Photoacoustic metrology of nanoimprint polymers

Nanoimprint lithography (NIL) is an alternative lithography method for patterning of thin polymer films using a rigid stamp, which is being developed as desired minimum feature sizes reduce to the scale of tens of nanometres. To characterise nanoimprinted structures, there is a need for more convenient and non-destructive wafer-scale metrologies to complement scanning electron microscopy and atomic force microscopy. The photoacoustic method, with a resolution in the range of 10 nm, and normally used to measure metal and dielectric layer thicknesses and physical properties, has been used for the first time to study nanoimprinting polymer layers. A good signal was obtained from the top and the bottom interfaces of two polymers, mr-I PMMA 75k300 and mr-NIL 6000.3, with thicknesses ranging from 100 to 500 nm. From the measured time of flight of the acoustic wave, and modelling physical parameters of the polymers, thicknesses calculated agree well with those measured by profilometry. The measurements are performed on a short pulse laser pump-probe setup, where bulk wave packets are excited and detected using near infrared laser pulses of less than 100 fs duration. The entire experimental setup is also simulated numerically.