Reinhard Völkel

Design, fabrication and testing of microlens arrays for sensors and microsystems

We report on our activities in design, fabrication, characterization and system integration of refractive microlens arrays for sensors and microsystems. Examples for chemical analysis systems (, blood gas sensor), neural networks and multiple pupil imaging systems for photolithography (microlens and smart mask lithography) are presented.

Artifical compound eyes - Different concepts and their application to ultra flat image acquisition sensors

Two different approaches for ultra flat image acquisition sensors on the basis of artificial compound eyes are examined. In apposition optics the image reconstruction is based on moire or static sampling while the superposition eye approach produces an overall image. Both types of sensors are compared with respect to theoretical limitations of resolution, sensitivity and system thickness. Explicit design rules are given. A paraxial 3x3 matrix formalism is used to describe the arrangement of three microlens arrays with different pitches to find first order parameters of artificial superposition eyes. The model is validated by analysis of the system with raytracing software. Measurements of focal length of anamorphic reflow lenses, which are key components of the superposition approach, under oblique incidence are performed. For the second approach, the artificial apposition eye, a first demonstrator system is presented. The monolithic device consists of a UV-replicated reflow microlens array on a thin silica-substrate with a pinhole array in a metal layer on the backside. The pitch of the pinholes differs from the lens array pitch to enable an individual viewing angle for each channel. Imaged test patterns are presented and measurements of the angular sensitivity function are compared to calculations using commercial raytracing software.

Novel optics/micro-optics for miniature imaging systems

The visual revolution triggered by the commercial application of digital image capturing devices generates the need for new miniaturized and cheap optical imaging systems and cameras. However, in imaging we can observe only a permanent miniaturization of elements but always similar optical principles are applied which are known to the optical designers for many decades. With the newly gained spectrum of technological capabilities it is the time to ask: Which vision principle should be used at which level of miniaturization and which technology has to be applied in order to achieve the perfectly adapted imaging system? In this paper we present an overview of two insect inspired artificial compound eye concepts for compact vision systems fabricated by lithographic technologies, one classical miniaturized objective and its wafer-scale fabrication and the use of variable focal length liquid lenses for miniaturized autofocus- and zoom objectives without moving parts.

Theoretical analysis of an artificial superposition compound eye for application in ultra flat digital image acquisition devices

A new concept for a flat digital image acquisition device for large field of views (FOV) has been developed. Antetypes for the optical system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the arrangement of three microlens arrays (MLA) with different pitches to find the first order parameters of the system. These considerations are extended to arrays of anamorphic lenses with variable parameters to achieve homogeneous optical performance over the whole FOV. The model is validated by implementation of different systems into commercial raytracing software. A trade-off between system length, sensitivity and diffraction limited resolution as well as aberrations is discussed.

Comparing glass and plastic refractive microlenses fabricated with different technologies

We review the most important fabrication techniques for glass and plastic refractive microlenses and we quantitatively characterize in a systematic way the corresponding state-of-the-art microlenses, which we obtained from selected research groups. For all our measurements we rely on three optical instruments: a non-contact optical profiler, a transmission Mach–Zehnder interferometer and a Twyman–Green interferometer. To conclude, we survey and discuss the different fabrication techniques by comparing the geometrical and optical characteristics of the microlenses, the range of materials in which the lenses can be produced, their potential for low-cost fabrication through mass-replication techniques and their suitability for monolithic integration with other micro-optical components.

Miniaturized imaging systems

Integrated digital micro-cameras are an important feature for next generations of customer products like mobile phones and computers. Key specifications of such micro-cameras are resolution, sensitivity, power consumption, manufacturing and packaging costs--as well as the overall size. Digital micro-camera used today are rarely smaller than 5 × 5 × 5 mm3. The recent improvements of CMOS image sensors would allow a further miniaturization. However, due to diffraction effects a miniaturization of the optics would drastically reduce resolution and image finesse. How to overcome these limitations of optics? A fascinating approach is to look how nature has successfully solved similar problems in the case of very small creatures. We will explain basics properties of miniaturized imaging systems and show how some of natures ideas might help further miniaturize micro-cameras. We will report on microfabrication of refractive microlens arrays, wafer-level packaging and present examples of array imaging systems used for micro-cameras and photolithography applications.

Fabrication of multilayer systems combining microfluidic and microoptical elements for fluorescence detection

This paper presents the fabrication of a microchemical chip for the detection of fluorescence species in microfluidics. The microfluidic network is wet-etched in a Borofloat 33 (Pyrex) glass wafer and sealed by means of a second wafer. Unlike other similar chemical systems, the detection system is realized with the help of microfabrication techniques and directly deposited on both sides of the microchemical chip. The detection system is composed of the combination of refractive microlens arrays and chromium aperture arrays. The microfluidic channels are 60 /spl mu/m wide and 25 /spl mu/m deep. The utilization of elliptical microlens arrays to reduce aberration effects and the integration of an intermediate (between the two bonded wafers) aluminum aperture array are also presented. The elliptical microlenses have a major axis of 400 /spl mu/m and a minor axis of 350 /spl mu/m. The circular microlens diameters range from 280 to 300 /spl mu/m. The apertures deposited on the outer chip surfaces are etched in a 3000-/spl Aring/-thick chromium layer, whereas the intermediate aperture layer is etched in a 1000-/spl Aring/-thick aluminum layer. The overall thickness of this microchemical system is less than 1.6 mm. The wet-etching process and new bonding procedures are discussed. Moreover, we present the successful detection of a 10-nM Cy5 solution with a signal-to-noise ratio (SNR) of 21 dB by means of this system.

Microoptical telescope compound eye

A new optical concept for compact digital image acquisition devices with large field of view is developed and proofed experimentally. Archetypes for the imaging system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the telescope arrangement of three microlens arrays with different pitch to find first order parameters of the imaging system. A 2mm thin imaging system with 21x3 channels, 70 masculinex10 masculine field of view and 4.5mm x 0.5mm image size is optimized and analyzed using sequential and non-sequential raytracing and fabricated by microoptics technology. Anamorphic lenses, where the parameters are a function of the considered optical channel, are used to achieve a homogeneous optical performance over the whole field of view. Captured images are presented and compared to simulation results.

Microoptical artificial compound eyes - from design to experimental verification of two different concepts

Two novel objective types on the basis of artificial compound eyes are examined. Both imaging systems are well suited for fabrication using microoptics technology due to the small required lens sags. In the apposition optics a microlens array (MLA) and a photo detector array of different pitch in its focal plane are applied. The image reconstruction is based on moire magnification. Several generations of demonstrators of this objective type are manufactured by photo lithographic processes. This includes a system with opaque walls between adjacent channels and an objective which is directly applied onto a CMOS detector array. The cluster eye approach, which is based on a mixture of superposition compound eyes and the vision system of jumping spiders, produces a regular image. Here, three microlens arrays of different pitch form arrays of Keplerian microtelescopes with tilted optical axes, including a field lens. The microlens arrays of this demonstrator are also fabricated using microoptics technology, aperture arrays are applied. Subsequently the lens arrays are stacked to the overall microoptical system on wafer scale. Both fabricated types of artificial compound eye imaging systems are experimentally characterized with respect to resolution, sensitivity and cross talk between adjacent channels. Captured images are presented.

Microlens array imaging system for photolithography

Keywords: flat panel displays ; compound eye ; microlenses ; micro-optics ; multi-aperture imaging ; optical images ; photolithography ; photoresists Reference EPFL-ARTICLE-183166 Record created on 2013-01-17, modified on 2017-05-10