Etch selectivity and etch rate
The trial # 1 (SF6) versus the trial# 3 (CF4) in table1 at the coil power 500W and platen power at 550W, it was found that SF6 plasma provide a lower etching rate (~ 2.8 x) and a lower etch selectivity (~1.38 x) compare to CF4 plasma at the same setting process.
The huge different of the etching rate between these two trials can be explained via the using different kind of plasma. While the plasma was forming, it would contain the etchant ion species such as SFx+ (x = 0-5) positive ions and the important
negative ions including SF6-, SF5- and F- [9]. While CF4 plasma was forming, it should contain with positive CF4+, CF3+, CF2+, CF+, C2+, F+ and negative ion F-.
The authors believed that the CF4 plasma would contain much more positive ion species which were responsible for physical etching (sputtering) than the SF6 plasma.
So it could lead to the higher etching rate of CF4 plasma.
The higher etching rate will not occur only on AlTiC substrate but also on the metal NiCr mask. The amount of the NiCr removal in CF4 plasma is higher than in SF6 plasma about 2 times but the amount of the metal mask removal is still lower than the amount of AlTiC substrate removal (2.8 times) so eventually the using the CF4 plasma provides a higher etch selectivity about 1.38 times over than using the SF6 plasma.
Considering only in SF6 plasma environment between the trial # 1 and the trial # 2, the trial # 2 was increased Coil power ~ 3 times (intended to increase chemical etching) and increased platen power ~ 2 times (intended to increase physical etching). The result showed that the etch rate of both metal mask and substrate are increased with ~ 2.27 times for metal mask and ~ 1.45 times for AlTiC substrate.
The etch rate of metal mask is higher than the etch rate of AlTiC substrate. It should relate to the hardness of both materials, the NiCr hardness is 8.5 GPa which is softer than the AlTiC substrate that has 17.6 GPa of hardness.
The increasing of metal mask etch rate is higher than AlTiC etch rate when increasing the coil power and the platen power, it effects to the lower etch selectivity in trial # 2.
Let’s consider the etch-by product of NiCr/AlTiC system which was contributed the etching rate to be higher if the etch by-products are in gas phase (chemical etching), for the metal mask NiCr, there are potentially occur CrF5 (bp = 117