Isolating Micro-Machined Accelerometer Zero-G Bias Shifts under Static Thermal Cycles
Static thermal zero-g bias shifts in MEMS accelerometers are isolated by enforcing prolonged thermal dwell periods to decouple stress creep from thermal gradients.
Anodic Glass Seal
Hermetic Interface
Hermetic bonding interfaces provide durable encapsulation for micro-electromechanical sensors by joining silicon substrates directly to glass wafers. This connection, known as an anodic glass seal, relies on both electrostatic forces and chemical bonding to create a tight joint without adhesives. The process occurs at elevated temperatures under a strong electric field, which drives mobile ions away from the interface.
A strong chemical bond is formed when oxygen ions migrate to the silicon surface and form a silicon dioxide layer.
Process Execution
Production of this joint requires precise control over temperature and applied voltage to ensure uniform contact across the entire wafer surface. The components are heated to a temperature between three hundred and four hundred degrees Celsius, which increases the mobility of sodium ions in the glass. A high voltage, typically between four hundred and one thousand volts, is applied across the assembly, creating a strong electrostatic force that pulls the surfaces into intimate contact.
This force must be maintained until the current decays to a stable baseline, indicating that the bonding process is complete.
Bond Evaluation
Optical microscopy and helium leak testing provide the primary methods for verifying the integrity and hermeticity of the sealed interface. The glass wafer allows direct visual inspection of the bond area, where a uniform gray region indicates a successful joint and dark spots reveal trapped air or contaminants. Fine leaks are quantified using a helium mass spectrometer, which detects trace gas escaping from the encapsulated cavity under high vacuum.
This verification step ensures that the sensor housing will protect the delicate internal components from moisture and environmental contaminants over a long operating lifetime.
Thermal Compatibility
Matching the thermal expansion coefficients of the glass and silicon substrates is necessary to prevent high residual stresses from causing mechanical failure. Specialized borosilicate glasses are chosen because their expansion curve closely matches that of silicon. This selection limits the residual stress to a level that the silicon can tolerate without fracturing.