Laser Voltage Probing and Imaging
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.
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.
The Benefits of Choosing Zurich Instruments
- The UHFLI Lock-in Amplifier offers the necessary capabilities for all LVP/LVI approaches in one box and a single user interface by integrating two lock-in amplifier units, a digital oscilloscope, a spectrum analyzer, two boxcar averager units (with the UHF-BOX Boxcar Averager option), and a periodic waveform analyzer (PWA) for visualizing and recording the reflected waveform or data pattern (with the UHF-BOX Boxcar Averager option).
- The UHF-AWG Arbitrary Waveform Generator option makes it possible to generate the excitation signals for the DUT without additional hardware. From pulse patterns with full control over all the pulse parameters to truly arbitrary waveforms to drive analog chips, the user-friendly C-like sequencer language of the UHF-AWG guarantees extreme timing accuracy.
- All functionalities of the UHFLI can be used in parallel, so that you can run all approaches (spectrum analyzer, lock-in, and boxcar) simultaneously and choose the best result for each case.
- It is possible to perform time-resolved measurements such as isolating specific bits in a pattern or highlighting only chip areas with a specific signal propagation delay: the two Boxcar averager units make the implementation of complex time-resolved measurements straightforward.
- The UHFLI can measure up to 8 harmonics simultaneously and produce one image for each. Further, the UHF-MF Multi-Frequency option enables parallel measurements of signals with completely different frequencies: this is important to characterize an analog circuit at multiple frequency points in parallel, for instance.
- With a dedicated imaging module, you can acquire LVI images directly in LabOne.