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1、薄膜淀积技术,半导体工艺中所涉及的常用薄膜：,Evaporation (蒸发）,Sputtering (溅射）,使用加热、等离子体或紫外线等各种能源，使气态物质经化学反应（热解或化学合成）形成固态物质淀积在衬底上的方法， 叫做化学汽相淀积(Chemical Vapor Deposition)技术，简称CVD技术。它与真空蒸发和溅射技术并列，是应用较为普遍的一种薄膜淀积技术。 特点： 1、淀积温度低； 2、可以淀积各种电学和化学性质都符合要求的薄膜； 3、均匀性好； 4、操作简便，适于大量生产；,CVD技术：,CVD的分类： 可按淀积温度，反应腔气压或淀积反应的激活方式分类 低温CVD (200
2、-500C) 中温CVD（500-1000C) 高温CVD（1000-1300C) 常压CVD 低压CVD 热CVD 等离子体CVD 光CVD 等等,热CVD系统：,等离子体CVD,Molecular Beam Epitaxy (MBE) 分子束外延技术,MBE自1960年开始就有人提出，是一种超精密和极精确的薄膜生长技术。其利用的是蒸发原理，将分子束射至单晶衬底上生长单晶外延层的方法。,MBE的特点： 超高真空；设备中外延生长室真空度可达5x10-11Torr,这样分子平均自由程L较大。例如：P=10-9Torr, L=5x106cm。这样大的自由程使分子碰撞几率很小，薄膜生长均匀，生长速率
3、和组分可精确控制。 可以实现低温过程；这样能减少杂质扩散和沾污的几率。利用MBE技术可生长出位错密度102cm-2的外延层。 原位监控；MBE设备上安装有许多原位监控仪器，可以实时监控外延薄膜的生长参数以及物理性能。 (UHV = Ultra High Vacuum),Photolithography Lithography is the process in which a microelectronics patterns are transfer to a substrate. This transfer can be aided by light, electron-beams, io
4、n beams, x-rays, etc. Without the techniques of pattern definition, the fabrication of multiple devices on one semiconductor would be impossible. Although the techniques of pattern definition seem simple they are the heart of modern IC fabrication.,半导体器件制作,Photoresist Photo lithography is a process
5、in which wafer is coated with a light sensitive polymer called photoresist. Polyisoprene is an example of a commonly used photoactive agent. A mask is used to expose selected areas of photoresist to UV light. The UV light induces polymerization in the exposed photoresist. UV causes it to cross link
6、rendering it insoluble in developing solution. Such a photoresist is called a positive photoresist. A negative photoresist shows an opposite behavior. That is exposure to UV makes the photoresist soluble in developing solution.,! Remember: There are two types of photoresist: * NEGATIVE - unexposed a
7、reas removed * POSITIVE - exposed areas removed Negative resist is the most often used because it is less affected by etchants although positive resist offers better resolution. Positive resists are more capable of producing the small size of modern device features which are typically below 1.0 m bu
8、t may be as small as 0.15 m.,光刻的大致工艺流程： 涂胶：一般从高温炉中 取出硅片立即涂胶或在180- 200C恒温干燥箱中烘烤 30分钟后再进行涂胶。要 求粘附性能良好，厚度均 匀适当。 前烘：在80 C恒温干燥箱中烘10-15分钟。目 的是使胶膜体内溶剂充分挥发，使胶膜干燥，以 增强胶膜与SiO2膜的粘附性和胶膜的耐磨,曝光与显影：在涂好光刻胶的硅片表面覆盖掩膜版(Mask)，一般利用紫外光进行选择性照射，使光照部分光刻胶发生光化学反应，经显影将部分光刻胶除去得到相应的图形。,坚膜：一般将显影后的硅片放在烘箱中热烘30分钟左右使经显影时软化、膨胀的胶膜坚固。这样
9、可使胶膜与硅片贴得更牢，同时也增强了胶膜本身的抗蚀能力。,腐蚀：在用正胶的情况下，利用适当的腐蚀液将SiO2或Al腐蚀掉，而有光刻胶覆盖的区域保存下来。 去胶：腐蚀结束后，利用湿法去胶，氧气去胶或等离子体去胶等方法将覆盖在硅片表面的保护胶膜去除。,Exposure Method: * CONTACT PRINTING - 1x mask required； * DIRECT STEP - 5x mask required； * E-BEAM - no mask required；,X-Ray Lithography X-Ray lithography (XRL) consists of pro
10、ximity printing of a mask onto a wafer. Advantages 1) resolution and process simplicity (linewidth1 m) 2) no need for multilevel resist systems used in e-beam lithography 3) XRL parallel writing process, e-beam is a serial.,Etch Process that follows immediately after photolithography step is the rem
11、oval of material from areas unprotected by photoresist. This process must be selective; that is SiO2 is removed while leaving photoresist and silicon intact. It must also be anisotropic; that is etching should be in one direction only.,Etch Method:,Two types of etching processes are used in practice
12、; namely, chemical and physical etching. In purely chemical etching material is removed by dissolution which is highly selective but not anisotropic. In purely physical method material is removed by bombardment of high energy ions which is inherently anisotropic but unselective. As an example, SiO2
13、which is used as a mask for drive in diffusion is removed by exposure to hydrogen fluoride. Hydrogen fluoride reacts with SiO2 to form volatile SiF4 which is swept away by inert argon gas.,湿法刻蚀特点： 选择性高； 生产量大；加工精度：3m 装置成本低；,干法刻蚀特点: 可控性好； 加工精度高,可达0.2m； 可加工设计形状；,对于硅系材料, 最常用的是用在CF4中放电所产生的 等离子体来腐蚀Si、多晶硅和
14、Si3N4。,主要反应：,在刻蚀过程中起主要作用的是原子态F和CF3游离基。近来人们发现在CF4中添加少量氧可使Si的腐蚀速率明显提高。这是因为O2与等离子体中的CF3、CF2或CF游离基作用而放出原子态F所致。,Doping (Diffusion and Ion Implantation),Doping is a general term which refers to the introduction of impurities into a semiconductor medium. Doping used independently is nonselective, whereas i
15、f used in conjunction with pattern definition, it can be selective; introducing impurities into only those areas that you desire.,THERE ARE TWO TECHNIQUES OF DOPING: * SOLID-STATE DIFFUSION * ION IMPLANTATION,Diffusion:,Diffusion is the process whereby particles move from regions of higher concentra
16、tion to regions of lower concentration. Although this includes the self diffusion phenomena, our interest is in the diffusion of impurity atoms. 硅平面扩散工艺是在硅集成电路中广泛使用的一种掺杂技术。其利用硅片表面的SiO2作为扩散掩膜，把待掺入的元素从窗口扩散到硅片内。,固态源扩散装置 气态源扩散装置 液态源扩散装置,扩散原理： 空位扩散 复合扩散 格子间隙扩散,Mathematical Model for Diffusion： The basic
17、one-dimensional diffusion process follows Ficks first law of diffusion: J = -D N/ x where J is the particle flux of the donor or acceptor impurity species, N is the concentration of the impurity, and D is the diffusion coefficient. Ficks second law of diffusion may be derived using the continuity eq
18、uation for the particle flux: N/ t = D 2N/ x2 in which the diffusion coefficient D has been assumed to be independent of position. This assumption is violated at high impurity concentrations.,Two specific types of boundary conditions are important in modeling impurity diffusion in silicon. The first
19、 is the constant-source diffusion(恒定表面浓度扩散), in which the surface concentration is held constant throughout the diffusion. N(x,t)=Noerfcx/2(Dt)1/2 Where erfc is complementary error function(相补误差函数).,The second is called a limited-source diffusion(限定源扩散), in which a fixed quantity of the impurity spe
20、cies is deposited in a thin layer on the surface of the silicon.,SOLID-STATE DIFFUSION USUALLY CONSISTS OF TWO STEPS: 1) PRE-DEPOSITION； 2) DRIVE-IN； During the pre-deposition step, impurities are introduced but typically do not diffuse very far into the substrate. 恒定表面浓度扩散 Before the drive-in, a la
21、yer of oxide is deposited to cap the wafer thus preventing impurities from escaping. During the drive-in, the wafer is heated and the impurities diffuse further into the wafer until the desired profile is reached.限定源扩散,在热扩散中，掺杂原子通过掩膜向硅片中扩散时，除了在深度方向形成一定分布外，在窗口边缘同样有横向扩散。另外，在大面积掺杂时的掺杂量、结深和浓度分布可控性重复性等方面
22、，高温扩散法有着一定不足之处，限制了其应用。目前，在低浓度高精度掺杂方面已被离子注入技术所替代。,Ion Implantation: Ions of dopant atoms are accelerated to a high velocity in an electric field and impinge on target wafer. The ions penetrate through the oxide layer and enter into silicon. Penetration depths of 500 to 5000 A are easily achieved. Pen
23、etration depth depends on size of ion and energy applied. The ions do not penetrate the photoresist layers which are typically 10,000 A thick. By manipulating the acceleration voltage, the average implantation depth can be precisely controlled. The dopant concentration can be carefully controlled by
24、 monitoring implantation current.,Advantages:,掺杂深度与浓度精确可控； 掺杂均匀性、重复性好； 掺杂温度低； 横向扩散小； 在真空中进行，对处理像As一类有害物质有利；,Disadvantage：,slow speed and cost； During high velocity impact of dopant atoms, the underlying crystal is damaged and a post implantation anneal at 800 C for 30 minutes is often required to re
25、move the created defects, prior to profiling dopant concentration by a diffusion step,Junction Depth and Sheet Resistance Ion implantation is often used to form shallow pn junctions for various device applications. The implanted profile approximates a Gaussian distribution, and the junction depth may be found by equating the implanted distribution to the background concentration.,