The Neon Gas Field Ion Source - Stability and Lifetime

Abstract

Soon after its development in 1955, the gas field ion source (GFIS) was pursued as the source of positive ions for focused ion beam (FIB) instruments [1]. Within the semiconductor industry, such FIB instruments are of critical importance for their failure analysis (FA), circuit edit (CE), and TEM sample preparation. However the GFIS development efforts were hampered by issues related to the source lifetime, and the short and long term temporal stability. The commercial gallium liquid metal ion source (Ga-LMIS) has served as the ion source of choice for the past 30 years with some recognized shortcomings arising from the probe size, electrical contamination, optical opacity, etc [2]. These shortcomings have produced a growing interest in FIBs with other ion species. In the past decade, the helium GFIS performance was vastly advanced -permitting the development of the helium ion microscope (HIM). In the past year, these same advances were applied to a neon GFIS.The challenges arising from a neon beam can be attributed to the fundamentally different characteristics of the neon atom compared to a helium atom. Neon atoms will field ionize at an electric field strength of 3.5 V/Å, or about 20% less than the field required for the field ionization of helium [3]. The reduced field strength gives rise to a limited lifetime of the emitter atoms due to field assisted chemical processes. Previous neon results show drastic changes in the apex of the emitter after typically a few minutes of operation [3]. The neon gas also is much more polarizable in the presence of a strong non-uniform electric field. This has the effect of making the gas atoms attracted to the emitter, and augmenting their normally small van der Waals forces to the point where they tend to adhere to the emitter [4]. As an indirect consequence, the emission current tends to fluctuateas muh as 30% over very short time periods (Figure 1a). Additionally, the location of the emission sites tends to jump between a small set of metastable states (Figure 1b).Most recently, the stability and lifetime of the neon GFIS has been improved to the point where it has become a suitable source for a neon ion microscope (NIM). Both the fluctuations in the brightness, and the motion of the emission sites, have been reduced by a factor of 5. Further improvements allow for the operation over long periods of time (~10 hours). Under these conditions, the NIM can be used for imaging (Fig. 2) and nano-machining applications.[1] A

The Prospects of a Subnanometer Focused Neon Ion Beam

“…During the last year, experiments were reported in which a trimer type of the GFIS was used for a neon ion source (Notte et al ., ). These experiments revealed millisecond fluctuations in the emission pattern.…”

Focused chromium ion beam

“…Alloys may be employed to broaden the species selection somewhat, but to achieve a monatomic beam then requires a mass selection filter 4 . Gas-phase ion sources are also available, but long lifetime, high-brightness operation has been elusive in species besides helium 5 .…”

Ion beams in SEM: An experiment towards a high brightness low energy spread electron impact gas ion source

“…1 As the dimensions of the chip components continue to shrink and the demand for more advanced tools to image and manipulate the materials to the atomic scale increases, there has been a growing interest in improving the performance of the FIB systems by developing an ion source with high brightness, low energy spread combined with operation with a broad range of noble ion species and ion currents. Over the course of the past decade, a number of research efforts have emerged to develop noncontaminating FIB source using various methods such as plasma, 5-7 laser photoionization, [8][9][10][11] and gas field ionization [12][13][14] techniques. Unfortunately, some fundamental problems exist in each of these techniques and so far none of these sources is posed to replace the current state-of-the-art Ga LMIS in the commercial FIB systems.…”

“…The development of a GFIS using a heavier gas such as neon has turned out to be technically difficult due to emission instability and short life time associated with the ionizing gas having a lower ionization energy compared to typical impurities found in the source region. 13,14 The reader is referred to the Ph.D. thesis of Tondare 3 for more in-depth discussions on the recent research efforts in developing a high performance noble gas ion source.…”