514
Anal. Chem. 1980, 52, 514-519
(10) Colovos, G.; Yokoyama. A.; Freiser, H. Anal. Chem. 1975, 4 7 , 2441. (11) Spink, D. R.; Okuhara, D. N. I n t . Symp. Hydrometall.,AIM€, New York, 1972, 497. (12) Fleming, C. A., “The Kinetics and Mechanism of the Solvent Extraction of Copper by LIX64N and Kelex loo”, N . I . M . Rep. No. 1793, 1976, Johannesburg, S. Africa. (13) Hanson, C.; Whewell, R. J.; Hughes, M . A. J. Inorg. Nucl. Chem. 1975, 3 7 , 2303. (14) Fiett, D.S.; Okuhara, D. N.; Spink. D. R. J . Inorg. Nucl. Chem. 1973, 35, 2471. (15) Flett, D. S.; Hartlage, J. A.; Spink, D. R.; Okuhara, D. N. J . Inorg. Nucl. Chem. 1975, 37, 1967. (16) Carter, S. P.; Freiser, H. Anal. Chem. 1979, 57, 1100.
Capeiios, C.; Bielski, B. H. J. “Kinetic Systems”; Wiiey: New Ywk, 1972. Chou, F. C.: Freiser. H., unpublished studies at University of Arizona. Budesinski, B.: Freiser, H., unpublished studies at University of Arizona. Wiikins, R. G. “The Study of Kinetics and Mechanism of Reactions of Transition Metal Complexes”: Allyn and Bacon: New York, 1974. (21) Bobtelsky, M.; Jungries, E. J . Inorg. Nucl. Chem. 1956, 3 , 38. (17) (18) (19) (20)
RECEIVED for review July 30, 1979. Accepted December 4, 1979. This work was conducted with financial assistance from the National Science Foundation.
Ion Implanted Standards for Secondary Ion Mass Spectrometric Determination of the la-7a Group Elements in Semiconducting Matrices D. P. Leta and G. H. Morrison* Department of Chemistry, Cornell University, Ithaca, New York 14853
Through the use of ion implanted standard materials the sensitivities and detection limits of 25 of the la-7a group elements have been investigated. Methods for the determination of the implanted concentration are presented, as well as results found in the Si, GaAs, Gap, Ge, and InP matrices. Although sensitivities were found to be extremely high, the elemental detection limits are in many cases determined primarily by mass interferences arising from matrix molecular ions and vacuum contaminant species.
Intimately linked with the goal of rendering secondary ion mass spectrometry (SIMS) capable of performing quantitative analysis on the micro level has been the difficulty of finding standard materials of both known concentration and homogeneity of trace element composition ( I ) . Though several such samples are currently available (notably the NBS SRM-660 steel and RM-30 glass series), these have not formed a broad enough d a t a base with which t o test the various hypotheses concerning t h e governing factors of ion formation in the sputtering process. We now have at our disposal the means t o remedy this situation through t h e use of the technique of quantitative ion implantation. In the ion implantation process, an ion beam of almost any element can be created, mass filtered and accelerated t o substantial energies, normally between 10 and 600 keV, and then “implanted” into the near surface region of a sample material. Additionally, t h e ion beam is rastered to ensure uniformity of doping in t h e lateral directions and, more critically, t h e current delivered t o a fixed area of the sample stage is 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 coverage in atom/cm2, the use of the SIMS depth profiling capabilities to determine the depth distribution a n d t h e integrated ion signal, combined with an accurate measurement of t h e depth of analysis allows the necessary conversion to concentration in atom/cm3. I n this study we have prepared ion implanted standard materials and applied them t o the determination of ion yields of various l a 7a group elements in semiconducting matrices. 600.3 2 i u O / 8 0 / 0 3 5 2 - 0 5 i4$0 1 O G / O
- ~ _ _ _
Table I. Ion Implantation Parameters elementmass
energy, keV
Li-7
100
Be-9 B-11 c-12
100
N-14 F-19 Na- 2 3
Mg-24 Si-28 P-31 S- 32 C1-35 K-39 Ga-69 Ge-7 4
As-; 5 se-8 0 Br--81 Rb--85 In-1 1 5 Sn-120
Sb-121 Te-128 1-1 27
cs-133
fluence, atm/cmz
100 200
zoo
200 200 200
200 200
200 200 200 250 250 250
300 300 250 250 250 230
250 260 250
1.0x 2.5 x
1 0 1 4 1013
1.0 x 1015 5.0 x lo1’ 5.0 x 1015 2.0 x 1.0x 5.0 x 1.3 x 2.0 x 5.0 x 2.0 x 2.0 x 2.0 x 7.3 x 1.0 x 1.0 x 2.0 x 2.0 x
1014
10” 1014
10” 10” 1015 1014 1014 1014
1014 1015
1015
1014 1014
5.0 x 1 0 1 4 1.0 X 10’’
2.0 x 1 0 1 5 9.9 x 1014 5.0 x 10’4 2.0 x 1 0 1 4
gas inlet source or solids material probe Lie1 Be BF,
eo;
N2 PF 5 Na C1 Mg Si F 4 PF 5 H2S
NaCl KC1 GaAs Ge GaAs H:Se Br, RbCl In
Sn Sb Te CSI csI
solid solid gas gas gas gas solid solid gas gas gas, solid solid solid
solid solid gas gas solid solid solid solid solid solid solid
Semiconductors have been chosen for study because much of the applied work in SIMS today is concerned with the analysis of materials for the electronics industry ( 2 , 3 ) . In the analysis of expitaxial growth layers, elemental diffusion profiles, and ion implanted microstructures, there is a strong need to determine not only the localization of a dopant species, but also the trace elemental concentrations involved. We have, therefore, developed the methodology for the use of ion implanted standards and applied it to the determination of useful secondary ion yields, sensitivities, and detection limits for 25 la-7a group elements in Si, GaAs, G a p , Ge, and I n P single crystal matrices. I t has been found that along with a species’ tendency to ionize in the sputtering process, a major determinant of its detection limit is the presence of mass spectral interferences arising from matrix molecular ion species and E 1980 Americar, incrfliial Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
from vacuum contaminant difficulties. EXPERIMENTAL Instrumentation. The ion implantation was accomplished using an ACCELERATORS INC. ion implanter model 300R. All of the implants were done using a hot filament ion source which was additionally equipped with a hollow molybdenum probe for the evaporation of solids directly in the filament chamber. The system was capable of accelerating ions to well-controlled energies up to 300 keV with unit mass resolution. The elements implanted, their energies, fluences (amount of surface ion flux), and source materials are presented in Table I. The single crystal samples were tilted at an angle of 7' to the incoming ion beam to prevent any substantial amount of channeling. An oscillographic display system was used to monitor the uniformity of the surface coverage during the implantation process, with the implantation ion bean1 being rastered over an aperture which provided for an implantation area of 28.1 cm2 on the sample stage. The implant current was monitored using the entire sample stage as a Faraday cup which was shielded from charged particle emission, and the incoming beam shuttered when a preset amount of total ion dose was reached. The SIMS analysis was performed using a CAMECA IMS-300 ion microanalyzer which has been previously described ( 4 ) . The size of the contrast aperture contained in the immersion lens was 200 pm, and the electrostatic analyzer sector was used with fully opened slits for maximum sensitivity. The instrument was interfaced with a DEC PDP 11/20 computer and a Nuclide D/A converter. Peripheral devices included a 1.2-M word cartridge disk used for the RT-11 operating system and for data storage, a GT-40 graphics display system, a Versatec high speed printer/plotter, and a Houston Instruments Complot x-y plotter for hard copy graphics. Computer Software. Analysis was performed under computer control using FORand ASSEMBLER level programming. During the analysis only the element of interest was monitored to ensure a sufficient number of data points in the implanted region, and the points were collected using 1-s integrations while staying on the relatively flat topped peaks. The depth profile data were stored on disk and later integrated and analyzed using FORTRAN level programs for the determination of implant concentration, background level, and elemental sensitivities. Sample Preparation. The semiconductor material used for this study was all electronic-grade bulk grown single crystals. All samples were cut in the (100) orientation with the exception of Ge which was (111) faced. Prior to ion implantation, the samples were cut into small pieces (approximately 1 cm2) and chemically/mechanically polished to a mirror-like surface. Before both ion implantation and SIMS analysis, all samples were rinsed with acetone followed by isopropanol to remove any traces of dust or adhering particulate matter. They were mounted in sets (one piece of each substrate) on 2-inch aluminum disks for implantation, and later removed and individually remounted on 0.5-inch aluminum disks for SIMS analysis. Procedure. The basic experimental parameters for the SIMS analysis are presented in Table 11. Following the ion implantation of a single dopant element, the samples were depth profiled using a positive oxygen primary ion beam. Since only one element was profiled during the actual analysis, the primary ion beam stability was monitored by determining the current both before and after each depth profile, and any cases where more than a 5% variation was found were rejected. The signal levels for the matrix ions were measured on each sample for each depth profile by sputter cleaning an adjacent region for a period of at least 5 min and then measuring an average cps for a 10-s interval. In cases in which the matrix ion signal was above the dynamic range of the pulse counting detector system, the signal strength was measured using a smaller acceptance area on the sample (40-pm diameter field of view) and later corrected for the reduced area of collection. Such a system of signal reduction was also used for several of the elemental analyses in which too much signal was received when monitoring the larger area chosen as the standard condition (see Table 11). Depth profiles of the species of interest were performed for both the positive and negative secondary atomic ions as well as for several negative molecular ions which were obsemed and found
515
_ ~ _ _ _ _ ~
.____.____~
Table 11. SIMS Operating Parameters instrument: CAMECA IMS-300 ion microanalyzer primary ion beam species: O!', 0' primary ion current: 1.0 p A mass resolution: 1 5 0 detection system: ion/electron multiplier pulse counting positive s c ~ nd o ar y ions
rastered crater size:
700 x 700
sampling area:
223-pm diameter circle 5.5 keV 3:3" from surf ace
primary ion energy: incidence angle: average sputter rates: Si Ga As GaP Ge InP
1-1.1 A / S 19.1 A / S 19.9 \ i s 113.9 A / S 2'1.5 11s
negative secondary ions 580 x 5 8 0 pm 190-pm diameter circle 11.5 k e V s- r i o from
surface 8.9 \ i s 22.4 j\ i s 23.1 > \ / s 26.9 A I S 30.6 i / s
to have sensitivities or detection limits better than those of the atomic ions. Following the SIMS analysis, the depth of each sputtered crater was measured using i~TALYSTEP stylus type surface profiler. The TALYSTEP is capable of less than 100-A depth measurements and was shown to have an accuracy of better than 5% when calibrated with depth standards. AH of the analysis craters were between 0.4 and 2.0 pm deep. RESULTS AND DISCUSSION Ion I m p l a n t e d Standards. Prior t o implanting samples to be used as S I M S standard materials, it is necessary t o decide both the energy and the fluence to be used. For the determination of implant energy, the primary consideration is that the chosen energy be sufficient to assure t h a t the near-gaussian depth distribution of the ion implant is essentially subsurface, Le., that the following tail of the distribution does not extend to the surface of the sample to any large degree. Additionally, it is not desirable to implant an element so deeply that the SIMS analysis (depth profile) takes an excessive amount of time. We have used energies in the 100to 300-keV range to satisfy both of these requirements. Since the penetration depth is a function of the implant energy, the mass of the implanted species, the substrate material, and the incidence angle, the general scheme was used in which the lighter elements were ion implanted a t 100 keV (below mass 12), and the heavier elements (up to mass 133) with 200- t o 300-keV energies. T o decide the fluence to be implanted, as ii general ruleof-thumb, it may be assumed that the maximum concentration (in atom/cm3) of the implanted species will be approximately 105 times the implant fluence (in atorn/cm'). This assumption proved to be legitimate within one order-of-magnitude for the energies used. For a more accurate approximation of the maximum concentration to be obtained, the theoretical range calculations of Lindhard, Scharff, and Schiott ( 5 ) (the LSS theory) may be used t o determine the standard deviation of the Gaussian distribution, u, and then applying the relationship t h a t E , the maximum implanted concentration (in atom/cm3), is approximately equal to the implanted fluence (in atom/cm2) divided by 2.5 u (in cm). Such calculations are generally accurate t o within about 30%. An upper limit of the concentrations which may be obtained by ion implantation is imposed by the sputter removal of the sample surface during implantation. If a large amount of sputtering occurs, some of the implanted dose will be lost; however, if the depth distribution of the implant does not appreciably extend to the sample surface, no major error is incurred at fluences less than 10l6atorn/cm2. A comprehensive
516
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
'! ,implant
-I
Table 111. Average Detected hiatrix I o n Yields (Used €or Normalization)
Profile
( Integral =
positive secondary ions
I)
ion-
substrate mass
1 -
a
Si GaAs
z l 2 1
"I
As'-75
,Hypothetical
.___......._.._._ /
I
i
'\
.............
C p s h e - Che
Equivalent - .- .- ...- .
\
Figure 1. Simulated SIMS depth profile of an ion implanted standard material showing the pertinent parameters. CPSk and Cb are the hypothetical equivalent of the integrated secondary ion signal and the average concentration of the implanted species in the profiled volume, respectively
theoretical treatment of the subject has been presented by Gries (6). After the samples have been ion implanted, there are several methods, with varying degrees of accuracy, with which t o determine t h e actual maximum concentration. T h e first method is t o assume that the previous theoretical calculations are correct a n d t h a t the distribution is actually a Gaussian, a n d t h a t no distortion of t h e implant profile such as channeling during implantation has occurred. T h e second level of approximation is to use the Gaussian assumption, but apply the SIMS depth profiling capabilities and a measurement of the analysis depth t o experimentally determine the standard deviation of the distribution. At a third level, the element of interest is depth profiled and the distribution obtained fitted t o a mathematical function such as t h e Pearson IV distribution (7). Such a method is quite accurate and only channeling effects are not corrected for. A fourth method, a n d t h e one used for this study, is t h e integration of the secondary ion signal derived from the implanted species and, using this integral, as well as a measurement of the analysis depth, t o determine t h e concentration of the implant. T h e assumptions inherent in this method of calculation are t h a t t h e S I M S ion intensities are linear with concentration (presumably valid at low concentrations), and t h a t an accurate measurement of t h e analysis crater is possible, i.e., t h a t a smooth bottomed crater is obtained. I t is also assumed t h a t t h e sputtering rate is constant throughout the analysis. T h e pertinent parameters for this method of calculation are presented in Figure 1. With the aim of calculating the peak concentration of t h e implanted standard in mind, it is inconvenient to perform calculations which depend on t h e exact shape of t h e depth distribution of the implant. This distribution is normally very close to a Gaussian function but may be somewhat distorted owing to the effects of channeling which can occur during implantation. Therefore, with the assumption of linearity of SIMS signal with concentration, t h e implanted quantity may be considered as hypothetical equivalent with an integrated ion signal which is equal to that of t h e actual implant distribution. If t h e hypothetical equivalent is considered to have a concentration which is homogeneous with depth and which ends abruptly a t a depth, D , which is defined as t h e measured depth of the analysis crater (in cm), it may be seen t h a t its concentration is (in atom/cm3) equal to t h e ion implanted fluence F (in atom/ cm2), divided by D. Therefore, the maximum implanted concentration
n- = -F . CPS,,,
D
st *
-
I
mass
cps
8 500 OOOn 18750000a 3 i 0 000 2 0 900 OOOa 318 000
Si--28 Ga -69
3 340 000' 3 800 400 000 1 4 100 2 450 000" 190000 2 9 600 1 2 4 0 000'
Ga+-69
Ge InP
P+-31 1 30O00Oa Ge'-74 I n + - l 1 5 17 000 OOOa P+-31 370 000
S+*D
ion yield,
CPS
GaP
... ....
T I M E .;DEPTH
Si--28 Ga'-69
ion yield,
negative secondary ions
____ ion-
Xs--75
Ga--69 P--31 Ge--'il In--ll5 P--31
" To avoid exceeding t h e instrumental d y n a m i c range,
these data were collected using a reduced signal acceptance area a n d t h e n normalized t o standard conditions. -_-__
-~
where n, F, and D have been previously defined, CPS,,, is the secondary ion intensity obtained a t the peak of the implant distribution (in counts per second), S , is the analysis time (in seconds), and I is the integrated ion signal arising from the implanted quantity. I t should be noted t h a t the term S,/I is the inverse of the average counts per second of the hypothetical equivalent. Since the accuracy of the implant fluence is better than 5 7 0 , the accuracy of such a calculation is dependent primarily on the depth measurement (assuming the SIMS depth profile represents an accurate picture of the actual concentration distribution of the trace element in the sample). I t is therefore our belief t h a t the concentrations found by such a method are accurate to better t h a n 15% a t a conservative estimate. Additionally, such a method of calculation is widely applicable t o many types of implanted materials because of its independence from t h e shape of the implanted distribution. In cases where extreme channeling or post implantation annealing have created "distorted" concentration distributions (differing from a near-Gaussian function), such a system of calculation will remain valid for the determination of concentrations a t any point in t h e distribution. In addition to the fact that ion implantation standards may be tailor-made for specific applications, they have several unique properties as compared to standard materials made by other processes such as alloying or elemental incorporation during single crystal growth. Because of the rastering of t h e ion implantation beam, the elemental distributions obtained are laterally homogeneous in homogeneous substrates. Since the implant profiles are subsurface in nature, assuming a high enough energy is employed, they are relatively free from surface contamination problems. The near surface placement of the known quantity is ideal for use as SIMS (or sputtering Auger) standard materials. The concentrations obtained are both accurate a n d controllable, within the limits previously discussed. I t is possible t o select a single isotope for implantation. Additionally, the instrumentation for ion implantation has advanced t o the point where most elements may be implanted into almost any material (8). A final advantage of ion implanted standards which merits individual attention is t h a t the implanted quantity may normally be distinguished from secondary ion signals arising from mass spectral interferences or from residual concentrations of t h e element of interest in t h e sample. Such a situation is shown in Figure 2. Here, sulfur a t mass 32 has been depth profiled using an oxygen primary ion beam. I t can be seen t h a t the ion signal arising from t h e implanted sulfur may be easily separated from t h a t ion signal derived mass from both the surface oxide of the I n P as well as the 02+
ANALYTICAL CHEMISTRY, VOL. 5 2 ,
NO.3,
MARCH 1980
517
Table IV. Ionization Probabilities, Sensitivities, and Detection Limits for Positive Secondary Ions element- s u b mass strate LI-7
Be-9
B-11
c-12
F-19
Na-23
Si GaAs GaP Ge InP Si GaAs GaP Ge InP Si GaAs GaP Ge InP Si GaAs GaP Ge InP Si GaAs GaP Ge InP Si GaAs GaP Ge
InP Mg-2 4
Si
Si-28
GaAs GaP Ge InP GaAs GaP Ge InP GaAs
P-3 1
Ge Si
T+
8 x
7x
sensiback- detection tivitv, ground, limit, at m s / i m 3 cps atmsicm3 2 2
x x
10'3 1013
4 X
3 x 1013
2x 3x 7x
8
5x 1 X 10.'
2x 1x 5X 1 X 10.' 1x 2 x 9 x lo-' 1x
6x 8
x lo-'
9 x lo-' 2x 3x 1 x lo-* 1x 8 X
3x
1x IO-' 2 x
9 x lo-) 1 x 10.' 5x 5 x low4
3x 3x 3 x
3 x lo-"
2x 2x
5 x 1x 5 x lo-' 3 x 2 x lo-'
x 10''
4 x 1013 2 x 1015 2 x 10'6
1 x loLh 1 x 10'5
'i 2
x
1 0 1 5
x 10'6 7 x 10'6 7 x 10'6 4 x 10'6 7 x 10'6 2x
2 x 2 x
