In the realm of material science and thin-film technology, molybdenum sputter targets hold a pivotal position. These targets, crafted from molybdenum, a transition metal known for its high melting point and remarkable thermal and electrical conductivity, are instrumental in the physical vapor deposition (PVD) process.
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This process is critical for applying thin film coatings on various substrates, which find applications across a multitude of industries. This article delves into the manufacturing process, applications, benefits, and challenges of molybdenum sputtering targets, providing a comprehensive understanding of their significance in contemporary technology.
The production of molybdenum sputter targets involves a series of sophisticated steps, each critical to ensuring the final product meets the stringent requirements for thin-film deposition applications.
The process starts with mining molybdenite ore, the primary source of molybdenum. The ore undergoes flotation to concentrate the molybdenite mineral, followed by a series of chemical reactions to purify the molybdenum. Roasting converts molybdenite to molybdenum oxide, which is then reduced to pure molybdenum metal through hydrogen reduction. This stage is crucial for removing impurities that could negatively impact the sputtering performance.
The pure molybdenum metal is then converted into fine powder. This powder metallurgy process involves atomization or mechanical milling, which breaks down the metal into a fine powder. The powder’s particle size and distribution are carefully controlled to ensure uniformity, a critical factor for achieving consistent density and mechanical properties in the final target.
The molybdenum powder is next pressed into a compact shape, often under high pressure, to form a green body. This body is then sintered at high temperatures in a protective atmosphere, causing the powder particles to bond together. Sintering not only densifies the material but also helps in achieving the desired purity and grain structure. The sintering conditions, including temperature and duration, are optimized to enhance the mechanical and physical properties of the ingot.
The sintered ingot undergoes rolling and forging processes to achieve the desired thickness and shape, improving its microstructure and mechanical properties. These mechanical deformation processes break up the grain structure, making it more uniform. The ingot is then machined to the precise dimensions required for sputter targets. This includes cutting to size, surface finishing, and sometimes drilling holes or adding features for mounting in sputtering systems. The machining process must be meticulously controlled to avoid introducing surface imperfections or stresses that could affect sputtering performance.
Throughout each stage of the manufacturing process, rigorous quality control measures are implemented. This includes inspections for purity, density, grain size, and absence of defects. Advanced techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) may be employed to analyze the material’s structural and compositional properties. Ensuring the molybdenum sputter target meets the specific criteria is vital for achieving uniform thin films with excellent electrical, thermal, and mechanical properties.
Molybdenum, with its unique set of properties, stands out as a sputtering target material for several reasons:
Molybdenum boasts a melting point of 2,623°C (4,753°F), which not only facilitates deposition processes at high temperatures but also ensures the structural integrity of the target under extreme conditions. This high melting point is instrumental in enabling high-temperature sputtering processes, which are often required to achieve optimal film adhesion and microstructural properties.
The thermal stability of molybdenum, characterized by its ability to retain its physical and chemical properties over a wide temperature range, plays a critical role in achieving uniform thin films with exceptional quality. This stability minimizes the risk of target degradation and contamination, leading to more reliable and consistent deposition outcomes.
The electrical conductivity of molybdenum is another key benefit, especially in applications requiring the deposition of conductive layers. Molybdenum films can effectively serve as conductive pathways in electronic devices, offering low resistance and high efficiency. This property is crucial in the fabrication of integrated circuits, thin-film transistors, and other components where electrical performance is paramount. The efficient conductive layers provided by molybdenum sputtering targets contribute to the overall performance, energy efficiency, and miniaturization of electronic devices.
The density and uniform grain structure of molybdenum targets are essential for achieving consistent sputtering rates and uniform film thickness across the substrate. A uniform grain structure ensures that the sputtering process proceeds at a steady rate, minimizing variations in film thickness and composition. This uniformity is critical for applications requiring precise control over the film’s optical, electrical, and mechanical properties. For instance, in the manufacturing of semiconductors and photovoltaic cells, where even minor inconsistencies can significantly impact device performance, the consistency offered by molybdenum targets is invaluable.
The advantages mentioned above directly contribute to the enhanced performance and longevity of devices and components coated with films deposited using molybdenum sputter targets. The high-quality films improve the durability, efficiency, and reliability of the final products.
For example, in the context of solar panels, molybdenum-based back contacts can improve the electrical conductivity and corrosion resistance of the cells, leading to higher energy conversion efficiencies and longer operational lifetimes. Similarly, in the realm of display technologies, molybdenum layers can enhance the performance and durability of the components, ensuring that they withstand the rigors of everyday use without degradation.
In the semiconductor sector, molybdenum sputter targets are indispensable for creating thin films that serve as barriers, adhesion layers, or conductive paths in integrated circuits (ICs). Molybdenum’s high melting point and excellent electrical conductivity make it suitable for forming the metallization layers that connect different parts of a semiconductor device, ensuring reliable performance even under high power and temperature conditions.
The use of molybdenum targets in the flat panel display industry is crucial for producing thin-film transistors (TFTs) and electrodes in liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and touchscreens. Molybdenum films contribute to the display’s functionality and durability, providing the electrical conductivity required for pixel activation while maintaining transparency and resistance to environmental degradation.
In solar energy applications, molybdenum sputter targets are used to deposit back contact layers on thin-film photovoltaic cells, such as copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) solar cells. These molybdenum layers are crucial for ensuring efficient electron flow and enhancing the cell’s overall energy conversion efficiency. The material’s stability under high temperatures and corrosion resistance contribute to the longevity and performance of solar panels.
Molybdenum is pivotal in manufacturing thin-film transistors, a fundamental component of many electronic displays and circuits. Its films serve as the gate electrode, providing the necessary conductivity and stability for the effective control of current flow within the device. This application underscores molybdenum’s role in advancing display technologies and flexible electronics.
The aerospace and defense industries leverage molybdenum coatings for their exceptional strength-to-weight ratio, high-temperature resistance, and corrosion resistance. Components such as turbine blades, engine nozzles, and heat shields benefit from molybdenum’s durability and performance in extreme conditions, contributing to the reliability and longevity of aerospace machinery and equipment.
In automotive applications, molybdenum sputter targets are used to deposit coatings on various parts, including electrical contacts, sensors, and protective layers on glass. These coatings enhance the components’ resistance to wear, corrosion, and high temperatures, improving vehicle performance and safety.
The biocompatibility and corrosion resistance of molybdenum make it ideal for medical device applications, including implants and surgical instruments. Thin molybdenum coatings can provide a barrier against corrosion and wear, extending the lifespan of medical devices while ensuring safety and reliability in medical procedures.
Beyond its electrical and thermal applications, molybdenum is used in protective coatings for industrial machinery and tools. Its resistance to wear, corrosion, and high temperatures protects equipment in harsh industrial environments, enhancing durability and reducing maintenance costs.
Read more: What Are the Uses of Molybdenum Sputtering Targets?
Molybdenum sputter targets are integral to the advancement of thin-film technologies, offering a unique combination of properties that make them indispensable in a wide array of applications. From enhancing the performance of electronic devices to enabling innovations in energy and aerospace, the contributions of molybdenum targets are vast and varied.
Despite facing challenges related to their production and use, ongoing research and technological developments are poised to overcome these obstacles, ensuring that molybdenum sputter targets continue to play a crucial role in the development of next-generation materials and technologies.
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The selection of EDM wire used to be easy as there were only a few choices; copper or brass. Today, there are more electrode choices for wire EDM than there are for sinkers. We have economy and utility-grade wires, wires for speed, wires for carbide and PCD’s, wires for tapering, and fine-wire machining. In this short study, we will first examine the different properties of EDM wire, then we will briefly examine the different types of wires and their applications.
EDM Wire Properties
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Tensile strength
Tensile strength is the maximum load-bearing capability given to a material based upon its ability to resist stretching and breaking. It is determined by the maximum load in pounds per square inch divided by the cross-sectional area of the wire. High-tensile EDM wire provides better edge straightness and so is a good choice for single-pass parts and in small- and fine-diameter wires to reduce wire breakage. Wirecutting tall parts is another application that requires good straightness and resistance to wire breaks. High-tensile wire is also good for skim cuts, aiding in part straightness and geometric accuracy.
Fracture resistance
There is no true index or “rating” of fracture resistance, so fracture resistance of an EDM wire might better be described as wire toughness or resilience. It is the ability of the wire to resist the effects of the incredibly dynamic environment of the spark gap.
Conductivity
This is the measure of a material’s ability to carry electrical current. In EDM, the higher the conductivity of the wire, the more power can be delivered to the workpiece. Since higher conductivity is more efficient, increased cutting speeds are the usual results.
Vaporization temperature
A low melting/vaporization temperature of the wire will prove the best for flushability. We want the wire surface to vaporize, to quickly turn into gasses, instead of contaminating the gap with resolidified “chips”. All chips have mass and therefore, will “take up room”. The spark gap is small enough already and we don’t need “chunks of stuff” floating around in it. When using longer on times (roughing), melting will occur. Even though melting produces chips, lowtemp electrode alloys (zinc-coated) will produce larger craters on the wire surface to carry more water and contaminants away from the gap, aiding flushing. Consequently, moly and tungsten wires, with high melt/vaporization temperatures cut slowly and flush poorly, but are satisfactory for skim cuts where speed is not an issue but part straightness and crisp geometry is.
Hardness
The term hardness is often mistaken for tensile strength. The hardness or temper of a wire refers to the wire’s ductility, or its ability to undergo elongation. In this category, EDM wires are called soft or hard. On closed-guide machines, a hard wire will thread better than a softer wire, but a soft wire will taper-cut better. A hard wire will also provide the best auto-threading reliability.
EDM Wire Types
Copper wire
The first wire used for wire EDM was copper. It’s availability and high conductivity (100% IACS) made it a logical choice for an EDM electrode at the time, but as generators became more powerful, copper’s limited cutting speed and low tensile strength (34,000-60,000psi) quickly revealed it’s limitations. Except in older machines that specify the use of copper wire, this wire is seldom recommended. In these machines, this wire must be used to wire-cut all materials simply because there is no other alternative.
Brass wire
The element zinc is added to copper to produce brass EDM wire, which is the most common EDM wire in use today. Brass wires for EDM are typically an alloy between 63/37 (American and European) to 65/35 (Asian), Cu/Zn ratio. Zinc has a lower melting/vaporization point which makes it a better electrode material than copper, so the more zinc in the surface of an EDM wire, the faster it will cut. However, manufacturing difficulties arise when the volume of zinc approaches 40% and its crystalline structure changes to a gamma phase, causing the wire to become very brittle and difficult to draw. However, there are two wires being produced with a 60/40 Cu/Zn content for faster cutting speeds. Tensile strengths of brass wires range from 54,000-173,000psi, depending upon the composition of the alloy and it’s temper. Brass wires are usually a shiny brass color. Brass wires with a dull, matte finish or with spots or discoloration are indications of oxidation or contamination. Brass wires are a good, all-around utility wire that can be used by most modern machines. Economical cutting of tool steels and most metals is possible with plain brass wire. Available in hardness (elongation) in ranges from <2% to over 30%. Wires with low percentages of elongation will thread reliably but are limited in tapering ability. Obversely, wires with high elongation can taper-cut to 45° but with much reduced threading relaibility.
Coated wire
Coated EDM wires are wires that have had a very thin layer (2-3μm) of pure zinc applied to a brass or copper core. Electro-galvanization is the most precise method of application, depositing pure zinc, atom by atom onto the body of the wire, insuring uniform thickness and surface uniformity. Hot-dipping is another method of coating the wire. It is a quicker and less precise method of coating and these wires will usually be less expensive. These wires are available in tensile strengths from 61,000-130,000psi and can have a pure zinc or zinc oxide coating depending upon the application. Pure zinc coatings will have a shiny, silver color while a zinc-oxide coating will appear dull gray. Zinc coated wires will cut much faster and break less often than plain brass wires and are recommended for fast roughing and finishing of steels. Zinc coated wires will produce a superior surface finish and better surface integrity, especially against carbides and PCD’s.
Diffusion-annealed wire
Wires with high zinc content make a better EDM electrode but EDM wire with zinc percentages approaching 40% are difficult to produce, so a special method of producing wire with a higher zinc surface has been devised. A heavy coating of pure zinc is applied to the outer surface (18-35μm) of a copper or brass core. Undergoing annealing in a special furnace diffuses the pure zinc coating into a mixture of nearly 50% brass and 50% zinc (actually, zinc content is approximately 45-47%), which is higher than the 40% threshold which was the previous manufacturing limitation. Diffusion-annealed wires are available in tensile strengths from 62,000-128,000psi and appear yellowbrown to brown in color. These wires are suited for fast cutting of tall workpieces, volume production and in poor flushing conditions. They can be used against a wide range of materials including tool steels, aluminum and graphite.
Molydebnum wire
This is a very high strength wire with tensile strength in excess of 275,000psi. but because moly wire has very high melting and vaporization temperatures, 4,757°F (2,625°C) and 10,040°F (5,560°C) respectively, it is a reletively poor electrode material. Because of it’s high working temperatures, the EDM craters left on the wire’s surface are very small compared to those left on a brass-based wire and therefore will not flush as efficiently. This wire is often used in the small, (0.006 to 0.004”), and fine, (<0.004”), diameters to meet blueprint requirements of narrow kerfs and near-sharp, inside corner radii. Because of its high tensile strength, moly wire aids in maintaining excellent wall straightness and reducing the number of wire-breaks common with many small and fine brass wires. In special cases, typically in medical and military applications, there can be no copper or zinc contamination of the wirecut surface. This criteria eliminates all copper-family wires and forces the use of a moly or tungsten wire. Note: Use of moly wire must be application-specific because of its high cost, reduced threadability and slow machining times.
Steel-core wire
This type of wire has a core of carbon steel for high strength and fracture resistance and is clad with a thick layer of zinc-rich, diffusion-annealed brass to provide good cutting qualities. The combination of break-resistance with the performance of 50/50 brass can make this wire a problem solver for difficult applications. Steel-core fine-wires (.001”-.004”), have a tensile strength of 290,000psi, exceeding that of moly, but with much better cutting characteristics. Larger diameters (.006”-.012”) have tensile strengths in the range of 116,000-145,000psi. Steel core wire is an option when attempting extremely tall parts, parts requiring good straightness, and where poor flushing conditions exist.
Tungsten wire
Tungsten wire is often one of the last choices when trying to solve a difficult application. Although this wire is very expensive, cuts poorly and is hard to work with, it is sometimes the only solution to an EDM problem. With even higher melting and vaporization temperatures than moly, 6170°F (3410°C) and 10,706 (5930°C) respectively, it is the least efficient wire electrode in regard to cutting performance. Available in small diameters (.001”-.002”), tungsten wire has the highest tensile strength of all EDM wires. This allows it to cut very fine, very straight-walled details with sharp definition. Like moly wire, tungsten wire is also used on applications that must be free from copper or zinc in the cut surfaces.
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