Sputtering sources include DC diode sputtering, tripolar sputtering, radio frequency sputtering, magnetron sputtering, beam sputtering, and reactive sputtering.
DC Diode Sputtering
The simplest and oldest source of sputter deposition is the DC diode. The two electrodes are usually parallel to each other, spaced 4 to 8 cm apart, and the substrate is placed on the anode, as shown in Figure 31.2. The applied potential is typically 1000 to 3000 Vdc and the argon pressure is about 0.075 to 0.12 Torr. The DC diode configuration has important disadvantages, including low deposition rates (~400 ô/min of metal), high operating pressure, limited electrical conductors on target, bombardment by plasma electrons and the substrate, resulting in substrate heating. The cathode system discussed next can be used to improve the performance of DC diodes.
Triple sputtering
A heated filament (Fig. 31.4) is discharged as a second source of electrons; an external magnet can also be used to increase the probability of isolating and confining the electrons. ADCs produce multiple triodes with high deposition rates, reaching several thousand Angstroms per minute, at lower pressures (0.5~ 1 × 10 –3 torr) and voltages (50~100 V). The use of triode vacuum tubes has been limited due to the difficulty in expanding the size of the cathode and corrosion of the firing gun filament.
RF sputtering
Non-conductive materials cannot be sputtered directly by applying a DC voltage, and positive charges accumulate on the target surface. If an AC potential of sufficiently high frequency is applied, an effective negative bias is created such that the number of positive electrons reaching the target is equal to the number reaching negative ions. Since the mass of the electrons is very small relative to the current ion, the target is only available for a short time, and the deposition rate of the RF diode is almost equivalent to that of the DC diode. The resulting negative bias allows sputtering of insulating targets. The frequency used in most practical applications is usually 13.56 MHz, a radio frequency band allocated by the Federal Communications Corps for industrial purposes. RF sputtering enables the deposition of insulators as well as conductors and semiconductors in the same equipment and also allows for lower sputtering pressures (5 to 15 × 10–3 torr). A major disadvantage of RF sputtering is the need for electromagnetic shielding to block RF radiation. In addition, power supplies, matching networks and other necessary components to realize a resonant RF network are very complex.
Magnetron sputtering
The magnetron cathode is essentially a magnetically enhanced diode. The magnetic field is used to form electron traps, which combine at the cathode surface, making B × E (electric field strength × magnetic flux density) electron drift current closed-loop path on the target surface. This "race track" effectively increases the number of ionizing collisions for electrons in the plasma. Magnetic confinement near the target results in achieving higher current densities at lower pressures (10 3 to 10 –2 torr), almost independent of voltage. This mode of cathode operation has been described as magnetron mode, and enables higher deposition rates (10 times that of DC diodes) and less electron bombardment of the substrate, thus less heating. Factors affecting the deposition rate are power density on the target, erosion area, distance from the substrate, target material, sputtering rate, and gas pressure. DC is usually used for magnetron sputtering, but RF can be used for insulators or semiconductors. When sputtering magnetic materials, a thinner target is usually required to maintain sufficient magnetic field strength. Three common magnetron cathode designs are described below, as shown in Figure 31.5.
1. Planar magnetron sputtering
A series of permanent magnets are placed behind flat, circular or rectangular targets. The magnets are arranged such that the region where the magnetic field lines are parallel to the target surface forms a closed loop on the surface. Around this ring, magnetic field lines enter the target, perpendicular to its surface. This produces an elongated electron raceway and an eroded pattern on the target surface. Due to the inhomogeneity of target erosion, the utilization rate of target material is very low, generally 26-45%. This also leads to uneven deposition on stationary targets. Uniformity is provided by substrate motion, usually linear or planetary, combined with uniform aperture shielding. Planar magnetron sputtering cathodes typically operate at 300 to 700 V to provide a power density of 60mA/cm2 or 4 at a current density of 36 W/cm2.
The deposition rate is generally proportional to the energy delivered to the target. Much of this energy is dissipated to heat the target. The main factor limiting the rate of magnetron deposition is the amount of power that can be applied to the target without melting, cracking or warping it. This is controlled by the cathode water cooling design and the thermal conductivity of the target, the backplate and the interface between them. Planar magnetron cathodes have been scaled up to several meters in length in production applications and are an important industrial coating tool.
2. Cylindrical magnetron
在一个圆柱形阴极设计的变化可以用于涂层大面积：圆柱磁控溅射后，向外从中央的目标后，和空心圆柱或倒置磁控管，这对一个圆柱靶内壁目标侵蚀。操作参数与平面磁控管相似。E×B电流关闭本身的绕柱或圆柱。静电或磁力控制通常用来减少终端损耗。沿柱或汽缸内部的侵蚀是均匀的。这使得没有基体运动的涂层相当均匀。空心阴极特别适用于复杂形状物体的涂敷。另一个圆柱形阴极，可旋转磁控管，使用类似于平面磁控管的磁体阵列，旋转目标或磁铁以获得均匀的侵蚀。
3、环形或磁控管磁控管
所述环形或枪磁控管源包括圆形阴极和同心中心阳极。与其他的磁控管，高沉积速率小的基板加热是可能的。由于圆形设计，行星基板运动对于沉积均匀性是必要的。这种设计广泛用于小规模的应用，但没有扩大到更大的尺寸。这些阴极阵列被用来覆盖大面积。
束溅射
一个单独的离子束源（图31.6），而不是辉光放电，可用于侵蚀目标的表面。离子束的能量、方向和电流密度可以独立地控制，并且可以在低于其他溅射沉积方法的背景压力下工作。使用离子束沉积有时可以获得独特的薄膜特性，它一般限于较小面积的覆盖率和较低的沉积速率。
反应溅射
在溅射沉积过程中，氩通常被用作工作气体。它是相对惰性的，被嵌入在生长的薄膜中，只有当它被困在或嵌入在它的表面时。其他更多的反应性气体如水蒸气、氧气和氮气通常在沉积室低水平的污染物，已脱气与衬底，目标，及室壁。这些气体可以通过冷凝原子在衬底表面反应纳入生长的薄膜，形成少量的氧化物，碳化物，和亚硝酸盐，溅射材料的其他类似的化合物。
在反应溅射中，气体有意引入沉积室，与形成的薄膜完全反应。一个气体歧管系统（图31.7）经常被用来提供反应气体在基底上的均匀分布，并在目标表面最小化反应气体。
Reactive sputtering is a very nonlinear process. The film grown on the substrate acts as a getter pump for the reactant gas up to the pressure of the stoichiometric compound. At this point, the pumping rate of the substrate drops drastically and the reaction gas pressure increases in the chamber. This gas may react with the target surface, resulting in a reduced sputtering rate, due to the reduced sputtering rate of the compound and other factors. Therefore, most reactive deposition processes attempt to work near the transition region of the target, where stoichiometric compounds are formed on the substrate and the cathode target is a metal. Thus, reactive sputtering can form compounds from simple metal targets in DC or rf mode. This process is widely used for the deposition of oxides and nitrites such as silicon oxide, silicon nitrite, titanium nitride, and indium tin oxide.
