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Nano-imprint lithography (NIL) has the advantages of high throughput, sub-10-nm feature and low cost. The residual layer is however often encountered in imprinting process. To remove the residual layer the reactive ion etching (RIE) is used. Namely, the quality of imprinted pattern is associated with the thickness of residual layer. The residual layer is currently measured by scanning electron microscopy (SEM), a destructive measurement. In this study, a non-destructive measurement of residual layer thickness is proposed. The authors apply the surface plasmon resonance (SPR) to monitor residual layer during imprint process. When the thickness of residual layer varies, it changes the resonance pattern, including the reflectivity and resonance angle. The optical analysis demonstrates this innovative method for monitoring of residual layer thickness is effective.