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1 Microsystem Technologies 10 (2004) 531535 _ 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.4ac(leftside).Surface profiles of microlens (PC with diameter (/) of 300 m). a effect of packing pressure, b effect of flow rate,ceffectofpacking time Fig.5ac.(rightside)Surface profilesofmicrolens (PMMA with diameter(/) of 300m). 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 300m 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 5m 5m. 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 200m 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; Gottert J; Ilie M; Muller A; Omann C (1997) Fabrication and characterization of microlenses realized by a modified LIGA process. Pure Appl Opt 6: 643653 2. Popovic ZD; Sprague RA; Neville Connell GA (1988) Technique for monolithic fabrication of microlens array. Appl Opt27: 12811284 3. Beinhorn F; Ihlemann J; Luther K; Troe J (1999) Micro-lens arrays generated by UV laser irradiation of doped PMMA. Appl Phys A68: 709713 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: 98103 5. Ong NS; Koh YH; Fu YQ (2002) Microlens array produced using hot embossing process. Microelectron Eng 60: 365379 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: 334340 7. Kim DS; Yang SS; Lee S-K; Kwon TH; Lee SS (2003) Physical modeling and analysis of m