Paper PS2-TuA4
Titanium Deep Etching for Medical Applications

Tuesday, October 29, 2013, 3:00 pm, Room 104 C

Titanium is a biocompatible material which is of great interest in the biomedical field and more especially for bio-MEMS, which have emerged recently. The fabrication of Ti platform based devices is adapted from micromachining techniques derived from microelectronics technologies. Most of the research work reported in the literature relies on a Cl₂/Ar chemistry to deep etch titanium, using masks like TiO₂, Ni or even SU8. Processes are performed at room temperature of the substrate with typical etch rates close to 1 µm/min. They provide rather smooth surfaces.

We report here the performances of deep titanium etching with SF₆ and Cl₂ based chemistries in an ICP etching tool. Samples are pieces of a patterned titanium wafer glued on a silicon carrier wafer. Mask is a 5 µm thick nickel layer.

Our preliminary results have shown that either a SF₆ plasma or a Cl₂/Ar plasma can be used separately to etch titanium. A SF₆ plasma helps to reach etch rates as high as 4 µm/min at higher pressures (a few Pa) but profiles are isotropic. A Cl₂/Ar chemistry at low pressure (almost 1 Pa) is preferred to get vertical sidewalls but the etch rate is reduced. This is why we proposed to mix these two chemistries to increase the etch rate while keeping vertical sidewalls. However, this process is not reproducible in our conditions since, in most cases, it leads to a very high roughness and a drop in the etch rate.

This non-reproducibility may be due to the use of a silicon carrier wafer: SiCl_x species, coming from the etch by-products of the silicon wafer, may participate to one passivation layer growth on the Ti surface as well as the chamber walls. This induces a micro-masking effect which subsequently leads to the formation of an unwanted roughness.

A SF₆ plasma appears to be a good way to remove (at least partially) the layer inducing roughness, which hence leads to a better reproducibility. As the process presented previously already contains SF₆, but is not reproducible, this means that the balance between etching and deposition becomes favorable to deposition from one process to another. This is why both the SF₆ injection and the whole process have been optimized to overcome this issue. It was then possible to etch 300 µm of Ti within nearly 3h30. Consequently, 300 µm have been etched with an average etch rate of 1.4 µm/min. The slope is slightly negative on the first half of the ring and tends to be more isotropic on the second half. A significant undercut can also be observed (a few 10s of µm). The estimated selectivity to the nickel mask is 35. The process has been repeated many times and the profiles were always reproducible, which is a significant improvement.