diffusion furnace

Diffusion furnaces (tube furnaces) are horizontal or vertical thermal processing systems that heat semiconductor wafers in controlled atmospheres at temperatures from 400°C to 1200°C for oxidation, diffusion, annealing, and low-pressure chemical vapor deposition (LPCVD). Furnace construction: (1) quartz process tube (high-purity fused silica tube 150-300mm diameter, 1-3m length—quartz is used because it withstands high temperature, introduces minimal contamination, and is transparent to infrared radiation), (2) resistive heating elements (SiC or MoSi₂ elements arranged in 3-5 independently controlled zones along the tube for temperature uniformity ±0.25-0.5°C across the flat zone), (3) gas delivery system (mass flow controllers meter O₂, N₂, H₂, HCl, and other process gases into the tube), (4) wafer loading system (boat/paddle loaded with 25-150 wafers in quartz carriers—batch processing is the primary throughput advantage). Process types: (1) thermal oxidation (dry O₂ or wet H₂O/O₂ at 800-1200°C—grow SiO₂ gate and field oxides), (2) dopant diffusion (drive-in of implanted or deposited dopants at 900-1100°C), (3) LPCVD (low-pressure deposition of Si₃N₄, polysilicon, SiO₂, and other films at 0.1-1 Torr), (4) annealing (stress relief, densification, and defect removal at 400-1000°C). Advantages: excellent temperature uniformity, high batch throughput (50-150 wafers simultaneously), well-established and reliable technology, low cost per wafer for long thermal processes. Vertical furnaces (used in modern fabs) offer a smaller footprint, reduce particle contamination (wafers face down, particles fall away), and provide better uniformity than horizontal designs. Temperature ramp rates are relatively slow (5-15°C/min) compared to RTP, making furnaces unsuitable for processes requiring rapid thermal transients but ideal for processes needing long, uniform thermal soaks.