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Bi-metal Canadian coin stitched in true color

Whether for reasons that are strictly functional, cosmetic, or a combination of both, inspection of surface characteristics is a critical step in the manufacture of a vast array of manufactured items.  From a lens surface in a cell phone camera, to a sealing surface in a precision fuel injector, or a solder bump on a semiconductor package, the surface topography and texture have significant bearing on what will ultimately be the difference between a good part and a reject.

It is common to think about optical profilers as instruments for measuring surface roughness.  However, today’s technology is capable of so much more.  At its heart, these systems are optical measuring microscopes for 3D dimensional surface structure analysis.  With a wide range of low-distortion magnifications available, 3D optical profilers can and do often double as vision inspection tools, measuring critical dimensions in addition to topographic information. In fact, one advantage such instruments have over traditional vision based tools is the use of the 3D surface data for measurement of 2D features.

Profiler montage
Zygo’s CSI based optical profilers range from workstations like the Nexview and NewView microscopes, to entry-level tools like the ZeGage system, and application specific instruments like the portable Nomad profiler.

This combination of 2D and 3D can be very useful for measurements where there is not good intensity contrast between the areas of interest, but a distinct and detectable height difference.

Coherence scanning interferometry

At the core, a coherence scanning interferometer (CSI) based profiler (as shown here)  is a high-performance automated optical microscope using special interferometric objectives that serve multiple functions.  First, they provide high-quality, low-distortion imaging and magnification.

The profiler illumination projects through the objective, where a beamsplitter sends a portion of the light to a reference mirror, and some of the light to the part being measured. When the optical distance in each of the legs’ interferometer is equal, the reflections from the two surfaces interfere, resulting in interference fringes on a detector. By changing the distance in one of the interferometer legs, the interference signal is ‘modulated’ and the signal at the detector can be converted into a height map. This map is the areal surface topography of our part.

A diamond turned optical surface measured with 10x magnification on the ZeGage Plus profiler

Topography measurement of a high aspect ratio microfluidic channel measured on a NewView 8000 3D measuring microscope with a 20x magnification shows a clear step height and sharp definition of the edges of the channels in the device.

Why non-contact CSI?

A tactile, contact-based probe for measuring cross-sectional surface profiles uses a small stylus to trace along the surface being inspected. This contact is can actually be a source of failure as a result of the contact probe scratching the measurement surface. In a high- pressure sealing application, such as in a fuel injector, a scratch can result in a fuel leak path, which leads to higher emissions. On a replacement joint, surface scratches can lead to premature wear based failure. On a precision optical surface such as a camera lens, scratches can lead to poor imaging. By using a non-contact technique, damage to the measurement surface is avoided.

Besides being non-contact, the CSI technique has some additional advantages as well.  First, it is an area-based technology - that is to say, rather than measuring a single line of data, or a single point, it scans a complete area in a single scan, usually with at least 1,000,000 data points per field of view. And each of these fields of view has at least nanometer-level vertical precision.

topography measurement of a high aspect ratio microfluidic channel

Topography measurement of a high aspect ratio microfluidic channel measured on a NewView 8000 3D measuring microscope with a 20x magnification shows a clear step height and sharp definition of the edges of the channels in the device.

Most CSI systems will easily measure surfaces with structure significantly smaller than a nanometer. These same systems also measure topography of exceedingly high slopes, with measurements beyond 87 degrees demonstrated. Finally, whether observing a 20mm or 20-micron field of view, the vertical precision of CSI tools does not depend on the optical magnification, unlike other microscope based non-contact technology. 

CSI technology can also be used to profile the topography and thickness of optically transparent films. These films have a wide variety of applications including protective (conformal encapsulation coatings), medicinal (drug delivery on an implant), sensor reagents, friction control, and optical to name a few.  Advanced CSI tools like the Zygo Nexview system can profile not only the top surface of the film, but also the interface between the sample and the coating as well as the thickness, potentially saving measurement time and expense that comes from inspecting the sample multiple times.

This all comes together to demonstrate the application flexibility of coherence scanning interferometry. This versatility makes the technology equally at home performing powerful analysis at the R&D phase as well as reliably and precisely performing higher volume process control in a production environment.

Flexibility and more

The non-contact area-based coherence scanning interferometry at the core of Zygo’s CSI profilers is an extremely flexible technology for evaluating surface structure across a huge range of applications. The area-based inspection and advanced algorithms provide a comprehensive 3D surface map - helping to ensure more complete characterization. And by removing contact from the metrology cycle, risks of damage to the sample are reduced - helping to maximize manufacturing yields. These advantages combined with excellent vertical resolution, a fast measurement speed, and application flexibility make these CSI tools attractive to researchers and manufacturers alike.

Written by Eric Felkel, Product Manager at Zygo Corporation

Top photo: A bi-metal Canadian coin stitched in true color using Zygo Corporation’s Nexview optical profiler.

Labels: surface inspection,non-contact,coherence scanning interferometry,CSI,Zygo,Ametek,Eric Felkel

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