Patent ID: 7476849

Claim:
A time-of-flight ion sensor for monitoring ion species in a plasma chamber, comprising: a drift tube; an extractor electrode and a plurality of electrostatic lenses disposed at a first end of the drift tube, wherein the extractor electrode is biased to attract ions from a plasma in the plasma chamber, and wherein the plurality of electrostatic lenses cause at least one portion of the attracted ions to enter the drift tube and drift towards a second end of the drift tube within a limited divergence angle; an ion detector disposed at the second end of the drift tube, wherein the ion detector detects arrival times associated with the at least one portion of the attracted ions; and a housing for the extractor, the plurality of electrostatic lenses, the drift tube, and the ion detector, wherein the housing accommodates differential pumping between the ion sensor and the plasma chamber, wherein the plasma chamber holds a wafer for processing in the plasma, wherein the ion sensor as positioned along a side of the wafer so that the ion sensor and the wafer share substantially the same vantage point with respect to the plasma, and the wherein the housing is biased at approximately the same potential as the wafer.