the solidified layer is delayed during the filling stage (Kim et al. 2004; Seong et al. 2001; Kang et al. 2000).
To control the stamper surface temperature, various methods have been adopted. Kim and Suh (1986), Liou and Suh (1989), and Kim and Niemeyer (1995) suggested an insulated mold insert to increase cavity wall temperature during polymer filling. Kang and coworkers (2002), Inoue et al. (2003), and Lee et al. (2003) replicated the optical disc substrates using an insulated stamper to improve the transcribability. Jansen and Flaman (1994) and Yao and Kim (2003) inserted an electric heating element into the injection mold to improve optical and geometrical properties. Also, Satoh and co-workers (2002) retarded the growth of the solidified layer using an IR laser.
Received: 29 June 2004 / Accepted: 10 January 2005 / Published online: 7 July 2005 Ó Springer-Verlag 2005
Abstract In this study, an injection mold equipped with a MEMS heater was designed and constructed to raise the stamper surface temperature over the glass transition temperature during the filling stage of the injection molding. First, high density optical disc substrates of ROM and rewritable types with a track pitch of 0.32 lm were replicated by conventional injection molding to analyze the effect of stamper surface temperature on the transcribability. Then the effect of heating on the replication process was simulated numerically. Based on the simulation results, we constructed the MEMS heater using joule heating and a MEMS RTD sensor. Finally, the replication quality of the substrate molded with the MEMS heater was compared with the case without MEMS heating.
Y. Kim Æ Y. Choi Æ S. Kang (&) School of Mechanical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul, 120-749, Korea E-mail: snlkang@yonsei.ac.kr Tel.: +82-2-21232829 Fax: +82-2-3622736
In this study, an injection mold equipped with a MEMS heater was designed and constructed to raise the stamper surface temperature over the glass transition temperature during the filling stage of the injection molding. Two different disc dimensions were used; the first case was a disc substrate with a diameter of 86 mm and a thickness of 1.1 mm, and the second case was a disc substrate with a diameter of 47 mm and a thickness of 0.4 mm. ROM and rewritable type high density optical disc substrates with a track pitch of 0.32 lm were replicated. The effect of heating on the replication process was simulated numerically. From the simulation results, the growth of the solidified layer and the viscosity were analyzed. Using the simulation results, we designed and constructed the MEMS resistance temperature detector sensor and the MEMS heater. Finally, the replication quality of the substrate molded with a MEMS heater was compared to the case without MEMS heating.
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the melting temperature of the polymer. To analyze the effect of the stamper surface temperature on replication quality, high density optical discs with a thickness of 1.1 mm and a diameter of 86 mm were fabricated at various mold temperatures. Optical grade polycarbonate, a common material for optical disc substrates, was used to replicate the substrate. Filling time and packing pressure were set to 0.1 s and 400 kgf/cm2. To raise the resin temperature to 300°C, the temperature distribution in the cylinder was divided into five sections. From the section nearest to the sprue, the temperature distribution was 300, 320, 360, 320, and 300°C. The mold temperatures were set to 90, 100, and 110°C to analyze the effect of stamper surface temperature. Figure 1 shows the AFM (atomic force microscope) images of stamper and molded substrates for a high density optical disc with a pitch of 0.32 lm. A stamper with a track pitch of 0.32 lm, a minimum pit length of 0.16 lm and a pit height of 85 nm was used. Figure 2 shows the height of pit and land-groove patterns of molded substrates for various mold temperatures. In the case of pit structures (ROM type), the height of molded patterns increased from 70 to 76 nm as mold temperatures increased from 90°C to 110°C. In the case of land-groove structures (rewritable type), patterns with heights of 26, 32.5, and 60.5 nm were replicated at mold temperatures of 90, 100, and 110°C, respectively. These results show that transcribability can be improved by increasing the mold temperature. However, considering that the height of the structures on the stamper is 85 nm, to guarantee the transcribability over 90%, the mold surface should be heated over the glass transition temperature (145°C) of polycarbonate to delay the growth of the solidified layer during the filling stage. There are many technical prob-