Method of manufacturing substrate for liquid ejection head, substrate for liquid ejection head, liquid ejection head, and printing apparatus

A method of manufacturing a substrate for a liquid ejection head including an ejection port forming member which has formed therein ejection ports opened to a surface for ejection liquid, the method comprising the steps of:

• forming a first layer by using a first layer forming member containing a solvent and a photo-acid-generating agent;
• forming a second layer on the first layer by using a second layer forming member containing a solvent and a photo-acid-generating agent;
• and
• hardening a partial region of each of the first layer and the second layer and removing a region different from the partial region so as to form the ejection ports, resulting in that the first layer and the second layer constitute the ejection port forming member,
• wherein a member containing a solvent to dissolve the first layer forming member and a photo-acid-generating agent having an acid strength weaker than an acid strength of the photo-acid-generating agent contained in the first layer forming member is used as the second layer forming member, and
• wherein an absolute value of a difference between a solubility parameter of the second layer forming member and a solubility parameter of the first layer forming member is a predetermined value that is four or less.