Handling the 450mm Wafer with Structural Ceramics
Whether we are considering wafer fab or wafer test and assembly, the handling issues are much greater for the 450 mm wafer but thankfully surmountable.
By Frank J. Ardezonne, President and CEO, F.J.A. Industries, Inc., Santa Clara, CA
It is not here, but it is coming! The 450 mm wafer is now spec'd and equipment for fabrication of the wafers is being built. The industry has changed its tune when it comes to handling the 450 mm wafer. Backside vacuum handling is now acceptable for the 450 mm wafer while edge gripping is discouraged for most operations, but not all.
The lessons learned from handling the 300 mm wafers should make manipulation of the 450 mm wafer a bit easier. There are still many issues which can cause concern for production equipment suppliers such as secure wafer holding, sagging wafers, and machine size related to the handling clearance areas required. The wafer hold becomes more of an issue for the 450 mm wafer because of the large area of the wafer. Movement of the wafer produces high loads from air resistance when the wafer is moved through the air while its flat surface is perpendicular to its travel. Without proper support the wafer can be broken from air pressure force alone. The higher the speed, the higher the force. Additionally a 5g hold will be required for rapid movement and accurate positioning. The equipment size also becomes an issue when the wafer must be moved and requires large clearances for the loaded as well as the empty end-effector in order to clear the equipment's structures.
In addition to equipment size and dynamic loads we are also faced with material limitation. Fabrication of ceramic end-effectors and wafer chucks become much more expensive and difficult to produce. The use of large sheet ceramics and "structural ceramics" techniques lessens the burden but does not eliminate it. The availability of 96.0% and 99.5% ceramic sheet material permits the fabrication of most of the handling and holding elements required by the machine manufacture.
Large ceramic sheets
In the 1960s, semiconductors were fabbed on one-inch wafers and any need for ceramic sheets was easily met. In 2011 we are contemplating 450 mm (17.7 inch) diameter wafers, which are larger than the readily available ceramic sheet often used by the industry. Herein lies the reason for the fabrication of large ceramic sheet stock.
|Figure 1. 300 mm, 450 mm ceramic discs and large ceramic sheets provide for chucks and end-effectors|
The fabrication rules used for production of small sheets of ceramic do not extrapolate out for producing large sheets. It is the general conclusion of most players in the industry that what was done for the fabrication of small ceramic sheet stock can be used to produce large thin sheet ceramic. Such is not the case from our findings. While sintering temperatures can be similar the firing techniques are very different. Let us first define what is generally available and what we call standard small sheet stock. Ceramic sheet material up to 4" (101.6 mm) square and up to .040" (1 mm) thick is available from 90% of the ceramic suppliers; in sizes up to 8" x 8" (314.9 x 314.9 mm) and thickness up to .040" (1 mm) it can be purchased from only about 10% of the ceramic suppliers. This size range represents the upper limit of what we call small and medium size ceramic sheet stock. The size beyond 8" x 8" up we call large ceramic sheet. The thickness of the large sheet stock can range from .020" thick to .50" thick and that thickness is related to size. The current maximum available size is 22" x 17" x .040" to .50" thick, with larger sizes contemplated.
|Figure 2. Functional components provide reliable operation while large thin stock permits fabrication of component parts and end-effector assemblies.|
Why larger sheet sizes? The need for tooling, rigid fixtures and handling equipment to service the semiconductor and photovoltaic industry has fostered the need. In the semiconductor industry we must handle, hold and process wafers, which are currently 300 mm (11.881") in diameter and soon there will be 450 mm (17.717") diameter wafers. In the photovoltaic industry we must process large numbers of photovoltaic cells (125 mm to 156 mm square) with gang tooling to meet production demands. In addition, the machine industry that produces the tools to fabricate these parts, needs to produce ever more precise equipment to meet fabrication requirements. The above needs can be filled by utilizing large ceramic sheet materials to produce the tools and fixtures required..
The use of structural ceramics techniques in conjunction with large pieces of thin ceramic sheets, allows the designer a freedom which cannot be achieved using any other technique to produce the part. Fabrication processes such as dry pressing, isostatic pressing or hot isostatic pressing injection molding and SLIP or JELL casting will not produce the size or quality needed to produce thin sheets. In simple terms, it is now possible to produce almost any shape that can be made in wood, plastic or metals using a ceramic sheet material in conjunction with structural ceramics assembly techniques. In most cases the cost of the part will be less than conventional methods and the quality and performance will be superior.
Industrial ceramic materials have unique properties which make them very desirable in the high-tech industries of the world. Such properties as selectable thermal expansion, low or high thermal conductivity, high and low operating temperatures, resistance to thermal shock damage, high strength to weigh ratios and several others make these materials unique.
Why enumerate the attributes of ceramic? Simply because the combining of these attributes with novel assembly techniques provides a discipline called "structural ceramics" which can supply parts and systems with unique and outstanding properties. The availability of large sheets of ceramic and laser fabrication of component parts along with unique assembly techniques permits the manufacture of almost any shape imaginable. Diamond tool machining can add extreme accuracy where needed although laser fabrication is sufficient in 90% of the project cases.
|Figure 3. A 450 mm end-effector which can be supplied In a vacuum grip format or as a Bernoulli type.|
While the high-tech semiconductor, photovoltaic and aerospace industries are the prime users of these products, the automobile industry, the medical industry and machine manufacturers are also availing themselves of ceramic technologies. The use of structural ceramics techniques can produce both simple and complex parts of lightweight and high strength with dimensional stability and at very reasonable pricing. The fabrication of structures which operate at high (+750??C and higher) and/or low temperatures (-200??C) are also possible.
The use of computer design techniques along with proprietary assembly techniques make almost any ceramic project possible and at acceptable costs. Whether you are cooling a laser diode, building a lightweight and rigid beam system, processing photovoltaic cells or testing and handling semiconductor wafers, ceramic materials and components will and are doing the job better than metals can.
|Figure 4. An example of structural ceramic technology|
Finished products can be fitted with simple mounting holes, screw threads, metal fittings and laminations of different types of ceramic. Issues dealing with differences in coefficient of expansion can usually be handled in several different ways dependent upon the components design criteria and operating temperatures. In severe requirements the material's CTE can be custom blended into the ceramic material.
Thicker parts can be made from laminations which will not de-laminate in use. The lamination of a 99.6% Al2O3 on a 96.0% Al2O3 structure is now a common occurrence and structurally sound. Thin wall structures such as nose cones can be produced to any complex configuration and literally in any size. The limitation on part size, whether a flat sheet or a complex shape is now only equipment restricted and no longer process restricted. Ceramic sheet sizes of 4 x 8 feet with thin cross sections could be fabricated if needed.
The semiconductor industry will be testing parts at higher temperatures and working with larger wafers (450 mm, 17.71"). The larger components required in ceramic, are now available. The photovoltaic industry which requires processing tools to ramp up production can now have these tools made. If you are manufacturing machine tools and require parts which are dimensionally stable or have high strength to weight ratios, you can now design your components using ceramic structures.
Frank J. Ardezonne is president and CEO, F.J.A. Industries, Inc., 1230 Coleman Ave., Santa Clara, CA 95050, 408-727-0100, www.fjaind.com.