The key to “ultra-clean” semiconductor clean vacuum equipment: the dual blessing of materials and processes
In semiconductor device manufacturing, cleanliness is one of the key factors that determine device performance. A vacuum environment, with its low suspended particles and residual gas content, is an ideal choice for achieving ultra-high cleanliness.
Producing and maintaining a clean vacuum environment is a critical issue in semiconductor production, intersecting multiple disciplines, including materials science, mechanical manufacturing, and vacuum technology. This article systematically analyzes the entire R&D process for semiconductor clean vacuum system process equipment, from core design to key technologies, comprehensively presenting the development logic behind this sophisticated system.
01 Core requirements of clean vacuum system
Semiconductor testing equipment places extreme demands on vacuum environments: not only must it achieve ultra-high vacuum (UHV) or even extreme high vacuum (XHV), but it also imposes stringent limits on environmental composition. For example, in ultra-high vacuum, the partial pressure of water must be below 1×10⁻⁹ Pa, and the partial pressure of hydrocarbons must be controlled between 1×10⁻¹⁰ and 1×10⁻¹¹ Pa, depending on their molecular weight. Large organic molecules are the primary contaminants, and their condensation on equipment surfaces can directly lead to chip defects. Therefore, particle size control focuses on ultra fine particles between 0.1 and 0.5 μm, and “large particles” larger than 0.5 μm are strictly prohibited. From atmospheric pressure (10⁵ Pa) to ultra-high vacuum (<10⁻⁹ Pa), the pressure span spans 10¹⁴ orders of magnitude. The molecular mean free path extends from nanometers to tens of thousands of meters, and the time it takes for a monolayer to form increases from nanoseconds to hours. This extreme environmental difference requires equipment to achieve all-round breakthroughs in material selection, structural design, and clean processing.
02 Typical configuration design of vacuum system
The configuration of semiconductor clean vacuum systems must take into account core indicators such as pumping efficiency, oil-free environment, and low vibration. The specific solution varies depending on the scale of the system.
Small system: Use a combination of a vortex dry pump and a magnetic levitation molecular pump, combined with a vibration isolation device to reduce vibration interference;
Large and medium-sized systems: Use screw pump unit + magnetic levitation molecular pump to meet large-volume pumping needs
Vacuum system diagram: P1 – screw pump unit; P2 – molecular pump; G1 – resistance gauge; G2 – ionization gauge; G3 – pressure switch; RGA – quadrupole mass spectrometer; V1 – flapper valve 1; V2 – plug valve; V3 – slow-draw valve; V4 – flapper valve 2; V5, V6 – diaphragm valves; V7 – manual regulating valve; V8 – pressure relief valve; F1 – high-purity filter
Valve selection must match process characteristics. Special testing equipment must be equipped with low-particle gate valves and vacuum transfer valves, and the outgassing rate of sealing materials must be strictly controlled. All interfaces utilize long-life valves sealed with oxygen-free copper gaskets or fluororubber O-rings. The overall baking temperature must be no less than 150°C, and the total system leakage rate must be less than 5×10⁻¹⁰ Pa・m³/s.
To prevent airflow from impacting precision components, a slow-pump valve must be activated during the roughing phase, and a precision filter must be used for buffer filtration during re-pressurization. Based on cleanliness zoning requirements, the main pump is placed in a Class 1000 or higher clean area (white zone), while the roughing pump and other equipment are placed in a lower-cleanliness gray zone, forming a hierarchical management and control system.
03 Precision control from materials to processes
1) Scientific Selection of Chamber Materials
Chamber materials are the foundation of clean vacuum systems, and their outgassing rate directly determines the stability of the vacuum environment. 316L stainless steel is the preferred metal material for ultra-high vacuum systems due to its low outgassing rate, easy welding, non-magnetic properties, and bake resistance. It is particularly well-suited to the low remanence requirements of semiconductor equipment.
The aluminum alloy chamber is machined from a forged blank to reduce weld seams and leak risks. The metal sealing flange utilizes a composite plate of explosively welded aluminum alloy and stainless steel, ensuring a balanced sealing performance and material compatibility.
2) Precision Chamber Design and Manufacturing Solutions
Chamber design must consider volume, shape, and process compatibility. Common structures include cylindrical, box-shaped, and polyhedral. Door opening options vary depending on operational requirements, including front-to-back (hinged, bolted), and top-to-bottom (manual, ceiling-mounted).
Cylindrical stainless steel chambers can be machined from standard seamless pipes, with polished interior surfaces ensuring a high finish.
Large and medium-sized box-shaped chambers are welded and annealed after welding to minimize deformation.
Polyhedral chambers are manufactured from single-piece forgings, achieving high-precision molding using a 5-axis CNC machining center.
Aluminum alloy chambers utilize a non-welding, integrated machining process to minimize welding deformation and leakage. O-ring grooves are machined simultaneously with the chamber body to ensure sealing accuracy.
3)The Importance and Methods of Cleanliness Control
Surface contamination of semiconductor equipment is the primary source of chip defects (accounting for over 50%). Therefore, the cleaning process must strictly adhere to semiconductor industry standards and is divided into two major steps: pre-cleaning and clean cleaning.
Pre-cleaning (assembly line operation): High-pressure washing → Degreasing cleaning → High-pressure washing → Acid cleaning → Pure water immersion → High-pressure washing
Clean cleaning (performed in a Class 1000 clean room): Ultrasonic water washing → Pure water immersion → Air drying → Drying
Cleaning parameters (pressure, concentration, temperature, time, etc.) must be strictly recorded. Multiple cleaning steps and repeated cycles ensure thorough removal of residues.
Post-cleaning inspection utilizes black light inspection (for fluorescence, fibers, and particles) and alcohol wipe testing (for sealing surfaces, blind spots, and threaded holes). Qualified products are immediately vacuum-packed and shipped to prevent secondary contamination.
4) Performance Verification Under Strict Indicators
Equipment testing is conducted in a graded clean room environment: assembly and commissioning in a Class 10,000 clean room → vacuum packaging → unpacking in a Class 1,000 clean room → transfer using dedicated equipment → testing in a Class 100 clean room. Specialized equipment such as cranes, suspended air cushions, and automated guided vehicles (AGVs) are used during the transfer process to minimize vibration and contamination.
Core testing metrics include:
- Leak rate test: After initial evacuation, a helium mass spectrometer leak detector is used to ensure a total leak rate of < 5×10⁻¹⁰ Pa・m³/s;
- Vacuum level test: Using a complex gauge with a lower range limit of 10⁻⁸ Pa, the ultimate vacuum level is tested under no-load and load conditions;
- Residual gas analysis: After bakeout, the gas composition is analyzed using an RGA system to ensure that process requirements are met;
- Pumping efficiency: Under load, the unit can quickly reach a working vacuum level of 1×10⁻⁴ Pa, reducing pumping time by 70% compared to standard requirements.
