Chemically enhanced focused-ion-beam (FIB) repair of opaque defects on chrome photomasks

Abstract

The characteristics of an ideally repaired opaque defect on a chrome (Cr) photomask are: (1) the total removal of the Cr defect, leaving no residual Cr; (2) a smooth, level quartz surface (no over-etch) after the Cr is removed; (3) minimal riverbedding of the quartz at the perimeter of the Cr defect and (4) maximum light transmission (%T) at the lithographic wavelength. Achieving these ideal repair characteristics is becoming increasingly difficult as the patterned features become smaller, as the lithographic wavelength becomes shorter and as phase shifting mechanisms are implemented. A chemical process has been developed to enhance the FIB (focused ion beam) etching of Cr defects. This chemical process enhances the FIB removal of a Cr defect 2.0 - 2.2 fold while inhibiting the removal of quartz by 60 - 80%. AFM (atomic force microscopy) indicates that (1) Cr is totally removed, (2) the quartz remains smooth and level (no over-etch) and (3) the riverbeds are 5 - 25 nm. If necessary, a second FIB-induced chemical process is used following the chrome etch process to reduce optical staining due to implanted gallium (a gallium ion beam is used in commerical FIB systems) such that the %T of the repaired areas at i-line(365nm) and DUV(248nm) wavelengths is 95%. In general, this second process is required at 248 nm but not at 365 nm. AIMS evaluations indicate a critical dimension variation between repaired and reference patterns of 10% at 35% light intensity at UV and DUV wavelengths. In summary: a. an FIB etch process has been developed which repairs opaque Cr defects, b. a second FIB etch process removes implanted gallium so that the %T is above 95% at i-line (if neccessary) and DUV wavelengths; c. these two etch processes are done sequentially, while the defect is positioned under the FIB column (post treatment processes are not required); d. clear defects can also be repaired at the same time by FIB-induced deposition of opaque carbon. Keywords: Mask repair, opaque defects, chrome defects, FIB