About determining reliable etching rates and the role of temperature in kinetic experiments on acidic wet chemical etching of silicon

Abstract

A characteristic of the wet chemical etching of silicon in concentrated HF–HNO₃ and HF–HNO₃–H₂SiF₆ mixtures is the release of a high reaction heat, without its numerical value being known. This liberated heat can lead to a significant temperature increase during the etching process, especially when the volume of etching solution provided is low. A noticeable increase in temperature not only leads to an increase in the etching rate, it simultaneously changes the concentrations of dissolved nitrogen oxides (e.g. NO, N₂O₄ and N₂O₃) and intermediary species (HNO₂), resulting in a change in the overall reaction process. The same parameters also influence the experimental determination of the etching rate. Further factors affecting the determination of the etching rate are transport phenomena as a result of the wafer positioning in the reaction medium and the surface properties of the Si used. As a result, etching rates determined from the mass difference of a silicon sample before and after etching are highly uncertain. This work describes a new method for the valid determination of etching rates using turnover-time curves that are calculated from the time-dependent temperature increase in the etching solution during the dissolution process. If only a slight increase in temperature is caused by the choice of proper reaction conditions, bulk etching rates representative for the etching mixture are obtained. Based on these investigations, the activation energy of Si etching was determined as a function of the concentration of the reactive species in the initial reaction step, the undissociated nitric acid (HNO₃, undiss). Based on a total of 111 investigated etching mixtures, a process enthalpy for acidic etching of Si was determined for the first time from the calculated adiabatic temperature increases. With a value of −(739 ± 52) kJ mol⁻¹, the determined enthalpy underlines the strongly exothermic character of the reaction.