- Email: [email protected]
Nano-pulsed laser irradiation scanning system for phase-change materials
ARTICLE IN PRESS Ultramicroscopy 108 (2008) 1110– 1114
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Nano-pulsed laser irradiation scanning system for phase-change materials Sookyung Kim a, Xue Zhe Li a, Sangbin Lee a, Kyung-Ho Kim b, Seung-Yop Lee b, a b
Nanostorage Inc., EMC Building #606, Sangam-Dong, Mapo-Ku, Seoul 120-090, Republic of Korea Department of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea
a r t i c l e in f o
PACS: 85.65.+h 73.61.Ph 73.40.Gk 85.65.+h 17 Keywords: Laser irradiation Phase change PRAM Crystallization Optical pick-up
a b s t r a c t Recently, the demand of a laser irradiation tester is increasing for phase change random access memory (PRAM) as well as conventional optical storage media. In this study, a nano-pulsed laser irradiation system is developed to characterize the optical property and writing performance of phase-change materials, based on a commercially available digital versatile disk (DVD) optical pick-up. The precisely controlled focusing and scanning on the material’s surface are implemented using the auto-focusing mechanism and a voice coil motor (VCM) of the commercial DVD pick-up. The laser irradiation system provides various writing and reading functions such as adjustable laser power, pulse duration, recording pattern (spot, line and area), and writing/reading repetition, phase transition, and in situ reflectivity measurement before/after irradiation. Measurements of power time effect (PTE) diagram and reflectivity map of Ge2Sb2Te5 samples show that the proposed laser irradiation system provides the powerful scanning tool to quantify the optical characteristics of phase-change materials. & 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of reversible light-induced crystallization of amorphous semiconductor films in the 1970s, phase-change optical recording has evolved to a mature technology that is applied in rewritable versions of optical data storage systems like compact disk (CD) and digital versatile disk (DVD) [1]. Recently, as the semiconductor memory scaling trend is slowing down, alternative memory concepts are being investigated to provide a better trade-off between scalability and reliability. Among them, phase change random access memory (PRAM), which takes electrical switching property between amorphous and crystalline states, is attracting growing interest [2]. The recording of information is based on writing and erasing amorphous marks in a crystalline layer of a phase-change material. Due to a difference in the optical properties of the crystalline and amorphous state, information can be read out as a change in the reflectance. Writing of amorphous marks occurs by a pulsed focused laser beam, which heats the phase-change layer locally above the melting point. After the pulse, the molten area is quenched to the amorphous state. The recorded information can be recrystallized (erased) by heating the material with the same focused laser beam above the crystallization temperature but below the melting tempera-
Corresponding author. Tel.: +82 2 705 8638; fax: +82 2 712 0977.
E-mail address: [email protected] (S.-Y. Lee). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.068
ture. Due to the increased mobility of the atoms at elevated temperatures, the amorphous state rapidly returns to the crystalline state. Materials that can be reversibly switched from an amorphous to a crystalline phase at an appropriate time scale for optical recording are composed mainly of elements from the groups IV to VI of the periodic table. Well-known phase-change materials are stoichiometric materials such as Ge2Sb2Te5 (GST), used, for example, in DVD-random access memory, and Sb-based compositions like AgInSbTe that are used in CD-RW and DVD+RW [3]. For development or optimization of write-once or rewritable materials, a static tester that irradiates the materials with a pulsed laser spot is required. Mansuripur et al. [4] have designed a static media tester for optical data storage using a commercially available polarized light microscope, employing two semiconductor laser diodes. TOPTICA Photonics has produced a commercial static tester (MediaTest-I) to experimentally characterize the recording performance of rewritable optical media using two laser sources [5]. Unlike the previous work using two laser sources, we propose a new laser irradiation system by using a commercially available DVD optical pick-up with one laser beam of wavelength 650 nm. The advantage of using the optical pick-up is that the system provides a precise measurement to quantify recording and reading characteristics of phase-change materials with the low cost. The laser irradiation system also provides various writing and reading functions as well as adjustable laser power
ARTICLE IN PRESS S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
and pulse duration. Experimental results of reflectivity changes before and after laser irradiations on a GST sample show that the proposed phase-change tester provides the powerful scanning tool to quantify the recording characteristics of phase-change materials.
2. Experiment
Halogen Lamp Source
Stage Controller
1111
Laser Irradiation System
Main Controller
2.1. Design of the laser irradiation scanning system We have developed a new static tester to irradiate phasechange materials with a pulsed laser spot. The laser irradiation system is based on a commercially available DVD pick-up. The system consists of three major parts: laser, monitoring and positioning units, as shown in Fig. 1.
2.2. Design of the laser irradiation scanning system: laser and detector unit Fig. 2 shows optical paths of the proposed system. A DVD laser diode generates a laser beam of wavelength 650 nm. The laser acts as writing source, heating the test sample with adjustable pulse intensity and time duration. The maximum laser power is 50 mW and the minimum time duration of a laser pulse is 5 ns. The laser power and pulse length are controlled by a control box and a personal computer. Most of the laser beam passes through a beam splitter, and the part of the laser beam reflected by the beam splitter is used to regulate the laser power by a front monitor sensor. The laser beam transmitted through the beam splitter is focused on the reflective layer by an objective lens with NA ¼ 0.6. The reflected beam is finally focused onto a four-quadrant photodiode (PD), which is used as a signal detector. A focus error signal (FES) is calculated by using the intensities of the four divided regions of the PD. The resulting FES signal is used to drive a VCM actuator in such a way that the objective lens is shifted to a point where its focal point falls upon the spot center. The laser beam focused on the sample surface writes or erases amorphous marks in a crystalline layer of the phase-change material. The advantage of using the DVD pick-up is that the real-time auto-focusing mechanism by the VCM actuator enables both high positioning accuracy and low cost. Before and after writing process at each spot, the laser operates at low power and the PD detects the laser intensity to measure the reflectance changes before/after phase change. On the reading process, the laser irradiation system stores graphic information corresponding to the reflection data.
CMOS camera
Zoom Lens
DVD Optical Pick-up
Motorized Stage
Fig. 1. Schematic of a new laser irradiation system (phase-change static tester), which consists of three major parts: laser, monitoring and positioning units.
2.3. Design of the laser irradiation scanning system: monitoring unit In order to measure the change of the reflectance for phasechange layer before/after the writing process, the system uses a regular microscope with an additional zoom lens and 1.3 Mpixel CMOS camera for direct observation of the recorded marks, The CMOS camera is connected to a personal computer via USB2.0 interface. After the writing process, the test sample is moved to the center of the microscope using index patterns which are marked around the recorded area. The halogen lamp source is used to illuminate the sample material. The images of the recorded area after irradiation are viewed on a PC monitor or easily saved in standard file formats.
2.4. Design of the laser irradiation scanning system: positioning unit The test sample is placed on a motorized XY-stage underneath the objective lens. The tracking motion of the sample is controlled by two actuators. One is the motorized stage for coarse actuation. The stage can move under computer control in the horizontal plane with 10 mm resolution. The other is a pick-up VCM actuator enabling fine positioning with 0.2 mm resolutions. The specifications of the laser irradiation system are summarized in Table 1.
ARTICLE IN PRESS 1112
S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
Fig. 2. Optical path of the laser irradiation system. A DVD laser diode generates a laser beam of wavelength 650 nm and the laser transmitted through the beam splitter is focused on the sample surface, writing or erasing optical data by an objective lens with NA ¼ 0.6.
Table 1 Specifications of the laser irradiation system Items
Parts
Specifications
Laser unit
Laser wavelength Maximum laser power Minimum laser pulse time NA of objective lens Motion of objective lens Camera sensor Objective lens Zoom lens Illumination Camera-PC interface Coarse positioning Fine positioning Tilt correction
650 nm 50 nW 5 ns (nano second) NA ¼ 0.6 Focusing/Tracking by DVD VCM actuator 1.3 M Pixel CMOS 20 /NA 0.46 (UM plan semi aprochromat) 12 ultra zoom lens with 3 mm fine focus 150 W Halogen lamp coaxial illuminator USB 2.0 XY motorized stage: 10 mm resolution DVD focusing/tracking servo: 0.2 mm resolution Enable, 2-D DVD pick-up actuator
Monitoring unit
Positioning unit
2.5. Experiments We have manufactured the experimental setup shown in Fig. 1. Various writing and reading functions as well as adjustable laser power and pulse duration are implemented in the static tester. It can be chosen the type of recorded pattern (dot, line, or area), the start and stop positions and recording areas can be chosen by a built-in program. In order to test these functions of the static tester, the GST was selected as phase change material. The multilayer structure, which were used for the measurements with the laser irradiation system, consists of SiO2 (20 nm)/GST (20 nm)/ SiO2 (20 nm)/Al (50 nm). The four-layer stack was deposited on glass substrate by using RF magnetron sputtering method.
3. Results and discussion Fig. 3 shows the recording results for various writing modes, where the laser beam writes amorphous marks in line, spot and rectangular shapes on a GST sample surface. The optical data recorded by the line mode are shown in Fig. 3(a), where the laser
power and pulse duration are fixed at 70 mW and 20 ms, respectively. The line width and interval are 0.2 and 2 mm. By using the line mode, two electrodes are linked together, forming a writing circuit. It is possible to investigate the electric switching characteristic of GST material by changing the resistance of GST when the electric current flows in the circuit. In the case of the spot mode, the optical data are recorded by changing the laser power and pulse width. Fig. 3(b) shows the mark patterns with the spot size of 1 mm. From the figure, the phase change properties of GST material from amorphous(no dots) to crystalline(white dots) and crystalline to ablation(black dots) can be easily observed with the increment of laser power and pulse width. Thus the optimal recording conditions of laser power and pulse width can be also determined from recording results using the spot mode function. On the other hand, mark patterns can be densely recorded at the optimum laser power and pulse width to generate the area mode. Fig. 3(c) shows the square-type recording pattern of 100 mm 100 mm. This area mode function can be used for the investigation of SEM and TEM and the measurement of complex refractive index of GST thin film. The minimum line width and
ARTICLE IN PRESS S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
2µm
1113
1µm
100µm
Fig. 3. Mark patterns written the GST sample surface with changing the laser power and laser pulse width in various writing modes, (a) line mode, (b) spot mode, (c) area mode, (d) repeated line mode.
spot size, that the static tester can make a pattern, are 0.1 and 0.3 mm. The dot size of the laser irradiation system will be reduced using object lens with higher NA and laser of shorter wavelength. The laser irradiation system also provides the repeated spot or line modes, which do continual records at a specific point or area. With these functions, it is possible to easily measure to changes of reflection rate after the repeated experiments under various conditions. Fig. 3(d) shows a mark pattern using the repeated line mode. Fig. 4(a) shows the reflectivity map of spot modes implemented under different laser powers and irradiation times. The laser power varies from 10 to 50 mW with an interval of 4 mW and the pulse duration of laser changes with an interval of 30 ns between 100 and 500 ns. The experimental results show that the irradiated laser pulse induces the thermal energy for crystallization between 13 and 34 mW for all pulse duration conditions. The reflectivity of the tested material changes remarkably when the phase transformation or ablation occurs [6]. When the laser power exceeds 34 mW, the ablation occurs where black marks are generated by overheating the surface. Fig. 4(b) shows a power time effect (PTE) diagram corresponding to the aforementioned reflectivity map. Optimized conditions for crystallization can be easily decided by the PTE diagram. Thus, the laser irradiation system provides a reasonable method to investigate the phase transition characteristics of the rewritable materials. To determine the optimal recording condition for the GST sample used in Fig. 4, the reflectivity changes are plotted in Fig. 5. Here the laser powers varies from 1 to 30 mW with an interval of 4 mW, and the pulse widths changes with an interval of 20 ns between 10 and 250 ns. The seven curves in Fig. 5 correspond to seven different laser powers, as follows: 1, 5, 9, 11, 13, 21 and 30 mW. The reflectivity of the phase-change material changes
dramatically when the laser power is larger than 9 mW and the pulse width is increased above 110 ns. From the results, by increasing the laser power and irradiation time leads to the increase of reflectivity. The observed phenomena are due to the crystallization of the amorphous layer as well as the change of the material’s optical constants with temperature. However, the reflectivity remains unchanged after the laser irradiation when the pulse width is above 200 ns and the laser power is higher than 21 mW. In this case, high temperature by the focused laser beam causes ablation of the phase-change layer. Therefore, the optimal value of laser power for crystallization of the GST sample is between 13 and 21 mW, and the optimal laser pulse width is 200 ns.
4. Conclusion In this study, a nano-pulsed laser irradiation system is developed to characterize the writing performance of phasechange materials by using a commercially available DVD optical pick-up. The laser irradiation system uses one laser beam of wavelength 650 nm with the maximum laser power of 50 mW and the minimum irradiation time (pulse width) of 5 ns. The precisely controlled focusing and scanning on the material’s surface with 0.2 mm resolution are implemented using the auto-focusing mechanism and a voice coil motor (VCM) of the commercial DVD pick-up. The laser irradiation system provides various writing and reading functions as well as adjustable laser power and pulse duration. Experimental results of reflectivity changes before/after laser irradiation on a GST sample show that the proposed phase-change tester provides the powerful scanning tool to quantify the recording characteristics of phase-change materials.
ARTICLE IN PRESS 1114
S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
1µm
Laser power (mW)
34 mW
13 mW
Irradiation time (ns) 500 -191.0 -154.3 -117.5 -80.75 -44.00 -7.250 29.50 66.25 103.0
Irradiation time (ns)
450 400 350 300 250 200 150 100 10
15
20
25
30
35
40
45
50
Laser power (mW) Fig. 4. (a) Reflectivity map of marks written on GST sample with increasing laser power from 10 to 50 mW with an interval of 4 mW and increasing laser pulse width from 100 to 500 ns with an interval of 30 ns in spot mode, (b) the corresponding PTE(power time effect) diagram.
Acknowledgments
100 1 mW 5 mW 9 mW 11 mW 13 mW 21 mW 30 mW
Reflectivity (%)
80
60
This study was supported by a grant from the Technological Development Program by the Ministry of Commerce, Industry and Energy (10006623-2006-22) and a grant from the Seoul Research and Business Development Program (10816). References
40 [1] [2] [3] [4]
T. Ohta, et al., Jpn. J. Appl. Phys. 39 (2000) 770. A.L. Lacaita, Solid-State Electron. 50 (2006) 24. L.V. Pieterson, et al., J. Appl. Phy. 97 (2005) 083520. M. Mansuripur, J.K. Erwin, W. Bletscher, P. Khulbe, K. Sadeghi, X. Xun, A. Gupta, S.B. Mendes, Appl. Opt 38 (1999) 7095. [5] A. Euteneuer, D. Fernandez, F. Lison, T. Weber, IEE Proc.-Sci. Meas. Technol. 150 (2003) 207. [6] T.-J. Park, S.-Y. Choi, M.J. Kang, Thin Solid Films 515 (2007) 5049.
20
0 0
50
100 150 Irradiation time (ns)
200
250
Fig. 5. Reflectivity change after laser irradiation with different laser powers and irradiation times. The laser power varies from 1 to 30 mW and laser pulse width change from 10 to 250 ns with intervals of 4 mW and 20 ns, respectively.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Nano-pulsed laser irradiation scanning system for phase-change materials Sookyung Kim a, Xue Zhe Li a, Sangbin Lee a, Kyung-Ho Kim b, Seung-Yop Lee b, a b
Nanostorage Inc., EMC Building #606, Sangam-Dong, Mapo-Ku, Seoul 120-090, Republic of Korea Department of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea
a r t i c l e in f o
PACS: 85.65.+h 73.61.Ph 73.40.Gk 85.65.+h 17 Keywords: Laser irradiation Phase change PRAM Crystallization Optical pick-up
a b s t r a c t Recently, the demand of a laser irradiation tester is increasing for phase change random access memory (PRAM) as well as conventional optical storage media. In this study, a nano-pulsed laser irradiation system is developed to characterize the optical property and writing performance of phase-change materials, based on a commercially available digital versatile disk (DVD) optical pick-up. The precisely controlled focusing and scanning on the material’s surface are implemented using the auto-focusing mechanism and a voice coil motor (VCM) of the commercial DVD pick-up. The laser irradiation system provides various writing and reading functions such as adjustable laser power, pulse duration, recording pattern (spot, line and area), and writing/reading repetition, phase transition, and in situ reflectivity measurement before/after irradiation. Measurements of power time effect (PTE) diagram and reflectivity map of Ge2Sb2Te5 samples show that the proposed laser irradiation system provides the powerful scanning tool to quantify the optical characteristics of phase-change materials. & 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of reversible light-induced crystallization of amorphous semiconductor films in the 1970s, phase-change optical recording has evolved to a mature technology that is applied in rewritable versions of optical data storage systems like compact disk (CD) and digital versatile disk (DVD) [1]. Recently, as the semiconductor memory scaling trend is slowing down, alternative memory concepts are being investigated to provide a better trade-off between scalability and reliability. Among them, phase change random access memory (PRAM), which takes electrical switching property between amorphous and crystalline states, is attracting growing interest [2]. The recording of information is based on writing and erasing amorphous marks in a crystalline layer of a phase-change material. Due to a difference in the optical properties of the crystalline and amorphous state, information can be read out as a change in the reflectance. Writing of amorphous marks occurs by a pulsed focused laser beam, which heats the phase-change layer locally above the melting point. After the pulse, the molten area is quenched to the amorphous state. The recorded information can be recrystallized (erased) by heating the material with the same focused laser beam above the crystallization temperature but below the melting tempera-
Corresponding author. Tel.: +82 2 705 8638; fax: +82 2 712 0977.
E-mail address: [email protected] (S.-Y. Lee). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.068
ture. Due to the increased mobility of the atoms at elevated temperatures, the amorphous state rapidly returns to the crystalline state. Materials that can be reversibly switched from an amorphous to a crystalline phase at an appropriate time scale for optical recording are composed mainly of elements from the groups IV to VI of the periodic table. Well-known phase-change materials are stoichiometric materials such as Ge2Sb2Te5 (GST), used, for example, in DVD-random access memory, and Sb-based compositions like AgInSbTe that are used in CD-RW and DVD+RW [3]. For development or optimization of write-once or rewritable materials, a static tester that irradiates the materials with a pulsed laser spot is required. Mansuripur et al. [4] have designed a static media tester for optical data storage using a commercially available polarized light microscope, employing two semiconductor laser diodes. TOPTICA Photonics has produced a commercial static tester (MediaTest-I) to experimentally characterize the recording performance of rewritable optical media using two laser sources [5]. Unlike the previous work using two laser sources, we propose a new laser irradiation system by using a commercially available DVD optical pick-up with one laser beam of wavelength 650 nm. The advantage of using the optical pick-up is that the system provides a precise measurement to quantify recording and reading characteristics of phase-change materials with the low cost. The laser irradiation system also provides various writing and reading functions as well as adjustable laser power
ARTICLE IN PRESS S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
and pulse duration. Experimental results of reflectivity changes before and after laser irradiations on a GST sample show that the proposed phase-change tester provides the powerful scanning tool to quantify the recording characteristics of phase-change materials.
2. Experiment
Halogen Lamp Source
Stage Controller
1111
Laser Irradiation System
Main Controller
2.1. Design of the laser irradiation scanning system We have developed a new static tester to irradiate phasechange materials with a pulsed laser spot. The laser irradiation system is based on a commercially available DVD pick-up. The system consists of three major parts: laser, monitoring and positioning units, as shown in Fig. 1.
2.2. Design of the laser irradiation scanning system: laser and detector unit Fig. 2 shows optical paths of the proposed system. A DVD laser diode generates a laser beam of wavelength 650 nm. The laser acts as writing source, heating the test sample with adjustable pulse intensity and time duration. The maximum laser power is 50 mW and the minimum time duration of a laser pulse is 5 ns. The laser power and pulse length are controlled by a control box and a personal computer. Most of the laser beam passes through a beam splitter, and the part of the laser beam reflected by the beam splitter is used to regulate the laser power by a front monitor sensor. The laser beam transmitted through the beam splitter is focused on the reflective layer by an objective lens with NA ¼ 0.6. The reflected beam is finally focused onto a four-quadrant photodiode (PD), which is used as a signal detector. A focus error signal (FES) is calculated by using the intensities of the four divided regions of the PD. The resulting FES signal is used to drive a VCM actuator in such a way that the objective lens is shifted to a point where its focal point falls upon the spot center. The laser beam focused on the sample surface writes or erases amorphous marks in a crystalline layer of the phase-change material. The advantage of using the DVD pick-up is that the real-time auto-focusing mechanism by the VCM actuator enables both high positioning accuracy and low cost. Before and after writing process at each spot, the laser operates at low power and the PD detects the laser intensity to measure the reflectance changes before/after phase change. On the reading process, the laser irradiation system stores graphic information corresponding to the reflection data.
CMOS camera
Zoom Lens
DVD Optical Pick-up
Motorized Stage
Fig. 1. Schematic of a new laser irradiation system (phase-change static tester), which consists of three major parts: laser, monitoring and positioning units.
2.3. Design of the laser irradiation scanning system: monitoring unit In order to measure the change of the reflectance for phasechange layer before/after the writing process, the system uses a regular microscope with an additional zoom lens and 1.3 Mpixel CMOS camera for direct observation of the recorded marks, The CMOS camera is connected to a personal computer via USB2.0 interface. After the writing process, the test sample is moved to the center of the microscope using index patterns which are marked around the recorded area. The halogen lamp source is used to illuminate the sample material. The images of the recorded area after irradiation are viewed on a PC monitor or easily saved in standard file formats.
2.4. Design of the laser irradiation scanning system: positioning unit The test sample is placed on a motorized XY-stage underneath the objective lens. The tracking motion of the sample is controlled by two actuators. One is the motorized stage for coarse actuation. The stage can move under computer control in the horizontal plane with 10 mm resolution. The other is a pick-up VCM actuator enabling fine positioning with 0.2 mm resolutions. The specifications of the laser irradiation system are summarized in Table 1.
ARTICLE IN PRESS 1112
S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
Fig. 2. Optical path of the laser irradiation system. A DVD laser diode generates a laser beam of wavelength 650 nm and the laser transmitted through the beam splitter is focused on the sample surface, writing or erasing optical data by an objective lens with NA ¼ 0.6.
Table 1 Specifications of the laser irradiation system Items
Parts
Specifications
Laser unit
Laser wavelength Maximum laser power Minimum laser pulse time NA of objective lens Motion of objective lens Camera sensor Objective lens Zoom lens Illumination Camera-PC interface Coarse positioning Fine positioning Tilt correction
650 nm 50 nW 5 ns (nano second) NA ¼ 0.6 Focusing/Tracking by DVD VCM actuator 1.3 M Pixel CMOS 20 /NA 0.46 (UM plan semi aprochromat) 12 ultra zoom lens with 3 mm fine focus 150 W Halogen lamp coaxial illuminator USB 2.0 XY motorized stage: 10 mm resolution DVD focusing/tracking servo: 0.2 mm resolution Enable, 2-D DVD pick-up actuator
Monitoring unit
Positioning unit
2.5. Experiments We have manufactured the experimental setup shown in Fig. 1. Various writing and reading functions as well as adjustable laser power and pulse duration are implemented in the static tester. It can be chosen the type of recorded pattern (dot, line, or area), the start and stop positions and recording areas can be chosen by a built-in program. In order to test these functions of the static tester, the GST was selected as phase change material. The multilayer structure, which were used for the measurements with the laser irradiation system, consists of SiO2 (20 nm)/GST (20 nm)/ SiO2 (20 nm)/Al (50 nm). The four-layer stack was deposited on glass substrate by using RF magnetron sputtering method.
3. Results and discussion Fig. 3 shows the recording results for various writing modes, where the laser beam writes amorphous marks in line, spot and rectangular shapes on a GST sample surface. The optical data recorded by the line mode are shown in Fig. 3(a), where the laser
power and pulse duration are fixed at 70 mW and 20 ms, respectively. The line width and interval are 0.2 and 2 mm. By using the line mode, two electrodes are linked together, forming a writing circuit. It is possible to investigate the electric switching characteristic of GST material by changing the resistance of GST when the electric current flows in the circuit. In the case of the spot mode, the optical data are recorded by changing the laser power and pulse width. Fig. 3(b) shows the mark patterns with the spot size of 1 mm. From the figure, the phase change properties of GST material from amorphous(no dots) to crystalline(white dots) and crystalline to ablation(black dots) can be easily observed with the increment of laser power and pulse width. Thus the optimal recording conditions of laser power and pulse width can be also determined from recording results using the spot mode function. On the other hand, mark patterns can be densely recorded at the optimum laser power and pulse width to generate the area mode. Fig. 3(c) shows the square-type recording pattern of 100 mm 100 mm. This area mode function can be used for the investigation of SEM and TEM and the measurement of complex refractive index of GST thin film. The minimum line width and
ARTICLE IN PRESS S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
2µm
1113
1µm
100µm
Fig. 3. Mark patterns written the GST sample surface with changing the laser power and laser pulse width in various writing modes, (a) line mode, (b) spot mode, (c) area mode, (d) repeated line mode.
spot size, that the static tester can make a pattern, are 0.1 and 0.3 mm. The dot size of the laser irradiation system will be reduced using object lens with higher NA and laser of shorter wavelength. The laser irradiation system also provides the repeated spot or line modes, which do continual records at a specific point or area. With these functions, it is possible to easily measure to changes of reflection rate after the repeated experiments under various conditions. Fig. 3(d) shows a mark pattern using the repeated line mode. Fig. 4(a) shows the reflectivity map of spot modes implemented under different laser powers and irradiation times. The laser power varies from 10 to 50 mW with an interval of 4 mW and the pulse duration of laser changes with an interval of 30 ns between 100 and 500 ns. The experimental results show that the irradiated laser pulse induces the thermal energy for crystallization between 13 and 34 mW for all pulse duration conditions. The reflectivity of the tested material changes remarkably when the phase transformation or ablation occurs [6]. When the laser power exceeds 34 mW, the ablation occurs where black marks are generated by overheating the surface. Fig. 4(b) shows a power time effect (PTE) diagram corresponding to the aforementioned reflectivity map. Optimized conditions for crystallization can be easily decided by the PTE diagram. Thus, the laser irradiation system provides a reasonable method to investigate the phase transition characteristics of the rewritable materials. To determine the optimal recording condition for the GST sample used in Fig. 4, the reflectivity changes are plotted in Fig. 5. Here the laser powers varies from 1 to 30 mW with an interval of 4 mW, and the pulse widths changes with an interval of 20 ns between 10 and 250 ns. The seven curves in Fig. 5 correspond to seven different laser powers, as follows: 1, 5, 9, 11, 13, 21 and 30 mW. The reflectivity of the phase-change material changes
dramatically when the laser power is larger than 9 mW and the pulse width is increased above 110 ns. From the results, by increasing the laser power and irradiation time leads to the increase of reflectivity. The observed phenomena are due to the crystallization of the amorphous layer as well as the change of the material’s optical constants with temperature. However, the reflectivity remains unchanged after the laser irradiation when the pulse width is above 200 ns and the laser power is higher than 21 mW. In this case, high temperature by the focused laser beam causes ablation of the phase-change layer. Therefore, the optimal value of laser power for crystallization of the GST sample is between 13 and 21 mW, and the optimal laser pulse width is 200 ns.
4. Conclusion In this study, a nano-pulsed laser irradiation system is developed to characterize the writing performance of phasechange materials by using a commercially available DVD optical pick-up. The laser irradiation system uses one laser beam of wavelength 650 nm with the maximum laser power of 50 mW and the minimum irradiation time (pulse width) of 5 ns. The precisely controlled focusing and scanning on the material’s surface with 0.2 mm resolution are implemented using the auto-focusing mechanism and a voice coil motor (VCM) of the commercial DVD pick-up. The laser irradiation system provides various writing and reading functions as well as adjustable laser power and pulse duration. Experimental results of reflectivity changes before/after laser irradiation on a GST sample show that the proposed phase-change tester provides the powerful scanning tool to quantify the recording characteristics of phase-change materials.
ARTICLE IN PRESS 1114
S. Kim et al. / Ultramicroscopy 108 (2008) 1110–1114
1µm
Laser power (mW)
34 mW
13 mW
Irradiation time (ns) 500 -191.0 -154.3 -117.5 -80.75 -44.00 -7.250 29.50 66.25 103.0
Irradiation time (ns)
450 400 350 300 250 200 150 100 10
15
20
25
30
35
40
45
50
Laser power (mW) Fig. 4. (a) Reflectivity map of marks written on GST sample with increasing laser power from 10 to 50 mW with an interval of 4 mW and increasing laser pulse width from 100 to 500 ns with an interval of 30 ns in spot mode, (b) the corresponding PTE(power time effect) diagram.
Acknowledgments
100 1 mW 5 mW 9 mW 11 mW 13 mW 21 mW 30 mW
Reflectivity (%)
80
60
This study was supported by a grant from the Technological Development Program by the Ministry of Commerce, Industry and Energy (10006623-2006-22) and a grant from the Seoul Research and Business Development Program (10816). References
40 [1] [2] [3] [4]
T. Ohta, et al., Jpn. J. Appl. Phys. 39 (2000) 770. A.L. Lacaita, Solid-State Electron. 50 (2006) 24. L.V. Pieterson, et al., J. Appl. Phy. 97 (2005) 083520. M. Mansuripur, J.K. Erwin, W. Bletscher, P. Khulbe, K. Sadeghi, X. Xun, A. Gupta, S.B. Mendes, Appl. Opt 38 (1999) 7095. [5] A. Euteneuer, D. Fernandez, F. Lison, T. Weber, IEE Proc.-Sci. Meas. Technol. 150 (2003) 207. [6] T.-J. Park, S.-Y. Choi, M.J. Kang, Thin Solid Films 515 (2007) 5049.
20
0 0
50
100 150 Irradiation time (ns)
200
250
Fig. 5. Reflectivity change after laser irradiation with different laser powers and irradiation times. The laser power varies from 1 to 30 mW and laser pulse width change from 10 to 250 ns with intervals of 4 mW and 20 ns, respectively.