Plasma spraying is gaining popularity as an economical way to deposit high performance ceramic coatings for use in the electronics and semiconductor industries.
Figure 1. A robotically controlled, mass-flow plasma spray operation.
Alumina-, zirconia-, and titania-based ceramics have become a key part of microelectronic and semiconductor components due to their ability to insulate and distribute heat. However, as the electronics industry continues to advance, improved methods are needed to increase both the flexibility and speed of ceramic processing in these applications.
Alumina-based ceramics with compositions greater than 95% alumina, called “high aluminas,” are often used for active and passive electronic components and are fabricated using powder-based processing techniques. The powder is produced with the desired chemistry, phases and morphology, and is then consolidated (compacted) and shaped through dry pressing, hydrostatic molding, extrusion, injection molding or hot pressing, depending on the dimensions, shape, quantity and requirements of the final product. A sintering process is required to provide the final properties and dimensions. These steps typically require a great deal of time to complete.
Tape casting and roll compaction processes are also used to manufacture microelectronics components. However, tape cast and dry pressed ceramics are generally limited to a thickness greater than 0.381 mm (0.015 in.), providing limited flexibility in manufacturing small components.
Ceramics can also be manufactured using non-powder based methods. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used for more than 20 years in the fabrication of microelectronics. However, these processes occur slowly and are restricted to relatively thin-film coatings.
One method that can be used as an economic alternative to these processes is plasma spraying. This process, which combines melting, rapid quenching, and consolidation into a single step, can be used for virtually any ceramic material that melts without decomposing. Plasma sprayed coatings can be applied in a thickness ranging from approximately 25 microns to a few centimeters, providing considerable flexibility in its applications. As a result of its increased speed and flexibility compared to other manufacturing methods, plasma spraying is gaining a great deal of interest in the high-tech ceramic field.
Plasma Spray Processing
One of the most popular plasma spray methods is the air plasma spray (APS) process. This spray process can melt and apply a tremendous variety of ceramic materials for the electronics and semiconductor industries at a spray rate ranging from 5-10 lbs per hour.
In Figure 1, a state-of-the-art, robotically controlled, mass-flow plasma spray technology is shown spraying aluminum oxide. The plasma spray system consists of an electronically controlled power supply, a PLC-based operator control station, a gas mass flow system, a closed-loop water chilling system, a powder feeder and a plasma gun. A primary inert gas, such as argon, is injected between two water-cooled electrodes in the gun, where it is ionized to form a plasma jet. Ceramic powder is injected into the plasma and is subsequently deposited onto the component.
The plasma gun is initiated and maintained with a steady direct current of several hundreds amperes and is operated in the range of 30-70 volts, depending on the material to be deposited. A thermal plasma is produced with a core temperature approaching 15,000C.1
A monatomic (helium) or diatomic (hydrogen or nitrogen) secondary gas is added to increase the heat content of the flame and thereby increase the power of the system. This increase in enthalpy to the plasma enables high melting point ceramics to be melted.
The ceramic powder is carried by a stream of inert gas and is injected into the flame either internally within the gun nozzle, or externally to the nozzle. The flame accelerates the particles, which are melted by its high temperature and turned into molten droplets. These droplets are then propelled onto the part surface, where they solidify and accumulate to form the coating.
A number of variables are selected to produce the desired coating, including the plasma spray, powder feedstock, material injection and processing variables. Plasma spray variables include the gun configuration, process gases, pressures, flow rates, voltage, amperage and carrier gases. The powder variables include chemistry, morphology, particle size distribution and method of manufacture. Material injection variables include powder feed rate, carrier gas flow, number of injectors, angle of injection and location of injection, while processing variables include the number of passes, spray distance, spray trajectory, traverse speed, tool fixturing and part cooling.
Plasma spray processes can also be conducted in low pressure, at atmospheric pressure in inert gaseous environments with the use of gas shrouds, or in chambers, and still provide potential improvements in the coating properties.
While the plasma sprayed coatings can be mechanically post-processed (through lapping, polishing, etc.) to obtain final dimensions and finishes they are commonly used in the “as-sprayed” condition. Chemical treatments to seal or treat the coatings can also be used. The as-sprayed coatings can also be thermally processed through sintering or laser processing to produce materials with properties similar to bulk sintered ceramics.
Properties of Plasma Sprayed Ceramic Coatings
Plasma sprayed coatings (as well as free-forms) are built-up by the impingement and solidification of molten droplets deposited in successive layers. Each layer consists of several lamella (thin layers), with each lamella approximately 3-10 microns thick, 60-180 microns in diameter, and typically containing columnar grains 0.1-0.3 microns. The solidification of these lamellae is dependent upon the particle size, velocity, temperature, substrate surface conditions and physical properties of the molten material.2
To determine the effect of plasma spraying on ceramic properties, coatings made from alumina (A1), alumina-3 wt% titania (AT3), alumina-25 wt% zirconia (AZ25), alumina-40 wt% zirconia (AZ40) and calcia-stabilized zirconia (CSZ) were sprayed to produce free-standing plates for examination.3 Tables 1-3 (p. 50) review the properties of the coatings in terms of their microstructure, phase composition and mechanical properties in both the as-sprayed and heat treated condition.
The structure and phase of each lamella depends on the cooling and solidification rates experienced during solidification. The cooling rate, in turn, depends on material properties such as the melting point, density and specific heat, and on the thermal contact between the lamella and previously deposited layer.4
Microstructures of these deposits are shown in Figure 2 using scanning electron microscope (SEM) imaging. The alumina and alumina-based deposits (AT3) exhibit columnar grains and lamellar boundaries. The alumina-zirconia deposits (AZ25 and AZ40) exhibit a glass-like morphology consisting of amorphous phases or a mix of crystalline and amorphous phases. The eutectic structure of Al2O3 and ZrO2 is shown in AZ40. Zirconia-based coatings also have lamellae boundaries and columnar grains.
These deposits were also heat treated (1450?C) to examine the properties that more closely resemble those of bulk sintered ceramics. The microstructures of the heat-treated deposits change significantly when compared to the as-sprayed condition. In alumina-based coatings, the grains became more equiaxed, and secondary grain growth produces larger grains. The amorphous phases in the alumina-zirconia deposits transform to crystalline phases, while the zirconia-based deposits contain only equiaxed grains.
The phase composition of the coating depends on the powder chemistry, the thermal histories of the particles in-flight, the solidification rates and the temperature of the deposit.
The phases produced as a result of the alloy composition are shown in Table 1. The alumina and alumina-based deposits consist of metastable gamma and delta phases that were formed during the plasma spray process. The phase composition of the as-sprayed AZ deposits are significantly different from that of either AT3 or CSZ. Crystalline and amorphous phases are present in the AZ deposits, and zirconia-based deposits consist of crystalline cubic and tetragonal phases.
Equilibrium phases are found in all deposits after heat treatment at 1450?C. In the alumina and alumina-based deposits, g alumina transforms to the equilibrium a-alumina. Alumina-zirconia deposits form a-alumina, and m-, t-, and c-ZrO2. The CSZ deposits also contained m-ZrO2 but in a lower amount due to stabilization of the tetragonal and cubic phases by CaO.
Porosity and Density
As shown in Table 2, the coating’s porosity is a function of the material, plasma spray parameters and process conditions. Open porosity in as-sprayed deposits can be produced in the range of 1-5% when the spray process is optimized. For the as-sprayed deposits, the porosity was the lowest for the alumina-25 wt% zirconia and the highest for the calcia-stabilized zirconia. The higher porosity in as-sprayed zirconia-based materials can be attributed, in part, to melting point differences of the alloys and the subsequent effects on solidification.
Heat treatment causes changes in the amount and shape of porosity due to phase transformations, grain growth and sintering. Phase transformations cause as-sprayed material to become more dense. In the alumina and alumina-based deposits, gamma-alumina transforms to the denser alpha-phase and results in the opening of voids between lamellae. In alumina-zirconia deposits, amorphous phases crystallize producing denser phases of material. Pores form as a result of this transformation and agglomerate during heat treatment. Sintering occurs to various extents in the deposits from the heat treatment at 1450C. As a result, the porosity is lowest for the alumina-based deposits, and highest for the alumina-zirconia samples.
Typical densities of the as-sprayed deposit are shown in Table 1. The apparent densities change for the ceramics after heat treatment. The apparent density of the alumina and alumina-based deposits increases slightly corresponding to the phase changes. The alumina-zirconia alloys increase in density as expected from crystallization.
The mechanical properties of bulk ceramic materials are strongly influenced by the porosity. Similarly, the strength for as-sprayed ceramic deposits is related to the contact area between lamella.
The modulus of rupture (MOR) for the as-sprayed deposits ranges from 22.9 to 50.5 MPa (see Table 2). In general, the MOR increases as the porosity decreases. For the heat-treated deposits, the strength depends on the chemical composition of the deposit.
In alumina, alumina-based and zirconia-based deposits, the strength increases significantly as the porosity decreases. In alumina-zirconia coatings, MOR increases slightly or remains relatively unchanged even though the porosity increases. Phase transformation from amorphous to crystalline, in addition to sintering, are responsible for these results.
Heat treating produces properties similar to bulk sintered ceramics. As shown in Table 3, experimenting with the heat treatment time and temperature significantly improves the strength of the deposit.
Coating Properties and Performance
The experiments detailed above proved that plasma spraying of ceramic oxides produces coatings (or free-forms) with the desired shape and dimensions, without the many steps associated with conventional processes (such as sintering). The plasma spray process results in a characteristic layered structure consisting of an anisotropic microstructure comprising lamellae and an interlamellar porosity. As a result of this unique microstructure, these coatings provide beneficial mechanical, thermal and chemical properties to the components they are protecting. The coatings are less prone to catastrophic brittle failure, are able to withstand high thermal gradients, and provide a tortuous path for chemical species attacking the coating. This quasi-layered process also provides tremendous flexibility in producing the desired thickness or surface texture required for the application. In as-sprayed coatings, some properties are inferior (e.g., mechanical properties) to those of bulk processed materials. However, in many cases, the flexibility of the plasma process compensates for this inferiority. When material properties similar to those of bulk processed ceramics are needed, the as-sprayed ceramics can be heat treated.
The electrical, thermal, mechanical and chemical properties of plasma sprayed ceramic coatings enable them to reduce part costs, increase product performance and lifetimes, and lower maintenance cycle times in a number of applications.
In the electronics industry, ceramic oxides are used to manufacture resistors, capacitors, hybrid circuits and sensors. Their low value for electrical conductivity and high dielectric strength make them popular for applications requiring electrical insulation.
In the semiconductor industry, various types of discrete packages are used to make electrical contact, dissipate heat and protect the semiconductor device. The thermal conductivity of the insulating materials is important for a high power device. Thermal dissipation is also crucial for the electrical efficiency and life of the package, with the thermal dissipation being directly proportional to the thickness of the material. In these applications, a dielectric coating can be plasma sprayed to a thickness that is unobtainable with traditional tape cast and dry pressed ceramics.5 Replacing the ceramic substrate with a plasma sprayed dielectric coating can provide a more cost-effective, less labor-intensive package with fewer pieces and less material handling.
Plasma sprayed ceramic oxides are also used in a broad range of semiconductor processing and equipment applications, where they enhance surface properties and serve as target materials, dielectric coatings, and chemical-, thermal- and erosion-resistant linings on products such as semiconductor sputtering targets, shield kits and a host of other components.
In physical vapor deposition (PVD) applications, the coatings are used to reduce particle generation in process chambers during thin film deposition.6 The coatings provide a highly textured surface that increases the mechanical bond between the coating surface and the depositing thin film, and leads to an increase in the process chamber lifetime, and extended chamber service time.7
Over the past several years, wafer manufacturers (often called “fabs”) have achieved tremendous cost savings with thermal-sprayed coatings. Their success has sparked an interest in expanding the use of the coatings in several other areas, including the manufacture of electrostatic chucks (ESCs),8 where the coatings’ dielectric constant, electrical resistivity and dielectric strength can be beneficial. The coatings are used on the lower electrode that controls the charge separation between the wafer and chuck. The dielectric constant is high enough that the applied voltage required to sustain the needed clamping pressure does not result in dielectric breakdown. ESCs used for etching, implantation, PVD and chemical vapor deposition (CVD) are candidates.
Interest in plasma spraying will undoubtedly continue to grow as an increasing number of applications experience success with this new technology.