Moire displacement detection by the photoacoustic technique

Moire effect produced by the two gratings under coherent illumination attracts a high level of interest in the mask and wafer alignment for X-ray lithographic system. In such reflection optical arrangement, the displacement signal is often affected by the light reflected directly from the mask grating. On the other hand, in the photoacoustic technique, the signal is generated by the absorbed light energy and thus it is not influenced by the direct reflection. We studied the detection method of the moire displacement signal by using the photoacoustic technique. Using the transmission and reflection (absorption) gratings with 25μm pitch, the lateral displacement was detected. The probe beam deflection method and the microphone method were used for sensing the displacement from the acoustic vibration and the acoustic pressure, respectively. The signals changed periodically by the pitch of the grating for the lateral displacement. The characteristics of the displacement signal were examined theoretically by the calculations based on the Fresnel diffraction theory.