Laser-Assisted Etching of Silicon in Hydrofluoric Acid

Laser-assisted wet etching (LAWE) has been considered for direct, non-lithographic creation of fine line patterns in microelectronics but not applied because of poor etch profiles. Two novel etching methods, laser-assisted aerosol jet etching (LAAJE) and laser-liquid film etching (LLFE), were investigated and developed in this study. The LAAJE process is not suitable for systems in which an electrochemical mechanism prevails while LLFE shows a profile improvement over LAWE technique. LLFE offers attractive possibilities for a high degree of anisotropic etching without causing radiation damage. Successful development of LAAJE and LLFE required a detailed understanding of the nature of laser-induced reactions and their effects on the properties of materials. In this study, silicon was etched with the 514.5 nm line from an argon-ion laser in the presence of an aqueous solution of hydrofluoric acid (HFA). A theoretical analysis of LAWE was performed and a 2-D mathematical model of LAWE was developed. The theoretical and mathematical results were confirmed by experimental results of the effects of laser fluence, exposure time, doping level, conductivity type, and etchant concentrations on etch rates of various n- and p- type silicon samples. Photovoltage and cathodic current measurements gave information that led to determination of the number of electrons consumed for hydrogen evolution and the quantum yield for silicon oxidation. From these measurements, the photocurrent doubling phenomenon was confirmed and an electrochemical mechanism of the reaction between silicon and HFA under illumination with photon energy larger than the band gap of Si was proposed. In this study, the results of direct patterning on n-Si(100) by the LLFE technique are presented. V-groove profiles with line widths of 20 to 50 mu m were obtained experimentally when the focused laser beam spot size was 1.5 mum. This orientation dependent etching results from the differential etch rate of the various crystallographic planes. The ratio of the etch rate in the < 100 > direction to the < 111> direction in Si, R(100)/R(111), was determined to be 1.94 in this study. The large etch profile of Si was due to the large hole diffusion length of Si and the small value of R(100)/R(111). For smaller etch geometries, and vertical profiles, III-V compounds with small values of hole diffusion length coupled with the use of short wavelength laser beam should be investigated.