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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 (&)
Department of Mechanical Engineering,
Pohang University of Science and Technology (POSTECH),
San 31, Hyoja-Dong, Nam-Gu, Pohang, 790-784, Korea
e-mail: thkwon@postech.ac.kr
Abstract Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varyingprocessing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in practical use.
1
Introduction
Microoptical products such as microlenses or microlens arrays have been used widely in various fields of microoptics, optical data storages, bio-medical applications, display devices and so on. Microlenses and microlens arrays are essential elements not only for the practical applications but also for the fundamental studies in the microoptics. There have been several fabrication methods for microlenses or microlens arryas such as a modified LIGA process [1], photoresist reflow process [2], UV laser illumination [3], etc. And the replication techniques, such as injection molding, compression molding [4] and hot embossing [5], are getting more important for a mass production of microoptical products due to the cost-effectiveness. As long as the injection molding can replicate subtle microstructures well, it is surely the most cost-effective method in the mass production stage due to its excellent reproducibility and productivity.
In this regard, it is of utmost importance to check the injection moldability and to determine the molding processing condition window for proper injection molding of microstructures. In this study, we investigated the effects of processing conditions on the replication of microlens arrays by the injection molding. The microlens arrays were fabricated by a modified LIGA process, which was previously reported in [6, 7]. Injection molding experiments were performed with an electroplated nickel mold insert so as to investigate the effects of some processing conditions. The surface profiles of molded microlens arrays were measured, and were used to analyze effects of processing conditions. Finally, a surface roughness of microlens arrays was measured by an atomic force microscope (AFM).
2
Mold insert fabrication
Microlens arrays having several different diameters were fabricated on a PMMA sheet by a modified LIGA process [6]. This modified LIGA process is composed of an X-ray irradiation on the PMMA sheet and a subsequent thermal treatment. The X-ray irradiation causes the decrease of molecular weight of PMMA, which in turn decreases the glass transition temperature and consequently causes a net volume increase during the thermal cycle resulting in a swollen microlens [7]. The shapes of microlenses fabricated by the modified LIGA process can be predicted by a method suggested in [7].
The microlens arrays used in the experiments were composed of 500μm -(a 2 × 2 array), 300μm -(2 × 2) and 200μm (5 × 5) diameter arrays, and their heights were 20.81, 17.21 and 8.06 μm, respectively. Using the microlens arrays fabricated by the modified LIGA process as a master, a metallic mold insert was fabricated by a nickel electroplating for the injection molding. Typical materials used in a microfabrication process, such as silicon, photoresists or polymeric materials, cannot be directly used as the mold or the mold insert due to their weak strength or thermal properties. It is desirable to use metallic materials which have appropriate mechanical and thermal properties to endure both a high pressure and a large temperature variation during the replication process. Therefore, a metallic mold insert is being used rather than the PMMA master on silicon wafer for mass production with such replication techniques. Otherwise special techniques should be adopted as a replication method, e.g. a low pressure injection molding [8].
The size of final electroplated mold insert was 30 × 30 × 3 mm. The electroplated nickel mold insert having microlens
arrays is shown in Fig. 1.
Fig.1.Moldinsert fabricated by a nickel electroplating (a) Real view of the mold insert (b) SEM image of 200 μm diameter microlens array (c) SEM image of 300 μmdiameter microlens array
3
Injection molding experiments
A conventional injection molding machine (Allrounders 220 M, Arburg) was used in the experiments. A mold base for the injection molding was designed to fix the electroplated nickel mold insert firmly with the help of a frametype bolster plate (Fig. 2). Shape of aperture of the bolster plate (in this study, a rectangular one) defines the outer geometry of the molded part on which the profiles of microlens arrays are to be transcribed. The mold base itself has delivery systems such as sprue, runner and gate which lead the molten polymer to the cavity formed by the bolster plate, the mold insert and amoving mold surface. The mold base was designed such that mold insert replacement is simple and easy. Of course, one may introduce an appropriate bolster plate with a specific aperture shape.
Fig. 2. Mold base and mold insert used in the injection molding experiment
The injection molding experiments were carried out with three general polymeric materials – PS (615APR, Dow Chemical), PMMA (IF870, LG MMA) and PC (Lexan 141R, GE Plastics). These materials are quite commonly used for optical applications. They have different refractive indices (1.600, 1.490 and 1.586 for PS, PMMA and PC, respectively), giving rise to different optical properties in final products, e.g. different foci with the same geometry.
The injectionmolding experiments were performed for seven processing conditions by changing flow rate, packing pressure and packing time for each polymeric material. Furthermore, same experiments were repeated three times for checking the reproducibility. It may be mentioned that the mold temperature effect was not considered in this study since the temperature effect is relatively less important for these microlens arrays due to their large radius of curvature than other microstructures of high aspect ratio. For high aspect ratio microstructures, we are currently investigating the temperature effect more closely and plan to report separately in the future. Therefore, flow rate, packing pressure and packing time were varied to investigate their effects more thoroughly with the mold temperature unchanged in this study. Table 1 shows the detailed processing conditions for three polymeric materials. Other processing conditions were kept unchanged during the experiment. The mold temperatures were set to 80, 70 and 60 _C for PC, PMMA and PS, respectively.
It might be mentioned that we carried out the experiments without a vacuum condition in the mold cavity considering that the large radius of curvature of the microlens arrays in the present study will not entrap air in the microlens cavity during the filling stage.
Table 1. Detailed processing conditions used in the injection molding experiments
Case
Flow rate (cc/sec)
Packing time (sec)
Packing pressure(MPa)
1
12.0
5.0
10.0
2
12.0
5.0
15.0
3
12.0
5.0
20.0
PS
4
12.0
2.0
10.0
5
12.0
10.0
10.0
6
18.0
5.0
10.0
7
24.0
5.0
10.0
PMMA
1
6.0
10.0
10.0
2
6.0
10.0
15.0
3
6.0
10.0
20.0
4
6.0
5.0
10.0
5
6
7
6.0
9.0
12.0
15.0
10.0
10.0
10.0
10.0
10.0
PC
1
6.0
5.0
5.0
2
6.0
5.0
10.0
3
5
6.0
6.0
9.0
5.0
10.0
15.0
5.0
6
5.0
5.0
7
12.0
5.0
5.0
4
Results and discussion
Before detailed discussion of the experimental results, it might be helpful to summarize why flow rate, packing
pressure and packing time (which were chosen as processing conditions to be varied in this study) affect thereplication quality. As far as the flow rate is concerned, there may exist an optimal flow rate in the sense that too small flow rate makes too much cooling before a complete filling and thus possibly results in so-called short shot phenomena whereas too high flow rate increases pressure fields which is undesirable.
The packing stage is generally required to compensate for the volume shrinkage of hot molten polymer when
cooled down, so that enough material should flow into a mold cavity during this stage to control the dimensional
accuracy. The higher the packing pressure, the longer the packing time, more material tends to flow in. However, too much packing pressure sometimes may cause uneven distribution of density, thereby resulting in poor optical
quality. And too long packing time does not help at all since gate will be frozen and prevent material from flowing into the cavity. In this regard, one needs to investigate the effects of packing pressure and packing time.
4.1
Surface profiles
Figure 3 shows typical scanning electron microscope (SEM) images of the injection molded microlens arrays for different diameters for PMMA (a) and different materials (b). Cross-sectional surface profiles of the mold insert and all the injection molded microlens arrays were measured by a 3D profile measuring system (NH-3N, Mitaka).
Fig. 3. SEM images of the
injection molded microlens
arrays and microlenses (a)
Injection molded microlens
arrays (PMMA) (b) Injection
molded microlenses of 300 μmdiameter for different materials
As a measure of replicability, we have defined a relative deviation of profile as the height difference between the molded one and the corresponding mold insert for each microlens divided by the mold insert one. The computed relative deviations for all the microlenses are listed in Table 2.
Diameter ( μm)
Relative deviation (%)
1
2
3
4
5
6
7
PS
200
300
500
-7.62
5.86
2.38
-7.59
2.03
-0.38
2.08
2.86
0.51
-
5.61
1.47
-8.66
6016
1.47
-11.44
4.29
1.47
-
5.73
1.95
PMMA
200
300
500
7.20
5.77
-0.66
1.31
5.60
-1.62
-3.88
6.45
3.98
-5.80
5.95
2.80
-0.97
5.95
-0.72
-8.53
6.68
-0.90
4.86
-2.62
-0.72
PC
200
300
500
23.02
6.20
-0.93
16.05
4.96
5.09
16.87
2.66
-1.86
19.66
4.53
1.88
33.97
4.78
6.96
18.67
1.79
2.43
-2.94
4.15
-1.55
It may be mentioned that the moldability of polymeric materials affects the replicability. Therefore, the overall relative deviation differs for three polymeric materials used in this study. It may be noted that PC is the most difficult material for injection molding amongst the three polymers. The largest relative deviation can be found in PC for the smallest diameter case, as expected. In that specific case, the largest value is corresponding to the low flow rate and low packing pressure. Packing time in this case does not significantly affect the deviation. The relative deviation for PS and PMMA with the smallest diameter is far better than PC case.
Table 2 indicates that the larger the diameter, the smaller the relative deviation. The larger diameter microlens is, of course, easier to be filled than smaller diameter during the filling stage and packing stage. Microlenses of larger diameters were generally replicated well regardless of processing conditions and regardless of materials. The best replicability is found for the case of PS with 500 μm diameter. Generally, PS has a good moldability in comparison with PMMA and PC.
It may be mentioned that some negative values of relative deviation were observed mostly in the smallest diameter case for PS and PMMA according to Table 2. In these cases, however, the absolute deviation is an order of 0.1 μm in height, which is within the measurement error of the system. Therefore, the negative values could be ignored in interpreting the experimental data of replicability.
Surface profiles of microlens of 300 μm diameter are shown in Figs. 4 and 5 for PC and PMMA, respectively. As shown in Fig. 4, the higher packing pressure or the higher flow rate results in the better replication of microlens for the case of PC, as mentioned above. Packing time has little effect on the replication for these cases. For the case of PMMA, the packing pressure and packing time have insignificant effect as shown in Fig. 5; however, flow rate has the similar effect to PC. It might be reminded that packing time does not affect the replicability if a 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 μm). a effect of packing pressure, b effect of flow rate, c effectof packing time
Fig.5a–c.(rightside)Surface profiles of microlens (PMMA with diameter(/) of 300 μm). a effect of packing pressure, b effect of flow rate,c effect of packing time
4.2
Surface roughness
Averaged surface roughness, Ra, values of 300 μm 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 μm · 5 μm. 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 μm diameter microlenses. a Nickel mold insert, b PS, c PMMA, d PC
4.3
Focal length
The focal length of lenses can be calculated by a wellknown equation as follows:
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 R1 0.624 mm and R2 11 ￥) for 200 μm diameter microlens, 1.130 mm (with R1= 0.662 mm and R2=∞) for 300 μm microlens and 2.580 mm (with R1=1.512 mm and R2=∞) for 500 μm 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 the molded microlens might be inversely deduced from an experimental measurement of the focal length.
5
Conclusion
The replication of microlens arrays was carried out by the injection molding process with the nickel mold insert which was 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.
References
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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 microlens formation fabricated by a modified LIGA process. J Micromech Microeng 13: 523–531
8. Bauer W; Knitter R; Emde A; Bartelt G; Go¨hring D; Hansjosten E (2002) Replication techniques for ceramic microcomponents with high aspect ratio. Microsyst Technol 7: 85– 90
微透鏡陣列注塑成型的復(fù)制
B.-K. Lee, D. S. Kim, T. H. Kwon
樸航科技大學(xué)(POSTECH) 機(jī)械工程學(xué)院
San 31, Hyoja-Dong, Nam-Gu, Pohang, 790-784, Korea
電子郵箱l: thkwon@postech.ac.kr
摘要 微透鏡陣列注塑成型，可作為一種非常重要的大量生產(chǎn)技術(shù)。因此我們?cè)诮鼇淼难芯恐蟹浅ｊP(guān)注， 為了進(jìn)一步了解注塑成型在不同的加工條件下對(duì)可復(fù)制的微透鏡陣列剖面的影響，如流量、填料壓力和填料時(shí)間，對(duì)3種不同的高分子材料(PS，PMMA和PC)進(jìn)行了大量的試驗(yàn)。 鎳金屬模具嵌件微陣列就是利用改良的LIGA技術(shù)電鍍主裝配的顯微結(jié)構(gòu)制造的。在表面輪廓得到測(cè)量的前提下，研究工藝條件對(duì)可復(fù)制的微透鏡陣列的影響。實(shí)驗(yàn)結(jié)果表明， 填料壓力和流速對(duì)注射模塑的終產(chǎn)品的表面輪廓有重要的影響。 原子力顯微鏡測(cè)量表明， 微透鏡陣列注塑成型的平均表面粗糙度值小于模具嵌件成型， 并在實(shí)際運(yùn)用中，能與精細(xì)的光學(xué)元件相媲美。
1 說明
微型光學(xué)產(chǎn)品，如微透鏡或微透鏡陣列已廣泛應(yīng)用于光學(xué)數(shù)據(jù)存儲(chǔ)、生物醫(yī)學(xué)、顯示裝置等各個(gè)光學(xué)領(lǐng)域。微透鏡和微透鏡陣列不僅在實(shí)踐應(yīng)用上，而且在微型光學(xué)的基礎(chǔ)研究上都是非常重要的。有幾種微透鏡或微透鏡陣列的制作方法，如改良的LIGA技術(shù)[1] ，光阻回流進(jìn)程[2]，紫外激光照射[3]等。還有復(fù)制技術(shù)，如注塑模壓成型[4]和熱壓[5]技術(shù) ，這種方法對(duì)于減少大規(guī)模生產(chǎn)的微型光學(xué)產(chǎn)品的成本尤為重要。由于其優(yōu)越的生產(chǎn)和再生產(chǎn)能力，只要注塑成型過程中能很好的復(fù)制微觀結(jié)構(gòu)，那么肯定是最適合于降低大量生產(chǎn)成本的方法。
基于這點(diǎn)，檢查注塑成型能力并確定成型加工條件是注塑成型微觀結(jié)構(gòu)過程中最重要的步驟。在本次研究中，我們考察了工藝條件對(duì)可復(fù)制的微透鏡陣列的注射成型的影響。微透鏡陣列是用之前介紹過[6，7]的改良的LIGA技術(shù)來編制的。注塑成型實(shí)驗(yàn)采用的是一種鍍鎳金屬模具，來探討了幾種不同工藝條件對(duì)成型的影響。通過對(duì)微透鏡陣列的表面輪廓測(cè)量，用來分析工藝條件產(chǎn)生的影響。最后，利用原子力顯微鏡(AFM)測(cè)量微透鏡的表面粗糙度值的大小。
2 模具嵌件的制造
利用改良的LIGA技術(shù)[6]，在一個(gè)有機(jī)玻璃板上制造出具有幾種不同直徑微透鏡陣列。此種技術(shù)是先用X光照射有機(jī)玻璃板，然后再進(jìn)行熱處理兩部分構(gòu)成的。X-射線照射引起有機(jī)玻璃分子質(zhì)量的減少，同時(shí)降低了玻璃化轉(zhuǎn)變溫度，并因此導(dǎo)致凈含量的增加，在熱循環(huán)的作用下，微透鏡發(fā)生微膨脹[7]。利用[7]中提出的方法，結(jié)合改良的LIGA技術(shù)可以預(yù)測(cè)微透鏡形狀的變化過程。
在試驗(yàn)中使用的微透鏡陣列，有500μm (2×2陣列)，300μm (2×2)和200μm (5×5)的直徑陣列，高分別是20.81μm，17.21μm和8.06μm。采用改良的LIGA技術(shù)制造微透鏡陣列作為一個(gè)主要的技術(shù)，用來制作鍍鎳的金屬模具的注塑成型。另一些特殊材料，因?yàn)樗鼈兊膹?qiáng)度不夠或熱性能差而不能直接進(jìn)行微細(xì)加工，當(dāng)作模具或金屬模具使用，如硅、光阻劑或高分子材料。盡量使用具有良好機(jī)械性能和熱性能的金屬材料，因?yàn)樗鼈兡茉诳蓮?fù)型加工過程中經(jīng)受高壓力和不斷變化的溫度。因此，為了利用這種復(fù)制技術(shù)進(jìn)行大批量生產(chǎn)，我們選擇使用金屬模具材料而不是有機(jī)玻璃硅晶體。一些特殊技術(shù)，如低壓注塑成型[8]技術(shù)，應(yīng)該作為良好的復(fù)制加工方法被采納。
電鍍模具的最終大小為30 mm×30 mm×3mm。鍍鎳金屬模具所具有的微透鏡陣列如圖1所示。
圖1 鍍鎳模具嵌件的制造 （a）直接觀察；（b）直徑為200μm
的微透鏡陣列電子顯微鏡圖像；（c）直徑為300μm的微透鏡陣列電子顯微鏡圖像
3 注塑成型實(shí)驗(yàn)
傳統(tǒng)注塑機(jī)(Allrounders 220 M，Arburg)多用做實(shí)驗(yàn)機(jī)。注塑模具設(shè)計(jì)的模架就是利用一塊框形支撐板固定鍍鎳模具(如圖2所示)。