Micro Pinhole Formation in Photoresist Multilayer Structure controlled with Hydrophilic Treatment

Abstract

The pinhole formation in photoresist multilayer structure acts as a critical defect for device fabrication. The purpose of this study is pinhole prevention by hydrophilic treatment and analysis of pinhole formation mechanism in terms of surface free energy. A SU-8 photoresist top layer is spin-coated on a SU-8 bottom layer as simplest multilayer structure. Pinholes are formed on bottom layer with no-treatment, however, pinholes cannot be observed when bottom layer is treated by hydrophilic process. Subsequently, the surface free energy and spread coefficients of each SU-8 layer are evaluated. Because the absolute value of spreading coefficient S top is small relatively in case of no-treatment, the pinning phenomenon can be caused by contaminants. A triple point, bottom layer, liquid top layer and air, appears at the pinning area. Air cannot intrude into interface between top-bottom layers due to the spreading coefficient "S air >0". Consequently, the pinhole expansion phenomenon would be dominated by tensional force and viscous-elasticity of liquid top layer. By hydrophilic treatment, surface free energy of bottom layer increases drastically, therefore the pinning phenomenon is prevented because of "S top <<0".

Not spreading in reverse: The dewetting of a liquid film into a single drop

“…This condition can be quantified in terms of the initial height-to-radius ratio of the droplet and the equilibrium contact angle, giving h 0 /R 0 << (1 − cosq e )/sinq e . These flattened films are not particular to drying situations but can arise in many situations involving fluid manipulation, such as spraying (27), drop impact (28)(29)(30), evaporation and condensation (31)(32)(33), convection (34), nucleate boiling (35), spin coating (36), and wetting on flexible surfaces (37)(38)(39).…”

Effect of Sub-Pattern on Guiding Liquid Propagation at Microchannel Junction

“…The microchannel was formed with SU-8 photoresist (3050, Kayaku Chemical Corp.) by the typical photolithography process. The details of fabrication processes are described in our previous report [3]. The width and height of the channel became 100 and 50 μm, respectively.…”

“…Recently, microfluidic devices have been actively studied in varieties of fields such as chemical, biological, and analytical industries [1][2][3]. The micro channel and tube application are recognized important key technologies.…”