Characterizing overlay registration of concentric 5X and 1X stepper exposure fields using interfield data

the cost advantages associated with implementing a mix-and- match photolithography process have led to a dramatic increase in the interest and development of these manufacturing environments. This is especially true for older fabs with high production lithography tools already in place but technology that has increased beyond the capability of the tools. For he process engineer the challenge is to define a method of optimizing the exposure field registration between each of the different imaging systems. In this paper a procedure used to evaluate intrafield and interfield overlay errors between six ASML 5X steppers and sixteen Ultratech 1X steppers is described. With this technique reticle data, stage registration and a commercially available software analysis package are used to model pattern displacement of each stepper within this population. Wafers from each steppes are first patterned with nine fields, each consisting f a 9 by 9 array of ASML alignment marks. The X and Y stage coordinate of each alignment mark is then measured using a standard ASML 5500/60 intrafield analysis routine. Spreadsheeting the resulting stage registration data, subtracting the expected or 'ideal' stage position and correcting for any reticle pattern shifts, grid and intrafield data are obtained. Using this process a data sheet for each stepper was developed and, once formatted properly, loaded onto the software analysis package for registration modeling. Use of multiple exposure fields per wafer enabled the software to characterize both intrafield and interfield registration by first modeling the grid errors, subtracting these values, and then performing intrafield analysis of the remaining data. Further, by collapsing the intrafield data into a single field a 'lens fingerprint' of each stepper lens was derived. Using vector subtraction a direct comparison was made between the lenses of each stepper and an indexed table of exposure field translation errors created. The stepper lenses were also sorted from best to worst matches. This approach generated the required 231 paired data sets needed to match each stepper to all others while exposing and measuring only 44 wafers and required no artifact wafers. Measured evaluation results will be reviewed and expansion of this procedure to mapping 1X wide field lenses and matching of non-concentric exposure fields discussed.