To optimize nanoimprint lithography (NIL), it is essential to be able to characterize and control the NIL process in situ and in real time. Here, we present a method for in situ real-time NIL process characterization using time-resolved diffractive scatterometry (TRDS). A surface relief diffraction grating is used as the imprint mold, and the diffracted light intensity is monitored continuously during the imprint process. We use a scalar diffraction model to calculate the diffraction intensity as a function of the mold penetration ratio. Simulations show good agreement with the experimental results. Our results indicate that TRDS offers a powerful characterization tool that can be used for in situ, real-time NIL process control. Nanoimprint lithography (NIL) is a promising technology that has the potential to become a low-cost high throughput lithography tool for the mass production of nanoscale devices and systems. 1 It has demonstrated sub-10 nm resolution and has been applied to the fabrication of a variety of nanodevices in different fields. [1][2][3][4] NIL patterns by physically deforming a polymer resist thin film using a mold. The resist can be either thermoplastic or photocurable. 1,5 A thermoplastic resist is initially in a solid form at a low temperature and becomes viscous when heated above its glass transition temperature ͑T g ͒ during imprinting. For a photocurable resist, it is initially in a liquid form and becomes solid after curing. Clearly, polymer deformation plays a critical role in nanoimprint lithography. To optimize NIL, it is essential to characterize and control the deformation of the resist in situ and in real time. However, currently, few studies on this subject have been carried out or reported.Here, we present a method for in situ real-time NIL process characterization using time-resolved diffractive scatterometry (TRDS). We report on its application in characterizing the nanoimprint process. In TRDS, a surface relief diffraction grating is used as the imprint mold, and the diffracted light intensity is monitored continuously during the imprint process. We use a scalar diffraction model to calculate the diffraction intensity as a function of the mold penetration ratio, simulations show good agreement with the experimental results. Our results indicate that TRDS offers a powerful characterization tool that can be used for in situ real-time NIL process control. Figure 1 shows a schematic of the TRDS setup we use to detect the NIL process. Previously, optical scatterometry was used in surface analysis, and diffraction-based interferometry was used in measuring the mask-wafer gap. [6][7][8] In the TRDS experiment, a surface-relief diffraction grating was patterned on an imprint mold made of a transparent fused silica substrate with a thickness of 0.5 mm. The grating mold is fabricated using a process combining interference lithography, NIL, wet etch, and reactive ion etching to achieve the desired sidewall smoothness and grating linewidth. 9 Figure 2 is a scanning electron micrograph (SE...