Transparent Substrate Inspection for Frontside-Backside Defect Separation
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Solution Overview
Problem
Existing optical inspection systems struggle to unambiguously distinguish defects on the frontside from defects on the backside of transparent substrates due to light scattering interference, leading to ambiguous defect identification.
Innovation Solution
An inspection system and method that uses intersecting light beams to create a measurement volume on the frontside of a transparent substrate, collecting and processing light scattered by defects to identify patterns with distinct intensity peaks, allowing discrimination between frontside and backside defects based on the separation interval of these peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If light beams illuminate both frontside and backside of transparent substrate, then inspection coverage is improved, but defect identification accuracy deteriorates due to light scattering interference
Solution Approach 1:
The patent segments the inspection process by creating distinct measurement volumes for frontside and backside inspection. By using two separate intersecting beam pairs, the system divides the inspection space into two independent zones, allowing defects on each surface to be detected and differentiated separately, thus resolving the ambiguity caused by simultaneous illumination of both surfaces.
Solution Approach 2:
The patent introduces asymmetry in the optical path configuration by positioning the two beam pairs at different angular orientations relative to the substrate normal. This asymmetric arrangement creates distinct scattering patterns for frontside versus backside defects, enabling the detection system to differentiate between the two based on the unique optical signatures produced by each surface's defects.
2Illumination intensity
If scattered light from backside defects is collected by mirrors, then signal intensity is improved, but signal ambiguity worsens
Solution Approach 1:
The patent uses the angular position and scattering characteristics of collected light as an intermediary parameter to distinguish between frontside and backside defects. By analyzing the directional information and angular distribution of scattered light rather than just its intensity, the system can identify the origin surface of each defect signal, eliminating ambiguity while maintaining signal collection efficiency.
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
Effectively discriminates between defects on the frontside and backside of transparent substrates, providing clear and accurate defect identification by filtering out patterns specific to each side, enhancing inspection precision.
Implementation Method 1
an optical source emitting a first light beam and a second light beam oriented in relation to one another so as to form, at their intersection, a measurement volume comprising a plurality of parallel interference fringes
Implementation Method 2
The scattered light is collected by collecting mirrors and directed to a detector for receiving the collected light
Implementation Method 3
A Doppler pulse is a signal that has a double frequency component: a low-frequency component, forming the envelope of the signal, corresponding to the mean light intensity scattered by the defect, and a high-frequency component, corresponding to the Doppler frequency containing the information on the velocity of the defect through the measurement spot
Data Source
AI summary
Method for discriminating defects present on a frontside of a transparent substrate from defects present on a backside of the substrate comprises disposing the substrate in an inspection system in which first and a second light beams intersect at a measurement spot on the frontside of the substrate. Relative movement of the substrate and measurement spot is controlled such that a reference plane is kept tangential to the measurement path. A first pattern is identified in a measurement signal, the first pattern corresponding to light scattered by a particle on the backside of the substrate and presenting two intensity peaks separated from each other by a determined separation interval corresponding to the time necessary for the defect to be moved over the distance separating two illumination spots on the backside of the substrate.