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“The MEEF Meter”: a Realistic Approach for Lithography Process Monitoring by Frank Schellenberg, Pat LeCour and Olivier Toublan, Mentor Graphics Geoffrey Anderson and Raymond Yip, KLA-Tencor Corporation
With the advent of sub-wavelength lithography, process control has taken on a whole new meaning. This article discusses a practical process monitor target for the low k1 lithography regime, labeled the MEEF meter. Its purpose is to allow accurate determination of the MEEF effect and a consistent monitor for any changes in the lithography process that can impact this effect. The investigation and characterization were followed from the design phase, through reticle fabrication and finally onto the wafer.
Introduction
CD metrology using top-down scanning electron microscopes (SEM) is a routine technique for monitoring IC wafer processes.1 When process CDs vary beyond established control limits, corrective action can be taken. Test structures which fit in the scribe lines between chips are routinely introduced to allow the metrology on periodic samples of production wafers or on all production wafers if so desired. However, with most contemporary processes (i.e., those targeted with minimum features at 180 nm or smaller) the combination of process factors is typically tuned to reduce any variation in CD, at least until the process fails catastrophically. Illumination conditions, reticle techniques, resist technologies, post-exposure baking processes, etch recipes, etc., are all chosen to work in tandem to reduce the variation of target CDs as much as possible.2 Many recent papers have discussed the mask error enhancement factor, or MEEF.3, 4, 5, 6, 7, 8, 9, 10, 11, 12 This represents an
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“amplification” of reticle errors by wafer process phenomena, producing changes larger than expected on the wafer. Perfect linearity in a process would give a MEEF of 1; but when process conditions significantly deviate from linearity (usually when attempts are made to create sub-wavelength lithography features), larger values are observed. In highly nonlinear conditions, MEEF values as large as 8 have been reported.8 Typical sub-wavelength MEEF values for binary lithography are in the range of 1.5-2.5.8, 11 On the other hand, MEEF values for special phase-shifting cases, or for certain dimensions of dense lines or with assist bars, have been predicted to be significantly less than 1.7, 12 Understanding MEEF for a process is, therefore, very important to any resolution enhancement technologies, such as OPC and PSM, that compensate for predicted wafer effects. What we propose here is that the MEEF, being essentially a derivative measurement of linearity conditions, may actually serve as a sensitive monitor for process variations. MEEF has been predicted to be a strong function of defocus,7 so this is something to examine under experimental conditions. To this end, we have created a MEEF meter and observed its performance under conditions of focus and exposure.