A Comparison of Optical vs. Tactile Measurement Methods

Which measurement method is best? What are the benefits and weaknesses? Which requirements can be achieved with each individual method? Both methods will continuously progress and will most likely exist alongside one another.

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Discover information about optical and tactile measurements in this article, along with the decisive framework requirements for selecting the correct method.

Optical Measuring Tools and Their Advantages

Present day optical measuring tools allow for non-contact and therefore non-destructive surface measurement. Profile lines can be set throughout the surface in any direction. Additionally, the contour or the entire topography of components can be analyzed in 3D mode. Layer thickness, flatness, waviness, or roughness can also be specified three-dimensionally.

Additional benefits of the optical measuring systems are the most efficient measuring times and an operation that is user-friendly. This allows the operator to undertake worker self-control with them.

Chromatic measurement with white light is a highly versatile method of quantitative surface measurement. This method exploits the inevitable color error of the optical lens system utilized for the measurement.

This color error results in stretching of the focus for the different colors of the light across the optical axis on the beam output side when illuminated with white light. If the system is properly calibrated, the wavelength of the reflected light from the surface is passed into a spectrometer and determines a height value of the sample.

This light spot can then be directed in a line across the surface and characterized in three-dimensional structures or lines. Therefore, the color of the surface does not impact the measurement result.

Stylus Instruments and Their Problems

As an established option, the classic stylus instruments can be used to measure contour or roughness. Although, the issues related to using profiles for the characterization of a surface are well documented. For example, the information is confined to the direction of the profile line. Only a small number of components have surface structures that are direction-independent.

Inevitably, this means that a line along a specific direction cannot make a definite statement concerning the structure or roughness of a surface. Additionally, the most frequently employed tactile devices have a resolution that is often too low. The trenches of the components are inaccessible to the stylus tip.

A quick measurement of surfaces rather than individual profiles is therefore impossible. Moreover, the unavoidable falsification of the surface by the contact force that is applied creates a problem in a large number of applications.

Comparability of Measurements

The comparability of all the measurements considered is critical. This is applicable to uncomplicated profiles along with complete three-dimensional surface areas. The processes and formulas for profile measurement outlined in the DIN/ISO regulations are similarly transferred to the surface.

The corresponding filter functions are also taken into consideration. The benefit here is a filter routine for measuring the corresponding values of roughness for the use of a probe tip with chosen geometry, as the optical data normally provides an extensively higher resolution than tactile data and therefore are not directly comparable with these.

Using probe tip simulation, the comparison values for customers or suppliers are given along with the better resolved data for their own production.

Production Environments in Technology-Intensive Industries

In production environments, the integration of optical surface measurement brings significant potential. Particularly in the high technology industry where the requirements on the surfaces of materials are constantly increasing and product complexity is consistently growing.

To name only three areas, medical technology, solar technology and microelectronics, all utilize surfaces as functional carriers, such as in regards to optical ad haptic properties, electrical conductivity, biocompatibility, and corrosion protection.

Manufacturing processes for example coating, joining, and moulding frequently take place in the range of micro and nanometers. If only slight deviations in the sub-nanometer range significantly influence the functionality of a product, for example in wafer technology, a consistent monitoring of the production process and related quality assurance is essential for a company’s success.

Reliable and accurate characterization through control measurements is invaluable in modern times. Optical surface measurement, which is non-destructive due to being non-contact, has shown itself to be the ideal solution.

The increasingly complex products such as microelectronic components, optical lenses, artificial knee joints or solar cells can only be analyzed with a highly flexible measuring technique. This is because one method is not enough in these cases.

Versatile Measurement Techniques

FRT multi-sensor measuring tools satisfy these needs. They are a combination of varied measuring sensors and methods that enable a wide range of surface characteristics such as 3D topography or geometry to be determined with high accuracy.

At wafer level, the manufacturers are interested in the characterization of waviness, bow, total thickness variation (TTV), roughness or the width and height of the electrical conductor paths. In this respect, multi-sensor measuring tools efficiently give critical information about the best manufacturing parameters.

Automation of the Measuring Process

Two aspects are essential for automation in quality assurance: the automation of the measurement process itself and integration into automated production processes. The former allows as many workers as possible to evaluate product quality.

One-button solutions have been created for this requirement: Automatic measuring programs for various ranges, procedures, and parameters which the operator can run at the push of a button after putting the sample on the machine.

With these kinds of solutions, even complicated measurements on solar wafers are simplified to easy to understand, good or bad evaluations, for example. The positioning of the sample is a further aspect of automation in the measurement process.

Particularly in the field of wafer technology, various grabber and handling systems can be used to make the loading process more efficient and simple. Advanced image acquisition hardware, automated measurement procedures, intelligent pattern recognition, and integrated calibration provide fast throughput times and trustworthy results.

Software at the Heart

The integration of results into production processes is also essential. For example, in the semiconductor industry, a powerful software platform of the measuring tools communicates the acquired data via a SEMI-compliant SECS/GEM interface to the next stage in the production line.

The reduction of material costs is made simpler by this feature. This is a critical benefit particularly for costly raw materials, for example those utilized in the solar industry.

The industry is increasingly attempting to integrate optical 3D measuring technology directly into the production line (also known as the inline area) to provide total control over the various parameters.

This is motivated by good reasons. An automated optical surface measurement makes sure that measuring processes are trustworthy, quick, reproducible and verifiable, which also brings a boost in development for quality assurance in production.

Further information can be found here.

For additional comments or questions, please contact FormFactor where the experts will be happy to provide solutions to measuring tasks.

This information has been sourced, reviewed and adapted from materials provided by FormFactor.

For more information on this source, please visit FormFactor.


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  1. Thomas Eisenberg Thomas Eisenberg Germany says:

    Excellent article. The measurment of surface structure across multiple scales is of growing importance, and there is high demand for better accuracy from mm to nm scales

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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