edfas.org 33 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 2 acquiring clean repeatable data sets from a reference unit and then extracting the equivalent data from the fail unit. If there is a soft failure or a failure mode that can be modulated by changing the value of the supply voltage at a given clock frequency, begin the analysis by probing the fail at a low Vdd and then raising the voltage to observe the pass behavior from the same target. This saves time where applicable by avoiding the need to swap samples and realign the probe tool. While this technique allows for qualitative comparisons, it has two main drawbacks: a) the waveforms are taken at different Vdd values and therefore will have slightly different timings, and b) the location of the probe relative to the target node cannot be exactly repeated. Extracting reference vs. fail waveforms from a transistor that is physically larger than the laser focus spot size is relatively easy, where waveforms are repeatable and exhibit only logically obvious high and low values. However, as gate pitch has scaled downward (for example, starting at about 28 nm technologies), the smallest drain layouts of transistors can result in multiple signals in the spot size of the probe laser. This laser spot size issue continues to be more problematic due to the endless pursuit of Moore’s Law, where device density per unit area has exponentially increased. The size of the probe targets versus the diffraction limited laser spot size on < 16 nm FinFET technology has made probing increasingly problematic due to a phenomenon known as waveform crosstalk. In simple terms, when there is more than one optical modulation source within the laser spot size, the collected waveform manifests a superposition of all of the potentially unique but physically adjacent Fig. 1 (a) Example of LVP logical waveforms acquired during a typical scan failure debug. (b) Example of a data vs. clock timing measurement data set used when analyzing a typical speed path within a CPU core. (a) (b) detected signals. Crosstalk makes interpretation of collected waveform data significantly more complicated and becomes progressively worse as technology shrinks.
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