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Design of a new MD-600C ZnO-MOCVD reaction cavity and study of process parameters
International Communications in Heat and Mass Transfer 110 (2020) 104394
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Design of a new MD-600C ZnO-MOCVD reaction cavity and study of process parameters Jian Lia,b,c, Tiecheng Luoa,c, Jie Wanga,b, Gang Wanga,b,c, Yanli Peia,c, , Jiajia Dengd, ⁎
T
⁎⁎
a
State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 51006, China Foshan Research Institute of Sun Yat-sen University, Foshan 528225, China c School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 51006, China d The School of Port and Transportation Engineering，Zhejiang Ocean University, Zhoushan 316000, China b
ARTICLE INFO
ABSTRACT
Keywords: MD-600C ZnO-MOCVD ZnO film Orthogonal experiments Deposition rate Uniformity
In this study, a low maintenance cost and high production efficiency MD-600C ZnO-MOCVD is developed based on the mass-produced MD-600B ZnO-MOCVD. Based on the design model of the MD-600C reaction chamber, the chemical reaction-transport model of ZnO grown with DEZn and O2 was used to analyze the deposition rate and uniformity of ZnO thin films by orthogonal experimental design. The effects of growth parameters on film quality were determined. The results show that the ZnO thin films prepared under the optimized conditions have good uniformity and a good deposition rate. The growth temperature and chamber pressure have the greatest influence on the deposition rate quality, followed by the inlet flow rate and the base speed, and the most important factors affecting film uniformity are the cavity pressure and inlet flow rate, followed by the base speed and growth temperature. The optimum combination process was simultaneously obtained by orthogonal analysis. The above results provide a solution for high quality epitaxy growth of MD-600C ZnO-MOCVD and a theoretical basis for equipment process improvement.
1. Introduction As a new third generation semiconductor material, ZnO has received extensive attention from researchers and become one of the focuses of research in the field of semiconductor materials [1–3]. ZnO is a multifunctional semiconductor material with wide bandgap and optimal photoelectric and piezoelectric properties. In addition, ZnO has several advantages such as the abundant raw materials, low price, strong radiation resistance, and environmental friendliness. It has been widely used in many fields such as transparent electrode materials, ultraviolet detectors, GaN buffer layers, surface acoustic wave devices, and light emitting devices [4–6]. At present, metalorganic chemical vapor deposition (MOCVD) is the most efficient way to realize the commercial production of III-Vand IIVI group compound semiconductor films [7,8]. It can grow in a large area, control composition and thickness accurately, have high repeatability, high growth rate, and cover complex substrate shape. The steep interface of multilayers can be prepared by quickly switching the gas path, and in situ annealing can be carried out. This makes it a preferred method for the growth of ZnO materials [9,10].
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Because of the complexity of MOCVD systems and the increase in film quality requirements, traditional design methods have been unable to address the high human and material costs and time cycle problems. Simulation has become an indispensable, powerful tool for reactor design [11–15]. Veeco Equipment Co. (USA), the world's most advanced manufacturer of MOCVD systems, has set up its own special simulation department, which greatly saves the design cost and improves the design efficiency [16,17]. Among them, B. Mitrovic [18] et al. analyzed the stability of the commercial E300GaN Veeco Turbodisc reactor and determined the typical flow states in the chamber (plug-in, stagnationin, buoyancy-induced, and rotation-induced flow regimes). The corresponding stability was deduced under different process parameters. Character maps provide guidance for the determination of MOCVD process parameters. C.H. Lin [19] and Li [20] found that gas-phase eddy current can be suppressed by adding a porous media region above the reaction chamber. The height of the porous media region has a positive relationship with the stability of the flow field. Hu Shaolin et al. proposed a novel BDS MOCVD reactor [21]; their findings show that with the increase of working pressure, the growth rate decreases, and with the increase of total gas flow rate, the growth rate increases
Corresponding author at: State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 51006, China. Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Y. Pei), [email protected] (J. Deng).
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https://doi.org/10.1016/j.icheatmasstransfer.2019.104394
0735-1933/ © 2019 Elsevier Ltd. All rights reserved.
International Communications in Heat and Mass Transfer 110 (2020) 104394
J. Li, et al.
Fig. 1. (a) ZnO-MOCVD chamber model, (b) Schematic diagram of gas flow, (c) Simulation model, and (d) Size and boundary.
significantly. The reaction chamber of a close-coupled showerhead (CCS) MOCVD was studied in detail by Zhi Zhang et al. [22,23]. The results show that the growth rate and uniformity are closely related to the nozzle height of the total gas flow rate and the inlet temperature. With the development of large-capacity commercialized MOCVD systems, the film uniformity problem has become increasingly prominent [24,25]. For this reason, the corresponding relationship between process parameters and film growth uniformity was established through mathematical modeling, and the process parameters were optimized using an optimization algorithm. The process control of complex MOCVD equipment was realized; this is expected to significantly promote the rapid development of equipment and materials. In this study, a new type of MD-600C ZnO-MOCVD is designed, which combines the advantages of GaN-K465i and the previous generation MD-600B ZnO-MOCVD. Compared with the previous generation of MD-600B, the results show that the deposition rate has been improved under the same process conditions. Based on computational fluid dynamic (CFD) software and the chemical reaction-transport model of a DEZn and O2 grown ZnO film, four process parameters, such as inlet flow rate, chamber pressure, rotational speed, and growth temperature, are selected as variables. A set of orthogonal experiments with four factors and four levels were designed based on other fixed parameters. The optimum technological conditions and process parameters for the preparation of ZnO thin films were obtained. Our findings lay a strong foundation for further epitaxy growth experiments on the basis of MD-600C ZnO-MOCVD.
2. Problem formulation 2.1. mathematical models The growth of zinc oxide by MOCVD is carried out under low pressure; a mixed gas is considered as an ideal gas, and the system is in a laminar flow state. The mathematical model is as follows [22,23,26–30]: The conservation equations can be described as (1)
( v) = 0 ( v v) =
p+
(2)
g
where ρ is the density, v is velocity vector of the gas mixture, is the shear stress tensor, and p and g are the pressure and the gravitational acceleration, respectively. The energy equation strongly coupled with fluid flow is given by N
Cp
( v T) =
(k T) + i=1
Hi Mi
N
Ji i=1
Hi,0 Ri Mi
(3)
where Cp is the specific heat capacity, k is the thermal conductivity, and T is the temperature.Hi, Hi, 0, Mi and Ji represent the molar enthalpy, the enthalpy of formation, the molar mass, and the diffusion flux of species i, respectively. Ri is the net volumetric generation rate of species i. For the epitaxial growth of ZnO, the species transport equations 2
International Communications in Heat and Mass Transfer 110 (2020) 104394
J. Li, et al.
model is generally in the following form: K
( v Y) i =
Ji + Mi
Rij
(4)
j=1
Rji
where Yi is the mass fraction of species i, and is the net volumetric rate of creation of the species i during the reaction j. For a system with N gas species, the gas reaction mechanism is described as. N
vik Si
kk
i=1
N
vik Si i = 1…N
(5)
i=1
where Si represents the ith species. vik′ and vik′′ are species stoichiometric coefficients of the ith reactant and product in the reaction k. The growth process includes complex surface reactions. The kth surface reaction can be described as Ng i=1
Nb
gik Gi +
Ng
Ns
bik Bi + i=1
sik Si i=1
i=1
Nb
gik Gi +
Ns
bik Bi + i=1
sik Si, k i=1
Fig. 2. Schematic diagram of global and local grids for MOCVD reaction chamber.
(6)
= 1…N
where Gi, Bi and Si are the gas phase, deposited phase, and surface phase species, respectively; Ng, Nb, and Ns represent numbers of the above corresponding species; gik′, bik′, and sik′ are the stoichiometric coefficients of the species in reactants, and gik′′, bik′′, and sik′′ are the coefficients of the products, respectively. The reaction mechanisms are calculated based on quantum chemistry, including ten gas and eight surface reactions [31]. A detailed understanding of this can be found in our previous work [20].
of the mathematical model of the system: 1: The growth of materials is considered to be stable and the flow is considered to be laminar because the Reynolds number is < 2300. 2: It is considered that the graphite disc has good thermal conductivity, the growth temperature is 773 K and the rotation speed is 750 rpm. The pressure inside the cavity is 10 Torr. 3: The inlet flow rate of the cavity is 4545.4 sccm, in which the flow rate of AR gas is 4441.7 sccm, the flow rate of DEZn is 5.1 sccm, and the flow rate of O2 is 98.6 sccm. 4: It is assumed that both heat and species mass transfer fluxes are zero at the exhaust.
2.2. Geometric model, mesh description, and boundary conditions Fig. 1(a) shows the geometric structure of the MD-600C ZnOMOCVD reaction chamber. The carrier gas (AR) carries the reaction gas (DEZn and O2) into the mixing chamber through different intake ports and mixes evenly, after which it sprays into the reaction chamber through a tubular conical hole. The flow diagram is shown in Fig. 1(b). The graphite disc base is heated by a heating sheet. Because of its good thermal conductivity, it can keep constant temperature during the growth process and rotate at high speed through electric control. Reactive gases enter the reaction chamber and react at high temperature; some of them are deposited on the surface of the substrate to produce ZnO, while others are discharged from the chamber through the exhaust holes. In order to ensure computational efficiency and neglect the structure above spray type, it is assumed that the gas mixes uniformly in the spray chamber and the flow velocity is the same. The simplified calculation model and size are shown in Fig. 1(c) and Fig. 1(d). The MOCVD reaction chamber model is meshed by a hexahedral mesh which is refined where the flow rate, temperature gradient, and the base surface change significantly. The grid independence is checked before calculation. The predicted sedimentation rate for each grid number is shown in Table 1; the relative deposition rate varies from 20.7 million to 32.8 million meshes, and the maximum error of the average deposition rate is 2.25%. This indicates that the numerical simulation results will change slightly with the further strengthening of the meshes. Considering the accuracy of the calculation results and the cost of calculation, 20.7 million meshes were selected for subsequent simulation. The grid model is shown in Fig. 2. The following boundary conditions have been used in the solution
3. Results and discussion 3.1. Model validation MD-600C ZnO-MOCVD is still in the design stage, and requires a certain amount of funds and time for prototype development. At present, the self-developed MD-600B ZnO-MOCVD has been industrialized. The prototype model is shown in Fig. 3(a) and Fig. 3(b). The system is mainly composed of five subsystems: the reaction chamber system, vacuum system and gas delivery system, growth loading and unloading system, and software and hardware automatic control system. The advantage of this model is that the mixing mode of the reaction source is easily realized, and the MO source controlled by five MFCs can achieve efficient film deposition, while the disadvantage is that the plate-like structure of fine mesh holes at the top of the reaction chamber is complex, difficult to process, and easy to deform when heated; additionally, because of the strong oxide pre-reaction, the nanoparticles produced are easy to deposit and clog the mesh holes, as shown in Fig. 3(c) and Fig. 3(d). MD600-C ZnO-MOCVD adopts the conical hole intake method, which avoids the adverse effects of the porous plate. It only needs to increase the carrier gas flow to remove nanoparticles, making the subsequent cleaning and maintenance convenient. In order to verify the quality of ZnO epitaxy grown by MD-600C ZnO-MOCVD, a comparison was made between MD-600B ZnO-MOCVD and MD-600B ZnO-MOCVD under the standard growth conditions. Both models are grown on 38*2 in. substrates. The distribution of substrates on graphite discs is shown in Fig. 4(a). Three points are selected uniformly in the radial direction for each substrate, and nine points are collected in the whole disk, as shown in Fig. 4(b). The evaluation of film quality includes the average deposition rate and film uniformity [12]. The uniformity of the films is inversely
Table 1 Average deposition rate for different grid numbers. Mesh number(106)
3.2
5.6
9.2
20.7
32.8
Average deposition rate (μm/h) Relative error
0.2304 12.1%
0.2397 8.51%
0.2486 5.11%
0.2554 2.25%
0.2620 /
3
International Communications in Heat and Mass Transfer 110 (2020) 104394
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Fig. 3. MD-600B ZnO-MOCVD: (a) Mass production machine, (b) Appearance of reaction chamber, (c) Top spray mesh plate, and (d) Local details of spray mesh orifice plate.
Fig. 4. Distribution of wafers above substrate. (a) Physical model and (b) Simulation model and point data.
proportional to the coefficient of variation; a coefficient of variation below 5% is generally considered acceptable. The film thickness at these nine points was measured by Filmetrics Optical Film Thickness Measuring Instrument F20 Thin-Film Analyzer. The experimental and numerical results of MD-600B ZnO-MOCVD and MD-600C ZnO-MOCVD are shown in Table 2 and Fig. 5. The results show that the uniformity of films grown by MD-600B ZnO-MOCVD under standard processing conditions is good; the coefficient of variation was 4.15%, which is the result of hundreds of process parameter adjustments. The experimental
results are in good agreement with the numerical simulation results. The maximum error of the deposition rate of monitoring points is < 4%, and the maximum error of average deposition rate of the whole disk is < 1%. The MD-600C ZnO-MOCVD simulation results show that its deposition rate is significantly higher than that of the MD-600B ZnOMOCVD equipment, the average deposition rate being 31.85% higher. It can be seen that the intake mode of the new model has been optimized not only in structure, but also the deposition rate has improved, 4
International Communications in Heat and Mass Transfer 110 (2020) 104394
J. Li, et al.
Table 2 Experimental data and numerical simulation results. Location of monitoring points
1 2 3 4 5 6 7 8 9 average
0.03 0.05 0.07 0.09 0.11 0.13 0.15 0.17 0.19 value
MD-600B ZnO-MOCVD Deposition rate (μm/h) Experimental data
Simulation data
0.1959 0.2067 0.2029 0.1921 0.1849 0.1827 0.1872 0.1932 0.1976 0.1937
0.1894 0.2010 0.1967 0.1948 0.1908 0.1898 0.1921 0.193 0.1947 0.1936
Relative error (%)
3.32 2.76 3.06 1.40 3.19 3.89 2.61 0.10 0.15 0.05
Table 3 Factor arrangement in orthogonal experiment. MD-600C ZnOMOCVD Deposition rate (μm/h) simulation data
Factor
Level
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm)
0.1930 0.2146 0.2348 0.2545 0.2658 0.2749 0.2822 0.2876 0.2910 0.2554
1
2
3
4
673 600 5 1500
773 750 10 2000
873 900 15 2500
973 1050 20 3000
Table 4 results of orthogonal experiment. Experimental number
Column number A
B
C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1
D
Average deposition rate (μm/h)
Film uniformity
1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3
0.4639 0.3360 0.5872 0.8474 0.2454 0.2115 0.7068 0.4837 0.3402 0.5756 0.2810 0.2151 0.4913 0.3223 0.1538 0.2056
0.3171 0.0829 0.0911 0.1123 0.1441 0.3961 0.0301 0.039 0.2017 0.1193 0.2869 0.0414 0.0910 0.0226 0.1915 0.2883
Bold type indicates that the process conditions have a significant impact on the film uniformity. Table 5 Analysis of variance of process parameters on film deposition rate. Fig. 5. Experimental and simulated results.
taking full advantage of the reaction source gas, saving resources, and reducing production costs. However, it can also be seen that the process parameters of MD-600C ZnO-MOCVD without mediation result in poor uniformity of the whole disk. The variation coefficient is 13.4%. The deposition rate of the films increases gradually from inner ring to outer
source
III Type squares
DF
mean square
F
P-value
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm) Error Optimization grouping
0.155 0.016 0.418 0.003 0.006 A1B3C4D1
3 3 3 3 3
0.052 0.005 0.139 0.001 0.002
26.849 2.848 72.281 0.564
0.011 0.206 0.003 0.675
Bold type indicates that the process conditions have a significant impact on the film uniformity.
Fig. 6. (a) Susceptor deposition rate distribution and (b) MO source velocity streamline. 5
International Communications in Heat and Mass Transfer 110 (2020) 104394
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4. Conclusion
Table 6 Analysis of variance of process parameters on film uniformity. source
III Type squares
DF
mean square
F
P-value
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm) Error Optimization grouping
0 0.009 0.153 0.034 0.001 A1B4C4D1
3 3 3 3 3
0 0.003 0.051 0.011 0
0.335 6.958 114.583 25.66
0.803 0.073 0.001 0.012
In this study, a new type of MD-600C ZnO-MOCVD reaction chamber was independently designed. The effects of different experimental parameters on the deposition rate and uniformity of films are studied. The following conclusions are obtained. (1) Under the same technological parameters, the deposition rate of model MD-600C is higher than that of model MD-600B. Technological parameters are required to meet the uniformity requirements with a coefficient of variation below 5%. (2) The growth conditions of ZnO thin films prepared by MOCVD technology were optimized using an orthogonal experimental design. The effects of growth temperature, base speed, chamber pressure, and inlet flow rate on the film quality were studied. The experimental results are evaluated by the significance test of variance analysis, which shows that all the errors in the experimental process are within the acceptable range, and the optimal film growth conditions under the current conditions are optimized. (3) For the deposition rate, the substrate temperature and cavity pressure play a significant role. With the decrease of base temperature, the deposition rate increases, and with the increase of cavity pressure above 10 Torr, the deposition rate increases. For film uniformity, cavity pressure and inlet flow rate play a primary role. With the increase of cavity pressure or the decrease of inlet flow rate, film uniformity improves.
Bold type indicates that the process conditions have a significant impact on the film uniformity.
ring as shown in Fig. 6(a). It can be seen from the AR + DEZn inlet streamline in Fig. 6(b) that the flow velocity of the MO source to the center of the substrate is very small under this process condition, resulting in the decrease of the DEZn concentration at the center of the substrate. The entire diffusion process takes place from the inner ring to the outside. If the process parameters are not appropriately matched, the film uniformity will be poor. This is the main explanation for the poor uniformity of the film. Because the thin film uniformity needs to be adjusted multiple times before the expectation can be reached, expensive manpower and material resources will be required to do so directly in the experiment. Therefore, based on the process conditions, the orthogonal film uniformity experiment was carried out by numerical simulation.
Declaration of Competing Interest
3.2. Analysis of design results based on orthogonal experiments
None.
The main factors affecting the growth of films by MOCVD include the flow rate of the source gas, the pressure inside the reaction chamber, the rotational speed of graphite disk and the temperature of graphite disk. Then, the optimum experimental conditions for the preparation of ZnO film with high uniformity and deposition rates were obtained by an orthogonal experimental design-assisted method through a series of experiments. The orthogonal experimental arrangement is shown in Table 3. The influencing factors are expressed as A, B, C, and D; each factor has four levels. According to the orthogonal table L16 (45), 16 groups of experiments were carried out. The orthogonal table and experimental results are shown in Table 4. The experimental results show that the uniformity of 7, 8, 12, and 14 in the orthogonal design scheme is < 5%, and the deposition rate is higher than MD-600B, which meets the expected optimization criteria. The variation of the film deposition rate and uniformity is a comprehensive result of the interaction of various experimental parameters. By using the intuitive analysis method of mathematical statistics, the optimum level of each factor can be preliminarily obtained, and the optimum combination of each factor in the experiment can be deduced. In Table 5, the results of variance analysis show that: (1) the P-value of parameters B and D are 0.206 and 0.675, respectively, which are than 0.05. The effect of parameters B and D on the deposition rate is not significant, that is, rotation speed and MO flow rate on deposition rate are not statistically significant; (2) the P-value of parameters A and C-Pvalue are 0.011 and 0.003, respectively, which are < 0.05. The parameters A and C have significant effects on the deposition rate, i.e., the substrate temperature and pressure have notable effects on the deposition rate. In Table 6, the variance analysis results shown in Table 6 show that: (1) the values of parameters A and B are 0.803 and 0.073, respectively, which are > 0.05. The effects of parameters A and B on film uniformity are not significant, that is, the effect of substrate temperature and rotation speed on film uniformity are not pronounced. (2) The values of parameters C and D are 0.001 and 0.012 respectively, which are < 0.05. The effects of parameters C and D on film uniformity are significant, that is, pressure and MO flow rate have clear effects on film uniformity.
Acknowledgments This study was partly supported by the Guangdong Province Key Fund (Grant no. 201804020051 and 2019B010132002), which we thank for providing financial support for this project. References [1] H. Hardhienata, I. Priyadi, B. Nurjanati, et al., a, J. Nonlinear Optical Phy. Mat. 27 (1) (2018) 1850025. [2] D.R. Yakovlev, V.V. Pavlov, A.V. Rodina, et al., Exciton spectroscopy of semiconductors by the method of optical harmonics generation (review)[J], Phys. Solid State 60 (8) (2018) 1471–1486. [3] K. Imasaka, S. Ashihara, T. Kaji, et al., Antenna-enhanced high harmonic generation in a wide-bandgap semiconductor ZnO[J], Opt. Express 26 (16) (2018) 21364. [4] F.F. Qin, C.X. Xu, Q.X. Zhu, et al., extra green light induced ZnO ultraviolet lasing enhancement assisted by au surface plasmons, Nanoscale 10 (2) (2017) 623–627. [5] Y.J. Guo, H.B. Lv, Y.F. Li, et al., High frequency microfluidic performance of LiNbO 3 and ZnO surface acoustic wave devices[J], J. Appl. Phys. 116 (2) (2014) 1458–1464. [6] A. Ghorai, S. Bayan, N. Gogurla, et al., Highly luminescent WS2 quantum dots/ZnO heterojunctions for light emitting devices[J], ACS Appl. Mater. Interfaces 9 (1) (2017) 558. [7] H.R. Qi, L.K. Yi, J.L. Huang, et al., Compensation of magnesium by residual carbon impurities in p-type GaN grown by MOCVD[J], J. Alloys Compd. 765 (2018) 245–248. [8] D. Wu, X.H. Tang, A. Olivier, et al., Free-standing GaAs nanowires growth on ITO glass by MOCVD[J], Mat. Res. Exp. 2 (4) (2015) 045002. [9] S. Pati, P. Banerji, S.B. Majumder, MOCVD grown ZnO thin film gas sensors: influence of microstructure[J], Sens. Actuat. A Phys. 213 (7) (2014) 52–58. [10] X.L. Chen, Y. Xu, J.M. Liu, et al., MOCVD-grown textured surface ZnO transparent conductive layers with modulated structure for thin film solar cells[J], Vacuum 109 (42) (2014) 74–77. [11] J. Li, Z.Y. Fei, Y.F. Xu, et al., Study on the optimization of the deposition rate of planetary GaN-MOCVD films based on CFD simulation and the corresponding surface model[J], R. Soc. Open Sci. 5 (2) (2018) 171757. [12] J. Li, J. Wang, J.D. Cai, et al., Numerical simulation and analysis of process parameters of GaN-MOCVD reactor[J], Int. Commun. Heat Mass Transfer 91 (2018) 64–76. [13] M.L. Tsai, C.C. Fang, L.Y. Lee, Numerical simulation of the temperature distribution in a planetary MOCVD reactor[J], Chem. Eng. Process. Process Intensif. 81 (2014) 48–58. [14] Z. Li, H. Li, J. Zhang, et al., A susceptor with partial-torus groove in vertical MOCVD
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Design of a new MD-600C ZnO-MOCVD reaction cavity and study of process parameters Jian Lia,b,c, Tiecheng Luoa,c, Jie Wanga,b, Gang Wanga,b,c, Yanli Peia,c, , Jiajia Dengd, ⁎
T
⁎⁎
a
State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 51006, China Foshan Research Institute of Sun Yat-sen University, Foshan 528225, China c School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 51006, China d The School of Port and Transportation Engineering，Zhejiang Ocean University, Zhoushan 316000, China b
ARTICLE INFO
ABSTRACT
Keywords: MD-600C ZnO-MOCVD ZnO film Orthogonal experiments Deposition rate Uniformity
In this study, a low maintenance cost and high production efficiency MD-600C ZnO-MOCVD is developed based on the mass-produced MD-600B ZnO-MOCVD. Based on the design model of the MD-600C reaction chamber, the chemical reaction-transport model of ZnO grown with DEZn and O2 was used to analyze the deposition rate and uniformity of ZnO thin films by orthogonal experimental design. The effects of growth parameters on film quality were determined. The results show that the ZnO thin films prepared under the optimized conditions have good uniformity and a good deposition rate. The growth temperature and chamber pressure have the greatest influence on the deposition rate quality, followed by the inlet flow rate and the base speed, and the most important factors affecting film uniformity are the cavity pressure and inlet flow rate, followed by the base speed and growth temperature. The optimum combination process was simultaneously obtained by orthogonal analysis. The above results provide a solution for high quality epitaxy growth of MD-600C ZnO-MOCVD and a theoretical basis for equipment process improvement.
1. Introduction As a new third generation semiconductor material, ZnO has received extensive attention from researchers and become one of the focuses of research in the field of semiconductor materials [1–3]. ZnO is a multifunctional semiconductor material with wide bandgap and optimal photoelectric and piezoelectric properties. In addition, ZnO has several advantages such as the abundant raw materials, low price, strong radiation resistance, and environmental friendliness. It has been widely used in many fields such as transparent electrode materials, ultraviolet detectors, GaN buffer layers, surface acoustic wave devices, and light emitting devices [4–6]. At present, metalorganic chemical vapor deposition (MOCVD) is the most efficient way to realize the commercial production of III-Vand IIVI group compound semiconductor films [7,8]. It can grow in a large area, control composition and thickness accurately, have high repeatability, high growth rate, and cover complex substrate shape. The steep interface of multilayers can be prepared by quickly switching the gas path, and in situ annealing can be carried out. This makes it a preferred method for the growth of ZnO materials [9,10].
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Because of the complexity of MOCVD systems and the increase in film quality requirements, traditional design methods have been unable to address the high human and material costs and time cycle problems. Simulation has become an indispensable, powerful tool for reactor design [11–15]. Veeco Equipment Co. (USA), the world's most advanced manufacturer of MOCVD systems, has set up its own special simulation department, which greatly saves the design cost and improves the design efficiency [16,17]. Among them, B. Mitrovic [18] et al. analyzed the stability of the commercial E300GaN Veeco Turbodisc reactor and determined the typical flow states in the chamber (plug-in, stagnationin, buoyancy-induced, and rotation-induced flow regimes). The corresponding stability was deduced under different process parameters. Character maps provide guidance for the determination of MOCVD process parameters. C.H. Lin [19] and Li [20] found that gas-phase eddy current can be suppressed by adding a porous media region above the reaction chamber. The height of the porous media region has a positive relationship with the stability of the flow field. Hu Shaolin et al. proposed a novel BDS MOCVD reactor [21]; their findings show that with the increase of working pressure, the growth rate decreases, and with the increase of total gas flow rate, the growth rate increases
Corresponding author at: State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 51006, China. Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Y. Pei), [email protected] (J. Deng).
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https://doi.org/10.1016/j.icheatmasstransfer.2019.104394
0735-1933/ © 2019 Elsevier Ltd. All rights reserved.
International Communications in Heat and Mass Transfer 110 (2020) 104394
J. Li, et al.
Fig. 1. (a) ZnO-MOCVD chamber model, (b) Schematic diagram of gas flow, (c) Simulation model, and (d) Size and boundary.
significantly. The reaction chamber of a close-coupled showerhead (CCS) MOCVD was studied in detail by Zhi Zhang et al. [22,23]. The results show that the growth rate and uniformity are closely related to the nozzle height of the total gas flow rate and the inlet temperature. With the development of large-capacity commercialized MOCVD systems, the film uniformity problem has become increasingly prominent [24,25]. For this reason, the corresponding relationship between process parameters and film growth uniformity was established through mathematical modeling, and the process parameters were optimized using an optimization algorithm. The process control of complex MOCVD equipment was realized; this is expected to significantly promote the rapid development of equipment and materials. In this study, a new type of MD-600C ZnO-MOCVD is designed, which combines the advantages of GaN-K465i and the previous generation MD-600B ZnO-MOCVD. Compared with the previous generation of MD-600B, the results show that the deposition rate has been improved under the same process conditions. Based on computational fluid dynamic (CFD) software and the chemical reaction-transport model of a DEZn and O2 grown ZnO film, four process parameters, such as inlet flow rate, chamber pressure, rotational speed, and growth temperature, are selected as variables. A set of orthogonal experiments with four factors and four levels were designed based on other fixed parameters. The optimum technological conditions and process parameters for the preparation of ZnO thin films were obtained. Our findings lay a strong foundation for further epitaxy growth experiments on the basis of MD-600C ZnO-MOCVD.
2. Problem formulation 2.1. mathematical models The growth of zinc oxide by MOCVD is carried out under low pressure; a mixed gas is considered as an ideal gas, and the system is in a laminar flow state. The mathematical model is as follows [22,23,26–30]: The conservation equations can be described as (1)
( v) = 0 ( v v) =
p+
(2)
g
where ρ is the density, v is velocity vector of the gas mixture, is the shear stress tensor, and p and g are the pressure and the gravitational acceleration, respectively. The energy equation strongly coupled with fluid flow is given by N
Cp
( v T) =
(k T) + i=1
Hi Mi
N
Ji i=1
Hi,0 Ri Mi
(3)
where Cp is the specific heat capacity, k is the thermal conductivity, and T is the temperature.Hi, Hi, 0, Mi and Ji represent the molar enthalpy, the enthalpy of formation, the molar mass, and the diffusion flux of species i, respectively. Ri is the net volumetric generation rate of species i. For the epitaxial growth of ZnO, the species transport equations 2
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model is generally in the following form: K
( v Y) i =
Ji + Mi
Rij
(4)
j=1
Rji
where Yi is the mass fraction of species i, and is the net volumetric rate of creation of the species i during the reaction j. For a system with N gas species, the gas reaction mechanism is described as. N
vik Si
kk
i=1
N
vik Si i = 1…N
(5)
i=1
where Si represents the ith species. vik′ and vik′′ are species stoichiometric coefficients of the ith reactant and product in the reaction k. The growth process includes complex surface reactions. The kth surface reaction can be described as Ng i=1
Nb
gik Gi +
Ng
Ns
bik Bi + i=1
sik Si i=1
i=1
Nb
gik Gi +
Ns
bik Bi + i=1
sik Si, k i=1
Fig. 2. Schematic diagram of global and local grids for MOCVD reaction chamber.
(6)
= 1…N
where Gi, Bi and Si are the gas phase, deposited phase, and surface phase species, respectively; Ng, Nb, and Ns represent numbers of the above corresponding species; gik′, bik′, and sik′ are the stoichiometric coefficients of the species in reactants, and gik′′, bik′′, and sik′′ are the coefficients of the products, respectively. The reaction mechanisms are calculated based on quantum chemistry, including ten gas and eight surface reactions [31]. A detailed understanding of this can be found in our previous work [20].
of the mathematical model of the system: 1: The growth of materials is considered to be stable and the flow is considered to be laminar because the Reynolds number is < 2300. 2: It is considered that the graphite disc has good thermal conductivity, the growth temperature is 773 K and the rotation speed is 750 rpm. The pressure inside the cavity is 10 Torr. 3: The inlet flow rate of the cavity is 4545.4 sccm, in which the flow rate of AR gas is 4441.7 sccm, the flow rate of DEZn is 5.1 sccm, and the flow rate of O2 is 98.6 sccm. 4: It is assumed that both heat and species mass transfer fluxes are zero at the exhaust.
2.2. Geometric model, mesh description, and boundary conditions Fig. 1(a) shows the geometric structure of the MD-600C ZnOMOCVD reaction chamber. The carrier gas (AR) carries the reaction gas (DEZn and O2) into the mixing chamber through different intake ports and mixes evenly, after which it sprays into the reaction chamber through a tubular conical hole. The flow diagram is shown in Fig. 1(b). The graphite disc base is heated by a heating sheet. Because of its good thermal conductivity, it can keep constant temperature during the growth process and rotate at high speed through electric control. Reactive gases enter the reaction chamber and react at high temperature; some of them are deposited on the surface of the substrate to produce ZnO, while others are discharged from the chamber through the exhaust holes. In order to ensure computational efficiency and neglect the structure above spray type, it is assumed that the gas mixes uniformly in the spray chamber and the flow velocity is the same. The simplified calculation model and size are shown in Fig. 1(c) and Fig. 1(d). The MOCVD reaction chamber model is meshed by a hexahedral mesh which is refined where the flow rate, temperature gradient, and the base surface change significantly. The grid independence is checked before calculation. The predicted sedimentation rate for each grid number is shown in Table 1; the relative deposition rate varies from 20.7 million to 32.8 million meshes, and the maximum error of the average deposition rate is 2.25%. This indicates that the numerical simulation results will change slightly with the further strengthening of the meshes. Considering the accuracy of the calculation results and the cost of calculation, 20.7 million meshes were selected for subsequent simulation. The grid model is shown in Fig. 2. The following boundary conditions have been used in the solution
3. Results and discussion 3.1. Model validation MD-600C ZnO-MOCVD is still in the design stage, and requires a certain amount of funds and time for prototype development. At present, the self-developed MD-600B ZnO-MOCVD has been industrialized. The prototype model is shown in Fig. 3(a) and Fig. 3(b). The system is mainly composed of five subsystems: the reaction chamber system, vacuum system and gas delivery system, growth loading and unloading system, and software and hardware automatic control system. The advantage of this model is that the mixing mode of the reaction source is easily realized, and the MO source controlled by five MFCs can achieve efficient film deposition, while the disadvantage is that the plate-like structure of fine mesh holes at the top of the reaction chamber is complex, difficult to process, and easy to deform when heated; additionally, because of the strong oxide pre-reaction, the nanoparticles produced are easy to deposit and clog the mesh holes, as shown in Fig. 3(c) and Fig. 3(d). MD600-C ZnO-MOCVD adopts the conical hole intake method, which avoids the adverse effects of the porous plate. It only needs to increase the carrier gas flow to remove nanoparticles, making the subsequent cleaning and maintenance convenient. In order to verify the quality of ZnO epitaxy grown by MD-600C ZnO-MOCVD, a comparison was made between MD-600B ZnO-MOCVD and MD-600B ZnO-MOCVD under the standard growth conditions. Both models are grown on 38*2 in. substrates. The distribution of substrates on graphite discs is shown in Fig. 4(a). Three points are selected uniformly in the radial direction for each substrate, and nine points are collected in the whole disk, as shown in Fig. 4(b). The evaluation of film quality includes the average deposition rate and film uniformity [12]. The uniformity of the films is inversely
Table 1 Average deposition rate for different grid numbers. Mesh number(106)
3.2
5.6
9.2
20.7
32.8
Average deposition rate (μm/h) Relative error
0.2304 12.1%
0.2397 8.51%
0.2486 5.11%
0.2554 2.25%
0.2620 /
3
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Fig. 3. MD-600B ZnO-MOCVD: (a) Mass production machine, (b) Appearance of reaction chamber, (c) Top spray mesh plate, and (d) Local details of spray mesh orifice plate.
Fig. 4. Distribution of wafers above substrate. (a) Physical model and (b) Simulation model and point data.
proportional to the coefficient of variation; a coefficient of variation below 5% is generally considered acceptable. The film thickness at these nine points was measured by Filmetrics Optical Film Thickness Measuring Instrument F20 Thin-Film Analyzer. The experimental and numerical results of MD-600B ZnO-MOCVD and MD-600C ZnO-MOCVD are shown in Table 2 and Fig. 5. The results show that the uniformity of films grown by MD-600B ZnO-MOCVD under standard processing conditions is good; the coefficient of variation was 4.15%, which is the result of hundreds of process parameter adjustments. The experimental
results are in good agreement with the numerical simulation results. The maximum error of the deposition rate of monitoring points is < 4%, and the maximum error of average deposition rate of the whole disk is < 1%. The MD-600C ZnO-MOCVD simulation results show that its deposition rate is significantly higher than that of the MD-600B ZnOMOCVD equipment, the average deposition rate being 31.85% higher. It can be seen that the intake mode of the new model has been optimized not only in structure, but also the deposition rate has improved, 4
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Table 2 Experimental data and numerical simulation results. Location of monitoring points
1 2 3 4 5 6 7 8 9 average
0.03 0.05 0.07 0.09 0.11 0.13 0.15 0.17 0.19 value
MD-600B ZnO-MOCVD Deposition rate (μm/h) Experimental data
Simulation data
0.1959 0.2067 0.2029 0.1921 0.1849 0.1827 0.1872 0.1932 0.1976 0.1937
0.1894 0.2010 0.1967 0.1948 0.1908 0.1898 0.1921 0.193 0.1947 0.1936
Relative error (%)
3.32 2.76 3.06 1.40 3.19 3.89 2.61 0.10 0.15 0.05
Table 3 Factor arrangement in orthogonal experiment. MD-600C ZnOMOCVD Deposition rate (μm/h) simulation data
Factor
Level
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm)
0.1930 0.2146 0.2348 0.2545 0.2658 0.2749 0.2822 0.2876 0.2910 0.2554
1
2
3
4
673 600 5 1500
773 750 10 2000
873 900 15 2500
973 1050 20 3000
Table 4 results of orthogonal experiment. Experimental number
Column number A
B
C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1
D
Average deposition rate (μm/h)
Film uniformity
1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3
0.4639 0.3360 0.5872 0.8474 0.2454 0.2115 0.7068 0.4837 0.3402 0.5756 0.2810 0.2151 0.4913 0.3223 0.1538 0.2056
0.3171 0.0829 0.0911 0.1123 0.1441 0.3961 0.0301 0.039 0.2017 0.1193 0.2869 0.0414 0.0910 0.0226 0.1915 0.2883
Bold type indicates that the process conditions have a significant impact on the film uniformity. Table 5 Analysis of variance of process parameters on film deposition rate. Fig. 5. Experimental and simulated results.
taking full advantage of the reaction source gas, saving resources, and reducing production costs. However, it can also be seen that the process parameters of MD-600C ZnO-MOCVD without mediation result in poor uniformity of the whole disk. The variation coefficient is 13.4%. The deposition rate of the films increases gradually from inner ring to outer
source
III Type squares
DF
mean square
F
P-value
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm) Error Optimization grouping
0.155 0.016 0.418 0.003 0.006 A1B3C4D1
3 3 3 3 3
0.052 0.005 0.139 0.001 0.002
26.849 2.848 72.281 0.564
0.011 0.206 0.003 0.675
Bold type indicates that the process conditions have a significant impact on the film uniformity.
Fig. 6. (a) Susceptor deposition rate distribution and (b) MO source velocity streamline. 5
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4. Conclusion
Table 6 Analysis of variance of process parameters on film uniformity. source
III Type squares
DF
mean square
F
P-value
A Temperature (K) B Rotation speed (rpm) C Pressure(Torr) D MO flow rate(sccm) Error Optimization grouping
0 0.009 0.153 0.034 0.001 A1B4C4D1
3 3 3 3 3
0 0.003 0.051 0.011 0
0.335 6.958 114.583 25.66
0.803 0.073 0.001 0.012
In this study, a new type of MD-600C ZnO-MOCVD reaction chamber was independently designed. The effects of different experimental parameters on the deposition rate and uniformity of films are studied. The following conclusions are obtained. (1) Under the same technological parameters, the deposition rate of model MD-600C is higher than that of model MD-600B. Technological parameters are required to meet the uniformity requirements with a coefficient of variation below 5%. (2) The growth conditions of ZnO thin films prepared by MOCVD technology were optimized using an orthogonal experimental design. The effects of growth temperature, base speed, chamber pressure, and inlet flow rate on the film quality were studied. The experimental results are evaluated by the significance test of variance analysis, which shows that all the errors in the experimental process are within the acceptable range, and the optimal film growth conditions under the current conditions are optimized. (3) For the deposition rate, the substrate temperature and cavity pressure play a significant role. With the decrease of base temperature, the deposition rate increases, and with the increase of cavity pressure above 10 Torr, the deposition rate increases. For film uniformity, cavity pressure and inlet flow rate play a primary role. With the increase of cavity pressure or the decrease of inlet flow rate, film uniformity improves.
Bold type indicates that the process conditions have a significant impact on the film uniformity.
ring as shown in Fig. 6(a). It can be seen from the AR + DEZn inlet streamline in Fig. 6(b) that the flow velocity of the MO source to the center of the substrate is very small under this process condition, resulting in the decrease of the DEZn concentration at the center of the substrate. The entire diffusion process takes place from the inner ring to the outside. If the process parameters are not appropriately matched, the film uniformity will be poor. This is the main explanation for the poor uniformity of the film. Because the thin film uniformity needs to be adjusted multiple times before the expectation can be reached, expensive manpower and material resources will be required to do so directly in the experiment. Therefore, based on the process conditions, the orthogonal film uniformity experiment was carried out by numerical simulation.
Declaration of Competing Interest
3.2. Analysis of design results based on orthogonal experiments
None.
The main factors affecting the growth of films by MOCVD include the flow rate of the source gas, the pressure inside the reaction chamber, the rotational speed of graphite disk and the temperature of graphite disk. Then, the optimum experimental conditions for the preparation of ZnO film with high uniformity and deposition rates were obtained by an orthogonal experimental design-assisted method through a series of experiments. The orthogonal experimental arrangement is shown in Table 3. The influencing factors are expressed as A, B, C, and D; each factor has four levels. According to the orthogonal table L16 (45), 16 groups of experiments were carried out. The orthogonal table and experimental results are shown in Table 4. The experimental results show that the uniformity of 7, 8, 12, and 14 in the orthogonal design scheme is < 5%, and the deposition rate is higher than MD-600B, which meets the expected optimization criteria. The variation of the film deposition rate and uniformity is a comprehensive result of the interaction of various experimental parameters. By using the intuitive analysis method of mathematical statistics, the optimum level of each factor can be preliminarily obtained, and the optimum combination of each factor in the experiment can be deduced. In Table 5, the results of variance analysis show that: (1) the P-value of parameters B and D are 0.206 and 0.675, respectively, which are than 0.05. The effect of parameters B and D on the deposition rate is not significant, that is, rotation speed and MO flow rate on deposition rate are not statistically significant; (2) the P-value of parameters A and C-Pvalue are 0.011 and 0.003, respectively, which are < 0.05. The parameters A and C have significant effects on the deposition rate, i.e., the substrate temperature and pressure have notable effects on the deposition rate. In Table 6, the variance analysis results shown in Table 6 show that: (1) the values of parameters A and B are 0.803 and 0.073, respectively, which are > 0.05. The effects of parameters A and B on film uniformity are not significant, that is, the effect of substrate temperature and rotation speed on film uniformity are not pronounced. (2) The values of parameters C and D are 0.001 and 0.012 respectively, which are < 0.05. The effects of parameters C and D on film uniformity are significant, that is, pressure and MO flow rate have clear effects on film uniformity.
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