Microwave-Induced Characteristics of ${({\rm NbN/TiN}_{\rm x})}_{N}/{\rm NbN}$ StackedJosephson Junction Arrays

Stacked (NbN/TiN_x)_N/NbN (N = 2, 3, 4, 5 and 10) Josephson junctions and their arrays have been fabricated on a Si chip and their microwave-induced characteristics were measured at a temperature around 10 K. To achieve a vertical and smooth edge of an NbN - TiN_x multilayer, an inductively coupled plasma reactive ion etching (ICP-RIE) technique was used. Current-voltage characteristics measured for stacks of (NbN/TiN_x)_N/NbN Josephson junctions without microwave indicated that critical currents for those junctions distribute in a certain range. We defined I_c - spread as (I_{c MAX} - I_{c MIN})/I_c Average to measure the distribution of Ic. The I_c - spread was minimized when the thickness of intermediate electrodes was d = 50 nm. We also evaluated values of I_c - spread for stacks prepared at different fabrication runs as a function of the number of junctions in a stack N. As a result, we found N = 3 is the maximum for practical use. For arrays, we obtained a constant-voltage step height of about 2 m A for 2 048 stacks of double-barrier junctions (N = 2), and about 1 mA for 1 024 stacks of triple-barrier junctions (N = 3).