Teiichirou Chiba

New microcharacters for wafer identification

Conventional identification (ID) systems use characters that are often illegible, with character recognition being difficult in as many as 20% of all processes. In contrast, new microcharacters in the V-shaped notch are clearly recognized throughout the process. The results can be explained by the following multiple effects. Marking location: These markings require only a small space. Hence, markings can even be in the beveled section of the V-shaped notch of a wafer. In the case of conventional ID systems, it is difficult to select an area that gives good readability during processes and wide enough for marking. Dot topography: A marking dot formed by conventional laser marking has a central depression due to the process of general heat distribution. In contrast, a marking dot formed by micro marking has a central peak protruding from the surface that is more easily distinguished than a dot that has a central depression. The contrast of the new dot is about two times greater than that of a conventional one. Contrast greatly influences readability.

Micropeak array in the scribe line on a wafer

Small dot matrix marking on a silicon wafer has been performed using an second-harmonic generation (SHG) laser of yttrium aluminum garnet (YAG), liquid-crystal-display (LCD) mask, and projection optics. A marked image was obtained after laser irradiation through the pattern on the LCD mask. The each dot is a square with sides of 3.6micrometers , the pitch of each dot is 4.5micrometers and the height (not the depth) of each dot is approximately 0.5micrometers . The topography of each dot is unique, and features a central peak and peripheral depression. We have named this topography micropeak and have proposed a hypothesis for the micropeak formation mechanism, based on the density of liquid silicon and the congelation of molten silicon. In this report, micropeaks were formed in the scribe line on a wafer covered with oxide layers. Without being torn, these oxide layers were pushed up by micropeak generation and rose. Silicon particle scattering around the laser irradiation area was prevented completely. Clear dot images were observed through the transparent oxide layers. The conditions forc lean marking by laser irradiation greatly depend on the thickness of the oxide layers.

New identification system for individual wafer management

A conventional identification (ID) has many illegible parts; ID recognition is difficult in approximately 20% of all processes. In contrast, new IDs in the V-shaped notch were clearly recognized up to the final process. The results can be explained in terms of marking dot topography and choice of the marking location. First, for marking the beveled part in a V-shaped notch on a wafer, known as finer marking, the influence of any semiconductor processes in which a conventional ID on the surface has only limited readability is kept to a minimum. Second, a marking dot formed by conventional laser marking has a central depression due to the process of general heat distribution. In contrast, a marking dot formed by finer marking has a central peak protruding from the surface, which is more easily distinguished than a dot which has a central depression. A difference in contrast has a great influence on readability for identification in semiconductor processes.