http://www.surfacetreatments.it/thinfilms Atomic Layer Deposition of NbN thin films for SRF applications (Larry Phillips - 15') Speaker: Larry Phillips - Jefferson Lab - Newport News - Virginia | Duration: 15 min. Abstract Niobium Nitride is a 17K superconductor investigated since early eighthies for Superconducting Radiofrequency applications. Atomic Layer deposition is instead a technique that only recently starts to be considered for industrial applications.

1. 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 Engineering Old Dominion University, Norfolk, Virginia 23529, USA and a ) Applied Research Center at Thomas Jefferson National Accelerator Laboratories Newport News, Virginia 23606, USA b ) Thomas Jefferson National Accelerator Facility Superconducting Radiofrequency Technology for Particle Accelerators Institute Newport News, Virginia 23606, USA c ) Black Laboratories L.L.C., Applied Research Center Newport News, Virginia 23606, USA Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu

2. Thomas Jefferson National Accelerator Facility in Newport News, Virginia a U.S. Department of Energy Lab Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu

3. APPLIED RESEARCH CENTER Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu OLD DOMINION UNIVERSITY College of Engineering and Technology

6. ALD Reaction Chamber to accommodate the 6 GHz Cavity Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu 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.

7. 6GHz Cavity from Legnaro Lab to fit the ALD Reaction Chamber Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu Schematic providing details of the dimensions of the 6 GHz cavity and a photograph of a complete cavity.

9. NbN Film Deposition by Atomic Layer Deposition from NbCl 5 Precursor Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu 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 Al 2 O 3 thin films were used for the NbN film deposition for characterization. ALD process parameters for NbN (using 99.999% N 2 as a carrier gas): Pulse time of NbCl 5 : 1 s Pump time following pulse of NbCl 5 : 15 s Pulse time of NH 3 : 0.01 s Pump time following pulse of NH 3 : 15 s The 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.

10. FE-SEM Micrograph of Cleaved ALD NbN Film Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu Cross-sectional FE-SEM micrograph of NbN film on Si substrate. The thick 100 nm ALD NbN film exhibits columnar structure .

11. Determine ALD NbN Film stoichiometry as a function of deposition parameters by Rutherford Backscattering Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu 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.

12. Cross-sectional TEM Analysis of ALD NbN Film of partially crystallized NbN Film at 450 °C Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu 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.

13. Cross-sectional TEM Analysis of ALD NbN Film Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu Cross-sectional TEM image showing a 10 nm ALD NbN thin film deposited on a Si substrate with 30 nm ALD Al 2 O 3 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 Al 2 O 3 on Si substrates.

14. X-ray Diffraction Confirmation of Superconducting cubic Phase of ALD NbN Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu 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 2 θ = 20.95, at 2 θ = 33.42, at 2 θ = 41.24, at 2 θ = 46.028 and at 2 θ = 55.98; one hexagonal phase peak at 2 θ = 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.

16. Future Research Directions: New Stable Organometallic Precursors for ALD Synthesis of NbN thin Films Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu Recent work has demonstrated that Nb(OEt) 5 is the ALD precursor of choice over NbCl 5 . However, Nb(OEt) 5 is an unstable compound when heated. The mixed imido-amido niobium complex (tBuN=)Nb(NEt 2 ) 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 Nb 2 O 5 . NbN films can be synthesized when the mixed imido-amido niobium complex (tBuN=)Nb(NEt 2 ) 3 is reacted with NH 3 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 NbCl 5 precursor and Cl incorporation into the NbN film. However, this new ALD precursor requires Plasma assisted ALD.

17. Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu

18. Frank Batten College of Engineering & Technology Old Dominion University: www.eng.odu.edu