Resist cross-sectioning using focused ion beams

摘要

This paper reports the results of efforts to find conditions under which FIB technology can be used to cross-section resist features. Evaluation of the accuracy of these cross-sections and the convenience or speed relative to cleaving is examined. FIB cross sections through a staggered window pattern on the edge of cleaved segment from a 0.4 $mu@m thick novolac (mp 2400-17) coated Si wafer showed that the ion beam milling relieves the taper present on the profile, and enlarges the dimensions of the smaller features (0.6 $mu@m windows). These effects are due to the near-maximum sputter yield at the angel that the resist side wall presents to the ion beam, the size of the ion beam relative to the feature size, and the dose used to cut the section. Alterations in the resist geometry can be minimized by using the lowest possible doses to image and cut the desired features. Redeposition of sputtered material in the window openings does not appear to be a problem. All of the work in this study involves uncoated resist, for both FIB cutting and SEM imaging. A technique for sectioning line and space patterns and then measuring the line profile at any point on a full wafer has been worked out. This technique fulfills the potential speed advantage of FIB ion milling over cleaving. Features in a 7 $MUL 7 array printed on a 12 cm wafer were ion milled at an angle of 45$DGR to the wafer surface by dose variation, giving a wedge-shaped trough in the wafer. When tilted to 45$DGR in the SEM, the angled wall of the wedge cut presents a surface that is normal to the electron beam, just as in the edge mounting of a cleaved section. Contrast enhanced digital images allow measurement of the line profile, which was found to be accurate within the limits imposed by the ion beam width.