Study of stress and adhesion strength in SU-8 resist layers on silicon substrate with different seed layers

Abstract

We present the details of our study on the internal stresses and adhesion strengths of SU-8 structures to different substrate seed layers. The effect of adhesion promoter-methacryloxy [propl] trimethoxysilane (MPTS), and OmniCoat-and different seed layer combinations (Ti/Cu/Ti, Ti/Cu, Cr/Au, and Cr/Au/Cr) was examined for internal stress and adhesion strength in 650-µm-thick SU-8 films. Increased stress and poor adhesion have led to the delamination of SU-8 in some cases. Adhesion and stress have proven to be the function of process parameters such as soft bake (time and temperature), exposure dose, post-exposure bake (time and temperature), and development time. We have found that a 100 silicon wafer containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer with MPTS as the adhesion promoter yielded a thick SU-8 film with a lower value of stress and consequently better adhesion for processing in deep x-ray lithography (DXRL). A detailed correlation of the effects of adhesion and internal stress on the SU-8 film is discussed. An analysis of the possible chemical bonding interactions occurring between SU-8, and its adhesion promoter and/or substrate is also presented.