A time-of-flight (TOF) ion sensor system for monitoring an angular distribution of ion species having an ion energy and incident on a substrate includes a drift tube wherein the ion sensor system is configured to vary an angle of the drift tube with respect to a plane of the substrate. The drift tube may have a first end configured to receive a pulse of ions from the ion species wherein heavier ions and lighter ions of the pulse of ions arrive in packets at a second end of the drift tube. An ion detector may be disposed at the second end of the ion sensor, wherein the ion detector is configured to detect the packets of ions derived from the pulse of ions and corresponding to respective different ion masses.