Microfabricated Retarding Potential Analyzers With Enforced Aperture Alignment for Improved Ion Energy Measurements in Plasmas

We report novel retarding potential analyzers (RPAs) intended to measure the ion energy distribution of cold, high-density plasmas (λ_D > 50 μm, i.e., T_e/n_i ≥ 5.25 × 10^-13 K·m^-3, e.g., T_e ~ 2 eV and n_i ~ 5 × 10¹⁶ m^-3; total pressure ~40 mTorr). By scaling down the electrode apertures and electrode separation, densely packing apertures in each electrode, and enforcing interelectrode aperture alignment, the sensor measures ion energy distribution with larger signal-to-noise ratio and resolution compared with a conventional RPA. Two implementations are demonstrated, i.e., a hybrid retarding potential analyzer (RPA) with microfabricated grids and precision-machined housing, and a fully microfabricated microelectromechanical systems (MEMS) RPA with deflection springs that enforce aperture alignment across the electrode grid stack. Using an ion source, the MEMS RPA generated data with an order of magnitude larger signal and one third the peak width compared with the data from a conventional RPA. Unlike the conventional RPA, the hybrid and MEMS RPAs were able to measure the ion energy distribution of high-density plasma from a helicon source over a wide range of power levels; however, the MEMS RPA generated data with larger signal strength and resolution, with as much as a 200-fold increase in signal strength and less than half the full width at half maximum, compared with the data from the hybrid RPA. The hybrid RPA and MEMS RPA plasma data are consistent with measurements from an independent double Langmuir probe installed in the facility.