Xiaomin Liu and Giovanni Zangari - MINT Center · 2012-12-19 · expansion (CTE) between the film and the substrate. bcc mixed fcc 350 400 450 500 550 600 650 700 750 800. along easy

Center For Materials For Information TechnologyAn NSF Materials Research Science and Engineering CenterThe University of Alabama

Corrosion Resistance and Mechanical Corrosion Resistance and Mechanical Properties* of Electrodeposited FeProperties* of Electrodeposited Fe--CoCo--Ni Ni

Films Films

Xiaomin Liu and Giovanni Zangari* in collaboration with Feng Huang and Mark Weaver

Center For Materials For Information Technology

Department of Metallurgical and Materials Engineering

The University of Alabama


• New, high moment magnetic alloys are currently utilized in thin film recording heads → possible additional processing and performance issues.

• Corrosion resistance is important in• head fabrication - lapping removes native oxide layer - intrinsic corrosion

resistance of the magnetic layer is required• head lifetime - ability to resist environmental degradation (Cl¯, SO2)• Avoiding localized corrosion (Cl¯ in particular: the main corrosion mechanism of

head materials): localized build-up of corrosion products, dimensional variations• Influence of mechanical properties:

• mismatch in the coefficient of thermal expansion CTE may lead to film delamination during processing or service;

• contact events may influence film magnetic properties due to stress-magnetostriction interaction

⇒ Investigate corrosion resistance and internal stresses, their variation upon thermal excursions, and the coefficient of thermal expansion.

MotivationMotivation


Sample PreparationSample Preparation

• Electrodeposition under current control at 25 οC

• Electrochemical cell containingabout 300 ml of solution

• Agitation (magnetic stirring)

• Applied magnetic field of ~ 600 Oe to induce a magnetic easy axis

• Pure Co counter electrode• Substrates:

– Cu/NiP (2 µm) – Si/Ti (10 nm)/Cu(200 nm)

Hot plate

Switch Voltage Current

InputInput Output

Anode (Co)

Magnetic stirrer

Cathode

Electroplating cell


CharacterizationCharacterization• Film composition: EDAX and XPS• Surface morphology was observed with SEM• Crystal structure and preferential orientation were evaluated by

XRD, using Cu Kα radiation• Corrosion resistance was investigated with an EG&G

potentiostat/galvanostat (model 273A)• Measurement of internal stresses: Flexus F2320 instrument was

used to measure wafer curvature before and after electroplating along two perpendicular directions (magnetically easy and hard axis)

• Hardness and reduced Young’s modulus were evaluated through nanoindentation performed on a Hysitron TriboScope®

nanomechanical testing system


Composition and Structure of FeComposition and Structure of Fe--CoCo--Ni FilmsNi Films

• With increasing current density the content of Fe and Co in the films decreases and the content of Ni increases → a corresponding transition in the phase structure from BCC to FCC is observed

• Films with BCC, FCC and mixed phase are investigated

0 10 20 30 40 5010

2030

40

5060

70

FCCBCC+FCC

BCC Fe Co Ni

Fe, C

o, N

i (at

%)

Current density (mA/cm2)42 44 46 48 50 52 54

FCC (200)

BCC (110)

FCC (111)

20 mA/cm2

15 mA/cm2

5 mA/cm2

Inte

nsity

(abs

.)

2θ (deg.)


Anodic PolarizationAnodic Polarization

-400 -200 0 20010-9

10-7

10-5

10-3

10-1

Epitting

BCC+FCC BCC FCC Permalloy

Curr

ent d

ensi

ty (A

/cm

2 )

Potential ( mV vs. SCE) Solution for corrosion experiments: aerated 2.5 wt% NaCl, pH~ 5

The phenomenon of electrochemical passivityprovides the basis for corrosion protection. The passive region under anodic polarization should be as wide and at low an anodic current as possible.

For films with BCC structure, the anodic polarization curve does not show an apparent passive region and the anodic current density is much higher than that for the other films at the same potential, ⇒ the BCC film is less resistant to corrosion, unlike FCC and mixed FCC-BCC films.


Corrosion PropertiesCorrosion Properties

bcc+fcc bcc fcc Permalloy

-400-400

-300

-200-200

-100

00

Ecorr Epitting

Pote

ntia

l (m

V vs

. SCE

)

bcc+fcc bcc fcc Permalloy0

5

10

15

20

25

I corr (

x10

-7) A

• The higher the pitting potential, the better the resistance to pitting corrosion.

• Bi-phasic FeCoNi films exhibit better corrosion resistance than single phase films in NaCl solutions.

• Bi-phasic FeCoNi is more noble and presents corrosion characteristic similar to NiFe Permalloy


Surface Morphology of Surface Morphology of FeCoNiFeCoNi films films before and after corrosion measurementsbefore and after corrosion measurements

FCC (Fe14Co31Ni55)BCC+FCC (Fe32Co47Ni21)BCC (Fe37Co48Ni15)Pits were observed in films with single fcc phase and mixed phases. The average size of the pits for film with mixed phases was about 1 mm, which is smaller than that with fccstructure,


Internal StressesInternal Stresses• Internal stresses

– Intrinsic stresses: originated during the growth process;– Extrinsic stresses: due to the difference in the coefficient of thermal

expansion (CTE) between the film and the substrate.

bcc mixed fcc350400450500550600650700750800

along easy axis along hard axis

Inte

rnal

stre

ss (M

Pa)

Maximum stress (760 MPa) was observed for films with BCC structure, which may be due to relevant hydrogen incorporation during deposition (current efficiency is lower at low cd).

Slightly anisotropic behavior is observed


Internal stress vs. temperature plotsInternal stress vs. temperature plots

0 50 100 150 200

400

500

600

700Cooling

Heating

BCC+FCC

1500C 2000C

Str

ess

(MP

a)

Temperature (0C)

0 50 100 150 2000

100

200

300

400

FCC

1500C 2000C

Heating

Cooling

Str

ess

(MP

a)

Temperature (C)0 50 100 150 200

600

700

800

900

Heating

Cooling

BCC

1500C 2000C

Str

ess

(MP

a)

Temperature (0C)

FCC films: pure elastic deformation resulting from CTE difference dominates up to 200oC. BCC and mixed phase films: deviation from a linear σ-T relation is observed at 140oC, due to diffusional-flow-induced relaxation in the films.


Typical LoadTypical Load--Displacement Curves, Young’s Displacement Curves, Young’s modulus and CTEmodulus and CTE

0 20 40 60 80 100 1200

200

400

600

800

1000annealed

as-deposited

Load

(µN

)

Displacement (nm)

The maximum load was 1000 µN to minimize the effects from both the surface oxides and the underlying substrateHardness of the as-deposited films was ~ 4.8 GPa, decreased to 3.6 GPa after annealing to 400oC for 30 min


Young’s modulus and CTEYoung’s modulus and CTEReduced Young’s modulus Ef /(1-υf

2) was determined from the slope of the initial unloading part of the loading-unloading cycle.

• 150 GPa for BCC structure• 160 GPa for mixed two phases• 180 GPa for FCC structure, close to that for bulk Ni78Fe22 (180-200 GPa)

The CTE can be calculated from the slope of the linear region of σ-T curves, which is

( ) ( )( )fsff

ffs

f

f EEdTd

αανν

ααν

σ−+

−=−

−= 1

1)1( 2

Assuming υf is the same as that for Ni-Fe alloys (υ ~0.3) and αs 2.6×10-6

oC-1, CTE is• 9.9×10-6 oC-1 for BCC phase, • 10.2×10-6 oC-1 for FCC+BCC, • 11.2×10-6 oC-1 for FCC phase

• 12×10-6 oC-1 for bulk Ni78Fe22


ConclusionsConclusions• The corrosion resistance of FeCoNi films has been investigated as a

function of their microstructure– the pitting potential of the two-phase films is more positive than that of the single

FCC phase films.– films with single BCC phase did not show any apparent passive region– the two-phase films exhibit better corrosion resistance than single-phase films in

NaCl solutions; this is also slightly superior to that of NiFe films

• The mechanical properties of electrodepositied Fe-Co-Ni films have been studied by stress measurements as a function of temperature and nanoindentation– The tensile internal stresses of Fe-Co-Ni films increase with increasing Fe content

in the films → Maximum stresses were observed in films with BCC structure – Hardness of as-deposited films was ~ 4.8 GPa, after annealing to 400oC hardness

was reduced to 3.6 GPa; hardness are lower than the corresponding value for NiFe Permalloy

– CTE was 10-11×10-6 oC-1, comparable to that of bulk Ni78Fe22

Documents

Xiaomin Liu and Giovanni Zangari - MINT Center · 2012-12-19 · expansion (CTE) between the film and the substrate. bcc mixed fcc 350 400 450 500 550 600 650 700 750 800. along easy