KEYWORDS: Helium, Gallium, Gold, Ions, Ion beams, Chemical species, Scanning helium ion microscopy, Scanning electron microscopy, Sputter deposition, Image resolution
Helium Ion Microscopy has been established as a powerful imaging technique offering unique contrast and high
resolution surface information. More recently, the helium ion beam has been used for nanostructuring applications
similar to a gallium focused ion beam. A key difference between helium and gallium induced sputtering is the less
intense damage cascade which lends this technique to precise and controlled milling of different materials enabling
applications. The helium ion beam has been used for drilling 5nm holes in a 100nm gold foil (20:1 aspect ratio) while
the gallium beam sputtered holes of a similar aspect ratio seem to be limited to a 50nm hole size. This paper explores
the drilling of nanopores in gold films and other materials and offers an explanation for the observed differences in
results between helium and gallium ions.
Recent helium ion microscope (HIM) imaging studies have shown the strong sensitivity of HIM induced secondary
electron (SE) yields [1] to the sample physical and chemical properties and to its surface topography. This SE yield
sensitivity is due to the low recoil energy of the HIM initiated electrons and their resulting short mean free path.
Additionally, a material's SE escape probability is modulated by changes in the material's work function and surface
potential. Due to the escape electrons' roughly 2eV mean energy and their nanometer range mean free path, HIM SE
mode image contrast has significant material and surface sensitivity. The latest generation of HIM has a 0.35 nanometer
resolution specification and is equipped with a plasma cleaning process to mitigate the effects of hydrocarbon
contamination. However, for surfaces that may have native oxide chemistries influencing the secondary electron yield, a
new process of low energy, shallow angle argon sputtering, was evaluated. The intent of this work was to study the
effect of removing pre-existing native oxides and any in-situ deposited surface contaminants. We will introduce the
sputter yield predictions of two established computer models and the sputter yield and sample modification forecasts of
the molecular dynamics program, Kalypso. We will review the experimental technique applied to copper samples and
show the copper grain contrast improvement that resulted when argon cleaned samples were imaged in HIM SE mode.
Focused ion beam systems have traditionally used liquid gallium as the ion source material. It may now be possible to have high current density focused beams of gas ions like hydrogen, helium, neon, and oxygen. This paper discusses the progress recently made toward the commercialization of an alternative to gallium, the gas field ion source, or GFIS.
Repair of photomasks by sputter removal of chrome and other opaque materials with a focused ion beam (FIB) of gallium results in the implantation of gallium and chrome ions into the quartz substrate. The effect is localized transmission loss in the regions where material was removed. Currently, these gallium and chrome `stains' are removed using blanket etching techniques of the complete quartz substrate, thereby restoring the transmission losses. However, these techniques are unacceptable for use with phase shift masks (PSMs). This paper describes a technique that was developed to restore the localized transmission losses to acceptable levels in situ at the FIB repair system. In particular, the development of a technique that restores transmission for i-line lithography phase shift masks for 0.50 micrometers and 0.35 micrometers technology requirements is described. Information is presented describing various applications of the process including etched glass and embedded shifter type phase shift photomasks.
The recent development of focused ion beam systems with image resolution in the 20 nm regime has made practical a new process monitoring discipline, in-line x-y-z metrology. At any step in the wafer fabrication cycle, it is now possible to rapidly image a top view or cross sectional profile of the exact location on a chip or test structure where monitoring is required. For example, one can examine metal and oxide step coverage, via dimensions, resist profiles, metal grain size, or film quality. Under computer automation, any region on an 8' wafer can be located. A hole several microns deep and wide can be milled at this site, the wafer tilted up to 60 degree(s), and the walls of this hole imaged at magnifications approaching 70,000 times. Any arbitrary sequence of steps may be linked together to define a procedure which could be applied to wafer after wafer. Image information available during such a sequence can be uploaded to a host computer and statistical process control methods applied to the image parameters of interest. In this paper we describe the characteristics of a new focused ion beam system, its hardware and software control, and typical results from the cross sectioning of a 4 Mb DRAM.
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