Anodic Bonding of Silicon to Glass Substrates
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Solution Overview
Problem
Hermetic sealing of microelectromechanical systems (MEMS) devices, particularly those with optically transparent substrates like glass, is challenging due to the high processing temperatures required for glass frit adhesives, which can exceed the limits of thin metal layers used in optical devices, and existing anodic bonding methods are not suitable for optically transparent substrates with low conductivity.
Innovation Solution
An anodic bonding method between a silicon wafer and an optically transparent wafer, such as Borofloat or Pyrex glass, using a raised feature to form a metal oxide layer, accompanied by a laterally adjacent bond to enhance hermeticity and electrical conductivity, employing materials like titanium, chromium, or silicon for forming covalent bonds and oxide layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If glass frit adhesive is used to hermetically seal a glass cavity, then hermetic sealing is achieved, but processing temperatures exceed 400C which damages thin metal layers in optical devices
Solution Approach 1:
The patent changes the bonding mechanism from thermal fusion (glass frit requiring >400C) to anodic bonding using electrical field and controlled oxidation. This allows hermetic sealing at temperatures below 400C, specifically in the range of 200-400C, preserving the thin metal layers while achieving the required hermetic seal with leak rates less than 1 Torr/hour
Solution Approach 2:
The patent replaces the mechanical/thermal bonding process (glass frit melting and fusing) with an electrochemical process (anodic bonding through oxide layer formation). The bonding is achieved by applying a voltage between the glass substrate and silicon wafer, promoting oxide growth that creates the hermetic seal without requiring excessive thermal energy
2Temperature
If anodic bonding is used to bond silicon to glass, then hermetic sealing is achieved at lower temperatures, but existing methods are not suitable for optically transparent substrates with low conductivity
Solution Approach 1:
The patent applies local quality by creating a raised feature (protrusion) on one of the substrates that concentrates the bonding action to a specific location. This localized approach allows anodic bonding to proceed effectively even when the overall substrate has low conductivity, as the bonding occurs at the concentrated interface region rather than requiring uniform conductivity across the entire substrate
Solution Approach 2:
The patent introduces a metal oxide layer as an intermediary bonding mechanism between the glass substrate and silicon wafer. This oxide layer forms during the anodic bonding process and serves as the actual bonding interface, enabling hermetic sealing of optically transparent substrates that would otherwise be incompatible with traditional anodic bonding methods due to their low conductivity
3Illumination intensity
If a hermetic seal is formed around a glass cavity, then electromagnetic radiation transmission is enabled, but the seal must withstand processing temperatures that may damage optical device components
Solution Approach 1:
The patent changes the bonding temperature parameter from >400C (glass frit) to 200-400C (anodic bonding), creating a temperature window that preserves thin metal layers and other heat-sensitive optical components while still achieving hermetic sealing. The glass substrate maintains its optically transmissive properties throughout this lower temperature process
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 achieves a hermetic and electrically conductive bond between silicon and optically transparent substrates, providing a protective environment for MEMS devices while allowing electromagnetic radiation to pass through, with leak rates less than 1 Torr/hour and particle impenetrability, and offering improved electrical conductivity.
Implementation Method 1
A layer of metal oxide is formed between the silicon substrate and the optically transparent substrate. The metal oxide may be the oxidation product of a metal material and the optically transparent substrate.
Implementation Method 2
The application of temperature and/or voltage to the substrate assembly causes the formation of an oxide layer between the raised feature and the other substrate, which bonds substrates anodically.
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 Second bond, for example a metal alloy, solder, eutectic and polymer bond. The two bonds may be used for the same or a different purpose, and may be selected for the following attributes: hermeticity, electrical conductivity, low RF loss, high adhesive strength, leak resistance, thermal conductivity. The attributes for each bonding technology may be the same, or they may be different.