Chemistry Reference
In-Depth Information
Primary ion beam
Target
lon detector
Reflector
FIGURE 7.16
Scheme of ToF-SIMS apparatus.
distinguishedbyahighevaporationenthalpyandastrongabsorptionfor thelaser radi-
ation, for example, for 337 nm of a nitrogen laser. Irradiation with the intensive short
(somens)laserpulseleadstodesorptionofthematrixwiththeanalyteandiscombined
with the ionization of desorbed material. These ions are analyzed in the ToF-MS. The
mass spectrum of the analyte is obtained after subtraction of the pure matrix spec-
trum. In particular, MALDI allows the investigation of molecules with mass larger
than 10
5
u.
Secondary ion mass spectrometry (SIMS) is a method for analyzing the com-
position of solid surfaces and thin films by mass spectrometric investigation of the
secondary ions (positive and negative) emitted from the target after irradiation with
a focused energetic ion beam [59,60] (see Figure 7.16). The chemical element or
molecular composition of the surface can be determined. The ion beam operates
typically in the keV energy range and is focused to less than 1 μm diameter at the
target surface. Various ions such as He
+
,Ne
+
,Ar
+
,Xe
+
,O
−
, and O
2
,aswellSF
5
are used. Cs
+
and Ga
+
ions enhance the emission of negative ions. Cluster ions like
C
60
increase the yield of emitted fragments with higher molecular weight. The time
of flight mass spectrometers are very suitable for SIMS because of their high mass
range and resolution, high transmission and parallel mass detection. SIMS represents
a very sensitive method with a detection limit of 10
12
-10
16
atoms per cm
3
. Static
SIMS operates in a slow sputtering mode with an ion dose lower than 10
13
ions/cm
2
and is used for study of the top two to three monolayers (nearly 10
20 Å). Dynamic
SIMS with continuous sputtering allows the investigation of depth profiles of mate-
rials. The charging of insulating samples by emission of secondary electrons and
the bombardment of the primary positive ions is avoided by pulses of low-energy
electrons between the pulses of primary ions.
The influence of substrate temperature during plasma deposition, the poly-
mer damage by plasma treatment, or the functionalization of polymers in plasmas
were studied by ToF-SIMS investigations, see [61]. The results are interpreted by
comparing the characteristic peak intensities of the relevant secondary ions.
−
