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