Method to solve alignment mark blinded issues and a technology for application of semiconductor etching at a tiny area

A method of unblinding an alignment mark, comprising the steps of:

• providing a substrate having a cell area and an alignment mark area;
• the substrate having an alignment mark within the alignment mark area;
• forming an SiN layer over the substrate and lining the alignment mark;
• forming an STI trench through the SiN layer and into the substrate within the cell area;
• forming a silicon oxide layer over the substrate, filling the STI trench and the SiN layer lined alignment mark;
• planarizing the silicon oxide layer to form a planarized STI within the STI trench and leaving silicon oxide within the alignment mark to form a blinded alignment mark;
• applying a wet chemical etchant within the alignment mark area over the blinded alignment mark to at least partially remove the silicon oxide within the alignment mark;
• and
• removing the SiN layer from over the substrate and within SiN layer lined alignment mark whereby the silicon oxide is also removed from within the blinded alignment mark to unblind the alignment mark;
• wherein the wet chemical etchant is applied to the blinded alignment mark in the form of a drop.