Conductive-atomic force microscopy has been successfully used for characterizing recorded marks on commercial digital versatile disk and Blu-ray disk. Nano recorded marks beyond diffraction limit are imaged with high spatial resolution and excellent contrast of conductivity. The smallest mark size resolved is around 23.5 nm which is limited by background spots around 18.5 nm. The results of different optical power and writing strategy on the size, shape, and close packed writing process of recorded marks clearly show the opto-thermal response of phase-change recording layer.
© 2006 Optical Society of America
In the digital information era, the technology of massive data storage has become more and more important. Optical storage technology, such as compact disk (CD) and digital versatile disk (DVD) has played an indispensable role in massive data storage for decades. Recently, the phase-change optical disks  such as the digital versatile rewritable (DVD±RW) disk enjoy a renaissance due to its rewritable properties in comparison with write-once optical disk. The subsequent Blu-ray optical disk [2–3] and the near-field optical disk [4–8] which have the advantage of ultrahigh data capacity are also expected to have benefits on reducing the cost of optical storage per byte. The aggressive growth of capacity on optical disk pushes the recorded marks smaller and smaller, even beyond the diffraction limit. Although optical readout signals and the spectrum of carrier-to-noise ratio (CNR) have long been used for testing the performance of recorded marks on optical disks and readout capability of disk drivers, it is always needed to further characterize the small recorded marks on optical disks. The size, shape, and optical response on phase-change recording layer are closely related to opto-thermal property of different phase-change recording media. Advanced characterization of recorded marks is required to provide vital information for the understanding and design of optical storage on disks and drivers. Several sophisticated microscopic techniques, such as scanning near-field optical microscopy (SNOM) , scanning surface potential microscopy (SSPM) , scanning electron microscopy (SEM)  and transmission electron microscopy (TEM) [12–13], have been used for studying recorded marks on phase-change recording layers, especially for those beyond diffraction limit. In this paper, we investigate the imaging of recorded marks on phase-change recording layer of optical disk by using conductive- atomic force microscopy (C-AFM). We found the simplicity and usefulness of C-AFM on characterization of recorded marks on the phase-change recording layer of commercial DVD and Blu-ray optical disks. Recorded marks beyond diffraction limit are resolved and studied under different writing strategies.
2. Experimental setup
Two types of commercial phase-change optical disks, DVD+RW  and write-once Blu-ray  optical disk, were used in the experiments. Recorded marks were prepared using commercial DVD+RW and Blu-ray optical disk drivers, respectively. A DVD dynamic optical disk-tester (Model DDU-1000, Pulstec, Japan) with a laser wavelength of 658±5 nm and numerical aperture of 0.65 was employed to record marks on phase-change layer of disks under different writing strategies as well. A simple and reliable mechanic striping procedure has been developed to separate the phase-change recording layer from the protective dielectric layer of the bonded disks. Conductive-atomic force microscopic images are scanned on the freshly cleaved phase-change recording layer subsequently.
The schematic of resolving recorded marks on the phase-change layer of optical disks is shown in Fig. 1. The C-AFM cantilever probe is coated with PtIr5 (Model Pointprobe cont-Pt, Nanosensors, Germany). The resistivity of probe is 0.01–0.02 Ω/cm. Two types of commercial C-AFM systems, Dimension 3100 SPM from Veeco Instrument and MFP-3D™ of Asylum Research, are used for C-AFM imaging, respectively. The conductive probe of C-AFM is virtually grounded to avoid oxidization. Samples of phase-change recording layers are biased at 10–100 mV through using silver paint or conductive carbon tape. Ultra-low current (10 pA to 1 µA) measurements between the probe and samples with high sensitivity (1 nA/Volt) provide excellent contrast of conductivity. High spatial resolution images of topography and conductivity can be acquired simultaneously. For better topographic images, the scanning direction is adjusted to be perpendicular to the direction of the grooves and lands of optical disk. High quality images are achieved at constant scanning speed of 10 µm/s in contact AFM mode with proper set point of contact force. Images of nano scale recorded marks on phase-change recording layer of optical disk can be obtained at fresh regions to avoid possible artifacts created by previous scanning.
3. Results and discussions
A 5µm×5µm scan on AgInSbTe phase-change recording layer of a commercial DVD+RW optical disk  is shown in Fig. 2. AFM topographic image and C-AFM intensity image acquired simultaneously are displayed in Figs. 2(a) and 2(b), respectively. The lower tracks shown in Fig. 2(a) are grooves of DVD optical disk where marks are recorded. The image also shows that the width of lands varies to provide low frequency wobble signals for the tracking of optical disks.
The average difference of height between lands and grooves in Fig. 2(a) is measured around 28±1 nm. Recorded marks in the grooves of Fig. 2(b) can be seen distinctly with the width of 354 nm and different lengths. The marks displayed in white color are the low conductive area of AgInSbTe phase-change layer. The contrast ratio of conductive current between marks and unmarked area is 2.82. Figure 2(b) evidently demonstrated the usefulness of C-AFM image of recorded marks on phase-change layers. C-AFM image can discern the length, width, intrinsic shape, perfection of boundary, covered area and conductive property of each individual recorded mark, and their relationship with respect to various writing strategies.
Figure 3 shows AFM and C-AFM images of recorded marks on the phase-change layer of a SONY blu-ray optical disk with scanning area of 2.5 µm×2.5 µm. The period of lands and grooves shown in Fig. 3(a) is 334±6 nm. The average difference of height between lands and grooves in Fig. 3(a) is measured around 25±1 nm. Figure 3(b) shows that the average width of recorded marks is 210 nm with two different lengths at grooves. The intrinsic shape of the head and tail of each recorded mark can be seen clearly. The smallest length displayed in Fig. 3(b) is around 128 nm. The precise control of the recorded marks is closely related to the thermal and optical properties of recording layers and optical writing strategies, including laser power, pulse duration, speed of disk, etc. Results of Figs. 2 and 3 fully demonstrate the capability of C-AFM for the characterization of nano recorded marks on DVD and Blu-ray disks.
For the understanding of the dependence of writing laser power on the mark size, recorded marks on a commercial DVD+RW (4X) optical disk are prepared by a dynamic optical disk tester (DDU-1000, Pulstec, Japan) with a constant linear velocity of 3.5 m/s and different laser writing power, 11 mW, 10 mW and 9 mW, respectively. Results of C-AFM images are shown in Figs. 4(a)–4(c). The recording spacing between marks is arranged to be 800nm to avoid the interference between adjacent marks. The average width of recorded mark displayed in Fig. 4(a) is 316±26 nm, which is smaller than the laser focusing spot size, 840nm. For the smaller writing power, 10 mW, the average width of marks shown in Fig. 4(b) is 208±14 nm, which is smaller than the diffraction limit, 616 nm. Results of Fig. 4(c) demonstrate the average width of 50±29 nm for 9 mW laser writing power. The smallest width of marks is around 23.5 nm which is limited by many (bright) spots with an average size around 18.5 nm in background. The area fraction of the spots in the background shown in C-AFM images is around 0.339. Resolving the spots in the background has never been reported through SEM or TEM images before. Apparently, the advantages of non-destruction, high spatial resolution, and excellent contrast on conductivity are the important capability of C-AFM.
The phenomena of spots in the background were observed on AgInSbTe as well as GeSbTe phase-change recording layers, and the resolvability of these nano background spots is limited by the size of C-AFM probe. The bright spots shown in C-AFM images are low conductivity area which is considered to be amorphous state of phase-change material. Results of Figs. 4(a) to 4(c) clearly show that nano recorded marks on phase-change recording layers can be achieved by proper control of writing laser power. A schematic of the dependence of mark size on writing power is shown in Fig. 4(d). The size of recorded mark depends on how much area of laser power is above the opto-thermal threshold plane, which is related to properties of phase-change recording layers and the structure of optical disks. The size fluctuation of nano recorded marks increased dramatically when the peak of the laser power is very close to the opto-thermal threshold plane as the results shown in Fig. 4(c). This could be one of the alternative ways to verify the uniformity of the phase-change property and stability of recording processes and optical recording systems.
Another interesting issue observed in C-AFM image is the influence of writing strategy on recorded marks. Figures 5(a) and 5(b) show a typical result of recorded marks with two different writing pulse intervals, 228 ns and 114 ns, at constant disk rotational speed of 3.5 m/s. Amorphous mark sizes of 307 nm×345 nm and 307 nm×210 nm, and mark spacings of 800 nm and 400 nm are distinctly shown in the crystalline background. Results of C-AFM image displayed in Fig. 5(b) evidently demonstrate the variations of spacing as well as shape of recorded marks. The elliptical shape of marks is changed to crescent shape for spacing of pulse of 114 ns. A schematic of writing strategy and results of recorded marks is shown in Fig. 5(c). Laser writing power of 10 mW with its pulse duration of 28.5 ns and temporal spacing of 228 ns or 114 ns is used for recording processes. A picture of the areas above opto-thermal threshold plane is sketched to illustrate the size and shape of recorded marks affected by thermal interactions with adjacent marks. Apparently, when the temporal spacing between two marks is small enough, the next writing pulse will keep providing thermal energy to overwrite certain area of the previous mark. Figure 5(c) demonstrates that the overlap range of this case is around 400 nm in radius from the center of recorded marks.
For the first two recorded marks shown in left, the center-to-center distance between marks is 800 nm; therefore, it can be regarded as no overlap writing process. For the other four recorded marks with 400 nm spacing, three of the recorded marks are deformed due to the overlap of writing processes generated by the subsequent recording laser pulse. The crescent shape of recorded marks shown in Figs. 5(b) and 5(c) is formed because part of the mark is re-crystallized to the background. This is a typical example that size and shape of recorded marks can be closely affected by thermal interactions with adjacent marks.
Characterization of recorded marks on phase-change recording layers of commercial DVD and Blu-ray optical disks has been successfully demonstrated by using C-AFM. Topographic images provide physical information of optical disk, such as pitch length, wobble, height difference and width of lands and grooves, etc. Conductivity contrast of images discerns length, width, intrinsic shape, perfection of boundary, covered area, and conductive properties of each individual recorded mark. Simple sample preparation, fast imaging, high spatial resolution, nondestructive method, inexpensive processes, excellent contrast and sensitivity of conductivity are the key advantages of C-AFM method. We found the smallest recorded mark observed through C-AFM on the DVD optical disk in the experiment is 23.5 nm which is limited by the fluctuation of conductivity in the background of AgInSbTe phase-change recording layer. Random spots with an average size around 18.5 nm and 33.9% area fraction in background were resolved through C-AFM, which have never been reported by SEM or TEM measurements before. High spatial resolution and excellent contrast on conductivity of C-AFM clearly demonstrates its significant contribution in comparison with previous SEM or TEM imaging methods.
The merits of C-AFM enable us to study the performance of different writing strategies on phase-change recording layers in depth. Results show the dependence of optical power and writing strategies on the size and shape of recorded marks can be comprehensively understood. Amorphous recorded marks formed on the crystallized phase-change thin film depend on not only the control of writing laser power, but also the timing of writing strategies. The size of discrete recorded marks can be determined by the recording laser power with respect to the virtual opto-thermal threshold plane. Various nano marks have been recorded and observed by proper control of writing laser power on the phase-change recording layer with homogeneous opto-thermal properties. For close pack recorded marks, conditions of properties of thermal conductivity, writing laser power, duration of writing pulse, phase-change opto-thermal properties and distance between marks are closely related to the formation of recorded marks [15–16]. To avoid overlap of recording marks and maintain uniform mark size and shape, proper writing strategy has to be designed according to properties of phase-change recording layers, optical disk structures and recording conditions. Results of this paper readily provide a novel means with profound information from C-AFM images to properly control the recording processes on the phase-change recording layer of optical disks, and will be very useful for the future developments of next-generation ultra-high density nano recording optical storage on phase-change recording layers.
The authors are grateful for the research support from the National Science Council of Taiwan, R.O.C., under project number NSC-94-2112-M-002-001 and the Ministry of Economic Affairs, R.O.C., under project number 94-EC-17-A-08-S1-0006. D. P. Tsai thanks the support from Center for Nano Science and Technology, National Taiwan University. Correspondence should be addressed to Din Ping Tsai, by phone: 886-2-3366-5100, fax: 886- 2-2363-9928, or e-mail: email@example.com.
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