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 V dc and the argon pressure is approximately 0.075 to 0.12 Torr. The DC diode configuration has important disadvantages, including low deposition rates (approximately 400 º/min for metals), high working gas pressures, targets limited to electrical conductors, and plasma electron bombardment of the substrate, causing heating of the substrate. The cathode systems discussed next can be used to improve the performance of DC diodes.
Figure 31.4 Schematic diagram of the triode sputtering process. (courtesy of WH Brady Co.)
Triode sputtering
A heated filament (Fig. 31.4) is used as an auxiliary electron source for the discharge; an external magnet can also be used to confine the electrons and increase the probability of isolation. Triodes can be produced at lower pressures (0.5 to 1 × 10–3 torr) and voltages (50 to 100 V). The usefulness of triodes is limited by difficulty Graduation up to larger cathode sizes and erosion of the emitter filament by chamber gases.
RF sputtering
Non-conductive materials cannot be sputtered directly with an applied DC voltage due to charge buildup 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 electrons reaching the target while it is positive is equal to the number of ions arriving when it is negative. Because the mass electrons are very small relative to the ions present, the target is only positive for a short time, and the deposition rate of RF diodes is nearly equivalent to that of DC diodes. This resulting negative deviation 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 Commission for industrial use. RF sputtering allows the deposition of insulators as well as conductors and semiconductors . The same equipment also allows sputtering at lower pressures (5 to 15 × 10-3 Torr). A disadvantage of professional RF sputtering is the need for electromagnetic shielding to block RF radiation. Additionally, the power supplies, matching networks, and other components required to implement a resonant RF network are complex.
Magnetron sputtering
The magnetron cathode is essentially a magnetically enhanced diode. The magnetic field is used to form an electron trap that, together with the cathode surface, confines the E × B (electric field strength × magnetic flux density) electron drift current to a closed-loop path on the target surface. This "race track" effectively increases the number of ionizing collisions per electron in the plasma. The confinement near the magnetic target leads to higher current densities (10-3 to 10-2 torr) at lower pressures, almost independent of voltage. This cathode mode of operation is described as magnetron mode and enables higher deposition rates (10 times that of DC diodes) to bombard the substrate with fewer electrons and therefore less heating. Factors affecting the deposition rate are power density on the target, etch area, distance to the substrate, target material, sputtering throughput, and gas pressure. DC is usually used for magnetron sputtering, but RF can be used for insulators or semiconductors. When sputtering magnetic materials, thinner targets are generally required to maintain sufficient magnetic field strength above the target surface. Three common magnetron cathode designs are described below and shown in Figure 31.5.
Figure 31.5 Clockwise from top left: planar magnetron, gun magnetron, and cylindrical rear magnetron sputtering source.
1. Planar magnetron
一组永磁体放置在平面、圆形或矩形目标后面。 磁铁是布置成使得磁场线平行于目标表面的区域形成闭合在表面上循环。 围绕这个环，磁场线通常进入目标，垂直于其表面。 这会在目标上产生细长的电子跑道和侵蚀图案表面。 由于靶材侵蚀的不均匀性，靶材的利用率很低，通常26% 到 45%。 这也导致在固定目标上的不均匀沉积。 均匀性由基板运动，通常是线性或行星运动，结合均匀孔径屏蔽。 平面磁控管阴极通常在 300 至 700 V 下工作，提供 4 至 60mA/cm2 的电流密度或 1 至 36 W/cm2 的功率密度。
沉积速率通常与传递到目标的功率成正比。 这种力量的很大一部分被耗散为目标加热。 限制磁控管沉积速率的主要因素是可以施加在目标上而不会使其熔化、破裂或变形的能量。 这是由阴极水冷设计，以及靶材、背板和靶材的导热系数它们之间的接口。 平面磁控管阴极已在生产应用中扩大规模长达数米，是一种重要的工业涂装工具。
2、圆柱形磁控管
Two variants of the cylindrical cathode design are available for coating large surface areas: a cylindrical- column magnetron, which targets outward sputtering from a central column, and a cylindrical hollow or inverted magnetron, which is on the inner wall of a cylindrical target There is target corrosion. The operating parameters are similar to planar magnetrons. The E × B current closes itself by going around the post or cylinder. Typically electrostatic or magnetic containment is used to minimize tip losses. Erosion is uniform inside the column or cylinder. This allows for a fairly uniform coating without substrate movement. Hollow cathodes are particularly effective in coating complex shaped objects. Another cylindrical cathode, a rotatable magnetron, uses a magnet array similar to a planar magnetron and rotates the target or magnet to obtain uniform corrosion.
3. Ring or gun magnetron
A ring or gun magnetron source consists of a circular cathode and a centrally located concentric anode. As with other magnetrons, very little substrate heating is required to achieve high deposition rates. Due to the circular design, planetary substrate motion is necessary for deposition uniformity. This design is widely used in small-scale applications, but has not been scaled up to larger sizes. These array cathodes have been used for large area coatings.
beam sputtering
In contrast to glow discharge, a separate ion beam source (Fig. 31.6) can be used to etch the surface of the target. A method that can control the ion beam energy, direction, and current density independently, and can work at lower background pressures than other sputter depositions . Unique thin-film properties can sometimes be obtained using ion beam deposition, which is usually limited to covering rather small areas and low deposition rates.
Figure 31.6 Schematic of an ion beam sputtering source showing the relative positions of the target and substrate.
Figure 31.7 Schematic diagram of reactive sputtering equipment.
reactive sputtering
Argon is commonly used as the working gas in sputter deposition processes. It is relatively inert and is only incorporated in the growing film when trapped or embedded in its surface. Other more reactive gases such as water vapor, oxygen, and nitrogen are often present in the deposition chamber as horizontal contaminants that have been exhausted from the substrate, target, and chamber walls. These gases can be incorporated into the surface of the growing film by reacting with condensed atoms on the substrate, forming small amounts of oxides, nitrites, carbides, and other similar compounds of the sputtered material.
In reactive sputtering, gases are deliberately introduced into the deposition chamber to fully react with the formed film. A gas manifold system (Figure 31.7) is typically used to provide uniform distribution of reactant gases over the substrate and minimize the amount of reactant gases on the target surface.
Reactive sputtering is a very nonlinear process. The growing film on the substrate acts like a getter to pump the reactant gas to the pressure at which the stoichiometric compound is formed. At this point, the pumping rate of the substrate is significantly reduced and the reaction gas pressure is increased in the chamber. This gas may react with the target surface, resulting in a lower deposition rate due to lower sputter yield 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 metallic. Reactive sputtering thus allows the formation of compounds from simple metallic targets in DC mode or RF mode. This process is widely used to deposit oxides and nitrites such as silicon oxide, silicon nitrite, titanium nitride and indium tin oxide.
