Argon ion polishing of FIB specimens in PIPS II system

Argon ion polishing of focused ion beam specimens in PIPS II system
Anahita Pakzad, Gatan, Inc.

As researchers push boundaries of elemental analysis and HR imaging with their transmission electron microscope (TEM), ultra-low damage specimens less than 40 nm thickness are frequently required. When evaluating preparation techniques for these thin lamellae, it is important to consider class of material being polished and whether the method is reproducible when applied across multiple samples. Here we will discuss broad argon (Ar) beam ion milling and focused ion bean milling (FIB). These two most common techniques are used for preparation of electron transparent specimens for a diverse class of materials, including semiconductors, metals and ceramics.

When assessing techniques, FIB is especially viable when specific sample orientation or site specific positional accuracy is required. However, amorphization, implantation and vacancies due to application of high energy gallium (Ga) ions, sample alteration due to beam heating in addition to re-deposition, surface roughness and selective abrasion are some of various damages caused by this method.¹

Amongst these problems, FIB-induced surface amorphization is the most extensive and limiting as it reduces the signal-to-noise ratio.² Additionally in order to avoid geometrical blurring for high resolution transmission electron microcopy (HRTEM) applications, sample thickness must be reduced by the square root of the feature size.³ A 25 keV Ga+ FIB column can generate ~20 nm per specimen side damage in silicon (Si).⁴ This can be specifically limiting in modern Si-based semiconductor devices, where minimum feature size is less than 20 nm. The depth of FIB induced amorphous layer depends on beam energy, beam angle and material being milled, different techniques commonly used to reduce this damage in TEM samples are:

• Gas-assisted etching: Method enhances the milling rate, however increases roughness of crystallineamorphous interface that further damages TEM images.⁵
• Low-energy FIB: Reduction of beam energy minimizes damage depth, although etching rate and positional resolution are also impacted at these energies.
• Wet etching: Single-layered materials benefit significantly when used with suitable solution³, but because of different etching rates, this method cannot be used for multi-layer semiconductor devices.
• Argon ion milling: Most promising method for multi-layer materials, as none of the drawbacks mentioned above is present. Here the original FIB damage layer is replaced by newly formed Ar ioninduced damage layer.^3,6 The thickness of this layer depends on the milling energy, angle and time, which are all parameters controlled by the user in the PIPS™ II system.

For the complete application note, click here.