Anal, Chem, 1993, 65, 3194-3198
3194
Laser Ablation/ Ionization Technique for Trace Element Analysis S. S. Alimpiev, M. E. Belov,' and S. M. Nikiforov Laboratory for Laser Diagnostics, General Physics Institute, Vavilov st. 38, 117492 Moscow, Russia
The laser ablation/ionization technique combined with a reflectron time-of-flight mass spectrometer was used for detection of trace elements in industry-made semiconductor samples of Si and GaAs. A sample atomization was carried out by a 10-ns &-switched Nd:YAG laser operated at a wavelength of 1064 nm. The ablated atoms were ionized by a two-color ( 1 + 1) REMPI technique. An abundance level of several ppb was determined for a number of elements (B, Al, Fe, Cr). The overall detection efficiency of the instrument was found to be The layer-by-layer analysis of Si sample doped with lo6 As was accomplished by a frequency-doubled 10-nsNd:YAG laser for sample ablation and a KrF excimer laser for ionization of ablated neutral species. The whole depth of the arsenic atom occurring was measured to be -3.6 Ccm. INTRODUCTION Resonance-enhanced multiphoton ionization (REMPI) combined with time-of-flight mass spectrometry has been proved to be a very sensible and selective technique in analysis of trace This approach is capable of eliminating a background ion signal from a matrix due to multiple resonant ionization of the impurity atoms and mass-selective detection of the ions. A detection limit down to parts in 10l2 was achieved with a beam of Ru atoms evaporated from a crucible.'j A significant point which is essential for detection of widespread elements at trace concentrations is the method of sample preparation. Any additional stage in this procedure (crushing, dissolving, melting, flame burning, etc.) almost inevitably provides an auxiliary source of impurities. From this point of view, direct mass spectrometry of the sample without any preliminary preparation seems to be preferable. Such an approach was accomplished with a laser microprobe LAMMA,T which is a versatile tool in quantitative element analysis at a concentrationlevel above 10" atoms/cm3(several ppm). The sensitivity of this instrument was raised when a tunable dye laser was applied to analysis of trace elements from thin films and detection limits ranging from 0.04 to 3 ppm were attained for copper impurities evaporated from (1) Hurst, G. S., Morgan, C. G., Eds.; Resonance Ionization Spectroscopy; Proc. 3rd Int. Symp. Reson. Ioniz. Spectrosc. Its Appl. 3rd; (1986), Institute of Physics Conference Series 84; Institute of Physics: Bristol, UK, 1987. (2) Donohue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mass Spectrom. Ion Phys. 1982, 43, 293-298. (3) Fasset, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983,55, 765-770. (4) Fasset, J. D.; Powel, L. J.: Moore, L. J. Anal. Chem. 1984,56,22282233. (5) Moore, L. J.; Fasset, J. D.; Travis, J. C. Anal. Chem. 1984, 56, 2770-2775. (6) Bekov, G. I.; Letokhov, V. S.; Radaev, V. N. J . Opt. SOC.Am. 1985, 82, 1554-1558. ( 7 ) Hillecamp, F.; Unsold, E.; Kaufmann, R.; Nitsches, R. Appl. Phys. 1975, 8, 341-346. 0003-2700/93/0365-3 194$04.00/0
epoxy resin and albumin, respectively.8 However, the background signal of the matrix ions, which are being detected simultaneously with the resonantly ionized neutral species, considerably restricts this technique's sensitivity. The alternative way is to use a pulsed ion gun followed by REMPI technique for trace analy~is.~-ll An abundance level of several ppm in the analog mode and several dozens of ppb in the ion counting mode was reached for a number of elements.12 To achieve a stoichiometric sample evaporation, a pulsed laser ablation technique is used.13-15 Combined with the REMPI technique, this method is capable of providing trace element analysis under conditions of saturation of the multiphoton ionization yield. Mayo et a1.16 have demonstrated some exciting results with a system which incorporated pulsed laser ablation with resonance ionization spectroscopy. In particular, a very small amount of sodium impurity ( 5 x 1011 atoms/cm3, 10 ppt) contained in the silicon matrix was detected when the sample was placed in the buffer gas a t a pressure of 95 Torr. This approach seems to be very attractive for analysis of trace elements with low-energyfirst excited states like alkali metals. However, use of the UV laser radiation to ionize the ablated neutral species under the absence of mass discrimination will lead to rising of the background signal from ambient gas and imperfection of the detection limit. Williams et al.17have described a Nd:YAG laser microprobe (laser spot of lo4 cm2) in which a dye laser and an excimer laser were involved in the resonance excitation and ionization steps, respectively. In this system the resulting ions were analyzed with a time-of-flight mass spectrometer. As a result, chromium with an abundance level of 0.3% was detected from a stainless steel sample. In this work the laser ablationiresonance ionization technique combined with RETOF mass spectrometry was applied to determination of trace amounts of widespread elements (iron, boron, chromium, aluminum) contained at ppb levels ( 5 x 1013-1015atoms/cm3) in industry-made semiconductor samples of Si and GaAs. Such an increase in the technique sensitivity in spite of the use of the ionizing lasers of UV (8) Verdun, F. R.; Krier, G.;Muller, J. F. Anal. Chem. 1987,59, 13831387. (9) Donohue, D. L.; Christie, W. H.; Goeringer, D. E.; McKown, H. S. Anal. Chem. 1985, 57, 1193-1197. (10) Towrie, M.; Drysdale, S. L. T.; Jennings, R.; Land, A. P.; Ledingham, K. W. D.; McCombes, P. T.; Singhal, P. R.; Smyth, M. H. C. Int. J. Mass Spectrum. Ion Phys. 1990,96, 309-320. (11) Parks, J. E.; Spaar, M. T.; Gressman, P. J. J.Cryst. Growth 1988, 89, 4-8. (12) Gelin, P.; Debrun, J. L.; Gobert, 0.;Inglebert, R. L.; Dubreuil, B. Nucl. Instrum. Methods 1989, 840/41, 290-292. (13) Venkatesan, T.; Wu, X. D.; Iham, A.; Wachhtman, J. B. Appl. Phys. Lett. 1988, 52, 1193-1195. (14) Beekman, D. W.; Callcott, T. A.; Kramer, S. D.; Arakawa, E. T.; Hurst, G. S.; Nussbaum, E. Int. J . Mass Spectrom. Ion Phys. 1980, 34. 89-97. (15) Nogar, N. S.; Estler, R. C.; Miller, C. M. Anal. Chem. 1985, 57, 2441-2444. (16) Mayo, S.; Lucatorto, T. B.; Luther, G. G. Anal. Chem. 1982, 54, 553-556. (17) Williams, M. W.; Beckman, D. W.; Swan, J. B.; Arakawa, E. T. ilnal. Chem. 1984, 56, 1348-1350.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
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spectral range was attained owing to the effective suppression of the background signal of matrix ions with a retarding electric field. Furthermore, becauseof a considerablematerial removal per laser pulse, this method was found to be very effective in the profiling of trace impurities located a t a depth of several microns.
EXPERIMENTAL SECTION The experimentalapparatus shown in Figure 1consists of a reflectron time-of-flight (RETOF) mass spectrometerequipped with a specimen insertion system and lasers for sample ablation and ionization. The analyzer had a fwhm resolution of about 600 and included a gridless ion mirror and a doubled-microchannel detector. When placed on the rotatable plate holder, the sample under investigation was maintained at ground potential. The ion extraction system positioned 5 cm apart from the target consisted of three high-transmissiongrids and a pair of deflection plates. The first and the third grids were grounded and the second one was biased at +1770 V. To decrease a fringe field distortion in the inlet region of the deflection system, voltages of opposite polarity were applied to the deflecting plates. The ablation laser was a 10-ns 50-mJ NdYAG laser with a Gaussian beam profile which operated in fundamental and double-frequency modes with a repetition rate of 5 Hz. The ablation laser radiation was focused on the sample surface by a 30-cm lens and a spot diameter was of 150pm. The laser system of resonance enhanced multiphoton ionization consisted of a 20ns 150-mJXeCl pump laser and a dye laser. A part of an excimer laser power output (about 10 mJ) was deflected by a splitter to provide a transition from an exited state into an ionization continuum for the atom of interest. The dye laser had a bandwidth of 0.5 cm-1 and a power output of 5 mJ per pulse in fundamental mode and 0.3 mJ per pulse in frequency-doubled mode. A frequency-doublingBBO crystal covered a wavelength range of 230-270 nm. Ablated neutral speciesenteredthe ion extractionsystem where they were resonantly excited and ionized via intermediateatomic levels by the ionization lasers. The signal of background ions emitted from an ablated plume was suppressed by a retarding electric field in the region between the first and the second grids of the ion extraction system. Further, the ions were accelerated to approximately 1700eV energy, passed through the deflection system, and directed toward the RETOF mass spectrometer. The ion signal detectedwas p r o c e d by a data acquisitionsystem which consisted of a transient recorder coupled to an IBM PC XT. The sample chamber was pumped with an oil diffusion pump to a residual pressure of lo-' Torr.
RESULTS AND DISCUSSION The overall detection efficiency of the laser ablation/ ionization technique has the following contributions: the
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Flguro 2. Yield of Iron atoms ablated from a silicon sample as a function of the time delay between the ablatlon and the ionization laser pulses. Dependence was obtained at the ablating laser fluence of 0.4 J/cm2. Distance from target to ionization region was - 5 cm. apparatus efficiency and the ionization efficiency. The former measures the probability with which laser-ablated atoms are brought into interaction with the ionization lasers, and the latter determines the portion of atoms intersected by ionization laser beams which can be resonantly ionized. To achieve maximum apparatus efficiency, the temporal and spatial overlap of the neutral species cloud and the ionization laser beams has to be optimized and therefore the delay between ablation and ionizationlaser pulses has to be adjusted properly. The dependence of the detected signal of Fe impurity in a Si sample on the time delay between the ablation and ionization pulses is presented in Figure 2 at the ablating laser fluence of 0.4 J/cm2. The data pointa represent a 50-lasershot average a t each delay time. The solid curve is a smooth interpolation of the data and is not meant to be a theoretical fit. Stemming from this distribution, all mass spectra were recorded at a delay time of 20 ps, corresponding to a velocity of 2.5 X lo6 cm/s for this experimental configuration. A spatial overlap of the ablated atom cloud and the ionizationlaser beams can be estimated by taking into account the velocity and angular distributions of ablated neutral species as well the dimensions of an ionization area. An angular spread of the expanding atomic beam with a Mach number of 7 was assumed to be 3Oola2l and an ionization area was 0.5 X 0.5 mm2. The length of the waist of the ionization laser beams where the ionization saturation was observed was measured to be 1cm, and hence the ratio of the ionization volume to one of the ablated atom cloud was estimated to be 3 X 104. The diameter of the ion beam on the ion detector was measured to be 3 cm a t a diameter of the acceptance area of 2.8 cm. Taking into account that the transmission of the grids of the ion extraction system was 0.5, one can conclude that the transmission of the time-of-flight mass spectrometer was -0.4 and the apparatus efficiency was -104. The obvious way to improve the apparatus efficiency is to decrease the distance between the target and the ionization area. However, without a change in the ion extraction system, this approach leads to plasma shielding of an ion source and a reduction of the signal-to-noise ratio (SNR). Hence, a refined construction of the extraction system is required to increase the sensitivity of the method, and this work is in progress. On the other hand, a Fabri-Perot intracavity etalon allows one to narrow the dye laser bandwidth to 0.1 cm-l and
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(18) Namiki, A.; Kawai, T.; Ichige, K. Surf.Sci. 1986,166,129-133. (19) Kelly, R. J. Chem. Phys. 1990,92,5047-5056. (20) Kelly, R. Phys. Rev. A 1992,46,860-874. (21) Kelly, R.; Braren, B. Appl. Phys. B 1991,53,160-169.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
MASS, a.m.u
Figure 4. Mass spectrum of the Si sample (sample 1) ablated by a 10-ns, 0.4 J/cm2Nd:YAG laser pulse. Ionization was carried out by a 248.33-nm, 1 mJ/cm2dye laser pulse and a 308-nm, 50 mJ/cm2 XeCl excimer laser pulse. Fe with an abundance level of 10 ppb was detected as well as Cu, Te, and Cs.
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Figure 3. (Top)Yield of resonantly ionized iron atoms from a Si sample as a function of the dye laser fluence providing a transition from the ground state into the x5F0 excited state. (Bottom)Yield of resonantly ionized iron atoms from a Si sample as a function of the XeCl excimer laser fluence providing a transition from the x5F0 excited state into an ionization continuum.
use unfocused laser beams that give rise to the increase in spatial overlap of ionization laser/atom beams. This experiment carried out in GaAs analysis has shown a growth of SNR by a factor of 5. To evaluate the efficiency of an ionization process, the dependences of the yield of resonantly ionized atoms on the ionization laser fluence were obtained. As an example, the number of Fe ions as a function of the ionization laser fluence is illustrated in Figure 3. The REMPI signal saturation is observed a t the dye laser fluence of 0.7 mJ/cm2 for the 5D4 x5F0, X = 248.33nm transition and at the XeCl excimer laser fluence of 50 mJ/cm2 for the transition from the x5FO excited state into the ionization continuum. Similar dependences were obtained for B and Cr ions and the thresholds of REMPI signal saturation were found. The ionization efficiency under conditions of strong saturation of both transitions is estimated to be about unity. It should be pointed out that the saturation effect was obtained when multimode laser beams were used to excite and ionize ablated neutral species. In this case, the increase in laser power leads to a negligibly small increase of ionization volume that differs considerably from the case of the Gaussian beam profile when the dimensions of the waist of the laser beam are a logarithmic function of the laser power. Therefore, the overall detection efficiency was calculated to be lo4. Knowing the crater dimensions, the number of evaporated particles per laser pulse was calculated to be 2 x 1014. This overall detection efficiency enables us to state that the isolated ions will be detected a t trace element concentrations of 1ppb; i.e., a detection limit down to ppb is available with this instrument. To confirm the previous estimations, the detection limit was measured directly with the use of a set of the reference samples (Si, GaAs) that were independently tested by neutron +
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and proton activation techniques, whose detection limits for a number of elements (B, Al, Cr) are in the range of 1-10 ppb.22,23Silicon samples 1cm long, 0.7 cm wide, and 0.05 cm thick were cut from float-zone single-crystal wafers of n-type (111)(30 Q-cm)silicon. A GaAs sample of the same size was cut from an n-type (10' Dcm) liquid-encapsulated Czochralski crystal of GaAs that was grown in the (100) crystallographic direction. All mass spectra averaged over 20 laser shots were obtained using the Q-switched Nd:YAG ablation laser which irradiated the polished face of the wafers at a wavelength of 1064 nm and a laser fluence of 0.4 J/cm2on the sample surface. The choice of the IR laser to ablate the specimen was due to the ion-neutral ratio in the ablated plume, which abruptly rises with a decrease of laser wavelength.24 The fluence threshold a t which the ablation process begins was found to be -0.15 J/cm2 for the GaAs sample which is in good accordance with the data obtained in ref 25 while for the silicon matrix this value appeared to be 0.2 J/cm2. Ionization of neutral species ablated from Si (sample 1, Figure 4) was accomplished by a two-step process: a doubledfrequency photon from Coumarin 302 (248.33 nm, 1mJ/cm2) for resonant excitation and a XeCl excimer laser photon (308 nm, 50 mJ/cm2) for ionization. As a result, Fe atoms at a concentration of 10 ppb were detected as well as Cu, Te, and Cs. Nonresonant ionization of the latter elements does not allow determination of their exact abundance level, and REMPI experiments will have to be done for every trace element. It should be pointed out that a low photoionization cross section of Te atoms at a wavelength of 248.33 nm was the reason for the distorted isotopic pattern of this impurity; i.e., the signals of isotopes with low natural abundances (120Te, lZ2Te,123Te, lZ4Te)are beyond the available detection limit. Figure 5 and Figure 6 present the results of analyses of Si (sample 2) when the following resonance ionization schemes were used: a doubled-frequency photon from Coumarin 307 (249.68 nm, 1 mJ/cm2) to excite the B atom, a doubledfrequency photon from Coumarin 307 (248.33 nm, 1mJ/cm2) to excite the Fe atom, and a XeCl photon (308 nm, 50 mJ/ cm2) for ionization of both atoms. This sample of Si appears to have a contamination of Fe with an abundance level of 27 ppb and B of 1ppb. Furthermore, an abundance level of 8 ppb was obtained for A1 atoms which were resonantly ionized by a XeCl excimer laser. (22)Engelmann, Ch. J. Radioanal. Chem. 1971,7,89-93. (23)Winchester, J. W.Radioactiuation Analysis in Inorganic Chemistry; Cambridge University Press: Cambridge, UK, 1960; Vol. 2, p 1. (24)Wiedeman, L.;Helvajian, H. J.Appl. Phys. 1991,70,4513-4523. (25)Mohlmann, G.R.;Kuzmin, V. A. Laser Chem. 1986,6,349-359.
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Fburo 7. Mass spectrum of GaAs sample ablated by a 1 0 ” 0.4 J/cm2 M Y A Q laser pulse. Ionlzatlon was carried out by a 425.43nm, 2 mJ/cm2dye laser pulse and a 308-nm, 50 mJ/cm2XeCl exclmer laser pulse. Cr with an abundance level of 10 ppb was detected.
A sample of GaAs was analyzed to detect Cr as a trace element, and a blue photon from POPOP (425.43nm, 1 mJ/ cm2) followed by a XeCl photon (308nm, 50 mJ/cm2) was used to excite and ionize the atom. Cr with an abundance level of 10 ppb was detected, as can be seen in Figure 7. The detection limits for the elements under study are summarized in Table I. These values were obtained from the experimental data under the assumption that SNR = 3. T w o significant points that need to be considered are the system background and the technique calibration. The
complete absence of the matrix ion signal in the mass spectra presented in Figures 4-7 allows the claim that the ablated plume was efficiently charge-separated and the background ions were retarded in the region between the first and the second grids of the ion extraction system. It should be mentioned that just owing to the suppression of the background signal of the matrix ions, the achieved detection limits turned out to be better than those obtained in the experiments when laser ablation was incorporatedwith inductively coupled plasma mass spectrometry.*a The matrix effect can also show itself as a variation of the ablation yield from matrix to matrix. In these experiments the different matrices were tested independently by a neutron (proton) activation technique that allowed us to avoid the problem of slightly different mass removal per laser pulse in analysis of the samples of different materials (Si, GaAs); i.e., for quantitative determination of element concentrations by this technique, calibrated samples are required when the different matrices are analyzed. Furthermore, the ratios of signal intensities obtained with the REMPI technique in determining B, Al, Cr, and Fe concentrations in the samples of Si and GaAS are in a good agreement with those obtained with neutron and proton activation methods. This permits us to conclude that the overall detection efficiencyof this instrument is constant from element to element. One more question is whether the Cu or Fe signals are coming from semiconductor samples but not from structural components of the mass spectrometer that are grazed by one of the lasers or from metallic grids that are bombarded by an ablated plume. These points were thoroughly examined. Switching off either the ablating laser or the ionizing one as well as use of different samples with a trace amount lower than the detection limit of this instrument resulted in the complete disappearance of the signal detected. Thus, the system background was determined only by the noise of the electrical circuit. Relative calibration implies the availability of standards calibrated by other independent analytical techniques with comparable sensitivity a t low impurity concentrations. However, manufacturing of the standards containingthe impurities of interest (iron, copper, aluminum, boron, chromium) at an abundance level within 1-100 ppb range is a very complicated problem. These elements could be injected into a semiconductor sample at such a level of concentrations only as uncontrolledimpurities. From this point of view, one possible solution is to measure a depth profiie of the analyte injected in a semiconductor whose concentration is ranging within 1 order of magnitude and compare the distribution obtained with the well-known theoretical one based on the diffusion model. The layer-by-layer analysis of an Si sample doped with As was performed with a Q-switched Nd:YAG laser operated in doubled-frequency mode (532 nm) to ablate the specimen and a KrF excimer laser to ionize the ablated plume. Arsenic ~~~
(26) Gray, A. L. Analyst 1985,110,551-556. 127) Arrowsmith. P. Anal. Chem. 1987.59.1437-1444. (28) Mochizuri, T.; S h h i t a , A.; Iw&, H:; KagGa, T.; Shimanurn, T.; Blair P. Anal. Sei. 1988,4,403-409. (29) Ho-ming Pang; Wieldering, D. R.; Houk, R. S.;Yeung, E. S. Anal. Chem. 1991,63, 39Cb394.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993 I
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atoms were injected in the n-type (111)silicon matrix by the thermal diffusion method at a temperature of 1250 "C and at a concentration of 10l6atoms/cm3 for 3 h. A fluence of the ablation laser and a spot diameter were optimized (i) to decrease rolls on the crater edges and (ii) to obtain the higher depth resolution and were found to be 0.5 J/cm2and 300 pm, respectively. The ionizing radiation of the KrF laser was focused by a 100-cm lens into an area of 3 X 3 mm2. Two photons of 248-nm wavelength appear to provide a transition from the ground state into the autoionization state of the As atom.30 That is why a KrF laser power output of 3 mJ was sufficient to saturate the ionization yield of impurity atoms. The analysis of craters was fulfilled with a profile meter of 200-8, depth resolution, and a linear dependence of the crater depth on the number of laser shots was observed.
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The results of layer-by-layeranalysis are presented in Figure 8. The depth of crater was measured to be 1200 A and the whole depth of As atom diffusion was found to be -3.6 pm. The solid curve depicted is the theoretical distribution of the (30)Bhatia, K.S.;Jones, W. E. Can. J. Phys. 1971,49, 1773-1776. (31) Brodie, I.;Murray, J. J. The Physics ofMicrofabrication;Plenum Press: New York, 1982; Chapter 1.
dopant based on the diffusion modelF
where No = 5 ~ 1 0 atoms/cm3, '~ the initial impurity concentration; D = 0.25 pm/h, the diffusion coefficient; t = 3 h, the diffusion time; and z is the coordinate perpendicular to the sample surface. A coincidenceof the theoretical distribution of the impurity occurring with the experimental one within experimental accuracy allows the claim that this technique can be successfully applied to the profiling of wide-spread impurities at a concentration level down to 1 ppb located at a depth of several microns. A rough estimation of the depth resolution gives the value of 1200 A, but for exact measurement of this parameter the analysis of thin films with an abrupt concentration step should be performed and this is the subject of further investigations.
CONCLUSION It has been shown with laser ablation followed by the REMPI technique combined with time-of-flight mass spectrometry that ppb sensitivity can be achieved for wide-spread elements contained in unprepared semiconductor samples. The technique makes it possible the layer-by-layer analysis of trace elements at a concentration level down to 1ppb and a depth of the impurity of several microns. A significant improvement of the overall detection efficiency of this instrument by 1-2 orders of magnitude seems to be attainable with the use of a refined ion extraction system as well as narrow-band ionizing lasers, and this work is in progress.
ACKNOWLEDGMENT This research was supported by the Ministry of Science, Higher School and Technical Policy, Russia, Grant N801F. The authors are grateful to Prof, A. M. Prokhorov for helpful discussion and to Dr. V. P. Kalinushkin for a set of the reference samples. Received for review May 5, 1993. Accepted July 15, 1993.8 Abstract published in Advance ACS Abstracts, September 1,1993.
