Electronics Guide

Cavity Design Considerations

Proper cavity design requires careful attention to multiple factors. The cavity volume affects outgassing behavior and getter lifetime. Smaller cavities reduce outgassing but may require more aggressive getter materials. Cavity geometry influences stress distribution around the die, with sharp corners concentrating stress and rounded transitions providing better stress relief.

Electrical feed-throughs must penetrate the cavity walls while maintaining hermetic sealing. Common approaches include brazed pins in ceramic packages, through-silicon vias in wafer-level packages, and molded interconnects in plastic cavities. Each feed-through represents a potential leak path that must be carefully designed and tested.

Optical windows in cavity packages require transparent materials like glass or sapphire with appropriate antireflective coatings. The window must be hermetically sealed yet maintain optical clarity and minimize stress on the MEMS device. Applications include optical MEMS, infrared sensors, and imaging devices.

Die Attach Strategies

The die attach layer is a critical element in stress isolation. Compliant die attach materials like silicone adhesives can absorb stress but may compromise thermal performance and hermeticity. Rigid materials like gold-silicon eutectic or solder provide excellent thermal and electrical performance but transmit more stress to the die.

Advanced approaches include selective die attach, where adhesive is applied only in non-sensitive regions, creating islands of attachment with stress-relieving gaps. Some designs use a rigid central attachment for mechanical support with compliant materials around the periphery. The die attach thickness also matters: thicker layers provide more compliance but reduce thermal conductivity.

For ultimate stress isolation, some MEMS devices use suspended die mounting where the active structures are completely released from the substrate, connected only through flexible tethers. This approach is common in gyroscopes and resonant sensors where mechanical isolation is paramount.

Substrate Design for Stress Isolation

The package substrate can incorporate stress isolation features. Slots or cutouts around the die attach area reduce stress transmission from the outer package to the die. Compliant regions in the substrate absorb thermal expansion differences. Some designs use a rigid island for die mounting, isolated from the main substrate by flexible connecting sections.

Multi-layer substrates can distribute stress more evenly through careful layer design and via placement. Symmetrical stackups minimize warpage and stress gradients. The use of core materials with appropriate stiffness helps control stress distribution while maintaining package integrity.

Getter Types and Selection

Evaporable getters, typically barium-based, are heated to evaporate a reactive film onto cavity surfaces. They provide excellent pumping speed and capacity but require careful placement to avoid coating sensitive structures. These getters are common in larger packages where space allows separation between the getter and active device.

Non-evaporable getters (NEGs) are solid materials that absorb gases through surface absorption and bulk diffusion without evaporation. Common NEG materials include titanium-zirconium-vanadium alloys and zirconium-aluminum alloys. They offer cleaner operation than evaporable types but generally lower capacity per unit volume.

Thin-film getters deposited directly on package surfaces provide space-efficient gas absorption in miniaturized packages. These films, typically 1-5 micrometers thick, can be patterned to specific areas and activated at temperatures compatible with wafer-level processing. They are ideal for wafer-level packaging where traditional getter pellets will not fit.

Getter Activation and Performance

Getter activation typically occurs at 300-450 degrees Celsius, requiring thermal processing after hermetic sealing. The activation temperature must be compatible with other package materials and the MEMS device itself. Some advanced getters activate at lower temperatures or can be pre-activated before sealing.

Getter capacity determines the vacuum lifetime. Capacity must exceed the total gas load from initial cavity pressure, outgassing from package materials, and permeation through seals over the required lifetime. For consumer MEMS, this might be 10-15 years; for aerospace applications, 20-30 years or more.

Getter pumping speed affects how quickly pressure equilibrium is reached after sealing and activation. Faster pumping speeds are important when outgassing rates are high or when rapid processing is required. The effective pumping speed depends on getter surface area, temperature, and gas conductance within the cavity.

Bonding Technologies

Anodic bonding joins silicon and glass wafers through the application of high voltage (200-1000V) and moderate temperature (300-450 degrees Celsius). The process creates a strong hermetic bond without intermediate materials. It is widely used for MEMS because it provides excellent hermeticity, high bond strength, and is compatible with getter activation temperatures. However, it requires conductive wafers and is limited to silicon-glass combinations.

Fusion bonding directly joins two silicon wafers or two oxide-covered surfaces through atomic-level bonding at high temperatures (800-1100 degrees Celsius). This process creates bonds as strong as the bulk material with no intermediate layer. While it provides excellent hermeticity and reliability, the high temperatures can be incompatible with some MEMS devices and metallization schemes.

Eutectic bonding uses metal alloys (commonly gold-silicon, gold-tin, or aluminum-germanium) that melt at specific temperatures to create hermetic seals. The process occurs at moderate temperatures (280-400 degrees Celsius depending on the alloy) and provides good electrical conductivity through the bond. Eutectic bonding is flexible regarding substrate materials but requires precise alloy composition and may introduce stress from the metal layer.

Glass frit bonding uses a low-melting-point glass paste deposited between wafers. Upon heating to 400-550 degrees Celsius, the glass melts, flows, and bonds the surfaces. Glass frit provides good hermeticity and can accommodate some surface roughness, but the process is sensitive to frit composition, layer thickness, and firing conditions.

Polymer bonding utilizes materials like epoxy, benzocyclobutene (BCB), or polyimide for lower-temperature sealing (150-350 degrees Celsius). While generally not hermetic, some advanced polymers can achieve near-hermetic performance for less demanding applications. Polymer bonding offers flexibility and low stress but typically shorter lifetime than ceramic or metal sealing.

Vacuum Sealing at Wafer Level

Creating and maintaining vacuum in wafer-level packages requires careful process control. The bonding process typically occurs in a vacuum chamber or controlled atmosphere, with the cavity pressure at bonding becoming the initial sealed pressure. For high-vacuum applications (below 1 Pascal), the process chamber must achieve ultra-high vacuum, and all materials must be thoroughly degassed.

Getter integration is critical for vacuum WLP. Thin-film getters can be deposited on the cap wafer before bonding, positioned to maximize pumping speed without interfering with the MEMS structures. The getter is activated after sealing, either through the cap wafer using laser irradiation or through bulk heating of the entire wafer stack.

Through-Silicon Vias for WLP

Through-silicon vias (TSVs) enable electrical connections through the cap wafer or device wafer, providing routing flexibility and enabling testing before dicing. TSVs are formed by deep reactive ion etching (DRIE) to create high-aspect-ratio holes through the silicon, followed by insulation layer deposition and metal filling.

TSV design for MEMS WLP must consider hermeticity, as each via penetrating the package represents a potential leak path. Careful attention to insulation layer quality and seal ring design around vias is essential. The thermal expansion mismatch between silicon and TSV fill materials can also induce stress affecting MEMS performance.

Achieving and Maintaining Vacuum

Creating vacuum begins with evacuating the package cavity before hermetic sealing. This can be done through pump-down ports that are subsequently sealed, by sealing in a vacuum chamber, or through getter pumping after an initial low-pressure seal. The method chosen depends on the required pressure, package type, and manufacturing constraints.

Maintaining vacuum over the device lifetime requires addressing three primary sources of pressure increase: outgassing from package materials, permeation through seals and package walls, and virtual leaks from trapped volumes. Outgassing is minimized through material selection, pre-bake processes to remove absorbed gases, and getter capacity to absorb released gases. Permeation is controlled through hermetic seal design and material selection, with metal and ceramic packages providing better performance than glass or organic materials.

Getter sizing must account for the total gas load over the required lifetime. This includes the initial cavity pressure after sealing, gases released through outgassing (which can continue for years at decreasing rates), and permeation through package interfaces. A typical rule of thumb allocates getter capacity at 5-10 times the expected gas load to ensure reliable long-term vacuum maintenance.

Leak Testing

Hermetic seal quality is verified through leak testing, typically using helium mass spectrometry. This technique can detect leak rates below one times ten to the minus twelve atmospheric cubic centimeters per second, small enough to maintain vacuum for decades. MIL-STD-883 defines standard leak rate requirements for hermetic packages based on cavity volume.

For vacuum MEMS packages, leak testing must distinguish between true leaks (continuous gas flow through defects) and virtual leaks (trapped gas slowly released into the cavity). Virtual leaks can be more problematic than small true leaks because they release gas directly into the sealed cavity, and getters may not be positioned to effectively capture it.

Sealing Controlled Atmospheres

Controlled atmosphere sealing requires backfilling the package with the desired gas before hermetic sealing. This can be accomplished by sealing in a controlled environment filled with the target gas, or by pump-purge cycles to remove air and replace it with the desired atmosphere. Multiple purge cycles improve atmosphere purity, with three to five cycles typically achieving better than 99% target gas concentration.

Maintaining atmosphere composition over time requires hermetic sealing to prevent air ingress and carefully selected internal materials to minimize outgassing of unwanted species. Some applications use getters that selectively absorb contaminants while leaving the desired gas intact. For example, reactive getters can remove water vapor and oxygen while allowing inert gases to remain.

Applications of Controlled Atmospheres

Thermal sensors and infrared detectors often use low-pressure inert gas fills to minimize convective heat transfer while preventing oxidation. The gas provides enough molecular conductance to equalize pressure during temperature changes while maintaining thermal isolation.

Chemical sensors may require specific atmospheres for calibration or to prevent degradation of sensitive materials. Some sensors are packaged with a reference chamber containing a known gas composition, providing a stable baseline for differential measurements.

Microphones and acoustic MEMS devices use atmospheric pressure packaging with controlled humidity to prevent corrosion while allowing acoustic coupling through precisely designed ports or membranes. The internal atmosphere must be dry to prevent moisture condensation on sensitive structures.

Design Strategies

Package design can incorporate particle-tolerant features. Generous spacing between moving parts and fixed surfaces ensures that typical particles do not cause jamming. Stop structures limit the range of motion, preventing particles from wedging into critical gaps. Smooth surfaces and radiused corners minimize particle generation from wear.

Some designs include particle traps: regions designed to capture any loose particles and prevent them from reaching sensitive areas. These traps might be physical barriers, adhesive surfaces, or simply low points where gravity causes particles to accumulate away from active structures.

Electrical design can provide particle tolerance through redundant sense elements and differential measurements that cancel common-mode effects of contamination. Some devices include built-in particle detection through resistance or capacitance monitoring.

Process Controls

Sealing processes are critical contamination sources. Solder and braze sealing can generate particles through flux residues or metal spatter. Glass frit sealing may produce glass fragments. Even clean bonding processes like fusion bonding can trap particles at the bond interface if surfaces are not perfectly clean.

Pre-seal cleaning processes remove particles from package cavities and die surfaces. Common techniques include nitrogen blow-off, plasma cleaning, wet chemical cleaning, and ultraviolet ozone treatment. The cleaning method must be compatible with the MEMS device and package materials while effectively removing contaminants.

In-process monitoring detects contamination before sealing. Optical inspection can identify visible particles, while automated vision systems can detect defects as small as 10 micrometers. Some processes use particle counters to verify cavity cleanliness immediately before sealing.

Coating Types and Application

Self-assembled monolayers (SAMs) are molecular coatings that form ordered arrays on surfaces through chemical bonding. Common SAM materials include fluoroalkylsilanes and perfluorinated compounds that provide extremely low surface energy (contact angles exceeding 110 degrees). SAMs are typically applied from vapor phase or solution, forming single-molecule-thick layers that do not significantly alter device dimensions.

The application process involves surface preparation (cleaning and activation), exposure to the coating precursor (vapor or liquid), and a curing or bonding phase. Surface cleanliness is critical: contaminants prevent SAM formation and create defects in the coating. Most SAM processes are performed in controlled atmospheres to prevent water or oxygen interference.

Polymer coatings like parylene or fluoropolymers provide thicker protection (0.1-10 micrometers) with excellent conformality and chemical resistance. These coatings are applied through vapor deposition, covering all exposed surfaces uniformly. The coating thickness must be controlled to avoid altering device gaps and mechanical properties.

Diamond-like carbon (DLC) coatings provide both anti-stiction properties and wear resistance, making them ideal for MEMS devices with contacting surfaces like switches and relays. DLC is applied through plasma-enhanced chemical vapor deposition, creating hard, smooth, low-friction surfaces. The process requires elevated temperatures (150-400 degrees Celsius) which may not be compatible with all device types.

Coating Durability and Testing

Anti-stiction coatings must survive not only subsequent packaging processes but also the operating lifetime in the field. Thermal cycling, humidity exposure, mechanical wear, and contamination can all degrade coating effectiveness. Long-term reliability testing subjects coated devices to accelerated aging under temperature, humidity, and mechanical stress to verify coating stability.

Coating effectiveness is measured through contact angle measurements (higher angles indicate lower surface energy), pull-off force testing (measuring the force required to separate surfaces that have been in contact), and actual device testing (measuring release performance and cycling behavior). These tests are performed both immediately after coating and after reliability testing to verify durability.

Stress Analysis and Simulation

Finite element analysis (FEA) is the primary tool for understanding package stress. Thermal-mechanical simulations model temperature changes from assembly temperatures to operating ranges, predicting stress distributions in the die, die attach, substrate, and surrounding structures. These simulations help optimize material selection, geometry, and assembly processes before committing to expensive prototyping.

Accurate simulation requires good material properties (Young's modulus, Poisson's ratio, CTE, yield strength) over the relevant temperature range, appropriate boundary conditions representing the actual package mounting, and sufficient mesh density to capture stress gradients. Nonlinear material behavior (plasticity, creep, viscoelasticity) should be included for polymers and solders.

Sensitivity studies identify which parameters most strongly influence stress, guiding optimization efforts. For example, simulations might reveal that die attach thickness has more impact than die attach modulus for a specific package, leading to process optimization focused on thickness control rather than material development.

Experimental Stress Measurement

While simulation provides insight, experimental validation is essential. Techniques for measuring package stress include strain gauge rosettes attached to the die surface, piezoresistive sensors integrated into the MEMS device itself, and optical methods like photoelasticity or digital image correlation. Each technique has advantages and limitations regarding sensitivity, spatial resolution, and ease of implementation.

Wafer curvature measurements before and after die attach quantify stress induced by the attachment process. The Stoney equation relates curvature change to film stress, enabling calculation of average stress from simple optical measurements. This technique is particularly useful for process development and qualification.

Device-level testing measures the impact of stress on actual performance. For sensors, this involves characterizing output versus temperature, mechanical loading, and humidity while monitoring for drift and hysteresis. Temperature cycling tests reveal whether package stress causes permanent performance changes or only reversible shifts.

Mitigation Strategies

Material matching is the most fundamental stress management approach. Selecting materials with similar CTEs minimizes thermal stress. For silicon MEMS (CTE 2.6 ppm per degree Celsius), good substrate matches include silicon itself, silicon carbide (CTE 4.0 ppm per degree Celsius), aluminum nitride (CTE 4.5 ppm per degree Celsius), and certain low-CTE organic materials (CTE 6-12 ppm per degree Celsius).

Compliant interfaces absorb stress through elastic deformation. Soft die attach materials (silicones, compliant epoxies) provide stress isolation but compromise thermal performance. Advanced approaches use patterned die attach, rigid in the center for thermal performance with compliant periphery for stress isolation.

Stress-free mounting techniques minimize external stress from PCB assembly. These include flexible interconnects that do not transmit PCB stress to the package, underfill materials with controlled modulus and geometry, and package designs that isolate the MEMS die from mounting stresses through compliant regions in the package itself.

Compensation techniques use electronics to correct for stress effects rather than preventing stress. Temperature sensors integrated in or near the MEMS device enable temperature compensation of stress-induced drifts. Multi-axis sensing with known stress response patterns can enable calculation of true input signals despite package stress. While compensation adds complexity, it can enable performance unattainable through mechanical design alone.

Moisture Protection

Moisture is the most common environmental threat to MEMS devices. Water vapor can corrode metal structures, cause stiction through capillary forces, alter electrical properties through charge accumulation, and freeze at low temperatures causing mechanical damage. Protection strategies range from hermetic sealing for the highest reliability to conformal coatings and hydrophobic treatments for less demanding applications.

Hermetic packages using metal, ceramic, or glass-sealed cavities provide the ultimate moisture barrier, with properly designed seals preventing water ingress for decades. Leak rates below five times ten to the minus eight atmospheric cubic centimeters per second are typical for qualified hermetic packages, corresponding to moisture levels below 5000 ppm even after years of operation.

For applications where hermetic packaging is too expensive, advanced polymer sealing with desiccants can provide excellent moisture protection. Multilayer barrier coatings using alternating organic and inorganic layers achieve moisture vapor transmission rates below one times ten to the minus four grams per square meter per day, adequate for many consumer applications. Integration of desiccant materials absorbs moisture that does permeate, maintaining low internal humidity.

Chemical Resistance

MEMS devices in chemical sensing, medical, or industrial applications may encounter aggressive chemicals that can degrade package materials, attack seals, or contaminate the MEMS structures. Material selection focuses on chemical compatibility: fluoropolymers for acid resistance, specialized epoxies for solvent resistance, ceramics for high-temperature oxidation resistance.

Selective exposure strategies protect sensitive package regions while allowing necessary environmental access. Membranes can permit gas permeation for chemical sensing while blocking liquids. Hydrophobic coatings prevent liquid ingress while allowing gas exchange. Sacrificial barrier layers absorb chemical attack, protecting underlying structures for extended periods.

Temperature Extremes

MEMS packages for automotive applications must survive negative 40 to positive 150 degrees Celsius, industrial applications may see negative 55 to positive 175 degrees Celsius, and aerospace applications can reach negative 65 to positive 200 degrees Celsius or beyond. These extreme temperatures challenge sealing materials, cause large differential thermal expansion, alter material properties, and can drive diffusion processes leading to long-term degradation.

High-temperature materials enable operation at temperature extremes. Ceramic packages with metal seals can operate above 300 degrees Celsius. Silicon carbide substrates extend temperature capability while maintaining semiconductor compatibility. Specialized high-temperature solders (gold-tin, gold-silicon eutectic) provide reliable interconnects at temperatures where standard solders would melt.

Thermal cycling resistance requires careful attention to CTE matching and fatigue-resistant designs. Solder joints are particularly vulnerable to thermal fatigue, with lifetime depending strongly on temperature range and cycle frequency. Stress modeling helps predict fatigue locations, enabling reinforcement through design changes or material selection.

Mechanical Protection

Shock and vibration protection is critical for automotive, aerospace, and portable applications. MEMS devices must survive shock events (thousands of g's for milliseconds during drops) and vibration (tens of g's at frequencies from Hz to kHz during operation). Protection strategies include mechanical stops limiting destructive motion, damping structures absorbing energy, and robust package designs distributing impact loads.

Over-range protection prevents mechanical damage during shock events that exceed normal operating ranges. Stops physically limit motion before structures reach breaking strain. Mechanical fuses are designed to fail predictably during extreme events, protecting the main sensor structure. Some designs incorporate shock absorption materials in the package to reduce transmitted forces.

Vibration testing verifies package and die attach integrity. Devices are subjected to sinusoidal vibration sweeps, random vibration spectra matching application environments, and combined stress tests including vibration with temperature cycling. Failure analysis of test failures guides design improvements in die attach, wire bonding, and package structure.