Diffusion Effect of Intermetallic Layers on Adhesion and Electrical

Properties of Electrical Contacts

Golnaz Bassiri

Abstract

Application of multilayer thin film MEMS

Reasons for different layers Ensure mechanical and electrical properties

Conduction layer Au, Cu, Ag

Adhesion layer/diffusion barrier Ti, Cr, Pt, Ta, Mo, Nb

Different factors in diffusion Heat treatment, oxygen treatment

Diffusion changes properties of the film Adhesion, contact resistance

Microelectromechanical systems (MEMS) are vastly used as an intelligent integrated electrical systems in contacts, relays, actuators, RF circuits, hybrid switches [1].

To ensure some of the properties of the electrical contacts and relays, multilayer thin films are prepared to provide the reliability, long term stability and the performance of these thin films [2].

The selection of the sublayers of the sandwich depends on the application of the films

Background and Introduction

Diffusion of intermetallic layers change the properties of individual layers, and the over all thin film

Diffusion effect of different intermetallic layers on the mechanical (adhesion) and electrical (mostly resistivity and contact resistance) to determine the most efficient layers in the electrical contact layer

Background and Introduction

Literature Review

Au, Ag and Cu are commonly used as a conduction layer for the multilayer thin films[3,4]

Cr, Pt, Ti, Ta, Mo, and Nb are usually used for an intermediate layer as an adhesion layer/diffusion barrier[4,5]

Holloway et al.(1976) studied the effect of oxygen treatment of the adhesion layer on the properties of films[4]

Literature Review

Understanding of this subject is derived from chemistry, physics, material science, electrical and mechanical science, and manufacturing process of these multilayer thin films.

Different fabrication methods has an impact on the effect of diffusion on the properties of thin films. Different methods such as electron beam evaporation, laser pulsed deposition, chemical vapor deposition, and Langmuir–Blodgett films changes some of the properties of the film.

Theoretical Background

Diffusion is intermingling of one or more substance into each other and diffusion is present in multilayer films[6]

To have the high conductivity and low electrical resistance and high corrosion resistance, Au, Ag is used on the silicon wafer substrate.

To increase the adhesion of noble metals to the silicon, few angstrom of adhesion layer is deposited on the substrate[5]

Application of the multilayer thin films are in MEMS such as in actuators, relays, ohmic or electrical contacts, RF switches, hybrid circuits[1]

Current Research and Applications

Current Research and Applications Holloway et al. studied that oxidation decreases

the diffusion effect of some of the intermetallic layers (Cr in Au) which can be used as a diffusion barrier[4]

The Cr forms the Cr2O3 with oxygen (diffusion barrier), and causes delamination of the surface (poor adhesion)

The oxygen treated Au/Cr/Si does not change the resistivity compare to the bulk gold

Ono et al. studied the diffusion effect of Cu/Cr/Si vs. Cu/Ta/Si as a function of heat treatment of the specimen[3]

The top image RBS result of Cu/Cr/Si and bottom is Cu/Ta/Si before and after annealing at 600°C for 1 hr

The different peaks represent different layers of deposited metals, annealed peak shows the diffusion of Cu,Cr and Si in the top image and insignificant change of Cu/Ta/Si after annealing (bottom image).

Current Research and Applications

He also compared the different intermetallic layers resistivity as a function of annealing temperature[3]

Ti and Cr abruptly increase in resistivity at about 500°C while Mo, Nb at about 600°C and Ta and W about 700°C

Current Research and Applications

Current Research and Applications Diffusion effect on mechanical properties

such as hardness and elastic modulus Metals for different sublayers of the

sandwich with the maximum desired properties

The most efficient method of fabrication and treatment to obtain the desired adhesion

Future Directions

Determination of the diffusion effect can resolve by:Selection based on the appropriate layers relative

to the application (mechanical properties i.e. hardness, elastic modulus or electrical properties)

Selection based on the fabrication methodSelection based on the thickness of the layersSelection based on the different treatment of

layers

Grand challengeDetermination of the most efficient multilayer

thin films with respect to the metal configuration, fabrication method, and required treatments

Significant technical challengeGeometry of the multilayer thin films changes

the properties

Future Directions

References

[1] D. C. Miller, C. F. Herrmann, H. J. Maier, S. M. George, C. R. Stoldt, and K. Gall. Intrinsic stress development and microstructure evolution of Au/Cr/Si multilayer thin films subject to annealing. Scripta Materialia, 52(9):873–879, 2005.

[2] M. Martyniuk, J. Antoszewski, B. A. Walmsley, C. A. Musca, J. M. Dell, Jung Yeon- Gil, B. R. Lawn, Huang Han, and L. Faraone. Determination of mechanical properties of silicon nitride thin films using nanoindentation. volume 5798, pages 216–225, USA, 2005. SPIE-Int. Soc. Opt. Eng.

[3] H. Ono and T. Nakano. Diffusion barrier effects of transition metals for Cu/M/Si multilayers (M=Cr, Ti, Nb, Mo, Ta, W). Applied Physics Letters, 64(12):1511, 1994.

[4] P. H. Holloway and C. C. Nelson. In situ formation of diffusion barriers in thin filmmetallization systems. Thin Solid Films, 35(1):L13–6, 1976.

[5] Milton Ohring. The Materials Science of Thin Films, volume 1. ACADEMIC PRESS LIMITED, San Diego, united kingdom edition edition, 1992.

[6] Diffusion, 01Dec. 2006, 2004. www.dictionary.com