Ion milling systems fire argon ions at samples until they are thin enough to achieve electron transparency. The ions bombard the material from an angle and sputter it from the surface. A transmission electron microscope (TEM) records an image of the sample.

Ion milling systems are used to prepare polished cross-sectional samples for analysis under a scanning electron microscope (SEM) when manual polishing proves difficult. This process exposes underlying material while protecting it from mechanical stress and is suitable for surface analysis performed in high-vacuum analytical experiments.

Employing small spot size ion guns to inspect submicron die level defects related to fabrication as well as artifacts is one of the more recent applications of ion milling. This technique is called focused ion beam (FIB) milling and is applied in electronics (particularly semiconductors) manufacturing and the biological field for analysis, material deposition, and ablation. While insights related to this technology reach back a number of years, FIB systems for commercial use are a recent development. 

System Operation

Ion milling systems bombard a surface with ions, resulting in sputtering of material from the surface. This process generates electron-transparent samples for the transmission electron microscope (TEM) imaging.

The TEM forms an image based on electron interaction and level of transparency after sputtering. The ion milling process uses Ar+ beam with diameters of up to 5 mm in bench-top instruments in large-scale applications. Ion milling provides high-quality views of gold, solder, and soft metals such as indium. This is accomplished by preparing the surface using flat milling through a mechanical grinding process and polishing in the bond of substance.

Ion Beam Figuring

Figuring and finishing of optical surfaces also employs ion milling—a technique called ion-beam figuring (IBF). Eastman Kodak introduced it in 1988 and applied it in 1990. IBF first requires polishing the optical elements using conventional methods and then milling the final element using the IBF technique.

Focused Ion Beam (FIB) Milling

Another recent application of ion milling is found in focused ion beam (FIB) systems, predominantly in semiconductor fabrication. These systems operate similarly to SEMs. However, they emit a focused ion beam using liquid-metal ion sources (LMIS) such as gallium, instead of firing an electron beam. These beams operate at low beam current levels for imaging purposes or at high beam current levels for sputtering or milling.

When a gallium primary beam strikes the sample surface, sputtering a small amount of material, it leaves as secondary ions or neutral atoms and releases secondary electrons. As a result, the ions and electrons generate a signal that forms the image.

Low beam currents cause sputtering of subtle amounts of material and allow FIB systems to generate a higher imaging resolution. With higher beam currents, the amount of sputtered material increases significantly, facilitating high-precision sample milling to a sub-micrometer or a nanoscale.

In non-conductive samples, low energy electron flood guns neutralize the charge. This technique accommodates the formation of images for highly insulating samples by imaging with positive secondary ions employing positive primary ion beams. It also promotes milling without the need for a conducting surface coating, as is required with an SEM.

Chemical etching is another method for sample preparation. Performing a chemical process for eliminating excess material to generate microwave circuits involves submerging a substrate in an etching solution. Undercutting of the lines on the sides occurs when the sides and top surface are exposed to the solution. This leads to undesirable variation in circuit-to-circuit repeatability.

FIB milling promotes etching without altering the circuit metallization. The ion beams remove excess material without undercutting sidewalls, leading to the continuous generation of repeatable circuits. Additionally, some metals (platinum, for instance) cannot be etched using a chemical process.

Imaging Capacity

Technological advancements gave rise to FIB systems with greater imaging capacity, eliminating the need for examining under stand-alone SEM instruments. For specimens requiring the highest possible resolution imaging to avoid damage, SEM imaging is necessary.

Helium Ion Milling

Some ion milling systems use helium as an ion source. Helium is less damaging to samples compared to gallium and sputters the material in small amounts. Helium ions are focused into small probe sizes and offer sample interaction that is significantly smaller than SEM electrons. A helium ion microscope produces images of superior resolution and acceptable material contrast at a higher focus depth.

Wien Filters

Milling with gallium ions leads to the embedding of gallium into the surface of the sample, which damages the material and presents challenges for semiconductor manufacturing. An alternative approach is to exploit the Wien filter technology in the milling process.

Use of other LMISs in combination with a Wien filter's ability to switch between larger and smaller ions limits the damage in the milling process. It also allows for the admixture of elements from alloy sources—a procedure helpful in examining magnetic materials and devices.

Applications

Ion milling systems have a broad range of imaging and manufacturing applications, including:

• Food
• Pharmaceuticals
• Scientific sample preparation
• Semiconductor manufacturing
• Optics manufacturing
• Micro-machining
• Nano-machining
• Secondary ion mass spectrometry (SIMS)
• Magnetics

Selecting Ion Milling Systems

Selecting an ion milling system requires a thorough understanding of the process requirements associated with the intended use. Devices offering higher precision in measuring and sputtering are more costly, so it is important to define the application parameters prior to the purchase. Advances in ion milling technology provide enhanced performance on tasks initially carried out by SEMs and should be taken into consideration when evaluating the system.

References

Image credit:

Oxford Instruments