Optical detection method of electrical signals inside integrated circuits based on lock-in amplifier

As the number of interconnected layers on the front side of integrated circuits increase， it becomes increasingly difficult to detect the internal electrical signals from the front side of integrated circuits. A probing optical path based on the common-path interferometer is designed in this study to use a laser for detecting the internal electrical signal waveform from the back side of an integrated circuit without making contact. To this end， a signal processing method based on a lock-in amplifier is proposed to extract the weak electro-optical signal of the device carried in the reflected light， which exploits the strong noise rejection and high sensitivity of the lock-in amplifier technique. Considering that the electro-optical signal of the device varies periodically with the electrical signal， the electrical signal from the photodetector is first processed by the lock-in amplifier to eliminate most of the noise， and then the averaging technique is employed to further suppress the external noise and improve the signal-to-noise ratio of the electro-optical signal of the device. Finally， the electro-optical signal of a device drowned in noise is extracted and the electrical information inside the device is reconstructed. The electro-optical signal of the circuit node with a dynamic operating current in the order of μA inside the chip is successfully detected using the proposed method. The signal-to-noise ratio of the extracted signal reaches 4.99 dB， while the signal-to-noise ratio of the signal obtained solely via the averaging technique is only -44.29 dB. This paper presents a novel method for the optical detection of electrical signals inside integrated circuits， which can be applied to perform dynamic defect detection of integrated circuits in the future. © 2022 Guangxue Jingmi Gongcheng/Optics and Precision Engineering. All rights reserved.