3D packaging technique on liquid crystal polymer (LCP) for miniature wireless biomedical sensor

This paper presents a novel 3D packaging technique for a miniature wireless biomedical sensor. The end goal of this sensor is for implantation into the eye of a mouse, therefore size is extremely limited. The application makes the packaging of the sensor and control of IC a difficult challenge. The thickness of the unit must be less than 300 microns total. It is demonstrated in this paper that the thickness requirement can be met using novel epoxy interconnects and that micro-vias can be implemented in the package to distribute signals vertically to limit the eventual area of the device. First, the layer-to-layer interconnection between silicon and liquid crystal polymer (LCP) layers is demonstrated using a magnetically aligned Z-axis anisotropic conductive adhesive (ACA). The total thickness of the IC and the packaging layer is less than 150 microns. The resistance through Z-axis ACA represented 1.15 Ω on average for 75 micron pads. Second, 3D transitions through LCP via holes of 20 μm are demonstrated, which are suitable to distribute signals through the small form factor unit. In this paper, we demonstrate transition from an antenna layer through the LCP to a layer above where a rectifier resides. RF power received by a loop antenna on the bottom LCP layer is transferred to the rectifier on the top layer and generates 5 volts of DC voltage. These miniature 3D packaging techniques could make it possible to integrate all components in the small area (500 μm × 500 μm) to implement an implanted wireless biomedical micro-sensor.