Practical Electron Microscopy and Database

In Electron Beam Probing (eBP) measurements, the clock is supplied by an external signal generator, which drives the test structures during the measurements. The eBP captures the signal transitions corresponding to the clock edges. These signal edges reflect the high-frequency clock behavior of the circuit. The ability to detect such signal transitions in the eBP indicates that the clock signal is being properly generated and propagated within the circuit, allowing for real-time analysis of clock cycles and signal switching behavior. The measured eBP signal is sensitive to voltage changes caused by the clock and other signal transitions on the chip. As the clock drives different parts of the circuit (e.g., flip-flops or logic gates), the resulting voltage changes produce charge redistribution at the transistor junctions, which the eBP detects as signal variations. The clear signal edges in the eBP measurement reflect the precise timing of clock-driven transitions, enabling the detection of clock behavior during functional analysis of the chip.

In the eBP measurements used for clock edge detection, the electron beam is typically discontinuous, or pulsed, rather than continuously on. This pulsing allows the beam to probe specific time intervals or transitions in the circuit, such as the rising or falling edges of the clock signal. By pulsing the electron beam in sync with the clock or other signals, the eBP system can capture dynamic behavior at different phases of the clock cycle. On the other hand, continuous electron beam exposure would result in a time-averaged signal and would not be suitable for capturing the high-speed transitions associated with clock edges.

In this setup, the external signal to the chip (e.g., the clock signal) operates at its designed frequency, while the electron beam is used to capture snapshots or transitions at key moments during the signal cycle. The eBP system captures these transitions by modulating the electron beam to match or synchronize with the timing of the signal, allowing it to detect the voltage changes as the clock signal drives the circuit. Thus, the electron beam is pulsed or timed to probe specific points of interest rather than matching the external signal's frequency directly.

When comparing the eBP signals of a bad transistor versus a good transistor, the differences in the signal would likely reflect the transistor's ability to switch and conduct properly:

• Good Transistor:
  • Clear Signal Transitions: For a good transistor, the eBP signal will show well-defined transitions corresponding to the switching events (e.g., from high to low or low to high). The signal should be strong and consistent with the applied clock or input signal.
  • Expected Amplitude: The amplitude of the eBP signal would match the expected voltage levels (e.g., Vdd and ground) when the transistor is turned on and off correctly.
  • Minimal Noise: The signal-to-noise ratio would be relatively high, and the noise margin would be low, allowing for clean detection of the transistor's switching activity.
  • Correct Timing: The signal would reflect proper timing behavior with respect to the input or clock signal, with transitions occurring at the correct moments.

• Bad Transistor:
  • Weak or Distorted Transitions: In a bad transistor, you might observe weaker or poorly defined transitions in the eBP signal. The signal might not show a clear shift between high and low states, or the transitions could be delayed, erratic, or incomplete.
  • Reduced Amplitude: The voltage levels detected via eBP might be lower than expected, indicating that the transistor is not fully switching on or off, possibly due to higher resistance, leakage, or failure in the transistor's gate, drain, or source.
  • Increased Noise: The eBP signal could exhibit more noise or instability, reflecting problems in the transistor's ability to switch cleanly. This noise could make it harder to distinguish valid transitions from spurious signals.
  • Incorrect Timing: The timing of the signal might be off, with transitions occurring earlier or later than expected. This could result from the transistor not properly following the clock signal, leading to phase shifts or missed transitions.