Alternated process for the deep etching of titanium

Abstract

Titanium is increasingly used as a platform material in microdevices dedicated to biological and bio medical applications. Existing processes for titanium deep etching use a chlorine based chemistry. This paper reports on a low reproducibility for such chemistries when titanium samples are glued onto a silicon carrier wafer. In this case, a SiOCl layer redeposits on the chamber walls as well as on the sample surface. This leads to a decrease of the etch rate and the formation of a very high roughness with a similar morphology as black silicon. The alternated process for the deep etching of titanium (APETi) described in this paper has been designed to improve the overall reproducibility by preventing high roughness formation. It is a time-multiplexed process where Cl2/Ar plasma steps are alternated with SF6 plasma steps. The first step aims at etching with vertical walls (anisotropy) while the second aims at reducing the roughness by removing SiOCl from the sample surface.

Ultrahigh Resolution Titanium Deep Reactive Ion Etching

“…It is also conceivable that sidewall passivation may arise from the deposition and oxidation of reaction products produced by the concurrent etching of the Si carrier wafer. Evidence for this supposition can be found in the study by Tillocher et al, which showed significant micromasking in Cl 2 -based Ti DRIE of chip-scale samples mounted on larger Si carrier wafers. The authors suggested that this may have been caused by the incomplete removal of a SiOCl-based passivation film produced by the oxidation of SiCl x reaction products deposited on the trench floors.…”

“…This process enabled fabrication of HAR structures with MFS as small as 750 nm in bulk Ti substrates, with smoother sidewalls, higher etch rate (2 μm/min), and comparable mask selectivity (40:1 Ti/TiO 2 ) . Since then, a number of other processes for Ti DRIE have been reported by others. − Collectively, these techniques have begun to enable the exploration of Ti MEMS in a wide variety of applications, including microneedles for transdermal and ocular drug delivery, , vascular stents, neural prosthetic interfaces, , resonant sensors, and microfluidic devices for dielectrophoretic particle manipulation, biomolecular network studies, , thermal ground planes, and photocatalytic microreactors …”

Reactive ion etching for fabrication of biofunctional titanium nanostructures

“…This is because the etching profile created using SF 6 is isotropic, making it unsuitable for solar and microelectronic applications ( i.e . the most common areas of applications for such profiles) 50 . Furthermore, the resulting nanostructures of the Cl 2 gas have vertical sidewalls and smoother surfaces 51 .…”

“…Ar is one of the gasses that is widely used to etch titanium due to its high selectivity against TiO 2 , SU8, and Ni that are suitable masks to etch titanium for solar cells and MEMS applications 50 . Furthermore, some studies have shown that the existence of inert gasses during the etching process leads to process uniformity and improves plasma stability without changing the chamber pressure 43,52 .…”

Combined analysis methods for investigating titanium and nickel surface contamination after plasma deep etching

“…The etch rate is much higher than that of the previous method, 2.2 μ m.min −1 instead of 0.5 μ m.min −1 , and the process allows vertical sidewalls to be obtained without scalloping. Furthermore, the Alternated Process for the deep Etching of Titanium (APETi) developed at the GREMI laboratory in 2013 by Tillocher et al on an ICP reactor, alternating a Cl 2 /Ar plasma and an SF 6 plasma, has shown its ability to deep etch titanium effectively by reducing the roughness at the etch front as well as improving the reproducibility 7,8 . A plasma combining both chlorine and fluorine chemistries has been reported in the literature to efficiently etch titanium 9 .…”