Mask CD control requirement at 0.18-μm design rules for 193-nm lithography

Abstract

In this paper, the mask critical dimension (CD) control budget for the 0.18 micrometers generation design rules are assessed via experimental based data. The lithography tool used for wafer exposure is 193 nm microstepper with NA of 0.6. The resist process used is the top surface imaging (TSI) resist process. In the study, the process CD variation due to mask CD error, mask CD error combined with process variation such as focus and exposure, mask CD error combined with low k<SUB>l</SUB> imaging limitation, and mask CD error combined with the characteristics of a given resist process, such as feature dependent silylation, are investigated. It is found that for the lines with a relative larger pitch (e.g., pitch is approximately 2X the line width), the resist CD responds to the mask CD linearly down to a 0.16 micrometers feature size. However, unlike the most single layer resist processes, the slope of the line, i.e., (Delta) CD(wafer)/(Delta) CD(mask), in the above mentioned TSI process is not equal to 1. Their values are between 1.2 - 1.7, depending upon the pitch. This means that for a given mask CD error, the corresponding wafer CD error will be 1.2 - 1.7 times larger. In other words, the mask CD control budget will not scale the same as that of wafer CD control budget when the design rule scales. In fact, the mask CD control budget should be at least 1.2 - 1.7 times tighter than that of the wafer CD budget scaling. In the case of contacts, the resist CD linearity only extends to the 0.375 micrometers feature size with a slope approximately equals to 1. For the contacts smaller than 0.375 micrometers , the slopes are greater than 1 depending upon the exact mask contact sizes. For the contacts smaller than 0.25 micrometers , the slopes are between 2 - 3, depending upon the pitch. In addition, it is found that the defocus increases the slope value significantly for the small contacts sizes. The slopes for the case of lines, however, are found much less sensitive to the defocus conditions (defocus equals +/- 0.2 micrometers approximately +/- 0.3 micrometers ). Our assessment showed that mask CD control budget (4X) for 0.18 micrometers design rules are necessary to be in a range of +/- 8 nm approximately +/- 12 nm (4X), assuming that +/- 25% of total wafer CD budget which is +/- 10% of the target CD is given to the mask.