Mode-Locked Laser Voltage Imaging for High SNR IC Testing
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Solution Overview
Problem
Current laser-based probing systems for integrated circuits face challenges with low signal-to-noise ratio (SNR) and the need for extensive averaging of measurements due to weak reflections and low modulation levels, limiting the temporal resolution and accuracy in testing and debugging processes.
Innovation Solution
A hybrid laser probing system employing mode-locked laser (MLL) pulses combined with moderately fast electronics, allowing all pulses to be digitized using high-speed analog-to-digital converters (ADCs) and processed within a Field-Programmable Gate Array (FPGA), enabling high-bandwidth signal capture and processing without the need for pulse picking or additional amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional laser-based probing systems are used with weak reflections and low modulation levels, then the system can operate with standard equipment, but the signal-to-noise ratio is low requiring extensive averaging
Solution Approach 1:
The patent employs mode-locked laser pulses that provide periodic, high-intensity illumination of the DUT. The pulsed nature of the laser creates strong periodic modulation signals that are easier to detect and process, reducing the need for extensive signal averaging while improving the signal-to-noise ratio in laser voltage probing measurements
Solution Approach 2:
The system changes the temporal parameters of laser illumination by using mode-locked pulses with specific duty cycles and repetition rates. This parameter optimization enhances the modulation depth and signal strength reflected from the DUT, thereby improving measurement precision without requiring excessive averaging time
2Measurement precision
If all mode-locked laser pulses are digitized using high-speed ADCs and processed in FPGA, then the temporal resolution is enhanced, but the device complexity increases
Solution Approach 1:
The patent replaces traditional mechanical or electronic pulse-picking mechanisms with a fully digital processing approach using high-speed ADCs and FPGA. The mode-locked laser pulses are directly digitized and processed in the digital domain, eliminating the need for complex optical switching or mechanical pulse selection while achieving superior temporal resolution
Solution Approach 2:
The FPGA serves multiple functions simultaneously: it digitizes the laser pulse signals, processes the voltage probing data, performs signal averaging, and controls the overall measurement sequence. This multi-functionality consolidates what would otherwise require separate dedicated components, managing device complexity through integration
3Measurement precision
If mode-locked laser pulses are used instead of continuous wave laser, then the modulation depth and signal strength are improved, but the system requires high-speed digitization capability
Solution Approach 1:
Mode-locked lasers produce periodic pulses with high peak power and well-defined timing. This periodic action creates strong, consistent modulation signals when the laser illuminates switching transistors in the DUT, significantly improving modulation depth compared to continuous wave operation while the regular pulse train simplifies synchronization with high-speed ADC sampling
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the SNR and reduces the need for extensive averaging, allowing for higher temporal resolution and more accurate waveform reconstruction, effectively improving the efficiency and accuracy of laser voltage probing and imaging of integrated circuits.
Implementation Method 1
the laser beam is modulated by the response of various elements (switching transistors) of the DUT to the test vectors. This has been ascribed to the electrical modulation of the free carrier density, and the resultant perturbation of the index of refraction and absorption coefficient of the material of the IC
Implementation Method 2
the laser beam is modulated by the response of various elements (switching transistors) of the DUT to the test vectors. This has been ascribed to the electrical modulation of the free carrier density, and the resultant perturbation of the index of refraction and absorption coefficient of the material of the IC
Implementation Method 3
Light reflected from the DUT is collected and is converted into an electrical signal by a photosensor
Data Source
AI summary
An apparatus and method for laser voltage testing of a DUT is disclosed. The system enables laser voltage probing and laser voltage imaging of devices within the DUT. A selected area of the DUT is illuminating a while the DUT is receiving test signals causing certain of the active devices to modulate. Light reflected from the DUT is collected and is converted into an electrical signal. The electrical signal is sampled by an ADC and the output of the ADC is sent to an FPGA. The FPGA operates on the signal so as to provide an output that emulates a spectrum analyzer or a vector analyzer.