An ECR microwave plasma etching system and an alternating bias voltage source with variable frequencies ( 10kHz to 10 MHz ) have been built for the study of the effects of the substrate bias on the submicron feature etching. A quadrupole mass spectrometer was used for the end-point monitoring and the plasma chemistry analysis. A Langmuir probe was used to characterize the etching plasma. The system can generate plasmas with densities ranging from 1×1015m−3 to 5×1016m−3 corresponding to gas pressures from 1×10−4 torr to 6×10−3torr at a fairly low microwave power. This condition results in a stable electron temperature. The magnetic field profile can be adjusted to obtain a constant plasma density of 2.8×1016m−3 and an electron temperature in kTe of 4 eV to 5 eV. The plasma beam is of 75mm in diameter with the ion current intensity fluctuation of about ±5% at the etching position for the gas pressures ranging from 4×10−4 torr to 3×10−3 torr at an absorbed power of 70Watts. The plasma density, the electron temperature, the breakdown power, and the sustaining power have been studied as functions of gas pressure in order to optimize the etching performance.