outgassing
**Outgassing** is the **release of volatile chemical compounds from solid materials into the surrounding environment** — where polymers (epoxies, adhesives, mold compounds), plastics, and organic materials release trapped solvents, unreacted monomers, plasticizers, and decomposition products as gases, creating contamination risks in vacuum systems (EUV lithography, spacecraft), cleanroom environments (wafer processing), and sealed packages (MEMS, image sensors) where even trace amounts of outgassed compounds can degrade optical surfaces, contaminate wafers, or cause device failures.
**What Is Outgassing?**
- **Definition**: The spontaneous release of gas or vapor from a solid material — driven by diffusion of trapped volatile species to the surface, desorption from the surface into the gas phase, and thermal decomposition of the material at elevated temperatures. The rate increases exponentially with temperature.
- **Volatile Species**: Common outgassed compounds include water vapor, solvents (NMP, PGMEA from photoresist), plasticizers (phthalates from PVC), silicone compounds (siloxanes from sealants), and decomposition products (formaldehyde from epoxies).
- **Vacuum Acceleration**: In vacuum environments, outgassing is accelerated because the external pressure is removed — molecules that would remain adsorbed at atmospheric pressure readily desorb into vacuum, making outgassing a critical concern for EUV lithography, electron beam systems, and spacecraft.
- **Condensation Risk**: Outgassed compounds can condense on cooler surfaces — creating contamination films on optical lenses (EUV), sensor surfaces (image sensors), and MEMS structures that degrade performance or cause failure.
**Why Outgassing Matters**
- **EUV Lithography**: EUV systems operate in high vacuum — outgassing from resist, pellicles, and chamber materials can deposit carbon contamination on the expensive EUV mirrors and mask, degrading reflectivity and imaging quality.
- **Spacecraft**: In the vacuum of space, outgassed compounds from structural materials, adhesives, and cable insulation condense on optical surfaces (telescope mirrors, solar cells, thermal radiators) — NASA requires all spacecraft materials to pass ASTM E595 outgassing testing.
- **MEMS Devices**: Hermetically sealed MEMS packages can trap outgassed compounds — these compounds can condense on MEMS structures, change resonant frequencies, cause stiction (surfaces sticking together), or degrade optical MEMS performance.
- **Cleanroom Contamination**: Outgassing from construction materials, furniture, packaging, and equipment introduces airborne molecular contamination (AMC) into cleanrooms — degrading wafer processing quality.
**Outgassing Testing Standards**
| Standard | Test Conditions | Metrics | Application |
|----------|---------------|---------|------------|
| ASTM E595 | 125°C, 24 hrs, vacuum | TML (< 1.0%), CVCM (< 0.1%) | Spacecraft materials |
| ECSS-Q-ST-70-02 | 125°C, 24 hrs, vacuum | TML, CVCM, RML | European space |
| SEMI E108 | Various temps, GC-MS analysis | Species identification | Semiconductor equipment |
| MIL-STD-883 (TM 1018) | 100°C, 24 hrs, sealed | Moisture + organics | Military IC packages |
**Outgassing is the invisible contamination source that threatens vacuum systems, cleanrooms, and sealed packages** — releasing volatile compounds from polymers and organic materials that can deposit on optical surfaces, contaminate wafers, and degrade device performance, requiring careful material selection, bake-out procedures, and outgassing testing to control this pervasive contamination mechanism.