Preparation Of Polycrystalline Silicon Film On Upgraded Metallurgical Silicon Substrate By ECR-PECVD

Almost all (＞85ï¼…) PV modules sold today use Si wafer solar cells and their PV modulesare still getting cheaper and more efficient every year, but cost become a big obstacle for itsdevelopment. Because the fabrication of Si wafers is both material and energy intensive, thescope for further cost reduction is rather limited. This is aggravated by the expected risingcost of energy in the near future. Besides cost saving on materials side, thin film technologyoffer benefits of large area processing, which have a huge potential to be transferred intoindustry.In 2004, a roadmap towards production of c-Si thin films was formulated by thecoordinators of 5 EU projects doing R&D on c-Si thin films. In this roadmap, the epitaxialwafer equivalents can be found to be closest to a possible production. The epitaxial waferequivalents is depositing active silicon layer on a low cost silicon substrate. Its structureresembles the structure of a classical, self-sustaining bulk c-Si solar cell, which would resultin a low acceptance threshold in the solar cells industry, reducing the cost further.Researchers abroad have been conducting experiments depositing silicon film byECR-PECVD on c-Si, high quality m-Si or special prepared seed layer. Up to now, depositingsilicon film on low cost silicon substrate has not been reported. That depositing poly-Si filmon SSP (silicon sheets from powder) only by RF-PECVD has been proved impossible byresearcher at home.For the first time, a layer of high quality poly-Si film has been deposited on upgradedmetallurgical Si substrate at 175℃by ECR-PECVD in this work. The effects of controllableparameters and their reciprocate influences in the processing on the quality of films have beeninvestigated. Based on such experiments, a hypothesis has been proposed by the author andhas been proved by further experiments. The work and related conclusions have beendescribed in detail below:1) The purity,surface morphology,grain size,crystal structure has been studied by EPMA,SEM,XRD,Step Profiler respectively. Meanwhile the optimal processing of cleansing toremove the impurities turns out to be putting the samples directly into the cp133 mixture.Based on information above, crystal silica (quartz) thin film has been deposited onupgraded metallurgical grade silicon by ECR-PECVD through nine orthogonalexperiments. A simple conclusion results from analyzing these data: the combinationbetween power and the flow ratio (F(O₂)/F(SiH₄))determines the crystal quality of the final film and the influence of the combination is larger than any single factor.2) Through a careful analysis of different steps concerning preparing such films, a fewcreative and explorative works have been done on preparing poly-Si film on UMG-Sisubstrates by ECR-PECVD. Another set of orthogonal experiments tell one that thecombination between the power and the flow ratio (Fr=F(H₂)/F(SiH₄)) determinesthe crystal quality of the film. As for the effect of a single factor, the large influencecomes from power, substrate temperature has less influence on the crystal quality of thefilm compared to flow ratio.3) To depositing thin film at high speed and understanding the effect of the combination ofsubstrate temperature and flow ratio, experiments has been conducted at differentsubstrate temperature and flow ratio with fixed power 600w and fixed high chamberpressure 1 Pa. It indicates that there is an optimal Fr at a particular temperature: it will be9.5/8 at 175℃; 20/8 at 350℃; 24/8 at 500℃. As the substrate temperature is lowered, thematched Fr becomes smaller, at the same time the crystal quality of the resulting film getbetter. The author believes that the secondary gas reaction could not be overlooked atsuch high pressure. This is one reason causing low quality of the films. In addition,substrate temperature takes effect on kinetics of surface reaction through thermalactivation. Most films exhibit strong (111) texture, believed to be the result of twinassisted growth mode. Moreover, long duration at high temperature, the diffusion of theimpurities especially O, C from the substrate to the film becomes apparent. The result ofthe primary verification experiments shows that high quality thin film can easily result atlow pressure.4) To realize the epitaxial wafer equivalents, meanly depositing high quality epitaxial thinfilm, depositing poly-Si film at low substrate temperature (175℃)and low pressure hasbeen tried. High quality film has been successfully prepared at suck low temperature. Itturns out that at a particular pressure, as the Fr increases, the crystal quality becomesbetter at first, but turns worse as it pass over a critical value. It means that an optimal Frexists at certain pressure, it will be 10/5 at the pressure of 0.16Pa, and 10/6.8 at 0.4Pa. Asthe pressure grows larger, the Fr becomes smaller.5) Based on data above, to explain the data reasonably, careful analysis and generalizationhave been made by the author on each controllable factor and their combinations, whichleads to a proposal of a hypothesis. The whole film preparation process consists of twocontinuous phases: gas reaction and surface reaction; The former determine the crystalquality of the film, a certain power corresponds to an optimal pressure, and at thispressure, gas reaction is determined by the combination between power and Fr. Thematch between power and Fr leads to high quality thin film at low temperature; Thematch at high temperature cause thin film low quality.6) The hypothesis above has been proved true by two series of experiments and relatedanalysis. The hypothesis is also enhanced by the measurement of electron density in theplasma by Langrnuir probe under different processing parameters.