NbN Nanostrip Phase Switch using e-beam lithography and reactive ion etching

(in scope of European project CLOVER - Telescope for Measuring  CMB Polarization).

Ernst Otto and Leonid Kuzmin

Niobium Nitride is a superconductor with the critical temperature of about 14 K and rather high-ohmic material in normal state, that is at temperatures above the critical one or when density of a current flowing through a sample of NbN exceeds the critical value.

The idea behind this project is to use the property of NbN to switch between normal and superconductive state when the current through the sample is switching between two different values, the first one below critical current, and the second value above the critical one.  The NbN nanostrip line is located between two parts of a finline and operates as a short in superconductive state or as high resistive line in the normal state.

As a result, the finline is reflecting all incoming microwave signal when the NbN nanostrip is a superconductive short or transmitting most part of the incoming signal when NbN finline is in the normal state and works as open circuit for the microwave signal because its wave impedance is much greater than the characteristic impedance of the finline in the middle.

The goal of the project is to manufacture on-chip finline with NbN nanostrip in the middle and to measure resistance and critical current of the nanostrip.

The work will be performed in the MC2 cleanroom using negative e-beam lithography and reactive ion etching of NbN in CF4 plasma as a first step and photo lithography with following thermal evaporation with lift-off for contact pads as a second step.

In the case of successful samples, the reflection and transmission coefficients will be measured in normal or superconductive state of nanostrip for incoming microwave signal at 220 GHz. The measurements will be done in Oxford and Cardiff Universities in scope of joint European project CLOVER - Telescope for Measuring  CMB Polarization.

The results will be summarized in the form of a report and could be presented for publication. It is possible to prolong this work with participation in ambitious European project CLOVER.

Contact:
Leonid Kuzmin
<leonid.kuzmin -et- mc2.chalmers.se>