A method for assembling a semiconductor device includes: receiving a first chip including a plurality of first bonding pads, a first standoff and a second standoff, wherein a first solder is deposited on each of the first bonding pads; depositing a second solder on each of the first and second standoffs; arranging a second chip over the first chip, wherein the second chip includes a plurality of second bonding pads, and at least one of the second bonding pads has a corresponding first bonding pad; heating the second chip over a melting point of the second solder to melt the second solder, and placing the second chip on the first chip to touch and solidify the second solder on each of the first and second standoffs; performing a reflow process to melt the first solder on each of the first bonding pads so that at least one of the first solders touches a corresponding second bonding pad; and waiting a predetermined period of time to allow the second chip to move until a side edge of the second chip touches a waveguide of the first chip.