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Electronic Imaging & Signal Processing

Ultra-thin, wafer-level cameras

Multi-aperture, wafer-level optics enables the thinnest possible camera modules using micro-optical fabrication technology.
27 August 2012, SPIE Newsroom. DOI: 10.1117/2.1201208.004430

The application of miniaturized camera systems to portable electronic devices, such as mobile phones and automotive sensors, require not only the shrinking of the opto-electronic and electronic systems but also the optical components. To achieve this, a short focal length and an optical system with low complexity are needed. However, the ongoing miniaturization of image sensors also creates a demand for higher image resolution and increased light sensitivity. Wafer-level fabrication techniques for camera lenses and modules have been promising candidates for high-volume production at low cost. Unfortunately, existing concepts for parallel batch processing apply complex aspherical lens profiles and restrictions in wafer stacking and achievable tolerances have put strong limits on the process yield. We propose an alternative approach using a multi-aperture imaging system. Instead of imaging through a single aperture, it transfers fractions of the full field of view (FOV) through different optical channels, thus requiring only a single lens with a short focal length for each. The optical system was inspired by the insect's compound eye, which enables the shrinkage of the imaging optics' track length to achieve optimum miniaturization. It can be manufactured using state-of-the-art micro-optical fabrication techniques.

As shown in Figure 1, each channel of an array of multiple optical systems transfers only a small part of the FOV, so a single lens is sufficient for creating a good image across the limited field of each optical channel. With this architecture, there is no conventional image formed on the image sensor's pixel array. Instead, different partial images are stitched together in a reconstructed final image of the full FOV using software processing (see Figure 2). The field segmentation, together with the braided sampling of adjacent optical channels, decouples the trade-off between the focal length and FOV size.1 Therefore, the total track length of the multi-aperture setup is reduced by about 50% compared to classical single-aperture optics of the same resolution, pixel size, and sensitivity.


Figure 1. Working principle and section of the layered structure of an electronic cluster eye (eCLEY).

Figure 2. Image of a closed-circuit television test chart recorded by the eCLEY prototype's image sensor. The inset shows a magnified section of the 17×13micro-images.

The fabrication of the micro-lenses is based on the UV-replication of a polymer, where the master wafer of the micro-lens arrays has been fabricated by binary mask lithography and reflow of the photoresist. This standard technology allows the fabrication of precisely determined micro-lenses within tight tolerances and with extremely smooth surfaces. To achieve optimal imaging performance across the FOV, we adapted the shape of each individual micro-lens to its specific angle of incidence within the individual channel. We used several steps of mask lithography and wafer stacking to form the different, partially buried diaphragm arrays that are required to suppress channel crosstalk. Finally, we integrated the micro-lens wafer upside-down on the diaphragm wafer using standoffs molded in polymer. The diced micro-lens objectives were actively aligned and bonded on commercial CMOS image sensors with 3.2μm pixel pitch (see Figure 3).


Figure 3. Prototype of an eCLEY with VGA resolution (right) compared to a conventional VGA-webcam lens (left).

The prototype captures an FOV of 52°×44° (70° on the diagonal) with an image resolution of 700×550 pixels (approximately VGA). The optical module's dimensions are 6.8×5.2×1.4mm3. We applied real-time software correction to each partial image so the final image is free of distortion at video frame rates.2

With a total track length of 1.4mm, the demonstrated electronic cluster eye (eCLEY) is, to our knowledge, the thinnest lens system that achieves VGA resolution. The micro-lens arrays are realized by state-of-the-art micro-optical fabrication techniques on a wafer level and are suitable for application in consumer electronics. Most recently, we integrated the eCLEY camera module with a wireless local-area network interface and standard mobile hardware (see Figure 4). We showed that the imaging principle and real-time image reconstruction can be performed on current smart phones.


Figure 4. Demonstration of a wireless camera implementation of the eCLEY with VGA resolution. The video that is acquired with the ultra-thin optics is transferred by a wireless local-area network interface and can be displayed on any portable electronic device (such as a smart phone or MP3 player, left) nearby. Image post processing is done on the fly using standard mobile hardware.

We are concentrating our future work on increasing the image resolution and reducing the module footprint size by tailoring our principle to the smaller pixel sizes that are common in mobile image sensors.

The authors acknowledge the funding by the Fraunhofer Future Foundation's Facetvision project.


Frank Wippermann, Andreas Brückner
Fraunhofer Institute for Applied Optics and Precision Engineering (IOF)
Jena, Germany

Frank Wippermann graduated from the University for Applied Sciences in Jena in 1999. In 2004, he joined the Fraunhofer IOF, and finished has thesis in 2007. His main professional area is imaging optics design, especially in the field of wafer-level fabrication.

Andreas Brückner graduated from Friedrich-Schiller-University in Jena in physics in 2006. He then began working as a scientist in micro-optical systems at the Fraunhofer IOF. His research includes optical design for miniaturized imaging systems, microtechnology, and image processing. In 2011, he received his PhD in optics, also from Friedrich-Schiller.


References:
1. A. Brückner, J. Duparré, R. Leitel, P. Dannberg, A. Bräuer, A. Tünnermann, Thin wafer-level camera lenses inspired by insect compound eyes, Opt. Express 18(24), p. 24379-24394, 2010.
2. A. Brückner, R. Leitel, A. Oberdörster, P. Dannberg, F. Wippermann, A. Bräuer, Multi-aperture optics for wafer-level cameras, J. Micro/Nanolith. MEMS MOEMS 10, p. 043010, 2011.