Etch mechanism of an Al2O3 hard mask in the Bosch process

Abstract

The etching of high aspect ratio structures in silicon via the Bosch process is essential in modern technologies such as microelectromechanical systems (MEMS) and through‑silicon vias (TSV) fabrication. The process can be very demanding on the mask selectivity due to long etching times, and it has been shown that an Al 2 O 3 hard mask is very suitable in this regard, as it offers significantly higher selectivity compared to the conventional SiO 2 or resist masks. In this work, we employ a combination of Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and X-Ray Photoelectron Spectroscopy (XPS) depth profiling to scrutinize the Al 2 O 3 mask etching mechanism and therefore the origin of the extraordinary high selectivity. We demonstrate that by increasing the passivation step time, a thicker fluorocarbon polymer layer is formed on the Al 2 O 3 , and Al 2 O 3 is then removed with a minuscule average etch rate of ~0.01 nm/min. XPS depth profiling reveals that during Deep Reactive Ion Etching (DRIE) using the Bosch process, an AlF x layer is formed between the polymer and Al 2 O 3 . As AlF x is non-volatile, it requires sputtering to be removed. If the polymer layer is thick enough to attenuate the incoming ions such that their energy is not sufficient to lead to desorption of AlF x , such as when using a longer passivation time, the mask is not eroded. By investigating the surface after different amounts of DRIE cycles, we also obtained information about the formation rate of AlF x and the changes in the Al 2 O 3 and polymer thicknesses over the course of a DRIE process. These findings further expand the knowledge of DRIE and can help process engineers to tailor the processes accordingly. • Selectivity of an Al 2 O 3 mask in the Bosch process becomes practically infinite with longer passivation step time • XPS depth profiling reveals that an AlF x layer is formed during DRIE between Al 2 O 3 and the fluorocarbon polymer on top • This non-volatile AlF x cannot be removed by sputtering if the polymer is thick enough, i.e. with longer passivation time • The dynamics of AlF x formation during DRIE was observed with time resolved measurements