Page 198 - Inorganic Mass Spectrometry - Fundamentals and Applications

I84 Cristy

I

Re
W
Hf
os

L). lh
P,
L Sn
lr

Cd
C
0 P
S

102
0 10 20 40 50 60 TO I
ATOMIC NU~BER (Z)
Relative secondary positive ion yield (MS) vs. atomic number of secondary
ion. 0" primary beam, *, pure element; A, compound. (From Ref. 82.)

The use of reactive gases for sputtering was a great advance for SIMS. Wen
sputtering with an inert gas beam, such as is
Ar+, an exponential fall in the signal
observed as the oxide layer is removed. Figure 4.27 compares the profiles experi-
enced by sputtering aluminum with Ar+ and with Q,+ [83]. With oxygen bom-
bardment, a small drop in the signal is observed between the surface oxide and the
its
The
depth where the implanted oxygen reaches e~uilib~um con~en~ation. high
and stable secondary ion signal obtained at this depth makes reactive gases, and in
particular oxygen, the bombardment gas of choice for most dynamic secondary
ion mass analyses. With static SIMS analysis, the primary ion current is kept low
enough to prevent significant change in the surface composition; thus, noble gas
ions such as Ar+ are still used.
The typical SIMS mass spectrum is dominated by singly charged atomic and
molecular ions. Doubly charged ions are typically two or more orders of magni-