Two-Photon Laser Fault Localization for IC Defect Detection
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Solution Overview
Problem
Current laser-based defect localization techniques in integrated circuits face limitations in spatial resolution due to the wavelength of the laser used, which is insufficient for modern chip designs, particularly at 22 nm design rule, where conventional methods struggle to distinguish between neighboring transistors.
Innovation Solution
The Two-Photon Laser Assisted Device Alteration technique employs femtosecond pulsed lasers with wavelengths below the silicon bandgap to induce two-photon absorption, allowing for higher resolution fault localization by precisely controlling the timing of laser pulses relative to the tester's clock signals, thereby increasing the precision of fault detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional single-photon LADA technique is used, then the system is simple to operate, but the spatial resolution is limited to about 240 nm
Solution Approach 1:
The patent changes the fundamental parameter of light-matter interaction from single-photon to two-photon absorption. This requires using femtosecond pulsed lasers with wavelengths below the silicon bandgap (e.g., 500-800 nm) instead of conventional continuous wave lasers at 1064 nm. The two-photon absorption process provides quadratic intensity dependence, enabling sub-diffraction limited spatial resolution of approximately 120 nm while maintaining system operability through synchronized pulsing with the tester clock
Solution Approach 2:
The patent employs periodic action by synchronizing femtosecond laser pulses with the tester clock signals. The laser pulses are timed to coincide with specific clock edges, creating a periodic interaction that allows precise temporal control of carrier generation. This periodic synchronization enables the complex two-photon process to be integrated into existing test flows without requiring continuous operation, thereby managing system complexity
2Measurement precision
If laser wavelength is reduced to improve resolution, then spatial resolution improves, but optical absorption in silicon increases making light delivery difficult
Solution Approach 1:
The patent exploits the nonlinear parameter of two-photon absorption probability, which scales with the square of light intensity. By using femtosecond pulsed lasers, the peak intensity is sufficiently high to enable two-photon absorption at wavelengths where single-photon absorption would be prohibitive. This allows using shorter wavelengths (500-800 nm) for improved resolution while the nonlinear process compensates for increased linear absorption through the I² dependence
Solution Approach 2:
The patent converts the harmful effect of increased optical absorption at shorter wavelengths into a beneficial feature. The high peak intensity of femtosecond pulses, which would normally cause excessive heating in continuous wave operation, instead enables two-photon absorption through the nonlinear I² dependence. The absorption becomes beneficial because it provides the necessary photon density for two-photon processes while the ultrashort pulse duration prevents thermal damage, effectively converting what would be a harmful high-absorption regime into a useful high-resolution imaging mode
3Measurement precision
If femtosecond pulsed lasers are used for two-photon absorption, then spatial resolution improves by factor of 1.21, but the carrier generation volume reduces by 2000 times requiring precise timing control
Solution Approach 1:
The patent simplifies the operation of femtosecond pulsed lasers by synchronizing them with the periodic tester clock signals. The laser pulses are triggered at specific clock edges, creating a regular periodic pattern that aligns with the test stimulus. This periodic synchronization transforms the complex timing requirement into a straightforward clock-synchronized operation, making the system easy to operate while maintaining the precise timing needed for the reduced carrier generation volume
Solution Approach 2:
The patent implements feedback by using the tester's own clock signals and test responses to control the laser timing. The laser pulses are synchronized to clock edges, and the test vectors are applied in response to detected faults. This closed-loop feedback mechanism automatically adjusts the timing and sequencing, eliminating the need for manual timing calibration and making the system with its precise timing requirements easy to operate
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 fault localization resolution by nearly a factor of 1.21 and reduces the carrier generation volume by up to 2000 times, enabling better identification of fault locations and severity in integrated circuits.
Implementation Method 1
The Two-Photon Laser Assisted Device Alteration technique employs femtosecond pulsed lasers with wavelengths below the silicon bandgap to induce two-photon absorption
Implementation Method 2
A continuous wave (CW) laser to generate localized photocurrents in an integrated circuit's backside... The laser is typically of a short wavelength variety on the order of 1064 nm, so that the photon energy is above the silicon indirect band gap
Data Source
AI summary
A Two-Photon Laser Assisted Device Alteration technique is presented. Fault localization is investigated by exploiting the non-linear two-photon absorption mechanism to induce LADA effects. Femtosecond laser pulses of wavelength having photon energy lower than the silicon bandgap are directed at the area of interest, while the DUT is stimulated with test vectors. The laser pulses are synchronized to the DUT stimulation, so that switching timing can be altered using the two-photon absorption effect.