ELSEVIER Microelectronic Engineering 30 (1996) 575-578 MICROELECTRONIC ENGINEERING Optimization of FIB Methods for Phase Shift Mask Defect Repair Zheng Cui, Philip D Prewett and John G Watson Central Microstrncture Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OXll 0QX, UK Two new methods have been proposed to optimize the focused ion beam (FIB) technique for repair of phase shift masks (PSMs). The alternating box method is used to eliminate the post FIB repair phase trench defect and the biased method to eliminate the gallium staining effect. Both methods require only a change of sputtering or beam scanning strategy. Rim, halftone and embedded PSMs for contact hole and poly line features with programmed defects have been fabricated and repaired by the new methods. Computer simulation and experimental photolithography have confirmed that opaque defects can be repaired by the optimized FIB technique without leaving any post repair "ghost" defects. 1. INTRODUCTION With the inclusion of the phase shift mask (PSM) in photolithography for 64 MB and 256 MB DRAM manufacturing, the repair of defects as small as 0.25~tm must be considered. To repair such small defects, focused ion beam (FIB) tools are preferred due to their high accuracy and high resolution with ion probes as small as 50nm and below [1]. However, when used to repair opaque defects in PSMs, the FIB method can leave residual or "ghost" defects of two types. The fn'st is the post-repair phase trench, which is associated mainly with the repair of rim or halftone PSMs. The second is the gallium ion stain, which is also a common post-repair defect of the binary intensity mask (BIM). Optimised focused ion beam methods have been developed to prevent both of these post- repair "ghost" defects. The new methods have been tested by both computer simulation and experiments. Rim, halftone and fully embedded PSMs have been fabricated for contact hole and line/space features with programmed defects of various types. The programmed opaque defects were repaired by a FIB tool with conventional methods and the new methods. The repaired PSMs were then printed using an i-line stepper. Comparisons of the results have confirmed that the new FIB methods can effectively eliminate the post FIB repair defects. They involve only changes of sputtering or beam scanning strategy, which requires no additional equipment and can be implemented in any existing FIB repair tools. 2. POST-REPAIR DEFECTS Opaque defects in rim and halftone PSMs normally take the form of bumps or protrusions formed during dry etch processing of quartz or spin-on-glass phase shift elements. Monte Carlo simulation of ion sputtering and experiments have revealed that, when using FIB tools to remove these bumps, the removal rate is faster at the bump edge than in the centre. This causes a post-repair defect in the form of a "phase trench", which is dependent on the sputtering depth and independent of defect size [2]. Fig.1 shows a phase trench created after FIB repair of a programmed opaque edge defect in a rim PSM. The post-repair phase trench defect is printable and becomes equivalent to the original defect if the printed feature on wafer is in the sub-half micrometer range [2]. Fig.1 A 3~tmx3~tm programmed defect after FIB repair, showing post-repair phase trench "ghost" defecL 0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. Al1 rights reserved. SSDI 0167-9317(95)00313-4