太阳能硅片电磨削多线切割技术基础研究

发布时间：2018-05-04 08:10

本文选题：太阳能硅片 + 电解磨削　；参考：《南京航空航天大学》2016年博士论文

【摘要】：多线切割是当前太阳能硅片的主要加工方法,该方法是通过线网的高速运动把磨料带入硅锭加工区域进行研磨,最终将硅锭切割成薄片。根据磨料的附着形式多线切割又分为游离磨料和固结磨料两种。当前单晶硅片基本采用固结磨料切割,切割后的硅片通过各向异性腐蚀在表面形成随机分布的金字塔绒面,该技术路线成熟,硅片制造效率高成本低。多晶硅由于晶粒取向的随机性,固结磨料切割后的多晶硅片无法沿用现有的酸制绒工艺,暂时只能采用成本较高的黑硅技术来获得较好的绒面结构。多晶硅片的加工目前主要还是采用游离磨料切割。但是游离磨料切割多晶效率较低,尤其是多晶硅锭中含有硬质点时,切割难度更大风险较高。针对目前多晶硅片加工所存在的困难和障碍,本文在现有的多线切割系统基础上,提出了一种电磨削多线切割复合加工方法,该方法在硅锭与切割线间加上连续或脉冲电源,利用切削液的弱导电性产生微区氧化或腐蚀,在机械磨削的同时复合电化学氧化(或腐蚀)实现材料去除。本文针对该加工方法的作用机理、关键技术等进行了深入研究和试验,主要完成了以下几个方面的研究内容。(1)对硅电极进行循环伏安测试发现,在含0.5mol/L KCl的乙二醇溶液中,正向扫描电压为5V时,P型多晶硅片的循环伏安曲线上出现了一个电流峰,峰电流密度约为0.07A/dm2。硅电极在含0.5mol/L KCl的水溶液中循环伏安扫描时,测得了与HF溶液中较为类似的循环伏安曲线,正向扫描电压为1.5V和4V时分别出现了电流峰,峰电流密度分别为0.03、0.08 A/dm2。结果表明,在阳极电场的作用下,硅电极会产生阳极氧化,在水溶液中还会伴随有微弱的阳极腐蚀。随着反应时间的增加,氧化层不断累积,阳极反应逐渐停止,若能及时去除氧化层,电化学反应可持续不断进行。20V直流电压下反应100s氧化层厚度大约为100nm,其表面存在孔洞,孔洞的直径大约为2-3μm,整个氧化层表面致密性差,并检测到氧元素的存在,进一步证明了硅片表面发生了阳极氧化反应。(2)采用分子动力学方法对硅以及硅氧化物进行纳米压痕仿真,结果表明在同样的载荷下疏松状硅氧化物的压痕深度更深,载荷卸载后的残余深度也更大。随后对原始硅片和阳极氧化后的硅片开展纳米压痕实验,结果表明:在4mN的载荷下,原始硅片的最大压入深度约300nm,卸载后的压痕深度约100nm;而阳极氧化后的硅片其最大压入深度约550nm,卸载后的压痕深度约200nm,实验结果与仿真结果基本一致。同时硬度检测结果也表明,阳极氧化后的硅片硬度要小,压头更容易压入其表面,电化学反应产生的氧化层有利于切割时磨粒“嵌入”这一瞬间过程。(3)针对该方法在工程应用中所存在的难题,对其中的关键技术展开研究。阳极进电设计了端部进电法,阴极进电设计了碳刷进电、滚筒进电、石墨块进电和双工位进电等多种线网进电方案。研制了电磨削专用电源,该电源可以设置单极性和双极性两种输出模式,输出的频率(0-500Hz)、脉宽可调,能量可控。并以该电源为载体,设计了一种分布式太阳能硅片电磨削多线切割远程监控系统,便于对加工过程中的电参数和设备状态的监测以及对电参数的远程控制。(4)在NTC 442DW多线切割设备上开展了游离磨料电磨削多线切割及其对比试验。结果表明,电磨削切割后硅片的TTV和BOW的均值分别为11.37μm和7.38μm,对比试验的TTV和BOW的均值分别为12.06μm和10.12μm,两项统计指标的均值和分布区间均优于对比试验。而且,电磨削能减少硅片表面线痕的出现以及颗粒脱落的发生,减小硅片亚表面损伤层,提高切片合格率。同时,该方法因为切割负载的降低能减小切割过程中对切割线的损伤,减少切割线上周向切痕出现,降低断线几率。基于HCT B5多线切割系统,将该方法分别应用于点杂多晶硅和结构线切割,均取得了较好的试验效果,综合良率分别提高了2.54%和2.08%。(5)通过单丝线锯金刚线电磨削对比切割试验,发现对比试验片表面光亮,而电磨削试验片表面颜色偏暗并存在腐蚀迹象。采用酸制绒后,电磨削硅片表面能形成均匀致密的绒面结构,制绒后的硅片反射率低于对比试验硅片,尤其在短波范围内(波长300-600nm)其反射率明显低于对比试验片。然而,电磨削金刚线多线切割后的硅片与普通金刚线切割无明显差异,表面仍然十分光亮,制绒后绒面结构较差。这是因为电磨削多线切割时电流密度由单丝切割时的3.4A/dm2急剧减小到0.18A/dm2,导致电化学腐蚀产生的缺陷中心大大减少,不利于制绒。后续可进一步研究和试验,攻克相关工程难题,提高多线切割时单根钢线的电流密度,促进多线切割过程中电化学缺陷中心的形成,该方法将有望为未来的金刚线切割多晶提供一条可能的技术路径。
[Abstract]:Multi wire cutting is the main processing method of solar silicon wafer at present. This method is to lapping the abrasive into the processing area of the silicon ingot through the high-speed motion of the wire mesh. Finally, the silicon ingot is cut into thin slices. According to the attachment form of the abrasive, the multi wire cutting is divided into two kinds of free abrasive and consolidation abrasive. The current single crystal silicon chip is basically used as a consolidation abrasive. The silicon wafer after cutting, after cutting, forms a random distribution of Pyramid suede on the surface through anisotropic corrosion. The technology is mature and the efficiency of silicon wafer manufacturing is high and low. Polysilicon, due to the randomness of grain orientation, can not follow the existing acid cashmere process by the consolidated abrasive cutting. Silicon technology is used to obtain better suede structure. The processing of polysilicon chips is currently mainly used by free abrasive cutting. However, when free abrasive cutting has low polycrystalline efficiency, especially in polysilicon ingot, it is more difficult and more difficult to cut. In view of the difficulties and obstacles existing in the processing of polysilicon chips, this paper is in the present. On the basis of multi wire cutting system, a multi wire cutting compound machining method is proposed. This method is combined with continuous or pulse power supply between the silicon ingot and the cutting line, using the weak conductivity of the cutting fluid to produce micro zone oxidation or corrosion, and the compound electrochemical oxidation (or corrosion) is used to remove the material at the same time in the mechanical grinding. The mechanism and key technology of the method have been studied and tested in depth. (1) the cyclic voltammetry test of silicon electrode has found a current peak in the cyclic voltammetry curve of the P polycrystalline silicon wafer when the positive scanning voltage is 5V in the glycol solution containing 0.5mol/L KCl. When the current density is about 0.07A/dm2. silicon electrode in the water solution containing 0.5mol/L KCl, the cyclic voltammetry curve is similar to that in the HF solution. The current peak of the positive scanning voltage is 1.5V and 4V respectively. The peak current density is 0.03,0.08 A/dm2., respectively, and the silicon electrode under the action of the anode electric field. There will be anodic oxidation and weak anodic corrosion in aqueous solution. As the reaction time increases, the oxide layer accumulates continuously and the anode reaction gradually stops. If the oxidation layer can be removed in time, the electrochemical reaction continues to react to the.20V DC voltage and the thickness of the 100s oxide layer is about 100nm, and the surface has holes and holes. The diameter of the hole is about 2-3 mu m, the surface of the whole oxidation layer is not dense, and the presence of oxygen is detected. It is further proved that the surface of the silicon surface has been anodized. (2) the molecular dynamics method is used to simulate the nano indentation of silicon and silicon oxide. The results show that the indentation depth of the loose silicon oxide under the same load. Further, the residual depth after loading is also greater. Then the nano indentation test of the original silicon wafers and anodized silicon wafers is carried out. The results show that the maximum penetration depth of the original silicon wafer is about 300nm under the load of 4mN, and the indentation depth after unloading is about 100nm; and the maximum pressure of the silicon wafer after the anodic oxidation is about 550nm, the pressure after unloading. The depth of the trace is about 200nm, and the experimental results are basically consistent with the simulation results. At the same time, the hardness test results also show that the hardness of the silicon wafer after anodic oxidation is smaller and the pressure head is more easily pressed into its surface. The oxidation layer produced by the electrochemical reaction is beneficial to the "embedding" of the abrasive particles in the cutting process. (3) the difficulty of the method in engineering application is difficult. The key technology is studied. The anode incoming power is designed by the end feed method. The cathode intake is designed for a variety of wire mesh intake schemes, such as carbon brush intake, roller power intake, graphite block intake and double station power intake. The electric grinding special power supply is developed. The power supply can set two output modes, single polarity and bipolar, and the output frequency (0-500). Hz), the pulse width is adjustable and the energy is controllable. And with the power as the carrier, a distributed multi wire cutting remote monitoring system for solar silicon wafer grinding is designed to facilitate the monitoring of electrical parameters and equipment state and the remote control of electrical parameters in the process of machining. (4) free abrasive cutting is carried out on the NTC 442DW multi wire cutting equipment. The results show that the mean values of TTV and BOW of the silicon wafers after cutting are 11.37 m and 7.38 m respectively. The mean values of TTV and BOW in the comparison test are 12.06 m and 10.12 u m respectively. The mean and distribution range of the two statistical indexes are all superior to those of the contrast test. The occurrence of particle shedding reduces the subsurface damage layer of silicon wafer and improves the slice qualification rate. At the same time, the method can reduce the damage to cutting line in cutting process, reduce the cutting line cut mark and reduce the broken line probability because of the reduction of the cutting load. Based on the HCT B5 multi wire cutting system, this method is applied to the point miscellaneous polysilicon and the junction respectively. Good experimental results were obtained. The comprehensive good rate increased by 2.54% and 2.08%. (5) through the single wire wire saw diamond wire electric grinding contrast cutting test. It was found that the surface of the test piece was bright, while the surface of the electric grinding test sheet was dark and there was a sign of corrosion. The surface of the silicon wafer was formed by the acid cashmere. With dense suede structure, the reflectivity of the silicon wafer after cashmere is lower than that of the contrast test silicon, especially in the range of short wave (wavelength 300-600nm), its reflectivity is obviously lower than that of the contrast test piece. However, there is no obvious difference between the silicon chip and the common diamond wire cutting after the electric grinding of the diamond wire multi wire cutting, and the surface is still very bright, and the structure of the wool after the cashmere production is poor. It is because the current density in the multi wire cutting is reduced sharply to 0.18A/dm2 when the 3.4A/dm2 is cut from the single wire. The defect center of the electrochemical corrosion is greatly reduced and it is not conducive to the cashmere making. Further further research and testing can be done to overcome the related engineering problems, improve the current density of the single steel wire in multi wire cutting and promote multi wire cutting. The formation of the electrochemical defect center will hopefully provide a possible technical path for future polycrystalline diamond wire cutting.

【学位授予单位】：南京航空航天大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TN305.1

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