Wafer Inspection Using Segmented Pulsed Laser Illumination
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Solution Overview
Problem
Current wafer inspection systems face challenges in achieving high sensitivity and throughput due to limitations in illumination and collection subsystems, particularly with spiral inspection sequences, where defects are not detected effectively due to coordinate mismatch and noise from two-dimensional sensors, and area inspection systems lack flexibility in illumination and collection.
Innovation Solution
A system that uses a Q-switched laser to direct multiple pulsed light beams with different shapes, sizes, wavelengths, and polarizations across the wafer, with a computer subsystem to determine the optimal power for illumination and prevent unnecessary illumination based on scattered light detection, employing a scanning and collection subsystem to image and process light for defect detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If spot-illuminated inspection systems are used, then sensitivity to particles and defects is improved, but inspection throughput is limited due to the need to scan across the wafer surface
Solution Approach 1:
The illumination is divided into multiple discrete spots arranged in a grid pattern across the wafer surface. Each spot is independently controlled and illuminated sequentially, allowing parallel detection across multiple locations while maintaining the sensitivity benefits of focused illumination. This segmentation enables the system to inspect multiple regions simultaneously, improving throughput compared to single-spot scanning.
Solution Approach 2:
The system uses pulsed illumination where each spot is illuminated in periodic sequences. The laser emits short pulses at each spot location, and the detector captures scattered light during these periodic illumination events. This periodic action allows for time-gated detection that separates signal from background noise, maintaining sensitivity while enabling rapid sequential scanning across multiple spots to improve overall throughput.
2Productivity
If area inspection systems with two-dimensional sensors are used, then inspection throughput is improved, but sensitivity is reduced due to additional noise from silicon-based sensors
Solution Approach 1:
Instead of using a single large two-dimensional sensor array, the system segments the detection function across multiple discrete detector elements that correspond to each illumination spot. This segmentation allows the use of high-sensitivity discrete detectors (such as photomultiplier tubes or avalanche photodiodes) at each spot location, maintaining sensitivity while achieving area inspection capability through parallel detection across multiple segmented channels.
Solution Approach 2:
The system introduces an intermediary optical system that couples the scattered light from each illumination spot to a corresponding discrete detector. This intermediary optical path includes collection optics and beam steering mechanisms that efficiently transfer light from each spot to its dedicated detector, enabling the use of high-sensitivity discrete detectors while maintaining area inspection throughput through parallel optical channels.
3Productivity
If spiral inspection sequence is used, then inspection throughput is improved compared to XY sequence, but coordinate mismatch between spiral trajectory and rectilinear sensors reduces detection accuracy
Solution Approach 1:
The system segments the wafer surface into a grid of discrete spot locations that are illuminated and detected in sequential groups. Rather than attempting to map continuous spiral trajectory data to rectilinear sensor coordinates, the system divides the inspection into discrete spot positions arranged in a pattern that can be systematically scanned using spiral motion, with each spot detected by its corresponding discrete detector element.
Solution Approach 2:
The system dynamically adjusts the illumination and detection parameters based on the spiral scan position. As the wafer rotates and translates during spiral scanning, the system dynamically steers the illumination spots and corresponding detection channels to track the spiral trajectory, maintaining optimal alignment between illumination, scattering, and detection throughout the dynamic scan process.
4Measurement precision
If high power density illumination is used, then sensitivity to small defects is improved, but wafer damage occurs due to excessive energy concentration
Solution Approach 1:
The total illumination energy is segmented and distributed across multiple discrete spots rather than concentrated in a single location. Each spot receives a controlled amount of power that is sufficient for detection but below the damage threshold. By segmenting the energy distribution across many spots and illuminating them sequentially or in parallel groups, the system achieves high sensitivity through sufficient local intensity while preventing damage through distributed energy delivery.
Solution Approach 2:
The system uses periodic pulsed illumination where each spot is illuminated for short durations at high power density during the pulse, followed by rest periods between pulses. This periodic action delivers the necessary energy for sensitive detection during brief intervals while allowing thermal dissipation during off-periods, preventing cumulative heating and damage. The duty cycle is controlled to ensure peak power provides sufficient sensitivity while average power remains below damage thresholds.
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 sensitivity and throughput by optimizing illumination patterns and reducing noise, allowing for more effective detection of defects on wafers, particularly in spiral inspection sequences, while minimizing wafer damage from excessive illumination.
Implementation Method 1
The illumination subsystem comprises a Q-switched laser
Implementation Method 2
a collection subsystem configured to image light scattered from the area on the wafer to one or more sensors
Data Source
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AI summary
Systems configured to inspect a wafer are provided.