Fabrication of a Terahertz Reciever Array for SOfIA Using Superconducting Hot- Electron Bolometer (HEB) Mixers and Micromachined Waveguide Components Aaron Datesman University of Virginia Microfabrication Laboratory With credit to Art Lichtenberger and Jon Schultz (UVA), Chris Walker and Dathon Golish (UAZ), and Jacob Kooi (CalTech)

OUTLINE: Motivation: Submillimeter Radio Astronomy and SOfIA Overview: Spectroscopy and Downconversion A Small Amount about Mixers and Superconductivity Design & Construction of the 1x5 Terahertz Array Two Significant Fabrication Issues HEB Physics and Operation FABULOUS ACRONYMS! SOfIA = Stratospheric Observatory for Infrared Astronomy HEB = Hot-Electron Bolometer FIB = Focused-Ion Beam

SOfIA’s Submm Science: Interstellar cloud physics and star formation in our galaxy. Proto-planetary disks and planet formation in nearby star systems. Origin and evolution of biogenic atoms, molecules, and solids. Composition and structure of planetary atmospheres and rings, and comets. Star formation, dynamics, and chemical content of other galaxies. The dynamic activity in the center of the Milky Way. Ultra-luminous IR Galaxies (ULIRGS) as a key component of the early universe.

5 – 300 microns wavelength 9 first-light instruments First light winter 2005!

Protostellar 4448-mm Nisini 1999 / CFA Spectroscopy – Astrochemistry: build and verify models of stellar creation and evolution, radiative processes and absorption, etc. Submillimeter wavelength regime: very rich in rotational and vibrational molecular transitions

Heterodyne Downconversion To detect a spectral line is easy. To distinguish it from all of the other radiation a detector absorbs at the same time is hard! By mixing the RF signal with a reference provided by the experimenter, the “Local Oscillator”, the spectrum is downconverted and may be analyzed.

Mixing Elements: Superconducting and Otherwise Gerecht, 1998

Why Superconductors? Superconductors have an energy gap of order ~meV (gap frequency for niobium is ~ 700 GHz). Cryogenic operation reduces noise. Mixing is a process which requires a non-linear I(V) characteristic. Many of the characteristics of superconductive devices, including the R(T) transition, are also very non-linear. Why Hot-Electron Bolometers (HEBs)? Better noise performance than Schottky diodes. SIS junctions become lossy above the gap frequency. Requires less LO power than either Schottky diodes or SIS junctions.

1.45 THz Receiver: Feedhorn Block HEB Block Backshort Block

1.45 THz Feedhorn Block Half Fabricated by laser microchemical etching of silicon

Laser Micromachining of Silicon Fast laser microchemical etching without reference to crystal planes

HEB Mixer: Waveguide probe, chokes, CPW output line Electromagnetic simulation using HFSS

HEB: A thin microbridge of Nb contacting Au on each end

Focused-Ion Beam: The FIB Series 200 from FEI Applications: Cross-sectioning / TEM prep Trimming and IC repair Compositional profiling Micro- and nano-fabrication Specifications: < 80 Angstroms spot size CAD control, 4096 x 4096 pixels Milling and deposition (SiO & Pt) SIMS and end-point detection Secondary electron imaging

HEB sits on a 0.75 micron thick silicon nitride membrane Placing the backshort underneath the mixer allows us to create an array

Bulk Micromachined Pyramidal Backshorts The actual backshort will be laser-micromachined into this pedestal.

Fabrication Issue #1

Fabrication Issue #2

How Does It Work? Bolometer is a thermal device Thin superconducting film absorbs RF radiation above gap frequency Diffusion of heat to contacts creates temperature profile Portion of microbridge is superconducting (no R), portion is normal – “Hot Spot” Power ~ V 2 creates beats Temperature distribution follows envelope at IF IF bandwidth ~ a few GHz  3 dB ~ 1/L 2