Catoptric Imaging System for UV Inspection
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Solution Overview
Problem
Optical inspection systems for integrated circuits face challenges in achieving high throughput and sensitivity due to limitations in refractive optics, which suffer from chromatic aberrations and poor image quality at high numerical apertures, especially in the ultraviolet range.
Innovation Solution
A three-reflector, all-reflective catoptric imaging system with concave and convex aspherical surfaces is used, eliminating refractive elements and chromatic aberrations, allowing for a broad numerical aperture and high resolution, while maintaining compactness and wavelength insensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If refractive (glass) elements are used in ultraviolet optical systems, then the system can be simpler in structure, but chromatic aberrations occur and image quality deteriorates at high numerical apertures
Solution Approach 1:
The patent removes refractive glass elements from the optical system entirely, extracting the problematic component that causes chromatic aberrations. The system uses only reflective elements (mirrors and beam splitters) to guide and focus ultraviolet light, eliminating material absorption and dispersion issues inherent in glass optics at UV wavelengths.
Solution Approach 2:
The patent substitutes refractive optics (based on material refraction properties) with catoptric optics (based on reflective properties). This replacement uses the law of reflection instead of Snell's law, avoiding chromatic aberrations since reflection is wavelength-independent. The mechanical arrangement of mirrors replaces the optical function of glass lenses.
2Measurement precision
If high numerical aperture optics are used to increase resolution, then the optical resolution improves, but chromatic aberrations and image quality degradation worsen with refractive elements
Solution Approach 1:
The patent replaces refractive optics with reflective optics to achieve high numerical aperture without chromatic aberrations. The catoptric design uses precisely positioned mirrors to focus UV light at high NA, maintaining image quality because reflection does not disperse different wavelengths differently as refraction does.
Solution Approach 2:
The patent changes the fundamental optical parameter from refraction index-based focusing to reflection-based focusing. This parameter change allows the system to operate at high numerical apertures with UV light without suffering from the wavelength-dependent refraction that plagues glass optics in the ultraviolet range.
3Measurement precision
If catadioptric systems are used to correct chromatic aberrations, then some correction is achieved within a small bandwidth, but the system complexity increases and the bandwidth remains limited
Solution Approach 1:
The patent takes out the refractive elements that cause chromatic aberrations in the first place, rather than attempting to correct them with additional optical components. This extraction approach is simpler than catadioptric systems because it eliminates the root cause rather than compensating for it.
Solution Approach 2:
The patent substitutes the catadioptric approach (combining refraction and reflection) with a purely catoptric system (reflection only). This substitution achieves complete chromatic aberration correction without the complexity of correcting for partial chromatic effects, and works across the entire UV bandwidth rather than a limited range.
4Productivity
If pulsed ultraviolet lasers are used to achieve high power and throughput, then the inspection throughput and sensitivity improve, but the wavelength bandwidth becomes wider causing chromatic aberrations
Solution Approach 1:
The patent replaces refractive optics with reflective optics to handle the wide bandwidth UV laser light. Since reflection is independent of wavelength, the system can accommodate the broader spectral output of pulsed UV lasers without suffering from chromatic aberrations, thereby maintaining both high throughput and precise imaging.
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
The system achieves high resolution and compactness, reducing chromatic aberrations and central obscuration, enabling efficient ultraviolet optical inspection with improved sensitivity and throughput.
Implementation Method 1
a back-plane reflector having a concave aspherical reflecting surface... a fore-plane reflector having a concave aspherical reflecting surface... and a central reflector having a convex aspherical reflecting surface for receiving light from the fore-plane reflector and discharging the light
Data Source
AI summary
An imaging system including a back-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than a first distance, with an aperture formed in the back-plane reflector, the aperture for admitting light from a field of view to the imaging system, a fore-plane reflector having a concave aspherical reflecting surface and an outer diameter that is no greater than the first distance, with an aperture formed in the fore-plane reflector, the aperture for discharging the light from the imaging system to an image plane, and a central reflector having a convex aspherical reflecting surface for receiving light from the fore-plane reflector and discharging the light from the imaging system through the aperture in the fore-plane reflector.