Abstract
In Manufacturing technology of LSI, exposure, etching and other processes are iterated using several kinds of photomasks in order to form electrical circuit pattern on the silicon wafer. Therefore mask alignment is indispensable.
We tried to detect the translational displacements of IC utilizing the laser holography technique. Then real time on-line positioning control by a minicomputer was performed. The laser beam is divided into two beams, one of which is projected to a model IC and the reflected light from the model IC is projected to a photographic plate after passing through a Fourier transform lens. The other beam is directly projected to the photographic plate after passing through a beam expander, and after development the hologram is obtained.
Next, the reflected beam by the IC chip to be positioned which has the same pattern as that in case of hologram making is porojected to the hologram, and the diffracted light from the hologram generates an optical spot which is an autocorrelation function of the IC pattern. As movement of the spot is proportional to the displacement of the IC chip, measurement of the displacement of the IC chip can be done by detecting the spot using a photoelectric microscope which has L-shaped slits. The measurement data are sent from the positioning pulse generator to a minicomputer, thus positioning control is done in accordande with its command.
The positioning accuracy obtained is ±0.18μm (confidence coefficient: 99.7%) which is monitored by a laser interferometer.
This method has the feature that any marking on the IC chip is not necessary and that positioning of the IC chip is done using its own pattern.
