Filamentless neutralization of broad ion beams

Abstract

A new, effective technique of broad ion beam neutralization, based on alternating ion and electron extraction via the same optic is presented and discussed. The screen and accelerator grids of a two-grid extraction system are biased with bipolar and unipolar pulses of varying amplitude and frequency, respectively. Pulse frequencies of 500 Hz–20 kHz have been tested. In each period ions are extracted with screen pulse amplitudes of 200–800 V and accelerator voltages kept at 10%–20% of the respective screen grid potential. Similarly, electrons are extracted with screen pulse amplitudes of −50–−200 V and a grounded or slightly negatively biased accelerator grid. An electrically nonconductive target bombarded subsequently by ions and electrons will charge and discharge periodically, possibly resulting in a zero target bias. The neutralization effect has been studied in two ways: (i) measurement of a metallic target bias being electrically insulated from ground by a 220 nF capacitor; (ii) etching experiments of 1.2 μm thick polyimide layers (HPR 204) deposited on a Si3N4/Si substrate. A high positive target bias resulting from insufficient beam neutralization correlates with low etch rate and poor quality of the etched substrate surface. A negative target bias (overneutralization) increases the energy of impinging positively charged ions resulting in increased etch rates. Although unwanted in general this effect may be used intentionally in some cases. Zero target bias in all cases assures good surface quality and high etch rates. The neutralization principle presented is of prime importance in reactive ion beam processes when filamentless ion sources are used. This holds especially when interactions of chemically reactive ions with electrically insulating surfaces are considered.