Trap Rich Polysilicon Layer Optimization for Harmonic Suppression in Front End Modules
Trap rich polysilicon layers freeze interface charges to hold high resistivity and suppress switch harmonics below minus eighty dBc.
Microwave Photoconductive Decay
Meaning
Non-contact characterisation method for measuring the excess carrier lifetime in semiconductor wafers utilizes optical excitation and microwave reflectivity monitoring. Applying microwave photoconductive decay avoids the need for fabricating test contacts on the wafer surface, preventing contamination of the sample. This technique is bounded by the injection level of the optical pulse and the surface recombination velocity of the wafer, which must be carefully passivated to extract the true bulk recombination rate.
Measurement Principle
Laser pulses generate electron-hole pairs within the semiconductor to increase its electrical conductivity. This transient change in conductivity modulates the reflection coefficient of a continuous microwave signal directed at the surface. Recording the decay of the reflected signal over time allows calculation of the carrier lifetime.
Interface Sensitivity
Recombination at raw wafer surfaces occurs rapidly due to the high density of dangling bonds. Passivation of the surface with a thin oxide or chemical solution is required to measure the true bulk lifetime. This step separates surface effects from bulk material properties.
Process Control
Automated systems on the cleanroom floor use this method to monitor contamination from furnaces and etchers. It detects trace metallic impurities that degrade device performance before wafers undergo expensive lithography steps. This rapid feedback loop is essential for maintaining high yield in semiconductor manufacturing.