Abstract: A method to fabricate bottle-shaped deep trench in a semiconductor substrate which mainly involves two substitute plasma etching steps from the conventional approach. After a neck profile is formed, instead of raising the plasma gas pressure while keeping the etching composition constant, as in the conventional approach, the plasma gas pressure is first maintained the same, then decreased substantially. On the other hand, the concentrations of HBr and NF.sub.3 are increased substantially in both new steps. The first substitute plasma etching step is conducted at a pressure of 100 mtorr an RF power of about 1,000 W, a magnetic field of 65 Gauss. The plasma gas composition consists of HBr, NF.sub.3, and (He/O.sub.2) a at a ratio of about 200:20:20. The second substitute plasma etching step is conducted at plasma gas pressure of 30 mtorr, an RF power of 600 W, a magnetic field of 65 Gauss. The plasma gas composition consists of HBr, NF.sub.3, and (He/O.sub.2) a at a ratio of about 150:13:20.