【作者】 许盛之；

【导师】 赵庚申； 赵颖； 张晓丹；

【作者基本信息】 南开大学 ， 微电子学与固体电子学， 2012， 博士

【摘要】 随着太阳电池技术的发展，迫切要求进一步提高硅基薄膜材料与器件的性能，降低其制造成本。研究与生产硅基薄膜太阳电池普遍采用以等离子体增强化学气相沉积（PECVD）为核心工艺的技术路线，掌握等离子体特性是深入理解薄膜生长机理的关键之一，也是研究与制备高性能硅基薄膜材料及电池的基础。为此，本论文采用多种等离子体诊断技术，对高速沉积硅基薄膜过程中的甚高频等离子体的微观电学参数、宏观放电特性以及气体组分等进行系列测量；结合理论分析，探究工艺参数对等离子体特性的调控作用及其机理，以利于理解宏观工艺参数如何调控材料性能，深化高性能硅基薄膜材料及电池的研究。本论文的具体研究内容与创新点如下：1、电子碰撞是气体分解所需能量来源，所产生的各种离子种类、密度与沉积薄膜的质量密切相关。为此，本论文首先测量分析了电子温度、电子密度、离子密度以及等离子体电势等参数，研究了不同工艺条件下的等离子体中电子与离子的能量与密度的变化规律。发现：硅烷浓度的变化，导致了等离子体中离子种类的变化，使得电子温度与电子密度在一定的硅烷浓度下达到极值；而对等离子体电势的影响不大。功率增加或反应气压增大时，等离子体电势随之更负。气体总流量对等离子体参数的均匀性分布影响较大。在低流量条件下，整个电极上的电子温度更容易达到均匀分布；在高流量条件下，电子密度更容易达到均匀分布。在电极中心处，流量的变化对电子密度与电子温度的影响不明显；在电极边缘处，流量增大时，电子密度降低。离子的能量由等离子体电势确定，本论文首次利用电探针测量了反应气体等离子体电势的实时振荡波形，有助于了解离子的微观动力学机制。发现在氢气等离子体中，等离子体电势的振荡幅度不大，约为几十个毫伏。当气体中混入硅烷后，等离子体电势的振荡幅度明显增大，振荡的周期与幅度受到硅烷浓度的调制；在一定的硅烷浓度下，振荡周期达到最小值。同时，不同的硅烷浓度下，由于气体分解后的离子种类与密度的分布发生变化，而不同质量的离子对等离子体内电场与探针表面电场的响应有差异，导致振荡波形显著改变，即谐波含量发生改变。若能够建立合适的离子收集模型以及离子在电场中的运动模型，对上述等离子体电势的实时振荡波形进行拟合，则可定量的获得反应过程中的离子种类与数量，这有可能成为一种新的离子定量测量方法。2、对高气压下辉光等离子体的宏观放电特性进行了研究，对放电参数、等离子体电抗、系统的寄生电抗以及功率利用效率等进行了测量分析。建立了用于分析表征系统寄生电抗的四参数等效电路模型，并给出了求解模型参数的线性拟合方法。结合对PECVD系统的电气连接与腔室物理结构的详细分析，指出寄生电抗对VHF-PECVD系统性能具有显著影响，其来源包括腔室内的气盒、电极等引入的寄生电容、寄生电感，以及馈入电缆等引入寄生电容；而后者是不可忽略的。对系统各部分功率损耗进行的测量表明：真正用于等离子体辉光的功率只占到电源输出功率的10%以下；原因是较大的寄生电容导致了寄生电抗与匹配器功率损耗过多。为此，设计了新的气盒结构，有效降低了寄生电容，大幅提高了等离子体功率耦合效率，最高效率超过60%。通过测量与分析放电电压与放电电流的关系，发现等离子体能量耦合效率与具体放电模式相关。在高电极电压下，放电从α模式转变到γ模式，耦合效率得以提高。放电模式的改变还带来了硅烷分解率的区别。在γ模式下，硅烷分解更充分。在薄膜沉积中，可通过调整工艺参数，改变放电模式，使尽可能多的功率用于辉光放电，提高薄膜沉积速率上限。而放电模式的转变可通过监测等离子体的电抗特性加以判断。这种方法直观且易于施行，对提高反应气源的利用率以及系统的能量效率，改善薄膜沉积工艺，具有一定的指导意义。3、对辉光放电等离子体瞬态过程中的放电参数与气体分布进行了测量。辉光等离子体的电抗幅值与相位受反应气压与硅烷浓度的影响较大，辉光过程中由于工艺条件的漂移或者气体浓度的变化，将不可避免地导致等离子体电抗特性的不断变化。实时监测发现：在起辉的瞬间，等离子体区的硅烷发生局部耗尽。在辉光的初期，无反应空间内的硅烷向等离子体反应区反向扩散，导致此过程硅烷浓度不稳定。在此过程中放电电压、电流等等离子体放电参数也对应的发生较明显的瞬态变化。根据不同的气压、硅烷浓度、辉光功率等条件，该过程可能持续几秒到几分钟。达到稳定所需的时间随功率增加、气压降低以及气体流量提高而缩短；这说明：辉光开始阶段的不稳定过程是由放电参数与气体分布状态的相互影响导致的。因此，可通过实时测量宏观放电参数，间接监测等离子体内部的微观状态，提高工艺过程的可控性以及重复性。在此观察实测的基础上提出：采用后通硅烷法，以解决辉光初期的硅烷浓度不稳定问题。采用后通硅烷法时，硅烷通入相对于辉光开始的时间差，对辉光过程中的硅烷浓度以及放电参数的变化趋势有明显影响。基于此，监测硅烷浓度以及放电参数皆可较方便可靠的优化时间差参数。通过优化，使辉光初期的硅烷浓度有序变化，从而控制微晶硅薄膜的生长初期状态，进而改善了微晶硅薄膜的纵向结构以及电池的性能。更多还原

【Abstract】 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.更多还原