thermal chemical vapor deposition
The three main chemical vapor deposition (CVD) techniques are thermal CVD, plasma enhanced CVD (PECVD) and laser CVD (LCVD). They all require volatile precursors whose chemical composition changes during the deposition process. In thermal CVD, precursors form deposits when they come into contact with a hot surface. In PECVD, the vapors of the precursors are decomposed by contact with the plasma. This may have occurred in the gas phase, or the precursors were adsorbed on the substrate and subsequently decomposed by altered particle or photon bombardment. In LCVD, the precursors decompose either photochemically or by pyrolysis when they come into contact with a laser-heated surface. Laser CVD opens up new opportunities, including localized deposition and customization of reaction paths.
CVD technology opens up the possibility of preparing new materials and structures for various applications. The scope of this part of the chapter is to consider plasma activated CVD (PECVD) by physical influences. For thermally activated CVD, the reader is referred to the literature.
The following precursors and overall reactions are used for CVD of TiC, TiN and Al2O3:
For CVD, a fully dense coating, heterogeneous reaction on the substrate surface is required. In addition, many processes require homogeneous reactions in vapor. In these reactions, species produce adsorbables; thus, these reactions prepare gas species for heterogeneous deposition reactions. For the CVD of the above compounds, a heterogeneous reaction is required. In CVD for TiC, TiN and Al2O3, it is important to generate CHx, NH3 and H2O in the vapor, respectively. This means that experimental setups that favor homogeneous reactions should be used. In this case a hot wall CVD reactor is preferred. In cold wall reactors, these reactions are inhibited and the deposition rate will be greatly reduced.
In conventional high-temperature CVD processes widely used today, single and multiple layers of TiC, TiN, HfN and Al2O3 are deposited onto the tool at temperatures between 900 and 1100°C. The coating deposition rate depends on the temperature and partial pressure of the gas in the reactor. Figure 32.10 shows a schematic diagram of such a reactor.
Higher temperatures favor higher coating rates, but also result in coatings with coarse grains and lower hardness. In addition, higher coating temperatures promote the tendency to decarburize and embrittle the carbide matrix through the formation of η phase. A solution to the toughness problem was found by applying TiN and HfN coatings and later TiC-Ti(CxNy)-TiN multilayer coatings without the formation of η phase at the coating-substrate interface.
Figure 32.10 CVD coating system
Moderate temperature CVD can be accomplished by using organic precursors. The medium temperature process allows the deposition of Ti(CxNy) coatings in the temperature range of 700 to 900°C. Organic precursors are sources of carbon and nitrogen, and are more efficient than CH4 and N2 used in high-temperature CVD.36 The reaction of medium-temperature CVD is
By combining this new coating technology with special substrate materials, the development of new grades for milling – dry and coolant – represents a technological breakthrough in this field.
The excellent performance observed is mainly due to the reduced thermal load on the material on the substrate, the absence of brittle phases, and the good bonding of the coating to the substrate.
Plasma Enhanced Chemical Vapor Deposition (PECVD)
CVD and PECVD can use the same precursors but different deposition mechanisms. CVD uses high temperatures, therefore, the thermodynamic process controls the nature of the deposits produced. The same precursors react in PECVD at temperatures hundreds of degrees Celsius lower, and the deposition process is governed by kinetics. Lower deposition temperatures may be an advantage for sensitive substrate materials, or may lead to the formation of metastable phases of the deposit. For example, yttria-stabilized zirconia (YSZ) can be prepared by PECVD using tetramethylheptadione complexes Zr(thd)4 and Y(thd)3 as precursors. According to the phase diagram, the cubic phase of YSZ passes through 10 to 40 mol% Y2O3 at 1000°C and through 15 to 30 mol% Y2O3 at 500°C. PECVD38 deposits formed at 500°C formed cubic Y2O3 in the range of 3.5 to 80 mol% and maintained this structure after annealing at 900°C for several hours. The extended range of compositions offers the possibility to tune the lattice constant over a wider range than YSZ is possible with other techniques.
In addition to microelectronics, a very strong drive to develop new systems comes from the field of hard and protective coatings. Many materials formed by this technique are in excellent condition. Continuous development of new precursors, coatings and deposition techniques. It is impossible to cover all recent results. Some possibilities and uses of plasmonic organometallic compounds39 will be demonstrated with selected examples. Current knowledge about the true nature of the chemical reactions involved in plasma CVD processes is insufficient. Important The mechanisms involved will be discussed later.
Deposit all materials required for the coating (eg TiN) as gases (eg TiCl4, H2, N2, Ar) in the same way as conventional CVD. Creation of depositable species is achieved by decomposing process gases in a glow discharge. Because the plasma volume response is necessary to generate deposited species, the process steps cannot be clearly separated. The free radicals and excited species produced by this are mostly polyatomic particles. Their kinetic energy corresponds to the temperature of the process gas. In many cases, the creation of these species occurs at the first contact of the process gas with the plasma. Therefore, the initial spatial distribution of radicals will be determined by the gas inlet and distribution system as well as the geometry. Plasma zone.
In the commonly used pressure range (10 to 103 Pa) and mean residence time (0.1 to 1 s), gas flow can be described as slow, viscous and laminar. The mean free path of the process gas species is only a fraction of a millimeter, much smaller than the flow channel dimensions. Typical diffusion times are a few milliseconds. Therefore, the migration of radicals to the substrate is dominated by diffusion and airflow.
Plasma volume responses are complex because of the large number of different species and possible response pathways. An important process is electron dissociation of polyatomic carrier gases affecting dissociation. Energetic electrons also generate some free radicals and ions, which are able to decompose neutral carrier gases and polyatomic free radicals through radical-molecule and ion-molecule reactions. The decomposition efficiency of process gases is usually very high. Typically, 10% to 100% of the carrier gas fed to the reactor can be decomposed.
Coating formation occurs on substrate and film surfaces by absorbing free radicals, by chemical bonding to neighboring atoms on the surface, and by desorbing volatile compounds. Temperature and the bombardment of the coating by photons, electrons and ions affect film growth. Ions in particular can gain significant energy in the cathodic descent. This leads to higher mobility of surface atoms and sputtering of weakly bonded atoms.
For the technical realization of the plasma CVD process, two parts of the deposition system are of great importance, namely the glow discharge configuration and the gas inlet and distribution system. As a power supply, direct current, pulsed direct current, radio frequency or microwave can be used. For depositing hard coatings especially TiN planar electrode systems powered by DC, pulsed DC or RF are often used. The standard equipment for PECVD is a parallel reactor (Fig. 32.11) with two 10 to 60 cm diameters spaced a few centimeters apart. In commercial equipment for anisotropic etching, the electrode carrying the substrate is smaller (to create a self-bias), while for deposition both electrodes have the same diameter. In deposition experiments, asymmetric arrangements are sometimes used. Temperature control of the electrodes carrying the substrate is important. For simplicity of construction, this electrode is usually grounded. More general devices have heated electrode insulation to take advantage of this bias. Apart from parallel plate reactors, occasionally used arrangements separate the plasma from the substrate.
The equipment around the reactor mainly depends on the vapor pressure of the precursor. If this is high enough, distillation or sublimation can simply be performed from a constant temperature reservoir (Fig. 32.11). If the precursor needs to be heated to achieve the desired vapor pressure, all tubing connections need to be heated to avoid condensation. For substances that are difficult to evaporate, the connecting pipe between the vaporizer and the reactor should be as short as possible.
Films coated using CVD techniques show excellent step coverage and adhesion to the base metal. In general, their disadvantage is that higher process temperatures are required to form ceramic thin films. In contrast, PVD technology enables films with good adhesion at low temperatures, although its step- coverage capability prevents even coating of ceramics on complexly shaped base metals. A significant expansion of the application of plasma CVD technology can be expected if PECVD technology can be used at temperatures as low as those used by CVD technology and if it produces high adhesive films.
Figure 32.11 Parallel plate reactor
To obtain highly viscous coatings at low temperatures, the triode method40 was developed using grid electrodes (Fig. 32.12). A grid electrode of 20 mesh stainless steel was placed between the anode and cathode , and radio frequency power was applied thereto. The anode electrode is the gas feeder and grounded. The cathode electrode is the substrate holder, and a DC bias voltage is applied to the substrate and the cathode.
Of particular importance for uniform deposition and high deposition rates are the precursors. Most compounds are transported by a carrier gas flow through it. In a flow system, the partial pressure of the precursor in the gas stream is well below the equilibrium pressure. Actual values may be as low as 1/1000 of the equilibrium pressure, depending on the size of the crystallites and the level of material in the reservoir.
Figure 32.12 Triode Plasma CVD System
使用液体前体更容易实现稳定蒸发。大体积和不对称取代基降低熔点。有时引入单个甲基会降低熔点充分地。例如，(C5H5)2Zr(CH3)2 是固体，而 (CH3–C5H4)2Zr(CH3)2 在室温下是一种液体，因此更容易涂抹。在所有参数中，有机金属的分压化合物是最难控制的。在大多数实验中，它的汽化速率是确定的通过实验过程中水库的重量损失。然后估计分压其他气体的流量和压力数据。这个程序是不确定的，因为有些物质在蒸发器的温度下分解。在这种情况下，需要不断调整其温度。为了实现这一点，可以通过质谱或光学监测气流可以使用来自放电的排放物。
为了在 PECVD 中实现合理的沉积速率，前驱体的蒸气压应为在室温下至少 10 Pa，或者它应该能承受加热而不分解，直到达到蒸气压。某些元素形成满足此条件的卤化物或杂化物（例如，WF6 或SiH4)，但对硬质涂层和其他应用感兴趣的大多数元素没有形成挥发性无机化合物。然而，所有元素的碳化合物都是已知的，其中，有些非常不稳定。
关于有机金属的挥发性或热稳定性或光稳定性知之甚少，但最近，一些关于结构-波动率关系的概念已经形成。改善波动性，趋势需要减少要结合的分子的数量。这可以通过引入庞大的基团来实现，通过使用不对称取代基，并通过引入氟原子而不是氢。例如，在β-二酮酸盐系列，挥发性从 acac 到 fod 增加（表 32.3）。文献综述了有机聚合物在 PECVD 中的应用。
四亚甲基广泛用于形成各种金属（铝、镓、铟、硅、锗、锡和铅）。 Ar-H2 中的 PECVD 导致锡膜；与 Ar–O2，形成 SnO2。
对于锗和铟，形成了类似的结果。铁、钴、镍、铬、钼、钨和锰的羰基化合物已用于 PECVD。34 Ni(CO)4 和 Co2(CO)8 在热 CVD 中产生纯金属薄膜；在 PECVD 中，沉积物被碳和氧污染。只有仔细调整参数和使用H2作为载气可以制作金属薄膜。铬的羰基化合物，钼和钨产生的薄膜含有不同量的氧和碳。例如，在氩等离子体中由 Mo(CO)6 制成的薄膜具有 MoC0.1O2.5 在 H2–Ar 中的成分 MoC0.3O0.3。这原因是 CO2 分解为 CO 和碳；后者被纳入成长的电影。
π-配合物对于 CVD 应用具有足够的挥发性。与烷基配体的配合物可能是相当不稳定。特别是通过PECVD将(π-C3H7)Pd(π-C5H5)和(π-C3H7)2Pd转化为钯膜。
对氧化物薄膜的兴趣刺激了β-二酮酸盐的发展。乙酰丙酮化物不是非常易挥发，但在某些情况下使用（铜和铝）。 hfa 和 htd 配合物因其较高的蒸气压而得到更广泛的应用。因此，Cu (acac)2 需要一个温度140°C，Cu (thd)2 –110°C，而 Cu (hfa)2 只需 40°C 即可达到足够高的蒸气压。 45二酮体中的氧气限制了它们的使用。仅晚期过渡金属的螯合物（例如铜或钯）可以转化为金属膜：所有其他的都倾向于形成氧化物。当使用含氟配体时，沉积物可能是氟化物（铁和镍）。钇的二酮酸盐，为了制备超导氧化物薄膜，已经研究了钡、铜和稀土46。
有许多出版物专门分析其薄膜和涂层的特性通过 PECVD。广泛用作硬涂层的 TiN 在许多出版物中进行了研究。通常是由 TiCl4 CVD 制备。由于使用卤化物作为前体所涉及的问题，CVD 工作正在寻找替代方案。表 32.4 显示了许多其他材料，它们可能同样与 TiN 一样具有吸引力。钛、锆、铪、钒、铌和钨、锌铪的氮化物和几种硼化物、立方BN、SiC、Al2O3和金刚石显示非常有趣的属性。
表32.4各种化合物的维氏硬度
使用 PECVD 获得金属氧化物、氮化物、硼化物和碳化物薄膜。铜、银、近年来制备了钯、金、铂、铑及其合金薄膜。 47,48 大多数这些元素的前体在氩气或 Ar-H2 等离子体中分解时会产生闪亮的金属薄膜，但它们通常包括碳污染。为了沉积纯金属，有必要去除所有有机配体。为实现这一点，沉积速率不应太高，基板温度不应太高和偏差需要适当调整。铌、钼、钨、铁、钴、镍、锌、铟和锡已在 H2 和 H2-Ar 等离子体中处理。他们的存款显示出可观的碳和氧的污染。
对于硬涂层，金属薄膜不能直接使用，但它们的柔软性使金属可用作中间层。如果基材和涂层的热膨胀系数不匹配，温度变化可能会导致裂纹或本体与涂层分离。中间层软镍等金属可以大大提高此类系统中的附着力。特定条件下的 W(CO)6产生含有百分之几碳化物的钨薄膜。众所周知，钨的硬度增加从4到8Mohs，晶格中有一些碳。
氧化膜很容易通过 PECVD 制备，因为配体可以通过氧化完全去除。几乎所有的挥发性有机金属都可以用来制备氧化膜。 这些过程进行在 O2 或 Ar-O2 混合物中。 一些含氧前体如 Cr(CO)6 和 Ti(OR)4 形成氧化物直接地。 大多数关于氧化物的研究都针对高 T 超导体（钡、锶、钇、和铜）、半导体（锡和铟）和光纤（硅、硼和锗）。
表 32.5 TiNa 的有机金属前体
作为硬涂层，Al2O3 和 ZrO2 可能很重要，它们的硬度分别为 9.5 和 7 到 9，在莫氏量表上。 Al2O3 薄膜可以由几种前体制备。 烷基化合物AlR3对氧气和水非常敏感，即使以氢气为载气也会形成氧化物，因为小泄漏为它们的反应提供了足够的氧气。 其他铝化合物更容易控制。Al(acac)3 和 Al(O–C3H7)3 可以在 170°C 时蒸发。
为了制备 ZrO2 薄膜，已经测试了几种前体。49 Cp2Zr(CH3)2 在 80°C 下蒸发，然后在 300°C 或更高的衬底温度下产生化学计量的 ZrO2 薄膜。 Zr(OCH(CH3)2)4，与纯氩作为载气，在 160°C 和衬底温度下使用蒸发器形成 ZrO2 薄膜300°C。在二酮酸盐中，Zr(hfa)4 可在 60°C 蒸发并在 25 至 150°C 沉积，但沉积物似乎是一种氟氧化物而不是氧化物。当 Zr(thd)4 用作起始材料时，只有在高温 (400°C) 和高功率密度 (5 W cm–2) 下才能获得纯 ZrO 薄膜。
TiN和其他氮化物已被广泛研究。 TiN 在 800 至 1000°C 温度下的 CVD需要对基材进行后处理。通过 PECVD，已使用 TiCl4 在较低温度下沉积 TiN。已经测试了几种有机金属钛化合物作为前体TiN 薄膜的沉积（表 32.5）。
Films deposited from compounds 1 and 2 initiated in Table 32.5 contained about 60 wt% Ti, 15 to 25 wt% C, and only 5 wt% N. Compound 3 produced films with 8 to 21 wt% C and 4 to 9 wt% N. 4 resulted in a film with 70 wt% Ti and 20 wt% C, but they partially decomposed during the distillation. Compounds In Figure 5, H2 was used as the carrier gas to deposit thin films with a Ti content of 55 wt% and a C content of 6 to 9 wt%. All five precursors are possible precursors to CVD. The possibility of borides as hard coatings has been discussed in several reviews. In most cases, such films are prepared by halide sputtering or thermal CVD. And these techniques use temperatures around 1000°C, TiB2 + PECVD from TiCl4 + BCl3 to H2 at 480 to 650°C. 52 Recently, more and more attention has been paid to the formation of cubic boron nitride. Coatings of this compound have been prepared by PECVD starting from B2H6, B10H14 and BHAl3.
Carbides are also promising as hard coatings. A better chance of forming carbides is with compounds containing only metal, carbon and hydrogen. The neogenyl derivatives of titanium, zirconium and hafnium that form carbides by CVD form carbides by PECVD.
