The effect of the MgO process on the properties of AC-PDPs
Young-Sung Kim*, Min-Soo Park, Byung-Gil Ryu
PDP Materials Gr., Digital Display Research Lab., LG Electronics Inc., Seoul, Korea E-mail: kys73@lge.com
Abstract
The effects of the MgO fabrication process on the properties of AC-PDPs were examined. MgO films were deposited by e-beam evaporation with various substrate temperatures and oxygen flow rates. MgO films were analyzed by XRD, CL and ellipsometer.
Panel properties such as luminance, efficiency, discharge voltage and discharge delay time were measured with test panels. MgO films with higher temperature, smaller oxygen flow rate showed shorter discharge delay time. Also they showed smaller XRD peak intensity. These results revealed that the discharge delay time was strongly influenced by temperature and oxygen flow rate of the MgO fabrication process.
1. Introduction
MgO thin film is widely used as the protective layer of AC-PDPs due to its abilities to lower the discharge voltage, shorten discharge delay time and protect the dielectric layer from the ion bombardment.
There have been many studies to improve the quality of the protective layer by changing material [1,2] and fabrication method. [3] However, there were hardly any materials and/or fabrication methods better than MgO / E-beam evaporator deposition.
In general, properties of thin films are changed by the deposition condition. For E-beam evaporation, substrate temperature and oxygen flow rate are considered to be the main factors for thin film property improvement.
In our work, effects of E-beam evaporation process on the characteristics of the panel were examined. The relationship between unit MgO film and discharge properties was also analyzed by X-ray diffraction, refractive index, and CL.
2. Experiments
MgO films were formed by electron-beam evaporator deposition with various substrate temperatures (200 ~ 350 oC) and oxygen flow rates (0
~ 50 sccm). Deposition rate and film thickness were
fixed at 6 angstrom/sec, 7000 angstrom, respectively.
Base pressure was 1.0xE-3 Pa.
Surface morphology and growth of films were analyzed using FESEM (Field emission scanning electron microscopy). Preferred orientation and crystallinity were examined by XRD (X-ray Diffraction), refractive index of films were measured by ellipsometer (Horiba, MM16) and prism coupler (Metricon 2010). In order to study the change of oxygen vacancies by defects in the band gap, CL (cathodoluminescence) analysis was carried out.
For the properties of panel, discharge breakdown voltages were measured with 2 inch chip samples in the chamber. Ne-Xe (10%) mixed gas was filled in the chamber. Base pressure was kept at 1.0xE-7 torr , distance between two electrodes was also fixed at 2 mm. Parchen curve was obtained as working pressure was changed from 10 to 60 torr by 10 torr. The schematics of the chamber system to measure discharge breakdown voltages are drawn in Figure 1.
Cu plate Al plate Sample
Chamber 2mm Cu plate
Al plate Sample
Chamber 2mm
Figure 1. Schematics of chamber system for measuring the breakdown voltages.
Discharge delay time was measured with test panels of 7.5 inch size.
3. Results
3.1. Effect of substrate temperature on MgO films.
XRD patterns of MgO films grown at substrate temperatures of 200, 250, 300, 350 oC are shown in Figure 2. There is no change of preferred orientation with temperature. However, as the temperature rises, P2-32 / Y.-S. Kim
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intensity of XRD is smaller. This result indicates that crystallinity decreases at the high temperature.
10 20 30 40 50 60 70 80
0 500
1000 350
oC
300 oC
250 oC (111)
200 oC
Intensity (a.u)
2 theta (degree)
Figure 2. XRD patterns of MgO films deposited at 200, 250, 300, 350 oC.
Table 1 shows refractive index of MgO films with various temperatures. Refractive index was measured with ellipsometer (Horiba, MM16). χ2 is the extinction coefficient, smaller χ2 represents better fitting. Refractive index increases with increasing temperatures. Therefore, it is possible to mention that MgO films of higher temperature have high density by Lorentz-Lorenz equation.
0.398 0.209
0.189 0.265
ȱ2
714 1.635 200 OC
742 770
Thickness 750 (nm)
1.657 1.639
1.637 Refractive
Index
350 OC 300 OC
250 OC
0.398 0.209
0.189 0.265
ȱ2
714 1.635 200 OC
742 770
Thickness 750 (nm)
1.657 1.639
1.637 Refractive
Index
350 OC 300 OC
250 OC
Table 1. Refractive index at 632.8nm and film thickness of MgO films with temperature.
The above two results are explained as follows. (1) In general, at low temperature, atoms have insufficient energy to arrive at the stable sites and mix to make dense film. In this case, it has been reported that film is grown in the form of first-layer oxygen and second-layer magnesium and so on, alternatively.
In the end, (111) orientation film is formed. At high temperature, atoms have enough energy to be mixed with each other densely. Hence (200) orientation film is formed. [4]. Consequently films deposited at high temperature have (200) orientation and large refractive index. In our experiment, (200) orientation was not shown due to insufficient temperature to
change to the preferred orientation. Instead (111) intensity becomes smaller.
Discharge characteristics of panels are shown in Figure 3. Discharge delay time decreases significantly as the temperature increases. On the contrary, refractive index becomes smaller. This result indicates there is an inverse relationship between discharge delay time and refractive index.
200 250 300 350
1 2 3 4 5 6 7
8 Delay Time
Discharge Delay Time (μsec)
Temperature (OC)
1.63 1.64 1.65 1.66 1.67
Refractive Index
Refractive Index
200 250 300 350
1 2 3 4 5 6 7
8 Delay Time
Discharge Delay Time (μsec)
Temperature (OC)
1.63 1.64 1.65 1.66 1.67
Refractive Index
Refractive Index
Figure 3. Discharge delay time and refractive Index of MgO films with temperature.
In Figure 4, we show the mean breakdown voltages with various temperatures.
200 250 300 350
260 270 280 290 300 310
Temperature (OC)
Mean Breakdown Voltage (V)
Figure 4. Mean breakdown voltages of MgO films with temperature.
Similar with discharge delay time, mean breakdown voltage also decreases at high temperature. It may be possible to mention that crystallinity and density of film affect the discharge voltage.
3.2. Effect of oxygen flow rate on MgO films.
Figure 5 shows XRD patterns of MgO films grown at various oxygen flow rates, 0, 30, 50 sccm. (111) intensity becomes lower with a decreasing oxygen
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flow rate. In the case of no oxygen flowed in, XRD peak is shifted into (200) orientation.
10 20 30 40 50 60 70 80
0 200 400 600 800 1000
(200) (111)
(111)
50 sccm
30 sccm
0 sccm
Intensity (a.u)
2 theta (degree)
Figure 5. XRD patterns of MgO films deposited at 0, 30, 50 sccm oxygen flow rate.
Similar results have been reported before. [5]
Ron et al. have reported that oxygen surface diffusion coefficient of MgO film is related with the formation of (111), (200) orientations and the shift to (200) orientation can occur with longer growth cycle time interval. [4] It indicates that flux is also very important to form different orientations. As flux becomes smaller, particles like magnesium and oxygen have sufficient time to form stable layers like (200) orientation. Thus, (200) orientation film can be obtained with no oxygen flowed in.
Refractive index of films with 0, 30, 50 sccm oxygen flow rate is summarized in Table 2. Prism coupler is used to measure it. For the reasons mentioned above, films of smaller oxygen flow rate have larger refractive index. We can somewhat anticipate the properties of the panel with these analyses.
819 765
Thickness 600 (nm)
1.588 1.640
1.708 Refractive
Index
50 sccm 30 sccm
0 sccm
819 765
Thickness 600 (nm)
1.588 1.640
1.708 Refractive
Index
50 sccm 30 sccm
0 sccm
Table 2. Refractive index and film thickness of MgO films with oxygen flow rate.
Figure 6 shows the dependence of discharge delay time on the oxygen flow rates. Discharge delay time becomes shorter as oxygen flow rate decreases. Also this result reveals that discharge delay time is inversely proportion to the refractive index of MgO films.
0 10 20 30 40 50
2.0 2.5 3.0 3.5 4.0
Delay Time
Discharge Delay Time (μsec)
O2 Flow rate (sccm)
1.55 1.60 1.65 1.70 1.75
Refractive Index
Refractive Index
0 10 20 30 40 50
2.0 2.5 3.0 3.5 4.0
Delay Time
Discharge Delay Time (μsec)
O2 Flow rate (sccm)
1.55 1.60 1.65 1.70 1.75
Refractive Index
Refractive Index
Figure 6. Discharge delay time and refractive Index of MgO films with oxygen flow rate.
Figure 7 shows the mean breakdown voltages with various oxygen flow rates. Different from our expectation, the relationship between breakdown voltage and oxygen flow rate is not linear. Mean breakdown voltage of 30 scccm MgO film is not lower than that of 50 sccm MgO film despite of larger refractive index and smaller (111) orientation intensity. Another mechanism seems to be needed to explain this work.
0 10 20 30 40 50
280 290 300 310 320
Mean Breakdown Voltage (V)
O2 Flow rate (sccm)
Figure 7. Mean breakdown voltages of MgO films with oxygen flow rate.
It has been previously reported that CL intensity decreases and firing voltage increases with a decreasing oxygen flow rates due to the oxygen vacancies. [6]
Figure 8 shows CL spectra of MgO films with 0, 30, 50 sccm oxygen flow rate. CL measurement was conducted with 5 kV acceleration voltage and 200 magnifications. Working distance was fixed at 16. CL intensity increases in proportion of oxygen flow rate.
Therefore, it can be explained that breakdown voltage P2-32 / Y.-S. Kim
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of 50 sccm MgO film went down due to high CL intensity even though it has lower refractive index and higher (111) orientation intensity.
200 300 400 500 600 700 800
0 10000 20000 30000 40000 50000
5kV, x200, WD16
0 sccm 30 sccm 50 sccm
CL intensity (a.u)
wavelength (nm) 0 sccm 50 sccm 30 sccm
200 300 400 500 600 700 800
0 10000 20000 30000 40000 50000
5kV, x200, WD16
0 sccm 30 sccm 50 sccm
CL intensity (a.u)
wavelength (nm) 0 sccm 50 sccm 30 sccm 0 sccm 50 sccm 30 sccm
Figure 8. CL spectra of MgO films with oxygen flow rate.
4. Conclusion
The effects of the MgO fabrication process on the properties of AC-PDPs were examined. MgO films were deposited by e-beam evaporation with various substrate temperatures and oxygen flow rates. The following conclusions were drawn from our work.
(1) As temperature increases, orientation is not shifted in the range of temperature of our work.
However, XRD peak intensity decreases. Refractive index increases and panel properties become better with an increasing temperature.
(2) As oxygen flow rate decreases, XRD peak intensity decreases, however, CL intensity also
decreases. In case of no oxygen flow rate, orientation was changed into (200). Refractive index goes up and discharge delay time becomes shorter with a decreasing oxygen flow rate. But, breakdown voltage of panel was not linearly related with oxygen flow rate.
(3) As substrate temperature becomes higher, the oxygen flow rate is smaller, more stable and denser films can be obtained. Refractive index and orientation influence the discharge delay time of panel directly. However, more factors like CL seem to be involved for the voltage characteristics. To investigate the relationship between film analyses (refractive index, orientation, CL and so on) and properties of panel more clearly, further studies and understanding are needed.
5. References
[1] M. S. Lee, Y. Matulevich, et al., SID 06 Digest, (2006)
[2] Y. Motoyama, T. Kurauchi, SID 06 Digest, (2006) [3] K. Uetani, H. Kajiyama, et al., Mat. Res. Soc.
Symp. Proc., Vol 1647, (2002)
[4] Ron Huang, Adrian H. Kitai, Appl. Phys. Lett. 61, No. 12, (1992).
[5] E. Y. Jung, G. P. Choi, et al., SID 02 Digest, (2002)
[6] Y. Motoyama, Y. Hirano, K. Ishii, Y. Murakami, and F. Sato, J. Appl. Phys., Vol. 95, No. 12, pp.
8419-8424 (2004).
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