SIMS

1. I S M S econdary on ass pectrometry SIMS Viraj Jayaweera Department of Physics & Astronomy Georgia state University

2. Outline • Theory • Strength • Accuracy • Limitations • Application using GaN as example

3. SIMS Theory A well established analytical technique that was first pioneered in 1949 Primary ion beam (O-, O2+, Ar+, Cs+, Ga+ are often used with energies between 1 and 30 keV) Primary ions are implanted and mix with sample atoms to depths of 1 to 10 nm. SIMS is generally used for surface, bulk, microanalysis, depth profiling, and impurity analysis. http://atomika.com/ The bombarding primary ion beam produces monatomic and polyatomic particles of sample material and resputtered primary ions, along with electrons and photons. The secondary particles carry negative, positive, and neutral charges and they have kinetic energies that range from zero to several hundred eV. The technique involves bombarding the surface of a sample with a beam of ions, thus emitting secondary ions. These ions are later measured with a mass spectrometer to determine either the elemental or isotopic composition of the surface of the sample.

4. Schematic Diagram of a SIMS instrument

5. Cameca IMS 6f secondary ion mass spectrometer Faraday Cup Florescence Screen

6. Secondary ion current (ions/s) Incident ion current (ions/s) Fraction of the particles sputtered as ions Concentration of the ith element in the sputtered volume Sputtering yield of both ions and neutrals (particles / incident ion) Collection efficiency of the SIMS instrument SIMS Theory The detected secondary ion current for ith element

7. Mass Spectrometer For an analogy, think of how a prism refracts and scatters white light separating it into a spectrum of rainbow colors. In a mass spectrometer, ions travel different paths through the magnet to the detector due to their mass/charge ratios. A mass analyzer sorts the ions according to mass/charge ratios and the detector records the abundance of each ratio.

8. Mass Spectrometer

9. Time-of-Flight Mass Analyzer Typical flight times 10 ns to 800 µs

10. The Quadrupole Mass Analyzer www.chm.bris.ac.uk/ms/theory/quad-massspec.html The two opposite rods in the quadrupole have a potential of +(U+Vcos(wt)) and the other two -(U+Vcos(wt)) where 'U' is the fixed potential and Vcos(wt) is the applied RF of amplitude 'V' and frequency 'w'. This results in ions being able to traverse the field free region along the central axis of the rods but with oscillations amongst the poles themselves. These oscillations result in complex ion trajectories dependent on the m/z of the ions. Specific combinations of the potentials 'U' and 'V' and frequency 'w' will result in specific ions being in resonance creating a stable trajectory through the quadrupole to the detector. All other m/z values will be non-resonant and will hit the quadrupoles and not be detected. The mass range and resolution of the instrument is determined by the length and diameter of the rods.

11. SIMS Imaging

12. Depth Profiling • The measurement of dopant and impurity concentrations with depth in compound semiconductor is often accomplished by SIMS. • Monitoring the secondary ion count rate of selected elements as a function of time leads to depth profiles. 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.

13. Depth Profiling To convert the time axis into depth, the SIMS analyst uses a profilometer to measure the sputter crater depth. Total crater depth divided by total sputter time provides the average sputter rate. Relative sensitivity factors convert the vertical axis from ion counts into concentration. Previous phosphorous depth profile plotted on depth and concentration axes.

14. Depth Profiling Depth Resolution Depth resolution depends on flat bottom craters. • Modern instruments provide uniform sputter currents by sweeping a finely focused primary beam in a raster pattern over a square area. • In some instruments, apertures select secondary ions from the crater bottoms, but not the edges. • Alternatively, the data processing system ignores all secondary ions produced when the primary sputter beam is at the ends of its raster pattern.

15. Sensitivity and Detection Limits The SIMS detection limits for most trace elements are between 1012 and 1016 atoms/cm3. In addition to ionization efficiencies (RSF's), two other factors can limit sensitivity. The dark current (or dark counts) arises from stray ions, electrons in vacuum systems, and from cosmic rays Count rate limited sensitivity occurs when sputtering produces less secondary ion signal than the detector dark current. If the SIMS instrument introduces the analyte element, then the introduced level constitutes background limited sensitivity. Oxygen, present as residual gas in vacuum systems, is an example of an element with background limited sensitivity. Analyte atoms sputtered from mass spectrometer parts back onto the sample by secondary ions constitute another source of background.

16. Advantages and Weaknesses

17. SIMS Application: using GaN as an Example Detection Limits of Selected Elements in GaN Source:

18. SIMS Application: using GaN as an Example Detection Limits of Selected Elements in GaN Chu, Gao, and Erickson: Characterization of III nitride materials J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998

19. Al Depth Profiles at the Interface of an AlGaN/GaN Al depth profiles at the interface of an AlGaN/GaN sample. One profile was acquired on a sample with a high density of visible surface pits, whereas the other curve was obtained on a sample with few if any visible Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998

20. Si and Mg Doping Profile of GaN SIMS depth profile of a p-n homojunction in GaN using Mg and Si as p- and n-type dopants. Common dopants in GaN such as O and C and some transition metals such as Fe, Mo, Cr, and Ni are also measured. Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998

21. GaN/InGaN/GaN LED Device Optical micrograph depicting the post-SIMS measurement crater on the lower left-hand side and the remnant of the setup crater on the upper right-hand side of the device. SIMS analysis of a finished LED chip after de-encapsulation Chu, Gao, and Erickson: Characterization of III nitride materials J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998

22. Depth Profiles of GaN/InGaN/GaN LED Device SIMS depth profiles of dopants compositional profile for the GaN/InGaN/GaN LED device. Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998

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