Through-hole forming method, inkjet head, and silicon substrate

A through-hole forming method comprising steps of:

• forming a first impurity region around a region where a through-hole is to be formed in a first surface of a silicon substrate, the first impurity region being higher in impurity concentration than the silicon substrate;
• forming a second impurity region at a position adjacent to the first impurity region in a depth direction of the silicon substrate such that the first impurity region has a thickness greater than the second impurity region, the second impurity region being higher in impurity concentration than the first impurity region, wherein the step of forming the second impurity region is performed separately from the step of forming the first impurity region;
• forming a first layer on the first surface of the silicon substrate;
• forming an etch mask layer on a second surface of the silicon substrate opposite to the first surface, the etch mask layer having an opening at a position corresponding to the through-hole to be formed;
• and
• etching the silicon substrate until at least the first layer is exposed via the opening, thereby forming the through-hole.