Development of UV-assisted ozone steam etching and investigation of its usability for SU-8 removal

Abstract

We have developed ultraviolet (UV)-assisted ozone steam etching as a novel SU-8 removal technology. This technology is expected to remove SU-8 at a higher etch rate compared to a conventional wafer cleaning process based on ozone water immersion. An etching apparatus prototyped in this study achieves an etch rate greater than 100 nm min−1 by intermittently rotating an SU-8-coated substrate on a spinner for spinning out the etching products under UV irradiation. Its complete removability is also verified by analyzing the surfaces of etched samples via scanning electron microscopy and x-ray photoelectron spectroscopy. In a micromolding process, the non-swelling removability is successfully demonstrated by removing a SU-8 sacrificial mold and leaving electroplated metal mushroom microstructures without destruction. Such a structure cannot be released by conventional resist strippers, because it is lifted up due to resist swelling. We believe this etching technology becomes a viable option to remove chemically-stable polymers such as SU-8 in microelectromechanical systems.

Review of polymer MEMS micromachining

“…Commercial SU-8 strippers are also available, including NANO ™ RemoverPG, ACT-1, QZ3322, MS-111, Magnastrip, RS-120, and K10 molten salt bath. Ozone solution can also remove SU-8 [174], but removal rates are slow; the rate can be improved by using ozone steam [175]. However the ozone solution, as well as other stronger solutions, may not be compatible with other materials present in a device; nickel corrosion was reported after exposure to ozone [175].…”

“…Ozone solution can also remove SU-8 [174], but removal rates are slow; the rate can be improved by using ozone steam [175]. However the ozone solution, as well as other stronger solutions, may not be compatible with other materials present in a device; nickel corrosion was reported after exposure to ozone [175]. Physical methods such as water jetting [32], pyrolysis [33], and excimer laser patterning [176] can also be used to remove SU-8.…”

Ether cleavage and chemical removal of SU-8

“…Unfortunately, these conditions would cause damage on a wide range of materials of technological interest, limiting the applicability of the technique. Examples of these methods are various types of burning, oxidizing plasmas [7,8], piranha etching [9], and ozone exposure [10]. Certain molten salts have also been found to completely remove SU-8 at temperatures above 300 • C, unfortunately, these extreme conditions are also incompatible with a wide range of applications [11].…”

Ether cleavage and chemical removal of SU-8

“…Unfortunately, these conditions would cause damage on a wide range of materials of technological interest, limiting the applicability of the technique. Examples of these methods are various types of burning, oxidizing plasmas [5,6], piranha etching [7], and ozone exposure [8]. Certain molten salts have also been found to completely remove SU-8 at temperatures above 300 • C, unfortunately, these extreme conditions are also incompatible with a wide range of applications.…”