The Use of Aqueous Enzymatic Polymerization of Amphyphilic Alkyl Tyrosine Derivatives as Environmentally Benign Coatings
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The Use of Aqueous Enzymatic Polymerization of Amphyphilic Alkyl Tyrosine Derivatives as Environmentally Benign Coatings in the Microelectronics Industry Anastasios P. Angelopoulos1, Kenneth A. Marx2, and Kyoung S. Oh Department of Chemistry, University of Massachusetts Lowell 1 University Avenue, Lowell, MA 01854 1 Center for Advanced Materials, University of Massachusetts Lowell 2 Center for Intelligent Biomaterials, University of Massachusetts Lowell ABSTRACT The surface coating properties of enzymatically polymerized decyl esters of d-tyrosine from aqueous solutions onto gold surfaces have been investigated utilizing the complementary techniques of Potentiometeric Titration, UV-VIS Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). The aqueous-based coatings are of interest as environmentally friendly and cost-effective replacements for epoxy-based coatings currently employed in the microelectronics industry for both chemical protection and electrical insulation of gold-covered metallic conductors. Experimental results with respect to polymerization pH, immersion pH, and immersion time are presented and compared to the ionization behavior of the monomers in solution. Optimum processing conditions have been established, which yield uniform aqueous-based polymeric coatings on gold conductor surfaces. INTRODUCTION Enzymatic polymerization of self-assembling amphyphilic alkyl tyrosine derivatives (decyl ester d-tyrosine (DEDT) and l- tyrosine (DELT) isomers) from emulsions results in pH-dependent water-soluble polymers which are more easily processable than highly crosslinked polyphenols produced by similar methods [1,2]. Polymers of DEDT and DELT yield thin films when spread on the surface of water [1]. In addition, such polymers are found to bind onto the surface of gold substrates [3]. This surface activity may be exploited to eliminate organic solvents and other toxic reagents currently used in polymer coating applications in the microelectronics industry. Such coatings provide both chemical protection and electrical insulation of gold-covered metallic conductors during subsequent processing [4]. The steps of the particular current coating process, which is the focus of the present study, are depicted in Figure 1 [5]. Step 1 in this figure is a cross section of a circuitized substrate. In Step 2 of the figure, a solution of various epoxy resins dissolved in methyl ethyl ketone (MEK) is applied over the surface of the substrate, encapsulating the gold-coated conductors. The epoxy is then cured at elevated temperatures. In Step 3 of the manufacturing sequence, additional conductors are fabricated on top of the epoxy. The epoxy layer resulting from Step 2 provides both insulation resistance between the two layers of conductors as well as protection of the initial conducting layer during the manufacturing sequence. The sequence depicted in Figure 1 may be repeated as many times as desired to form a multilayer microelectronic package. Key requirements for the process depicted in Figure 1 include com
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