Retarding field energy analyzer
Retarding field energy analyzer for ion energy and ion flux measurements
The retarding field energy analyzer (RFEA) is a plasma diagnostic to measure the ion energy distribution function (IEDF) and the ion flux onto a surface in low-pressure/ low-temperature plasmas. After passing the entrance aperture of an RFEA sensor, the ions must overcome an electric field that is caused by an applied discriminator voltage. If the kinetic energy of the ions is sufficient, they can overcome the potential barrier and reach the collector, where the ion current is measured. If the electric field strength is raised by an increasing discriminator voltage, even the high-energetic ions can no longer reach the collector at a certain point and the ion current then drops to zero Ampere. The IEDF is then proportional to the first derivative of the measured current-voltage characteristic according to the discriminator voltage or the ion energy. With the retarding field analyzer only the kinetic energies and particle fluxes of all ions occurring in the plasma process can be measured. An identification of the participating ion species is not possible.
The AEPT has four different RFEA systems from the company Impedans Ltd. With the Semion pDC, a single sensor system, the IEDF and the ion flux can be measured time-averaged and time-resolved in a DC or pulsed low-pressure plasma. The newer Semion system, which is also available, can also be used in RF plasmas. Furthermore, the institute has two systems with sensor arrays, which can measure the IEDF and the ion flux spatially resolved on large electrodes (200 mm and 450 mm electrode diameter). The smaller sensor array can also be used for time-resolved measurements in pulsed plasmas.
Concerning the scientific investigation of the ion flux inhomogeneity in large-area plasma processes both a relative and/ or absolute calibration of each RFEA sensor is a current challenge. Recently, the AEPT has developed and tested a new method for the relative calibration of the sensor arrays for a correct interpretation of the radially and azimuthally measured ion flux profiles.