Advanced Fabrication Processes for Superconducting Very Large Scale Integrated Circuits
We review the salient features of two advanced nodes of an 8 Nb layer fully planarized process developed recently at MIT Lincoln Laboratory for fabricating single flux quantum (SFQ) digital circuits with very large scale integration on 200 mm wafers: the SFQ4ee and SFQ5ee nodes, where "ee" denotes that the process is tuned for energy efficient SFQ circuits. The former has eight superconducting layers with 0.5 mm minimum feature size and a 2 O/sq Mo layer for circuit resistors. The latter has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm and a thin superconducting MoNx layer (Tc 7.5 K) with high kinetic inductance (about 8 pH/sq) for forming compact inductors. A nonsuperconducting (Tc 2 K) MoNx layer with lower nitrogen content is used for 6 O/sq planar resistors for shunting and biasing of Josephson junctions (JJs) Another resistive layer is added to form interlayer sandwich type resistors of milliohm range for releasing unwanted flux quanta from superconducting loops of logic cells. Both process nodes use Au/Pt/Ti contact metallization for chip packaging. The technology utilizes one layer of Nb/AlOx Al/Nb JJs with critical current density Jc of 100 mA/mm2 and minimum diameter of 700 nm. Circuit patterns are defined by 248 nm photolithography and high density plasma etching. All circuit layers are fully planarized using chemical mechanical planarization of SiO2 interlayer dielectric. The following results and topics are presented and discussed: the effect of surface topography under the JJs on the their properties and repeatability, Ic and Jc targeting, effect of hydrogen dissolved in Nb, MoNx properties for the resistor layer and for high kinetic inductance layer, and technology of milliohm range resistors.
IEEE Transactions on Applied Superconductivity