Yukihiro Shimogaki

Cu Wettability and Diffusion Barrier Property of Ru Thin Film for Cu Metallization

The main issue of Cu metallization is the electromigration of Cu through the interface between Cu and the barrier or capping layer. To improve electromigration resistance at the Cu and barrier metal interface, insertion of a glue layer which enhances the adhesion of Cu onto the under layer may be effective. The wettability of Cu on Ru and Ta glue layers was evaluated as the index of Cu adhesion strength onto glue layers. The wetting angle of Cu (43°) on a Ru substrate was three times lower than that of Cu (123°) on a Ta substrate after annealing. Lower wetting angle of Cu on a Ru substrate indicates a good adhesion property between Cu and Ru and may imply a high electromigration resistance. The better Cu wettability of Ru compared to Ta can be explained by the concept of lattice misfit. A Ru(002) plane has lower lattice misfit, which suggests lower interface energy, and enhanced the adhesion of Cu onto Ru. However, the Ru film showed poor Cu diffusion barrier properties, which suggests Ru should be used as a glue layer in combination with another barrier layer.

Preparation of low-dielectric-constant F-doped SiO2 films by plasma-enhanced chemical vapor deposition

The delay due to the dielectric constant of an interlayer film results in the limited performance of very large-scale integrated circuits (VLSI). One solution to this problem is the use of a low-dielectric-constant interlayer film such as F-doped SiO2. We were able to obtain F-doped SiO2 films with dielectric constants as low as 2.3 and good step coverage by adding CF4 to SiH4/N2O plasma-enhanced chemical vapor deposition (PECVD). Our study focuses on the mechanism of the decrease in the dielectric constant and that of process improvement. It appears that a decrease in the dielectric constant is due to the decrease in the ionic polarization. The change in the Si–O stretching mode due to CF4 addition seems to be the most important factor in the decrease in the dielectric constant. The improvement of the step coverage and the decrease in the film growth rate are due to the decrease in the sticking probability of the film-forming species.

Surface Reaction Kinetics in Metalorganic Vapor Phase Epitaxy of GaAs through Analyses of Growth Rate Profile in Wide-Gap Selective-Area Growth

Wide gap selective area growth was proposed as a probe for obtaining information of surface kinetics in the metalorganic vapor phase epitaxy (MOVPE) of GaAs. GaAs was grown over patterned substrates using a SiO2 mask. The characterized growth-rate enhancement profiles for several total pressures were successfully fitted with two-dimensional simulation, taking only gas phase diffusion into account, except for the profiles in the vicinity of the mask edge, which seemed to reflect surface diffusion. The only parameter governing the profile was D/ks, the ratio of the gas-phase diffusion coefficient (D) to the surface reaction rate constant (ks) of a film precursor. The value of D can be estimated using the Chapman–Enskog kinetic theory, the accuracy of which was validated by observing a reactor-scale growth-rate profile under the mass-transfer-limited regime, and thus we obtained the value of ks. The sticking probability of a precursor, which was converted from ks, ranged between 0.06 and 0.33 for the temperature range of 823 to 973 K. The values are smaller than unity which was often assumed in growth simulations. The value of ks was measured as a function of growth temperature. Its activation energy changed at around 873 K: 8 kJ/mol for higher temperature and 121 kJ/mol for lower temperature. This information is important not only in the design of a selective-area growth process but also for constructing a more realistic reaction model of MOVPE.

Chemical reaction engineering in the design of CVD reactors

The current status of modeling on CVD processes to produce thin films is summarized in this review. The experimental methodologies for extracting the essential chemistry in CVD reactors had been developed to efficiently design CVD reactors. Tubular wall deposition method provides information on the rate limiting step of the deposition process and the rate constants. Molecular size of film forming species can be estimated from the deposition rate profile in the tubular reactor if the process is limited by the gas phase diffusion. Microcavity-deposition method provides the tool to analyze the surface reactions. The microcavity-deposition method can be combined with macrocavity-deposition method to extract the important reaction pathways like as the intermediate deposition. The use of computer chemistry is also useful in detecting key reactions and it may be used to construct new deposition chemistry. The computer fluid dynamics coupled with the chemistries obtained from these approaches will be a powerful tool to design CVD reactors.

Shortest intersubband transition wavelength (1.68 μm) achieved in AlN/GaN multiple quantum wells by metalorganic vapor phase epitaxy

Two-hundred-period high-quality AlN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy are studied using high-resolution x-ray diffraction, transmission electron microscopy, and optical transmission spectroscopy. Excellent interfaces of the MQWs are confirmed by these measurements. The strong intersubband absorption peak at a wavelength of 1.68 μm is achieved for AlN (1.6 nm)/GaN(1.7 nm) MQW. The full width at half-maximum of the absorption peak is estimated to be 27 meV.

X-ray photoelectron spectroscopic characterization of the adhesion behavior of chemical vapor deposited copper films

Adhesion behavior of chemical vapor deposited (CVD) copper (Cu) films on various barrier substrates was studied using a pull-off test. The mean adhesion strength of Cu films on air-exposed physical vapor deposition (PVD) Ta, air-exposed CVD TiN, in situ PVD TiN, and in situ CVD TiN were determined to be 3.42, 4.74, 4.64, and 10.24 MPa, respectively. The adhesion of Cu films on the in situ PVD TiN exhibited almost the same strength as the adhesion on the air-exposed CVD TiN. Root-mean-squared (rms) roughness values for air-exposed CVD TiN and in situ PVD TiN are 8.06 and 1.18 nm, respectively. The rougher surface morphology of the air-exposed CVD TiN films was considered to improve adhesion due to mechanical anchoring. The adhesion strength of the Cu film on the in situ CVD TiN was two times higher than that on the air-exposed CVD TiN. In order to examine the adhesion difference, x-ray photoelectron spectroscopy (XPS) measurement was performed on the exposed interface after pulling off the Cu film. The XPS...

Conformal Deposition and Gap-Filling of Copper into Ultranarrow Patterns by Supercritical Fluid Deposition

Supercritical fluid deposition (SCFD) of Cu onto ultranarrow vias (50 to 220 nm and 1 µm depth) was studied with using angled polishing for future ultralarge scale integration metallization. SCFD conformally fabricated a smooth, continuous, and 10-nm-thick Cu film in ultranarrow vias. Excess H2 compared with the precursor as well as surface saturation of the precursor enabled uniform nucleation and conformal deposition. Highest H2 concentration in this study (0.39 mol/L) promoted the nucleation density, resulting in formation of a smooth and continuous film. In conclusion, SCFD successfully achieved complete filling without any voids onto via patterns.

Reduction Mechanism in the Dielectric Constant of Fluorine‐Doped Silicon Dioxide Film

One solution to signal delay in very large-scale integrated circuits is to use a low dielectric constant interlayer film, such as F-doped silicon dioxide (SiO 2 ). By adding CF 4 to SiH 4 /N0 plasma-enhanced chemical vapor deposition, we obtained F-doped SiO2 films with a dielectric constant as low as 2.6. We studied the mechanism behind this decrease in the dielectric constant by estimating the constants due to each polarization component (ionic, electronic, and orientational) using capacitance-voltage (C-V) measurements, Fourier transform infrared spectroscopy spectra combined with the Kramers-Kronig relation, and spectroscopic ellipsometry. The Kramers-Kronig calculations showed that ionic polarization decreased with increasing CF 4 concentration, whereas the electronic polarization remained almost constant. However, the low dielectric constant obtained by the C-V measurements could not be completely explained by the reduction in ionic and electronic polarizations but probably resulted from a decrease in the orientational polarization. Orientational polarization may be caused by Si-OH bonds and is mainly decreased by adding CF 4 . Therefore, an effective way to reduce the dielectric constant in F-doped SiO 2 films is to reduce the orientational polarization.

Diffusion barrier property of TiN and TiN/Al/TiN films deposited with FMCVD for Cu interconnection in ULSI

Abstract Flow modulation chemical vapor deposition (FMCVD) with titanium tetrachloride (TiCl4) and ammonia (NH3) is effective for depositing titanium nitride (TiN) films with conformal morphology, good step coverage, low electrical resistivity, and low chlorine residual contamination. It means that FMCVD TiN film is a good candidate of diffusion barriers for copper interconnection technology in ULSI. But the diffusion barrier property of FMCVD TiN film against Cu diffusion has not been confirmed. So, firstly, we deposited Cu (100 nm)/FMCVD TiN (25 nm)/Si multilayer films and investigated the thermal stability of Cu/TiN/Si structure. Vacuum annealing was done at 400, 500, 550 and 600 °C. For films annealed for 30 min at 400 °C, Cu diffused through the TiN layer and formed copper silicides on the surface of Si substrates. Therefore, FMCVD films formed under such conditions are unsatisfactory diffusion barriers. To enhance the diffusion barrier property of FMCVD TiN films, we used sequential deposition to introduce a monolayer of Al atoms between two TiN films. Etch-pit tests showed that for TiN films with Al interlayer, Cu diffusion through the barrier occurred at 500 °C and that is 100 °C higher than TiN film without Al interlayer. Al atoms formed AlOx with oxygen atoms present in the TiN films as impurities, and fill up the grain boundaries of TiN film, thereby blocking the diffusion of Cu atoms.

Kinetic studies on thermal decomposition of MOVPE sources using fourier transform infrared spectroscopy

A kinetic study was done on the decomposition of source materials used in metalorganic vapor phase epitaxy (MOVPE) such as trimethylgallium, trimethylindium, tertiarybutylarsine, tertiarybutylphosphine and dimethylzinc. The purpose of this study was to construct reaction models with accurate rate constants, which are required for the numerical analysis of MOVPE process and computer assisted process optimization. For the measurements we employed a quartz-tube cracking reactor and Fourier transform infrared spectrometer (FT-IR) as the gas monitoring system. First, the decomposition rate of each source was measured and the ability of the system to determine source gas decomposition rates was validated. Next the effect of substrate surfaces on the decomposition rates and the effect of gas mixing were examined. We observed that surface reaction rates were not negligible in the decomposition of some sources, and that the decomposition rates of group III sources increased when they were mixed with group V sources. The results of this study showed that the effect of substrates and gas mixing need to be properly accounted for in numerical models to accurately simulate epitaxial growth process.