Anal. Chem. 1992, 64, 1100-1105
1100
Neutron Activation Analysis for Reference Determination of the Implantation Dose of Cobalt Ions Rainer P. H. Garten* Mar-Planck-Institut far Metallforschung, Laboratorium fur Reinststoffanalytik, Bunsen-Kirchhoff-Strasse 13, 0-4600Dortmund 1, Germany
Henning Bubert Institut fiir Spektrochemie und angewandte Spektroskopie, Bunsen-Kirchhoff-Strasse 11,D-4600Dortmund 1, Germany
Leopold Palmetshofer Institut fiir Experimentalphysik, Johannes-Kepler- Uniuersitat, A-4040 Linz, Austria
RellaMe quantlficatlon of methods for surface and depth proflllng analysis depends on the avallablllty of reference materlak for callbratbn. Slnce predskn of depth prolillng analyds procedures reaches the 1-2% level, the elemental contents In reference materials should be characterized wlth according accuracy. As an example, we prepared depth proflllng reference materlals by cobatl Ion lmplantatlon at an Ion energy of SO0 keV Into n-type rlllcon; the Implanted Co dose was monltored by Ion current measurement (ICM). The total Implanted Co Ion doses wore determlned by Instrumental neutron actlvatlon analysls (NAA) In the standard comparkon mode, withln a dynamic range of nearly 5 decades. The uncertalnty amounted to less than 1.5 % It was found that the relathre Mas was (10 f S)% of the lmplantatlon dose as measured by ICM In the dose range from 10l2 to lo1' Co lons/cm2. Sources of error (beam spreadlng, mlsallgnment) can be corrected lor In thls way. The advantages of this approach with slmllar samples of thls type Is outllned. The detect1011IWIwas 5 x loe Co 10ne/cm~.It can be hproved to lower than 10' lons/cm2 for 27 elements to be Implanted In hlgh-purity materials.
.
INTRODUCTION The continuing trend of miniaturization in technology challenges and drives the development in surface and thin-fii depth profiling analysis methods.14 The prominent methods Auger electron spectrometry (AES), X-ray photoelectron spectrometry (XPS), secondary ion mass spectrometry (SIMS),ion scattering spectrometry (ISS),and Rutherford backscattering spectrometry (RBS) are generally capable of a relative precision on the 1-2% level,1~510under favorable conditions. Reliable quantification, however, of these instrumental spectroscopic methods depends on the use of calibration samples that can be prepared with a well-defined near-surface composition and distribution by means of ion implantation. Definite absolute calibration of such materials requires determination of the total implanted ion dose density D, abbreviated as dose D (this term is more widely accepted" than the physically more exact term fluence; see ref 12, p 780), by an independent means, i.e., by precise and accurate determination of the elemental contenti This applies to the case of the so-called integration method (see refs 13,14, cf., refs
* Corresponding author. 0003-2700/92/0364-1100$03.00/0
12,15), and also to the general case of calibration of the molar fraction scale and of the corresponding sensitivity factors16 by comparison of signal intensities. To fully exploit the capabilities of the depth profiling methods, the total implanted dose should be determined at an accuracy levePJ7J8of 1-2% accordingly. Ion implantation is further used for the modification of surface layers and the preparation of special subsurface layers in materials.l9a In particular, implantation of metal ions (e.g. Co, Ta, Mo, Ti, Ni) in semiconductor devices is used in very large scale integration (VLSI) technology to form conductive silicide lines as interconnects!p21-B Buried Co silicide layers of this type have been characterized e l s e ~ h e r e . ~ ~ * ~ ~ - ~ ~ In matrices like high-purity target materials, instrumental neutron activation analysis (INAA) is a powerful methodm with high sensitivity for a number of elements that are to be quantified after their ion implantation. Basically, INAA is a very simple two-stage process consisting of (1)irradiation and (2) separated measurement. Matrix effects, especially of chemical origin, are often very low or negligible in INAA. Hence, with a number of high-purity materials, including metals, semiconductors,and ceramics, freedom from bias can be achieved by careful control of the identifiable sources of uncertainty (refs 31-34 and 35 pp 445-502), viz. (1) sample contamination, blanks, and homogeneity, (2) contamination and losses of volatile compound due to heating during irradiation, and contamination due to recoil of fission fragments from wrapping material, (3) preparation of standards and calibration procedure, (4) differences between the effective neutron flux densities that are incident onto samples and standards, (5) interfering nuclear reactions yielding the indicator nuclide from other parent elements in the sample, (6) counting geometry, dead time, and pile-up losses, attenuation of y-rays, and (7) counting statistics. For example, Table I givea a survey of the most appropriate nuclides and the relevant figures of merit for a silicon matrix. Furthermore, INAA is generally nonconsumptive and causes only minor or negligible damage to the samples considered here. Samples of very low element content (including any possible additional impurities) can be used subsequently in any other analytical procedure. Samples of moderate element contents require cooling during about 10 half-lives, regarding also activity produced by interfering reactions from sample constituents, e.g. %Na activity from primary interference reaction 27Al(n,a)aNain Al-based matrices. Samples of higher activity have to be considered definitely as nuclear waste, but those samples are needed only in small amounts due to the high sensitivity of NAA. 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
1101
Table I. Survey of Most Appropriate Nuclidesa (See Discussion: Detection Limit) for Combined Ion ImplantatiodNAA Calibration in High-Purity Silicon and Relevant Figures of Merits
element
implanted nuclide
natural relative abundance
Na Mg Al sc Mn Fe co Ni cu
Na-23 Mg-24 Al-27 sc-45 Mn-55 Fe-56 CO-59 Ni-58 CU-63
1.0 0.79 1.00 1.00 1.00 0.92 1.00 0.68 0.69
Na-24 Na-24 Na-24 SC-46 Mn-56 Mn-56 CO-60 CO-58 CU-64
15 h 15 h 15 h 84 d 2.6 h 2.6 h 5.3 y 71 d 13 h
Zn Ga Ge AS Se Br Kr Rb Sr Y Zr
Zn-64 Ga-71 Ge-76 AS-75 Se-82 Br-81 Kr-84 Rb-85 Sr-86 Y-89 Zr-94 Zr-96 Mo-98 Ru-102 Ru-104 Pd-108
0.49 0.40 0.077 1.00 0.090 0.49 0.57 0.72 0.099 1.00 0.17 0.028 0.24 0.32 0.19 0.27
Zn-65 Ga-72 Ge-77 AS-76 Br-83 Br-82 Kr-85m Rb-86 Sr-87m Y-90m Zr-95 Zr-97 Mo-99 Ru-103 Rh-105 Pd-109
244 d 14 h 11 h 26 h 2.4 h 36 h 4.5 h 19 d 2.8 h 3.2 h 65 d 17 h 66 h 40 d 36 h 13 h
Pd-110 Ag-109 Cd-114 In-115 Sn-116 Sb-123 Te-130 Xe-132 Xe-130 CS-133 La-139 Ce-140 Ce-142 Pr-141 Nd-146 Nd-150 Sm-152 Eu-151
0.12 0.48 0.29 0.96 0.14 0.43 0.35 0.27 0.041 1.00 1.00 0.88 0.11 1.00 0.17 0.056 0.27 0.48
Ir Pt
Gd-158 Tb-159 Dy-164 HO-165 Er-170 Tm-169 Yb-174 Lu-175 Lu-176 Hf-180 Ta-181 W-186 Re-187 OS-192 os-190 Ir-191 Pt-196
0.25 1.00 0.28 1.00 0.15 1.00 0.32 0.97 0.026 0.35 1.00 0.29 0.63 0.41 0.26 0.37 0.25
Ag-111 Ag-llOm Cd-115 In-115m Sn-ll7m Sb-124 5-131 Xe-133 Xe-l3lm (3-134 La-140 Ce-141 Ce-143 Pr-142 Nd-147 Pm-151 Sm-153 Eu-152ml Eu-152 Gd-159 Tb-160 Dy-165 HO-166 Er-171 Tm-170 Yb-175 Lu-176m Lu-177 Hf-181 Ta-182 W-187 Re-188 OS-193 os-191 Ir-192 Pt-197
7.5 d 250 d 53 h 4.5 h 14 d 60 d 8.0 d 5.3 d 12 d 2.1 y 1.7 d 32 d 1.4 d 19 h 11 d 28 h 46 h 9.3 h 13 Y 19 h 72 d 2.4 h 27 h 7.5 h 130 d 4.2 d 3.7 h 6.7 d 42 d 114 d 1.0 d 17 h 30 h 15 d 74 d 18 h
Au Hg Th U
Au-197 Hg-202 Th-232 u-238
1.00 0.30 1.00 0.99
Au-198 Hg-203 Pa-233 Np-239
2.7 d 47 d 27 d 2.4 d
Mo Ru Pd Ag Cd In Sn Sb Te Xe
cs
La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hf Ta W Re
os
y-radiating activation product
half-life 7
overall sensitivity of INAAb
detection limit in log ions implanted per omz this workf other authors"
0.48 0.00022d 0.00012d 3.9 0.38 0.00013d*' 0.23 0.0032d3' 0.00048 (0.58)c 0.00092 1.4 0.038e 0.89 0.00021e 1.9 0.020e 0.0074 0.021e 0.00012 0.0039 0.011 0.16 0.14 0.072 0.0014 (0.059)c 0.0026s 0.13 0.089 0.00lOd*~ 0.0029 0.56 0.093' 0.85' 3.9" 0.31 2.6 0.067 0.17 0.041 0.050 0.061 36 120 7.5 0.055 0.94 0.46' 1.5 1.3 0.17 4.5 0.08 150e 5.8 0.78 14 17 0.078 0.32 280 0.023 (0.094)c 0.24 0.29 0.35 0.28
2 w 1.4
3 w 0.03' 3.E4k
6
0.6' 404
308 50 108 408 308
3d 9'
358 0.18 240 708 1000
38 30
0.6'
50k
0.9
0.2k lj
lok 3' 0.4k
2008 20 16 138 5000 30 15 15 0.3 2 10 90 308 208 50 78 20 0.088 0.09 2008 2 158 128 8 7 0.7 2.6 28 0.68
100 12 0.013 3 0.25% 10 5 46
.w
3k 0.5k 0.7k 0.u'
0.5k 0.2k 0.03k 0.5" 2k 0.5k 0.002k
0.09 0.04k 0.05' 1,5' 3' 1.5' 0.005k 0.0004k 0.03k 0.2' 0.0~3~
0.009k 0.02k 0.0006'
0.0003i* 0.06k 0.06k
0.1k
"Boundary conditions for selection: a 2 0.1, 7 > 2 h, sensitivity s/tm 2 lo4 counts s-l per 10l2atoms. Elements prominent in ion implantation work are highlighted by boldfaced lettering. bEstimated figures of merit: sensitivities per counting interval s/tm quoted to read roughly as counts per s and 10l2implanted atoms, assuming a cooling period less than 12 h, under the experimental conditions as applied in this work. 'Numbers in parentheses referencing to y-lines which are frequently subject to interferences by other nuclides. dSensitivities for (n,p) and (n,n') reactions are based on irradiation a t an epithermal flux of 1.5 X 1Ol2 ns-l cm-l, available at channel BE20 of the same reactor FRJ-2 (cf. refs 36,37). OBased on nuclear data taken from refs 38, 39. fDetection limits as measured from blanks after a cooling period of 10 days. #Extrapolated detection limita regarding radioactive decay, assuming a cooling period of more than 15 h, by scaling from blank measurement after 4 days. hoptimum detection limits, recalculated from bulk data obtained from ultra-hieh-ouritv material (nonimdanted) given by. 'Verheijke et al.N 'B6ttger et al.41 kVerheijke et al.42
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ANALYTICAL CHEMISTRY, VOL. 84, NO. 10, MAY 15, 1992
Several Co ion implants in high-purity silicon wafers (listed in Table 111) were analyzed for the total implanted ion dose D by INAA in this work, using the standard comparison method. The results were compared with the results from initial determination by integration of the implanted ion current at the accelerator during the preparation of the samples, and from determination of the dose by X-ray fluorescence analysis (XRFA), flame atomic absorption spectrometry (FAAS), and RBS,as published r e ~ e n t 1 y . l ~ ~ ~ ~
EXPERIMENTAL SECTION
Table 11. Nominal Co Mass m of the Prepared Standards and Mean Relative Signal Intensities y To Establish the Calibration Function and RSD of the Intensities prepn probable nominal Co relative mass m (ng) error (%) 6000 2899 lo00 301 100.3
0.9 0.6 1.1
0.6
no. of replicas, n
Y
RSD (%)
4
5985 2908 990 299 100.6 50.1 19.9 5.04 1.03 0.203
0.4 0.6 3.4
6 8 5
1.2
Specimen Processing. The implantation by a 350-kV low1.4 7 2.5 current implanter has been described e1~ewhere.l~ Briefly, the 50.5 0.6 3 0.6 vacuum in the target station was about 2 X lo4 Pa during the 20.2 5 0.6 2.5 implantation. Wafers of n-type silicon (Wacker Chemitronic, 5.05 1.7 5 8.9 Burghausen/D), (100) oriented, measuring 30 X 30 mm2in size, 1.02 1.1 4 8.0 were implanted with single-charged 5sC0 ions of kinetic energy 4 3.2 11.9 0.203 of 300 keV at room temperature. To avoid channeling, an azicoupons (rods) were then wrapped in Al foil, and identification muthal angle of 14" and a polar angle of 7.5O were chosen. The (see Table I11 column 2 and ref 15) were nunberings were written on these Al foils using a hard pencil. implantation doses DICM determined by integration from ion current monitoring (ICM) After irradiation, standards were left in its Al cover and packed using an arrangement4 of 3 conventional Faraday cups." After separately in polyethene bags. Solutions were dosed to 0.5,1,2, 4,6, and 10 pL to prepare the standards listed in Table 11. implantation, the samples were cut into piece for depth Standards of about 6 pg of Co were prepared by cutting 5-mg and calibration analysis described in this paper. Reagents. Handling of samples, standards, and Al strips was pieces from standard reference material 953 (NIST, Gaithersburg, throughout done using thoroughly precleaned tweezers and cutting MD/USA) "neutron density monitor wire (cobalt in aluminum)". tools. Double distilled water from a quartz glass device, and pro Blanks on filter coupons and graphite rods were prepared by analysis (Merck, Darmstadt/D) acids and solvents were used dosing 7 pL of the HN03 used for dilution, according to the standards preparation procedure. Element contents and probable throughout the procedures. For wrapping of samples and errors of calibration samples are given in Table 11. standards, aluminum foil (high-purity Kryal OZ, Vereinigte Irradiation. The vial containing samples and standards was Aluminiumwerke, Cologne/D, Co content of 5 ng/g 4 0.3 X 10l2 Co atoms/cm2 at 24-pm thickness, U content of 50 ng/g; or, as irradiated in the research reactor FEW-2 (23 MW) at Jtilich, far as appropriate for the standards of higher Co contents, channel BE 4, at a thermal flux of (9.04 0.05) X 10l2n cme2s-l technical purity degree Al foil, Melitta, Minden/D, Co content (center of vial; epithermal flux of (7.2 0.3) X los n cm-2 s-l, nominal temperature of 90 "C) for a time duration ti, of 20 days. of 0.6 pg/g 4 16 X 10l2Co atoms/cm2 at 10-pm thickness) was Measurements. During handling of irradiated samples, the covered with clean paper and cut into strips of suitable size (ca. highest radiation dose after a cooling period of 10 days appeared 20 X 40 mm2; ca. 70 mg or 30 mg each) using a paper-cutting machine. Strips were stored in a polystyrene box until use. All with the standards of 6 pg of Co. At a working distance of 30 cm, the highest radiation dose was leas than 2 p Sv h-l, and it could fused silica flasks, pipets, and vials were cleaned by steaming for be reduced to less than 0.1 p Sv h-l by using a shielding wall of 6 h using HN03 and for 2 h using H20, dried at 105 "C. Sample Preparation prior to Analysis. Samples, nonimlead of 5-cm thickness. A high-resolution y-spectrometer was used, consisting of a planted Si blank wafers of ca.15 X 15 mm2 (8 X 8 mm2of samples coaxial intrinsic germanium detector 157 cm3 (PGT IGC 40, Co-A2 and Co-A3), and standards were wrapped into A1 foil. Princeton Gamma Tech, Wiesbaden/D), 8K multichannel anaSamples were then sandwiched between prepared standards of lyzer (ND 66, Nuclear Data, Schaumburg, IL/USA), on-line adapted Co contents and placed in a precleaned vial made of spectrum analysis and data handling system (ND neutron actihigh-purity fused silica (Suprasil of Hereaus, Hanau/D): The vation analysis package on VAX/VMS). System resolution was vial was sealed by means of an oxyhydrogen flame using a burner 1.9 keV at 1332 keV, at a peak-to-Compton ratio of 701; efficiency made of fused-silica. In this way, maximum care was taken to avoid any contamination from the sample preparation ~ t e p . ~ , ~ ' was 40% relative to a 3- X 3-in. NaJ(T1) detector. Detector Standards. The standard comparison method was used for shielding: 5 cm of lead with inner lining of Cd/Cu/Al/PE at a Co determination by NAA (see refs 35, 36). Standards were minimum distance of 17 cm from detector and sample. Standards and samples were covered by thin PE foils and were prepared from cobalt spectrometric standard solution (SRM 3113, NIST, Gaithersburg, MD/USA) 10 mg/mL, freshly diluted to either (1)pressed between two lucite sheets of 1-mm thickness 500 pg Co/mL, 50 pg Co/mL, 5 pg Co/mL, and 0.4 pg Co/mL and fiied at a distance of 150 i 0.1 mm to the detector by means in 10% HNOP All solutions were prepared by gravimetric control of a lucite mount, or (2) gently pressed to the center of the using fused silica flasks and pipets, from HN03 freshly distilled detector's end cap by means of a lucite mount. In this way the by subboiling distillation. total countrate was adjusted not to exceed 4000 counts/s. Co standards were prepared on a clean-bench (US standard All samples and standards were measured in duplicate, after 100). Filter paper coupons of 15 X 15 mm2 and 8 X 8 mm2were a cooling period of 10 days and 40 days,respectively, for measuring cut from ashless paper blue ribbon (Schleicher and Schiill, times t , from 10 min to 20 h, to obtain statistical counting errors of less than 0.2% with Co masses m I50 ng (counting error of Dassel/D). For cutting,the filters were sandwiched between clean fiiter papers. These covering papers were discarded. Filter paper less than 0.3% with m I1ng of Co and of less than 0.5% with m 5 0.2 ng of Co). The s u m of y-lines 1172 and 1332 keV emitted coupons were stored in polypropylene bottles until use. Another set of standards was prepared using graphite rods, spectroscopy from 6oCo (half-life = 5.272 y) was used for evaluation. grade, 6.15-mm diameter 1 X 1 mm (RW-1) grade, Ringsdorff, Control of Surface Contamination. To remove any posaible Bad Godesberg/D), in the same way. contamination from the surfaces (cf. ref 42) of blanks, and of All standard solutions were dosed using a microburette assamples CeL1 and CeL2 (i.e., those of low Co content),an etching sembly (Beckmann Instruments/USA) with a measured relative digestion was applied, which was adjusted to remove a layer of uncertainty of less than 0.5-1.1% in the volume range 10 to 2 pL. 1.1 0.5 pg cm-2 (44.7 nm Si) thickness from nonimplanted Si (Precisions and uncertainties are throughout quoted as 1s standard wafers. Within this thin surface layer, the relative Co amount deviations obtained by error propagation from all contributing is less than 0.3% of the total implanted Co dose D (for implansources of error.) The solutions were manually blotted to be tation profiles produced by 300 keV Co+ and at total doses below dispensed uniformly over the filter coupon area (graphite rod, ioi7 Co ions/cm2). resp), soaked up, and allowed to dry in the airstream of the clean These samples were digested, using ultrasonic agitation, in the bench under mild radiation heating up to 60 "C by a bulb. Filter following sequence: (a) degrease in toluene at 70 O C for 10 min,
* *
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
1103
Table 111. Results from Individual Implanted Samples As Measured by INAA (Column 5), Its Relative Standard Deviation RSD, and the Dose D Deduced from Measurements (Column 7)"
sample code blank A blank B blank C blank D co-L1
co-L2 CO-A~
nominal dose, DICM (Co/cm2)
mass (g)
area (cm2)
1 x 10'2 1 x 1014 1.6 X 10l6 5 x 10'6
0.2515 0.2209 0.2563 0.1884 0.2739 0.2663 0.06964 0.07447
2.091 1.836 2.131 1.566 2.277 2.214 0.5789 0.6190
Co mass by
INAA (ng)
RSD (%)
D (Co/cmz)
relative systematic deviation (%)
15. 5.1 1.4 1.3 1.2
9.8 x 109 8.91 X 10" 9.02 x 1013 1.379 X 10l6 4.645 X 10l6
-10.9b f 5 -9.8 -13.8 -7.1
Co ion dose
