Time of Flight ( TOF)Spectrometer ToF operates in a pulse mode Electron beam During a short pulse of E, ions are accelerated and acquire a constant AB kinetic energy kinetic energy mv2/2 >-0→0 but have different m/g and vs Thus they arrive to the detector in time sequence after travel the same distance. Time required to travel distance from the ion origin to the detector is p alse width cInsec 2qU amu kHz Schematic of TOF spectrometer with a spectrum The light ions with higher Vs arrive In order to provide higher resolution the pulse to the detector first should be as narrow as 1-10 ns. The pulse repetition frequency is usually in a kHz range
Time of Flight (TOF) Spectrometer TOF operates in a pulse mode. pulse width In order to provide higher resolution the pulse should be as narrow as 1-10 ns. The pulse repetition frequency is usually in a kHz range. During a short pulse of E, ions are accelerated and acquire a constant kinetic energy: kinetic energy = mv2 /2 but have different m/q and Vs. Thus they arrive to the detector in time sequence after travel the same distance. Time required to travel distance l from the ion origin to the detector is: The light ions with higher Vs arrive to the detector first. Schematic of TOF spectrometer with a spectrum
Quantitative Analysis using SIMS Primar Area of Instrumental Ion current analvSIs transmission factor for a+ densi IA+=jxA×YA+xfA+×CA+1 Measured secondary Secondary ion yield Atomic concentration of a on current of a+ in the matrix T in the matrix T in the matrix T L T=S,T A+×CA+ A+ Sensitivity factor for A in the matrix T Very difficult to calculate SA+T. It depends on the 1. Element and matrix 2. SIMS instrument 3. System parameters Standards are normally used Ire Standard Use s,I from standard Sample Measure IA+T the same matrix From known CAT Find CT Find, T
Detection Limits for some Elements in silicon SIMS can do trace element analysis WDS-100ppm Primary lon Beam O, or Cs EDS 1000ppm Element Detection limit Element Detected Ion atom/cm3 B 0.002 <1014 BPAScoNH 31P/31P+ 0.1 <5×1015 75AS- 0.2 <1016 121Sb 1.0 5×1016 C 1.0 5×1016 16O 10 5×10 5×10 H 100 5×1018 Detection limit Count rate limited is affected by Instrument transmission or ionization efficiency INstrumental backgrounds Residual vacuum or components of the instrument lon interferences matrix, primary beam and instrumental species
SIMS can do trace element analysis Detection limit is affected by WDS ~100ppm EDS ~1000ppm
Common Modes of Analysis 1. Bulk analvsis mode is used to obtain sensitivity to trace-level components, while sacrificing both depth and lateral resolution e Linear scan: spectrometer preset for a single element(peak )of interest 2. Mass scan mode is used to survey the entire mass spectrum within a certain volume of the specimen 1 and 2 Static SIMS 3. Depth profiling mode is use to measure the concentration of specific preselected elements as a function of depth from the surface 3 Dynamic sims 4. Imaging mode is used to determine the lateral distribution of specific preselected elements. In certain circumstances, an imaging depth profile is acquired, combining the use of both depth profiling and imaging
1 and 2 Static SIMS 3 Dynamic SIMS
Static Secondary Mass Spectrometry o Static SIMS involves the bombardment of a sample with an energetic(1-10 kev)beam of particles, which may be either ions or neutrals. As a result of the interaction of these primary particles with the sample, species are ejected that have become ionized. These ejected species, known as secondary ions are the analytical signal in SIMS In statIc SIMS, the use of low dose of incident particles (<5x102 atoms/cm2) is critical to maintain the chemical integrity of the sample surface during analysis. A mass spectrometer sorts the secondary ions with respect to their charge-to-mass ratio, thereby providing a mass spectrum composed of fragment ions of various functional groups or compounds on the sample surface. The interpretation of these characteristic fragmentation patterns results in a chemical analysis of the outer few monolayers the ability to obtain surface chemical information is the key feature distinguishing static SIMS from dynamic SIMS, which profiles rapidly into the sample destroying the chemical integrity of the sample