ultrasonic transducers and tomographic techniques determine the temperature of a semiconductor substrate holder at all points on the substrate holder , thereby allowing comprehensive real - time control of the temperature of the substrate holder during a process , such as a semiconductor wafer etching process . an apparatus for measuring temperatures of respective portions of a substrate holder that supports a substrate on which a process is carried out , and for controlling the temperatures of the respective portions in response to the measured temperatures , includes : an arrangement of at least one ultrasonic transducer arranged and configured to transmit ultrasonic energy through the substrate holder , and a data processor configured to calculate , during the process , the temperatures of the respective portions of the substrate holder based on respective propagation time delays of the ultrasonic energy through the respective portions .