Electrochemical Endpoint Lithography Techniques Applied to PMMA LIGA Fabrication of Xpci Gold Gratings
Authors: Arrington Christian L. , Hollowell Andrew E , Finnegan Patrick Sean , Baca Kalin R , Pillars Jamin R. , Dagel Amber L , Singh Varshni , Nguyen Quoc 
Published: February 2020

X-ray Phase contract imaging (XPCI) is a promising technique for medical and non-destructive testing[1]. XPCI requires High Aspect Ratio gratings made of high Z materials, e.g. gold. The preferred method of fabrication to realize the very high aspect ratio tall structures is X-ray based lithography, molding, and electroforming process (LIGA) capable of creating high density 1.2 micron features with straight sidewalls that are 120 micron in height [2]. One critical step to pattern such structures is to accurately develop the photoresist mold. This is so because the window is narrow between development and over development. We have proposed and demonstrated the use of a novel electrochemical technique to accurately develop LIGA molds with high repeatability prior to subsequent electroplating.

The number and period of development cycles often can be a critical and subjective step where operator error and human judgement can play a significant role drastically affecting the outcome. The analysis must be performed when the sample is still immersed in DI water to avoid capillary action destroying the photoresist molds at high aspect ratios. Some experts have implemented drying techniques to avoid the collapse of high aspect ratio polymer molds when they are dried. Some scientist show a direct phase change from liquid to gas using cyclohexane to mitigate this phenomenon [3].

Sodium chloride (NaCl) salts have been added at ~100 mM to a GG developer LIGA based chemistry. After x-ray exposure of the PMMA the GG developer removes regions where x-ray have severed the PMMA backbone of the polymer mold resulting in a patterned mold that can then be filled with gold to create a high aspect ratio grating. LIGA substrates were prepared using high molecular weight PMMA, x-ray exposed, and actively monitored utilizing electrochemical technique during the development process to search for an endpoint where subsequent gold plating can then occur. As the high aspect ratio mold is developed, the electrochemical interactions are measured between the conducting seed layer and NaCl. We will discuss conductivity of the chemistry, optimizing the process for repeatability, the benefits, and the challenges. Results show an increase in impedance as the exposed PMMA is chemically removed and the conductive NaCl salts get closer to the conductive seed layer at the base of the LIGA PMMA mold. This process uses first of its kind real time electrochemical data capable of detecting more accurately the endpoint of the development process. Previous endpoints were determined by visual inspection, destructive inspection after the fact, large lot burn ins, and single use chemical processes.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Momose, A. and W. Yashiro, X-ray Phase Measurements with Talbot Interferometry and Its Applications. International Conference on Advanced Phase Measurement Methods in Optics an Imaging, 2010. 1236: p. 195-199.

Mohr, J., et al., High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. International Workshop on X-Ray and Neutron Phase Imaging with Gratings, 2012. 1466: p. 41-50.

Koch, F., et al., Increasing the aperture of x-ray mosaic lenses by freeze drying. Journal of Micromechanics and Microengineering, 2015. 25(7).