Quantifying Cluster Beam Etch Rates on Amorphous Surfaces
Quantifying cluster beam etch rates requires integrating beam current, cluster size distribution, and optical step metrology while neutralizing charge.
Quartz Crystal Microbalance
Mass Measurement
A highly sensitive mass-sensing instrument that measures changes in the resonant frequency of an oscillating quartz crystal determines the thickness of deposited metal films. In vacuum deposition systems, the quartz crystal microbalance tracks the rate of copper or gold sputter coating on the circuit board substrate. This measurement ensures that conductive layers are within the sub-nanometer tolerances required for high-frequency signal lines.
Thin Film Calibration
The instrument relies on the piezoelectric effect, where the resonant frequency of the quartz crystal decreases in direct proportion to the mass deposited on its surface. When the quartz crystal microbalance is exposed to the deposition stream, the frequency shift is monitored in real time and converted into a thickness measurement. This data is fed back to the control system to terminate the sputtering process once the target thickness is reached.
Placing the sensor close to the substrate ensures that the measured thickness matches the film deposited on the actual boards.
Rate Monitoring
Regular calibration of the quartz crystal is necessary because accumulation of metal shifts the base resonant frequency. If the sensor becomes saturated, its sensitivity decreases, leading to deposition errors. This maintenance step ensures that the metallization step remains accurate over multiple production runs.