Origin and Standard System of Cleanrooms

The feature size of semiconductor chips is extremely tiny — for example, the line width in modern integrated circuits is less than microns. To prevent chips from being contaminated by dust, cleanroom technology emerged in the semiconductor industry

In the late 1950s to early 1960s, Willis Whitfield and others invented the cleanroom design with constant unidirectional airflow, which is also regarded as the origin of modern cleanrooms. As industries like semiconductors and microelectronics needed to meet complex process requirements, multi-level vertical unidirectional airflow cleanrooms were gradually built.

The concept of early cleanrooms originated from practices in other fields — for example, the demand for sterile and dust-free environments in hospital operating rooms and spacecraft assembly settings. In the mid-20th century, the U.S. Air Force and nuclear industry also laid the foundation for this standardization effort. The Federal Standard 209 (Fed Std 209), published in 1963, specified the maximum allowable dust particle count. Since then, this standard has undergone multiple revisions, and by 1992, its final version (209E) introduced metric units and the "Class M" classification. By the late 1990s, the international community began to seek a unified standard: the ISO/TC 209 Technical Committee released the ISO 14644-1 standard in 1999, which defines 9 cleanliness classes based on particle concentration per cubic meter of air. Since then, the ISO system has gradually replaced national standards (such as the U.S. Federal Standard, UK BS, German VDI, etc.) and become the authoritative basis for cleanroom classification worldwide.

The cleanliness of a cleanroom is essentially measured by the concentration of dust (particles) in the air. Simply put, it involves controlling the number of particles of different sizes per unit volume of air. The early U.S. Federal Standard (FS-209E) classifies levels based on the number of particles ≥0.5 μm per cubic foot of air (0.5 μm is usually used as the reference particle size). For example, Class 100 means no more than 100 particles larger than 0.5 μm per cubic foot of air; Class 1,000 corresponds to ≤1,000 particles, and so on.

The ISO 14644-1 standard, however, uses per cubic meter of air as the benchmark. For instance, ISO Class 5 allows no more than 10⁵ particles (0.1 μm and larger) per cubic meter. The approximate correspondence between the two systems is: FS Class 1 ≈ ISO 3, Class 10 ≈ ISO 4, Class 100 ≈ ISO 5, Class 1,000 ≈ ISO 6, Class 10,000 ≈ ISO 7, Class 100,000 ≈ ISO 8 (Note: ISO 14644-1 does not provide an official conversion table; the above is only a commonly used industry approximation).

For ISO standards, the smaller the number of the "cleanliness class", the higher the requirements (and the better the cleanliness). The U.S. Federal Standard 209E was discontinued in 2001, but many industries still habitually use its traditional designations like "Class 100" and "Class 10,000".

Cleanroom class requirements vary significantly across different semiconductor manufacturing processes.

Impact of AMHS on Cleanliness

The Automated Material Handling System (AMHS) is a key component of automated production lines in modern semiconductor factories, encompassing equipment such as transport rails, rail-guided vehicles (RGV), automated forklifts, and front-opening unified pods (FOUP). Since these systems operate within cleanrooms, their design and material selection have a direct impact on cleanliness.

1. Four Key Pathways by Which AMHS Impacts Cleanliness

• Mechanical friction and vibration generating particles
• Particle release from material surfaces and migration of volatile organic compounds (VOCs)
• Micro-vibration, which can cause resuspension of deposited particles, affecting equipment stability and leading to poor contact
• Electrostatic adsorption and discharge issues

2. Key Measures to Reduce Contamination in AMHS Design

3. Industry Standards Related to AMHS Design and Cleanliness

• ISO 14644 series: Governs cleanroom classification and equipment adaptation.
• FED-STD-209E: Historically common classification (discontinued but still used in the industry).

Additionally, the industry has established dedicated semiconductor equipment standards to regulate the clean design of AMHS—for example, SEMI Standards S2/S8, which require "no measurable additional contamination shall be introduced" and reference ISO 14644 as the verification method, ensuring that the entire production line does not introduce excessive contaminants due to material handling equipment.