A microelectronic unit includes a semiconductor element having a front surface to which a packaging layer is attached, and a rear surface remote from the front surface. The element includes a light detector including a plurality of light detector element arranged in an array disposed adjacent to the front surface and arranged to receive light through the rear surface. The semiconductor element also includes an electrically conductive contact at the front surface connected to the light detector. The conductive contact includes a thin region and a thicker region which is thicker than the thin region. A conductive interconnect extends through the packaging layer to the thin region of the conductive contact, and a portion of the conductive interconnect is exposed at a surface of the microelectronic unit.