This paper describes a new approach for mask-to-wafer alignment and gap control using modified moir technique. The alignment marks are in the form of gratings. A laser beam is passed through the mask grating and reflected from the wafer grating. A relative displacement, say in X-direction, between mask and wafer gives rise to periodic intensity variation called a moire signal. In the present method, the alignment is automatically achieved in a region of higher slope instead of at peak or trough of the moir signal where the slope is zero. This gives greater alignment accuracy. This was done earlier by using two pairs of l800 spatially phase shifted gratings. We have simplified this technique by using only one pair of grating to obtain the moir signal. The other moir signal is the electronic inversion of the former. Difference between the two moir signals is zero at a point in the higher slope region, which is considered as correct alignment position. Automatic alignment can also be achieved by using difference between the moir signal and a d.c. voltage equal to its average value. At the aligned position the mask grating is approximately quarter pitch shifted with the wafer grating. In a similar way an orthogonal grating pair gives automatic alignment in the "Y" direction. It has been observed that the gap characteristics for a quarter pitch shift are best suited for non-contact gap (Z-direction) measurement. This can be conveniently done by simply counting the number of cycles of the intensity variation with gap. The gap control is obtained between any two of the Fresnel image positions by using electronically inverted moir signal similar to that for X and Y alignment. The estimated accuracies for X and Y alignment are 32 nm and for Z control is 500 nm.

Date Published: 1 January 1991
PDF: 10 pages
Proc. SPIE 1332, Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection, (1 January 1991); doi: 10.1117/12.51112

Published in SPIE Proceedings Vol. 1332:
Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection
Chander Prakash Grover, Editor(s)