Preface: The Rise of Dry Vacuum Pumps

Over the past decade, the escalating demand for clean vacuum environments in high-end manufacturing sectors—such as semiconductors, pharmaceuticals, and microelectronics—has propelled dry vacuum pumps​ to the forefront. Their core advantage lies in not requiring lubricating oil in the compression chamber, eliminating the risk of oil contamination and significantly reducing maintenance compared to traditional oil-sealed pumps.

This evolution has led to the design and mass production of various dry pump types. They can be categorized into four main groups based on mechanical structure and working principle: lobe-type, claw-type, combined-type​ (Roots + claw), and screw-type. The first three, which employ a multi-stage series compression structure, are collectively known as multi-stage dry vacuum pumps. The screw-type, relying on a single-cavity compression design, is classified as a single-stage dry vacuum pump.

Their fundamental operational logic differs significantly. Multi-stage pumps achieve a high vacuum by compressing gas step-by-step through a series of chambers. In contrast, screw-type pumps complete the entire process of suction, compression, and discharge within a single chamber. The following sections will detail the physical characteristics, advantages, disadvantages, and ideal application scenarios for each type.

1. Multi-Stage Dry Vacuum Pumps: A Detailed Look

Multi-stage pumps share a common principle of achieving vacuum through step-by-step compression in series-connected chambers. This design inherently leads to a more complex gas flow path and longer residence time inside the pump.

1.1 Dual-Lobe and Tri-Lobe Types

Core Structure:​ The core structure is highly homologous to the Roots vacuum pump. Essentially, it is a multi-stage superposition of Roots-type rotor chambers connected in series to improve the ultimate vacuum degree.

Performance:

• High Vacuum:​ Can achieve an ultimate vacuum of 10⁻² ~ 10⁻³ Pa.
• Energy Efficiency:​ The high compression ratio from multi-stage superposition can result in 15-25% lower power consumption compared to single-stage pumps under the same pumping speed.

Inherent Challenges:

• Complex Flow Path:​ The multi-stage design creates a polygonal gas flow path, increasing the time gas remains in the pump body.
• High Nitrogen Consumption:​ Requires a large flow of nitrogen (typically 5-10% of the pump's rated speed) to be injected into each chamber to dilute process gases, isolate cross-contamination between stages, and prevent polymerization or condensation.
• High Precision Required:​ The gaps between rotors and chambers at all levels must be strictly controlled within 0.05-0.1 mm; otherwise, compression efficiency drops sharply due to leakage.
• Tri-Lobe Enhancement:​ The tri-lobe design, with three rotor blades, divides the gas into three parts per revolution. This results in smaller gas pulsation, lower operating noise (3-5 dB lower than dual-lobe), and better flow stability at the same speed. However, it shares the same challenges of a complex flow path and high nitrogen demand.

1.2 Combined-Type (Roots + Claw)

• Design Philosophy:​ This is an optimized design for wide pressure range conditions. It combines a "Roots stage" for high pumping speed in the medium-low vacuum range with a "claw stage" for high compression efficiency in the medium-high vacuum range.
• Performance Optimization:​ Some manufacturers modify the final claw stage into a star-type design for even smaller pulsation and a more stable compression process, suitable for precision processes.
• Inherent Characteristics:​ It continues the multi-stage series logic, inheriting the challenges of long gas residence time and the requirement for substantial nitrogen protection across all chambers.

2. Single-Stage Dry Vacuum Pumps (Screw-Type)

Screw-type vacuum pumps utilize a pair of helical screws that rotate in opposite directions to move gas linearly from the intake to the exhaust port. This design offers the shortest and simplest gas flow path among dry pumps. The key distinction lies in the rotor design, which divides them into two categories.

2.1 External Compression Screw Type

• Core Structure:​ Uses a pair of constant-pitch screw rotors. The meshing gaps between the rotors and between the rotors and the cylinder walls are uniform (typically 0.1-0.2 mm), ensuring minimal reverse leakage.
• Working Principle:​ Its hallmark is "no internal compression." The gas is primarily transported through the pump chamber, with compression occurring via back pressure in the external exhaust pipeline. This minimizes the internal compression effect.

Key Advantages:

• Short Gas Residence Time:​ The linear flow path reduces the time gas is inside the pump to 1/3-1/5 of that in multi-stage pumps, drastically reducing the risk of physical or chemical changes in the gas.
• High Stability:​ Excellent for complex processes like etching and thin film deposition, effectively avoiding common multi-stage pump failures like chamber blockage and rotor bonding.
• Low Nitrogen Dependence:​ Due to the short residence time and simple path, nitrogen is only needed in small amounts at the exhaust port (flow <1% of rated speed) to prevent backflow. In clean processes, non-nitrogen operation is possible, greatly reducing operational costs.
• Main Drawback:​ The non-internal compression design results in a lower compression ratio, leading to 20-30% higher power consumption than multi-stage pumps at the same pumping speed.

2.2 Internal Compression Screw Type

• Core Structure:​ Utilizes a pair of variable-pitch screw rotors. The closed volume formed during meshing gradually decreases, compressing the gas insidethe pump chamber.
• Performance Characteristics:​ The internal compression design yields a high compression ratio, reducing energy consumption to levels comparable with multi-stage pumps.
• Core Challenge:​ The temperature and pressure changes are concentrated inside the pump chamber. Similar to multi-stage pumps, this makes it susceptible to liquefaction, solidification, or polymerization when handling condensable or polymerizable gases, leading to rotor bonding and chamber blockage. Its suitability for different process gases is therefore narrower.

3. Key Selection Factors: Beyond the Brochure

When selecting a dry vacuum pump, consider these critical factors that impact total cost of ownership and process reliability.

Operational Stability vs. Cost:​ While operational cost (energy + nitrogen + maintenance) is a traditional primary factor, operational stability​ is now equally critical in high-end manufacturing. Unplanned pump failures can halt entire production lines, leading to scrap losses of high-value products (e.g., wafers) and associated equipment damage. The single-stage design of screw pumps, especially the external compression type, offers superior stability with 40-60% fewer core components and a significantly lower failure rate than multi-stage pumps.

Nitrogen Consumption and Environmental Cost:​ High nitrogen consumption in multi-stage pumps is not just an operational expense but also an environmental concern, as it generates large volumes of exhaust gas requiring treatment. With tightening environmental regulations, the cost of managing this exhaust is rising. The extremely low nitrogen requirement of the external compression screw pump presents a significant long-term advantage.

The Small-Size vs. High-Speed Trend:​ The drive for equipment miniaturization conflicts with the need for high pumping speed (which is proportional to pump volume). The industry solution is to increase rotor speed​ to compensate for smaller size. This is achieved through either variable frequency drives​ (offering precise control but at a higher cost and potential torque loss) or by optimizing gearbox transmission ratios​ (a lower-cost, reliable method that provides only a fixed speed).

4. Conclusion and Selection Suggestions

For controlling energy consumption​ and when process gases are clean and non-reactive, the internal compression screw vacuum pump​ is a suitable choice.

For the highest operational stability, minimal nitrogen use, and adaptability to multiple semiconductor processes, the external compression screw vacuum pump​ is the superior option. Its wide adaptability and simplicity allow a single unit to switch between different processes (etching, deposition, packaging) rapidly, reducing inventory costs and downtime.

From an industry trend perspective, the move towards environmental sustainability and lower cost of ownership is expanding the application scenarios for external compression screw vacuum pumps.

Disclaimer: This website respects intellectual property rights. If any infringement is found, please contact this website in a timely manner for handling.