Study and Design of a Micro-Fabricated DC Substitution Calorimeter for RF Power Measurement.

摘要

Diode detection and bolometric detection have been widely used to measure radio frequency (RF) power. However, flow calorimeters, in particular micro-fabricated flow calorimeters, have been mostly unexplored as power meters. This thesis presents the design, simulation, optimization, micro-fabrication and characterization of a DC substitution flow calorimeter. This novel device is capable of measuring power from 100 microW to over 200 mW. It has a 50-Ohm load that is heated by the RF source, and the heat is transferred to the fluid in a microchannel. The temperature change in the fluid is measured by a thermistor that is connected in one leg of a Wheatstone bridge. The output voltage change of the bridge corresponds to the RF power applied to the load. The ANSYS simulation results demonstrated the operation of the proposed calorimeter and proved that deionized water gives better performance than mineral oil in term of heat transferring from the load to the temperature sensor. Advanced Design System (ADS) has been used to create a model for the proposed coplanar waveguide and load and to optimize it in order to match 50-ohm impedance. S11 results showed that using the proposed structure, over 99.99% of the power could be transmitted at about 4.2GHz when the load thickness is around 140nm and the coplanar waveguide thickness is around 500nm. E-beam evaporator and DC sputtering metal deposition processes have been optimized to maximize the conductivity of deposited aluminum and tantalum nitride. The obtained optimized aluminum resistivity is 31.72 E-9 ohm.m, and tantalum nitride resistivity is 5.42 10E-6 ohm.m. This optimization results to an effective efficiency of 99.9% from 0 to 1GHz and more than 97.5% at frequencies up to 4 GHz. The measured reflection coefficient of the load and coplanar wave guide is less than --25 dB from 0 to 2GHz and less than --16 dB at 2GHz to 4GHz. The final microfabricated device measures 25.4 mm x 50.8 mm, excluding the power supplies, microcontroller, flow sensor and fluid pump. Experiments demonstrated that the micro-fabricated sensor has a sensitivity up to 22 E-3 V/W. The typical resolution of this micro-calorimeter is on the order of 90 microW, and the best resolution is around 10 microW. In order to accurately measure an unknown RF power, DC substitution has been used and resulted in a measurement accuracy above 99.7% excluding the error coming from coplanar waveguide and load impedance mismatch.