Sputtering Target Innovations
New applications in the photovoltaic, thermoelectric, storage, and semiconductor markets are spurring innovation in ceramic and semiconductor sputtering targets.
Magnetron sputtering was initially developed using metal or alloy targets with materials having high electrical conductivity (e.g., Al, Ag, Au, Cu, Ti, Mo, etc.). In order
to achieve acceptable deposition rates, the target material needed to be electrically and thermally conductive. Ceramic targets were developed for transparent conductive oxides (TCOs) and usually consisted of films made from compositions of ZnO:Al2O3 (2% wt) or In2O3:SnO2 (10% wt). However, as the name implies, the materials were fairly conductive and were suited for DC magnetron sputtering.
Pulsed-DC and RF magnetron sputtering allows for the deposition of materials with poor electrical conductivity. Semiconductor materials with better electrical conductivity can be sputtered with pulsed-DC power supplies, while insulating materials (mainly ceramics) require RF sputtering. The deposition rates for RF sputtering are generally much lower than with pulsed-DC. Also, pulsed-DC sputtering has a lower deposition rate than DC sputtering. New applications in photovoltaic, thermoelectric, storage, and semiconductor markets are spurring innovation in ceramic and semiconductor sputtering targets.
DC sputtering with metallic targets has fewer process problems since the metals are ductile and the materials feature high conductivity. Conversely, semiconductor and ceramic targets are more prone to process difficulties due to the brittle nature of the materials and the poor electrical and thermal conductivities. In order to achieve consistent sputtering over the life of the target, it is essential to have a well-sintered target material with high density. Voids and cracks in the material can propagate and lead to sputtering problems such as arcing, target cracking, and particle generation.
Metallic targets are typically made from cast ingot. The metal or alloy is melted and solidified to ensure full density with a minimum amount of voids. The casting process can be performed in a vacuum or inert atmosphere to reduce the presence of oxides. The majority of metals can form oxides that have much higher electrical resistance. The presence of these oxides can create non-uniform sputtering since there is a significant difference between the electrical properties of the metal and oxide. Metallic targets can be shaped to the final target dimensions through thermomechanical processing (e.g., forging, rolling and heat treatment).
Ceramic and semiconductor targets are typically made by sintering powders due to the difficult nature to cast these materials into the necessary target dimensions. Oxides, sulfides, selenides, and tellurides are brittle and feature poor electrical conductivity. These materials can be crushed into powders and then sintered into high-density targets. While oxides such as Al2O3 and ZnO have been studied for many years, new applications have sparked the development for various sulfides, selenides and tellurides. A small sampling of materials is listed in Table 1.
These compounds are typically reacted using high-purity starting materials. Most reactions between metals and O, S, Se, and Te are violent and exothermic. In addition, compound formation is generally done in vacuum to reduce the presence of oxides for non-oxide compounds. These reactions can be dangerous since S, Se and Te vapors can form toxic gases. The safety of the production staff comes first, with the secondary goal being to maintain the compound composition as closely to the ordered ratio and desired purity level as possible. Typical purity levels of 99.95-99.999% purity can be achieved using this process.
The materials are sized down into fine powders with a key emphasis in avoiding contamination. The powders are then loaded into a graphite mold and introduced into a vacuum hot press. The vacuum hot press subjects the powders to high temperature and high pressure while in a vacuum environment (see Figure 1). The vacuum ensures a limited opportunity for oxidation while the temperature/pressure allows the powders to sinter into a high-density press. Due to the stresses induced during pressing, careful monitoring of the thermal profile is necessary to reduce the likelihood of cracking in the target material.
The powder is sintered into a near-net shape and then ground/machined into the desired dimensions. The target must then be attached to a copper or molybdenum backing plate so it can be installed in the sputtering system. Metallic targets such as aluminum alloys, titanium or copper could be machined into the appropriate shape for direct install into the sputtering system. However, ceramic and semiconductor targets require bonding to a copper backing plate since the mounting screws and fixtures could easily create cracks in the brittle target material. Since the materials are not thermally and electrically conductive, care must be taken in the bonding process. Figure 2 shows the interface of a Cu(In,Ga)Se2 target bonded using indium solder to a copper backing plate.
The key to achieving high density during the hot press operation is to press the material at high temperatures. However, oxides, sulfides, selenides and tellurides
The key to achieving high density during the hot press operation is to press the material at high temperatures.
can out-gas at elevated temperatures due to the volatile nature of O, S, Se, and Te. Each compound has an optimal temperature for sintering, where high density can be achieved without triggering the evaporation of the material. This optimal temperature is lowered also due to the fact that pressing occurs in a vacuum.
One key aspect to lowering the propensity of the material to out-gas is to work with a stable compound. Stable compounds can be determined in binary, ternary, quaternary and five-element systems, depending on the line compounds present in phase diagrams.
Figures 3 and 4 shows the difference in the materials sintering property between the cross-section of a Sb2Te3 target (used for thermoelectric application) with a higher temperature sintering process and a lower temperature sintering process. At higher temperatures, the Te begins to out-gas, creating micro-cracks in the material. While density measurements show that the two materials have equivalent densities, the cross-section clearly shows problems with micro-cracks in the target materials that can create issues during sputtering.
Physical vapor deposition can stress the target surface immensely. Any differences in electrical and thermal characteristics in the target material can be exacerbated by poor thermal conductivity for many ceramic and semiconductor targets. For this reason, many targets exhibit poor performance at thicknesses much greater than 6 mm. Metallic targets can be sputtered at thicknesses greater than 20 mm with no issues due to the metals’ excellent thermal conductivity.
The poor thermal conductivity can affect how much power can be applied during sputtering. While some metallic targets can be sputtered at high powers of 10-30 kW, many semiconductor targets sputtered by pulsed-DC can only be sputtered at < 5 kW. Insulating ceramic targets sputtered by RF are at even lower powers of < 1 kW. The lower power levels and smaller duty cycles result in reduced deposition rates. The target surface can still be exposed to temperatures of several hundred degrees C despite the circulation of cooling water on the backing plate. This temperature can be even higher with materials with poor thermal conductivity. Target cracking can occur when the temperature of the target surface gets too high. Depending on the desired film thickness, the deposition rate can be a determining factor in the viability of the material for use in commercial production.
Another determinant of the target microstructure and the quality of the sintering process is the powder size. Finer powders can lead to higher density targets. However, oxidation can be increased as well since finer powders are associated with larger surface area. Figures 5 and 6 show the difference between an (In,Ga)2Se3 target cross-section for an average powder size of 150 µm and a target with an average powder size of 75 µm.
The strength of the materials is also directly related to the quality of the sintering and the material microstructure. Various techniques can be employed in the powder processing steps to minimize the oxidation of the materials. One of the drawbacks of finer powder sizes is the loss in materials, leading to higher costs.
One of the key benefits of RF and pulsed-DC magnetron sputtering is the wider array of materials available to sputtering, including fully insulating ceramics and semiconductors. Challenges must be overcome due to these materials’ brittle nature and poor electrical and thermal properties. Achieving a well-sintered, fully dense target results in uniform performance throughout the target life.
Key requirements for being able to achieve this goal include understanding the materials’ characteristics and combining the optimal compound formation parameters, powder size, sintering temperature, and sintering pressure while maintaining the desired purity and composition. Finally, the target assembly must be well constructed to ensure adequate heat flow between the target surface and the backing plate.