Method and apparatus for determining process rate

An apparatus for processing a substrate, comprising:

• a processing chamber;
• a substrate support for supporting the substrate within the processing chamber;
• a gas source;
• a gas inlet connected between the gas source and the processing chamber for providing gas from the gas source into the processing chamber;
• a RF power source for providing RF power into the processing chamber;
• an exhaust pressure system, comprising;
• an exhaust pump, with an inlet connected to the processing chamber and an outlet for directing exhaust away from the processing chamber and the exhaust pump, wherein the outlet is an exhaust pipe extending from the exhaust pump, wherein the exhaust pipe has an interior;
• a multipass gas cell with an interior, wherein the interior of the multipass gas cell comprises at least a portion of the interior of the exhaust pipe;
• an IR laser positioned to direct an IR beam into the multipass gas cell;
• and
• a sensor for detecting the IR beam after the IR beam has made multiple passes within the multipass gas cell and providing sensor output;
• and
• a controller connected to receive the sensor output from the sensor and controllably connected to the gas source and RF power source, wherein the controller comprises:
• at least one processor;
• and
• non-transitory computer readable media, comprising:
• computer readable code for measuring a concentration of at least one gas byproduct based on the sensor output from the sensor with respect to time, wherein the concentration of the at least one gas byproduct with respect to time has a plurality of peaks and valleys;
• and
• computer readable code for using times between peaks and valleys to determine transitions between each of a plurality of bilayers on the substrate.