Page 209 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ion Mass Spectrometry 195

instrument has additional features such as primary beam filtering and focusing, en-
ergy filtering of the secondary ions, counting systems for highest detection sensi-
for
tivity, and sometimes an electron flood gun charge neutralization on insulators.
SIMS instruments may be conveniently divided into three general types: (1)
(3) microscope imaging. Nonimag-
nonimaging, (2) scanning probe imaging, and
ing instruments are concerned only with surface or profile analysis within the area
covered by the primary beam, irrespective of the precise location of the emitted
secondary ion. Scanning probe imaging instruments such as the ion microprobe
mass analyzer (IMM~), Fig. 4.5, and the NanoSIMS 50, Fig. 4.14, form second-
ary ion images of the surface by scanning a focused ion beam over the sample sur-
face and displaying the detector signal on a synchronously scanning oscilloscope
or as a function of scan position with a computer. The image resolution in these
instruments depends on how finely the ion beam can be focused on the sample sur-
face. Microscope imaging instrume~ts such as the IMS-3f, Fig. 4.9, and the triple-
electrostatic-sector TOF, Fig. 4.12, form secondary ion images by stigmatically
transferring each secondary ion from the sample surface through the mass analyzer
to the detector. The detector may be a video camera that records the image formed
on a fluorescent screen when the secondary ions are converted to electrons by a
channel plate, or, in an alternative arrangement, a dual-microchannel plate elec-
tron multiplier used with a resistive anode encoder
[ 1031 to record counts as a func-
tion of position. Some modern commercial instruments have capability for both
probe and microscope imaging, which allows the analyst select the type of meas-
to
urement needed for optimal analysis.
Imaging instruments offer the advantage of analysis of much finer details on.
nonhomogeneous samples. Scanning probe imaging instruments have the advan-
tage of being able to achieve the highest spatial resolution; 20 nm has been achieved
[3]. Microscope imaging instruments have the advantage simultaneous transfer
of
of all ions of the analyzed mass from the total image area. Thus, images are formed
in much shorter times. In the microscope imaging instruments, large-diameter ion
beams with larger currents mean that profiling and fomation of images as a func-
tion of depth are much faster. The best image resolution achieved with microscope
imaging instruments is about 200 m.

Current density and €he type of primary ion have a critical effect on S
ses. High current densities are desirable for rapid profiling and high-sensitivity
analysis, whereas low current densities are chosen when the analyte layer is thin
or when using static SIMS. Exotic polyatomic ion sources are an area active re-
of
search today; Reo,- [104], SF,+ [105], SF6-, NO,', CF,+, C,o+, and many others
[ 106,1071, have been reported to provide exceptional enhancement of secondary
ion yields and ultrashallow depth profiling. However, the most heavily used ion
sources today are the hollow cathode duoplasmatron (Fig. 1081, the thermal
4.3 1) [