Method and apparatus using an infrared laser based optical probe for measuring electric fields directly from active regions in an integrated circuit

摘要

A method and an apparatus for detecting an electric field in the active regions of an integrated circuit disposed in a semiconductor. In one embodiment, a laser beam is operated at a wavelength near the band gap of a semiconductor such as silicon. The laser beam is focused onto a P-N junction, such as for example the drain of an MOS transistor, through the back side of the semiconductor substrate. As a result of photo- absorption, the laser beam is partially absorbed in the P-N junction. When an external electric field is impressed on the P-N junction, such as when for example the drain of the transistor switches, the degree of photo-absorption will be modulated in accordance with the modulation in the electric field due to the phenomenon of electro- absorption. Electro- absorption also leads to electro-refraction which leads to a modulation in the reflection coefficient for the light reflected from the P-N junction/oxide interface. The laser beam passes through the P-N junction region, reflects off the oxide interface and metal behind the junction, and returns back through the P-N junction and back out of the silicon surface. Any amplitude modulation in this reflected laser beam is detected with an optical detection system, and is attributed to a corresponding modulation in the electric field in the P-N junction due to the combined effects of electro-absorption and electro-refraction.