Abstract
This experimental study shows the validity of Sheridan's method in determining plasma density in low pressure, weakly magnetized, RF plasmas using ion saturation current data measured by a planar Langmuir probe. The ion density derived from Sheridan's method which takes into account the sheath expansion around the negatively biased probe tip, presents a good consistency with the electron density measured by a cylindrical RF-compensated Langmuir probe using the Druyvesteyn theory. The ion density obtained from the simplified method which neglects the sheath expansion effect, overestimates the true density magnitude, e.g., by a factor of 3 to 12 for the present experiment.
Highlights
The electrostatic probe known as the Langmuir probe has been extensively used for plasma diagnostics in systems spanning electric propulsion [1, 2] down to semiconductor processing [3, 4] and biomedical applications [5, 6], due to its advantages of simple construction and accurate spatial positioning
When the plasma density is sufficiently high such that the Debye length is small compared to the tip radius, the sheath expansion effect is negligible and Equation (1) is a valid method to obtain the ion density
The present study verifies the validity of Sheridan’s method in determining plasma density in low pressure RF plasmas, by comparing the ion density obtained from Sheridan’s method using a planar LP, and the electron density obtained from the Druyvesteyn theory using a cylindrical RF-compensated Langmuir probe (CP) [16, 17]
Summary
The electrostatic probe known as the Langmuir probe has been extensively used for plasma diagnostics in systems spanning electric propulsion [1, 2] down to semiconductor processing [3, 4] and biomedical applications [5, 6], due to its advantages of simple construction and accurate spatial positioning. When the plasma density is sufficiently high such that the Debye length is small compared to the tip radius (λD ≪ rp), the sheath expansion effect is negligible and Equation (1) is a valid method to obtain the ion density.
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