Root Causes of Etch Undercut and Countermeasures
蚀刻侧蚀（Undercut）的根因与对策
标签：侧蚀, undercut, 抗蚀层附着力, 线宽控制
侧蚀使蚀线比图形宽、深槽口沿塌边，根因有三：①蚀速过快——42°Bé 三氯化铁在 45 ℃ 的侧向钻蚀接近 25 ℃ 的 3 倍，精细线宽应降到 25–30 ℃；②抗蚀层附着力不足——铜面残油或氧化使侧壁失去保护，蚀刻前用 5% 硫酸活化 30 s 并禁止徒手触膜；③单纯静浸——侧壁与底面同步受蚀，改喷淋或侧向曝气，气泡冲刷侧壁可减缓钻蚀。经验值：静浸侧蚀比约 1:1（深 0.2 mm 单边扩 0.2 mm），喷淋可压至 1:0.4；设计端预留补偿量最可靠。
Undercut widens lines beyond the artwork and collapses slot walls. Three roots: etch too hot—42°Bé FeCl3 at 45 °C undercuts nearly 3× the rate at 25 °C, so drop fine work to 25–30 °C. Poor resist adhesion—oil or oxide leaves sidewalls unprotected; activate 30 s in 5% sulfuric and never touch the film bare-handed. Pure immersion, where walls etch as fast as floors—switch to spray or lateral aeration, bubbles scrubbing the walls. Rule of thumb: immersion undercuts about 1:1 (0.2 mm lateral at 0.2 mm depth), spray down to 1:0.4. Artwork compensation is the most reliable fix.