materials technology
These disadvantages are basic and severely limiting. The need for higher productivity in LED production is a real- world requirement. Overcoming any the limits of flow rate scalability, bulk material contamination, waste, performance, and safety would be a positive step. Overcoming them all would be a breakthrough.
A newer approach to bulk ammonia supply, Liquid Extraction and Total Vaporization (LETV) is the breakthrough technology that overcomes all of the weaknesses of conventional BSGS systems. First, we will describe the detail of the BSGS limitations; after which we will provide a description of the LETV technology and its advantages.
Flow limitations in conventional bulk delivery systems A conventional BSGS is comprised of a bulk container
and a gas piping system. Liquid NH3 vapourises within the bulk container and is piped to the point of use as a gas. Limitations associated with heat control of the bulk container (explained below) constrain the size of the bulk container, and impose limitations on liquid ammonia surface area – in turn limiting the flow capacity, and constraining the scalability of the system.
So, even with a bulk delivery system in place, fabricators who continue to expand by the addition of multiple GaN reactors place consequent additional burdens on their
NH3 delivery systems, and face the need to deploy additional BSGS systems.
Impurity issues with BSGS The mechanism employed in a conventional BSGS is, in effect, a single plate distillation. As with all distillations, the composition of what remains inside the container changes over time. As the ammonia is consumed, the concentration of impurities increases. Under process conditions, the overall composition of the bulk supply can change to the extent that it no longer meets the specifications required, and its use must be terminated. Depending upon process requirements and end-user SOP, the unusable supply can be as much as 20% of the original volume; this is an obvious and immediate target for cost reduction.
Wasted supply is not the only problem. Water is a common impurity for ammonia, and oxygen is the cause of critical defects related to LED brightness. As described
above, as the NH3 is consumed, the concentration of impurities increases. Well before the point at which the ammonia supply is rendered completely unusable, the drift in moisture contamination from the BSGS is a process variable that impacts product quality (see Figure 1).
Removal of moisture and impurities from the exiting gas is possible, but because the moisture and impurity content changes dynamically as the bulk supply is consumed, purification under these changing conditions represents a challenge to most purification systems. The resulting
November / December 2010
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inconsistency in the quality of the gaseous ammonia output can lead to process and product variability.
Heating the bulk container
An additional complication is that a conventional BSGS requires that heat be applied to either to the entire bulk
NH3 supply or to the liquid surface through microwave technology in order to induce vaporization and to maintain the required flow rates.
The application of heat to ammonia presents control problems as well as safety risks. Heating any size vessel poses a control problem, because the amount of heat must be continuously monitored and adjusted as the container is emptied in order to maintain the delivered gas at a constant flow.
Larger vessels respond to temperature changes more slowly, and are difficult to control with precision. Larger vessels also pose a larger safety risk when electric heaters and microwave energy are present. Smaller vessels reduce the control problem, but suffer the obvious disadvantage of low capacity. Large or small, vessels of any size present a temperature control problem. In the quest for capacity, there are practical limits to the
size of a bulk liquid NH3 container that can be efficiently and safely heated.
Phase change dynamics and moisture NH3 has a high latent heat of vaporization and keeping the phase change continuous and controlled in the face of
varying demand from the process is non trivial. From a conventional BSGS the concentration of the water impurities increases with exiting gas flow rate; it reaches a maximum; then declines. This behavior is a result of the combined effects of thermodynamics, fluid dynamics and heat transfer. As smooth evaporation
occurs, an increase in moisture develops at the NH3 liquid-vapor interface. Surface enrichment increases with an increased flow rate and therefore the moisture concentration increases (see Figure 2).
Figure 2. Representation of
contamination profiles as a function of gas output flow rate. Flow rate changes are to be expected in a real-world production setting – variable contamination as a function of flow rate is a real-world problem.
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