Converting oil-sealed pumps to dry pumps on older process tools

Many process tools built in the 1990s and 2000s left the factory with oil-sealed rotary vane pumps as roughing and backing pumps. They are simple, cheap to buy and easy to service, and plenty of them still run. On an etcher, a CVD system or a load-locked PVD tool, though, an oil-sealed pump is also a source of hydrocarbon contamination, a consumer of expensive oil and a frequent cause of unplanned downtime.

A dry pump conversion replaces it with a pump that has no oil in the vacuum path. This post covers when the conversion is worth doing, how to choose and size the replacement, and what else on the tool and in your facility has to change.

Why labs move away from oil-sealed pumps

The case for a dry pump is usually built from several smaller problems rather than one big one:

• Backstreaming. Oil vapour from a rotary vane pump can migrate up the foreline into the process chamber, especially when the pump runs at low pressure for long periods or when a valve sequence lets the foreline sit under vacuum with no gas flow. Traps and foreline purges reduce this but do not remove it.
• Oil degradation. Corrosive and reactive process gases, such as chlorine, boron trichloride, fluorine compounds and ammonia, attack hydrocarbon oils. The oil darkens, thickens and turns acidic, and the pump's internal parts corrode. Oil change intervals on a busy etcher can drop to weeks.
• Oxygen service. Pumping high oxygen concentrations through a hydrocarbon-oil pump is a fire risk. Tools running oxygen plasmas or ashing processes are typically fitted with perfluoropolyether (PFPE) oil, which is inert but costly, and whose disposal needs care.
• Waste and handling. Contaminated oil is hazardous waste. Every oil change means draining, collecting, labelling and disposal, plus exposure risk for the technician.
• Maintenance labour. Oil checks, filter changes and oil mist filter replacements add up across a cleanroom with many pumps.

A dry pump removes the oil from the vacuum path entirely. It does not remove maintenance, but it changes it from frequent small jobs to scheduled overhauls at longer intervals.

Types of dry pump and where they fit

"Dry pump" covers several mechanisms, and they are not interchangeable:

• Scroll pumps are compact and quiet, and work well on clean applications such as load locks, SEM columns, metrology tools and evaporators without reactive gases. They are generally not suited to corrosive gases or particle-laden exhaust, because the tip seals wear quickly.
• Multi-stage roots and claw pumps are the usual choice for process chambers. They tolerate corrosive gases and some particles, and many models have nitrogen purge and corrosion-resistant coatings for harsh chemistries.
• Screw pumps handle high gas loads and particles well, and are common on CVD and furnace exhausts where by-products condense.

For heavy deposition by-products, such as the ammonium chloride produced by LPCVD silicon nitride or the residues from some metal etch processes, look for pumps that run hot enough to keep by-products in the vapour phase, combined with a heated foreline where needed.

Sizing the replacement

The most common mistake is matching the nameplate pumping speed of the old pump and stopping there. Pumping speed curves differ between pump types, and what matters is speed at your operating pressure, not peak speed.

To size a dry pump properly, collect:

• Process pressure range and total gas flow for the heaviest recipe, including purge and dilution flows.
• Chamber volume and target pump-down time, if the pump also roughs the chamber.
• Whether the pump backs a turbomolecular pump, and the turbo's maximum allowable foreline pressure.
• Gas chemistries and by-products, so the pump maker can confirm materials, coatings and purge.
• Foreline length and diameter. A long, narrow foreline can cut effective speed far more than the pump choice. Conversion is a good moment to shorten or enlarge it.

With these, the pump supplier's application team or a service engineer can check the pump curve against your conditions. For most single-wafer research tools, a dry pump of similar nominal size to the old rotary vane pump works, but the check is worth doing for CVD and high-flow etch processes.

What changes on the tool

A dry pump is not always a straight swap. Expect to deal with:

• Interlocks and signals. The tool controller often expects a "pump running" or "pump fault" signal. Dry pumps provide status outputs, but the wiring and logic need to match. On older tools this can mean adding a relay interface.
• Start-up sequence. Some dry pumps need a warm-up period before they reach operating temperature. The tool's pump-down sequence may need a delay or a manual step.
• Soft vent and foreline valves. Dry pumps can be more sensitive to sudden pressure bursts. Check that roughing valves open in a controlled way.
• Exhaust connection. Dry pumps exhaust a gas stream that still contains process by-products, without oil to trap them. The exhaust must go to the right abatement or scrubbed exhaust, and an oil mist filter is no longer the right fitting.

What changes in your facility

Dry pumps typically need more than a single-phase socket and a drip tray:

• Electrical supply. Many process-rated dry pumps need a three-phase supply, often at a higher current than the rotary vane pump they replace.
• Cooling water, on water-cooled models, with the flow and temperature the pump maker specifies.
• Nitrogen purge, at a regulated pressure and flow, for pumps on corrosive or condensable processes.
• Space and noise. Roots and screw pumps are larger and heavier. Check sub-fab or chase space, floor loading and access for future overhauls.
• Exhaust routing to scrubbed or general exhaust, depending on chemistry and your site rules.

Facilities work is often the longest part of a conversion. Planning it before the pump arrives keeps tool downtime short.

Costs and downtime

There are no universal figures, but the cost drivers are predictable: the pump itself, any foreline and exhaust changes, electrical and cooling water work, interlock integration, and the time to requalify the process. Against that, set the oil, filters, waste disposal and labour you no longer spend, plus the unplanned downtime you avoid.

On a typical single-chamber tool with facilities ready, the physical swap and requalification usually fit within a few days. Requalification should include base pressure, pump-down time, leak-up rate and a reference process run, compared against the numbers from before the change.

What we can do

We survey the tool and facilities, size the replacement with you and carry out the swap, interlock integration and requalification, with before-and-after data in the report. See our dry pump conversion service for scope and process, or vacuum system refurbishment if the rest of the vacuum system also needs attention. Etch tools are where most conversions start; see the etch equipment we support.