Surfaces and Contact Mechanics, Part One, Page 6

Sputtering was previously mentioned in conjunction with depth-profiling in Auger electron spectroscopy (AES). The technique may also be used for surface studies directly. Figure 27 shows a typical experimental setup for the technique called secondary ion mass spectroscopy (SIMS). The experiments are carried out under ultrahigh vacuum. The primary ions are accelerated to energies on the order of 10 eV and focused before the ions are energy separated by passing through a magnetic field normal to the ion beam. Ions of desired energy and mass are then directed through beam centering plates to the sample surface. The secondary ions emitted from the sample are then accelerated by an applied voltage toward a quadrupole mass filter. Upon mass separation, the ions are reflected electrostatically into an electron multiplier. The off-axis location of the detector prevents neutral particles (such as photons) from adding to the signal.

Figure 27. The experimental configuration for SIMS is shown. In part (a), a schematic of the whole apparatus is shown. The primary components are an ion source, a magnetic mass separator, the sample on a rotating stage, a quadrupole mass analyzer, and a detector. Part (b) shows the components of the quadrupole mass analyzer.

The quadrupole mass analyzer or spectrometer (QMS) is a very important component in the SIMS system. There are four rods that are biased as pairs with both dc and ac voltages. For mass analysis, both V_dc and V_ac are scanned. The maximum of V_ac is typically 1 kV; and V_dc is usually (1/6) of V_ac for optimum performance. The transmission through the QMS is inversely proportional to the mass and directly proportional to the ion charge. Figure 28 illustrates the dependence of transmission on the ion mass.

Figure 28. The transmission ratio of a quadrupole mass filter as a function of mass number is shown. Some important ions are indicated.

The energy transfer of the primary ion to an atom near the surface occurs via a chain-reaction of two body collisions. This process is more or less destructive of the surface since lattice defects are produced, ions are implanted in the upper layers of the sample, and atoms are removed from the sample. Figure 29 illustrates the complexity of this process.

Figure 29. The sputtering process is illustrated. The primary ion impacts the surface of the sample setting off a chain-reaction of collisions, the formation of defects, ions become implanted, and atoms from the sample are ejected in either the neutral or ionized state.

As an example of the output from the SIMS system, consider Figure 30 that shows the signal intensity versus mass for both positive and negative ion bombardment of polymethylmethacrylate (PMMA). Some of the peaks correspond to fragments of the basic polymer unit. The fragmentation patterns may be used to study the structure of organic molecules.

Figure 30. Positive and negative SIMS data is shown for a sample of polymethylmethacrylate (PMMA). [Figure 15.11 from reference 3].

SIMS data are taken in one of three operating modes: static, dynamic, or scanning. In static mode, the ion bombardment rate is kept small, so that the surface is not rapidly damaged. This mode is useful for studying thin film layers on a bulk material. In dynamic mode, one rapidly bombards the surface and collects spectra for greater and greater depths into the sample. One can also focus on one or more particular species present; and thus, perform depth profiling, an example of which is shown in Figure 31. In scanning mode, the SIMS system is used to raster the beam over the surface and form an image. The output is a map of the surface concentration of a desired species. Successive scans may then be performed to map other species. The typical resolution of such instruments is on the order of one micron.

Figure 31. Positive Cs ion SIMS data for hydrogen implanted silicon. [Figure 15.12 from reference 3].