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Old 04/01/2019, 03:05 AM   #1
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If you're a fundamental scientist who's never had to design or order vacuum chambers or valves, it's very easy to run into problems that affect the results of your process. It's even easy for experienced vacuum users to overlook basic vacuum practices or take shortcuts that affect their process.

We've asked leading manufacturers of high-vacuum valves and chambers for their recommendations regarding the most common problems that their customers experience. The result is a list of 10 problem areas and solutions that should help you avoid running into similar difficulties.

1 Differential Pressure. A recurring topic mentioned in our interviews concerned problems associated with differential pressures on opposing sides of a valve. "Opening a valve under excessive differential pressure can, at the least, cause a valve to jam," says John Freeman, vice president of VAT-North America, Woburn, MA. "At the worst, it can structurally damage the valve, costing thousands of dollars to repair and forcing you to shut down your system."

Look at a 200-mm-dia all-metal gate valve, for example, that opens a vacuum chamber to atmospheric pressure. This situation places the equivalent of a 335-kg load on the valve face--large enough to distort the face. When it opens, the distorted valve face will scrape across the body and damage the seal. Gate valves, while being the most practical configuration for material transfers between chambers, are the most susceptible to these conditions.

"You have to assure, through procedures or interlocks, that pressure is reduced on both sides of the plate to within the manufacturer's limits before opening the valve," says Freeman.

2 Virtual Leaks. A virtual leak is caused by entrapped air within the vacuum system, commonly between two mating metal surfaces that are not welded or brazed. "Although virtual leaks are studiously avoided by designers, some always remain," says Phil Danielson, president of Danielson Associates, Lisle, IL. "Gaps at feedthrough flanges are an obvious example."

Most established vacuum chamber manufacturers use standard vacuum welding practices, which means that, whenever possible, welds are made on the inside of the chamber or as full-penetration welds to eliminate entrapment and virtual leaks.

Similarly, vacuum valve manufacturers eliminate the problem of virtual leaks by vacuum-furnace brazing any exposed joints. "The bodies of our valves are brazed at 1100 C," says Tony Semaan, a sales engineer at High Vacuum Apparatus, Hayward, CA. "It eliminates entrapment areas and minimizes body distortion."

3 Cost-Cutting. "Researchers who think they need to save money sometimes will have their vacuum valves and chambers fabricated in a small job shop," says Danielson. This is very risky, according to most manufacturers with whom we talked.

"Large nonvacuum manufacturing companies with substantial resources, such as Applied Materials, have been able to design and fabricate valves for their equipment that work well," says VAT's Freeman. "Other smaller companies have not been as successful."

Some of the problems you might run into by having your vacuum systems fabricated by an inexperienced company are poorly cleaned interior surfaces, that either take too long to pump down or contribute particulates to the system, and poorly welded joints and flanges, that contribute to virtual leaks.

"Established vacuum fabricators also have the ability to inspect welds with helium mass spectrometers and use residual gas analyzers to check for cleanliness," says Thomas Deany, vice president of marketing for NorCal Products, Yreka, CA.

4 Particulates. As the size of integrated circuits shrinks to submicron levels, the need for further reductions in particulate generation becomes imperative, says Semaan.

This is especially true in transfer valves, where particulates generated by the valve could fall onto a wafer being transferred. "We have to use valve components that generate very few particles," Semaan says.

The Modular Equipment Standards Committee, a division of SEMI, has established standards (SEMI STD 1796) for gate-type transfer valves to be used in cluster-type tools for fabricating thin films on semiconductor wafers.

These standards have provided manufacturers with the impetus to develop whole new valve systems. High Vacuum Apparatus, for example, has developed a valve/interface system that has been beta tested at more than 1 million cycles. The valve has fewer components, which increases its life and reliability.

5 Conductance. Conductance is defined as the throughput of gas through any conducting element, such as a valve, tube, or manifold.

"Selecting a valve that does not have enough conductance is not a common mistake," says Freeman. "More often the plumbing connected to it has been undersized."

Flow rates for the process must be calculated and the appropriate size plumbing and valves selected. "There are many software packages available that can be used to calculate flow," says Ken Kissane, director of marketing at Huntington Mechanical Laboratories, Mountain View, CA.

It should be remembered that conductance is predominantly a function of tubing and valve lengths. For similar sizes, an angle valve, which has a long flow length, will have less conductance than a thin gate valve.

"Most system designers choose valves with sufficient conductance, but a few will choose a valve that is too small, or has too low a conductance," says Danielson.

6 Design Assistance. While most users have relied on valve manufacturers for their expertise in designing and fabricating valves, many users have successfully designed their own conventional chambers. But the current trend is toward less conventional or off-the-shelf systems and toward more complex or custom-designed systems. "Over 50% of our business is in custom products and supported services," says Huntington's Kissane. "Funded scientists, in general, don't buy off-the-shelf systems."

This is forcing users to rely more on the custom fabricator's expertise. Huntington Labs has developed a software design program for UHV chambers based on an Autocad platform. You input parameters on the basic chamber plus the associated ports and flanges, and the software translates them into a format that provides 3-dimensional views for an examination of interferences, a complete set of drawings, and a quotation.

7 Life Cycles. Users also are concerned about valve life cycles, according to the manufacturers we interviewed. Improvements in valve design and materials has resulted in some valves that go more than [10.sup.6] cycles without failing. On a system cycling once aminute, a half-million cycles a year is possible. While valves with [10.sup.5] cycle lifetimes are more abundant, user demands are forcing steady improvements.

"The recent innovation of vulcanizing viton seals directly to a sealing plate, instead of using a separate Oring, improves UHV sealing in several ways," says Freeman. "Vulcanizing the seal results in very consistent vacuum characteristics from batch to batch. It also eliminates the risk of pulling the O-ring out of the groove, using an improper seal, or causing damage during seal replacement."

Improvements in the seal compounds, such as DuPont's Kalrez, have resulted in products that degas less than viton and have higher bakeability limits--from 200 C for viton to 300 C for Kalrez.

8 Sizing. One of the most common problems that even experienced vacuum fabricators encounter is in sizing a flange and not having it fit because the bolt pattern on the matching part is different, says Freeman.

"Up to 200-mm ID there are standards on metal knife-edge seals that most manufacturers adhere to," he says. "Over 250-mm there are no standards, and sizes change from one manufacturer to another. You also have to be careful because ASA-type flanges don't match the size by which they are described; a 6-in. ASA flange really fits a part with a 7/87-in. ID."

You also have to be careful when mounting thin gate valves mounting thin gate valves between chambers, says Freeman. "If there is a misalignment between the two chambers, the valve will distort to accommodate the misalignment. Gate valves are hollow, and a torque applied to the body will cause it to leak or jam," he says.

9 Materials. "Pressure, temperature, and corrosion resistance determine the materials needed for vacuum systems," says NorCal's Deany. "For most applications AISI-type 304 stainless steel is the most cost effective choice."

AISI 304 is an austenitic chromium nickel alloy particularly well suited where welded construction is required and where the finished product must resist most severe forms of corrosion and chemical attack.

"The desirable properties of 304 are improved by electropolishing," says Deany. "Electropolishing attacks the microscopic peaks of the surface more rapidly than the valleys, thereby maintaining critical dimensions while improving surface finish. Iron also is removed, which leaves a surface rich in chromium and nickel which have low vapor pressures.

"Other types of stainless steel, such as 304L, 316, or 316L, have lower carbon contents and more corrosion resistance and can be used for specialty applications."

Type 304L has better resistance to intergranular corrosion after welding. Type 316 is more resistant than 304 to hot and cold solutions of sulfuric and hydrochloric acids and alkaline chlorides. Type 316L is similar to 316 but is more resistant to intergranular corrosion following welding.

"Customers may choose to select aluminum alloy valves, such as those made from alloy 6061, because they are much less expensive," says VAT's Freeman. "However, they can't reach as good a base pressure, they're not as rugged, and they are supplied only with viton seals--they won't work with metal seals."

10 Structural Integrity. "Most vacuum designers know how to calculate the structural requirements of their chambers," says Huntington's Kissane.

But most will also overlook a decimal point sometime in their careers, according to Danielson. "When you pump down the system for the first time and hear that characteristic 'plink' indicating that something moved when it shouldn't have, then you know someone should have checked your calculations," he says.

Experienced chamber fabricators will generally find those errors. In situations where precise alignment is necessary, such as in the construction of analytical surface science housings, experienced fabricators also will be able to fabricate the product faster and less expensively than small shops.

"Most researchers usually know what their vacuum system can tolerate in terms of process gases and flows," says Freeman. Here too, though, you have to consider the worst case scenario and consider the effect of all process gases on the integrity of all component parts.

"Additionally, don't design and build a UHV chamber for a process that is going to be carried out at low vacuum pressures," says Deany. "It will be overdesigned and unnecessarily expensive."

Following these 10 checkpoints won't guarantee errorless vacuum designs. However, talking to experienced colleagues and choosing an experienced fabricator will minimize your problems.

Last edited by sage12; 04/03/2019 at 04:50 PM.
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Old 04/01/2019, 05:48 AM   #2
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Try one of these three mirrors. Hopefully one will work for you.
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Old 04/01/2019, 11:09 AM   #3
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I suppose calling your parents you internet service provider is technically correct, if a little cold.
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Old 04/01/2019, 11:45 AM   #4
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Why would your ISP block samp download? you should contact them
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