POTENTIAL MEASUREMENTS ON FLOATING-FIELD-LIMITING RINGS IN A POWER MOSFET BY AN ELECTRON-BEAM PROBE

Potentials on the floating guard rings in a power vertical-channel dou ble-diffused MOS(VDMOS) transistor were measured for the first time us ing an electron-beam probing system which incorporates a hemispherical retarding field energy analyser. Problems in measuring the potential on the VDMOS by an electron beam probe are that strong local fields on the specimen cause the primary electron beam to shift from the target position and depress the saturation level of the S curve due to forma tion of a potential barrier in front of the specimen. In the measureme nt procedure the primary electron beam shift is corrected by measuring the shift each time an experimental condition is changed. The influen ce of depression in the saturation level of the S curve is eliminated by setting the slice level of the S curve at a low level of less than the maximum depressed saturation level. Measured results well reflect the function of the floating guard ring structure.