H. Okano

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.

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.

Contrast enhancement pattern transfer etching of phosphorus-doped polycrystalline silicon

Pattern transfer etching of heavily phosphorus‐doped polycrystalline silicon has been developed using a KrF excimer laser through a 1:1 reflective optics and a new contrast enhancement scheme employing methylmethacrylate. With laser power at more than 0.5 mJ/cm2 and Cl2 gas as etchant, etching of n+ poly‐Si proceeded in the illuminated (bright) region, while the dark region was protected from etching by a polymer film formed from methylmethacrylate. As a result, a 0.9‐μm resolution pattern has been achieved.