Apparatus and method for measuring charge density distribution

Apparatus for measuring charge density distribution at the surface of a material sample, comprising:

• an electric potential sensor, wherein the electrical potential sensor comprises a probe for capacitively coupling the electric potential sensor to the surface of a material sample, an amplifier for generating a measurement output, the probe being connected to an input of the amplifier and the measurement output being supplied at an output of the amplifier, and a feedback arrangement driven from the output of the amplifier for enhancing the input impedance of the amplifier;
• a positioning system configured to mount the probe of the electric potential sensor above the surface of the material sample and to move the probe at a constant height over the surface;
• and
• a processor configured to:
• repeatedly cause the positioning system to step the probe from a first to a second position;
• record a transient signal from the measurement output as the probe moves from the first to the second position for each step;
• determine a value proportional to a difference in charge density at the first and second positions from the transient signal for each step;
• and
• integrate the determined values with respect to position to obtain a profile of charge density against position.