Vibrations can present considerable challenges during phase-shifting laser interferometer measurements. Today, state-of-the-art interferometers can measure with nanometer-levels of precision. In return, they can be susceptible to errors if used in loud and highly dynamic manufacturing environments where significant vibration is present.
Why Vibration Matters
Measurements taken in vibration-prone environments typically encounter fringe print-through errors — resulting in bad data or a signal that is impossible to acquire. There are solutions that attempt to correct a poor environment in order to mitigate its impact undesirable effects — and for instance, sometimes facilities are completely renovated in order to establish a “metrology” grade environment. Another method to overcome the negative effects is to take many repeated measurements over time and average them. This method has limited effectiveness and can take a long time.
QPSI Technology Enables Reliable Data in Harsh Environments
High-powered laser sources can be a part of an anti-vibration solution as they provide enough power to shutter the camera faster as there is more light. This means faster data acquisition and less sensitivity to vibration because the measurement takes less time.
Over the last several years, ZYGO has worked to eliminate the negative effects of vibration — resulting in the development of acquisition technology solutions that can be used in relatively hostile manufacturing environments.
In addition to providing proprietary high power HeNe lasers in its interferometers, ZYGO has developed the QPSI and FT-QPSI solutions. QPSI acquisition is a model-based approach that addresses rigid body vibrations and increases vibration tolerance without sacrificing lateral resolution. It uses an identical set up to traditional Phase Shifting Interferometry (PSI) acquisition.
QPSI technology eliminates noise and ripple print-through in phase data due to small vibrations and provides reliable data that would otherwise be “noisy” with traditional PSI acquisition. QPSI measurements require no special setup or calibration, and cycle times are typically equivalent to standard PSI measurements. In the presence of vibration from manufacturing equipment, QPSI enables reliable high-precision measurements.
QPSI acquisition, however, only supports two surface cavities, meaning a single interference pattern. To address this, ZYGO has recently introduced FT-QPSI acquisition on MST interferometers, a measurement mode which includes simultaneous front and back surface measurements, optical thickness variation measurements, and homogeneity measurements.
Summary
It is the very precision of interferometry that can be its Achilles heel, as even the minutest of environmental vibration can distort measurements. As interferometry along with other highly accurate metrology solutions are is increasingly being used in-process and on the production floor, vibration will remain a top-of-mind issue for engineers and quality managers.