Charge neutralization, SIMS

Figure 5. FAB SIMS Spectra of Polyethylene Showing Thermal Damage with e Beam Charge Neutralization...

Polymer Surface Analysis. The major technique used for the surface analysis of polymers has been X-ray photoelectron spectroscopy (XPS or ESCA). However, this technique is often not adequate to determine the molecular structure of polymers. This has prompted many workers to explore the potential of SIMS for this work (11-16). Significant problems encountered with ion beam bombardment in conjunction with electron beam charge neutralization have been drift in the polymer surface potential and thermal damage from the combined effects of the electron and ion beams. These problems do not exist when utilizing FAB in conjunction with photoelectron charge neutralization. 

Figures 7a, b, and c show the SIMS spectra of known polymers PTFE, FEP, and PVF2. Briggs and Wootton (13) report that radiation from electron beam charge neutralization...

FAB SIMS in conjunction with charge neutralization utilizing mercury discharge lamp induced photoelectrons permits low-damage highly reproducible analysis of electrically insulating surfaces. Stable spectra can be obtained from polymeric materials such as polyethylene for periods of an hour or more. Minor spectrum differences between samples such as the various oxides of cobalt, which may have previously been due to thermal or surface potential effects, can now be more confidently assigned to compositional differences. 

SIMS Analysis. The equipment and instrumental conditions have been described in detail (7.121. Mostly, a model 6300 Perkin Elmer spectrometer was used which was equipped with a 0-255 amu quadrupole mass analyzer. The primary beam of 4.5 keV Xe+ and 60 pA current was rastered in a 2x2 mm2 area. The total ion dose was not more than 3xl012 ions per cm2, which is considered a practical limit for static conditions (5rZ)- Charge neutralization was done by means of 500 eV electrons. Spectral optimization has been described (7.121. 

Some spectra were recorded on a non-commercial SIMS instrument designed and built at EG G in Idaho Falls, ID. It has been described (13.141. Essentially, it is equipped with a primary source of SF6 neutrals and anions and a 0-2000 amu quadrupole mass analyzer. Qualitatively, the spectra obtained were identical to those of the commercial instrument, but the secondary ion yield was higher and charge neutralization was not required. 

Ion Bombardment Conditions. A base pressure of 10 9 Torr is maintained in this chamber the noble gas pressure (He, Xe) rises to about 10"7 Torr during the ion bombardment. The ion beam is rastered on (1.5 x 1.5) mm2 areas at normal incidence 4He-2 keV ions are used for the ISS analysis when Xe-4 keV ions are used for SIMS. The incident ion current is measured with the aid of a moveable Faraday cup. Since the investigated samples are electrical insulators, charge neutralization is performed with low energy electrons ( 10 eV) emitted from a heated W filament. 

Figure 14.40 Effect of charge neutralization on the SIMS spectrum of soda lime glass. The bottom spectrum, taken with the use of a flood gun, shows improved resolution and a flat baseline compared with the top spectrum.

One of the reasons why UHV conditions are required in SIMS is that this controls the re-adsorption of gas phase contaminants onto the surface of interest during analysis. Note Such adsorption issues are accelerated over regions experiencing electron (for charge neutralization) or ion impact, particularly if freshly sputtered (a pure Silicon surface, for example, is highly reactive, hence the reason for the native oxide formation that occurs within seconds on air exposure). A particular instance where this is noted is in the analysis of elements present in the gas phase,... 

In Dynamic Secondary Ion Ma s Spectrometry (SIMS), a focused ion beam is used to sputter material from a specific location on a solid surface in the form of neutral and ionized atoms and molecules. The ions are then accelerated into a mass spectrometer and separated according to their mass-to-charge ratios. Several kinds of mass spectrometers and instrument configurations are used, depending upon the type of materials analyzed and the desired results. 

We will first consider, however, Secondary Ion Mass Spectroscopy (SIMS) in which both neutral and charged species are sputtered from the surface, and detected by means of a mass spectrometer. This involves ion beams of lower energy than in the techniques described previously. 

Figure 2.8. Schematic of a sputtering event in secondary ion mass spectrometry (SIMS). Mainly neutral species are ejected, but also some positively charged and negatively charged ions. For samples containing analyte with relatively low masses, intact molecular ions can be desorbed. The greater portion of ejected compounds is, however, fragments.

Other types of mass spectrometry have also been used to examine ambient particulate samples. One such technique is secondary ion mass spectrometry (SIMS) in which the surface of the sample is bombarded with a beam of ions or neutral atoms that cause ejection of fragments from the surface. The fragments may be neutral atoms or molecules, positively or negatively charged species, electrons, or photons. The... 

This paper focuses on special ionization methods such as secondary ion MS (SIMS) (1, 13, 24-28) and ZCf plasma desorption (PD), and on MS/MS methods for characterizing primary ions, such as surface induced dissociation (SID), laser photodissociation, and neutralization of multiply charged ions. A Hadamard transform method for more efficient recording of multiple MS-II spectra is also proposed. 

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