Low Stress LOCA Additive for Optical Substrate Bonding
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Solution Overview
Problem
Existing bonding processes for optical substrates in waveguide display systems face challenges in achieving low bowing and minimal internal stress, leading to optical artifacts and performance degradation due to high internal stress and thermal expansion coefficient mismatches.
Innovation Solution
A siloxane-containing epoxy-based liquid optically clear adhesive (LOCA) with a reactive plasticizer, such as the additive of Structure 1, is used for bonding optical substrates, which reduces internal stress and bowing by incorporating a flexible alkyl chain that improves adhesion and stress reduction during curing and thermal treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional bonding processes are used for optical substrates, then bonding strength is achieved, but high internal stress and bowing occur leading to optical artifacts
Solution Approach 1:
The patent modifies the chemical composition parameters of the LOCA by incorporating specific ratios of siloxane oligomers (1-10 wt%), epoxy oligomers (1-10 wt%), and reactive plasticizers (1-7 wt%). This chemical parameter optimization enables the adhesive to achieve low internal stress while maintaining bonding strength, resolving the contradiction between strength and stress generation.
Solution Approach 2:
The patent creates a composite adhesive system combining siloxane-containing epoxy adhesive with reactive plasticizers and crosslinking agents. This composite formulation leverages the complementary properties of each component: the bonding strength of epoxy, the flexibility of siloxane, and the stress-reducing effect of reactive plasticizers, thereby achieving both strong bonding and low internal stress.
2Strength
If conventional bonding processes are used for optical substrates, then bonding is achieved, but thermal expansion coefficient mismatches cause performance degradation
Solution Approach 1:
The patent adjusts the thermal and mechanical parameters of the LOCA formulation by controlling the molecular weight and composition of siloxane and epoxy components. This parameter optimization enables the adhesive to accommodate thermal expansion differences between substrates with varying thermal conductivity and expansion coefficients, maintaining bonding reliability under thermal cycling.
Solution Approach 2:
The patent specifically addresses thermal expansion mismatch by incorporating reactive plasticizers that provide flexibility and stress relief. This allows the adhesive layer to expand and contract with substrate movements during thermal cycles, preventing delamination and maintaining bonding reliability despite mismatched thermal expansion coefficients.
3Illumination intensity
If high refractive index LOCA is used to match optical substrates, then optical performance is improved, but adhesion strength may be compromised
Solution Approach 1:
The patent develops a composite LOCA system where siloxane oligomers provide high refractive index (1.6-1.7) for optical performance, while epoxy components and reactive plasticizers ensure strong adhesion. The synergistic combination of these materials achieves both high refractive index and high bonding strength simultaneously, resolving the contradiction between optical performance and adhesion.
4Reliability
If LOCA thickness is increased to compensate for bowing, then bonding coverage is improved, but optical absorption increases
Solution Approach 1:
The patent optimizes the optical parameters of the LOCA by selecting materials with low optical absorption coefficients and controlling the curing process to minimize polymerization-induced stress. This enables the use of thinner adhesive layers (reducing absorption) while maintaining adequate bonding coverage through improved adhesive formulation and application techniques.
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 results in a bonded substrate stack with a refractive index of 1.6 or higher, low optical absorption, and high adhesion strength, maintaining optical performance and minimizing bowing to below 20 micrometers, even with different thermal expansion coefficients of the substrates.
Implementation Method 1
curing the substrate stack using ultraviolet (UV) light to crosslink the LOCA material
Implementation Method 2
thermally curing the substrate stack to transform the LOCA material into a thermoset state
Implementation Method 3
reduces internal stress and bowing by incorporating a flexible alkyl chain that improves adhesion and stress reduction during curing and thermal treatment
Implementation Method 4
When cured, the LOCA may have a refractive index equal to or greater than about 1.6 at 450 nm
Implementation Method 5
a UV-activated photo-acid generator
Data Source
AI summary
A liquid optically clear adhesive (LOCA) for bonding optical substrates includes siloxane and epoxy-containing oligomers, a UV-activated photo-acid generator, a cross-linker additive, a solvent; and a reactive plasticizer, such as an additive of Structure 1. In one example, the additive of Structure 1 constitutes about 1-7% of a total mass of the LOCA excluding the solvent. R1, R2, and R3 of Structure 1 include methoxide, ethoxide, propoxide, or a combination thereof. R4 of Structure 1 includes an alkyl chain that is linear or branched and includes 2-8 carbons. The LOCA material is characterized by a refractive index equal to or greater than about 1.6 at 450 nm and an optical absorption below about 0.1% per micrometer of a thickness of the LOCA material.