Etching Kinetics of Silicon Dioxide in Aqueous Fluoride Solutions: A Surface Complexation Model

Abstract

A molecular-level model of Si02 dissolution in aqueous fluoride solutions is presented. The model recognizes the fact that the SiO,/water interface contains neutral hydroxylated surface groups (Si-OH) which can protonate to give positively charged sites (Si-OIlfl or deprotonate to give negatively charged sites (Si-0), depending on the pH. The adsorption of the fluoride ion, which is formulated as a surface ligand exchange reaction (Si-OH + HF = Si-F + EI,O), results in the polarization of the underlying Si-O bonds. The subsequent detachment of the surface Si-F complex constitutes the effective dissolution event. It is shown that the effects of aqueous phase variables (e.g., pH) on the dissolution rate are correlated with the effects of these same variables on the surface concentration of adsorbed fluoride. The model is compared with published experimental results, and it is demonstrated that a unified theory is obtained when the role of surface complexation is incorporated into the dissolution mechanism. The frequently reported observation of a decline in etching rate at elevated NH4F concentrations (and therefore high pH) is attributed to a competition between OW ions and F ions for adsorption sites. InfrocluclionRemoval of silicon dioxide films via dissolution in aqueous fluoride solutions is a key processing step in practically all silicon-based microfabrication technologies. The premier and most established field of application is the manufacture of integrated circuits, where silicon dioxide etching is exploited in wafer cleaning and in pattern delineation.'' A more recent application is in the fabrication of precision quartz-and silicon-based micromechanical structures, where chemical etching provides a convenient method for removing undesirable surface layers, for preparing thin quartz blanks, and for surface micromachining of the desired microstructures.6" It is a remarkable fact that, inspite of the tremendous technological importance of silicon chips, and the numerous publications already available on various aspects of aqueous chemical processing, the physicochemical details of the SiO,-H,O-HF reaction are still waiting to be unravelled."4' Reaction mechanisms have been proposed, but few attempts have been made to use these to derive appropriate rate equations. On the other hand, rate equations have been offered, but the connection between the associated rate parameters and physicochemical events at the solid/water interface has typically been left unresolved. Recent developments are however forcing a change from this empirical approach to etching kinetics modeling. The advent of ultralarge scale integration (ULSI), with its attendant dramatic reductions in device features, has drastically decreased the tolerance of microelectronic devices for surface contamination and microroughness."9'4' As a result, the ability to engineer ultraclean silicon wafers and to attain atomic-level control of etch rates has become a major agenda item for those engaged in the advanced processing of semiconductor device In th...

Effect of Hydrofluoric Acid in Oxidizing Acid Mixtures on the Hydroxylation of Silicon Surface

“…12B. 27 This Si-F bond induces polarization of underlying Si-O bonds leading to the detachment of the surface silicon atom ͑c͒ of Fig. 12B with the attack of H 2 O.…”

Solubility and dissolution rate of silica in acid fluoride solutions

“…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).…”

“…In our study, and all of those that used quartz grains, the grain surfaces displayed an ensemble of bonding geometries so that the measured rate is an average of the crystallographically different rates. Osseo-Asare (1996) proposed a surface complexation model that is based on his analysis of the Judge (1971) rate data. He suggests that the surface silanol groups (Si-OH) protonate and deprotonate depending on the pH to give positively charged >Si-OH þ 2 and negatively charged >SiO À sites, respectively.…”

“…One of the most important applications of HF solutions is in the semiconductor industry in the manufacture of integrated circuits, where it is used in silicon wafer cleaning and pattern delineation by silicon dioxide etching (Osseo-Asare, 1996). HF solutions are also used to chemically clean silicon semiconductor surfaces for selective growth of silicon and metallization, and this is achieved by rinsing the surface in a HF solution or by exposing the surface to a HF/H 2 O vapor that condenses on the surface (Kang and Musgrave, 2002).…”

Fabrication of three-dimensional stamps for embossing techniques by lithographically controlled isotropic wet etching

“…For the etching of structures with fine details, less concentrated HF solution ͑15 wt %͒ was used in order to lower the etching rate. 7 After PMMA and chromium mask removal in acetone and ammonium cerium ͑IV͒ nitrate, respectively, additional features can be obtained by further processing. We introduced for example at the surface of existing structures new structures by a focused Ar + beam at 30 kV acceleration voltage and a subsequent step of isotropic wet etching.…”