Disclosed herein is an apparatus suitable for detecting X-ray, the apparatus comprising: a first substrate comprising a plurality of first electric contacts; a first chip layer comprising a plurality of first chips, wherein each of the first chips comprises a first electrode and is bonded to the first substrate such that the first electrode is electrically connected to at least one of the first electrical contacts; a second substrate comprising a plurality of second electric contacts; and a second chip layer comprising a plurality of second chips, wherein each of the second chips comprises a second electrode and is bonded to the second substrate such that the second electrode is electrically connected to at least one of the second electrical contacts, wherein the first chip layer and the second chip layer are bonded to each other such that at least two second chips are bonded to a same first chip.