technology materials
The vaporizer is isolated from the bulk supply, therefore vaporization conditions are constant, and are easier to control – and the process is, by definition, a safer one.
The flow rate limitations of existing designs due to heat flux limitations are not present in the LETV. Everything that reaches the vaporizer is vaporized and there is no “concentration effect” on impurities in the bulk container.
The flow rate from a single LETV system vaporizer is easily increased to support 50 GaN MOCVD reactors with flow rates up to 5,000 slpm achievable by adjusting the heat flux.
Figure 3 – Output from liquid delivery system and NH3 vaporizer at 250 slpm, showing no change in moisture
concentration as the bulk supply is consumed
At a certain flow rate, boiling becomes vigorous enough to disrupt and mix the liquid-vapor interface which starts to lower the surface enrichment of moisture. At high flow rate, a film boiling regime develops and heavy mixing is found within the container. As a result, the surface enrichment is not capable of developing; and a lower moisture level is observed. In the real-world setting of LED production, reactors and processes start and stop requiring on-the-fly changes in
demand for gaseous NH3. Under this circumstance, levels of moisture will always vary in the output flow of
conventional NH3 delivery systems - a result of fundamental physical properties of NH3 with H2O contamination no matter what the purity.
In the final analysis, the instability and variability of the vaporization process in the typical BSGS presents barriers for the development of a repeatable, high volume production process.
Liquid extraction and total vaporization (LETV) LETV technology circumvents the problems observed with gas phase delivery from a bulk supply. In a critical departure from conventional BSGS technology, LETV technology isolates the storage vessel from the vaporization process. The design concept is to push liquid from a bulk container to an external vaporizer. In doing so,
the system is able to achieve total vaporization of the NH3 while maintaining constant temperature, pressure and therefore flow.
Figure 4. Gaseous NH3 purity extracting then vaporizing a UHP liquid source –
changes in flow rate over time do not impact purity of output
26
www.compoundsemiconductor.net November / December 2010
The LETV technology overcomes the problem of moisture impurity spikes and drift through the life of the entire bulk container and across a wide range of flows.
Because vaporization is isolated as a separate and
controlled step, and because all of the liquid NH3 is vaporized and exits the system, the stability of moisture
levels in NH3 delivered by the LETV is better than 2% across a widely varied flow rate range, and throughout the depletion life of the bulk supply.
LETV: No increase in water contamination during bulk supply depletion
The stability of moisture contamination in NH3 is represented in Figure 3. The moisture content was constant as the bulk vessel was depleted. There was no “concentration effect” of impurities as the supply was consumed. The standard deviation in the data was less than 1%.
Tests of the LETV technology have shown that the standard deviation was less than 2% even when the flow rate was changed, and regardless of the liquid volume in the bulk container. In fact, LETV is so effective throughout the depletion of the bulk container that the amount of
wasted bulk NH3 is limited only by the extent to which the bulk vessel and dip tube enable the emptying of the bulk supply.
LETV: Purity of gas supply from UHP liquid bulk supply In conventional BSGS designs the concentration of water contamination will increase as the bulk supply is consumed (to a point where the bulk supply becomes unusable). In an LETV system the purity of the gas output is wholly dependent upon the purity of the bulk supply – and the impurity profiles of both the bulk liquid and the gas output remain essentially flat during use. Maintaining UHP performance from source to delivery is a high demand on any liquid/gas delivery system.
For NH3 with 100 ppb moisture levels in liquid phase the effect of the drying down of the tubing is important in the
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