Optical versus tactile - Comparison of measurement methods

Which method is the better one? What are the advantages and disadvantages? Which requirements can be fulfilled with the individual methods? Both methods will continue to develop and probably exist side by side. Nevertheless, there are more limits to tactile measurement technology. Find out more about tactile and optical measurements, as well as the decisive framework conditions for choosing the right method.

Optical measuring tools and their advantages

Modern optical measuring tools enable non-contact and thus non-destructive surface measurement.  Profile lines can be laid through the surface in any direction. In addition, the contour or the complete topography of components can be determined in 3D mode.  Roughness, waviness, flatness or layer thickness can also be evaluated three-dimensionally.

Further advantages of the optical measuring systems are the short measuring times and the user-friendly operation, which makes it possible for the operator to carry out worker self-control with them. A very versatile method of quantitative surface measurement is chromatic measurement with white light. This takes advantage of the unavoidable color error of the optical lens system used for the measurement. When illuminated with white light, this color error leads to stretching of the focus for the various colors of the light along the optical axis on the beam output side. If the light reflected from the surface is passed into a spectrometer, the wavelength of this light indicates a height value of the sample if the system is correctly calibrated. This light spot can now be guided in a line across the surface and characterized in lines or three-dimensional structures. The color of the surface therefore does not influence the measurement result.

Stylus instruments and their problems

The classic stylus instruments are an established option for measuring roughness or contour. However, the problems associated with the characterization of a surface using profiles are well known: The information is limited to the direction of the profile line. Only a few components have direction-independent structures on the surface. This inevitably means that a line along a certain direction cannot make a significant statement about the roughness or structure of a surface. In addition, the resolution of the most commonly used tactile devices is often too low. The components have trenches that are inaccessible to the stylus tip. A fast measurement of surfaces instead of individual profiles is therefore not possible. Also the non-avoidable falsification of the surface by the applied contact force becomes a problem in many applications. 

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Comparability of measurements

However, the comparability of all the measurements considered is essential. This applies to simple profiles as well as to complete three-dimensional surface areas. The formulas and processes for profile measurement specified in the DIN/ISO regulations are transferred analogously to the surface. Of course, the corresponding filter functions are also taken into account. The highlight here is a filter routine for calculating the corresponding roughness values for the use of a probe tip with selected geometry, since the optical data usually provide a significantly higher resolution than tactile data and are therefore not directly comparable with these. With the simulation of a probe tip, the comparison values for suppliers or customers are provided in addition to the better resolved data for the own production.

Production environments in technology-intensive industries

The integration of optical surface measurement in production environments offers significant potential. Especially in the high technology industry, the demands on the surfaces of materials are permanently increasing and the complexity of products is constantly growing. Solar technology, microelectronics or medical technology, to name just three areas, use surfaces as functional carriers, for example in terms of corrosion protection, biocompatibility, electrical conductivity or optical and haptic properties. Manufacturing processes such as joining, moulding and coating often take place in the micro- and nanometer range. If only minimal deviations in the sub-nanometer range have a significant impact on the functionality of a product – as in wafer technology, for example – a permanent monitoring of the production process and corresponding quality assurance is important for a company’s success. Precise and reliable characterization by means of control measurements is nowadays indispensable. Optical surface measurement, which is non-contact and thus non-destructive, has proven to be the perfect solution.

The more and more complex products such as solar cells, artificial knee joints, optical lenses or microelectronic components can only be inspected with a highly versatile measuring technique. A single method is not enough in such cases.

Versatile measurement techniques

FRT multi-sensor measuring tools fulfill these requirements. They combine various measuring methods and sensors that allow a broad range of surface properties such as geometry or 3D topography to be measured with high precision. At wafer level, the manufacturers are interested in the characterization of roughness, total thickness variation (TTV), bow, waviness or the height and width of the electrical conductor paths. Here, multi-sensor measuring tools quickly provide important information about the ideal manufacturing parameters.

Automation of the measuring process

Automation in quality assurance includes two aspects: automation of the measurement process itself and integration into automated production processes. The former enables as many workers as possible to monitor product quality. One-button solutions have established themselves for this purpose: Automatic measuring programs for different procedures, parameters and ranges which the operator runs at the push of a button after placing the sample on the machine. With such solutions, even complex measurements on solar wafers are abstracted to understandable “good/bad evaluations”, for example. A second aspect of the automation in the measurement process is the positioning of the sample. Especially in the field of wafer technology, different handling and grabber systems can be used to simplify and accelerate the loading process. Powerful image acquisition hardware, intelligent pattern recognition, integrated calibration and automated measurement procedures ensure short throughput times and reliable results.

Software at the heart

It is also important to integrate the results into production processes. In the semiconductor industry, for example, a powerful software platform of the measuring tools transfers the acquired information via a SEMI-compliant SECS/GEM interface to the next step in the production line. This also makes it easier to reduce material costs. This is an important advantage especially for expensive raw materials, such as those used in the solar industry. The industry is more and more trying to integrate optical 3D measuring technology directly into the production line (the so-called inline area) and thus enable 100% control of different parameters. This is for a good reason: An automated optical surface measurement ensures that measuring processes are reliable, fast, reproducible and verifiable. This means a development boost for quality assurance in production.

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