Influence of different initiators on the adhesion properties of photopolymerized epoxides on gold and silicon

Photocuring of epoxides proceeds by a cationic mechanism. The required photoinitiators are iodonium or sulfonium salts with non-nucleophilic anions. The influence of different photoinitiators on the adhesion properties of both a rigid and a soft epoxide is examined. Depending on the substrates to be joined, different decomposition products of the initiators are concentrated in the interphase. This accumulation of decomposition products in the interphase leads to a decreased adhesion. On gold surfaces the sulfur of sulfonium salts is concentrated as expected. But surprisingly the iodine of the iodonium salts is also concentrated on gold. It could be shown by immobilization experiments that organic iodine compounds with the iodine in any oxidation state reacted with gold surfaces. Due to this reaction iodide anions are formed. On silicon surfaces the fluorine of the anions PF6- and SbF6- is concentrated due to the high affinity of silicon for fluorine. The iodonium salt with [B(C6F5)(4)](-) anion contains no fluorine able to react with the silicon. Therefore, the fluorine cannot be concentrated in the interphase and the joints prepared with the soft epoxide containing this initiator have a higher shear strength compared to adhesives with conventional anions.