New gap detection method using two lasers optical heterodyne interference for X-ray lithography

A new gap detection method for synchrotron radiation (SR) X-ray lithography has been developed. Two laser lights with different wavelengths lambda(1) and lambda(2) are used as light sources. The proximity gap between a mask and a wafer was measured by noting the phase difference of the four heat signals obtained from two optical heterodyne interference lights. The absolute gap is shown as the phase difference of the synthetic wavelengths Lambda = (lambda(1) . lambda(2))/(lambda(1) - lambda(2)) of wavelengths lambda(1) and lambda(2). Direct gap detection of the absolute value is established by measuring the phase difference in the optical heterodyne interference beat signal. A repeatability of 5 nm (3 sigma) or less was obtained.