The Simons Observatory: Characterization of All DC/RF Routing Wafers for Detector Modules

TL;DR

The paper addresses the need for uniform TES biasing across a large set of DC/RF routing wafers that couple detector arrays to the SQUID multiplexer in the Simons Observatory. It employs cryogenic screening at $T \approx 100\,\mathrm{mK}$ using four‑wire resistance measurements to determine the average shunt resistance $R_{sh}$ for each bias line, and it tracks yield and shorts. The key findings are a mean $R_{sh}$ of $396\,μΩ$ with a standard deviation of $16\,μΩ$ (about 4%), a small but systematic radial dependence from wafer center, and a strong agreement between cryogenic $R_{sh}$ and room-temperature metrics ($R_1$ thickness and $R_{sheet}$). The results validate the routing-wafers design and screening process, enabling reliable NEP performance and deployment in SO UFMs and the ASO upgrade.

Abstract

The Simons Observatory (SO) is a cosmic microwave background experiment with over 67,000 polarization-sensitive transition-edge sensor (TES) detectors currently installed for use in observations and plans to increase the total detector count to ${\sim}$98,000 detectors with the Advanced SO upgrade. The TES arrays are packaged into Universal Focal-Plane Modules (UFMs), which also contain the multiplexing readout circuit. Within a readout module, a DC/RF routing wafer provides a cold interface between the detectors and the readout multiplexing chips. Each routing wafer hosts twelve bias lines, which contain the ${\sim}$400 $μΩ$ shunt resistors that are part of the TES bias circuitry. More than 70 routing wafers have been fabricated and tested both at room temperature and 100 mK before integration into UFMs. The lab measurements for all screened wafers have been compiled to show the distribution of measured average shunt resistance Rsh for each bias line, both across bias lines on a single routing wafer and across all routing wafers. The mean average shunt resistance for all wafers was found to be 396 $μΩ$ with a standard deviation of 16 $μΩ$, or ${\sim}$4%. For each wafer, we note good uniformity of average Rsh between bias lines, with a slight downward trend with increasing distance from the center of the wafer. The fabrication data collected at room temperature shows agreement with the cryogenic measurements of Rsh distribution.

Figures (7)

• Figure 1: Exploded view of an SO UFM showing the optical coupling and multiplexing readout componentsmccarrick202190150ghzuniversalHealy_2022 including the location of the routing wafer, shown in green. The cutouts in the routing wafer design leave open space for the multiplexer chips as well as PCBs and dowel pins for mechanical assembly (see Figure \ref{['fig:waferdiagram']}).
• Figure 2: The TES bias circuitry and routing wafer bias line shunt resistors. Each TES is in parallel with a low-impedance shunt resistor so that the bias voltage can be set with a current, $I_{bias}$, flowing into the circuit and in series with an inductor which couples to an RF-SQUID so that the current of the TES is read out.
• Figure 3: A routing wafer mounted onto a PCB and installed in a Bluefors LD400 dilution refrigerator for cryogenic testing.
• Figure 4: The distribution of all measured average bias line shunt resistances. Each average $R_{sh}$ measurement is weighted by the number of shunt resistors in the bias line (see Table \ref{['tab:bl_channels']}), and the y-axis is normalized such that the area under the curve is one. The distribution has a mean value of 396 $\mu \Omega$ and with a standard deviation of 16 $\mu \Omega$, or $\sim$$4 \%$. This is within agreement of the nominal 400 $\mu \Omega$$R_{sh}$ for SO TES detectors.
• Figure 5: Routing wafer diagram showing approximate locations of shunt resistors for each bias line, highlighted in color by bias line. Bias lines were divided into pairs based on relative distance from the center of the wafer so that we could analyze radial dependence of $R_{sh}$.