Insights from industry

Innovative and Versatile Optical Profilers

Gerald Nitsch, CEO of Sensofar Metrology, has worked in the thin-film industry for over a decade. AZoM spoke to Gerald about Sensofar's optical, multipurpose surface profilers, the comprehensive range of measurements they are capable of and the versatility they provide for their customers.  

What are the key application areas for Sensofar’s technology?

Our technology can be used in all industry and research segments where 3D surface metrology at the nm to µm level forms an integral part of the manufacturing, development or testing process.

The requirement to characterize, or perhaps quality assure, a surface has many origins such as;

  • The examination of wear on a multitude of diverse surfaces and tools
  • The quality control of optics profiles and of microstructured or micromachined surfaces
  • Establishing structures to achieve the desired haptic on surfaces
  • The identification and characterization of marks and/or defects
  • Establishing thin film thickness and mapping its uniformity
  • The list goes on!

Which industries do Sensofar supply systems to and what are the key types of system you offer?

It is fairly well known that the need to characterize surfaces at the nm to µm level is a growing requirement in many industry segments.

There is no particular industry focus – the potential markets are very fragmented, and are likely to remain so. So, while we find established applications in optics, medical technology, display technology, semiconductor and solar, microelectronics and MOEMS and so on, we are also encountering increasing interest from the automotive, micromachining and consumer electronics sectors. And then there are the less expected applications, for example in forensics and micropaleontology.

Much as for anyone else in this field, we’re discovering new applications for our systems all the time.

The fact that technology allows a surface to be analyzed optically, i.e. with a non-contact approach, is a major benefit for many applications.

Our product systems can be separated into three categories, which differ in terms of their optical technology.

We have the S line of surface metrology systems, available in flagship, compact and industrial formats, which are designed with versatility in mind to cater to a broad range of surface metrology tasks.

Then we have two other systems that have been developed for more specific tasks – the PLu apex was developed for aspherics and freeform optics metrology, and the Q six system was developed for medical technology, specifically for assisted stent inspection.

Last but certainly not least, we also provide in-line variants for OEM customers, with throughput and software algorithms designed for the task at hand.

We implement optical surface metrology techniques in all our systems, which are rapidly gaining ground over conventional stylus techniques. The key reasons that optical techniques are becoming increasingly prevalent for 3D areal surface metrology is the speed at which measurements can be taken, the high resolution data and the non-contact nature of the methods.

Optical techniques are becoming so ubiquitous that they have recently been adopted into the ISO standards (specifically ISO 25178), making them auditable under ISO 9000.

Which optical metrology technologies are provided by Sensofar’s surface profilers?

Sensofar’s S line of metrology systems all incorporate our ‘3-in-1’ technology approach – the integration of three different measurement techniques into a single sensor head: confocal, interferometry and focus variation.

Confocal, interferometry and focus variation are all well established optical metrology techniques, and all are recognized within ISO 25178. Each has their own set of strengths and more or less critical weaknesses, and so we find that each technique is best suited to a broad, but nonetheless restricted class of application requirements.

We can also offer a 4-in-1 approach too, with the addition of spectroscopic reflectometry to extend the capabilities of thin film characterization, but that is an optional extra.

What are the key differences between confocal, interferometric and focus variation techniques?

The differences in the techniques are not critical to the end-user. All require a microscope arrangement along the optical path, a light source for illumination and a high-resolution camera for signal acquisition. At Sensofar, we have condensed this down to a single sensor head for all 3 techniques, where the only interchangeables are the objectives, the light source (in our case R, G, B and white LEDs) and the appropriate algorithms.

Confocal profilers measure the surface height of smooth to very rough surfaces, with spatial sampling as low as 0.10 μm and vertical repeatability on the nm scale. High NA (0.95) and high magnification (150x) objectives are available to measure steep, >70°, local slopes on smooth surfaces and up to 86° on rough surfaces.

Phase shifting interferometry (PSI) can measure the surface height of very smooth and continuous surfaces, providing sub-nm vertical resolution regardless of the objective’s NA. Very low magnifications (2.5x) enable a large field-of view (FOV) to be acquired with no compromise in the height resolution.

Vertical scanning interferometry (VSI) uses white light to measure smooth to moderately rough surfaces, also providing nm vertical resolution regardless of the objective’s NA. The VSI technique belongs to a class of surface measurement techniques known as coherence scanning interferometry (CSI) – CSI has been recognized/adopted by ISO.

And lastly, focus variation is an optical technology that has been developed to measure the shape of large rough surfaces. Sensofar’s implementation of this approach has been specifically designed to complement confocal measurements at low magnification. Highlights of the technology include high slope surfaces (up to 86°), highest measurement speeds (mm/s) and a large vertical range.

What is more important for the end-user is in which instance one chooses a particular technique. Interferometry in its various guises is an extremely well-established tool for surface metrology, but it nonetheless quickly looses ground on very rough surfaces. Focus variation is an excellent tool for establishing shape and form, for example for tooling, but it doesn’t provide enough detail for true high-resolution metrology.

And then we have confocal, which is the technique of choice for detail and for very rough surfaces, but speed very quickly becomes an issue over very large FOVs.

Why is it beneficial to have complementary technologies in a single device? How easy is it to swap between the different techniques?

We regularly encounter a number of applications where a single optical technique does not provide enough versatility when studying varying surface structures and across all surface scales. Consider the intricacies in MOEMS devices nowadays, the increasing precision of laser micromachining, the optical quality requirements for microlenses or the complexity of freeform optics. The fact that any one technique is often not enough to cover all the bases is the raison d’être for our approach.

In our systems a programmable microdisplay is the key element that, in combination with flexible illumination options and swappable objectives, delivers a sensor head with the necessary versatility.

For example, a variable slit pattern programmed into the microdisplay and combined with a brightfield objective are the essential ingredients for confocal microscopy. Reprogram the slit pattern for a flat mirror, and the system is now capable of focus variation. Now swap the brightfield objective for an interferometric objective, and the system is capable of interferometry.

Simply by being able to swap the light source and the objective lets one measure considerably more types of surface. This easy switching means researchers and quality control managers are prepared for any task regardless of roughness, waviness, structure, size (area), smoothness, reflectivity, scaling and more.

Swapping between the techniques is as simple as choosing the appropriate objective and selecting the corresponding measurement technique in SensoSCAN, the user interface software. SensoSCAN then does the rest, automatically reconfiguring the system (algorithms, LED, available measurement parameters) according to the type or measurement required (image, 3D surface, 2D profile or thickness).

Not everyone is going to need all of these options – how does Sensofar cater to in-line industrial applications requiring only a single technique?

Take our flagship product, the S neox. A component such as the microdisplay is only actively used for confocal measurements, but it also does double duty as a passive mirror when the system is configured for interferometry or for focus variation. Aside from 3 out of the 4 installed LEDs, and the multiplicity of objectives, little or nothing is actually sitting idle in the sensor head during data acquisition, regardless of the chosen technique.

All of Sensofar’s S line systems are based on this common sensor head platform, and exploiting any available versatility in the system is then down to the choice of light source, objective and appropriate software parameters (data acquisition and measurement algorithms).

For more general, non in-line applications, where the S neox is perhaps ‘too much’ both in terms of format and features, we have the new compact S lynx. It comes with only a single LED, and does not provide PSI, but it does still afford multiple objectives. The demand for this system came initially from our partners in the Far East, but we have since realized that there is considerable potential elsewhere too.

For applications in in-line process control, we have our S mart system. The whole assembly is packaged in an industrially compatible format. Once the customer has identified and thoroughly characterized the application, the S mart is equipped with a dedicated LED, an appropriate objective and is effectively configured as a ‘one trick pony’, optimized for this particular task.

We counter the comment that the system then has 2 idle technologies with the fact that Sensofar doesn’t need to sustain and support 3 separate in-line systems, and also that the S mart can always be reconfigured at a later date.

An additional aspect of the common sensor head approach is that nothing is ‘de-specified’ that might, for example, yield lower quality results – given a single common measurement task and the proper LED and objective, all 3 S line systems are capable of the same performance. Differentiation between the systems is instead made by software and additional or optional hardware features, which can of course be determined by the customer.  

You mentioned optical and medical systems?

Our PLu apex is able to measure any optical surface, from aspheric to flat surfaces, machined and freeform optics, molded optics and even the prized molds themselves.

Its innovative design – adapted from Sensofar’s core technology, confocal metrology – allows straightforward non-contact and high-precision measurements. Measurements are not limited by the shape of the sample and are independent of sample reflectivity. It is difficult to find this feature set in any other system designed for the same application.

The Q six started development as an adapted S neox with a special xyz stage. It has now evolved into a dedicated system for assisted stent inspection with trick lighting options and a highly specialized motion system. High-resolution 2D microscopic imaging allows imaging of the outer AND of the inner AND of the cut surfaces (side walls), and can also be used for defect identification and classification.

VSI can then be used to perform measurements of surface roughness and for mapping any coatings (for example, for drug eluting stents), and can also be used to obtain a 3D profile of any defects. Again, this is a rather unique feature set for this application.

How will these technologies develop in the future?

Generally, the expectations we see from our customers can be reduced to 3 aspects – speed, size/format and FOV-capability.

Faster measurements are of course of particular benefit in industrial in-line applications as they enhance the throughput. The ability to acquire a larger FOV would reduce the need to acquire multiple measurements and so reduce stitching and speed up acquisition. And more compact and more deeply integrated systems will naturally benefit their amalgamation into larger process systems.

But the innovation doesn’t end there, at least not at Sensofar. For example, it is no secret that there are several established workarounds to greater resolution in optical microscopy, so we are currently exploring potential routes to greater lateral resolution. And we may yet encounter new applications that cause us to reassess and reconfigure the technologies we currently implement.

About Gerald Nitsch

Gerald Nitsch

Gerald Nitsch has been associated with Sensofar for over a decade. Initially as an OEM supplier of thin-film metrology engines to Sensofar during his role as CEO/President of Mikropack and until Mikropack’s acquisition by Ocean Optics in 2006 as part of the establishment of Ocean Optics EMEA.

After leaving Ocean Optics EMEA in 2009, Gerald Nitsch became a minor stakeholder and Member of the Board at Sensofar and took over the role of VP Sales & Marketing.

Since Fall 2014, Gerald Nitsch is now CEO of Sensofar Metrology, his task being to ensure a healthy operational transition during this continued period of rapid growth for the company.

At the same time he retains his position as VP Sales & Marketing for the Sensofar Group.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

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