EWLT EW
6.3 Cm 2 Proof-of-Concept Device
6.3.2 Results
After fabrication, the devices were tested in an Oxford Instruments Kelvinox 25 dilution refrigerator (TBase ≈ 70 mK). The readout system utilized the FPGA- based Reconfigurable Open Architecture Computing Hardware (ROACH) devel- oped by the Collaboration for Astronomy Signal Processing and Electronics Re- search (CASPER) [116]. This system utilizes dual digital-to-analog converters (DAC’s) which generate a comb of tones at baseband (fDAC < 500MHz) to ex- cite the resonators. These signals are then up-converted to the 3 GHz band at which the MKID’s operate using a customIQmixer. After being run through our device, this signal is then down-converted using the same model IQ mixer and digitized
9Specifically, the Cannon EX3 stepper system. The thickest wafer this system can easily align to is 1 mm. Similarly, a single stepper-field was 22 mm by 20 mm when projected on the substrate.
Figure 6.4: Schematic of the mask used to pattern our proof-of-principle device.
The layout consists of a simple rectilinear grid of 20 resonators. Inset detail: each MKID is roughly 0.8 mm by 1.5 mm although the exact dimensions are slightly different from one another to ensure different Lm, and achieve frequency-domain multiplexing.
using dual analog-to-digital converters (ADC’s). Custom firmware developed for this ROACH system then implements a 20-channel digital downconverter (DDC) to isolate each frequency channel. The system provides the amplitude and phase of the signal transmitted past each resonator at 1.3 MHz per channel, with on-board buffering and triggering. The total system bandwidth is 340 MHz, easily encom- passing the 200 MHz bandwidth of the array. For a schematic of this please see figure 6.5. TiNx films proved problematic for these devices. Changing the amount of nitrogen in the TiNxfilms changes the gap, and allowsTcto be tuned from 4 k for stoichiometric TiN to∼0.5 K for highly substoichiometric films. Collecting ballis- tic phonons required films withTc < 2K, which is well into the substoichiometric regime. Each MKID’s responsivity is highly sensitive to this exact chemistry, and it proved impossible to deposit films with a high enough uniformity to be useful.
An example of the phase response for each of the 20 resonators following a 200 keV interaction in the substrate from a cosmic-ray can be seen in Fig. 6.6. The location of the interaction can be reconstructed from the relative fraction of energy parti- tioned into each of the 20 MKIDs (just as in section 2.4.5). This allows for a local
Figure 6.5: (a) Schematic of our ROACH-based readout system. Many hundreds of MKIDs can be read out simultaneously using these relative simple and cheap room temperature electronics, which are shown in (b) [49].
energy recalibration to be preformed as seen in Fig. 6.7. The best-fit resolution after the position dependent correction is preformed and the periphery of the device is excluded isσ = 0.55 keV at 30 keV (from an129I line). This is almost a factor of 2 better than the uncorrected resolution and brings us within 40% of the baseline resolution ofσ=0.38 keV.
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Figure 6.6: Example phase response of a 200 keV cosmic ray event. The pulses have all been normalized against their respective MKID’s relative respon- sively, and an anti-aliasing filter has been applied. The resonator closest to the interaction site (thick black pulse) has both the greatest energy partition as well as the most prompt response. (in- set) This is used to construct a two tem- plate optimal filter fit to the data (solid fit). This contains a prompt component (dotted-dashed) that reflects the quasi- particle lifetime in the MKID as well as a slower component (dashed) represent- ing the phonon lifetime in the substrate.
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Figure 6.7: Observed spectrum when the substrate face opposite the res- onators is illuminated with x-rays from an 129I source. The light histogram shows the reconstructed energy for all events prior to correcting for position- dependent smearing of the response; the dark histogram is after. The best fit resolution at 30 keV is σE=0.55 keV.
(inset) Fit to the reconstructed energy spectrum for randomly triggered noise traces. The inferred baseline resolution isσE=0.38 keV.
After the success or our proof-of-concept devices discussed in the last chapter, we next turned to prototyping an iZIP that would utilize MKIDs as the phonon-sensing element. The most obvious change here would be one of scale. The proof-of- concept was a 2 cm x 2 cm x 1 mm chip with twenty resonators, while iZIPS are single-wafer devices that each use an entire◦76 mm x 25 mm thick substrate. Aside from a simple scaling-up of our device, as we have seen in chapter 2, iZIPs contain a few important design features not present in our proof-of-concept. In what proved to be a somewhat ambitious decision, we choose to tackle all of these considerations simultaneously. As a result, the initial series of full-wafer devices we produced very much resemble an iZIP with it’s QET’s replaced by MKIDs and will be referred to as “iZIP-MKID” or “iZID” devices.