Time-of-flight secondary ion mass spectrometry (TOF-SIMS)

The modern Time-of-flight secondary ion mass spectrometry (TOF-SIMS) originated in the 1970s. One of its characteristics is high sensitivity. It can detect trace amounts of almost all elements at the level of parts per million (ppm), and some can even reach parts per billion (ppb). Another characteristic is high longitudinal resolution. The latest generation of TOF-SIMS can achieve a resolution of two to three atomic layers. Additionally, with technological improvements, the analyzed area is becoming increasingly smaller. These characteristics make TOF-SIMS indispensable in the analysis of materials' composition, doping, and impurity contamination.

Principles

1. A focused primary ion beam bombards the sample, causing stable impacts. Primary ions may undergo backscattering from the sample surface (with low probability) or penetrate through several atomic layers into a certain depth, undergoing a series of elastic and inelastic collisions during penetration. Some of the primary ion's energy is transferred to lattice atoms, causing some of them to move to the surface and transfer energy to surface ions, resulting in particle sputtering. Other physical and chemical processes may occur during primary ion bombardment, such as lattice distortion and chemical reactions on surfaces covered with adsorption layers. Most sputtered particles are neutral atoms and molecules, with a small portion being positively or negatively charged atoms, molecules, and molecular fragments.

Figure 1: Identifying the chemical species of surface materials by measuring the mass of secondary ions.

2. The ionized secondary particles (sputtered atoms, molecules, and clusters, etc.) are separated by mass-to-charge ratio for mass spectrometry.

3. Collecting the mass-separated secondary ions reveals the elemental composition and distribution of the sample surface and bulk. During analysis, the mass analyzer not only provides multi-element analysis data for each moment of the fresh surface but also offers secondary ion images of surface element distribution.

4. The uniqueness of TOF-SIMS lies in its ion flight time, which depends solely on their mass. Because a full spectrum can be obtained with a single pulse, TOF-SIMS achieves the highest ion utilization and can effectively conduct non-destructive static analysis of samples. Its most important feature is that the mass range can be expanded by simply reducing the pulse repetition rate, theoretically without limitations.

Classification

Secondary ion mass spectrometry (SIMS) can be primarily categorized into two types:

1. Based on the differences in the mass spectrometer, there are three types:

   • Quadrupole Secondary Ion Mass Spectrometry (Q-SIMS)
   • Magnetic Sector Secondary Ion Mass Spectrometry (MS-SIMS)
   • Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)

   Quadrupole SIMS primarily utilizes electric field scanning;

   Magnetic sector SIMS primarily utilizes electric and magnetic field scanning;

   TOF-SIMS utilizes ion beam sputtering.

2. Based on the intensity and density of primary ions, SIMS can be divided into two types:

   • Static SIMS and Dynamic SIMS

   Static SIMS operates at low energy, typically less than or equal to 10^13 primary ions/cm^2; Dynamic SIMS operates at high energy, typically greater than or equal to 10^17 primary ions/cm^2.

   Quadrupole and Magnetic Sector SIMS belong to Dynamic SIMS.

Characteristics

TOF-SIMS combines the features of secondary ion mass spectrometry and time-of-flight analysis techniques.

1. High sensitivity detection at ppm/ppb levels.

2. Depth profiling capability.

3. Detection of elements and isotopes, including hydrogen.

4. Quantitative analysis possible with standard samples.

5. High lateral resolution (< 60 nm).

6. Depth resolution better than 1 nm.

7. High precision scanning (pixel resolution up to 1024 × 1024).

8. Rapid detection and image acquisition (pixel frequency up to 50 Hz).

9. Sputtering rates up to 10 μm/h.

10. Imaging area ranges from μm^2 to cm^2.

11. Low sample consumption.

12. Detection of atoms, functional groups, fragments, and molecules.

13. Wide mass range and high mass resolution.

However, TOF-SIMS has some limitations:

1. Slight sample damage due to sputtering.

2. Currently limited to solid samples: thin films or bulk materials.

3. Powder samples can only be tested for spectra, not for depth analysis.

Applications

Time-of-flight secondary ion mass spectrometry primarily analyzes the surface elemental composition and distribution of samples through mass spectrometry, surface imaging (2D imaging/3D imaging), depth profiling, and other functions.

1. Mass Spectrometry

The initial and fundamental function of time-of-flight secondary ion mass spectrometry (TOF-SIMS) is mass spectrometry. By bombarding the sample surface with primary ions, secondary ions are generated, extracted, and analyzed to obtain mass spectra of different ions. Analysis of the mass spectra provides information on the elemental composition and distribution of several atomic layers on the sample surface.

Peng Huang et al. utilized the mass spectrometry function of TOF-SIMS to analyze the surface composition of a mixture of chalcopyrite and galena after treatment with chitosan. The experiment explored the mechanism of using chalcopyrite as a capturer and chitosan as a suppressor of chalcopyrite to selectively enrich galena. A mixed ore containing chalcopyrite and galena in a 1:1 ratio was selected for the experiment. The experimental results are as follows:

Figure 2: Mass Spectrometry Analysis Results of TOF-SIMS

From the analysis of Figure 2, it can be seen that the CuNH4+ ion content is the highest and significantly greater than the PbNH4+ content on the galena surface. CuNH4+ is a secondary ion generated from the sputtering of CuNH3+.

The high content of CuNH4^+ indicates that the Cu+ on the mineral surface has chemically reacted and combined with the amino groups in chitosan. The hydrophilic groups in chitosan interact with water molecules, making chalcopyrite hydrophilic and thus suppressed, achieving the goal of selectively floating galena .

2. Two-Dimensional Imaging

Two-dimensional imaging is also a primary function of TOF-SIMS. By using an ion source to emit a focused primary ion beam in a "scanning" manner across the sample surface, and collecting and recording the resulting secondary ions with a mass spectrometer detector, a two-dimensional image can be obtained.

Two-dimensional imaging can analyze the intensity of element distribution and chemical composition information.

The lateral resolution of two-dimensional imaging is less than 60 nm, with image acquisition rates of up to 50 Hz pixel frequency, and imaging areas ranging from μm² to cm².

Yang Ou et al. used a TOF-SIMS 5-100 time-of-flight secondary ion mass spectrometer to characterize the surface elemental distribution of PM 2.5 particles.

During testing, the sample size was 5 mm × 10 mm, and the sample was adhered to the SIMS sample stage with conductive adhesive. A test area of 100 μm × 100 μm was selected. Some of the secondary ion imaging results are shown in Figure 3.

Characterizing quartz filter membrane PM 2.5 particles with TOF-SIMS can avoid the interference of complex particulate matter, thereby obtaining information on the morphology and chemical composition of individual particles, and it is not limited by ultra-light elements (H, O, N, etc.).

Figure 3:Secondary Ion Imaging of Quartz Filter Membrane PM 2.5 Sample (Note: d. SO3-; m. SiHO+; o. O-; h. Na+)

3. Depth Profiling

In time-of-flight secondary ion mass spectrometry (TOF-SIMS), a low-energy ion beam is applied to the sample, etching it to form a small sputtered crater. Simultaneously, a pulsed ion beam is used to analyze the center of the sputtered crater, which constitutes depth profiling.

Depth profiling uses dual ion beams: a sputtering ion source and an analysis ion source. Through depth profiling, researchers can obtain the ion intensity distribution of the sample as a function of depth.

Depth profiling can achieve resolutions better than 1 nm, and the sputtering rate can reach up to 10 μm/h.

Xiao Heping and colleagues used a TOF-SIMS 5-100 instrument to study the depth distribution of elements within metal-semiconductor materials.

The experiment involved depth profiling of chips both before and after annealing. The authors focused on the depth distribution of Au, O, Be, P, and Ga elements. The experimental results are shown in Figure 4.

Figure 4:Depth Distribution Curves of Unannealed (a) and Annealed (b) Materials)

4. 3D Analysis By integrating data from mass spectrometry, two-dimensional imaging, and depth profiling, three-dimensional imaging of the sample composition can be achieved, resulting in a 3D map. A 3D map allows for the analysis of sample structure, defects, and other information. Figure 5 shows a 3D schematic of an unknown sample.

Figure 5: 3D Schematic of Unknown Sample

Additionally, time-of-flight secondary ion mass spectrometry is also used to analyze surface wettability, mechanisms of inhibition in certain flotation processes, isotope analysis, and more.