As semiconductor industry moves towards advanced technology node, requirement for tighter Critical Dimension (CD) control constantly raises the bar for CD metrology. Yet despite various intrinsic bias origins, CD-SEM is still serving as the workhorse and ‘go to’ metrology mean for inline CD control in modern IC fabrication day in and day out. Such confidence comes from extensive studies around the underlying physics of SEM as major bias types are all marked as 'accountable' and some even 'predictable' nowadays. Still there are times when unexpected metrology results slip through with no obvious trace leading to any well established theories. And it is none the less necessary and challenging to single out the root cause from the complex physics models. Such a case is presented in this work. A reproducible CD diving behavior on the scale of 0.4~0.8nm during the very first one or two measurements by SEM on Poly-Si sample is described and verified. Various experiments are conducted to identify the physical origin. We propose that this ‘first measurement effect (FME)’ is related to SEM proximity shadowing and e-beam seasoning on pattern sidewall material.
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.