Elsevier

Thin Solid Films

Volume 519, Issue 7, 31 January 2011, Pages 2093-2097
Thin Solid Films

Influence of the surface roughness on the properties of Au films measured by surface plasmon resonance and X-ray reflectometry

https://doi.org/10.1016/j.tsf.2010.10.059Get rights and content

Abstract

Thickness and refractive index of Au films thermally evaporated onto glass substrates and with an underlayer of Cr are determined from surface plasmon resonance. The results for the thickness are found to agree very well with those from X-ray reflectivity when a simple model of layers with flat interfaces is used. Plasmon propagation along thin films is influenced by radiative damping due to scattering from surface roughness. To study this influence the surface roughness of the glass substrate, Cr an Au layers are measured by X-ray reflectometry and the results used to introduce three intermediate layers with effective refractive indices and thicknesses corresponding to the roughness. Then Fresnel's equations are used to fit the reflectivity and to deduce the layer properties. It is found that the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm. In particular, the absolute value of real part of the dielectric constant decreases while its imaginary part increases when those effects are not taken into account.

Introduction

Using surface plasmons (SP) to determine optical constants and/or thickness of metal films is important from both fundamental and industrial point of view. The method is a part of the more general issue of optical characterization of thin films and structures [1]. Thin metal films are an integral part of SP based biosensors [2] and Au films are by far the most common. The thickness and optical properties of the Au layers determine the SP sensor performance. Therefore, it is important to know how accurately these parameters can be deduced from SP measurements and what factors affect the precision of the measurements.

Several papers in the 1980s were devoted to the problem [3], [4], [5], [6]. It was established that two different values for the complex dielectric constant and layer thickness fit equally well the measured reflectivity data near the plasmon resonance angle but only one of them corresponds to the actual values of these parameters. In a later paper [7], it was demonstrated that when using a wider range of angles including the critical angle the ambiguity was resolved. All these papers used a one-layer model — the metal film was considered homogeneous and the interfaces perfectly flat. Recently, there is a renewed interest in characterizing Au films suitable for SP biosensors. Atomic force microscopy [8], [9], X-ray studies [8] and transmission electron microscopy [10] have been used to study their structure and topography, and relate them to the optical properties of the films. It was found that surface roughness and heterogeneity of the films significantly affect their properties and the models used in the simulation procedures became more complex. The influence of the surface roughness and inhomogeneity of Au layers on their optical properties were extensively studied in Zhang et al. [9]. Zhang's model included an intermediate layer to account for surface roughness. Its optical constants are modeled with the effective medium approximation [11]. However, the authors included only one such layer — between the gold and the air while the roughness of the substrate was not included into the model. The influence of very thin Cr and Ti layers deposited on the substrate before the gold on the optical performance of the structures was also studied [8], [10], [12]. These layers are very important for various applications, including sensors, because they increase the Au film adhesion to the substrate. However, in all these papers, the thickness of the Au layers was not measured by another method and the accuracy of the SP methods and the validity of the models were not verified independently.

In our paper, we compare results from SP experiments and X-ray reflectivity measurements [13], [14] from thin film structures consisting of two-metal films — a very thin Cr film and an Au film. Our main interest is to investigate how accurately the thickness of the Au film can be determined from SP measurements in the presence of the Cr underlayer and how the inclusion in the model of several intermediate layers to account for the roughness of the substrate and the films affects the dielectric constants of the Au layers obtained from fitting the experimental data.

Section snippets

Sample preparation

For the purpose of the present work, three samples, further denoted as Au1, Au2 and Au3 were studied. The metal films were deposited on 170 nm thick BK7 glass cover slips cleaned by consecutive soaking in acetone and isopropyl alcohol for 20 min and rinsed with distilled water. The samples were evaporated by resistive heating from 99.99% pure gold in an Edwards thermal evaporator E12E. The residual vacuum was 1–2 × 10 2 Pa. The deposition rate was 0.5–1.2 nm/s for all three samples and their

Results from X-ray

The X-ray reflectivity curves from the three samples are shown in Fig. 1. All data sets were modeled as a two layer system on top of a semi-infinite substrate (glass). Each of the layers is described by 3 parameters: thickness, scattering length density and a Gaussian roughness between the layers. The scattering length density is proportional to the number of electrons in a unit volume. The data were fitted using a least squares cost function (on a log10 scale) using the stochastic differential

Discussion

From Table 2 it can be seen that several similar sets of parameters corresponding to different initial values of the sample thickness and/or optical constants are obtained from the minimization procedures for the SPR data. This is not surprising since with six or seven free parameters the surface defined by χ2 is likely to have several local minima in the space of the free parameters. For a given sample the values of χ2 corresponding to different minima are very close to each other and it is

Conclusion

In this paper we compared results obtained from X-ray reflectometry and SP measurements of Au films evaporated on top of a thin Cr layer. It was found that the thickness obtained from optical measurements when the roughness of the films is neglected coincides very well with the one obtained from X-ray measurements. On the other hand, the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm.

Biosensor sensitivity increases when surface

Acknowledgment

The Bulgarian National Science Fund supported this research through grant no. VUF 203/06.

References (17)

  • J. Homola et al.

    Sens. Actuators B

    (1999)
  • S. Cowen et al.

    Opt. Commun.

    (1990)
  • H. Neff et al.

    Thin Solid Films

    (2006)
  • B.A. Sexton et al.

    Sens. Actuators A

    (2008)
  • O.S. Heavens

    Rep. Prog. Phys.

    (1960)
  • W.P. Chen et al.

    J. Opt. Soc. Am.

    (1981)
  • F. Yang et al.

    Appl. Opt.

    (1988)
  • W.M. Robertson et al.

    J. Opt. Soc. Am. B

    (1989)
There are more references available in the full text version of this article.

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1

Present address: PANalytical B.V., Lelyweg 1, 7602 EA Almelo, The Netherlands.

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