Nanoindentation measurements of the mechanical properties of polycrystalline Au and Ag thin films on silicon substrates: Effects of grain size and film thickness

doi:10.1016/j.msea.2006.04.080

Materials Science and Engineering: A

Volume 427, Issues 1–2, 15 July 2006, Pages 232-240

https://doi.org/10.1016/j.msea.2006.04.080 Get rights and content

Abstract

This paper presents the results of nanoindentation experimental studies of the contact-induced deformation in Au and Ag thin films. The paper examines the effects of film thickness and substrate deformation restraint on the mechanical properties of electron beam (e-beam) deposited Au and Ag films. Following a brief description of film microstructure, surface topography, and contact-induced pile-up deformation, film mechanical properties (hardness and Young's modulus) were determined using nanoindentation techniques. The indentation size effects (ISE) observed in films with different thicknesses were explained using a mechanism-based strain gradient (MSG) theory. The intrinsic film yield strengths and hardnesses extracted from the MSG theory are shown to exhibit classical Hall–Petch dependence on the inverse square root of the average film grain size. Displacement bursts were also found to occur in Ag films at indentation load levels of 100 μN. These were attributed to the initial onset of dislocation slip activity, when the shear stress exceeds the estimated theoretical shear strengths of the materials.

Introduction

In recent years, significant efforts have been made to develop microelectronics and microelectro-mechanical systems (MEMS) structures in which face-centered cubic (fcc) films have been used as metallic contacts at the micro- and nano-scales [3], [4], [5]. However, the physics of contact-induced deformation of fcc films has not been fully investigated, since the mechanical properties of the small structures may be significantly different from those of bulk materials [6].

The differences between the mechanical properties of small structures and bulk materials may be attributed to: differences in microstructure due to the fabrication [6]; possible substrate effects on thin films [1], [2], [7], [8] and film size effects due to strain gradient plasticity phenomena [9], [10], [11]. There is, therefore, a need to study the mechanical properties of fcc films at the appropriate scale.

A number of experimental researchers [12], [13], [14], [15] have made efforts to study the nano/micro-scale mechanical properties of polycrystalline fcc metals. However, a complete understanding of mechanical properties of fcc films is yet to emerge. In particular, there is a need to develop a basic understanding of the effects of film thickness, microstructure and substrate modulus.

This paper presents the results of an experimental study of the effects of film microstructure/thickness on contact-induced deformation in polycrystalline Au and Ag thin films produced by electron-beam deposition on silicon substrates. Following a brief description of sample preparation and surface microstructure, contact-induced material pile-up and film mechanical properties (hardness and Young's modulus) are characterized using nanoindentation techniques. The effects of Au and Ag film thicknesses and microstructure on indentation size effects (ISE) are analyzed within the framework of the mechanism-based strain gradient (MSG) theory.

The intrinsic yield stress/hardness, extracted from the MSG theory, is shown to exhibit a Hall–Petch dependence on grain size. The strengthening increase with decreasing film thickness is also explained by the measured microstructural length scales obtained from the MSG theory. Subsequently, Young's moduli are determined using the conventional Oliver–Pharr method and models by King [1], and Saha and Nix [2] that account for substrate effects. Finally, displacement bursts are observed to occur when the shear stress underneath the indenter just exceeds the theoretical shear strength of the material.

Section snippets

Theory

Doerner and Nix [7], and later on Oliver and Pharr [16], [17] developed a most comprehensive method for determining the hardness and modulus from depth sensing indentation (DSI) load–displacement data. In the theory, the Meyer's definition of hardness, H was adopted. This is given by: $H = \frac{P_{max}}{A}$ where P_max is the maximum load and A is the projected contact area. The recorded load–displacement data were used to relate the stiffness, S, from the slope of the initial unloading curve, to the reduced

Sample preparation

Polycrystalline Au and Ag thin films with different thickness were deposited onto substrates of Si. The Si substrates were obtained from Silicon Quest International Company, Santa Clara, CA. Prior to deposition, the silicon wafers were prepared by submerging them in a 4:1 mixture of sulfuric acid and hydrogen peroxide in order to clean the wafer. They were then rinsed in hydrofluoric acid to remove the remaining oxide layer, before rinsing with water to remove any residual impurities.

Experimental techniques and calibrations

The measurements of hardness and modulus were performed using a TriboScope (Hysitron Inc., Minneapolis, MN) Nanomechanical Testing System integrated with a DI Dimension 3100 AFM frame. A three-plate capacitive transducer was used by the TriboScope system to control the applied load and displacement. The low spring mass (200 mg) of the transducer's center plate minimized the instrument's sensitivity to external vibrations. Within the system, the standard cantilevered AFM tip was replaced with a

Materials pile-up

Significant material pile-up was found to occur in the fcc polycrystalline Au and Ag thin films with different thicknesses. Typical images showing pile-up as well as the relevant section analysis are presented in Fig. 5(a) and (b), respectively. The pile-up is due to the severe constrained plastic deformation in the films.

The extent of material pile-up increases monotonically with increasing indentation depth and increasing film thickness (Fig. 6(a)). It is important to note that, without

Concluding remarks

This paper presents the results of an experimental study of the microstructure and mechanical properties of polycrystalline Au and Ag thin films on silicon substrates. The salient conclusions arising from this study are summarized below:

1.
The grain sizes of the e-beam deposited Au and Ag films increase with increasing film thickness from 100 to 2000 nm.
2.
The intrinsic hardnesses of the Au and Ag films decrease with increasing film thickness. The strengthening increase with decreasing film thickness

Acknowledgments

This work was supported by the National Science Foundation (Grant Nos. DMR 0213706 and DMR 0231418). Appreciation is extended to the Program Managers (Dr. Ulrich Strom and Dr. Carmen Huber) for their encouragement and support. The authors would like to thank Ms. Zong Zong for useful technical discussions, Prof. Jeffrey Kysar and Mr. Yong Gan for providing help with EBSD facilities at Columbia University.

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Nanoindentation measurements of the mechanical properties of polycrystalline Au and Ag thin films on silicon substrates: Effects of grain size and film thickness

Abstract

Introduction

Section snippets

Theory

Sample preparation

Experimental techniques and calibrations

Materials pile-up

Concluding remarks

Acknowledgments

Int. J. Solids Struct.

Sens. Actuators A: Phys.

J. Mech. Phys. Solids

Surf. Coat. Technol.

Sens. Actuators B: Chem.

Appl. Surf. Sci.

Mater. Sci. Eng. B

Acta Mater.

Curr. Appl. Phys.

Mater. Sci. Eng. A

J. Mech. Phys. Solids

Acta Mater.

Scripta Mater.

Surf. Sci.

Acta Mater.

Silicon VLSI Technology: Fundamentals, Practice and Modeling

J. Appl. Phys.

The MEMS Handbook

J. Mater. Res.

J. Mater. Res.