Quantitative CD-SEM Resist Shrinkage Study and Its Application for Accurate CD-SEM Tools' Matching

Photo resist shrinkage during CD-SEM measurements has been a well known phenomenon. It presents great difficulties and challenges in CD-SEM metrology, for example, in the CD-SEM tool matching. The "check board" method currently popular in CD-SEM tool matching avoids measuring the same location twice and resorts to the statistical averaging out the CD difference in each paired measurements. This paper presents a new cross sampling method which inherently eliminates the CD difference in each paired measurement. The pro and con of these two sampling methods are studied with resort to a resist shrinkage model. A conceptual equation of the real tool difference on ADI pattern is proposed. Through this equation, we are able to determine the magnitude of the real difference via the shrinkage model combining the measurements from the cross sampling method. A quantitative study of the resist shrinkages with multiple subsequent measurements is carried out. An exponential model is assumed and proved to have good fit with the experimental data. From such resist shrinkage model, we are able to deduce the original resist line/hole CD size without any e-beam disturbance. The relative change of the CD after the first measurement is revealed quantitatively from such a model with very good accuracy. Combing the fitted models with the cross sampling measurement results we are able to determine the real CD difference, if they were measured by these two CD-SEM tools, which cannot be obtained by direct measurements because of the memory of any previous measurements.