MEASURING SUPER SMOOTH SURFACES BY SENSOFAR | 1Measuring Super Smooth Surfaces
Exploiting the potentialof 3D optical profilometryMEASURING SUPER SMOOTH SURFACES BY SENSOFAR | 2INTRODUCTION

3What is a smooth surface?
Where do we find smooth surfaces?
Technologies for measuring super smooth surfaces

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INTERFEROMETRY TECHNIQUES12

How to select the proper Interferometry technique?
How to choose the proper Interferometry system?
Importance of calibration and traceability

131718BEST PRACTICES FOR MEASURING SMOOTH SURFACES

19Measurement workflow21CONCLUSIONS

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Where do we find smoothsurfaces?
Smooth and super smooth surfaces find extensive
applications across diverse fields and markets. The
optics market prominently utilizes smooth surfaces,
evident in the lenses, mirrors, and various other opticalcomponents used. Similarly, consumer electronics
heavily rely on smooth surfaces, particularly insemiconductors’ wafers, displays, and other criticalcomponents where functionality and assembly depend
on impeccable smoothness. The medical industryalso leverages smooth surfaces in medical implants,recognizing the vital role of roughness in ensuring
compatibility and minimizing friction betweenelements.
Other fields that significantly benefit from smoothsurfaces are advanced manufacturing, aerospace,and automotive sectors, as the roughness ofnumerous components directly influences the final
functional performance of the respective workpieces.
The multifaceted applications of smooth and supersmooth surfaces demonstrate their indispensability indriving innovation and facilitating advancements acrossvarious industries.

CONTACTMEASURING SUPER SMOOTH SURFACES BY SENSOFAR | 11

NON-CONTACT (optical)
SINGLE-POINT

Holography

Coordinate Machines

Triangulation

Stylus profilers

ConfocalScanning probe
microscopy(AFM, STM,...)

Chromatic Confocal

Damage on the surface
Low sampling of the surface
Slow methods

IMAGING

Fringe projection

Interferometry

Focus variation
ConfocalInterferometry
Best accuracy& repeatability

Low sampling

Figure 5. Classification of profilometry techniques based on contact with the surface.

They can all be classified according to different criteria, one
being the distinction between contact and non-contacttechniques [3]. Contact-based options encompass, forexample, Coordinate Measuring Machines (CMMs) or stylus
profilometers. Non-contact techniques can be further
classified into single-point or imaging techniques basedon their rastering technique. Optical techniques include
Confocal, Focus Variation, or Interferometry, for example.
Considering the previous aspects, contact techniques
are suboptimal for measuring smooth surfaces
due to potential surface damage, limited surface

sampling, and relatively slower speeds. Single-point
imaging approaches are non-damaging but offer lowsampling, providing an incomplete picture of surface
characteristics.
Hence, imaging techniques are the preferredchoice for characterizing smooth surfaces. Theyoffer fast, non-contact measurement capabilities
with improved surface sampling [4]. Among thesetechniques, Interferometry stands out for smoothsurface measurement, delivering superior accuracy andrepeatability.

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SIC mirror
Sa = 0.1120 nm
Noise 0.01 nm

PSI

WHEN TO USE PSI?
To compare the capabilities of CSI and PSI, let us
consider an example involving the measurement ofsurface roughness on super smooth surfaces. Figure 6
shows the resulting measurements on a silicon carbide
mirror with both CSI and PSI. The results are very
different as they can be extracted from the averageroughness parameter (Sa).

CSI

However, it is crucial to note that the CSI value
lies within the range of noise associated with thistechnique; therefore, the measurement’s accuracy iscompromised.
On the other hand, PSI offers a significantly lower
measurement noise, potentially as low as 0.01 nm. In
this case, the Sa value is well above the backgroundnoise, indicating a more accurate measurement.
This is the reason why PSI is the preferred technique formeasuring surface roughness of super smooth surfaces.

Sa = 0.5385 nm
Noise 1 nm
Figure 6. Measurement of a SiC mirror with CSI and PSI. Both topographies have been obtained withthe same acquisition and filtering settings for comparison.

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FLATNESSIn the case of flatness measurements, the hardware
setup plays a critical role since flatness errors of thesystem will be the limiting factor for our measurements.
These errors can come from the optical system or themoving stage (if image stitching is used). In the caseof the example topography that we report here, weused a specialized configuration featuring a 0.65X
magnification objective lens, a high stand, a granitetable, and a high-precision XY stage.
The chosen technology here is ePSI, which offers thebest possible vertical resolution across a wide range.
We conducted a scan of a 100 mm diameter wafer,completing it in just four minutes. We applied the ISO
12781 standard to assess the flatness, which enabled us
to obtain various parameters characterizing the surface.

HARDWARE

Large standGranite table
0.65X lensHigh precision stage

ACQUISITIONePSI
100 mm x 97 mm
4’21 μm / pixel
20 μm

It’s worth mentioning the significance of the hardware
utilized in this scenario. The Sensofar lens with 0.65X
magnification offers a generous field of view while
maintaining minimal flatness error. Combining this lenswith a high-precision stage is crucial to ensure accurate
flatness measurements. Standard stages can introduce
errors on the micron scale, which can interfere with theflatness measurement.

Figure 13. Flatness characterization ofa SiC wafer using S neox with a highprecision stage, 0.65X Michelson lens,and ePSI technique. Flatness parameters
were obtained following ISO 12781 with
an S-filter of 8 mm and a leveling plane as
the F-operator.

ANALYSIS

S-filter:
8 μm
F-operator: Leveling plane
L-filter:
–2 μm

-2 μm

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