Ultra-Low Frequency Laser Voltage Imaging of Mixed-Signal Designs

During the last years, laser reflectance modulation measurements (i.e. LVI, CW-SIP etc.) have become indispensable tools for the analysis of logic circuits at frequencies in the megahertz range. In this paper we present a method to extend the usefulness of these methods to mixedsignal circuits driven at ultra-low frequencies in the kilohertz range. We show that by toggling the main power supply, information of the electric behavior can be easily obtained from analog structures, removing the need for tester-based stimulation. This method proved especially useful for the debugging of chip startup failures. We demonstrate this with two case studies. In a first case, a defect in the analog part shut down the digital part of the chip. This prevented the use of debugging methods such as the read-out of error registers or the use of scan chains. Conventional methods like photon emission microscopy and thermal laser stimulation were also not successful at finding the problem. However, laser-voltage imaging (LVI) of the analog circuit at key locations while toggling the chip power supply in the kilohertz range led us to the failing net. In a second case on a different product, we similarly identified a failing capacitor in the error logic by modulating the chip enable pin in the kilohertz range.