Birefringence Properties of Polymeric Substrate Materials
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éfringence Properties of Polymeric Substrate Materials Ramesh M. Pisipati, Helmut Schmid, and Gùnther Kàmpf Introduction Depending upon the nature of the application, three materials currently dominate the optical substrate market today. Polycarbonate (PC), the most commonly used material, is typically used in disks with diameters up to
120 mm. For larger diameters, the inhérent biréfringence exhibited by PC interacts with the read laser and results in errors in the read-back signal. For 200 mm and greater, both polymethylmethacrylate (PMMA) and glass are used. Because PMMA is low in inhérent
Table 1. Spécial Requirements for Optical Data Storage Substrates. 1 Properties Optical Properties Light transmission at 630 nm Biréfringence (optical path différence, single path) Optical defects (size >200 um) Mechanlcal Properties Thickness (nominal Thickness tolérance Distortion Roughness on surface Rockwell hardness Impact strength Thermal Properties Vicat B Melt index Physlco-Chemlcal Properties Water absorption (at 100% RH) Water permeation coefficient (water, 23°C)
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M2-, M3-Birefringence [nm/mm] (average of |M2|- and |M3| - values; single pass) Figure 5. Dependence of Mlc and LFP values on primary values of Mlp and M2, M3.
injection pressures are employed to encourage fast, controllable injection rates. The full injection p r e s s u r e is never felt on a disk because the holding pressure is switched on as soon as the cavity fills. The effect of injection rate on biréfringence is seen in Figure 6. The m a g n i t u d e and duration of holding pressure are critical to disk quality. This combination is set so that a filled disk of proper dimension is produced at the
lowest possible level of molded-in stress, i.e., to minimize biréfringence and maximize stamper surface replication. If hold time is too long, high biréfringence will be produced near the center and midsection of the disk; if hold pressure is too high, flashing may occur producing a disk with high biréfringence over the entire disk. The effects of holding pressure and cooling time on biréfringence can be seen in Figure 6.
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Biréfringence Properties of Polymeric Substrate Materials
Influence of Injection Rate on Biréfringence
Influence of Melt Température on Biréfringence
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Biréfringence at 25-mm Radius
Maximum Allowable Level
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Biréfringence at25-mm Radius'
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Influence of Holding Pressure on Biréfringence 50
Influence of Cooling Time on Biréfringence 50
Maximum Allowable Level
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Biréfringence at 56-mm Radius
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