有害ガスの Point‑of‑Use（PoU）アベートメントにおける新コンセプト

The problem was previously solved by separating the exhaust of each tool chamber into pipes for alkaline, acidic or organic exhaust, as shown in Figure 1. The separation into these three types of exhaust was realized by connecting the exhaust pipes from several chambers of a wet-clean tool to a so-called switching box, where large dimensioned valves directed each exhaust flow from a single chamber into one of the three specific central exhaust pipes, depending on the actual process step in each chamber. Each exhaust pipe was then treated by a specific central scrubber.

Although the switching boxes performed well, several drawbacks forced chip manufacturers to search for a modified solution: to avoid pressure modulations in the exhaust pipes the switching boxes feed air into the dead ends that are temporarily not connected with a process chamber. Each exhaust pipe therefore has to conduct the maximum possible flow for all process chambers at all times. Since the boxes have to be located close to the tool, inside the clean environment, a high volume of costly clean air is lost and has to be replaced. Downstream of each central exhaust pipe, the central scrubber systems are always loaded with the maximum flow of air, requiring a scrubber design capacity that is a multiple of the actual exhaust from the process tools. The area where the auxiliary equipment is installed usually corresponds to the clean room square footage and is therefore limited and densely packed with equipment. Due to the large diameter of pipes and valves, the switching boxes occupy a large footprint. Signals have to be generated to direct the flow from each process exhaust into a suitable central exhaust pipe. However, with a point-of-use wet-scrubber these disadvantages can be overcome.

Generally, wet-scrubbers are well suited to remove soluble gases from a gas flow. Acidic and basic gases (here HF and NH3) can be reduced to low levels by chemical absorption with alkaline or acidic scrubbing liquid (see Equations 2 and 3). For solvents that follow Henry’s Law the lowest possible exhaust concentration post scrubber is limited by the effective concentration in the scrubbing liquid of the final scrubber stage (see Equation 4).

But these theoretical limits require that the scrubber is large enough, specifically, the residence time of the gas is long enough to get the gas and liquid phase into equilibrium, and the flow of liquid is high enough so that concentrations in the liquid remain relatively stable along the scrubber column. The challenge in the design of the SALIX was a trade-off to meet the specific restrictions concerning size, efficiency and pressure drop.

HF + OH– ↔ F– + H2O (2)

NH3 + H3O+ ↔ NH4+ + H2O (3)

kH = p / caq (4)

where caq is the concentration in the liquid phase

p is the partial pressure in the gas phase

Figure 2 shows the schematic diagram of a new point-of-use wet-scrubber solution called SALIX. All process chambers from one process tool are connected into one wet-scrubber, which is connected to only one central exhaust pipe. An additional feature, requested by customers is a bypass line around the scrubber to keep the tool ventilated during scrubber maintenance or in case of failure. Located directly at the entrance of the exhaust pipes into the wet-scrubber are spray nozzles that initially reduce the concentration of soluble gases before the different exhaust flows get mixed. The gas then flows through two packed scrubbing stages, which can be operated with different scrubbing liquids. At the end of each scrubber stage a demister removes droplets from the gas flow. The pressure in the system is controlled and kept constant by a fan with an electronic frequency converter. The gas can bypass the two scrubbing stages in case of failure or maintenance.