Elimination Of Mask-Induced Defects With Vote-Taking Lithography

The problem of ensuring adequately low density of defects in lithographic masks is becoming increasingly serious as circuit patterns become denser and more extensive. This paper discusses a radically alternative strategy to eliminate the effect of random defects on reticles. In this method, a number of reticle fields containing nominally identical patterns are aligned and exposed in sequence at the same site, each with an equal fraction of the nominal exposure dose. The optical intensity distribution impinging on the resist is the sum of the aerial images from these exposures. As a result, a random defect unique to a single reticle field affects only a minor part of the total exposure. The effect of this exposure deviation can then be minimized with an adequate resist contrast and a properly adjusted exposure dose. With the lithographic tools and resist process technology presently available, gross reticle defects can generally be reduced to minor distortions in the resultant features. A series of experiments have been performed with Shipley Microposit 1470 photoresist exposed with an Ultratech 900 1x wafer stepper, and demonstrated the feasibility of this technique. The effects of misalignment among fields, resist contrast, exposure dose, and defect size and type have been studied in particular. A novel etching process that permits the electrical detection of defects due to photolithography alone is being used to evaluate the effectiveness of this vote-taking scheme in VLSI patterning, and has demonstrated its capability of eliminating mask-induced defects, and no noticeable loss of lithographic yield when defect-free masks are used.