Design and fabrication of through-glass via (TGV) based 3D spiral inductors in fused silica substrate

Design and fabrication of the through-glass via (TGV) based three-dimensional (3D) spiral inductors in the fused silica substrates by a simpler electrochemical discharge machining (ECDM) is reported for the first time. The design of the spiral inductor was simulated using a high-frequency structure simulator to predict the S-parameters, inductance, and quality factor for a frequency up to 800 MHz. The 3D spiral inductors were designed with variable coil widths, i.e., 100 μm, 150 μm, and 200 μm, and a different number of turns from 3 to 6 turns. The through-holes having a diameter of 320 μm were etched in highly insulative 500 μm thick fused silica substrates in less than 7 min using a cost-effective ECDM technique. The through-holes were then completely filled with copper by bottom-up electrodeposition. Top and bottom side redistribution lines (RDL) were fabricated by standard through-photoresist electrodeposition process. Nanoindentation was carried out to ascertain the mechanical properties of the electrodeposited copper. The I–V characteristic showed perfect ohmic contact between the TGV and RDL. The quality factor and inductance of the fabricated 3D inductors were measured using a vector network analyzer. For an inductor coil having four turns and 150 μm width, the maximum quality factor was measured to be 11.6 at 440 MHz, while the measured inductance was 114.9 nH at 800 MHz. The experimental results were compared with the simulation results and were observed to be in good agreement. The fabricated inductors in fused silica substrates can be used in radio-frequency (RF) applications as an alternative to silicon-based inductors since it will have low substrate losses.