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Trends in Optical Measurement Technology

Optical metrology is one of the key technologies in manufacturing today. It can generally be defined as the science of making measurements with light, and is widely used to assess the physical properties of a product (or some of its parts or components), as well as to monitor large-scale infrastructure and equipment. According to Memes Consulting, recently, Antonio Castelo-Porta of the European Photonics Industry Consortium (EPIC) published an article entitled "The future of optical measurement technology" on PhotonicsViews, summarizing the current different manufacturing industries. Some of the optical measurement techniques in , as well as new developments and trends related to the continuous demand for high-precision and efficient solutions.

Metrology and Industrial Digitalization

Recent technological developments have enabled innovations such as smart multi-sensor systems or virtual metrology, shifting the role of metrology from a post-production activity to a real-time inspection and analysis process. Improve the digitalization of factories through "Industry 4.0" related technologies, so as to realize 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 industrial robots as a fast control and verification solution. These measuring devices can work close to assembly and production cells, perform inspections before, during and after production, and store data related to each product. In this way, all relevant information on workpiece properties can be collected during the manufacturing process and then explicitly assigned to the digital quality control facility for quality control.

Following this trend, Sensofar Metrology (Talassa, Spain) recently launched the only autonomous surface confocal profiler Smart 2 (Fig. 1) on the market. Its powerful functions and compact design make it a breakthrough in the field of optics. . In order to scan with the most suitable technology, the Smart 2 is equipped with three systems for measurement within the same probe head: active illumination focus change, confocal and interferometry. The solution is designed to automate the kind of automation typically required on production lines and is very easy to integrate. All electronics are contained within the narrow probe head so that it can be mounted in an area that will not interfere with user or manufacturing operations.

Figure 1 Sensofar's autonomous surface confocal profiler S mart 2

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

Figure 2 New ROBOTAG system for high-speed online detection and measurement

New Solutions for Optical Component Manufacturing

The production of optical components requires not only precise manufacturing and polishing, but also precise measurements. Improvements in the efficiency and repeatability of manufacturing methods mean little if the final workpiece cannot be precisely measured. The industry has created various 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 a very common element 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 as functional coatings on the surface of optical components (e.g. protective or anti-reflective coatings) or to manufacture different types of filters and mirrors. Metrology is crucial to ensure the quality of the final product produced using the different thin film deposition technologies available in the market. A key need in this industry is the spectroscopic measurement of existing and newly developed components with optical coatings.

EssentOptics Europe (Vilnius, Lithuania) offers different solutions for completely unattended spectral measurements of planar components.

And Xinxin GEM provide AR coating TO Can, optical windows or lenses (including aspheres). These devices measure transmission and reflection over a wide range of wavelengths from ultraviolet (UV) to visible (VIS), mid-wave infrared (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 devised a new technology: it is being tested and fine-tuned in cooperation with Omega Optical.

When it comes to the production of lenses, an important parameter to control is the center thickness, as it can significantly affect the optical path of light through the component. From a manufacturer's perspective, controlling the repeatability of this parameter across a set of lenses with the same specification is critical to ensure a high quality end product. Trioptics (Wedel, Germany) has developed OptiSurf LTM (Lens Thickness Measurement), a precision central thickness measurement system with ±0.5 μm accuracy 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 vibration-damping and self-centering mechanical grippers, making operation simple and independent of the operator. Another advantage is the software's optimized user interface, which enables OptiSurf LTM to be seamlessly integrated into any production process.

Metrology for the Semiconductor and Consumer Electronics Industries

Production requirements in the semiconductor industry are high. Optical metrology solutions are perfect for high-speed measurement and defect detection, and some technologies have been adapted in recent years to meet the unique requirements of this industry. Optical metrology equipment has now become an important tool in semiconductor production, enabling the inspection of increasingly complex and miniaturized 3D structures and the production of thin layers requiring thicknesses down to nanometers.

An 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, where important parameters such as wafer temperature, reflectivity, growth rate and layer thickness, chemical composition of growth 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 algorithms and a materials database for analyzing the measurement data (Fig. 3). LayTec's tools are integrated into deposition systems, such as Metal Organic Chemical Vapor Deposition (MOCVD) systems, and 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 the growth layer on the wafer(source: LayTec)

Also relevant to the semiconductor and consumer electronics industries, a recent case study by Sensofar Metrology focused on the effect of temperature changes on the evolution of silicon wafer shape and texture. A key way to assess the effects of temperature changes 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 hot cell, the roughness can be observed by interferometry (Fig. 4). On the one hand, the combination of these two systems enables the temperature to be raised precisely to similar values as in the manufacturing process when the sample is viewed through a microscope; on the other hand, it eliminates problems related to spherical aberration to obtain 3D profiles precise measurement.

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

To sum up, optical measurement technology is being increasingly used in various industries and has proven to be the most effective and versatile tool for 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, more demanding products and functions with greater accuracy in the semiconductor, consumer electronics, automotive, optical components, and medical industries. 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.

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