Optimization of lenslet arrays for PRIMA Kinetic Inductance Detectors

TL;DR

PRIMA aims for high-sensitivity far-infrared observations with FIRESS, requiring efficient coupling of fore-optics to ~11,000 background-limited KIDs. The authors develop monolithic kilo-pixel silicon lenslet arrays fabricated by grayscale lithography and DRIE, optimize per-band lens geometries (including hexagonal corners for Band 4), implement stepped Parylene-C AR coatings with a planned four-thickness scheme, and refine epoxy bonding to the KID wafer. Key results include improved lens-profile fidelity (RMS $<0.9 \mu$m), a ~14% gain in light collection from hexagonal-corner designs, and bond layers kept within a 1–6 µm range to satisfy a ≤5% loss budget, validated by transmission measurements and destructive metrology. These advancements enable high-efficiency, robust lenslet-KID integration for PRIMA FIRESS and provide broadly applicable techniques for other far-infrared instruments, with the assemblies undergoing flight-like environmental testing and in-cryostat optical characterization.

Abstract

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is a cryogenically cooled 1.8-m space telescope designed to address fundamental questions about the evolution of galactic ecosystems, the origins of planetary atmospheres, and the buildup of dust and metals over cosmic time. PRIMA will achieve unprecedented sensitivity in the 24 - 261 $μ$m wavelength range, enabled by background-limited kinetic inductance detectors (KIDs) cooled to 120 mK. For PRIMA's Far-InfraRed Enhanced Survey Spectrometer (FIRESS) instrument, we have developed monolithic kilopixel silicon lenslet arrays to efficiently couple incident radiation from the telescope's fore-optics onto the KID absorber elements. These three-dimensional lenslet arrays are fabricated using grayscale lithography, followed by deep reactive ion etching (DRIE), and are anti-reflection (AR) coated with a quarter-wavelength thick deposition of Parylene-C. The lenslet arrays are aligned and bonded to the KID arrays using a thin layer of epoxy through a flip-chip bonder. In this work, we report on the optimized fabrication, lens design, AR coating, and bonding processes developed for the FIRESS lenslet arrays. We characterize brassboard lenslet arrays fabricated to meet the specifications of the FIRESS low and high spectral bands, demonstrate stepped-thickness AR-coatings to achieve high efficiency across broad wavelength ranges, and present spectral transmission measurements of the AR coating and the epoxy bonding layers.

Figures (5)

• Figure 1: Top: Illustration of PRIMA lenslet-detector array configuration. The lenslets focus the incoming radiation from the telescope fore-optics onto the KID absorber elements. The lenslet and the detector arrays are fabricated on separate silicon substrates and then bonded together with a thin adhesive layer. Updated figure from cothard2024. Bottom: Photograph of a PRIMA FIRESS long waveband lenslet array. The 10.613 mm wide $\times$ 77.05 mm long die consists of 12 $\times$ 84 lenslets, hexagonally packed with a 900 µ m pitch.
• Figure 2: Profilometer measurements of a FIRESS Band 1 array. The top plot compares the design with the measured lens profiles from five different regions of the array. The corresponding residuals in the bottom plot show RMS error $<$ 0.9 µ m for all regions, except 1.5 µ m for the bottom of the array. Compared to our prior work (see Figure~2 in cothard2024), our improved fabrication process has further reduced differences between as-fabricated and designed profiles, especially at larger radii.
• Figure 3: Top: Scanning electron microscope (SEM) image of a FIRESS Band 4 array from our prior work cothard2024. Middle: SEM image of an upgraded Band 4 array with reduced flat spaces between the lenses for increasing light collecting area and controlling stray light. The concentric steps visible towards the center of the lenses are due to discrete grayscale exposure levels. Bottom: 3D profilometry of one of the lenses from the upgraded array. With a resist profile compensated for lateral etching effect (see Section \ref{['sec:fab']}), the lenses could be etched deep enough to produce the desired profile with hexagonal pockets.
• Figure 4: Measured spectral transmission for free-standing Epo-Tek 301 and Parylene-C samples cooled to 5 K. These data were used to extract the complex dielectric function of the materials at PRIMA wavelengths to optimize the bonding and the AR-coating thicknesses. The actual thicknesses (see Sections \ref{['sec:ar_coating']} and \ref{['sec:bonding']}) used for PRIMA lenslets are smaller, providing correspondingly higher transmission than the thicker free-standing samples shown here for optical characterization of the materials.
• Figure 5: Demonstration of two-thickness Parylene-C AR coating on a silicon test wafer. Top: Photograph of the test wafer with two thicknesses visible on the left and the right halves of the wafer. Bottom: Profilometry of the AR-coating thickness.