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/μm
²
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/μm
²
. 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 ω, σ(ω) = σ
₀
/(1 + iωτ), where σ
₀
is the static conductivity and r 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/μm
²
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/μm
²
JJs in very large scale integration single flux quantum circuits and 0.5-mA/μm
²
JJs in high-density integrated circuits without shunt resistors.