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Four Advanced Techniques in Focused Ion Beam (FIB) Microscopy

2021/07/15

The Focused Ion Beam (FIB) Microscope uses gallium (Ga) or xenon (Xe) ions, which have a much greater mass than electrons, to perform Selective Milling at selected positions on a sample. It uses bombardment or sputtering to complete fixed-point processing or cutting to the test pieces.

With a Scanning Electron Microscope (SEM) electron gun configured to the FIB equipment, it becomes a Dual-Beam System, or DB-FIB System, capable of both ion beam cutting and electron beam observation. The "real-time analysis" feature of the DB-FIB shortens the time between the processing of fixed points on a sample and the observation, greatly reducing the amount of time needed for the analysis.

 

 

Dual-Beam System Focused Ion Beam (FIB) Microscopy

FIB is widely used in fixed-point cross section and SEM analyses, IC circuit repair, TEM sample preparation and more (See [ Principles of FIB Technology ] to learn more). In addition to its applications in precision cutting, FIB allows for several advanced analysis methods, which as the descriptions below.

Let MA-tek help you master FIB analysis!

 

 

 

Advanced Analysis Type I

I-Beam View — The lattice interface on the surface of the sample can be presented clearly

Since ions and electrons are both charged particles, they can be focused by electromagnetic lenses. Using a focused ion beam to scan the surface of the sample, you can perform imaging analysis like SEM to observe surface morphology. To different surface materials and regional lattices, different image contrasts can be created by adjusting the energy of the incident ions due to the various degrees of ion Tunneling. In this way, you can observe the Grain Boundary on the surface of the sample.

 

The image below shows the difference between an I-beam View and E-beam View of copper lattice images. The I-beam View is a copper lattice image scanned by an ion beam. Note that the Grain Boundary is clearly visible. In contrast, using the E-beam View’s SEM scan, the lattice interface on the image is not so clear.

 

I-beam View

E-beam View

 

 

Advanced Analysis Type II

Auxiliary Gas Etching — Further enhance the differences by micro-etching on samples

In addition to cutting samples and obtaining images, the Focused Ion Beam Microscope also has another powerful function: FIB Circuit Repair (FIB-CKT). In the IC industry, it is often used in the pre-verification stage for changes in circuit design. It reduces the research and development costs of recreating new versions of the masks.

 

The principle involves passing an organometallic precursor gas over the surface of the sample then using an ion beam to break the precursor’s organic bonds. Conducting metal wiring is then deposited in order to meet various circuit repair needs. Learn more about the underlying technical principles and see examples of these applications at [ FIB Technical Principles ]!

 

In addition to the organometallic complexes used to deposit metal wires, corrosive auxiliary gases can be introduced (e.g. I2, XeF2, etc.). Although the original purpose for introducing such corrosive gases was to increase the etching rates of metals or oxides during circuit repair, they can also be used to help cut out the cross section of an IC component by injecting gases with selective etching characteristics. It can also perform micro-etching on the surfaces of samples to produce effects like Junction Staining to enhance image contrasts.

 

The following figure shows the difference at the test position once the sample has been enhanced using auxiliary gas micro-etching. The surface of the sample was selectively etched using the auxiliary gas XeF2. Once the Poly-Silicon is removed, the structure of the target area can be observed more clearly. With this same mechanism, you can also use other gases with high selectivity to metals or oxides to enhance the contrast in target areas. This approach to Failure Analysis is extremely helpful when observing subtle differences in the target structure.

Before Etching

After XeF2 Gas Micro-Etching of the Poly-Silicon Area

 

 

Advanced Analysis Type III

Eliminate the Curtain Effect — Reduce image interpretation errors

The Curtain Effect is when, after FIB cutting, streaks parallel to the ion incident direction appear in the lower half of the cross section of the sample (shown on the left). This is due to the different materials above the cutting surface (such as metal and oxide layers), which are affected by the uneven etching rate of ion milling. This creates streaks in specific directions that, in addition to being unsightly, sometimes cause problems in interpretation. As such, they need to be eliminated.

 


The Curtain Effect can be eliminated by adjusting the ion dose and other parameters

 

Common methods for improvement include the following:

  1. Avoid areas with uneven etching rates:If you can choose the location, try to avoid graphically Dense Areas. Instead, choose Iso Areas.
  2. Add a protective layer to the surface of the sample:By adding a dense, protective layer to the surface, you can reduce factors that lead to uneven etching rates when the ion beam comes into direct contact with different materials.
  3. Change the ion Milling Rate:Try to choose milling rates according to the different materials involved and make flexible adjustments. Refer to the following table for common cutting materials and the corresponding milling rates.

 

 

 

Advanced Analysis Type IV

Be Careful When Interpreting Signals - Don’t be blinded by cutting-caused splashing and backfilling

Sometimes you encounter voids of delamination in the material while using FIB to perform ion milling. When there is a void, the material near the cavity is often splashed during cutting, resulting in Re-Sputtering and Re-Deposition into the hollow. This results in a layer of residue of the same composition that can be observed at the edge of the hole.

 

For example, on the left of the picture below, it is a cross-section view containing a hollow structure. There is gallium (Ga) above the hole (see the middle figure below) and copper (Cu) under the hole (see the right figure below). They have accumulated around the hole due to backfilling during cutting. This may lead to the misunderstanding that these materials were present from the beginning, which then causes the wrong inferences to be drawn from the analysis results.

 

The signals from these backfills need to be carefully inspected by experienced engineers who can filter them appropriately. Sometimes it is also possible to fill the cavity in advance with plastic materials or a known material that is significantly different in composition from the target. This helps you avoid having your interpretations influenced by the backfill effect.

 


Cutting spatter backfill on the edge of the cavity

 

 

If you compare different sample preparation and observation tools, Focused Ion Beam Microscopy (FIB) is like delicate, minimally invasive surgery. It performs preparation and observation only at the targeted position on the sample, thus maintaining the original form of the sample. It is an analysis technology with a very high case rate received. If you want to review other sample preparation and observation tools, please read [ How do you choose sample preparation and observation tools when observing the cross section of a sample? ].

 

We hope that the four FIB analysis methods as discussed in this article help you make good use of Focused Ion Beam microscopy and thereby solve your analysis needs accurately and effectively!