Gas delivery: Modifications yield serviceability improvements
07/01/2002
Going modular
by John Baxter
Nearly every process that adds or alters material on silicon wafers uses process gases. These gases are contained under pressure in cylinders and distributed through stainless steel tubing, while valves, filters, gas purifiers, pressure regulators and transducers measure and control flow. The layout configuration of these combined components-the gas stick-has undergone various design changes in recent years.
As the semiconductor industry moves from conventional gas sticks to modular gas systems, the issue of serviceability has become paramount. While reduced weight and footprint are important, the gas stick's serviceability, accessibility and versatility are equally important.
This latest generation of modular gas sticks is referred to as "top-mounted" systems because it can be configured, reconfigured and serviced from the top. These sticks should not have to be removed from the gas box, wall or back plate when changing or rearranging components or adding a manifold.
These recent design changes were required to keep pace with fast-changing technology and to meet the semiconductor industry's needs associated with advanced wafer processing.
This latest generation of modular gas sticks consists of three parts: (1) substrates, (2) manifolds, and (3) top-mounted components, including valves, filters, pressure regulators, gauges and transducers.
 Four contiguous modular gas sticks have top-mounted components attach to the substrate with a metal-to-metal seal and four screws. |
Of these three parts, top-mounted components have changed the least from generation to generation of gas-stick system technology. Their function is to regulate, control, process and purge gases. The challenge for the industry centers on how to connect and arrange these components with the following objectives in mind: to minimize the amount of space gas sticks occupy in the chase, the area adjacent to the cleanroom where the chips are man ufactured; and to enhance the serviceability and design flexibility of gas sticks.
With conventional gas sticks, each component contains an inlet and outlet port, which is connected to other components via a weld or fitting. These connections are in-line; therefore, adequate space must remain between components to accommodate the tools associated with either welds or tube fittings.
By comparison, modular components offer a footprint approximately 40 to 60 percent smaller than conventional systems. The inlet and outlet ports are on the bottom of the components, and components are connected to each other-not via a weld or fitting-but via a substrate. The substrate provides both the channel for the gas flow and the mechanical support for the configuration of components.
Modular components connect to the substrate with a clean, metal-to-metal seal, usually a C-seal. An additional substrate layer-the manifold-is also standard on most systems. It provides the ability to move gas parallel or perpendicular to the process stream gas flow, allowing for bypass ability.
Serviceability and versatility
The ability to easily service and reconfigure gas sticks is important to the semiconductor industry for four main reasons: (1) it reduces labor and assembly costs; (2) it enables technicians to make quick, economical repairs to existing gas sticks; (3) it allows an OEM or designer to move quickly toward building a new tool or modifying an old one; and (4) it speeds up the supply chain.
With modular gas systems, labor and assembly costs are reduced because trained personnel for welding and tube fitting applications are not required. Almost all parts of a modular gas system are fastened together with screws.
Modular gas systems currently come in two standard sizes, 1 1/8 inches and 1 1/2 inches. The measurement refers to the footprint or width of the standard component. Although the 1 1/8-inch footprint results in a reduction in space and weight, there have been some issues with gas flow.
Both 1 1/8-inch and 1 1/2-inch systems employ a nominal 1/4-inch flow path, but the top-mounted components themselves are smaller with the 1 1/8-inch system. As a result, some flow problems have arisen, not so much in the wafer-etching process, but in the chemical vapor deposition (CVD) and the physical vapor deposition (PVD) processes, where relatively large flow rates are required. In these applications, the 1 1/2-inch system is preferred.
In addition, some modular gas systems come with the option of computer software that enables gas sticks and components to be configured using the drag-and-drop capabilities of Microsoft Windows. The designer enters the desired flow path and components to be utilized and then receives a complete bill of materials and an assembly diagram detailing the selected configuration of substrates, manifolds and components.
While the designer or software operator must have an advanced understanding of the requirements for safe product selection, the program saves a considerable amount of time and calculation in determining exactly which components and parts are required for assembly. With the aid of computer-generated assembly diagrams, layout of the gas stick becomes a relatively straightforward process.
When gas sticks require service on site, after installation, modular systems are preferable because a single component can be changed out for another without any disturbance to the other components on the gas stick. Further, changing a component is a relatively simple matter, just as simple as the original assembly, with no welding or tube fittings.
Ease of assembly is particularly important to OEMs pursuing niche markets under tight deadlines A modular gas system provides the flexibility and customization desired, without increasing assembly time. Some OEMs are also manufacturing tools that are designed to accommodate different applications, with only slight modifications. In such cases, the modification may require a quick reconfiguration of the gas stick, which is now practical because of modular gas system technology.
One well-known design creates the substrate from a series of stainless steel pieces that fit together like a jigsaw puzzle, with a male on one side and female on the other. This system provides optimum versatility in controlling the length of the substrate; but since the puzzle pieces are screwed together from underneath, no adjustments to length can be made without taking the substrate off the back plate.
In addition, a break in the substrate would accommodate a mass flow controller, but if one wanted to move a smaller component into the same spot, the substrate would need to be pulled off the back plate and reconfigured from underneath.
Another common design uses a series of stainless steel cubes to make up the substrate. Unlike the system described above, these substrate pieces are not attached or screwed to one another directly. Rather, they are screwed from the top directly down onto the back plate. Then, the top-mounting components are screwed onto the cubes, bridging the connection from substrate cube to substrate cube.
Unfortunately, this substrate is configured to accommodate a particular arrangement of components and if that arrangement changes, so must the arrangement of cubes. Because the individual cubes are mounted directly onto the back plate, it's possible that when the substrate is reconfigured, the holes may not line up, especially since the cubes are not uniform in size.
A third design diverges from the other two in the make-up of the substrate. In this system, the substrate consists of two types of parts: (1) a substrate channel (made of lightweight aluminum); and (2) stainless-steel flow components that slide into the substrate channel.
While the substrate channel provides mechanical support, the flow components handle the flow of gas. Consisting of about one inch of stainless steel tubing with two ports (one at each end), the flow components can accommodate any conceivable combination of top-mounted valves, transducers and filters. Meanwhile, the substrate channel-a fixed length piece of solid aluminum, mounted from the top down onto the back plate-remains stationary and unchanging during any reconfiguration of the components.
John Baxter is an engineer specializing in the semiconductor industry for Swagelok Company, Solon, Ohio, U.S.A.