Secondary ion mass spectrometry (SIMS) ELEC-L3211 Postgraduate Course in Micro and Nanosciences Department of Micro and Nanosciences
Personal motivation and experience on SIMS Offers the possibility to measure doping profiles in silicon (total dopant concentration vs. electrically active concentration given by ECV) Study of impurity concentrations in thin films No experience on performing SIMS measurements but analyzed doping profiles obtained with the technique. Interested in learning the capabilities of the method (e.g. is it possible to analyze silicon nanostructures?) 2
Contents Introduction Instrumentation Analysis modes Static mode Imaging mode Dynamic mode (depth profiling) Summary Homework 3
Ion beam techniques Incident ions absorbed, scattered or reflected light, electrons, X-rays, secondary ions, Ion beams used also in ion implantation Image: Martinavicius, A. Ion Beam Analysis Techniques 4
Secondary Ion Mass Spectrometry (SIMS) Ions in secondary ions out (destructive) Element specific: capable of detecting all elements as well as isotopes and molecular species The most sensitive method of all beam characterization techniques Detection limit for some elements in the range of 10 14-10 15 cm -3 (1 ppm or 1 ppb) Lateral resolution even 0.1 µm Depth resolution in the order of 1 nm 5
Instrumentation Sample surface bombarded by primary ions (energy 1-25 kev) Sample atoms and ions (only ~1 %) are removed by sputtering Mass/charge ratio of the ions is analyzed Mass spectrum is detected as a count or displayed on a fluorescent screen High vacuum (<10-6 Torr) required Image: Schroder, Semiconductor material and device characterization. John Wiley & Sons, 2006. 6
Selection of primary ions Selection of the ions source depends on required current (pulsed or continuous) required beam dimensions the sample (which ions are to be analyzed) Electronegative primary ions for elements which produce positive secondary ions and vice versa Oxygen (O 2+ ) for e.g. B and Al in Si (positive secondary ions) Cesium (Cs + ) for e.g. P and As in Si (negative secondary ions) Inert gas ions (e.g. Ar +, Xe + ) can be used to minimize chemical modification of the surface 7
Mass analyzers Quadrupole Sector mass spectrometer Time-of-flight mass analyzer 8
Quadrupole mass analyzer Four parallel rods with an oscillating electric field Less expensive but lower resolution Cannot distinguish between ions with close mass/charge ratios Image: http://www.murr.missouri.edu/ps_analytical_icp_quadrupole.php 9
Sector mass spectrometer Based on the Lorenz force: = + Electric field selects ions with a certain kinetic energy Subsequent field selects ions with a certain m/q ratio Different masses detected by scanning and fields Narrow slits required for high resolution sensitivity reduced Image: http://www.analyticalspectroscopy.net/ap8-22.htm 10
Time-of-flight SIMS Ions sputtered in short bursts, even < 1 ms Time of secondary ions to travel to the detector is measured = Both very large and small ions can be detected Unlimited mass range Parallel detection of all masses Low primary ion current static SIMS, depth profiling only in ~10 nm range Image: https://www.aif.ncsu.edu/equipment/time-of-flightsecondary-ion-mass-spectrometry/ 11
Analysis modes Static SIMS Surface sensitive: surface concentrations of elements and molecules without significantly altering the sample Imaging SIMS Similar to static mode, but generates 2D images or maps of samples based on concentration of ions with a certain mass Dynamic SIMS Primary ions with higher energy are used to dig a crater into the sample depth profiling Images: http://pubs.rsc.org/en/content/articlehtml/2014/cp/c3cp54337d 12
Static SIMS Slow sputtering rate (<0.1 nm/h) Only a small portion of an atomic layer typically removed Allows characterization of organic surface layers Complete mass spectrum of the surface Dwivedi, N. et. al. Graphene/Fullerene-like Nanostructures: A Durable Protective Overcoat for High Density Magnetic Storage. Scientific Reports, 5, 2015. doi: 10.1038/srep11607 13
Imaging SIMS Spatial resolution determined by the spot size of the primary ion beam (~100 nm diameter with Ga + ) Absolute quantity difficult to measure, suitable for relative concentration differences Since only the very first monolayer is analyzed, surface has to be cleaned in UHV (e.g. by sputtering the contaminants) Hofmann, J. et. al. Recent advances in secondary ion mass spectrometry of solid acid catalysts: large zeolite crystals under bombardment. Phys. Chem. Chem. Phys., 2014,16, 5465-5474. doi: 10.1039/C3CP54337D 14
Dynamic SIMS: Depth profiling High sputtering rate (~10 μm/h) Intensity of a single peak corresponding a certain element monitored as a function of time Time depth Final depth of the crater measured with a profilometer Average sputtering rate (assumes homogeneous material) Secondary ions (1/s) Concentration (atoms/cm 2 ) Time (s) Depth (μm) 15
Dynamic SIMS: Depth profiling Ion count concentration In principle, can be calculated Factors poorly known sputter yield secondary ion current primary ion current Use of standards with matrices similar to the unknown concentration of detected species ionization efficiency = ( ) transmission of energy and mass filters Secondary ions (1/s) Time (s) Concentration (atoms/cm 2 ) Depth (μm) 16
Dynamic SIMS: Depth profiling Calibration standards An accurately known dose of ions are implanted The secondary ion signal from the standard is integrated over the entire profile Measured total concentration must equal to that of the dose Secondary ions (1/s) Concentration (atoms/cm 2 ) Time (s) Depth (μm) 17
Relative sensitivity factor (RSF) total impurity ion counts (no unit) known concentration (cm -3 ) total matrix ion counts (no unit) = Number by which the measured ratio of impurity/dopant ion intensity to matrix ion (ion/element selected for the reference) intensity is multiplied to give the impurity atom concentration. Unit: cm -3 18
Secondary ion yield Average number of secondary ions sputtered per incident primary ion Depends on Sample material Matrix effect: yield of the same element from different types of samples varies (e.g. yield of Si from SiO 2 ~100 times higher than that from Si substrate) Crystallographic orientation Type, energy and incident angle of primary ions Some knowledge about the sample must be available 19
Challenges Edge effect: Atoms from the crater sidewalls contribute to the measured signal. Primary ions (e.g. O 2 ) are implanted into the sample and subsequently sputtered Cascade mixing: Primary ions displace sample atoms from their lattice positions dopant profile will give a deeper distribution than it in reality is Primary ion penetration depth should be kept minimum for shallow dopant profiling High vacuum required: if arrival rate of gaseous species from the vacuum chamber is larger than the primary ion current, vacuum contamination is measured instead of the sample Most sputtered material is neutral and cannot be detected 20
Summary- SIMS Capable of detecting all elements as well as isotopes and molecular species Most sensitive of all beam characterization techniques Detection limit: 10 14-10 15 cm -3 Different ion sources and mass analyzers for different purposes Static/imaging mode with low sputtering rate for surface characterization Dynamic mode with high energy primary ion beam for depth profiling 21
Homework a) A SIMS standard is made by implanting phosphorus into a silicon wafer with a dose of 10 cm and energy of 100 kev. A dynamic SIMS measurement produced profiles for Si and phosphorus shown in the next slide. After the measurement, a crater depth of 502.9 nm was measured with a profilometer. Determine the relative sensitivity factor (RSF) and with the help of that, plot the doping profile in format: phosphorus concentration (cm -3 ) as a function of depth (μm). You can assume the sputtering rate to be uniform (i.e. constant as a function of depth) for each sample. Note: For separate samples, the sputtering rate may vary. b) Another set of data was measured with the same equipment and measurement conditions from another wafer with phosphorus diffused near the surface. Plot the phosphorus diffusion profile in a format: concentration (cm -3 ) as a function of depth (μm). The measured crater depth was 1.215 μm. What is the surface concentration in this sample? Please show in your answer how you have determined the profiles. If possible, return also the code or Excel-sheet you have used for the calculations. 22
Profiles for Homework (Provided in a separate Excel-file) Standard: measured data Unknown profile: measured data 1.00E+09 1.00E+09 1.00E+08 1.00E+08 Secondary ion count (1/s) 1.00E+07 1.00E+06 1.00E+05 1.00E+04 1.00E+03 1.00E+02 Si P Secondary ion count (1/s) 1.00E+07 1.00E+06 1.00E+05 1.00E+04 1.00E+03 1.00E+02 Si P 1.00E+01 1.00E+01 1.00E+00 0 50 100 150 200 250 Sputtering time (s) 1.00E+00 0 100 200 300 400 500 Sputtering time (s) 23
Additional slides 24
Ion sources 1. Duoplasmatron Mainly for noble gas (Ar + or Xe + ) or oxygen ions Roughly focused, high current ion beams A cathode filament emits electrons into a vacuum chamber. A gas (e.g. Ar) is introduced in very small quantities into the chamber, where it becomes charged or ionized through interactions with the free electrons from the cathode, forming a plasma. The plasma is then accelerated through a series of at least two highly charged grids, and becomes an ion beam, moving at fairly high speed from the aperture of the device. (https://en.wikipedia.org/wiki/duoplasmatron) Image: https://en.wikipedia.org/wiki/duoplasmatron 25
Ion sources 2. Surface ionization source For Cs + ions Fine focus or high current can be obtained Cesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Image: http://www.eaglabs.fr/cm/sources-ions-primaires.html 26
Ion sources Operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten tip and forms a cone under influence of an intense electric field => ions are emitted. 3. Liquid metal ion gun (LMIG) Typically Ga is preferred due to its low melting point Tightly focused beam (<50 nm) enhanced lateral resolution Moderate intensity: can be combined with an O 2+ or Cs + gun for elemental depth profiling Able to generate short pulses of ion beams time-of-flight SIMS Image: Arunbalaji, S. Masked ion beam lithography http://www.slideshare.net/ramyakannan1/masked-ion-beamlithography 27