(53) Violet-Light Phase Shifting Interferometry for Surface Metrology

Since Albert Michelson developed his namesake interferometer in the early 20th century, we have been developing new and clever ways of using the interference of individual light waves to measure all kinds of physical... [ view full abstract ]

Since Albert Michelson developed his namesake interferometer in the early 20th century, we have been developing new and clever ways of using the interference of individual light waves to measure all kinds of physical phenomena. Interferometers can measure effects as diverse as the separation of sodium doublets and the minute ripples in spacetime from coalescing celestial objects, as measured at the LIGO observatories. Given proper measuring equipment, interferometers of this type can also measure the depth of nanoscale features on a surface with great precision. I built a Phase Shifting Interferometer, which is capable of shifting the phase of light incident upon a surface by one quarter of the light’s wavelength. Using a published algorithm, I developed a program in MATLAB to turn my images of interference fringes (interferograms) into a 3D sheet plot showing the dimensions and nanoscale features of each surface. Now, beyond merely “plugging and chugging,” this program needed to overcome an inherent problem with PSI algorithms. Due to the sinusoidal nature of light waves, any algorithm which calculates depth from a set of interference fringes will encounter a phase-wrapping error in the arctangent function. This occurs when the bounds of the data exceed the range of validity of the arctangent function. Thus, the measured depth of a surface feature may seem choppy and incoherent. So, phase unwrapping methods must be considered after applying the algorithm. The MATLAB program I wrote addresses this by allowing me to choose a threshold phase and align many phase-jumped sections into one continuous sheet. I performed this experiment on a variety of objects including a commercial mirror, a diffraction grating, a Ronchi ruler, and a thin film of corroded silver. These methods are invaluable in the world of precision machining, and can be applied to a variety of fields. Precision optics manufacturers and ball bearing manufacturers, for example, need to know the dimensions and surface qualities of their products with great precision, and Phase Shifting Interferometry allows them to take nanoscale measurements without the monetary cost of electron microscopy or the time cost of atomic force microscopy.