Effects of mask error factor on process window capability

In the photolithographic process, critical dimensions (CD) of exposed features in photoresist need to be controlled to within a specified tolerance related to the nominal feature size. A portion of this tolerance budget is consumed by variations in CD on the photomask. At low k1 factor, a number of parameters in the lithography system impact linearity including lens aberrations, defocus, exposure, partial coherence, and photoresist contrast. The combined effect of these parameters is that errors in the mask CDs are not transferred to the wafer in direct proportion to the optical reduction value of the lithography system. This Mask Error Factor (MEF) becomes a significant problem as it consumes a larger than anticipated portion of the CD tolerance budget. This paper will discuss experimentally evaluated MEF using a 4X i-line stepper for a range of feature sizes from subwavelength to approximately twice the exposure wavelength. A test reticle was built with isolated lines from 200 nm to 600 nm in 12.5 nm increments at 1 X. CD measurements on the reticle were compared to corresponding CD measurements on the wafer in order to establish both linearity and MEF curves for the lithography system. MEF values were also determined across a process window for multiple feature sizes. The MEF was observed to be less than 1.4 for CDs greater than 330 nm (k1 equals 0.5) throughout the process window. However, the MEF rises rapidly to over 3 for CD values smaller than 300 nm (k1 equals 0.45) at nominal focus and exposure. Changes in exposure were not observed to have a noticeable impact on MEF while focus offsets were observed to result in significant increases in MEF. These results indicate that MEF has a much larger impact on focus latitude than on exposure latitude. As a result the process window will be compressed more in focus than in exposure.