NUCLEATION AND GROWTH OF Cu THIN FILMS ON SILICON ...

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72 Noriah Bidin And Siti Noraiza Abd. Razaksize and the microstructure of poly-Si thin film which is the most important characteristics ofexcellent built-in poly-Si devices [17]. The principle advantage of excimer lasers is the strongabsorption of ultraviolet (UV) light in silicon beside having larger beam size and high energydensity than other laser light sources [18]. Brended et al. [19], have reported the influence of metalcoatings of glass substrates on the laser crystallization process of amorphous silicon. Amorphoussilicon was deposited on top of the metal and crystallized using a XeCl excimer laser. They foundthat the average grain size varies with the laser fluence.Crystallized by single shot ELA at different laser energy densities, have been investigated byvarious researchers [14, 20, 21]. Another crystallization scenario for energy densities below thecomplete melting was realized due to recalescence effect [20]. It plays an important role than thesuper lateral growth-regime which normally represents the upper energy density limit of partialmelt crystallization regime. Similar recalescence after bulk solidification in germanium filmsmelted by ns laser pulses has been reported by Armengol, et al. [22]. The recalescencecharacteristics in rapid solidification of copper was studied previously by Zhang and Atren [23].The effects of the heat transfer coefficient, the melt thickness and the nucleation temperature wereinvestigated. They claimed that lower nucleation temperature and thinner melt lead to a longerrecalescence effect while larger heat transfer coefficient results in a weaker recalescence effect.In this study, thin copper films were thermally evaporated onto the a-Si base and recrystallizationby ELA. The structural evolution of polycrystalline film growth was monitored via Atomic ForceMicroscope. The growth of poly-Si grains was demonstrated as a function of energy density E L .Base on the AFM results, the recalesence effect was identified and the explosive crystallizationwas found at super-lateral growth region. Furthermore, the stress remaining in the films wasmeasured for various energy densities E L by means of surface roughness measurement.2. Experimental Set-upInitially, silicon thin films were prepared by using PVD technique via a thermal evaporator Edwars360 with low pressure down to 10 -4 Pa. A 0.3 g silicon powders were deposited on 7010microscope glass substrate at a thickness of 100 nm. The a-Si film was doped with a copper, Cu at10% of silicon film thickness. In order to dry out the water contain the silicon film was placed intoa carbon block and underwent conventional annealing using Tube Furnace model F21100. Theheat treament was carried out almost four hours with temperature of 350 °C under atmosphericpressure. This particular temperature was chosen for consideration not to overcome the meltingpoint of the glass substrate.Figure 1. The schematic diagram of the experimental set-up.Later the a-Si films were crystallized by GAM LASER ArF excimer laser. The laser wavelengthwas 193 nm with pulse duration of 10 ns, operating at ambient temperature with gas pressure of
Nucleation and Growth of Cu Thin Films on Silicon base Induced by Excimer Laser Annealing 733350.0 Torr. The laser capacitor voltage was kept constant at 12 kV. The laser was externallytriggered via a AFG310 function generator to control the number of pulses bellow 10 pulses. Thedimension of laser beam spot was 2 × 4 mm 2 . The a-Si films was exposed with different number ofpulses, n. Finally, nanostructure of the crystallized silicon was examined by AFM. Figure 1 showsthe schematic diagram of the experimental setup.3. Results and DiscussionThe nucleation and the growth of SICu thin film was observed via AFM. The typical ofobservation results are shown in Figure 2. The SiCu crystallization are arranged in the increasingorder of the film crystallization. Prior annealing, the grain size of the amorphous silicon thin filmwas measured. The average nanocrystal size, G av of the a-Si film before doping and heating asshown in Figure 2(a) was measured to be approximately 17 nm. After experienced four hours withconventional annealing, the nucleation of crystal was observed to form new nanostructure. Thecrystallization pattern formation almost uniform with an increment of nanocrystal size up to 56nm as depicted in frame (b) of Figure 2.Immediately after exposed by a single shot of excimer laser, with corresponding energy density of65.50 mJcm -2 , the crystal growth was observed having new pattern nanostructure. Thecrystallization of the SiCu film was accelerated to be 75 nm as illustrated in Figure 2(c). Thecrystallization of SiCu film was observed continuously increasing after received the sequentialnumber of pulses from excimer laser.The maximum crystallization of SiCu film occurs after received five number of pulses as depictedin Figure 2(g). The enlargement achieved approximately as 143 nm corresponding to the fluenceof 345 mJcm -2 . Beyond this critical energy for example after treatment with six number of pulseswhich corresponding to total energy density of 389 mJcm -2 , the crystallization reduced drasticallyas illustrated in Figure 2(h). Further decreasing was observed with higher number of pulses. Thesummarization profile of nucleation and the growth of SiCu crystal is presented in Figure 3.The surface roughness due to the melted SiCu thin film is an alternative parameter to indicate theintergration of crystalization formation. It is used to quantify the stress develop duringsolidification. The example of surface roughness measurement is shown in Figure 4. It comprisedthe agglomeration and coalesence of grain that cause even greater surface roughness especially athigher laser fluence. Consequently the surface roughness is increasing but not beyond the SLGlimit. The maximum surface roughness was obtained as 3 nm.In general the excimer laser was partially crystallize the SiCu film at low energy density or at lownumber of exposure. As the energy density increases the film was totally melting and freezing.The UV light of excimer laser with temperature of 6.42 eV or equivalent with 7.5 × 10 4 K wasused to melt the film and activate the Cu to diffuse into silicon base. Considering the temporallength of pulse duration each of laser pulse was only 10 ns, this means the SiCu film wasexperience ultra-speed heating and freezing. The energetic excimer can melted higher than adecalesence point. Consequently during rapid solidification its meet a recalesence point, wherebyspontaneous pattern formation occurs and begins crystallization. The higher the heat transferredinto the SiCu film the greater the crystallization formation. But if the delivered energy greater thansuper lateral growth of the SiCu film, the solidification occurs before meet the recalesence point,as a result no spontaneous pattern formation consequently no crystallization. Thenanocrystallization formation indicates the solidification form in an homogenous pattern.
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