TABLE OF CONTENTS (Edited By K.R. Williams)
K.R. Williams
July 1998
Our emphasis in the Datareviews in this chapter is on technologically important etches used in microelectronics fabrication, micromachining and materials characterization.
Wet single-crystal silicon etchants can be divided into isotropic etchants, which etch at nearly the same rate in all directions, and orientation-dependent etchants, which etch at rates that vary with the crystallographic direction. The latter will be discussed in the next Datareview.
The most popular etchant for isotropic wet etching is the HF + HNO 3 + diluent system. In these solutions, etching proceeds in two steps: silicon is oxidised by HNO 3, followed by etching of the oxidation product by HF. These two acids are usually diluted by either H 2O or CH 3COOH. The advantages of a CH 3COOH diluent have alternatively been given as preventing dissociation of HNO 3 (undissociated HNO 3 is the primary oxidant) [1,2] and as producing better wetting and therefore smoother etched surfaces [3,4]. We refer to any combination of these reagents as 'HNA' for hydrofluoric-nitric-acetic.
At low HF concentrations, the etch rate is limited by the etching of the oxide, which is typically limited by diffusion of HF to the reaction site above room temperature (the transition point depends on the concentrations) [5]. This diffusion-limited process has a lower activation energy (around 0.18 eV) and therefore a weaker dependence on temperature. Because the etch is...
