Semiconductor wafers are manufactured using a variety of Chemical Vapor Deposition (CVD) processes. The Silicon structure of a wafer is built-up using Silane (SiH4), and semiconductor properties are created by interstitially depositing elements such as Arsenic and Phosphorous. The sources for these elements, commonly called doping agents, are the respective hydrides Arsine (AsH3) and Phosphine (PH3). All of these compounds are pyrophoric and toxic and, therefore,
need to be scrubbed when evacuated from the CVD reactor.

All of these compounds hydrolyze too slowly in water for water scrubbing to be very effective. Caustic scrubbing, such as with Sodium Hydroxide, works a little better, but to achieve high removal efficiencies and oxidizing agent needs to be used. Common scrubbing oxidizers include Sodium Hypochlorite, Hydrogen Peroxide, and Potassium Permanganate. Two of the
by-products, Silicic and Arsenious Acids, are insoluble in water but soluble in strong bases. Therefore, to minimize solids accumulation within the scrubber, the scrubbing solution should be maintained at high pH.

Since these gases spontaneously ignite in air, carrier gases are inert such as Nitrogen, Argon, or Hydrogen, and certainly air cannot be used as a means of diluting high inlet concentrations. To accomplish dilution, scrubbed gas may be recycled back to
the scrubber inlet. This also has the advantage of ensuring good gas distribution, turbulence, and contacting in the scrubber by increasing the scrubber flow beyond what otherwise might just be a “trickle” of gas.

AAT, Inc. designs with many different scrubber types, including packed columns, tray columns, spray towers, and ejector-venturis. This latter device is especially suited for low volume, high concentration gas streams, as is typically the case with CVD exhaust streams. Its openness facilitates handling of scrubbing liquor high in suspended solids. It not only scrubs using an extremely high liquid to gas ratio, but it also provides motive for exhausting the CVD reactor(s). We offer complete wafer scrubber packages, including instrumentation and controls, skid-mounting of equipment, and start-up.