Process for the fabrication of hollow core solenoidal microcoils in borosilicate glass

We report the fabrication of solenoidal microcoils with hollow core embedded within two 100 mu m thick borosilicate glass wafers. The main process steps are the reactive ion etching of borosilicate glass, anodic wafer bonding, copper metal organic chemical vapor deposition (Cu MOCVD) and electroless galvanization. Our motivation stems from the need for a reliable, precise fabrication method of microcoils for high-resolution magnetic resonance imaging (MRI). For reduced loss at high-frequency operation, glass, with a lower dielectric constant as compared to silicon, was chosen as a substrate material. Simultaneously, this offers MRI sample observation owing to its optical transparency. Further essential parameters for the coil design were the need for small coil dimensions, a high filling factor (region of interest within the coil occupied by the sample/overall coil volume), and low-loss electrical connectability to external devices. In an attempt to achieve those requirements, the reported process demonstrates the combination of front-and backside borosilicate glass RIE of small dimensional features (down to 10 mu m wall thickness) with subsequent conformal metallization of the 3D solenoidal coil by means of Cu MOCV and electroless galvanization.