Hard copies with higher spatial resolution and better quality full color pictures are always expected in electrophotography technology. The photoreceptor is a key device to acquire the high quality hard copies. While on the one hand it is required to make a precise measurement of charge distribution on a photoreceptor drum, on the other hand the spatial resolution of currently available apparatus is fairly low. Charge distribution measurement with a very high spatial resolution is required both in electrophotography and in semiconductor research. It would be desirable to realize a measurement system which enables the charge distribution measurement to have a spatial resolution less than 10 xcexcm in diameter with utilizing the electrostatic force. Laser printers may already have the spatial resolution of 600 dpi or higher, which indicates that each pixel has approximately 21 xcexcm in diameter. Studies have been made relating to the scanning electrostatic force microscope, however, the theoretical aspects of those studies were only extended to the analysis of a parallel plate model and no further discussion was made relating how the detector needle would affect the charge distribution measurement.
A cantilever shaped sensor is normally used for atomic force microscopes, electrostatic force microscopes and similar critical dimension measurement instruments. The cantilever for those applications always consists of a needle or tip detector part and an arm part. If an electrostatic force appears at the needle part, other electrostatic force which is caused by the same electrostatic field appears at the arm part which should generate measurement error. It would, therefore, be highly desirable to shield the arm part to prevent the electrostatic force from appearing on the arm part, so that the accuracy of the measurement can be improved.
The present invention provides an electrostatic force microscope for measuring electrostatic force of a sample under test including a detector comprising a cantilever arm having a tip formation at one end and located so that electrostatic force is induced at the tip due to electrostatic charge on the sample under test, an optical system for transforming bending of the cantilever arm due to electrostatic force induced at the tip into an electrical signal containing a frequency component of the electrostatic force induced at the detector tip, means for applying bias voltage to the detector, means for detecting the frequency component of the electrostatic force induced at the detector tip so that a measurement of electrostatic force on the sample under test can be obtained, and an electrostatic shield operatively associated with the cantilever arm. The shield is located between the cantilever arm and the sample under test, in particular in close spaced relation to the arm. The cantilever arm and the shield are maintained at the same electrical potential so that lines of electrostatic force are terminated at the shield.