Stepper focus characterization using diffraction from latent images

We present a novel technique for the rapid, nondestructive evaluation of the contrast of the latent image in photoresist. Measurements are made of the intensity of light diffracted from a grating pattern in exposed, undeveloped photoresist, the so‐called latent image. Optimum exposure tool parameters such as exposure tool dose and focus can be determined based on the intensity of light diffracted into specific orders. Employing the proposed technique, measurements can be made sufficiently fast to allow rapid control of the exposure conditions, such as focus, which influences image contrast. The measurement can be made after an exposure, and corrections to focus for subsequent exposures may be derived to compensate for changing optical or mechanical properties of the wafer. The technique may be used with a variety of photoresist materials on different substrates.