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A study on the improved performances of OLED using CMP process parameters determined by DOE method
Microelectronic Engineering 85 (2008) 1776–1780
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A study on the improved performances of OLED using CMP process parameters determined by DOE method Yong-Jin Seo a,1, Gwon-Woo Choi b,1, Woo-Sun Lee c,* a
Department of Electrical Engineering, Daebul University, 72, Sanho, Samho, Youngam, Chonnam-do 526-702, Republic of Korea Research Institute of Energy Resources Technology, Chosun University, Gwangju 501-759, Republic of Korea c Department of Electrical Engineering, Chosun University, Gwangju 501-759, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 November 2007 Received in revised form 12 April 2008 Accepted 5 May 2008 Available online 16 June 2008 Keywords: Chemical mechanical polishing (CMP) Indium–tin oxide (ITO) Organic light emitting diode (OLED) Design of experiment (DOE) Current–voltage (I–V) Photoluminescence spectrum
a b s t r a c t In order to study the chemical–mechanical polishing (CMP) characteristics of indium–tin oxide (ITO) thin film with a sufficient removal amount and a good planarity, the optimal CMP process conditions were determined by using a design of experiment (DOE) approach. The electrical and optical properties, such as current–voltage (I–V) curve and photoluminescence spectrum, were discussed in order to evaluate the possibility of the CMP application for an organic light emitting diode (OLED) device using an ITO film. The electrical I–V characteristics and optical properties of ITO thin film were improved after the CMP process using optimized process conditions compared to that of as-deposited thin film before the CMP process. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the organic light emitting diode (OLED) was firstly reported by Tang and Vanslkye [1], the OLEDs have been intensively investigated driven by their potential applications in the fields of flat-panel display (FPD), light emitting diode (LED) and liquid crystal display (LCD) [2]. OLED devices are nowadays one of the most attractive devices based on the organic semiconductors due to their successful application in the display technology [3,4]. The OLED display features a broader viewing angle, thin type, high brightness, faster response and low power consumption. The pure red, green and blue emissions of OLEDs are necessary to manufacture the full color display [5]. A great benefit of OLED displays beyond traditional LCDs is that OLEDs do not require a back-light function. The OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays [6–8]. On the other hand, indium–tin oxide (ITO) film has a high optical-transmittance property in the visible wavelength range and low electrical resistivity [2–5]. Therefore, it has been widely used in many applications such as transparent electrodes for solar cells, * Corresponding author. Tel.: +82 62 230 7024; fax: +82 62 230 7023. E-mail addresses: [email protected] (Y.-J. Seo), [email protected] (G.-W. Choi), [email protected] (W.-S. Lee). 1 These authors contributed equally to this work. 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.05.022
LCD, plasma-display panel (PDP), and OLED [6–8]. Also, ITO is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer [9–13]. The interface between the electrode and organic layer in OLEDs has been reported as an important factor to influence the electrical and luminescent properties [14]. However, this film has a problem because peaks and hillocks form on the surface of thin film [15]. The chemical mechanical polishing (CMP) process is one of the suitable solutions, which could solve the above-mentioned surface problems [16–24]. The CMP process is a polishing method, where a rotating wafer is pressed against a rotating polishing pad on a platen, with slurry in between. The CMP process has been widely used in the semiconductor fabrication and microelectronics industry. The CMP process must provide a high removal rate (RR) and a low nonuniformity (NU) through the simultaneous action of chemical dissolution with a mechanical abrasion. The CMP performance can be optimized by several CMP components, such as process equipment and consumables (e.g. polishing pad, backing film, and slurry). In this work, the optimum process parameters with a sufficient removal amount and a good planarity were determined by design of experiment (DOE) method. And then, we studied the electrical and optical properties of OLED with the structure of Al/ MEH-PPV[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene]/ ITO/glass using polished ITO surface as a bottom electrode (anode).
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2. Experiments The experimental equipments and procedures used in present study have been reported elsewhere [16–20]. In brief, the fabricated procedure of OLED structure used in this experiment is as follows. ITO thin film was deposited on the glass substrate by radio frequency (RF) magnetron sputtering in order to fabricate a high polymer OLED with Al/MEH-PPV/ITO/glass structure. The conditions of RF sputtering were as follows; flow rate of argon (Ar) gas, RF power, substrate temperature, and working pressure were 30 sccm, 100 W, 35 °C, and 5.2 10 2 torr, respectively. In order to make an emitting layer, a toluene was used as a solution and stirred for 12 h at room temperature with the rate of 0.2 wt% for MEH-PPV. The completed MEH-PPV was deposited on ITO thin film by the spin coating method at a rotation speed of 1000 rpm for 20 s, and it was annealed at 75 °C in a vacuum and removed volatile ingredient. A negative electrode, consisting of aluminum (Al) with a work function of 4.08 eV, was deposited on emitting layer by evaporation. After making a device accord-
ing to the above-mentioned process conditions, we investigated the properties of OLED device with polished ITO thin film by CMP technique. All test wafers were polished by a POLI-380 CMP polisher of G&P Technology. The double pad and silica-based oxide slurry of Rohm & Hass Company were used as the CMP consumables. The process parameter ranges of slurry flow rate, platen table speed and polishing time were varied to obtain the optimal ITO-CMP performance as shown in Table 1. Other process parameters were fixed as follows: head speed, slurry temperature, and down force were 60 rpm, 30 °C, and 300 gf/cm2. In order to compare the electrical characteristics before and after ITO-CMP process, the current–voltage (I–V) relations were measured by using a semiconductor parameter analyzer (HP4155, Hewlett-Packard Corp.). And we observed the surface morphology of polished ITO thin film and MEH-PPV layer using an atomic-force microscope (AFM). Finally, the optical properties of ITO thin film were measured by UV-Spectrophotometer (Varian Techtron, Cary500scan) in the range of 200–800 nm.
Table 1 Measurement data and their real curves according to the typical CMP process conditions such as table speed, polishing time, and slurry flow rate
50
2000
45
1800
40
1600
Post-CMP Thickness
35
1400
30
1200
25
1000
20
800 Sheet Resistance
15
600
10
400
5
200
Non-Uniformity
0
Measurement data Rs (X/°C)
NU (%)
THK (Å)
(a)
0 20 40 60 80
8.4 10.0 11.3 13.2 18.2
7.1 6.3 7.9 6.2 10.2
1800 1503 1335 1139 827
(b)
0 30 60 90 120
8.4 9.9 13.2 18.2 28.4
7.1 6.0 6.2 12.8 19.7
1800 1525 1139 827 528
(c)
20 40 60 80 100
12.7 12.6 12.9 13.2 13.3
8.7 5.2 6.6 6.2 5.9
1187 1195 1165 1139 1130
Post-CMP Thickness [A]
Sheet Resistance [Ohm/Sq] Non-Uniformity [%]
Typical CMP process conditions versus real curves
0 0
20
40
60
80
(a) Platen Table Speed [rpm]
2000 Post-CMP Thickness
45
1800
40
1600
35
1400
30 25
1200 1000
Sheet Resistance
20
800
15
600
10 5 0
400 200
Non-Uniformity 0
20
40
60
80
Post-CMP Thickness [A]
Sheet Resistance [Ohm/sq] Non-Uniformity [%]
50
0 100 120 140 160
50
2000
45
1800 1600
40 35
Post-CMP Thickness
1400 1200
30
1000
25 20
Sheet Resistance
15
800 600
Non-Uniformity 400 200
10 5
Post-CMP Thickness [A]
Sheet Resistance [Ohm/sq] Non-Uniformity [%]
(b) Polishing Time [sec]
0
0 20
40
60
80
100
(c) Slurry Flow Rate [ml/min]
The sheet resistance (Rs), non-uniformity (NU), and post-CMP thickness (THK) were presented in right three columns.
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3. Results and discussion 3.1. Determination of CMP process parameters by DOE approach Table 1 shows the measurement data and their real curves of sheet resistance, post-CMP thickness and non-uniformity of ITO thin film according to the variation of CMP process parameters, such as platen table speed, polishing time and slurry flow rate. The sheet resistance, post-CMP thickness and non-uniformity have different dependencies according to the typical CMP process parameters as shown in left column of Table 1. Therefore, in order to evaluated the correlations between the CMP characteristics and the process conditions, the DOE approach was performed by normalization of each measurements data. Fig. 1 represents the DOE trend curves. The lines (a–c) of Fig. 1 correspond to the sheet resistance, non-uniformity and post-CMP thickness of ITO thin film according to the variation of platen table speed without any changes to other process parameters, respectively. The sheet resistance increased and the post-CMP thickness of ITO thin film decreased according to increasing platen table speed. The lines (d–f) show the (d) sheet resistance, (e) non-uniformity, and the (f) post-CMP thickness of ITO thin film as a function of polishing time. The sheet resistance increased and the post-CMP thickness of ITO thin film decreased according to increasing polishing time. The lines (g–i) correspond to the (g) sheet resistance, (h) non-uniformity, and the (i) post-CMP thickness of ITO film as a function of slurry flow rate. This result indicates that the slurry flow rate did not directly affect the sheet resistance and postCMP thickness. We concluded that the CMP performance has no connection with the slurry amount. Using a DOE approach, we investigated the trade-off between the various parameters such as platen table speed, polishing time, and slurry flow rate. A better understanding of the trade-off behavior between the various parameters and the effect on the post-CMP thickness, non-uniformity, and sheet resistance is achieved by using the normalization method. The optimized combination of CMP process conditions for polishing of ITO/MEH-PPV structure was determined for higher removal rate, lower non-uniformity, and sheet resistance of below 15 X/°C through the above DOE results. The DOE region was indicated as a dotted box in Fig. 1. In summary, the optimal CMP process conditions were as follows: platen table speed, head speed, slurry flow rate, polishing time, and down-force were 60 rpm, 60 rpm, 60 ml/min, 60 s, and 300 gf/cm2, respectively.
Fig. 2 shows the AFM images of (a) as-deposited ITO film and (b) polished film by the aforementioned optimum process parameters, respectively. After the CMP process, all bumps, large particles and peaks in Fig. 2a were removed, and then the surface morphology was improved as shown in Fig. 2b. Any scratches were not observed before and after CMP process. Fig. 3 compares the AFM surface morphology of as-deposited MEH-PPV with and without CMP process of underlying ITO film. Fig. 3a shows the surface morphology of as-deposited MEH-PPV before the CMP process of ITO thin film. Fig. 3b shows the surface morphology of as-deposited MEH-PPV after CMP process of ITO thin film. The surface morphology of MEH-PPV on polished ITO thin film was more uniformly coated as shown in Fig. 3b with compared to Fig. 3a. This result was originated from the improvements of interface property between polished ITO film and as-deposited MEH-PPV. From this result, we can expect that the photoluminescence and transmittance properties of emitting devices will be affected by improved interface property. It will be verify in the results of Figs. 5–7 of Section 3.3. 3.2. Electrical characteristics In our previous work [18,19], we reported the Hall mobility, carrier density, and sheet resistance of ITO thin films before and after the CMP process. After CMP, the carrier density decreased, but the Hall mobility increased. The sheet resistance of polished ITO film was higher because the carrier density was lower, probably due to the lower density of oxygen vacancies in the film [25]. The lower density of oxygen vacancies also explained the higher Hall mobility measured in the polished film [26]. The density of oxygen vacancies was thought to be lower due to the chemical reactions with the slurry in CMP process. Fig. 4 shows the I–V curves of the OLED device before and after the CMP process [17]. Before the CMP process, the threshold
DOE Region
Normalized Value [a.u.]
1.0
(g)
(i)
(b)
(h)
(a)
(c)
(e) (d)
(f)
0.8
0.6
0.4
0.2 Table Speed : 1(0 rpm), 2(20 rpm), 3(40 rpm), 4(60 rpm), 5(80 rpm) Polishing Time : 1(0 sec), 2(30 sec), 3(60 sec), 4(90 sec), 5(120 sec) Slurry Flow Rate : 1(20 ml/min), 2(40 ml/min), 3(60 ml/min), 4(80 ml/minc), 5(100 ml/min)
0.0 1
2
3
4
5
Process Parameters (Table Speed, Polishing Time, Slurry Flow) Fig. 1. DOE trend curves according to the typical CMP process parameters such as platen table speed, polishing time and slurry flow rate.
Fig. 2. AFM analysis of ITO thin film (a) before CMP and (b) after CMP.
Y.-J. Seo et al. / Microelectronic Engineering 85 (2008) 1776–1780
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3.3. Optical characteristics
Fig. 3. AFM analysis of MEH-PPV thin film (a) before CMP and (b) after ITO-CMP.
Post-CMP : Threshold Voltage=12.5 V Pre-CMP : Threshold Voltage=14.5 V
Current Density [mA/mm2]
0.7 0.6 0.5 0.4 0.3 0.2
Fig. 5 compares the photoluminescence (PL) spectrum of unpolished ITO/MEH-PPV and polished ITO/MEH-PPV structure. After the CMP process, the intensity of PL peak was higher compared with PL peak before the CMP process. When the MEH-PPV was deposited on unpolished ITO surface, the two peaks were observed. The first peak showed a maximum value of 195 at the range of 540nm wavelength. And then, the second peak was 131 at the range of 680 nm. When the MEH-PPV was deposited on polished ITO surface, the first peak showed a maximum value of 211 at the range of 540 nm. And then, the second peak was 181 in the wavelength of 692 nm. After the CMP process, the intensity of PL peak was increased, and the peak of emitting region as shown in dotted line of Fig. 5 moved from 680 nm to 692 nm toward high wavelength region. This is caused by change of interface state between the ITO film and the MEH-PPV layer by CMP technique. Therefore, we expect that the emitting efficiency will be high when the device is fabricated by CMP process. Fig. 6 compares the optical transmittances of the polished and unpolished ITO thin film. Also, the optical transmittances of MEH-PPV layers deposited on polished and unpolished ITO film were compared. When the CMP process was performed on ITO thin film, the optical transmittance was slightly increased, and the peak was also increased in comparison with before CMP process. The transmittance curve of the emitting region as shown in dotted lines of Fig. 6 was not steep. These results indicate that the dispersion and the extinction of light were decreased due to the improvement of surface quality or interface state by CMP process. When the MEH-PPV was deposited on polished and unpolished ITO thin film, both the optical transmittance was similar in the range of 430 nm. However, the optical transmittance of the polished ITO thin film in the light-emitting peak region of above 680 nm was increased. This result is also due to improvement of interface property between the polished ITO thin film and MEH-PPV layer. This means that the light-emitting property of OLEDs device will be improved because the optical transmittance is increased in the light-emitting region of MEH-PPV. The optical transmittance in the range of below 550 nm was remarkably decreased. However, when the MEH-PPV was deposited, the optical transmittance in the range of above 600 nm was increased more than 80%. When the ITO thin film was polished, the optical transmittance was increased more than 1% in comparison with unpolished ITO thin film. The optical transmittance is increased more than 85% in the 690 nm range, where the intensity of PL peak is the maximum.
0.1 0.0 -2
0
2
4 6 8 10 12 14 Operating Voltage [V]
16
18
20
Fig. 4. I–V curve before and after CMP process.
voltage was 14.5 V when the ITO thin film was used as a bottom electrode of OLED. However, when the polished ITO thin film was used, it was 12.5 V. Therefore, after the CMP process, the operating voltage was decreased because the contact area with the MEH-PPV layer was advanced by CMP planarization of the ITO surface. It is considered that the operating voltage was also decreased because the thickness of ITO thin film was reduced. That is to say, the decrease of operating voltage indicates the increase of Hall mobility in the OLED applications. In this point, we anticipate that our first approach, the CMP application of anode (ITO) layer as a bottom electrode, will make the operating speed of an OLED device faster.
Fig. 5. PL spectrum of ITO/MEH-PPV structure.
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4. Conclusion
Fig. 6. Optical transmittance of ITO/MEH-PPV monolayer before and after CMP.
We studied the effect of CMP process on the performances of OLED device with Al/MEH-PPV/ITO/glass structures. The optimum conditions for CMP process were determined by DOE method as follows; slurry flow rate, slurry temperature, polisher pressure, platen speed, and polishing time were 60 ml/min, 30 °C, 300 g/ cm2, 60 rpm, and 60 s, respectively. According to AFM analysis, the surface quality of ITO thin film was drastically elevated by CMP technique. As the interface state between ITO thin film and MEH-PPV was improved, the PL intensity was increased in the 680-nm wavelength range. Transmittance of low wavelength region was reduced. However, it was increased in the range of more than 690 nm where the intensity of PL peak is the maximum. Also, after CMP process, the operating voltage was decreased because the contact area with PPV layer was advanced by CMP planarization of ITO surface. The device characteristics will be advanced in the near future as both the PL property and the optical transmittance in the range of red wavelength is increased. Acknowledgment This work was supported by Korea Research Foundation Grant (KRF-2006-005-J00902). References
Fig. 7. Optical absorption of ITO/MEH-PPV monolayer before and after CMP.
Fig. 7 shows the optical absorption of ITO thin film and ITO/ MEH-PPV structure with and without CMP process, respectively. The absorption rate of polished ITO thin film is reduced. When the MEH-PPV is deposited on the polished ITO thin film, the absorption rate was decreased in comparison with unpolished ITO thin film. The decrease of the absorption makes the increase of the optical transmittance. The absorption is remarkably decreased in the range of more than 680 nm, which is a light emitting region.
[1] C.W. Tang, S.A. Vanslkye, Appl. Phys. Lett. 51 (1987) 913. [2] J. George, C.S. Menon, Surf. Coat. Tech. 132 (1) (2000) 45. [3] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bardley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [4] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884. [5] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [6] M. Bender, J. Trube, J. Stollenwerk, Thin Solid Films 354 (1999) 100. [7] J. Ma, D. Zhang, J. Zhao, C. Tan, T. Yang, H. Ma, Appl. Surf. Sci. 152 (1999) 239. [8] T. Futagami, Y. Shigesato, I. Yasui, Jpn. J. Appl. Phys. 37 (1998) 6210. [9] H. Kudoh, T. Sasuga, T. Seguchi, Y. Katsumura, Polymer 37 (1996) 2903. [10] T. Minami, H. Sonohara, T. Kakumu, S. Takata, Thin Solid Films 270 (1995) 37. [11] B. Chiou, S. Hsidh, Thin Solid Films 229 (1993) 146. [12] M.J. Alam, D.C. Cameron, Thin Solid Films 377/378 (2000) 455. [13] J.S. An, S.C. Kim, S.H. Hahn, S.K. Ko, E.J. Kim, J. Korean Phys. Soc. 45 (2004) 1629. [14] S.C. Graham, R.H. Friend, S. Fung, S.C. Moratti, Synthetic Met. 68 (1997) 903. [15] C. Heck, T. Seki, T. Oosawa, M. Chikamatsu, N. Tanigaki, T. Hiraga, J. Matsuo, Nucl. Instrum. Meth. B 242 (1/2) (2006) 140. [16] Y.J. Seo, Microelectron. Eng. 77 (2005) 263. [17] Y.J. Seo, G.W. Choi, W.S. Lee, Microelectron. Eng. 84 (2007) 2896. [18] G.W. Choi, N.H. Kim, Y.J. Seo, W.S. Lee, Electron. Lett. 42 (2006) 487. [19] G.W. Choi, K.Y. Lee, N.H. Kim, J.S. Park, Y.J. Seo, W.S. Lee, Microelectron. Eng. 83 (2006) 2213. [20] G.W. Choi, Y.J. Seo, K.Y. Lee, W.S. Lee, J. Vac. Sci. Technol. A 25 (2007) 999. [21] H. Lei, J. Luo, Wear 257 (2004) 461. [22] K. Qin, B. Moudgil, C.W. Park, Thin Solid Films 446 (2004) 277. [23] Y.J. Seo, N.H. Kim, G.W. Choi, E.G. Chang, J. Park, W.S. Lee, J. Vac. Sci. Technol. A 23 (2005) 1133. [24] N.H. Kim, Y.J. Seo, W.S. Lee, Jpn. J. Appl. Phys. 44 (2005) L1256. [25] Y. Han, D. Kim, J.S. Cho, S.K. Koh, Thin Solid Films 473 (2005) 218. [26] E. Terzini, P. Thilakan, C. Minarini, Mater. Sci. Eng. B 77 (2000) 110.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A study on the improved performances of OLED using CMP process parameters determined by DOE method Yong-Jin Seo a,1, Gwon-Woo Choi b,1, Woo-Sun Lee c,* a
Department of Electrical Engineering, Daebul University, 72, Sanho, Samho, Youngam, Chonnam-do 526-702, Republic of Korea Research Institute of Energy Resources Technology, Chosun University, Gwangju 501-759, Republic of Korea c Department of Electrical Engineering, Chosun University, Gwangju 501-759, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 November 2007 Received in revised form 12 April 2008 Accepted 5 May 2008 Available online 16 June 2008 Keywords: Chemical mechanical polishing (CMP) Indium–tin oxide (ITO) Organic light emitting diode (OLED) Design of experiment (DOE) Current–voltage (I–V) Photoluminescence spectrum
a b s t r a c t In order to study the chemical–mechanical polishing (CMP) characteristics of indium–tin oxide (ITO) thin film with a sufficient removal amount and a good planarity, the optimal CMP process conditions were determined by using a design of experiment (DOE) approach. The electrical and optical properties, such as current–voltage (I–V) curve and photoluminescence spectrum, were discussed in order to evaluate the possibility of the CMP application for an organic light emitting diode (OLED) device using an ITO film. The electrical I–V characteristics and optical properties of ITO thin film were improved after the CMP process using optimized process conditions compared to that of as-deposited thin film before the CMP process. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the organic light emitting diode (OLED) was firstly reported by Tang and Vanslkye [1], the OLEDs have been intensively investigated driven by their potential applications in the fields of flat-panel display (FPD), light emitting diode (LED) and liquid crystal display (LCD) [2]. OLED devices are nowadays one of the most attractive devices based on the organic semiconductors due to their successful application in the display technology [3,4]. The OLED display features a broader viewing angle, thin type, high brightness, faster response and low power consumption. The pure red, green and blue emissions of OLEDs are necessary to manufacture the full color display [5]. A great benefit of OLED displays beyond traditional LCDs is that OLEDs do not require a back-light function. The OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays [6–8]. On the other hand, indium–tin oxide (ITO) film has a high optical-transmittance property in the visible wavelength range and low electrical resistivity [2–5]. Therefore, it has been widely used in many applications such as transparent electrodes for solar cells, * Corresponding author. Tel.: +82 62 230 7024; fax: +82 62 230 7023. E-mail addresses: [email protected] (Y.-J. Seo), [email protected] (G.-W. Choi), [email protected] (W.-S. Lee). 1 These authors contributed equally to this work. 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.05.022
LCD, plasma-display panel (PDP), and OLED [6–8]. Also, ITO is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer [9–13]. The interface between the electrode and organic layer in OLEDs has been reported as an important factor to influence the electrical and luminescent properties [14]. However, this film has a problem because peaks and hillocks form on the surface of thin film [15]. The chemical mechanical polishing (CMP) process is one of the suitable solutions, which could solve the above-mentioned surface problems [16–24]. The CMP process is a polishing method, where a rotating wafer is pressed against a rotating polishing pad on a platen, with slurry in between. The CMP process has been widely used in the semiconductor fabrication and microelectronics industry. The CMP process must provide a high removal rate (RR) and a low nonuniformity (NU) through the simultaneous action of chemical dissolution with a mechanical abrasion. The CMP performance can be optimized by several CMP components, such as process equipment and consumables (e.g. polishing pad, backing film, and slurry). In this work, the optimum process parameters with a sufficient removal amount and a good planarity were determined by design of experiment (DOE) method. And then, we studied the electrical and optical properties of OLED with the structure of Al/ MEH-PPV[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene]/ ITO/glass using polished ITO surface as a bottom electrode (anode).
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Y.-J. Seo et al. / Microelectronic Engineering 85 (2008) 1776–1780
2. Experiments The experimental equipments and procedures used in present study have been reported elsewhere [16–20]. In brief, the fabricated procedure of OLED structure used in this experiment is as follows. ITO thin film was deposited on the glass substrate by radio frequency (RF) magnetron sputtering in order to fabricate a high polymer OLED with Al/MEH-PPV/ITO/glass structure. The conditions of RF sputtering were as follows; flow rate of argon (Ar) gas, RF power, substrate temperature, and working pressure were 30 sccm, 100 W, 35 °C, and 5.2 10 2 torr, respectively. In order to make an emitting layer, a toluene was used as a solution and stirred for 12 h at room temperature with the rate of 0.2 wt% for MEH-PPV. The completed MEH-PPV was deposited on ITO thin film by the spin coating method at a rotation speed of 1000 rpm for 20 s, and it was annealed at 75 °C in a vacuum and removed volatile ingredient. A negative electrode, consisting of aluminum (Al) with a work function of 4.08 eV, was deposited on emitting layer by evaporation. After making a device accord-
ing to the above-mentioned process conditions, we investigated the properties of OLED device with polished ITO thin film by CMP technique. All test wafers were polished by a POLI-380 CMP polisher of G&P Technology. The double pad and silica-based oxide slurry of Rohm & Hass Company were used as the CMP consumables. The process parameter ranges of slurry flow rate, platen table speed and polishing time were varied to obtain the optimal ITO-CMP performance as shown in Table 1. Other process parameters were fixed as follows: head speed, slurry temperature, and down force were 60 rpm, 30 °C, and 300 gf/cm2. In order to compare the electrical characteristics before and after ITO-CMP process, the current–voltage (I–V) relations were measured by using a semiconductor parameter analyzer (HP4155, Hewlett-Packard Corp.). And we observed the surface morphology of polished ITO thin film and MEH-PPV layer using an atomic-force microscope (AFM). Finally, the optical properties of ITO thin film were measured by UV-Spectrophotometer (Varian Techtron, Cary500scan) in the range of 200–800 nm.
Table 1 Measurement data and their real curves according to the typical CMP process conditions such as table speed, polishing time, and slurry flow rate
50
2000
45
1800
40
1600
Post-CMP Thickness
35
1400
30
1200
25
1000
20
800 Sheet Resistance
15
600
10
400
5
200
Non-Uniformity
0
Measurement data Rs (X/°C)
NU (%)
THK (Å)
(a)
0 20 40 60 80
8.4 10.0 11.3 13.2 18.2
7.1 6.3 7.9 6.2 10.2
1800 1503 1335 1139 827
(b)
0 30 60 90 120
8.4 9.9 13.2 18.2 28.4
7.1 6.0 6.2 12.8 19.7
1800 1525 1139 827 528
(c)
20 40 60 80 100
12.7 12.6 12.9 13.2 13.3
8.7 5.2 6.6 6.2 5.9
1187 1195 1165 1139 1130
Post-CMP Thickness [A]
Sheet Resistance [Ohm/Sq] Non-Uniformity [%]
Typical CMP process conditions versus real curves
0 0
20
40
60
80
(a) Platen Table Speed [rpm]
2000 Post-CMP Thickness
45
1800
40
1600
35
1400
30 25
1200 1000
Sheet Resistance
20
800
15
600
10 5 0
400 200
Non-Uniformity 0
20
40
60
80
Post-CMP Thickness [A]
Sheet Resistance [Ohm/sq] Non-Uniformity [%]
50
0 100 120 140 160
50
2000
45
1800 1600
40 35
Post-CMP Thickness
1400 1200
30
1000
25 20
Sheet Resistance
15
800 600
Non-Uniformity 400 200
10 5
Post-CMP Thickness [A]
Sheet Resistance [Ohm/sq] Non-Uniformity [%]
(b) Polishing Time [sec]
0
0 20
40
60
80
100
(c) Slurry Flow Rate [ml/min]
The sheet resistance (Rs), non-uniformity (NU), and post-CMP thickness (THK) were presented in right three columns.
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3. Results and discussion 3.1. Determination of CMP process parameters by DOE approach Table 1 shows the measurement data and their real curves of sheet resistance, post-CMP thickness and non-uniformity of ITO thin film according to the variation of CMP process parameters, such as platen table speed, polishing time and slurry flow rate. The sheet resistance, post-CMP thickness and non-uniformity have different dependencies according to the typical CMP process parameters as shown in left column of Table 1. Therefore, in order to evaluated the correlations between the CMP characteristics and the process conditions, the DOE approach was performed by normalization of each measurements data. Fig. 1 represents the DOE trend curves. The lines (a–c) of Fig. 1 correspond to the sheet resistance, non-uniformity and post-CMP thickness of ITO thin film according to the variation of platen table speed without any changes to other process parameters, respectively. The sheet resistance increased and the post-CMP thickness of ITO thin film decreased according to increasing platen table speed. The lines (d–f) show the (d) sheet resistance, (e) non-uniformity, and the (f) post-CMP thickness of ITO thin film as a function of polishing time. The sheet resistance increased and the post-CMP thickness of ITO thin film decreased according to increasing polishing time. The lines (g–i) correspond to the (g) sheet resistance, (h) non-uniformity, and the (i) post-CMP thickness of ITO film as a function of slurry flow rate. This result indicates that the slurry flow rate did not directly affect the sheet resistance and postCMP thickness. We concluded that the CMP performance has no connection with the slurry amount. Using a DOE approach, we investigated the trade-off between the various parameters such as platen table speed, polishing time, and slurry flow rate. A better understanding of the trade-off behavior between the various parameters and the effect on the post-CMP thickness, non-uniformity, and sheet resistance is achieved by using the normalization method. The optimized combination of CMP process conditions for polishing of ITO/MEH-PPV structure was determined for higher removal rate, lower non-uniformity, and sheet resistance of below 15 X/°C through the above DOE results. The DOE region was indicated as a dotted box in Fig. 1. In summary, the optimal CMP process conditions were as follows: platen table speed, head speed, slurry flow rate, polishing time, and down-force were 60 rpm, 60 rpm, 60 ml/min, 60 s, and 300 gf/cm2, respectively.
Fig. 2 shows the AFM images of (a) as-deposited ITO film and (b) polished film by the aforementioned optimum process parameters, respectively. After the CMP process, all bumps, large particles and peaks in Fig. 2a were removed, and then the surface morphology was improved as shown in Fig. 2b. Any scratches were not observed before and after CMP process. Fig. 3 compares the AFM surface morphology of as-deposited MEH-PPV with and without CMP process of underlying ITO film. Fig. 3a shows the surface morphology of as-deposited MEH-PPV before the CMP process of ITO thin film. Fig. 3b shows the surface morphology of as-deposited MEH-PPV after CMP process of ITO thin film. The surface morphology of MEH-PPV on polished ITO thin film was more uniformly coated as shown in Fig. 3b with compared to Fig. 3a. This result was originated from the improvements of interface property between polished ITO film and as-deposited MEH-PPV. From this result, we can expect that the photoluminescence and transmittance properties of emitting devices will be affected by improved interface property. It will be verify in the results of Figs. 5–7 of Section 3.3. 3.2. Electrical characteristics In our previous work [18,19], we reported the Hall mobility, carrier density, and sheet resistance of ITO thin films before and after the CMP process. After CMP, the carrier density decreased, but the Hall mobility increased. The sheet resistance of polished ITO film was higher because the carrier density was lower, probably due to the lower density of oxygen vacancies in the film [25]. The lower density of oxygen vacancies also explained the higher Hall mobility measured in the polished film [26]. The density of oxygen vacancies was thought to be lower due to the chemical reactions with the slurry in CMP process. Fig. 4 shows the I–V curves of the OLED device before and after the CMP process [17]. Before the CMP process, the threshold
DOE Region
Normalized Value [a.u.]
1.0
(g)
(i)
(b)
(h)
(a)
(c)
(e) (d)
(f)
0.8
0.6
0.4
0.2 Table Speed : 1(0 rpm), 2(20 rpm), 3(40 rpm), 4(60 rpm), 5(80 rpm) Polishing Time : 1(0 sec), 2(30 sec), 3(60 sec), 4(90 sec), 5(120 sec) Slurry Flow Rate : 1(20 ml/min), 2(40 ml/min), 3(60 ml/min), 4(80 ml/minc), 5(100 ml/min)
0.0 1
2
3
4
5
Process Parameters (Table Speed, Polishing Time, Slurry Flow) Fig. 1. DOE trend curves according to the typical CMP process parameters such as platen table speed, polishing time and slurry flow rate.
Fig. 2. AFM analysis of ITO thin film (a) before CMP and (b) after CMP.
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3.3. Optical characteristics
Fig. 3. AFM analysis of MEH-PPV thin film (a) before CMP and (b) after ITO-CMP.
Post-CMP : Threshold Voltage=12.5 V Pre-CMP : Threshold Voltage=14.5 V
Current Density [mA/mm2]
0.7 0.6 0.5 0.4 0.3 0.2
Fig. 5 compares the photoluminescence (PL) spectrum of unpolished ITO/MEH-PPV and polished ITO/MEH-PPV structure. After the CMP process, the intensity of PL peak was higher compared with PL peak before the CMP process. When the MEH-PPV was deposited on unpolished ITO surface, the two peaks were observed. The first peak showed a maximum value of 195 at the range of 540nm wavelength. And then, the second peak was 131 at the range of 680 nm. When the MEH-PPV was deposited on polished ITO surface, the first peak showed a maximum value of 211 at the range of 540 nm. And then, the second peak was 181 in the wavelength of 692 nm. After the CMP process, the intensity of PL peak was increased, and the peak of emitting region as shown in dotted line of Fig. 5 moved from 680 nm to 692 nm toward high wavelength region. This is caused by change of interface state between the ITO film and the MEH-PPV layer by CMP technique. Therefore, we expect that the emitting efficiency will be high when the device is fabricated by CMP process. Fig. 6 compares the optical transmittances of the polished and unpolished ITO thin film. Also, the optical transmittances of MEH-PPV layers deposited on polished and unpolished ITO film were compared. When the CMP process was performed on ITO thin film, the optical transmittance was slightly increased, and the peak was also increased in comparison with before CMP process. The transmittance curve of the emitting region as shown in dotted lines of Fig. 6 was not steep. These results indicate that the dispersion and the extinction of light were decreased due to the improvement of surface quality or interface state by CMP process. When the MEH-PPV was deposited on polished and unpolished ITO thin film, both the optical transmittance was similar in the range of 430 nm. However, the optical transmittance of the polished ITO thin film in the light-emitting peak region of above 680 nm was increased. This result is also due to improvement of interface property between the polished ITO thin film and MEH-PPV layer. This means that the light-emitting property of OLEDs device will be improved because the optical transmittance is increased in the light-emitting region of MEH-PPV. The optical transmittance in the range of below 550 nm was remarkably decreased. However, when the MEH-PPV was deposited, the optical transmittance in the range of above 600 nm was increased more than 80%. When the ITO thin film was polished, the optical transmittance was increased more than 1% in comparison with unpolished ITO thin film. The optical transmittance is increased more than 85% in the 690 nm range, where the intensity of PL peak is the maximum.
0.1 0.0 -2
0
2
4 6 8 10 12 14 Operating Voltage [V]
16
18
20
Fig. 4. I–V curve before and after CMP process.
voltage was 14.5 V when the ITO thin film was used as a bottom electrode of OLED. However, when the polished ITO thin film was used, it was 12.5 V. Therefore, after the CMP process, the operating voltage was decreased because the contact area with the MEH-PPV layer was advanced by CMP planarization of the ITO surface. It is considered that the operating voltage was also decreased because the thickness of ITO thin film was reduced. That is to say, the decrease of operating voltage indicates the increase of Hall mobility in the OLED applications. In this point, we anticipate that our first approach, the CMP application of anode (ITO) layer as a bottom electrode, will make the operating speed of an OLED device faster.
Fig. 5. PL spectrum of ITO/MEH-PPV structure.
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4. Conclusion
Fig. 6. Optical transmittance of ITO/MEH-PPV monolayer before and after CMP.
We studied the effect of CMP process on the performances of OLED device with Al/MEH-PPV/ITO/glass structures. The optimum conditions for CMP process were determined by DOE method as follows; slurry flow rate, slurry temperature, polisher pressure, platen speed, and polishing time were 60 ml/min, 30 °C, 300 g/ cm2, 60 rpm, and 60 s, respectively. According to AFM analysis, the surface quality of ITO thin film was drastically elevated by CMP technique. As the interface state between ITO thin film and MEH-PPV was improved, the PL intensity was increased in the 680-nm wavelength range. Transmittance of low wavelength region was reduced. However, it was increased in the range of more than 690 nm where the intensity of PL peak is the maximum. Also, after CMP process, the operating voltage was decreased because the contact area with PPV layer was advanced by CMP planarization of ITO surface. The device characteristics will be advanced in the near future as both the PL property and the optical transmittance in the range of red wavelength is increased. Acknowledgment This work was supported by Korea Research Foundation Grant (KRF-2006-005-J00902). References
Fig. 7. Optical absorption of ITO/MEH-PPV monolayer before and after CMP.
Fig. 7 shows the optical absorption of ITO thin film and ITO/ MEH-PPV structure with and without CMP process, respectively. The absorption rate of polished ITO thin film is reduced. When the MEH-PPV is deposited on the polished ITO thin film, the absorption rate was decreased in comparison with unpolished ITO thin film. The decrease of the absorption makes the increase of the optical transmittance. The absorption is remarkably decreased in the range of more than 680 nm, which is a light emitting region.
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