Upload
maritza-campen
View
222
Download
3
Tags:
Embed Size (px)
Citation preview
ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS): A TOOL FOR THE
CHARACTERIZATION OF SPUTTERED NIOBIUM FILMS
M. MusianiIstituto per l’Energetica e le Interfasi, CNR,
C.so Stati Uniti 4, 35127 Padova, Italy
Work performed in collaboration with V. Palmieri, D. Tonini (INFN LNL) and with S. Cattarin (IENI CNR)
Contents
Electrochemical Impedance Spectroscopy (EIS)EIS, Capacity and Surface RoughnessEIS of porous/rough electrodes
Magnetron Sputtering of NbEIS results: - Surface roughness as a function of Target-Substrate angle
- Attempts to control surface roughness
Conclusions
Electrochemical Impedance Spectroscopy
EIS employs sine-wave modulations (of either potential E or current I). If E is modulated
and the system is linear and stationary, the current is
The impedance is the transfer function between E(t) and I(t)
tsin|E|EEE)t(E ss
)tsin(|I|III)t(I ss
jexp|Z|
IE
)(Z
0 2 4 6 8 10 12
0
I
EE
o
r I
/ a
.u.
time / a.u.
The ratio |E|/|I| and the Phase angle depend on the system and vary with the frequency.
EIS measures |E|/|I| and in a wide frequency range.
Representation of Impedance Data
Bode Diagrams: log|Z| and vs. log (or logf)Nyquist Diagrams: Im(Z) vs. di Re(Z) (frequency as parameter)
sin|Z|)ZIm(
cos|Z|)ZRe(
)ZRe()ZIm(
arctg
)ZIm()ZRe(|Z| 222
The “electrical double layer”
The interface between an electronic conductor (generally a metal) and an electrolytic solution can store a charge.
Due to the presence of the electrical double layer a “blocking” electrode behaves like a capacitor in series with the electrolyte resistance.
The double layer capacity is easily measured by EIS
Impedance of a porous/rough electrode
De Levie (1963): Model of a porous electrode.Semi-infinite identical pores, homogeneous in diameter, with no cross links.
Experimental verification Pt brush electrode
Impedance of a porous/rough electrode
0 5 10 150
2
4
6
8
10
f1 > f
2
f2
f1
-Im
(Z)
/ Arb
itrar
y U
nits
Re(Z) / Arbitrary Units
Real pores are not semi-infinite.
When frequency is low enough that the modulated potential penetrates the whole pore depth, a “classical” capacitive behaviour is observed.
Impedance of a porous/rough electrode
Keddam et al. (1981) “When a porous electrode is constituted of a three dimensional combination of small occluded pore units, the electrode impedance becomes very similar to that of a cylindrical pore electrode”
Keddam et al. (1982) “At sufficiently low frequency, the impedance of a cylindrical pore is equal to that of the flat electrode of the same area as the developed pore surface”
Impedance of a porous/rough electrode
The double layer capacity of a porous electrode can be measured (in the low frequency range).
The Surface Roughness of a metal (defined as the ratio between its real area and its geometric area) can be obtained as the ratio between its double layer capacity and the double layer capacity of an ideally flat sample of the same material with identical geometric area.
Rough
Flat
Flat
RoughC Z
Z
C
CR
)Im(
)Im(
Magnetron Sputtering of Nb
Whatever the geometry of the cavity cells, an Nb film with identical properties must be deposited onto all points of the inner cavity wall
Magnetron sputtering of Nb thin films onto Cu is proposed as an approach alternative to construction of resonators consisting of bulk Nb.
Magnetron Sputtering of Nb
Nb atoms impinge the substrate under variable angles after travelling across variable distances.
There are evidences that superconducting properties of the Nb deposit are the best when =0° and worsen as increases from 0 to 90°.
In a real cavity
Magnetron Sputtering of Nb
The holder can carry seven samples at a time, with angles varying from 0° (parallel to the target) to 90° (normal to the target).
Same process conditions: the film properties depended only on the substrate orientation.
On a specifically designed sample holder
Magnetron Sputtering of Nb
The deposition system works at a pressure of ca. 2 10-3 mbar (Ar). A plasma is generated by a 400 V potential difference and confined next to a high purity Nb target by a magnetic field.Nb atoms are sputtered from the target by Ar ions and, after travelling mostly in a straight line, reach the substrate and form the deposit.
EIS Results
Quartz substrates 75 mm x 25 mm (WE area 1.54 cm2)Quartz substrates 9 mm x 9 mm (WE area 0.28 cm2)
Nb film thickness: variable or constant
Electrolyte: 0.2 M Na2SO4
Open circuit potential
EIS Results1st Series: large quartz substrates (75 mm x 25 mm)Porous electrode behaviour for 45°Increase in hf resistance at large
0 20 40 60 800
20
40
60
A
23.7 Hz
161.5 Hz
= 0 degrees = 45 degrees
-Im
(Z)
/ Ohm
Re(Z) / Ohm
0 100 200 300 400 5000
100
200
300
400B
1.615 Hz3.48 Hz
= 75 degrees = 45 degrees
-Im
(Z)
/ Ohm
Re(Z) / Ohm
EIS Results
Very sharp maximum in the capacity- curveStrong dependence of hf resistance on
0 15 30 45 60 75 900
100
200
300
400 constant deposition time
Cap
acity
/ F
cm
-2
/ degrees
0 15 30 45 60 75 9010
100
1000
10000
constant deposition time
hf R
esis
tanc
e / O
hm
/ degrees
EIS Results2nd Series: small quartz substrates (9 mm x 9 mm) constant deposit thicknessMaximum in the capacity- curvehf resistance quasi independent of
0 15 30 45 60 75 900
100
200
300
400B constant deposition time
constant deposit thickness
Cap
acity
/ F
cm
-2
/ degrees0 15 30 45 60 75 90
10
100
1000
10000
constant deposition time constant deposit thickness
A
hf R
esis
tanc
e / O
hm
/ degrees
EIS Results2nd Series: small quartz substrates, constant deposit thickness
Effect of potential in the EIS experiments
0 15 30 45 60 75 90
1
10
100
1000
open circuit potential 4 V
Cap
acity
/ F
cm
-2
/degrees
Comparison EIS-AFM
0 15 30 45 60 75 900
2
4
6
8
10
EIS Open Circuit EIS 4 V AFM Surface Profiling
Rel
ativ
e S
urfa
ce R
ough
ness
/ degrees
Shape of the roughness- curves
The mean free path and mean deflection path of Nb atoms are comparable to the target substrate distance
A large fraction of Nb atoms impinge onto the substrate after travelling along a straight line. This fraction decreases as increases
As increase, outstanding features of the substrate prevent Nb atoms (travelling along a straight line) from impinging onto some substrate areas (shadowing) As approaches 90°, the Nb film is formed mainly by scattered atoms; shadowing is not important.
Nb deposition under pulsed condition
3rd Series: small quartz substrates Constant deposition timeFrequency 70 kHz, duty cycle 10%
0 15 30 45 60 75 900
100
200
300
400 pulsed deposition continuous deposition
Cap
acity
/ F
cm
-2
/ degrees
Nb deposition onto heated substrates
4th and 5th Series: small quartz substrates Constant deposition timeTemperature: 400 or 600°C
0 15 30 45 60 75 900
50
100
150
200
400°C 600°C
hf R
esis
tanc
e / O
hm
Cap
acity
/ F
cm
-2
/degrees
0
50
100
150
200
250
300 400°C 600°C
Conclusions
•EIS provides useful data on the surface roughness of Nb deposits obtained by magnetron sputtering
•A marked increase of surface roughness occurs when the target-substrate angle exceeds 45°
•The increase in roughness correlates with a deterioration of the superconducting properties of the Nb deposits
•Heating of the substrates limits the development of rough/porous structures