Page 213 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ion Mass Spectrometry l99

ally bias the sample to accelerate the secondary ions. A Faraday cup designed to
is
trap the beam and any sputtered particles often provided to measure the primary
beam current at the target. Sample current is not an accurate measure of the beam
current, as significant numbers of charged particles are emitted in the sputtering
process and add to or subtract from the primary current (depending on their charge
be
polarity). The beam diameter on the sample in scanning probe instruments can
measured by scanning the beam across the Faraday cup and measuring the distance
required for the beam current to drop from the level with the beam outside the cup
to that inside the cup.
The sample mount also positions the sample properly for analysis. This in-
cludes x and y lateral motion, z height, and angle with respect to primary beam and
secondary extraction. It may vary from a simple mechanical mount to a sophisti-
cated stage with stepping motors under computer control.

SIMS instruments are generally grouped by type of secondary analyzer as well as
by imaging type. Three general types enjoy wide popularity, and each has its dis-
tinct advantages. They are the magnetic sector, the quadrupole, and the time-of-
flight (TOF).
Magnetic sector instruments are popular because of their high mass resolu-
tion and high transmission of the secondary ions. Transmission figures of 10% to
40% at low mass resolution are common. Most SIMS magnetic sector instruments
are designed to be double-focusing, combining the direction focusing the mag-
of
netic sector with the velocity focusing of an electrostatic sector. Useful mass res-
olution in current magnetic sector SIMS ins~ments ranges from about 5000 to
30,000, Examples of magnetic sector instrumentsJ~ include nonimaging (the
SHRIMP I1 in Fig. 4.13), microscope imaging (MS 3OO”Fig. 4.4 and
Fig. 4.9), and scanning probe imaging (IMMA-Fig. 4.5, and NanoSIMS 50-
Fig. 4.14). Secondary ion extraction elements, apertures, and lens elements vary
widely, depending on the requirements.
A novel way of providing electron charge compensation for insulators was
developed for the f-series Cameca instruments, Fig. 4.34 [l lo]. Electrons from a
gun are turned 90” to pass coaxially through the objective lens of the secondary
ion extractor, which works as an electron mirror to provide a self-regulated po-
tential on the sample surf‘ace.
Simultaneous secondary ion detection is limited to the number of detectors
placed in the transmission plane of the magnetic sector. ~uasi-si~ultaneous de-
tection of two or more ions may be achieved by programming electrostatic de-
flection plates to switch ions rapidly within a fairly narrow mass range (see Fig.
4.6). Simultaneous or quasisimultaneous collection of ions is especially helpful for
measurement of isotopic ratios.