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In order to get a small-sized pit, the reaction threshold of a photoresist is enhanced by increasing the prebake temperature and time. With the improved photoresist, a pit size corresponding to or even smaller than the minimum pit on a blue laser disc can be obtained on an industrial DVD product line. The improved photoresist may serve as the master material of future multilevel blue laser discs, which might provide a potential solution for the fabrication of multilevel blue laser discs.
©2007 Optical Society of America
1. Introduction
Compared to a DVD, a blue laser disc (HD-DVD or Blu-ray) increases disc capacity with a decrease in pit size. For example, the minimum pit length or width of a DVD is about 400 nm (3T), while those of HD-DVDs and Blu-ray discs are about 200 and 150 nm (2T), respectively. Multilevel (ML) technology can increase the capacity of a disc without changing the optical and mechanical units [1] and is compatible to other technology such as run-length-limited (RLL) or even reducing RLL channel-bit length technology. ML technology has been used on DVDs to increase the ML-DVD capacity [2–4]. For a read-only disc, the reflection signal increases when the pit volume decreases, the ML-DVD reduces the pit to a multistage pit volume, and Multi-level reflection signal can be obtained [2–4]. Therefore, the multistage pits on ML-DVD discs are smaller than the pits on common DVD discs.
In this paper, we try to use the ML technology on a blue laser disc to increase the disc capacity further. Similar to a ML-DVD, the pit size (width and depth) on a ML blue laser disc is also smaller than that on a conventional blue laser disc. Therefore, in making a ML blue laser disc we must find a way to make pits even smaller, and shaping the accuracy of the ML patterns is the key point of the ML blue laser disc.
The mastering process is the core of the fabrication of a read-only disc. After applying a special coating of photoresist on a glass substrate, a Laser Beam Recorder (LBR) is used to record data onto the glass master. Once formed, the glass master is eventually metallized to produce the metallic master, which has the negative impression of the disc data and can be used to do actual disc replication via the process of injection molding of molten polycarbonate.
At present, the mastering of a blue laser disc requires a short-wavelength laser (for example, an ultraviolet laser) and a high-resolution mastering material, so the mastering cost of a blue laser disc is much more expensive than that of a DVD. It sounds reasonable then that the mastering of a ML blue laser disc may have more rigid requirements for the mastering equipment. In this paper, we try to explore the possibility of fabricating a ML blue laser disc using an industrial DVD product line. The key issue in the fabrication of a ML blue laser disc is how to make the master disc. In order to obtain the fine structure patterns of a ML blue laser disc, both the exposing laser power and the rate of photochemistry are reduced. Some physical processes are done during the mastering process; for instance, raising the temperature or extending the time of preprocessing to make the solvent vaporize so as to reduce the rate of photochemistry, and then the exposure volume on the photoresist decreases with both the depth and width of the pit decreased. In our experiment, using the improved photoresist, a minimum pit with a width and depth of 140 and 23 nm is obtained on a commercial DVD mastering system with a laser wavelength of 405 nm, which might largely reduce the fabrication cost and provide a potential approach to fabricating future ML blue laser discs.
2. Model and simulation
2.1 Model
In this paper, we use the model in [2,5–7] as the conclusion in [2]; the depth of any point on the photoresist surface after development is given by
where, h(x, y) is the depth of dissolved film thickness, T is the development time, and the exposure dose E(x, y) can be calculated from Eq. (2):
where I(x, y, z, t) is the exposure intensity.
Based on Eq. (1), we give some discussion for the characterization change of the photoactive compound (PAC). In this paper, the PAC consists of several DiazoNaphthoQuinone (DNQ) moieties bound to a common ballast group, i.e., a poluhydroxybenzophenone via a sulfonic acid esterlinkage. Prebaking in a high temperature (exceeding 120°) makes the solvent (2-methoxy-1-methyl-ethyl acetate) vaporize so as to reduce the rate of photochemistry. As a result, the activity of the PAC is depressed, the effective exposure threshold is elevated, and the effective exposure intensity I is reduced correspondingly. As a result, the exposure volume on the photoresist decreases with both the depth and width of the pit decreasing. A new parameter a is introduced to express the threshold, and I’ is the effective exposure intensity:
Here, I is the normalized intensity [Fig. 1(a)], a is from 0 to 1, and for the prebaked PAC the threshold a is elevated, as shown in Fig. 1(b).
Fig. 1. (a) 3D view of normalized light intensity [2]. (b) Sketch map of normalized light intensity and threshold a. The x-axis is on the focal plane with units of nm; the y-axis is the normalized light intensity (1 stands for maximum intensity).
2.2 Simulation
In this paper, the model and the experiment equipment are the same as those in [2], so we chose the same parameters. The correctness and accuracy of the model was demonstrated in [2].
Table 1 shows the size of standard DVD disc pits. Table 2 shows the parameters in the model for the simulation of a standard DVD.
Table 1. Size of standard DVD disc pits
Table 2. Simulation parameters
In Eq. (2), I is replaced by I’ as expressed in Eq. (2). The threshold parameter a is added in our model as in Eq. (3), which is different from the work in [2].
On a ML-DVD disc [2–4], the multilevel pit is achieved by reducing the pit size, as both the pit width and depth of multistage pits are smaller than those of common DVDs. Similarly, the multistage pits on a ML blue laser disc are also smaller than those on a common blue laser disc. For example, the minimum pit dimension in a HD-DVD is 2T, with the pit width and depth at about 200 and 80 nm, respectively. Therefore, to achieve a ML blue laser disc, we must try to obtain pits even smaller than the minimum pit size on a blue laser disc.
In the following simulation, we try to change some parameters in order to get small-sized pits.
- Changing the mastering laser power. We change the normalized laser power from 1 (power for the mastering of a standard DVD disc) to 0.5. The relation between the normalized laser power I and the depth (and width) is shown in Fig. 2.
Figure 2 shows that when reducing the laser power, both the pit depth and width decrease, but the pit depth decreases faster than width. Therefore, solely reducing the laser power may not be enough to get a satisfactory multistage pit.
- Elevating threshold a. We change threshold a from 0.0 to 0.4, with normalized power I=1. The relationship of pit width (depth) vs. threshold a is shown in Fig. 3.
Figure 3 shows that the variation of the threshold parameter a greatly affects the pit size (depth and width). When threshold a is elevated, both the pit width and depth reduce synchronously, which hints that satisfactory multistage pits might be obtained. If we can elevate threshold a, small-sized pits can be obtained, which may provide a way to make a ML blue laser disc.
3. Experiment
The experiment is carried out on the commercial DVD mastering system AM200 manufactured by a Dutch company, ODME. Some key parameters during the mastering process are showed in Table 3.
Table 3. Key parameters during the mastering process
Unlike the normal use of a positive photoresist in the common DVD mastering process, if we enhance the temperature and time of the preprocess, the activity of the photoresist would be reduced and the threshold a would be increased. As a result, a small-sized pit may be obtained. In our experiments, we try to enhance the prebake temperature or time in the mastering process. As a contrast, we also prebake the photoresist in the normal DVD condition. The prebaking temperatures and times are shown in Table 4.
Table 4. Prebaking temperatures and times
In our experiment, we set the normalized laser power as 1 (normal DVD mastering power), 0.8, and 0.6, respectively. Using the three kinds of photoresist in Table 4, three kinds of discs are fabricated, and the pit size of these discs are measured by the atomic-force microscope (AFM). The relation between the measured width (depth) and the normalized power I is shown in Fig. 4.
From Fig. 4 we can see that the width and depth of the pit with normalized power 0.8 and prebaking condition (120°C for 240s) are about 190 and 73 nm, which correspond to the minimum pit on a HD-DVD disc. Furthermore, the width and depth of the pit with normalized power 0.6 and prebaking condition (130°C for 240s) are about 140 and 23 nm, which are much smaller than that of a blue laser disc.
The AFM pictures of a common DVD and the improved photoresist disc (normalized power I=0.6, prebaking at 130°C for 240s) are shown in Fig. 5.
Fig 5. (a). AFM picture of DVD. (b). AFM picture of improved photoresist disc (normalized power I=0.6, prebaking in 130°C for 240s). (c). Vertical section of Fig. 5(a) with depth of 125 nm. (d). Vertical section of Fig. 5(b) with depth of about 23 nm.
Figure 4 shows that when the prebaking temperature and time are enhanced, the pit width and depth both decrease. When the laser power is reduced to 80% of the DVD mastering power, a pit with a width of 190 nm and a depth of 73nm can be obtained, which corresponds to a HD-DVD. When the laser power is reduced to 60% of the DVD mastering power, a pit with a width 140 nm and a depth 23 nm can be obtained, which hints that the improved photoresist might serve as the mastering material of future ML blue laser discs.
Owing to the restriction of our experimental conditions, the readout performance of the abnormal prebaking disc cannot be carried out; only trends of the improved photoresist are shown. In future experiments, we expect to investigate further the stability, reproducibility, and process margins of the improved photoresist.
However, there may be some disadvantages of the abnormal prebaking method. Since abnormal prebaking enhances the threshold of the photoresist, the improved photoresist becomes more inert. As a result, the adjusting of the mastering parameter becomes more difficult. For example, to change the asymmetry in the same magnitude, the mastering parameter adjustment in the abnormal photoresist process is greater than that in the normal photoresist process. Furthermore, other parameters (such as laser power, developing time, and focus of the laser) also need to be adjusted in the mastering system, which may increase the complexity for a practical production process.
4. Conclusion
In this paper, experiments show that abnormal prebaking with a high temperature and an extended time can reduce the activity of the photoresist and enhance the threshold of reaction. With the abnormal prebaked photoresist, a pit size corresponding to or even smaller than that of a blue laser disc can be obtained on an industrial DVD product line. The experimental data is in accord with the simulation results. This work may provide a potential cheap solution for the fabrication of future ML blue laser discs.
Acknowledgments
This work is supported by the National Natural Science Foundation of China grant 60677036.
References and links
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