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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Assessment of the Composite Behavior of Different Ni/Cu Multilayer Composite Systems

J. Lamovec, V. Jović, I. Mladenović, M. Sarajlić, V. Radojević

Abstract – Nickel and copper thin films were alternately electrodeposited from sulphamate and sulphate-based electrolytes respectively, on different substrates: polycrystalline cold rolled copper, single crystal (111)-oriented Si and 100 µm-thick electrodeposited nickel films. Electrodeposition of layers at a very narrow spacing contribute to high strength of the composites. The influence of the substrate and plating conditions on the mechanical properties of these composite structures were investigated by Vickers microhardness testing for different loads. Composite hardness models were applied to the experimental data in order to obtaine absolute film hardness.

I. INTRODUCTION

Thin metallic multilayer composite films have attracted great attention for their promising mechanical, electrical and magnetical properties, especially for applications in fabrications of MEMS structures. Multilayers consisting of nanometer layer thickness have shown enhanced mechanical properties. The mechanical properties of multilayers depend on the processing conditions, and microstructure and interfacial structure of the component materials. Ni/Cu is a technologically interesting system, because of the ease with which coherent multilayers of Ni/Cu can be fabricated and the fact that both Ni and Cu have a fcc structure and the lattice mismatch is only 2.5% [1].

Electrodeposition technique is fully compatible with MEMS technologies and has several advantages in relation to another deposition processes: it is a low-temperature deposition technique with easy control of high deposition rate allowing a wide thickness range, the easy control of thickness and residual stresses, chemical composition of the layer, microstructure of the deposits. Electrodeposition is a simple, inexpensive and versatile method to produce dense fine-grained films of many different metals and alloys. Choice and optimization of the electrodeposition process parameters lead to ordered grain size and microstructure and because of that materials can be strengthened with little or no loss ductility [2].

Vickers hardness testing is a reliable test method for the evaluation of mechanical properties of the films. The

evaluation of the absolute hardness of thin films is difficult, because the influence of the substrate must be taken into account. The measured hardness values vary continuously with the indentation depth, the film thickness and the hardness of the film and of the substrate. There is a critical indentation depth when the measured hardness named “composite hardness” includes, except film hardness, also a component of the substrate hardness. The composite hardness models of Korsunsky and Chicot-Lesage were chosen and applied to experimental data for analysis of two different composite system types

Change of the composite and film hardness with applied load depends on the composite structure (especially on the substrate type). Composite Mayer's index m (composite work hardening exponent) characterizes the way in which the composite hardness varies with load and (t/d)m is a parameter that can express the difference in tendency of the composite hardness with the indentation load (t is film thickness, d is indent diagonal and m is work hardening exponent) [3,4].

II. COMPOSITE HARDNESS MODELS A. Korsunsky model

Korsunsky model has interesting approach in analysis of the hardness data for thin films on the substrates. The model is based on the assumption that the total “work of indentation” during a hardness test is consisted of two parts: the work of deformation in the substrate WS, and the deformation and/or fracture energy in the film, WF. This model introduces the constant a, that represents the fraction of the film involved in the hardness test and is given by:

211βk

a+

= (1)

Parameter k represents a dimensionless materials parameter related to the composite response mode to indentation and ß, the relative indentation depth (ß=d/7t). The composite hardness, according to this model is given by:

( ) ( )

tkk

HHtdk

HH SFSC

49

;/1

1

'

2'

=

−

+

+= (2)

J. Lamovec, V. Jović, I. Mladenović and M. Sarajlić are with the IChTM-CMT, Njegoševa 12, 11000 Beograd, Serbia, E-mail: jejal@nanosys.ihtm.bg.ac.rs,

V. Radojević is with the Department of Materials Engineering, Faculty of Technology and Metallurgy, UB, Serbia

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It is not possible to compute the film hardness at each indentation diagonal value since the magnitude of k should also be determined simultaneously from the experimental measurements of the composite hardness. This model does not allow computing the change in the film hardness with the indentation diagonal from the individual measurements of this property.

B. Chicot-Lesage model

The model proposed by Chicot and Lesage avoids the knowledge of any other data than that obtained easily from standard measurements (thickness and apparent hardness). They have constructed a model based on the analogy between the variation of the Young’s modulus of reinforced composites in function of the volume fraction of particles, and the variation of the composite hardness between the hardness of the substrate and that of the film.

Hardness value deduced from an indentation test is not constant because hardness is load-dependent. Meyer’s law express the variation of the size of the indent in function of the applied load P. The evolution of the measured diagonal and the applied load can be expressed by a similar relation as is Meyer’s: ∗∗= ndaP (3)

The variational part of the hardness number with load

is represented by the factor n* in the following expression:

*1

nmwheref

dt

dtf

m

==

=

(4)

Composite hardness can be expressed by the next relation:

( )

( )( )SFS

SFSC

HHfHf

HHfHfH

−⋅+⋅+

+

−⋅+−=

11/1/1

(5)

Hardness of the film is the positive root of the equation:

02 =+⋅+⋅ CHBHA FF ;

( )( ) ( )

( ) 22

23

2

1

1122

1

SSC

CS

HffHHfC

HfHffB

ffA

⋅−⋅+⋅⋅=

⋅−+⋅−+−=

−⋅=

(6)

The value of m (composite Mayer’s index) is

calculated by a linear regression performed on all experimental points obtained for a given film substrate couple and deduced from the relation:

bPmd +⋅= lnln (7)

With the known value of m, only the hardness of the

films remains to calculate.

III. EXPERIMENTAL Three different substrates were prepared for the

experiments: cold-rolled polycrystalline copper pieces, single crystal Si(111)-oriented wafers and 100-µm thick films of nanocrystalline electrodeposited Ni.

The plating base for the Si wafers were sputtered layers of 100 Å Cr as the adhesion film and 800 Å Ti as the nucleation film. Electrochemical deposition (ED) was performed using dc galvanostate mode. Copper is electrodeposited from a sulfate bath consisting of 240 g/l CuSO4 ·5H2O, 60 g/l H2SO4 , 40 mg/l thiourea and Ni from a sulphamate bath consisting of 300 g/l Ni(NH2SO3)2·4H2O, 30 g/l NiCl2·6H2O, 30 g/l H3BO3, 1 g/l sacharine. The process temperatures were maintained at 40 °C (sulphate bath) and 50 °C for the sulphamate bath.

The current density values were maintained at 10 mA·cm-2 for the electrodeposition of the thin Ni/Cu films and 50 mA·cm-2 for the thick films of Ni. According with plating surface, projected thickness of deposits was determined.

The mechanical properties of the films were characterized using Vickers microhardness tester “Leitz Kleinharteprufer DURIMET I” with load range from 1.96 N down to 0.049 N. Three indentations were made at each indentation load from which the average composite hardness value could be calculated. Experimental data were fitted with GnuPlot, v4.0.

IV. RESULTS AND DISCUSSION

A. Absolute hardness of the substrates

Indentations were performed with Vickers diamond pyramidal indenter both on uncoated substrates and on different composite systems. Proportional specimen resistance model of Li and Bradt is suitable for analysing the variation of substrate microhardness with the load:

22

01 )/( ddPdaP c+= (8) Here Pc is the critical applied load above which

microhardness becomes load independent and d0 is the corresponding diagonal length of the indent. A plot of P/d against d will give a straight line, the slope of which gives the value for the calculation of load independent microhardness.

The average values of the indent diagonal d (in m), were calculated from several independent measurements on

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every specimen for different applied loads P (in N). The absolute substrate hardness and composite hardness values, H (in GPa), were calculated using the formula:

201854.0 −⋅⋅= dPH (9) where 0.01854 is a constant, a geometrical factor for the Vickers pyramid.

On Fig.1, P/d values are plotted against d for three tested substrates: polycrystalline cold-rolled Cu (Fig.1.a), single crystal (111)-oriented Si and 100-µm thick electrodeposited Ni film with current density of 50 mA/cm2 (Fig.1.b). HS is 0.37 GPa for cold-rolled Cu substrate, 8.71 GPa for (111)-oriented Si single crystal substrate and 5.38 GPa for thick ED Ni film substrate. B. Composite hardness and film hardness

Three different composite systems were investigated: hard ED Ni/Cu film on soft substrate of Cu and soft film of Ni/Cu electrodeposited on hard substrates of Si(111) and thick EDNi film. Our intention was to analyse the composite hardness response of the mentioned composite systems.

Fig.1. PSR plot of applied load (in N) through indent diagonal (in µm), P/d, versus indent diagonal, d, for soft cold-rolled Cu substrate (a) and for (111)-oriented Si single crystal and thick ED Ni films as examples of the hard substrates (b).

Change of the composite hardness (HC) of the system EDNi and EDNi/Cu films on Cu substrate with relative indentation depth h/t, is shown on Fig.2.

Fig.2. Change in composite hardness with relative indentation depth for different composite systems. Total film thickness of the film is 5µm and current density value was 10mA/cm2.

With increasing the sublayer thickness ratio (Ni/Cu from 1:1 to 1:4), the hardness of the composite system increases and becomes higher than the hardness of monolayered Ni film (according to Korsunsky model, HF = 1.58GPa) approaching the film-dominant region (h/t < 0.1) [5].

Change of the composite hardness Hc with relative indentation depth h/t, for the “hard film on soft substrate” composite system type (Ni/Cu film on Cu substrate) and the “soft film on hard substrate” composite system type (Ni/Cu film on Si(111) and thick ED Ni-film substrates are shown on Fig.3.

Fig.3. Change in composite hardness with relative indentation depth for different composite systems. Total film thickness of the film is 5µm and current density value was 10mA/cm2.

The microstructure of the substrate plays key role for

the composite hardness response and because of the work

(a)

(b)

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hardening of the EDNi substrate and according to Korsunsky and Chicot-Lesage models, the system EDNi/Cu on EDNi substrate has the highest values of hardness and film hardness of all the analysed systems. This system has the most expressive composite character of mentioned composite systems.

Fig.4. Change in composite hardness with relative indentation depth for different composite systems. Total film thickness of the film is 5µm and current density value was 10mA/cm2.

The difference in tendency of the composite hardness with load can be expressed through (t/d)m parameter [6]. When the composite hardness tends to that of the film (for low loads), parameter (t/d)m is almost independent of the substrate type. With increasing load, influence of the substrate becomes dominant and parameter (t/d)m depends mostly on the substrate type (Fig.5).

Fig.5. Comparison of the parameter (t/d)m with h/t, for ED Ni/Cu multilayer films on different substrates. Ni/Cu 5µm-thick films are grown with 10 mA⋅cm-2 current density.

III. CONCLUSION

It was shown that the tendency of the composite hardness depends on the type of the composite system, i.e. the differences in the mechanical properties of the film and of the substrate: the hardness of the substrate, the hardness of the film, and their relative difference.

Korsunsky and Chicot-Lesage models were applied to experimental data in order to obtain the film hardness values.

Layer thickness, sublayer thickness ratio Ni:Cu, and also the microstructure of the substrate are important parameters for the mechanical strength of the composite system.

Parameter (t/d)m deserves more attention in analysis of the hardness of the composite systems, because of its dependence on the substrate type for the high loads applied.

ACKNOWLEDGEMENT

This work was funded by Republic of Serbia – Ministry of Education, Science and Technological Development through the project TR 32008 named: “Micro and Nanosystems for Power Engineering, Process Industry and Environmental Protection – MiNaSyS”.

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