Analysis of TMGa output of on-board cylinders for chemical vapor deposition

The delivery rates of liquid or solid MOCVD precursors depend on the temperature of the liquid precursor inside its container which is also referred to as bubbler, the total pressure inside the container and the carrier gas flow. For use in the CVD process, the liquid precursor is evaporated and transported to the process by use of a carrier gas. The evaporation process requires heat. Unless this heat is provided through a heater, the precursor will cool and the vapor pressure will drop. For high carrier gas flows and large capacity, short bubblers that are commonly used in III-V semiconductor epitaxy production today, we found that the temperature drop and the associated vapor pressure drop will change considerably over the life of the bubbler as the precursor liquid is depleted. For best control of the process (and highest yield), it is beneficial to have a mathematical expression that predicts the expected output concentration over the life of the bubbler. The mathematical expression that we present here uses the carrier gas flow, the precursor level and the aspect ratio of the bubbler as input to predict the expected concentration shift and an associated compensation factor for the carrier gas flow. The mathematical expression uses two fitting parameters. We determined these parameters for TMGa by the measurement of the output concentration from cylinders of different shape under various flow conditions and liquid levels.