Introduction

Secondary Ion Mass Spectroscopy (SIMS) is an advanced and accurate surface-sensitive material characterization technique, in which the mass to charge ratios of so-called secondary ions are analyzed to achieve precious information about surface chemistry of a sample. The term surface sensitivity in an instrumental technique is a criterion indicating the depth from which the signals are generated through interaction of sample with an incident beam. Thus, the lower the interaction depth, the higher the surface sensitivity. First sparks for designing secondary ion mass spectrometers goes back to 1910 when J. J. Thomson first reported secondary ion emission from a surface, as a result of being bombarded by an ion beam. However, the instrument was first practically designed and employed in the University of Vienna, Austria by Herzog and Viehböck^[1][2].

Scientific principles

Sputtering and secondary ion generation

To put it simply, secondary ions are charged particles, ejected from the sample, as a result of interaction with a beam of heavy high-energy charged species which is applied to the sample by instrument, known as primary ion beam. The ejection of particles from a solid is generally known as sputtering. This process starts with a so-called collision cascade upon hitting an incident beam, here primary ion beam, to the surface of the material. Collision cascade is referred to numerous sequential collisions in a solid material which involves lots of atoms. As a simplifying assumption, these collisions are considered separate from each other, resulting in a model of linear collision cascade^[3]. Throughout these collisions, whenever sufficient energy for scaping the solid matrix is transferred to an atom, ejection occurs. This can be observed as neutral particles, individual atomic ions, or molecular ions. More specifically, interaction of an incident ion beam (primary ion beam) with solid’s atoms may happen through elastic and inelastic collisions, which result in energy transfer from primary beam to the solid in different mechanisms. In elastic collisions, momentum transfer is responsible for depositing energy from primary ion to the atoms, leading to a change in the primary beam trajectory^[4]. On the other hand, inelastic collisions are those in which energy is transferred to the atoms through electronic excitations of the atoms which does not result in any deflection of primary ions from their initial trajectory.  Accordingly, three different kinds of sputtering can be observed, based on the energy transfer mechanism, including kinetic sputtering (pure momentum transfer), potential sputtering (pure electronic excitation modes), and kinetic-assisted potential sputtering (momentum-induced excitations). During the SIMS characterization all different types of sputtering occur and both the neutral and charged species are ejected from the sample, from which only the charged species- secondary ions- are measurable by the instrument. However, the ejected population mostly consists of neutral atoms and molecules. Thus, it is extremely important to collect as much secondary ions as possible and transfer them to the detection system^[1][3].

Operational modes

Generally, the SIMS analysis is classified into two categories, based on the volume of the sample which is analyzed per each data collection cycle, including Static SIMS and Dynamic SIMS. Accordingly, in Static SIMS secondary ion signals are collected and analyzed from less than 1% of the outermost monolayer of the sample’s surface, which is typically below 5 nm. In dynamic SIMS, on the other hand, the analysis is carried out on many atomic layers, and the mass spectroscopy data can be obtained as a function of sputtering time or depth from the sample’s surface, known as depth profiling^[3]. Moreover, recent developments in the SIMS instruments have provided the capability of molecular depth profiling by using large cluster primary ion beams. Although this can be technically considered as a special form of dynamic SIMS, in some references has been introduced as a separate operational mode, called Cluster Ion SIMS^[5][1].

An important point about static SIMS is that the ion beam-induced damages on the sample should be minimized to achieve accurate data of a few atomic layer on the sample’s surface. In fact, sputtering is intrinsically a destructive phenomenon, and impacts of ions in the primary beam can considerably alter the underlaying layers. For instance, different kinds of segregation, recrystallization, implantation, and so on may occur due to the primary beam impacts. This accordingly results in unreliable data, specially about surface molecular ions. To minimize this effect, the current density of the primary beam in the static mode is kept in an extremely low value. For example, the current density of a primary ion beam used in static SIMS is typically kept in the range of 1 nA/cm², while its energy is in the keV range. This enables the instrument to collect secondary ions from undisturbed regions and prevent primary beam from impacting the same area on the surface for several times. In dynamic SIMS, however, this is not an issue and the instrument is supposed to collect data from different depths by removing many surface layers throughout the analysis. Accordingly, the elemental compositions can be accurately analyzed against sputtering time or depth at the cost of missing the data of molecular ions. Thus, the primary ion beam employed in the dynamic SIMS is typically in the range of 0.1 to over 30 keV with current densities much higher than those used in static SIMS. This results in obtaining the depth profile of present elements in a range of a few nanometers up to several micrometers with a resolution of around 1 nm and detection limit of 1 ppb, regardless of sample’s damage, in contrast to static SIMS^[1][6].

Moreover, the reason for designing large cluster ion SIMS is to achieve information on molecular ions in a dynamic SIMS. This becomes possible using large cluster ions as the primary beam because it, from one hand, minimizes the damage during sputtering process, and from the other hand, increases the sputtering rate, leading to whole the damaged area to be removed in each sputtering process. Accordingly, in contrast with the case of using atomic or small molecular ions, the underlying substrate which is newly exposed to the primary ions remains without damage due to the change in sputtering mechanisms. It is worth mentioning that this technique has been mostly successfully applied on soft materials so far, but it is still being developed to become applicable on larger range of samples^[1].

Components

Figure 1 illustrates a simplified schematic of SIMS instrument with its main components.

Figure 1. Simplified schematic illustration of a typical SIMS instrument Figure: Parham Koochak

Primary ion column

Generally, the primary ion column consists of different components, including one or more ion guns, electrostatic lenses, and a mass filter to obtain a pure beam.

Ion guns or ion sources

Typically, five kinds of ion sources may be found in SIMS instruments, including Duoplasmatron ion source, RF plasma ion source, Electron Ionization (EI) source, surface Ionization source, and field Ionization source.

EI sources produce positive primary ion beams from inert gases and can also be used in generation of large cluster molecular ion beams. Although these sources do not provide smallest spot sizes, are almost the most commonly used sources in conventional SIMS instruments, particularly for dynamic SIMS analyses.

Both the Duoplasmatron and RF sources employ plasma to generate almost same ions as EI sources, in addition to some negatively charged ion beams, such as O^-. These ion sources are used mostly due to their higher secondary ion yield, specifically for positive secondary ions, compared with EI sources, particularly for electronegative elements.

Surface Ionization sources are commonly employed to generate alkali primary ion beams, and results in higher secondary ion yield for negative secondary ions. The most common ion source in SIMS instruments in the Cs+ source which is a kind of this category. Generally, the surface ionization sources are good choices for analysis of electropositive elements^[4].

Field Ionization sources, also known as Liquid Metal Ion Guns, are used to generate Ga+ and In+ primary ion beams with extremely small spot sizes.  LMIGs are mostly used in pulsed primary ion Time-of-Flight SIMS analysis (TOF-SIMS).^[3][6][1]

Electrostatic lenses and mass filter

After the primary ions are generated by the ion gun, they should be focused and hit the sample in a controlled condition. For this purpose, the so-called primary ion column in the instrument is equipped with deflection plates, lenses, mass filter, and apertures, in order to filter the primary ion beam, raster it, and control its shape, position, and diameter. The mass filter is required to prevent impurities or unwanted charged species from hitting the sample and being implanted into it^[1].

Extraction of secondary ions

Bombarding the sample’s surface by primary ion beam, results in generation and ejection of secondary ions which should be immediately extracted and collected to be transferred towards the detection system. For this purpose, an extraction (or immersion) lens can be used which extract the formed secondary ions due to a high potential difference with sample. It is worth mentioning that polarity of the secondary ions is not dependent on the primary ion beam, meaning that either positive or negative ions can be extracted from the sample, depending on its polarity, regardless of the primary beam. An important point is that the potential difference between sample and extraction lens should be kept constant in order to achieve a stable current of secondary ions. In the case of insulating samples this can be more or less done by applying a thin layer of carbon or gold on the sample’s surface, through which the accumulated charges can be transferred to the ground^[1].

Secondary ion transfer

Extracted secondary ions by extraction lens should be transferred into the mass spectrometer. This is carried out by another electrostatic lens, known as transfer lens. This lens is used to create a focused beam from secondary ions towards the entrance slit of spectrometer, and to create a real magnified image of the sample’s surface. The combination of extraction and transfer lenses, accompanied with a detector in the position of image formation (field aperture) could be considered and used as an ion microscope, providing magnified images from sample’s surface^[3][1].

Ion energy filter

The energy filter in the SIMS instrument consists of a curved path between outer and inner spherical surfaces with opposite potentials. The working principle of this electrostatic energy filter is based on the relationship between energy of the charged particles and their deflection in an electric field between to surfaces with a potential difference. In fact, secondary ions with higher energies experience less deflection than those with lower energy levels. Accordingly, a movable slit after this filter path, enables the instrument to separate secondary ions with specific energy levels which are then passed through a mass filter. As mentioned before, during the sputtering process, molecular ions are generated and can be found mostly with lower energies compared with atomic ions which are abundant in higher energy levels. Thus, moving the filter’s slit in a way to exclude lower-energy species, provides the capability to remove molecular, and typically unwanted, ions from the secondary ion beam before entering the mass filter^[3][1].

Mass filter

As suggested by its name, the most important component in the SIMS instrument is mass filter in the secondary ion column. In fact, the type of this mass analyzer defines the design of the SIMS instrument. Generally, three different types of mass filters are available in SIMS instruments, including Quadrupole mass filter, magnetic sector mass filter, and time-of-flight mass filter.^[3][1][3]

Quadrupole mass filter

Quadrupoles have been the first mass filters employed in SIMS instruments, working based on the relationship between mass to charge ratio of secondary ions with the change in their trajectory in a two-dimensional oscillating electric field with tunable DC and RF voltages. These filters are still being used in some applications, due to their simplicity and small size. However, relatively low mass resolution and transmittance have led them to be replaced with two other mass filters^[4][1].

Magnetic sector mass filter

Magnetic sectors work in accordance with almost same principles as quadrupoles, whereas they filter the ions by applying a magnetic field in perpendicular direction with respect to the secondary ion beam, resulting in a deflection in their direction which is determined by their mass to charge ratio and the strength of the magnetic field. Accordingly, the mass spectra of secondary ions can be obtained by changing the magnetic field’s strength. Although these filters are the largest available ones, their high mass resolution and transmittance have made them the most common choice for conventional dynamic SIMS analyses^[6][1].

Time-of-Flight (ToF) mass filter

Time-of-Flight mass filters are the simplest ones in terms of working principles. However, the need for more accurate and advanced electronics has led them to be the latest developed filters. ToF filters simply analyze the secondary ions, based on the fact that for ions with same charges, heavier ones travel the same distance in longer times. In order to use these analyzers, a pulsing system is required either in the primary ion beam (for static analysis) or in the secondary beam (for dynamic analysis). Time of Flight mass filters have been attracting more and more attention throughout the recent decade, due to their considerable higher transmittance and mass resolution, compared to other commercially available mass filters^[3][1].

Detector

In SIMS analysis, the detector is a relatively simple component which is responsible for recoding and counting the number of secondary ions which are already separated by the mass analyzers, per unit time. Accordingly, the detector converts the number of secondary ions to a current of electrons which can be easily measured and then again convert it to different values as the analysis output, such as counts per second. Common detectors, used in SIMS instruments are Faraday Cup, Micro-Chanel Plate Electron Multiplier, and Discrete Dynode Electron Multipliers^[1].

Vacuum

Finally, the vacuum design in SIMS instruments, similar to many other surface characterization techniques, such as XPS and AES, is an extremely essential parameter that should be taken into consideration, enabling the secondary ions to travel relatively long distances in the instrument through the secondary ion column and reach the mass analyzer and eventually get detected in the detector. Additionally, the importance of a contamination-free surface analysis is another reason for need of an Ultra-High Vacuum (UHV) condition (even lower than 10^-9 Torr)^[3].

Applications

As mentioned before, surface chemical characterization of solid materials and thin films is considered as the most common application of SIMS. Obtained data from a conventional SIMS analysis could be in the form of intensity-mass, counts-mass, or concentration-mass spectra, where the mass/charge ratios correspond to different ejected atomic or molecular secondary ions^[4]. Additionally, intensity, counts, or concentration of secondary ions can also be analyzed against depth or sputter time in the dynamic SIMS analysis. Moreover, 2D and even 3D images can be obtained by processing data over different area on the sample to visually illustrate from which position on the surface each secondary ion has ejected (SIMS imaging)^[5]. For instance, figures 2, 3, and 4 represent typical SIMS data in the form of intensity-mass spectra, intensity-depth, and SIMS imaging, respectively. 

Figure 2. Typical mass spectra, representing different isotopes of gold (a-d), Cr (e-h), and Si (i-l), obtained from Tof-SIMS analysis^[7]. (License CC BY 4.0)

Figure 3. A conventional depth profiling for a polydimethylsiloxane-like coating, obtained from SIMS analysis ^[8](License CC BY 4.0)

Figure 4. Tof-SIMS imaging (mapping) of a meteorite for secondary ions ejected from different regions of the sample. Color scales represents the intensity of ion emissions in accordance with mass spectra (counts per second) ^[9]. (License CC BY 4.0)

References