Investigation of Laser Voltage Probing Signals in CMOS Transistors

The increasing number of interconnection layers is making failure analysis of integrated circuits (ICs) increasingly complex. In addition, the decreasing supply voltage is a challenge for failure analysis equipment, as minimum detection levels are reached. Laser voltage probing (LVP) is a common method for probing internal nodes through the silicon back side, and is particularly suited to low-voltage measurements. However, to date there has been little work on detailed understanding of the physical origin of LVP signals. This article presents measurements of LVP signal strength on large test structures over a wide range of parameters as a first approach to detailed understanding of LVP physics and future scaling. All the measurements we present were performed using a continuous-wave 1319 nm laser. In addition to the usual time-domain LVP measurements, the authors demonstrate rapid data acquisition scheme using frequency-domain measurements. This method allows rapid voltage sweeps and also allows for (x, y) mapping of signal strength across device structures. The signal strengths are in accord with qualitative models.