The formation mechanism and step‐coverage quality of films formed by the pyrolysis of tetraethylorthosilicate (TEOS) were studied, using a novel experimental technique called the "multi‐layered micro/macrocavity method." The growth rate profiles at millimeter (macrocavity) and submicron (microtrench) sales deposited under a total pressure ranging from 2 to 760 Torr were simultaneously analyzed. The step coverage approaches conformal deposition either with decreasing volume‐to‐surface ratio (V/S) of the macrocavity reaction zone or with increasing total pressure. Combining these results with the growth‐rate profiles of the macrocavity shows that two kinds of intermediate species participate in deposition. One is a high‐activity species with a surface sticking probability near 1, and the other is a low‐activity. A nonlinear increase of the growth rate with the macrocavity V/S ratio indicates that a polymerization reaction occurs in the gas phase. A comprehensive model of the deposition kinetics is presented to correlate the step coverage quality and the growth rate uniformity with the operating conditions.
Selective tungsten chemical vapor deposition was carried out on three kinds of substrates: hydrogen‐terminated silicon (H‐Si), monolayer nitrided silicon (N‐Si), and thermally grown silicon oxide. X‐ray photoelectron spectroscopy (XPS) confirmed that the H‐Si substrates differ from the N‐Si substrates only by the monolayer of nitride on their surface. Field‐emission scanning electron spectroscopy and XPS showed that tungsten does not deposit on the N‐Si substrates but does deposit on the H‐Si substrates. Monolayer nitridation therefore has the potential for improving and optimizing the thin‐film preparation processes, because it provides a means for altering the surface reactivity while keeping the bulk properties unchanged.
A hydrogen-terminated silicon surface was successfully converted to a surface covered with a monolayer of nitrogen. Nitridation was carried out in a vacuum chamber using either dimethylhydrazine [H2N–N(CH3)2] or ammonia at a pressure of 1 mTorr and at temperatures ranging from 400 to 600 °C. In situ x-ray photoelectron spectroscopy measurements revealed that the binding energy and the full width at half-maximum in the nitrogen spectra are the same as those in bulk Si3N4. Nitrogen content at the surface increased as the nitridation time increased and, below 500 °C, saturated at a value that approximately corresponds to a monolayer thickness. These results show the effectiveness of dry chemical processes for preparing uniform Si surfaces terminated with specific atoms or molecules other than hydrogen.
In this work, deposition of TiO2 was performed as a typical case study to demonstrate the concept of area-controlled CVD process. As the reactant gases, TiCl4, H2 and CO2 were used. In this reaction system, TiO2 is deposited by the reaction of TiCl4 with H2O which is formed by the reverse water gas shift reaction. Hence, the formation and deposition zone of TiO2 was controlled by regulating the position of the catalyst of the reverse water gas shift reaction and the concentration of reactant gases.At 443–473 K, TiO2 was deposited on a inner surface of tetrafluoroethylene tube, where a certain amount of catalyst pieces were set at the close end. The deposition profile was measured with SEM. The deposition area depended on the length of the Teflon tube, the reaction temperature, the number of catalyst pieces and the concentration of reactant gases. The relationship between the deposition area and the above parameters could be interpreted by a model, which is based on an assumption that the rate-limiting step in the CVD process is TiCl4 diffusion process. It was found to be possible to control the deposition area by the proposed CVD method.
Mesoporous SiO2-P2O5 films were synthesized from the vapor phase onto a silicon substrate. First, a precursor solution of cetyltrimethylammonium bromide (C16TAB), H3PO4, ethanol, and water was deposited on a silicon substrate by a spin-coating method. Then, the C16TAB-H3PO4 composite film was treated with tetraethoxysilane (TEOS) vapor at 90-180 degrees C for 2.5 h. The H3PO4-C16TAB composite formed a hexagonal structure on the silicon substrate before vapor treatment. The TEOS molecules penetrated into the film without a phase transition. The periodic mesostructure of the SiO2-P2O5 films was retained after calcination. The calcined films showed a high proton conductivity of about 0.55 S/cm at room temperature. The molar ratio of P/Si in the SiO2-P2O5 film was as high as 0.43, a level that was not attained by a premixing sol-gel method. The high phosphate group content and the ordered periodic mesostructure contributed to the high proton conductivity.
