Wafer-level Optics Research Articles

Laser-Assisted Bonding Approach for Photonic Integration Processes

Current development trends concerning miniaturizing of electronics and photonics systems are aiming at assembly and 3D co-integration of a broad range of technologies including MEMS, microfluidics, wafer level optics, and silicon photonics. To this end, on-chip integration using siliconphotonics platform offers a wide range of possibilities addressing passive optics functionality, active optoelectronic devices, and compatibility with CMOS fabrication. On the other hand, the hybrid technology enabling volume manufacturing of such system-on-chip components it is still in an early development stage. In this work, the original LAB setup with a coaxial bottom irradiation architecture was developed. The setup allows a rapid and energy-effective process with the ability to produce successful electrical bonds with negligible thermalinduced stress and warpage to bonded surfaces. The proposed machine-vision based temperature sensor is validated for photonic integration assembly processes.

Machine Vision System Utilizing Black Silicon CMOS Camera for Through-Silicon Alignment

Current development trends concerning miniaturizing of electronics and photonics systems are aiming at assembly and 3-D co-integration of a broad range of technologies including MEMS, microfluidics, wafer level optics, and silicon photonics. To this end, on-chip integration using silicon-photonics platform offers a wide range of possibilities addressing passive optics functionality, active optoelectronic devices, and compatibility with CMOS fabrication. On the other hand, the hybrid technology enabling volume manufacturing of such system-on-chip components, it is still in an early development stage. Here, a new type of machine vision system enabling precision stacking and bonding processes of III–V components on silicon photonics chips is introduced. In particular, we focus on the ability to see through substrates with high resolution, which is crucial for the alignment of the markers used for assembly of integrated components on silicon wafer. The system is based on the use of a black silicon enhanced CMOS sensor with extended wavelength response beyond transparency cutoff wavelength for silicon substrate. We study the use of the camera system as a microscope with bottom coaxial infrared (IR) illumination scheme and demonstrate the ability of through-silicon vision for one- and two-layered silicon photonic integrated circuit (PIC) samples. The ability of observing objects with dimensions down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2 \mu \text{m}$ </tex-math></inline-formula> is confirmed. This resolution is close to diffraction limit and corresponds to the dimensions of optical waveguide structures on the PICs surface. In addition, we demonstrate the implementation of a 2-D Fourier transform-based autofocusing technique for through-silicon IR microscopy. These building blocks offer a solution for advanced photonic integration processes and other through-silicon vision-related applications, which is instrumental for a large variety of assembly, lithography, and wafer bonding setups.

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(Invited) Miniaturized Infrared Lenses Realized By Wafer Bonding Technologies

All surfaces in our surrounding radiate heat as an electromagnetic wave - similar to light, but invisible to our eyes. The ability to sense and quantify thermal radiation opens up a vast field of application scenarios such as person detection, thermal analysis of buildings or equipment, and health monitoring.Especially the latter has become a matter of utmost importance with the sudden outbreak of the SARS-CoV-2 pandemic in the beginning of 2020. In such an event, the most important task is to closely observe the spreading of the infection and to initiate adequate measures for social distancing.In many cultures, monitoring the health of citizens is a legally delicate issue, and prescribing confinement to an individual would raise difficult questions when it comes to compensate for lost wages. However, having precise instruments at hand to monitor and control the pandemic would allow governments to provide individual support rather than bringing the whole economy to a shutdown.The widespread availability of medical grade temperature sensors could contribute valuable information to a multi-parametric pandemic monitoring. Many infections are accompanied by an elevated body temperature, and often fever is not only one of the most obvious symptoms but also an early stage indicator. Hence, there is a renewed interest in ultra-small medical grade IR sensors which can be integrated into mobile consumer products such as wearables and smart phones.In order to produce IR sensors for a global mass market, a thorough consideration of all aspects on the system level is necessary to optimize cost, size and performance for the targeted application. Infrared detectors for temperature measurement have not yet reached high-volume applications for two main reasons: (1) The optical design necessitates far-infrared transparent materials like ZnSe, CaF or Ge, which are expensive and not always robust for the target application. (2) The state-of-the-art assembly and packaging mostly uses large metal housings (“TO cans”) for their vacuum capability. The size and price of such housings definitely prevents their integration into extremely compact, mobile devices.We are presenting a technological approach that allows effective scaling of production volumes into high quantities by using a fully wafer-based optical assembly and integration using spherical silicon lens preforms. The complete vacuum packaging and optical alignment are performed as a CMOS post-process in our MEMS cleanroom, using industrial state-of-the-art equipment for 200 mm wafers.Considering a spot thermometer where light just needs to be concentrated, convex lenses can either be manufactured as Fresnel lens or as a spherical lens. Both types can be produced only laboriously with standard Silicon technology using dry etching processes and generally multiple lithography cycles when it comes to larger apertures. For a small F number, the Fresnel lens would rather generate stray light than concentrate incoming radiation on the detector element. Additionally, it is not sufficient to just produce a lens - obtaining a precise spherical shape with defined radius of curvature with good uniformity across the wafer is challenging with etching processes. Expensive equipment would be needed for 100% optical test coverage and any lens failure or parametric deviation would equally bring down the CMOS yield when using wafer-to-wafer bonding.In our paper, a detailed review of the IR packaging solutions, lens materials and their fabrication process will be given. A novel technology platform being developed at Fraunhofer ISIT together with the end user will be presented. At the core of the fabrication process, preformed Si spheres with a defined curvature are used that are subsequently grinded to form multiple plano-convex lenses on the wafer level. The process is a new element in our glass forming process portfolio that already enabled many technological innovations in wafer-level optics. Using spherical preforms results in excellent optical properties for IR spot detectors and eliminates the need for expensive 100% optical testing. Although certain limitations with respect to the pitch and the availability of intermediate sphere sizes exist, a clever design (e.g. focal length adjustment, placement of IR pixels etc.) allows covering a commercially very relevant spot on the IR detector market. Figure 1

Tolerancing the surface form of aspheric microlenses manufactured by wafer-level optics techniques

Tolerancing is an important step toward the fabrication of high-quality and cost-effective lens surfaces. It is critical for wafer-level optics, when up to tens of thousands microlenses are fabricated in parallel and whose surfaces cannot be formed individually. However, approaches developed for macro-optics cannot be directly transposed for microlenses because of differences in fabrication and testing techniques. In particular, microlens surfaces are usually limited to conical surfaces. Here, we study the connection between the microlens optical performance and the form of its surface, suggesting surface form representations suited for tolerancing purposes. Then, we compare them with common representations for tolerancing real optical systems. Measured surface forms of microlenses are also provided to make the tolerancing procedure realistic. In addition, we propose term definitions for micro-optics, complements to typical terms for macro-optics, to ease the communication between optical designers and manufacturers. Based on the results presented in this paper, guidelines are proposed for tolerancing microlenses. We suggest applying them as a first step toward a more effective and comprehensive tolerancing procedure.

Microprism for a capsule endoscope with wafer level optics

Capsule endoscopes are pill-sized devices that physicians can utilize for diagnosis of a patient’s small intestine. For diagnostic requirements, excellent image quality, large field of view (FOV) and compact size are crucial. In this paper, a multi-camera capsule endoscope (MCCE) composed of two wafer level optic modules and microprism elements which can offer both a large FOV and clear image quality within limited space conditions is described. The properties of the microprism needed to ensure the necessary image quality are further discussed, including working efficiency, the ability to increase the FOV, and the inhibition of chromatic aberration. Finally, the functioning of the MCCE is verified through an optical simulation and preliminary experiments. The results show that the max half FOV of the MCCE could be increased from 28° to 37°, and the relative illumination could be kept at 80% without a dark corner effect.

Sustainable Manufacturing of High-Precision, Heat-Resistant Aspherical Lenses Using Ultraviolet Illumination With Prognosis of Remaining Useful Life

Recently, carbon emissions and global warming have become major issues, and efforts are being made to develop sustainable manufacturing systems and improve product lifespans. Waste and greenhouse gases created during manufacturing can be minimized using sustainable processes and by proactively considering the environment during product design and fabrication. Miniaturization of optical parts is key in the maturing mobile device market; the demand for ultra-small light-emitting diodes (LEDs) and aspherical lenses is growing rapidly. Small aspherical lenses are created using injection molding, wafer-level optics, and glass molding. Traditionally, injection molding was associated with excellent transferability, and is suitable for mass production. However, considerable energy is required to create high internal cavity pressures and high temperatures. Furthermore, a great deal of waste such as runners is created, and the lenses are unstable at high temperature. We sought to resolve these issues by using sustainable manufacturing concepts in the design stage. To this end, we used ultraviolet (UV)-curable resin to mold high-precision lenses exhibiting excellent heat-resistance. We proposed a methodology to mold ultra-small optical lenses using UV-curable resin to improve material and energy efficiency compared with the traditional injection molding process. We employed a prognostics to predict the life cycle of the system and improve sustainability.

UV-Nanoimprint Lithography: Structure, Materials and Fabrication of Flexible Molds

Large-area nanopatterning technology has demonstrated high potential which can significantly enhance the performance of a variety of devices and products such as LEDs, solar cells, hard disk drives, laser diodes, wafer-level optics, etc. But various existing patterning technologies cannot well meet industrial-level application requirements in term of high resolution, high throughput, low cost, large patterned areas, and the ability to pattern on non-ideal surfaces or waters. Soft UV-nanoimprint lithography (UV-NIL) by using a flexible mold has been proven to be a cost-effective mass production method for patterning large-area structures up to wafer-level (300 mm) in the micrometer and nanometer scale, fabricating complex 3-D micro/nano structures, especially making large-area patterns on the non-planar surfaces even curved substrates at low-cost and with high throughput. In particular, it provides an ideal solution and a powerful tool for mass producing micro/nanostructures over large areas at low cost for the applications in compound semiconductor optoelectronics and nanophotonic devices, especially for LED patterning. That opens the way for many applications not previously conceptualized or economically feasible. The flexible mold is the most critical elements for soft UV-NIL. The performance of the flexible mold has a decisive effect on the soft UV-NIL in term of resolution, patterning area, throughput, uniformity of the imprinted patterns, and repeatability of multi-imprinting. The key enabler that can fulfill mass production of micro-and nanostructures over large areas by NIL is the continual advancement of mold techniques (structures, materials and fabrication processes) towards higher resolution over a larger area at a lower cost. This paper provides a comprehensive review on the structural types, materials used and fabrication methods of various flexible molds in soft UV-NIL, surveys major progress in various flexible molds, particularly highlights some concluding remarks and generalizations. Two key issues for flexible molds, deformation mechanism and controlling solution of soft molds as well as fabrication of large area (wafer level) master template, are described in detail. Furthermore, prospects, challenges and future directions for flexible molds are addressed. Finally, some potential or promising solutions for improving the performance of flexible molds and soft UV-NIL process, as well as some important conclusions are presented.

Resolution and sensitivity of wafer-level multi-aperture cameras

The scaling limits of multi-aperture systems have been widely discussed from an information-theoretical standpoint. While these arguments are valid as an upper limit, the real-world performance of systems for mobile devices remains restricted by optical aberrations. We argue that aberrations can be more easily controlled with certain architectures of multi-aperture systems, especially those manufactured on wafer scale (wafer-level optics, WLO). We complement our analysis with measurements of one single- and one multi-aperture WLO camera. We examine both sharpness and sensitivity, giving measurements of modulation transfer function and temporal noise, and showing that multi-aperture systems can indeed reduce size without compromising performance.

Next-Generation Cell Phone Cameras

The mobile camera market is being driven by consumers’ desire for big imaging capability in smaller and smaller packages. Despite the limitations imposed by these competing demands—or perhaps because of them—many companies are unleashing creative solutions that incorporate wafer-level optics, liquid lenses and LED flashes.

Multi-aperture optics for wafer-level cameras

Wafer-level optics is considered to yield imaging lenses for cameras of the smallest possible form factor. The high accuracy of the applied microsystem technologies and the parallel fabrication of thousands of modules on the wafer level make it a hot topic for high-volume applications with respect to quality and costs. However, the adaption of existing materials and technologies from microoptics for the manufacturing of millimeter scale lens diameters led to yield problems due to material shrinkage and z-height accuracy. A multi-aperture approach to real-time vision systems is proposed that overcomes these issues because it relies on microlens arrays. The demonstrated prototype achieves VGA (Video Graphics Array, 640×480 pixels) resolution with a thickness of 1.4 mm, which is a thickness reduction of 50% compared to single-aperture equivalents. The partial images that are separately recorded in different channels are stitched together to form a final image of the whole field of view by means of image processing. Distortion is corrected within the processing chain. The microlens arrays are realized by state-of-the-art micro-optical fabrication techniques on wafer level that are suitable for a potential application in high volume, e.g., for consumer electronic products.

Optical Cluster Eye fabricated on wafer-level

Wafer-level optics is considered as a cost-effective approach to miniaturized cameras, because fabrication and assembly are carried out for thousands of lenses in parallel. However, in most cases the micro-optical fabrication process is not mature enough to reach the required accuracy of the optical elements, which may have complex profiles and sags in the mm-scale. Contrary, the creation of microlens arrays is well controllable so that we propose a multi aperture system called "Optical Cluster Eye" which is based on conventional micro-optical fabrication techniques. The proposed multi aperture camera consists of many optical channels each transmitting a segment of the whole field of view. The design of the system provides the stitching of the partial images, so that a seamless image is formed and a commercially available image sensor can be used. The system can be fabricated on wafer-level with high yield due to small aperture diameters and low sags. The realized optics has a lateral size of 2.2 × 2.9 mm2, a total track length of 1.86 mm, and captures images at VGA video resolution.

Solutions for Wafer-Level Packaging of High Brightness LEDs

High brightness LEDs (HB-LEDs) carry a high prospect for general lighting applications. Competing with the cost/performance ratio of current light sources demands an increase of the overall efficiency as well as the reduction of the device cost. Since packaging accounts for 30%–50% of the cost of HB LED manufacturing, moving from die- level to wafer-level processes is one likely potential solution for reducing cost per lumen. Silicon-based WLP, using the established processing technology of the MEMS and IC industry, offers high fabrication reliability, high yield and the direct integration of the driver IC in the package. The already small form factor of WLP can be further reduced using Through-Silicon-Vias (TSV), increasing the maximum amount of chips per wafer. Silicon WLP also offers superior thermal management, with the relatively high thermal conductance of silicon. Redistributing LED dies on silicon wafer submounts, with metal bonding and copper TSVs, further improves the heat conductance away from the active region of the chip, resulting in increased device performance. Wafer-level optics can further improve performance and reduce packaging costs. Wafer-level lens molding based on imprint lithography is in high volume manufacture for cell phone camera modules. It allows creation of spherical and a-spherical lenses as well as lens stacks with minimized form factor. In contrast to the currently applied drop dispensing technique for LED lens fabrication, the shape of the lens can be accurately tailored and the decrease of the lens size results in lower absorption and higher light output. Most of these technologies are already in high volume production in other sectors. We will discuss the field proven solutions at each process step, from the formation of the silicon interposer, through the chip-to-wafer bonding, to the final imprinting of the wafer-level optics.

Advances in Wafer Level Processing and Integration for CIS Module Manufacturing

This article presents the advances in wafer-level processing and integration techniques for CMOS image sensor module manufacturing. CMOS image sensors gave birth to the low-cost, high-volume camera phone market and are being adopted for various high-end applications. The backside illumination technique has significant advantages over the front-side illumination due to separation of the optical path from the metal interconnects. Wafer bonding plays a key role in manufacturing backside illuminated sensors. The cost-effective integration of miniaturized cameras in various handheld devices becomes realized through the introduction of CMOS image sensor modules or camera modules manufactured with wafer-level processing and integration techniques. We developed various technologies enabling wafer-level processing and integration, such as (a) wafer-to-wafer permanent bonding with oxide or polymer layers for manufacturing backside illuminated sensor wafers, (b) wafer-level lens molding and stacking based on UV imprint lithography for making wafer-level optics, (c) conformal coating of various photoresists within high aspect ratio through-silicon vias, and (d) advanced backside lithography for various metallization processes in wafer-level packaging. Those techniques pave the way to the future growth of the digital imaging industry by improving the electrical and optical aspects of devices as well as the module manufacturability.