Overview of Optical Measurement Technology Trends

Metrology is one of the key technologies in today's manufacturing industry. It can generally be defined as the science of measuring with light and is widely used to evaluate the physical characteristics of products (or some of their parts or components), as well as to monitor large infrastructure and equipment. According to MEMS Consulting, Antonio Caslo-Porta of the European Photonics Industry Consorum (EPIC) recently published an article titled "The future of optical measurement technology" on PhotonicsViews, reviewing some of the current optical measurement technologies in different manufacturing industries, as well as new developments and trends related to the continued demand for high-precision and efficient solutions.

Metering and digitization

Recent technological developments have made possible innovations such as multi-system or virtual metrology, transforming the role of metrology from a post-production activity to a real-time inspection and analysis process. The digitalization of factories through the "Industry 4.0" related technologies enables the collection and use of data from different production equipment, machines or processes. In this environment, optical metrology technology is often combined with automatic positioning systems or as a fast control and verification solution. These measuring devices can work near assembly and production units, perform pre-, mid- and post-production, and store data related to each product. In this way, all relevant information about the characteristics of the workpiece can be collected during the manufacturing process and then clearly assigned to the digital quality control facilities for quality control.

Following this trend, Sensofar Metrology (Tarasa, Spain) recently launched the only autonomous confocal profiler on the market, Smart 2 (FIGURE 1), whose powerful features and compact design make it a breakthrough in the field of optics. In order to use the most appropriate technology for scanning, Smart 2 is equipped with three systems for measurement in the same probe head: active illumination focus variation, confocal and interferometry. The solution is designed to achieve automation commonly required in production lines and is very easy to integrate. All components are contained in a narrow probe head so that it can be installed in an area that will not interfere with users or manufacturing operations.

Manufacturer Mitutoyo (Kawasaki, Japan) also mounted one of its most interesting optical measurement devices on an arm to improve the accuracy and speed of measurement. The new ROBOTAG solution integrates a vision system with the well-known Tunable Acoustic Gradient Index Lens (TAGLENS). The product combination provides sharper images due to improved focus depth, excellent repeatability, and higher efficiency. This is thanks to the ultra-fast zoom characteristics of TAGLENS. ROBOTAG systems will soon be equipped with broadband pulsed light sources (PLS) to perform precise 3D shape detection and improved online measurement (Figure 2).

New solutions for optical component manufacturing

The production of optical components requires not only precise manufacturing and polishing, but also precise measurement. Improving the efficiency and repeatability of the manufacturing method means little if the final workpiece cannot be accurately measured. The industry has created a variety of metrology techniques such as profilometry, confocal microscopy, ellipsometry or interferometry to measure different key parameters (radius of curvature, flatness, roughness, film thickness, transmittance...).

Thin films are very common elements in the optical components industry and a lot of work has been done on their quality control. In recent years, thin films have been widely used for functional coatings on the surface of optical components (such as protective or anti-reflective coatings) or for the manufacture of different types of filters and mirrors. Metrology is essential to ensure the quality of the final product produced using the different thin film deposition technologies available on the market. A key need of the industry is the spectroscopic measurement of existing and newly developed components with optical coatings.

EssentOptics Europe (Vilnius, Lithuania) offers different solutions for fully unattended spectral measurements of flat components and lenses, including aspheric surfaces. These devices can measure transmission and reflection over a wide wavelength range from ultraviolet (UV) to visible (VIS), mid-wave (MWIR), and soon long-wave infrared (LWIR). One of the most interesting applications of these devices is the characterization of linear variable filters, an optical component with many applications in spectroscopy for biological and life science research. EssentOptics Europe has designed a new technology that is being tested and fine-tuned in collaboration with Omega Optical.

When it comes to the production of lenses, an important parameter to control is the center thickness, as it strongly affects the optical path of light through the component. From the manufacturer's point of view, controlling the repeatability of this parameter within a set of lenses with the same specifications is crucial to ensure the high quality of the final product. Trioptics (Wedel, Germany) has developed OptiSu LTM (Lens Thickness Measurement), a precision center thickness measurement system with an accuracy of ±0.5 μm for single and double lenses up to 150 mm thick. The technology behind the solution is high-precision, low-coherence interferometry, which is equipped with a vibration-damped and self-centering mechanical fixture, making operation simple and operator-independent. Another advantage is the software's optimized user interface, which enables OptiSurf LTM to be seamlessly integrated into any production process.

and industry measurement

The semiconductor industry has very demanding production requirements. Optical metrology solutions are perfect for high-speed measurement and defect detection, and in recent years some technologies have been adapted to meet the special requirements of this industry. Optical metrology equipment has now become an important tool in semiconductor production, allowing the inspection of increasingly complex and miniaturized 3D structures, as well as the production of thin layers with thickness requirements down to nanometers.

One interesting application is the in-situ control of the epitaxial growth of thin-film layer structures on semiconductor wafers. This process is indispensable for the manufacture of products such as VCSELs, μLEDs, or power transistors, and important parameters such as wafer temperature, reflectivity, growth rate and layer thickness, chemical composition of the grown material, and wafer bow need to be controlled. LayTec (Berlin, Germany) has developed different integrated optical metrology solutions for this application, including optical tools, special materials, and material databases for analyzing measurement data (Figure 3). LayTec's tools are integrated into deposition systems, such as metal-organic chemical vapor deposition (MOCVD) systems, and are used at the front end of the semiconductor device manufacturing process. They are integrated into control loops for real-time feedback control, batch control, and fault detection in the process.

Figure 3 In-situ measurement of growth layers on wafers (Source: LayTec)

Also relevant to the semiconductor and consumer electronics industries, a recent case study conducted by Sensofar Metrology focused on the impact of temperature variations on the evolution of shape and texture of silicon wafers. A key way to assess the impact of temperature variations during manufacturing is to measure the surface roughness of the wafer as a function of temperature, but imaging issues due to spherical aberration have been a challenge. Using Sensofar Linnik objectives and in combination with a thermal chamber, the roughness can be observed using interferometry techniques (Figure 4). On the one hand, the combination of these two systems enables the temperature to be accurately raised to values ​​similar to those during the manufacturing process when observing the sample through a microscope; on the other hand, it eliminates the problems associated with spherical aberration to obtain accurate measurements of the 3D profile.

Figure 4 Setup for measuring wafer surface roughness during rapid thermal processing (Source: Sensofar)

In summary, optical measurement technology is being increasingly used in a variety of different industries, and it has proven to be the most effective and versatile tool for performing quality and process control in a variety of applications. Recent technological developments have focused on overcoming some of the limitations of previous systems and measuring new and more demanding products and functions in the semiconductor, consumer electronics, automotive, optical components and medical industries with higher accuracy. There is a clear trend in the market to integrate these optical measurement solutions into robotic arms and other positioning systems to perform in-situ measurements and provide valuable information in real time during the manufacturing process.

Review editor: Liu Qing