An optical method for determination of the mass thickness of thin gold films with arbitrary morphology

Highlights

• Laser and sputtering techniques were used for gold films synthesis.

• Gold film morphology does not influence optical properties in ultraviolet range.

• Ultraviolet light attenuation allows determination of the gold films thicknesses.

• The calibrated technique for thickness measurement in range of 1–20 nm was suggested.

Abstract

The mass thickness of thin metal films is an important parameter determining the film structure, however a robust routine method for its determination is still missing. In this work, we have developed a simple optical method for determining the mass thickness of gold films with arbitrary morphology on transparent substrates in the range 1–20 nm. The method is based on film spectrophotometry in the far-UV range when the light attenuation is due to interaction with core electrons and thus the influence of the film morphology is negligible. The calibration was performed at a wavelength of 200 nm for gold films of known thickness and a good agreement with the Beer–Lambert law was obtained. The feasibility of the method was demonstrated with gold films of various morphologies produced on quartz substrates by pulsed laser deposition.

Introduction

Supported gold nanoparticles, i.e., nanostructured films, have unique properties including catalytic activity [1,2], plasmon response [3], high absorption of X-rays [4] that makes them attractive for various applications in medicine, optoelectronics, sensors etc. The application fields are dependent on nanoparticles properties [5] determined by film structure and morphology. A gold thin film can be continuous, percolating, island or an assembly of nanoparticles while its constituent parts can be amorphous, polycrystalline, monocrystalline or embedded nanocrystals [6], [7], [8], [9], [10], [11], [12]. The final morphology of the film can be controlled by synthesis method and experimental conditions [10,11]. Various physical [13], chemical [14,15] and biological [16] methods are presently used for synthesis of thin metal films. The physical methods include thermal deposition, pulsed laser deposition, electron deposition, sputter deposition and so on) [17], [18], [19], [20], [21], [22], [23], [24] and appear to be relatively simple in terms of describing processes occurring during synthesis. Hence they should provide high predictability of the deposition results. From this point of view, gold is one of the most attractive materials for understanding the thin film formation processes due to its high chemical inertness and a large amount of information available on gold film properties [25], [26], [27], [28], [29]. However, many aspects of gold behavior at the nanoscale remain unclear. For example, there are still no unambiguous relationships between final film properties (nanoparticle size, shape and surface density) and synthesis conditions (substrate type and temperature, deposition rate and time) which are usually obtained empirically by the trial-and-error method [22,24,30,31]. Typically, structural characteristics of island films are analyzed in such experiments using mainly imaging techniques such as transmission and scanning electron microscopy (TEM and SEM) and atomic force microscopy (AFM). In contrast, the mass-equivalent thickness (or simply mass thickness) of nanostructured films is rarely reported despite high importance of this parameter for reproducible synthesis and control of film properties [32,33].

Generally, there are a variety of physical and chemical methods for determination of the film mass thickness. The most common physical method is apparently the quartz crystal microbalance technique [34,35]. The method is characterized by good accuracy, and allows to monitor the mass thickness during the deposition process. However, for substrates different of the quartz crystal, extrapolation of the obtained data is not always straightforward. In addition, the method does not allow to control the deposition process at elevated substrate temperatures. A widespread used non-destructive technique is ellipsometry which enables determination of both film thickness and optical constants [36]. The method is universal with respect to materials but, for correct measuring the mass thickness, additional information on the film structure is needed. Other physical methods such as X-ray diffraction (XRD) [37], Ruherford back scattering (RBS) [38], X-ray photoelectron spectroscopy (XPS) [39], radiotracer method [40] or a combination of several techniques [41] have been used for determination of the mass thickness of metal films but they are rather complicated and not practical for routine monitoring of the deposition process. Chemical methods for metal analysis in solutions are quite precise but require removing the film from the substrate [32] that is often undesirable. Also, optical measurements are widely used to determine film properties. Thus, the thickness of semiconductor coatings is often monitored by the interference technique in the transmission [42,43] and reflection [44,45] spectra. However the interferometric methods can be applied only for relatively thick films, thicker than ~ 100 nm [44]. Recently, terahertz spectroscopy in reflection geometry was used for conductivity mapping of thin graphene films [46].

In this work, we propose a simple optical method for determination of the mass thickness of thin metal films with arbitrary morphology on transparent substrates. Such film systems are used in various applications including flexible electronics [47], laser-induced forward and backward transfer [48,49], biosensing [50], fabrication of metamaterials [51]. The method is based on film spectrophotometry in the far-UV range when the light attenuation is due to interaction with core electrons and thus the influence of the film morphology is negligible. The method was tested using gold films produced by pulsed laser deposition (PLD) on quartz substrates at different substrate temperatures resulting in different film structures. The calibration of the method was performed with gold films of known thicknesses deposited by the ion beam sputtering (IBS) technique.