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1、第四章 微纳制造工艺半导体材料与制备,1 结合键 原子间的结合力称为结合键，它主要表现为原子间吸引力与排斥力的合力结果。根据不同的原子结合结合方式，结合键可分为以下几类：,1：离子键 大多数盐类、碱类和金属氧化物主要以离子键的方式结合。离子键键合的基本特点是以离子而不是以原子为结合单元。一般离子晶体中正负离子静电引力较强，结合牢固。因此。其熔点和硬度均较高。另外，在离子晶体中很难产生自由运动的电子，因此，它们都是良好的电绝缘体。但当处在高温熔融状态时，正负离子在外电场作用下可以自由运动，即呈现离子导电性,2：共价键 共价键的实质就是两个或多个电负性相差不大的原子间通过共用电子对而形成的化学键。
2、共价键键合的基本特点是核外电子云达到最大的重叠，形成“共用电子对”，有确定的方位，且配位数较小。共价键的结合极为牢固，故共价晶体具有结构稳定、熔点高、质硬脆等特点。共价形成的材料一般是绝缘体，其导电性能差。,3：金属键 金属中的自由电子和金属正离子相互作用所构成键合称为金属键。金属键的基本特点是电子的共有化。既无饱和性又无方向性，因而每个原子有可能同更多的原子相结合，并趋于形成低能量的密堆结构。当金属受力变形而改变原子之间的相互位置时，不至于使金属键破坏，这就使金属具有良好延展性，并且，由于自由电子的存在，金属一般都具有良好的导电和导热性能。,4：范德华键 属物理键，系一种次价键，没有方向性和
3、饱和性。比化学键的键能少12个数量级。主要由静电力、诱导力和色散力组成。5：氢键 它是由氢原子同时与两个电负性很大而原子半径较小的原子（，等）相结合而产生的具有比一般次价键大的键力，具有饱和性和方向性。氢键在高分子材料中特别重要。,与四种键型相联系的物理性质和结构性质,锗（Ge）：1947-1958，但耐高温和抗辐射性能较差。硅（Si）：1962-砷化镓（GaAs）：1970-宽带材料：ZnSe(1990),SiC(1992),GaN(1994),ZnO(1996)高分子材料？稀土材料？无定形材料？,2.半导体材料,根据各自所具有的原子有序的大小，可分为三类：单晶：几乎所有的原子都占据着安排良
4、好的规则的位置，即晶格位置；有源器件的衬底 非晶：如SiO2,原子不具有长程有序，其中的化学键，键长和方向在一定的范围内变化；多晶：是彼此间随机取向的小单晶的聚集体，在工艺过程中，小单晶的晶胞大小和取向会时常发生变化，有时在电路工作期间也发生变化。,单晶,非晶,多晶,半导体,导电能力介于导体和绝缘体之间的物质。,导体：金属（Ag,Au,Al,etc),10-6 1-cm;绝缘体：陶瓷（石英，氧化铝等），107-cm;半导体：Si,Ge等，10-2 106-cm.,欧姆定律：,R L/A,=1/,半导体的导电机理不同于其它物质，所以它具有不同于其它物质的特点。例如：,当受外界热和光的作用时，它的
5、导电能 力明显变化。,往纯净的半导体中掺入某些杂质，会使 它的导电能力明显改变。,*元素半导体*化合物半导体：IV-IV,III-V,II-VI,3.晶体和晶格,晶格,If a solid has atoms in periodic positions then it can be described by a set of vectors These vectors can then be used to build an array describing the position of every atom in a perfect crystal In 2-dimensions the
6、lattice can be described by two vectors,Origin,a,b,基矢,Gives the number and positions of the atoms INSIDE the repeating unit cell.2D examples shown below.,e.g.Basis of 1Each atom has basis vector of 0 0,e.g.Basis of 2Green atom at 0 0Red atom at 0.5 0.5,3D 基矢,If we take a face centred cubic lattice,w
7、ith a basis of 1 we can describe a material such as copper.,+,2 atom basis at 0 0 0 and,元胞,=,描述四面体结构如硅,简立方结构,In this case,the positions are simply represented by atoms at each corner of a cube.This is the most basic structure and is generally used for understanding the basics of crystallography.Only
8、 one element,Polonium(Po)adopts this structure.,a,a is known as the lattice constant and represents the length of each side of the cube.The lattice constant for Po is 3.35.Units of angstroms(1=110-10m)are used extensively in crystallography.,Hexagonal Close Packed,Each layer consists of atoms arrang
9、ed in a hexagonal arrangement.The next layer sits in the three-fold hollow sites of the layer below.The third layer then sits in sites directly above the first layer.If each layer is labelled by a letter,then the HCP structure is said to show an ABABABAB stacking sequence.,This structure is found in
10、 materials such as cobalt(Co),magnesium(Mg)and zinc(Zn).,Face Centred Cubic,The fcc arrangement can be represented by a cube with an atom at each corner and an atom at the centre of each face of the cube.,Elements adopting this structure include copper(Cu),nickel(Ni),silver(Ag)and aluminium(Al).,Fac
11、e Centred Cubic 2,Can consider the fcc structure as a stacking of hexagonal planes.It has a different stacking sequence to the hcp structure,leading to a different layering.The fcc sequence is ABCABCABC as the third layer does not now lie directly above the first layer,but instead sits in the altern
12、ative three fold hollows of the second layer.,Clear Layer 1Grey Layer 2Red Layer 3,Body Centred Cubic,Again this structure is based on a cube.In this case,the structure is made by placing an atom at each corner and then one at the centre of the cube.,Elements adopting this structure include iron(Fe)
13、,sodium(Na),tungsten(W)and chromium(Cr).,元胞,The unit cell is any representation of the structure that when systematically stacked together will yield the position of every atom.For structures such as the fcc lattice,the traditional view is based on a cube with atoms at the corners and face centres.I
14、t is equally valid to represent the structure by the greyed section,the primitive unit cell.,金刚石晶格,硅,Vector Notation,a1,a3,a2,Right hand axes,Vectors give components for each direction and by convention are written in square brackets,1 0 0 direction,0 1 0 direction,0 0 1 direction,1 1 0 direction,1
15、1 1 direction,晶向指数,密勒指数,a1,a3,a2,ua1,wa3,va2,Using a right hand set of axes,a plane is characterised by its intersections with the principal axes.In the figure to the left the intersections occurs at ua1,va2 and wa3.The reciprocal values of these intersections are then taken;1/u,1/v and 1/w and redu
16、ced to their simplest integer set,labelled h,k and l.The plane is then denoted by these Miller indices in round brackets(h k l).,密勒指数 2,In a cubic system,a vector h k l is always perpendicular to the plane with a set of Miller indices(h k l).Using geometry it can be shown that the spacing between th
17、e planes of Miller indices h k l is given by:,where a is the lattice constant of the unit cell.,a1,a3,a2,e.g.0 0 1 direction red arrow and(0 0 1)plane-green,晶体平面,a1,a3,a2,(1 0 0)plane,(0 1 0)plane,(0 0 1)plane,By convention round brackets are used to represent planes(h k l),晶体平面 2,a1,a3,a2,(1 1 0)pl
18、ane,(0 1 1)plane,(1 0 1)plane,Families of planes h,k,l represented by curly brackets h k l,晶体平面 3,a1,a3,a2,(1 1 1)plane,晶体平面4,0 0 0,(1-1-1)planes conventionally written as(1 1 1),a1,a3,a2,晶体平面 5,a1,a3,a2,The plane shown here runs parallel to the a1 and a3 direction and intercepts a2 at 0.5It is ther
19、efore the(0 2 0)plane,This is related to the(0 1 0)plane in that it runs parallel to it,but the separation of the planes is half that of the(0 1 0)plane.,周期表,Crystal Lattices,硅的晶体结构,晶体缺陷,点缺陷：影响掺杂和扩散；线缺陷：影响热处理；面缺陷和体缺陷：影响成品率,点缺陷(0-dimensional),An avenue for atomic motion within the lattice,in response
20、 to an external mechanical or electrical load,In stainless steel,carbon,which makes it a steel,is an interstitial impurity in the iron lattice(and chromium,which makes it stainless,is a substitutional impurity),In semiconductors,substitutional impurities are called dopants,and control the amount of
21、charge carriers,Intrinsic(vacancies)Extrinsic(interstitial and substitutional impurity atoms)Alter the mechanical properties(by affecting slip and dislocation motion),electronic properties(doping in semiconductors),etc.,Grain Boundaries,Microstructure&Macrostructure:planar defects(2-dimensional),Dur
22、ing solidification,Boundary between two different phases or materials,or between two crystallites of the same material but oriented wrt each other Affects mechanical properties by affecting point and line defect motion Other defects:voids,porosity,precipitates,secondary inclusions,位错:线缺陷(1-dimension
23、al),Extra“half-plane”of atoms in a crystalDislocations make slip 1000 times easier,which is why metals deform easilySlip of atom planes over each other due to deformation occurs one atom row at a time,analogous to caterpillar motion or moving a pile of bricks one at a time,mediates,4.本征半导体和掺杂半导体,半导体
24、,导电能力介于导体和绝缘体之间的物质。,本征半导体,纯净的半导体。如硅、锗单晶体。,载流子,自由运动的带电粒子。,硅(锗)的原子结构,简化模型,硅(锗)的共价键结构,自由电子,(束缚电子),空穴可在共价键内移动,电子：Electron，带负电的导电载流子，是价电子脱离原子束缚 后形成的自由电子，对应于导带中占据的电子空穴：Hole，带正电的导电载流子，是价电子脱离原子束缚 后形成的电子空位，对应于价带中的电子空位,本征激发：,复 合：,自由电子和空穴在运动中相遇重新结合成对消失的过程。,漂 移：,自由电子和空穴在电场作用下的定向运动。,在室温或光照下价电子获得足够能量摆脱共价键的束缚成为自由电
25、子，并在共价键中留下一个空位(空穴)的过程。,两种载流子,电子(自由电子),空穴,两种载流子的运动,自由电子(在共价键以外)的运动,空穴(在共价键以内)的运动,结论：,1.本征半导体中电子空穴成对出现，且数量少；,2.半导体中有电子和空穴两种载流子参与导电；,3.本征半导体导电能力弱，并与温度有关。,硅在 300K,ni 1.6 X 1010 电子/cm3,温度越高，载流子的浓度越高。因此本征半导体的导电能力越强，温度是影响半导体性能的一个重要的外部因素，这是半导体的一大特点。,本征半导体的导电能力取决于载流子的浓度。,本征半导体中电流由两部分组成：1.自由电子移动产生的电流。2.空穴移动产生
26、的电流。,在本征半导体中掺入某些微量的杂质，就会使半导体的导电性能发生显著变化。其原因是掺杂半导体的某种载流子浓度大大增加。,掺杂半导体,P 型半导体：空穴浓度大大增加的杂质半导体，也称为（空穴半导体）。,N 型半导体：自由电子浓度大大增加的杂质半导体，也称为（电子半导体）。,在硅或锗晶体中掺入少量的五价元素磷（或锑），晶体点阵中的某些半导体原子被杂质取代，磷原子的最外层有五个价电子，其中四个与相邻的半导体原子形成共价键，必定多出一个电子，这个电子几乎不受束缚，很容易被激发而成为自由电子，这样磷原子就成了不能移动的带正电的离子。每个磷原子给出一个电子，称为施主原子。,N 型半导体,硅或锗+少量
27、磷 N型半导体,掺杂浓度远大于本征半导体中载流子浓度，所以，自由电子浓度远大于空穴浓度。自由电子称为多数载流子（多子），空穴称为少数载流子（少子）。,在硅或锗晶体中掺入少量的三价元素，如硼（或铟），晶体点阵中的某些半导体原子被杂质取代，硼原子的最外层有三个价电子，与相邻的半导体原子形成共价键时，产生一个空穴。这个空穴可能吸引束缚电子来填补，使得硼原子成为不能移动的带负电的离子。由于硼原子接受电子，所以称为受主原子。,P 型半导体,空穴,P型半导体,硼原子,硅原子,空穴被认为带一个单位的正电荷，并且可以移动,杂质半导体的导电作用,I,IP,IN,I=IP+IN,N 型半导体 I IN,P 型半导
28、体 I IP,5.晶体制备,主流技术:直拉生长法：Czochralski发明，用于从熔融金属中拉制细灯丝。Teal在1950s用于半导体晶体的生长。,ccw,cw,melt,crystal,Rf coil,seed,Graphite susceptor,Fused quartz silica,interface,Argon Flow,CzochralskiProcess,Process Control,TemperatureCCW RateCW RatePull RateGas FlowExternal Mag FieldDoping Conc.,PuritySizeOrientationDe
29、fect DensityGrowth RateDoping Profiles,Czochralski Process Control,Single Crystal,Wafer,Grind Ingot to defined diameterSaw off Ingot ends using a diamond sawSaw ingot into wafers(0.5mm 0.75mm)Edge Grind wafersLap wafers flaten and ensure faces are parallelWet Etch to remove surface damagePolish remo
30、ve residues and planarise locallyFinal Wet Clean,硅的电阻测量,Polysilicon(多晶硅)-MEMS中基本的结构材料 生长方法：LPCVD（高于晶化温度）特性：压阻,热阻 应用：微机械结构（微梁，横隔膜）,p-n结二极管，加速度计，非晶硅 生长方法：LPCVD（低于晶化温度）特性：压阻,光敏，气敏 应用：太阳能电池、气敏传感器、光传感器等多孔硅 生长方法：电化学 特性：易腐蚀 应用：牺牲层,其他材料,Silicon oxide and nitride-SiO2 生长方法：CVD，溅射 绝热，绝缘材料，掩膜材料（碱性溶液湿法腐蚀Si），支撑结构Polymers-Polyimides,photoresist,epoxy resin，BCBGlass and quartz substrate,Thin metal films,Silicon carbide and diamond 具有压阻特性GaAs,AlGaAs,GaN 光电子器件Shape-memory alloys actuatorsCarbon nanotubes,
