Wavefront sensors are strongly associated with adaptive optics due to their ability to measure the phase of light waves as well as their intensity. Moreover, since adaptive optics is tightly tied to telescopes and imaging satellites, wavefront sensors are often referenced in the context of astronomical instruments. Wavefront sensors are integral to the fine-tuning of these systems. Courtesy of Optocraft. But the use of a single tool to obtain precise measurement values of the shape, orientation, brightness, and amplitude of light waves benefits a range of fields. Through advancements in CMOS technology and advanced phase retrieval algorithms, wavefront sensors are overcoming spatial resolution limitations and finding opportunities beyond microscope, telescope, and satellite imaging. Industrial applications are now within the orbit of these metrology devices, with an emphasis on extreme-ultraviolet (EUV) and deep-ultraviolet (DUV) wavelengths. “The days when metrology was performed almost exclusively at helium-neon wavelengths (e.g., 632.8 nm) are long gone,” said Philippe Clemenceau, CEO of Axiom Optics, a Massachusetts-based provider of optical equipment, including wavefront sensors and wavefront metrology instruments. “Wavefront sensors — among them Shack-Hartmann sensors — are versatile and can measure at any wavelength, particularly those used by the components or subsystems they are testing or aligning. Therefore, it is no surprise that we find more and more applications beyond the visible spectrum, whether in the UV and DUV or SWIR, MWIR, and LWIR regions,” Clemenceau said. As a result of these advancements, wavefront sensors are increasingly competitive in industrial markets traditionally dominated by high-precision — but wavelength-limited — interferometric technologies. The semiconductor industry, and particularly its manufacturing sector, where engineers perform critical tests and measurements, is among the most prominent adopters of wavefront sensors. “The renewal of the semiconductor industrial ecosystem is strongly developing the trend toward UV optical system production,” said Nolan Chan, a sales director at Phasics, a developer of optical testing and metrology instruments. “As such, wavefront sensors and dedicated metrology systems in general are surfing this wave.” Industrial applications related to defense (LWIR) imaging, meta-optics, lidar, and AR/VR are also emerging, along with those in semiconductor and advanced manufacturing. Wavefront testing Wavefront sensors based on the Shack-Hartmann technique have become commonplace in optical testing labs. This approach originated from the Hartmann test, developed around 1900 by Johannes Franz Hartmann, its namesake inventor. Hartmann measured wavefront slopes by placing a metal sheet with tiny holes in the pupil of a telescope and focusing part of the wavefront on photographic plates. Wavefronts without distortions or aberrations align with the pinhole grid. Those with aberrations shift from their expected spots within the grid. In 1971, Roland Shack and Ben Platt upgraded the mechanism; the pair swapped out the plate and the existing hole design concept for a microlens array. The reconceptualization led to the rise of the Shack-Hartmann technique. Compared to the earlier Hartmann approach, Shack-Hartmann sensing increases efficiency: More light is concentrated, and less is blocked. Eventually, designers began replacing photographic plates and analog detectors with CCD detectors. The durability and versatility of wavefront sensing, particularly Shack-Hartmann wavefront sensing, are evident in the range and types of optics to which it can be applied. The schematic shows how wavefront sensors measure geometrically flat (plano) and irregular (aberrated) wavefronts. Courtesy of Optocraft. Industry has continued to innovate its designs. Wavefront sensing and metrology leader Optocraft, for example, favored CCD detectors for its Shack-Hartmann wavefront sensors until 2020. It has since gravitated toward CMOS sensors. Other system designers have continuously refined their product architectures as well. Still, neither wavefront sensing nor the Shack-Hartmann approach achieved immediate widespread use in industrial settings. Boston-based Optikos is among the companies that championed the shift from interferometry to wavefront testing about 25 years ago. At that time, the company deployed the method to test large volumes of contact lens molds. According to Optikos president and founder Stephen Fantone, wavefront sensors have distinguished themselves in specific areas due to, among other factors, their low cost and resistance to vibration-related operational issues. In contrast, interferometers require bulky isolation equipment, and multiple lasers are needed to test at multiple wavelengths. This drives up costs and can increase the need for customization. Shack-Hartmann deployments UV-CMOS technology is an area of considerable technological progress; for UV wavelengths, CMOS detectors with greatly improved light sensitivity have emerged in the market. According to Christian Brock, head of technical sales at Erlangen, Germany-based Optocraft, these detectors also achieve a slower or minimized degradation of the pixels due to UV light irradiation. The technology is gaining traction in the semiconductor industry. For example, Optocraft wavefront sensors are commonly used for the alignment of optical systems, laser beam characterization, and the testing of objective lenses. For these applications, semiconductor test engineers often use DUV wavelengths, particularly 266 nm and 193 nm. Optocraft’s Shack-Hartmann wavefront sensors measure wavefronts with extremely strong aberrations — for example, 100 waves of spherical aberration, Brock said. This performance quality is necessary for measuring aspheric lenses, such as single lens elements from a smartphone lens assembly. “And when aligning optical systems, the large dynamic range of >10° wavefront tilt and Phasics’ quadriwave lateral shearing interferometry (QWLSI) wavefront sensor is based on a diffractive grating design that creates an interferogram on the detector. The solution addresses the limited lateral resolution that hinders typical Shack-Hartmann wavefront sensor designs. Courtesy of Phasics. “Furthermore, the high measurement speed of up to 30 Hz and the high intrinsic stability make this technology suitable for applications in industrial production environment,” Brock said. The challenges that hinder interferometers are paramount in this context. Pulstec Industrial Co., based in Hamamatsu, Japan, is preparing to launch a line of DUV wavefront sensors for the semiconductor industry. The evaluation of optics in photolithography tools is one target application for this technology, since interferometers face challenges measuring actual wavelengths between 100 and 300 nm. Although data from interferometers theoretically offers much higher resolution than that from wavefront sensors, software algorithms that calculate values for Zernike polynomials and other Shack-Hartmann parameters, for example, can render the precision of these competing technologies comparable, according to Shoji Suzuki, a Pulstec sales and service specialist. Adding flexibility and durability As Suzuki said, Shack-Hartmann wavefront sensors have limited lateral resolution. This is due to the number of microlenses that comprise these devices. This constraint puts the traditional Shack-Hartmann technique at a disadvantage compared with high-lateral-resolution techniques, such as Fizeau interferometry. Last year, Phasics aimed to address this constraint. The company commercialized its new quadriwave lateral shearing interferometry (QWLSI) wavefront sensor. Phasics bases the product on a smart diffractive grating design. This diffractive grating creates an interferogram on the detector by replicating the incident beam into four identical waves that overlap and interfere. This QWLSI technique achieves high spatial resolution of up to 19.5 µm/phase pixel, which is much smaller than the diameter of a Shack-Hartmann microlens. The technique also supports achromaticity across the entire spectral range of the detector and enables simplified single-pass metrology through a large wavefront dynamic range while maintaining subnanometer phase resolution. According to Phasics, semiconductor industry players use its QWLSI wavefront sensors for the characterization of wafer surfaces and lithography systems. Using the technology for lithography system characterization involves monitoring the beam inline (with the focusing system alignment) as well as characterization of the beam path, according to Chan. “The new players in the domain value flexibility and versatility, as they need to be able to pivot when needed, and this mindset is also found in their requirements for solutions, both in measurement configurations and wavelengths,” Chan said. Further, wavefront sensors can be used across entire spectral domains, he said. This includes, for example, the UV range from 190 to 400 nm. According to Chan, this contrasts with historically used UV Fizeau interferometers, which can be used only at a single unique wavelength. Linearized focal plane technique Despite their advantages and ongoing progress in product design, wavefront sensors with megapixel resolution may offer low measurement accuracy and stability, according to Brock. Companies are working to improve the spatial resolution of wavefront sensors, according to Fantone. A scientist tests Wooptix’s wavefront phase imaging (WFPI) technique. For semiconductor applications, the technology offers high phase sampling, real-time processing, and absolute wavefront accuracy measurements on the order of λ/20 root mean square — with performance comparable to high-end interferometers used commercially. Courtesy of Wooptix. But this comes at a cost. “That trade-off between resolution and dynamic range is very real,” Fantone said. “If you’re testing systems with very small aberrations, you want to have long focal length lenses, but that means you can’t have too many of them because a change in wavefront error may cause the spots to overlap.” France-based Imagine Optic is pushing the limits of this trade-off through its linearized focal plane technique (LIFT) approach. This technology is based on standard Shack-Hartmann sensor hardware architecture but uses a different phase reconstruction method. The result is a wavefront sensor with up to 680 × 504 sampling points. “The principle is to analyze the centroid intensity distribution created by each microlens and use phase retrieval techniques to reconstruct complex wavefronts at the scale of each microlens. This provides the ability to reconstruct higher phase frequency patterns compared to standard Shack-Hartmann, resulting in a 16-fold improvement in resolution,” Axiom Optics’ Clemenceau said. Axiom Optics is a supplier of wavefront sensing and adaptive optics technology, including products from Imagine Optic. The LIFT algorithm bypasses the trade-off that the Shack-Hartmann technique faces, in which more microlenses are needed for more resolution, which in turn results in less dynamic range due to more focal spots to track and a smaller range of spot displacement for effective tracking, Clemenceau said. Rather than using more focal spots, the LIFT technique uses additional information found in the structure or shape of each focal spot to extract higher resolution. Imagine Optic and Axiom Optics offer sensors for use in semiconductor industry applications, covering both EUV and DUV wavelengths. This Shack-Hartmann technology is of interest for EUV lithography and optical projection lithography. And the DUV wavefront sensors, for example, are used to measure and correct aberrations in the objectives and projection optics and to align the optical path and source. According to Clemenceau, the solution is an alternative to conventional interferometry, which is accurate but more difficult to calibrate. It is also less dynamic in measuring aberrations and is sometimes limited by the coherence of the beams being tested, he said. Wavefront phase imaging In Madrid, the semiconductor metrology company Wooptix developed a wavefront phase imaging (WFPI) technique specifically for semiconductor applications. Wooptix’s technology offers high phase sampling of 1000 × 1000 phase points and real-time processing at 30 fps. With absolute wavefront accuracy measurements on the order of λ/20 root mean square, WFPI delivers functional performance comparable to high-end interferometers and features greater speed and reduced sensitivity to vibrations. “Its full-field, single-shot acquisition enables the measurement of an entire 300-mm wafer in about 1 s, significantly faster than traditional scanning techniques,” said Sebastien Pauliac-Vaujour, vice president of product management at Wooptix. When the WFPI technique is applied using Wooptix’s metrology equipment, it enables direct measurements of wafer freeforms. It can also widen the warpage measurement range to several millimeters, which is 250 µm with standard WFPI configurations. Wooptix’s WFPI technology is undergoing testing for advanced packaging, hybrid bonding, and semiconductor process monitoring and control — including wafer bonding and grinding and chemical-mechanical polishing. With WFPI, Wooptix aims to dethrone dual-Fizeau interferometers. These instruments are considered the reference for wafer warpage measurements and can be used to obtain simultaneous front-side and backside wafer measurements. According to Pauliac-Vaujour, the company has already shown that WFPI can compete with this approach by rotating the wafers. This emphasis on high-precision warpage measures is especially relevant to tasks involving the fabrication and testing of high-performance AI chips. These chips require thinned wafers and advanced bonding and use novel wafer materials. To prevent overlay misalignments, yield losses, and tool downtime during the production of these devices, test engineers must control warpage, stress, and defects. Pauliac-Vaujour identified WFPI as a useful solution for process control in AI chip production because it delivers rapid, full-field wafer shape maps or high-resolution measurements of specific areas at pre- and post-fabrication stages. Additionally, for DUV and EUV lithography, WFPI helps to improve accuracy by providing precise, high-resolution maps of wafer deformations, also referred to as in-plane distortion. In materials science, WFPI can also optimize the quality control of silicon carbide and gallium nitride wafers, owing to its utility for glass and semitransparent substrate measurements. Emerging opportunities Whether for standard Shack-Hartmann or WFPI instrumentation, wavefront sensor manufacturers see several horizonal applications for the technology. Wooptix, for example, has identified emerging opportunities for its technology in advanced packaging, 3D integration, memory devices, and semiconductor process control. “For example, hybrid bonding, whether wafer-to-wafer or die-to-wafer, demands flat and controlled surfaces without defects, and WFPI is ideally positioned to provide the rapid global metrology needed to qualify and control bonding processes,” Pauliac-Vaujour said. Meanwhile, Optikos’ Fantone anticipates that Shack-Hartmann wavefront sensors will achieve wider use with infrared cameras, especially within the defense industry. “As infrared cameras become less and less expensive, there is an opportunity to build very compact wavelength sensing that will combine very well with infrared,” he said. Phasics sees potential in metasurfaces for its QWLSI wavefront sensors; the company has established several industrial and academic partnerships. And, according to Phasics, its solutions are already compatible with all metasurface designs that engineer phase. “We see that, at the current level, the industrials are focusing on using metalenses, which can be characterized in one go with our wavefront sensors,” Chan said. Shack-Hartmann sensing could benefit from metasurfaces, too; metalenses and printed optics could help designers to create microlens arrays with arbitrary designs, Optocraft’s Brock said. “As of today, the optical quality of printed microlens arrays does not reach the quality of the arrays resulting from the traditional lithographic processes,” he said. “Still, we are curious about what the future brings.”