An ion energy analyzer is described for use in diagnosing the ion energy distribution (IED) of ions incident on a radio frequency (RF) biased substrate immersed in plasma. The ion energy analyzer comprises an entrance grid exposed to the plasma, an electron rejection grid disposed proximate to the entrance grid, and an ion current collector disposed proximate to the electron rejection grid. The ion current collector is coupled to an ion selection voltage source configured to positively bias the ion current collector by an ion selection voltage, and the electron rejection grid is coupled to an electron rejection voltage source configured to negatively bias the electron rejection grid by an electron rejection voltage. Furthermore, an ion current meter is coupled to the ion current collector to measure the ion current.