Breaking A14 Node Barriers! How APT Transforms Materials Analysis

Atom Probe Tomography (APT) is currently one of the most advanced materials characterization techniques. It provides the ability to map the distribution of atoms within a small 3D region with sub-nanometer spatial resolution. Its detection limit reaches approximately 10 ppm within a 20 x 20 x 10 nm^³ volume, making it an invaluable tool for precise dimensional and doping concentration control. Recognizing its importance, leading semiconductor companies such as TSMC, Samsung, and Intel have adopted this cutting-edge technique within their operations.

APT works on principles fundamentally different from electron microscopy, which relies on interactions of electron-beam with the specimen. Instead, APT utilizes field evaporation, a process in which atoms on the tip of a needle-shaped sample are ionized and projected onto a position-sensitive detector, to view the atomic arrangement in the sample. So, it bypasses optical diffraction limit. This technique analyzes 3D microstructure of devices or materials, and offers key data for research.

Key Steps in APT Analysis:

1. Field Evaporation of Atoms:
   The sample is prepared into a needle-like shape with a tip curvature radius must be smaller than 100 nm. By applying high voltage or laser pulses, atoms are evaporated from the tip surface, one by one.
2. Ion Trajectory and Detection:
   Ionized atoms fly toward a position-sensitive detector, which captures their impact positions and time-of-flight.
3. Mass Spectroscopy:
   Time-of-flight data are used to characterize the type of ionized atoms, such as iron or nickel, through mass spectrometry analysis.
4. 3D Reconstruction:
   These collected data are combined to generate a three-dimensional atom map of the material.

For R&D and failure analysis of devices, a technique capable of analyzing local compositional variations—such as in thin films, grain boundaries, and junctions—is often essential. While Secondary Ion Mass Spectrometry (SIMS) offers nanoscale depth resolution and a detection limit in the parts-per-million range, it cannot effectively resolve 3D nanoscale structures. TEM can perform compositional analysis using Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS), but it faces challenges with 3D nanoscale structures, detection limits, and varying elemental sensitivities. These limitations prevent TEM from providing comprehensive analytical evidence and data.

A research team from National Taiwan University (NTU) and the Taiwan Semiconductor Research Institute (TSRI) has investigated doping segregation and diffusion in phosphorus-doped polycrystalline silicon (poly-Si) using APT. They observed a significant drop in carrier concentration in the poly-Si following rapid thermal annealing (RTA). Previously, this reduction in carriers was attributed to incomplete recrystallization or crystal defects. As shown in Figure 1, however, APT analysis revealed that phosphorus atoms segregated at grain boundaries and diffused into the substrate along these boundaries after annealing. The APT data provided 3D distributions of elements, which were further analyzed to generate 2D maps of specified cross-sections and 1D line profiles along designated traces to investigate the segregation phenomenon. The findings of this study highlight that the activation rate of carriers in poly-Si is influenced by the segregation of dopants. Importantly, the segregation of doping elements can be mitigated by optimizing the parameters and methods used during thermal annealing. Once segregation of phosphorus is reduced by optimizing annealing process parameters or methods, the electrical performance, process stability, and device performance can all improved.

Each of these three techniques, APT, TEM, SIMS, has its own advantages and limitations. You can select the most suitable one based on requirements in R&D and characteristics of samples. (If you need a comparison table, please contact the iST representative listed below this article.)

For APT analysis, the sample must be prepared as a tiny tip specimen with a tip radius of curvature smaller than 100 nm. This sharp tip enhances the electric field effect, reducing the potential difference (approximately several thousand volts) required to evaporate and ionize atoms from the surface. Currently, semiconductor device or thin-film samples can only be prepared using Dual Beam Focused Ion Beam (Dual Beam FIB) techniques. iST’s Materials Analysis (MA) lab has collaborated with National Taiwan University to develop a standardized method for preparing tip specimens. These include thin-film samples, MRAM samples, AlGaN optoelectronic devices, FinFET semiconductor devices, and various metals. Figure 2 illustrates a FinFET needle specimen prepared by iST alongside its corresponding APT data. Notably, iST is the first company in Taiwan to be trained and certified by the national APT team to prepare APT tip specimens, establishing a significant milestone in advanced materials analysis.