Surface Morphology of Gold Thin Films using RF Magnetron Sputtering Moniruzzaman Syed1, Caleb Glaser2 and Muhtadyuzzaman Syed3 1
Division of Natural and Mathematical Sciences, Lemoyne-Owen College, Memphis, TN, USA 2 Sandia National Laboratories, Albuquerque, NM, USA 3 Department of Electrical and Computer Engineering, Purdue University, IN, USA
Corresponding author: Moniruzzaman Syed, Division of Natural and Mathematical Sciences, LeMoyne-Owen College, 807 Walker Ave, Memphis, TN 38126, USA, Tel: 1(901) 435-1424; Email:
[email protected]
Abstract In this study, Gold (Au) thin films were deposited on glass (SiO2) and silicon (100) substrates at room temperature (RT) in an argon (Ar) gas environment as a function of sputtering time [TS]. The structural and surface morphological properties of Au films have been studied using an Atomic Force Microscope (AFM), X-ray diffraction (XRD) and Raman Scattering. It is found that the deposition rate (DR) on silicon substrate decreases while it increases on glass substrate. This result can be explained by simple Kinetic Theory where sticking probabilities and surface coverage of Au atoms must be considered. AFM micrographs of Au films show that the films on glass substrates, homogenous structures at lower [TS] has observed, which changed to polygonal-like, by aggregating Au atoms into larger island-type clusters with different sizes are clearly observed with increasing [TS]. However, Au coverage of the silicon substrate is rather homogeneous, consisting of tiny gold grains, but a worm-like structure is formed at lower sputtering time [TS]. The growth mechanisms of early stages with the structural behaviors are also discussed.
Introduction Gold (Au) thin films offer a wide range of applications in many fields and may be used for memory storage, energy harvesting [1-3] and storage, nanosensors, optics, corrosion prevention, ware protection and biosensing devices [4-5]. Various approaches such as sputtering, pulsed laser deposition, vacuum evaporation and molecular beam epitaxy have been used for thin film growth. Deposition technique, properties of substrate, adsorbed atoms and their interaction strength with the substrate surface are playing very important role on the growing rate, micro structure and morphology of the thin metal films. In physics of thin films and also in a wide range of technological applications, initial phase of the film growth and its morphology is a persisting problem. Venables JA et. al have been observed and discussed three basic mechanisms of the film growth[6]. Au thin films are currently being studied more closely for many critical applications, as they are highly conductive and yet not easily oxidized. It is
also exhibiting interesting physicochemical properties of gold nanoparticles. Therefore, it is necessary to understand the structural, surface morphology and growth mechanisms of film at early stage of deposition on various substrates. The structural properties of films are showing crucial role on the film quality, which may affect its’ optical properties as well as the sensing capabilities of the device. Many research groups published the Au deposition [7-9] but there is no report on the systematic investigation of the structural and surface morphological properties of Au film on various substrates. Therefore in this study, we deposited the Au thin film on glass and silicon (100) substrates by RF magnetron sputtering with various sputtering time [TS]. Raman scattering, X-Ray diffraction and Atomic Force Microscopy (AFM) methods are used for the study of growth, morphology and the structural properties of thin gold layers.
Experimental Method Thin films of Au were deposited by rf magnetron sputtering onto unheated glass plates in a versatile deposition system with a base pressure of 100 mT of argon gas. Target of 99.99% (MTI Corporation, Density 19.30 g·cm−3, Liquid density at m.p. 17.31 g·cm−3, Melting point 1337.33 K, 1064.18 °C, 1947.52 °F, Boiling point 3129 K, 2856 °C, 5173 °F, Heat of fusion 12.55 kJ·mol−1, Heat of vaporization 324 kJ·mol−1, Molar heat capacity 25.418 J·mol−1·K−1) pure Au was positioned 13 cm above the substrates. All depositions were made at room temperature. The power of the sputter plasma was kept in the range of 50-90 W. Glass and n-type silicon (100) substrates were used to deposit Au film. Substrates were prepared from larger glass microscope slides and cut with a scribe of 1x1 cm. Silicon substrates were also cut and cleaved to this size from larger wafers. The cut substrates were then cleaned with an ultrasonic cleaner. The substrates were subjected to a basic cleaning process before they were entered to the sputtering chamber for deposition. All substrates were cleaned by acetone, isopropanol, and deionized water for 15 minutes respectively (Table 1). After allowing the substrates to air dry, tape was covered to a small area of both glass and silicon substrates to form a mask for deposition in order to measure the film thickness. Magnetron
sputtering is a physical vapor deposition (PVD) technique and the depositions are performed at low pressure in a versatile system. The method is reliable, the results can easily be reproduced and the technique is frequently used for large-scale coatings of thin films. Sputtering means that atoms from a target are knocked out by energetic ions, the released atoms are deposited onto a substrate and a layer is built up. The working gas is usually argon and the ionization is caused by a strong electrical field applied between the target and the grounded chamber. When the field is applied, electrons are emitted from the target and they accelerate towards ground, they collide with and ionize argon atoms. The positive argon ions are forced to impinge on the target and the substrate atoms are knocked out as well as secondary electrons. The electrons are trapped near the target by a magnetic field from a magnetron placed behind the target, and these electrons create further ionization of the gas and increase the efficiency of the sputtering. Thin films deposited using this ion-assisted method become continuous at lower mass thickness compared to when the coating is done by chemical vapor deposition (CVD). Power Supply
Magnetron Targe t
Ar O2
Ar+ ions Atoms from target
Arplasma
Substrate Substrate holder
Fig. 1 shows the schematic diagram of Magnetron sputtering used in this experiment A Hummer VI Sputtering System was used for deposition of the gold film using argon (Ar) gas. The chamber was cleaned prior to each deposition to prevent contamination from each consecutive use. Deposition was carried out at a constant pressure and current of 100 mTorr and 5 mA respectively. The various films deposited on the Si (100) and glass substrates were made as a function of sputtering time only (Table 2). After deposition, the mask was removed and the samples were prepared for characterizations. The deposition rate for each sample was determined by measuring the thickness of the film using an Ambios XP-1 profilometer. Several scans were taken on each sample to obtain a more accurate thickness. The deposition rate was found by dividing the average thickness with the deposition time. The AFM is a scanning probe microscope. It works by scanning an extremely fine probe on the end of a cantilever across the surface of a material, profiling the surface by measuring the deflection of the cantilever. This allows a 2D/3D profile of the surface to be produced at magnifications over one million times, giving much more topographical information than optical or scanning electron microscopes. Its limitation is that the surface to be observed needs to be very flat or the tip will crash into the ‘hills’ as it is scanned. The microscope can run in two modes, contact and close contact. Contact mode scans the probe across the surface, keeping a constant force between tip and
sample, maintained by a feedback control. The amount of movement required to keep the constant force is then used to create the image. Close contact mode, often called tapping mode, uses a vibrating cantilever. Simple height data can be obtained from the changes in Z-axis displacement, but phase Table 1: Substrates Cleaning Conditions Substrate
Acetone min 15 15
Glass Silicon
Isopropanol min 15 15
DI water min 15 15
Table II: Gold Thin Film Deposition Conditions Substrate Glass Silicon
Gas Pressure mT 100 100
Current mA 5 5
Voltage V 5 5
data can also be obtained. AFM images were recorded with a Veeco CP-II atomic force microscope (AFM) to analyze the topography of the film surface. Images taken with the AFM were analyzed using Gwyddion software. Tapping mode was used in this experiment. The root mean square (rms), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variants are positive and negative. It can be calculated for a series of discrete values or for a continuously varying function. Its name comes from its definition as the square root of the mean of the squares of the values. (Zi−Zave)2
rms = √
N
(1)
The XRD measurements were carried out using an XRD apparatus (Shimadzu XD-D1), employing a diffractometer with a slit width of 0.1mm set at the front of the detector. Diffractograms were registered at the angle range of 2θ = 100850. The average grain size, δ in the depth direction was estimated from the half-width value of the X-ray spectra by the Scherrer formula, 𝛿 = 0.9×λ/B×cosθ (2) In this formula, B is the corrected width: B = (B S – BM), where BS is the half width values measured and BM the width due to instrumental broadening. Wavelength of the x-ray used is λ = 1.5418 Å. The structural properties were also characterized by Raman scattering measurements. The Full width half maximum (FWHM) of the films was estimated from the Raman spectra. The stress was estimated from changes in the curvature of the substrate/film system using Stoney’s formula.
Results and Discussion Evaluation of surface morphology based on AFM measurements Fig. 2 shows AFM micro graphs of the Au films deposited on glass substrates as a function of sputtering time [TS]. In these diagrams, the degree of surface roughness is the root mean square (rms) value of the roughness heights. Uniformly distributed Au grains were observed for the films (rms = 1.47
2
nm) at lower sputter time [TS] of 15 min. However the presence of polygonal-like islands with a larger aspect ratio (height/width) with respect to sample 2a were noticed at [TS] = 25 min. Moreover the later displayed the presence of three dimensional individual Au particles were perceived at [TS] = 30 min. Finally, at [TS] = 35 min, the Au particles are pulled together from the nearest surface to form the cluster and/or island type structure (rms = 5.25 nm) because Au has a rather high surface tension [4-7].
surface roughness increases sharply (Fig. 4). In the layer deposited for [TS] = 30 min, homogeneous structures with larger grains are formed (see Fig. 3). This stage of gold layer growth coincides with the transition phase from the electrically discontinuous to continuous gold layer. At later deposition stages, [TS] = 35 min, a homogenous globular structure (rms = 2.72 nm) of the gold layer is observed. Fig. 4 shows the rms value by AFM as a function of sputtering time [TS]. Glass surface exhibits higher surface roughness compared to Si surface (see Fig. 4). A rather complex surface morphology is observed on the gold-covered glass. The presence of isolated gold grains of various sizes is observed for [TS] = 30 min. With increasing sputtering time [TS] the mean size and the density of these grains increase, but the initial size differences are gradually smeared out. The later effect is reflected in the evolution of the surface roughness which increases rapidly in initial deposition stages. With increasing [TS], the gold layer becomes electrically conductive. The appearance of the gold grains and their growth may be due to the above-mentioned preferential capture of the incoming gold atoms on the already existing gold islands or to surface diffusion of deposited gold atoms and their aggregation into larger grains. 6
Fig. 2 shows AFM micrographs of Au thin films on Glass substrates as a function of sputtering time [TS].
Au/Silicon Au/Glass
rms (nm)
5
4
3
2
1 15
20
25
30
35
[TS (min)]
Fig. 3 shows AFM micrographs of Au thin films on Silicon substrates as a function of [TS]. Fig. 3 shows AFM micro graph of Au films deposited on silicon (100) substrates as a function of sputtering time [TS] Surface morphology of the bare glass and silicon substrates were examined by AFM method before [10]. The typical AFM images of bare Silicon surface exhibits flat morphology with low surface roughness not exceeding 0.2 nm. The gold layers, deposited on the silicon substrate, are rather homogenous consisting of small worm-like gold particles (rms = 1.1 nm). With increasing sputtering time (TS), the layer morphology changes to cluster type structure at [TS] = 25 min and the
Fig. 4 AFM surface roughness (rms) of Au thin films on Glass and Silicon substrates as a function of [TS]. It may be concluded that the initial morphology of the substrate affects the morphology of the deposited Au layer significantly. Dependence of deposition rate on Au layers Fig. 5(a) shows the dependence of the deposition rate of Au thin films as a function of sputtering time on glass and silicon substrates. Au films on silicon substrates, deposition rate decreases with increasing [TS]. However, on glass substrates, deposition rate increases sharply with increasing [TS] has been observed as shown in Fig. 5(a).
3
14
8
1.0
(b)
0.8
0.6
12
10
8
0.4
6
0.2 15
20
25
30
[TS (min)]
35
15
20
25
30
35
[TS (min)]
Fig. 5 Dependence of (a) Au deposition rate and (b) Stress on Glass and Silicon substrates as a function of TS. It is evident that the overall deposition rate on silicon substrate is faster than that of on glass substrate at [TS]
