Importance of the molecular identity of ion species in reactive ion etching at low energies

Abstract

Surface reactions on Si and SiO2 induced by CF+ and CF+2 from a mass-separated ion-beam system were studied with x-ray photoelectron spectroscopy. The bombardment energies were varied from 100 to 2±0.6 eV. We found that the surface reactions induced by fluorocarbon molecular ion bombardment largely depended on the molecular identity of the ion species, its degree of molecular dissociation upon impact, and the reactivity of the substrate surface. Four examples are shown to illustrate this new approach in altering reaction pathways. (1) CF+ bombardment at 2 eV: This led to molecular adsorption on both Si and SiO2and to SiFx formation on Si but little reaction with SiO2. (2) CF+/20 eV: This led to a higher probability of molecular dissociation upon impact and thereby opened the channel for CF disproportionation. The energy carried by the fragment was sufficiently high that the reaction channels with SiO2 was also opened. (3) CF+/100 eV: This resulted in efficient molecular dissociation and incorporation of atomic fragments rather deeply into the substrate. The channel for fluorocarbon formation was closed and those for SiC and SiF formation were opened. A silicon compound layer was formed on both substrates. (4) CF+2/100 eV: This resulted in molecular dissociation but the average energy per fragment was low. The channel for deep penetration was closed and most reactants were confined in the very top surface. CF disproportionation became a viable channel on both Si and SiO2 and a new channel was opened for etching SiO2 due to the availability of a sufficiently high local concentration of fluorine.

Vibrational electron energy loss studies of surface processes on Si(111)7×7 and vitreous SiO2 ion-mediated in CF4 and CH2F2

“…Some attempts to understand the mechanisms of plasma-mediated processes have been made under simplified conditions using XeF 2 and F 2 [5,7]. Mass-separated low-energy molecular ion beams have been used to study specific ionmediated surface processes [8], and the effects of ion bombardment on the reaction layer under realistic plasma conditions have been investigated [9]. Since manufacturing of modern microelectronic devices often involves such key steps as selective etching of SiO 2 over Si with a high aspect ratio, a better understanding of the complex plasma chemistry and its effects on the substrate would be of great interest.…”

“…A better understanding of the fluorocarbon-surface interaction may provide new insight into the early stage of formation and the subsequent reactions of this type of films. In spite of the extensive studies on the effects of ion bombardment on silicon [7][8][9]11], the interactions of fluorocarbon ions with Si remain poorly understood, especially for bombardment energy below 100 eV. Many surface diagnostic tools including X-ray photoelectron spectroscopy, Auger electron spectroscopy (AES), and Fourier transform infrared spectroscopy have been used to investigate the surface processes induced by ion irradiation of halocarbons [8,[12][13][14][15].…”

“…In spite of the extensive studies on the effects of ion bombardment on silicon [7][8][9]11], the interactions of fluorocarbon ions with Si remain poorly understood, especially for bombardment energy below 100 eV. Many surface diagnostic tools including X-ray photoelectron spectroscopy, Auger electron spectroscopy (AES), and Fourier transform infrared spectroscopy have been used to investigate the surface processes induced by ion irradiation of halocarbons [8,[12][13][14][15]. Recently, we have investigated the surface products on clean and oxidized surfaces of Si(1 1 1)7 Â 7 generated by ion irradiation in CF 4 at 50 eV impact energy by using the more surface-sensitive vibrational EELS technique [16].…”

Chemical Modification of Polystyrene Surfaces by Low-Energy Polyatomic Ion Beams

“…Interest in low-energy ion beam−surface interactions is driven by two general applications: (1) the development of analytical techniques such as low-energy ion scattering spectroscopy, secondary ion mass spectrometry, and surface-induced dissociation mass spectrometry − and (2) materials processing. ,, Ion beam processing of materials involves the modification of surfaces by direct exposure to energetic ions. Ion beams have been used to graft specific functional groups to polymer surfaces for biomaterials, adhesion, and printing applications. − Ion beams have also been used to synthesize or deposit thin films on metal and semiconductor surfaces with chemical and physical properties different from the bulk substrate. , Finally, ion beams have been used to etch features on surfaces for pattern generation …”

“…However, the extreme complexity of the plasma environment makes it difficult to gain a detailed understanding of the fundamental plasma−surface interaction, especially for plasmas of organic molecules ,, nitridation, , hydroxylation, amination, and reactive ion etching. ,, These and other model systems revealed some of the mechanistic details in the reaction processes, particularly the importance of energetic ion bombardment of the surface during plasma treatment . For example, specific chemical functional groups can be grafted onto the surface of polystyrene by bombardment with OH + and NH + ions at specific kinetic energies (<100 eV) and fluences .…”