Solids Analysis Using Energetic Ion Bombardment and Multiphoton

electron bombardment (8), glow discharges (9), and plasmas. (10) have been ... progress of chemical reactions (12) and as a structural probe. (14). Us...
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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 (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%

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-2 10c a

t r . I v

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t

x x x x

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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, 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

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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

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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 I

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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-

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3140

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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

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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984

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3193 3194 3195 WAVELENGTH

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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.

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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.

LITERATURE CITED (1) Ens, W.; Standing, K. G.; Westmore, J. 6.; Ogllvie, K. K.; Nemer, M. J. Anal. Chem. 1982, 5 4 , 960-9136, (2) Glbbs, R. A.; Holland, S. P.; Foley, K. E.; Garrison, B. J.; Winograd, N. J . Chem. Phys. 1982, 7 6 , 684-695. (3) Wlttmaack, K. Appl. Phys. Lett. 1978, 2 9 , 552-554. (4) Wittmaack, K.; Clegg, J. B. Appl. Phys. Lett. 1980, 378, 265-287. (5) Clegg, J. 6.; Scott, G. B.; Hallals, J.; Mlrcea-Roussel, A. J . Appl. Phys. 1981, 52. 1110-1112. (6) Pellln, M. J.; Wright, R. B.; Gruen, D. M. J . Chem. Phys. 1981, 7 4 , 6448-6457. (7) Yu, M. L.; Grischkowsky, D.; Balant, A. C. Phys. Rev. Lett. 1982, 48, 427-430. (8) Honlg, R. E. "Advances In Mass Spectrometry"; Pergamon Press: New York, 1962; Vol. 2. (9) Coburn, J. W.; Kay, E. Appl. Phys. Lett. 1971, 19, 350-352. (10) Oechsner, H.; Oerhard, W. Surf. Sci. 1974, 44, 480-488. (11) Winograd, N.; Baxter, J. P.; Klmock, F. M. Chem. Phys. Lett. 1982, 8 8 , 581-584. (12) Klmock, F. M.; Baxter, J. P.; Winograd, N. Surf. Sci. 1983, 124, L41L46. (13) Hurst, G. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. PhyS. 1979, 5 1 , 767-819. (14) Kobrin, P. H.; Baxter, J. P.; Winograd, N., unpublished work. (15) Parks, J. E.; Schmltt, H. W.; Hurst, G. S.;Fairbank, W. M. SPIE27th Ann. Tech. Symp. Proc., 27th 1883. (16) Fassett, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983, 5 5 , 765-770. (17) Donohue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mass Spectrom. Ion PhyS. 1982, 4 3 , 293-307.

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(18) Miller, C. M.; Nogar, N. S. Anal. Chem. 1983, 5 5 , 1606-1608. (19) Carter, G.; Colligon, J. S. "Ion Bombardment of Solids"; American Elsevler: New York, 1968; Chapter 7. (20) Coburn, J. W. Thin Solid Films 1979, 6 4 , 371-382. (21) Coburn, J. W. J . Vac. Sci. Techno/. 1976, 13, 1037-1044. (22) Oechsner, H.; Schoof, H.; Stumpe, E. Surf. Sci. 1978, 7 6 , 343-354. (23) Wright, R. B.; Pellin, M. J.; Gruen, D. M.; Young, C. E. Nucl. Instrum. Methods 1980, 170, 295-302. (24) Pellin, M. J.; Young, C. E.; Calaway, W. F.; Gruen, D. M. Surf. Sci., in press.

(25) Wlttmaack, K. Nucl. Instrum. Methods 1980, 168, 343-356. (26) Fassett, J. D.;Moore, L. J.; Travis, J. C.; Lytle, F. E. Int. J . Mass Spectrom. Ion Procs. 1983, 5 4 , 201-218. (27) Onderdelinden, D. Can J . Phys. 1988, 46, 739-745. (28) Thompson, M. W. Phllos. Mag. 1968, 78, 377-414. (29) Morellac, J.; Normand, D.; Petite, G. Adv. At. Mol. Phys. 1982, 18, 97- 164. (30) Young, J. P.; Donohue, D. L. Anal. Chem. 1983, 5 5 , 88-91. (31) Mamyrln, B. A.; Karataev, V. I.; Shmlkk, D. V.; Zagulin, V. A. Sov. Phys.-JETP (Engl. Transl.) 1973, 3 7 , 45-48. (32) Becker, C. H.; Glllen, K. T. Anal. Chem. 1984, 9 , 1671-1674. (33) Mayo, S.; Lucatorto, T. 6.; Luther, G. G. Anal. Chem. 1982, 5 4 , 553-556.

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