1. A semiconductor processing tool, comprising: a chamber; a gas line for providing a source gas to the chamber; and a gas concentration sensor in the gas line, wherein the gas concentration sensor comprises: a first electrode, wherein the first electrode comprises first fingers; and a second electrode, wherein the second electrode comprises second fingers that are interdigitated with the first fingers, the first electrode comprising a first plate along a vertical direction relative to the substrate, the second electrode comprising a second plate along the vertical direction, and the first fingers and second fingers laterally between the first electrode and the second electrode along a horizontal direction relative to the substrate, and wherein the second electrode is capacitively coupled to the first electrode to provide a capacitance to measure a gas concentration.