Accurate Control of Ion Bombardment in an Expanding Thermal Plasmas

Remote plasmas are extensively used in industry for both etching and deposition of materials. As ion bombardment has been found to be crucial for controlling deposited material properties or enhancing etch rate and anisotropy during ion induced etching, an additional bias voltage is often applied to the substrate to control the energy of the bombarding ions and/or enlarge their flux onto the substrate. Recently, a pulsed bias scheme became subject of increased interest as a promising technique to reach narrow, almost mono-energetic ion energy distribution (IED) when dielectric substrates are being processed, especially considering that the conventionally used radio frequency bias inherently leads to a bimodal IED. Moreover it offers an ultimate control of the ion flux onto the substrate by varying the duty cycle. Experiments were run in a remote expanding thermal plasma (ETP) reactor, in Ar and Ar/H2 gas mixture compositions. The substrate holder was negatively biased (up to -100V) by means of a home designed pulsed power supply operating with a frequency up to 200 kHz and a variable duty cycle. Ion energy distributions have been measured by means of a planar gridded retarding field energy analyzer. Two pulsed biasing approaches will be presented (asymmetric rectangular pulses and modulated pulses with a linear voltage slope during the pulse) and their applicability is discussed on the basis of an intrinsic capacitance of the processed substrate-layer system. The substrate voltage and current waveforms were measured and mutual relations with the obtained ion energy distributions will be shown for both aforementioned cases. To demonstrate the IED control achieved, the effective carrier lifetime of n-type c-Si wafers, passivated by an a-Si:H thin film, as a function of the flux and energy of bombarding argon ions was determined. The ion energy and ion flux was independently varied and threshold ion bombardment characteristics leading to degradation of the effective lifetime will be presented