Silicon Piezoresistive Pressure Diaphragm Encapsulation Selection Criteria
Encapsulation selection balances gel potting low cost against oil-filled cell thermal stability to control long term zero drift and field recalibration expenses.
Silicon Diaphragm Encapsulation
Protective Layering
Isolating micromachined silicon pressure elements from corrosive process fluids prevents chemical etching and electrical shorting across piezoresistive bridges. Implementing silicon diaphragm encapsulation involves applying a thin conformal layer of parylene or organic gel over the exposed silicon face. Protective coatings prevent oxidation and ion migration without altering mechanical stiffness significantly.
Thin film deposition methods guarantee uniform thickness across deep micromachined cavity sidewalls. Material selection depends on fluid compatibility requirements and operating temperature limits.
Dynamic Compliance
Coating thickness determines the added mass and bending stiffness of the flexible sensor structure. Minimal thickness preserves high frequency response characteristics required for dynamic pressure measurement. Elastic modulus stability across operating temperature spans prevents thermal zero shift errors.
Compliant encapsulation compounds absorb mechanical impact from high velocity fluid bursts.
Chemical Permeation
Solvents and gases slowly diffuse through polymeric encapsulation layers over extended exposure periods. Hydrogen ion penetration through protective coatings degrades underlying silicon piezoresistors through chemical reduction. Permeability rates increase non-linearly with ambient temperature increases.
Fluorinated coatings reduce hydrocarbon gas diffusion rates compared to standard dimethyl silicone gels.
Viscous Damping
High viscosity encapsulation gel alters dynamic pressure step responses by introducing fluid drag forces. Dynamic response bandwidth decreases as gel thickness increases over the active diaphragm surface. Temperature dependent viscosity changes shift sensor time constants between cold start and steady state operating conditions.
Metrological characterization requires dynamic frequency response testing at maximum and minimum operational temperatures.