Chem. Mater. 1993,5, 1636-1640
1636
TpHydridospherosiloxanes: Novel Precursors for Si02 Thin Films. 1. Precursor Characterization and Preliminary CVD M. D. Nyman,*J S. B. Desu,* and C. H. Pengg Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060 Received April 28, 1993. Revised Manuscript Received August 12, 1 9 9 9
High-quality Si02thin films were deposited from the novel precursor T8-hydrido~pher~siloxane [H8Si8012]. Films were specular, smooth, amorphous, stoichiometric SiO2. The T8-hydridospherosiloxane precursor was characterized in terms of its volatility and decomposition properties. The precursor was volatile (1-100 mTorr) and thermally stable over 80-140 "C and has an enthalpy of vaporization of 78 kJ/mol. Decomposition of Ts in an 0 2 environment was initiated at 250 "C and completed by 350 "C at a decomposition time of 30 min. The CVD experiments were carried out in a hot-wall reactor. Source temperature was 80 "C, deposition temperatures ranged from 450 to 525 OC, carrier gas was 10 sccm 0 2 , chamber pressure was 10 Torr, and substrate was [ 1001 p-type Si wafer. Film characterization techniques included XRD, FTIR, ellipsometry, AES, ESCA, and SEM. Volatility was characterized by isothermal TGA experiments. The precursor was heated in a vertical tube furnace with a N2 purge rate of 25 mL/min. Vapor pressure was calculated using gas diffusion equations. Thermal decomposition experiments were carried out in quartz flasks which were sealed under an 0 2 environment. Products of decomposition were examined by 29Sisolid-state NMR and FTIR.
Introduction
Recently,we reported chemical vapor deposition of Si02 thin films from hydridospherosiloxanes, a novel class of precursor materials.' Hydridospherosiloxanesare cagelike polyhedra with a general formula HnSinOl.Sn,where n is an even integer ranging from 8 to 20. The polyhedral cage is made up of Si-0-Si linkages, with Si-H at the apex of each silicon atom in tetrahedral coordination. The hydridospherosiloxanesare quite intriguing as Si02 precursors because the Si-0-Si structure of silica exists in the polyhedral cages. Therefore, it is feasible that thermal decomposition of the hydridospherosiloxanes in oxygen to form silica involves simple activation of the Si-H bond and subsequent linkage of the polyhedra by oxygen bridges. Additionally, since the size of the polyhedra varies with n, silica films deposited from pure hydridospherosiloxane sources will vary in porosity, density, and other related properties, if the polyhedra are indeed preserved during the CVD process. To examine the feasibility of varying film properties by varying the polyhedra size of the hydridospherosiloxane, Si02 films must be deposited from pure hydridospherosiloxanes precursors. In the preliminary CVD experiments, we used a mixture Of Oh-H8Si&12 (T8)and &,h-H10Si10015 (TIC,)as the precursor material. Both T8 and Tlo are air stable, white crystalline solids which sublime at C50 "C (0.01 Torr), and are synthesizedtogether in a single reaction which yields 36% Tlo and 64% T B . Deposition ~ of Si02 films from the Ts/Tlomixture was successful,and the films * To whom correspondence should be addressed.
8 Current address: Department of Chemistry, University of New Mexico, Albuquerque, NM 87131. i Current address: CERAM Inc., Virginia Tech Corporate Research Center, Blacksburg, VA 24060. e Abstract published in Advance ACS Abstracts, October 1, 1993. (1) Desu, S.B.; Peng, C. H.; Tim, S.;Agaskar, P. A. J.Electrochem. SOC. 1992,139, 2682-2685. (2) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707-2708.
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were of excellent quality.' In this present study, we have deposited and characterized Si02 films from pure T g precursor. Additionally, we have characterized volatility and decomposition properties of T g precursor in order to better understand the decomposition mechanism and to optimize CVD conditions for Si02 deposition from this precursor. Experimental Section Synthesis of Precursor. The O,,-HaS&012 (Ta) compound was synthesizedby a method reported byAgaskare2Hydrochloric acid (20 mL), 3 g of sodium dodecyl sulfate, 100 g of anhydrous ferric chloride, 80 mL of methanol, and 800 mL of pentane were vigorously stirred in a three-neck, 3000-mL Morton type flask. A mixture of 400 mL of pentane and 40 mL of trichlorosilane was gradually added (over 5 h) to the flask. The pentane layer and some yellow solid were siphoned into a flask, and stirred over K2C03 (28 g) for 12 h. Products of this reaction include pentane-soluble T8, Dm-HloSilo015 (TI& polymeric byproduct, and an insoluble yellow powder. The insoluble yellow powder was removed by filtration and discarded. Remaining products were concentrated by evaporation of pentane. The polymer was removed by washing with 50 mL of pentane. The T8 was isolated from T1o by repeated washings with 15 mL of pentane. Approximately 150 mg of Tlo and 60 mg of Ta were removedwith each 15-mLwashing; therefore, washings were repeated until a weight loss of only 60 mg was noted (3-4 washings). Separation of Tlo from T8 was confirmed by lH NMR spectroscopy. Spectra were obtained from a 200MHz Bruker instrument equipped with a 5-mm probe. Samples were dissolved in benzene-de with 1% TMS. Chemical shifts of T8 and Tlo, referenced to the TMS peak at 0 ppm, are 4.203 and 4.244 ppm, respectively.2 The yield is approximately 3 g of pure Ta. Pure Ta is an air stable, crystalline solid, which sublimes at
