Final answer:
The etch rate of positive photoresist when using Cl₂ for ScAlN varies and must be defined experimentally or by the manufacturer. For etching 6μm of ScAlN, a thicker etch mask like metal hard masks or silicon dioxide should be used, ideally with a thickness at least 3 times the depth of the desired etch.
Step-by-step explanation:
When using a Cl₂ based etch for ScAlN (Scandium Aluminum Nitride), the etch rate of positive photoresist (PR) typically depends on the specific etching conditions and the composition of the photoresist used. The exact etch rate needs to be determined experimentally or provided by the manufacturer. However, as a general guideline, the selected etch rate should allow the thickest layer of ScAlN that can be etched with 4μm of positive PR to be removed without completely etching through the PR itself.
For etching a 6 μm layer of ScAlN, a more appropriate etch mask would likely be required due to the insufficiency of 4 μm of positive PR in terms of thickness and etch resistance. A common choice for a thicker etch mask could be metal hard masks, such as chrome or aluminum, or even silicon dioxide (Σ2O₃) if available. The thickness of such an etch mask would need to be determined based on its etch selectivity relative to ScAlN and the desired etch depth. A rule of thumb is to have the mask thickness to be at least 3 times the desired etch depth to account for the mask erosion during the etching process, but this factor can vary depending on the mask material and etching conditions.