KIPM-DETECTOR DEVELOPMENT
4.3 Early Device Tests
4.3.1 Feedline-only Tests
Figure 4.4: An example KIPM-detector design with CPW feedline and MKIDs.
The feedline widens near either end to make wire-bonding possible. A zoom-in on the central MKID shows (in red) the parts that would be fabricated out of Nb (feedline, ground shield, and capacitor). Only the inductor (green) is sensitive to energy dispositions and would therefore be fabricated out of Al. In this design, only one MKID was intended to be sensitive. The others would be used for calibration purposes.
Figure 4.5: The mask design for producing a feedline-only device. This device was used to produce the results in Figure 4.7. The solid red trace going up and down the wafer is the CPW feedline (the separation between signal and ground is impossible to see at this scale). The wider pads at the bottom were used for wire-bonding.
After fabrication, the device was mounted in our 3" wafer testing box. The box is shown in Figure 4.6. The primary purposes of the box are thermal sinking, connecting coaxial cables to the feedline, and minimizing optical loading. The 3"
box is gold-plated copper.
Figure 4.6: A feedline-only device (using the mask from Figure 4.5) mounted in the 3" device box. The device box is mounted in our 4-Kelvin cryostat. In this photo, the box lid was removed. The box lid is a flat hexagon which would otherwise close the box, blocking our view of the device and providing some protection from light.
After cooling the device in our cryostat, the test consisted of measuring RF trans- mission and reflection as functions of frequency using a vector-network analyzer (VNA). The setup only required connecting the two VNA ports to the two device ports using coaxial cables and cryostat feed-throughs.
Shorter feedlines with fewer turns were observed to have more uniform transmission [69]. Unfortunately, short feedlines are not sufficient to construct a multi-MKID KIPM capable of accurate position reconstruction. We suspect the uneven trans- mission to be (at least partially) due to feedline coupling to the resonant modes of the cavity formed by the device box. The copper device box creates E=0 bound- ary conditions and therefore defines the length-scale of the coupled modes. We experimented with minimizing the coupling by comparing the transmission of the device using various configurations of the box: box with both lids attached, box with both lids removed and raised off the cold-plate surface, and box with both lids attached and microwave-absorbing Eccosorb foam placed between the device and the feedline-facing lid. Changing the box configurations altered the boundary
Figure 4.7: The feedline-only device transmission in various states of enclosure.
The red transmission is mounted in the box with both lids attached. The green transmission is mounted in the box with both lids removed and raised off the cold-plate surface. The blue transmission is mounted in the box with both lids attached and microwave-absorbing Eccosorb foam placed between the device and the feedline-facing lid. Due to uncertain attenuation before and after the device, the transmissions should only be considered relative to each other.
The results confirmed that coupling to the box modes was one major contributor to feedline-transmission structure. Removing the lids removed structure at some frequencies and added structure at others. The structure that disappeared with the lids was likely caused by box-mode coupling. Structure that appeared when the lids were removed was likely caused by coupling to the resonant modes of the cavity formed by the cryostat itself (no longer shielded by the lids). Structure that appeared with and without the box lids may have been the result of coupling to modes defined by the non-lid parts of the box. While it is difficult to assign each transmission feature to a specific coupling, the results confirm that the box does have a large effect.
The Eccosorb configuration was included to approximate replacing the lid with infinite free space (another boundary condition of interest). Since Eccosorb is a microwave absorber, we would expect the box-mode coupling to disappear without exposing the device to additional cryostat-mode coupling. This seems to agree with the last result in Figure 4.7. The sharpest structures in the transmission disappeared without significant new structure appearing.
An outstanding concern is the temperature of the Eccosorb foam during the test. We did not have an adequate method to thermally sink the foam to the device box. The foam was therefore at a completely unknown temperature during the test. It may have remained significantly above 4 Kelvin. This would have caused the foam to radiate infrared (IR) photons directly at the substrate and feedline. IR photons should not affect the transmission except to lower it overall due to the feedline possibly having an effective temperature above 4 Kelvin. That said, the overall transmission being similar in magnitude to both other tests confirms the Nb remained superconducting.
Radiating Eccosorb would not be acceptable for use near an actual device. Raising the effective temperature of the MKIDs would degrade their internal quality factors and quasiparticle lifetimes5. Eccosorb is also too radioactive for placement adjacent to a detector searching for dark matter.
Satisfied that the worst transmission features were not inherent to the CPW feedline design or fabrication, we proceeded to make (and test) full devices with MKIDs and ground shields included. It is possible that the transmission features may be removed in the future using a non-radioactive heat-sinkable absorber instead of Eccosorb.
Another option may be to update the feedline with special corner designs intended to keep the signal and ground lines in phase thus minimizing the distance over which the feedline can couple [72].
4.3.2 80 MKID, Nb-feedline KIPM
One of our most exhaustively studied devices was fabricated by Yen-Yung Chang beginning on July 26th, 2018. As the second of two devices started by Yen-Yung on that date, it received the identifier YY180726.2. This device used 80 Al MKIDs with designed frequencies between 3.05 and 3.45 GHz. The feedline was fabricated out of Nb to minimize wasted phonon energy (as described in Section 4.2.3). The device substrate is a 1-mm thick, 3"-diameter Si wafer. The Al MKIDs are 30 nm thick. The Nb feedline is 300 nm thick to ensure good continuity across the wafer.
An image of the mounted device YY180726.1 can be seen in 4.8. After fabrication, this device was used to for many of the initial MKID-characterization tests described below.
5We did see degraded internal quality factors when the Eccosorb test was repeated using a KIPM with previously characterized MKIDs.
Figure 4.8: The device referred to with the identifier YY180726.2 mounted in a device box (with lid removed) in our Oxford Kelvinox 25. This device has 80 Al MKIDs and a Nb CPW feedline.