Diamond lumped elements and multi-fingers MESFETs

Abstract

Improving in growing large area polycrystalline diamond films by CVD process has highlighted the possibility of producing integrated circuits for specific applications. In order to create a Monolithic Microwave Integrated Circuit (MMIC) based on diamond, we need to design and develop all fundamental components for this goal, from power active device to matching or polarization networks based on passive elements: inductors, capacitors and resistors. For these reasons, during the doctorate research activity, we’ve designed and constructed multi-finger MESFETs, rectangular multi-turn inductors and square capacitors. To obtain these devices, we’ve developed and optimized the use of air-bridge technique and gold electroplating on polycrystalline diamond substrate with small dimensions (1 cm2). This technique is a standard process in GaAs and Si MMIC manufacturing, but it was never used before on diamond devices. Multi-finger MESFETs were realized on H-terminated polycrystalline diamond. The adopted device structure is a typical coplanar multi-finger gate RF layout fabricated by optical photolithography except for gate realization which is performed by e-beam lithography. In this structure, Ti/Au source and drain ohmic contacts are realized by thermal evaporation. Device active area is then defined by reactive ion etching in oxygen and argon plasma, accomplishing at the same time electrical isolation among FET structures. It is worth to note that the isolation pattern was established in order to force the drain to source current flow underneath the gate, thereby avoiding parasitic resistance effects. I-shaped Al-gate electrode was defined by a single-layer e-beam lithography process in order to achieve different gate lengths (0.5, 1.0, 2.0, 4.0 μm) followed by e-beam evaporation.Referring to polycrystalline diamond inductors, we’ve constructed multi-turn devices with conductor width W=20 μm, spacing between turns S=20 μm and inner diameter Di =100 μm, varying the number of turns n from 1.5 to 5.5. The innermost turn is connected to other circuitry by using a conductor that passes under air-bridges. Instead, for diamond capacitors, we’ve chosen the square configuration having the following parameters: dielectric thickness d = 2 μm height, material dielectric constant εr = 2.6 (PMMA) and plates area that varies from 50x50 μm2 to 200x200 μm2. We’ve simulated the frequency behavior [0-20 GHz] either of diamond inductors or capacitors using the SONNET electromagnetic simulator. For inductors having W=S=20 μm and Di =100 μm, we expect inductance values from 0.66 nH (for 1,5 turns) to 9.3 nH (for 5.5 turns) while for diamond capacitors with square configuration and d = 2 μm, εr =2.6 (PMMA) we expect capacitance values from 0.04 pF (for 50x50 μm2 area) to 0.52 pF (for 200x200 μm2area).