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Measuring Thin, Parallel Optics

Dan Musinski is the Vice President of Strategic Business Development at Zygo Corporation in Middlefield, CT.
By Dan Musinski Vice President of Strategic Business Development , Zygo Corporation

Driven by demand for smaller consumer products and semiconductor devices, manufacturers require thin planar optics for many applications. This means the optics manufacturers must ensure the glass is flat and free of material deformities, which can distortion and make the glass unusable.

This places a huge burden on metrology tools that need to measure and confirm the uniformity of thin planar optics.

Measuring thin parallel optical surfaces can be taxing. Such optics are less than a few millimeters thick, and this means that the front and back surfaces are very close together. Because of this, it is difficult to use standard mechanical phase shifting interferometry (PSI) to separate between the surfaces.

A more advanced solution is Fourier-transform phase-shifting interferometry (FTPSI), which enables easy characterization of the front and back surface, optical thickness variation, and material homogeneity of thin plane parallel glass. FTPSI makes it possible to distinguish between the front and back surfaces and characterize the quality of both in a single measurement, even if they are less than a millimeter thick.

Why FTPSI?

To understand why FTPSI is the preferred technique for measuring thin parallel optics, we need to see where traditional measurement techniques fall short.

PSI works by passing a light beam through an ideal reference optic (called a transmission flat [TF]), to the part under test. This technique cannot distinguish between the front and back surfaces of a thin parallel optic.

fig 3.jpg

Figure 3. The simplest FTPSI measurement is a three-surface configuration that consists of the TF — surface 1 ‚ and the test part — surfaces 2 and 3 (See Figure 3). In this configuration, a back-surface result is provided but it includes material non-uniformities due to the measurement beam passing through the material of the test part. Example of a 3-surface and 4-surface configuration.

When properly aligned, the TF and the part under test create an interference pattern, recorded as an interferogram. The metrology software analyzes the height variations produced by the phase shifts and reconstructs the surface wavefront, which represents the difference in height between the TF and the test part.  

When the front surface of a thin, parallel part is aligned, a second reflection is typically returned to the interferometer from the back surface. This results in a complex fringe pattern created by multiple, overlapping interferograms that cannot be accurately analyzed using PSI.

There are actions that could improve the situation, including painting black the back surface to extinguish its reflection, coloring with a dark colored marker, or spreading petroleum jelly on the surface.

The FTPSI method negates the necessity to manually manipulate the back surface of the thin optic to get an accurate measurement. Instead, FTPSI uses the refection from the back surface to gain more information about the thin optical component in a single measurement.

Why? FTPSI does not require mechanical motion within the test cavity to create the interferograms. Instead, FTPSI relies on modulation of the wavelength of the laser source to enable the measurement. Each cavity in the optical path in an FTPSI acquisition produces a unique interference frequency that defines its cavity length, and this enables a clear delineation and accurate characterization of the surface. Algorithms can then analyze both surfaces and characterize their form independently.

Accuracy

As with all interferometric test methods the measurement uncertainty is based on a number of factors including the quality of the reference optics, stability of the measurement environment and mounting techniques.

For parts less than 6 inches in diameter (150mm) the reference optic peak-to-valley surface form can be of the order of 2.5% of the wavelength of the light used to make the measurement — λ/40. If the system, for example, has a laser emitting red light at a wavelength of 633 nm this corresponds to approximately 16 nm. In most cases this enables the resultant measurement to be well within the tolerance bandwidth for thin glass applications.

How the part is held in the test cavity is probably the most critical factor when measuring thin optics, more specifically the mounting technique and the mounting orientation. Simply clamping a thin optic can induce unwanted stress and cause the optic to bend. Differences in orientation can yield very different measurement results, especially for thin parts, due to gravity affects. Ideally, the part should be mounted in the same configuration in which it will be used in its end-use application to avoid unexpected differences between the designed intent and actual performance.

Summary

FTPSI is a compelling choice for optics manufacturers who need to ensure the quality of thin, parallel optics. Unlike conventional mechanical PSI, FTPSI can distinguish the front and back surfaces and characterize their corresponding surface information in a single, repeatable measurement. Thanks to advances in both equipment and algorithms FTPSI can characterize surface form, thickness deviation, and material homogeneity of optics that are less than 1 mm thick. Faced with the growing demand for thin, parallel optics, and the challenges involved in accurately measuring those optics, FTPSI overcomes the limitations of previous methods. Its strength in characterization along with its ease-of-use makes it a good choice for optical metrology.

Dan Musinski is the Vice President of Strategic Business Development at Zygo Corporation in Middlefield, CT. He can be contacted at dan.musinski@ametek.com.

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