Basic Introduction to Vacuum Pumps and Vacuum Equipment

1 Classification and Application of Vacuum Ranges

Vacuum is graded by pressure magnitude. Each order-of-magnitude improvement in vacuum level corresponds to a sparser gas molecular density and purer physical environment, covering full scenarios from conventional industrial processing to cutting-edge fundamental physics research.

• Rough Vacuum (10⁵ ~ 10² Pa) Applications: Material handling via industrial suction cups, food vacuum packaging, vacuum forming, rough vacuum filtration and other general civil and light industrial processes.
• Medium Vacuum (10² ~ 10⁻¹ Pa) Applications: Vacuum annealing heat treatment, vacuum freeze-drying of materials, vacuum distillation and purification in chemical industry, vacuum impregnation of workpieces.
• High Vacuum (10⁻¹ ~ 10⁻⁵ Pa) Applications: Optical and magnetron sputtering coating, electron microscopy observation, vacuum metallurgical smelting, semiconductor device fabrication.
• Ultra-High Vacuum (UHV, ＜ 10⁻⁵ Pa) Applications: Surface physical analysis (SEM/XPS), particle accelerator chambers, aerospace space environment simulation, vacuum chambers for nuclear fusion devices.
• Extreme High Vacuum (XHV, ＜ 10⁻⁹ Pa) Applications: Gravitational wave detection instruments, cutting-edge fundamental physics experiments, ultra-pure surface science research, ultra-precision instrument manufacturing.

2 General Classification Framework of Vacuum Pumps

As core equipment for vacuum generation, vacuum pumps are categorized by three dimensions: operating principle, working pressure range, and sealing/lubrication medium. Model selection requires comprehensive evaluation of cleanliness requirements, target vacuum level, pumping speed and production costs.

2.1 Classification by Operating Principle

• Gas Transfer Pumps Gas is continuously conveyed via mechanical motion, representing the most widely adopted category in industry. It is subdivided into positive-displacement pumps (rotary vane pumps, reciprocating pumps) and momentum-transfer pumps (turbomolecular pumps, jet pumps).
• Capture Pumps Gas molecules are immobilized through cryogenic adsorption or chemical reactions without continuous gas exhaust, primarily deployed for UHV and XHV environments. Typical types include cryopumps and sputter ion pumps.

2.2 Classification by Operating Pressure Range

• Forepumps / Roughing Pumps: Evacuate atmospheric pressure to establish initial rough vacuum, serving as the foundational unit of all vacuum systems.
• Main Pumps: Deliver the high/ultra-high vacuum required by technological processes, acting as the primary vacuum source of equipment.
• Booster Pumps: Installed in series between forepumps and main pumps to enhance pumping efficiency within the medium vacuum range; Roots pumps are the representative type.

2.3 Classification by Sealing and Lubrication Medium

• Oil-Sealed Vacuum Pumps: Vacuum oil provides sealing and lubrication, featuring large pumping speed and low procurement cost. The drawback is backstreaming of oil vapor, which necessitates oil mist separators. Suitable for metallurgy, mechanical processing and other applications with non-stringent cleanliness requirements.
• Dry Vacuum Pumps: Oil-free sealing and lubrication eliminates oil vapor contamination entirely, with simplified post-operation maintenance. They are the preferred choice for high-cleanliness processes such as semiconductor manufacturing, pharmaceutical production and precision coating.

3 Mainstream Positive-Displacement Vacuum Pumps

3.1 Oil-Sealed Rotary Vane Pumps

3.1.1 Operating Principle and Structure

Key components consist of a stator, eccentric rotor and elastic vanes. Specialized vacuum oil fills the pump cavity, forming an oil film to seal mechanical clearances and provide lubrication. Rotation drives centrifugal motion of vanes to complete sequential suction, compression and exhaust cycles. Two-stage rotary vane pumps adopt series compression to achieve superior ultimate vacuum compared with single-stage models. Gas ballast valve accessory: Dry air is introduced into the compression chamber to reduce partial pressure of water vapor, preventing vapor liquefaction and oil contamination for water-vapor-laden processes.

3.1.2 Performance Characteristics

Pumping speed remains stable across a wide pressure band and gradually decays as inlet pressure decreases. Two-stage units deliver an ultimate pressure of 10⁻³~10⁻⁴ Pa, making them the primary equipment for rough and medium vacuum and standard forepumps for high-vacuum systems.

3.1.3 Applications and Selection Criteria

• Typical applications: Forestage rough pumping for all high-vacuum systems, refrigerant filling for refrigeration equipment, food freeze-drying, vacuum packaging of electronic components, basic coating processes.
• Selection indicators: Effective pumping speed (determines evacuation duration), ultimate pressure, gas ballast configuration, oil mist separation assembly.
• Operational limitations: Cold traps or molecular sieves must be installed to block oil backstreaming for cleanliness-sensitive processes; pre-filters are mandatory for dust-laden gas streams; corrosive and flammable/explosive gases cannot be directly pumped without pre-treatment units.

3.2 Roots Pumps (Booster Pumps)

3.2.1 Operating Principle

A pair of figure-eight rotors inside the pump cavity rotate synchronously in opposite directions driven by precision timing gears without physical contact, conveying gas via periodic volume variation. No internal compression occurs within the pump body; exhaust pressure differential is fully supplied by forepumps (rotary vane pumps or dry pumps), hence Roots pumps must operate in series with forepumps as a pump set. An internal bypass valve automatically relieves pressure when differential pressure exceeds the threshold to avoid rotor overheating and seizure. Each Roots pump has a critical compression ratio; gas backflow occurs if the actual pressure ratio exceeds this limit.

3.2.2 Performance Characteristics

Maintains stable large pumping speed within the medium vacuum range of 10³~10⁻² Pa, significantly boosting the overall pumping efficiency of combined vacuum systems as standard booster equipment.

3.2.3 Applications and Selection Criteria

• Typical applications: Vacuum metallurgical smelting, optical/electronic vacuum coating, vacuum heat treatment of workpieces, rapid evacuation of large vacuum chambers.
• Selection requirements: The pumping speed ratio of Roots pump to forepump is recommended as 5:1~10:1; startup and continuous operating differential pressure must be strictly controlled; water cooling is adopted for high-power continuous operation, while air cooling suffices for small and medium-duty conditions.

3.3 Series of Dry Vacuum Pumps (Oil-Free Clean Pumps)

Dry pumps operate without oil-based sealing or lubrication, completely eliminating oil vapor contamination and catering to high-cleanliness scenarios including semiconductor manufacturing, pharmaceutical production and precision analytical instruments. Four mainstream types are listed below:

• Claw Pumps Gas is compressed in multiple stages by a pair of non-contacting claw-shaped rotors. Characterized by stable operation, tolerance to minor particulate dust and optional nitrogen purging for explosion protection, suitable for 24-hour continuous rough pumping in rough and medium vacuum regimes.
• Screw Dry Pumps Twin screw rotors mesh to realize isothermal compression, achieving high compression ratios in a single stage with direct atmospheric exhaust capability. Pump cavities can be coated with anti-corrosion materials to handle process streams containing vapor and corrosive gases, widely applied in semiconductor etching, CVD thin-film deposition, photovoltaic coating and pharmaceutical freeze-drying.
• Scroll Dry Pumps Relative orbital motion between fixed and orbiting scroll plates forms continuous compression chambers. Compact footprint, low noise and vibration, and low failure rate make scroll pumps ideal for laboratory mass spectrometers, helium leak detectors and other small precision instruments.
• Multi-Stage Roots Dry Pumps Multiple pairs of Roots rotors are connected in series for staged gas compression, delivering large pumping speed and direct atmospheric exhaust performance, widely deployed in large-scale industrial production lines for semiconductors and photovoltaics.

4 Momentum-Transfer Vacuum Pumps (Main Pumps for High Vacuum)

4.1 Turbomolecular Pumps

4.1.1 Operating Principle

Multi-stage high-speed rotating rotor blades and stationary stator blades are arranged alternately. Rotors spin at tens of thousands of revolutions per minute, transferring momentum to gas molecules via collision to drive directional gas transport. Most commercial designs adopt a composite turbine-Holweck structure to balance high pumping speed and high compression ratio. Compression ratios are extremely high for heavy gases such as argon but relatively low for light gases (hydrogen, helium), mandating pre-evacuation by matched forepumps. Two bearing configurations are available: low-cost mechanical bearings lubricated with grease/oil wicks, and advanced fully magnetic levitation bearings with contact-free operation, low vibration and extended service life.

4.1.2 Applications and Selection Guidelines

Ultimate vacuum ranges from 10⁻² to 10⁻⁹ Pa. Oil-free and clean, turbomolecular pumps serve as the core main pump for high-vacuum processes, including semiconductor PVD coating, transmission/scanning electron microscopy, mass spectrometry and aerospace space simulation chambers. Key selection considerations: matching forepump pumping speed, limiting maximum inlet pressure to avoid overload, water cooling for high-power models, vibration isolation and magnetic shielding during installation. Direct exposure to atmospheric pressure is prohibited, as it will cause permanent damage to turbine blades.

4.2 Oil Diffusion Pumps

4.2.1 Operating Principle

Silicone oil or polyphenyl ether working fluid is heated to generate supersonic directional vapor jets, which entrain and compress gas molecules via viscous collision for pumping. Multi-stage nozzle fractionation suppresses oil backstreaming. No moving mechanical components ensure stable operation, low maintenance cost, large pumping speed and high cost-performance ratio. The primary drawback is oil vapor backstreaming, requiring cold traps or high-efficiency baffles to protect vacuum chambers.

4.2.2 Applications and Selection Guidelines

Achieves an ultimate vacuum of 10⁻⁶ ~ 10⁻⁹ Pa, widely used for large-area decorative coating, optical lens coating, vacuum heat treatment furnaces and large aerospace space simulation chambers. Selection criteria: matching chamber nominal diameter, integrated temperature-controlled heating system; a Roots-rotary vane pump set is required as forestage to maintain pre-vacuum below 10 Pa. Silicone oil is preferred for high-vacuum applications, while mineral oil is limited to low-standard industrial processes. Industry trend: Turbomolecular dry pumps gradually replace oil diffusion pumps in precision semiconductor production lines, yet oil diffusion pumps remain irreplaceable for large-volume chambers and industrial processes with moderate cleanliness requirements due to their large pumping speed and economic advantages.

4.3 Steam Jet Pumps

High-pressure steam is converted into supersonic jet flow through a Laval nozzle, which entrains and mixes target gas to realize pumping. Free of moving mechanical parts, jet pumps feature ultra-low failure rates and can directly process gas streams containing particulates, corrosive media and flammable/explosive substances. Multi-stage series assembly achieves rough vacuum levels of hundreds of Pascals. Major applications cover vacuum distillation in petrochemicals, molten steel vacuum refining in metallurgy, large-scale material vacuum drying and power plant vacuum systems. Selection parameters: steam pressure and consumption, cooling water flow rate, customized pump material according to gas corrosivity and processing throughput.

5 Capture Pumps for Ultra-High Vacuum

5.1 Cryopumps

5.1.1 Operating Principle

A G-M cryocooler generates two-stage cryogenic cold plates: the primary stage (50–80 K) traps water vapor and hydrocarbons; the secondary stage (10–15 K) condenses nitrogen, oxygen and argon. Activated carbon coated on secondary cold plates adsorbs light gases such as hydrogen and helium via physical adsorption. Once saturated with adsorbed gas, cold plates are heated to 100–200 °C for regeneration to recover pumping capacity, with oil-free operation and zero backstreaming throughout the cycle.

5.1.2 Applications and Operational Specifications

Ultimate vacuum reaches 10⁻⁹ Pa, deployed in semiconductor ion implantation, magnetron sputtering, optical evaporation coating, nuclear fusion and surface physics research. Prior to startup, forepumps must rough the chamber to below 1 Pa and fully dry the vacuum system; direct startup under high pressure with abundant water vapor will lead to cold head icing and permanent damage. Selection factors include chamber thermal load, regeneration cycle duration and anti-backstreaming baffle design.

5.2 Sputter Ion Pumps

5.2.1 Operating Principle

Penning discharge is induced under orthogonal electromagnetic fields to ionize gas molecules. High-energy ions bombard titanium cathodes to sputter fresh active titanium films. Reactive gases form stable compounds via chemical reaction with titanium films, while hydrogen and other light gases are trapped within titanium crystal lattices. Free of rotating mechanical parts, ion pumps deliver vibration-free operation with all-metal oil-free sealing and ultra-long maintenance-free continuous runtime. Limitation: Low pumping speed for inert gases (He, Ar), requiring supplementary combination with titanium sublimation pumps or cryopumps.

5.2.2 Applications and Selection Guidelines

Ultimate vacuum can extend to 10⁻¹¹ Pa, suitable for electron microscopes, particle accelerators, XPS/AES surface analytical instruments and extreme high vacuum research facilities. Built-in permanent magnets generate stray magnetic fields, requiring safe installation distances from precision electron-beam equipment. Pre-evacuation to 10⁻² Pa is mandatory for startup. Direct pumping of dust and hydrocarbon vapor is forbidden to prevent electrode poisoning and failure.

5.3 Titanium Sublimation Pumps (TSP) and Non-Evaporable Getter Pumps (NEG)

Both serve as auxiliary pumps for UHV/XHV systems, commonly combined with ion pumps and cryopumps:

• Titanium Sublimation Pumps (TSP): Titanium filaments or pellets are heated with high current to sublime titanium atoms and form adsorption films on cold walls for reactive gas capture. Operate intermittently with periodic regeneration; incapable of pumping methane or inert gases.
• Non-Evaporable Getter Pumps (NEG): Zr-V-Fe alloy is activated at 400–450 °C to form porous structures, realizing room-temperature chemical adsorption of hydrogen, carbon monoxide, carbon dioxide and water vapor. Vibration-free and oil-free with extended service life, ineffective for methane and inert gases. Primary applications: Synchrotron radiation storage rings, particle accelerators and high-end semiconductor equipment, used to construct clean UHV/XHV systems and guarantee stable beam quality and experimental reliability.

6 General Selection Methodology for Vacuum Pumps

Vacuum pump selection cannot rely solely on target vacuum level; multi-dimensional cross-evaluation is required:

• Target Vacuum Level: Match main pump and forepump combinations according to rough/medium/high/ultra-high vacuum grading of technological processes.
• Cleanliness Standards: Dry pumps, cryopumps and ion pumps are prioritized for semiconductors, pharmaceuticals and precision analysis; oil-sealed rotary vane pumps and oil diffusion pumps are acceptable for metallurgy, packaging and large-area industrial coating with relaxed cleanliness constraints.
• Process Gas Conditions: Gas-ballast-equipped rotary vane pumps for water-vapor-laden streams; pre-filters for dust-containing gas; anti-corrosion screw dry pumps for corrosive media; cryopump integration for high inert gas loads; nitrogen purging explosion-proof assemblies for flammable and explosive media.
• Pumping Speed Matching: Larger chamber volume and leak rate demand higher pumping speed; a 5:1~10:1 pumping speed ratio must be maintained between Roots booster pumps and forepumps; turbomolecular pumps and cryopumps require matched forepump throughput.
• Capital and Maintenance Costs: Oil-sealed rotary vane pumps offer low upfront cost for short-term intermittent operation; dry pumps are optimal for long-term continuous clean production; oil diffusion pumps are cost-effective for large chambers with moderate cleanliness requirements; scroll dry pumps fit small laboratory equipment.

In summary, vacuum technologies rely on combined configurations of diverse pump types to cover full industrial chains from basic manufacturing to cutting-edge fundamental research. Continuous improvements in vacuum level and environmental purity drive technological iteration across manufacturing, materials science, semiconductors, aerospace and fundamental physics.