Methods and systems for monitoring film thickness using a sensor assembly include a process chamber having a chamber body, a substrate support disposed in the chamber body, a lid disposed over the chamber body, and a sensor assembly coupled to the chamber body at a lower portion of the sensor assembly. The sensor assembly is coupled to the lid at an upper portion of the sensor assembly. The sensor assembly includes one or more apertures disposed through one or more sides of the sensor assembly, and the one or more sensors are disposed in the sensor assembly through the one or more of the apertures.