In high-precision manufacturing fields such as semiconductors and flat panel displays, the cleanliness of the production environment directly determines product yield and performance. For a long time, physical filtration technologies, represented by HEPA (High-Efficiency Particulate Air) and ULPA (Ultra-Low Penetration Air) filters, have built a solid barrier against airborne particles. However, as chip manufacturing processes enter the nanoscale, a more insidious threat—airborne molecular contaminants (AMCs)—has become a key bottleneck affecting production. The SEMI F21-1102 standard aims to systematically define, classify, and control these molecular-level contaminants that even HEPA/ULPA filters cannot block, thereby elevating cleanroom environmental control to a new level.
1. Limitations of Physical Filtration and Definition of Molecular Contaminants
AMCs are typically measured in angstroms (Å) or nanometers (nm), with 1 Å equal to only 0.1 nanometers. These contaminants have molecular sizes much smaller than the pores between filter fibers, allowing them to penetrate any physical filter medium freely, just like air. AMCs can originate from outdoor atmosphere, raw materials, building materials, and even equipment and personnel themselves.
2. Classification System and Control Logic of SEMI F21-1102 Standard
Based on the chemical properties of AMCs and their hazard mechanisms to semiconductor processes, this standard classifies them into four main categories:
Molecular Acids : Corrosive gaseous substances capable of accepting electrons, such as hydrochloric acid (HCl), hydrofluoric acid (HF), sulfur dioxide (SO₂), and nitrogen oxides (NOx). They primarily cause corrosion of metal circuits and chemical changes on material surfaces.
Molecular Bases : Corrosive gaseous substances capable of donating electrons, most typically ammonia (NH₃) and its derivatives (amines). Alkaline contaminants readily react with photoresist, leading to pattern formation defects (such as T-top defects), making them a critical factor requiring strict control in photolithography areas.
Condensable substances: These are organic compounds with high boiling points at room temperature and pressure that readily condense into thin films on clean, cold surfaces (such as silicon wafers and optical lenses), such as siloxanes and phthalates (plasticizers). This film severely affects the light transmittance of optical systems and the interface characteristics of silicon wafer surfaces.
Dopants: These are chemical elements that can alter the electrical properties of semiconductor materials, such as boron, phosphorus, and arsenic. Uncontrolled diffusion into the wafer can cause drift in device electrical parameters, leading to product failure.
It is worth noting that releases from some filter materials themselves can also be sources of contamination. For example, some HEPA filter adhesives or sealing materials may release trace amounts of boron, falling under the category of dopants; while plasticizers are condensable substances. Therefore, high-end filter manufacturers like German company Trenntech[11] must strictly select low-emission, high-purity materials at their R&D center in Frankfurt to ensure that their HEPA/ULPA filters, while efficiently intercepting particles, do not themselves become a source of AMC (Aerobic Methane Contamination).
3. Achieving Molecular-Level Cleanliness: A Systems Engineering Approach from Monitoring to Filtration
Meeting the requirements of the F21-1102 standard requires a comprehensive solution that surpasses traditional air purification methods. Its core control strategy can be summarized as “source control and process interception.”
1. Source Control and Isolation: This is the most economical and effective method. By using low-volatile building materials and chemicals, AMC-generating process equipment (such as etching and cleaning equipment) is placed in a negative pressure isolation chamber, and the fresh air supplied to the cleanroom undergoes deep pretreatment, reducing the introduction of pollutants at the source.
2. Chemical Filtration Technology: This is the core technology for intercepting AMC. Completely different from the physical interception principle of HEPA/ULPA, chemical filters rely on adsorption or chemical reactions. Chemical filter media such as activated carbon, impregnated activated alumina, and ion exchange resins can selectively adsorb or neutralize acidic, alkaline, and organic molecules in the air. These chemical filters are typically installed in modular form in the fresh air and return air sections of cleanroom air conditioning systems, or directly integrated into the microenvironment control systems of critical process equipment (such as lithography machines).
The SEMI F21-1102 standard marks a shift in cleanroom technology from the macroscopic “particulate cleanliness” era to the microscopic “molecular cleanliness” era. It reveals that in modern advanced manufacturing, relying solely on HEPA/ULPA filters for physical cleanliness is far from sufficient. AMC control is a complex system integrating chemistry, materials science, automation control, and building environmental engineering. Molecular-level cleanliness technologies built around the F21-1102 standard will continue to be a powerful force supporting the advancement of the information technology industry.