Study for aluminum metal patterning process with oxide hard mask in 90-nm s-flash memory device fabrication

Abstract

It is necessary to make small sized flash cells for high density flash products and it is indispensable to minimize the line and space widths of Metal lines in such a high density Flash chips. To minimize a line and space widths, the thickness of photo resists must be reduced for the process margin of photo lithography. but the reduced thickness of photo resists is normally not enough for metal etch. So hard masks are needed for metal etch. In this study we have made a metal-etch process with KrF photo resists and oxide hard mask which has lines and spaces with 120 by 120nm. We have used TEOS film as an oxide hard mask for metal etch and have made an ex-situ process to open the hard mask for metal etch, in this ex-situ hard-mask-open process we have had bad line-edge-roughness (LER) problems but we have found out the main effective parameters for LER by DOE methods and various experiments and have finally optimized the process conditions. After the hard-mask-open process, metal line can be patterned with photo resist and oxide hard mask. The side wall profile of metal lines made by oxide hard mask are different from those made by normal photo resist mask because the types and quantities of by-products from oxide masks are different from those from photo resists during etch process. Normally by-products or polymer could be re-deposited to the side wall of metal lines, so they could be protected from possible side-wall-attacks. However due to the lack of by-products from oxides, the side wall of metal lines could be damaged if they were etched only with oxide-hard-masks, so we have had a experiment of metal lines' profile characteristics according to the ratio of photo resist's thickness to oxide hard mask's thickness, and have found out that the ratio of photo resist to oxide hard mask has a significant role in metal line profiles. Despite those side wall profile problems, eventually we have optimized a condition by adjusting the proper thickness of oxide-hard-masks and by tuning the process gas in the series of etch process. Finally in 90nm s-Flash device fabrication, we have made the metal lines with the same high qualities as those of 130nm devices from the view point of profiles and Electro Migration characteristics.