Stepped back side deep trench isolaton structure

An image sensor comprising:

• a semiconductor substrate including a front side and a back side;
• photodetectors arrayed within the semiconductor substrate;
• and
• a back side isolation structure extending into the back side between the photodetectors, wherein in a first area the back side isolation structure has an upper part and a lower part both of which are within the semiconductor substrate;
• wherein the lower part is narrower and has a higher aspect ratio than the upper part;
• and
• the semiconductor substrate forms a ledge at a juncture between the upper part and the lower part.

A method comprising:

• providing a semiconductor substrate including a front side, a back side, an image sensing area, a peripheral area, and photodetectors in an array within the image sensing area;
• forming trenches, wherein within a first area the trenches are formed by both a high-aspect ratio etch and a mouth etch, the high-aspect ratio etch comprises etching through first slits having a first width, the mouth etch comprises etching through second slits having a second width, and the second width is greater than the first width;
• and
• filling the trenches to form a back side isolation structure.