Etch Resistance Of Focused-ion-beam-implanted SiO/sub 2/

Abstract

The etch rate of CVD Si02 in buffered hydrofluoric acid (BHF) is reduced after implantation with a 15 to 25 keV gallium focused ion beam (FIB). This effect is of interest for direct-write patterning of submicron Si02 features for semiconductor strain-confined quantum wires, quantum dots, for impurityor vacancy-induced disordering in optoelectronic integrated circuits and for fabrication lithography for other devices. Direct write into Si02 eliminates several lithographic steps and, if combined with dry etch development techniques, is of interest for use in multi-chamber all-vacuum processing.1 This approach also minimizes implant damage in the semiconductor sample. We investigated the effect of gallium ion dose on the etch rate of CVD Si02 in 2: 1,7: 1, and 20:l BHF. Regions of 5x70 pm2 area on these samples were implanted with the Micrion 808 FIB using a 0.15-pm FWHM beam. The dose was varied from 0.01 to 0.16 nC/pm2. The samples were etched in BHF, rinsed in deionized water, and profiled using a Tencor Alpha-Step 200 profilometer. The thickness of the unimplanted Si02 and the step height, or contrast, of the implanted region relative to the unimplanted region were measured as a function of etch time.The data obtained with a 7: 1 BHF etchant show the step height is a function of implant dose, with an optimum dose of about 0.04 nC/pm2 = 2 . 5 ~ 1 0 ~ ~ ions/cm2 (see Fig. 1). The effect of implantation is essentially to delay the onset of the normal etch rate. An optimum dose exists because of two competing effects: f i i t , a dose-dependent substantial reduction of the etch rate in the thin (hundreds of Angstroms) implanted Si02 layer and second, FIB milling removal of the implanted SiO2. The transition between reduced and normal etch rates was also investigated using other strengths of BHF solution at an FIB energy of 25 keV. 20:l BHF is shown in Fig. 2 Comparable step heights were observed for a 15 keV FIB energy. The optimum-dose Si02 step heights for the 25 keV data were 985, 1625, and 3500 Angstroms for 20:1,7:1, and 2: 1 BHF, respectively. This phenomenon appears similar to etch rate reduction found in boron doped glass (BSG) etched in 1O: l BKF 2. Gallium oxides, if present, would be expected to have a corresponding effect.