188.8.131.52 Ion sources
Before gases can be analyzed in a mass filter, they must first be ionized in an ion source by means of electron bombardment.
Electrons are emitted from an electrically heated cathode (filament). A voltage is applied between anode and cathode, which accelerates the electrons. Neutral gas molecules that are present in the formation space between anode and cathode are ionized by collisions between electrons, forming single and multiple positive ions. The energy of the colliding ions exerts a significant influence on both the number and type of ions that will be formed.
Ionization of the neutral particles commences at a minimum electron energy of between 10 and 30 eV (appearance potential). The number of formed ions quickly increases as electron energy rises (acceleration voltage), reaches a maximum at 50 to 150 eV depending upon the type of gas in question, and slowly declines again as energy continues to rise. Since the yield in ions, and thus the sensitivity of the mass spectrometer, should be as large as possible, electron energies between 70 and 100 eV are typically used.
The ion current IK+ of a gas component K can be calculated from the following relationship:
|i−||=||electron current (emission current), in A|
|le||=||mean path length of the electrons, in cm|
|s||=||differential ionization effect cross section K, in 1 /(cm · mbar)|
|pK||=||partial pressure of the gas component K, in mbar|
Many types of ions are formed when ionizing complex molecules. In addition to single- and multiple-charge molecule ions (ABC+, ABC++) fractal ions also occur:
- ABC+ + 2e−
- ABC++ + 3e−
- AB+ + C + 2e−
- BC+ + A + 2e−
- A+ + BC + 2e−
- C+ + AB + 2e−
- B+ + A + C + 2e−
In addition to these types, it is also possible for recombination ions, such as AC+, to form. The occurrence and relative frequency of individual types of ions are characteristic of a certain type of molecule and serve as an important aid in identifying the molecule, and thus as an aid in qualitative gas analysis. Figure 4.7 shows the fragment ion distribution (cracking pattern or fractal pattern) of the simple molecule CO2, recorded at 70 eV of electron energy.
Selection of the ion source and the optimal filament material is based upon the requirements imposed by the measurement task. Applications often impose contradictory requirements on the ion source. In order to achieve optimal results, the ion source must be matched to the task at hand. This has resulted in the development of different types of ion sources that can almost all be equipped with cathodes made of rhenium (Re), tungsten or yttriated iridium (Y2O3/ Ir).
T-cathodes are preferred in the UHV range or where the vapor pressure of Re could have a disturbing effect. However the brittleness of tungsten cathodes due to the tungsten /carbon/ oxygen cycle must be taken into account; i.e. due to the formation of W2C. Yttriated iridium is increasingly being used today instead of the pure metal cathodes that were used in the past. The advantages offered by these cathodes are significantly lower operating temperature and relative insensitivity to air inrush. Consequently, the preferred fields of implementation for these cathodes are analysis of temperature-sensitive substances, e.g. metal-organic compounds, or analysis of contaminants in gas mixtures containing a high concentration of oxygen.
The various ion sources are described below on the basis of their attributes and fields of application. What all ions have in common is that they can be electrically biased up to 150 V. This avoids signal background due to ESD ions. This technology will be explained in detail later.
Axial ion source
This ion source is characterized by its extremely robust mechanical design and high sensitivity. It is primarily employed for residual gas analysis in high vacuum systems due to its open construction. Figure 4.8 shows a schematic diagram of an axial ion source.
The cathode (1) is arranged within a hole in the Wehnelt electrode (2) and is connected with this electrode on one side. The electrons accelerated toward the anode (3) ionize gas molecules in the formation area (4). The positive ions reach the mass filter through the extraction orifice (5). Due to its relatively open construction, only minor falsification occurs through desorption and surface reactions.
Lattice ion source
A lattice ion source is used to examine residual gas in UHV or even XHV applications. Its extremely open construction and the selection of materials ensure extremely low internal gas emission. This ion source is equipped with two tungsten filaments that can be heated simultaneously for degassing. If work is to be performed at temperatures below 10-11 mbar, rod systems that have been highly degassed especially for this purpose would be used. Measurements in the pressure range below 10-10 mbar can be falsified by so-called EID (electron impact desorption) ions. These (H+, O+, F+, Cl+) ions are directly desorbed by electron bombardment of surfaces, often with a high yield. EID ions come from adsorbed coatings that originate earlier in the history of the UHV equipment or the ion source, and usually have an initial energy of several eV. This attribute is utilized by skilled selection of the field axis voltage to suppress the EID ions relative to ions from the gas phase having energy of approximately eV, (Figure 4.10).
Crossbeam ion source
A crossbeam ion source (Figure 4.11) allows direct passage of molecular beams perpendicular and parallel to the system axis. The system emits electrons with pre-selectable energy (90 – 120 eV) into the formation area (3), from either the left or the right filament (1). The Wehnelt cylinder (4) at filament potential prevents the electrons from scattering to the environment. Because the electron energy can be set within broad limits, this ion source can be used to determine the appearance potential. The crossbeam ion source very precisely maintains the shot conditions of the ions into the mass filter.
Crossbeam ion sources are used for diagnosing bundled molecular beams. In this process, the molecular beam is shot into the formation area perpendicular to the axis of the plane of projection (Figure 4.11). After passing through the ion source (7), un-ionized neutral gas molecules are either channeled into a pump or into a cold trap for condensation. Mass spectrometers with this type of ion source are also used as „rate meters” for molecular beam epitaxy.
Gas-tight ion sources
Some of the above-described ion sources are also available in gas-tight versions. Gas-tight ion sources are used if only small quantities of gas samples are available, or if the signal background generated by residual gas needs to be effectively suppressed. In this connection, the gas inlet system (e.g. a heated capillary) and the ion source must be matched to one another. The inflowing gas volume will determine the pressure in the formation area, which can be a multiple of the pressure in the surrounding vacuum chamber, by means of the conductivity of the ion source. The operating principle will now be presented using an axial ion source by way of example (Figure 4.12).
The gas to be analyzed is introduced directly into the formation area via a metal capillary (6) that is at ground potential and an insulating spacer (5). The conductivity to the vacuum chamber is approximately 1 l / s.
Sputter process monitor (SPM) ion source
In this ion source, the formation area (7) communicates directly with the process chamber. The analyzer is equipped with a small turbopumping station (1), which also evacuates the cathode space (5) to approximately 10-5 mbar. Electrons are shot into the formation area (7) from the low pressure side through small holes in order to ionize them.
The ions thus formed are also extracted to the mass filter through a small opening to the lowpressure side. This ion source offers two crucial advantages for examining the composition of the gas in sputtering processes. On one hand, the analysis is performed at an ion source pressure that is up to three orders of magnitude higher; i.e. a higher concentration of residual gas can be tolerated in the vacuum chamber. On the other hand, the hot filament is not in direct contact with the sputtering process. This avoids contamination by the hot cathode for sensitive processes.
Standard PrismaPlus™ ion source
The PrismaPlus mass spectrometer from Pfeiffer Vacuum is equipped with this robust and highly sensitive ion source. It is an ion source that is especially suitable for residual gas analysis. Its design is comparable to that of a lattice source; like the lattice ion source, it has two cathodes, thus affording particularly secure operation. Both an open version as well as a gas-tight version with gas inlet in the axial direction are available.
All ion sources described here ionize by means of electron collision. The ion sources can be categorized into two groups:
- Open ion sources are used if the process gas is to be analyzed and additional pressure reduction is not required
- Closed ion sources are used in analytical applications, for example, in order to require only small volumes of gas or to increase sensitivity relative to the substrate of the vacuum system
Closed ion sources are used in combination with a differentially pumped system (Figure 4.13) in order to analyze higher-pressure gases.
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