Micro-fabricated flow calorimeter for RF power measurement
Microfluidic devices have been extensively used for determination of thermo chemical values and reaction kinetic studies; however, as power sensors, they have been mostly unexplored. Among heat based sensors, calorimetric technique is known to be the most accurate method for power measurement and insensitive to modulation of the signal. Using a micro-fabricated device with an embedded microfluidic system reduces the amount of time needed to reach equilibrium and difficulty in measuring low power since the device has a very low thermal mass. In this work, we present and characterize a novel micro-fabricated and microfluidic-based calorimeter for RF power measurements, with a focus on the microfluidic control for improved accuracy. In our system, a RF power is applied to a thin-film load causing a temperature rise. A microfluidic channel with a serpentine structure in contact with the load carries the heat to temperature sensors where a temperature rise is measured. The measured temperature change is directly related to the power applied, which allows for power measurement. To increase the measurement accuracy, a micro pump and a flow meter are integrated with the microfluidic device to create a closed loop flow control system. We demonstrate the performance of the system using an open-loop control and then by introducing a micropump and flow meter are able to reduce variations in flow rate by a factor of five. This also results in the ability to produce highly reproducible and controllable flow rates, which are needed for consistent RF measurements. The work presented here will lead to a valuable tool for accurate RF power measurement and calibration.