Utilization of cross‐correlation function for assessment of replication quality in ultrasonic embossing of microchannels on polymethyl methacrylate

Abstract

AbstractUltrasonic embossing—a process where a horn vibrating at 20–30 kHz compresses on a thermoplastic substrate sitting on a mold to replicate the features on the mold to the substrate—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 to be vertical in microchannels. We propose using cross‐correlation—a method that quantifies the similarity between two signals by measuring the overlap of their patterns—between the derivative of the mold profile and the embossed profile as a figure of merit that accurately reflects the fidelity of the side wall. For testing purposes, we embossed straight microchannels on polymethyl methacrylate (PMMA) using micron‐scaled protrusions on a brass mold fabricated by CNC milling. The mold comprised 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 derivatives of the profiles 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 derivatives. 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.Highlights Parameters' effect on replication of varied aspect ratio features was assessed. Assessment was carried out by cross‐correlating mold and channel profiles. Cross‐correlating derivatives of profiles provide a more sensitive means. General rules for replication quality in ultrasonic embossing were driven.