An embodiment of the invention provides an interposer which includes: a substrate having a first surface and a second surface; a first hole extending from the first surface towards the second surface; a second hole extending from the first surface towards the second surface, wherein a width of the first hole is different from a width of the second hole; an insulating layer located on the substrate and extending onto a sidewall of the first hole and a sidewall of the second hole; and a conducting layer located on the insulating layer on the substrate and extending onto the sidewall of the first hole, wherein there is substantially no conducting layer in the second hole.