4 mdmin to a surfacepressure of 25 mN/m where deposition took place at a vertical dipping speed of 10 1 . The transfer ratios for these films were all unity.
Results and Discussion The thickness of a single monolayer of diacPDMS on a solid substrate corresponds well to a model where the PDMS chains configure as close-packed 6/1helices standing on end.13 The model prediction for diacPDMS (38A for M , = 2100)agrees fairly well with film thicknesses measured by ellipsometery and XPS (29 and 28 A, respectively). After 15 min of UV-ozone exposure, the film thickness decreased to 14-15 A, representing a thickness loss of 50%. Grazing-incidence infrared spectroscopy (GIR) was carried out on multilayer LB films of diacPDMS (t = 232 A) deposited on silver, in order to monitor the UV-ozone reaction process. Figure 1 shows a plot of the GIR curves versus UV-ozone exposure time. The absorbance peaks of interest are those for Si-0-Si asymmetric and symmetric stretch at 1109 and 1063 cm-', respectively, and the -CH3 symmetric bend at 1263cm-l. The appearance of the strong Si-0-Si absorbance bands indicates that the diacPDMS chains are oriented perpendicular to the substrate surface. As the W-ozone exposure time increases, the -CH3 absorbance bands, including the bands for asymmetric stretch at 2963 cm-l and rock at 824 cm-', all decrease. This indicates either a loss of methyl side groups or a loss of chain orientation. XPS measurements show that the C/Si ratio for unreacted diacPDMS is 24,which is in (13)Lee, D. H.T. Ph.D. Thesis, University of Connecticut, Storrs, CT, 1989.
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agreement with the stoichiometric ratio for diacPDMS of 2.2/1. For UV-ozone exposeddiacPDMS, after correcting for airborne carbon contamination, the C/Siratio decreases to 0.22/1 corresponding to a 90% loss of carbon atoms. While the magnitudes of the -C& IR absorbance bands decrease significantly during UV-ozone exposure, the Si0-Si bands do not. However, the Si-0-Si symmetric stretch band merges with the asymmetric stretch band to give a rather featureless broad absorbance band from 1250 to 1050 cm-1 typical of a quartz-like material.s The frequency ofthe Si-0-Si asymmetric stretch band shifts to a higher value (1142 cm-9 after 30 min of UV-ozone exposure, suggesting that the Si-0-Si packing ofreacted diacPDMS is less dense than that of unreacted diacPDMS. Another interesting feature of the GIR curve for diacPDMS at the 30-min exposure time is the appearance of a large broad peak a t 3500-3100 cm-l. This is due to Si-OH stretch vibrations and is evidence for the conversion of a portion of the methyl side groups to hydroxyl groups during UV-ozone exposure. At the end of 30 min of W-ozone exposure, there is still unreacted diacPDMS below the quartz-like layer as shown by the presence of the -CH3 bending band in the GIR spectrum. Ellipsometry of this same sample shows that the thickness of unreacted diacPDMS decreases from 232 to 91 A. If we define the depth of reaction or ozone penetration as the difference between the thickness of the unreacted diacPDMS before and after UV-ozone exposure, then for this s a m le, the UV-ozone has penetrated to a depth of 141 This distance is much smaller than the penetration limit found by Kalachev et al.8 for either argon (~OO-IOOO A) or oxygen plasma (in excess of 1000A). UV-ozone treatment therefore appears to be a much more surface sensitive chemical modification technique. It is generally accepted that the rate limiting process for preparing thick oxide films on silicon surfaces is the diffusion of molecular oxygen into the silicon.2 By analogy, the kinetics of the reaction of diacPDMS with UV-ozone to form an SiO, layer is most likely limited by the diffusion rate of ozone into the LB film. To model the UV-ozone reaction a s a diffusion process, the intensity for -CH3 symmetric bend absorbance band, which is proportional to the concentration, is first normalized by dividing it by the Si-0-Si asymmetric stretch absorbance at each exposure time. The fractional change in -CHs concentration, given by (C - CdC, - C, (where C is the concentration at time t, COis the concentration at t = 0, and C, is the concentration at t = 30 min) is then plotted versus tm. The diffusion plot shown in Figure 2 exhibits a distinct sigmoidal shape.14 A possible explanation for this behavior is as follows: a t t < 3 min, the generated ozone concentration is low and only the outer surface of diacPDMS reacts giving a slow reaction rate. As the generated ozone concentration increases, it diffuses into the unreacted layers below the surface and the rate of -CH3 reaction increases (3 t < 10 min). As more diacPDMS reacts, the buildup of a thicker SiO, layer hinders further ozone transport to the lower layers and the rate of reaction drops until, a t t = 30 min, the ozone can no longer penetrate into the film. To determine the composition of the W-ozone reacted diacPDMS a m , angle-resolved XPS was carried out on monolayer films deposited on gold. Table 1compares the binding energies found for the Si(2p), C(ls), and O(ls) core level photoelectrons from unreacted and reacted diacPDMS. All of the binding energies were corrected for
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Table 1. X P S Binding Energies for Monolayer Films of Diacid PDMS and W-Ozone Exposed Diacid PDMS on Gold" binding energies (ev) monolayer film element experimentally determined reported diacid PDMS Si(2p) 102.2 102.2b C(lS) 284.6,288.3 2a4.6b O(1s) 531.9 532.3b diacid PDMS Si(2p) 103.5 103.6c W-ozone C(1s) 286.3 to 289.3 om) 532.4 532.5c a Takeoff angle for photoelectrons was 62". For poly(dimethylsiloxane), ref 15. For Si02 ,refs 16 and 17.
static charging of the samples by referencing them to the C(1s) peak arising from airborne carbon contamination at 284.6 eV. Also included in the table are the literature values of the respective binding energies for PDMS and amorphous SiO2. Before exposure to UV-ozone, the binding energies in diacPDMS are found to be in good agreement with the reported values for PDMS.16 The C(1s) signal for diacPDMS has a n additional peak at 288.3 eV corresponding to the carboxylic acid endgroups. After UV-ozone exposure, curve fits of the C(ls) spectrum showed that it consists of peaks from 286.3 to 289.3 eV corresponding to -COH and -COOH chemical species which indicate incomplete oxidation of the -CH3 side groups. The Si(2p) peak shiRed to 103.5 eV, close to that found in amorphous Si02 (103.6eV),16which suggests that the stoichiometry of the UV-ozone exposed diacPDMS film is very nearly that of silicon dioxide. Figure 3 shows a plot of the atomic ratios Si/Au, C/Au, and O/Au for a monolayer of diacPDMS after UV-ozone treatment, versus sin 8, where 8 is the photoelectron takeoff angle. It is clear from the plot that oxygen is the most abundant element in the film, followed by silicon and carbon. Figure 4, which shows the C/Si and O/Si atomic ratios for thisfilm, as a function of the photoelectron takeoff angle, indicates that there is more carbon on the free surface of the W-ozone treated diacPDMS film and more oxygen a t the gold substrate interface. The higher (15)Wagner, C.; Passoja, D.; Hilley, H.; Kinisky, T.; Six,H.; Jansen, T.; Taylor, J. J. Vac. Sci. Technol. 1982,21, 993. (16)Wagner, C.; Riggs, W.; Davis, L.; Moulder, J.; Mullenberg, G. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp., Physical Electronics Division; Eden Prairie, MN, 1974.
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1052 Langmuir, Vol. 11, No. 4, 1995
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sin e Figure 4. Angle-resolved XPS for a monolayer of diacPDMSW-ozone exposed on gold. Plot is for C/Si and O/Si atomic ratios at the photoelectron takeoff angles of 15",30", and 62". amount of carbon a t the air-film interface is most likely the result of airborne carbon contamination while the higher amount of oxygen near the gold surfaceis associated with the partially oxidized -CHs groups as discussed previously. High-resolution XPS (pass energy 8.95 ev) was carried out a t 15" takeoff angle on the W-ozone exposed diacPDMS film to identify the components and determine the composition of the Si(2p) and O(ls) photoelectron peaks. Curve fitting ofboth spectra was done using 100% Gaussian curves. The best fit for Si(2p) spectra reveals two silicon peaks a t the binding energies of 102.8 and 103.5 eV. The higher binding energy peak, accounting for 77% of the silicon, has already been identified as the
Si(2p) in Si02. The lower binding energy peak is most likely due to 0-Si-C groups present in the film due to the incomplete oxidation of the -CH3 side groups. Using the corrected C/Si ratio (0.22/1) and assuming that all of these carbon atoms are bonded to silicon atoms, the calculated percentage of Si-C bonds for all the silicon atoms is 22%,which agrees with the Si(2p)curve fit results of 23%. The best fit for the O(1s)peak shows three peaks. Their binding energies and atomic percentages are as follows: 531.6 eV, 11%;532.4 eV, 75%; 533.2 eV, 14%. The 532.4eV peak is consistent with the O(ls) peak in amorphous Si02 for 0-Si-0 groups (532.5 ev), while the peak a t 531.6 eV is in the range for carbon-oxygen bonded species.16J7 The peak a t 533.2 eV is possibly that for oxygen present in Si-OH groups. From GIR, we know that a portion of the -CH3 groups in diacPDMS is transformed into -0Hgroups during W-ozone exposure. The higher binding energy of this O(1s)peak is due to the higher electronegativity of the bonded hydrogen atom compared to that for silicon bonded to oxygen.'* From the results for Si(2p)and O(ls) curve fits we can deduce the following structure for UV-ozone exposed diacPDMS film: from the C/Si ratio of 0.22/1, and assuming no loss of silicon during UV-ozone exposure, the number of carbon atoms per chain left after exposure is approximately six. These are all composed of carbonoxygen species. The O/Si ratio after neglecting C-0 and Si-C species is found to be 2 I 1 , close to that found for stoichiometric Si02 films. The slightly higher oxygen content derives from the Si-OH groups present in the film. The arrangement of atoms in an amorphous Si02 film consists of a three-dimensional network where each silicon atom is bound to four oxygens and each oxygen atom is bound to two silicon atoms. From the binding energies and atomic percentages ofthe HRES O(1s)peak, 75% of the oxygen atoms bound to silicon are incorporated into an Si02 network structure, while 14% as Si-OH groups are not. The remaining 11%of the oxygen atoms present in the film are bonded to carbon and most likely nonbridging as well. This amount of network formation is significant given the fact that, for photo-induced transformation of polysiloxanes layers, temperatures on the order of 350-800 "C are required to produce the h a l condensation reactions leading to the creation of stoichiometric Si02.6J
Acknowledgment. C.L.M. wishes to thank the Eastman Kodak Company-Fellows Program, the U.S. Army Research Office, and the Connecticut Department of Higher Education under Grant No. 631606 for their financial support during the preparation of this work. LA940973A (17)Shalvoy, R.;Reucrofk, P.;Davis, B.J. Catul. 1977, 66, 336.
(18)Andrade, J. Surface and Interfmial Aspects of Biomedical Polymers; Plenum Press: New York, 1985;Vol. 1, p 146.
