Fabrication and Characterization of Superconducting Resonators

Summary

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Abstract

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Introduction

Advances in astrophysical instrumentation have recently introduced superconducting microwave resonators for the detection of infrared light.^1-4 A superconducting resonator will respond to infrared radiation of energy E = hv > 2Δ (where h is Planck's constant, v is the radiation frequency and Δ is the superconducting gap energy). When the resonator is cooled to a temperature well below the superconductor critical temperature, this incident radiation breaks Cooper pairs in the resonator volume and generates quasiparticle excitations. The increase in the density of quasiparticle excita....

Protocol

1. Microstrip Line Resonator Fabrication¹⁴ (Figure 3)

1. Clean a silicon-on-insulator (SOI) wafer, which has a 0.45-µm-thick silicon device layer, with freshly mixed H₂SO₄:H₂O₂ (3:1) for 10 min. Then rinse the wafer in deionized water for 10 min and dry with a nitrogen gun. Immediately prior to subsequently processing, dip the wafer in H₂O:HF (10:1) for 10 sec and rinse in deionized water for 5 min.
2. Fabricate a lift-off mask, which consists of a germanium (Ge)/positive photoresist such as S-1811.¹⁵
   1. Spin-coat the wafer with thinned positive photoresist ....

Results

The response of a half-wave Mo₂N resonator (Figure 5) fabricated on a 0.45-µm single-crystal silicon dielectric was validated with this methodology. In this instance, coupling to a Nb coplanar waveguide (CPW) feedline for read-out is achieved via capacitive coupling through a sputter-deposited SiO₂ dielectric, in the "H" shaped region at one of the open ends of the resonator (see Protocol section 1.6). In other instances, capacitive coupling to the f.......

Discussion

The single-flip fabrication process provides a means for realizing superconducting resonators on both sides of a thin 0.45-µm single-crystal Si substrate. One may be motivated to use a single-crystal Si dielectric because it has more than an order of magnitude lower loss than deposited dielectrics (such as Si₃N₄) with loss tangents in the 4.0-6.5-GHz range < 1 x 10^-5. ^23-24 The ability to pattern features on both sides of this substrate allows one to employ a microstri.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Protocol Section 1
Microposit S-1811 Photoresist	Shipley
BCB	Dow		3022-35
SOI wafers	SOITec		Fabricated with SmartCutTM process
Mo	Kamis		99.99%
Nb	Kamis		99.95% (excludes Ta)
E-6 metal etch w/AES	Fujifilm		CPG Grade
Acetone	JT Baker	9005-05	CMOS Grade
HF dip (1:10)	JT Baker	5397-03
PMMA	Microchem		950 PMMA A2
Protocol Section 2
GE 7031	General Electric		Low-temperature adhesive
Protocol Sections 3-4
Cryogenic Microwave Amplifier	MITEQ	AF S3-02000400-08-CR-4	2-4 GHz, gain ~30 dB
NbTi Semi-rigid SMA cables	Coax. Co.	SC-086/50-NbTi-NbTi
Circulator	PamTech	CTD1229K	return loss > -20 dB from 2-4 GHz
RF attenuator	Weinschel	Model-4M	7 dB attenuation
Flexible SMA cables	Teledyne-Storm	R94-240	ACCU-TEST
Vector Network Analyzer	Agilent	N5242A PNA-X
Liquid He-4 cryogen	Praxair
Liquid N2 cryogen	Praxair