Figure 1. Examples of confocal applications. (Image: Micro-Epsilon)

Light is used in many ways in sensor technology for high precision applications. For example, white light technology can be used for confocal chromatic sensors and interferometers that can make extremely precise and accurate measurements of distance and thickness down to the sub-nanometer range. This makes them suitable for production monitoring in different industries, including semiconductor fabrication. However, even though both sensor types work with white light technology, the two measurement methods differ significantly, although they complement each other.

Confocal chromatic sensors from Micro-Epsilon provide sub-micrometer precision and are used in automation and production monitoring in dynamic industrial inline applications. Interferometers are used in applications that require nanometer precision in combination with maximum signal stability, which is required in the semiconductor industry or precision glass production.

Interferometer vs. confocal chromatic sensor.

Confocal chromatic systems are the more flexible solution, at least in terms of the number of different sensors and thus the variety of applications. They can measure either the thickness of transparent objects or distances on numerous surfaces. Since Micro-Epsilon’s devices in the confocalDT series have a tilt angle of up to 48° they can reliably detect curved and structured surfaces. They are also very flexible in terms of the base distance and measuring ranges.

The interferoMETER series, on the other hand, with a measuring angle of 4°, is best suited for flat surfaces such as those found on semiconductor wafers. The interferometer measuring range is specified by the controller and therefore cannot be changed as flexibly as with the confocal measuring system. However, it is an extremely precise optical system, with resolution of up to 30 picometers. This is compared with the limit of the confocal system at three nanometers.

High-precision confocal measurements are possible on reflective surfaces, such as highly polished metals or liquids, on matte surfaces, such as plastic or black rubber, and on transparent materials, such as glass or plastic sheets. Thanks to the very fast exposure time control, these sensors can also measure stably when the material changes from matte to glossy and vice versa. The extremely small measuring spot, which covers only a few μm, depending on the system, enables measurements on tiny objects, such as IC pins on circuit boards, bonding wires, or small contours of mechanical parts.

The confocal measuring system also enables thickness measurements of transparent materials such as glass, to micrometer accuracy with a single sensor, using the reflections from the front and rear sides of the material. These reflections generate peaks on the CCD array based on which, the corresponding distance and thickness are calculated.

Characteristics of Confocal Measurement

  • Surface-independent, also valid for mirrored and glass surfaces

  • High measuring rates for dynamic monitoring tasks

  • One-sided thickness measurement of transparent materials

  • Large tilt angle up to 48° so that curves and profiles can be measured reliably

  • Resolution 3 nanometers

Confocal Chromatic Measuring Principle

The confocal chromatic measuring principle is based on polychromatic light (white light), which is divided into separate spectral colors by being focused through a multi-lens optical system at different distances from the sensor. Shortwave, blue light (400 nm) is refracted more than long-wave, red light (700 nm). The start of the measuring range is with blue light and the end is with red light. A specific distance to the target is assigned to each wavelength by a factory calibration of the controller. Only the wavelength that is exactly focused on the target is used for the measurement in the sensor system. The light reflected from this point is imaged by an optical arrangement onto a light sensitive sensor element, on which the associated spectral color is detected and evaluated. In the case of thickness measurements, several distance points are evaluated accordingly.

Figure 2. Multi-layer measurement: The thickness of materials such as laminated glass can be evaluated. Up to six peaks can be measured synchronously. (Image: Micro-Epsilon)

Multipeak Measurement

The confocal chromatic measuring principle enables thickness measurements of transparent materials such as glass. The thickness is detected to micrometer accuracy using one single sensor that uses the reflections of the front and rear sides of the material. These reflections generate peaks on the CCD array based on which, the corresponding distance and thickness are calculated. Therefore, the refraction index of the transparent material needs to be known.

White Light Interferometers for Nanometer Precision

The interferoMETER series white light interferometers are the most precise sensors in Micro-Epsilon’s overall portfolio. They too, are available as multi-peak systems, which opens up numerous new potential applications. Interferometers are used when high-resolution distance and thickness measurements are required.

Figure 3. White light interferometers offer picometer-level resolutions, ideal in the semiconductor industry. When loading wafers, white light interferometers are used to measure the horizontal tilt of wafers. The interferometers provide absolute distance values at a subnanometer resolution. The measurement ensures the greatest possible positional accuracy when picking up and removing wafers. (Image: Micro-Epsilon)

Multi-peak versions enable high-precision detection of transparent layers. Depending on the controller model, this is achieved via calculations based on the respective distance values and taking the refractive indices into account. The thickness measuring system, however, does not perform distance measurements — it measures the thickness of the individual layers and their combinations. For industrial use, the interferometers are equipped with protection class IP65 sensor housings.

Thanks to absolute distance measurement, the scanning of steps with white light interferometers takes place with high signal stability and precision. When measuring on moving objects, the height differences of heels, steps, and depressions can thus be detected without additional referencing.

It is also suitable for precise thickness. The large thickness measuring range allows the measurement of thin layers, flat glass, and films. Since the white light interferometer works in the near infrared range, thickness measurement of anti-reflective coated glass is also possible.

Finally, it is suitable for distance measurements with picometer resolution. Thanks to the absolute measurement, sampling is performed without signal loss. When measuring on moving objects, this enables the differences in the height of heels, steps, and depressions to be reliably detected. The measuring system offers sub-nanometer resolution with a large offset distance in relation to the measuring range.

In addition to excellent linearity, the white light interferometers offer a resolution up to < 30 picometers. Configuration is possible via an integrated web interface.

Characteristics of Interferometer Measurement

  • Absolute distance measurement with nanometer accuracy

  • Precise thickness measurements independent of the distance from the sensor

  • Multi-peak distance measurement and multi-layer thickness measurement

  • With a measuring angle of 4°, ideally suited for flat surfaces

  • Resolution up to 30 picometer

interferoMETER Measuring Principle

The measuring principle of an interferometer is based on the wave nature of light. This leads to the fact that overlapping waves can either intensify or cancel each other, depending on whether wave crest meets wave crest or wave crest meets wave trough. If a light beam is split so that it takes different paths and the two partial beams subsequently overlap again, interference occurs that depends on the difference between the two paths. If the length of one of the two paths changes by half a wavelength of the light used, this leads to a complete change from positive interference (amplification) to negative interference (extinction). This enables accuracy in the nanometer or even sub-nanometer range. In order to measure distances using this method, for example, one of the two partial beams is reflected at the measuring object and then superimposed on the reference beam. If the distance from the target changes, this change in distance can be detected very sensitively by the interference.

When measuring the thickness of films or glasses, it takes advantage of the fact that both the front and back of the measuring object reflect. Thickness changes then also cause the interference signal to change, so for that application, a separate reference beam is not necessary. A special feature of thickness measurement is that since the two interfering partial beams originate from the upper and lower surfaces, the measurement result is independent of the distance from the measuring object.

In Conclusion

Confocal chromatic sensors and interferometers using white light can perform numerous extremely precise measurements even under continuous manufacturing scenarios.

This article was written by Martin Dumberger, Managing Director, Micro-Epsilon America. For more information, contact Mr. Dumberger at martin. This email address is being protected from spambots. You need JavaScript enabled to view it..