etchstop

An etch-stop, or etch-stop layer, is a material intentionally incorporated into a microfabrication stack to have a significantly lower etch rate in a chosen etchant. As etching proceeds, the stop layer acts as a barrier that halts material removal at its interface, enabling precise control of feature depth and protecting underlying structures during subsequent processing. This concept is central to reliable depth control in semiconductor and MEMS fabrication.

Common stop materials include silicon nitride (Si3N4) and silicon dioxide (SiO2). Si3N4 is widely used as an etch stop in plasma etching and in certain wet chemistries, while SiO2 can serve as a stop in processes where the etchant attacks silicon but not oxide. In silicon-on-insulator devices, the buried oxide layer often acts as an intrinsic etch stop during bulk silicon removal with alkaline solutions such as KOH or TMAH.

Etchant choice and film thickness determine effectiveness. The etch selectivity, defined as the ratio of the etch rate of the target material to that of the stop layer, guides material choice and thickness. High selectivity reduces the risk of over-etching the stop layer or damaging underlying structures.

Applications include backside etching to create MEMS membranes and cavities, through-wafer etch depth control, and release steps where fragile structures must be preserved. After it finishes its role, the stop layer may be removed or incorporated into the final device, depending on the process and design.

Design considerations include mechanical stress, thermal budget, adhesion to adjacent materials, and compatibility with subsequent deposition or etching steps. Selection of the stop layer and its thickness balances etch control with device performance and manufacturability.