Vacuum: A High-Tech Commodity?

		At a Glance
	Increasingly, chip makers are viewing vacuum as a commodity, not unlike water or electricity. Pump suppliers have recognized this and are working to extend their service capabilities while developing new technologies that minimize cost-of-ownership.

Most semiconductor manufacturing processes require some level of vacuum or reduced pressure to operate. This can range from a pressure slightly below atmospheric pressure to what is known as 'ultra-high vacuum,' where pressures are measured in the 10^-9-10^-10 Torr range.

Low- to mid-levels of vacuum (a few milliTorr) usually are achieved by evacuating the process chamber with a 'dry' vacuum pump. For ultra-high vacuum, a dry pump is used in combination with a turbomolecular pump (turbopump) and/or a cryogenic pump (cryopump). Cryopumps are used primarily for applications such as sputtering, where the largest gas load to be pumped is water vapor (see sidebar, 'Vacuum Pumping Basics'). A large fab may have on the order of 400-500 dry pumps, 250 turbopumps and 100 or so cryopumps, depending on the equipment design.

Vacuum pump suppliers introduced a variety of innovations over the last several years, including new high throughput turbo pumps with pumping speeds up to 3000 liters/sec; advanced screw designs; pumps that run at higher temperatures to prevent condensation of particles; and pumps with special coatings to prevent internal corrosion.¹ These kinds of high-tech advancements will continue. But pump suppliers increasingly are finding their customers more focused on business-related issues, such as the degree of service, available support and cost-of-ownership of the pump. 'Nobody likes to hear it, but vacuum is getting close to being a commodity,' said Roland Hellmer, marketing manager at Pfeiffer Vacuum. 'What differentiates a company is your service -- how you stand behind your products -- and your know-how.'

Mark Perry, vice president of field operations at Ebara Technologies, added: 'We're expected to provide capital equipment level of support or better, anywhere in the world, 24 hours a day. Whether it's Oregon or Malaysia, we're no different than major equipment suppliers. We've got to respond, respond now, have the product there and up and running, and have local service and support capabilities.'

Fig 1 Although vacuum is increasingly thought of as a commodity (far-left bubble), pump suppliers are working to provide a broader range of services. (Source: Leybold Semiconductor Vacuum Solutions)

This trend is part of a general movement in the semiconductor equipment industry, where tool performance is largely taken for granted, especially in well-established technologies. Instead of performance, customers appear focused on service, support and such issues as uptime, throughput, cost of consumables, electricity consumption, warranties, spare-part supply, time-to-repair and overall equipment effectiveness (OEE).

Of course, vacuum is not really a commodity, since it is an integral part of the process equation. But just as tool suppliers are being asked to supply guaranteed process solutions, pump suppliers are now expected to provide guaranteed vacuum solutions. 'The overall involvement of pump suppliers is much more intensive than it has been in the past,' said Hellmer. 'Pump suppliers have to deliver not just hardware, but a solution. In a sense, we don't sell pumps anymore -- we sell our expertise.'

Fig 2 As part of the drive to reduce pump cost-of-ownership and facilitization costs, pump suppliers have developed new designs that can be located immediately next to the tool and use little or no water for cooling.

One example of the type of solutions pump suppliers are working to provide is the vacuum abatement process for copper that Leybold introduced in 1998. This solution consists of an inlet reactor, optimized DRYVAC system, cooled exhaust collector and a resin bed abatement device from CS Clean Systems. The inlet reactor removes elemental copper before it enters the dry pump, where it would plate on the pump's rotor, thus damaging the pump. The exhaust collector is collecting Cu(hfac)₂ which not only simplifies the abatement system but also can be returned to the manufacturer for recycling as a valuable resource for a new Cu precursor. 'This is an excellent example of how the scope of vacuum changed over the years,' said Dr. Hans-Jürgen Mundinger, vice president of innovations and solutions at Leybold. 'What was once just a vacuum pump became process compatibility and cost-of-ownership. Now our customers ask for Uninterrupted Vacuum Supply (UVS) and extended process compatibility. In the future, customer demands will be for total sub-fab solutions (minimizing environmental impact) and recycling of valuable gases.'

Fig. 1 illustrates how the role of pump suppliers is changing. The far-left bubble is what is considered a commodity, and the other bubbles show the other areas for which a pump supplier can be responsible.

To service the hundreds of pumps in a large fab, some IC manufacturers have established a contract with their pump supplier to provide a guaranteed uptime, which can be as little as 2.7 hours per month, which equates to about 99.5% uptime, according to Ebara's Perry. To achieve that, the supplier provides on-site personnel to handle any emergencies that arise.

New pump innovations

The trend to provide improved service, maintainability and cost-of-ownership also is reflected in new pump designs. One way some suppliers have been able to reduce operating cost is to cut the amount of nitrogen purging needed, shrink power consumption and eliminate or reduce water cooling. Hellmer said Pfeiffer's new Unidry pump, for example, has an annual operating cost of $620, compared to $2300 for claw designs, $2397 for Roots types and $2793 for screw pumps. Lowering consumption of nitrogen purge gas has a do uble benefit: not only does the user save on the cost of nitrogen, but it puts less of a load on the scrubber/gas abatement system located downstream from the pump.

Fig 3 The uniform temperature distribution, without cold spots, inside a Varian V1000 ICE turbo-molecular pump is shown. The inset chart shows the temperature over time measure inside the pump at spots 1 (high-pressure stages) and 2 (lower-pressure stages).

To save on tool footprint, pumps are being made smaller while delivering the same speed. Some suppliers also have integrated the pump controller electronics within the pump, eliminating the need for costly rack space. Another important trend is toward increased pump networking and remote diagnostic capabilities, designed to allow potential pump failures or problems to be detected in their earliest stages. Such capabilities allow suppliers to 'maximize process equipment uptime and, by reducing the need for on-site interventions, cut the overall service cost,' said Varian's Marco Perini.

Pump suppliers also have developed new dry pumps that can be placed very close to the tool, instead of the sub-fab, which eliminates long forelines. BOC Edwards' new IPX100, for example, is said to increase productivity, reliability and pumping speed while reducing facilitization costs, commissioning time and cost, and sub-fab footprint. Fig. 2 shows how the IPX100 (identified as Pump 1) compares with other pumps in terms of cost-of-ownership and facilitization costs.

In the semiconductor industry, another important feature of a vacuum pump is its ability to handle corrosive and/or particle-laden byproducts found in processes such as low-pressure chemical vapor deposition (LPCVD), tungsten CVD, phosphorus silicate glass (PSG) and borophosphorus silicate glass (BPSG) deposition, metal etch and even ion implant. Each supplier has addressed this in different ways. For example, Varian's ICE turbo pumps line is designed specifically for ion implanters, CVDs and etchers (hence the ICE name). It features a special protection system against corrosives based on the 'Armorr coating' and the 'Dynamic Seal,' while the 'Dust Centrifugator,' another ICE feature, allows reliable pumping in presence of highly concentrated particles and condensed byproducts. The uniform temperature distribution around 110°C, without cold spots, minimizes byproducts condensation (Fig. 3).

New screw pumps also are well equipped to handle particle-laden processes. Ebara's new AAS Series of dry vacuum pumps, for example, features a two-stage screw for reduced power consumption, a patented zero theoretical error screw and NiResist, a nickel-rich iron alloy that offers corrosion resistance for the screw and pump casing.

These are just a few examples, of course. The wise pump purchaser will want to gather data from each of the many suppliers, including Alcatel, APD Cryogenics, BOC Edwards, Busch, CTI Cryogenics, Danielson, Ebara, Kashiyama, Leybold, Mitsubishi, Osaka Vacuum, Pfeiffer (formerly Balzers), Seiko Seiki, Stokes, Ulvac and Varian. For detailed information on the many different pumps offered by these companies try the American Vacuum Society's (AVS) on-line buyer's directory at www.vacuum.org. The Association of Vacuum Equipment Manufacturers (AVEM) also has detailed information on vacuum pumps at its Web site: www.avem.org.

References

1. P. Singer, 'Vacuum Pumps Now Run Hotter and Better,' Semiconductor International, Oct. 1998 , pg. 62.

Suggested additional reading:

1. J. O'Hanlon, A User's Guide to Vacuum Technology, John Wiley & Sons.

Vacuum Pumping Basics

Three basic pump types are used in the semiconductor industry: dry pumps, turbomolecular pumps (turbopumps) and cyrogenic pumps (cryopumps). Dry pumps generally are mid-range pumps, in that they can pump a chamber from atmospheric pressure (760 Torr) to about 10-5 Torr. Pump suppliers report that dry pumps, introduced about 10 years ago, have almost exclusively replaced oil-sealed rotary vane pumps, which once were the vacuum workhorses of the semiconductor industry. This switch occurred because the latter require frequent oil changes and can cause contamination due to oil backstreaming. Turbopumps and cyropumps are high- to ultra-high vacuum pumps, able to pump to much lower pressures -- about 10-9-10-10 Torr. Pumping beyond that, to what some call extremely high vacuum (XHV), is impractical. Above 10-9, the pump actually is pumping light gases such as hydrogen that penetrate through the wall of the process chamber. Dry vacuum pumps work by trapping and moving small pockets of gases from the inlet to the outlet, often in several stages, with each stage successively compressing the gas pocket. Compression can be achieved in several ways; the most common is with intermeshing lobes (i.e., 'blowers'), scrolls or screws, or with a hook-and-claw design. By comparison, the pumping action in a turbopump is done at a molecular level by bouncing gas molecules off rotors -- angled surfaces moving at high speeds -- and fixed stators. In cryopumps, molecules are 'pumped' by trapping or adsorbing them on extremely cold surfaces. Most cryopumps also rely on a cooled charcoal array to trap gases with high partial pressures (i.e., argon, helium and hydrogen). There has been a prominent shift from regular turbomolecular pumps to so-called 'compound' pumps that combine a molecular drag type pump with a regular turbopump. The molecular drag pump is similar to the turbopump in that it involves bouncing gas molecules off a rotating surface; but instead of rotors and stators, it employs a rotating drum or disc. The main advantage of these pumps is that they can exhaust at relatively high pressures. In many semiconductor processes, this means a smaller and less expensive backing pump can be used, and the diameter of the forelines can be smaller. Another critical part of turbopump design is the type of bearing system used. Magnetically levitated (mag-lev) pumps are now the preferred option, over ceramic or steel bearings, due to longer life and because the bearing design allows the pump to be heated.