Vibrations can produce a number of significant challenges when conducting phase-shifting laser interferometer measurements. Today’s advanced interferometers have the capacity to measure with nanometer-levels of precision.
Conversely, this makes them vulnerable to error if deployed in a highly dynamic manufacturing environment where noise levels are high, leading to considerable amounts of vibration.
Why Vibration Matters
Taking measurements in vibration-prone environments usually presents fringe print-through errors — this may lead to bad data or an unworkable signal acquisition.
There are solutions that try to mitigate a detrimental environment in order to lessen the impact of undesirable effects. For instance, facilities may at times conduct a refurbishment in order to guarantee a “metrology” grade environment.
Another technique used to circumvent the negative effects is to take numerous repeated measurements over a period of time and find the average. However, this method can be time-intensive and have limited effectiveness.
QPSI Technology Enables Reliable Data in Harsh Environments
High-powered laser sources may form part of an anti-vibration solution as they deliver a sufficient level of power to shutter the camera faster as there is additional light. This takes less time and leads to faster data acquisition and reduced sensitivity to vibration as a result.
In recent years, ZYGO has worked to eradicate the negative effects of vibration, which has resulted in the emergence of acquisition technology solutions that can be deployed in adverse manufacturing environments.
Further to offering proprietary high-power HeNe lasers in its interferometers, ZYGO has designed the QPSI and FT-QPSI solutions.
QPSI acquisition is a model-based approach that takes rigid body vibrations into account and enhances vibration tolerance without compromising lateral resolution. It uses the exact same assembly as traditional Phase Shifting Interferometry (PSI) acquisition.
QPSI technology eradicates noise and ripple print-through in phase data caused by small vibrations and offers reliable data that would otherwise be impacted by noise as with conventional PSI acquisition.
QPSI measurements need no esoteric setup or calibration, and cycle times are generally the same as conventional PSI measurements. When vibration is present due to the manufacturing equipment, QPSI facilitates reliable high-precision measurements.
QPSI acquisition, however, is only compatible with two surface cavities, meaning a single interference pattern.
To tackle this, ZYGO has recently developed FT-QPSI acquisition on MST interferometers, a measurement mode that has the capacity to conduct front and back surface measurements simultaneously, as well as synchronous optical thickness variation measurements and homogeneity measurements.
Summary
Due to the precise nature of interferometry, even the tiniest amount of environmental vibration can be an Achilles heel and result in distorted measurements.
As interferometry is becoming increasingly popular, along with other highly accurate metrology solutions, it is now deployed in-process and on the production floor; thus, vibration will remain a core issue for engineers and quality managers.
This information has been sourced, reviewed and adapted from materials provided by Zygo Corporation.
For more information on this source, please visit Zygo Corporation.