Piezoresistive Silicon Diaphragm Doping and Strain Gauge Adhesion Defect Modes
Optimal piezoresistive stability requires heavy surface boron doping above 2e19 cm-3 combined with plasma-activated silane coupling to prevent delamination.
Titanium Diffusion Barrier
Metal Isolation
A thin metallic film prevents the interdiffusion and reaction between copper or aluminum interconnects and the underlying silicon substrate. The titanium diffusion barrier is deposited between the metal routing lines and the silicon contact regions to ensure that the metal does not migrate into the active junction area. Without this barrier, the metal atoms would diffuse rapidly into the silicon, causing junction leakage and device failure.
Sputtering Process
Physical vapor deposition is typically used to sputter a thin, uniform layer of titanium or titanium nitride onto the contact vias. This layer exhibits a dense, grain-boundary-free microstructure that physically blocks the migration of metal atoms even at elevated temperatures. The thickness of the deposited barrier must be carefully controlled, as a layer that is too thin will fail to block diffusion, while a layer that is too thick will increase the resistance of the contact via.
Diffusion Control
This barrier layer also improves the adhesion of the subsequent metal layer to the surrounding dielectric materials. In-line metrology uses sheet resistance measurements and X-ray reflectivity to monitor the thickness and density of the deposited barrier. These measurements confirm that the barrier film is continuous and free of voids or pinholes before the main metallization step begins.
Contact Resistance
Excessive processing temperatures can cause the titanium barrier to react with the silicon to form titanium silicide, which can alter the contact properties. The thermal budget of subsequent processing steps must be kept below the temperature where this reaction occurs.