High vacuum pumps provide evacuation of chambers or systems into the high vacuum (10^-3 to 10^-8 torr) or ultra-high vacuum (<10^-8 torr) range of pressure. Mechanical pumps and venturi generators operate in the medium to rough vacuum ranges.

Types of High Vacuum Pumps

There are a number of pump designs made which can generate high or ultra-high vacuums. Each of these types operates using either the gas-transfer or gas-capture principle. A summary of all the types is listed in the table below:

High vacuum pump spec comparison. (Data compiled from Lesker Company and other sources.)

Gas-Transfer

Vacuum pumps that operate on the gas-transfer principle create vacuums by moving gases through the pump. These pumps can be used continuously without the need for regeneration. All gas-transfer pumps are either positive displacement or kinetic (momentum transfer). Positive displacement (gas-displacement) pumps displace gas from sealed areas to the atmosphere to a downstream pump stage. Kinetic pumps displace gas by accelerating it in the pumping direction, either mechanically (mechanical vacuum pumps) or via an adjacent vapor stream (venturi jet vacuum pumps).

Turbomolecular pumps use a rotating disk with a series of rotors and flow stabilizing, stationary stators to impart a preferential motion to gas molecules and create molecular flow through the pump. The drive shaft of the disk may operate at speeds from 25,000 to 90,000 rpm. They are often described as "molecular bats." They are also known as axial flow turbines. Turbomolecular pumps can pump into the 10^-10 Torr range on a suitable system, and are known as the work horses of high vacuum technology. 

Molecular drag pumps (also called molecular pumps or drag pumps) are similar to turbomolecular pumps, except a rotor drum with a ridged surface and cylindrical stator are used in place of stator and rotor blades to impart a preferential motion to gas molecules and create molecular flow through the pump. Through this mechanism, gas molecules are physically dragged along the surface, hence the name molecular "drag" pump. These pumps can only reach ultimate pressures as low as 10^-6 Torr. These pumps are used in applications with limited conductance (pressure drop or resistance through the system) and for frequent cycling operations.

Selection Tip: Drag pumps will start pumping at inlet pressures several torr higher than turbo pumps, and they do not require as low backing pressures for full operation. They also have much lower pumping speeds than turbomolecular pumps with the same inlet size.

Turbo/drag pumps are hybrid vacuum pumps which combine design features of turbomolecular and molecular drag pumps. Using combinations of blades and ridged drums, these pumps are made to harness the strengths of of both types of pumps: namely the turbo pump's lower ultimate pressure and the drag pump's allowance for higher backing pressure.

Gas-Capture

Vacuum pumps that operate on the gas-capture or gas-binding principle create vacuums by physically capturing gas molecules, holding them in a frozen state. These pumps typically have less moving parts than gas-transfer pumps, but must be periodically regenerated because they become saturated with captured material over time.

Diffusion Pumps

Diffusion pumps (more aptly named gas-jet pumps) use high speed jets of diffusion vapor to impart momentum and sweep gas molecules from the system. They are among the most widely used high vacuum pumps. Diffusion pumps are very reliable, have no moving parts, and have long service lives. They provide the highest pumping speed for lighter gases like helium and hydrogen. Process contamination and backstreaming of oil in oil diffusion pumps can be a problem, but baffles and traps can be installed used to reduce this. These pumps can reach ultimate vacuums of 10^-10 torr under the right conditions.

In oil diffusion pumps, oil is heated to a vapor and is directed out into the gas stream. This oil vapor entrains gas molecules and directs them to the foreline port towards the backing pump before recondensing into in the boiler.

Ion Pumps

Ion pumps are the primary choice for most ultra-high vacuum systems. They provide clean, dry, and vibration free operation in the vacuum range of 10^-6 to 10^-11 Torr . These pumps utilize a sputtering process to ionize gas molecules through collisions with electrons. Ionized gas molecules are then drawn to and embedded on getter plates made of titanium or tantalum.

Cryogenic Pumps

Cryogenic pumps (also known as cryopumps) utilize extremely cold (liquid nitrogen and liquid helium temperature) surfaces which absorb gas molecules by freezing or trapping them. Operational errors are less likely with these pumps, since temperature rise and a temporary stall of pumping action are the only consequences of overloading the vacuum chamber. Cryogenic pumps can operate with relatively high force or exhaust pressures. Cryogenic pumps must be periodically generated to purge the frozen or trapped gases. They are particularly suited for pumping atmospheric gases and high melting point vapors (H2O) in the 10-6 to 10-9 Torr range. They are used commonly in non-aggressive semiconductor processes where oil-free operation and high pumping speeds are necessary.

Cryosorption Pumps

Cryosorption pumps evacuate gas molecules from a volume by adsorbing them on the chilled surface of a molecular sieve. These molecular sieves are designed to have a large surface area-to-volume ratio to maximize the adsorbing area. Single cryosorption pumps can achieve ultimate pressures of 10^-4 Torr within a few minutes, but must be regenerated after each use. These simple pumps are low-cost and completely dry. Standalone cryosorption pumps are used as roughing or backing pumps for high vacuum pumps, but higher vacuums can also be achieved by using multiple units in series.

Getter Pumps

Getter pumps entrain gas molecules in a getter (a deposit of reactive material) in order to absorb or capture the molecules and embed them on the cold outer wall of the chamber. Evaporable getter pumps, including titanium sublimation pumps (TSP), use getters which are heated to the point of evaporation or sublimation where it subsequently condenses on the chamber surface. Non-evaporating gettering (NEG) sorption pumps use getters which remain in the solid state as pellets or thin films, providing a large-surface-area porous matrix for entrainment. Getter pumps provide very economical and compact pumping in many applications when used properly.

Mechanical Pump Technology

Some mechanical pumps, when supported by a backing pump, are also capable of achieving pressures in the high vacuum region. Types of high vacuum mechanical pumps include regenerative blowers, rotary piston, rotary vane, and scroll pumps.

Backing Vacuum Pumps

Integral "backing" vacuum pumps, also called roughing pumps, are those specifically designed to support high vacuum pumps. High vacuum pump systems may include or contain a roughing pump designed to fit the backing requirements of the primary (high vacuum) pump. In many cases, backing pumps must be sourced separately from high vacuum pumps, in which case backing pump selection must correspond to the application and performance requirements of the high vacuum pump purchased.

Specifications

There are a number of important specifications provided by manufacturers for sourcing high vacuum pumps.

Performance

Performance specifications define the performance capabilities of high vacuum pumps.

Ultimate (maximum) operating vacuum or ultimate pressure is the lowest pressure which the vacuum pump can generate (typically within a set time). Buyers should note the conditions or assumptions used to obtain this value, since manufacturers may provide this rating using assumptions that are not realistic under normal operating conditions (e.g. ignoring the pressure of condensable gases like water vapor). Vacuum ratings which follow the ISO 21360-1 standard adhere to standard methods for measuring vacuum-pump performance.

Pumping speed or vacuum flow is the volumetric rate at which gas molecules are removed from the vacuum chamber, typically given in ft³/min (cfm), m³/s, L/min, or gal/min (gpm). It is defined mathematically by the equation:

S = Q/P

where S is pumping speed, Q is the throughput of the gas load (described below), and P is the partial pressure of the gas at or near the pump inlet. A pump's maximum achievable pumping speed (over its entire pressure range) is always referred to as its rated pumping speed, and pumping speeds listed by manufacturers are typically referenced to STP (standard temperature and pressure). Pumping speed needs to be matched according to the needs of the application, which are dependent on the system's chamber volume, desorption, and process gas loads. Keep in mind that the speed of the pump itself is seldom the actual pumping speed in the system's chamber. In blowers and non-positive displacement vacuum pumps, speed also varies with the type of gas material.

Selection Tip: Pumping speed is generally defined under the same ideal conditions as the ultimate pressure (minimum volume, right at the pump inlet, lowest possible outgassing rate, etc.). Care should be taken on these details when considering these specifications for pump performance assessment. 

Throughput or gas load is the quantity of gas (i.e. the volume of gas at a known pressure) that passes through the pump inlet in a known time. In SI units, throughput is often given in Pa-m³/s. It defines the energy required to transport the gas molecules across a plane in the system or chamber. At a specified temperature, throughput is proportional to the mass flow rate of the pump. When discussing a system leak or backstreaming, throughput can also refer to the volume leak rate multiplied by the pressure at the vacuum side of the leak. This leak rate throughput can be compared to the throughput of the pump.

The pumping speed vs. pressure curve shows how the pressure generated by a vacuum pump varies with pumping speed. It describes the pump's performance throughout its probable application range, allowing users to assess the pump's capability at specific operating conditions for different gases. A high vacuum pump may have a performance curve like this one:

Turbomolecular pump performance curves for different gases. Image Credit: Osaka Vacuum Inc.

Motor power is a reference value used to characterize vacuum pump size. Vacuum pumps are available in a number of different power sources, including compressed air, DC power, single-phase or three-phase AC power, or internal combustion. Magnetic drive pumps driven by AC or DC power are specialty pumps which allow for seal-less construction to eliminate oil contamination of the fluid in the pump.

Lubrication

High vacuum pumps may be either dry (oil-less) or oil-sealed based on the means of lubrication.

Dry or oil-less pumps use permanently sealed bearings or other isolation technology to eliminate oil in the fluid train. Some dry pumps use oil-lubricated bearings, so they are not always truly oil-free. However, they do minimize the potential for oil contamination within the system because they do not rely on oil for sealing. Dry pumps are more tolerant of particulates and vapors than other types. Most high vacuum pumps (besides diffusion, rotary vane, and rotary piston pumps) are dry pumps.

Wet or oil-sealed pumps use oil to lubricate and seal bearings and parts. These types of pumps may leak small amounts of oil into the flow chamber, and should not be used if oil contamination presents a problem. Diffusion, rotary vane, and rotary piston pumps are examples of wet high vacuum pumps. There are a variety of different types of lubricants available; properties to consider in selection include lubricity, chemical stability, viscosity, and material compatibility. There are also a number of lubrication styles for oil-sealed pumps:

• Splash lubrication styles splash oil onto components from an oil bath.
• Oil-flooded lubrication, used in some mechanical pump styles, involves the heavy application of oil for moving parts. Oil contamination is likely in pumps incorporating this lubrication style.
• Positive pressure styles maintain oil pressure for the highest level of lubrication.

Mounting

Based on construction and weight, vacuum pumps can be mounted in a variety of different ways. The mounting style needed will depend on the pump's location and the application.

Benchtop mounting is used for pumps small enough to mount or be placed on a bench or table. These models may also be portable.

Carts or other portable mounts include wheels, casters, or easily movable frames for frequent relocation of the pump.

Larger vacuum pumps may rest on the floor or on a skid.

Permanent installation is used with very large systems or stations that are installed permanently in one place.

Configuration

Vacuum pumps can be purchased in a number of different configurations depending on the needs of the client.

Individual vacuum pumps are typically for insertion into or used with a larger system or process. When a high vacuum pumps is purchased individually, a backing pump(s) which meets its performance requirements must be separately acquired or purchased to complete the system.

Vacuum pumping units consist of two or more pumps or stages using different technology, which are coupled or stacked together to increase capacity or take advantage of the features of each type. The most typical combination is a high vacuum pump supported by a mechanical backing pump.

Vacuum systems can include multiple pumps and associated piping, valves, controls, receivers, etc. These pumps can also indicate centralized vacuum sources for manufacturing or automation cells or plants.

Material Compatibility

High vacuum pumps, including their lubricants and materials of construction, need to be compatible with the fluids they move or capture to prevent wear or corrosion and ensure safe and clean operation. The type and consistency of gas being handled also has an effect on the pump's performance, since most pumps will pump different gases at different speeds. For example, a cryopump will pump water vapor much faster than an equivalently sized turbomolecular pump, but both pumps might have exactly the same speed when pumping nitrogen.

Features

High vacuum pumps may incorporate a number of additional features. The IEEE GlobalSpec SpecSearch Database contains a number of additional features.

Filters, baffles, and separators - used to protect vacuum pumps against wear and corrosion. They separate or filter out particles or vapors from a fluid stream that may otherwise damage parts of the pump.

OEM controller or control panels - used to adjust controls and provide additional functions above that of a simple regulator knob.

Valve sequencing control - used to control the state of valves in a vacuum pump system, often from a central panel or location.

Vacuum gauge - provides a dial, numeric, or other type of readout of the pressure at points in the vacuum system.

Integral trap - used to prevent backstreaming and resultant contamination from roughing pumps in high vacuum or sanitary applications. There are various trap types or technologies designed for the prevention of chamber entry by oil or water vapor, hydrocarbons, etc.

Gas ballast - allows atmospheric air into the compression chamber to minimize condensate in the oil and prevent corrosion.

Magnetic bearing - used to lift the pump rotor through magnetic levitation to eliminate contact and contact friction between the two surfaces.

References

Vacuum Pumps - Kurt J. Lesker Company

Vacuum Technology: Oil Diffusion Pumps - Society of Vacuum Coaters (pdf)

Image credit:

Tuthill Corporation / Vacuum & Blower Systems