Flex-to-rigid (F2R) is a technology platform for the fabrication of miniature partially flexible sensor systems for minimally invasive medical instruments. Because of the flexibility, these sensor systems can be wrapped around cylindrical medical instruments such as catheters or guidewires. F2R is a postprocessing technology whereby flexible interconnects, consisting of an AlCu routing layer sandwiched between two layers of flexible polyimide, are fabricated on a silicon wafer. This is followed by a separation step whereby individual elements are singulated by Deep Reactive Ion Etching (DRIE) and Reactive Ion Etching (RIE). The F2R fabrication is based on standard IC fabrication technology, which allows the devices to scale down so that all kind of sensing functionalities and electronics can be integrated into the tip of a catheter. 

This thesis focuses on the improvement of the F2R technology. One of the objectives was to increase the density of the devices. By patterning the metal interconnects by RIE, instead of wet etching, very high density interconnects are fabricated. Corrosion-free interconnects down to 0.8 μm width were fabricated using this new developed process. Furthermore, the vias between the two metal layers in F2R were improved. Small pitch vias down to 3 μm diameter were fabricated by etching small tapered holes in the polyimide layer and sputtering metal in these contact holes. 

Because of the in-body application of F2R, a perfect adhesion between the several layers is required. The polyimide-polyimide adhesion is known to be troublesome. Therefore, improving the adhesion of this interface was explored in this work. A perfect adhesion was obtained by a novel method which combines a thin layer of SiC and SiO2 into an adhesion improvement layer between the polyimides.

The developed fabrication processes, high density interconnects, small pitch vias and the adhesion improvement, were integrated into the F2R process flow. Test devices, consisting of a silicon island with bond-pads and test structures and a flexible part with meandering interconnects, were fabricated using this improved process flow. Bending tests showed that bending radii of the flexible interconnects down to 10 μm are achievable while not damaging the interconnects. This extreme flexibility is required to wrap a high density imaging system around a guidewire to fabricate a 360 μm diameterF2R IVUS catheter. An analysis is presented that describes the relation between the geometry of the F2R technology and how such a device can be folded around a cylindrical instrument. It is found that using the same F2R technology, such a system can be scaled down to a smaller diameter relatively straightforward.