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This work presents an interface circuit for low-frequency dithering measurements of resistor-based transducers. It is demonstrated in the context of a polyimide-shank scanning thermal microscopy probe which provides high thermal sensitivity and spatial resolution, but has a low bandwidth from both mechanical and thermal perspectives. These pose challenges in temperature dithering and control, as well as noise immunity. The circuit includes a proportional-integral controller and a demodulator, along with appropriate amplifier and filter blocks. It keeps the average temperature of the probe tip constant while synchronously detecting variations in the second harmonic of the modulated signal as the tip is scanned across the sample surface. Strategic choices in the circuit architecture and topology are evaluated, and the overall system including the sensor and the circuit is simulated. Measurements of the implemented system show that a signal-to-noise ratio (SNR) of 15.7 is achieved while scanning a photoresist sample of 218 nm thickness on a silicon substrate, and that the detection limit for variations in thermal conductance is <3 pW/K.
Date of Publication: Feb. 2005