Metrology

To make it, it must be measured

What is Metrology?

Metrology is defined as the scientific study of measurement. The ability for Nanophorm to produce useful products is ultimately limited by its ability to measure them. This is particularly true with the production of optical devices where the dimensional requirements are extremely demanding. Metrology facilitates two critical outcomes: First, it allows two people to independently measure an optic to the desired level of precision, and second, it allows those two people to agree on the results. To accomplish this, the science of metrology is concerned with three concepts:

The establishment of common, consistent units of measurement, i.e. definitions of length, mass, etc.
The methods of realizing those units of measurement in the real world, i.e. measurement devices and
methods
Connecting one measurement to another by tracing them to a common master reference, i.e. “traceability”

Metrology at Nanophorm

In order to make products that meet or exceed customer expectations, Nanophorm continues to make significant investments in metrology. We can properly measure the form, the fit, and the finish of everything that we make. We can provide this measurement data as part of the deliverable to our customers. The types of measurements that we perform include:

Interferometry
Profilometry
Coordinate / angular measurements
Coating reflectivity and durability

Fizeau Interferometry

Many characteristics of optical devices are conveniently measured by interferometry. Interferometry is a technique whereby the interference of light is used to measure distances with extreme precision. A Fizeau interferometer is a type of interferometer that is especially suited for measuring the form or figure of optical surfaces. It does this by comparing the surface of the optic-under-test to a master reference surface. The difference in shape between these surfaces can be observed as an interference pattern at the image plane of the interferometer.

Modern phase- shifting interferometers carefully move the reference optic relative to the optic-under-test and capturing images of the interference pattern with a digital camera. By way of digital image processing one can detect differences spanning many waves of light without ambiguity with extremely high precision. Surface height differences of one one-hundredth of a wave of light can be detected (~6 nanometers).

Nanophorm uses a 4” ADE-MiniFiz Horizontal Phase Shifting Interferometer and full range of Zygo lambda/20 transmission reference optics to make a variety of measurements. By comparing to a master reference flat, we can measure the flatness of flat mirrors or plano transmission surfaces, or the wedge of an optical window. If we compare to one of several master transmission spheres, we can measure spherical irregularity or mild aspherical departure. We can extend this technique to measure radius of curvature.

Scanning White Light Interferometry

Scanning white light interferometry (SWLI) is a technique where the focus height of a white light microscope equipped with a beam splitting objective lens is scanned over a surface of interest. The image is captured repeatedly during scanning. Where the reference and test beams are very close to the same length there is enough coherence to see interference between the beams. By studying this interference pattern as a function of the height of the objective lens, a 3D topographical map of the surface can be made with sub-nanometer resolution. This is useful for observing small-scale waviness or roughness of the optical surface. It can also be used for looking at the structure of diffraction gratings, micro lens or mirror arrays, or other microstructures.

This nano or micro-scale data is critical for controlling the performance of optics and our manufacturing process. For example, light scatter is a function of the high frequency errors in the surface, typically called “roughness”. We can measure and control this down to the nanometer level. Image quality and contrast is related to the mid-frequency errors, often called “mid-spatial error” or “waviness”. This is measured by the SWLI. Fourier transform techniques can be used to determine where the errors are coming from and fix them. With diffractive optics such as gratings, the blaze angle and pitch of the features on the surface affect the dispersion of light and the efficiency of light throughput through the device.

Nanophorm uses a Zygo New View SWLI with a full range of objective lenses to measure surface roughness and diffractive structure. We use this information to control roughness and waviness of diamond turned optics, the blaze angle and pitch of diffraction gratings, and the general process control of our diamond turning machines.

Profilometry

For many aspherical, free-form, or hybrid-shaped optics or opto-mechanics, the best way, or perhaps the only way to measure the surface form is via profilometry.

Profilometry is the collection of coordinate measurements of the surface by tracing a probe over a surface. The position of the probe and part is measured to high accuracy during the process. Both contact and non-contact probes are possible. Surface scanning can be in two dimensions (a line trace) or in three dimensions (an area trace). Repeatability of better than 20 nanometers is possible.

Nanophorm utilizes a Taylor-Hobson PGIOptics contact profilometer as well as its own Nanophorm Scanning Confocal profilometer to measure arbitrary surface figure and diffractive structure to the nanometer level. This data is fed back into the diamond turning process in order to produce correctly-shaped optics and opto-mechanics.

Coordinate / Angular Metrology

Nanophorm has wealth of additional metrologycapabilities including:

Keyence LM-1100 Optical CMM
Keyence VHX-970FN Digital Microscope
Customized PneumoCentric roundness gauge
McBain Optical CMM with Heidenhain metrology and Nikon 5X, 10X, 20X, 40X, 60X, and 100Xobjectives, CCD Camera with 0.3-micronrepeatability
Mitutoyo ELWD 50X, 20X, 5X, 2X CCD Microscopes for in-situ metrology and tool setting, 0.1-micronrepeatability
Zygo ZMI displacement measuring interferometers,2.5 nanometer resolution
Nikon autocollimators, sub-arcsecond
Boom stereoscopes
NIST-traceable master gauge blocks and pins, surface plates, Mitutoyo Master Height Gauge, Heidenhain Metro Height Gauges, Rank Taylor Hobson LVDT’s, Mitutoyo Micrometers and Calipers, Drop Gauges, etc., Davidson arcsecond master polygon, Davidson arcsecond master pentaprism
High precision voltage sources, voltmeters, data acquisition systems, oscilloscopes, thermistors, airflow meters, accelerometers, HeNe lasers, etc.