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Simulation of the thin-film thickness distribution for an OLED thermal evaporation process
Vacuum 83 (2009) 848–852
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Simulation of the thin-film thickness distribution for an OLED thermal evaporation process Eungki Lee* Division of Mechanical and Automotive Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2007 Received in revised form 23 August 2008 Accepted 26 August 2008
A design of an OLED (organic light-emitting device) fabrication system strongly depends on a thermal evaporation process. In an OLED evaporation process, the essential requirements include good uniformity of the film thickness over a glass substrate. In this paper, a process simulation model was developed to predict the film thickness distribution by understanding system design parameters that affect the uniformity of film thickness. Based on the method developed in this study, the uniformity of the thickness in an organic layer was successfully controlled. The developed method allowed the manufacture of high quality OLED displays. Ó 2008 Elsevier Ltd. All rights reserved.
Keyword: OLED (organic light-emitting device) Vacuum thermal evaporation Thin film Thickness Uniformity PACS: 81.15.Ef Vacuum deposition 85.40.Sz Deposition technology
1. Introduction The trend in information display is moving from CRT to FDP, and there is much interest in information display research because of the significant potential of OLED (organic light-emitting device) for a number of consumer applications, and because of its many advantages, such as its simple structure, high reply speed, wide viewing angle, and low driving voltage [1,2]. OLEDs are expected to be used in a place of liquid crystal display (LCD) because of its desirable benefits [3]. A vacuum thermal evaporation process is the most mature and common method in the production of OLED displays. It allows the deposition of pure organic materials on a glass substrate without concerning the contamination of water or solvents [4]. Trends in display sizes have hauled the enlargement of mother glass substrates. The enlargement of substrates requires the improvement of the evaporation system design to control the uniformity of the organic film thickness for large glass substrates. The control of film thickness as a function of cell source in a thermal evaporation process is important in both industry and research community. Many investigations have been done to optimize the uniformity of such thickness in substrates for a sputtering process, pulsed arc process, and other processes [5,6]. However, few theoretical and * Tel.: þ82 41 550 0250; fax: þ82 41 550 9123. E-mail address: [email protected] 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.08.007
experimental researches have been conducted to produce uniformed films on a large deposition area in a thermal evaporation process. Some work has been reported in the prediction of film uniformity regarding the range of sources to substrate distances and in the use of computer programs to simulate the uniformity of film thickness [7]. In general, the molecular emission of a cell source can be described by using a cosine evaporation law. In the Pulker’s theory, the film thickness distribution was calculated for the cell source that has the integral exponent number of the cosine law. In practical applications in a thermal evaporation process, a cell source usually represents a real exponent number of the cosine law. Thus, it is required to expand the theory for the cell source that has a real exponent number of the cosine emission law. In this paper, the experimental determination of n was performed by measuring the thickness profile of substrates and fitting the measured profiles with those calculated. The main purpose of this paper is to simulate the film thickness in a thermal evaporation process for the fabrication of industrial OLEDs. In this paper, the thickness uniformity of the organic film deposited by a thermal evaporation process with the rotation of a glass substrate was studied. Also, a practical methodology in the design of an evaporation process was introduced. The evaporating deposition process has been simulated to improve the thickness uniformity according to the various arrangements in cell sources. Based on the method developed in this study, the user-defined thickness uniformity was successfully achieved.
E. Lee / Vacuum 83 (2009) 848–852
4
Nomenclature d d0 D D0
film thickness film thickness at the substrate rotation center film thickness under rotation conditions film thickness at the substrate rotation center under rotation conditions
2. Simulation and experiments A point-type source used in a conventional OLED fabrication system does not represent a uniform distribution in the emission of molecules. The deposited layers should be as uniform as possible, and therefore it is important to know the directional distribution of the molecules evaporated from an evaporation source. With the cosine evaporation law, the relation of the thickness D in a random point to the thickness D0 of the substrate center point on a rotating substrate is represented as Eq. (1). [7]
Z q2 nþ3 2 D 1 p ¼ 1þ p 0 h r 2 D0 h h
d4 2 inþ3 2 q r 2 h h cos 4 þ hq þ1
(1)
According to Eq. (1), the shape of the thickness profile is strongly influenced by the geometrical position of the evaporation source and the value of n. The geometrical position of the evaporation source is easy to establish, but the n-value for characterizing the shape of the deposition profile must be determined experimentally. For the determination of the cosine exponent n, the film thickness profile of the substrates was measured and fitted with the calculated profile. In this research, the experimental film was deposited by the center-located source with the distance of 480 mm from the substrate in the vacuum environment of 1.2 106 torr. The film thickness profile is shown in Fig. 1.
h n q r
849
substrate rotation angle distance between the cell source and the substrate cosine exponent cell source offset from the substrate rotation axis distance from the substrate rotation center on the substrate
As shown in Fig. 1, the thickness profile is asymmetric. It may be caused by the asymmetric arrangement of source heaters and the asymmetric charge of organic material powder. In this paper, the average value was considered to be the primary value as the cosine exponent n. The average value was calculated from the four cosine fitting values of four directions. An example for these four fitting directions is shown in Fig. 2. The least-square fitting method was applied for obtaining the best fit between the experiment and the calculation results. Fig. 3 shows the examples of cosine fitting along the four directions. One result of four examples is enlarged and presented in Fig. 4. As shown in Fig. 4, the calculated value of n was 4.5804 along the direction 3 of the substrate. The calculated four values and its average value are shown in Table 1. As shown in Figs. 3 and 4, the results of the cosine fitting are real numbers as shown in Table 1. For the case of the cell source and the chamber in use, the cosine law was decided to be appropriate for modeling the organic material evaporation by the n-value fitting results. With the calculated n-value, the thickness distribution profile can be predicted. The analytic solution of the integral does not exist for all possible n-values of the evaporation because the n-values are usually real numbers. Therefore, a numerical calculation method was used in this paper. The problem of the applied integration is to get the evaluation of a definite integral of Eq. (1). A numerical integration method may be obtained by approximating an integrand function using
Fig. 1. Experimental film thickness profiles for the determination of cosine exponent n.
850
E. Lee / Vacuum 83 (2009) 848–852
Fig. 2. Four cosine fitting directions.
Fig. 3. Four cosine fitting results along each four direction. (a) Cosine fitting along the direction 1. (b) Cosine fitting along the direction 2. (c) Cosine fitting along the direction 3. (d) Cosine fitting along the direction 4.
E. Lee / Vacuum 83 (2009) 848–852
851
Fig. 4. Results of the cosine fitting.
Table 1 Fitted n-values and its average value Direction
1
2
3
4
Average
n-value
4.9213
5.9493
4.5804
5.6010
5.2630
polynomials. In this paper, the trapezoidal rule was used to solve Eq. (1) with the real value n. As represented in Ref. [8], it may be helpful for that purpose. For example, thickness profiles were calculated using some of real-number values as shown in Fig. 5. To verify the algorithm used in the calculation of the thickness profile, experiments were performed. Table 2 represents the results of the experiments on the comparison of the film thickness. Also, it shows the thickness calculation error. The maximum error was 4.66 %.
Fig. 5. Numerically calculated thickness distribution profiles with various real-number exponents.
852
E. Lee / Vacuum 83 (2009) 848–852
Table 2 Thickness comparison between the predicted and the experimented values No.
Predicted thickness (A)
Experimented thickness (A)
Difference (%)
1 2 3 4 5 6 7 8 9 10 11
2132.7 3214.4 4601.8 6085.3 7228.8 7573.0 7228.8 6085.3 4601.8 3214.4 2132.7
2061 3299 4827 6120 7108 7573 7230 6284 4817 3217 2068
3.48 2.56 4.66 0.57 1.70 0.00 0.02 3.16 4.47 0.08 3.13
The shape of the resulting thickness profile in the direction of the radius of the substrate D/D0 was simulated with the calculated cosine exponents and the geometrical parameters q and h of the vapor source (Eq. (1)). The geometrical parameters q and h were used and allowed to vary the values in a computer simulation under defined geometrical boundary conditions, such as the dimension of the chamber and the glass area that is to be coated. The uniformity of the film thickness can be characterized by typical quantities, such as Max–Min, which are determined by the following Eq. (2).
Max Min Uniformity ð%Þ ¼
Max Min 100 Max þ Min
(2)
In Eq. (2), the elements of Max and Min on the right side in Eq. (2) are the maximum and minimum values of the film thickness, respectively [9]. Fig. 6 shows the trend of the thickness uniformity according to the various positions of the evaporation source. Briefly, a decrease in the film thickness according to the increase in the distance of the source from the rotation axis is more marked at the higher values of the exponent n of the vapor cloud characteristics. Utilizing the proposed methodology, the point-type cell source was located at h ¼ 520 mm and q ¼ 360 mm for depositing the organic film on a 550 650 mm2 of glass substrate. Then, the thickness uniformity was achieved as 8.5%.
1000
900
800
3. Conclusions 700
A computer simulation method for an OLED evaporation process was developed to improve the thickness uniformity and the evaporating efficiency. The evaporation process was simulated as the various arrangements for cell sources. The shape of the resulting thickness profile in the direction of the radius of the substrate D/D0 was strongly influenced by the geometrical position of the vapor source and the exponent n. The developed method was verified by the evaporation experiments and related measurements. With the presented method, OLED displays were manufactured as a high level of performance. The measured and calculated thickness was also compared. It showed that the measured thickness distribution can be successfully described through applying a cosine evaporation law. The proposed geometric simulation was able to build an OLED evaporator designer that is able to check the process performance of the evaporating system mentioned in this study. Thus, the performance of OLED displays can be precisely improved by applying the allowable specification of the evaporation process.
h (mm)
600
500
400
300
References 200
100 0
100
200
q (mm)
300
400
Fig. 6. Film thickness uniformity (%) as a function of cell source position q and h.
[1] [2] [3] [4] [5] [6]
Kin Z, Kajii H, Ohmori U. Thin Solid Films 2006;499:392. Lee J, Wu M, Chao C, Chen H, Leung M. Chem Phys Lett 2005;416:234. Iwama Y, Itoh T, Mori T, Mizutani T. Thin Solid Films 2006;499:364. Glang M. Handbook of thin film technology. McGraw-Hill; 1970. Silva M, Nicholls JR. Surf Coat Technol 2001;142:934. Fuchs H, Engers B, Hettkamp E, Mecke H, Shultz J. Surf Coat Technol 2001;142:655. [7] Pulker HK. Coating on glass. Elsevier Sicence Publishing Co.; 1987. [8] Kreyszig E. Advanced engineering mathematics. John Wiley & Sons; 1983. [9] Fu C, Yang C, Han L, Chen H. Surf Coat Technol 2006;200:3687.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Simulation of the thin-film thickness distribution for an OLED thermal evaporation process Eungki Lee* Division of Mechanical and Automotive Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2007 Received in revised form 23 August 2008 Accepted 26 August 2008
A design of an OLED (organic light-emitting device) fabrication system strongly depends on a thermal evaporation process. In an OLED evaporation process, the essential requirements include good uniformity of the film thickness over a glass substrate. In this paper, a process simulation model was developed to predict the film thickness distribution by understanding system design parameters that affect the uniformity of film thickness. Based on the method developed in this study, the uniformity of the thickness in an organic layer was successfully controlled. The developed method allowed the manufacture of high quality OLED displays. Ó 2008 Elsevier Ltd. All rights reserved.
Keyword: OLED (organic light-emitting device) Vacuum thermal evaporation Thin film Thickness Uniformity PACS: 81.15.Ef Vacuum deposition 85.40.Sz Deposition technology
1. Introduction The trend in information display is moving from CRT to FDP, and there is much interest in information display research because of the significant potential of OLED (organic light-emitting device) for a number of consumer applications, and because of its many advantages, such as its simple structure, high reply speed, wide viewing angle, and low driving voltage [1,2]. OLEDs are expected to be used in a place of liquid crystal display (LCD) because of its desirable benefits [3]. A vacuum thermal evaporation process is the most mature and common method in the production of OLED displays. It allows the deposition of pure organic materials on a glass substrate without concerning the contamination of water or solvents [4]. Trends in display sizes have hauled the enlargement of mother glass substrates. The enlargement of substrates requires the improvement of the evaporation system design to control the uniformity of the organic film thickness for large glass substrates. The control of film thickness as a function of cell source in a thermal evaporation process is important in both industry and research community. Many investigations have been done to optimize the uniformity of such thickness in substrates for a sputtering process, pulsed arc process, and other processes [5,6]. However, few theoretical and * Tel.: þ82 41 550 0250; fax: þ82 41 550 9123. E-mail address: [email protected] 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.08.007
experimental researches have been conducted to produce uniformed films on a large deposition area in a thermal evaporation process. Some work has been reported in the prediction of film uniformity regarding the range of sources to substrate distances and in the use of computer programs to simulate the uniformity of film thickness [7]. In general, the molecular emission of a cell source can be described by using a cosine evaporation law. In the Pulker’s theory, the film thickness distribution was calculated for the cell source that has the integral exponent number of the cosine law. In practical applications in a thermal evaporation process, a cell source usually represents a real exponent number of the cosine law. Thus, it is required to expand the theory for the cell source that has a real exponent number of the cosine emission law. In this paper, the experimental determination of n was performed by measuring the thickness profile of substrates and fitting the measured profiles with those calculated. The main purpose of this paper is to simulate the film thickness in a thermal evaporation process for the fabrication of industrial OLEDs. In this paper, the thickness uniformity of the organic film deposited by a thermal evaporation process with the rotation of a glass substrate was studied. Also, a practical methodology in the design of an evaporation process was introduced. The evaporating deposition process has been simulated to improve the thickness uniformity according to the various arrangements in cell sources. Based on the method developed in this study, the user-defined thickness uniformity was successfully achieved.
E. Lee / Vacuum 83 (2009) 848–852
4
Nomenclature d d0 D D0
film thickness film thickness at the substrate rotation center film thickness under rotation conditions film thickness at the substrate rotation center under rotation conditions
2. Simulation and experiments A point-type source used in a conventional OLED fabrication system does not represent a uniform distribution in the emission of molecules. The deposited layers should be as uniform as possible, and therefore it is important to know the directional distribution of the molecules evaporated from an evaporation source. With the cosine evaporation law, the relation of the thickness D in a random point to the thickness D0 of the substrate center point on a rotating substrate is represented as Eq. (1). [7]
Z q2 nþ3 2 D 1 p ¼ 1þ p 0 h r 2 D0 h h
d4 2 inþ3 2 q r 2 h h cos 4 þ hq þ1
(1)
According to Eq. (1), the shape of the thickness profile is strongly influenced by the geometrical position of the evaporation source and the value of n. The geometrical position of the evaporation source is easy to establish, but the n-value for characterizing the shape of the deposition profile must be determined experimentally. For the determination of the cosine exponent n, the film thickness profile of the substrates was measured and fitted with the calculated profile. In this research, the experimental film was deposited by the center-located source with the distance of 480 mm from the substrate in the vacuum environment of 1.2 106 torr. The film thickness profile is shown in Fig. 1.
h n q r
849
substrate rotation angle distance between the cell source and the substrate cosine exponent cell source offset from the substrate rotation axis distance from the substrate rotation center on the substrate
As shown in Fig. 1, the thickness profile is asymmetric. It may be caused by the asymmetric arrangement of source heaters and the asymmetric charge of organic material powder. In this paper, the average value was considered to be the primary value as the cosine exponent n. The average value was calculated from the four cosine fitting values of four directions. An example for these four fitting directions is shown in Fig. 2. The least-square fitting method was applied for obtaining the best fit between the experiment and the calculation results. Fig. 3 shows the examples of cosine fitting along the four directions. One result of four examples is enlarged and presented in Fig. 4. As shown in Fig. 4, the calculated value of n was 4.5804 along the direction 3 of the substrate. The calculated four values and its average value are shown in Table 1. As shown in Figs. 3 and 4, the results of the cosine fitting are real numbers as shown in Table 1. For the case of the cell source and the chamber in use, the cosine law was decided to be appropriate for modeling the organic material evaporation by the n-value fitting results. With the calculated n-value, the thickness distribution profile can be predicted. The analytic solution of the integral does not exist for all possible n-values of the evaporation because the n-values are usually real numbers. Therefore, a numerical calculation method was used in this paper. The problem of the applied integration is to get the evaluation of a definite integral of Eq. (1). A numerical integration method may be obtained by approximating an integrand function using
Fig. 1. Experimental film thickness profiles for the determination of cosine exponent n.
850
E. Lee / Vacuum 83 (2009) 848–852
Fig. 2. Four cosine fitting directions.
Fig. 3. Four cosine fitting results along each four direction. (a) Cosine fitting along the direction 1. (b) Cosine fitting along the direction 2. (c) Cosine fitting along the direction 3. (d) Cosine fitting along the direction 4.
E. Lee / Vacuum 83 (2009) 848–852
851
Fig. 4. Results of the cosine fitting.
Table 1 Fitted n-values and its average value Direction
1
2
3
4
Average
n-value
4.9213
5.9493
4.5804
5.6010
5.2630
polynomials. In this paper, the trapezoidal rule was used to solve Eq. (1) with the real value n. As represented in Ref. [8], it may be helpful for that purpose. For example, thickness profiles were calculated using some of real-number values as shown in Fig. 5. To verify the algorithm used in the calculation of the thickness profile, experiments were performed. Table 2 represents the results of the experiments on the comparison of the film thickness. Also, it shows the thickness calculation error. The maximum error was 4.66 %.
Fig. 5. Numerically calculated thickness distribution profiles with various real-number exponents.
852
E. Lee / Vacuum 83 (2009) 848–852
Table 2 Thickness comparison between the predicted and the experimented values No.
Predicted thickness (A)
Experimented thickness (A)
Difference (%)
1 2 3 4 5 6 7 8 9 10 11
2132.7 3214.4 4601.8 6085.3 7228.8 7573.0 7228.8 6085.3 4601.8 3214.4 2132.7
2061 3299 4827 6120 7108 7573 7230 6284 4817 3217 2068
3.48 2.56 4.66 0.57 1.70 0.00 0.02 3.16 4.47 0.08 3.13
The shape of the resulting thickness profile in the direction of the radius of the substrate D/D0 was simulated with the calculated cosine exponents and the geometrical parameters q and h of the vapor source (Eq. (1)). The geometrical parameters q and h were used and allowed to vary the values in a computer simulation under defined geometrical boundary conditions, such as the dimension of the chamber and the glass area that is to be coated. The uniformity of the film thickness can be characterized by typical quantities, such as Max–Min, which are determined by the following Eq. (2).
Max Min Uniformity ð%Þ ¼
Max Min 100 Max þ Min
(2)
In Eq. (2), the elements of Max and Min on the right side in Eq. (2) are the maximum and minimum values of the film thickness, respectively [9]. Fig. 6 shows the trend of the thickness uniformity according to the various positions of the evaporation source. Briefly, a decrease in the film thickness according to the increase in the distance of the source from the rotation axis is more marked at the higher values of the exponent n of the vapor cloud characteristics. Utilizing the proposed methodology, the point-type cell source was located at h ¼ 520 mm and q ¼ 360 mm for depositing the organic film on a 550 650 mm2 of glass substrate. Then, the thickness uniformity was achieved as 8.5%.
1000
900
800
3. Conclusions 700
A computer simulation method for an OLED evaporation process was developed to improve the thickness uniformity and the evaporating efficiency. The evaporation process was simulated as the various arrangements for cell sources. The shape of the resulting thickness profile in the direction of the radius of the substrate D/D0 was strongly influenced by the geometrical position of the vapor source and the exponent n. The developed method was verified by the evaporation experiments and related measurements. With the presented method, OLED displays were manufactured as a high level of performance. The measured and calculated thickness was also compared. It showed that the measured thickness distribution can be successfully described through applying a cosine evaporation law. The proposed geometric simulation was able to build an OLED evaporator designer that is able to check the process performance of the evaporating system mentioned in this study. Thus, the performance of OLED displays can be precisely improved by applying the allowable specification of the evaporation process.
h (mm)
600
500
400
300
References 200
100 0
100
200
q (mm)
300
400
Fig. 6. Film thickness uniformity (%) as a function of cell source position q and h.
[1] [2] [3] [4] [5] [6]
Kin Z, Kajii H, Ohmori U. Thin Solid Films 2006;499:392. Lee J, Wu M, Chao C, Chen H, Leung M. Chem Phys Lett 2005;416:234. Iwama Y, Itoh T, Mori T, Mizutani T. Thin Solid Films 2006;499:364. Glang M. Handbook of thin film technology. McGraw-Hill; 1970. Silva M, Nicholls JR. Surf Coat Technol 2001;142:934. Fuchs H, Engers B, Hettkamp E, Mecke H, Shultz J. Surf Coat Technol 2001;142:655. [7] Pulker HK. Coating on glass. Elsevier Sicence Publishing Co.; 1987. [8] Kreyszig E. Advanced engineering mathematics. John Wiley & Sons; 1983. [9] Fu C, Yang C, Han L, Chen H. Surf Coat Technol 2006;200:3687.