Design and Fabrication of High Qi 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 80305-3328, 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 , 093918 2010

Internal Quality Factor is High For Resonators Made From TiN

Fitting Data for Half Wave TiN CPW Resonator Tc = 5.1 K

J. Gao, et al. Appl. Phys. Lett. 92, 152505 (2008)H. Paik, et al., Phys. Lett. 96, 072505 (2008).

• We perform high power fitting to get Qi

• 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) Qi(x10-6)1/4 1/4

wavewave5.93 1.15

1/4 1/4 wavewave

5.96

1.78

LumpeLumped d

6.18 6.52

LumpeLumpe

dd6.54 1.49

LumpeLumpe

dd6.76 11.18

LumpeLumpe

dd6.87 0.95

LumpeLumpe

dd7.07 0.21

Lumped Element Resonators are Sensitive to Photons

X-Ray Diffraction of TiN as a Function of N2

Concentration

M. Vissers, et al., Appl. Phys. Lett. 2010 vol. 97 pp. 232509

Si(200)Ti(200)

(deg)

Tc is Can Be Tuned by Varying the N2 Concentration During Deposition

N2 (sccm) (sccm) Tc (K) (K)

0.7 1.5-2.5

1.0 1.7-2.7

1.5 2.1-2.4

2.0 4.5

2.5 4.7

10.0 5.1

H. Leduc, Appl. Phys. Lett. 2010 vol. 97 pp. 102509

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

CF4/O2 Nb Etch Low Power SF6 Nb Etch

Scanning Electron Microscope Images of the Different Processing Techniques

High Power SF6 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:•Superconducto

rs – Nb, Al

•Re, TiN – reduced oxidation

•Substrates – Si, sapphire

•SiOX, AlOX, 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

Dielectric Loss (10-6)

a-AlOX 900

a-SiOX 700

a-B4C 151

a-SiN 100

a-Si 22

Reference Loss (10-6)

Nb on silicon 7

Al on sapphire 6

Re on sapphire 3

TiN on silicon 1

T1

5 - 30 μs

0.04 - 1 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

Rrms= 10.8 nm

Rrms= 11.0 nm

Rrms= 0.8 nm

Rrms= 45.0 nm

Loss (1/Q) of Different Metals

1/Q

i=ta

nδi

Multiplexed Resonators as a Tool to Study Materials

TLS Loss for B4C vs. SiOx

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 (B4C) has less loss due to materials than amorphous silicone oxide (SiOx)

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 (B4C) 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 ProcessingSurface roughness in the gap of CPW does not affect the loss due to TLS

CF4/O2 Nb Etch High Power SF6 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 B4C and SiOxReasons for B4C:

•Strong intericosahedral bonding and

• Weak polarizability

•B4C is extremely physically hard

•Easy to grow using sputter deposition

1/Q= 3.2*10-4

1/Q= 1.7*10-4

Loss Increase with Thickness

Microwave Modeling Software is Used to Design Resonant Cavities

A= 4.01×10-6 GHz-1 µm-1

B= -3.35×10-8 µm-1

C= 2.60×10-5 GHz-1

D= 4.55 ×10-8.

Lc

The Quantum Computing Challenge

Systems:Superconductors

Phase, charge, flux ~~~~~~~~~~

Semiconductor spinQuantum dot~~~~~~~~~

NMRNeutral atoms

IonsPhotons

Coupling

Isolation• Initialize single photons• Interact• Readout

Quantum Computing with Trapped Ions at NIST

• 9Be+ 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:

• Qr – observed resonance Q

• QC – feedline-resonator coupling

• fr – resonance frequency

Get Qi => internal quality factor from:

Lossresonance = Losscoupling + Lossinternal

Where are Two Level Systems Located?

Gao and colleagues showed the observed loss in their multiplexed CPW corresponds to Fδ=3*105 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, 152505 (2008)