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1 Microsystem Technologies 10 2004 531 535 Springer Verlag 2004 DOI 10 1007 s00542 004 0387 2 Replication of microlens arrays by injection molding B K Lee D S Kim T H Kwon B K Lee D S Kim T H Kwon however flow rate has the similar effect to PC It might be reminded that packing time does not affect the replicabilityifa gate is frozen since frozen gate prevents material from flowing into the cavity Therefore the effect of packing time disappears after a certain time depending on the processing conditions Fig 4a c leftside Surface profiles of microlens PC with diameter of 300 m a effect of packing pressure b effect of flow rate ceffectofpacking time Fig 5a c rightside Surface profilesofmicrolens PMMA with diameter of 300 m a effect of packing pressure b effect of flow rate c effectof packing time 6 4 24 24 24 2 SurfaceSurfaceSurfaceSurface roughnessroughnessroughnessroughness Averaged surface roughness Ra values of 300 m diameter microlenses and the mold insert were measured by an atomic force microscope Bioscope AFM Digital Instruments The measurements were performed around the top of each microlens and the measuring area was 5 m 5 m Figure 6 shows AFM images and measured Ra values of microlenses PMMA replicas of microlens have the lowest Ra value 1 606 nm It may be noted that AFM measurement indicated that Ra value of injection molded microlens arrays is smaller than the corresponding one of the mold insert The reason for the improved surface roughness in the replicated microlens arrays is not clear at this moment but might be attributed to the reflow caused by surface tension during a cooling process It may be further noted that the Ra value of injection molded microlens arrays is comparable with that of fine optical components in practical use Fig 6 AFM images and averaged surface roughness Ra values of the mold insert and injection molded 300 mdiametermicrolenses a Nickel mold insert b PS c PMMA d PC 4 34 34 34 3 FocalFocalFocalFocal lengthlengthlengthlength The focal length of lenses can be calculated by a wellknown equation as follows 1 12 111 1 n fRR where f nl R1 and R2 are focal length refractive index of lens material two principal radii of curvature respectively For instance focal lengths of the molded microlenses were approximately calculated as 1 065 mm with R10 624 mm and R2 for 200 m diameter microlens 1 130 mm with R1 0 662 mm and R2 for 300 m microlens and 2 580 mm with R1 1 512 mm and R2 for 500 m microlens according to Eq 1 These calculations were based on an assumption that microlenses are replicated with PC nl 1 586 and have the identical shape of the mold insert It might be mentioned that the geometry of themolded microlens might be inversely deduced from an experimental measurement of the focal length 5 5 5 5 ConclusionConclusionConclusionConclusion The replication of microlens arrays was carried out by the injection molding process with the nickel mold insert which was 7 electroplated from the microlens arrays master fabricated via a modified LIGA process The effects of processing conditions were investigated through extensive experiments conducted with various processing conditions The results showed that the higher packing pressure or the higher flow rate is the better replicability is achieved In comparison the packing time was found to have little effect on the replication of microlens arrays The injection molded microlens arrays had a smaller averaged surface roughness values than the mold insert which might be attributed to the reflow induced by surface tension during the cooling stage And PMMA replicas of microlens arrays had the best surface quality i e the lowest roughness value of Ra 1 606 nm The surface roughness of injection molded microlens arrays is comparable with that of fine optical components in practical use In this regard injection molding might be a useful manufacturing tool for mass production of microlensarrays ReferencesReferencesReferencesReferences 1 Ruther P Gerlach B Go ttert J Ilie M Mu ller A O mann C 1997 Fabrication and characterization of microlenses realized by a modified LIGA process Pure Appl Opt 6 643 653 2 Popovic ZD Sprague RA Neville Connell GA 1988 Technique for monolithic fabrication of microlens array Appl Opt27 1281 1284 3 Beinhorn F Ihlemann J Luther K Troe J 1999 Micro lens arrays generated by UV laser irradiation of doped PMMA Appl Phys A68 709 713 4 Moon S Lee N Kang S 2003 Fabrication of a microlens array using micro compression molding with an electroformed mold insert J Micromech Microeng 13 98 103 5 Ong NS Koh YH Fu YQ 2002 Microlens array produced using hot embossing process Microelectron Eng 60 365 379 6 Lee S K Lee K C Lee SS 2002 A simple method for microlens fabrication by the modified LIGA process J Micromech Microeng 12 334 340 7 Kim DS Yang SS Lee S K Kwon TH Lee SS 2003 Physical modeling and analysis of