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Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry in laboratory

What is secondary ion mass spectrometry?

Secondary ion mass spectrometry(SIMS) is the process of sputtering secondary particles from the surface of a sample by bombarding the surface with a high-energy primary ion beam, which causes the atoms or groups of atoms on the surface to absorb energy, and these charged particles pass through a mass analyzer to obtain a map of the sample surface.
In conventional SIMS experiments, energetic primary ion beams, such as Ga, Cs, or Ar ions, are focused on the surface of a solid sample under ultra-vacuum conditions. The primary ion beam interacts with the sample and secondary ions are sputtered and desorbed from the material surface. These secondary ions are then extracted into a mass analyzer, which presents a mass spectrometric map with the characteristics of the analyzed surface and produces elemental, isotopic, and molecular information with sensitivity in the ppm to ppb range. There are three basic types of SIMS instruments most commonly used in this field, each using a different mass analyzer.

Application of secondary ion mass spectrometry

The field of SIMS mass spectrometry is growing rapidly and has very important applications in the fields of elemental doping in semiconductor manufacturing, component measurements of thin films and other inorganic materials, isotopic ratios in the universe, and trace elements in the Earth.
The analysis of thin film structures by secondary ion mass spectrometry is a unique analytical tool, especially for the analysis of materials in different thin layers and the interaction between two adjacent layers. The analysis of features, defects or contamination at the sub-micron scale is crucial for many industrial applications, such as: semiconductor device processing, hard disk head processing, special reflective surfaces, composite materials, etc. composite materials, etc.
In addition, the use of secondary particle mass spectrometry is a powerful tool for exploring the nature of frontier issues in science and technology, such as the visualization of single cells in the life sciences, where information on drug uptake, distribution, and metabolism in cells can be obtained, and the localization of drugs in tissues or cells can be studied, which is important for improving drug targeting and rational drug design.
In conclusion, secondary ion mass spectrometry has a wide range of applications: metals, semiconductors, microelectronics, aerospace, automotive, chemical, biomedical, single-cell, environmental and other fields, especially in the traditional semiconductor industry, integrated circuits, solar photovoltaic cells, etc. Analysis has a very important role in it.

Secondary ion mass spectrometry working principle

Secondary ion mass spectroscope is a mass spectrometry-based surface analysis technique. The principle of operation of secondary ion mass spectrometry is based on the phenomenon of primary ion interaction with the sample surface. Primary ions with an energy of several kilo electron volts bombard the surface of the sample, triggering a series of physical and chemical processes in the bombarded area, including primary ion scattering and sputtering of surface atoms, atomic clusters, positive and negative ions, and surface chemical reactions, resulting in secondary ions, which are analyzed by mass analysis to obtain a mass spectrum of information about the sample surface, referred to as secondary ion mass spectrometry.
Mass spectrometry can be used to obtain molecular, elemental, and isotopic information on the sample surface, to detect the distribution of chemical elements or compounds on the surface and inside the sample, and to image the surface or internal chemical composition of biological tissues and cells, and to obtain a three-dimensional image of the surface or internal chemical composition of the sample with sample surface scanning and stripping (sputtering stripping speed can reach 10 μm/hour). images. Secondary ion mass spectrometry has a high sensitivity to ppm and even ppb levels, and can also be used to image micro-zone composition and depth profiling.

Composition of secondary ion mass spectrometry

Secondary ion mass spectrometer mainly consists of three parts: primary ion emission system, mass spectrometer, secondary ion recording, and display system. The first two are in a vacuum chamber with a pressure of <10-7 Pa.
Primary ion emission system
The primary ion emission system includes an ion generator ( or ion guns) and a lensing lens. The ion source is a device that emits primary ions, usually by bombarding gas molecules (such as inert gases helium, neon, argon, etc.) with an electron beam of several hundred volts, which ionizes the gas molecules and produces primary ions. Under the action of voltage, ions are emitted from inside the ion gun and then passed through several electromagnetic lenses to focus the ion beam and irradiate on the surface of the sample to excite secondary ions. The secondary ions are introduced into the mass spectrometer using a lead electrode with a voltage of about 1 K V. The primary ion sources of SIMS are divided into gas emission sources (O2+, O-, N2+, Ar+), surface ionization sources (Cs+, Rb+) and liquid metal field ion emission sources (Ga+, In+).

Mass spectrometer
A mass spectrometer consists of a sector-shaped electric field and a sector-shaped magnetic field, where secondary ions first enter a sector-shaped electric field called an electrostatic analyzer. In the electric field, the ions move along a circular orbit of radius r. The electric field produces a force equivalent to the centripetal force.
The radius of the moving orbit r is equal to mv2/eE, which is proportional to the energy of the ion. So the sector electric field can make the same energy of ions for the same degree of deflection. The secondary ions deflected by the electric field then enter the sector magnetic field (magnetic analyzer) for a second focus. The Lorentz force generated by the magnetic flux is equal to the centripetal force.
Ions with different mass-to-charge ratios are focused on different points of the imaging surface. If the C slit is fixed and the strength of the sector magnetic field is changed, ions of different masses enter the detector through the C slit, and the B slit is called the energy slit, and changing the width of the slit selects secondary ions of different energies to enter the magnetic field.

Ion detection system
The ion detector is a secondary electron multiplier tube with curved electrodes inside, and a voltage of 100-300 V is applied between each electrode to accelerate electrons step by step. The secondary ions pass through the mass spectrometer and collide directly with the primary electrode of the electron multiplier tube, producing secondary electron emission. The secondary electrons are attracted and accelerated by the second stage electrode, on which more secondary electrons are bombarded, thus multiplying step by step, and finally entering the recording and observation system.
The recording and observation system of secondary ions is similar to the electron probe, which can display the secondary ion image on the cathode ray tube, give the surface distribution of an element, or draw the secondary ion mass spectrum of all elements on the recorder.

Secondary ion mass spectrometry advantages

1. The secondary ion mass spectrometer is capable of analyzing all elements including hydrogen and can monitor isotopes (m different).

2. Secondary ion mass spectrometers can obtain information on the most superficial layer of a sample from 1-3 atoms in-depth, and even the most superficial atoms of a single layer can be detected.

3. SIMS can analyze compounds and obtain information about their molecular weight and molecular structure.

4. The secondary ion mass spectrometer has a high sensitivity for many components, and some impurities are detected up to ppm (10-6) and ppb (10-9) levels, which is one of the most sensitive for surface analysis (trace B, O, etc.).

5. SIMS mass spectrometry can be used for micro-area composition analysis and depth profiling, and can also obtain a certain degree of lattice information.

Types of secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometer(ToF-SIMS)

In TOF-SIMS, the secondary ions are extracted to the field-free drift tube, and the secondary ions arrive at the ion detector along the established flight path. Since the speed of a given ion is inversely proportional to its mass, its time of flight will vary accordingly, with heavier ions arriving at the detector later than lighter ones. This type of mass spectrometer can detect secondary ions of all given polarities simultaneously with excellent mass resolution.
In addition, because such mass spectrometers are designed to utilize pulsed ion beams operating in the very low current (pA) range, they are useful for analyzing surfaces, insulators, and soft materials that are vulnerable to chemical damage from ions.

Fan-field secondary ion mass spectrometer

Fan-field secondary ion mass spectrometers usually use electrostatic and fan-field analyzers to analyze the velocity and mass of sputtered secondary ions. The fan-shaped magnetic field deflects the beam so that lighter ions deflect more than heavier ones, which have more momentum. As a result, ions of different masses will separate into different beams. Electrostatic fields are also applied to secondary beams to eliminate chromatic aberrations. These instruments are useful for deep analysis because of their higher operating currents and continuous beams. However, when these instruments are used for surface analysis and characterization of samples prone to charge and/or damage, they are not effective.

Four poles secondary ion mass spectrometer

A four-stage pole secondary ion mass spectrometer uses a resonant electric field in which only ions of a specific mass can stably pass through the oscillating field. Similar to fan-field instruments, these instruments operate at higher ion currents and are often referred to as "dynamic secondary ion mass spectrometry" instruments. For example, for sputter depth profiling and/or for gross analysis of solid samples.

How to maintain secondary ion mass spectrometry?

a. When we use a secondary ion mass spectrometer, it is particularly important to develop regular maintenance habits, which can not only make full use of its value but also extend the service life of the equipment. Therefore, for secondary ion mass spectrometer maintenance, we need to pay attention to the following points:

b. In the early stage, organic solvents will enter the pump with the detection of samples in the mechanical pump, so it is necessary to regularly open the gas shock valve and shock the gas for about 20min. The gas shock time should not be too long, because too long will lead to too fast consumption of pump oil. According to the amount of detection to adjust the frequency of gas, is generally recommended once a week.

c. The ion source cavity, sampling cone hole, and cover should be cleaned with dust-free paper and 50% methanol-water, usually once a week.

d. The pump oil of the mechanical pump needs to be replaced regularly, usually once every six months. If the pump oil looks dirty, it needs to be replaced in advance. When replacing the pump oil, all the pump oil needs to be poured out and then replaced with new pump oil. Different brands of pump oil cannot be mixed. Most laboratory mass spectrometers undergo heavy maintenance during long holidays.

e. General maintenance in addition to replacing the oil pump, does not need to discharge under the vacuum condition, internal cleaning and maintenance of mass spectrometry are generally less do, suggest to maintenance engineer to do, internal metal parts can be polished with alumina powder, other components be careful not to run into organic reagents, especially the sealing ring and so on.

f. The above is the regular maintenance method of secondary ion mass spectrometer, regular maintenance can reduce the frequency of failure of the instrument to extend its service life.

How to order secondary ion mass spectrometry?

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