Development and evaluation of the porous Au structure for the thin film encapsulation

This paper presents the application of the Nano Porous Gold (NPG) for encapsulating the MicroElectroMechanical System (MEMS) devices. The NPG was realized by selective etching of the Sn from electroplated AuSn alloy on a particular seed layer. However, it is very difficult to form through pores during selective etching process as seed layer does not contain Sn. This seed layer acts as barrier for the application of Thin Film Encapsulation (TFE) which need through etch holes in cap layer to remove the sacrificial layer underneath. So in this paper, different seed materials were studied to find comptability of the material with AuSn alloy as well as their suitability in easy etching to form the through etch holes after NPG formation. Cu and Ni were found the suitable seed layer for forming the AuSn alloy. During the above study, it was found that current density for electroplating of AuSn alloy is also very important parameter to fabricate the uniform AuSn alloy. It was found that 1.25 mA/cm² is the optimum current density to achieve the AuSn alloy for TFE application. These parameters were used for demonstration of TFE.