Clearing resist from alignment mark areas using an excimer laser

Abstract

An increasingly important problem in various lithographies is accurate alignment. Detection of alignment marks can be obscured by different resist coatings covering the marks. To circumvent this problem, a technique has been developed to clear resist from alignment mark areas. The technique uses ablative photodecomposition (APD) with an excimer laser to clear the resist thereby facilitating alignment. A potential problem using APD for semiconductor processing is the well-known phenomenon of debris formation. It has already been shown that only a small percentage of the products derived from ablation are volatile and that large amounts of solid debris are formed under a variety of ablation conditions. Though some postablation wafer cleaning techniques may be possible, these involve extra process steps and are therefore undesirable. The technique described here uses a localized vacuum system to collect the debris. A marked improvement in debris containment has been demonstrated using this system. The alignment mark clearing technique has been implemented in a prototype tool consisting of an automated x,y stage driven by linear motors for wafer positioning, a personal computer driven laser shutter to control the number of laser pulses, alignment optics, and the vacuum system for debris containment. The wafer is loaded on the stage after which alignment is performed using an off-axis alignment system. The software removes wafer rotation and controls stage positioning and laser output to ablate the desired number of alignment marks. The prototype tool has been used to clear alignment marks for both optical and electron beam alignment systems.