Fabrication and Characterization of Superconducting Resonators

The overall goal of this procedure is to fabricate superconducting microwave resonators and characterize their scattering parameters. This method enables one to design and implement superconductors insrumentation thorough the catheterization of superconducting lines on the electric substrate. For example, the internal quality factors, loss fractions and connecting that infractions can be The main advantage of these techniques is that they allow realization of superconducting resonators on both sides of a silicon substrate, which enables precise measurements.

The implications of this technique extend to a wide range of applications, including kinetic inductance detectors for the detection of faint astrophysical signals as well as quantum computing applications and materials characterization. To begin, onto a freshly-cleaned silicon on insulater wafer. Spin coat a photo-resist layer at 4, 000 rpm for 30 seconds.

Next, electron-beam deposit germanium onto the wafer. Next, spin on a layer of thin depositive photo resist at 2, 000 rpm for 30 seconds and then bake the wafer on a 110 degree celsius hotplate for one minute. Now, use a mask aligner to expose the photo-resist.

And spray on the develop resist TMAH-based solution. The next step is to reactive ion etch the fabricated germanium layer with sulpher hexaflouride and oxygen plasma at 70 watts. Now, ash the underlying photo-resist layer with oxygen plasma inside a reactive ion etcher to get an undercut of photo-resist.

Next, using a DC Magnetron, sputter deposit a niobiam ground plane onto the wafer with 3.7 millitor of argon at 500 watts. Then, lift off the ground plane, by submerging the wafer in acetone for four hours. Next, spin coat BCB onto the niobium-coated wafer at 4, 000 rmp for 30 seconds and then do the same with a new silicon wafer.

Now, bond the two BCB-coated surfaces together with three bars of pressure at 200 degrees celsius. Next, flip the wafer stack over, and begin processing the backside of the silicon on insulator wafer. Etch the silicon handle wafer, using mechanical lapping with an aluminum oxide slurry composed of 150 milliliters of aluminum oxide powder, 30 micron diameter, and 1, 500 milliliters of water.

Set the rotation of the lapping plate to 45 rpm and let this go for two to four hours. The most critical step is thinning the wafer via mechanical lapping. This is because during the first fabrication run, 50%of the wafers were broken.

The breakage problem was mitigated by applying heat-released tape to the back of the wafers. Next, use the Bosh Process to deeply reactive ion etch the remaining silicon handle wafer. Etch the buried silicon oxide layer with hydrofluoric acid diluted one-to-ten in water for 20 minutes.

Continue using the same techniques to deposit and etch a molybdenum nitride layer, followed by adding another germanium layer. Thus fabricate the microstrip line resonator. For this procedure, have prepared a gold-coated copper cavity test package and a controlled impedance microwave fan-out board to root the signals between the chip and sub-miniature version A or SMA connectors.

The SMA connectors were previously inserted into the input and output of this test package with the center conductor pin aligned and then soldered to the corresponding fan-out board contact pad. The resonator chip was previously mounted into the gold-coated copper package cavity with the on chip feed line output and input pads next to their corresponding fan-out boards CPW lines. The chip should be secured with copper clips, which make contact at the edges of the chip.

Super conducting aluminum wire bonds were previously placed between the fan-out board and on chip contact pads, with as many bonds as needed to impedance match between the SMA connector input and outputs and the on chip CPW feed line. After wire bonding, with a multi-meter, check the DC resistance between the center pins of the input and output connectors. And between a center pin and ground, to confirm there is an electrical connection across the two center pins and an open connection between the center line and ground.

The test bed is assembled with a series of SMA cables rooted from room temperature, to a 0.3 kelvin cold stage where the device will be mounted. Copper and super conducting niobium titanium cables are used to minimize microwave loss. For a thermal break between the two kelvin and 0.3 kelvin stages, niobium titanium cables are used.

For low noise amplification in the band of the resonator device a cryogenic high-electron mobility transistor amplifier is mounted to the two-kelvin stage on the output line. Also note, that on the output line, at the input to this amplifier, there is a cryogenic circulator. At the 0.3 kelvin cold stage, mount the packaged resonator devices onto the bolted down bracket.

Next, connect a microwave attenuater on the input side of the package to provide a matched termination. Then, connect the appropriate SMA cables to this attenuator input and package output. These terminations define the device calibration plane.

Now close up the cryostat and follow the standard procedure used to cool the devices to 0.3 kelvin. To begin, set up the VNA. Scan between 10 megahertz and 8 gigahertz or a similarly wide band, ensuring that the power level does not exceed the critical current of the superconducting microwave resonator and superconducting feed line.

Adjust the power level to around minus 30 decibel milliwatts, or a similarly suitable level that provides an adequate signal to noise ratio. Next, calibrate the flexible radio frequency cables following standard short open load through procedure. Using the VNA software as directed by the manufacturer.

This calibration defines the instrument reference plane. After the calibration, verify its fidelity, by confirming the transmission S21 with the through-line connected. The measured VNA response should have low residual errors, near 0 decibels an S11, an S22 should both measure below minus 50 decibels.

Next, connect the flexible cables to the input and output lines of the cryostat. Then, turn on the cryogenic microwave amplifier by applying the manufacturer-specified DC bias voltage for the microwave amplifier. Then, begin the measurements.

First, complete a wide-band scan, to observe the S21 baseline structure and to look for any sharp high-Q structures, which are indicative of microwave resonators. Then, narrow the frequency band range. Make it wide enough to provide an adequate base line for later fits and inseech-you calibration.

Then adjust the number of data points. In this case, to about 30, 000. A half-wave molybdenum nitride resonator was fabricated on both sides of a 0.45 micron single crystal silicon dielectric.

The dielectric was coupled to a niopium co-planer wave-guide feed line for readout via a capacitive coupling through a sputtered deposited silicon oxide dielectric. This is see as the eight shaped region at one of the open ends of the resonator. The magnitude of the measured transmission coefficient, S21 of this feed line, at the VNA reference plane was measured as a function of frequency.

At each resonator's resonance frequency a dip in the transmission magnitude indicates that the microwave power was coupled to the resonators. Following these procedures, the data gathered can be analyzed following a inseech-you calibration method to extract the detailed resonator in electromagnetic parameters of interest. After its development, this fabrication technique paved the way for researchers in the field of microwave kinetic conductance detectors to develop novel ultra-low loss detector arctic, architectures.