SETRA PRESSURE TRANSDUCERS: Solution Helps New Gas Flow System

CUSTOM TRANSDUCER SOLUTION HELPS UNIT INSTRUMENTS REDUCE SPACE, INCREASE RELIABILITY OF NEW GAS FLOW CONTROL PANEL

Retooling for the next generation of wafer processing equipment makes cleanliness in semiconductor fabrication more critical than ever. It will also improve the efficiency of IC chip production by processing more 300mm (12-inch) diameter wafers than 200 mm (8-inch) diameter wafers in each process load. A self-contained, automobile-sized unit called a semiconductor process tool handles the complete operation of fabricating patterns on wafers, from the loading of pure silicon wafers to outputting a patterned wafer.

Due to the costly premium on workspace for these process tools, wafer fabrication laboratories are telling process tool makers to minimize any increase in their tool size, while accommodating their redesign for larger loads of 300 mm wafers. In response, the fabrication support industry is seeking ways to reduce the volume of all subsystems and components where possible, while simultaneously increasing their reliability to reduce the cost of downtime.

A semiconductor process tool contains many subsystems. Key examples are wafer transporting, loading chamber, wafer process chamber, gas delivery system, and vacuum system. Specialized subsystems include: light projection systems (photolithography tool), plasma generator (plasma etch/CVD), ionization system (ion implant) and coat/develop system (tracking system).

The gas delivery system is critical to the IC pattern development and must deliver clean and controlled gases in a reliable and maintainable manner. Although taking up only 10% to 20% of the process tool volume, any reduction in the size of the gas delivery system is beneficial. This reduction helps offset the necessary expansion of other process tool components for the 300 mm wafer production, such as wafer transport and the process chamber.

In response to present retooling pressures for compactness, cleanliness and easy maintenance, manufacturers have devised a gas delivery system based on gas sticks constructed in the form of channeled stainless steel blocks. These gas stick designs are referred to as top-mounted or flat-bottom systems. The gas flow control components need only be attached directly to the channeled block on one side to complete the gas flow channels that are drilled into the blocks. The ability to better drill channels into stainless steel blocks and improvements made in metal seals over the past 20 years, have made this approach to gas stick design viable.

Unit Instruments, Inc., of Yorba Linda, California, a leading manufacturer of mass flow controllers, ultra-high purity gas isolation boxes, and ultra clean gas panels for semiconductor processing tools, recently introduced its own version of this gas stick design: the Z-Bloc Modular Gas System. This top-mounted gas stick design not only addresses volume reduction issues, but utilizes seals and components that make it extremely compact, clean and accurate.

Pressure sensors are key components in any gas flow control system. When the idea for the modular gas system was developed, Unit began working with Setra Systems, Inc., of Boxborough, Massachusetts, a designer and manufacturer of highly accurate capacitive-based pressure sensors. David Sheriff, Vice President and General Manager of the Z-Bloc Modular Gas Group, said, "Setra's involvement in the early research and development of the Z-Bloc, and their willingness to work with us and adapt their product to fit our needs, allowed us to enhance the cleanliness, accuracy and compactness of the product."

Tiago Anes, National Sales Manager at Setra, explained that working with a customer in the development stage of their product and providing a customized transducer solution is far from unusual for Setra. "Before spending millions of dollars, our customers generally need the assurance that the solution we provide will meet their requirements," he said. "The amount of time varies, but we are quite accustomed to working with them well before their product enters final development and full production. In addition, we cater to many different industries and provide a tremendous volume of custom products," he added. "If you look at our top customers, probably seven out of ten are using products that were customized to meet their specialized requirements."

Before explaining in greater detail how Setra's custom solution played such a key role in the development of the Z-Bloc, it is important to understand how the design and characteristics of the Z-Bloc itself will accommodate the current demands of semiconductor process retooling - that is, increased reliability with decreased component size.

First, Z-Bloc makes use of 316L VIM/VAR stainless steel for its block with its internal channels passivated by a chromium oxide layer to minimize specialty gas corrosion. Components on a single "stick" are top-mounted and consecutively positioned along it's extended steel block. The "in" and "out" ports of each component are positioned to the block's internal "V-shaped" channel configuration (see Figure 1). This allows internal connections of each neighboring component to complete the flow path through the gas stick and eliminates the need for the space required for tubing and fittings. As a result, the size of the gas panel decreases up to 75%. This configuration still allows access to each component directly. Mounting and removing components require only a hex key "Allen" wrench. With direct access to components, it is also possible to make repairs simply by removing only the damaged component, thereby reducing downtime. Since component mounts are standardized, the Z-Bloc design is able to preserve the design flexibility inherent in conventional welded systems, in which the user has the flexibility to position components anywhere on the stick.

Second, increased reliability depends on improving the intricate sealing techniques of conventional face seal fittings. Unit developed Z seal joints which are effectively sealed simply by tightening four socket-head capscrews without any complicated tightening sequence or torquing. In the sealing process, both the component mount and the block incorporate machined glands, which compress a malleable nickel seal to produce a leak-free system. Under installation, the seal produces no particles greater than 0.1 microns and less than 10 particles greater than 0.02 microns. Furthermore, these particles are typically purged out of the system in less than one minute.

Unit Instruments incorporates components of high stability, accuracy and cleanliness into their Z-Bloc Modular Gas System to enhance reliability. "Pressure transducers were critical and Setra's Model 215 Ultra High Purity Pressure Transducer offers the kind of stability, accuracy and cleanliness that ensures our highest quality product," states Sheriff. "And, the Model 215 offers superior stability and accuracy of a quarter percent full scale, which goes a long way to reducing maintenance downtime to recalibrate." According to Anes, "We modified the 215 pressure transducer for Unit by welding the transducer to a flange, so that it would easily mount to the Z-Bloc." Eric Redemann, Technical Director at Unit Instruments, said, "Setra's willingness to redesign their pressure transducer to adapt to the Z-Bloc also enhances our product for our customers, since they see that more component manufacturers support our product."

The Model 215 incorporates Setra's patented variable capacitance sensor technology, which features a 316L stainless steel diaphragm and an insulated electrode plate. A variable capacitor is formed between the diaphragm and the electrode plate. When pressure is applied, it causes a slight deflection of the diaphragm which decreases the capacitance. The change in capacitance is detected and converted to a highly accurate linear analog signal by Setra's unique custom integrated circuit, which utilizes a patented charge balance principle. The pressure transducer's output signal is fed to the process tool computer, which monitors the status of all gas sticks. With such monitoring, the computer can shut down the wafer processing when pressure status throughout the gas delivery system implies a malfunction. If a malfunction occurs, the repair technician can analyze the pressure status and determine the location of the component failure. The repair technician can also view local pressure readings on Setra's swivel-base LD330 display, which is directly mounted to the Model 215 Pressure Transducer.

With the cooperative help of Setra in modifying its high purity, stable, and accurate transducers to the Z-Bloc design, Unit is better able to fulfill the semiconductor retooling needs by producing a leading-edge, compact gas delivery system. According to Anes, "The cooperative effort, as was exhibited by Unit Instruments and Setra, is the key to future technological innovations and improvements in this industry."

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