Creating Superwicking Surfaces on Aluminum Nitride Using Femtosecond Laser Surface Processing

Ryan R. Healey, University of Nebraska-Lincoln

Document Type Thesis

Copyright 2025, Ryan R. Healey. Used by permission

Abstract

Aluminum nitride (AlN) is a high-performance ceramic with excellent thermal conductivity and electrical insulation properties, making it a promising material for use in next-generation electronic devices. However, its naturally low surface wettability provides a challenge for integrating the material into applications that require fluid spreading, adhesion, or thermal management based on two-phase processes. This thesis serves to explore the use of femtosecond laser surface processing (FLSP) to modify the surface of AlN and enhance its wetting characteristics. A thorough laser parameter study was conducted using a high-power, high-repetition rate Amplitude Tangor 300 femtosecond laser system. AlN surfaces were processed under a range of fluence values (0.66-1.45 J/cm²) and pulse counts (800-25,600). The resulting micro- and nano-scale surface structures were characterized using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). Structural metrics such as relative peak and valley positions and surface structure height (R_z) were analyzed against the processing parameters of fluence and pulse count, as well as a normalization metric of accumulated fluence. It was found that structure height generally increased with both fluence and pulse count but plateaued at high accumulated fluences due to peak erosion. Low pulse count values were considered ineffective at any fluence value to create dense or tall surface structures.Notably, results from lower accumulated fluence values suggested that fluence and pulse count can both distinctly govern structure formation more so than the absolute amount of energy accumulation from the laser. The enhanced surfaces were evaluated for their fluid handling performance through wicking diffusion rate (WDR) testing. Nine FLSP-processed samples were prepared across the fluence values of 0.79, 0.92, and 1.19 J/cm², as well as pulse counts of 3,200, 6,400, and 25,600. All processed surfaces exhibited superhydrophilic behavior, with contact angles of 0° and significantly increased wicking rates compared to untreated AlN. Samples with higher pulse counts consistently exhibited faster wicking rates. When comparing samples with the same pulse counts, it was found that an increase in fluence results in an increase in WDR. These results confirm that FLSP is an effective and tunable technique for improving the wetting behavior of AlN.

Advisor: Craig A. Zuhlke