Shun Momose

Microdischarge Optical Emission Spectroscopy as a Novel Diagnostic Tool for Metalorganic Chemical Vapor Deposition of (Ba,Sr)TiO3 Films

Microdischarge optical emission spectroscopy (µD-OES) was developed as a diagnostic tool for analyzing chemical reactions in metalorganic chemical vapor deposition of (Ba,Sr)TiO3 films. The degree of thermal decomposition of the CVD source molecules was obtained from the observed emission spectra of the small plasma excited at the OES sensor head. According to the observation of SrII and BaII emission lines, Sr(DPM)2 and Ba(DPM)2 decompose at the gas temperatures of 270°C and 280°C, respectively. The temperature dependence of the observed emission intensity showed a behavior similar to that of the deposition rate. The oxidation of the CVD source molecules in the gas phase was also investigated by observing the change in the emission spectra due to the addition of oxygen gas. We also measured the spatial distribution of the source molecules by radially moving the OES sensor head, and compared it with the uniformity of the film thickness.

Effects of gas-phase thermal decompositions of chemical vapor deposition source molecules on the deposition of (Ba, Sr)TiO3 films: A study by in situ Fourier transform infared spectroscopy

We studied the thermal decompositions of Ti(t-BuO)2(DPM)2, Sr(DPM)2 and Ba(DPM)2 under actual chemical vapor deposition (CVD) conditions by in situ Fourier transform infrared spectroscopy (FT-IR). From the temperature dependence of the IR absorbance, we investigated the thermal stability of the chemical bonds in the source molecules. The obtained FT-IR data were correlated with the characteristics of the deposited BST films. Although Sr(DPM)2 and Ba(DPM)2 molecules decompose completely in the gas phase before Sr and Ba atoms are incorporated in the film, the deposition of Ti atoms does not require the complete decomposition of Ti(t-BuO)2(DPM)2 molecules in the gas phase. In this study, we discuss the effects of these thermal decompositions on the deposition mechanism of (Ba,Sr)TiO3 films.

Diagnosis of oxidation reactions in metalorganic chemical vapor deposition of (Ba,Sr)TiO3 films by in situ Fourier transform infrared spectroscopy

We studied the oxidation reactions of source Ti(t-BuO)2(DPM)2 and Sr(DPM)2 molecules during metalorganic chemical vapor deposition (MOCVD) by in situ Fourier transform infrared spectroscopy (FT-IR). We used O2 and N2O gases as oxidizing agents and investigated the difference in oxidation effect between the two gases. From measurements of IR absorption spectra, we found that O2 is more reactive than N2O in the gas phase, and selectively attacks low-electron-density sites in the source molecules. We deposited strontium oxide and (Ba,Sr)TiO3 films in O2 and N2O ambients, and investigated the relationship between the qualities of deposited films and the gas-phase reactions measured by FT-IR spectroscopy. In terms of the suppression of carbon contamination and the increase in the Ti/(Ba+Sr) atomic ratio, we found that O2 is more suitable than N2O for the deposition of (Ba,Sr)TiO3 films.

Reaction Mechanism of Alkoxy Derivatives of Titanium Diketonates as Source Molecules in Liquid Source Metalorganic Chemical Vapor Deposition of (Ba,Sr)TiO3 Films: A Study by In Situ Infrared Absorption Spectroscopy

The effect of the difference in the reaction mechanisms of the titanium source molecules on film quality was investigated in liquid source metalorganic chemical vapor deposition of (Ba,Sr)TiO3 (BST) films. The optimum deposition condition was significantly dependent on the alkoxy moiety of the mixed titanium alkoxide/diketonate compounds such as Ti(t-BuO)2(DPM)2 [bis(tert-butoxy)bis(dipivaloylmethanato)titanium] and Ti(i-PrO)2(DPM)2 [bis(iso-propoxy)bis(dipivaloylmethanato)titanium]. The liquid source delivery was found to be accompanied by local distortion and/or decomposition in the iso-propoxy moieties of Ti(i-PrO)2(DPM)2; no corresponding phenomena occurred in the case of Ti(t-BuO)2(DPM)2. In order to clarify the origin of this dependence, we analyzed the reaction mechanisms of the individual titanium sources by in situ infrared absorption spectroscopy. The chemical stability of the alkoxy moiety of the titanium source determines the reactivity of the whole molecule, which significantly influences on the crystallinity of the resultant BST films.

Film precursor formation in metalorganic chemical vapor deposition of barium strontium titanate films: A study by microdischarge optical emission spectroscopy

We investigated the gas-phase reactions of source molecules for metalorganic chemical vapor deposition (MOCVD) of barium strontium titanate (BST) films to study how the precursors for each element are formed from the source molecules in the gas phase. By microdischarge optical emission spectroscopy (µD-OES) and in situ infrared absorption spectroscopy under actual CVD conditions, we found that the addition of oxidation gas has more effects on the creation of Sr precursors than that of Ti precursors. Moreover, we deposited strontium oxide, titanium oxide and strontium titanate (STO) films to investigate the deposition mechanism of BST films from various angles. The results of these deposition experiments were consistent with the precursor formation mechanism suggested from the spectroscopic diagnoses.

Formation Mechanism of Strontium and Titanium Oxide Films by Metalorganic Chemical Vapor Deposition: An Isotopic Labeling Study Using 18O2

Isotopic labeling experiments using 18O2 were carried out to understand the decomposition and oxidation reactions of source molecules in the metalorganic chemical vapor deposition (MOCVD) of strontium and titanium oxide films. The isotopic ratios of oxygen incorporated in the deposited films were determined by time-of-flight secondary ion mass spectrometry (TOF-SIMS) in both positive and negative secondary ion detection modes. The obtained M18O+/M16O+ (M=Sr, Ti) ratios showed good agreement with the corresponding 18O-/16O- ratios. The oxygen incorporation from the oxidant gas (18O2) to the strontium oxide films is dominant under typical deposition conditions, while the majority of oxygen in the titanium oxide films originates from the ligands of the source molecules.

Effects of O2 Gas on Reaction Mechanisms in the Chemical Vapor Deposition of (Ba, Sr)TiO3 Thin Film

Effects of O2 gas on reaction mechanisms in the chemical vapor deposition (CVD) of (Ba, Sr)TiO3 [BST] film were studied by investigating the atomic incorporation rates of Ba, Sr, and Ti. The atomic incorporation rates were measured using an X-ray fluorescence method for BST film prepared for several molar source supply ratios with different O2 flow rates of 0.1, 0.5, and 1.0 slm. In the experiments, these rates were found to increase monotonically with increasing O2 flow rate in flux regions where incorporation reactions might be dominated kinetically. This suggested that the supplied O2 gas might affect the adsorption of film precursors onto the film surface in the CVD of BST. From the obtained experimental results, we proposed a CVD model, in which some precursors adsorbed on the film surface form adsorptive sites, and that successive precursors are adsorbed thereon. The supplied O2 gas contributes towards the formation of the adsorptive sites; the Ba and Sr precursors release their own oxygen and receive oxygen from the supplied O2 gas, and then the oxidized precursors form adsorptive sites. On the other hand, the Ti precursors form adsorptive sites by holding their own oxygen on the film surface. Then, under the proposed model, atomic incorporation rates and overall sticking coefficients for BST film depositions were numerically calculated. The calculated results were found to be in good agreement with the experimental results for several molar source ratios with different O2 flow rates of 0.1, 0.5, and 1.0 slm.

In Situ Infrared Spectroscopic Study on a Titanium Source in MOCVD

The optimum deposition condition in liquid source metalorganic chemical vapor deposition (MOCVD) of (Ba,Sr)TiO 3 films was significantly dependent on the chemical structures of titanium source molecules. To clarify the origin of this dependence, we investigated the reaction mechanisms of the individual titanium sources by in situ infrared absorption spectroscopy. A systematic study on the reaction mechanism of MOCVD source molecules was carried out using a novel titanium source molecule, Ti(MPD)(DPM) 2 [methylpentanediolbis(dipivaloylmethanato)titanium]. The obtained data on Ti(MPD)(DPM) 2 were discussed by comparing with those on the conventional titanium CVD sources such as Ti(t-BuO) 2 (DPM) 2 [bis (tert-butoxy)bis(dipivaloylmethanato)titanium] and Ti(i-PrO) 2 (DPM) 2 [bis(iso-propoxy)bis(dipivaloylmethanato)titanium]. Ti(MPD) (DPM) 2 is about as thermally stable as Ti(t-BuO) 2 (DPM) 2 and more stable than Ti(i-PrO) 2 (DPM) 2 . The thermal stability of the organometallic molecules as a whole is determined by the stability of the moieties other than the DPM ligands. The addition of O 2 have little influence on the temperature dependence of the infrared absorption of Ti(MPD)(DPM)2 in the gas phase under the actual CVD conditions. The competition between thermal decomposition and oxidation of titanium sources is also sensitive to the structures of their ligand groups other than DPM groups.