Abstract: In manufacturing industry, bonding wires are used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip. With advance of factory automation, it is necessary to introduce intelligent sensor to monitor breakage of bonding wire. As chip size and diameter of bonding wire become smaller and smaller, current monitoring methods encounter a lot of limitations. In this paper, a method to monitor breakage of fine bonding wires using single-slit diffraction is proposed, where irradiance variation of the corresponding diffraction pattern at the special receiver could be observed, due to the presence of a wire. Experimental results demonstrate that the proposed method renders satisfactory monitoring performance for bonding wire with a diameter of 25 μm or less.