u u2032 and 612 l u2032 for lower grating 604 l u2032 offset bias of grating stack 202 x is defined as the value of the offset 614 i e x 612 u u2212 x 612 l the difference in the x coordinates of the symmetry planes 612 u and 612 l when the lithography process is in perfect alignment similarly offset bias of grating stack 202 x u2032 is defined as the value of offset 614 u2032 x 612 u u2032 u2212 x 612 l u2032 when the lithography process is in perfect alignment upper grating 604 u and lower grating 604 l are offset so that the difference between offset biases of 202 x and 202 x u2032 is equal to pitch 4 i e this can be seen for example in fig6 a where lithography alignment is perfect and offset bias of grating stack 202 x is equal to offset 614 which is equal to pitch 8 offset bias of grating stack 202 x u2032 is equal to offset 614 u2032 which is equal to u2212 pitch 8 the difference between offset 614 and offset 614 u2032 is constant and is not affected by changes in the alignment between upper layer 608 u and lower layer 608 l alignment changes do however change the values of offset 614 and offset 614 u2032 this is evident in fig6 b where upper layer 608 u has been shifted to the left with the result that offset 614 is now pitch 4 and offset 614 u2032 is now zero in general small changes in alignment between upper layer 608 u.