Advances in Vacuum Pumping

Working behind the scenes in many semiconductor manufacturing processes — physical and chemical vapor deposition, etch and ion implantation, for example — is a vacuum pumping system.

Wafers are typically brought into a load-lock chamber that is pumped down with one pump, moved into a transfer chamber that is pumped with a second pump and then into the process chamber where a third pump is employed. Each pump has different requirements in terms of the number of cycles, the gas load (volume and type of gases to be pumped) and, in the process chamber, the type of byproducts generated.

Three different types of vacuum pumps are generally used: mechanical pumps, turbomolecular pumps (turbopumps) and cryogenic pumps (cryopumps).

Mechanical pumps (also called roughing pumps) are used to pump from atmospheric pressure to about 10^-3 Torr. Although they can vary greatly in design, ranging from intermeshing hook-and-claw to Roots to screw products, they all work in basically the same way to mechanically "push" gases through the pump (hence the name). If higher pumping speeds are required, mechanical pumps are combined with another pump, commonly called a Roots blower. The industry has standardized on "oil-free" designs for mechanical pumps to eliminate the danger of oil backstreaming into the process chamber.

Turbopumps and cryopumps are used to achieve higher levels of vacuum, going from 10^-2-10^-3 to ~10^-9 or 10^-10 Torr. Pressures below 10^-10 Torr can only be achieved with very high-speed pumping and special attention to chamber materials and seals; these lower pressures have not yet been required for current production processes. These types of pumps work at the molecular level — cryopumps trap molecules by cryoadsorption on very cold surfaces and in a charcoal trap; turbopumps use a series of blades spinning at very high speeds to "bat" molecules through the pump. Turbopumps and cryopumps are always backed with a mechanical pump because they can only operate under vacuum, after mechanical pumps have finished evacuating the process chamber.

1. Information on the health of vacuum pumps is a key element of an e-diagnostic program. The ultimate goal is to detect failures before they occur, and that the parts are available to fix the problem. (Source: BOC Edwards)

Several interesting trends have emerged in the way vacuum pumps are used. They have become a focal point in the move to e-diagnostics, where process tools and components are remotely monitored. Due to the often harsh conditions in which they must operate, it's not uncommon for vacuum pumps to require relatively high maintenance, including occasional replacement. Careful monitoring is required to know when such maintenance is needed and to ideally catch failures before they occur.

Another key trend is the integration of mechanical pumps into the process tool, resulting in what's being called a point-of-use or on-tool pump. Traditionally, mechanical pumps are housed not in the tool but in another part of the fab, usually the sub-fab or basement, and connected to the tool through a series of pipes called the foreline. The advantage of integrating them into the tool is that these pipes and associated conductances are eliminated, allowing smaller pumps to be used to achieve the required pumping speeds, while gaining cost reductions associated with reduced power consumption and a reduction in the sub-fab floor space required.

2. One of the greatest inefficiencies of traditional pumping strategy, where pumps are located in the sub-fab or basement, is that conductances dramatically reduce the pumping speed at the tool. (Source: Pfeiffer Vacuum)

This move to point-of-use, on-tool pumps is largely driven by the move from 200 to 300 mm wafer processing, where the inefficiences of locating a pump in the basement would be even more severe, because 300 mm processes require higher pumping speeds. Another pumping challenge associated with 300 mm is that most 300 mm systems are designed to process wafers one (perhaps two) at a time through the load lock. This is a significant departure from 200 mm systems, where cassettes of 25 wafers were typically pumped down in the load lock and then transferred individually to the process chamber. This means the load-lock pump and valving must deal with a much higher number of pumping cycles in a given timeframe.

In a broader sense, changes in process technologies have also had an impact on pump requirements. Processes such as metal etch and nitride CVD have always been difficult applications due to the high volumes of particles and nasty byproducts produced. Newer processes, such as high-density plasma CVD and etch, operate at higher levels of vacuum and have higher gas flows, which has resulted in a demand for turbopumps with pumping speeds upwards of 2000 L/sec, placed very near the wafer (in some designs the wafer practically sits on top of the turbopump). Vacuum pumping technology has also had to adapt to a variety of new materials such as copper (i.e., copper seed layers are deposited by PVD), and the use of new gases such as xenon in ion implantation (xenon is used to neutralize the beam charge).

Figure 1 illustrates vacuum pump monitoring as part of an e-diagnostics program. BOC Edwards (Wilmington, Mass.), which is also involved with gas and chemical supply, provided the illustration, showing how complex e-diagnostics can be, and the variety of sources from which information is generated.

One goal of pump e-diagnostics is to detect potential failures before they occur. "We never want a process to stop because a vacuum pump has failed," said Kevin Hill of Applied Materials (Santa Clara, Calif.). "That requires monitoring and diagnostics, and the integration of those diagnostics into a Web-based format with integrated paging and notification techniques so that the appropriate service technician is notified."

Remote monitoring is certainly not a new concept for pump suppliers. Helix Technology (Mansfield, Mass.), for example, has offered such capabilities on its CTI cryopumps for years. In fact, the company claims its GOLDLink support network is the most broadly deployed e-diagnostic solution in the entire semiconductor manufacturing equipment world. With more than 35 sites in more than 10 countries, the GOLDLink support system is currently improving factory operations on more than 2000 vacuum nodes on 300 process tools. Countries supported include Korea, Singapore, Japan, Ireland, Scotland, France, the United States, Taiwan, Germany and the Netherlands.

"With GOLDLink, we can talk to the pumps over the Internet, and by analyzing the data we can detect a problem that's coming up before it causes a problem in the fab," said Gary Ash of Helix. "Then we call the customer and say we think there's an issue and we have a repair part on the way, before it ever gets the system shut down."

"The goal of the GOLDLink effort is to monitor more than just cryopumps," added Helix's Shaun Wilson. "In fact, monitoring the status and the health of the roughing pumps, vacuum gauges and mass flow controllers . . . is all possible using GOLDLink e-diagnostics. GOLDLink can monitor entire subsystems with a tool."

3. Applied Materials developed specifications for an on-tool pump and worked with various pump suppliers to develop the iPUP (integrated point-of-use pump), which was launched in July. (Source: Applied Materials)

In practice, e-diagnostics requires that the appropriate sensors be built in the tool, as well as capabilities to communication using the latest protocol, such as DeviceNet. Most pump suppliers are already able to monitor key pump parameters, and have (or will soon have) DeviceNet communication capabilities. Varian Vacuum Products (Lexington, Mass.), for example, has developed the MoniTorr system that, according to Randy Pfenninger, measures a number of the pump parameters like vibration, temperature and power draw. "As it measures changes in those under various process conditions, it can predict within some level of confidence when the pump is going to fail, or if the pump is near end-of-life condition."

On the communication side, Varian has been working with Brooks Automation (Chelmsford, Mass.) to develop a solution using DeviceNet. "We have integrated the pumps with the vacuum controllers and are now integrating those device drivers into our system software," said Krzystof Majczak of Brooks Automation. "Putting the turbopumps as well as the other pumps on DeviceNet is just a step away from the next consecutive solution to the problem, which is hooking up to the Internet and opening up the window of opportunity to diagnose the systems, record the data . . . It's simply a new generation of products."

What could delay the acceptance of such e-diagnostic tools is how comfortable semiconductor companies feel about the security provided. There are also concerns about cost and complexity. "There is a reluctance by some customers to allow that level of communication into their site," said Nigel Gibbins of BOC Edwards. "They're worried about the security of the data and their process recipes escaping. The take-up will depend on how open the customers are and how much they trust the level of security you can put in place to protect their data."

Dan Mrotek of Leybold Vacuum Products (Export, Pa.) said that many end users feel like they are getting too much diagnostics pump information, and would like to push the responsibility of interpreting that data back on the OEM. The OEM, in turn, may only have the ability to monitor three or four parameters, such as pump temperature, exhaust pressure and motor current draw. "It is up to the pump manufacturers to develop intelligent algorithms that can predict pump health with a minimum of information," he said.

Considering that large fabs have upwards of 500 pumps, what end users really want from pump suppliers is to know when those pumps need to be serviced. "They want to know how many they need to pull out next week, and the week after that, so they can coordinate their pump service program with their scheduled tool downtime and set up a maintenance schedule with minimal impact on fab production," Mrotek said. "And, of course, there has to be a paging system, so if anything does go wrong on a pump the system pages the serviceman's beeper and he's in there. He's not sitting there looking at the Internet."

In the end, however, no remote capability will ever replace the need to have a service technician on site to follow up vacuum operating parameters and prevent catastrophic failure from occurring. "Today a large number of failures are still due to leaky vacuum lines," said Alcatel's Frederic della Faille. "Leak testing should be performed each time a line is being opened; on top of that periodic line test is required as seals degrade over time (process effluent dependent). The number of required sensors to fully follow up the product will not provide a cost-effective solution. E-diagnostics will allow experts to analyze failure mode or to understand unexpected parameter behavior, putting intelligence where needed and when needed."

The move from 200 to 300 mm wafer processing has, of course, had a major impact on how semiconductor manufacturing equipment is designed, and that impact has carried over to vacuum pumping technology. "The increased volume of the chamber and the increased gas flow of 300 mm systems was going to require a bigger remote dry pump to handle the processes," said Applied Materials' Hill. "When you normally run a process with an 80 m³/hr pump, and you have to upgrade to a 500 m³/hr pump or even higher to a 1000 m³/hr and beyond pump, it not only becomes a significant cost obstacle, but it drives an increase in sub-fab space."

About four years ago, Applied Materials started developing an alternative strategy. Instead of locating the pumps in the sub-fab and connecting them with long forelines that can measure 5-20 m, locate them close to the tool, or within the tool. Due to conductances, long forelines dramatically reduce pumping speed at the tool (Fig. 2). "The thought process was, 'Let's eliminate those long lines and then, instead of going to a bigger pump, let's use a smaller pump for the 300 mm systems,'" Hill said.

The move to single-wafer processing on 300 mm tools has also been a driver for point-of-use pump technology, said Mike Percy of BOC Edwards. "The first bridging tools, and now the 300 mm tools, mostly have single-wafer load locks taking one or two wafers at a time. Those are therefore much smaller, typically around 5 L. When you're down to that sort of volume, it makes real sense to integrate the load-lock pump into the tool. By eliminating the foreline down to the basement, you can achieve shorter roughing times."

On-tool pumps required a significant rethinking in how vacuum pumps were designed. "Typically, pumps are big and noisy and have a lot of vibration," Hill said. Applied Materials developed a set of new pump specifications that required, for example, that the vibration had to be no bigger than what a turbopump has, and that the noise generated had to be lower than the ambient noise level. "Most importantly, it had to have a footprint that we could integrate right into the tool or place it in the proximity of the tool in a local configuration," he said.

"Today, OEMs are requesting smaller point-of-use pumps designed to be installed directly on the tool without the common vacuum foreline plumbing currently used," said Marcus Snow of Busch Semiconductor. "Many pump manufacturers have developed smaller pumps to facilitate this emerging requirement. These pumps are available in the multi-stage compression design as well as the single-stage, dry screwpump design."

In July, Applied Materials also jumped into the vacuum pump business by formally launching what it calls the iPUP (integrated point-of-use pump), for a variety of process equipment pumping applications (Fig. 3). The company reports that the iPUP enables a dramatic reduction in fab vacuum cost by cutting power consumption more than 50% and reducing facility costs in new fabs by up to $100,000 per system over traditional vacuum pumps. All iPUP service and maintenance is handled by Applied Materials' Customer Productivity Support organization.

Applied has standardized on the iPUP A100 (manufactured by Alcatel), and is planning to offer the iPUP T100 (manufactured by Toyota Industries) and iPUP E100 (manufactured by BOC Edwards) as additional options. Each offers a slightly different variation but has the same overall dimensions, weight and pumping speed, making them fully interchangeable. The clean-duty version of these pumps, designated the L version, is designed for load locks, transfer chambers, PVD and metrology inspection systems, and P versions are designed for process applications.

Also in July, Applied Materials introduced the Pegasys plasma exhaust gas abatement system, designed to work concurrently with the iPUP. "It was designed to be put underneath the floor or tucked away in a small footprint location, and it goes between the chamber and the vacuum pump. We call it the pump that abates because now you have a vacuum pump integrated with an abatement system," Hill said. The Pegasys is applicable to the dielectric oxide etch processes, and the company is working to adopt it to other processes as well. Alcatel also reports it is working on a point-of-use scrubber.

This is an important development because many processes have traditionally required the use of heated exhaust gas lines to minimize the condensation of byproducts within the line, and sophisticated gas scrubbing/abatement systems, usually located in the sub-fab near the pumps. (For a good overview of such challenges in a PECVD process, see "Vacuum Pump Change Increases PECVD Uptime").

"The semiconductor industry requirements for vacuum pump designs have been evolving. To facilitate this, vacuum pump manufacturers are continuously working on new developments that will stay ahead of pending process changes and industry requirements," said Busch's Snow.

To address changing requirements, vacuum pumps of all types are designed to be more flexible. "Cryopumps are becoming more tunable to the process," said Marc Jalbert of Helix. "Where we're headed is to make cryopumps where all the performance parameters are decoupled and independently controlled," added Helix's Wilson.

In the end, however, vacuum supplied to a process is still viewed as a basic commodity, not unlike nitrogen or water. The optimal pumping technology or package, therefore, is typically viewed largely in terms of cost, which some jokingly refer to as "suck per buck." "This can only be achieved through innovative, sometimes revolutionary, technologies and products that at the same time provide the highest level of reliability," said Roland Hellmer of Pfeiffer Vacuum (Nashua, N.H.).

Alcatel www.alcatelvacuum.com
APD Cryogenics www.apdcryogenics.com
Applied Materials www.appliedmaterials.com
BOC Edwards www.edwards.boc.com
Busch www.buschsvg.com
Danielson Vacuum www.danvac.com
Ebara www.ebara.co.jp
Helix Technology www.ctihelix.com
Kashiyama www.kashiyama.co.jp
Kurt J. Lesker www.lesker.com
Leybold Vacuum www.leyboldvacuum.com
Mass-Vac www.massvac.com
Osaka Vacuum www.osakavacuum.com
Pfeiffer Vacuum www.pfeiffer-vacuum.com
Seiko Seiki www.seiko-seiki.com
Stokes www.stokesvacuum.com
Toyota Industries www.toyota-industries.com
Ulvac www.ulvac.com
Varian Vacuum Products www.evarian.com
Welch Vacuum Products www.welchvacuum.com