A mask-to-wafer fine gap setting method

This paper describes a mask-to-wafer fine gap setting method for an X-ray stepper. A capacitance micrometer on the wafer chuck and those on the mask chuck are used in this method. The mask and wafer surfaces are positioned parallel to the travel plane of the wafer x-y stage so that the gap does not vary with the step and repeat motion of the wafer stage. Therefore, gap setting is required only once for each wafer, and the throughput becomes better. This method has the following two additional merits. A complex gap measurement optics is not required and the setting value of the gap is variable. The gap setting error caused by mask unflatness, wafer unflatness and travel error of the wafer x-y stage was analyzed. Experiments were carried out using mask and wafer stages both with 6-axes. A gap setting accuracy better than ± 1.5 μm at the exposure positions of 5 wafers has been achieved for 30 μm and 20 μm gaps. (author)