Advances in the Fabrication of On-chip Superconducting Integral Field Units for CMB and Line-Intensity Astronomy

Abstract

Studying the polarization and spectral distortion of the Cosmic Microwave Background (CMB) in tandem with intensity fluctuations of the Cosmic Infrared Background (CIB) allows us to verify our assumptions on cosmic inflation and investigate the dynamics and evolution of galaxy clusters in the last 10 billion years. Because of its broadband emission and being an all-sky extended source, observing the entire CMB in detail is a very time-consuming and expensive exercise. Fortunately, in the last few years, the on-chip superconducting spectrometer technology has moved out of the lab and into the telescope. With its compact size and background-limited sensitivity, this family of instruments is particularly well-suited for fast and large area observations in a relatively unexplored range of the electromagnetic spectrum. However, recent examples of this technology do not yet reach the requirements needed for large spectroscopic and polarimetric surveys of the CMB. We formulate several of these requirements and introduce novel on-chip components and fabrication techniques. We introduce a cross-over to enable distinguishing signal polarization, minimize signal loss by locally optimized lithography of a coplanar-waveguide (CPW), lower the spectral resolution of microstrip filters by deposition of a dielectric layer, and increase the yield of the spectrometer array by removing individual line shorts. These together have culminated in the successful fabrication of a fourteen-spaxel IFU.

Figures (5)

• Figure 1: An overview image of the on-chip improvements discussed in this article. a. shows a schematic of a spaxel above a successfully fabricated fourteen-spaxel IFU. The four advances presented in this paper are b. the microstrip crossing to enable dual-pol reception, c. the CPW me mbrane-substrate transition (a. shows a microstripline instead),d. a dielectric-covered microstrip power coupler, and e. a repaired section of readout line. The colored lines indicate their usual on-spaxel position. More detailed images will be presented in the following sections.
• Figure 2: a. The signal lines from both sides of the dual-pol Leaky Lens antenna will inevitably cross, indicated by the circle. b. Using a polyimide (red) and aluminum (light grey) bridge style structure, the lines can cross with minimal cross-coupling. c. SEM image of a coupler that showed excellent transmission. The dashed line indicates the cut of the cross-section in b.
• Figure 3: a. The slanted resist layer causes scattered electrons (blue), during e-beam patterning, to overexpose the area around the transition (purple). b. shows an optical image of the step as a result of this overexposure. The red square highlights the overexposed area and the shorted line. c. shows a SEM image of the NbTiN CPW transmission line (orange) across the step at the edge of the SiN membrane (green) after the exposure dose is reduced from 1100 to 500 $\mathrm{\mu C}/\mathrm{cm}^2$ during patterning.
• Figure 4: a. the PECVD aSi:H coverlayer (blue) on top of a NbTiN microstrip filter and coupler (yellow) can be used to increase $\epsilon_{eff}$. As can be seen in this FIB image, cavities (red) have formed between the filter and the coupler due to the semi-isotropic growth of the film. b. shows a top view of the structure. The dashed line represents the location of the cross-section. The top of this line corresponds to the right side of the a. For clarity, the cover layer is not shown.
• Figure 5: Spaxels on a multispaxel-IFU share long readout lines that, if shorted, will render the detector useless. a) shows a potentially shorted CPW line. b) shows the same area with part of the NbTiN ground plane etched away using microscope exposure. Removing this material isolates the defect and prevents shorting of the line. c) emphasizes the length of the readout line (blue) in comparison to an optical image of a successfully fabricated fourteen-spaxel array. The red circles highlight several potential short sites.