Oblique Defect Inspection Optics With Dynamic Focus Compensation
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Solution Overview
Problem
The resolution of defect inspection images is compromised by height variations of the sample surface during inspection, particularly when the detection optical system is inclined with respect to the sample surface, due to factors like sample warpage, air flow, and rotational vibrations.
Innovation Solution
The defect inspection device incorporates a height measuring unit to measure sample height variations, a focus actuator to adjust the focusing position, and a computer to control the actuator based on height measurements, ensuring the imaging sensor's light receiving surface aligns with the illumination spot, and combines data sets from multiple sensors to compensate for height deviations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the detection optical system is arranged to detect the image of the linear illumination spot obliquely, then scattered light detection capability is improved, but resolution decreases due to operating distance variation across the illumination spot
Solution Approach 1:
The light receiving surface of the imaging sensor is inclined relative to the optical axis of the detection optical system, creating a dimensional change in the sensor orientation. This inclination allows the entire illumination spot to be imaged within the depth of focus range, maintaining resolution while preserving scattered light detection capability. The sensor plane is angled to match the oblique illumination geometry, ensuring all points on the extended illumination spot remain in focus.
2Productivity
If the sample rotates at high speed during inspection, then productivity is improved, but height variation of the sample surface occurs due to warpage, air flow, and vibration
Solution Approach 1:
A height measuring unit continuously measures the actual height of the sample surface during rotation, and this measurement feedback is used by a focus actuator to dynamically adjust the focusing position of the detection optical system. This closed-loop feedback mechanism compensates for height variations caused by high-speed rotation, warpage, and vibration, maintaining image resolution and inspection quality throughout the inspection process.
Solution Approach 2:
The focus actuator dynamically adjusts the focusing position in real-time based on the measured height variations of the rotating sample. Instead of using a static focus setting, the system adapts the focal plane continuously to track the sample surface, allowing high-speed rotation without degradation of image quality. This dynamic focus adjustment maintains the sample surface within the depth of focus range throughout the inspection.
3Manufacturing precision
If the light receiving surface of the sensor is inclined to match the optical axis inclination, then the entire illumination spot falls within depth of focus, but device complexity increases
Solution Approach 1:
The inclination angle of the light receiving surface is specifically set to match the inclination angle of the detection optical system's optical axis with respect to the sample surface. By changing the sensor orientation parameter to correspond to the optical system geometry, the entire illumination spot is captured within the depth of focus range. This parameter matching simplifies the overall alignment requirements compared to using multiple sensors or complex optical paths.
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 suppresses resolution loss caused by sample surface height variations, maintaining image clarity and accuracy in defect inspection.
Implementation Method 1
capture scattered light that is generated at the defect
Implementation Method 2
form an image on a light receiving surface of a sensor
Implementation Method 3
a focus actuator configured to relatively move a focusing position of the optical image
Data Source
AI summary
A defect inspection device in which an optical axis of a detection optical system is inclined with respect to a surface of a sample, and an imaging sensor is inclined with respect to the optical axis, a height variation amount of an illumination spot in a normal direction of the surface of the sample is calculated based on an output of a height measuring unit, a deviation amount of the focusing position with respect to the light receiving surface in an optical axis direction of the detection optical system is calculated based on the height variation amount of the illumination spot, the deviation amount of the focusing position being generated accompanying a height variation of the illumination spot, and the focus actuator is controlled based on the deviation amount of the focusing position, and scattered light intensities at the same coordinates of the sample are added.