ATTEMPTS TO DEPOSIT NbATTEMPTS TO DEPOSIT Nb33Sn BY MOCVDSn BY MOCVD

Giovanni Carta, Gilberto Rossetto, Pierino Zanella, Laura Crociani,

Vincenzo Palmieri, Francesco Todescato

CNR- INFM

Istituto di Chimica Inorganica e delle Superfici

C.so Stati Uniti,4 35127 Padova, Italy

Laboratori Nazionali di Legnaro

V.le dell’Università 2, 35020 Legnaro, Padova, Italy

Superconductivity Nb3Sn

A15 superconductor with a very low resistivity and a high transition temperature (Tc=18K).

Nb3Sn

object of great interest for its applications in superconducting magnets, and promising candidate for superconducting cavities.

Sputtering : - high purity of the films;

- very expensive system;

- difficulty to cover complex shapes.

(MO)CVD:- cheap system;

- possibility to cover also complex shapes;

- several industrial applications;

- difficult to control several parameter processes;

- choice of the right precursor.

Deposition methods

CVD and MOCVD

CVD Chemical Vapor Deposition

Process in which one or more precursors, present in vapor phase, chemically react on an appropriate warm substrate,

giving rise to a solid film

Chemical reactions Several energetic activations

- Thermal CVD;

- Plasma enhanced CVD;

- Photo assisted CVD;

- Laser CVD.

MOCVD (Metal Organic Chemical Vapour Deposition) is a particular case of CVD in which the precursor is a metallorganic compound

MOCVD process

1) Production (evaporation or sublimation) of precursors in gas phase.

2) Trasfer of the reactants into the reaction chamber by the gas carrier (N2, H2, He..).

3) Gas phase reactions: (a) homogeneous, with formation of powders films with poor adhesion

(b) heterogeneous, films with good adhesion

4) Adsorption of the gaseous reactants, throught the boundary layer, on the warm substrate with heterogeneous reaction at the interface gas-solid.

5) Diffusion along the surface of active species towards the growth sites; chemical reactions on the surface with crystallization and growth of the film.

6) Deadsorption of reaction by-products.

7) Transfer of the gaseous not reacted precursors and of the by-products out from the reactor chamber by the carrier gas.

Precursor properties

• Good volatility (better liquid than solid);

• good thermal stability during the evaporation or sublimation and during the transfer into the reactor;

• high purity;

• clean decomposition to avoid contamination of the film;

• not toxic;

• easy to synthetize, with high yield and low production costs;

• good stability during the time conservation.

Niobium precursorsSeveral precursors have been synthetized in order to choose those with the best properties (in particular with good volatility) :

As the compounds are air and moisture sensible, the synthesis has been made inside dry-boxes under a nitrogen atmosphere :

1- Pentakis(dimethylamide)niobiumNb(NMe2)5

BuHLiNMeLiButNHMe 22

Me2NH gas bubled into 50 mL LiBut 1,6 M (80mmol) for 90 min;

added 4,3 g NbCl5 (16 mmol)

LiClNMeNbLiNMeNbCl 55 52

pentane

25

after filtering LiCl the solution was dried under vacuum obtaining a dark brown solid;

purification by sublimation at 130°C under vacuum;

NMR 1H and 13C characterization: good product but with a low volatility.

This compound was not used for the MOCVD deposition.

2- Bis(cyclopentadienyl)niobium dimethylCp2NbMe2

LiClNbMeCpLiMeNbClCp 22 22

OEt

22

2

1,2 g Cp2NbCl2 (4,0 mmol) in Et2O (-30°C);

5 mL LiMe 1,6 M in Et2O (8,0 mmol).

After filtering LiCl, the solution is dried under vacuum to give a brown solid;

low volatility;

the compound is dangerous because it explodes at 128°C;

low yield.

For these reasons this compound was not used for the MOCVD depositions.

3- Bis(cyclopentadienyl)niobium boroydrideCp2NbBH4

3,0 g Cp2NbCl2 (10 mmol) in DME;

1,2 g NaBH4 (20 mmol);

Stirring for 90 min.

62242555

DME

422555 2442 HBHNbBHHCNaClNaBHNbClHC

Filtering NaCl2, solution dried under vacuum;

Green powder compound;

Sublimation temperature, 120°C.

4- Bis(methylcyclopentadienyl)niobium boroydride(MeCp)2NbBH4

Two ways to obtain the compound

FIRST WAY

Preparation of (MeCp)2NbCl2 is similar to that of Cp2NbCl2;

1,0 g (MeCp)2NbCl2 (3 mmol) in DME;

0,35 g NaBH4 (6 mmol).

62242

DME

422 4)(242 HBHNaClNbBHMeCpNaBHNbClMeCp

Stirring for 90 min, filtering NaCl, solution dried under vacuum;

black sticky solid.

4- Bis(methylcyclopentadienyl)niobium boroydride(MeCp)2NbBH4

SECOND WAY

Different synthesis of (MeCp)2NbCl2:

1,8 g NbCl5 (6,7 mmol) in 30 mL of toluene at low T;

2,0 g di Bu3SnH (6,9 mmol) in 3,1 mL THF (38,8 mmol);

NbCl4(THF)2 + 0,2 g di (MeCp)Na (1,6 mmol) (MeCp)2NbCl2;

62242

DME

422 4)(242 HBHNaClNbBHMeCpNaBHNbClMeCp Drawbacks:

low yield;

black sticky solid;

sublimation temperature not higher than Cp2NbBH4.

For these reasons this compound was not used for the MOCVD depositions.

2324

toluene

35 212 HSnClBuTHFNbClTHFSnHBuNbCl

NaClNbClMeCpMeCpNaTHFNbCl 22 22

THF

24

then

5- Bis(methylcyclopentadienyl)niobium dimethyl(MeCp)2NbMe2

1,0 g (MeCp)2NbCl2 (3,1 mmol) in Et2O at 0°C;

3,9 mL LiMe 1,6 M (6,2 mmol) in Et2O.

LiClNbMeMeCpLiMeNbClMeCp 22 22

OEt

22

2

Filtering NaCl, solution dried under vacuum;

sticky dark green solid;

the solid explodes at 128°C;

very low yield.

For these reasons this compound was not used for the MOCVD depositions.

6- (Cyclopentadienyl)niobium tetramethylCpNbMe4

LiClCpNbMeLiMeCpNbCl 44 4

OEt

4

2

1,0 g CpNbCl4 (3,3 mmol) in Et2O at low T;

added dropwise 8,3 mL LiMe 1,6 M (13,3 mmol) in Et2O.

Filtering LiCl, solution dried under vacuum and purified with hexane;

sticky dark green solid;

sublimation at 80°C;

high volatility;

low yield.

Purifying by hexane solution

- 1 . 5- 1 . 0- 0 . 50 . 00 . 51 . 01 . 52 . 02 . 53 . 03 . 54 . 04 . 55 . 05 . 56 . 06 . 57 . 0

( p p m )

non purific a to

purific a to

Not purified

purified

1H-NMR analysis of CpNbMe4

/ 8

4.9

99

9

12

.11

0

Inte

gr

al

7.1

59

3

5.1

88

6

1.4

13

5

( p p m )

- 0 . 50 . 00 . 51 . 01 . 52 . 02 . 53 . 03 . 54 . 04 . 55 . 05 . 56 . 06 . 57 . 07 . 58 . 08 . 5

Element Theoretic%Experimental

%

Carbon 49,96 % 49,22 %

Hydrogen 7,85 % 7,82 %

Elemental analysis of CpNbMe4

1H-NMR spectrum of CpNbMe4

5,2 ppm Cp peak;

1,4 ppm methyl peak (broadened for Nb paramagnetic nucleus.

Final considerations on the precursors

Among all the precursors synthetized, the MOCVD of metallic niobium has been carried out using the following:

Bis(cyclopentadienyl)niobium boroydride Cp2NbBH4

(Cyclopentadienyl)niobium tetramethyl CpNbMe4

For tin deposition the precursor was the commercial liquid with b.p.=80°C (at 0,5 mbar):

Tributyltin hydride Bu3SnH Tributyltin hydride Bu3SnH

For tin deposition the precursor was the commercial liquid with b.p.=80°C (at 0,5 mbar):

Tributyltin hydride Bu3SnH

The MOCVD old apparatus

reactor

ovenbath

trap

Vacuum pump

The MOCVD new apparatus

MOCVD Deposition (old app.) from Cp2NbBH4

Cp2NbBH4

Experimental conditions

Precursor quantity 0,230 g

Bath temperature 120°C

Line temperature 130°C

Deposition temperature 550°C

Pressure 7,110-1 mbar

Carrier gas flux Ar : 20 scc/min

Dilution gas fluxAr : 250 scc/min or

N2/H2(10%): 250 scc/min

Deposition time 180 min

Substrates Glass and (001) silicon

Characterization SEM, XPS

Black deposit

SEM analysis:

C 16,00 % ; O 69,07% ; Nb 14,93 %.

Thickness: 125 ± 25 nm.

XPS analysis : presence of Nb(V) (207.1 eV), C and O

RBS analysis :Nb and O in atomic ratio to give Nb2O5

The film is Niobium (V) oxide Nb2O5

XPS analysis from Cp2NbBH4

B.E. Nb 3d

207,1 eV

B.E. O 1s

530,5 eV

Inte

nsi

ty (

a.u

.)

120010008006004002000BE (eV)

O1s

Nb3d

Na(A

uger)

OK

LLN

b3p

Nb4p

Nb4s

Na1s

C1s

campioni febbraio 2004pezzo scuro

superficie 5' erosione 3 kV

Nb3s Surface

5’ sputtering 3KeV

Inte

nsit

y (a

.u.)

215210205200BE (eV)

Nb3d campioni febbraio 2004pezzo scuro

superficie 5' erosione 3 keV

O/Nb surface

2,9

O/Nb after sput.

1,4

Inte

nsit

y (a

.u.)

540535530525BE (eV)

O1s

superficie 5' erosione 3 keV

campioni febbraio 2004pezzo scuro

MOCVD deposition (old app.) from CpNbMe4

CpNbMe4

Experimental conditions

Precursor quantity 0,050 g

Bath temperature 120°C

Line temperature 140°C

Deposition temperature 520°C

Pressure 2,7mbar

Carrier gas flux Ar : 40 scc/min

Co-reactant gas flux N2/H2(10%) : 250 scc/min

Deposition time 100 min

Substrates Glass, quartz, silicon (001)

Characterization XRD, RBS

Black deposit

Thickness: 137 ± 25 nm.

RBS analysis (atomic ratio) : Nb = 1 ; O = 2,38 ; C = 1,58

XRD: monoclinic Nb2O5

XRD from CpNbMe4

The spectrum agrees with that of monoclinic Nb2O5 .

Crystallites mean size :25 ± 7 nm.

20 30 40 50 60 70 80 902Theta (°)

0

500

1000

1500

2000

Inte

nsity (

counts

)

(-1,1,1)

(-1,1,2)

(-3,1,4)

(-5,1,3)

(-2,1,6)

(0,1,6)

(-6,1,8)

(3,1,6)

(-4,2,3)

(-6,0,12) (-5,2,7)

(-11,0,13)

(-9,2,1)

(3,2,8)

Deposition with the new MOCVD apparatus

Cyclopentadienyl)niobium tetramethyl CpNbMe4

The MOCVD of metallic niobium have been carried out using the precursor:

Cyclopentadienyl)niobium tetramethyl CpNbMe4;

Tributyltin hydride Bu3SnH

Some co-deposits of metallic niobium and tin have been carried out using as precursors:

Deposition (new app.) from CpNbMe4

CpNbMe4

Experimental conditions

Bath temperature 80°C

Line temperature 100°C

Deposition temperature 520°C

Pressure 3mbar

Carrier gas flux Ar : 40 scc/min

Co-reactant gas flux N2/H2(10%) : 250 scc/min

Deposition time 80 min

Substrates Quartz, copper

Characterization SEM, RBS

Grey, metallic mirror, deposits.

SEM analysis of the films on:quartz and copper:

Copper : Cu 85,34 % ; Nb 14,66 %

Quartz : Si 85,49 % ; Nb 14,51 %

The relative atomic ratio by RBS analysis is on:

Quartz: Nb = 0,315; O = 0,382; C = 1,83

Copper: Nb = 0,217; O = 0,139; C = 0,454

Presence on the film of NbO and Nb2O

Co-depositions (new app.)Co-deposit

Tin from Bu3SnH Niobium from CpNbMe4

Bath temperature 70°C Bath temperature 80°C

Line temperature 100°C Line temperature 100°C

Co-reactant gas flux N2/H2(25%) : 10 scc/min Co-reactant gas flux N2/H2(25%) : 60 scc/min

Deposition temperature 550°C

Pressure 2,710-0 mbar

Carrier gas flux N2/H2(25%) : 250 scc/min

Deposition time 45 min

Substrates Glass, silicon (001) , copper

Characterization SEM, RBS

The films are grey metellic mirror

SEM analysis has given the following atomic percentages on:

(001) silicon : Nb 5,50 % ; Sn 2,13 %

copper: Nb 3,46 % ; Sn 3,43 %

glass : Nb 16,62 % ; Sn 10,56 %

RBS analysis on (001) silicon gives the following relative ratios:

Nb = 0,683 ; Sn = 1,00 ; O = 1,58 ; C = 0,84

There is presence of a mixture of NbO, NbO2 and SnO

SEM micrograph of a co-deposit on copper

SEM analysis:

Area 2 : Nb 4,43 % ; Sn 14,16 % .

Spot 1 : Cu 75,99 % ; Sn 24,01 % .

Conclusions

Several attempts to deposit Nb3Sn by MOCVD have been carried out using both an old and a new apparatus. Unfortunately all the deposits, characterized by XRD, SEM and RBS analyses, showed, in

these preliminary results, presence of niobium (I, II, V) and tin (II) oxides in the surface.

MAIN PROBLEMS: - great oxophylic character of Nb

- not perfect vacuum tightness of the employed systems

Good future outlooks for the MOCVD technique for the depositions of niobium and tin thin films as long as in the reaction chamber no oxygen traces are present.

The technique opens a new research path in the field of thin films for coating large area and tricky shapes as that of a superconducting cavity.

Bis(cyclopentadienyl)niobium dichlorideThis is an intermediate compound necessary for the synthesis of two precursors:

Cp2NbMe2 Cp2NbBH4

NaClHCHCNbHCNaHCNbCl 55 552552555

toluene

555

11,2 g NaCp (127,1 mmol) in 250 mL toluene

6,2 g di NbCl5 (22,8 mmol)

filtered from NaCl solution dried black powder= NbCp4

6522555

OEt

2552555 22

2

HCNbClHCηHClHCσNbHCη

NbCp4 in Et2O

160 mL HCl 1 M in Et2O solution dried brown powder= Cp2NbCl2

Cyclopentadienylniobium tetrachlorideCpNbCl4

NaClSiCpMeNaCpSiClMe 3

hexane

3

SiClMeCpNbClNbClSiCpMe 34

ClCH

53

22

7,6 g NaCp (86,8 mmol) in hexane a low T (-30°C);

11 mL Me3SiCl (86,8 mmol) diluted in hexane;

filtering NaCl, yellow solution, distillation of hexane (b.p. 83°C);

Me3SiCp, yellow liquid with b.p. 134°C.

2,7 mL di Me3SiCp (16,3 mmol) in CH2Cl2;

4,2 g di NbCl5 (15,5 mmol);

reflux for 10 min;

filtering and dried the solid good elemental analysis.

dark red powder , CpNbCl4.

Elemental analysis Teoretic %

Experimental %

Carbon 20,03 % 19,82 %

Hydrogen 1,68 % 1,16 %