Nobuo Hayasaka

Water absorption properties of fluorine-doped SiO2 films using plasma-enhanced chemical vapor deposition

The water absorption properties of a PE-CVD (plasma-enhanced chemical vapor deposition) fluorine-doped SiO2 film with a low dielectric constant were studied. It was concluded that highly stable F-doped SiO2 film was obtained at F contents from 2.0% to 4.2% (3.2≤k≤3.6) using high-density plasma CVD. However, at F contents higher than 4.2% (k<3.2), the amount of water absorption was markedly increased due to the presence of Si–F bonds, such as Si(–F)2 bonds, which are highly reactive with water. On the other hand, water absorption was observed at every F content for conventional plasma CVD films. Through gas phase component analysis and investigation of the incident ion energy distribution using a quadrupole mass spectrometer, it was confirmed that a high efficiency of gas dissociation and high-energy ion bombardment are the keys to obtaining high-quality films with a high resistance to water absorption.

Damage‐free selective etching of Si native oxides using NH3/NF3 and SF6/H2O down‐flow etching

Damage‐free selective etching of Si native oxides against Si has been achieved by NH3/NF3 and SF6/H2O down‐flow etching. In the NH3/NF3 etching, the wafer was covered with a film, and after its removal by heating above 100 °C, only SiO2 was found to be etched with an extremely high selectivity with respect to Si. Selective etching of Si oxides has also been obtained for SF6/H2O microwave discharge. In this case, a film of liquid solution containing HF and H2SOx is considered to form on the wafer surface. The selective etching of SiO2 takes place by the dissolved HF just as in the wet etching by an HF solution. The mechanisms of these selective reactions are discussed in detail based on the covalency of Si and SiO2 bondings.

Excimer-laser etching on silicon

Studies have been made of poly- and single Si etching induced by excimer-laser irradiation of the silicon surfaces in halogenated gases. Etching was investigated for different conduction types, impurity concentrations and crystallographic planes. Chlorine atoms accept electrons generated in photoexcited, undoped p-type Si, thus becoming negative ions which are pulled into the Si. However, the n+-type Si is etched spontaneously by Cl− as a result of the availability of conduction electrons. Fluorine atoms, with the highest electronegativity, take in electrons independent of whether the material is n- or p-type. And thus, the easy F− ion penetration into Si causes spontaneous etching in both types. New anisotropic etching for n+ poly-Si is investigated because of its importance to microfabrication technology. Methyl methacrylate (MMA) gas, which reacts with Cl atoms, produces a deposition film on the n+ poly-Si surface. The surface, from which the film is removed by KrF (5 eV) laser irradiation, is etched by Cl atoms, while the film remains on the side wall to protect undercutting. However, with the higher photon energy for the ArF (6.4 eV) laser, the Si-OH bonds are broken and electron traps are formed. These electrontrapping centers are easily annealed out in comparison to the plasma-induced centers. Pattern transfer etching for n+ poly-Si has been realized using reflective optics. The problems involved in obtaining finer resolution etching are discussed.

Synchrotron radiation-excited etching of SiO2 with SF6 at 143 and 251 Å using undulator radiation

Photoexcited etching of SiO2 surface with SF6 gas is studied using undulator radiation at 143 and 251 A as an extreme ultraviolet light source. The SF6 pressure and the wavelength dependences of the etch rates have been measured for SiO2 in the pressure region between 0.016 and 0.50 Torr. We find that, at these wavelengths, the etch rate is proportional to the intensity of the light absorbed by the surface species, most probably SiO2 in the pressure region studied.

Synchrotron radiation-assisted etching of silicon surface

The photo-assisted etching of heavily phosphorous-doped polycrystalline silicon by chlorine was studied using synchrotron radiation as an extreme ultraviolet (EUV) light source. The quantum yield for the removal of the Si atoms at a chlorine pressure of 0.3 Torr was found to be about 0.5% photon-1 using the Ti-filtered light, which is mostly in the EUV region, 1-20 nm. Formation of electronically excited Cl+ ions upon EUV irradiation was confirmed by emission spectroscopy. Negative bias applied to the Si crystal was found to increase the etch rate.

Silicon interposer technology for high-density package

The achievement of rapid advances in integration density and performance of LSI devices is predicated on increasing the total number of Input/Output (I/O) and Power/Ground (P/G) terminals, which, in turn, leads to shrinking design rule of wiring and bump pitch on the organic substrate of a flip-chip package. However, decreasing the bump pitch and wiring rule raises the process cost of fabricating organic substrate. Moreover, it is difficult to obtain highly reliable connections between chip and organic substrate with smaller bumps due to the mismatching of the coefficient of the thermal expansion (CTE). To overcome these problems, a new interposer using silicon (Si) substrate with through plug is developed.

Properties of high-performance porous SiOC low-k film fabricated using electron-beam curing

In this paper, we describe the effect of electron-beam (EB) curing on ultra-low-k dielectric porous SiOC material (k=2.2) and the application of this technology to the 90-nm-node Cu/low-k multilevel damascene process. A significant improvement of dielectric porous SiOC films with EB curing has been demonstrated. The mechanical and adhesion strength of these films were increased by a factor of 1.5–1.6 without degrading the films k. This result can be explained by the reconstruction of a Si–O random network structure from cage Si–O bonds and Si–CH3 bonds through EB curing. Additionally, the EB curing of spin-on dielectric (SOD) porous low-k films contributes to a decrease in their curing temperature and a decrease in their curing time. Under optimum EB curing conditions, no degradation of transistor performance was revealed. The excellent adhesion strength obtained by EB curing, has contributed to the success of multilevel damascene integration. On the basis of our findings, this EB curing technology can be applied in devices of 65-nm-node and higher.

Beam induced deposition of an ultraviolet transparent silicon oxide film by focused gallium ion beam

We have deposited a silicon oxide (SiOx) film with a high optical transmittance in the DUV region by a focused ion beam induced deposition technique using a gallium ion beam and a mixture of oxygen and TMCTS(1,3,5,7‐tetramethylcyclotetrasiloxane) as a source gas. The optical transmittance of a 0.3 μm thick film is higher than 90% at the wavelength of 250 nm. The transmittance of the deposited SiOx film depends on both the source gas and ion beam irradiation conditions. A scaling to explain the transmittance along with the ion beam conditions is proposed.

Photo‐assisted anisotropic etching of phosphorus‐doped polycrystalline silicon employing reactive species generated by a microwave discharge

An anisotropic etching of heavily phosphorus‐doped polycrystalline silicon is achieved by protecting the etched sidewall with polymerized film which results from the reaction of chlorine species and methyl methacrylate. Chlorine species are generated by a microwave discharge in Cl2 in the portion separated from a reaction chamber with a laser beam irradiation system. The laser beam enhances the removal of the polymerized film on the illuminated surface except the sidewall. And the Si/SiO2 etch rate ratio is infinite in this system.

Structural Studies of High-Performance Low-k Dielectric Materials Improved by Electron-Beam Curing

With the use of a newly developed electron beam (EB) curing process, an advanced methylsilsesquioxane (MSQ) low-k dielectric (LKD) film of k=2.9 was developed. It is noteworthy that the EB curing process can drastically improve the mechanical strength of LKD film and reduces the thermal budget without increasing the k value. The X-ray absorption fine structure (XAFS) study on the LKD was conducted to clarify the structural change upon EB curing. The structure of the film was compared with those of two different types of other MSQ films, the ladder-network structure and the random-network structure, and a chemical vapor deposition (CVD) film. The Si–O–Si bond angle and Si–O (Si–C) bond length were determined by fitting the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra. Si–O–Si bond angle of LKD film was found to be between those of the ladder and the random structure, which are 135° and 147°, respectively. The X-ray absorption near-edge structure (XANES) spectra of LKD film revealed two broad features corresponding to a mixture of the two structures. In contrast, Si–O–Si angles of the EB-cured LKD film and the CVD film were similar, and the XANES features of both films were almost identical with those of the random structure. The electronic structure as determined from XANES spectra was also discussed by comparing three-dimensional-linkage models obtained by ab initio calculations. We confirmed that the EB curing process of LKD film causes a drastic structural change. The change from the mixture of ladder and random structures to the completely random structure was caused by C–H bond breaking followed by the formation of new polymer-like clusters with C–C bonds.