This paper presents microlenses (MLs) with low f-number made of AZ4562 photoresist for integration on optical microsystems. The fabrication process was based on the thermal reflow and rehydration. Large series of MLs were fabricated with a width of 35 μm, a thickness of 5 μm, and spaced apart by 3 μm. The MLs were fabricated directly on the surface of a die with type n+/p-substrate junction photodiode fabricated in a standard CMOS process. The measured focal length was 49 μm with a tolerance of ±2 μm (maximum error of ±4%), resulting in a numerical aperture of 33.6 × 10-2 (±1.3 × 10-2). The measurements also revealed an f-number of 1.4.This work was sponsored by the Brazilian agency Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) under the grant 400110/2014-8: Ciencia sem Fronteiras, Bolsa Pesquisador Visitante Especial (PVE) 2014. The associate editor coordinating the review of this paper and approving it for publication was Prof. M. Nurul Abedin

Rocha, Rui Pedro

Belsley, M.

Correia, J. H.

Carmo, João Paulo Pereira

IEEE Sensors Journal

Universidade do Minho: RepositoriUM

IEEE SENSORS JOURNAL, VOL. 15, NO. 7, JULY 2015 4073Low f -Number Microlenses for Integration onOptical MicrosystemsRui Pedro Rocha, Michael S. Belsley, José Higino Correia, and João Paulo CarmoAbstract— This paper presents microlenses (MLs) with lowf -number made of AZ4562 photoresist for integration on opticalmicrosystems. The fabrication process was based on the thermalreflow and rehydration. Large series of MLs were fabricatedwith a width of 35 µm, a thickness of 5 µm, and spaced apartby 3 µm. The MLs were fabricated directly on the surface of adie with type n+/ p-substrate junction photodiode fabricated in astandard CMOS process. The measured focal length was 49 µmwith a tolerance of ±2 µm (maximum error of ±4%), resultingin a numerical aperture of 33.6 × 10−2 (±1.3 × 10−2). Themeasurements also revealed an f -number of 1.4.Index Terms— OPTO, micro-optics, microlenses, optical devicefabrication, f-number, focal length characterization.I. INTRODUCTIONSTANDARD microfabrication processes (more precisely,photolithography and photoresist (PR) thermal reflow)were used in the fabrication of spherical refractivemicrolenses (MLs) that are presented in this paper. Thesemethods and technologies (see Fig. 1) allow the fabricationof three dimensional microstructures in a reproducible wayand, at the same time, to customize high-quality and costeffective optical microcomponents. The fabrication settingswere selected to obtain MLs measuring 35 μm in the interfacewith the substrate. Each ML is spaced apart to its adjacent byapproximately 3 μm with their apexes presenting a thicknessof 5 μm. The thermal reflow (TR) is based on the surfacetension phenomenon [1] and it was applied to obtain theactual lens profile. This technique allows the fabrication ofhigh quality spherical and parabolic shaped MLs by heatingup the PR above its glass transition temperature [2]. Oncethis temperature is reached, the PR starts to melt with thesurface tension effect causing the fabricated microstructure togain the desired spherical lens profile [3]. The entire processManuscript received October 24, 2014; revised December 13, 2014;accepted December 17, 2014. Date of publication December 23, 2014; dateof current version May 20, 2015. This work was sponsored by the Brazilianagency Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq) under the grant 400110/2014-8: Ciência sem Fronteiras, BolsaPesquisador Visitante Especial (PVE) 2014. The associate editor coordinatingthe review of this paper and approving it for publication was Prof. M. NurulAbedin.R. P. Rocha and J. H. Correia are with the Department of IndustrialElectronics, University of Minho, Guimarães 4800-058, Portugal (e-mail:rrocha@dei.uminho.pt; higino.correia@dei.uminho.pt).M. S. Belsley is with the Department of Physics, University of Minho,Braga 4710-057, Portugal (e-mail: belsley@fisica.uminho.pt).J. P. Carmo is with the Department of Electrical Engineering, São CarlosSchool of Engineering, University of São Paulo, São Carlos 13566-590, Brazil(e-mail: jcarmo@sc.usp.br).Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JSEN.2014.2385476Fig. 1. Microlenses array fabrication steps: (a) PR spin coating, prebakeand rehydration; (b) UV exposure for transferring the mask pattern; (c) thedevelopment to extract the PR that was exposed; and (d) representation of thethermal reflow for obtaining the desired lens profile.is relatively simple to perform and the TR, specifically, isvery well controlled without the need of high-technologyequipment, guaranteeing good dimensional control and asmooth homogeneous surface. Moreover, this is an importanttopic because it provides the fabrication of MLs arrays in areproducible way and at low-cost. The benefits of fabricatingMLs directly on the surface of a die with type n+/p-substratejunction photodiode fabricated in a standard CMOS processwas already demonstrated as an interesting solution for opticalmicrosystems [4]. Microlenses are especially important inlight-dependent sensors [5] and devices [6] for enhancingthe amount of light reaching the photosensitive area thusincreasing their efficiency. The actual values of the severalprocess parameters will obviously depend on the desired finalsize, requirements and applications of the micro-optical device.In the context of the focus of this paper it is described,from what is to the authors’ knowledge, perhaps the simplestexperimental method yet employed to characterize the focallength of microlenses.II. DESIGN AND FABRICATION SUMMARYThe AZ4562 [7] PR allows the microlenses array fabricationby thermally reflowing the three dimensional structures thatwere obtained as a result of the photolithographic process.The selected PR was chosen due to the fabrication require-ments. Emadi et al. have proven [2] that this PR is idealfor coating thicknesses above 3-5 μm without the need toincrease the exposure energy considerably and still providingenough energy down to the substrate of the AZ material.Fig. 1 illustrates an overview of the MLs’ fabrication. Note thaton this fabrication method, particular attention was given to1530-437X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.4074 IEEE SENSORS JOURNAL, VOL. 15, NO. 7, JULY 2015Fig. 2. SEM images of the selected MLs array prototype used in themeasurements: photos before (left) and after (right) the TR.the prebake, used to evaporate the solvents off the PR. If notall the solvent is cleared off, at least most of it should bein order to create a better PR profile as well as a decreaseof the developer etch to the unexposed photoresist. Moreover,the trapped solvent may form bubbles and lift the resist filmcausing adhesion failure. Therefore, the prebake phase consistsof applying 100 °C for 5 minutes using a computer controlledhotplate with proportional-integral-derivative (PID) control.The hotplate was used due to its ability to evaporate thesolvent as it ramps up to the final selected prebake temperature.Nevertheless, the prebake has the drawback of also evaporatingthe PR’s water content (the PR literally dries out), so it ismandatory to rehydrate the PR afterwards. The rehydrationfacilitates the fabrication of pre-thermal reflow structures witha profile closer to the theoretical parallelepipedic shape ratherthan a trapezoidal one.Thus, the rehydration is the key factor for obtaining thevery smooth and homogeneous structures seen in the pre-vious images which are essential for good optical qualitymicrolenses. In Fig. 2 is possible to observe the selectedprototype which was used in the measurements with the pre-MLs structures seen on the left and the actual MLs achievedafter the TR are seen on the right.III. EXPERIMENTALAn experimental setup was designed and assembled fordetermining the focal length of the cylindrical MLs inthe array. The Gaussian beam from a He-Ne laser withλ = 632.8 nm (Melles Griot model 05LHP171) was focusedto a beam waist with a 1/e2 diameter of approximately 4.2 μmusing an aspheric lens (New Focus model 5724) with aneffective focal length of 8 mm and a numerical aperture of 0.5.This reduced beam width allowed to probe the response ofindividual microlenses in the array. A CCD camera, withouta glass cover filter designed for laser beam characterization(DataRay WinCamD), was then placed with the CCD surfaceapproximately 1.2 cm downstream from the beam waistcreated by the aspheric lens.Fig. 3 shows a photograph of the measurement setup. It isobserved that the glass slide with the fabricated microlensesarray was mounted on a micrometer driven xyz translationstage and placed in the normal direction to the beam withthe microlenses array on the exit side.Fig. 4, on the left, shows the beam profile captured bythe CCD camera when the slide was positioned so that onlyFig. 3. Photograph of the measurement setup that was used.Fig. 4. On the left is the spatial distribution of the He-Ne laser beamafter being focused by the aspheric lens and then passing through the glasssubstrate that serves as a support for the microlenses array. The beam has a1/e2 diameter of 2.55 mm. On the right is the image obtained after the beamprofile changed by one of the cylindrical lenses in the microlenses array. The1/e2 diameters have been calculated to be 1.17 mm and 2.56 mm for thevertical and horizontal axis, respectively.Fig. 5. Variation of the beam magnification as the microlenses array istranslated near the best focus position. Dots: measurements; continuous line:eq. (1) fit, resulting in f = 49 ± 2 μm in the air for the ML.the glass substrate was illuminated. The MLs array was thenshifted into the beam and translated until a minimum spot sizealong the direction of the cylindrical lens axis (the verticaldirection) was obtained as shown in the right side of Fig. 4.Using the micrometer stage, the MLs array was then trans-lated about the position of the best focus and the respectivediameters along the vertical direction were determined usingthe DatarayR beam analysis software, with the results beingdisplayed in the plot of Fig. 5. Simultaneously, the beamwidth along the horizontal axes was also measured and wasfound to remain nearly constant at 2.56 ± 0.01 mm. The blueROCHA et al.: LOW f -NUMBER MLs FOR INTEGRATION ON OPTICAL MICROSYSTEMS 4075curve in this figure is a theoretical fit assuming Gaussian beampropagation and treating the MLs array as a thin lens with aneffective focal length f . The predicted variation of the beam1/e2 diameter on the fixed CCD camera as the MLs array istranslated near the point of best focus is given by the followingequation:M =√(1 − z0+zf )2 + ( zRf )2 (1)where M is the effective magnification (ratio between thevertical and horizontal beam diameters), f is the effectivefocal length of the MLs array, z0 + z is the distance betweenthe lens and the beam waist created by the aspheric lens, whilezR = πw20/λ is the Rayleigh length of the beam focused bythe aspheric lens with w0 the corresponding 1/e2 beam radius.This equation is valid in the limit that the distance betweenthe CCD camera plane and the beam waist created by theaspheric lens is much greater than either the Rayleigh lengthor the effective focal length of the MLs array.The take of the measurements started by placing the array ofMLs close to one effective focal length behind the initial beamwaist (z0 represents this initial distance) and then translatedthe array through several different positions (by varying thez parameter). A nonlinear least squares fit to the data resultedin the following values: f = 49 ± 2 μm, zR = 22 ± 1 μmand z0 = 44 ± 4 μm. This value for the Raleigh distanceis consistent with that estimated from the divergence of theHe-Ne laser beam (λ = 632.8 nm) after being focused by theaspheric lens. The main focusing mechanism in the ML isrefractive in nature although the spot size at the focus willbe limited by diffraction. For circular MLs this diffractionlimit is typically given by (1.22 × λ × f )/d , where λ is thewavelength in the image space, f and d are the ML’s focallength and diameter, respectively. In this case the diffractionlimit is sufficiently smaller than the size of the individualphotodiode in the die. Moreover, the error obtained for thefocal length resulted in values that did not exceed 4.1% of thenominal f , which is very good for lenses whose dimensionsare in the range of few dozen micrometers.An important dimensionless characteristic is the numericalaperture (NA) because it is possible to obtain, in a straightfor-ward way, the range of angles over which a lens (and thus,a ML) can accept or emit light. Very often, the NA is obtainedin term of both the index of refraction n of the medium inwhich the lens is working (in this case, the air with n≈1) andin terms of the half-angle of the maximum cone of light thatcan enter or exit the lens θ , e.g., NA = n · sin(θ). Additionally,the NA can be obtained by calculating the ratio between thefocal length f and the diameter of the entrance pupil D, whichin the present case is the ML’s width. Therefore, this resultsin NA = n · [D × (4f 2 + D2)−1/2] ≈ (33.6 ± 1.3) × 10−2,with f = 49 ± 2 μm and D = 35 μm. Finally, the f -number(defined as the ratio f /D) of these MLs is equal to 1.4.IV. CONCLUSIONThis paper presented a simple experimental method tocharacterize the focal length of microlenses with lowf -number made of AZ4562 photoresist for integration onoptical microsystems.REFERENCES[1] L. Feng, C. Sihai, L. Huan, Z. Yifan, L. Jianjun, and G. Yiqing,“Fabrication and characterization of polydimethylsiloxane concavemicrolens array,” Opt. Laser Technol., vol. 44, no. 4, pp. 1054–1059,2012.[2] A. Emadi et al., “Fabrication and characterization of IC-compatiblelinear variable optical filters with application in a micro-spectrometer,”Sens. Actuators A, Phys., vol. 162, no. 2, pp. 400–405, 2010.[3] M. Tekin and C. Tarimci, “Study of the structure and refractive para-meters of diethylamine and triethylamine,” Opt. Laser Technol., vol. 38,no. 7, pp. 498–505, 2006.[4] R. P. Rocha, M. J. Maciel, J. M. Gomes, J. P. Carmo, andJ. Higino Correia, “Fabricating microlenses on photodiodes to increasethe light-current conversion efficiency,” IEEE Sensors J., vol. 14, no. 5,pp. 1343–1344, Feb. 2014.[5] W. Wang and J. Fang, “Variable focusing microlens chip for potentialsensing applications,” IEEE Sensors J., vol. 7, no. 1, pp. 11–17,Jan. 2007.[6] X. Zeng, C. T. Smith, J. C. Gould, C. P. Heise, and H. Jiang, “Fiberendoscopes utilizing liquid tunable-focus microlenses actuated throughinfrared light,” J. Microelectromech. Syst., vol. 20, no. 3, pp. 583–593,Jun. 2011.[7] Clariant GmbH. [Online]. Available: http://www.clariant.com, accessedOct. 2014.Rui Pedro Rocha received the M.Sc. degreein industrial electronics and computer engineeringfrom the University of Minho, Portugal, and thePh.D. degree in leaders for technical industriesof the engineering design and advanced manufac-turing focus area of the MIT-Portugal Programwith the thesis entitled Microlenses for OpticalMicrosystems in 2013. His main research inter-ests are on microfabrication, micro-optics, andmicrotechnologies.Michael S. Belsley received the Ph.D. degree fromthe University of Colorado at Boulder, in 1986.Since 1992, he has been an Associate Professor withthe Department of Physics, University of Minho,Portugal. His research interests include all thingsrelated to optics and have lately focused on non-linear optical material characterization and ultrafastspectroscopy.José Higino Correia received the Degree in physicalengineering from the University of Coimbra, Portu-gal, in 1990, and the Ph.D. degree from the Labora-tory for Electronic Instrumentation, Delft Universityof Technology, The Netherlands, in 1999, workingin the field of microsystems for optical spectralanalysis. He is currently a Full Professor with theDepartment of Industrial Electronics, University ofMinho, Portugal. He was the General-Chairmanof Eurosensors 2003 and MME 2007, Guimarães,Portugal. His professional interests are in microma-chining and microfabrication technology for mixed-mode systems, solid-stateintegrated sensors, microactuators, and microsystems.João Paulo Carmo (S’02-M’08) was born in Maia,Portugal, in 1970. He received the degree and theM.Sc. degree in electrical engineering from the Uni-versity of Porto, Porto, Portugal, in 1993 and 2002,respectively, and the Ph.D. degree in industrial elec-tronics from the University of Minho, Guimarães,Portugal, in 2007. From 2008 to 2013, he wasa Principal Researcher with the CMEMS Center,University of Minho. Since 2014, he has been aProfessor with the University of São Paulo, Brazil.He is involved in research on micro/nanofabricationtechnologies for mixed-mode/RF systems, solid state integrated sensors,microactuators, micro/nanodevices and microprocessors for use in wireless,and biomedical and industrial applications.

Low f-number microlenses for integration on optical microsystems

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Low f-number microlenses for integration on optical microsystems

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