Laser-Assisted Defect Localization Using Synchronized Pulses
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Solution Overview
Problem
Conventional laser-assisted defect localization techniques in integrated circuits face limitations in spatial resolution, particularly at smaller design rules, due to the wavelength of the laser used, which prevents accurate identification of defects among closely spaced transistors.
Innovation Solution
The use of a pulsed laser synchronized with the device's clock to enhance spatial resolution, either through single-photon or two-photon absorption mechanisms, allowing for precise timing of laser pulses to alter transistor responses and improve fault localization beyond the limitations of continuous wave lasers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a continuous wave laser is used for LADA, then the device can operate at standard clock frequencies, but the spatial resolution is limited to 240 nm due to the laser wavelength
Solution Approach 1:
The patent applies periodic action by using pulsed laser operation synchronized to the device clock frequency. Instead of continuous wave illumination, the laser delivers periodic pulses that coincide with specific clock cycles, enabling temporal gating of the LADA signal. This periodic excitation allows the system to resolve individual transistor switching events within the laser beam path, effectively achieving sub-240 nm spatial resolution through time-domain multiplexing while maintaining compatibility with standard device operating frequencies
Solution Approach 2:
The patent transitions from spatial-domain resolution limits to temporal-domain discrimination by introducing time as an additional dimension for defect localization. By synchronizing pulsed laser excitation with the device clock and analyzing LADA signals across multiple clock cycles, the system distinguishes between transistors that are spatially overlapping but temporally separated in their switching behavior. This dimensional transformation from purely spatial to spatio-temporal analysis resolves the fundamental wavelength-based resolution barrier
2Measurement precision
If the laser wavelength is shortened to improve spatial resolution, then resolution increases, but optical absorption in silicon increases, preventing backside illumination
Solution Approach 1:
The patent employs periodic pulsed laser operation at wavelengths that would otherwise be too strongly absorbed for backside illumination. By delivering energy in synchronized pulses rather than continuous wave, the system achieves sufficient peak power for LADA signal generation while the duty cycle remains low enough to manage thermal effects. The temporal synchronization with clock cycles allows discrimination of signals from individual transistors despite the broader spatial illumination footprint caused by longer wavelength usage
Solution Approach 2:
The patent introduces dynamic control of laser parameters including pulse width, repetition rate, and synchronization phase relative to the device clock. This dynamic adjustment allows optimization of the balance between achieving sufficient LADA signal strength (requiring higher peak powers) and managing optical absorption effects (requiring lower average powers). The system adapts pulse characteristics based on the specific device under test and target transistor location, enabling effective operation at wavelengths that would be problematic for static continuous wave illumination
3Measurement precision
If pulsed laser synchronization is implemented to improve spatial resolution, then individual transistors can be identified, but the system complexity increases due to synchronization requirements
Solution Approach 1:
The patent leverages the existing device clock signal, which is already present and operating in the device under test, to drive the laser synchronization. Rather than requiring an external reference or additional clock generation hardware, the system uses the device's own operational clock as the synchronization reference. This multi-functional use of the clock signal for both device operation and laser timing eliminates the need for separate synchronization infrastructure, reducing overall system complexity while achieving the desired temporal resolution
Solution Approach 2:
The patent implements feedback mechanisms where the LADA signal response is monitored and used to adjust and refine the laser pulse timing and synchronization phase. By analyzing the correlation between laser pulse timing and observed LADA signal strength across different clock cycles, the system automatically optimizes the synchronization parameters. This closed-loop feedback approach simplifies the synchronization process by allowing the system to self-adjust rather than requiring manual calibration or complex pre-synchronization procedures
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 enables higher spatial resolution in fault localization, allowing for the identification of individual transistors within the laser beam, even at smaller design rules, by synchronizing laser pulses with the device's clock, thereby improving the accuracy of defect detection and characterization.
Implementation Method 1
either through single-photon or two-photon absorption mechanisms
Implementation Method 2
either through single-photon or two-photon absorption mechanisms
Data Source
AI summary
A pulsed-laser LADA system is provided, which utilizes temporal resolution to enhance spatial resolution. The system is capable of resolving CMOS pairs within the illumination spot using synchronization of laser pulses with the DUT clock. The system can be implemented using laser wavelength having photon energy above the silicon bandgap so as to perform single-photon LADA or wavelength having photon energy below the silicon bandgap so as to generate two-photon LADA. The timing of the laser pulses can be adjusted using two feedback loops tied to the clock signal of an ATE, or by adjusting the ATE's clock signal with reference to a fixed-pulse laser source.