2782
Anal. Chem. 1984, 56, 2782-2791
Solids Analysis Using Energetic Ion Bombardment and
Multiphoton Resonance Ionization with Time-of-Flight Detection
Fred M. Kimock, James P. Baxter, David L. Pappas, Paul H. Kobrin, and Nicholas Winograd*
Department of Chemistry, T h e Pennsylvania State University, 152 Davey Laboratory,
University Park, Pennsylvania 16802
Recently multiphoton resonance ioniratlon (MPRI) has been
coupled with energetlc Ion bombardment to yleld a highly
efficient and selective tool for solids analysls. Although this
method promises to yield sub-part-per-billion determlnations
for many elements wlthout chemlcal alteratlon of the matrlx,
there are a number of experlmental factors which may ultlmately llmlt the sensltlvlty of the technlque. Among these
factors are (a) duty cycle, (b) primary Ion current, (c) sputter
yield, (d) useful fraction of ejected particles, and (e) detectlon
efficlency. I n this paper we discuss the origln of these factors
and their Influence on the use of MPRI of sputtered neutrals
as a tool for the elemental analysls of solids.
The use of energetic ion beams as probes for the analysis
of solids is now well established. Detailed measurements of
charged matter ejecting from ion bombarded surfaces as in
secondary ion mass spectrometry (SIMS) have been employed
for a wide variety of analytical applications. Among these
applications are the molecular weight determination of nonvolatile molecules ( I ) , determination of surface structure (2),
and trace analysis of solids (3). As a trace analysis tool, SIMS
has exhibited a unique ability to examine surfaces, thin films,
and interfaces. The SIMS technique has also shown considerable dynamic range ( lo6) ( 4 ) and can achieve sensitivity for
detection of certain elements down to the few parts-per-billion
level in favorable cases (5). However, determination of elemental concentrations in parts-per-billion regime is seldom
realized by SIMS. The detection limit arises from several
sources. First, the ion fraction of ejected particles is often
or less. In these cases, the great maority of all ejected
species are neutral and go undetected by the mass spectrometer. Next, in order to obtain the mass resolution necessary
for SIMS analysis, quadrupole or magnetic sector mass
spectrometers are typically employed which have ion transmissions in the range of 10-1 to
Thus, the useful ion
fraction (number of ions detected/number of particles ejected)
can easily be
or less. Although not directly related to the
detection limit, a final problem with SIMS analysis is that
secondary ion formation is strongly influenced by electronic
effects arising from the sample matrix, making it extremely
difficult to quantify most measurements. Clearly, a method
to efficiently detect neutrals ejected from an ion bombarded
surface would overcome these problems and could be a major
breakthrough for the application of ion beam methods to
chemical analysis.
Several attempts have been made to directly monitor the
flux of sputtered neutrals (in both ground and excited states),
although none has exhibited the sensitivity to operate in the
low dose regime (<1010 incident ions/cm2.s) (6, 7). Experiments involving postionization of the ejected neutrals using
electron bombardment (8), glow discharges (9),and plasmas
(10)have been moderately successful, but none has achieved
ionization efficiencies much greater than 1%.A major difficulty inherent in these methods is that the ionization
probability is a function of particle velocity.
Recently, we have demonstrated the selective ionization of
neutral atoms sputtered from solids by multiphoton resonance
ionization (MPRI) (11,12). The theory behind MPRI is well
documented (13). Basically, three salient features of MPRI
coupled to ion bombardment make it attractive as an analytical tool. (i) All elements but He and Ne can be detected.
(ii) MPRI can be made selective to a single element by a
judicious choice of the excitation wavelength. (iii) Laser
technology has advanced to the point where the MPRI process
can be saturated using pulsed lasers, i.e., with adequate photon
fluxes every atom in the laser beam can be ionized. In this
work, we demonstrate that MPRI of sputtered neutrals can
be employed to overcome the three main difficulties inherent
in SIMS analysis. First, for the majority of cases, MPRI of
sputtered neutrals will sample the major fraction of ejected
particles. With the appropriate experimental geometry, a high
fraction of the sputtered atoms can be made to interact with
the laser beam, resulting in efficient sampling. Second, since
the ionization is selective, a high mass resolution detector is
not needed; thus the photoionized neutrals can be detected
by a low resolution time-of-flight detector with high efficiency.
Finally, examination of the neutral flux should dramatically
reduce matrix effects and improve the prospects for quantitative analysis.
When operated with a low primary ion dose, the MPRI
experiment may be more sensitive than static SIMS by several
orders of magnitude and will be useful for monitoring the
progress of chemical reactions (12) and as a structural probe
(14). Used with higher primary ion doses, MPRI of sputtered
neutrals should find application in the areas of depth profiling
and trace analysis of solids. For example, the detection of 0.5
ppm of Ga sputtered from a Si matrix has recently been
achieved by workers at Atom Sciences, Inc., using MPRI (15).
To date, multiphoton resonance ionization has been applied
to over 40 elements, mostly by investigators using hot filament
evaporation techniques (16, 17).
This paper evaluates the utility of MPRI coupled with
energetic ion sputtering as a tool for the elemental analysis
of solids. Generally, our calculations indicate that subpart-per-billion determinations of many elements will be
possible, The sensitivity of the technique is presently limited
chiefly by the laser duty cycle and ion source brightness. Using
model systems of elemental targets, we illustrate the influence
of the sample matrix, experimental operating conditions, and
atomic photoionization spectroscopy on the sensitivity of
MPRI of sputtered neutrals.
EXPERIMENTAL SECTION
All experiments are conducted in an ion pumped Perkin-Elmer
Ultek TNB-X ultrahigh vacuum chamber with a base pressure
torr after bakeout. A primary beam of Ar" ions is
of
generated by a Danfysik 911A hollow cathode source. The primary
beam kinetic energy can be varied from several hundred eV t o
-30 keV. After extraction from the source, the ion beam is mass
selected by a 90° magnetic sector, and passed through two stages
of differential pumping si' ultrahigh vacuum conditions can be
maintained in the analysis chamber. The beam is pulsed at 30
Hz (defined by the laser) by deflecting it through an aperture with
a 200-V pulse applied to a set of parallel plates located within
0003-2700/84/0356-2782$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
ELECTRON
MULT IP t I ER
2783
p m
1
J
Generators
EXTRACTION
ELECTRODE
X
I
Trigger
Trigger
B O X C A Q #2
G&TE
TIWL-CF-FLIGHT
SPECTRUM
n n
A U’L
Secondary
Y
TIME
LASER
Flgure 1. Experimental configuration of the MPRI experiment on
sputtered neutrals. Atoms are sputtered from the sample by a pulse
of primary Ar+ ions, and then postionized by the laser. All ions are
extracted and detected by an electron multiplier. Secondary ions can
be distinguished from laser-produced ions on the basis of their timeof-flight to the detector.
the beam line. Typically, a primary ion beam current of 2 pA
(at 5 keV beam energy) can be delivered to the sample at an
incident angle of 45O into a spot size of less than 1mm in diameter.
Upon impact, the pulse of primary ions produces a distribution
of atoms localized in space a few millimeters above the target.
A few hundred nanoseconds after the ion pulse has ended, the
volume of sputtered neutrals is irradiated with a laser pulse of
the appropriate wavelength(s) to cause MPRI of the desired
atomic species. Ions generated in the experiment are then extracted and detected with a copper-beryllium electron multiplier,
an arrangement which serves as a low resolution time-of-flight
analyzer. The experimental geometry is illustrated in Figure 1.
It is useful to be able to distinguish between the ions produced
directly at the surface (SIMS) and those produced by interaction
with the photon field. Since the secondary ions and MPRI ions
are born in distinctly different regions of the same extraction field,
they will have different velocities at the time they reach the
detector. This is to say that the SIMS and MPRI ions can be
discriminated with an electrostatic energy filter (11), or more
simply on the basis of their time-of-flight to the detector. As we
will show, for maximum analytical sensitivity, it is necessary that
the laser be fired <500 ns after the primary ion pulse has ended.
Thus, in order to ensure both maximum MPRI yield and minimum overlap between the laser produced ions and any “tail” of
secondary ions at the detector, it is critical that the fall time of
the primary ion pulse be on the order of 100 ns or less. For
primary ion pulses having longer fall times, the laser firing time
will have to be further delayed and sensitivity will be sacrificed.
The laser system has been previously described (12). Briefly,
the Quanta-Ray DCR-2A Nd:YAG/dye laser system is capable
of generating tunable laser light from 260 nm to -800 nm. The
laser is pulsed (at 30 Hz, with -66-11s pulses) and typically -5
mJ/pulse of ultraviolet light and/or -15 mJ/pulse of visible light
can be obtained. The laser beam which is shaped by filled-in beam
Ions
MPRI
Ions
Flgure 2. Schematic of pulse timing and data collection in the experiment. The timing sequence is initiated by the internal flashlamp
trigger of a Nd:YAG laser. All other pulses are triggered externally by
time delay generators. Boxcar integrators are used to monitor the
primary ion current and MPRI signal after a time-of-flight. Typical
values for delay times are D1 = 200 ps, D2 = 5 ps, D3 = 3 ps, and
D4 = 4 ps. Tis the primary ion flight time from the pulsing aperture
to the sample, 7 is the primary ion pulse width, and t is the laser firing
time. The laser pulse width is - 6 ns. The time axis is given for
illustration only; no absolute scale is Intended.
optics in the YAG is -6 mm in diameter in the unfocused condition and exhibits a near-Gaussian profile.
The instrument is interfaced to a Digital Equipment Corp. LSI
11/23 microcomputer which is equipped with a 40 megabyte
Winchester hard disk and 256 kilobytes of random access memory.
Computer control of the experiment is afforded through a BiRa
Systems Model 5000 Powered CAMAC crate. The crate contiiins
a card rack with 25 positions for plugging in CAMAC modules.
Three positions are occupied by a BRQ bus controller for communication between the CAMAC modules and the computer.
The laser power, primary ion beam current, and MPRI signal
can be continuously monitored by applying dc voltage signals to
a Kinetic Systems 3560 Quad voltage to frequency converter (V-F).
The output frequency of the V-F is integrated by a BiRa Model
2101 scaler/timer. The scaler output is then relayed to the
computer for disk storage and observation on a terminal screen
or immediate plotting. In the case of the laser power, the V-F
receives an amplified voltage from a Scientech Model 36-2002
power and energy indicator. The output from a picoammeter can
be sent to the V-F to monitor primary ion current in the dc mode.
When the ion beam is pulsed, the pulsed output of the picoammeter can be measured with a boxcar integrator, whose output
voltage is then sent to the V-F. For the MPRI signal, a boxcar
integrator is employed as a signal averager to measure the
preamplified signal of an electron multiplier operated in the analog
mode. The CAMAC crate is also equipped with two LeCroy
Model 2323 programmable dual gate and delay generators which
are responsible for triggering the ion beam pulse, the laser, and
the boxcars. In addition to the above, the CAMAC crate has been
equipped with a Standard Engineering Corp. SMC-406/H stepping motor controller which can be used for preprogrammed
scanning of the dye laser monochromator. The pulsing sequence
of our experiment is illustrated in Figure 2.
Exciton DCM laser dye in methanol solution was used to
generate all wavelengths for the photoionization experiments on
In, AI, Co, and Mo atoms. Gallium photoionization was accomplished by using the output of Exciton Rhodamine B laser dye
in methanol solution.
The In foil was cleaned before all experiments by Ar+ ion
sputtering under vacuum. Sputter cleaning was terminated when
2784
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
the In,/In ratio as determined by MPRI reached a maximum
value (12). Oxidation experiments employed Matheson reagent
grade oxygen of 99.9995% purity. The AI,Co, and Mo foil samples
were cleaned by acid etching in a hot solution of 50% glacial
CH,COOH, 30% HNO,, 10% H2S04,and 10% H3P04,followed
by Ar+ ion sputtering under vacuum. All of the foil samples had
a purity of 99.9995%. Gallium atoms were sputtered from single
crystal GaAs which was cleaned under vacuum by alternatelycycle
heating to 500 "C and Ar+ ion sputtering.
RESULTS AND DISCUSSION
Three features of MPRI of sputtered neutrals reveal the
potential sensitivity of this experiment. (i) Ion beam sputtering is a very efficient atomization source. For example, a
1mA primary beam of Ar+ ions could conceivably liberate 10l6
atoms/s from a solid. Also, by focusing the ion beam, atomization can be made to arise from a microscopic region within
the solid. (ii) With adequate photon fluxes, nearly every atom
which is resonantly excited can be ionized. Also, the ionization
probability will be independent of particle velocity for the
majority of sputtered partiels. (iii) Either ions or electrons
generated by MPRI can be detected with high efficiency. The
lower limit to such a measurement, the detection of single
atoms, has been demonstrated by using MPRI (13). Indeed,
there are many limitations to the detection of one atom in 1OI6
as alluded to in the above hypothetical experiment. For MPRI
of sputtered neutrals, the measured signal intensity, I , can
be expressed as a product of experimental parameters
I = DPSCUE
(1)
where D is the duty cycle, P is the primary ion current, S is
the sputter yield of secondary particles, C is the concentration
of the element to be determined in the solid, U is the fraction
of ejecting analyte atoms which are in the electronic state that
is being probed by MPRI, and E corresponds to some measure
of detection efficiency. In eq 1, E is influenced by the ability
to inject atoms into the photon field, ionize these atoms, and
subsequently detect them. In the remainder of this paper,
we discuss the origin of these factors and their influence on
the use of MPRI of sputtered neutrals as a probe for the
elemental analysis of solids.
Duty Cycle. One of the chief shortcomings of the use of
MPRI is the need to employ pulsed lasers in order to approach
saturation of the ionization process. As a result, in our configuration the primary ion beam is fired at the pulse repetition
frequency (prf) of the laser (30 Hz) in order to ensure efficient
sampling of the sputtered particles by the photon field.
Calculations and experiments (discussed below) indicate that
the maximum MPRI signal can be achieved using a primary
ion beam pulse width of 110 ps, making the sampling duty
cycle of the experiment 3 x
at best. This problem could
be significantly improved by increasing the prf on the laser.
In fact, the use of continuous wave (CW) lasers for MPRI has
been shown to result in total ion yields (counts/s) which are
comparable to those obtained in pulsed experiments (18).
However, in CW experiments the photon flux is considerably
reduced, so the high degree of ionization efficiency and hence
sampling efficiency is lost. Thus, further evolution of laser
technology will be necessary before the compromise between
duty cycle and ionization efficiency can be overcome.
Although the use of lasers with short pulses (ns) can be
viewed as a disadvantage in terms of duty cycle, it is a direct
consequence of these short photon pulses that the postionization probability for the sputtered atoms is independent of
particle velocity. For example, a 100-eV In atom will move
only 0.007 cm during a 6-ns laser pulse. A lighter atom, for
example, 100 eV B, will travel 0.03 cm during the laser pulse.
Since the vast majority of sputtered atoms have kinetic energies less than 30 eV, most particles are virtually frozen inside
the photon field (mm in diameter) for the duration of the laser
pulse.
Primary Ion Beam Current. In sputtering experiments,
the primary ion beam current is the major factor governing
the number of particles which are atomized. For MPRI, the
ionization signal is proportional to the quantity of atoms in
the ionization volume. Thus, to overcome the duty cycle
problem in the MPRI experiment on sputtered neutrals, the
number of incident primary ions per pulse must be relatively
high. For example, in the case of low dose experiments, a duty
cycle of 3 X
and a primary ion beam current density of
lom6A/cm2 still provides static bombardment conditions.
With similar primary ion fluxes, the yield of neutrals ejected
from both a clean and chemically reacted surface has been
measured with good sensitivity (12).
If MPRI is to exceed the capabilities of SIMS for trace
analysis, primary ion currents should be in the range of 0.01
to 1 mA or greater. Dynamic SIMS studies normally employ
about 10 pA of primary beam current. Note that when a
source emitting 33 mA of ion current is pulsed at 30 Hz with
10-ps pulses, the average ion beam current is 10 pA. Although
ion sources which emit such high currents are not standard
equipment on most surface analysis systems, they are available. With duoplasmatron type sources, up to 10 mA of Ar+
ion current can be delivered to the target at ion beam energies
of -25 keV, into a spot size of several millimeters.
To maximize the spatial overlap between the ejected neutrals and the photon field, it is important to operate the MPRI
experiment with an ion beam spot which is small relative to
the diameter of the laser beam. Unfortunately, it is difficult
to transfer high ion currents (mA) to the sample in spot sizes
much smaller than 1 mm or a t low kiloelectronvolt beam
energies due to space-charge factors. The space-charge effect
has two important consequences for the MPRI experiment.
First, in situations where multiphoton ionization must be
accomplished using a focused laser, there will be a maximum
useful ion beam spot size that will cause the sputtered atoms
to be injected into the photon field. Due to space-charge
limitations, the current which can be delivered into this spot
is not arbitrarily large. Secondly, considering microprobe
analysis via MPRI, primary ion beam currents are already
space-charge limited and cannot be increased to offset the duty
cycle.
Sputter Yield and Analyte Concentration. Knowledge
of the influence of experimental parameters upon the sputter
yield is important when choosing the optimum analysis conditions for the MPRI experiment. Several trends regarding
the sputter yield for a solid under energetic ion bombardment
must be considered. First, for most metals, an examination
of the sputter yield as a function of primary ion energy shows
a monotonic increase up to about 5 keV, where the yield either
levels off or begins to decrease (19). Secondly, the sputter
yield increases as ion bombardment is performed in a nonperpendicular direction and is usually maximized by bombarding the target a t angles between 45' and 60' from the
surface normal. Last, for monoenergetic noble gas ion bombardment, primary ion particle mass has only a minor influence on the sputter yield. For example, changing the primary
ion from Ar+ to Xe+ generally results in approximately a 2-fold
increase in the sputter yield (19). However, for a given ion
beam energy and beam diameter, the maximum obtainable
ion current density varies inversely with the ion mass. Noble
gas ions are useful for our experiment since (i) they allow clean
operation of the ion source and (ii) their interaction with the
solid is thought to have little influence on surface electronic
properties. The latter is in sharp contrast to many dynamic
SIMS experiments in which bombardment is performed with
an 02+
beam to enhance positive secondary ion yields, or a
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Cs+ beam to enhance negative secondary ion yields. For the
above reasons, many of our initial investigations have employed a primary beam of 5-keV Ar+ ions, incident on the
sample a t 45’.
The sputter yield is also not free from matrix effects. It
is often observed that as a multicomponent surface is exposed
to a flux of energetic ions, initially the species with the highest
effective sputtering yields will be preferentially desorbed,
resulting in the near-surface region becoming enriched in
species with low effective sputtering yields (20). For our
purposes, we employ Coburn’s definition of “preferential”
which is referenced to the bulk composition of the solid (21).
Once steady-state conditions have been reached, the surface
concentrations of the residing elements are proportional to
their bulk concentrations and inversely proportional to their
effective sputter yields. At that point, the concentrations of
atoms which are sputtered from the surface are equal to the
atomic concentrations in the bulk. Thus, preferential sputtering (according to the above definition) occurs only during
the time required to establish the nonstoichiometric surface
layer, and there is no preferential sputtering under steady-state
conditions. This means that for MPRI of sputtered neutrals,
ion intensities will be directly proportional to the bulk concentrations of the corresponding elements, and no knowledge
of effective sputtering yields is required for quantitative
analysis. In light of the formation of a nonstioichiometric
surface region, the combination of MPRI and ion bombardment has an advantage in terms of its ability to make quantitative measurements (e.g., in sputter profiling) over techniques like XPS and Auger electron spectroscopy, which
directly examine the surface layer, rather than the sputtered
particles.
For bulk analysis of major constituents, the need to reach
steady-state concentrations at the sputtered surface is a minor
problem. For binary and ternary systems, this condition is
often achieved by sputtering for several minutes with removal
rates of tens or hundreds of angstroms per minute. However,
it is not yet clear how difficult it will be to establish the
enriched phase for the analysis of trace constituents, or even
if the existence of such a phase will be necessary at all. Also,
the accuracy and resolution of depth profiling analysis is, in
part, determined by the thickness of the layer to be examined
relative to the amount of sample which must be removed in
order to achieve steady-state sputtering conditions. The need
to form an “equilibrium” nonstoichiometric phase also increases the difficulty of accurately profiling constituents which
have a steep concentration gradient in the solid.
Useful Fraction of Ejected Atoms. For the situation of
a solid having a sputter yield of 5 atoms per incident ion, a
10-pA ion beam pulsed at 30 Hz with pulses of 10 ps duration
would generate a reservoir of about lo1’ atoms/s. Under ideal
detection conditions an impurity at a concentration of 1ppb
(atomic) would produce 100 counts/s. In reality, however,
only atoms which are in the electronic state that is being
probed by MPRI are analytically useful.
Generally, for an element M in the atomized volume, one
can express the concentration of M as
c(M) = c(M0) + c(M*) + CC(M,*) + n C c ( M n )
i
n>l
nCCc(M,X,)
n m
+
+ ... (2)
where Mo represents ground state atoms, M* represents
monomeric secondary ions, M,* represents atoms in long-lived
excited states, and M, and M,X, are examples of molecular
species which can exist in ground or excited states, or as ions.
If the element to be determined is present in trace quantities,
n = 1. For the systems we have studied so far, the majority
of ejected particles have been monomers. For example, from
a clean indium surface bombarded by 5-keV Ar’ ions, -80%
1
-
I
I
-2 10c
a
t r . I
v
I
t
x x x x
I
4
I
x x x x x x
I
I
x x x x x
Co MPRI
X
X
X
X
WAVELENGTH
2785
13
-
->
E
(4)
Flgure 3. Ion intensity vs. excitation wavelength for one-color, single
resonance MPRI of sputtered Co atoms. The five peaks labeled G are
ground state orlglnating. Experimental conditions were as follows:
5-keV Ar+ (2 pA, 5-ps pulses, 30 Hz), -25 mJ/(cm*.puise) of laser
light. The laser doubled dye output curve is also presented (X).
-
of the ejected In atoms are in the ground state, 10% of all
In atoms reside in the first excited state, <lo% of the
sputtered In atoms form dimers, and <1% of all secondary
particles are In+ (12). This observation is consistent with
previous measurements on neutrals sputtered from clean metal
targets which indicated that 5 to 10% of the neutral flux may
be composed of dimers and small cluster species (IO).
Oechsner et al. (22) have reported that for some oxides, the
major sputtered species are of the form MO, so in certain
circumstances molecule formation may be significant. Since
MPRI couples ionization to a resonant bound-bound transition, it is both atom- and state-selective. Therefore, generally
only one of the species in eq 2 can be detected at a time. The
result of this selectivity is that for MPRI, the useful fraction,
U , for most experiments is given by
U = c(Mo)/c(M)
(3)
or if an excited state is being probed
U = c(M,*)/c(M)
(4)
The excited-state populations which can be detected by
MPRI are those with relatively long lifetimes, i.e., metastables.
During sputtering, fluorescent decay from atoms in high-level
excited states can often be observed in the region of space just
above the solid. The lifetimes of these states are typically very
short (nanoseconds or subnanosecond regime), and consequently atoms in these states will not undergo efficient
multiphoton ionization. However, after cascading, most atoms
should exist in the ground state or in one of several low-lying
metastable states which can be probed by MPRI. In our
configuration, since the laser pulse is fired several hundred
nanoseconds after the primary ion pulse has ended (producing
a temporal separation of the secondary ions and the laserproduced ions at the detector) nearly all atoms in high-lying
excited states should have adequate time to decay before they
are exposed to the photon field.
The existence of metastable populations of sputtered atoms
is illustrated in Figure 3 for Co atoms ejected from Co metal.
For the wavelength region shown in the figure, we have
identified 5 transitions which originate from the ground state
and 18 which originate from five low-lying excited states.
Although the largest peaks in the spectrum arise from MPRI
of ground state atoms, the signals resulting from ionization
of excited state atoms are significant. For this spectrum, the
laser was unfocused and none of the ion signals exhibited
saturation. No detailed analysis of the electronic partitioning
of the Co excited states was attempted since the ionization
efficiency for each individual transition was not determined.
Indium presents itself as a convenient model system to
2786
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
"''-7
g 1.0
>
*.
k t
O
1 n+
1
Ar+ BEAM ENERGY ( k e V )
effect of prlmary Ion beam energy on the relative
population of metastable ('P3/2) I n atoms to ground state (2P1,2)In
atoms. The plot Is normalized to the maximum value. Error bars
present the relative standard deviation of the data.
Figure 5. The
0
1000
2000
3000
OXYGEN EXPOSURE (L)
Fraction of the total monomer yield for (0)ground state
(2P112)In atoms, ( 0 )metastable (2P3,2) In atoms, and (A)In+ secFigure 4.
ondary ions vs. oxygen exposure. Approximately one monolayer of
In203 is formed after 800-L exposure. Error bars present the relative
standard deviation of the data. 1 L = 10" torvs. Data were collected
with a quadrupole mass spectrometer as described in ref 12.
study the population of metastable states. The electronic
structure is simple, with only one low-lying excited state (2P312,
0.27 eV). Transitions between the 2P3/zstate and the ground
state (2Pl/2)are forbidden, thus the 2P3/z state must be
long-lived. The second lowest excited state lies greater than
halfway between the ground state and the ionization continuum, and atoms excited into this state (or any higher states)
could be nonresonantly ionized by a single ultraviolet photon.
Since we have never detected a measurable signal which could
be attributed to nonresonant photoionization of In atoms, we
believe that for sputtered In, the 2P,/2and 2P3/2states are the
only long-lived states that are significantly populated.
The photoionization schemes used to study In have been
previously published (12). It is worthwhile to note that we
have observed saturation of the ion yields for two-color MPRI
of both the 2P1/2
and 'P3/2 states with a laser beam diameter
of -0.6 cm. An ionization efficiency of 100% is consistent
with our measurements at saturation powers, and a realistic
measure of both the 2P1/2and 'P3/2 populations is achieved.
In the following section, we present results of investigations
on polycrystalline In to begin evaluating the influence of the
sample matrix and primary beam energy on the useful yield
of sputtered atoms.
The yield of secondary ions is known to exhibit a strong
dependence on the sample matrix. For example, the presence
of surface oxygen is often observed to enhance positive ion
yields by several orders of magnitude over the clean surface.
It has been conjectured that for most systems, the secondary
neutrals would be less subject to these matrix effects, and thus
measurements on the neutrals would provide a more quantitative measurement of surface concentrations.
In order to test this theory and evaluate the influence of
the sample matrix on the useful yield, we have chosen to
examine the effect of oxygen chemisorption on the sputter
yield of In+ ions and both ground and excited state In atoms.
In Figure 4, the population of each In monomer species,
plotted as a fraction of the total monomer yield, i.e., 1(2P1/2
In) 1(2P3/2
In) I(In+),vs. oxygen exposure is presented.
Note that from the clean surface nearly all ejected atoms are
found to reside in the ground electronic state. In fact, from
the clean surface, the yield of 2Plj2In atoms is >lo00 times
that of In+ secondary ions (12). From the oxygen saturated
surface, however, nearly 50% of all sputtered atoms are observed as excited neutrals or as ions. The formation of ap-
+
+
proximately one monolayer of InzO3 enhances the yield of
secondary In+ by a factor of >200 over the clean surface.
However, the yield of ground state In atoms decreases to -0.4
of its original value as monolayer coverage is approached (12).
Thus, the neutral yield seems to be a much more direct reflection of surface composition than does the secondary ion
yield. Although the yield of neutrals is not completely free
from matrix effects, these effects are much less serious than
those observed in SIMS. There is a noteworthy ramification
of this experiment for the use of MPRI as a tool for solids
analysis. No chemical alteration of the sample (e.g., dosing
the surface with oxygen or cesium) is necessary to enhance
the sensitivity of the MPRI experiment. The integrity of the
chemical environment of the solid can be maintained during
the analysis.
Previously, we have discussed the influence of primary ion
beam energy on both the sputter yield and attainable primary
ion current density. The primary ion beam energy can also
influence the useful yield in the MFRI experiment by affecting
the relative population of excited state atoms. We have examined the magnitude of this effect for atoms sputtered from
a clean In surface. The relative population of metastable In
atoms (plotted as the 'P3/2:2P1/2 ratio) as a function of primary
Ar+ ion beam.energy from 2 to 12 keV is presented in Figure
5. Accurate measurements could not be made below 2 keV
as a result of a significant loss of primary ion current due to
space-charge blow-up in the beam line and magnified effects
of stray electrical fields from the detector which steered the
ion beam away from the sample. The plot reveals a dramatic
increase in the 'P3/2 population up to 4-keV beam energy,
followed by a plateau region. The absolute 2P3/z:2P,/2ratio
for In sputtered by 5-keV Ar+ has previously been measured
to be -0.1 (12). The shape of the curve reveals a possible
trade-off for optimizing analysis conditions in the MPRI experiment. At least for this case, the relative population of
excited state atoms is minimized by decreasing the primary
ion energy. This increases the useful yield in the experiment
by maximizing the fraction of atoms which are in the ground
electronic state. However, at low kiloelectronvolt energies,
reducing the primary ion beam energy will necessarily decrease
both the sputter yield and primary ion current density, thereby
reducing the total number of particles which can enter the
photon field. Because the number of atoms in the photon field
is the overriding concern, most analyses via MPRI will be
conducted using 5- to 20-keV ion beams.
It is interesting to compare the excited state populations
we have measured to those observed by other workers. For
example, Wright et al. (23) and Pellin et al. (24) examined
the populations of metastable states of sputtered uranium and
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
iron atoms, respectively, using laser induced fluorescence. In
their investigations of uranium bombarded by 500 to 3000-eV
Art ions (23), the authors found the populations of ejected
metastable U atoms to obey a near-Boltzmann distribution
which was characterized by a temperature of 920 f 100 K.
Similarly, the populations of excited state Fe atoms (sputtered
from Fe foil by 3-keV Art) were characterized by a temperature of -700 K (24). We point out that this approach is
merely an attempt to describe a population distribution long
after the bombardment event has occurred. As such, these
temperatures should not be confused with the very high effective sputtering temperatures ( lo4 K) which have been
assigned to occur at the impact event, resulting in surface
ionization (25). Using our data for clean polycrystalline In
bombarded by 5 keV Ar+, we find the sputtered In atoms to
be characterized by a Boltzmann temperature of 1180 K.
Similarly, using data from Figure 5, we calculate a Boltzmann
temperature of -725 K for In atoms sputtered by 2-keV Ar'.
These temperatures are in reasonable agreement with those
calculated for sputtered U (23)and Fe (24). It is worthwhile
to note that the temperatures we have calculated are on the
order of the actual temperatures employed in hot filament
evaporation techniques. In the hot filament experiments up
to 40% of the evaporated atoms have been observed in
metastable states (26). Clearly, the mechanism that is responsible for populating metastable excited levels in hot filament evaporation is quite different from that in sputtering.
However, in terms of influencing analytical capabilities, only
the relative yield (not the mode of population) of long-lived
excited states is significant, and it appears that at the very
worst, sputtering is an atomization source that is a t least as
cool as filament evaporation. Although the population of
metastable excited state atoms will decrease the useful yield
in many cases, it opens the possibility for multielement
analysis with a single laser dye.
Spatial Overlap between the Sputtered Atoms and the
Photon Field. Thus far, we have discussed the major factors
which influence the number of neutrals which are desorbed
from an ion bombarded surface. The remainder of this paper
focuses on the detection of these particles. Detection efficiency
is affected by the ability to (i) achieve overlap between the
spatial distribution of sputtered atoms and the photon field,
(ii) ionize atoms which interact with the photon field, and (iii)
extract and identify particles once they have been ionized.
In order to determine the number of sputtered atoms which
are detectable, it is necessary to understand the energy and
angular distributions of secondary particles. It is chiefly the
energy distribution which establishes the optimum timing
relationship between the primary ion beam pulse and the laser
pulse. As depicted in Figure 2, the experimental timing sequence is characterized by a primary ion pulse of width 7 ( ~ s ) ,
and a 6 4 s laser pulse fired at a delay time t - 7. The probability, p, of an ejected particle being at some point in space
above the surface is a function of its distance r from the
ejection point, the time t after the particle has left the surface,
and the azimuthal and polar angles of ejection, 4 and 8, respectively. (See Figure 1.) It has been shown for single crystal
surfaces that there are preferential azimuthal angles of ejection
for secondary particles (2,27). However, for polycrystalline
surfaces, p is not a strong function of 4. With the additional
approximation that the angular and velocity distributions are
independent, we have
-
-
P = f(r,t)-f(o)
(5)
Generally, the kinetic energy distribution of secondary
neutrals can be estimated (28) as
N(E) =
CE
( E -!-
ION PULSE WIDTH,
T
2787
( p ~ )
Flgure 6. (A)Calculated fraction of all sputtered I n atoms which are
in the laser beam, (0)calculated relative yield of sputtered In atoms
in the laser beam, and (0)measured MPRI signal vs. primary ion pulse
width. For each calculation and the experiment, f is varied for each
T
to glve the maximum intensity.
where N ( E ) is the yield of particles having kinetic energy E,
c is a constant, and E b represents the surface binding energy
of the ejected species. For a system in which all secondary
particles have the same mass (e.g., In atoms) and eject from
a point source, a t any time t the energy distribution can be
expressed as a radial distribution, N(r). From this radial
distribution, one can calculate the fraction of all particles
which will exist between rl and r2 at any fixed time, t. This
fraction corresponds to the probability p(rl,r2,t)that a particle
will lie between rl and r2 and can be expressed as
where
In order to include primary ion pulses of finite width, eq
7 must be convoluted over the width of the ion pulse, resulting
in
after normalization. Since the actual width of the laser pulse
is small (-6 ns) relative to the width of the primary ion pulse
(microsecond regime), it can be ignored in this calculation.
Expansion of eq 9 from one to three dimensions is accomplished by incorporating a cylindrical laser beam and calculating 1000 azimuthal trajectories for each of nine polar angles.
This results in an average entrance and exist distance (rent,
rex)through the photon field for particles ejecting at each polar
angle as well as the percentage of all trajectories which intersect the laser. (Note from Figure 1 that particles which
desorb with large polar angles, 8, may not intersect the laser
beam.) For the final step, a near-cosine polar angle distribution (shifted off-normal by loo in the specular direction)
for the sputtered atoms is assumed. The product of the
fraction of atoms, p , between rentand rex,the percentage of
all trajectories intersecting the photon field, and the polar
angle distribution function yields the fraction of all sputtered
atoms which should lie in the laser beam that is fired at t T after the primary ion pulse has ended.
The results of calculations we have performed for sputtered
In atoms are presented in Figures 6-9. The laser beam is
positioned parallel to the surface directly above the ejection
point such that the x axis (Figure 1)intersects the center of
the beam. Note that in the three-dimensional calculation the
particle mass, m, is implicit in vb. For all f i i e s except Figure
7 (in which t is varied) the laser firing time, t , is optimized
so that the maximum number of atoms lie in the ionization
volume. For these calculations, we have employed a source
2788
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
(a) x ,
4
z
LASER FIRING TIME, t (ps)
15
:0.1
cm
0.05 c m beams
ION PULSE WIDTH, r ( p s )
figure 7. The effect of laser firing time, t , on the total intensity of
sputtered I n atoms localized in (a) a 1.9 cm diameter laser beam, and
(b) a 0.6 cm diameter laser beam, each positioned with Its lowermost
edge 0.1 cm from the sample. For both cases, T = 6 1s.
ION PULSE WIDTH, T (ps)
Flgure 8. Calculated number of In atoms in the photon fieid vs. primary
ion pulse width for five laser beams positioned directly above the atom
ejection point and intersecting the x axis between the followlng x
coordinates: (a) x 1 = 0.1 cm, x z = 2.0 cm; (b) x , = 0.1 cm, x : , =
0.7 cm; (c) x , = 0.2 cm, x:, = 0.8 cm; (d) x , = 0.5 cm, x:, = 1.1
cm; (e)x 1 = 0.1 cm, x:, = 0.15 cm.
emitting 10 pA of primary ion current (which is pulsed in
microsecond intervals), a sputter yield of five In atoms per
incident ion, and a surface binding energy E b = 5 eV. All
atoms are assumed to eject from a point source.
The calculated fraction of all sputtered atoms which lie in
the ionization volume, as well as the total number of atoms
in that volume are plotted in Figure 6 as a function of primary
ion pulse width. The measured ion intensity for two-color
MPRI of ground state In atoms vs. primary ion pulse width
is also illustrated. Both the calculation and experiment were
performed using a laser beam of 0.6 cm diameter positioned
with its center 0.4 cm above the bombarded surface. Two
regimes of the plot are of special interest. First, for a primary
ion pulse width T = 200 ns, -70% of the ejected atoms should
lie in the photon field; however, the total number of atoms
in this region is quite low. For T = 5 ps, the fraction of all
sputtered atoms which reside in the laser beam has decreased
to about 25%, but the absolute number of particles in the
beam has almost reached a maximum value. For primary ion
pulse widths >5 p s , the total intensity of atoms localized in
the photon field reaches a steady state. This is the rationale
for our comment that for maximum analytical sensitivity,
primary ion pulse widths need to be no greater than 10 ps.
For operation at 30 Hz, this ion pulse width fixes the maximum sampling duty cycle at about 3 X lo4.
For several reasons, it is difficult to make a rigorous comparison between the experimental and calculated results
presented in Figure 6. First, for the calculation, E b must be
assumed, and the energy distribution is assumed to fall off
-
Flgwe 9. Calculated number of I n atoms in the photon field vs. primary
ion pulse width for three 0.05 cm diameter laser beams positioned with
the lowermost edge of the beam (a) 0.1 cm, (b) 0.5 cm, and (c) 1.O
cm directly above the atom ejection point. Curve (a) is identical with
Figure 8e.
as E'. For example, increasing the value Of E b to 10 eV results
in a curve which more closely resembles the experiment. By
the same token, using values slightly greater or less than 3
in eq 6 results in minor changes in the curve shape. From
the point of view of the experiment, it is difficult to know the
position of the laser to within h0.5 mm. During the analysis,
the laser beam is moved toward the sample until it begins to
ablate the surface, at which point it is backed away. Finally,
the laser beam has a Gaussian profile, which results in different degrees of ionization for different locations within the
beam. The beam profile is not included in the model. Due
to these uncertainties, one can conclude only that reasonable
qualitative agreement between the experiment and theory is
achieved.
To achieve maximum sensitivity in the MPRI experiment,
it is important to know when to fire the laser pulse relative
to the primary ion pulse. This question can be answered by
choosing a laser beam size, a value for r, and then solving for
the number of atoms in the laser beam as a function oft. The
calculated result for two different diameter laser beams (0.6
cm and 1.9 cm) positioned with their lowermost edges 0.1 cm
above the sample and fired both during and after a 6 - p s ion
pulse is presented in Figure 7. For each curve the maximum
intensity occurs for t - T of about 100 ns. Note the formation
of a steady-state population of atoms in the photon field that
occurs for the 0.6 cm diameter laser beam. Also, notice that
the number of atoms in the ionization volume is still quite
high for t - r of several hundred nanoseconds, which is when
the laser is fired during the experiment. A steady-state particle
flux is not observed for the 1.9 cm diameter laser beam, which
encompasses greater than 60% of all sputtered atoms a t t =
6 ps.
The effect of laser beam size on the calculated number of
In atoms in the photon field is illustrated in Figure 8. From
this figure, it is obvious that the laser beam volume is a critical
factor in determining the number of atoms which can be
photoionized. In fact, for a IO-ps primary ion pulse, the 1.9
cm diameter laser beam (Figure 8a) overlaps 70 times more
atoms than does the 0.05 cm diameter laser beam (Figure 8e).
Increasing the beam diameter from 0.6 cm to 1.9 cm results
in a more modest gain of a factor of 3 in atom intensity.
Figures 8 and 9 also illustrate the effect of displacing the laser
beam along the surface normal. For laser beams placed
nearest to the surface (Figures 8b and 9a), a steady-state atom
flux is achieved after comparatively short primary ion pulses.
Also, the number of atoms available for photoionization is
increased by positioning the laser beams nearer to the ejection
point. Thus, not surprisingly, the number of atoms in the
photon field is maximized by using the largest diameter laser
-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
I
I
I
I
I
I
I
I
1
I
'
I
I.o
I
I
'
I
2789
'
Mo MPRI
nI G
L
5
I-
a
-1
W
U
0.0
I
3110
2942
2943
2944
WAVELENGTH
2945
2946
(A)
Flgure 10. Ionization signal vs. excitation wavelength for two-color,
single resonance MPRI of
Ga under two photon fluxes: (upper
spectrum) 25 rnJ/(cm2.pulse) of 2944-A 29 d/(crn2.pulse) of 5888-A
light; (lower spectrum) 3.6 mJ/(cm2.pulse) of 2944-A light, 29 mJ/
(cm*-pulse) of 5888-A light. Each spectrum is normalized to the
maximum intensity. Gallium atoms were sputtered from GaAs by 5keV
Ar+ (2 pA, 5-ps pulses, 30 Hz).
beam oriented as close to the sample as is possible.
Ionization Efficiency. Once the appropriate configuration
has been chosen to localize the maximum number of atoms
inside the photon field, the next task is to accomplish efficient
photoionization of these atoms. Now, a conflict is apparent.
If the MPRI process is to be saturated, certain minimum
photon flux and fluence requirements must be met (13).
Typically, greater than 100 mJ/(cm2-pulse) is needed to
saturate the ionization of atoms. The relevance of this condition to our experiment is as follows. By use of the laser
beams in Figure 8, 1 mJ/pulse in a 0.6 cm diameter beam
corresponds to 3.6 mJ/(cm2.pulse), while the same pulse energy corresponds to 510 mJ/(cm2.pulse) in a beam 0.05 cm
in diameter. Recall that decreasing the laser beam size from
0.6 cm to 0.05 cm leads to a loss in atom intensity in the
photon field of a factor of -25. Therefore, especially for atoms
which are difficult to photoionize, a compromise must be
reached between maximizing the number of atoms in the
photon field and maximizing the ionization efficiency.
Since the velocity of sputtered atoms can be appreciable,
the possibility of signal loss due to Doppler shifting of atomic
transitions must be considered. Fortunately, power broadening of absorption lines caused by resonance interaction of
intense laser light with the atomic energy levels overcomes
this problem. For example, the Doppler shift of a 100-eV Ga
atom (moving parallel to the laser beam) at 2944 A is 0.17 A,
but only 0.07 A for 15-eV Ga. We have often measured power
broadened line widths on the order of 0.5 8, for this wavelength
regime. The effect of power broadening on the 2P31z 2D312
and 2P3/2 2D5/2transitions for MPRI of metastable Ga
atoms sputtered from GaAs is illustrated in Figure 10.
In order to achieve maximum ionization efficiency, it is
important to employ the correct ionization scheme. Although
a table of ionization shcemes for MPRI of ground state atoms
has been published by Hurst et al. (13),these schemes are not
necessarily the most efficient. Generally, we have observed
that for schemes involving a single resonance, two-color MPRI
(employing low power ultraviolet light for the resbnance step,
followed normally by high-powered visible light for the ionization) is far more efficient than one-color MPRI. When
MPRI employs a transition through a virtual level to reach
an intermediate bound state, much higher photon fluxes are
necessary to saturate the process. For a laser with 6-11s pulses,
-2000 mJ/(cm2-pulse) is required to saturate a virtual
transition in atoms (13). Initial investigations on As atoms
sputtered from GaAs indicate that almost no As+ ion signal
is obtained until the laser beam is tightly focused. This behavior should be typical of most elements that require tran-
-
-
I
I
3120
I
I
3130
I
I
3140
I
I
3150
-I
I
3160
WAVELENGTH ( A )
Flgure 11. Ion Intensity vs. excitation wavelength for one-color, single
resonance MPRI of Mo atoms sputtered from Mo foil by 5 keV Ar'
(2 pA, 5-ps pulses, 30 Hz): (upper spectrum) laser tightly focused,
> 1500 mJ/(cm2.pulse); (lower spectrum) laser unfocused, 11 mJ/
(cm2.pulse). Each spectrum is normalized to the 3133-A peak.
sitions through virtual states for MPRI. This need to tightly
focus the laser may be the limiting factor in using MPRI for
trace detection of elements such as 0, C, P, As, S, and the
halogens. In the case of nonresonant multiphoton ionization
of atoms, still higher photon fluxes are required (29) to approach saturation of the ionization; thus the effective ionization volume will be smaller than for MPRI and reduced
signal intensities can be expected.
Several workers using multiphoton resonance ionization
have noted the appearance of untabulated spectral lines which
exhibit significant intensity when the laser is focused (16,30).
An example of this behavior is illustrated in Figure 11, which
presents a wavelength spectrum for one-color MPRI of Mo
atoms sputtered from molybdenum foil. The peaks labeled
"G" correspond to ground state originating transitions. All
other peaks remain unidentified. Note that when the laser
is focused, several of the untabulated peaks are nearly as
intense as those which have been identified. Fassett et al. (16)
have reported a nearly identical spectrum for thermally vaporized Mo, with unidentified peaks at 3131.9 A, 3137.6 A,
and 3140.1 8,. The appearance of untabulated peaks in MPRI
experiments means that having a detector with some mass
resolving capability is a necessity in order to verify the element
being detected. This is especially important during an analysis
for unknown elements, in which wavelength scanning could
be employed to give multielement capability.
Finally, it is important to know when 100% ionization
efficiency is being achieved. One should be able to make this
identification by monitoring the ion intensity as a function
of laser power of the ionizing wavelength. The variation of
ion signal with laser power should exhibit a positive nonzero
dependence on the laser power as the power is increased until
100% ionization efficiency is achieved, after which the dependence should be almost zero. The results of studies of this
type for single resonance MPRI of sputtered In, Al, and Mo
atoms are presented in Figure 12 along with the wavelength
spectrum for each resonance transition.
Panels a-d of Figure 12 were obtained for In atoms. Note
that for both the two-color MPRI (a-b) and one-color MPRI
(c-d) the ion intensities are observed to saturate as the laser
power is increased. However, the ion yield obtained using
resonance excitation by 3039.4-A light is 33 times greater than
that measured with 4102-A light. In addition, one-color MPRI
by 3039.4-A light gives an ion yield similar to that of the
two-color experiment. So far, we have not been able to explain
this phenomenon; however, if it is a common observation it
is obviously important for quantifying the MPRI method.
Incidentally, we believe that the background in spectrum (c)
is due to the photodissociation of Inz+to In and In+, caused
by the absorption of a 4102-A photon after resonant ionization
2790
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
u
0
go.01
-
4100
,
,
4101
,
4102
,
,
4103
,
25
4 1,
4104 0
, ,
I
5
75
2
3
12.5 15
IO
,
,
4
,
,
5
,i
00
0'5v
UP?
00
_.
3192
3193
3194
3195
WAVELENGTH
(a)
3196 0
0.5 1.0 1.5 2.0 2.5 3.0
LASER ENERGY (rnJ/pulse)
Flgure 12. Wavelength spectra and associated power spectra for
sputtered (a-b) ('PI,,) In using two-color MPRI with the laser unfocused, (c-d)(2P,,2) In using one-color MPRI with the laser unfocused,
(e-f) (*Pql2)AI using two-color MPRI with the laser tightly focused, and
(9-h) (7S,) Mo uslng one-color MPRI with the laser tightly focused.
Each wavelength scan was taken at the maximum laser power shown
in the corresponding power spectrum.
of Inz by 4102-A photons. To determine if either of the above
measurements is consistent with 100% ionization efficiency,
we have used a Faraday cup to measure primary ion current,
and a hemicylindrical collector to measure the sputter yield
by efficiently capturing the laser-produced ions. For the
two-color ionization (Figure 12a,b),we measure a sputter yield
of 5.7 for In bombarded at 4 5 O by 5-keV Ar+. Although this
measurement has never been made for In, it is consistent with
other sputter yield measurements (19),so we believe that an
assumption of 100% ionization efficiency for this scheme is
good to better than a factor of 2. From the above data, it
appears that saturation of the ion yield is a necessary but not
sufficient condition to establish 100% ionization efficiency.
Figure 12e,f was obtained for two-color ionization of A1 atoms
using a tightly focused laser beam. The laser was focused from
0.6-cm diameter to a tight focus (using a 30-cm lens) directly
above the primary ion impact zone of the target. Note that
the Al+ ion yield does not saturate, even though the energy
density of the laser at the focal point is greater than 6000
mJ/(cmz-pulse). This apparent lack of saturation is not a
function of the expanding conical-shaped profile of the tightly
focused laser (which would result in a slope of 312 in the power
spectrum), as illustrated by panels g-h for MPRI of Mo atoms.
The power spectrum for Mo was obtained with the laser focused to the same degree as for the A1 spectrum. Thus, it is
not always a straightforward procedure to saturate ion yields
or to determine if 100% ionization efficiency is achieved.
Extraction and Identification. Since there are many
variables which can reduce ion formation in the MPRI experiment, it is crucial to extract and identify the majority of
ions that are formed. A high transmission detector is required
if trace level analysis is to be performed. For example, if a
detector with a transmission of lo4 is used, then the duty cycle
problem will offset any gains which are realized by monitoring
neutral atom ejection rather than secondary ion ejection. One
solution to this problem is to use a time-of-flight (TOF) detector. With appropriate construction, the TOF device should
have nearly 100% transmission, and the transmission should
be independent of ion mass and energy. A detection efficiency
of >40% can be achieved by using state-of-the-art microchannel plate electron multipliers. We are presently constructing a reflecting-type TOF mass spectrometer (31, 32)
which will provide the necessary mass resolution for ion indentification and isotope ratio measurements. For this device,
transmission losses can be expected from the multiple grids
and from the effects of the transverse velocity of the sputtered
particles. However, a transmission of -50% should be reasonable depending on the design and the amount of energy
focusing which is required. An attractive feature of the ion
reflector is that it provides a convenient way to reject SIMS
ions which may interfere with trace analysis measurements
via MPRI (32). An additional advantage of the TOF device
is that it is ideally suited to pulsed experiments and allows
for signal-to-noise enhancement by gating the detector.
Prospects for Ultrasensitive Solids Analysis. Using
eq 1,we now perform two example calculations to demonstrate
the potential sensitivity of MPRI of sputtered neutrals as a
tool for the detection of elements at trace level concentrations
in solids. First, assume an element of low ionization potential
(e.g., Ga) is present in a silicon matrix at a concentration of
1 ppb (5 x 1013atoms/cm3). Consider a 1-mA primary ion
beam, fired at 30 Hz with 5-ps pulses, a sputter yield of 5, and
a useful fraction of 50% ground-state Ga atoms. Also assume
that the laser beam is 0.6 cm in diameter such that 28% of
all ejected atoms will be in the photon field, the ionization
efficiency is loo%, and the extraction/detection efficiency
is 20%. Then from eq 1,I = 150 counts/s for 1 ppb Ga. With
a gated detector and single ion counting capability, the lower
limit of detection for this case could be as low as 0.01 ppb,
or 5 X 10l1atoms/cm3. Now, consider the same bombardment
conditions, analyte concentration, and useful fraction, only
assume the detection of an atom that is difficult to ionize (e.g.,
As). Since the laser will have to be tightly focused, assume
that 1% of all sputtered As atoms will lie in the photon field,
and that 25% will be ionized. For an extraction/detection
efficiency of 20%, 5 X 1013As atoms/cm3 would produce about
1count/s. With a gated detector, this is about the lower limit
of detection. If detection of elements at these trace levels can
be demonstrated, MPRI of sputtered neutrals would be an
improvement over most existing methods by several orders
of magnitude and would have a sensitivity comparable to that
demonstrated by the laser ablation/resonance ionization
spectroscopy technique (33). Increasing ion source brightness
and/or the laser duty cycle would enhance the sensitivity of
the experiment even further.
Due to the sensitivity enhancement over SIMS (as well as
the reduction in the magnitude of matrix effects), surface
analysis by MPRI of sputtered neutrals should find application in the study of adsorbate systems relevant to heterogeneous catalysis. In addition, the MPRI approach will prove
to be a viable alternative to high-resolution SIMS analysis for
cases in which isobaric interferences seriously limit the sensitivity of SIMS. For example, selective ionization of P atoms
sputtered from Si would eliminate the mass interference which
occurs between P+ and SiH+ in SIMS.
-
ACKNOWLEDGMENT
The authors are grateful for the encouragement of Sam
Hurst and many fruitful discussions with Jim Parks. Thanks
are also due to Dick Hockett for stimulating us to pursue
analytical aspects of our approach.
Anal. Chem. 1904, 56, 2791-2797
Registry No. Co, 7440-48-4;In, 7440-74-6; Ga, 7440-55-3; Mo,
7439-98-7; Al, 7429-90-5.
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RECEIVED
for review April 30, 1984. Accepted July 23, 1984.
The authors are grateful for the financial support of the
National Science Foundation (Grant No. CHE 81-08382),the
Office of Naval Research (Grant No. N00014-83-K-0052),the
Air Force Office of Scientific Research (Grant No. AFOSR82-0057), and the donors of the Petroleum Research Fund,
administered by the American Chemical Society.
Low Temperature Ashing Preconcentration for Elemental
Localization in Biological Soft Tissues by Ion Microscopy
J. T. Brenna and G. H.Morrison*
Department of Chemistry, Cornell University, Ithaca, New York 14853
Low temperature oxygen plasma ashlng (LTA) was Investigated as a preconcentration method for major and trace eiementai iocaiiratlon in biological soft tissue sections. I t was
found that LTA pretreatment provides satisfactory preservation of elemental morphology. Experiments with fabricated
standards show that LTA enhances elemental sensitlvlties 30
to 1500-fold dependlng on the element. Copper and aluminum
ion micrographs, whlch are unobtainable in intact plastic
Sections, were generated from ashed sections of intestine
taken from normal healthy mice. These data suggest a unique
applicability of LTA in ion microscopical studies of trace e i e
ment dlstrlbutlon in bloiogical samples.
The elemental microcharacterization of thin-sectioned biological tissue is a subject of intense interest. Ion microscopy
via secondary ion mass spectrometry (SIMS) (1) has been
shown to be a useful tool for this purpose. Among its advantages as an analytical technique are high sensitivity and
the ability to distinguish isotopes of the same element. Major
elemental constituents of tissue such as Na, K, Ca, Mg, and
C1 are routinely localized by SIMS (I,2); however, studies on
transition metals and other minor elements have generally
been limited to cases in which the target element concentration
has been artifically raised to toxicological or pharmacological
levels. These trace elements at their ambient levels are of
sufficiently low concentration and ionization probability as
to preclude imaging from intact resin embedded thin sections.
Low temperature oxygen plasma ashing (LTA) is a wellknown and well-characterized technique used for the preconcentration of inorganic constituents from organic material
(3-6). LTA treatment consists of exposing an organic sample
to a stream of oxygen excited by radio frequency to the singlet
state (02,
A): and free atoms (0,3P) which react with and
remove organic material (C, H, N) at relatively low temperatures (7, 8). In high doses, LTA is known to completely
remove organic material while giving quantitative retention
for most elements with no detectable contamination (4,9,10).
For resin embedded biological thin sections mounted on
smooth surfaces, LTA treatment in sufficient doses produces
ash patterns (spodograms) of high morphological integrity (3,
11-14).
Ion microscopy applied to the determination of elemental
distributions in ashed sections has not previously been investigated. Thus, the purpose of this study was to characterize
the usefulness of LTA pretreatment for the ion microscopic
localization of biologically important trace elements at their
normal levels in thin tissue sections. Mouse intestine prepared
by use of conventional fixation procedures and embedment
in plastic served as a model system. The data from this study
indicate signal enhancements of a minimum of 30-fold for Ca
to 1500-fold for Co are obtained upon ashing. The absence
of serious spectral interferences at masses 63,65, and 27 allows
the direct imaging of Cu and A1 at their physiological concentrations.
0003-2700/64/0356-279 1$0 1.50/0 0 1984 American Chemical Society