Paper PS2-WeA1
Novel On-Wafer RF-Current Sensor: Sheath Impedance and Plasma Density

Wednesday, October 22, 2008, 1:40 pm, Room 306

“On-wafer” plasma sensors are new metrology tools that provide spatially-resolved wafer-state and/or plasma information. Measuring the temporal and spatial evolution of wafer-surface and adjacent plasma characteristics is the key to developing advanced plasma tool control schemes. One such commercially available sensor is the PlasmaVolt^TM device, developed by KLA-Tencor. We utilize a 150 mm diameter version of the commercial product consisting of an on-board electronics module with wireless communication that allows data storage of 2 RF-current sensors embedded on the wafer at different radial positions. We report results using this device in an inductively coupled plasma with RF-biased substrate under a range of conditions. Electron density and temperature are independently measured above the wafer using a Langmuir probe and the positive ion current to chamber walls is measured with a shielded planar probe. The RF-voltage and current waveforms applied to the substrate are measured with a digital oscilloscope. The quantitative relation between the PlasmaVolt^TM sensor wafer measurement and the adjacent electron density is established using a fluid sheath model. We demonstrate with this combined experimental-modeling approach that the plasma density scales with the RF-current sensor measurements but the nature of the scaling is dependent on the sheath impedance. When the sheath impedance is predominately capacitive (corresponding to relatively low electron density and high RF-bias voltage) sensor measurements are proportional to the square-root of the electron density. When sheath impedance is more resistive (corresponding to relatively high electron density and low RF-bias voltage), the sensor measurement is proportional to electron density.