Miniaturization of Imaging Systems R

M MST/MEMS FOR PRODUCTION ENGINEERING

Miniaturization of Imaging Systems R. Völkel, M. Eisner and K.J. Weible

Micro-cameras integrated into mo- How did Mother Nature solve minia- 2) The next step is to derive the max- bile phones or computers are very turization problems in optics? For imum lens diameter of the system popular these days. Such micro- large vertebrates, Nature implement- from the desired overall thickness of cameras operate with low-priced ed single-aperture eyes. Here the vol- the camera system. For an F/2.4 sys- lenses made of plastics and provide ume of the eye is a free design pa- tem, the image distance is 2.4 times a decent image quality. Having a rameter - the optical performance is the aperture diameter. The overall closer look at the camera, we dis- the key issue. For small invertebrates, thickness is the image distance plus cover that the optical part is usually evolution preferred to distribute im- lens and detector thickness. a bulky block of some 5x5x5 mm3 age capturing to a matrix of small on top of a very thin electronic im- eye sensors [1]. Usually the resolution 3) Knowing lens diameter and stop age sensor. Why does optics remain of such so-called compound or fly's number F, the maximum number of so huge compared with highly eyes is pretty poor. For small animals, transferred pixel M is derived. If the miniaturized electronics? Is there a this is the only way to avoid a flood- number of image pixels of one single fundamental problem with the ing of the animal's neural system. imaging channel is not sufficient, miniaturization of a lens system? However, the poor image quality multiple channels have to be used. Yes, there is. The number of image makes the fly's eyes concept useless Each imaging channel should only pixels a lens can transmit scales for technical applications. image a limited angular section. A with the square of the lens diame- superposition of the partial images is ter. If the lens diameter is getting The most promising natural ap- performed either within the signal- smaller, the image quality will be proach for miniaturization is the eye processing unit or by spatial superpo- reduced drastically. This is a funda- system of jumping spiders. Jumping sition in the image plane (see Fig. 1). mental limitation to the further spiders have opted for single-lens For spatial superposition, erect imag- miniaturization of imaging systems. eyes, but eight of them. Jumping spi- ing is required. Only next neighbor Is there any chance to overcome ders have two high-resolution eyes, images should superimpose to limit this problem? A fascinating ap- two wide-angle eyes and four addi- off-axis aberrations. proach is to look how Mother Na- tional side eyes. The two high-resolu- ture has found solutions for very tion eyes provide a magnified image small creatures. Nature has dis- at a high resolution for a rather tributed the imaging task to an ar- small visual field. Jumping spiders ray of lens systems. Microfabrica- use these eyes for detailed inspection tion capabilities now allow imple- of objects of interest. The two wide- mentation of similar design ap- angle eyes provide a large visual proaches to imaging systems used field at a reduced resolution. The e.g. for micro-cameras and pho- four small side eyes cover the large tolithography machines. field left and right of the spider. Figure 1: Miniaturized imaging systems based Jumping spiders do not have com- on spatial superposition of the partial erect im- Miniaturization of Lens Systems pound eyes. Their resolution would ages created by adjacent imaging channels. Both, the stop number F and the be too poor to identify a target Each lens channel images only a limited angular section. diffraction-limited spot size of a lens worth to jump on. To have single- are independent of lens scale. A lens eyes common to vertebrates, spi- Micro-Optics downscaling of a lens does not influ- ders are too small. The spider uses a Today's micro-optical design and ence the size of the image pixel; cluster of single-lens eyes, each pair manufacturing is closely tied to however, downscaling drastically re- tailored for a different task. Spiders ideas, concepts and technologies de- duces the number of transported pix- see almost as sharp as we do and veloped for the semiconductor indus- els. For a diffraction-limited lens sys- likewise have a good idea of what is try [2]. Three main categories of tem, the number of transported im- going on in their surroundings. miniaturized lenses are available: age pixels scales with the square of diffractive, refractive and graded in- the lens diameter. Table 1 gives the Learning from the jumping spider we dex microlenses. number of image pixels M for an can derive the following design strat- F/2.4 diffraction-limited system of egy for miniaturized imaging sys- Although animal eyes are based on different lens diameters. A miniatur- tems: graded index lenses, the difficult ized imaging system is able to image manufacturing process severely limits fine details of a scene, but not many. 1) The first step is to choose the F- their availability for technical imag- number on the basis of the ing. For diffractive microlenses, focal detector resolution, the length and efficiency depend strong- desired light gathering ly on the wavelength. The use of ability and the numerical diffractive lenses is restricted to aperture of the available monochromatic imaging applica- Table 1: Number of image pixels for an F/2.4 diffraction-limited single-aperture system of different lens diameters. optical sub-components. tions. Refractive microlenses seem to

mstnews 2/03 36 MST/MEMS FOR PRODUCTION ENGINEERING M be the best solution. A standard "classical" mounting is not practical sition of the partial images created manufacturing technique for refrac- and is too expensive. The preferred by adjacent imaging channels seems tive microlens arrays is the reflow solution is manufacturing on the ba- to be a promising approach. Each technique. Photoresist is micro-struc- sis of a wafer-scale and a wafer-level imaging channel images only a very tured by photolithography and melt- packaging approach for mounting. A limited angular section. Elliptical lens ed. The lens profile is formed by sur- Mask Aligner is used to align a stack bases are used to correct astigmatism face tension during melting. The of planar wafers containing both im- for oblique incidence. Wafer-scale melted resist lens serves as a master age sensors and optics. The different lens manufacturing and wafer-level for subsequent transfer processes layers are bonded together by using packaging are the key objectives of like reactive ion etching or replica- epoxy, thermal or fusion bonding, or this project (see Figure 2). tion. Aspheric lens profiles are ob- thick-film solder glass bonding. A tained by varying the etch parame- subsequent dicing step is used to ters. separate the wafer stack into the in- dividual systems or modules. This Apertures, stops, baffles and filters method allows a cost-efficient are other essential parts of every op- mounting of some hundreds of mi- Figure 2: Wafer-scale lens manufacturing and wafer-level packaging for imaging system. tical system. They are necessary to cro-cameras in one step. improve the image contrast by block- Microlens Projection Lithography ing aberrant rays, adapting the In the following we will give two ex- Microlens Projection Lithography wavelength spectra and reducing amples of miniaturized imaging sys- (MPL) is a contact-less photolitho- straylight. For miniaturized imaging tems based on the array concept. graphic technique that has been de- systems, structured wavelength fil- veloped for SUSS MicroTec Mask ters (IR- or color filters) and aperture Miniaturized Imaging System Aligners [4]. MPL uses an ultra-flat arrays are realized by thin film depo- Wafer Level Optics for CMOS Imagers microlens-based projection system sition, photolithography and conse- Miniaturized imaging systems based consisting of some 100,000 side-by- quent etch or lift-off steps. on the above design rules and multi- side identical lens channels. Each lens Packaging and alignment of minia- ple imaging channels are currently channel consists of 4 microlens layers turized lens systems is a rather diffi- being investigated within the EU-IST (see Figure 3). Wafer-level packaging cult task. For micro-optics, standard Project WALORI [3]. Spatial superpo- of the different optical layers ensures

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square of the lens diameter. If the lens diameter is getting too small, the image quality will be reduced drastically. A possible way out of this dilemma is to use bio-inspired array imaging systems. Highly miniaturized imaging systems based on this concept offer an enormous potential for applications from electronic imaging to high-resolution photolithography.

Figure 4: Ultra-flat microlens projection system integrated into the SUSS MicroTec MA150-MPL References Mask Aligner. Microlens Projection Lithogra- [1]M. F. Land, D. E. Nilson, "Animal phy allows photolithography on curved or Eyes", Oxford Animal Biology Se- non-planar substrates, in V-grooves, and holes. Figure 3: A microlens based projection system ries, ISBN 0 19 8575645 (2002). projects a photomask onto a resist layer. Each images from different channels over- [2]H. P. Herzig (editor), "Micro-op- lens channel images a small part of the pho- lap consistently and form a complete tics", Taylor & Francis, ISBN 0- tomask pattern onto the wafer. The partial images overlap consistently and form a complete aerial image of the photomask. 7484-0481-3 (1997). aerial image of the photomask. Front- and [3]European Commission, Project backside telecentricity provides equal line Microlens Projection Lithography No.: IST-2001-34646, WALORI - width over the whole depth of focus DOF. provides a free working distance of WAfer Level Optic solution foR a precise alignment of the projection WD = 0.8 mm and a depth of focus compact CMOS Imager system. Figure 4 shows the ultra-flat DoF > 50 µm for a resolution of 5 [4]W. B. Hugle, R. Dändliker, H. P. projection system within a SUSS µm. The extended DoF allows pho- Herzig, "Lens array photolithogra- MA150-MPL Mask Aligner. A fully tolithography on curved or non-pla- phy", GB patent appl. 9224080.3 symmetrical optical design eliminates nar substrates, in V-grooves, and (1992), U.S. pat. 08/114,732 (1993). coma, distortion and lateral color. holes. The lens system is frontal- and back- Contact: side telecentric to provide a unit Conclusion Reinhard Völkel magnification over the whole depth A miniaturization of imaging systems SUSS MicroOptics SA, Neuchatel of focus DoF. Each lens channel im- cannot be done by simply downscal- www.suss-microoptics.com ages a small part of the photomask ing the lens. The number of image Phone: +41 32 7205103 pattern onto the wafer. The partial pixels a lens transmits scales with the E-Mail: [email protected] The Yin and Yang Strategy for a more Economic Hybrid Microsystem Assembly U. Gengenbach, A. Hofmann, F. Engelhardt and R. Scharnowell

A substantial number of microsys- Unlike microelectronics, microsys- nents is eventually based on highly tems is manufactured in small and tems technology is not very much in automated processes (e.g. CNC micro medium series where manual as- the public eye. Since there are no re- milling, microinjection moulding or sembly still dominates. The main al, flashy "microsystem inside" prod- hot embossing), assembly is often reasons for the reluctant introduc- ucts, but microcomponents are based on manual labour. There are tion of automatic assembly systems rather hidden in cars, printers etc., two main causes for this situation, are design unsuitable for automat- this technology does not receive which are to some extent interrelat- ed assembly as well as high invest- much attention. Apart from a few ed: ment and still low flexibility of as- high volume components mostly 1. Technical reasons: While the assembly equipment. Design for as- based on silicon microtechnology sembly of silicon microcompo- sembly is still a methodology that is (e.g. sensors for automotive industry nents boils down to the placement rarely used. Recent years have seen and components for IT industry), pro- and bonding of planar, chip- increasing efforts to modularise duction volumes are comparatively shaped parts with well-established equipment. Adopting only one of small. Several promising microsys- processes, assembly of hybrid mi- these approaches will fall short. tems, such as micro-optical sensors, crosystems frequently requires Hence a holistic strategy is pro- micro-fluidic devices, are only pro- handling of complex shapes and posed that combines measures on duced in small and medium batches application of novel, non-standard the product design side with mea- up to a few 10,000 per year. Quite a bonding processes. sures on the equipment side, the number of these products are based 2. Economic reasons: The small batch Yin and Yang strategy for micro-as- on materials other than silicon such sizes of hybrid microsystems sembly. as glass, metals, polymers. While the scarcely ever justify investment in manufacturing of the microcompo- automatic assembly equipment

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