FIB Focused Ion Beam Microscope

The specific applications of FIB equipment can be roughly classified as:

1. Circuit editing of integrated circuits
2. Localized profiling and SEM observation
3. Ion Channeling Contrast
4. Transmission Electron Microscopy (TEM) sample preparation

Figure 1 Focused Ion Beam Microscope

(a) Liquid Metal Ion Source

(b) Single beam system, equipped with various reaction gases such as: Br₂、XeF₂TEOS, specialized for circuit editing.

(c) Dual beam system, integrating electron beam and ion beam in one machine, specializing in precise positioning for profile cutting and TEM sample preparation.

In the development stage of integrated circuit products, when the first batch of chips is produced, if there are design errors or malfunctions on the circuit, circuit modifications are needed to verify the circuit design. In the early days, modifying the photomask and re-fabricating the chips for circuit modification and verification was quite time-consuming and costly, especially as the integrated circuit process continued to miniaturize, the expenses required for this approach increased dramatically. Therefore, when FIB was able to perform the deposition and etching of metal and insulating layers, it became an essential part of the backend production line in miniaturized wafer fabrication, providing quick circuit editing or photomask repair at an extremely low cost to expedite product verification; at most, only one revision of the photomask is needed to complete product development. Depending on the different packaging of integrated circuit products and the locations that need editing, circuit editing can be divided into two main categories: Front-side Editing and Back-side Editing.

 

 

Generally speaking, the difficulty of editing varies depending on the differences in circuit structure. The larger the available excavation space at the editing location, the lower the construction difficulty; editing of upper metal wires is easier than that of lower metal wires; aluminum metal wires are easier to work with than copper wires; front editing is easier than back editing. Commonly used editing projects include:

• Excavation of the insulation layer deep well
• Cutting or piercing of metal lines
• Metal filling of the insulation layer deep well
• Metal connections, metal pads for probing.
• Capacitor and resistor manufacturing

 

The deposition of the insulating layer is facilitated by the ion beam to promote the reaction gas dissociation, thereby generating SiO.₂The commonly used gases are TEOS or TMCTS. The reaction gases for metal deposition include (CH for depositing platinum (Pt).₃)Pt(CpCH₃), deposition of tungsten (W) W(CO)₆ 。

 

FIB deposited W has a lower resistivity than Pt, and its filling capability is better, but the deposition rate is relatively slow, requiring a longer construction time. The deposition gas for carbon film is etched with C10H8 aluminum metal, and iodine (I) can be used.₂），Bromine (Br₂）或氯（Cl₂）to achieve. Copper etching uses gallium and moisture to sputter copper metal; the etching of the insulating layer uses XeF₂to achieve the chemical etching reaction.

 

Left image 2 Example steps for front circuit editing:

(a) IC Decapsulation

(b) Opening and Filling Metal

(c) Connection and Disconnection (Plan)

(d) Pt connection

(e) M2 cut-off (result after completion)

FIB can precisely excavate at the location where cross-sectional cutting is needed. Failure analysis or process monitoring often uses dual-beam FIB for specific point observation, first using electron beam imaging (i.e., SEM imaging) to search for the desired cutting location, and after positioning, cutting is performed with FIB; after the cross-section is completed, electron beams are used to obtain images of the cross-section, as shown in Figure 4(a). Figure 5(b) shows the cross-sectional structure of an integrated circuit in the copper wire process.

When observing solid crystal samples, due to the regular arrangement of atoms, many regular lattice gaps similar to long channels are formed between specific crystal directions and crystal planes. When the ion beam is aligned with these directions, ions can penetrate directly without colliding with the surface atoms in the sample, resulting in no secondary electrons or backscattered incident ions being generated, thus the signal received by the signal detector is relatively weak.

Figure 6 Schematic diagram of ion alignment and misalignment crystal channel direction (change "tunneling Ga+" to "channel Ga+")

 

When observing solid crystal samples, due to the regular arrangement of atoms, many regular lattice gaps similar to long channels are formed between specific crystal directions and crystal planes. When the ion beam is aligned with these directions, ions can penetrate deeply without colliding with the surface atoms in the sample, resulting in no secondary electrons or backscattered incident ions being generated, thus the signal received by the signal detector is weaker.

 

However, if the ion beam is not aligned with these directions, it will collide with the sample atoms at the surface, resulting in a greater number of secondary electrons or backscattered incident ions being produced, leading to a stronger signal received by the signal detector. Therefore, the obtained image will have a significant contrast in brightness, which is known as the ion channel contrast effect, as shown in Figure 6.

 

 

 

 

 

Figure 7 (a) Optical image of the cross-section of the solder ball; (b) FIB ion channel image of the solder ball

The ion channel imaging contrast effect can clearly show the layered structure of different crystals, and thus can be used for the analysis of grain size and orientation. In Figure 7(a), the optical microscope OM image shows the contrast formed by different materials due to differences in reflectivity and refractive index, or the differences in hardness caused by cross-section grinding of the sample, which can be used to distinguish different material compositions; however, the FIB ion channel image in Figure 7(b) can further reveal the grain structure of Pb-Sn within the solder ball, and the distribution of these grain sizes reveals the temperature gradient during the thermal treatment process of the solder ball joining, representing the temperature difference between the PCB and the IC.

In the preparation of TEM samples from FIB cross-sections, there are three methods: Pre-Thin, Lift-out, and Omni-probe. The choice of FIB depends on the analysis requirements of the sample.

pre-thinning method

Figure 8(a) and (b) show the sample preparation using the pre-thinning method, where the sample is first thinned to 5-10 um by grinding, and then further thinned to a thickness of 0.1 um suitable for TEM observation using FIB. The advantage of this method is that it can produce very large area (~50 um) and uniformly thick TEM samples; since the surrounding area of the thin region is still supported by the same material, there is no concern for deformation or curling of the thin region sample. However, this method requires grinding followed by FIB cutting, making it more labor-intensive and time-consuming, and there is still a risk of grinding failure.

Figure 9 Electrostatic Lift-out Method

 

 

 

 

Electrostatic suction method

Figure 9 shows the sample preparation using the suction method. First, the sampling area is thinned using FIB, then the thin slice is separated from the sample using U-shaped cutting, and finally, it is extracted with a glass probe using electrostatic adsorption and placed on a copper grid with a carbon film. This is currently the fastest and most time-saving method for TEM sample preparation, with each sample taking less than 1 hour to produce. Therefore, the production of a large number of TEM samples uses this method. However, once these samples are placed on the carbon film, no further processing or rework can be done, which means that the optimal quality of the samples cannot be guaranteed. Ultimately, the judgment of the final sample thickness still relies on the work experience of the FIB engineer.

 

 

 

 

 

 

 

Probe Extraction Method

Figure 10 shows the probe extraction method, where the specimen is roughly cut to about 1-2 mm away from the sample using FIB, then the probe is welded to the specimen with Pt deposited by FIB. The probe is then moved to transfer the specimen to the specimen holder; after welding the specimen to the holder with Pt deposited by FIB, the probe is cut away from the specimen using FIB, and finally, the specimen is finely polished to a thickness suitable for TEM observation using FIB.

 

This is the most complex and time-consuming method for preparing TEM samples, with a total working time of about 1.5 to 2 hours. However, this method allows for repeated entry and exit from FIB for further work if any local adjustments to the sample thickness are needed after TEM observation. Therefore, it ensures zero errors and zero risks in the preparation of TEM samples, and this method is usually adopted for very important sample analyses.

 

Left image 10: Steps record of the probe extraction method (Omni-probe) for TEM sample preparation:

(a),(b) Stick the probe to the cut sample.

(c) Extracted specimen

(d) Attach the sample to the sample holder.

(e) Cut the probe on the sample

(f) Place the sample holder and the sample into the TEM for observation.