thermal evaporation

Thermal evaporation is one of the simplest PVD techniques, using resistive (Joule) heating to raise a source material above its melting and vaporization temperature in a high-vacuum environment (10⁻⁵ to 10⁻⁷ Torr). The source material is placed in a resistively heated element — typically a tungsten, tantalum, or molybdenum boat, basket, coil, or crucible — through which a high current (50-300A at low voltage) is passed. The resistive element heats to temperatures of 1,000-2,000°C, melting and evaporating the source material. The vaporized atoms travel in straight-line trajectories through the vacuum (mean free path >> source-to-substrate distance) and condense on the cooler substrate to form a thin film. Thermal evaporation is limited to materials with relatively low melting points and high vapor pressures at accessible temperatures, including aluminum, gold, silver, chromium, germanium, indium, tin, and many organic materials. Refractory metals like tungsten and tantalum have evaporation temperatures too close to or exceeding the melting points of available heater materials, making e-beam evaporation necessary. Key advantages of thermal evaporation include equipment simplicity, low cost, minimal substrate damage (no plasma or energetic particles), high deposition rates for suitable materials (up to 100 nm/min for aluminum), and compatibility with lift-off patterning due to the highly directional deposition. The technique is widely used for depositing metal contacts in research and development, organic light-emitting diode (OLED) fabrication where organic materials are thermally evaporated in sequence, and thin film resistor and sensor fabrication. Disadvantages include poor step coverage due to line-of-sight deposition, limited material selection, difficulty depositing alloys with controlled composition (co-evaporation from separate sources is needed), and potential contamination from the heater element. Deposition rate is monitored in real-time using quartz crystal microbalances (QCM) positioned near the substrate, providing sub-angstrom thickness resolution. Source-to-substrate distance, substrate rotation, and geometric source positioning are optimized to achieve thickness uniformity within ±5% across the substrate area.