Research Of Plasma Characteirstic For High Rate Silicon Thin Film Deposition Process

With the rapid development of solar cell technology, it is an urgent requirementto improve the performance of silicon-based thin film materials and solar cells withreduced cost. The technology of plasma enhanced chemical vapor deposition(PECVD) is commonly used in silicon thin film solar cell research and productionline.Therefore masteringthe characteristics of plasma in-depth is the key tounderstandthemechanism of thin film growth, whichis the fundamental to prepare high-performancesilicon-based thin film materials and solar cells. Soseveralplasma diagnostictechniques in this thesis were used tomeasure and analyze boththe micro-and themacro-discharge characteristics and gas composition of plasma excited by very highfrequency signal, which is used for high-rate deposition of silicon thin film process,to explore the process parameters effects on the regulation of plasma characteristicsand its mechanism.This work helps understandthe relationship between the processconditionsand the material properties, and would be beneficial to the growth of highperformance silicon thin film material and solar cell.Specific research and innovationof this thesis is as follows:1, the properties of deposited films are closely related to the ions and neutralradicals decomposed from gas molecule under impact with electrons. Therefore, theelectron temperature, electron density, ion density and the plasma potentialweremeasured and analyzed under vary process conditions, in order to study theenergy and density distribution of electrons and ions in plasma.We found that theplasma ion species changes with silane concentration variation, leading to significantchanges of electron temperature, electron density and plasma potential. While theelectron temperature and electron density reach a peak atthe certain silaneconcentration, the plasma potential changes a little with silane concentrationvariation.Plasma potential is increased with more discharged power or higher reactionpressure. And total gas flow rate affects a lot on the plasma parameter distribution.Atthe center of reaction region,flow rate effect on the electron density and electrontemperature is not obvious; at the edge of the electrode, the flow rate increases, the electron density reduces.At lower flow rate conditions, the electron temperature iseasier to achieve uniform distribution.Ion energy is decided by plasma potential, soreal-time oscillation waveform ofthe silane plasma potential is measured in this thesis using electrical probes for thefirst time, and we found in hydrogen plasma, the plasma potential oscillationamplitude is about dozens of millivolts. When mixed with silane gas, the plasmapotential oscillation amplitude is significantly increased, and period and amplitude ofthe oscillations is controlledby the silane concentration. The oscillation period of theminimum is got at certain silane concentration. Different silane concentrationchangeions distribution inkinds and density, resulting in the change of oscillation waveformin harmonic contentsignificantly, since ions with different mass actdifferently to theelectric field of plasma and electric field near the probe tip surface. Provided withsuitable ions collection model, and by fitting the oscillation waveform, ionic speciesand density maybe calculated quantitively, which may be one new method to measureions.2, macroscopic properties in high pressure glow discharge plasma werestudied.The discharge parameters, plasma impedance, system of a parasiticimpedance and power utilization efficiency were measured and analyzed.Weestablished four-parameter equivalent circuit model to characterizethe parasiticimpedance, and the method to solve model parameter was given.Based on theanalysis to the electrical connection and the physical structure of chamber, we pointedout that parasitic impedance has a significant impactionon the performance ofVHF-PECVD system, and the parasitic impedance consists of theparasiticcapacitance from chamber gas box, the parasitic inductance from electrodes, andtheparasitic capacitance from thepower feeding cable, while the lattercannot beignored. Power consuming measurement shows that the real power coupled to glowdischarge account for only less than10%, while most of the rest power output fromthe VHF power supply was wasted by matching network and parasitic impedance.One new gas distribution box with effectively reduced parasitic capacitance wasdesigned;which substantially increased the plasma power coupling efficiency and thehighest value of over70%. By measuring and analyzing the relationship between discharge voltage anddischarge current, we found that the plasma energy coupling efficiency is related tospecific discharge modes. High electrode voltageleads to the improvement ofcouplingefficiency by the transition ofdischargemode fromαto γ. The variation of dischargemodes also brought the different of silane decomposition rate, and more silane wasdecomposed byγdischarge. Duringthe film deposition, by adjusting the processconditions, changes in discharge mode, leading to more power coupled into glowingplasma, which improvesupper limit of thin film deposition rate. And the dischargemode transitioncan be found by monitoring plasma impedance, which is intuitive andeasy to beimplemented, with some instructive significance to improve the utilizationratio of reaction gas and the energy efficiency of the system, therefore to improve thefilm deposition process.3, the transient parameters and gas distribution of the glow dischargeplasmawere measured.We found that the impedance was influenced mainly byreaction pressure and silane concentration, and the drift of the process conditions orthe change of gas concentrations in the glow process will inevitably lead to thechanging characteristics of the plasma. Through real-time measurement, we foundthat the silane in the plasma regionlocally depletedinstantaneouslyafter the discharge.In the initial stage ofglow, silane out of the plasma region started to diffuse reverselyto take part in reaction, resulting in the instability of the silane concentration duringthis process. In this process, accompanied with the instability of the glow dischargeparameters, meanwhiledischarge voltage and current has also underwent a moresevere transient change. Depending on the different pressure, silane concentration,discharge power and other parameters, the process may last from a few seconds to afew minutes. The time required to reach the stable status shortenedwith the increasesof the power, the decreases of pressure as well as the increase of gas flow rate; Thisshowed that the instability of initial glowis a result of the mutual influence of thedischarge parameters and gas distribution, which means through real-timemeasurement of macroscopic discharge parameters, one can indirectly monitorplasma internal microscopic state, by which to improve the repeatability of theprocess. Delayed silane feeding method is proposed on the basis of the above observationand analysis, in order to solve the problem ofsilane concentration instability attheinitialglow. The time difference of delayed silane feeding method, which isbetween the silane input tick and discharge starting point, had a significant effect onthe silane concentration and the trend of the discharge parameters of the whole glowprocess. Based on this, it should be convenient and reliable to optimize the timedifference by monitoring either the silane concentration orthe discharge parameters.After optimization, by varying the initial silane concentration in order, the growth ofinitial state of microcrystalline silicon film has been controlled, and then thestructuralhomogenous of the microcrystalline silicon thin films as well as theoverallperformance has been improved.