Anal. Chem. 1980, 52, 277-280
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Ion Implantation for in-situ Quantitive Ion Microprobe Analysis D.
P. Leta
and G.
H. Morrison*
Department of Chemistry, Cornell University, Ithaca, New York 74853
A technique is described for the direct quantitative ion implantation of unknown samples and the subsequent quantitative determination of the trace elements Originally present in the sample by the use of the depth profiling Capabilities of the ion microprobe. The proposed method of solid-state standard addition analysis is verified by the application of the system to the quantitative determination of AI and Si in steel and of Te in GaAs.
The ion microprobe is presently one of the most sensitive methods for the in-situ characterization of elemental distributions in samples of limited dimensionality, often with detection limits in the ppm range. The capabilities of lateral elemental imaging (extremely useful in defining microfeatures of interest) and of elemental depth profiling with resolution of less than 100 A make the ion microprobe unsurpassed in the field of three-dimensional trace element localization. The expansion of the general utility of the ion microprobe, however, has been hampered by the difficulty in obtaining quantitative measurement of concentration in samples not extremely well characterized. This difficulty is due to several considerations: secondary ion yields vary over more than four orders-ofmagnitude from element to element, each element’s ion yield is affected by the matrix in which it is contained, and instrumental parameters and ion collection efficiencies vary from case to case. These factors have, in general, limited the ion microprobe analysis of unknown samples to qualitative elemental characterization. Several methods of semiquantitative ion probe analysis are currently in use, ranging from semitheoretical modeling of the ion surface interaction to the direct application of standard materials for instrumental calibration. The most widely used semitheoretical method to date has been the local thermal equilibrium (LTE) model as exemplified by Andersen and Hinthorne’s computer program CARISMA ( I ) . Although the model has been very successful in some cases, in general the accuracy of the results cannot be stated to be better than a factor of 2 or 3 ( 2 , 3 ) . In addition, the necessity for prior determination of several internal standard elemental concentrations within the specific sampling area and the requirement of known instrumental transmission factors for each element ( 4 ) have made practical applications of such models difficult. The empirical calibration approach, used in many other analytical techniques, has been shown to provide substantially more accurate results (5),but often suffers from the lack of standard materials suitable for microanalysis which provide a sufficiently close sample/standard match. I t has been demonstrated that ion implantation techniques may be used to incorporate a known amount of almost any element into the near surface region of a sample (6). In the ion implantation process, an ion beam of the desired element is created, mass filtered and accelerated to substantial energies, normally between 10 and 600 keV, and then directed onto a sample surface where the dopant species comes to rest a t a depth of less than several micrometers. The ion beam is rastered over the target area to ensure uniformity of doping in the lateral directions, and the current delivered to the 0003-2700/80/0352-0277$01.00/0
sample stage may be monitored and integrated to provide an accurate and controllable dopant concentration in the implanted sample. Although this concentration is originally known only as the amount of surface coverace (fluence), in atm/cm2, the use of the ion microprobe’s depth profiling capabilities to determine the “shape” of the concentration distribution with depth, which nominally resembles a Gaussian distribution in small polycrystalline or amorphous materials, combined with an accurate measurement of the depth of analysis allows the necessary conversion in concentration in atm/cm3. Owing to the characteristic shape of the depth concentration distribution of an ion implanted species, it is possible to distinguish the secondary ion signal arising from the implanted quantity from that due to the same element originally present in the sample. In the absence of mass spectral interferences, it is therefore possible to use ion implantation tx, perform solid state standard addition analysis to obtain quantitative elemental concentrations in microsamples, such as epitaxial layers, or in a particular microfeature of a surface. The present work describes the applications of such an ion implantation doping approach for the determination of aluminum and siliccai in polycrystalline iron, and of tellurium in single crystal gallium arsenide. For the analysis of the steel samples, which generally sputtered nonuniformly, the accuracy was consistently better than 50%, while 15% accuracy was demonstrated for the more ideal semiconductor matrix. The requirements and methodology of the analysis system are described as well as its inherent advantages.
EXPERIMENTAL Instrumentation. The ion implantation was accomplished using an Accelerators Inc. ion implanter 300R. The ion probe analysis was performed using a Cameca IMS-300 which has been previously described ( 7 ) . Computer Software. Data collection was accomplished using peak top hopping from mass to mass and one-second integrations per point during the depth profiles. Ion implant and background concentrationswere calculated after the analysis from stored data with the use of Fortran level programs. Standards. The standards used in this study for the determination of aluminum and silicon were the NBS low alloy steels SRM-661, 662, and 663. For the determination of tellurium, a section of a large single crystal of gallium arsenide, known to be doped with tellurium, was sliced into 15-mil thick wafers, and the adjacent sections to that used were dissolved and analyzed for bulk composition by atomic absorption spectroscopy. Sample Preparation. The steel samples were polished and etched to provide clean, planar surfaces. The tellurium containing gallium arsenide was sliced from the large single crystal in the (100) orientation using a diamond saw and then chemically/ mechanically polished on a rotating paper-covered wheel under a flowing solution of 1% bromine in methanol. Procedure. The basic experimental parameters are presented in Table I. Prior to being implanted, each sample was depth profiled for the elements of interest to establish the residual concentration distribution with depth of the species to be determined. In all cases for the standards, this was found to be homogeneous. Mass scans were also collected to determine the presence of spectral interferences on the masses to be analyzed. For the cases of Si and A1 in steel, some interferences were detected from the doubly charged iron species from the matrix, but
B 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
Table I. Experimental Parameters Ion Implantation instrument: Accelerators Inc. Model 300 R available ion energies: 1 0 t o 300 keV mass resolution: 1 2 0 ion source: hot filament plasmatron with solids probe Analysis instrument: Cameca IMS-300 ion probe primary ions: 0,’ and 0’ primary ion energy: 5.5 keV for positive secondary ions 14.5 keV for negative secondary ions primary current density: 5 x to 3 x A/cm2 primary crater size : rastered, 1 mm2 sampling area: 30- to 225-pm diameter circle mass resolution: 200 detection system: ion-electron multiplier pulse count rate depth measurement instrument: TALYSTEP surface pro filer were found to be less than 10% of the secondary ion signal for masses 27 and 28 and were ignored in the treatment of the data. No interference was present for Te and GaAs as determined by isotope ratio measurement. From the amount of signal collected, combined with a previous knowledge of the general range of secondary ion yields for each of the elements, an order-of-magnitude approximation of the concentration present was calculated. This was used as a guide to control the amount to be implanted, so that the maximum concentration of the implanted species would be from one to ten times that of the element originally present. The samples were then ion implanted in the following manner: one set of steel samples were doped with 27Alat 150 keV to a fluence of 5 X 10’j atm/cm2,another set was implanted with ‘%i a t 150 keV to a fluence of 1 X 10l6atm/cm2,and the GaAs sample was implanted with 130Te at 250 keV to a fluence of 3 X 1013 atm/cm2. The single crystal GaAs was tilted a t an angle of 7’ to the incoming implantation ion beam to reduce the amount channeling by presenting an apparently amorphous structure to the incident ion beam and provide for an approximately Gaussian concentration distribution with depth. Following implantation, the samples were again depth profiled for the elements of interest. The A1 and Si were monitored by following their positive secondary ion signals and for Te the negative spectrum was used. Where possible, i.e., for Si and Te, several isotopes were monitored, only one of which had been implanted, to aid in the identification and separation of the signal arising from the added implanted species from that due to the elemental concentration already present. (For monoisotopic elements, if a varying concentration with depth is originally present, difficulties may be encountered because the implant profile will mask out the original profile.) In this way, and relying on the distinguishability of the Gaussian implant from the steady-state background signal, the integrated secondary ion signals for the amount implanted and the signal level for the background (the amount originally present) were determined. To obtain concentration measurements from the implant fluence and the signal levels, it was then necessary to determine the depth of analysis. This was accomplished using a stylus type surface profiler (Talystep) device. This critical measurement tends to be more inaccurate for the steels than for the semiconductor material owing to the nonuniformity of the sputtering rates for the polycrystalline iron, a rough bottomed crater often being obtained.
CALCULATIONS T o obtain a final depth profile in which the signal from the implant is easily separable from that of the “background”, it is necessary t h a t the concentration added by ion implantation be approximately equal to or slightly greater than the original amount to be determined. If the added concentration is too low, the small variations in the background will make
the determination of the integrated signal from the implant inaccurate, and if the amount implanted is excessively high, i.e., greater than ten times the original concentration, the high concentration “tail” from the implant distribution will make the measurement of the background signal difficult, requiring that a very deep profile be obtained t o reach a region of the sample which is unaffected. Therefore, after estimating the order of concentration in the sample, it is necessary t o calculate the ion implant fluence (atm/cm2) which will give the desired maximum concentration for the implant profile. The actual maximum concentration of the implanted element, which normally has a near-Gaussian distribution with depth, may be calculated from the implanted fluence by several methods. T o match the concentration obtained to that desired, it is necessary t o know in advance the standard deviation of the Gaussian concentration distribution and make use of the relationship
where A is the maximum concentration in atm/cm3, F is the implant fluence in atm/cm2, and cr is the standard deviation of the Gaussian distribution in cm (equal to l / z the peak width a t 0.606 maximum). Values for the implant distribution parameter, a, may be calculated with a fair degree of accuracy from the theory of Lindhard, Scharff, and Schiott (LSS) (8) and are often available for certain implanted species/substrate combinations in tabular form as a function of the implant energy. The direct use of the full calculations, however, where the desired species/substrate combination values are unavailable, is quite lengthy and a usable numerical approximation may be obtained from the relationship (9)
a=
+
F(l M2/3Ml)gZ2/3lo8 130E(1 + M2/M1)
where A is the maximum concentration in atm/cm3, F is the implant fluence in atm/cm2, M 2 is the average mass of the substrate atoms in amu, M1 is the mass of the implanted element in amu, g is the density of the substrate in g/cm3, 2 is the atomic number of the implanted element, and E is the implant energy in keV. This equation assumes that M1 > M 2 and M1/Z1 = 2.2 = M 2 / Z 2and is generally reliable t o within 50%. Once a suitable fluence for ion implantation has been determined and the sample implanted, it is then depth-profiled to a depth at least 3 t o 4 times that of the concentration maximum of the implanted quantity. At such a depth, all of the ion signal obtained will be originated from the residual concentration of the species of interest (at f 3 a the peak concentration is down by approximately A simulation of such a depth profile is shown in Figure 1,with the graphical equivalents of the pertinent parameters included. The total depth of the analysis, D (in cm), must be determined, perferably from direct measurement where at all possible, since the accuracy of the analysis is critically dependent on this parameter. The next parameters which must be determined are the integrated secondary ion signal arising from the implanted quantity, I , and the signal level of the element which comes only from the residual amount present, S b . These quantities may be determined in any consistent units, and as long as any dead time in the data collection of the integrated ion implant is corrected for t o give the integral which would result from continuous monitoring, the corrected sum of the counts per second (cps) may be used, giving I units of counts if Sb is in cps. As long as the assumption of linearity of secondary ion signal with concentration is correct, which is quite reasonable a t low concentrations, the implanted quantity may be considered as a hypothetical equivalent with an area
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
279
Table 11. Analysis Results and Error Analysis ~
sample
A1 deter. actuala concn., concn., atm/cm3 atm/cm3 x 10-20 x
SRM 661 steel
0.37
SRM 662 steel
1.7
SRM 663 steel
4.2
Si actuala deter. concn., concn., atm/cm3 atm/cm3 x lO-’O x
error factor
error factor
-1.06 1.51
3.8
0.56
4.8 3.3
1.26 -1.15
1.2 2.6
-1.42
6.6
4.8 5.8
-1.38 -1.14
0.35
5.7 6.3
1.53
12.4
1.36 1.50
Te actualb ‘deter. concn., concn., atm/cm3 atm/cm3 x lo-’’ x
12
-1.03 -
9.8
-1.27 9.2
GaAs
a
Calculated using 7.85 g/cm3density for all steel samples.
error
factor
8.0 8.5 8.6
-1.15 -1.08 -1.07
Value for Atomic Absorption Spectroscopy.
A I iri S t e e l IO5[\
I
D
0
,
1
I
I
:I
I
,
;
I
I
1000
Time (secll
Figure 1. Simulated depth profile for a sample which has been ion
implanted for solid-state standard addiion analysis, showing the important parameters (defined in text) equal to I and a uniform concentration to the depth D, where it abruptly ends. T h e concentration of this hypothetical implant profile, Che (in atm/cm3), will be the implanted fluence, F (in atm/cm2),divided by the depth of analysis, D, and the corresponding signal level will be I , divided by the time of analysis, t (in s). Therefore, the concentration of the element originally present, Cb (in atm/cm3), will be SbChe cb=--- SbtF she
,
0
Depth
ID
(3)
RESULTS AND DISCUSSION The results of the solid-state standard addition analyses performed are presented in Table 11. The deviations in accuracy are represented by error factors which are calculated as the obtained concentration divided by the actual concentrations of the samples for cases where the determined results are too high, and as the actual concentration divided by the determined value where the results are too low (denoted with a minus sign). Positive error factors are equal to one plus percent error. The numerical part of a negative error factor, however, truly represents the factor by which the result obtained must be multiplied by to equal the true answer unlike a negative percent error which may be misleading for large deviations. For the A1 and Si determination in steel a large degree of variance was obtained, believed to be due t o the inaccuracy of the measurement of the analysis depth because of the extremely rough bottomed craters which are often created by the sputtering of polycrystalline alloys. The craters, which were approximately 1 mm2, were generally less than 2 pm in depth and showed a roughness of up to 7000 A. The
Figure 2. Ion microprobe depth profile obtained for the standard addiion analysis of AI in NBS SRM-661 steel
results however may be seen to be within a factor of 1.5 of the certified concentrations. For the determination of T e in GaAs very little scatter in the determined concentrations was found, in this case attributable to t.he well defined craters formed in the single crystal material. From Equation 3, it may be seen that the important parameters are the measurement of the background signal, the time and depth of analysis, the integral of the implant profile and the implant fluence. The background signal measurement will be affected only be the presence of mass interferences. This added error to the A1 and Si determinations because of the presence of Fez+isotopes on the monitored masses, but was less than 10%. Time was measured directly by the computer clock and is considered to have negligible error. The measurement of the analysis depth, D , is considered to be the most inaccurate parameter in this case. In general, the TALYSTEP surface profiler is accurate to less than 10% for depth measurements greater than 100 A; however, crater bottom roughness caused by sputtering of polycrystalline samples and additionally by “cone” formation (IO)may cause the depth measurement to be inaccurate by as much as 30%. The measurement of implant fluence is considered accurate to better than 5% and does not constitute a large error provided the implantation sample stage is shielded against secondary electron and ion emission during the current monitoring procedure. Finally, the determination of the integral of the ion implant profile adds another degree of inaccuracy. The error incurred will be greatest where the peak base line must be approximated, Le., for Al, where no undoped isotope is available. Figure 2 shows an analysis depth profile obtained from Al+ in NBS SRM-661. The accuracy of the determination in cases
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 A
3"""-
I 0
1
,
I
I
400
T i m e (sec)
Flgure 3. Depth profile of Te isotopes 128 and 130 in GaAs. The sample was ion implanted with 13?e only
such as this, where no isotopes are available for background monitoring, was accomplished by obtaining an average residual signal level in a region relatively deep in the sample. This average signal was subtracted from the signal levels obtained in the implanted region, which was determined to begin at the point where any obvious surface contamination signal stopped and the signal level began to rise because of the implant, and was defined as ending when the signal obtained a steady level. For cases where several uninterfered-with isotopes of an element were available for analysis, i.e., for Te and Si in the samples studied, the measurement of the integral of the implanted signal was simplified by matching the residual signal levels of the doped and undoped isotopes in the "background region" of the depth profile and using the shape of the undoped isotope in the implanted region to define its base line. Figure 3, a depth profile obtained for T e in GaAs, demonstrates this possibility where in this case only the '30Te isotope was ion implanted. Although the concentrations of the standards analyzed were homogeneous with depth, the use of such isotope monitoring should enable accurate determinations of concentration in samples where the element of interest exhibits a varying concentration with depth. The use of solid-state standard addition analysis has several advantages over quantitation using either theoretical methods or empirical external calibration schemes. These advantages arise from the fact that when the unknown sample is ion implanted and analyzed with the ion microprobe, both the standard quantity and the residual signal level are determined simultaneously and in exactly the same matrix. The effects of any instrumental parameters such as ambient pressure, primary beam density and composition, and ion transmission and counting efficiencies are automatically eliminated, provided that conditions are held stable during the collection of the depth profile. The fact that the ion implant and the residual concentration are present in the same matrix will serve to eliminate inaccuracies due to matrix effects on the secondary ion formation probabilities. This holds true provided that the element of interest is contained in solid solution in the sample, a condition which is closely approximated by the individual atom positioning of the implanted quantity. In the use of ion implantation for solid-state standard addition analysis, attention must be paid to several requirements.
The monitored mass peak, which could be a molecular species in some cases, must be free of mass spectral interferences in order to obtain a measurement of the signal arising only from the element to be determined. Care must be taken to ensure that the energy of the ion implant is sufficient to position the implant distribution deep enough in the sample to avoid interferences arising from the surface contamination. Also, there is an upper limit of the implanted fluence which may be used owing to the sputter removal of the surface during the ion implantation process (11). Lastly, it should be noted that the implanted quantity will be homogeneously distributed on an atomic scale so that it will have the same secondary ion yield as that of the originally present element only if the element is also homogeneously distributed, and not present in microprecipitates (although in any case the method will remove the effects of instrumental conditions). Much of the methodology developed here is additionally applicable to the formation and use of separate (external) ion implanted standard materials (12). In cases where many similar samples must be analyzed, use of the solid-state standard addition analysis method is impractical owing to the expense of implanter time and often a better approach is the formation of a single external standard for repeated use. The technique of ion implantation for standard addition analysis, in which an unknown sample is implanted with a known quantity of the element to be determined and then depth profiled using the capabilities of the ion microprobe, should prove to be very useful and versatile for the analysis of microsamples. Of particular application will be the quantitative determination of trace elemental concentration in thin-layer systems such as epitaxial growth layers of semiconductor materials. The technique should also prove applicable in other depth profiling methods of analysis such as Auger microprobes with sputtering capabilities.
ACKNOWLEDGMENT The authors acknowledge the assistance of James Comas in providing the implantation of the steel samples at the Naval Research Laboratories, and of Charles Lee and Gary Harris in the use of the ion implantation facilities of the National Resource and Research Facility for Submicron Structures at Cornell.
LITERATURE CITED (1) Andersen, C. A,; Hinthorne, J. R. Anal. Chem. 1976, 4 5 , 1421. (2) McHugh, J. A. "Secondary Ion Mass Spectrometry" in "Methods of Surface Analysis", Wolsky, S. P., Czanderna, A. W. Eds.; Elsevier: New York, 1976. (3) Simon, D. S.;Evans, C. A., Jr. Anal. Chem. 1976, 48, 1341. (4) Rudat, M. A,; Morrison, G. H. Anal. Cbem. 1979, 5 7 , 1179. (5) Ganjei, J. D.; Leta, D. P.; Morrison, G. H. Anal. Chem. 1978, 5 0 , 285. (6) Gries, Vi. H. Int. J . Mass Spectrum. Ion Phys. 1979, 30, 113. (7) Scilla, G. J.; Morrison, G. H. Anal. Chem. 1977, 4 9 , 2322. (8) Lindhard, J.; Scharff, M.; Schiott, H. E. Mt.-Fys. Medd. Dan VU. Selsk. 1963, 33, No. 14. (9) Mayer, J. W.; Erikson, L.; Davies, J. A. "Ion Implantation in Semiconductors"; Academic Press: New York, 1970, 19-37. (IO) Gvosdover, R. S.;Efrernenkova, V. M.; Shelyakin, L. 8.;Yurasova. V. E. Radiat. Eff. 1976, 27, 237. (11) Gries, W. H. Int. J . Mass Spectrom. Ion Pbys. 1979, 30, 97. (12) Leta, D. P.; Morrison, G. H. Anal. Cbem., in press.
RECEIVED for review September 9,1979. Accepted November 15, 1979. This work was supported under a grant from the National Science Foundation No. CHE77-04405 and through the Cornel1 Materials Science Center.
