Optical Filter Warpage Suppression via Stress Balancing Layers
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Solution Overview
Problem
Existing near-infrared cut filters face challenges with substrate warpage and high reflectance due to the thickness of optical multilayer films, which complicates manufacturing and affects spectral characteristics, especially at large incident angles.
Innovation Solution
An optical filter design featuring a refractive index adjusting layer and a characteristic improving layer with a physical film thickness of 600 nm or more, along with an optical multilayer film, helps alleviate stress and reduce warpage, while maintaining good spectral characteristics by balancing refractive indices and using a near-infrared absorbing layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If an optical multilayer film is provided on a substrate with small plate thickness to achieve thin optical filter, then the optical filter thickness is reduced, but substrate warpage occurs noticeably
Solution Approach 1:
The patent applies the counterweight principle by providing an optical multilayer film on both the front and rear surfaces of the substrate. The film on the rear surface acts as a counterweight to balance the stress caused by the front surface film, thereby suppressing substrate warpage while maintaining thin overall thickness.
Solution Approach 2:
The patent applies local quality by providing an anti-reflection film specifically on the rear surface of the substrate, where it is needed to balance stress. This localized addition of film material with specific optical properties (refractive index between substrate and front multilayer film) addresses the warpage issue without affecting the overall thin design.
2Shape
If the number of layers of optical multilayer film is increased to suppress warpage, then substrate warpage is reduced, but reflectance increases due to film thickness error
Solution Approach 1:
By providing optical multilayer film on both surfaces with balanced layer numbers (front surface: 1-3 layers, rear surface: 1-2 layers), the patent achieves stress balance to suppress warpage while limiting the total number of layers to minimize reflectance accumulation from thickness errors.
Solution Approach 2:
The patent applies parameter changes by carefully controlling the refractive index of the rear surface film to be between the substrate and front multilayer film, and by limiting the number of layers within specific ranges (1-3 layers front, 1-2 layers rear) to optimize the balance between warpage suppression and reflectance control.
3Illumination intensity
If the number of layers of anti-reflection film is increased to suppress reflection, then transmittance is improved, but manufacturing complexity increases and productivity decreases
Solution Approach 1:
The patent applies local quality by providing anti-reflection film only on the rear surface of the substrate where it is most effective for stress balancing and reflection suppression, rather than on both surfaces. This localized approach achieves the desired transmittance improvement with minimal added complexity.
Solution Approach 2:
The patent applies parameter changes by using a single layer with optimized refractive index (between substrate and front multilayer film) and controlled thickness (600-1500 nm) to achieve effective reflection suppression, avoiding the need for multiple thin layers and simplifying manufacturing.
4Shape
If optical multilayer film is provided on both front and rear surfaces to balance stress, then substrate warpage is suppressed, but spectral characteristic dependence on incident angle increases
Solution Approach 1:
The patent applies local quality by providing optical multilayer film only on the front surface (1-3 layers) and a different film structure on the rear surface, creating asymmetric stress balance that suppresses warpage while minimizing the impact on spectral characteristics at large incident angles.
Solution Approach 2:
The patent applies parameter changes by carefully selecting the refractive index of the rear surface film to be between the substrate and front multilayer film, and by controlling the thickness (600-1500 nm) to optimize the balance between stress balancing for warpage suppression and maintaining spectral characteristic stability.
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 solution effectively suppresses substrate warpage and maintains excellent spectral characteristics, improving manufacturing efficiency and reducing reflectance issues, especially at large incident angles.
Implementation Method 1
by setting composing materials, thicknesses, a layer number, or the like of the high-refractive index layer and the low-refractive index layer appropriately, selectively transmits light by using interference of light
Implementation Method 2
there was a problem that providing an optical multilayer film on a substrate whose plate thickness is small causes distortion (warpage) of a substrate shape noticeably. To cope with such a problem, there is proposed a method of suppressing warpage of a substrate by setting a ratio of the number of layers of optical multilayer films formed on both front and rear surfaces of the substrate to a predetermined range
Implementation Method 3
an optical filter with small substrate warpage and a near-infrared cut filter
Data Source
AI summary
An optical filter has a substrate, a refractive index adjusting layer on the substrate, a characteristic improving layer having a stress adjustment function on the refractive index adjusting layer, and an optical multilayer film on the characteristic improving layer, wherein the refractive index adjusting layer is constituted by one to three layers, and the characteristic improving layer is a single-layer film whose physical film thickness is 600 nm or more. The optical filter whose substrate has small warpage and which has a good spectral characteristic is provided.