Electron cyclotron resonance (ECR) plasma reactors are being used for ultralarge scale integrated circuit fabrication to meet the stringent requirements on submicron feature etching. Three issues are critical for ECR reactor design: plasma uniformity, ion energy control, and wafer temperature control. Plasma uniformity is important for minimizing over etch times and reducing the probability of producing charging damage. Ion energy control is needed to optimize etching rate, anisotropy, and selectivity without compromising device yield. Wafer temperature control is important because large ion currents at low pressure can result in wafer heating and thereby alter the rates of surface chemical processes. An ECR plasma reactor is described that is designed to etch compound semiconductors and Si at low temperatures (-170 to 20°C), where superior selectivity and linewidth control are achievable. By measuring dc bias, floating potential, and ion saturation current densities it is shown that ion energies in this system can be controlled by applying an rf bias to the sample. To characterize plasma uniformity, the radial ion density profile is measured using a fast injection Langmuir probe. Hollow, peaked, or uniform radial plasma density profiles can be obtained depending on microwave power, pressure, and magnetic field. Plasma density profiles are influenced by microwave absorption and refraction which, in turn, are influenced by both the magnetic field and plasma density profiles. The net result is a strong coupling between wave propagation and charge particle transport. To control wafer temperature a cryogenic electrode capable of maintaining a wafer temperature to ±2.5°C at -170°C is used while the wafer is exposed to an electron cyclotron resonance plasma. The sample temperature is monitored using infrared laser interferometric thermometry and the heat flux to the wafer surface in an Ar plasma is measured as a function of operating parameters by monitoring temperature transients as the discharge is gated on and off.