high transparent light guiding plate for single-sided light emission
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Microelectronic Engineering xxx (2014) xxx–xxx
MEE 9453 No. of Pages 4, Model 5G
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Contents lists available at ScienceDirect
Microelectronic Engineering
journal homepage: www.elsevier .com/locate /mee
High transparent light guiding plate for single-sided light emission
http://dx.doi.org/10.1016/j.mee.2014.05.0040167-9317/� 2014 Published by Elsevier B.V.
⇑ Corresponding author. Tel.: +49 231/7556660; fax: +49 231/7554631.E-mail address: [email protected] (M. Jakubowsky).
Please cite this article in press as: F.-C. Tengler et al., Microelectron. Eng. (2014), http://dx.doi.org/10.1016/j.mee.2014.05.004
Florian-Carsten Tengler a, Michael Jakubowsky a,⇑, Andreas Neyer b
a RIF Institut für Forschung und Transfer e.V., Joseph-von-Fraunhoferstr. 20, D-44227 Dortmund, Germanyb AG Mikrostrukturtechnik, Faculty of Electrical Engineering and Information Technology, Dortmund University of Technology, D-44221 Dortmund, Germany
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a r t i c l e i n f o
Article history:Received 28 October 2013Received in revised form 29 April 2014Accepted 4 May 2014Available online xxxx
Keywords:LGPPDMSMicroopticsMicrostructureMicroreplication
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a b s t r a c t
The paper demonstrates a novel light guiding plate (LGP) mainly for illumination purposes with two spe-cial features: (1) Light emission is basically single-sided. (2) The device has a high transparency in thesense that clear, distortion free vision perpendicular through the plate is provided due to the flat micro-structures responsible for the homogeneous and planar light emission.
Using ray tracing simulations cylindrical microstructures with planar top covers have been identified tofulfil perfectly the above mentioned requirements.
Experimental demonstration of single-sided light emitting LGPs could be performed by implementingcylindrical microstructures with 50 lm and 100 lm diameter and a pitch-diameter ratio of 3:1. Replica-tion technology using thick photolithographic moulds (Ordyl FP450) and PDMS (Polydimethylsiloxane)casting has been applied to produce first prototypes. The ray tracing simulation predictions could be con-firmed by optical measurements very well.
� 2014 Published by Elsevier B.V.
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1. Introduction 2. Basic principle and applications66
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In the field of architecture, illumination and light art planar,single-side emitting light sources are highly desired. The basic ideafor the realization of planar light sources is the application of LED-coupled light guiding plates (LGP) with optical microstructures forextracting the light. These types of light sources are state of the artin TV and monitor backlight units (BLU) [1]. However, thesedevices are in general two-side emitting (using reflection foils toredirect the light into only one direction) and not well transparentin the sense that they provide a clear, distortion free view perpen-dicular to the plate. For BLUs this is not of importance, however inlighting applications like facade sheathing, interior walls, skylightsand all kind of ultra-flat lamps high transparency and single-sidedemission are highly desirable. State of the art systems of planaremitting LGPs are manufactured e.g. by burning ceramic coloursdots into glass as defects [2]. Diameters of these dots are in the mil-limeter range and they disturb a clear view through the plate. Thehere presented LGP is using flat microstructures with lateraldimensions near to the resolution limit of the human eye and istherefore highly transparent. There are LGP designs also usingmicrostructures with those lateral dimensions, e.g. V- or U-grooves, with partly similar emission characteristics, but withoutany criteria of high and distortion free transparency [3].
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Because of the small size and the good efficiency the penetra-tion of LEDs in the lighting market grows more and more. A verypromising application in LED lighting is the edge coupling ofLED-light into flat and highly transparent polymer plates wherebythe light is guided within the plate (LGP) by the principle of totalinternal reflection (TIR). The great advantage of the LGP-principleis the conversion of the point source characteristic of the LED intoa uniform and homogeneous two dimensional plane. All rayswhich are coupled into the plate at angles below the critical angleof total reflection will be guided – in principle without losses. Thecritical angle depends on the refractive index of the material. In thecase of PDMS (Polydimethylsiloxane) as LGP (n = 1,43 atk = 500 nm) and air as cover material – the system which has beenused in our experiments –, the critical angle is 42,1�. Taking intoaccount the radiation characteristic of the here used LED (OSRAMLED LW T673), the geometrical arrangement of the system andthe Fresnel loss a coupling efficiency of 90% has been calculated.A ray tracing simulation confirms this result.
Only defects in the LGP material or at the two interfaces withthe surrounding air can change the direction of the rays in a waythat they can leave the plate. In this work we introduce surface-microstructures as artificial ‘‘defects’’ to provoke an intended lightloss at only one of the two interfaces. The exit angle is determinedby the geometry of the microstructures.
The goal of this work is an efficient and single-sided exit of lightin combination with a distortion-free transparency. Moreover the
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Fig. 2. Calculated influence of the scaling of structure diameter and plate thicknesson the light emission. Scale of the radius of the cylinders at the top (light grey);scale of the plate thickness at the bottom (dark grey).
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plate should be produced in an economic way. Another benefit ofthe developed microstructure based system is that the angle ofthe emitted light can be adjusted by choosing the right microstruc-ture, their dimensions and angles. The emission intensity can betuned by the dimensions of the microstructures and distancesbetween the structures.
The chosen structures will lead to a narrow emission angle ofabout 20�, which is highly desirable for example for indirect roomlightning and light spots. In this case there is no blooming and thetransparency of the LGP is not affected, if the viewer is not standingin the spot. Some other examples of LGP applications in architec-ture are: transparent balustrades, roof lights, inserted ceiling orfacade illumination. Applications in lighting are e.g.: lamps for ceil-ing, wall and workplace illumination, reading lights, cockpit arma-tures, advertisement illumination.
3. Micro structures and simulations
In order to estimate the light extraction effect of differentmicrostructures, ray tracing simulations with Radiant ZEMAX havebeen employed. Microstructures like prisms, star structures, rect-angles and cylinders have been implemented as surface perturba-tions. As light source the OSRAM LED LW T673 has been used in thesimulations as well as in the experiments. This LED is well docu-mented and source files for Zemax exist [4].
In the simulations the outer plate dimensions were 50 mm �50 mm � 4 mm. In the simulation environment the plate is sur-rounded by a polar detector with a radius of 50,000 mm. The rea-son of the large radius is that in this case the plate can be assumedas a point source.
The simulations show that cylinders with flat top covers arebest suited for single-sided emission with near 100% efficiency.Moreover they are easy to fabricate by photo-lithography and rep-lication e.g. casting in highly transparent PDMS. In order to get thehigh single-side emission efficiency, the ratio of the pitch (D) to thethickness (d) of the cylinders (Fig. 1) should not be smaller than 3and the aspect ratio of the microstructures (height:diameter) notsmaller than 1:1 to guarantee, that all rays travelling within theLGP will fall on the sidewalls of the microstructures and contributeto emission and no ray will fall on the flat top surface of the micro-structure, which would result in reflection, refraction and possiblyin an undesired backside emission.
The simulations show, that cylindrical structures emit lightunder an angle of about 145� with respect to the propagationdirection (Fig. 4a). The emission side is that where the microstruc-tures are placed. Simulations of plates with different thicknessesshow, that the thickness of the plate is inversely proportional tothe emitted light intensity. Whereas simulations of different geo-metrical dimensions of the microstructures show, that the radius
Fig. 1. Principle of light propagation in LGP by TIR with light extraction caused byflat cylindrical micro structures.
Please cite this article in press as: F.-C. Tengler et al., Microelectron. Eng. (201
affects the intensity in a proportional way, while the angular emis-sion is independent of all scaling (Fig. 2).
4. Fabrication
A 100 � 100mm2 sized prototype of a light guiding plate hasbeen fabricated featuring cylindrical structures with diameter of50 lm and 100 lm and a pitch-diameter rate D/d of 3:1. The heighth of the cylinders was 42 lm and is identical with the thickness ofthe used dry photoresist Ordyl FP450. A 600-Si-Wafer is used assubstrate. The resist was laminated on the wafer in a hot roll
Fig. 3. Fabrication process of LGP prototype: hot roll lamination of dry resist onsilicon wafer, exposure, development and casting of PDMS.
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lamination step. For a more homogeneous surface a post lamina-tion bake at 115 �C was necessary. The exposure is done on aMA-6 with a chrome-glass mask. The exposure time is 3 s by19 mW/cm2, which is unusually short but required because of agap between mask and substrate. A development with natron car-bonate (Na2CO3) follows. Before casting, the mould is coated withhexamethyldisilazane (HMDS) in a chemical vapour depositionstep. The developed and coated resist structures could be useddirectly as robust casting mould [5] (Fig. 3). The LGP prototypeitself was fabricated by casting of a 4 mm thick plate of PDMS(Elastosil RT601, Wacker) [6,7]. PDMS has got some advantages.It has got a high flexibility, which made it easy to unmould. More-over it features a high transparency and a low optical absorption.The casting was performed in a closed moulding frame with atrough for the wafer and polished surfaces (Ra = 97 nm) (Fig. 3).After the unmoulding an optical inspection follows (Fig. 5). Ra oncylinders is 54 nm, on the surface around the microstructures68 nm and on the unstructured side 88 nm (Measured with WykoNT1000 from VEECO).
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Fig. 4. Simulated (a) and measured (b) light distribution curves; cylindricalstructure diameters 50 lm and 100 lm, pitch D in X and Y direction: three timesthe diameter; light enters the plate from the right (arrow); lower side of the LGP isthe structured one.
Please cite this article in press as: F.-C. Tengler et al., Microelectron. Eng. (201
5. Measurements
The two special features of this work – single-side emission andhigh transparency – are characterised by two measurement sys-tems. To specify single-sided light emission, the two dimensionallight distribution curve (LDC) has been measured by rotating agoniometer 360� around the plate. The 4 mm thick LGPs are stableenough to be measured without any stabilizing substrate. The totalpower is calculated with the characteristic spectral curve of theused LED (intensity maximum at 450 nm).
As expected, the results confirm the ray tracing simulations forthe cylindrical microstructures with light emission under an angleof around 150�. However, in contrast to the simulated values theexperiment shows that 30% of the whole emitted light is emittedon the back side (Fig. 4b). This deviation is caused by diffusion cen-tres in the used PDMS. The best results are produced by the struc-tures of 50 lm diameter and a height of 42 lm. The maxima of theLDC are in the angular range of 150�. The results show a good coin-cidence with the simulations (145�) with a deviation of only 5�which is mainly due to imperfections in the lithographic process
Fig. 5. Image of cylindrical microstructures on a LGP taken from a 3D LaserScanning Microscope with side view from an optical microscope.
Fig. 6. Photos of the LGP taken at the angle of maximum light emission (150�) withLEDs turned off (a) and on (b).
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Fig. 7. Photos of a logo without LGP (a), with inserted LGP (b) and with inserted LGPand LEDs turned on (c).
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(Fig. 5). Photos of the LGP taken at the angle of maximum lightemission with LEDs turned on and off are presented in Fig. 6.
The second feature is the distortion free transparency. Fig. 7demonstrates the transparency qualitatively by photographic pic-tures of the MST – Logo (a) without the perturbation by a LGP,(b) by inserting a LGP in the light pass and (c) by inserting theLGP and turning the LED emission on. A more detailed quantifiedstudy of the transparency using the modulation transfer function(MTF) including the subjective quality factor (SQF) are subjects ofupcoming investigations.
Other detailed experimental research with focus on input andoutput coupling efficiency and uniformity of the illumination overthe plate surface are a complex thematic in consideration of differ-ent structures, material and measurement methods, which will beperformed in near future.
6. Summary and outlook
In this work the fabrication process of a highly transparent lightemitting plate was developed. It is based on thick film photoresist
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lithography and casting technology with PDMS. The optimummicrostructures for a single-sided light emission and a high trans-parency are cylindrical with a diameter of 50 lm and height of42 lm.
A ratio of 3:1 between the pitch and the diameter and a ratio ofnearly 1:1 between height and diameter of the cylinders haveshown to be the best conditions for high single sided emission.The maximum of the emission happens under an angle of 150�.The emission on the unstructured backside is still significant andseems to be a problem of diffusion centres in PDMS. Reduction ofbackside emission to near zero – which is feasible according tothe simulation results – is one of the project goals.
The presented work describes research at PDMS prototypes. Thefinal goal of the project is enabling an industrial production of theproposed LGP in PMMA by injection moulding or hot embossing.This will result in less scattering and higher transparency but alsogreater demands on the unmoulding process.
Acknowledgement
The authors gratefully acknowledge the financial support fromAiF Projekt GmbH as part of a ZIM-Project.
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