Anodic Glass Bonding Thermal Expansion Strain Reduction Techniques
Anodic glass bonding strain is minimized by combining matched non-linear CTE glass selection, thin compliant interlayers, and controlled cool-down ramps.
Pyrex 7740
Substrate Material
Borosilicate glass composed primarily of silica and boric oxide exhibits excellent thermal stability and high chemical resistance. In wafer-level microfabrication, pyrex 7740 is a standard substrate for creating microfluidic channels and capping sensor cavities. Its properties ensure durability in corrosive measurement environments.
Anodic Bonding
The thermal expansion curve of the glass must match that of silicon to prevent fracture during cooling from high bonding temperatures. When pyrex 7740 is bonded to silicon, the residual stresses are minimized due to the close match in expansion coefficients. A strong hermetic seal is formed as sodium ions migrate under the influence of an applied voltage.
Metrological Quality
Surface roughness and total thickness variation are measured using optical profiling before the wafer bonding process begins. Any surface defect or contamination preventing intimate contact will lead to unbonded regions or voids. To maintain high yields, the wafer flatness must be controlled to within three micrometers across a four-inch wafer.
Electrical Isolation
High dielectric strength and low electrical conductivity are required to prevent signal leakage between silicon structures across the glass substrate. Utilizing pyrex 7740 provides excellent electrical isolation, which reduces parasitic capacitance in high-frequency sensor applications. This isolation improves the signal-to-noise ratio of capacitive sensors, allowing for the detection of extremely small physical displacements.
By isolating the electrode structures from the surrounding packaging, the material protects the sensor against electromagnetic interference.