Measurement of Downstream Charge Transport During Plasma Cleaning of Vacuum Chambers

Vacuum chambers suffer deleterious effects from the presence of adventitious hydrocarbons (AHCs). These AHCs originate from various sources including samples under analysis and can cause distortion of both images (blurring) as well compositional detection (via interference with secondary signals). A compounding factor is that AHCs typically possess very fast surface mobility such that a cleaned surface begins to show measureable re-contamination soon afterward (ones of hours). A common method to remediate AHC contamination in situ is through the use of plasma-based methods wherein an electrical discharge is ignited in a suitable precursor gas and the plasma products perform cleaning. Because of the sensitive nature of parts typically found in these tools it is highly desirable to utilize plasma products that are electrically neutral but chemically reactive. To achieve this goal it is usually arranged to have a plasma generation volume on an upstream port of the microscopy tool and to allow neutral plasma products flow downstream to work-pieces. Charged species more than 10 cm from the plasma are assumed not to occur, and we have measured negligible charge collection in hydrogen discharges using a Langmuir probe [1] at 200-600 mT under typical safe plasma cleaning powers (20 W maximum)