Page 217 - Inorganic Mass Spectrometry - Fundamentals and Applications

202 Cristy

to
onto an aluminized surface from which electrons are liberated and accelerated
The
a scintillating phosphor that emits light. light pulses are piped outside the vac-
uum system to a photomultiplier. Analogue currents from either electron multipli-
X-Y plotters, strip chart
ers or photomultipliers may be amplified and displayed on
of
recorders, or oscilloscopes. For greatest sensitivity, pulse counting the individ-
ual ion-produced cascades is done. In this mode signals ranging from a few ions
per second to over lo7 per second may be detected. To cover the high counting
rates that are not uncomon in SIMS, 200-MHz or faster counters should be used.
At the higher counting rates (>lo5 sec-l), deadtime corrections need to be made
for accurate counting.
In microscope imaging instruments the detectio~recording system requires
the ~plification and collection of the ion signal from all portions of the detector
plane while m~nt~ning the position information. One technique is to focus the
to
secondary ion image onto a microchannel plate that converts the ions electrons;
these electrons are then projected onto a fluorescent screen. The image on the flu-
orescent screen may be viewed, recorded on photographic film, or recorded a
by
sensitive CCD video camera. These images on film may be digitized with an op-
tical scanner. The video camera images may be digitized by using an analog-to-
digital converter. The digitized image frames are sent to a computer for frme image
integratio~, quantification, storage, and display. alternate method is to focus the
An
secondary ion. image on a dual-~crochannel plate electron multiplier that provides
pulse counting and, in conjunction with a resistive anode encoder, position infor-
mation [ 1031. These count-quantified images may be displayed on an oscilloscope
and/or stored and displayed by computer.
The detector for time-of-flight SIMS must be capable of counting pulses very
of
rapidly and accurately recording the time arrival of each pulse. The time func-
tion is usually handled by a time-to-digital converter that must have 4. l-nsec time
resolution, In Schueler’s microscope imaging TOF [55], ions are detected by using
a dual-microchannel plate (DMCP). The arrival time of the ion is extracted from
the second plate of the DMCP. The pulse is amplified and routed to a time-to-dig-
ita1 converter. A resistive anode encoder that determines position information for
the pulse follows the DMCP. Arrival time (mass) and position for each secondary
ion detected are stored in a computerized data system. Integration of data over a
su~cient number of primary pulses results in count-quantified secondary ion im-
ages for every ion mass collected.

The idea in static SIMS (SSIMS) is to analyze only surface areas that have not been
affected by prior ion bombardment. Thus, the SS~S experimenter is limited to