Real time observation of surface potential distribution with an EFM on CTL

Experiments were conducted with an EFM (Electrostatic Force Microscope) to measure surface potential distribution on CTL (Carrier Transport Layer) having two different charge mobility characteristics. One CTL has lower charge mobility characteristic and other CTL has higher charge mobility characteristic. Four electrodes were placed on each CTL with 100 micrometers and 200 micrometers separation respectively. We had a detector of the EFM scan over three electrodes to measure surface potential distribution on the surface of each CTL. We applied a known voltage on two electrodes and remaining electrodes were connected to ground. We consequently could observe significant surface voltage fluctuation real time basis on both CTL films. The substantial fluctuation of surface voltage distribution could explain charge migration in CTL toward horizontal direction, which is perpendicular to applied electric field. The thickness of each CTL was only 5 micrometer therefore the electric field which was perpendicular to the CTL films was supposed to be very strong and uniform. Background Measurement of surface voltage distribution on electrophotographic photoreceptor is commonly made with using an electrostatic voltmeter. Electrostatic voltmeter is conventionally used with a non contacting method with taking a fairly large spacing (around 3 mm) between detector probe and surface of photoreceptor, therefore the expected spatial resolution from measurements with an electrostatic voltmeter is extremely large so it is inadequate to accomplish a direct measurement of electrostatic latent image on a photoreceptor. Surface voltage measurement on photoreceptor with much higher spatial resolution than conventional electrostatic voltmeter has been a critical demand for electrophotography for a long time. Trek Japan K.K. and Nihon University have been working jointly on a research for the development of electrostatic voltage measurement apparatus having relatively high spatial resolution. Surface voltage on electrophotographic photoreceptor is relatively high namely in the range between the absolute value of 200 V and 800V and area needs to be measured is fairly large. What required on a new surface voltage distribution measurement apparatus, i.e. an electrostatic force microscope (EFM) are (1) having a spatial resolution of 10 μm in diameter, (2) having a capability to measure wide area such as it should cover an area as large as a few 10 cm, and (3) the measurable surface voltage should be as high as 1kV. For the sake of understanding our goals clearly, we have done numerical simulations with the Finite Element Method (FEM) to start with. [1] Principle of Electrostatic Force Microscope Figure 1. Principle of EFM Tip of detector on an optical cantilever is placed close to surface under test. DC bias voltage (VDC) and AC bias voltage (VAC sin t) are applied to cantilever including detector. If the surface under test is charged, we should be able to expect either attractive or repelling electrostatic force appeared on the detector. Either attractive or repelling force can be detected through measuring bending amount of the cantilever with an optical leverage method. The electrostatic force appeared on the detector consists of two different force components F as well as and F2 where F has the same frequency component as the applied AC bias voltage whereas F2 has twice higher frequency component as the applied AC bias voltage. If we estimate a parallel plane model on the apparatus as shown in Figure 2, those two forces are obtained with the following equations [2], [3], If the distance between detector and surface under test (d d0) as well as the dielectric constant of a material under test ( ) are evident, we can measure F with applying a pre-set VDC to cantilever and detector so that we can simply calculate the voltage on surface under test ( d0/ ) with aforementioned equation (1). However, since high voltage is in existence on the surface under test, an arcing between detector and surface under test may occur. To prevent from the occurrence of any contingent arcing between detector and surface under test, we discussed a zero voltage method. We obtained a VDC with which F became to be zero in advance. 2 2 cos } ) 1 ( { 4 1 1 sin } ) 1 ( { 2 0 2 0 0 2 0 2 0 0 0 0 t SV d d F t SV d d d V F