Design and Fabrication of High Q i Titanium Nitride Resonators David S. Wisbey, Jiansong Gao, Michael Vissers, Jeffery Kline, Martin Weides, and Dave Pappas National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado , USA

CPW and Lumped Element TiN Resonators Half wave coplanar waveguide (CPW) resonators and lumped element were fabricated using a low power SF6 etch TiN was deposited on HF dipped Si(100) to eliminate loss at the interface David Wisbey, et al., J. Appl. Phys. 108,

Internal Quality Factor is High For Resonators Made From TiN

Fitting Data for Half Wave TiN CPW Resonator T c = 5.1 K J. Gao, et al. Appl. Phys. Lett. 92, (2008) H. Paik, et al., Phys. Lett. 96, (2008). We perform high power fitting to get Q i TiN was deposited on HF dipped Si(100) to eliminate loss at the interface

Increasing Trench Depth Decreased the Low Power Loss Removing the dielectric in the gap of the resonator decreased the loss and shifted the resonance frequency. Over etch depth

Internal Quality Factor is High For Resonators Made From TiN f 0 (GHz)Q i (x10 -6 ) 1/4 wave /4 wave Lumped Lumped Lumped Lumped Lumped Lumped Lumped Lumped Lumped

Lumped Element Resonators are Sensitive to Photons

X-Ray Diffraction of TiN as a Function of N 2 Concentration M. Vissers, et al., Appl. Phys. Lett vol. 97 pp Si(200)Ti(200) (deg)

T c is Can Be Tuned by Varying the N 2 Concentration During Deposition (sccm) N 2 (sccm) (K) T c (K) H. Leduc, Appl. Phys. Lett vol. 97 pp

Conclusions: Kinetic inductance and Tc of TiN films can be costume tailored to preferred values by changing the N2 concentration in the films Lumped element TiN LC resonators are promising as single photon detector As the stress decreases, the Tc also decreases As the trench depth increases due to over etch the internal loss decreases

Fitting Data for Tc = 5.1 K Quantum Computing with Molecules N. Gershenfeld, I. L. Chuang, Sci. Amer. 278, 66 (1998).

CF 4 /O 2 Nb Etch Low Power SF 6 Nb Etch Scanning Electron Microscope Images of the Different Processing Techniques High Power SF 6 Nb Etch

A Microwave in a Resonator Acts Like a Laser Bouncing Between two Mirrors

Different Types of Microwave Resonators Multiplexed resonators have an advantage because if one fails, it is still possible to measure other devices. 1/2 wave single resonators disadvantageous because if failure occurs, additional cool down is required.

Demonstrated T < 100 mK 1 day turnaround Adiabatic Demagnetization Refrigerator

Superconducting Microwave Resonators to Study Loss Absorption: e.g. Nb on Si Q=267k Loss =1/Q ~7x10 -6 Test: Superconduct ors – Nb, Al Re, TiN – reduced oxidation Substrates – Si, sapphire SiO X, AlO X, SiN, a-Si

Rhenium After Exposure to Atmosphere Gimpl et al., Trans. Metallurg. Soc. AIME 236, 331 (1966). 10 µm

Annealing Rhenium Changes the Type of Surface Oxide Formed Whiskers: ~100 µm long & 0.3 µm tall AES: Re, C, & O Can rinse away with Acetone, IPA, or water Spots cover surface: ~100 µm diameter & 1.3 µm tall. Cannot remove with Acetone, IPA, or water Chemical reaction

Single photon loss measurements in microwave resonators 1/26/11 DielectricLoss (10 -6 ) a-AlO X 900 a-SiO X 700 a-B 4 C151 a-SiN100 a-Si22 ReferenceLoss (10 -6 ) Nb on silicon7 Al on sapphire6 Re on sapphire3 TiN on silicon1 T1T μs μs

Difference Between Low and High Power Loss is Due to Materials

Loss can by Caused by Polar Impurities Temperature and intensity dependence of the dielectric absorption of vitreous silica at 10 GHz. The dashed line indicates the contribution of the relaxation process. [Von Schickfus, Phys. Lett. 64A, 144 (1977)] Schickfus et. al found that loss originated from polar impurities such as OH -, F -, or Cl - in the insulating material.

Processing Affects the Loss (1/Q) HF dipped resonators have less impurities between metal and substrate and have lower loss Surface roughness and loss not always correlated R rms = 10.8 nm R rms = 11.0 nm R rms = 0.8 nm R rms = 45.0 nm

Loss (1/Q) of Different Metals 1/Q i =tanδ i

Multiplexed Resonators as a Tool to Study Materials

TLS Loss for B 4 C vs. SiO x

Summary of Findings Overall loss is affected by the surface roughness, but loss from materials is independent of surface roughness Improving the material by eliminating defects and impurities decreases loss Oxide at the interface between the superconducting metal and the dielectric substrate greatly increases the loss Titanium nitride is a very low loss material and is promising candidate for qubits Low temperature boron carbide (B 4 C) has less loss due to materials than amorphous silicone oxide (SiO x )

Future Plans: Find new materials that allow quantum states to be stored longer New materials should be grown and tested as solid state devices Purposefully induce defects and add impurities to study the effect New possible materials include boron carbide (B 4 C) and boron nitride (BN) Study the electronic structure of the superconductor dielectric interface

Using HF Clean Significantly Reduces Loss in Rsonators Gap roughness does not significantly reduce TLS loss Careful substrate preparation interface between Nb and Si reduces TLS loss This means TLS reside primarily at the metal/substrate interface Fr vs. T

Loss Tangent as a Function of Processing Surface roughness in the gap of CPW does not affect the loss due to TLS CF 4 /O 2 Nb EtchHigh Power SF 6 Nb Etch

df0/f0 1/Q=1.93*10-4 1/Q=1.3*10-4 1/Q=8.6*10-6 df0/f0 Temperature (K) Resonance Frequency vs. Temperature For B 4 C and SiO x Reasons for B 4 C: Strong intericosahedral bonding and Weak polarizability B 4 C is extremely physically hard Easy to grow using sputter deposition 1/Q= 3.2* /Q= 1.7*10 -4

Loss Increase with Thickness

Microwave Modeling Software is Used to Design Resonant Cavities A= 4.01 × GHz -1 µm -1 B= × µm -1 C= 2.60 × GHz -1 D= 4.55 × LcLc

The Quantum Computing Challenge Systems: Superconductors Phase, charge, flux ~~~~~~~~~~ Semiconductor spin Quantum dot ~~~~~~~~~ NMR Neutral atoms Ions Photons Coupling Isolation Initialize single photons Interact Readout

Quantum Computing with Trapped Ions at NIST 9 Be + ions in RF traps Addressed with focused lasers Ions are moved into 150 zones Long coherence times ~ seconds Challenges –ion heating from fluctuations on surfaces –Two level charge systems

CPW resonant cavities are superconducting LC circuits used to store single photons CPWs act as quantum information buses between qubits. A simpler tool to measure loss in supercondcuting circuits than a qubit in terms of fabrication and measurement If a squid loop is added, could be used as a qubit also What Are CPW Resonant Cavities Used for in the Context of Quantum Information Processing?

* 6 Resonator Q measurement Fit each peak for: Q r – observed resonance Q Q C – feedline-resonator coupling f r – resonance frequency Get Q i => internal quality factor from: Loss resonance = Loss coupling + Loss internal

Where are Two Level Systems Located? Gao and colleagues showed the observed loss in their multiplexed CPW corresponds to Fδ=3*10 5 for a 3µm resonator which is consistent with a ~2nm layer of TLS loaded material on the metal surface or ~3nm layer on the gap surface J. Gao, et. al, Appl. Phys. Lett. 92, (2008)