Ceramics In The Semiconductor Industry
The manufacture of ceramics for the semiconductor industry is typical of standard structural ceramics manufacturing, using traditional forming and densification methods. The biggest difference is the high purity requirement of the ceramic, and occasionally the very tight control over electrical properties.
Oxide Ceramics (Quartz/SiO2, Al2O3)Fused quartz is the most widely used ceramic in the semiconductor industry. Quartz is traditionally thought of as crystalline, but in this sense it refers to high purity silica glass. Its high purity, chemical resistance, and thermal and electrical insulation make it the preferred material for a full range of semiconductor process applications, including diffusion and oxidation equipment, crucibles for drawing silicon ingots, epitaxial silicon deposition reactors, wafer carriers, single wafer process equipment, wet etch tanks and complex machined components (see Figure 1).
Quartz parts production can be split into two areas: blank or ingot forming, and fabrication. Ingots are made from a high purity silica powder, generally from a melt. The powder is placed inside a large zircon bowl and melted at over 2000°C via gas firing. This melt pool is then drawn through a zircon die to create a large continuous ingot. The ingots are then cut, inspected and machined to create a uniform surface. Removal of a thin reaction layer by acid cleaning is necessary to take out any impurities introduced during the process.
Other alternative methods of forming ingots are chemical vapor deposition (CVD) and electric fusion. CVD yields ingots of extremely high purity (ppb range). Electric arc fusion can be used to create both the large crucibles used in Czochralski silicon ingot drawing and hollow tubes.
Ingots at this point can be made into tubes and rods of different sizes on resizing lathes and draw towers. Fabrication of actual parts from ingots requires a number of different machining, forming and welding capabilities. CNC machines, wire saws and grinders are used to "cold fabricate" quartz rods and tubes into pieces used in the "hot fabrication" process, where flame welding techniques (hydrocarbon fuel torches) are used to join cold fabricated pieces into the final part. Flame polishing is used to "heal" the surfaces of parts, while anneal ovens are used to relieve any internal stresses introduced during the fabrication process. Repair of damaged quartz parts can also be accomplished in this fashion.
High purity alumina is used extensively in semiconductor wafer processing equipment. Use of alumina ranges from standard purity (99.5%) to ultra high purity (99.9%+). Alumina is electrically and thermally insulating; has relatively high strength, hardness and stiffness; and is corrosion resistant to a wide range of chemistries. This combination of properties makes it useful in a large number of applications in the semiconductor industry.
Alumina parts are generally formed from blanks or near net shape pieces by machining. Blanks can be formed from high purity raw materials by CIP (cold isostatic pressing) or dry pressing, and then densified by pressureless sintering. Machining is the most important step in the production of high purity alumina components in that the sintering of blanks is relatively straightforward. With the transition from 200 mm silicon wafers to 300 mm wafers, larger components (up to 24 in. in diameter) are necessary and must be machined to the same tight tolerances and exacting dimensions as the smaller components used in 200 mm equipment.
Carbide Ceramics (SiC and B4C)Silicon carbide is a material gaining more usage in semiconductor single wafer fabrication processes. It has been used extensively in diffusion furnaces for over 25 years due to its high purity and high temperature strength (see Figure 2). It has also been used as a material for large vacuum chucks and CMP blocks due to its thermal expansion match to silicon, high elastic modulus (stiffness), high thermal conductivity and chemical inertness. However, its plasma corrosion resistance and high purity also make it an ideal candidate for use in plasma etch chambers.
Fabrication of complex large parts for 200 or 300 mm silicon wafer process chambers can be accomplished through a combination of slip casting and machining. Beginning with high purity silicon carbide powder, silicon carbide slip is mixed with a binder to provide green body strength. Custom-made plaster molds are used for rough green forming. After casting is completed, parts are typically green machined to exact specifications using a CNC milling machine. Firing is done at temperatures in excess of 2000°C in an electric periodic furnace. During this recrystallization stage there is no fired shrinkage. Instead, the material remains essentially the same size but develops a large interconnected pore structure. To make a solid component, high purity polysilicon is filled into the pore structure. Final machining is completed to achieve the correct tolerances on the part. For use in plasma etch environments, the silicon carbide etches more slowly than silicon, thus a protective layer of CVD SiC is coated on the surface. This protective coating can be placed on both large surface areas and on the inside walls of tubes and partial enclosures.
For fabrication of simpler shapes, dry pressing, CIP and extrusion can be used. A silicon carbide slip is created as before, with the exception of sinter aids added to the slip. But instead of being cast, the slip is spray dried into a very flowable powder that is amenable for pressing. The powder is placed into a flexible mold and sealed. High fluid pressure is applied isostatically to the CIP mold, allowing shapes that cannot be made by traditional uniaxial pressing. Large parts can be produced by CIP, depending on the size of the equipment, and certain features can be formed by the mold. In contrast to the previous process, SiC parts produced by this method can be easily pressureless sintered with the addition of a boron carbide sintering additive.
Boron carbide ceramic is similar to silicon carbide except for its lower resistivity and thermal conductivity. Forming of boron carbide parts can be done by hot pressing blanks, with dense machining used to provide large intricate shapes. Hot pressing limits the shapes available in boron carbide parts, relying mainly on dense grinding to bring the blanks to the desired shape. Smaller shapes can be formed by injection molding and HIP (hot isostatic pressing). This method is good for high volume runs where the shape is fixed. A plastic binder is blended into the powder to enable good flow characteristics. Upon heating, the mixture deforms easily and flows into the cold mold under high pressure. The resultant parts are exactly alike in shape and size. Removal of the binder process requires a burnout procedure, the length of which is directly proportional to the cross sectional thickness of the part. Boron carbide is not used extensively in the semiconductor industry, but has potential in plasma etch applications due to its composition and corrosion resistance.
Nitride Ceramics (Si3N4, AlN, BN)While silicon nitride is beginning to see use in the semiconductor industry as a low thermal expansion/high corrosion resistance substitute for alumina, its traditional use in the semiconductor industry has been in the form of ceramic ball bearings (see Figure 3). The ceramic balls provide a number of features over standard ball bearings in the form of less lubrication, lower friction, electrical insulation and good corrosion resistance. Silicon nitride's resistivity and dielectric constant are similar to that of alumina, but the material itself can be far stronger due to its microstructure. Resistance to certain etch environments is another advantage of silicon nitride over alumina.
The manufacturing of large intricate parts is difficult due to the temperatures and pressures needed for densification of high purity silicon nitride. Silicon nitride can be green formed by CIP or dry pressing, with final densification provided under high temperatures and pressures for high purity, or by liquid phase sintering with the use of ceramic sintering aids such as MgO, Y2O3, or other rare earth oxide additions.
The use of aluminum nitride as a material that has tightly controlled electrical properties is critical for applications such as electrostatic chucks (ESC). Its high thermal conductivity and its good thermal expansion match to silicon make aluminum nitride a desirable material for use in ESC, which provide fixturing for the silicon wafer in vacuum environments without the use of mechanical clamps. In creating an aluminum nitride puck, a high purity powder is once again obtained and processed. Various processes are used to fabricate embedded electrodes, co-firing being the most common approach.
Hexagonal boron nitride for the semiconductor processing industry is used mainly in diffusion furnaces in which a BN wafer is used to actively dope silicon wafers (see Figure 4). Boron nitride is also used as an insulator in ion implant equipment. Among its desirable properties are its high dielectric strength, thermal conductivity, machinability and low dielectric loss. Unlike other ceramic components, boron nitride solid components are all machined directly from a large nitrided hot pressed billet that has been synthesized from B2O3 raw materials.
Ceramic CoatingsCeramic coatings on a variety of substrates, ceramic and non-ceramic, are also used in wafer fabrication equipment. The benefit for metallic substrates is offering the performance of ceramics, while using the ease of manufacture of the metal component. Ceramic coatings are usually applied on ceramic substrates when the highest purity ceramic is desired. Thermal spray, CVD or PVD (physical vapor deposition) methods can be used for creating ceramic coatings on the substrates. Depending on coating method, thickness and uniformity of the coating can vary.
Thermal spray coatings are typically line-of-sight processes. Coating rates are fast, and production costs can be relatively low compared to other coating methods. Disadvantages include a porous microstructure and less uniform films than those produced by CVD and PVD.
CVD methods produce a dense, high-purity coating, but are much slower and more expensive. PVD coatings use thermal evaporation, electron beam evaporation or sputtering to create a dense, high-purity coating. Disadvantages of PVD include a slow deposition rate and difficulty in applying oxide coatings well.