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

Part 1

Electrochemical Impedance Spectroscopy

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 Capacitor

1

11

dldl

j

CjR

CjRZ ee

eRZr

-Zj

C dlR e

Rough and Porous electrodes

Electrodes are not always flat!

B

A

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(

Part 2

Magnetron Sputtering of Nb

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

Definition of

AFM images

= 15° = 45°

= 60° = 90°

The roughness of Nb deposits depends on

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

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