Corona ionizers collect material from the environment much like a dust precipitator. That material lowers the efficiency of the ionizer and must be removed. Also as tools and fixtures are moved in the cleanroom, the balance of the ionizer is effected. For both of these reasons and because the control circuitry in the ionizer drifts over time, the ionizers require maintenence. Corona ionizers need dramatically more frequent cleaning and adjustment in environments where liquid is dispensed, This includes wet cleaning, wet etch and photolithography.
LBL has also used x-ray and alpha ionizers which do not acquire material from the environment. In some cases, these are a better choice for the process.
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