Yoriyuki Ishibashi

A mask‐to‐wafer alignment and gap setting method for x‐ray lithography using gratings

A new optical‐heterodyne interferometry alignment and gap setting method for x‐ray lithography is developed, where the phases of beat signals are used for detection. The fact that the lateral displacement and the gap between mask and wafer can be detected independently is shown, based on the results of analysis and experiments. Two pairs of gratings are arranged as detection marks. One pair forms a window on the mask and a checkerboard grating on the wafer, while another pair forms two linear gratings at right angles to each other. Two He‐Ne laser beams with slightly different frequencies illuminate the gratings from the ±1st‐order diffraction light directions. The lateral displacement is detected by measuring the beat signal phase difference between the two (0,1)th‐order diffraction lights from two pairs of gratings. To detect the gap, the signal phase difference between the (0,1)th‐ and (1,1)th‐order diffraction lights from the pair of linear gratings is used. Better than 0.02 μm (3σ) alignment repeatability and 0.1 μm gap setting accuracy have been achieved.

X‐ray stepper aiming at 0.2 μm synchrotron orbital radiation lithography

A vertical x‐ray stepper has been developed for 0.2 μm synchrotron orbital radiation lithography. The key features of this prototype stepper are a new gap setting algorithm, an optical heterodyne alignment system, and a newly developed fine motion mask stage. Gap setting is executed so as to make the mask and wafer parallel to the travel plane of the wafer x–y stage so that only one gap setting per wafer is required. The gap setting accuracy between 20 and 50 μm gaps is better than ±1.5 μm (3σ) for each exposure position. The optical heterodyne alignment signal obtained by detecting the diffraction beams from two checkerboard gratings has a detectable resolution of better than 0.01 μm and has only a small dependence of ±0.02 μm on gap variation. The alignment signals are fed back to the mask stage which can align the mask and wafer with a resolution of 5 nm. In exposure experiments, 0.15 μm lines and spaces were printed on a negative resist (SAL 601) and a 0.05 μm overlay accuracy has been obtained.

The effect of intensity distribution in the reflected beam on the detection error of monochromatic optical autofocus systems

The measurement error induced by the intensity distribution in the reflected beam from the surface of a substrate was studied to improve the focal accuracy of an autofocus system. It is shown that scanning of the probe beam across the wafer surface reduces the effect of nonuniform reflectivity. An accuracy of 0.6 μm can be obtained. A signal processing circuit which accommodates 40 dB of incident intensity range is developed.