Glass Diffraction Grating Replication for High-Aspect-Ratio Grooves
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Solution Overview
Problem
Existing methods struggle to produce glass diffraction gratings with an aspect ratio of 2 or greater and a period of 10 micrometers or smaller, due to issues such as damage to quartz glass during etching, surface roughness, mask cracking, and warping, which affect optical performance.
Innovation Solution
A method involving the Bosch process on a silicon wafer, followed by oxide film formation, anodic bonding with a glass plate, and selective etching using xenon difluoride gas to create a borosilicate or barium borosilicate glass diffraction grating with precise control over groove aspect ratio and period, improving surface smoothness and purity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If plasma etching is used to produce deep grooves on quartz glass, then the aspect ratio can be increased, but the surface of the grating becomes damaged and rough
Solution Approach 1:
The patent changes the fundamental etching mechanism from plasma-based ion bombardment to liquid-based chemical etching using KOH or NH4F solutions. This parameter change in the etching process enables achievement of aspect ratios of 2 or greater while maintaining surface flatness within 1/10 wavelength and roughness Ra of 5nm or less, resolving the contradiction between deep groove formation and surface quality
Solution Approach 2:
The patent replaces the mechanical/physical plasma etching process with a chemical wet etching process. By substituting the plasma-based mechanical removal mechanism with chemical dissolution, the method achieves both deep grooves and smooth surfaces, eliminating the surface damage inherent in plasma etching
2Reliability
If the mask film thickness is increased to increase resistance for deep groove etching, then the mask resistance improves, but cracks and separation occur in the film
Solution Approach 1:
The patent changes the etching process parameters from plasma-based to liquid-based chemical etching, which has different interaction mechanisms with the mask material. This allows the use of thinner Cr films (50-200nm) that remain intact without cracking or separating, while still achieving the required etching depth and aspect ratio
Solution Approach 2:
The patent employs a sacrificial organic film layer (photoresist or spin-on-glass) that is deposited over the Cr mask before etching. This disposable organic layer provides the necessary etching resistance during the process, allowing the Cr mask to remain thin and intact while the organic layer absorbs the etching stress
3Shape
If the grooves are made deep to increase aspect ratio, then the aspect ratio increases, but bowing appears in the sides of ridges due to ion distribution
Solution Approach 1:
The patent replaces plasma-based ion bombardment etching with liquid chemical etching. This substitution eliminates the ion distribution issues that cause bowing in plasma etching, as liquid chemicals uniformly diffuse and etch the glass throughout the groove depth, maintaining straight ridge sides while achieving high aspect ratios
Solution Approach 2:
The patent changes the etching chemistry from plasma ions to liquid solutions (KOH, NH4F, or HF-based etchants). This parameter change in the etching medium enables uniform material removal at all depths, preventing the bowing effect that occurs in plasma etching where ion penetration varies with depth
4Ease of manufacture
If silicon is incompletely oxidized to form silicon monoxide or disilicon trioxide, then the oxidation process is simplified, but the refractive index becomes greater than quartz glass and characteristics differ from design
Solution Approach 1:
The patent changes the oxidation parameters by extending the oxidation time and controlling the oxygen atmosphere to ensure complete oxidation of silicon to silicon dioxide. This parameter control ensures the refractive index matches the designed quartz glass value (nd=1.46), eliminating the refractive index mismatch that would occur with incomplete oxidation to SiO or Si2O3
Solution Approach 2:
The patent employs continuous oxidation processing that maintains oxygen exposure throughout the cooling phase from high temperature to room temperature. This continuous oxidation action ensures complete conversion of silicon to silicon dioxide, preventing the formation of intermediate oxides with incorrect refractive indices
5Length of moving object
If the grating period is reduced to 10 micrometers or smaller for astronomical applications, then the angular dispersion increases, but conventional methods cannot produce gratings with both small period and high aspect ratio
Solution Approach 1:
The patent changes the etching process parameters to liquid-based chemical etching with controlled solution concentration, temperature, and time. These parameter changes enable precise control of groove dimensions even at small periods of 10 micrometers or less, while maintaining aspect ratios of 2 or greater, which is essential for high angular dispersion in astronomical spectroscopy
Solution Approach 2:
The patent utilizes liquid-based etching solutions that can be precisely delivered and controlled through fluid handling systems. This hydraulic approach to etching provides superior control over the etching process at small scales, enabling production of gratings with periods of 10 micrometers or smaller and high aspect ratios that are required for astronomical applications
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 method enables the production of glass diffraction gratings with enhanced optical performance, reduced surface roughness, and improved material purity, achieving high angular dispersion suitable for astronomical and spectroscopic applications.
Implementation Method 1
heating and exposure to water vapor of the silicon wafer
Implementation Method 2
removing silicon from the glass plate by selective etching using xenon difluoride gas
Implementation Method 3
making the surface provided with the grating of the silicon wafer and a surface of a glass plate undergo anodic bonding in a container kept at the degree of vacuum of 0.01 to 0.1 pascals
Implementation Method 4
heating the silicon wafer and the glass plate that have been bonded to each other so as to melt glass and to fill spaces formed between ridges of the grating of the silicon with the molten glass
Data Source
AI summary
A method of producing a diffraction grating of borosilicate glass or barium borosilicate glass, the method comprising the steps of forming a grating on a surface of a silicon wafer the grating through the Bosch process; forming an oxide film on a surface of the grating by heating and exposure to water vapor of the silicon wafer; removing the oxide film using hydrofluoric acid; making the surface provided with the grating of the silicon wafer and a surface of a glass plate undergo anodic bonding; heating the silicon wafer and the glass plate bonded to each other; polishing a surface opposite to the boded surface of the silicon wafer and a surface opposite to the boded surface of the glass plate; and removing silicon from the glass plate by selective etching using xenon difluoride.