Plane Mirror Illumination Position Control for Optical Defect Inspection
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Solution Overview
Problem
Optical defect inspection apparatuses face challenges in maintaining stability and reproducibility due to laser beam illumination on the same position on plane mirrors, leading to reduced reflectance and increased frequency of mirror replacement, as well as dust generation from actuators affecting semiconductor device processing and inspection accuracy.
Innovation Solution
Implementing a mechanism to move the laser beam illuminated position on reflecting mirrors without displacing the optical axis, using a combination of translation and rotation mechanisms, and incorporating a dust collection system to minimize dust generation from actuators, ensuring the optical axis remains stable and reducing the need for frequent mirror replacement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a laser beam with high illumination density is illuminated to the same position on a plane mirror for a long time, then the detection sensitivity is improved, but the reflectance of the mirror is reduced and the mirror surface is deteriorated
Solution Approach 1:
The patent introduces a mirror surface moving mechanism that dynamically moves the illuminated position on the plane mirror surface. This prevents the laser beam from continuously illuminating the same spot, thereby avoiding mirror surface deterioration while maintaining high detection sensitivity. The mechanism transforms the static illumination system into a dynamic one where the illumination position changes over time.
Solution Approach 2:
The patent extends the problem from one-dimensional beam direction control to two-dimensional illumination position control on the mirror surface. By introducing movement in the lateral direction across the mirror surface, the system distributes the laser illumination across multiple areas, preventing localized deterioration while maintaining the required detection sensitivity.
2Duration of action of stationary object
If the illuminated position on the plane mirror is changed to avoid mirror deterioration, then the mirror life is prolonged, but the optical axis displacement occurs and inspection stability is reduced
Solution Approach 1:
The patent employs a dynamic control system that coordinates mirror surface movement with optical axis compensation. The mirror surface moving mechanism is paired with an optical axis displacement compensation mechanism that actively adjusts other optical components to maintain the optical axis position, thereby allowing mirror position changes without compromising inspection stability.
Solution Approach 2:
The patent implements a feedback control system where the optical axis position is monitored and used to control the adjustment of optical components. This closed-loop system ensures that when the mirror illuminated position is changed, any resulting optical axis displacement is detected and compensated, maintaining inspection stability throughout the extended mirror usage period.
3Duration of action of stationary object
If actuators are used to move the illuminated position, then the mirror life is extended, but dust is generated from the actuators affecting inspection accuracy
Solution Approach 1:
The patent introduces a protective cover as an intermediary element between the actuator and the inspection environment. This cover prevents dust generated by the actuator from contaminating the optical path and inspected objects, while still allowing the actuator to perform its function of moving the mirror illuminated position to extend mirror life.
Solution Approach 2:
The patent separates the dust-generating actuator mechanism from the clean inspection environment by using protective covers and isolation structures. This extraction removes the harmful dust-generating element from the sensitive inspection area while preserving the functional benefits of actuator-driven mirror position adjustment.
4Measurement precision
If a thin beam with high illumination density is used, then the detection sensitivity is improved, but the apparatus size increases due to ND filter mechanism
Solution Approach 1:
The patent replaces the static ND filter mechanism with a dynamic mirror surface movement mechanism. Instead of using a large ND filter to control light intensity, the system uses the moving mirror surface to distribute and control the high-density laser beam, achieving the same light quantity control function with a more compact structure that maintains detection sensitivity.
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 prolongs the life of plane mirrors, reduces dust generation, and enhances the stability and reproducibility of the inspection apparatus by maintaining the optical axis alignment and preventing dust from affecting the inspection process, thereby improving the detection sensitivity and accuracy of defects on semiconductor devices.
Implementation Method 1
the inspection apparatus is usually constructed such that a thin beam with a high illumination density is reflected several times within an apparatus housing by reflecting mirrors (plane mirrors)
Data Source
AI summary
A laser beam oscillated from a laser source is folded in its path by first and second plane mirrors and enters a beam expander. The surface of each plane mirror is deteriorated with illumination by the laser beam and the reflectance is reduced. To avoid a light quantity of the laser beam entering the beam expander from being reduced below a reference value, when the laser beam is illuminated over a certain time, a position on each of the first and second plane mirrors at which the laser beam is illuminated is changed by a structure for rotating and/or translating a reflecting surface of each plane mirror on a plane, which includes the plane mirror, while an optical axis is kept same. Thus, the useful life of each plane mirror can be prolonged without displacing the optical axis.