Etching Rate and Mechanism of Doped Oxide in Buffered Hydrogen Fluoride Solution

Abstract

Etching rates of borosilicate, borophosphosilicate, phosphosilicate, and arsenosilicate glass films in various buffered hydrogen fluoride solution (BHF) have been studied. Etching rates were found to depend on the glass composition and dopant concentration as well as the concentration of ammonium fluoride in the BHF. This phenomenon is clearly different from the etching rate of thermal SiO2 oxide which is hardly affected by the concentration of ammonium fluoride in excess of equimolar to the HF concentration. The etching rates of the doped films may sometimes be higher than that of a nondoped SiO2 film but the rate is reduced when certain other dopants are present. This phenomenon is attributable to valence differences between silicon and the dopant elements. If the doped film is electrophilic compared to silicon, the etching rate is higher; if it is nucleophilic, the etching rate is lower. By controlling the type and concentration of the dopant and the concentration of NI-I~ in BHF, it is possible to etch the doped film and thermal oxide at either the same rate or selectivity with respect to one another.

Wet chemical etching of silicate glasses in hydrofluoric acid based solutions

“…Recent studies of the etching in HF solutions buffered with NH4F [16,17] also report the relation in Equation 5 at low NH4F concentrations.…”

“…The dissolution mechanism, in particular the role of the various fluorine-containing species, has been studied in more detail by Judge [6], ProkopowiczPrigogine [9][10][11], Kline and Fogler [12,13], Spierings [14], Kikuyama et al [15,16] and Proksche et al [17]. The most detaikM model has been presented by Prokopowicz-Prigogin~e~ [ll], in which the adsorption processes of HF molec'Me~s, HF~ and H + ions determine the reaction rate.…”

“…Etching solutions prepared by mixing a concentrated 40 wt % NH4F solution with a concentrated 49 wt % HF solution in various ratios (usually from 6:1 to 10:1) are used extensively for etching SiO2 and doped SiOa thin films in IC processing (see Section 5.2) [15][16][17]. They are often referred to as buffered oxide etches (BOEs) or buffered HF (BHFs).…”

Solubility and dissolution rate of silica in acid fluoride solutions

“…He also suggested that at higher total fluoride concentrations, the polymeric HF species of the form H n F nþ1 À may be involved in the dissolution reaction. Kikuyama et al (1992) and Proksche et al (1992) suggested similar dissolution mechanisms involving HF and HF 2 À . Spierings (1993) and Spierings and van Dijk (1987) concluded that the dissolution of silicate glasses in aqueous hydrofluoric acid solutions is controlled by the adsorption of HF and HF 2 À and the catalytic action of H + ions.…”

“…Dissolution rates in pure water predicted by Rimstidt and Barnes (1980) are shown by the tic marks on the right axis. Kikuyama et al, 1992;Kline and Fogler, 1981;Knotter, 2000;Osseo-Asare, 1996;Prokopowicz-Prigogine, 1989;Proksche et al, 1992;Spierings, 1993;Spierings and van Dijk, 1987;Verhaverbeke et al, 1994) or ab initio quantum mechanics models (Hoshino and Nishioka, 1999;Kang and Musgrave, 2002;Trucks et al, 1990).…”