Secondary Ion Mass Spectrometry TOF-SIMS Zaahir Salam
Basic OverviewWhat is Spectroscopy? It is the study of the interaction between matter and radiation.What is an Ion? It is a charged atom or molecule. It is charged because the number of electrons are not equal the number of protons in the atom or molecule. An atom can acquire a +ve or a -ve charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.
When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ION. If the atom has more electrons than protons, it is a negative ion, or ANION. If it has more protons than electrons,it is a positive ion.
Secondary IonA small percentage of these ejected atoms leave as eitherpositively or negatively charged ions, which are referred to assecondary ions’
Mass Spectrometry (MS) It is an analytical technique used for the determination of qualitative and quantitative information about both atomic and molecular composition of inorganic and organic compounds. Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other.
Mass Spectra• When the electron beam ionizes the molecule, the species that is formed is called a radical cation, and symbolized as M+•.• The radical cation M+• is called the molecular ion or parent ion.• The mass of M+• represents the molecular weight of M.• Because M is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M+•.
• The mass spectrometer analyzes the masses of cations.• A mass spectrum is a plot of the amount of each cation (its relative abundance) versus its mass to charge ratio (m/z, where m is mass, and z is charge).• Since z is almost always +1, m/z actually measures the mass (m) of the individual ions. Mass Spectrometry• Though most C atoms have an atomic mass of 12, 1.1% have a mass of 13. Thus, 13CH is responsible for the peak at m/z = 17. This is called the M + 1 peak. 4
Why SIMS Is Used The technique offers the following advantages: The elements from H to U may be detected. Most elements may be detected down to concentrations of 1ppm or 1ppb. Isotopic ratios may be measured, normally to a precision of 0.5 to 0.05%. 2-D ion images may be acquired. Using a high-energy and high primary beam densities (dynamic SIMS) a volume of a 100 to 1000 μm is analysed. In contrast, using low energy and low primary beam densities (static SIMS) the material sputtered is exceedingly small, with surface mono-layers lasting hours or days.
Advantages Continued… 3-D ion images may be acquired by scanning (rastering) the primary beam and detecting the ion signal as the sample is gradually eroded. Little or no sample preparation may be needed. For all semiconductor products, from IC’s to solar cells to LED’s, it is crucial to control the concentration depth profiles of dopants and contaminants. Secondary Ion Mass Spectrometry (SIMS) is the most important technique for measuring depth profiles of all elements with extreme sensitivity, huge dynamic range and very good depth resolution. The technique is also used to measure dilute element profiles in specialty glasses, ceramics, and metal alloys, and –by measuring isotope ratios– in art history, geology and extra-terrestrial research.
SIMS is an analytical technique based on the measurement ofthe mass of ions ejected from a solid surface after the surfacehas been bombarded with high energy (1-25 keV) primary ions.
Collision CascadeThe primary ions deposit energy into the surface layers. Around the impact site andto the depth of ~3 nm, many bonds are broken and there is much randomdisplacement and movement of atoms. This region is called the collision cascade• The physical effects of primary ion bombardment: implantation and sputtering
SPUTTERING• The sputtering, or ejection, of target atoms and molecules occurs because much of the momentum transfer is redirected toward the surface by the recoil of the target atoms within the collision cascade• The bombardment of a solid surface with a flux of energetic particles can cause the ejection of atomic species, it causes erosion or etching of the solid.• The incident projectiles are often ions, because this facilitates production of an intense flux of energetic particles that can be focused into a directed beam.• However, in principle, sputtering (and secondary ion emission) will also occur under neutral beam bombardment.
Depth Profiling• Monitoring the secondary ion count rate of selected elements as a function of time leads to depth profiles. The following figure shows the raw data for a measurement of phosphorous in a silicon matrix. The sample was prepared by ion implantation of phosphorous into a silicon wafer. The analysis uses Cs+ primary ions and negative secondary ions.
• To convert the time axis into depth, the SIMS analyst uses a profilometer to measure the sputter crater depth. A profilometer is a separate instrument that determines depth by dragging a stylus across the crater and noting vertical deflections. At the end of the above phosphorous depth profile, profilometry gives 0.74 um for the crater depth. Total crater depth divided by total sputter time provides the average sputter rate.• Relative sensitivity factors (RSFs) convert the vertical axis from ion counts into concentration. The appropriate RSF value for the above phosphorous implant is 1.07E23 atoms per cubic centimeter and the matrix current (IM) is 2.2E8 silicon ion counts per second. The following figure shows the above phosphorous depth profile plotted on depth and concentration axes.
Secondary Ion Energy Distributions• The sputtering process produces secondary ions with a range of (translational) kinetic energies.• Molecular Ions (internal vibrational and rotational modes)• Atomic Ions (only in translational modes)
Species Monitored• The choice for impurity species for analysis is usually concerned with detection limits, but also involves sensitivity and dynamic range. The best detection limit can be found with knowledge of the mass interferences and the relative sensitivity factors for atomic and molecular ions. The use of rarer isotopes for analysis can be an aid.
Ion Implantation• The primary ion undergoes a continuous energy loss due to momentum transfer, and to the electronic excitation of target atoms.• Thus, the primary ion is eventually implanted tens to hundreds of angstroms below the surface.• In general, then, the ion bombardment of a solid surface leads not only to sputtering, but also to electronic excitation, ion implantation, and lattice damage.
• Under typical SIMS conditions most of the sputtered material (>95%) emanates from the uppermost two atomic layers, so the sampling depth can be taken to be ~1 nm.• Large organic fragments are more likely to come predominantly from the surface monolayer, whilst the most energetic atomic fragments may come from significantly deeper layers.• Primary ion bombardment also causes the emission of low energy electrons and these can be detected to provide sample visualization (topographic information) in a manner analogous to a Scanning Electron Microscope (SEM).
Sputtering YieldSputtering yield is the average number of sputtered particles per incident ion.• Depends on the ion incident angle, the energy of the ion, the masses of the ion and target atoms, and the surface binding energy of atoms in the targetIon sputtering yield is the average number of Secondary ion cluster ions emitted per incident primary ion. spectrum from Ar ion Many factors affect the ion yield. The bombardment of Al. Note most obvious are that the ordinate is in a log scale. Predominant species• Intrinsic tendency to be ionized are Al+ ions; Al2+ and Al3+ are Positive ion : Ionization potential (IP) also abundant Negative ion: Electron affinity (EA)• Matrix effects Al+ from Al2O3 versus Al+ from Al metal
The 3 SIMS Analysis Modes• Static SIMS are used to determine surface concentrations of elements and molecules without significantly altering the analyte.• Imaging SIMS like static SIMS does not alter the analyte appreciably. This mode is used to generate images or maps of analytes based upon concentrations of one secondary ion representing either an element or molecule.• Dynamic SIMS involves the use of a much higher energy primary beam (larger amp beam current). It is used to generate sample depth profiles.
Static SIMS• Low ion flux is used. This means a small amount of primary ions is used to bombard the sample per area per unit time. Sputters away approximately only a tenth of an atomic monolayer.• Ar+, Xe+, Ar, and Xe are the commonly used particles present in the primary particle beam, which has a diameter of 2-3 mm.• The analysis typically requires more than 15 minutes.• This technique generates mass spectra data well suited for the detection of organic molecules.
Static SIMS: Organic Analytes • Fragmentation and subsequent ion formation of the sample can reveal the overall structure of the molecule through mass spectrometry. Polymethylmethacrylate SIMS (a) positve and (b) negativeSIMS data. -http://www.siu.edu/~cafs/surface/file6.html
Imaging SIMS• The mass spectrometer is set to only detect one mass.• The particle beam traces a raster pattern over the sample with a low ion flux beam, much like Static SIMS.• Typical beam particles consists of Ga+ or In+ and the beam diameter is approximately 100 nm.• The analysis takes usually less than 15 min.• The intensity of the signal detected for the particular mass is plotted against the location that generated this signal.• Absolute quantity is difficult to measure, but for a relatively homogeneous sample, the relative concentration differences are measurable and evident on an image.• Images or maps of both elements and organics can be generated.
Images created using the Imaging SIMS mode. Scanning ion image of granite from the Isle of Skye. -University of Arizona SIMS 75 x 100 micrometers.
Dynamic SIMS• The higher ion flux used in dynamic SIMS eats away at the surface of the analyte, burying the beam steadily deeper into the sample and generating secondary ions that characterize the composition at varying depths.• The beam typically consists of O2+ or Cs+ ions and has a diameter of less than 10 μm.• The experiment time is typically less than a second.• Ion yield changes with time as primary particles build up on the analyte effecting the ejection and path of secondary ions.
Dynamic SIMS generated depth Profile Depth profile of hydrogen embedded in Silicon.The intensity of a secondary ion signal as a function of sputtering time is measured. Bysuitable calibration measurements, this can later be translated into a concentrationdepth profile.
The active part of a fast-switching bipolar transistor contains a single crystalline stack of Si / SixGel-x/Si (see figure 1). The p-type dopant Boron is included during layer growth. The n-type dopantArsenic is introduced by deposition of an As-doped poly –crystalline Si layer. Upon a thermalanneal, the As and B dopants diffuse, yielding the profiles shown in figure 1. The shape of the Asand B dopant profiles and their position with respect to the SixGe1-x layer are very critical for theperformance of the transistor. To define the position of the pn-junction, it is essential to measurethe n- and p-type dopant depth profiles simultaneously with the Ge concentration depth profile.
Instrumentation Ion Sources• Ion sources with electron impact ionization - Duoplasmatron: Ar+, O2+, O-• Ion sources with surface ionization - Cs+ ion sources• Ion sources with field emission - Ga+ liquid metal ion sources Mass Analyzers• Magnetic sector analyzer• Quadrupole mass analyzer• Time of flight analyzer Ion Detectors• Faraday cup• Dynode electron multiplier
Mass Analyzers• The secondary mass spectrum is obtained by collecting the secondary ions and subjecting them to mass filtration prior to detection.• Three types of mass filter (or mass analyser) are used in SIMS – 1) Magnetic sector. 2) Quadrupole. 3) Time-of-flight.
Magnetic Sector Analyzer R 2000 Capable: R ~ 105 High transmission efficiency High mass resolution Imaging Capability
• The mass filter must be capable of separating secondary ions that differ in mass by one atomic mass unit (amu or dalton) over the whole mass range. The mass resolution or resolving power (R) of the mass spectrometer is given by R= m/∆m• where m is the mass of the detected species and ∆ m is the peak width. ∆ m is often quoted as the full width at half maximum peak height (fwhm) but in the case of the quadrupole it is more useful to quote ∆ mb, the full width at 10% maximum peak height i.e. the width at the base of the peak.
Quantitative Analysis• The secondary ion current Is for a selected ion of mass m (or more correctly of mass/charge ratio m/z, where z is almost always unity in SIMS) is given by Is(m) = Ip y αT C(m) where Ip is the primary ion current, y is the sputter yield, α is the ionization probability, T is the overall transmission of the energy and mass filters (i.e. the fraction of the sputtered ions of a given mass which are actually detected), and C(m) is the concentration of the detected species in the sputtered volume.
• For a quadrupole ,T is of the order 0.1% and is approximately inversely proportional to m. Therefore the sensitivity of the instrument decreases as higher mass ions are selected by the mass filter.• y is the total yield of sputtered particles, neutral and charged, of mass m per incident primary ion; y is typically between 1 and 20 for atomic species and it is a function of the mass and energy of the primary ion and its angle of incidence (peaking strongly at ~ 60º to the surface normal).• Under the constant primary beam conditions in the MiniSIMS, y only varies by a factor of 5 for different elements.
• α+ and α - are the respective probabilities that the sputtered particle will be a positive or negative ion. Ionization probabilities vary dramatically across the elements and are additionally very sensitive to the electronic state of the surface.• The secondary ion yield (y) can therefore vary by over four orders of magnitude for different elements and may be very matrix sensitive. – For example, the yield of Mg+ from clean Mg versus MgO is 0.01/0.9 (= 0.01) whereas the yield of W+ from clean W versus WO3 is 0.00009/0.035 (= 0.003).
Time-of-Flight Mass Spectrometer (TOF-MS)• In a more sophisticated design, the TOF analyser corrects for small differences in initial energy and angle in order to achieve high mass resolution. Combinations of linear drift paths and electrostatic sectors or ion mirrors are used and results with mass resolutions, M/∆M, above 10,000 can be achieved.• Major advantages of this approach over quadrupole and magnetic sector type analysers are the extremely high transmission, the parallel detection of all masses and the unlimited mass range.
SECONDARY ION DETECTORS• Most modern mass spectrometers have more than one detector.• The Cameca ims-4f has four secondary ion detectors; an electron multiplier, a Faraday cup, an image plate and a resistive anode Encoder.• In the Cameca ims-1270 the instrument is equipped with eleven detectors some of which can be used simultaneously to give higher precision and rapid acquisition.
Electron Multiplier• An electron multiplier consists of a series of electrodes called dynodes. Each dynode is connected to a resistor chain.• The first dynode is at ground potential, so that both positive or negative ions may be detected. The last dynode can be between +1500 to +3500 V depending on the age and type of multiplier.• When a particle (electron, neutral, ion etc.) strikes the first dynode it may produce a few (1, 2 or 3) secondary electrons.• These secondary electrons are accelerated to the second dynode that is held at a slightly higher positive potential. On impact more secondary electrons are generated and a cascade of secondary electrons ensues.
• For optimum performance, the electron multiplier should operate at sufficiently high voltage so that every ion arrival produces a pulse. This pulse is then amplified and as long as it is above a set threshold, it will be passed to the counting circuit.• The time taken for the multiplier, amplifier and discriminator, to process a pulse is known as the dead time (t). With fast pulse-processing circuitry, this is in the order of 15 to 20 ns and limits the electron multipliers maximum count rate to about 5x106 c/s if the dead time correction is to be kept low.• At a count rate of 5x105 c/s and a dead time of 25 ns the percentage correction is about 1.3%.• The true count rate (n) may be calculated from the observed count rate (no) by the equation:
• Pulse counting detectors follow Poisson statistics which require that each ion arrives independently of all other ions. If, in a fixed interval of time, n counts are detected, the standard deviation of the measurement is given by:• and the relative standard deviation is given by:
Faraday Cups• A Faraday cup detector can detect count rates from 5x104 c/s upwards. Unlike the electron multiplier it does not discriminate between the type of ion or its energy. It is simple and cheap, but its response time is slow
• The Faraday cup detector consists of a hollow conducting electrode connected to ground via a high resistance. The ions hitting the collector cause a flow of electrons from ground through the resistor.• The resulting potential drop across the resistor is amplified. A plate held at about -80 V in front of the collector, prevents any ejected secondary electrons from escaping and causing an anomalous reading.• A single charge on a single ion is 1.6x10-19 C. Therefore a count rate of 1x106 c/s would produce a current of 1.6x10-13 Amps. With a resistor of 10 MW connected to ground, the amplifier must be able to detect a potential drop of 1.6x10-6 (0.0016 mV).• The detection limit of the Faraday cup is limited by the thermal noise in the resistor and the quality of the amplifier. Often these components will be enclosed within an evacuated, thermally controlled chamber.
Technique Comparison AES XPS D-SIMS TOF-SIMSProbe Beam Electrons Photons Ions IonsAnalysis Beam Electrons Photons Ions IonsSpatial 8nm 9µm 2µm 0.1µmResolutionDetection 0.1-5 atom% 0.01-.1 atom% 1ppm 1ppmLimitsQuantification Good Excellent Challenging ChallengingInformation Elemental Elemental Elemental ElementalContent Chemical Molecular bondingDept Profiling Excellent for Excellent for Excellent for Excellent for small areas insulating Speed and sensitivity materials Sensitivity
Surface Imaging using SIMS• If the aim of the measurement is to obtain compositional images of the surface formed from the secondary ion spectrum with minimum possible damage to the surface, then the main problem is to ensure that sufficient signal is obtained at the desired spatial resolution whilst minimizing the ion flux incident on any part of the surface.• This is most easily achieved by switching from the traditional instrumental approach of using continuous-flux ion guns and quadrupole mass spectrometer detectors, to using pulsed ion sources and time-of-flight (TOF) mass spectrometers. The TOF mass spectrometers are a much more efficient way of acquiring spectral data, and also provide good resolution and sensitivity up to very high masses. Using such instruments, SIMS images with a spatial resolution of better than 50 nm are obtainable.
ImagingSome instruments simultaneously produce high mass resolution and high lateralresolution. However, the SIMS analyst must trade high sensitivity for high lateralresolution because focusing the primary beam to smaller diameters also reducesbeam intensity. High lateral resolution is required for mapping chemical elements. 197 AU 34 SThe example (microbeam) images show a pyrite (FeS2) grain from a sample of goldore with gold located in the rims of the pyrite grains. The image numerical scales andassociated colors represent different ranges of secondary ion intensities per pixel.
Cross-Sectional ImagingCross-sectional 27Al- Image depth profile of SiO2 capped GaAs/AlGaAssuperlattice with a 4 micrometer laser melt strip