Utilization of Cross-Correlation Function for Assessment of Replication Quality in Ultrasonic Embossing of Microchannels on Polymethyl Methacrylate (PMMA)

Abstract

Ultrasonic embossing is a promising method for fabricating microchannels on thermoplastics due to reduced cycle time and relatively low equipment cost. Replication quality in ultrasonic embossing has been investigated in the literature by primarily referring to the replication depth. However, this approach does not fully reflect the fidelity between the mold and the replicated feature, especially at the side walls, which are typically designed as vertical in microchannels. We propose to utilize cross-correlation between the gradients of the mold profile and the embossed profile as a figure of merit sensitive to the side wall’s fidelity. For testing purposes, we embossed straight microchannels on polymethyl methacrylate (PMMA) using a brass mold comprising 100, 300, and 1000 µm-wide protrusions of same designed height of 300 µm, resulting in aspect ratios (AR) of 3, 1, and 1/3, respectively. Cross-correlations between the gradients of the pofiles ranged between 0.50 and 1.00, while cross-correlations between the profiles were greater than 0.97 in all cases, showing the sensitivity imposed by utilizing the gradients. Setting average of all cross-correlation values (0.8) as the quality threshold, we observed that high AR (3) features were replicated at low quality, while low AR (1/3) features were replicated at high quality, regardless of the process parameters. In replicating unit AR features, the process parameters were observed to affect the quality, where the combination of vibration time and pressure was the most significant compared to individual effects of the parameters.