Direct Liquid Injection Vaporizers for High-Efficiency, Cost-effective Photovoltaic Manufacturing

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1 Direct Liquid Injection Vaporizers for High-Efficiency, Cost-effective Photovoltaic Manufacturing By Scott Benson, Director of Sales - Americas, Brooks Instrument January 2012 Photovoltaic manufacturers work with expensive and challenging liquid chemistries every day in order to manufacture state-of-the-art solutions for the solar industry. So, it s imperative that PV developers harness the most reliable and cost-efficient technology during the manufacturing process to produce modules of the highest quality at the lowest cost. One way PV companies can improve their operations is to optimize the liquid chemical-to-vapor, or vaporizer, sub-system in manufacturing lines that utilize vaporized liquid precursors. Manufacturers use vaporizer technology in processes that chemically deposit layers, such as in the production of thin-film modules or creating a transparent conducting oxide layer. Currently, many manufacturers use vaporizing technologies that have not been fully-optimized for the requirements of PV manufacturing. This can result in: inconsistent layer uniformity and performance, excessive production of waste gases for disposal, excessive liquid precursor consumption, slow deposition speed, and inefficient use of expensive deposition equipment. However, by integrating a modern direct liquid injection (DLI) vaporizer, manufacturers can achieve the following in order to have the most efficient operations, least waste, and highest product quality: 100% conversion of the liquid feed into the vapor phase with no possibility of liquid carry-over to the deposition chamber Precise, repeatable stoichiometry in the deposition chamber Elimination of thermal decomposition of the liquid precursor which eliminates the possibility of introduction undesirable (or harmful) chemical species Obtain vapor-on-demand the ability to generate the precise amount of vapor exactly when it s needed without wasteful, expensive divert steps

2 Overcoming Limitations DLI vaporizer technology overcomes the limitations of more conventional vaporizer technologies. For example, bubblers also called vapor-draw systems are difficult to start and stop, with large bubblers requiring hours to equilibrate. They require close control of temperature and pressure, and they are inefficient at generating well-controlled vapor mass flow. Heated bubblers can promote precursor decomposition for liquids that are thermally sensitive. Vaporizing valves, hot frit, heat tracing and other flash vaporizers that apply heat to the liquid using a hot metal surface are also inefficient at generating vapor mass. They frequently can cause thermal decomposition of the liquid precursor leading to undesirable chemical species or particles. None of these conventional technologies can eliminate the potential for liquid carryover and its corresponding problems. When liquid enters the (typically) hot deposition chamber, it explosively vaporizes leading to pressure transients that impact layer uniformity, repeatability, and performance. Figure 1 shows pressure fluctuations in a chemical vapor deposition (CVD) chamber that are caused by incomplete vaporization and precursor carryover from a conventional vaporizer. With conventional vaporizers, it s challenging to determine precisely how much vapor mass is actually being delivered so that the correct chamber stoichiometry is maintained. In photovoltaic applications that require precise vapor mass addition, users must employ secondary measurement or control devices like vapor mass flow controllers, which increase the cost and complexity of the vapor module. Because of these known limitations of conventional vaporizer technologies, some CVD processes use a gas precursor instead of liquid in certain PV applications, even though the gas analogue can be more dangerous, toxic, expensive, or all three. With the reliability and precision of DLI vaporizer technology, PV manufacturers can rethink their precursor strategy and select the precursor that best fits their manufacturing strategy and objectives. State-of-the-Art DLI Technology Unlike bubblers and hot surface vaporizers, DLI vaporizers are extremely efficient at producing vapor from liquid. DLI vaporizers use hot gas instead of a hot metal surface to accomplish liquid vaporization. As liquid enters the hot gas chamber, a carrier gas stream atomizes it. When the atomized liquid contacts the hot gas, it immediately changes to vapor. The result is chemically pure vapor that is free of decomposition byproducts or liquid carryover. The most advanced DLI vaporizers can accept multiple liquid inlets and will generate perfectly mixed vapors. To accommodate a wide variety of liquid properties extremely low vapor pressures (sub 1 torr), very low flow rates (sub 5 grams per hour) and very high flow rates (more than 15 kg per hour) the vaporizers offer several heat exchanger designs. 2

3 Some DLI vaporizers are close to being true vapor on demand systems because increasing, decreasing or stopping the incompressible liquid flow quickly results in the desired change in vapor flow. For example, Figure 2 illustrates a common application depositing silicon oxide films from tetraethyl orthosilicate (TEOS). It shows a DLI vaporizer s fast response to 900 gram per hour TEOS step challenges. By comparison, the large bubblers commonly used for methyltrichlorosilane, BCL3, TiCl4 and similar liquids are difficult to start, run and stop effectively. Many DLI vaporizer solutions feature Coriolis liquid flow controllers. Because it uses the Coriolis measurement principle, this flow controller provides ultra-precise mass delivery to the hot gas chamber without regard to chemical composition or flow rate. By definition, the vapor mass that exits the vaporizer is identical to liquid mass that entered it. This eliminates the need for additional vapor measurement and control devices, which reduces the complexity of the system. DLI vaporizers are also designed to operate in a wide range of process pressure regimes, from sub-atmospheric to many atmospheres. Convenient, modular construction makes it easy to adapt them to any photovoltaic processing conditions, include those that impose ultra-high purity conditions. Applications Deposition A complete PV deposition system typical includes a DLI vaporizer, Coriolis liquid mass flow controller, gas thermal mass flow controller, and temperature and setpoint controllers for the heated vaporizer. DLI vaporizers are ideal for CVD applications such as depositing silicon carbide, tin oxide, titanium nitride, boron nitride, zinc oxide and diamond-like carbon. It is effective whether the material is deposited as layers or bulk material, or is infused into a matrix. 3

4 In CVD, DLI vaporizers provide excellent layer morphology. They significantly increase deposition rates, reduce precursor consumption and produce less waste gas than conventional vaporizers. In these applications, the return on investment of a DLI vaporizer system is especially rapid. Figure 3 shows the remarkable improvement in layer-tolayer reproducibility reported by one CVD customer. Unlike large and complex control bubbler systems or vapor-draw systems, which can be difficult to control and require a temperature-controlled supply vessel, DLI vaporizers respond almost instantaneously to changes in vapor demand commands. They are also physically smaller devices and simpler for a given vapor mass flow capacity. In vacuum polymer deposition applications, DLI vaporizers can accept more than one liquid feed. This facilitates vaporizing monomers with dissimilar vapor pressures. Thin Films Another application of DLI vaporizers is vaporizing thin film precursors, including advanced thermally sensitive precursors containing lanthanum, zirconium, strontium or hafnium. Unlike flash vaporizers, bubblers and vapor draw systems, DLI vaporizers deliver fast and repeatable flow rates, in this case, in excess of 15 grams per minute for silicon oxide films. TEOS vapor reaches full concentration in less than three seconds. This fast response time, combined with a Coriolis mass flow controller with 0.2% flow rate accuracy and wafer-to-wafer repeatability of better than 0.2 grams per hour, can produce high quality repeatable films at very high deposition rates. This eliminates time wasted during vapor divert steps for liquid and vapor flow stabilization. In these applications, DLI permits flexible operation and repeatable delivery of liquid precursors. DLI technology ensures complete vaporization, low transit times, and quick start-up and shutdown. Additionally, DLI vaporizers are engineered using a modular design that supports a wide range of process fluids and operating conditions. Conclusion The PV industry is growing rapidly and shows great promise to be a source of clean energy in decades to come. A key factor for the long-term success of this industry is the continual reduction in the manufacturing costs of new solar technology. Chemically depositing layers from chemical precursors is a critical process in many of these new technologies, and the efficient and effective production of these films is a critical production step in the goal of reducing the cost of production. DLI vaporizer technology is the way of the future for PV manufacturers looking to achieve cost savings and efficient processes that result in high-quality products. Transparent Conducting Oxides One of the most important films in a thin film solar cell is the transparent conducting oxide. Metal organic precursors are used to deliver conducting atoms to the substrate. Bubblers have been used to vaporize these liquids, but their use is challenged by minimal flexibility and relatively low mass delivery. 4

5 About Brooks Instrument Every day, customers turn to Brooks Instrument for solutions to their flow, pressure, vacuum and level challenges. In industries as diverse as biopharmaceuticals, oil and gas, fuel cell, solar cell, chemicals, medical devices, analytical instrumentation, and semiconductors, Brooks provides the broadest array of flow products in the market. Our awardwinning flow meters and flow controllers consistently rank at the top of their category for accuracy, reliability, and user preference, as judged by the audience that matters users of flow instruments. But Brooks products are only half the story. Our customers are backed by unsurpassed technical expertise in virtually every corner of the planet. The local Brooks product and application specialist brings to each customer priceless application experience. They have been extensively trained to help you select the optimal solutions for your measurement or control needs, and offer years of experience solving application problems just like yours. Toll-free (USA): FLOW 5