Application Description

Laser voltage probing (LVP) and laser voltage imaging (LVI) are techniques used in electronic failure analysis to investigate single devices and full chips while they are operated in a controlled mode.

The device under test (DUT) is excited with a specific signal, such as a clock or data pattern for digital chips and a controlled waveform for analog ones. The DUT – usually prepared by thinning its substrate to minimize light absorption – is then illuminated by a continuous-wave laser. A photodiode records the reflected light, which is modulated by the carrier density and, as a consequence, by the electrical signal in the interaction spot. The signal detected by the photodiode can thus be compared to the expected chip behavior. Infrared lasers were the preferred choice for LVP and LVI, but decreasing transistor sizes and increasing transistor densities call for a shift to shorter wavelengths to get access to higher resolution.

LVP requires the laser spot to be focussed at a specific point on the chip, whereas in LVI the laser beam moves relative to the chip following a raster pattern to create a 2D image of the chip. The objective of the former technique is to obtain high-precision measurements at specific chip locations: looking at the signal phase, for example, is useful to derive the propagation delay of the signal. On the other hand, LVI aims at creating 2D images of the chip to compare themse to CAD drawings and evaluate the behavior of the chip under given operating conditions.

Measurement Strategies

The modulation of the reflected signal is rather weak, and the signal itself is absorbed by the residual substrate thickness. Such challenging conditions require sensitive and low-noise instruments for signal measurement and analysis, particularly when using visible light for which substrate absorption is stronger.

The signal from the photodiode is acquired with a spectrum analyzer centered at the data pattern repetition rate or at the clock frequency and with a measurement bandwidth chosen in the light of the required measurement speed and of noise considerations. Additionally, LVP/LVI systems include an oscilloscope to visualize the reflected waveform pattern and measure the signal phase for propagation delay measurements.

A lock-in amplifier brings several advantages over a traditional LVP/LVI system:

• Higher signal-to-noise ratio (SNR), thanks to lower input noise and a finer adjustment of the filter properties.
• Phase information, useful to separate inverting and non-inverting areas on the chip and distinguish between p- and n-doped areas.

• High-precision signal propagation delay measurements through phase information.
• Analog and digital interfacing.

A higher SNR leads to clearer images and shorter recording times; access to phase information helps to increase the effective obtainable resolution.

A boxcar averager also offers advantageous capabilities for LVP/LVI measurements:

• Even higher signal-to-noise ratio (for a comparable measurement duration).
• The measurement of signals with a duty cycle much lower than 50%.
• High time-domain selectivity, useful for measurements requiring precise time discrimination (with long bit patterns or specific signal propagation delays, for instance).
• Distinction between p- and n-doped areas.

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