Precise alignment through thick wafers using an optical copying technique

Submicrometer alignment of two mask patterns on the top and the bottom surface of a thick glass substrate is possible by using optical copying. The optical copying step transfers an alignment pattern from one surface to the other by shadow casting or Fresnel propagation. In a demonstration experiment, the alignment between the two surfaces of a 3-mm-thick quartz glass substrate was achieved with a precision of 0.5-0.7 microm.