Biomedical Engineering Reference
In-Depth Information
might be followed as a function of time. If the ion beam is
well controlled and the sputtering rate is constant, the
sodium ion signal intensity measured at any time will be
directly related to its concentration at the erosion depth of
the ion beam into the specimen. A concentration depth
profile (sodium concentration versus depth) can be
constructed over a range from the outermost atoms to
a micron or more into the specimen. However, owing to
the damaging nature of the high-flux ion beam, only atomic
fragments can be detected. Also, as the beam erodes
deeper into the specimen, more artifacts are introduced in
the data by ''knock-in'' and scrambling of atoms.
Static SIMS, in comparison, induces minimal surface
destruction. The ion dose is adjusted so that during the
period of analysis less than 10% of one monolayer of sur-
face atoms is sputtered. Since there are typically 10
13
-
10
15
atoms in 1 cm
2
of surface, a total ion dose of less than
10
13
ions/cm
2
during the analysis period is appropriate.
Under these conditions, extensive degradation and rear-
rangement of the chemistry at the surface does not take
place, and large, relatively intact molecular fragments can
be ejected into the vacuumformeasurement. Examples of
molecular fragments are shown in
Fig. 3.1.4-10
. This
figure also introduces some of the ideas behind SIMS
Fig. 3.1.4-10 Static positive and negative ion SIMS spectra of a poly(ethylene glycol)-poly(dimethyl siloxane) copolymer containing
disulfide side groups on a gold surface. The primary peaks are identified. The low-mass region of the negative ion spectrum offers little
insight into the polymer structure, but the high-mass region is rich in information. In this case, the low-mass positive spectrum is rich in
information. Further details on this class of polymers can be found in Macromolecules 27: 3053 (1994). (Figure supplied by D. Castner.)
