When etching an oxide contact hole with a given process, the etch selectivity compared to photoresist is found to be 2.5. If the oxide thickness to be etched is 140 nm and a 50% over etch is used, what is the minimum possible photoresist thickness (that is, how much resist will be etched away)?
To find the minimum possible photoresist thickness that will be etched away, we need to calculate the over etch thickness for the oxide contact hole and then divide that by the etch selectivity ratio.
Let's break down the steps:
1. Calculate the over etch thickness:
Over etch refers to the additional thickness you want to etch beyond the desired thickness. In this case, the over etch is 50% of the oxide thickness to be etched.
Over etch thickness = 50% of the oxide thickness
= 0.5 * 140 nm
= 70 nm
2. Calculate the photoresist thickness:
The photoresist thickness that will be etched away is the over etch thickness divided by the etch selectivity ratio.
Photoresist thickness = Over etch thickness / Etch selectivity ratio
= 70 nm / 2.5
= 28 nm
Therefore, the minimum possible photoresist thickness that will be etched away is 28 nm.