We present the design, fabrication, and characterization of surface micromachined on-chip 3-D air-core arch-shape solenoid microinductors. Combinations of unique surface micromachining fabrication process techniques, such as deformation of polymeric sacrifical molds and conformal electrodeposition of photoresist molds on nonplanar sacrificial polymer mounds, are utilized. An air gap inserted between the inductor's body and the substrate is used to reduce the degradations of high-frequency inductor performances. Fabricated inductors are characterized and modeled at high frequencies from S-parameter measurements. ABCD parameters, derived from measured S parameters, are translated into a simplified physical π model. The resulting 2-, 3-, and 5-turn arch-shape suspended air-core solenoid inductors have inductances between 0.62 to 0.79 nH, peak quality (Q) factors between 15.42 to 17 at peak-Q frequencies between 4.7 to 7.0 GHz, and self-resonant frequencies between 47.6 to 88.6 GHz.
This work reports a new approach to fabricate spiral on-chip inductors with suspended dome-shape metal tracks. The new fabrication techniques are aimed at reduction of substrate losses by fabrication of spiral metal tracks on top of a sacrificial polymeric dome so that the inductor is suspended in the air when the dome is finally removed. In addition, the reduction of fringing capacitance between the spiral metal tracks is expected since the tracks sidewalls are not fully overlap to each other. Dome-shape spiral inductors made of copper were demonstrated. Currently, high-frequency characterizations of these inductors are under way.
A novel on-chip 3D air core micro-inductor, utilizing deformation of sacrificial thick polymer and conformal photoresist electrodeposition techniques, is reported. The bottom conductors are formed on silicon or glass substrate by metal electroplating through SU-8 polymeric mold. A thick SJR 5740 photoresist is then spun on and patterned to be a supporting mesa. Hard curing of such polymer mesa could significantly deform it into a cross-sectional bell-shape sacrificial core with graded profile in which is used to support top conductors formation. A layer of conformal electrodeposited photoresist (PEPR 2400) is then coated along the core's surface profile, patterned by standard optical lithography and filled up by metal electroplating. Finally, all polymeric molds including significantly deformed sacrificial core and electroplating bases are removed, resulting in an on-chip solenoid-type 3D air core micro-inductor. Since this new inductor has an air core and has only two contact points per turn, the core loss and equivalent series resistance are expected to be small, and hence, to give higher quality factor at high-frequency operation. Currently, high-frequency characterization of this on-chip inductor is under way.