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Ind. Eng. Chem. Res. 2009, 48, 5992–5999
Scaleup of Cu(InGa)Se2 Thin-Film Coevaporative Physical Vapor Deposition Process, 2. Evaporation Source Design Kapil Mukati and Babatunde A. Ogunnaike* Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716
Erten Eser, Shannon Fields, and Robert W. Birkmire Institute of Energy ConVersion, UniVersity of Delaware, Newark, Delaware 19716
We recently presented the development and experimental validation of two models that are essential for effective commercial-scale source design for Cu(InGa)Se2 thin film coevaporative physical vapor deposition processes: a three-dimensional thermal model of the evaporation source, and a Direct Simulation Monte Carlo (DSMC)-method-based effusion model. We showed that these models can be used to obtain reasonably accurate melt temperature dynamics and nozzle effusion flow properties. We now present how these simulation tools are used to develop a scale-up methodology for the effective commercialization of the pilot-scale physical vapor deposition (PVD) process at the University of Delaware’s Institute of Energy Conversion (IEC). We illustrate the methodology using two commercial-scale source design studies: a three-nozzle single source and a four-nozzle modular source. We also show that the proposed source designs are robust to modeling errors and the important process parameter of the source-to-substrate distance. 1. Introduction The Institute of Energy Conversion (IEC) at the University of Delaware uses an inline thermal coevaporation process for Cu(InGa)Se2 deposition on flexible substrates. The pilot-scale process is shown schematically in Figure 1. The Cu(InGa)Se2 thin film is deposited onto a 6-in.-wide flexible substrate in a roll-to-roll technique. The copper, gallium, and indium sources with two nozzles each are placed sequentially, as shown in the figure. Selenium is provided in excess via sparger pipes. The flexible substrate (or “web”) is wound onto two rolls that can move over the sources in either direction, with the speed being controlled by direct current (DC) motors. The desired web temperature is achieved with two large resistively heated and individually controlled steel plates placed near the top of the web. The desired elemental effusion rates are obtained by controlling the temperatures of the evaporation sources. The deposition zone is 15 in. long in the web-motion direction and 5 in. wide across the web. The IEC’s PVD process works reasonably well at pilot scale for substrates that are up to 6 in. wide, providing acceptable film thickness uniformity and composition uniformity across the substrate.2 However, commercially viable production is possible only by achieving high film throughput, using wider substrates, and by operating the process at higher web speeds. The difficult part in such a process scaleup is mainly the design of evaporation sources that achieve the following critical objectives (discussed in some detail in the first part of this series of papers1): (i) Deposit uniformly thick high-quality film on large area substrate, (ii) Minimize the effect of melt depletion on the film quality, and (iii) Improve material utilization efficiency. Achieving these objectives requires knowledge of the source temperature profile, as well as accurate estimation of nozzle * To whom correspondence should be addressed. E-mail address:
[email protected].
effusion rates and vapor flux distribution; therefore, the development of accurate evaporation source thermal and effusion models is essential. We have already developed and validated these models in the first part of this series of papers:1 a finite-element electrothermal model, and a Direct Simulation Monte Carlo (DSMC)-based effusion model. In this paper, we present a methodology using the aforementioned two models to design evaporation sources that achieve the necessary scale-up objectives. To illustrate the proposed methodology, we design two commercial-scale evaporation sources: a three-nozzle single source, and a four-nozzle modular source. It is also shown that the proposed source designs are robust to modeling errors and the important process parameter of the source-to-substrate distance. The paper is organized as follows: Section 2 lists the specific scale-up objectives for the IEC’s PVD process, and the design parameters that have to to be fixed based on the system’s physical constraints. The design procedure is discussed in Section 3. As an illustration, three-nozzle source design is presented in detail in Section 4. The robustness of the design is also discussed. In Section 5, the design of the four-nozzle modular source is summarized, and finally, the paper is concluded in Section 6.
Figure 1. Schematic of the pilot-scale inline coevaporative Cu(InGa)Se2 thin-film physical vapor deposition (PVD) process.
10.1021/ie801596a CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
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Table 1. Comparison of Commercial-Scale Process Design Objectives with the Corresponding Pilot-Scale Values Value parameter
pilot scale
commercial scale
film thickness web width web speed film thickness nonuniformity material utilization efficiency
2 µm 6 in. (5 in. usable) 0.75 in./min 7% 30%
2 µm 12 in. 12 in./min 5% >50%
2. Process Scaleup 2.1. Objective. The objective of the scaleup is to obtain a 2-µm-thick CIGS films with 5% thickness nonuniformity over a 12-in.-wide substrate that is moving at a speed of 12 in./min. Such a design will increase the throughput of the process from 3.75 in.2/min to 144 in.2/minsa scaleup of ∼38 times greater than that of the pilot-scale process. In addition, the material utilization efficiency should be >50%, and the maximum operating temperature of the evaporation sources should be 12 in.) by including more standardized boats in series. (3) To the authors’ knowledge, the largest commercially available boron nitride (BN) block has dimensions of 16 in. × 2 in. × 2 in. Therefore, constructing large multinozzle singleblock sources for substrates greater than 15 in. in size (after accounting for the wall thickness and nozzle-wall spacing) is not feasible. For such cases, modular design is the only option. The same design steps that are listed in Section 3, and discussed in Section 4 for the three-nozzle single source, are performed for the design of the four-nozzle modular source. The design is summarized as follows. 5.1. Prespecified Design Parameters. The prespecified process design parameters are listed in Table 6. The outer nozzle diameter is restricted to D ) 0.5 in., because the pilot-scale source width is Wb ) 0.75 in. Moreover, the minimum nozzle spacing between the two inner nozzles is set at 3 in. by accounting for the wall thickness and allowing a 1-in. separation between the wall and the nozzles (see Figure 16). 5.2. Design Summary. The design parameters that must be determined for the modular source are the outer nozzle-to-nozzle spacing (douter NN ) and the inner nozzle diameters (Di). The optimum outer and the relative flow rate (ε) that maximize the values for dNN material utilization efficiency while achieving 5% thickness outer ) 11 in. and ε ) 0.56. uniformity were determined to be dNN Therefore, the effusion rate from the inner nozzle should be 56% of that for the outer nozzle. The maximum material utilization efficiency was Mu ) 62%. The source design is summarized in Table 7. The film thickness and composition profiles are plotted in Figure 17. Figure 18 shows the estimated operating temperatures as a function of web speed. Given that the maximum nozzle diameter is restricted to D ) 0.5 in., because of the smaller dimensions of the pilot-scale sources, the melt operating temperatures must be increased to TCu ≈ 1480 °C, TGa ≈ 1180 °C, and TIn ≈ 1110 °C to achieve the specified 2-µm film thickness for the desired web speed of 1 ft/min. 6. Summary and Conclusions The high throughput required for a cost-effective commercial roll-to-roll copper-indium-gallium diselenide deposition process can be achieved by increasing both the substrate width and speed. This must be achieved while still maintaining robust and tight control of the mean values of film thickness and composi-
Figure 18. Plot of melt operating temperature versus web speed.
tion, as well as their uniformity across a large-area substrate, for long deposition times. In this paper, the commercial process is considered to have a 12-in.-wide substrate with translation speeds >6 in./min, running continuously for a duration of 8 h or more. A design methodology is developed using the thermal and effusion models of the evaporation source. This design procedure is then illustrated via two design studies: a three-nozzle single source and a four-nozzle modular source. The latter is proposed because it is scalable for webs wider that 12 in., because of its modular structure and ease of implementation in the pilot-scale physical vapor deposition (PVD) vacuum chamber. The material utilization efficiency is maximized by optimizing the nozzleto-nozzle separation and nozzle diameters. Investigation of the issue of melt depletion shows that, although the film thickness composition and uniformity is robust to the decrease in melt level, the film thickness value varies with time; consequently, the use of an outer multivariable controller is essential, which manipulates the setpoints for the elemental source temperatures to maintain the film thickness at the desired setpoint. The proposed methodology can be used to design evaporation sources for any given substrate width, source-to-substrate distance, evaporant, and source shape.
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Literature Cited (1) Mukati, K.; Ogunnaike, B. A.; Eser, E.; Fields, S.; Birkmire, R. W. Scale-up of Cu(InGa)Se2 Thin Film Co-evaporative Physical Vapor Deposition Process, 1. Evaporation Source Model Development. Ind. Eng. Chem. Res. 2009, 48, DOI: 10.1021/ie8015957. (2) Hanket, G. M.; Singh, U. P.; Eser, E.; Shafarman, W. N.; Birkmire, R. W. Pilot-Scale Manufacture of Cu(InGa)Se2 Films on a Flexible Polymer
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Substrate. In Proceedings of the Twenty Ninth IEEE PhotoVoltaic Specialists Conference, New Orleans, LA, 2002; p 567.
ReceiVed for reView October 21, 2008 ReVised manuscript receiVed April 13, 2009 Accepted April 14, 2009 IE801596A
