attempts to deposit nb 3 sn by mocvd giovanni carta, gilberto rossetto, pierino zanella, laura...
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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 %