章化学气相沉淀.doc

14章 化学气相沉淀141介绍化学气相沉淀（CVD）被广泛地用于微电子集成制造领域，用以沉淀薄模。它是一种可以制造导体与非导体的多功能技术。CVD型薄模有非常好的填充能力。在现代化电子集成制造中它们被用于填充，深度非常高的过孔与沟槽。1411CVD基础介绍在CVD工艺中，反应气体通过硅芯片表面。这些气体被吸附于芯片表面，反应而形成薄模，而反应的附加产物以气体的形式脱离表面，被真空泵吸走。化学反应可由两种方法来激发：通过加热而产生的热能，通过等离子体而产生的射频（RF）（极少数也有采用其他方法，如激光）。在第二种方法中，RF能量在反应舱中激发出等离子体，用于生成带能量的电子用于推动反应发展。一个典型的CVD反应腔如图14。1所示：气体通过入口进入反应腔，通过加热后的晶片表面，然后通过泵抽出反应腔。1412历史介绍由于大多数非反应所需的物质在一个大气压下为气态，所以极少数的CVD在大气压下工作，因此，直到19世纪后期，随着稳定的真空泵设备的发明，化Pyroetic学气相沉淀才发展起来，1880年Savycv和Man 成功地淀积出Pyroetic 碳，1896年Aylesworth 通过氢气还原氯化物的方法，淀积出多种金属膜，现代CVD反应腔是在二十世纪中期发展出来的。1959年，Noyce和Houvni 发明了p;anar工艺，使得集成电路制造成为可能。这种工艺在薄膜淀积后，通过光割与刻蚀工艺，制造出形成二极管的金属与绝缘层。第一个在半导体集成制造中使用的CVD型薄膜是Vapox,一种在一个大气压下由silane与氧气反应淀积的Sio2 膜，主要用于金属面介电质与钝化保护型。1413CVD工艺与其他工艺的比较有许多工艺方法可用于薄膜淀积，其中一些在集成电路制造中使用，当铝成为主要的内部连接的导体物质时，一个nice division of labor 在CVD 与PVD工艺的形成。PVD用于淀积铝。这种工艺的纯净度可精确控制Si与cu的掺杂比例，而产生很低的电阻率，薄膜间的绝缘层是由等离子体enhanced CVD工艺淀积的，连接不同金属层的过孔是CVD工艺淀积的金属钨。近几年中，有一种用铜来做内部连接的金属的趋势。它具备较低的电阻率，加上对铜扩散有较好的抑制，成为高性能器件中的导体选择，铜淀积CVD工艺比电镀工艺更经济实用。表141 列出各种半导体制造中通用的淀积技术。1414典型应用在整个电路生产中，CVD薄膜被一直应用，从二极管的形成，到连接二极管的内部连接层，直到保护器件的钝化层。14.2 Theorythe theoretical study of CVD focuses on two areas。the transport of gases to and from the wafer, and the chemical reactions that take place both on the wafer and in transit CVD 的理论研究集中于二个区域。气体到晶片和气体离开晶片的传输过程, 和发生在晶片上的和在气体传输过程中的的化学反应14.2.1 Mass Transferin order to deposit a CVD film, the reactants must be transported from the inlet to the wafer surface。for many processes, the deposition rate is limited by how fast this takes place 。additionally, film uniformity can be strongly dependent on mass transfer rates 为了淀积CVD 薄膜, 反应剂必须从入口被运输到晶片表 面。在很多工艺中，淀积率由反应速度来限制的。另外, 薄膜均匀度在很大程度上是由质量传输率决定的consider a tungsten CVD process in which hydrogen and tungsten hexafluoride flow into a chamber and react to form tungsten考虑氢和六氟化钨流入反应腔并起反应形成钨 的一个钨CVD 过程the volatile HF is pumped away。if the wafer were maintained at room temperature, the reaction would proceed at near zero rate. as the temperature is increased, the deposition rate would increase exponentially。at sufficiently high temperature, the rate would slow down and level off, as illustrated in Fig. 14.2。What is going on? at low temperatures, the reactants are being consumed at a very slow rate, much slower than they are entering the chamber. thus the deposition rate is completely governed by the reation rate on the wafer surface。this reaction rate is very temperature sensitive, and so increases rapidly with temperature。Under these conditions, the process is said to be kinetically controlled. That is, the deposition rate is governed by the chemical kinetics or the reaction rate. As the temperature is increased further, the reaction rate eventually reaches and then exceeds the rate at which WF6 or H2 arrive at the wafer surface. Once that happens, the process is no longer kinetically controlled; it is now mass-transfer controlled or mass-transfer limited. At this point, further increases to the temperature cannot increase the deposition rate because there is not enough material to react. The only way to increase the rate at this point is to turn up the flow.挥发性HF 用泵抽掉。如果晶片在室温保存, 反应将很难进行。随着温度被增加, 淀积率以指数级地增加。在充足地高温下, 淀积率会减慢而持平, 依照说明在图 14.2。到底什么情况发生了呢？在低温, 反应剂在以非常慢的速率消耗, 比他们正在进入反应腔时慢得多。因而淀积率由在表面的反应完全地控制。这反应率对温度非常敏感, 而且随着温度的上升迅速地增加。在这些情况下, 这个过程被认为是动力控制的。也就是，淀积率由化学动力学或反应率控制。随着温度进一步被提高, 反应率最终达到和然后超出WF6 或H2 到达晶片表面的速率 。到那时, 这个过程不再被动力控制; 它将是物质传输控制或物质传输限制。这时, 进一步增加温度不可能增加淀积率，因为反应材料已不足够。唯一的方式增加淀积率就要提高流量。Does it matter if the process is kinetically controlled or mass-transfer limited? Yes. When mass-transfer limited, the surface is starved for at least one of the reactants. That means the sticking coefficient for that reactant will be high. As we will see later in the chapter, high sticking coefficients result in poor step coverage. On the other hand, in the kinetic regime, the surface is saturated with reactants. Under these conditions, by-products can become trapped into the film, reducing its purity. In the case of the tungsten reaction, the level of fluorine incorporated into the film is much higher when the process is kinetically controlled.这个过程是由动力控制的或由物质传输限制，有关系吗? 是 。当由物质传输限制的, 表面至少缺少反应剂当中的一个。那意味粘附的系数为那反应剂是高。因为我们看见以后在这个章节, 上 流粘附的系数导致粗劣的步覆盖面。另一方面, 在这个运动政体, 表面饱和与反应剂。在这些情况下, 副产物可能成为设陷井入这张 胶片, 减少它的纯净。在钨反应的情况, 氟素的水平合并入这张胶 片是比较高当这个过程是运动控制的。Gas Utilication.For many CVD processes, reactants are expensive and so we want to use them most efficiently. Unfortunately, the most efficient operating conditions are those that produce the lowest deposition rate, i.e., flow the gases into the chamber very slowly, to allow lots of time for the reactants to diffuse to the wafer. 为许多CVD 过程, 反应剂是昂贵的和如此我们想最高效率 地使用他们。不幸地, 最高效率的操作条件是这些生产最低的证言 率, 即, 非常慢慢流动气体入分庭, 允许许多时刻为反应剂散开这 个薄酥饼。Mass-transfer limited processes will be more efficient than kinetically controlled ones. The low sticking probabilities of kinetically controlled processes mean that some molecules will hit the wafer, bounce off, and be pumped out. The utilization can be easily calculated if you know the flow and deposition rates.比运动被控制的那些大量转移有限的过程是高效率。运动 控制的过程的低落粘附的可能性意味有些分子将击中这个薄酥饼, bounce, 和用抽机抽。运用可能容易地被演算如果你知道流程和证 言率。Transport mechanisms: Diffusion, convection, thermal diffusion, E Fields, Pe Number. Reactants are transported from the inlet to the wafer primarily by convection and diffusion. Several secondary mechanisms may also be present. In convection, the reactants are transported by the gas velocity. In diffusive transport, the reactants migrate from areas of high concentration to low. The rate of convective transport is given by运输机制: 扩散, 对流, 热扩散, E 领域, 氢化数字。反 应剂被运输从入口对这个薄酥饼主要由对流和扩散。几次要机制也 许并且是存在。在对流, 反应剂由气体速度运输。在diffusive 运 输, 反应剂移居从高浓度区域低落。对流运输的率给Where Y is the mass fraction of species I, p is the density of the gas, and u is the velocity. The diffusive flux is given by那里Y 是种类i 的许多分数, p 是气体的密度, 并且u 是 速度。Diffusive 涨潮给Where D is the diffusion coefficient of species i in the mixture. The relative importance of convection and diffusion is determined by the dimensionless Peclet number那里D 是扩散系数种类i 在这个混合物。对流和扩散的相 对重要性由无维Peclet 数字确定Where d is the characteristic length of the chamber (typically the distance from showerhead to wafer). If Pe 1, it is by convection. For most CVD reactors, Pe is between 1 and 10, indicating that convection is somewhat stronger, but both mechanisms are important. This can have implications for process development. For instance, if a process engineer changes a carrier gas from helium to argon, as a cost-saving measure, there will likely be the unintended consequence of lowering the deposition rate because of the lower diffusivity of argon. Diffusive transport can also lead to nonuniformities when there are purge flows near the wafer surface. The reactants diffuse out of the reaction zone, following the concentration gradient, leading to a thin deposition at the wafer edge.那里d 是分庭(典型地距离的典型长宽从showerhead 到薄 酥饼) 。如果氢化 1, 它是 由对流。为多数CVD 反应器, 氢化是在1 和10 之间, 表明对流有些 比较坚强, 但两机制是重要的。这可能有涵义为处理发展。例如, 如果一位工艺工程师改变运载气体从氦气到氩, 作为一个 cost-saving 措施, 那里可能是不愿意的后果的降下证言率由于氩 比较低的扩散性能。Diffusive 运输可能并且导致 nonuniformities 当有清除流程在薄酥饼表面附近。反应剂散开出 于反应区城, 跟随浓度梯度, 导致稀薄证言于薄酥饼边缘。The Pe number can be used to estimate the gas utilization of a process, at least for the rate-limiting reactant. Figure 14.3 shows the gas utilization versus Pe number for a mass transfer limited reaction in a showerhead reactor like Fig. 14.1. In the typical range of 1 to 10, the efficiency is between 25 and 68 percent.氢化数字可能被使用估计一个过程的气体运用, 至少为率 限制的反应剂。图14.3 显示气体运用对氢化数字为质量传递被限 制的反应在一台showerhead 反应器象图14.1 。在典型的范围的1 到10, 效率是在百分之25 和68 之间。Additional transport mechanisms may also be important. In plasma processes, electron and ion transport are governed by the electric fields in the chamber. The chamber surfaces are typically somewhat negative due to higher electron mobilities. This leads to ion bombardment of those surfaces, including the wafer.另外的运输机制也许并且是重要的。在血浆过程中, 电子 和离子运输由电场治理在分庭。分庭表面典型地有些消极归结于比 较高的电子mobilities 。这导致那些表面离子炮击,包括这个薄酥 饼。Thermal diffusion is a weak transport phenomenon that only becomes important where there is a large difference in the molecular weights of the gases and large temperature gradients. For instance, in CVD tungsten where H2 and WF6 are the reactants there is a difference of 298/2 = 149 in the molecular weight. Under these conditions, thermal diffusion causes the heavier molecule to move toward the colder surface and the lighter one to move to the hotter surface. In tungsten this can reduce the deposition rate by 20 percent. Even for smaller mass differences, the effect can be important. In polysilicon deposition when H2 or He is the carrier gas and silane is the reactant thermal, diffusion has a noticeable effect.热扩散是只变得重要的的一种微弱的运输现象有在气体和 大温度梯度的分子量上的一个大区别的地方。例如, 在CVD H2 和 WF6 是反应剂.C 的钨.C 有区别298/2 = 149 在分子量。在这些情 况下, 热扩散导致比较重的分子行动朝比较冷的表面和比较轻的对 移动热表面。在钨里这可能减少证言率由百分之20 。为比较小的许 多区别, 这个作用可能是重要的。在polysilicon 证言当H2 或他是 运载气体并且硅酮是反应剂上升暖流, 扩散有一个引人注目的作用 。14.2.2 KineticsWhile the laws of mass transfer are universal, the kinetics of a reaction is specific to the process under consideration. Thus the reaction mechanisms must be worked out for each chemistry.当质量传递法律是普遍, 反应的动能学是具体对这个过程 在研究中。因而反应机制必须被制定出为各化学。Surface chemistry. Often there are many reactions occurring on the substrate surface during CVD film growth. Broadly speaking, these can be broken down into three sets of reactions adsorption, reactions, and desorption.表面化化学。经常有许多反应发生在基体表面在CVD 胶片 成长时。宽广地讲话, 这些可能为三套被划分反应.C 吸附, 反应, 和解吸附作用。The first step in the surface chemistry is adsorption of the reacting species onto the substrate. The simplest form of chemisorption is there the adsorbing molecule attaches to an open site on the substrate surface. This is the Langmuir-hinshelwood mechanism. Some molecules decompose during adsorption, requiring multiple open sites for adsorption (Eley-Ridel mechanism).第一步在表面化化学是起反应的种类的吸附这个基体。 chemisorption 的最简单的形式是那里adsorbing 分子附上对一个 开放站点在基体表面。这是Langmuir-hinshelwood 机制。有些分子 分解在吸附时, 要求倍数开放站点为吸附(Eley-Ridel 机制) 。The adsorbed species react with one another, often through multiple pathways, to produce the desired film and by-products. The by-products must desorb off the wafer surface and reenter the gas phase, where they are pumped away.被吸附的种类起反应互相, 经常经由多条路, 生产渴望的 胶片和副产物。副产物必须放出薄酥饼表面和再进入气体阶段, 他 们用抽机抽。The surface chemistry of most CVD reactions is extremely complex and unfortunately theoretical methods are not as advanced as for gas-phase reactions. Nevertheless, computational chemistry has made great strides in recent years and surface reaction pathways are being worked out. For instance, the effect of germane on Si deposition has been analyzed by Hierlemann et. Al. Computers will probably have to advance by another couple of orders of magnitude before surface kinetics can be routinely determined by these methods. Until then, a semiempirical approach is often used. This consists of using chemical intuition to postulate some chemical pathways. One then looks at the thermodynamics of the system and rejects the paths that are not emergetically favorable. Careful experiments can then be used to estimate the rate coefficients for the rest. For CVD tungsten, this has been done to varying levels of detail.多数CVD 反应表面化化学是极端复杂的并且理论方法不幸 地不是一样先进象为gas-phase 反应。然而, 计算化学近年来有了 不起的进步并且表面化反应路正在被制定出。例如, germane 的作 用在硅证言由Hierlemann 分析et 。Al 计算机可能将必须前进由数 量级其它夫妇在表面化动能学可能由这些方法常规确定之前。到那 时, 一种semiempirical 方法经常被使用。这包括使用化工直觉假 设有些化工路。你看这个系统的热力学和然后拒绝不是 emergetically 有利的的道路。仔细实验可能然后被使用估计率系 数为休息。为CVD 钨, 这做过对细节的变化的水平。Gas-phase chemistry. Up until now we have discussed reactions occurring on the wafer surface. However, in many CVD processes, reactions occur in the gas prior to the reactants reaching the wafer. This is particularly true for high-pressure processes (for instance, those operating at 1 atm) and for plasma processes. A classic example of a thermal gas-phase reaction isGas-phase 化学。我们直到现在商谈反应发生在薄酥饼表 面。然而, 在许多CVD 过程中, 反应发生在气体在反应剂之前到达 这个薄酥饼。这是特别真实为高压过程(例如, 这些运行于1 自动付 银机) 并且为血浆过程。热量gas-phase 反应的一个经典例子是In polysilicon deposition. Coltrin, Kee and Miller found that this reaction plays a key role in the deposition of Si on the wafer surface. Deposition comes from both SiH4 directly and from SiH2. As the temperature increases, the gas-phase reaction becomes increasingly important and a larger fraction of the deposition is from SiH2. Interestingly, adding H2 to the mixture suppresses, to some degree, the formation of SiH2, making SiH4 the dominant surface reactant.在polysilicon 证言。Coltrin, Kee 和米勒发现这反应 充当在硅的证言的一个关键角色在薄酥饼表面。证言自SiH2 直接地 来自两SiH4 和。因为这个温度增加, gas-phase 反应变得愈来愈重 要的并且证言的一个比较大的分数是从SiH2 。Interestingly, 增 加H2 来这个混合物压制, 对某一程度, SiH2 的形态, 制造SiH4 统 治表面化反应剂。Thermodynamics versus Kinetics. For a thermal CVD process to move forward, it must be energetically favorable. That is, the energy level of the final state must be lower than the initial state. This is a problem in thermodynamics and is solved by minimizing Gibbs free energy. However, just because a reaction is thermodynamically favorable, it doesnt mean it is useful for CVD. Thermodynamics doesnt consider the rate of reaction. It only says that eventually the reaction will occur. It is kinetics that deals with the rates of reactions. In order to be useful for CVD applications, a reaction must be both thermodynamically and kinetically favorable (i.e. occur at a fast rate). When investigating a new chemistry, it is common to begin with a thermodynamics study to confirm its viability. Middleman and Hochberg provide a detailed example of this type of calculation.热力学对动能学。使一个热量CVD 过程前进, 它必须是精 力充沛地有利的。那是最后的状态的能级必须是低比初始状态。这 是一个问题在热力学方面和由减到最小解决Gibbs. 任意能量。然 而, 正因为反应热力学上是有利的, 它doesn.t 手段它是有用为 CVD. doesn.t 考虑反应的率的Thermodynamics 。它只认为反应 最终发生。这是处理反应的率的动能学。为了是有用为CVD 应用, 反应必须是热力学上和运动有利的(即发生以一快速速度) 。当侦查 新化学, 它是共同开始从热力学研究证实它的生活能力。中间人和 Hochberg 提供这类型一个详细的例子演算。14.2.3 PlasmaPlasma processing is widely used in semiconductor processing, particularly in deposition, etching, and photoresist stripping. In CVD, the energy in the plasma supplements the thermal energy present, allowing a much wider range of chemistries to be used while maintaining moderate wafer temperatures. Plasma can also produce ion bombardment of the wafer, which can be beneficial for improving feature filling. 血浆处理广泛被应用在半导体里处理, 特别在证言, 蚀 刻, 和photoresist 剥离。在CVD, 能量在血浆补充热量能量礼物, 允许化学的范围广被使用当维护适度薄酥饼温度。血浆可能并且生 产薄酥饼的离子炮击, 可能是有利为改进特点装填。E-energy instead of thermal. To prevent excessive diffusion of dopants and other problems, CVD processes generally cannot be run at temperatures over 400 C. Yet, some very useful chemistries dont have practical deposition rates in this temperature range. For instance, thermal deposition of SiO2 from TEOS and N2O requires temperatures of 700 C or higher. So how can one take advantage of the excellent step coverage of this film without damaging the device being fabricated? The answer is plasma processing. In typical plasma, the electrons have temperatures of 10,000 K or higher, while the wafer remains cool.E 能量代替上升暖流。防止dopants 和其它问题过份扩散 , CVD 过程不可能一般跑于温度400 C. Yet, don.t 有实用证言 率在这温度范围的一些非常有用的化学。例如, SiO2 的热量证言从 TEOS 和N2O 要求温度700 C 或比较高。如此怎么你可能利用这张 胶片优秀步覆盖面没有损坏设备被制造? 这个答复是血浆处理。在 典型血浆, 电子有温度10,000 K 或比较高, 虽然这个薄酥饼保留 凉快。Allows lower temperatures. The energy required to break chemical bonds is typically a few electron volts (eV). The average electron energy in plasma is typically in this range, and at the high energy tail of the electron distribution, the energies are much higher (more than 10 eV). These electrons collide with neutral gas molecules and either excite or ionize them. Their tremendous energy allows reactions that would normally require hundreds of degrees to occur at room temperature. In the TEOS case mentioned previously, the addition of plasma allows the process to be operated in the range of 300 to 400C. This process can take place at room temperature, but it is found that the film quality is greatly improved by heating the wafer.准许低温。能量要求打破化学键典型地是几电子伏特 (eV) 。平均电子能量在血浆典型地是在这个范围, 并且于电子分布 的高能尾巴, 能量是比较高(超过10 eV) 。这些电子与中立气体分 子碰撞并且也激发或电离他们。他们的巨大能量允许通常要求上百 程度发生于室温的反应。在TEOS 事例早先提及, 血浆的加法允许这 个过程被管理在范围的300 对400C 。这个过程可能发生于室温, 但它被发现胶片质量经过供热很大改进这个薄酥饼。Ion bombardment. If the wafer platen is negative biased, positive ions will be drawn out of the plasma and will bombard the wafer surface. For low-density plasma, such as typical capacitively coupled systems, ion fluxes are not high enough to provide significant etching. However, they can densify the film and improve the quality of the film. Ion bombardment can also be used to adjust the stress of a film. For instance, plasma-enhanced chemical-vapor-deposited (PECVD) silicon nitride films are typically very tensile. Under ion bombardment, this high stress can be reduced or even make compressive. In high-density plasma, the high ion flux causes sputter etching of the deposited film. The sputter yield is highest at about 45 to the incoming ion direction. Over time, this preferential etching can produces 45 facets at feature corners. When the deposition and etch rates are will balanced this can be used to prevent trenches from pinching off, resulting in improved feature fill.离子炮击。如果薄酥饼台板是消极偏心的, 正面离子将被 画出于血浆和将炮击薄酥饼表面。为低密度血浆, 譬如典型 capacitively 被结合的系统, 离子涨潮不将足够高提供重大蚀刻。 然而, 他们能densify 这张胶片和改进这张胶片的质量。离子炮击 可能并且被使用调整胶片的重音。例如, 血浆改进的化学制品蒸气 被放置的(PECVD) 硅氮化物胶片典型地非常拉伸。在离子炮击之下 , 这高重音可能被减少甚至使压缩。在高密度血浆, 高离子涨潮起 因飞溅这张被放置的胶片的蚀刻。飞溅出产量是最高在45 对接踵而 来的离子方向。随时间,这个优先蚀刻罐头生产45 个小平面于特点 拐角。当证言和铭刻率是将平衡这能被使用防止沟槽捏, 造成改进 的特点积土。Methods of plasma coupling. The plasma energy is brought into the chamber by one of two methods capacitive coupling or inductive coupling. This is shown schematically in Fig. 14.4. In capacitive discharges, two surfaces in the chamber form the two plates of a capacitor. These are typically the showerh