The effects of deposition parameters on microstructure, grain size and crystallographic orientation of silver thin films with various thicknesses by X-ray diffraction and scanning electron microscopy (SEM) of samples has been studied. The influences of substrate and annealing temperature on microstructure, orientation and grain size variations have been investigated. The results show that, grain size increases with film thickness and substrate temperature and the produced Ag films exhibit (111) peak as preferred growth orientation in all cases. Grain size variation with deposition temperature has been explained by structural zone model (SZM).
In recent years thin metal films specially silver films attracted great interest, due to their unique micro-electronic and optical properties (lowest electrical resistivity â‰ˆ1.6Î¼Î©.cm and the highest reflectance â‰ˆ 95% in visible and IR region of spectrum) [1,2,3]. Ag reflects heat on automotive glazing, and for windshield only silver can be employed as the reflecting metal layer because of its low light absorption compared whit Au, Cr, Cu,â€¦and other noble metals[4,5]. Silver thin films exhibit sufficient long-term chemical stability also Ag is usually used in surface-enhanced scattering because of its rough surface due to its high reflectance and energy-saving application like solar control system for heat rejection[6,7]. Since the physical properties of silver films were found to be strongly dependent on their density, microstructure and morphology, variety of methods have been proposed for the deposition of silver thin films[8,9]. But some of the methods used are good only for small size substrate, and some are not suitable to achieve a good homogeneity of the film layer properties across the whole deposition width, which is very important for most optical and electrical applications. Also since Ag is a noble metal, these films sometimes exhibit poor adhesion to some metals, semi-conductors and specially dielectrics like glass[10,11]. Magnetron sputtering which has been employed in this work has many advantages compared with other methods of coating, high deposition rate, and very good homogeneity of film and very suitable for coating large areas. Since high energy particles ejected from sputtering target arrive at the substrate surface with a mean energy much higher than other methods of PVD. This kinetic energy can enhance the interaction between condensed species, and substrate surface leading to much better adhesion, and high packing density of atoms in the films. Also heat load of the substrate is small compared to that in evaporation processes, which can be important for deposition on plastic and substrate materials with very low melting points, which may deform (polycarbonate, Pc) due to heat from high temperature of evaporation sources. Recently considerable works have been directed toward detailed understanding of metal-semiconductor systems with large lattice mismatch such as Al/Si, Cu/Si, Ag/Si,â€¦, for both fundamental and practical application[13,14,15,16]. The metal-semiconductor systems have potential in production of stable Schottky barriers, and conducting electrodes in large scale integrated circuits and for this reason in this work deposition of silver on glass and Si has been compared.Since in many applications in different technologies, the distribution of grain size and crystalline orientation as well as their variations strongly can affect the various properties performance, reliability and stability of thin films in optical and electrical devices. In this work the influence of different deposition parameter (film thickness, substrate, annealing temperature) on microstructure and morphology of Ag thin films have been investigated. For coating metals by sputtering the structural zone model (SZM), consisting of three zones separated by two boundary temperature proposed by Movchan, for polycrystalline film structure has been refined by Thronton, in which an additional zone (transition zone, T) appears between zone I and zone II. In our earlier works on residual stress in Cu and Ti thin films at different substrate temperature the results showed good agreement with SZM. In present work also this model has been employed for description of the effect of deposition temperature. Although many works have been carried on Ag thin films, but either film thickness has been very low, so discontinuous films take place or film thickness range is very limited[20,21,22,23].To the best of our knowledge comparison of annealing and substrate temperatures effects on different substrate materials has not been reported.
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Silver thin films has been deposited on silicon and glass substrate under various experimental conditions, a vacuum system (Hind High Vacuum, H.H.V), with base pressure of 10-6 mbar was employed, thus the background pressure prior to silver coating was around this value. Circular Ag target disc with 125mm diameter and 3mm thickness was made of pure silver (99.9%). For plasma formation research grade argon (99.99%) was used in pressure range of 2Ã-10-2 - 2Ã-10-1 mbar. To vary the deposition rate discharge current was changed from 0.2 to 1.2 A, while Ar pressure was kept constant, voltage changes was in range of 300-450 Volts, depending on Ar pressure. For sputtering the optimum distance between Ag target and substrate (glass, Si) was found to be 10 cm and this distance was kept fixed for all samples in this work. Deposition rate and film thickness were measured by use of vibrating quartz crystal thickness monitor, Ag film with various thickness (50-1000nm) with optimum coating rate of 4.2nm/s were deposited. The substrates were fixed on stainless steel holder, which could be heated to a set temperature as required in each run of coating. The substrate temperature (Ts) was controlled by programmed thermostats and digital thermocouples fixed inside holes on the surface of substrate holders. Just before use of substrates they were cleaned and made ready for deposition so the native oxide on the surface of substrate was subsequently removed. Si(100) P type wafer was cut to required dimensions (1Ã-1cm) and circular glass substrate with 2 cm diameter and 1mm thickness were used for coating Ag. Before start coating discharge was runned for few minutes and the produced plasma was checked by spectrometer with high resolving power, the shutter was removed when only line spectra belonging to Ag and Ar atoms and ions were observed and no band spectra due to contamination was observed (this checking was carried out via window of viewing port of vacuum chamber). For annealing the Ag film samples a vacuum and gas flow (Ar) oven was employed. To investigate the effects of deposition parameters on microstructure, crystallographic orientation and grain size studies, X-ray diffraction (model PW30 Philips) was used. Scanning electron microscopy (SEM) of the produced Ag thin films also was carried out (Model VEGA/XMU) to give better view and information.
rESULTS & DISCUSSION
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Silver (FCC, noble metal) was coated on glass and Si substrate with deposition rate of 4.2 nm/s at room temperature (25oC), Fig(1a,b) show XRD pattern of Ag/Si and Ag/glass respectively for different Ag film thickness.
Fig1. XRD pattern of Ag thin films for different Ag film thickness (a) Ag/Si ,(b) Ag/Glass
In both cases (111) of silver peak is very intense compared with (200) peak, which exhibit that (111) is the preferred orientation for both substrate used. Since effective way of reducing electron migration in metallic films Cu, Al, Au, Ag,â€¦ is production of thin films with strong(111) orientation , so these results can be very important in that respect. (111) orientation in thin metal films can enhance their electromigration lifetime, as an important feature for device applications, so variation of (111) intensity (I111) and (200) (I200) against thickness of Ag was plotted for both Ag/Si, Ag/glass samples, Figs (2a,b).
Fig2. Variation intensity (111), (200) against Ag thickness for (a)Ag/Si, (b)Ag/Glass
Intensity of (111) peak increases much sharper than intensity of (200) peak with film thickness and for both substrates this rise of intensity is very similar, which means different type of substrate do not change this situation. For calculation of grain size and studying its variation with the deposition parameters, Scherrer equation has been used .
Where D is grain size diameter, is X-ray wavelength (),is the Brrage angle, B is the full half width of peak maximum and k is a dimensionless constant which for metal films usually is taken as unity . Although this equation for exact calculation grain size is not very suitable and adequate, but for comparing the results is good enough especially when SEM of the samples also is carried out. When variation of grain size versus Ag thickness was plotted (Fig 3) , grain growth for Ag/Si sample with silver thickness can be observed , while for Ag/glass after certain Ag thickness, a sharp rise of grain is happened, followed by fast reduction of grain growth for 700nm thickness of silver. SEM pictures of these samples also showed this situation, for Ag film with 750nm thickness many large grain can be observed while for lower and higher thickness of Ag very smooth film with small grains is obtained, Figs (4a,b,c).
Fig3. Grain size of Ag films versus the films thickness
Fig4.Electron micrographs from Ag films on Si at different thickness: (a) 250nm, (b)750nm ,(c)1000nm
To investigate the effect of substrate temperature on microstructure, preferred orientation and grain growth variation, Ag films with film thickness of 250nm with coating rate of 4.2nm/s were deposition on Si and glass substrate at different deposition temperatures. Fig(5a,b) show XRD pattern of the Ag samples (Ag/Si, Ag/Glass) for different substrate temperatures , in both cases (111) peak is again the preferred orientation. Fig(6) gives plot of grain size variation against reduced temperature (),(where TS is substrate temperature, and TM is the melting point temperature of coating material, both in Kelvin degree).
Fig5. XRD pattern of Ag films deposited at various substrate temperature for , (a)Ag/Si, (b)Ag/Glass
Fig6. Grain size of Ag films as a function reduced temperature (Ts/Tm).
For Ag/Si grain growth increase with reduced temperature, while for Ag/glass after a small increase for , a sharp rise of grain growth takes place, followed by fast reduction at higher temperature. Since deposition of thin metal films by sputtering on glass substrate obey the structural zone model, this behavior of grain growth is due to change of Thornton zones and also it can be due to thermal grooving which stops and reduced growth of grains. SEM pictures of these samples also show this situation , in high substrate temperature large grains can be observed, Fig(7). To study how annealing temperature can affect the structural and grain growth of Ag films, two samples of silver films with 250nm thickness with 4.2nm/s coating rate were deposited on glass and Si at room temperature. Both of these samples were annealed up to temperature of 325oC in vacuum for 60 minutes Figs (8a,b) show XRD pattern of Ag/glass, Ag/Si respectively. Once again (111) peak is preferred orientation in both cases, plot of grain size variation against annealing temperature has been given in Fig (9).
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Fig7. SEM picture of Ag/Si film on deposited 400K
Fig8. XRD pattern of Ag thin films against annealing temperature (a) Ag/Si, (b)Ag/Glass
Fig9. plot of grain size as annealing temperature
For Ag/Si thin film grain growth increases with temperature up to 250oC and then grain growth stops and even shows a little reduction at higher temperature. But for Ag/glass sample at low temperature grain size rises with temperature up to 150oC, then a sudden decrease occurs and after that remains roughly constant at higher temperature. This is saying that in low temperature region for both samples grain rise takes place, but at higher annealing temperatures large grains break to smaller one, but for Ag/glass this happens at lower temperature. This can be due to change of adhesion force between thin film (Ag) and substrate, which means a change of stress and strain between film and substrate took place at these annealing temperatures.
To compare the annealing and substrate temperature effects on microstructure of Ag thin films which have been coated on Si, Fig (10) shows how grain growth takes place for these samples with temperature. But for the annealed samples grain are larger which can be due to agglomeration of Ag mean while after temperature of 225oC this grain rise stops and show a reduction at higher temperature. To see the effect of different substrates, and surface condition, 250nm Ag was coated on glass, and on smooth and rough surface of Si, SEM pictures of these samples which have been coated under similar experimental condition is shown in Figs (11a,b,c) on smooth surface of Si, very uniform Ag thin films with small grains can be observed, while for rough surface of Si, larger grains with few agglomerations can be seen. But for glass substrate number of large grains due to agglomeration is observable which can be due to smoothness of glass surface, the grain growth from XRD results agree with these SEM observations.
Fig10. compare grain size of annealing and deposition temperature of Ag/Si
Fig11. SEM picture of Ag films on different substrates (a):on smooth surface of silicon, (b):Rough surface of Si, (c):Glass
SUMMERY & CONCLUSION
For characterization of Ag thin films coated on Si and glass substrate and studying the effects of deposition parameters on microstructure and crystalline orientation and grain size variations various silver thin films under different conditions were investigated. Grain size for Ag/Si sample increases very little with film thickness, but for Ag/glass grain growth is sharp and then at higher Ag thickness a reduction of grain size takes place. This can be due to condition and smoothness of glass surface compared with Si surface and also adhesion of Ag to Si and glass is very different. Increase of film thickness did not change the preferred orientation ((111)), also peak (111) was the intense peak for both substrate which means preferred orientation is independent of used substrates. Changing deposition temperature was described by SZM, for Ag/glass samples, but Ag/Si did not obey this structural model. Increase of substrate temperature did not change the preferred orientation for both substrates, but thermal grooving for Ag films on glass takes place, and also for Ag/Si increase of (111) peak intensity is not the same for glass substrate which is amorphous material. For annealing temperature once again (111) is the preferred orientation, for Ag/Si grain growth with temperature is observed, which is due to expansion and smaller grains get together to make larger grains. But for Ag/glass samples grain growth stops at higher annealing temperature and followed by a reduction which is due to breaking of larger grains to smaller ones, and this can occur because of changes of adhesion force between thin films (Ag) and substrate. Type and surface roughness of substrate can vary the growth of grains and agglomeration for Ag thin films. At very low deposition rate adhesion of film to substrate (specially glass) was not that good and very high deposition rate 14 nm/s produced amorphous Ag film, so optimum rate of 4.2nm/s was used. Heating can improve the adhesion of film to substrate, specially for samples with higher deposition temperature which produces thin films with good quality and stability.