Meaning
Measurement technique determining the absorption coefficient of a sample based on the decay rate of light trapped in an optical resonator. In cavity ring down spectroscopy, a high-finesse cavity lengthens the effective path of light through the medium to several kilometres. Monitoring this decay allows for the detection of trace contaminants at parts per billion levels during semiconductor gas delivery audits.
Decay Measurement
Laser energy enters the cavity until a threshold is reached and the source is then abruptly shuttered. The intensity of light leaking from the cavity ring down spectroscopy system follows an exponential decay curve as the signal bounces between mirrors. By measuring the time constant of this decay, the total optical loss is calculated without needing a reference beam.
Variations in the laser power do not influence the result because the measurement is based on time.
Sensitivity Threshold
Success depends on the reflectivity of the mirrors which must exceed 99.99 percent.
Production Yield
High-purity gas suppliers use these instruments to verify that moisture or oxygen levels stay below the tolerance of a specific fabrication process. A failure to detect these impurities before the production run begins leads to wafer contamination and costly scrap. Standardizing the decay monitoring period across all sites ensures that a pilot result matches the performance of a high-volume facility.
Demonstrated rates of gas purity are checked against the throughput requirements of the fab.
