Microfabrication of polymer with laser-induced backside wet etching

MAY 08, 2020

A new study demonstrates that laser-induced backside wet etching can successfully work on a polymer substrate, which has implications for manufacturing microfluidic chips.

Meeri Kim

Microfabrication of polymer with laser-induced backside wet etching internal name — Microfabrication of polymer with laser-induced backside wet etching lead image

Polymethyl methacrylate (PMMA), a transparent thermoplastic, is widely used as a substrate for microfluidic chips in biological and chemical applications. Traditionally, trench micromachining on a PMMA substrate involves direct ablation with a carbon dioxide laser. However, this method limits the channel size to around 85 microns in order to retain good surface quality.

An alternative method for PMMA micromachining, known as laser-induced backside wet etching (LIBWE), has been previously employed for use with glass and offers several advantages over direct laser machining methods, including smaller feature size. The findings reveal that LIBWE with a visible laser can successfully perform continuous 12-micron-wide trench etching on PMMA.

LIBWE works by focusing a laser beam on the backside of the transparent substrate, which is in contact with a highly absorbing liquid. Etching results from intense temperature and pressure increases at the thin interface between the substrate and liquid due to strong absorption.

The experimental setup included a PMMA sheet in contact with a potassium permanganate absorber solution within a liquid chamber. A 532-nanometer Q-switched laser with a pulse duration of approximately 10 to 15 nanoseconds was then focused on the PMMA-potassium permanganate interface. The researchers obtained continuous and crack-free etching of the PMMA sheet with the lowest possible scanning speed of 2 millimeters per second. In addition, a smooth surface required a laser fluence of around 14 joules per square centimeter.

For future work, they hope to perform surface microfabrication on other polymers, such as polystyrene, polycarbonates and cyclic olefin copolymers, with visible LIBWE.

Source: “Surface micromachining on a polymethylmethacrylate substrate using visible laser-induced backside wet etching with a KMnO₄ solution as an absorber,” by Hui-Fang Chang, Wing Kiu Yeung, Wei-Chen Kao, Martin Ehrhardt, Klaus Zimmer, and Ji-Yen Cheng, Journal of Laser Applications (2020). The article can be accessed at http://doi.org/10.2351/1.5114659 .