Properties of Unshunted and Resistively Shunted Nb/AlOx-Al/Nb Josephson Junctions With Critical Current Densities From 0.1 to 1 mA/μm2

We investigated current–voltage characteristics of unshunted and externally shunted Josephson junctions (JJs) with high critical current densities J_c in order to extract their basic parameters and statistical characteristics for JJ modeling in superconducting integrated circuits as well as to assess their potential for future technology nodes. Nb/AlO_x-Al/Nb junctions with diameters from 0.5 to 6 μm were fabricated using a fully planarized process with Mo or MoN_x thin-film shunt resistors with sheet resistance R_sq = 2 Ω/sq and R_sq = 6 Ω/sq, respectively. We used our current standard MIT Lincoln Laboratory process node SFQ5ee to fabricate JJs with J_c = 0.1 mA/μm² and our new process node SFQ5hs (where “hs” stands for high speed) to make JJs with J_c = 0.2 mA/μm ² and higher current densities up to about 1 mA/μm². Using LRC resonance features on the I–V characteristics of shunted JJs, we extract the inductance associated with Mo shunt resistors of 1.4 pH/sq. The main part of this inductance, about 1.1 pH/sq, is the inductance of the 40-nm Mo resistor film, while the geometrical inductance of superconducting Nb wiring contributes the rest. We attribute this large inductance to “kinetic” inductance arising from the complex conductivity of a thin normal-metal film in an electromagnetic field with angular frequency $\boldsymbol{\omega },$ ${{\mathbf \sigma }}(\boldsymbol{\omega }) = {\boldsymbol{\sigma }_0}/({1 + \boldsymbol{i\omega \tau }})$, where ${{{\mathbf \sigma }}_0}$ is the static conductivity and $\boldsymbol{\tau }$ the electron scattering time. Using a resonance in a large-area unshunted high-J_c junction excited by a resistively coupled small-area shunted JJ, we extract the Josephson plasma frequency and specific capacitance of high-J _c junctions in 0.1–1 mA/μm² J_c range. We also present data on J_c targeting and JJ critical current spreads. We discuss the potential of using 0.2-mA/μm² JJs in very large scale integration single flux quantum circuits and 0.5-mA/μm² JJs in high-density integrated circuits without shunt resistors.