半导体硅材料基础知识.1

半导体硅 材料 关于××同志的政审材料调查表环保先进个人材料国家普通话测试材料农民专业合作社注销四查四问剖析材料 基础知识.1 半导体硅材料基础知识讲座 培训大纲 一、 什么是半导体, 1、 导体(Conductor) 导体是指很容易传导电流的物质 2、 绝缘体(Insolator) 是指极不容易或根本不导电的一类物质 3、 半导体(Semiconductor) 导电性能介于导体和绝缘体之间且具备半导体的基本特性的一类材料。 二、 半导体硅材料的电性能特点 硅材料的电性能有以下三个显著特点: 一是它对温度的变化十分灵敏; 二是微量杂质的存在对电阻率的影响十分显著; 三是半导体材料的电阻率在受光照时会改变其数值的大小。 综上所述，半导体的电阻率数值对温度、杂质和光照三个外部条件变化有较高的敏感性。 三、 半导体材料的分类 1、 元素半导体 2、 化合物半导体 3、 有机半导体 4、 无定形半导体 迄今为止，工艺最为成熟、应用最为广泛的是前两类半导体材料，尤其是半导体硅 材料，占整个半导体材料用量的90%以上。硅材料是世界新材料中工艺最为成熟、使 用量最大的半导体材料。它的 实验室 17025实验室iso17025实验室认可实验室检查项目微生物实验室标识重点实验室计划 纯度可接近本征硅，即12个“九”，即使是大工业 生产也可以到7—9个“九”的纯度。 四、 半导体硅材料的制备 1、 冶金级硅(工业硅)的制备 冶金级硅是将比较纯净的SiO矿石和木炭或石油焦一起放入电弧炉里,在电孤加热2 的情况下进行还原而制成。 其反应式是: SiO+2C ? Si+2CO 2 普通冶金级硅的纯度大约是2,3个“九”。目前市面上也有号称4,5个“九”纯 度的冶金级硅，那是通过多次“冶金法”或称为“物理法”提纯后获得的。 2、 多晶硅的制备 目前全世界多晶硅的生产 方法 快递客服问题件处理详细方法山木方法pdf计算方法pdf华与华方法下载八字理论方法下载 大体有三种:一是改良的西门子法;二是硅烷法;三 是粒状硅法。 implementation of corrective tasks for the new problems found in a timely manner into the special rectification and rectification category. In-depth summary of good practices and good experience in educational practice, good around the typical, continue to promote the rectification, to prevent repeated rebound, use of the results of the activities, timely organization look back again. The theoretical results, continuously consolidate and deepen and expand educational practice to achieve the system results, practical results. Three, the main problems and rectification measures focus on rectification goals, determined to honor the commitment rectification measures to build long-term mechanism style construction bureau Party, improve the ideological style The construction and working level, according to the list of issues to sort it out, with overall requirements to carry out the mass line of educational activities, focus on the implementation of the rectification, enhance team ability to discover and solve their own problems, focus on building a political firm, pioneering and innovative, wholeheartedly for the people, strive for unity, harmony, strong collective leadership (a honest and clean.) there lack of global awareness and innovation problem in carrying out the democratic centralism of the party. The party will discuss the main points of the 2014 Bureau of safety production and internal functional components in the labor bureau held a bureau and executive, and discuss specific personnel in 2013, major capital spending No, strictly implement the decision procedures of "three major" matters "collective leadership, democratic centralism, individual consultations, the meeting decided the working principle of the tight loose, not very good to play the role of all members of the leadership team, in the global consultation is not enough, did not fully consider the actual situation of the individual, but by the executive sure, the lack of democracy in different opinions fully discussed and demonstrated. In relation to personal adjustment and the functional group work, individual party members and cadres are not good put forward their views and opinions, or dare to express their views, the crowd, the lack of global awareness and sense of innovation. Some leading cadres are too strong Adjust the local and local interests, the tendency to ignore the overall interests, focus on immediate interests and local interests, in the implementation of the decision and task, and even the existence of "do not want to do, want to do such a backlash and cope with the negative (1) 改良的西门子法生产半导体级多晶硅: 这是目前全球大多数多晶硅生产企业采用的方法，知名的企业有美国的 Harmlock、日本的TOKUYAMA、三菱公司、德国的瓦克公司以及乌克兰和MEMC 意大利的多晶硅厂。全球80,以上的多晶硅是用此法生产的。其工艺流程是: 原料硅破碎 筛分(80目) 沸腾氯化制成液态的SiHCl3 粗馏提纯 精馏提纯 氢还原 棒状多晶硅 破碎 洁净分装。 经验上，新建设一座多晶硅厂需要30—36个月时间，而老厂扩建生产线也需要大 约14—18个月时间，新建一座千吨级的多晶硅厂大约需要10—12亿元人民币，也就 是说每吨的投资在100万元人民币以上。 (2) 硅烷法生产多晶硅 用硅烷法生产多晶硅的工厂仅有日本的小松和美国的ASMY两家公司，其工艺流程是: 原料破碎 筛分 硅烷生成 沉积多晶硅 棒状多晶 破碎、包装 (3)粒状多晶硅 全球用此法生产多晶硅的仅有美国休斯顿的PASADENA工厂，它的生产流程与硅 烷法生产多晶硅的工艺大体相似，所不同的是它沉积出来的多晶硅不是棒状，而是直径 仅为φ1—3mm的硅粒。 、 单晶硅的制备 3 根据单晶硅的使用目的不同，单晶硅的制备工艺也不相同，主要的制备工艺有两 种: (1) 区域熔炼法(简称区熔法或FZ法，Float Zone)。 这是制备高纯度，高阻单晶的方法，区熔法既可以提纯，又可以成晶。 它是利用杂质在其固体和液体中分凝系数的差异，通过在真空下经数次乃至数十次的区域熔炼提纯，然后成晶而制成。 (2) 切克劳斯基法(简称直拉法。CZ法，Czochralski) 这是将清洗好的多晶硅块料(块径,5mm以上)装入石英坩埚 再把装好料的石 英坩埚放入直拉单晶炉内置的石墨托碗上,?抽真空—? 充氩气—? 高频加热石墨 托碗使石英坩埚内的多晶料熔化成液体(需要在1430?以上)—? 降下预先置于炉顶 部的籽晶,?引晶,? 缩颈,? 放肩,? 等径生长,?收尾 等一系列复杂的工艺 而制成。 五、 半导体硅材料的加工 这是指由Ingot ,? wafer的过程。硅片的加工大体包括:硅棒外径滚磨、硅切片、倒角、硅磨片、硅抛光等几个过程 1、 硅切片 硅切片是将单晶硅锭加工成硅片的过程，通常使用的设备有两种: , 内圆切片机:一般加工直径?6″的硅单晶锭。片厚300—400μm，刀口厚度在 300—350μm，加工损失在50%以上。 用这种设备加工的硅切片一般有划道、崩边、且平整度较差，往往需要研磨后 才可使用。 e lack of democracy in different opinions fully discussed and demonstrated. In relation to personal adjustment and the functiobal consultation is not enough, did not fully consider the actual situation of the individual, but by the executive sure, ththe glg decided the working principle of the tight loose, not very good to play the role of all members of the leadership team, in ision procedures of "three major" matters "collective leadership, democratic centralism, individual consultations, the meetinhe decr bureau held a bureau and executive, and discuss specific personnel in 2013, major capital spending No, strictly implement tThe party will discuss the main points of the 2014 Bureau of safety production and internal functional components in the laboarty. a honest and clean.) there lack of global awareness and innovation problem in carrying out the democratic centralism of the pcal firm, pioneering and innovative, wholeheartedly for the people, strive for unity, harmony, strong collective leadership (olitithe implementation of the rectification, enhance team ability to discover and solve their own problems, focus on building a p o the list of issues to sort it out, with overall requirements to carry out the mass line of educational activities, focus onterm mechanism style construction bureau Party, improve the ideological style The construction and working level, according t-long s and rectification measures focus on rectification goals, determined to honor the commitment rectification measures to buildroblemly consolidate and deepen and expand educational practice to achieve the system results, practical results. Three, the main peated rebound, use of the results of the activities, timely organization look back again. The theoretical results, continuousrectification, to prevent rep depth summary of good practices and good experience in educational practice, good around the typical, continue to promote the-new problems found in a timely manner into the special rectification and rectification category. In implementation of corrective tasks for thetiveion of the decision and task, and even the existence of "do not want to do, want to do such a backlash and cope with the negainterests, the tendency to ignore the overall interests, focus on immediate interests and local interests, in the implementat local ews, the crowd, the lack of global awareness and sense of innovation. Some leading cadres are too strong Adjust the local androup work, individual party members and cadres are not good put forward their views and opinions, or dare to express their vional g2 , 线切割机:一般用于加工直径?6″的单晶(如8″、12″等),片厚最薄可达 200μ—250μ，刀口厚度?200μ，加工损失在40%左右，较内圆切割机可多 出5~10%左右的硅片，用这种设备加工的硅切片表面光滑，平整度好，不用经 过磨片工序即可投入太阳电池片的生产。 但线切割机较为昂贵，单机价格是内圆切片的8—10倍。 2、 硅磨片 100μ时大约需要15—20分钟， 一般是双面磨，用金刚砂作原料，去除厚度在50— 用磨片的方法可去除硅片表面的划痕，污渍和图形等，可提高硅片表面平整度。凡用 内圆切片机加工的硅片一般都需要进行研磨。 3、 硅抛光片 这是只有大规模集成电路工业才用的硅片，这里不述及。 六、 半导体硅材料的主要性能参数 1、 导电类型 这是讲半导体内部导电是以什麽类型的载流子在载带电荷。这里我们介绍三种情况: 本征硅:习惯上我们把绝对纯净而没有缺陷的半导体叫作本征半导体。通常纯净而没有缺陷的硅晶体叫作本征硅。 N型硅:若在纯硅中掺入V族元素(如磷、砷等)以后，由于V族元素最外层是5个价电子，当这5个价电子中4个与硅原子最外层的4个价电子形成共价键时，就会有一个多余的电子脱离出来成为自由电子，从而就提供了同等数量的导电电子，这种能提供自由电子的杂质统称为施主杂质(如P、AS等)，掺入施主杂质、以电子为多数载流子的硅叫N型硅。 P型硅:如果在纯硅中掺入III族元素(如硼)以后，由于硼原子的最外层是3个价电子，当它进入硅的晶体构成共价键时，就缺少了一个电子，因而它就有一种从别处夺来一个电子使自己成为负离子，并与硅晶体相匹配的趋势，因此我们可以认为硼原子是带有一个很容易游离于晶体间的空穴。在半导体中，这种具有接受电子的杂质称为受主杂质。掺入受主杂质、以空穴为多数载流子的硅叫作P型硅。 2、 晶体结构 自然界中的固体可以分为晶体和非晶体两大类，晶体是指有固定熔点的固体(如:Si、GaAs、冰及一般的金属等)，而没有固定熔点，加热时在某一温度范围内逐渐软化的固体叫作非晶体。(如:松香、玻璃、橡胶等) (1)单晶、多晶和无定形: 晶体又可以分为单晶体、多晶体和无定形体。 现有的晶体都是由原子、离子或分子在三维空间上有 规则 编码规则下载淘宝规则下载天猫规则下载麻将竞赛规则pdf麻将竞赛规则pdf 的排列而形成的。这种对称的有规则排列叫作晶体的点阵或叫晶格。最小的晶格叫晶胞，晶胞的各向长度叫晶格常数。将晶格周期性的重复排列，就可以构成整个晶体，这就是晶体的固有特性，而非晶体则没有这种特征。那种近程有序而远程无序排列的称为无定形体。 一块晶体如果从头至尾都按同一种排列重复下去叫作单晶体，由许多微小单晶颗粒杂乱地排列在一起的称为多晶体。 (2)晶格结构: 我们对一些主要的晶体进行研究后发现，其中的晶胞多不相同，常见的晶胞有:简单立方结构、体心立方结构、面心立方机构、金刚石结构(如:Si、Ge等)，闪锌矿结构(如 atic centralism, individual consultations, the meeting decided the working principle of the tight loose, not very good to plamajor capital spending No, strictly implement the decision procedures of "three major" matters "collective leadership, democr2013, uction and internal functional components in the labor bureau held a bureau and executive, and discuss specific personnel in carrying out the democratic centralism of the party. The party will discuss the main points of the 2014 Bureau of safety prodm in e for unity, harmony, strong collective leadership (a honest and clean.) there lack of global awareness and innovation problesolve their own problems, focus on building a political firm, pioneering and innovative, wholeheartedly for the people, striv er andout the mass line of educational activities, focus on the implementation of the rectification, enhance team ability to discovstyle The construction and working level, according to the list of issues to sort it out, with overall requirements to carry term mechanism style construction bureau Party, improve the ideological -honor the commitment rectification measures to build long m results, practical results. Three, the main problems and rectification measures focus on rectification goals, determined tosystelook back again. The theoretical results, continuously consolidate and deepen and expand educational practice to achieve the continue to promote the rectification, to prevent repeated rebound, use of the results of the activities, timely organizationdepth summary of good practices and good experience in educational practice, good around the typical, -tion category. Inimplementation of corrective tasks for the new problems found in a timely manner into the special rectification and rectificaant to do such a backlash and cope with the negativedo, w iate interests and local interests, in the implementation of the decision and task, and even the existence of "do not want toding cadres are too strong Adjust the local and local interests, the tendency to ignore the overall interests, focus on immede leaheir views and opinions, or dare to express their views, the crowd, the lack of global awareness and sense of innovation. Somrelation to personal adjustment and the functional group work, individual party members and cadres are not good put forward t ed. Intion of the individual, but by the executive sure, the lack of democracy in different opinions fully discussed and demonstratrole of all members of the leadership team, in the global consultation is not enough, did not fully consider the actual situay the 3 GaAs、Gap、Iusb等)，一般元素半导体多为金刚石结构，III—V族化合物半导体多为闪锌矿结构。 金刚石结构是两个面心立方结构的晶胞在对角线上滑移1/4距离后形成，而闪锌矿结构是在金刚石结构中把相邻两个Si原子分别换作Ga和As而形成。 简单立方体心立方面心立方 (3)晶面和晶向: 晶体中那些位于同一平面内的原子形成的平面称为晶面。晶面的法线方向称为晶向。单晶常用的晶向有(100)、(111)和(110)，晶体在不同的方向上具有不同的性质，这就是晶体的多向异性。 ZZZ (100) aaa (100) 000aaaYYY a(100)a(110)a(111)XXX 晶向示意图 晶向示意图 (4)晶体中的缺陷: 当晶体中的原子周期性重复排列遭到破坏或出现不规则的地方就形成了缺陷，硅单 晶中常见的缺陷有:点缺陷、线缺陷、面缺陷、孪晶、旋涡、杂质条纹、堆垛层错、氧 化层错、滑移线等等。 3、 电阻率 电阻率是半导体材料的一个极其重要的参数，前面我们已经提到电阻率是区分导体、 绝缘体和半导体的关键因素。不同的器件要求不同的电阻率。 4、 少子寿命 所谓少子寿命是指半导体中非平衡少数载流子平均存在的时间长短，单位是μs(1 the implementation of the rectification, enhance team ability to discover and solve their own problems, focus on building a p o the list of issues to sort it out, with overall requirements to carry out the mass line of educational activities, focus onterm mechanism style construction bureau Party, improve the ideological style The construction and working level, according t-long s and rectification measures focus on rectification goals, determined to honor the commitment rectification measures to buildroblemly consolidate and deepen and expand educational practice to achieve the system results, practical results. Three, the main peated rebound, use of the results of the activities, timely organization look back again. The theoretical results, continuousrectification, to prevent rep depth summary of good practices and good experience in educational practice, good around the typical, continue to promote the-new problems found in a timely manner into the special rectification and rectification category. In implementation of corrective tasks for thetiveion of the decision and task, and even the existence of "do not want to do, want to do such a backlash and cope with the negainterests, the tendency to ignore the overall interests, focus on immediate interests and local interests, in the implementat local ews, the crowd, the lack of global awareness and sense of innovation. Some leading cadres are too strong Adjust the local androup work, individual party members and cadres are not good put forward their views and opinions, or dare to express their vional ge lack of democracy in different opinions fully discussed and demonstrated. In relation to personal adjustment and the functiobal consultation is not enough, did not fully consider the actual situation of the individual, but by the executive sure, ththe glg decided the working principle of the tight loose, not very good to play the role of all members of the leadership team, in ision procedures of "three major" matters "collective leadership, democratic centralism, individual consultations, the meetinhe decr bureau held a bureau and executive, and discuss specific personnel in 2013, major capital spending No, strictly implement tThe party will discuss the main points of the 2014 Bureau of safety production and internal functional components in the laboarty. a honest and clean.) there lack of global awareness and innovation problem in carrying out the democratic centralism of the pcal firm, pioneering and innovative, wholeheartedly for the people, strive for unity, harmony, strong collective leadership (oliti4 -6微秒是10秒)。所谓非平衡载流子是指当半导体中载流子的产生与复合处于平衡状态时，由于受某种外界条件的作用，如受到光线照射时而新增加的电子——空穴对，这部分新增加的载流子叫作非平衡载流子。 对于P型硅而言:新增加的电子叫作非平衡少数载流子;而新增加的空穴叫作非平衡 多数载流子。 对于N型硅而言:新增加的空穴叫作非平衡少数载流子;而新增加的电子叫作非平衡 多数载流子。 当光照停止后，这些非平衡载流子并不是立即全部消失，而是逐渐被复合而消失，它 们存在的平均时间就叫作非平衡载流子的寿命。 非平衡载流子的寿命长短反映了半导体材料的内在质量，如晶体结构的完整性、所含杂质以及缺陷的多少，因为硅晶体的缺陷和杂质往往是非平衡载流子的复合中心。 , 少子寿命是一个重要的参数，用于高能粒子探测器的FZ硅的电阻率高达上万 Ωcm，少子寿命上千微秒;用于IC工业的CZ硅的电阻率一般在5—30Ωcm 范围内，少子寿命值多要求在100μs以上;用于晶体管的CZ硅的电阻率一般 在30—100Ωcm，少子寿命也在100μs以上;而用于太阳能电池CZ硅片的电 阻率在0.5—6Ωcm，少子寿命应?10μs。 5. 氧化量:指硅材料中氧原子的浓度。 183太阳能电池要求硅中氧含量,5×10原子个数/cm。 6. 碳含量:指硅材料中碳原子的浓度。 173 太阳能电池要求硅中碳含量,5×10原子个数/cm。 7、晶体缺陷 另外:对于IC用硅片而言还要求检测: 微缺陷种类及其均匀性; 电阻率均匀性; 氧、碳含量的均匀性; 硅片的总厚度变化TTV; 硅片的局部平整度LTV等等参数。 七、 我公司在采购中常见的几种硅材料 1.Cell:称为电池片，常常是电池片厂家外销的产品，它实际是一个单元电池。 2.Wafer:这通常指的是硅片，可能是圆片，也可能是方片。 圆片包括:硅切片，硅磨片、硅抛光片、图形片、污渍片、缺损片。 3.Ingot:常常指的是单晶硅锭，且是圆柱形的硅锭，也有用指多晶硅铸锭的。 4.Polysilicon:通常是指多晶硅料，它又分为棒料、块料、碎料。 5.碳头料(goods with carbon): 通常指多晶硅棒的下部接近石墨头的部分 6.横梁料(beam):通常是指多晶硅棒最上部的横梁，由于其处在硅棒上部，靠近炉 顶部，且过热(生成温度超过1100?)，也常是金属杂质较多的部分，常不 适合于IC工业，而作为太阳电池材料。 7.头尾料(top and tail):这是指拉制单晶锭的头部和尾部的部分，它由于电阻率 范围不在IC适用范围内，杂质浓度高(如尾料)，或缺陷密度大(如头部料) 而被切下报废，但可作太阳电池的原料。 8.埚底料(Pot scrap):这是指CZ单晶拉制结束后残留于石英埚底部的余料，常用 作太阳电池片的原料。 role of all members of the leadership team, in the global consultation is not enough, did not fully consider the actual situay the atic centralism, individual consultations, the meeting decided the working principle of the tight loose, not very good to plamajor capital spending No, strictly implement the decision procedures of "three major" matters "collective leadership, democr2013, uction and internal functional components in the labor bureau held a bureau and executive, and discuss specific personnel in carrying out the democratic centralism of the party. The party will discuss the main points of the 2014 Bureau of safety prodm in e for unity, harmony, strong collective leadership (a honest and clean.) there lack of global awareness and innovation problesolve their own problems, focus on building a political firm, pioneering and innovative, wholeheartedly for the people, striv er andout the mass line of educational activities, focus on the implementation of the rectification, enhance team ability to discovstyle The construction and working level, according to the list of issues to sort it out, with overall requirements to carry term mechanism style construction bureau Party, improve the ideological -honor the commitment rectification measures to build long m results, practical results. Three, the main problems and rectification measures focus on rectification goals, determined tosystelook back again. The theoretical results, continuously consolidate and deepen and expand educational practice to achieve the continue to promote the rectification, to prevent repeated rebound, use of the results of the activities, timely organizationdepth summary of good practices and good experience in educational practice, good around the typical, -tion category. Inimplementation of corrective tasks for the new problems found in a timely manner into the special rectification and rectificaant to do such a backlash and cope with the negativedo, w iate interests and local interests, in the implementation of the decision and task, and even the existence of "do not want toding cadres are too strong Adjust the local and local interests, the tendency to ignore the overall interests, focus on immede leaheir views and opinions, or dare to express their views, the crowd, the lack of global awareness and sense of innovation. Somrelation to personal adjustment and the functional group work, individual party members and cadres are not good put forward t ed. Intion of the individual, but by the executive sure, the lack of democracy in different opinions fully discussed and demonstrat5 9.边皮料(Side walls): 目前理解方法有两种，一种是单晶锭劈成方锭时取下的料，这应是一种比较好的原料，可用于回炉再次拉制单晶锭。 另一种是浇注硅的大方锭六面劈下的废料(由于上方有浮渣，其余五面接触石英，故需剖下)，腐浊清洗后才可再用于浇注硅方锭。 10. 硅渣:这通常是指在对多晶棒进行破碎加工时，粒径,2cm的渣料，不可再用作拉 单晶使用，但可用于浇注硅，用于太阳能电池工业。 11. 瓜子料(Small pot scrap):这是在清理埚底料时的残渣,大约有1/3以上是石英碎片, 其余 的硅粒上都存有或多或少的石英渣残留物。如果我们把那些, 1cm的硅粒选出来的话，也必须进行腐浊处理，去掉石英渣， 作成免洗料以后才能送去作浇注用料。 思考题: 1、 什么是半导体, 、 半导体硅材料的电性能有哪些特点, 2 3、 简述半导体硅材料的制备工艺(分为多晶硅、单晶硅及硅片三部分), 4、 什么是N型半导体,什么是P型半导体, 5、 半导体的少子寿命指的是什么, 6、 请正确区分Cell Wafer Ingot和头尾料、埚底料、边皮料、碳头料、横樑料 等硅原料。 7、 在硅材料采购中应注意哪几个主要参数, o the list of issues to sort it out, with overall requirements to carry out the mass line of educational activities, focus onterm mechanism style construction bureau Party, improve the ideological style The construction and working level, according t-long s and rectification measures focus on rectification goals, determined to honor the commitment rectification measures to buildroblemly consolidate and deepen and expand educational practice to achieve the system results, practical results. Three, the main peated rebound, use of the results of the activities, timely organization look back again. The theoretical results, continuousrectification, to prevent rep depth summary of good practices and good experience in educational practice, good around the typical, continue to promote the-new problems found in a timely manner into the special rectification and rectification category. In implementation of corrective tasks for thetiveion of the decision and task, and even the existence of "do not want to do, want to do such a backlash and cope with the negainterests, the tendency to ignore the overall interests, focus on immediate interests and local interests, in the implementat local ews, the crowd, the lack of global awareness and sense of innovation. Some leading cadres are too strong Adjust the local androup work, individual party members and cadres are not good put forward their views and opinions, or dare to express their vional ge lack of democracy in different opinions fully discussed and demonstrated. In relation to personal adjustment and the functiobal consultation is not enough, did not fully consider the actual situation of the individual, but by the executive sure, ththe glg decided the working principle of the tight loose, not very good to play the role of all members of the leadership team, in ision procedures of "three major" matters "collective leadership, democratic centralism, individual consultations, the meetinhe decr bureau held a bureau and executive, and discuss specific personnel in 2013, major capital spending No, strictly implement tThe party will discuss the main points of the 2014 Bureau of safety production and internal functional components in the laboarty. a honest and clean.) there lack of global awareness and innovation problem in carrying out the democratic centralism of the pcal firm, pioneering and innovative, wholeheartedly for the people, strive for unity, harmony, strong collective leadership (olitithe implementation of the rectification, enhance team ability to discover and solve their own problems, focus on building a p 6