Superconducting niobium integrated circuit technology employing Josphson tunnel junctions is well established, reliable, and reproducible. It has been used to make circuits with 70,000 junctions on a chip and has the potential for making single flux quantum (SFQ) circuits that operate at 50 GHz and above with extremely low power dissipation. However, it is not the ideal technology for these circuits, since the tunnel junctions must be shunted with resistors to yield the needed I-V characteristics. The goal of this project is to provide a replacement for the tunnel junction and its shunt resistor in the form of an internally shunted Josephson junction. The elimination of the external shunt not only saves space, but also avoids steps in the fabrication process and minimizes parasitic inductances that complicate circuit design and are detrimental to performance.
We are working in collaboration with a group at Arizona State University to form Josephson junctions by subtractive etching of the so-called “pentalayers.” A pentalayer covers an entire 4” silicon wafer and has the thin film form Nb/NbTiN/TaN/NbTiN/Nb, in which the TaN layer is the junction barrier. In varying the nitrogen content in TaN, the resistivity passes through the metal-insulator transition. By judicious choice of barrier thickness and resistivity, we can control the junction current density and shape of its I-V characteristic.
The experimental results are extremely encouraging. The I-V characteristics show a typical resistively shunted junction behavior. Critical current density of about 20 kA/cm2 has been obtained. This value is close to the desired critical current density for niobium-based junctions. Also, the correlation between the barrier resistance and the critical current has been observed. A parameter that determines the upper speed of superconducting SFQ digital circuit is the product (IcR) of the maximum zero-voltage current Ic and junction resistance R in the absence of supercurrent. We have obtained IcR products that will allow circuit operation at over 100 GHz and are quite similar to junction with such ideal properties. Our current effort is focused on optimizing the deposition parameters of barrier TaN and improving the reproducibility of junctions. The next step will be directed toward demonstrating various circuits using this new technology.