Nanoscale Microscopy with Gallium FIB SEM

Focused ion beam scanning electron microscopes for precision defect analysis

In the first blog in this series, we discussed laser and plasma focused ion beam (PFIB) scanning electron microscopes and their ability to remove large volumes of material quickly. On the other end of the spectrum is the need to remove thin layers of material at a very precise, nanometer scale for structural analysis, failure analysis, and metrology.

For instance, you are trying to analyze a sub-nanometer defect in a certain location in a die, but you don’t know exactly where the defect is. In this case, you need a focused ion beam scanning electron microscope (FIB-SEM) with the ability to thinly slice and image the sample repeatedly, until you find the defect. For this application, and others that require nanoscale precision, a gallium FIB-SEM (Ga⁺ FIB-SEM), such as the Thermo Scientific Helios 5 DualBeam, is the recommended technology of choice due to its superior FIB and SEM performance and broad application flexibility.

In this blog, we’ll focus on Ga⁺ FIB-SEMs and key capabilities that are needed to support small scale milling and nanoscale microscopy.

The Ga⁺ FIB-SEM

Introduced approximately 30 years ago, the Ga⁺ FIB-SEM combines a gallium focused ion beam and a scanning electron microscope in one instrument. The result is a powerful, flexible tool for analyzing and milling semiconductor devices that supports many applications. These include SEM imaging, cross-sectioning, defect analysis, in-situ sample preparation, 3D reconstruction, atom probe tomography preparation, and lamella preparation, the most common application and the subject of a subsequent blog.

To support these applications, key capabilities to look for in a Ga⁺ FIB-SEM are precise beam energy control, high resolution, navigation accuracy, endpointing, and spot size consistency.

Precise FIB energy control

Today’s semiconductors are comprised of complex structures which may include hard and soft materials and increasingly, materials which may be beam sensitive. The energy of the applied ion beam determines both the penetration depth and the amount of material that can ultimately be removed.

FIB induced amorphous damage comparison in nanoscale microscopy

FIB induced amorphous damage comparison at 5 kV, 2 kV, 1 kV and 500V.

While maximizing the beam current increases material removal speeds, if the beam energy is too high, it can result in damage to the region of interest and surrounding circuits. When this occurs, it makes it difficult, or impossible, to obtain accurate information about the sample’s structure and composition. On the other hand, if the beam energy is too low, it may not be able to remove the desired amount of material or produce the necessary imaging contrast to correctly position the FIB over the region of interest (ROI).

Therefore, it is important that a Ga⁺ FIB-SEM supports a range of beam energies and provides the ability to change voltages to best suit the application.

The critical value of high-resolution SEM

SEM resolution is critical for multiple reasons.

First, as semiconductor devices are becoming smaller and more complex, the features and defects that need to be studied have shrunk to a few nanometers in size. Resolving features on this scale requires a SEM resolution of around 1nm or even less.

Comparing image resolution of a specimen at 1kV for nanoscale microscopy

Comparing image resolution of a specimen at 1kV, 500V and 200V. Low kV imaging performance is particularly important in the analysis of surfaces or beam sensitive materials.

Secondly, as more materials sensitive to ebeam induced damage are integrated into semiconductor devices, the need for stable, high-resolution imaging at low (<1kV) accelerating voltages takes on more importance.

Extreme resolution is vital in semiconductor applications. However, the SEM imaging performance must also be repeatable, from system to system and sample to sample. Automated SEM calibration and alignment is required. This is one of the key characteristics of Thermo Fisher Scientific’s Helios DualBeam family of instruments.

Exact navigation and end-pointing

As we reach the most advanced technology nodes, the margin for error in creating site-specific data has shrunk proportionally. Precise navigation to the ROI and exact end-pointing are now critical to capturing the correct feature(s) for analysis.

Navigation refers to the ability to move the sample and the ion beam precisely and accurately to the ROI. In semiconductor analysis, this ROI could be as small as a few nanometers or tens of thousands of times smaller than a human hair. Performing this sort of navigation accurately on one sample is challenging for the most experienced engineer, but doing this on many hundreds or even thousands of samples demand the use of automated applications software.

End-pointing is the process of halting the material removal process, or shutting off the FIB beam at the precise location to capture the structures or materials to be studied. Precision end-pointing now routinely enables the capture of nm scale buried defects, or a single FinFET or GAA structure in a sample which may then be sent on to a TEM or Atom Probe instrument for atomic-scale analysis. In addition, accurate end-pointing is critically important to get the best possible TEM data on advanced semiconductor devices without any obfuscation by projection effects.

Accurate endpointing is required to isolate the feature of interest and avoid projection effects.

Together, navigation and end-pointing enable users to precisely target specific features and control the semiconductor sample preparation process with a FIB-SEM, providing the best possible SEM, STEM or TEM answers.

FIB beam consistency

Spot size consistency and repeatability of the FIB beam is also important as it fundamentally affects the resolution, precision, and accuracy of the FIB process.

A smaller FIB beam size results in a higher resolution image and the ability to remove ultra-thin layers of material. This is especially important as you move to lower beam energies. A larger beam size, on the other hand, provides faster removal rates but results in a lower resolution image with less precise details and milling precision.

Today’s broad range of semiconductor FIB-SEM applications require a range of milling parameters depending on the structures being analyzed – from trailing edge logic or power semiconductor devices which can have feature sizes in the hundreds of nm’s to the most advanced logic analysis which may require the generation of sub-10nm thickness samples. Maintaining spot size consistency can be a challenge when moving across a range of samples. Today’s DualBeam systems contain optimized and automated FIB and SEM columns which have the ability to handle a broad range of accelerating voltages from 30 kV to a few hundred volts, while maintaining alignment and beam consistency.

The Helios 5 DualBeam family: semiconductor sample preparation for nanoscale microscopy and analysis

Nanoscale sample preparation, imaging and analysis continue to play a critical role in structural analysis, defect analysis, and metrology of today’s semiconductors. With each generation of semiconductors becoming more complex with smaller features, new materials, and high aspect ratio 3D architectures, it is important to have tools that offer beam energy control, high resolution, advanced navigation and endpointing, and spot size consistency.

Thermo Fisher offers a broad range of gallium focused ion beam scanning electron microscopes to meet the small-scale material removal needs of the semiconductor industry. These include the Thermo Scientific Helios 5 FX DualBeam, the Thermo Scientific Helios 5 UX DualBeam, and Thermo Scientific Helios 5 CX DualBeam. These systems provide industry leading, precise, repeatable milling and high-resolution imaging capabilities for nanoscale sample preparation, imaging, and analysis.

Our next blog post in this series is focused on lamella preparation and challenges that may be encountered.