Analytical Chemistry of Surfaces Part Ill. Ion Spectroscopy David M. Hercules and Shirley H. Hercules University of Pittsburgh, Pittsburgh, PA 15260
The previous two parts in this series have dealt with general aspects of surface analysis ( I ) and how electron spectroscopy (ESCA and AES) can be applied to surface characterization (2).Here we treat the remaining two of the four main surface techniques: secondary-ion mass spectrometry (SIMS) and ion-scattering spectroscopy (ISS). As before, we will look a t the fundame&& of the-two techniques and then a t some examples of how they have been applied to surface prob- lems. The fundamental processes important for ISS and SIMS are summarized in Figure 1.A primary ion, usually an inert gas ion, having kinetic energy of 0.3-5 keV is incident on a surface. One of two ~ h e n o m e n awill occur. The nrimarv ion can be elastically scat~eredby a surface atom, resulting in a reflected nrimarv ion. It is this ion which is measured in ISS. ~lthou~h-w wilidiscuss e the details of ISS later, it should he noted that this is an elastic scatterina process. and the kinetic energy of the reflected primary ionwill depend on the mass of the surface atom involved in the scattering process. Another possible phenomenon is that the primary ion can penetrate the surface and become imbedded in the solid. w h e n the primary ion penetrates the lattice, considerable disruption occurs by transfer of momentum to lattice atoms (or molecules). This is the sputtering process discussed in Part I (I).For -1 keV ions, disruption occurs only several atomic layers deep. Stopping such an energetic ion in such a short distance can be visualized as erupting the surface, expelling atomic and molecular fragments. These fragments can he either neutral atoms (or clusters) or ions, as indicated in Figure 1; both positive and negative ions are produced. The ions are referred to as secondary ions, thus the term secondary-ion mass spectrometry. Sputtering from surfaces produces primarily neutrals; secondary ion yields are 1%or less for most materials. However. even for a verv low. ~rimarv-ioncurrent ~ l c m qthe , number i f secondary ions produced density is sufficientlv large for mass spectra with reasonable sianalto-noise ratibs t o b e obtained.. A block diagram of a typical ISS/SIMS instrument is shown in Figure 2. As with ESCA and Auger spectrometers, a SIMS instrument must be capable of operating in high vacuum. In
Surface
IrnYoy
Figure 1. Fundamental processes importam for SlMS and ISS. 592
Journal of Chemical Education
this example, an ion gun is coaxial with a cylindrical mirror analyzer and bombards the sample with the primary ion beam. T o obtain ISS spectra, the backscattered primary ions are sampled by the cylindrical mirror analyzer and their kinetic energies are measured. The secondary ions emitted from the sample are focused through a lens and a prefilter system (a discriminator being used to select ions having a particular kinetic energy range) and passed on to a quadrupole mass spectrometei: The ions exiting from the mass sprrtrumeter are detected by a channeltron detector (or other multiplier), and the sienal is amnlified and nresented bv a read-out svstem. I t is Got necessary to do ISS and SIMS;~the particuiar configuration shown above; other configurations have been used. However, the above represents a reasonably convenient experimental set-up. Secondary-Ion Mass Spectrometry (SIMS) Here we will consider how SIMS is used in the analytical chemistry of surfaces. Figure 3 shows a SIMS spectrum of a surface-contaminated aluminum sample. In the lower tracing one sees mainly the aluminum peak, but when magnified 32X, one detects a number of surface impurities including Li, B, F, Na, Mg, K, Ca, and Cr. The major peak at 20 (next to F) is due to 20Newhich was used as the sputtering gas. On amplifying the spectrum further (256X), one detects the presence of other impurities such as Si, P, S, Fe, Cu, and Sn. Both metal and metal oxide peaks are frequently seen in SIMS spectra; note the presence of A10+, A120+, CrOf, and CuO+ peaks. Although it is possible from Figure 3 to verify the presence of surface impurities i t is not possible to obtain even a semiquantitative estimate of their amounts.
Detector
Quadrupole Mass Spectrometer
Arnpl~fler
Detector
Flgure 2. Block diagram of typical ISSISIMS insnument
and Reod-out
Table 1. Analyllcal Characteristics of SlMS Spectral Range: 0-500 a m Analysis Depth: 40A (dynamic): monolayer (static) Chemical information: mass spectrum hom surface layer Elemental Sensitivity Elements: All Specificily: good (some overlap) Sensitivity Variations: lo5 Quantitative Analysis Absolute: not possible Relative: 1.50% Detection Limit: lo-'% monolayer Matrix Effects: severe Vacuum: 10F torr of ionizing gas Depth ProfilingCapability: yes, rapid, "dynamic" SIMS x-yResalution: 1 p with ion microprobe; X-Y rasterin0 far "Static" SlMS ~ l e i e n t a~l a b i n g as : a microprobe Speed: fast, most spectra take minutes Sample Destruction: yes, spunering of surface
Figure 3. SlMS spectrum of a surfs-maminated aluminum sample (3).(See Fig. 13 for m s p o n d i n g ISS spectrum.)
Table 1presents a summary of the analytical characteristics of SIMS. Most SIMS instruments use a quadrupole mass soectrometer oneratine -un.to 5W1000 m u . Resolution of the spectrometer is usually 1amu, comparable to medium-resolution. oreanic mass snectrometrv. The chemical information obtainabre from SIMS is pote&ially very high because it nresents a mass snectrum characteristic of the surface. However, it must b;realized that although a SIMS spectrum is related to the com~ositionof a surface. i t is not a mass spectrum of that surface. This point will be discussed in detail I~elow. The analysis depth of SIMS varies depending on whether one is o ~ e r a t i n ein the "dvnamic" or "static" mode. In the dynamii mode'the primary ion current density is approximately A/cm2 and scrambling occurs in a layer a t least -40 Athick. Thus, dynamic SIMS has a depth resolution of -40 A a t best. When operating in the static mode (-lo@ A/cm2), each sputtering event comes from a virgin surface. Here one can assume that the SIMS spectrum is characteristic of the top few atomic layers. Of the surface analysis techniques, SIMS is the only techniaue sensitive to all elements. SIMS shows eood snecificitv alt'hough there is some overlap between peiks ofbiffereni elements. For example, the Si2+peak (56) overlaps with Fe+ (56), rendering it difficult to detect small amounts of iron in the presence of silicon. Sensitivity variations in SIMS are extremely high. Because of severe matrix effects, sensitivity variations between the most sensitive and least sensitive elements are ahout lo5. As a rule of thumb, the more electropositive elements show hieher sensitivity in ~ o s i t i v eSIMS. and the more electronegat&e elements show high sensitivity in negative SIMS. ~ e c a u s eof sensitivity variations due to matrix effects, performing absolute analyses with SIMS generally is not possible. Also, because of intensity changes which can occur due to minor changes in the matrix, the relative standard deviations even for calihirated systems are poorer than for the other surface analytical techniques. Thus, one can say that SIMS suffers from severe matrix effects. However, a very important feature of SIMS is its ability to detect very small amounts of materials on the surface. In the more sensitive cases it is quite possible to achieve a discernible SIMS signal (1 n ~ m of ) a monolaver! Therefore. althoueh from SIMS is the leasiquantitative of the surface anal&cal teckniques, it can be the most sensitive.
Other aspects of SIMS which deserve mention are summarized in Table 1.Freauentlv SIMS exneriments are nerformed by backfillmg thespectiometer to approximately h5 torr of the ionizine ....eas to oroduce the ~ r i m a r vions. In so doine it is imperative that exceptionally pure gas be used or the SIMs spectrum will Contajn peaks from surface conmminantn due t o impurities in the ionizing gas. I t is possible to obtain depth profiles with SIMS, and this can be done rapidly in the dynamic mode. Dynamic SIMS in fact is used for bulk analysis of alloys and refractory materials. I t is also Dossible todo x-Y resolution and to achieve approximately 1 resolution with the ion microprobe. Generally, analytical SIMS instruments have a minimum beam diameter of -100 pm, and, although a beam of this diameter can he used for elemental mapping, spatial resolution is poor relative to Auger spectroscopy. However, with the ion microprobe, spatial resolution comparable to Auger spectroscopy can be achieved for mapping of specific mass fragments (note that the ion microprobe operates in the dvnamic mode). In general, SIMS is a relatively rapid technique and a snectrum coverine 0-100 amu can be run in annroximatelv 10 &in. I t must be realized that SIMS disrupts'ihe sample surface due to sputtering. But once again it must be emphasized that under static SIMS conditions i t is possible to obtain a SIMS spectrum that can be considered characteristic of the undisturbed surface. Before lookinn at e x a m ~ l eof s how SIMS is a ~ p l i e dto surface analysis, i t j s important to investigate somi of the parameters discussed above in greater detail. Specifically,we will discuss sputter yields and ion yields, to see how these parameters impose restrictions on both the detection limits of SIMS and its spatial resolution. The primary process of SIMS is penetration of the primary ion into the solid lattice causing expulsion of atoms and ions from the surface. This can be expressed as a current. (i,) observed in the detector of the mass spectrometer where i, is the primary ion current; S, the overall sputtering yield; a A , the ion yield of the sputtered species A; CA the concentration of A; BA the fractional monolayer coverage, and q~ the ion collection efficiency. The primary ion current is related to the beam size and current density by where D, is the current density, and d is the beam diametel Typical values for the parameters in these equations arc shown in Table 2. Equations (1) and (2) define the limitations of SIMS, particularly when operated in the static mode. The rate at which Volume 61
Number 7 July 1984
593
0"
Table 2.
INCIDENT ION CUWKNT WNSITY l r n ~ / c r n ~ l 10-' 10-2 I 0 2 I I 1
Typical Values ot Some SlMS Variables (used In eqns. (1) and (2))
Variable
Symbol
Ionization Efficiencyb Sputlering Yieldb Ion Collection Efficiency Primary Ion Current Density Beam Diameter
Valuesa
0 1 ~
S q~
4
.
d
10-5-10-' 1-10 IS] 10-~-10-~ [lo-3] 10-8-10-2 [lo-'] 10-%lo-'
'Values in brackns are hew used !n c.+latlon. Sreple~emshe total panicles(ions neuvs1s)ofan element spunwed per insidem ion, n, is me number of secondary Ions of an element divided by total panicles lions + neutrals)of that element sputtered. It is also called the Ion yield.
+
Table 3. Absolute Secondary Ion Yields for Clean and Oxygen-Covered Suriaces (5)
S (ion yield) Metal
Clean Surface
OxygenGovered Surtace
S(oxide)l Wean)
.
I 01 0-4 0-2 ATOMIC LAYERS REMOVED PER SECOND Figure 4. Relationship between incident ion current. beam aimmeter, spuner rate. and detection llmits in SIMS (4.
a laver can be removed from a surface is . ~rooortional to the . primary ion current density (at a given energy), whereas the minimum detectable ouantitv of an element is inverselv proportional to the incihent ion current. In static SIMS, i t is ~articularlvimoortant to keeo the rate of soutterine from the . . surface at a minimum, thus placing restrictions on detection limits. ficure 4 shows the relationshivs between incident ion current, SIMS detection limits and
