Secondary ion mass spectrometry
Encyclopedia
Secondary ion mass spectrometry (SIMS) is a technique used in materials science
and surface science
to analyze the composition of solid surfaces and thin film
s by sputtering
the surface of the specimen with a focused primary ion beam
and collecting and analyzing ejected secondary ions. These secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion range.
observed a release of positive ions and neutral atoms from a solid surface induced by ion bombardment. Improved vacuum pump
technology in the 1940s enabled the first prototype experiments on SIMS by Herzog and Viehböck in 1949, at the University of Vienna
, Austria. Then in the early 1960s two SIMS instruments were developed independently. One was an American project, led by Liebel and Herzog, which was sponsored by NASA
at GCA Corp, Massachusetts, for analyzing moon rock
s, the other at the University of Paris-Sud in Orsay by R. Castaing for the PhD thesis of G. Slodzian. These first instruments were based on a magnetic double focusing sector field mass spectrometer
and used argon for the primary beam ions. In the 1970s, K. Wittmack and C. Magee developed SIMS instruments equipped with quadrupole mass analyzer
s. Around the same time, A. Benninghoven introduced the method of static SIMS, where the primary ion current density is so small that only a negligible fraction (typically 1%) of the first surface layer is necessary for surface analysis. Instruments of this type use pulsed primary ion sources and time-of-flight mass spectrometers
and were developed by Benninghoven, Niehus and Steffens at the University of Münster
, Germany
and also by Charles Evans & Associates. Recent developments are focusing on novel primary ion species like C60
or ionized clusters of gold
and bismuth
.
Typically, a secondary ion mass spectrometer consists of:
with pressures below 10−4 pascal
(roughly 10−6 mbar or torr
). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector (i.e. the mean free path
of gas molecules within the detector must be large compared to the size of the instrument), and it also prevents surface contamination by adsorption
of background gas particles during measurement.
s or by electron ionization
, for instance noble gas
es (Ar
+, Xe
+), oxygen
(O-, O2+), or even ionized molecules such as SF5+ (generated from SF6
) or C60+. This type of ion gun is easy to operate and generates roughly focused but high current ion beams. A second source type, the surface ionization source, generates Cs+ primary ions. Caesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Depending on the gun design, fine focus or high current can be obtained. A third source type, the liquid metal ion source
(LMIG), operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten
tip and emits ions under influence of an intense electric field. While a gallium source is able to operate with elemental gallium, recently developed sources for gold, indium
and bismuth use alloys which lower their melting point
s. The LMIG provides a tightly focused ion beam (<50 nm) with moderate intensity and is additionally able to generate short pulsed ion beams. It is therefore commonly used in static SIMS devices.
The choice of the ion species and ion gun respectively depends on the required current (pulsed or continuous), the required beam dimensions of the primary ion beam and on the sample which is to be analyzed. Oxygen primary ions are often used to investigate electropositive elements due to an increase of the generation probability of positive secondary ions, while caesium primary ions often are used when electronegative elements are being investigated. For short pulsed ion beams in static SIMS, only LMIGs are deployable, but they are often combined with either an oxygen gun or a caesium gun for sample depletion.
uses a combination of an electrostatic analyzer and a magnetic analyzer to separate the secondary ions by their mass to charge ratio. A quadrupole mass analyzer
separates the masses by resonant electric fields, which allow only the selected masses to pass through. The time of flight mass analyzer
separates the ions in a field-free drift path according to their kinetic energy
. It requires pulsed secondary ion generation using either a pulsed primary ion gun or a pulsed secondary ion extraction. It is the only analyzer type able to detect all generated secondary ions simultaneously, and is the standard analyzer for static SIMS instruments.
measures the ion current hitting a metal cup, and is sometimes used for high current secondary ion signals. With an electron multiplier
an impact of a single ion starts off an electron cascade, resulting in a pulse of 108 electrons which is recorded directly. A microchannel plate detector
is similar to an electron multiplier, with lower amplification factor but with the advantage of laterally-resolved detection. Usually it is combined with a fluorescent screen, and signals are recorded either with a CCD-camera or with a fluorescence detector.
s for most trace elements are between 1012 and 1016 atoms per cubic centimeter, depending on the type of instrumentation used, the primary ion beam used and the analytical area, and other factors. Samples as small as individual pollen grains and microfossils can yield results by this technique.
The amount of surface cratering created by the process depends on the current (pulsed or continuous) and dimensions of the primary ion beam. While only charged secondary ions emitted from the material surface through the sputtering process are used to analyze the chemical composition of the material, these represent a small fraction of the particles emitted from the sample.
process, using a DC primary ion beam and a magnetic sector or quadrupole mass spectrometer.
will be the first instrument to determine the composition of cometary dust in situ with secondary ion mass spectrometry in 2014.
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
and surface science
Surface science
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid-gas interfaces. It includes the fields of surface chemistry and surface physics. Some related...
to analyze the composition of solid surfaces and thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
s by sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
the surface of the specimen with a focused primary ion beam
Ion beam
An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
and collecting and analyzing ejected secondary ions. These secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion range.
History
In 1910 British physicist J. J. ThomsonJ. J. Thomson
Sir Joseph John "J. J." Thomson, OM, FRS was a British physicist and Nobel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer...
observed a release of positive ions and neutral atoms from a solid surface induced by ion bombardment. Improved vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
technology in the 1940s enabled the first prototype experiments on SIMS by Herzog and Viehböck in 1949, at the University of Vienna
University of Vienna
The University of Vienna is a public university located in Vienna, Austria. It was founded by Duke Rudolph IV in 1365 and is the oldest university in the German-speaking world...
, Austria. Then in the early 1960s two SIMS instruments were developed independently. One was an American project, led by Liebel and Herzog, which was sponsored by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
at GCA Corp, Massachusetts, for analyzing moon rock
Moon rock
Moon rock describes rock that formed on the Earth's moon. The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.The rocks collected from the Moon are measured by radiometric dating techniques...
s, the other at the University of Paris-Sud in Orsay by R. Castaing for the PhD thesis of G. Slodzian. These first instruments were based on a magnetic double focusing sector field mass spectrometer
Sector instrument
A sector instrument is a general term for a class of mass spectrometer that uses a static electric or magnetic sector or some combination of the two as a mass analyzer. A popular combination of these sectors has been the BEB...
and used argon for the primary beam ions. In the 1970s, K. Wittmack and C. Magee developed SIMS instruments equipped with quadrupole mass analyzer
Quadrupole mass analyzer
The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...
s. Around the same time, A. Benninghoven introduced the method of static SIMS, where the primary ion current density is so small that only a negligible fraction (typically 1%) of the first surface layer is necessary for surface analysis. Instruments of this type use pulsed primary ion sources and time-of-flight mass spectrometers
Time-of-flight mass spectrometry
Time-of-flight mass spectrometry is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined via a time measurement. Ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has...
and were developed by Benninghoven, Niehus and Steffens at the University of Münster
University of Münster
The University of Münster is a public university located in the city of Münster, North Rhine-Westphalia in Germany. The WWU is part of the Deutsche Forschungsgemeinschaft, a society of Germany's leading research universities...
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
and also by Charles Evans & Associates. Recent developments are focusing on novel primary ion species like C60
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
or ionized clusters of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
.
Instrumentation
- primary ion gun generating the primary ion beam
- primary ion column, accelerating and focusing the beam onto the sample (and in some devices an opportunity to separate the primary ion species by Wien filterWien filterA Wien filter is a device consisting of perpendicular electric and magnetic fields that can be used as a velocity filter for charged particles, for example in electron microscopes and spectrometers. It is named for Wilhelm Wien who developed it in 1898 for the study of anode rays. It can be...
or to pulse the beam) - high vacuumVacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
sample chamber holding the sample and the secondary ion extraction lens - mass analyser separating the ions according to their mass to charge ratio
- ion detection unit.
Vacuum
SIMS requires a high vacuumVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
with pressures below 10−4 pascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(roughly 10−6 mbar or torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector (i.e. the mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
of gas molecules within the detector must be large compared to the size of the instrument), and it also prevents surface contamination by adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
of background gas particles during measurement.
Primary ion sources
There are three basic types of ion guns. In one, ions of gaseous elements are usually generated with DuoplasmatronDuoplasmatron
Duoplasmatron, an invention of Manfred von Ardenne, is a type of ion beam source. The duoplasmatron operates as follows: a cathode filament emits electrons into a vacuum chamber. A gas such as argon is introduced in very small quantities into the chamber, where it becomes charged or ionized through...
s or by electron ionization
Electron ionization
Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...
, for instance noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
es (Ar
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
+, Xe
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
+), oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O-, O2+), or even ionized molecules such as SF5+ (generated from SF6
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
) or C60+. This type of ion gun is easy to operate and generates roughly focused but high current ion beams. A second source type, the surface ionization source, generates Cs+ primary ions. Caesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Depending on the gun design, fine focus or high current can be obtained. A third source type, the liquid metal ion source
Liquid metal ion source
In a Liquid metal ion source , a metal is heated to the liquid state and provided at the end of a capillary or a needle. Then a Taylor cone is formed under the application of a strong electric field. As the cone's tip get sharper, the electric field becomes stronger, until ions are produced by...
(LMIG), operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
tip and emits ions under influence of an intense electric field. While a gallium source is able to operate with elemental gallium, recently developed sources for gold, indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
and bismuth use alloys which lower their melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
s. The LMIG provides a tightly focused ion beam (<50 nm) with moderate intensity and is additionally able to generate short pulsed ion beams. It is therefore commonly used in static SIMS devices.
The choice of the ion species and ion gun respectively depends on the required current (pulsed or continuous), the required beam dimensions of the primary ion beam and on the sample which is to be analyzed. Oxygen primary ions are often used to investigate electropositive elements due to an increase of the generation probability of positive secondary ions, while caesium primary ions often are used when electronegative elements are being investigated. For short pulsed ion beams in static SIMS, only LMIGs are deployable, but they are often combined with either an oxygen gun or a caesium gun for sample depletion.
Mass analyzers
Dependent on the SIMS type, there are three basic analyzers available: sector, quadrupole, and time-of-flight. A sector field mass spectrometerSector instrument
A sector instrument is a general term for a class of mass spectrometer that uses a static electric or magnetic sector or some combination of the two as a mass analyzer. A popular combination of these sectors has been the BEB...
uses a combination of an electrostatic analyzer and a magnetic analyzer to separate the secondary ions by their mass to charge ratio. A quadrupole mass analyzer
Quadrupole mass analyzer
The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...
separates the masses by resonant electric fields, which allow only the selected masses to pass through. The time of flight mass analyzer
Time-of-flight mass spectrometry
Time-of-flight mass spectrometry is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined via a time measurement. Ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has...
separates the ions in a field-free drift path according to their kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
. It requires pulsed secondary ion generation using either a pulsed primary ion gun or a pulsed secondary ion extraction. It is the only analyzer type able to detect all generated secondary ions simultaneously, and is the standard analyzer for static SIMS instruments.
Detectors
A Faraday cupFaraday cup
A Faraday cup is a metal cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup...
measures the ion current hitting a metal cup, and is sometimes used for high current secondary ion signals. With an electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
an impact of a single ion starts off an electron cascade, resulting in a pulse of 108 electrons which is recorded directly. A microchannel plate detector
Microchannel plate detector
A micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...
is similar to an electron multiplier, with lower amplification factor but with the advantage of laterally-resolved detection. Usually it is combined with a fluorescent screen, and signals are recorded either with a CCD-camera or with a fluorescence detector.
Detection limits and sample degradation
Detection limitDetection limit
In analytical chemistry, the detection limit, lower limit of detection, or LOD , is the lowest quantity of a substance that can be distinguished from the absence of that substance within a stated confidence limit...
s for most trace elements are between 1012 and 1016 atoms per cubic centimeter, depending on the type of instrumentation used, the primary ion beam used and the analytical area, and other factors. Samples as small as individual pollen grains and microfossils can yield results by this technique.
The amount of surface cratering created by the process depends on the current (pulsed or continuous) and dimensions of the primary ion beam. While only charged secondary ions emitted from the material surface through the sputtering process are used to analyze the chemical composition of the material, these represent a small fraction of the particles emitted from the sample.
Static and dynamic modes
In the field of surface analysis, it is usual to distinguish static SIMS and dynamic SIMS. Static SIMS is the process involved in surface atomic monolayer analysis, usually with a pulsed ion beam and a time of flight mass spectrometer, while dynamic SIMS is the process involved in bulk analysis, closely related to the sputteringSputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
process, using a DC primary ion beam and a magnetic sector or quadrupole mass spectrometer.
Applications
The COSIMA instrument onboard RosettaRosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...
will be the first instrument to determine the composition of cometary dust in situ with secondary ion mass spectrometry in 2014.
External links
- Annual SIMS workshop - site includes a literature database.
- Tutorial pages for SIMS theory and instrumentation
- Time-of-Flight SIMS - tutorial slide presentation.
- Presentation by Dr. Charles Magee A Historical Perspective of SIMS.