The ion flux, total ion energy reaching the substrate and the ion energy distribution function (IEDF) can be measured by a simple system called retarding field electrostatic analyzer as shown in Figure 7.5 . It consists of a 4-grid stack installed in the middle part of the substrate holder, ensuring minimal disturbance of the plasma (by leveling the surface of the analyzer with the substrate holder’s top surface). The ion-current values are recorded simultaneously with the analyzing voltage applied to the third grid. The resulting retarding curves are differentiated with respect to voltage and normalized in mAcm-2 eV-1 to obtain the IEDF. Consequently, the integral of the IEDF in the form,
ffi (e)de (7.8)
gives the ion-current density to the grounded electrode surface. However, to get more insight into the density and energies of individual ions a mass spectrometer (MS) needs to be used . This technique, for example, can be used to study the kinetics of the plasma species in a pulsed gas or layer by layer (LBL) deposition process . In a kinetic mode MS can detect all the plasma transients occurring even in microseconds range.
Figure 7.5 Schematic of a retarding field electrostatic analyzer at the grounded electrode of a PECVD assembly.