Conductive Anodic Bonding for Hermetic Optical Encapsulation
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Solution Overview
Problem
The packaging of optical devices in a hermetically sealed, optically transmissive glass cavity remains an unresolved challenge due to the inability to apply voltage for anodic bonding between glass wafers, which are insulating, and the need for conductive bonding to provide an electrical pathway.
Innovation Solution
An electrically conductive anodic bond is formed between two optically transparent wafers using ion-rich glass substrates, such as Borofloat or Pyrex, by depositing a silicon layer on one substrate and applying temperature and voltage to create a bonding oxide layer, with additional bonds like thermocompression or metal alloy bonds for enhanced attributes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If glass wafers are used for optical device encapsulation, then optical transparency is achieved, but voltage cannot be applied for anodic bonding because glass is insulating
Solution Approach 1:
A conductive adhesive layer is introduced as an intermediary between the glass substrates. This layer enables voltage application and anodic bonding to occur while maintaining the optical transparency of the overall structure, since the adhesive layer is thin and transparent. The intermediary resolves the contradiction by providing the necessary electrical conductivity without compromising the optical function.
Solution Approach 2:
The bonding structure uses a composite approach combining glass substrates (optically transparent) with a conductive adhesive layer (electrically conductive). This composite material solution allows both optical transparency and electrical conductivity to coexist, enabling anodic bonding between glass wafers that would otherwise be impossible.
2Reliability
If glass frit adhesive is used for hermetic sealing, then hermetic seal is achieved, but processing temperature exceeds 400 C. which may damage thin metal layers
Solution Approach 1:
The bonding method changes the temperature parameter by using anodic bonding at lower temperatures (below 400 C.) instead of traditional glass frit bonding. The voltage applied during anodic bonding enables bonding at reduced temperatures, preventing damage to temperature-sensitive thin metal layers while still achieving hermetic sealing.
Solution Approach 2:
The patent replaces the thermal-mechanical bonding process (glass frit melting and fusing) with an electrochemical process (anodic bonding using voltage-induced oxide growth). This substitution eliminates the need for high temperatures, allowing hermetic sealing without damaging temperature-sensitive components.
3Strength
If voltage is applied between glass wafers for anodic bonding, then bonding is achieved, but glass wafers must be conductive which they are not
Solution Approach 1:
A conductive adhesive layer serves as a mediator that enables voltage application between glass substrates. The adhesive layer provides the necessary electrical conductivity pathway while the voltage still drives the anodic bonding process. This intermediary allows both bonding strength and electrical conductivity requirements to be satisfied simultaneously.
4Reliability
If getter material is used to maintain vacuum, then vacuum level is maintained, but high temperature activation is required which conflicts with optical device temperature limits
Solution Approach 1:
The bonding process parameter (temperature) is changed to a lower range through anodic bonding, which enables the vacuum package to be sealed without activating the getter material. The getter remains inactive at these lower temperatures, avoiding the conflict between vacuum sealing and optical device temperature limits.
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 method enables the formation of a conductive, hermetic bond between glass substrates, allowing for the encapsulation of optical devices while maintaining transparency and providing electrical conductivity.
Implementation Method 1
Anodic bonding of a glass substrate to a silicon substrate is known, wherein voltage and heat are applied between the glass wafer and the silicon wafer. The voltage applied promotes the growth of the oxide layer between the silicon and the glass, which adheres the materials together.
Implementation Method 2
The voltage applied promotes the growth of the oxide layer between the silicon and the glass
Implementation Method 3
Because of the reactive nature of these materials, they also tend to oxidize spontaneously at the surface, forming an oxide layer that must be removed in order to activate the getter. Activation of the getter may require exposure to high temperatures
Implementation Method 4
a hermetic seal around a glass cavity typically requires a glass frit adhesive, which may require processing temperatures in excess of 400 C. to melt and fuse the frit
Implementation Method 5
The application of temperature and voltage to the substrate assembly and the resultant formation of a second oxide layer that bonds the silicon to the second optically transparent substrate
Data Source
AI summary
A bonding technology is disclosed that can form an anodic, conductive bond between two optically transparent substrates. The anodic bond may be accompanied by a metal alloy, solder, eutectic and polymer bond. The first anodic bond may provide one attribute such as hermeticity, whereas the second bond may provide another attribute, such as electrical conductivity.