  A modified AC method based on micro-fabricated heater and resistive
thermometers has been applied to measure the thermopower of microscale samples.
A sinusoidal current with frequency {\omega} is passed to the heater to
generate an oscillatory temperature difference across the sample at a frequency
2{\omega}, which simultaneously induces an AC thermoelectric voltage, also at
the frequency 2{\omega}. A key step of the method is to extract amplitude and
phase of the oscillatory temperature difference by probing the AC temperature
variation at each individual thermometer. The sign of the thermopower is
determined by examining the phase difference between the oscillatory
temperature difference and the AC thermoelectric voltage. The technique has
been compared with the popular DC method by testing both n-type and p-type thin
film samples. Both methods yielded consistent results, which verified the
reliability of the newly proposed AC method.
