Front-to-Back

TTV can only be calculated if the front and back measurements are performed without translating or moving the part. While this restriction poses challenges for traditional laser Fizeau systems, this is accomplished easily with SCI as the fringes are moved rather than the part.

The front surface is measured, named, and stored in REVEAL’s clipboard

The back surface is measured, and a correction is applied for the index of refraction.

The two measurements are added to report the TTV.

An advantage of Front-to-Back TTV parts as thin as 100 µm OPL can be characterized.

Direct

The Direct measurement is accomplished by locating the SCI

interference fringes inside the substrate itself. These fringes are called “internal Fizeau” fringes because the fringes are between the front and back surfaces or internal to the substrate. Once the SCI has localized fringes inside the substrate, the interference phase is directly measured and hence the TTV acquired. Phase measurement of internal Fizeau fringes is only possible by wavelength modulation (wavelength phase shifting) and SCI phase shifting.

The OPL limitations for Direct TTV measurement are:

For wavelength modulation: OPL must be >2 mm and phase measurements can take more than 60 seconds

For SCI: OPL of 500 µm are possible with measurement times measured in milliseconds.

Summary

Measurement of a glass plate has many complications mostly due to interfering back reflections as found in a laser interferometer. SCI solves this problem and expands what can be measured on a substrate in one setup and with no preparation.

Plane-Parallel Substrates

What could be easier than measuring a plate of glass? Actually, almost all interferometric measurements are easier! Measuring a plate of glass, or a substrate, is one of the most difficult interferometric measurements. In a laser interferometer, the two surfaces of the plate simultaneously interfere with the reference surface and each other creating three sets of fringes. When multiple sets of fringes exist it is impossible to make a measurement.

Laser Fizeau a Compromised Measurement

The laser Fizeau interferometer produces fringes off every surface. Therefore, back reflections off the unwanted surfaces must be suppressed in order to take an accurate measurement. The typical suppression method is to apply a foreign material to the back surface to reduce the magnitude of the back reflection and decrease these fringe’s contrast. This foreign material might be blue paint, grease, index matching fluid or tape.

Accuracy Degraded

These methods cannot completely diminish the fringe; their contrast is just reduced. By setting a modulation (contrast) threshold the laser Fizeau nominally ignores these fringes during measurement. Unfortunately thresholding is not foolproof. The low contrast secondary surface fringe intensities add to the higher contrast front surface fringes distorting the phase measurement. Therefore even though the results can improve, the back surface fringes still degrade the measurement.

The back surface coating can also warp the part, depending on the thickness to diameter ratio of the part. Most users assume the paint does not warp the front surface, but drying paint induces stresses and hence it degrades the accuracy, the larger the diameter to thickness ratio the more severe the potential warping.

Time Wasted/Costs Increased

The largest associated cost of measuring plano optics is the time wasted during sample preparation. Painting can take minutes to a few hours for the paint to dry¹ and while the measurement can be fast, removing the paint can add hours to the processing.

Substrate Measurement with SCI

Front Side Measurement with SCI

Spectrally Controlled Interferometry (SCI) solves this problem. First, in the long coherence “laser mode” SCI allows for easy alignment, just like in a laser Fizeau. Then by electronically controlling the spectrum of the illumination, the coherence of the Fizeau interferometer is narrowed and positioned on the surface of interest, thereby limiting the interference to only one cavity. Therefore, substrates can be measured without preparation and to higher accuracy.

Standard SCI can isolate surfaces separated as thin as 150 µm Optical Path Length (OPL), where:

OPL = n (index of refraction) X d (physical thickness)

With custom designed SCI systems even thinner substrates are possible to be measured. Once the fringes are found a standard phase measurement is performed taking only milliseconds.

Back Side Measurement with SCI

Without moving the part, the fringes are electronically moved to the back of the substrate. Now the front surface is invisible and only the backside measured. The distance the fringes are moved is the OPL, making finding the second surface easy. A standard phase measurement is again performed.

This measurement was made through the material so the index of refraction must be accounted for. ÄPRE’s back side measurement profile (application in REVEAL) makes this correction. The REVEAL profile compensates for the index of refraction correcting the surface heights, while reversing the sign of the surface heights, as if it was measured face-on (air to glass).

Summary

Spectrally controlled interferometer coupled with a standard Fizeau interferometer enables direct measurement of the front and back surfaces of a substrate without requiring treating the surfaces to suppress back reflection interference. With easy alignment in a coherent “laser” mode and electronically isolated fringes SCI lowers the cost of measurement and improves accuracy.

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¹ Users of SCI have noted they are saving hours-per-part switching from painting to SCI.