ADVANCES IN SPUTTERING COATING
Magnetron sputtering is a vacuum coating process for depositing thin films on glass. Since their invention in the late 1960s, sputtering electrodes have undergone a developmental revolution. The most significant technological advances are rotating cylindrical magnetrons and advanced rotating cylindrical sputtering targets. These two parallel developments have enabled manufacturers to boost coating throughput and reduce cost, while maintaining layer quality and thickness consistency.
We know that rotatable magnetron sputtering is the most economical and results-driven process available today because of remarkable R&D advancements in technology, process, and engineering. The many shortcomings of planar magnetron sputtering techniques can be overcome by the adoption and implementation of rotating cylindrical technology. There are three significant advantages to adopting the rotating cylindrical magnetron sputtering method, they include: superior material inventory, a higher degree of utilization, and the possibility to triple the power density, resulting in much faster sputter rates or in more complex stacks.
Rotatable sputter targets
As the market interest in vacuum coating by magnetron sputtering grows, target manufacturing is consequently expanding. Thermal spray is the preferred technology to manufacture sputtering targets, because it offers a broad range of capabilities to meet these very complex manufacturing demands. Three parameters directly impact total cost of ownership:
✦ Material composition: Doped materials can be produced in both stoichiometric and nonstoichiometric compositions without the limits of phase diagrams, allowing operators to develop specific coatings that cannot be made via classic target casting technologies. Thermal spraying does not need to take possible restrictions of limited solubility into account with thermal spraying: Any mixture of two materials can be processed by simply mixing the appropriate fractions together before spraying.
✦ Expanded coverage: Nearly all materials can be sprayed, from low-melting point metals to high-melting point ceramics.
✦ Target flexibility: Long-life (dog-bone shaped) targets increase thickness of the material at both ends. As a result, high target material utilization is possible with most materials and for different target lengths (up to 152 inches), and are easily produced.
✦ Film composition: Typical thin films and coating stacks, such as SnO2, TiO2, SiO2, and Si3N4, can be made via advanced cylindrical target tubes.
Here are some special rotary cylindrical targets which are widely used in thin films coating industry:
Silicon aluminum targets
Thin films of SiO2 and Si3N4 are sputtered from Si(Al) targets. The successful production of Si(Al) targets by thermal spray takes advantage of key spray process features. Its inherent flexibility for target geometry allows a wide range of target diameter, length, and straight or dog-bone target ends, while maximizing target sputter capacity by increasing the target layer thickness up to 9 mm. Aluminum dopant levels can range from 0 wt% to 19 wt%, with strict controls over the final chemical composition. By changing from standard 6 mm thick targets to the new 9 mm targets (containing 50% more material), coating cost can be reduced by up to 3%, and uptime can be raised by 5% because of fewer target swaps.
Standard thermal-sprayed tin targets have 90% of the required theoretical density, with an estimated oxygen content of 2000 ppm. However, advances in thermal-spray technology have resulted in a new high-density tin target, reaching more than 98% of the required theoretical density, combined with oxygen content below 250 ppm. This advance combines the benefits of thermal spray technology with high-density structures. Defined in terms of arc rate, burn-in behavior, deposition rate, and current/voltage characteristics, the sputter behavior of the high-density tin target demonstrates superior performance. In addition, advanced thermal spraying allows for precise tuning of the grain morphology, grain orientation, and material density. These flexible adjustments optimize performance to provide specific sputter or coating characteristics, resulting in significant cost savings.
A perfect illustration of how thermal spraying results in a value-added target product is the production of TiOx targets. First, the high process temperatures allow the ceramic titanium oxide to melt. Simultaneously, the titanium oxide undergoes partial reduction with the process gases, transforming it into an electrically conductive phase. At high cooling rates, it remains conductive at room temperature. This material greatly enhances stability during reactive processes, without requiring a feedback loop process control system, yet it still improves sputter deposition speed.
Indium tin oxide
Indium tin oxide is one of the top performing transparent conductive oxides available to the display market. Applications include flat panel displays, such as LCD, PDP, and OLED, in which the indium tin oxide layer serves as a transparent
electrode. Planar ceramic targets consist of one or more tiles bonded to a metallic backing plate. Today, reactive DC magnetron sputter deposition from a planar ceramic target is the most widely deployed technique for deposition of indium-tinoxide (ITO) coatings on glass and plastic substrates. In spite of their popularity, planar targets have several intrinsic restrictions because of their planar structure.
Rotating cylindrical ITO targets resolve many of the limitations of planar ceramic ITO targets. Some of its inherent advantages include:
✦ Larger useful target inventory and increased target material utilization, both of which lead to reduced machine downtime.
✦ Increased process stability for reactive deposition.
✦ Improved target cooling, which increases power density and raises the deposition rate.
✦ Preliminary field tests have shown that total cost of ownership can be reduced by more than 40% per square meter while doubling the utilization of targets.