Paper PS2-TuM1
Wafer-level Plasma Parameters Measurements in a Multi-Frequency Capacitively Coupled Plasma Discharge

Tuesday, October 19, 2010, 8:00 am, Room Galisteo

Two complications with wafer-level measurements in a capacitively coupled plasma (CCP) discharge are very high DC (~ -1kV) and radio-frequency (RF) (~2kV peak-to-peak) voltages of the substrate, and the lack of theoretical basis for interpretation of volt-ampere characteristics (VACs) of the probes inside the RF sheath. In this work, we present the diagnostic apparatus that measures ion current to the wafer, along with near-sheath plasma density (n_e) and electron temperature (T_e). Particle-in-cell (PIC) and fluid plasma simulations are used to help interpret collected VACs. Measurements are performed using a set of radially distributed planar double probes (DP). The electronic circuit located outside of the discharge chamber provides: (1) DC isolation and RF filtering of the high voltage, (2) biasing voltage to the probes, (3) switching between the probes, and (4) probe current measurements. Electrical signals are brought in and out of the chamber using a specially designed feed-through and a low-profile connection to the substrate. Results of wafer-based measurements performed in an Applied Materials CCP chamber at a variety of rf-frequencies (2, 13, 162 MHz), rf power levels (300 – 1000 W), neutral fill pressures (30 – 100 mT), chemistries (Ar, O2, Ar/CF₄), and magnetic field configurations are presented. At low frequency, pressure was found to have stronger effect on ion current and plasma density than that at intermediate and high frequencies; in all chemistries. The effect of mixing low and medium frequencies with the high frequency was found to be most pronounced at the periphery of the discharge. Magnetic field was confirmed to be a powerful knob for controlling radial uniformity of the discharge at all frequencies; namely, edge current and density tend to increase with application of the magnetic field. To interpret VACs, a 2-dimensional fluid plasma model was developed for the CCP chamber with a DC-biased pad on the substrate. This model was used to calculate current at the DC-biased pad versus applied DC voltage, i.e. single probe (SP) VAC. The SP VACs for a variety of discharge rf-voltages and neutral pressures were then used to derive the DP VACs, which were in turned analyzed using standard experimental techniques to obtain plasma parameters. Those were found to be in a good agreement with near-sheath plasma parameters calculated self-consistently by the fluid model. PIC simulations confirm the results of fluid simulations, but also highlight the highly non-equilibrium nature of electron energy distribution at the electrode.