Vacuum Pumps Now Run Hotter and Better

With the push to smaller geometries, larger wafer sizes, new materials and increased tool productivity, the semiconductor industry has presented an unprecedented challenge to vacuum equipment suppliers. "The semiconductor industry is experiencing an evolution of vacuum requirements, which in any other industry, might be considered a vacuum revolution," said John Little of Alcatel. "The vacuum demands of the semiconductor industry very closely track the SIA roadmap. With each new smaller critical dimension, new metal or dielectric process or increased substrate diameter, the vacuum systems must meet increasingly challenging performance, and they must do so at ever decreasing price to the equipment manufacturer."

The push to smaller geometries means, among other things, increased use of high-density plasma sources for chemical vapor deposition (CVD) and etch. This means lower pressures (higher levels of vacuum) and higher gas loads (high-density sources work best at low pressures, and the best film quality is achieved when large amounts of reactive gases are impinging on the wafer). Smaller geometries also mean greater attention to particle generation, of which vacuum pumps are one potential source due to a phenomenon known as "backstreaming."

The semiconductor industry's push to 300 mm wafer processing has led to larger chamber sizes and consequently a need for higher pumping speeds, since pumpdown times must be kept short.

Vacuum pumps are being asked to handle some new materials also, as the industry works to lower interconnect capacitances and resistances through the use of low dielectric constant materials and copper. If these materials are to be deposited by CVD (which is in question), then it will be necessary for vac-uum pumps to handle some new and unusual byproducts. The pumping of copper CVD byproducts has proven troublesome already, since it is possible for copper to plate out inside the pump.

Lead Photo: New turbopumps are designed to run at higher temperatures, helping to minimize condensation inside the pump. This is made possible through new high-temperature aluminum alloys. (Source: BOC Edwards)

They are asking for smaller pump packages also, including pump control electronics, as well as improved pump monitoring, so they can be alerted to potential failures before they occur. "A clear trend is the process monitoring at the pump in order to predict downtime and/or when preventive maintenance is needed," said Balzer's Roland Helmer. "Overall, the contents of electronic components, data storage and communication capability accompanied with software issues are increasing in vacuum pumps. In order to arrive at meaningful MTTF or MTBF numbers, this is the only way to go." Figure 1 shows various components of Pfeiffer Vacuum's turbomolecular drag pump with an integrated drive system. The pump and drive assembly is in the middle, and from the upper left and clockwise are: the power supply with integrated display control, a power supply for in-rack or 19 in. front panel mounting (both shown), display control unit, water cooling, air cooling, heating, relay box, venting valve, backing pump control, purge gas valve, vacuum gauges and PC/serial interface.

Some pump basics

Fig. 1. The complexity of a complete turbopumping system is illustrated well with Pfeiffer Vacuum's turbopump with an integrated drive system.

Turbopumps and cyropumps are high- to ultra-high vacuum pumps in that they can pump to much lower pressures, to about 10^-9-10^-10 Torr. Pumping beyond that, to what some call extremely high vacuum (XHV), is impractical. Above 10^-9 Torr, the pump actually is pumping light gases such as hydrogen that penetrate through the wall of the process chamber.

The two most common configurations in the semiconductor industry are as follows. A dry pump is used alone, or if higher levels of vacuum are required, a dry pump is used in conjunction with a turbopump. Cryopumps are used primarily for applications such as sputtering, where the largest gas load to be pumped is water vapor.

Fig. 2. Alcatel's new ACP 20 dry roughing pump.

In alphabetical order, suppliers of dry pumps include Alcatel, BOC Edwards, Busch, Ebara, Kashiyama, Leybold, Pfeiffer (formerly Balzers), Stokes and Varian. Suppliers of turbopumps include Alcatel, BOC Edwards (which markets the Seiko Seiki line of pumps), Busch (which markets the Mitsubishi line of pumps), Leybold, Osaka Vacuum, Pfeiffer and Varian. Makers of cryopumps include APD Cryogenics, CTI Cryogenics and Leybold. For more detailed information on the different pumps offered by these companies, check out their web sites listed at the end of this article; visit our web site at , or try out the AVS (American Vacuum Society) 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.

Dry vacuum pumps (Fig. 2) 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 different ways, the most common ones being 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 the 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).

The total pressure in the chamber is a combination of the partial pressures of each type of gas contained in the chamber. The pump's effectiveness at removing a gas is dependent upon the partial pressure of that gas; it takes longer to pump "lighter" gases with high partial pressures - such as helium, hydrogen, oxygen, nitrogen and argon - than it does "heavier" compound gases (i.e., ClF₃) to achieve the same pressure.

Fig. 3. BOC Edwards' iH dry pumps are designed for harsh applications.

There has been a prominent shift from regular turbomolecular pumps to so-called "compound" pumps that combine a molecular drag 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 that 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 option allows the pump to be heated. "The industry has standardized on magnetic bearings, because that allows you to do rotor heating," said Mike Percy of BOC Edwards. "Ceramic bearings and conventional steel bearings were limited in that if you tried to run the pump hot, you'd just get early bearing failures."

"Seven years ago, turbopumps, if used at all in dry etch processes, were almost exclusively ball-bearing type pumps, and very often their pumping speed was in the 400 liter per second range," Little said. "Today, there are virtually no ball-bearing pumps used in contemporary etch applications, because ball-bearings need to be replaced during PMs (preventive maintenance), they can represent unscheduled downtime risk and also present a possible source of particle generation or bearing lubricant backstreaming contamination."

Another interesting technology is called a cryoturbo or "turbo chiller," where a cryogenic water pump is joined with a turbomolecular pump. Here, the superiority of the cyropump in pumping water vapor is combined with the turbopump's higher pumping speeds for lighter gases. CTI-Cryogenics, for example, recently introduced the ThinLine waterpump systems, designed for turbopumped applications such as loadlocks and process or transfer chambers, where fast pumpdowns are required. CTI claims the addition of a ThinLine waterpump can increase pumping performance by more than 5X while decreasing pumpdown times by as much as 70%. CTI also just introduced an On-Board TurboPlus vacuum pumping system that incorporates the waterpump and (Fig. 4)turbo in a single package.

Fig. 4. CTI's new OnBoard TurboPlus system.

Another option is the AquaTrap offered by APD Cryogenics, which is said to increase water pumping by a factor of up to 10 when combined with a turbopump. The AquaTrap features a low profile, compact refrigerator body; refrigeration achieved right at the pump eliminates cumbersome cold lines. The in-chamber design of an AquaTrap can be installed with the cold plate inserted directly into the vacuum chamber.

General considerations

Today, about 60% of processes used in the semiconductor manufacturing industry employ some level of vacuum. These include etch (metal, oxide and polysilicon), chemical vapor deposition (CVD), ion implant, sputtering and resist ashing. Each of these applications has a fairly unique set of vacuum requirements, governed by pumping speeds and pressures required for various gases used, the types and amounts of process byproducts that must be handled and the size of the process chamber. Gas loads vary from a few sccm (standard "cc"s/min) in ion implantation systems to hundreds of sccm in CVD and etch systems. Pumps also are used in analytical instrumentation such as scanning electron microscopes as well as for crystal growers.

A fully loaded cluster tool can require as many as seven cryopumps, two turbopumps and four mecahnical pumps to evacuate process chambers, the wafer handler, transport module and loadlocks. Most cryopumps and turbopumps are designed into the system by equipment suppliers, whereas most dry pumps are purchased by the end user. It is not unusual in a large fab to have 300 or more dry pumps in a single facility. Usually, they are located in the "basement" of a fab (or in a parallel corridor) and connected to the process tool through piping.

Fig. 5. The BOC Edwards line of turbopumps.

The main considerations when selecting a pump must go far beyond such considerations as pumping speed and pressure to include: lifetime and reliability (warranty), safety, COO (including power consumption, costs of water cooling, nitrogen purging, size, etc.), monitoring and control, networking capabilities, space, vibration, spare parts availability and particle generation. Wilson said that it is also important to consider the type of exhaust gas treatment system to be connected to the pump and if the supplier offers an integrated pump/exhaust treatment package.

"Increased pumping performance, small package size, particle reduction, high reliability and, of course, price are the critical factors which pump manufacturers must address as we move in to the future," Little said. "Pump users are very interested in reducing the size of pump packages while increasing pumping speed in specific process regimes. Pump users have become sophisticated and have identified the vacuum issues affecting their business, and they focus on improvement. In fact, the pump user wants the pump to be optimized to a particular pump process gas, which is most efficiently pumped between some pressure range, usually in the millitorr range, without any contribution to particles or system downtime."

Indeed, some pump suppliers have gone so far as to design a line of pumps for specific processes. Leybold Vacuum (Export, Pa.) for example, has different lines of turbopumps for etch, CVD and ion implant. "We have come up with a way of optimizing the turbo performance based on blade angle and drag stages in the pump to actually pump gases differently for each one of those applications," Mrotek said. "We change how the rotor is milled based on the application." The pumps also have different foreline and inlet pressure specifications. More specifically, the company's MAG 1600 I MAG 2000 C/CT turbopumps (Fig. 6) have a magnetic bearing active along five axes. The rotor is suspended along two orthogonal axes in each of two radial planes and completely in the axial direction by means of electromagnets. The bearing's design makes it possible to implement two different rotor designs in which all important components, the connection geometry and the bearing control system are identical. Thus, the turbomolecular pump is available both with the advanced rotor, specially developed with the high throughput required for many applications, a fore-vacuum resistance of 1 Torr and with a compound rotor (turbine blades and threading in combination) suitable for higher fore vacuum pressures.

Fig. 6. Leybold Vacuum's M2000 turbopump.

"Nowadays turbomolecular pumps manufacturers can count on significant technological improvements, including high strength aerospace alloys, high-precision ceramic ball bearings and ultra-low vapor pressure lubricants," said Giuseppe Signorelli, head of R&D for Varian Vacuum (Leini, Italy). "These, along with advanced CAD-CAM systems, can be utilized to design sophisticated vacuum pumping solutions." Varian recently introduced a line of turbopumps, ranging from 300 to 2000 I/s, called ICE (Ion Implantation, CVD and Etching). The ICE pumps, designed for 120,000 hours MTTF, are protected by a patented coating and spiral seal and come in a system with a set of built-in accessories mastered by an "electrical shock- proof"control.

"Technology can also assist the after-sales support to approach 100% uptime," Signorelli said. "It is now possible, by monitoring continuously the vibrations and all the significant pump parameters, to have a picture of the bearings condition and replace the pump at the next system scheduled maintenance, before it fails."

Handling process byproducts

Process byproducts, especially in etch and CVD, can include fairly large amounts of solid, liquid and/or corrosive materials that are carried in the gas stream. These also can be near-solids or near-liquids that condense inside the pump during compression. This is by far what usually leads to pump failure and/or process contamination. Some of the newest pumps on the market are designed to reduce that problem by allowing the pump to run hotter. For example, Alcatel just introduced a new dry pump line, the Series Two, that according to Bertrand Seigeot, product manager for dry pumps at Alcatel, offer multi-processes capacity with a temperature management system allowing exhaust temperature up to 250°C. The pumps also offer a reduced footprint with a possibility to configure utilities either on the rear side or top of the unit and a new monitoring system including a smart hand-held remote control able to store and download running parameters in the pump . Seigeot said the new series offers "lower cost of ownership with mean time between maintenance extended between two and four years depending on application, lower nitrogen consumption, lower water cooling consumption, fewer and less expensive parts and lower labor cost for maintenance and rebuild."

BOC Edwards also has introduced turbopumps and drypumps (Figs 3 and 5) designed to run at a higher temperature. "We've introduced a new line of pumps that use a different type of aluminum alloy for the rotor that allows it to operate at a considerably higher temperature than our previous range of pumps," Percy said. "That is going to give you more effective minimization of byproduct condensation. That's really important for minimizing particle counts in the chamber." Percy said the new pumps can run as hot as 150°C, where the older designs probably should not be run over 100-110°C. BOC Edwards adopted a similar philosophy in its newly introduced dry pump iH series (the "H" stands for harsh), which also is able to run at a higher temperature compared to previous models, "preventing by-product condensation within the pump, often the root cause of mechanism failure in drypumps," according to company literature. Three pump models are available: the iH80, the iH600 and the iH100 with pump speeds from 60 to 590 cfm. The company also offers an iQ and iL series of drypumps, the iQ with enhanced monitoring and control and the iL for light duty applications.

Other pump suppliers have taken different approaches to handling difficult byproducts such as nitride CVD, which can dump the equivalent of "six pounds of sand a day" down the inlet of the pump, according to Woody Farrow, general manager of Kashiyama in the United States. "Since the introduction to the USA market seven years ago, Kashiyama (pumps) have always incorporated corrosive resistant materials, operated at a high operating temperature and been able to be interfaced with an external computer," Farrow said. Kashiyama also recently introduced a lower cost HC series of dry pumps for use on ashers, ion implanters and load locks.

Another pump designed to handle difficult semiconductor processes is the Stokes "Stealth" pump, which is said to produce more pumping capacities in a smaller size than other dry pumps. The "Stealth" Pump uses a clamshell design with a water jacket for cooling and a special hermetic design that places the rotor inside a buffered vacuum space, eliminating the need for atmospheric seals in the motor.

Conclusion

To meet the increasing needs of the semiconductor industry, vacuum pump suppliers have made a variety of new advances aimed at improving pump reliability and performance while reducing particles and overall cost of ownership. Some new pumps are designed to run hotter for example, through new designs and materials. This reduces condensation inside the pump, helping to minimize particle generation, improve pump lifetime and reduce the need for nitrogen purge gas. Many new pumps also have more advanced controllers and monitors, enabling faster troubleshooting and maintenance, earlier failure warnings and easier integration. Some pumps now are custom-designed for specific semiconductor manufacturing processes such as etch, CVD (chemical vapor deposition) or ion implant.