6.3 Hermetic sealing and environmental protection

Hermetic sealing and environmental protection are crucial for MEMS/NEMS devices. These techniques shield sensitive components from moisture, contaminants, and physical damage, ensuring long-term reliability and performance in harsh environments.

Various bonding methods and protective coatings create robust seals and barriers. Getter materials absorb residual contaminants inside sealed packages, maintaining a clean internal environment. These strategies are essential for successful MEMS/NEMS integration and packaging.

Bonding Techniques for Hermetic Sealing

Glass Frit Bonding Process and Applications

• Glass frit bonding creates a hermetic seal between two surfaces using a glass paste that melts at a lower temperature than the bonded materials
• Involves screen printing a glass paste onto one surface, aligning the second surface, and heating to melt the glass and form a bond
• Commonly used to seal glass to glass, ceramics, or metals in MEMS packaging (pressure sensors, accelerometers)
• Provides a strong, hermetic seal resistant to moisture and contaminants
• Requires careful control of the bonding temperature and pressure to ensure a uniform, void-free bond

Anodic Bonding Mechanism and Advantages

• Anodic bonding, also known as field-assisted bonding, creates a hermetic seal between glass and silicon or metal surfaces using an applied voltage
• Process involves heating the glass and silicon to 300-500°C, applying a high DC voltage (500-1000V) across the stack, and allowing mobile ions to migrate and form a chemical bond at the interface
• Offers a strong, hermetic seal with minimal thermal stress due to the lower bonding temperature compared to fusion bonding
• Widely used in MEMS packaging for inertial sensors, microfluidic devices, and vacuum packaging
• Requires a smooth, clean bonding surface and careful control of the applied voltage and temperature to prevent dielectric breakdown or residual stress

Eutectic Bonding for Wafer-Level Packaging

• Eutectic bonding uses a low-melting-point alloy (eutectic) to create a hermetic seal between two metal surfaces or a metal and semiconductor
• Common eutectic alloys include gold-silicon (Au-Si), gold-tin (Au-Sn), and aluminum-germanium (Al-Ge) with melting points below 400°C
• Process involves depositing the eutectic alloy on one surface, aligning the second surface, and heating to melt the alloy and form a bond upon cooling
• Enables wafer-level packaging of MEMS devices with high bond strength and hermetic sealing
• Suitable for packaging devices sensitive to high temperatures (RF MEMS switches, resonators)
• Requires careful control of the bonding temperature and pressure to prevent voiding or non-uniform bonding

Protective Coatings and Barriers

Conformal Coatings for Environmental Protection

• Conformal coatings are thin, protective layers applied to PCBs and electronic components to protect against moisture, dust, chemicals, and mechanical damage
• Common conformal coating materials include acrylics, epoxies, silicones, and urethanes, each with different properties and application methods
• Acrylic coatings offer good moisture and dielectric protection, ease of application and repair, but limited chemical and abrasion resistance
• Epoxy coatings provide excellent moisture, chemical, and abrasion resistance, but are harder to apply and repair
• Silicone coatings have excellent high-temperature and flexural properties, but lower dielectric strength and abrasion resistance
• Urethane coatings offer good moisture, chemical, and abrasion resistance, but can be difficult to apply and repair

Parylene Coating Properties and Applications

• Parylene is a family of polymeric conformal coatings known for their excellent moisture barrier properties, biocompatibility, and pinhole-free coverage
• Deposited via a vapor deposition polymerization (VDP) process, resulting in a uniform, conformal coating on complex geometries and surfaces
• Common types include Parylene C (chlorinated), Parylene N (non-chlorinated), and Parylene HT (high-temperature)
• Parylene C is the most widely used, offering excellent moisture, chemical, and dielectric barrier properties
• Parylene coatings are used in medical devices (pacemakers, catheters), automotive sensors, and aerospace electronics for protection against harsh environments
• Provides a thin (1-50 μm), transparent coating that minimally impacts device dimensions and performance

Moisture Barrier Films and Encapsulation

• Moisture barrier films and encapsulation techniques are used to protect sensitive electronic components and devices from humidity and water ingress
• Common moisture barrier materials include inorganic films (silicon nitride, silicon oxide, aluminum oxide) and organic films (polyethylene, polyimide, epoxy)
• Inorganic films offer excellent moisture barrier properties but can be brittle and prone to cracking
• Organic films provide flexibility and ease of processing but have lower moisture barrier performance compared to inorganic films
• Multilayer barrier films combining inorganic and organic layers offer the best of both worlds, with high moisture barrier properties and mechanical flexibility
• Atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) are common techniques for depositing thin, conformal moisture barrier films
• Encapsulation techniques, such as glob top and dam-and-fill, use polymeric materials (epoxies, silicones) to protect components from moisture and mechanical damage

Getter Materials

Getter Functions and Types

• Getter materials are used in hermetically sealed packages to remove moisture, oxygen, and other contaminants that can degrade device performance or reliability
• Getters work by chemically absorbing or adsorbing the contaminants, maintaining a clean, dry environment within the package
• Two main types of getters: evaporable getters and non-evaporable getters (NEGs)
• Evaporable getters are materials (barium, titanium) that are heated to evaporate and deposit a thin, reactive film on the package interior, absorbing contaminants
• NEGs are porous materials (zirconium, titanium, or alloys) that absorb contaminants without evaporation, activated by heating during packaging
• NEGs offer higher absorption capacity and compatibility with lower-temperature packaging processes compared to evaporable getters

Getter Activation and Placement

• Getter activation is the process of heating the getter material to a specific temperature to initiate its absorption properties
• Evaporable getters are activated by heating to their evaporation temperature (700-900°C for barium) during the packaging process
• NEGs are activated by heating to a lower temperature (300-500°C) to desorb surface contaminants and increase porosity
• Getter placement within the package is critical for effective contaminant removal
• Evaporable getters are typically placed in a separate compartment or on a dedicated substrate to prevent contamination of the device during evaporation
• NEGs can be placed in direct contact with the device or on a nearby surface, as they do not require evaporation
• Getter size and surface area are designed based on the expected contaminant levels and desired lifetime of the packaged device
• Proper getter selection and placement ensure a clean, stable environment within the hermetically sealed package, extending device reliability and performance