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This study describes the brightness field distributions of microlens arrays fabricated by micro injection molding (μIM) and micro injection-compression molding (μICM). The process for fabricating microlens arrays used room-temperature imprint lithography, photoresist reflow, electroforming, μIM, μICM, and optical properties measurement. Analytical results indicate that the brightness field distribution of the molded microlens arrays generated by μICM is better than those made using μIM. Our results further demonstrate that mold temperature is the most important processing parameter for brightness field distribution of molded microlens arrays made by μIM or μICM.
©2010 Optical Society of America
1. Introduction
In recent years, microlenses, especially microlens arrays, have become an important optical component in many fields such as optical fibers, optical data storage, flat panel displays and optical communication.
Many production methods have been developed for fabricating microlens arrays, including the microplastic embossing process applied to a microlens array mold [1], replication of microlens arrays by injection molding with an Ni mold fabricated using the modified LIGA method [2], the thermal reflow sol-gel method [3], femtosecond photopolymerization process [4], self-patterning and digital holography method [5], hybrid extrusion rolling embossing [6], gas-assisted embossing [7], UV molding method [8], and the micro dispensing process [9].
Among these methods, hot embossing, rolling embossing, gas-assisted embossing, compression molding and injection molding are regarded as low-cost mass-production processed suited to replicating microlenses and microlens arrays. However, E-beam, focused ion beam and modified LIGA techniques are expensive, complicated and not easily accessible to scientists and industrialists.
This work fabricates microlens arrays using a novel and effective procedure. A microlens array master is generated using room-temperature imprint lithography and the thermal reflow process. Electroforming is then applied to fabricate the Ni mold insert from the microlens arrays master. Finally, micro injection molding (μIM) and micro injection-compression molding (μICM) are utilized to fabricate the microlens arrays.
An experimental study is conducted to characterize the effects of different processing parameters on micro-molding of molded microlens arrays. The micro-molding processes in this work are μIM and μICM. The optical properties of molded microlens arrays are measured and analyzed.
2. Experimental approach
The microlens arrays mold insert is electroformed from a master whose surface was patterned with a microlens array structure (Fig. 1 ). The original microlens design had a diameter of 150 μm, height of 11.35 μm and pitch of 200 μm. The mold insert is installed in a fan-gated mold base for micro molding. The mold has two cavities symmetrically located on opposite sides of the sprue. One cavity has microlens arrays, the other has none. The detailed steps in mold insert fabrication are as follows.
Room-temperature imprint lithography is applied in the first step. A silicon mold insert with micro-hole arrays is fabricated by conventional photolithography and deep reactive ion etching (DRIE). Figure 2 shows these steps.
In conventional photolithography, patterns on a mask are transferred onto a photoresist layer on top of a (100)-oriented silicon wafer. The SiO2 layer is thermally grown on top of the (100)-oriented silicon wafer. A 1.5-μm-thick AZ 5214 positive photoresist is spun over the wafers at 4000 rpm followed by a softbake at 100°C for 2 min. The wafer is then exposed through a mask with circular holes for 10 sec. The resist patterns are then developed using AZ 400k developer, diluted to a ratio of 1:4 with de-ionized (DI) water, followed by a thorough rinse in DI water. Following the definition of resist patterns, the wafer is baked in an oven at 120°C for an additional 15 min to harden the resist and form feature patterns. Second, an etching mask is fabricated by etching a silicon dioxide layer via selective reactive ion etching (RIE). The thermoplastic photoresist layer is then coated onto a substrate, which is then pressed against by the silicon mold insert at room temperature and then baked in a vacuum. After the silicon mold insert is peeled off the substrate, circle columns array photoresist patterns are obtained.
The second process is photoresist thermal reflow (Fig. 3 ). After the circle columns array photoresist structures are patterned on the substrate, the substrate is heated to a temperature slightly higher than the glass-transition temperature of the photoresist. The surface tension effect transforms the photoresist column surface into a spherical profile. The master of the photoresist microlens array is available. The final step is gold-coating and Ni-electroforming (Fig. 3). The 50-nm-thick gold seed is deposited on the microlens arrays master. This gold-coated master with the microlens arrays structure is then placed into an electroforming bath for several hours to form a 5-mm-thick Ni mold insert. Figure 4 shows the CCD image of the Ni mold insert.
The μIM and μICM processes are carried out using a commercial injection molding machine (220S, Arburg). The optical grade (PC) material (H3000R, Mitsubishi) is used for μIM and μICM. Figure 5 shows a CCD image of molded microlens arrays produced by μICM.
Identifying the effects of different process parameters on the optical quality of molded microlens arrays is extremely important. Four different process parameters—mold temperature, injection speed, packing pressure and melt temperature—are selected as factors for evaluating μIM. The process parameters of μICM are mold temperature, injection speed, compression speed and melt temperature. Table 1 lists the process parameters and parameter levels selected for the principal experiment.
Table 1. Parameters and levels selected in the main experiment. (μIM/μICM)
This work uses a He-Ne laser projected onto the nine points of the mold microlens array and forms the diffraction diagram on the screen. This study uses a CCD to capture the nine diagrams (Fig. 6 ). The diagram is 640 × 840 pixels. The color photos of these nine diagrams are transformed in to a grey scale diagram using MATLAB software. This study analyzes the brightness field of each pixel point on the grey scale diagram and conducts a histogram analysis. Analytical results of histogram analysis plot the curve of the histogram distribution. If the distributions of nine curves are close to each other, the brightness field is uniform for molded microlens arrays. Figure 7 shows the CCD image of an optical photograph of molded microlens arrays. The brightness field distribution is related to the pixel point (Fig. 8 ). This study discusses the brightness field distribution of molded microlens arrays with different processing parameters for different micro moldings. In this research, the molded microlens arrays as the light guiding plate of liquid crystal display (LCD). The light uniform is very important on LCD. The brightness field distribution of molded microlens arrays is the key point for optical analysis on this study.
3. Results and discussion
Figure 9 shows the brightness fields at different mold temperatures during μIM. Analytical results indicate that the distributions of the brightness field are not uniform and the brightness field distributions are uniform at high mold temperatures. The reason is that the plastic melt front of microlens arrays cannot fill the mold cavity due to the high viscosity of plastic melt at low mold temperatures. Figure 10 shows the brightness fields at different melt temperatures during μIM. The experimental results demonstrate that the brightness field distributions are very similar regardless of different melt temperatures. That is, the brightness field of molded microlens arrays is independent of melt temperature. Figures 11 and 12 show the brightness fields of molded microlens arrays at different injection speeds and packing pressures during μIM. The distributions of brightness fields of molded microlens arrays are uniform despite different injection speeds and different packing pressures. In summary, mold temperature is the most important processing parameter for the brightness field of molded microlens arrays during μIM.
Fig. 9 The brightness field for different mold temperatures on μIM. (a) Mold temp. = 100°C (b) Mold temp. = 120°C.
Fig. 10 The brightness field for different melt temperatures on μIM. (a)Melt temp. = 300°C (b) Melt temp. = 320°C.
Fig. 11 The brightness field for different injection speeds on μIM. (a)Injection speed = 160mm/s (b) Injection speed = 200mm/s.
Fig. 12 The brightness field for different packing pressures on μIM. (a)Packing pressure = 160MPa (b)Packing pressure = 200MPa.
Figure 13 shows the brightness fields of molded microlens arrays for different mold temperatures during μICM. The distributions of brightness fields of molded microlens arrays are not uniform at low mold temperatures and they change to uniform when mold temperatures increase. This is because low mold temperatures increase the viscosity of the plastic melt front; consequently, the plastic melt of molded microlens arrays cannot easy fill the mold cavity. Thus, the molded microlens arrays cannot form the original design shape, and the brightness field of molded microlens arrays cannot reach the value of the original design. Thus, the distributions of brightness fields of molded microlens arrays are not uniform. Figures 14 and 15 show the brightness fields of molded microlens arrays at different melt temperatures and injection speeds during μICM. The distributions of brightness fields of molded microlens arrays do not change regardless of different melt temperatures and different injection speeds during μICM. Figure 16 shows the brightness fields of molded microlens arrays at different compression speeds during μICM. The distributions of the brightness fields of molded microlens arrays are not uniform under low compression speeds and change to very uniform at high compression speeds. The high compression speed helps the plastic melt of molded microlens arrays fill the mold cavity and form the shape of molded microlens arrays. The distributions of the color photo brightness fields are very uniform under high compression speeds. In summary, mold temperature and compression speed are the most important processing parameters for the brightness fields of molded microlens arrays during μICM.
Fig. 13 The brightness field for different mold temperatures on μICM. (a)Mold temp. = 100°C (b)Mold temp. = 120°C.
Fig. 14 The brightness field for different melt temperatures on μICM. (a)Melt temp. = 300°C, (b)Melt temp. = 320°C.
Fig. 15 The brightness field for different injection speeds on μICM. (a)Injection speed = 160mm/s (b)Injection speed = 200mm/s.
Fig. 16 The brightness field for different compression speeds on μICM. (a)Compression speed = 8mm/s (b)Compression speed = 24mm/s.
Figures 9 and 13 show the brightness field distributions for different mold temperatures during μIM and μICM, respectively. The brightness field distributions of molded microlens arrays for a mold temperature at 100°C during μICM is more uniform than that during μIM.
4. Conclusions
This work developed a novel and effective method for fabricating a microlens arrays master using room-temperature imprint lithography and the thermal reflow process. This microlens array master is used to electroform the nickel mold insert with microlens array cavities. The μIM and μICM processes are then used to transfer microlens arrays structures from the mold insert. Mold temperature is the most important process factor for the brightness field distributions of molded microlens using PC material. These experimental results show that the μIM and μICM processes have great potential for mass production of microlens arrays.
The findings of this study indicate that the replication and surface roughness of microfluidic chips are influenced by silicon mold insert for micro-hot embossing. The results showed that silicon mold inserts are effective for replicating molded microfluidic chips. Embossing temperature is the most important factor for replicating molded microfluidic chips on different processing parameters. Optimal parameters for processing molded microfluidic chips are embossing temperature 190 °C, embossing pressure 3.125 MPa, embossing time 120 s and de-molding temperature 80 °C.
De-molding temperature is the most important factor in the surface roughness of molded microfluidic chips under varying processing parameters. The experimental results also indicated that the hardness of the silicon mold insert has the good value. The silicon mold insert must have low roughness and high hardness. Bonding temperature is the most important factor in the bonding strength of sealed microfluidic chips.
Acknowledgements
The authors would like to thank the National Science Council of the Republic of China, Taiwan for supporting this research under Contract No. NSC 93-2218-E-262-001. The authors also would like to thank the Taipei Medical University, Taiwan for supporting this research under Contract No. TMU96-AE1-B17.
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