Pulse Laser Surface Inspection Synchronization
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Solution Overview
Problem
Conventional surface inspection methods using pulse lasers suffer from reduced detection sensitivity due to the ripple component caused by the pulse oscillation of the light source, which is not effectively integrated by the photodetector, leading to noise in the scattered light intensity signal.
Innovation Solution
A surface inspection method and apparatus that utilizes a pulse laser with a sampling interval for A/D conversion synchronized with the pulse oscillation repetition period, setting the maximum response frequency of the detection system lower than the pulse laser's repetition rate, and sampling at intervals equal to or related to the pulse oscillation period to minimize the influence of the ripple component.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a pulse laser is used as the light source to achieve higher sensitivity, then the detection sensitivity for minute contaminants is improved, but the ripple component caused by pulse oscillation introduces noise that deteriorates signal quality
Solution Approach 1:
The patent applies periodic action by synchronizing the A/D conversion sampling intervals with the pulse laser's oscillation period. The sampling is performed at specific phases of the pulse cycle (when the scattered light signal is strongest) rather than continuously, creating a periodic sampling pattern that naturally filters out the ripple component while maintaining detection sensitivity.
Solution Approach 2:
The patent implements feedback by using the pulse laser's oscillation timing information to control the A/D conversion sampling timing. The sampling clock is synchronized with the laser pulse sequence, allowing the system to adaptively adjust sampling moments based on the known pulse characteristics, thereby eliminating the harmful ripple effect.
2Measurement precision
If the sampling interval is set to integrate scattered light pulses completely, then the signal accuracy is improved, but the ripple component from pulse oscillation remains integrated as noise
Solution Approach 1:
The patent applies partial action by sampling the signal only at specific critical moments within the pulse cycle rather than continuously integrating the entire pulse. The sampling is performed at phases where the scattered light signal is strongest, capturing sufficient signal information while deliberately excluding the ripple component that persists throughout the pulse duration.
Solution Approach 2:
The patent extracts only the useful signal portion by timing the A/D conversion sampling to occur specifically when the scattered light pulse is at its peak intensity, while ignoring the rest of the pulse cycle where the ripple component exists. This selective extraction removes the harmful ripple noise from the measurement.
3Speed
If the photodetector's response frequency is increased to capture faster pulses, then the time resolution is improved, but the ripple component is not effectively integrated and remains as noise
Solution Approach 1:
The patent applies dynamics by making the sampling timing adaptive to the pulse characteristics. Rather than using a fixed sampling rate, the system dynamically adjusts the sampling moments based on the actual pulse timing, allowing optimal capture of the scattered light signal while automatically excluding the ripple component through synchronized 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 effectively reduces the noise caused by the ripple component, enhancing the detection sensitivity for minute contaminants and defects on semiconductor substrates by accurately integrating the scattered light pulses and eliminating residual noise in the signal.
Implementation Method 1
scattered/diffracted/reflected light detection means for detecting light generated by scattering/diffracting/reflecting the irradiating light in the illumination spot and converting the light to an electric signal
Implementation Method 2
If in this state a contaminated particle which has adhered to the surface of the semiconductor wafer or a defect on the surface crosses the illumination spot, it generates scattered light
Data Source
AI summary
When detecting light scattered by an object to be inspected by using a pulse laser as a light source, noise increases unless a sampling repletion period of an A/D converter is determined so as to be related to a pulse oscillation repetition period of the light source. (1) The sampling repletion period of the A/D converter is set equal to the pulse oscillation repetition period of the light source or an integer times thereof, and the sampling is synchronized with oscillation of the light source. Or (2) the sampling repletion period of the A/D converter is set equal to a half-integer times the pulse oscillation repetition period of the light source. Even if a ripple component resulting from emission pulses of the light source remains in the scattered light signal supplied to the A/D converter remains, therefore, its influence can be eliminated or reduced.