One aspect of this description relates to a testing apparatus including an advance process control monitor (APCM) in a first wafer, a plurality of pads disposed over and coupled to the APCM. The plurality of pads are in a second wafer. The testing apparatus includes a testing unit disposed between the first wafer and the second wafer. The testing unit is coupled to the APCM. The testing unit includes a metal structure within a dielectric. The testing apparatus includes a plurality of through silicon vias (TSVs) extending in a first direction from the first wafer, through the dielectric of the testing unit, to the second wafer.