Performance of X-ray stepper for next-generation lithography

Next-generation lithography must have the capability of replicating patterns with critical dimensions of 100 nm and below. Proximity X-ray lithography is one of the leading candidates because of its demonstrated high resolution, wide exposure latitude and simple wafer processing. However, overlay accuracy is not yet sufficient for the requirement due to pattern placement errors in X-ray masks and the lack of alignment accuracy with magnification errors in exposure systems. Intensive efforts have been making to improve the pattern placement accuracy through the development of advanced e-beam mask writers and better membrane and absorber materials. Several methods of magnification correction have already been proposed, but none has actually been demonstrated. NTT and ASET has developed a twin X-ray stepper (SS-3 and XS-1, respectively) that employs a two-wavelength (685 nm and 830 nm) optical heterodyne alignment system and a vertical xy-stage using air guides and air lead screws. The design of the stepper is based on the concept that good overlay accuracy can be obtained by making accurate through-the-mask (TTM) measurements of the relative position of a mask and wafer and by feeding the data back to a high-repeatability stage positioning system. The X-ray steppers employ a method of magnification correction involving the adjustment of the wafer temperature to improve the overlay accuracy. It enables corrections of up to 4 ppm. This paper discusses the performance of the X-ray steppers, which demonstrate the capabilities of X-ray lithography; this work is the result of a collaborative effort between NTT and ASET.