Towards self-alignment with copper pillars

In this paper we study the assembly of chips with copper pillars and SnAg bumps onto chips with Ti/Ni/Au pads. This with the aim to optimize self-alignment of micro optics and photonic chips. Different flux and fluxless processes of assembly are studied and we compare the yield, cleanliness and the quality of the metallic joint. The latter is evaluated by observation of cross sections and by shear strength measurement. During this work, we developed a fluxless method of assembly for self-alignment of photonic devices which uses a pre-treatment step in order to improve wettability. Compared to other fluxless processes (formic acid, plasma, vacuum ultra violet), the method developed only requires low cost equipment and performs more efficiently than formic acid in term of wettability for SnAg bumps onto Au pads. We discuss the potential consequences of the different processes on the self-alignment for optical chips.