The industry's worship of Ra in gas system plumbing
08/01/2003
By Charles Drexel, DXL USA, Torrance, California
In our modern world of semiconductor manufacturing, Ra is the symbol for average roughness of a surface (Roughness average). Often Ra has taken on godlike significance in the lexicon of engineers that design plumbing or anything that process gases touch on their brief journey to a reaction chamber (see "Ra: The ancient god of the sun"). This importance stems from the need to keep silicon wafer surfaces free of foreign materials that can get stuck in minute surface imperfections in the plumbing where they may later come loose, be swept up by the gas stream, and sprinkled on the wafers. It is difficult to flush out particles because they are in the gas boundary layer, but they are usually dislodged if the plumbing is vibrated, such as when a valve is closed.
Surface roughness
Surface roughness is measured by an instrument such as a tracer-point analyzer or a laser, which can detect the number and size of surface anomalies relative to a hypothetical perfect surface, and then compute an average value (Ra). If this value is consistent over the total wetted area (TWA), then the total trapping volume (TTV) in which particles can reside equals the Ra value times the TWA. Since the TWA is seldom calculated or estimated, a low value of Ra is assumed to indicate a low number of particles can be trapped. It also is a convenient way to compare competing surface-.finishing methods and gas component designs. The goal is to reduce the number of resident particles in the plumbing system to a minimum.
In the pursuit of minimum TTV, engineers have concentrated on ways to produce highly polished surfaces, usually ignoring the other factor in the equation — TWA. In the detailed specifications of any semiconductor plumbing component, a valve for instance, the Ra value is always specified as usually "single digit," meaning <10, but the total wetted area is not. To check on the significance of this oversight, we disassembled six mass flow controllers of different design and measured the total wetted area. The variation was from 4.448–33.371 in2, a ratio of 7.5:1. Although all MFCs specify single-digit Ra, they will hardly have equal particle-shedding performance unless they have the same TWA.
A side issue is the cost of achieving a low Ra value. It is not uncommon for gas system components with a specified surface finish of single digit to be more expensive than ones with a higher value. But without knowing the total wetted area, how can an equipment engineer make a reasonable choice when he wants to minimize particle shedding? He often goes for the higher price to be safe, but he may be buying more particle exposure instead of less.
Another view
Let's look at another aspect of Ra. After we measure Ra and TWA, that's not the end of the story. We should additionally look at cracks and crevices in the flow path that can also trap particles. Crevices created by mating surfaces, seals, threads, and dead-end cavities are prime examples.
Consider mating surfaces. Where two parts are bolted together (e.g., an MFC body and end fitting), the mating corners cannot be perfectly square (see figure). There must be an allowance for "breaking" sharp edges to eliminate burrs. The least break that is practical is 0.010 in. Such breaks form grooves that become particle traps with a cross-section area of 0.0001 in2. If the inside diameter of the flow channel is 0.25 in., the total trap volume formed by these mating breaks is 0.000079 in3 (0.25π x 0.0001). For a surface finish of Ra 10 (10μin.) the extra effective surface area caused by this one mating groove is equal to 7.9 in2 (0.000079/0.000010), almost twice as much as the lowest value measured in a complete MFC. Typically, end fittings are assembled with metal O-rings, which form two such grooves at each seal, or four grooves in just the MFC body. This effectively adds 31.6 in2 of wetted area that must be considered in evaluating particle entrapment. Most MFCs have five metal seals, making the total additional effective wetted area between 50 and 100 in2, depending on the detail design. In other words, the total volume of particle voids in MFCs can vary by as much as 20 to 1.
 Where two parts come together, the mating corners cannot be perfectly square. |
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It is suggested that gas component vendors specify the effective TWA of their products and that equipment engineers encourage them to do so. This has other benefits because the lower the TWA, the faster the purging and the less moisture trapped, so equipment will have longer life when flowing reactive gases. The differences in TTV between different component designs may be huge in terms of particle reduction.
Chuck Drexel received his BS from Colorado University and MS from UCLA. He was the founder of Tylan Corp., Nippon Tylan (now Aera), three MFC companies in Europe, and, most recently, DXL. In 1987, Drexel (with Dick Blair and Dan LeMay) received the Semi Award North America for the MFC. He holds 15 patents in the field of fluid control. Drexel is CEO at DXL USA, 960A Knox St., Torrance, CA 90502; ph 310/784-5455, fax 310/784-5464, e-mail [email protected].
Ra: The ancient god of the sun
Ra was the god of the sun in ancient Egypt, the supreme deity in its array of idols. He is depicted with the body of a man and the head of a hawk, crowned by a disk representing the sun, and a cobra. Ra was worshiped as the most powerful of the Egyptian gods, with the ability to cure ills, assure long life, and even reserve a place in the afterworld.
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When it comes to gas handling in wafer processing, however, there are good reasons why engineers should not make Ra their supreme deity.