Atomic Layer Deposition of NbN Thin Films for Superconducting Radiofrequency (SRF)

Accelerator Technology

Diefeng Gu a), Helmut Baumgart a), H. L. Phillips b) and Roy Crooks c)

a ) Department of Electrical and Computer EngineeringOld Dominion University, Norfolk, Virginia 23529, USA

anda ) Applied Research Center at Thomas Jefferson National Accelerator Laboratories

Newport News, Virginia 23606, USA

b ) Thomas Jefferson National Accelerator FacilitySuperconducting Radiofrequency Technology for Particle Accelerators Institute

Newport News, Virginia 23606, USA

c ) Black Laboratories L.L.C., Applied Research CenterNewport News, Virginia 23606, USA

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Engineering & TechnologyOld Dominion University: www.eng.odu.edu

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Thomas Jefferson National Accelerator Facility in Newport News, Virginiaa U.S. Department of Energy Lab

Frank Batten College ofEngineering & TechnologyOld Dominion University: www.eng.odu.edu

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3

APPLIED RESEARCH CENTERAPPLIED RESEARCH CENTER

Frank Batten College ofEngineering & TechnologyOld Dominion University: www.eng.odu.edu

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OLD DOMINION UNIVERSITYCollege of Engineering and Technology

OutlineOutline

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Experimental ALD Reaction Chamber to accommodate 6 GHz

Cavity

Precursor Chemistry for ALD NbN thin Films

Materials Analysis and Physical Characterization of ALD NbN

Cross-sectional SEM

Rutherford Backscattering (RBS)

High Resolution Transmission Electron Microscopy (HR-

TEM)

X-ray Diffraction (XRD) Analysis of ALD NbN

Summary of Achievements

Future research directions

Construction of custom made heated ALD Reaction Chamber

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Engineering & TechnologyOld Dominion University: www.eng.odu.edu

Custom built ALD system consists of a “top-hat” furnace machined from 316 stainless steel, a band heater and controller, insulation, and a high-temperature O-ring. The specifications are shown below: a. Adequate for 2 inch x 4 inch cylindrical 6GHz cavity b. Maximum temperature of 537°C c. Stainless Steel Cylindrical Chamber, 5.625 I.D., 6 “ O.D., 2.5 inch height, 3/16” wall thickness, 7.375 lower flange (smooth for O-ring) d. Omega 750 W Band Heater for 6” O.D. cylinder e. Omega controller, 240V f. Insulation shell over bands and chamber

The unusual high temperature for the ALD chamber was required for two reasons:

•because the chemical precursors NbCl4 and NH3 react only at elevated temperatures into the superconductive a-phase of NbN and• higher ALD deposition temperature drives out any residual chlorine from the grown NbN film. The retention of residual chlorine is a function of temperature. At 550 C there is no detectable chlorine remaining in the NbN films.•at lower ALD deposition temperature residual chlorine can be found in the films, which in the long term would deteriorate and corrode the NbN film.

ALD Reaction Chamber to accommodate the 6 GHz Cavity

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7/6/2010 3

ALD Reaction Vessel - AssembledSide View (Cross-Section)

+ +

1/16” d ia. ho les (2)

for therm oc ouples, dri ll 6” deep

CF200 Blank Flange, PUR CHA SED S TO CK

ITEM

1.50” 1.50”

7.75” I.D.

8 .00” O.D.

10.00” O.D.

4.00” O .A. he ight3 .00” in terior depth

1.00”

Note: Drawing is NOT to scale.

Mat ’l: 304 Stainless Steel

CF200 Hal f-n ipple , pipe

end length is 5.5”, PUR CHA SED S TO CK ITEM

H erm etic weld, enti re c i rcum ference.

2 .00”

Schematic cross-section of the specially designed stainless

steel ALD Reaction chamber.

View of the special high temperature ALD reaction chamber constructed to accommodate a 6 GHz cavity and achieving ~ 500°C sample temperature inside the chamber.

6GHz Cavity from Legnaro Lab to fit the ALD Reaction Chamber

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Schematic providing details of the dimensions of the 6 GHz cavity and a photograph of a complete cavity.

NbN Film Deposition by Atomic Layer Deposition from NbCl5 Precursor

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NbN thin films were deposited ALD at 500 °C in order to achieve minimum Cl content and cubic phase. Si substrates and Si substrates with 30 nm of ALD Al2O3 thin films were used for the NbN film deposition for characterization.ALD process parameters for NbN (using 99.999% N2 as a carrier gas):Pulse time of NbCl5: 1 sPump time following pulse of NbCl5: 15 s Pulse time of NH3: 0.01 sPump time following pulse of NH3: 15 sThe long pump time is due to large chamber volume. All the non-reacted precursor chemicals and the by-products need to be pumped out before the next chemical precursor comes into the chamber.

FE-SEM Micrograph of Cleaved ALD NbN Film

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Cross-sectional FE-SEM micrograph of NbN film on Si substrate. The thick 100 nm ALD NbN film exhibits columnar structure.

Determine ALD NbN Film stoichiometry as a function of deposition parameters by Rutherford Backscattering

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RBS analysis of ALD NbN films showing a ratio of Nb to N of 1. A minor contamination with Fe and Cl in the film was also detected. However, high Fe contamination was detected due to the corrosion of the stainless steel chamber by the chlorine reaction by-products.

Cross-sectional TEM Analysis of ALD NbN Film of partially crystallized NbN Film at 450 °C

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Cross-sectional TEM image showing partially crystallized NbN thin film of 10 nm on Si substrate deposited by ALD at relatively low temperature of 450 °C. Small grains of NbN were found embedded in the amorphous phase. The relatively low ALD deposition temperature of 450°C is the main reason why the NbN film was not fully crystallized during initial growth. By increasing the ALD deposition temperature, crystallization of ALD NbN films occurs at the expense of the amorphous phase.

Cross-sectional TEM Analysis of ALD NbN Film

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Cross-sectional TEM image showing a 10 nm ALD NbN thin film deposited on a Si substrate with 30 nm ALD Al2O3 film. The ALD NbN film has similar structure as shown in Figure 3 exhibiting NbN crystallites surrounded by amorphous NbN because the film was deposited at 450 °C. Furthermore superconductor-insulator (S-I-S) multilayers were realized by ALD. The TEM micrograph demonstrates that an ALD NbN film was successfully deposited onto ALD Al2O3 on Si substrates.

X-ray Diffraction Confirmation of Superconducting cubic Phase of ALD NbN

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XRD scan of NbN film deposited on a Si substrate at a temperature close to 500 °C. The following crystalline phases were observed: cubic phase at 2θ = 20.95, at 2θ = 33.42, at 2θ = 41.24, at 2θ = 46.028 and at 2θ = 55.98; one hexagonal phase peak at 2θ = 48.357. The crystalline structure was analyzed by XRD. The data show various phases of the NbN thin films deposited at a temperature close to 500°C. A majority of superconducting cubic α‘ and α‘‘ peaks were detected in the ALD NbN films and evidence of the presence of hexagonal NbN structures were also observed from XRD scan. A critical ALD deposition temperature close to 500°C is required to transition to the superconducting cubic crystalline phase of NbN films.

Summary and Conclusions

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NbN thin films were successfully deposited by thermal atomic layer deposition (ALD) in the temperature range of 450°C – 500°C by reacting the NbCl5 precursor with ammonia (NH3) gas.

The XRD analysis of the ALD films exhibits the crucial α’ and α” peaks indicative of cubic NbN, which proves experimentally that the superconducting phase of NbN was achieved.

Crystallization of the resulting NbN films is a sensitive function of ALD deposition temperature.

The phase transition to crystalline cubic NbN occurs around 500°C.

The feasibility of growing superconductor-insulator-superconductor (SIS) multilayer structures was demonstrated by depositing NbN on ALD Al2O3 films.

Future Research Directions: New Stable Organometallic Precursors for ALD

Synthesis of NbN thin Films

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Recent work has demonstrated that Nb(OEt)5 is the ALD precursor of choice over NbCl5. However, Nb(OEt)5 is an unstable compound when heated.

The mixed imido-amido niobium complex (tBuN=)Nb(NEt2)3 is a stable and volatile complex, showing ideal ALD behavior with both water and ozone as the oxidants for ALD in order to synthesize Nb2O5.

NbN films can be synthesized when the mixed imido-amido niobium complex (tBuN=)Nb(NEt2)3 is reacted with NH3 as a the second precursor.

The new Stable Organometallic Precursors for ALD Synthesis of NbN thin Films avoids all of the problems of the corrosive by-products of the former NbCl5 precursor and Cl incorporation into the NbN film. However, this new ALD precursor requires Plasma assisted ALD.

OOlldd DDoommiinniioonn UUnniivveerrssiittyy AApppplliieedd RReesseeaarrcchh CCeenntteerr Frank Batten College of

Engineering & TechnologyOld Dominion University: www.eng.odu.edu

OOlldd DDoommiinniioonn UUnniivveerrssiittyy AApppplliieedd RReesseeaarrcchh CCeenntteerr Frank Batten College of

Engineering & TechnologyOld Dominion University: www.eng.odu.edu