Microanalysis of Flourine by nuclear reactions

NUCLEAR

INSTRUMENTS

AND

METHODS

168 ( 1 9 8 0 ) 9 3 - 1 0 3 ;

(~) N O R T H - H O L L A N D

PUBLISHING

CO.

MICROANALYSIS OF FLUORINE BY NUCLEAR REACTIONS*

I.

19F(p, ~0)160 and 19F(p, ~ 7 )160 reactions

D. DIEUMEGARD Thomson-C.S.F., Laboratoire Central de Recherche, Domaine de Corbeville, 91401 Orsay, France B. MAUREL and G. AMSEL Groupe de Physique des Solides de I'Ecole Normale Supdrieure, Tour 23, 2 Place Jussieu, 75005 Paris, France

The quantitative determination and depth profiling of fluorine in the surface region of various substrates using nuclear reaction microanalysis was studied. Differential cross sections and excitation functions for the reaction 19F(p, ~0)160 and 19F(p, 0:y)160 were investigated in great detail, especially in the vicinity of resonances, where automatic energy scanning was used. Resonance widths and positions were measured precisely, in particular for the narrow resonances of 19F(p, ~,)160. A very narrow, F = 150_+50 eV, resonance was found at 1088 keV. The excitation curves shown for various resonances allow one to carry out the computer calculations required for converting yield curves into depth profiles. The paper discusses how to choose among the nuclear reactions and resonances available to achieve best sensitivity, selectivity, depth resolution, ease of operation or speed of measurements, according to the problem considered, and to define the corresponding optimal experimental conditions. It appears that, in addition to the strong 872 keV, F = 4.2 keV resonance of 19F(p, o:7)160, the 340 keV, F = 2.4 keV resonance may be especially useful. The results are also of interest for hydrogen depth profiling when using the reverse reaction 1H09F, 77)160.

1. Introduction Microanalysis by nuclear reactions applied to 19F (the only isotope of fluorine) has received increasing attention during the past decade, since the pioneer work of M611er and Starfeltl). This technique has been used in our laboratory for many years, and a large amount of data has been accumulated on cross sections and excitation curves for nuclear reactions induced on 19F by proton and deuteron beams in the 300 keV to 2000 keV range. It is the aim of these papers to present in great detail yield curves for various emitted particles, especially in the vicinity of resonances, and to define criteria for choosing among the many nuclear reactions and resonances available to achieve best sensitivity, selectivity, depth resolution, ease of operation or speed of measurements, according to the problem considered, and to define the corresponding optimal experimental conditions. In the first paper the 19F(p, 0~)160 reactions are considered. These are the mo~t convenient when the determination of 19F alone is of interest. When other light nuclei like 160, 14N, etc. also require measurement it might be advantageous to resort to 19F + d reactions, which may allow the results to be obtained simultaneously. In this case, however, * Work supported D.G.R.S.T.

by

C.N.RS.

(R.C.P.

No.

157)

and

complex spectra arise and the ensuing overlap problems must be considered carefully. 19F + d reactions will be studied in the second paper. Cross section curves for 19F(p, oe0)160 and 19F(p, oc~')160 have been studied by several authors. The extensive data found in the last compilations of Ajzenberg-Selove are valuable for nuclear physics2). For nuclear micro analysis the precise shape of the cross section tr(E), more than its absolute value, is of central interest, as the usual practice is to use reference standards for converting countings to absolute quantities of nuclei per cm 2. Yield curves for 19F(p,¢(.0)160 w e r e studied among others by Lerner and Marion 3) who produced absolute cross sections and by Golicheff et al.4). Yield curves for 19F(p, ot'~,')160 w e r e investigated among others by Stroobants et al. 5) and Chandri et al. 6) who measured thick target yields, and by Golicheff and Engelmann 7) and more recently by Jarjis s) who recorded a(E). 19F(p,p'7)19F was studied for instance in ref. 5. However this reaction yields low energy gamma rays and its use is less general than that of 19F(p, o~7)160; it will not be considered in this paper. Some applications of these methods are cited. In metallurgy, M611er and Starfelt 1) studied fluorine contamination of zircaloy undergoing various heat treatments, as did Jarjis 8) who worked on magnesium alloy scales. The mechanisms of surface 11. CROSS S E C T I O N S

94

D. D I E U M E G A R D et al.

contamination by fluorine due to various polishing or oxidation procedures were studied by Maurel et al. 9) for tantalum, Croset and Dieumegard l°) for silicon, and by Golicheff and EngelmannT). Croset and Dieumegard l~) used nuclear reaction analysis to calibrate SIMS .measurements in SiO2. An interesting application in the field of medicine was the study of human dental enamel, particularly in vivo measurements by the group of Namur 5'12) and Lyon~3). An application to dating methods in archeology by Taylor ~4) is most exciting. Finally it should be noted that a precise knowledge of the 19F(p, a~?')160 resonant cross sections is also of prime importance for hydrogen profiling, using the reverse reaction 1H(19F, ~zy)160. A large part of this work was the subject of the thesis of Dieumegard~S); some data were reported in ref. 16.

inch NaI(TI) detector at a laboratory angle of 90°, at 7 cm from the beam spot, the targets being irradiated normally. No attempt was made to determine absolute cross sections for 7-ray production: all our results are given in counts for this detection geometry. All pulses corresponding to 7-ray energies above ET=4.7MeV were registered; in this way natural radioactivity background was very low. Detection efficiency could be increased in other experiments by lowering this threshold. Various targets were used, according to the experimental requirements. For ~ detection LiF targets were prepared, as the 7Li(p, c~)4He reaction was studied simultaneously. For 7-ray detection CaF2 targets were used, as according to Butler 19) the contribution of 4°Ca(p, 7) is negligible with respect to that of ~9F(p, o~7). Both types of targets were produced by vacuum deposition• LiF was deposited on pure tantalum and silicon backings. 2. Experimental The latter were used above energies Ep-1.6 MeV, The measurements were carried out using the to lower the energy and flux of the backscattered 2 MeV Van de Graaff nuclear microanalysis facility protons, which could not be completely stopped in of the Ecole Normale Sup6rieure described in the absorber. CaF2 targets were deposited on pure ref. 17. Beam currents were around 1 ~A, on a tantalum. The uniformity of the deposits was 1 mm 2 spot. Energy spread of the incoming parti- checked with the 19F(p,o:'0) reaction by careful •beam scanning: the spread was better than _+2%. cles was better than i70 eV fwhm at 1 MeV. One of the main features of our results is the Impact points were changed frequently, so as to very great detail of the excitation curves; this was eliminate carbon build-up effects. The stability of obtained using a special automatic energy scanning the targets under a 1/~A proton beam on 1 mm 2 system devised in our laboratory, the details of was checked similarly and found to be better than which will be published soon~8). This system based 1% over one hour. on electrostatic principles is strictly hysteresis-free, K~, ~0 ~ 16 , ~ E62o m 19 very stable and linear; its scanning range is ~?- ~. . . . . . . . ~ F (p,%) 0 ....... 7," j OLAS =150o ; _+60 keV. Energy calibration could thus be obtained with high precision for those resonances which are close enough to well known resonances; we used those from 27Al(p, y) as references. For chimers some resonances interpolation was used between the closest reference energies, using magnetic field measurements with a VARIAN FR-40 Hall effect gaussmeter. The detailed excitation curves were obtained directly as multichannel spectra, with fixed channel widths calibrated in eV. ... ". Alpha-particle detection was with an OR'. ,,..~-*" TEC 3 cm 2, 100/~m depletion zone surface barrier 0.~""'0'6 0.~ 6 s "~9 1.0 u 1.2 1.3 1.4 is t6 t7 is ~.9 2.0 detector, fitted with a 16 mm diam. diaphragm at E Mev 50ram from the beam impact point (0.08sr). Fig. 1. a(E, 150 ° ) for the ~z 0 group from 0.5 to 2MeV. The Various thicknesses of Mylar absorber were inserted in front of the detector, according to the proton counts at the 1.35 MeV resonance m a x i m u m were 68 000 for 250~C. The insert shows a detailed scan of the 760 keV to energy, to stop the backscattered protons~7), while 860 keV relatively weak resonant structure, recorded with a letting through the desired u-particles (see below). target containing 9.3btg/cm 2 F (3keV thick at 800keV); 250/_zC/channel; 1000 counts are equivalent to 0.047 mb/sr. Gamma-ray detection was with a 3 " × 3 "

x.\~

,,/vV

MICROANALYSIS

Targets were absolutely calibrated through a thick CaF2 reference standard. Its 19F content was estimated in 3 ways, by weighing chemical analysis and 7-activation analysis (see ref. 15). The three methods gave the same results, with an overall estimated precision of _+6%. Better precision was not aimed for, as the shape of the excitation curves was of principal interest. The thinner targets were calibrated in relative terms, using the 19F(p,~z0) reaction. Target thicknesses rated in terms of energy loss were calculated using the data of ref. 20. For very thin targets these calculated thicknesses serve only as an indication, since microscopic non-uniformities may increase their effective thicknesses; the latter may be extracted from the analysis of very sharp resonance excitation curves recorded with these targets, as will be shown below. 3. Results 3.1. ' 9 F ( p , o~0)160 This reaction has a Q value of 8.119 MeV yielding typically an ~z group with E ~ = 6.93 MeV at a laboratory angle 0 = 150 °, at Ep= 1.26 MeV. The only nucleus which produces ~z-particles with higher energy is 7Li, with Q = 1 7 . 3 4 7 MeV, while the next below is ~ I B , Q = 8.582 MeV, yielding E~= 5.68 MeV. Thus the ~z's from 19F are very well separated: their detection is practically background free, even for thick targets. Figure 1 shows the excitation curve for the ~z group at 0 = 150 °. LiF targets on tantalum containing (17_+1) x 1016 a t o m s / c m 2 F ( 5 . 3 / l m / c m 2 F, 1.5 keV thick a t E p = 1 MeV) were used for Ep_< 1.6 MeV and above CaF2 targets on silicon

95

OF F L U O R I N E

containing (11.4±0.7) × 10 •6 a t o m s / c m 2 F (3.6 # g / cm 2 F, 1.3 keV thick at Ep = 1 MeV). Mylar absorber thicknesses ranged from 25 # m to 44 ~m. Energy steps were 2.5 or 10 keV according to regions, with 250 # C per point. W h e n changing targets the yield curves were joined by overlapping in the vicinity of peaks. The cross section scale was set with a calibrated target at the 1.35 MeV resonance. Absolute cross sections are to within ± 8 % , the target 19F content dominating the errors. It appears that the cross section below 700 keV is very small. No real plateau appears in this curve but the region from 1100 keV to 1260 keV approximates one best. A very strong and relatively narrow peak appears at 1.35 MeV as shown below in great detail (see fig. 10). The observed resonance peaks, neither very narrow nor isolated, are listed in table 1. Figure 2 and table 1 show similar results at 0 = 9 0 °. The cross sections are smaller, except around 1700 keV. Some peaks are markedly shifted, indicating strong nuclear level interference effects. The cross section of the 1.35 MeV resonance is found to be 0.98___0.08 mb/sr, an excellent agreement. A stationary region exists at this angle between 1430 keV and 1550 keV. The narrow resonance at 777 keV appears narrower and more symmetrical than at 0 = 150 °. --

I

•

r

T

1

T~--I

F I g ( p . ~ ) O 16 I

TABLE 1

@'led = 9 0 0

oL

..

F 19(p,(z O) 016

° "

OLAB = 9 0 "

. . . . . E k~

Resonance peaks from 19F(p, %)160.

7~ Energies (keY) 0 = 150 ° 0 = 90 °

777_+ 3 777±2 840+2 843_+ 3 1347+3 1354_+ 3 1652-+5 1713+_ 6 1842_+7 1901 _+ 10

Widths (keY)

~r at the maxima (mb/sr)

7+1 10_+1 32_+2 17.5-+1 36_+1 36_+1 90-+5 72+2 122_+5

0.075 0.17 0.22 0.13 3.2 1.05 1.4 2.9 3.2 1.6

E

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b

° °

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i 09

i

i 11

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i

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13

i 1.5

i

i 1.7

i

i

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19

E MeV

Fig. 2. a(E, 90 °) for the % group from 0.7 to 1.95 MeV. Same conditions as in fig. 1. The insert shows a blow-up of the yield curve in the 700 keV to 900 keV region. 11. C R O S S S E C T I O N S

96

D. D I E U M E G A R D

The target thicknesses in the above measurements were sufficiently small to insure that the cross section curves in figs. 1 and 2 were not deformed by target thickness effects. 3.2.

]9F(p, &'7)160

This reaction has very high resonant cross sections but as the excited states of ~60 are above 6.06 MeV the corresponding at's have a too low energy to be detected easily. The annihilation 7's from the ~z= decay were not detected; this reaction has however relatively low cross sections, like the ]9F(p, y)2°Ne reaction2). Thus the y-rays of interest pertain to the ~z], ~z2, ~z3 groups, with energies 6.13, 6.92 and 7.12 MeV. There is no need to use Ge-Li detectors, except when high resolution is required to discriminate against y-rays from other low Z nuclei in the target; this however entails a large loss of counting efficiency. Figure 3 shows an overall view of the excitation curve from 0.3 MeV to 2 MeV at 0 = 90°. Various target thicknesses were used according to the width of the resonances under study. The counts in fig. 3 are normalized to the CaF2 target containing 3.8/,tg/cm 2 F (1.2 keV thick at Ep = 1 MeV) used for 0.7 MeV
et al.

was recorded by manually varying the accelerator energy. Figure 4 shows a typical absolute energy calibration, for the 1371 keV resonance, recording- in turn the y's from 19F(p, ~y) and 27Al(p, 7) for which the well known resonances at 1363.72__0.07 keV ( F = 7 0 e V ) ; 1381.3_+0.3keV ( F = 6 4 0 e V ) and 1388.4+0.3 keV ( F = 550 eV) 21) are seen (thick target yield). In this experiment the electrostatic energy scanning system was used, which, being hysteresis-free, provides a constant energy scale in successive runs. The results for the 19F(p, g7)160 reactions are summarized in table 2. Excitation curves reflect the shape of ty(E) only if the maximum energy loss in the thin target is much smaller than the width F of the resonance considered. For narrow widths this creates a difficult problem as reducing the thickness of the target rated in keV (i.e. the energy loss calculated assuming a uniform film containing a given number of atoms/cm 2) also reduces the counting rate. For very sharp resonances, this becomes too small to obtain precise measurements. Moreover the effective thickness of very thin targets may be much larger than their rated thickness, as thin deposits (in the 100 A range), are not uniform, the atoms having a marked tendency to form aggregates when deposited under vacuum. The production of uniform thin targets is the main problem in narrow resonance measurements. Even so, for uniform thin E 1360

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Fig. 3. Overall view of the 19F(p, ~y)160 excitation curve at 0 = 90 °, normalised to a 3.8 # g / c m 2 F targeL 60 uC per point; Y ' x 3" NaI(TI) detector at 7 cm : detected E~ >~4.7 MeV.

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20 410 CHANNELS

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Fig. 4. Example of an energy calibration, using the electrostatic scanning device, of a 19F(p, 0r7)160 resonance v.s. well known 27Al(p,y) resonances. Targets: CaF2, 3.4,zg/cm 2 F (15/aC/ channel); thick aluminum deposit on tantalum (50/~C/channel).

M I C R O A N A L Y S I S OF F L U O R I N E

97

TABLE 2 19F(p, :x7) resonances at 01ab = 90°; Ey_>4.7 MeV.

Results of literature (ref. 2) Resonance Resonance energies widths (keV) (keV)

340.46_+0.04 483.8 _ + 0 . 3 596.8 _+1.0 671.6 -+0.7 834.8 _ + 0 . 9 872.11_+0.20 902.3 _+0.9 935.4 +1.3 1090 -+1.0 1140. +1.0 1283 _ + 1 . 4 1347.7 _+1.0 1373.0 --1.0 1607 -+1.6 1694 ±1.7 1949 +2.5 2030 _+3.0

2.4+ 0.2 0.9_+0.1 30.0_+ 3 6.0_+ 0.7 6.5_+1.0 4.7_+ 0.2 5.1_+ 1.0 8.1_+ 0.5 0.7_+ 0.3 2.5 18.6_+ 1.0 4.9_+ 0.7 12.4_+ 1.0 6.0_+ 1.0 35-+ 3 40_+10 120_+20

Resonance energies in keV measured by Electrostatic Magnetic calibration calibration

Present results Measured widths in keV of yield peaks registered with CaF 2 targets of thickness thick 1.1/gg 19F/cm2 3.4/zg 19F/cm2 >25 keV c 400 eV a 1200 eVa 2.5 _+0.2 0.8 _+0.1

483.7_+0.5

2.9-+0.2 1.5_+0.2 25-+3 6.7-+0.3

594 _+3 667.5_+2 832.l_+1 872.1_+0.5 989.8_+1 933.6_+1 1087.7-+1 1135.6-+1

5.6___0.5 4.2-+0.4 3.5_+0.4 8.0-+0.4 0.15_+0.05

0.7_+0.1 3.1-+0.2 15.6-+0.8 4.9-+0.4 11.4-+0.4 4.5-+0.3 23 ___2

1280 _+1 1344.5_+1 1370.9-+0.8 1603 1690 1935 2014

_+2 _+2 -+3 _+6

Relative intensity

(%) b

11.9 (4.6) 1.4 7.7 2.5 100 3.8 38.6 (2.6) 4.9 4.6 30.4 77.4 5.5 17.1

a Rated thickness at 1 MeV. b As recorded with the targets reported in the adjacent columns. Brackets indicate resonances for which the target thickness effects strongly decreased the peak maximum to a(E) maximum ratio (see text). c Widths extracted from thick target data, yielding the true resonance width F for very narrow resonances.

targets beam energy straggling has a major effect in deforming the recorded resonance shape. In fact, as the energy loss spread mean square deviation s~ varies with depth x as x/x while the mean energy loss AE varies as x, for low values of x, sE > AE. Thus it is impossible to measure the shape of very sharp resonances with a thin target, the m a x i m u m energy loss being in practice always larger than the resonance width F, due either to target non-uniformity or to statistical fluctuations of the energy loss, or both. Consequently, the procedure used here was as follows. In a survey, corresponding to fig. 3, the narrowest peaks were picked out and were further studied by detailed energy scanning using the 1.1 lzg/cm 2 F target with an energy rated thickness of 4 0 0 e V at 1 MeV and a geometric thickness below this value would not produce further reduction of the effective thickness. The narrowest peak found in the 0.3 to 2 MeV range, at 1088 keV, is shown in fig. 5(a) with the neighbouring 1136 keV narrow resonance and in great detail in fig. 5(b). The measured width is 700 eV whereas the rated target thickness is only 370 eV at the resonance.

The marked asymmetry towards high energies suggests a broadening of statistical origin. This resonance was hence further investigated with a thick target. In fact the only way to measure very thin resonances consists of using the thick target method. The excitation curve N®(E) is recorded here with a target of thickness x>> F; if the statistical fluctuations of the beam particle slowing-down process are not taken into account it is clear that N=(E) is the integral of the ideal excitation curve No(E) corresponding to an infinitely thin target. In other terms: Woo ( E )

~

f;

- 4E(x)

No (E) dE,

(1)

where AE(x) is the thickness in energy units of the target. Now No(E) is the convolution of a(E), the true resonance shape with the energy spread distribution f(E) of the reacting particles as seen by the 19F atoms, which is itself the convolution of the mean energy spread distribution fb(E) and of the gaussian Doppler broadening distribution fo(E) due to the thermal agitation of the atoms. Hence the lI. CROSS SECTIONS

98

D. D I E U M E G A R D

derivative near the low energy edge of the step of

No(E) gives: dN~ (E) dE

~r(E) • [ L ( E ) * fD(E)] •

(2)

In our case fb(E) was Measured to be nearly gaussian with width 170 eV fwhm. The estimated overall beam contribution is a gaussian of width 200 eV fwhm. The shape of a narrow isolated resonance is taken as a Breit-Wigner function, i.e., a ( E ) ,-, [ ( ½ F ) 2 +

(E-ER)2]-~,

(3)

where ER is the resonance energy. The true a(E) may thus be deduced from N=(E) without target thickness effects, using eq. (2). In reality the statistical nature of the slowing down process induces an effect, the Lewis effect, which produces an overshoot of the step in N=(E) at ER, at least for resonances with F smaller than ~ 5 0 0 eV. The above calculations and the Lewis effect will be considered in detail in a forthcoming paper22). E KeY 1105 1125

1085 19

1145

15

F ( p,c,~)O 15

768

e V / / c h o , n he[

%

O

E) O O

Q

O

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et al.

Fig. 5(c) shows the high resolution excitation curve corresponding to the 1088 keV resonance recorded with a CaF2 target of thickness x > 2 5 keV. After subtraction of the continuum clue to a nonzero off-resonance cross section (see fig. 5(a)) a step shaped curve is obtained, exhibiting a marked Lewis effect. This curve was fitted with curves calculated from equations 1 to 3 using the above parameters forfb(E)*ft~(E) and letting F be a free parameter. The best fit, displayed in fig. 5(c) yields F = ( 1 5 0 _ 50) eV. This simplified procedure is incorrect since the Lewis effect is ignored and the fit does not reproduce the overshoot. A more precise calculation, as outlined in ref. 22, would however not give a result very far from that obtained with this method. It thus appears that there is a very narrow, although rather weak, resonance in the 19F(p, ~7)]60 reactions at 1088 keV. The very narrow nature of this resonance does not seem to have been reported in the literature (see table 2). This resonance, which may be used for very high resolution depth profiling of 19F (or IH), allowed us to estimate the effective thickness of our 1.1/2g/cm 2 F target: the width in fig. 5(b) is effectively due to the t a r g e t thickness, which is ~700 eV. Effective thicknesses at other energies were calculated by scaling this value with the dE/dw of protons in CaF2.

g

z

1084

1088

1086

19F(p, ~'~)160

E •

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0 I I T 50 100 CHANNELS Fig. 5. Scans of the region of the very sharp resonance at 1088 keV. (a) target 3.6/,tg/cm 2 F (1.2 keV thick at 1100 keV); 50~tC/channel. (b) detailed scan of the 1088keV resonance: target 1.1 Fig/cruZ F (370eV rated thickness at l l00keV); 100/~C/channel. (c) High resolution thick target yield curve of the 1088 keV resonance: target 5000 A CaF2 ( - 2 5 keV thick); the lower curve is corrected for the assumed continuum, as indicated; the solid line is the calculated step for a resonance width F = 150 eV and an energy spread of 200 eV fwhm. From 5(b) and (c) the effective thickness of the 1.1/2g/cm 2 F target may be deduced: 700 eV at 1100 keV. 2

2LO

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E keY

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CHANNELS

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MICROANALYSIS

The other narrow resonances were investigated in the same way. If the resonance width measured with the 1.1/xg/cm 2 F target was comparable to the effective target thickness a thick target yield curve was recorded and the resonance width extracted from N=(E). It should be noted that for F>_ 0.8 keV, the second narrowest resonance width found, beam energy spread and Doppler effects are negligible and the Lewis effect hardly shows up. F is then just the 25% to 75% of N=(E) as is easily calculated from eq. 3. Figure 6 shows the second narrowest resonance, at 484 keV. The width of the peak in fig. 6(a), 1.5 keV, is much larger than the 800eV found from the thick target yield of fig. 6(b) (see table 2); this is in agreement with the effective target thickness at resonance, 1100 eV. The slight asymmetry is probably due to energy straggling. Figure 7 shows similar results for the 340 keV resonance; no attempt was made to determine its energy since it is a very well known resonance, used routinely for accelerator energy calibration2). E

This is the second resonance of the 19F(p, ~))160 reaction, the first at 227 keV being very weak23). The continuum below the resonance is also very low, as shown by the thick target yield in fig. 7(b) which starts practically from zero (the figures are not corrected for background). The resonance peak in fig. 7(a) is only slightly broadened here, as F is E (keV) 19 .

330

340

350

360

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384 eV/channe[ lO

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OF F L U O R I N E

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19F (p, ~,# )160 2

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Fig. 6. Scans of the 484 keV narrow resonance. (a) target 1.1 ~ g / cm 2 F (600eV rated thickness, 1100 eV effective thickness at resonance); 150/xC/channel; (b) thick target (5000A CaF 2, 40 keV thick at 500 keV); 20/xC/channel.

eeoe • * I

2O

~0 CHANNELS

I

60

Fig. 7. Scans of the 340 keV narrow resonance. (a) target 1.1/~g/ crn 2 F (700eV rated thickness, 1300eV effective thickness at resonance); 100/xC/channel; (b) thick target (6000,~ CaF2, 57 keV thick at resonance); ]0HC/channel. I1. C R O S S S E C T I O N S

100

D. D I E U M E G A R D et al.

twice the effective target thickness at resonance, 1300 eV (see table 2); its long-tailed Breit-Wigner shape appears quite strikingly. Fig. 7(b) shows the usefulness of thick target yield curves: the slight slope above the resonance indicates a continuum in a(E), hardly visible in .fig. 7(a); this is most useful information for depth profiling. Figures 8 to 10 show typical scans in the vicinity of strong, relatively broader resonances; target thickness effects are practically absent, the peak widths in table 2 being equal to the resonance widths r. The resonance at 872 keV, the strongest below 2 MeV and the most used for depth profiling E

82o

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4. Discussion and applications -r" /

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80

4O

both 19F and 1H, is not well isolated (see fig. 8), Figure 9 shows the details of the second part of the strong doublet around 900 (see fig. 3); a third resonance, of significant intensity, shows up at 899 keV, right in the middle of the two others. Figure 10 shows, in the vicinity of the strong doublet around 1350keV, a truly simultaneous recording of 19F(p,(zy)I60 and 19F(p,~0)160 (at 150 °) in two subgroups of the multichannel analyzer memory. The relative positions of the resonances and the relative counting rates in typical experimental conditions are illustrated. It should be remembered that the 1.35MeV resonance of 19F(p, ~0)160 is the strongest at 150°; this underlines the very high yield of the t 9 F ( p , ~Z7)160 reaction, as compared t o 19F(p, ~0)160. In conclusion, the present data contain an almost complete set of points of the a(E) curve for 1 9 F ( p , ~ y ) I 6 0 . Near very narrow resonances the experimental curves should, however, be replaced by Breit-Wigner functions with E R and F as listed in table 2 (with intensities deduced from the thick target yields), and joined off-resonance to the cross section continuum.

160

CHANNELS

Fig, 8. Scan of the region of the very strong resonance at 872keV. Target 3.4~g/cm 2 F; 1.3 keV thick at resonance; 40 ~C/channel. The peak near 900 keV is detailed in fig. 9.

4.1, JgF(p, ~o)160 REACTION This reaction is best for the determination of total amounts of F/cm 2 in thin films. At 0 = 1 5 0 °, an optimal geometry, a good bombarding energy is E KeV

E KeY 900

920

19

1320

940

1340

1360

1380

1400

960

~6

F( p,c~5~0

(~

768 eV/chcmne~

960 eV/channel 19 16

F~p,ot6)O

jf~

8=150"

@ F;~,~)0"

/

'

C3 O C3

,,

~2

i

t

0

I

z ,° °•

1 "x5

•• "•.

-. ., "*..°

°, ..°

•~.° 0

°

.

i

i

i

i

20

40

60

80

0

20

100

CHANNELS Fig. 9. Scan of the region of the strong resonance at 934 keV, Same conditions as in fig. 8; the two figures represent contiguous scans in a single run.

Fig. 10. Scan 19F(p, ~y)160 19F(p,~,0)I60, 3.4 #g/cm 2 F,

40

60

80

CHANNELS in the region of the very strong doublet in the reaction; the strongest resonance in at 150 °, was recorded simultaneously. Target 1 keV thick at 1350 keV; 30/~C/channel.

M I C R O A N A L Y S I S OF F L U O R I N E

1260 keV, since use of the stationary part of a(E) extending down to 1100 keV is convenient. As this is not a real plateau, cross section corrections must be made for thick targets if high precision is aimed at; from this point of view a bombarding energy of 1550 keV at 0 = 90 ° is more favourable. The depths corresponding to the quasi-plateau at 0 = 150 ° are typically 1 m g / c m 2 in silicon, i.e. - 4 / z m or - 2 m g / c m 2 in zirconium, i.e. 3/,tm. The - 0 . 5 rob/st cross section gives 0.36 counts per ~C fbr 1015atoms/cm2F using a 3 c m 2 detector at 5 cm. This figure is rather low as compared for example to 160 or 180 analysis~7). However, these counts are absolutely background free. Measurement of a monolayer to within ± 7 % with a 1/~A beam requires - 1 0 r a i n ; the ultimate practical sensitivity is -1013 at./cm 2. Such a sensitivity was reached in the F contamination studies of silicon reported in ref. 10. The 872keV resonance of ~gF(p,~zT)160 gives a counting rate - 2 5 0 times larger. However, this is not background-free, especially for low-Z samples, and the advantage in sensitivity is only realisable if the F atoms are in a thin layer (for example less than - 500 A, of silicon) because of the narrow resonance width, 4 keV. Thus for thick F containing films ~z detection is best (see ref. 9 to 11), as well as for samples rich in F. For films of intermediate thickness the 1347 keV resonance peak in 19F(p,~'0)160 at 150 ° gives - 6 times higher sensitivity than the plateau. Since it has a width of 36 keV, it allows one to measure - 1 0 times thicker targets than the 872 keV resonance of ~gF(p, ~zy)160. The latter resonance, however, yields 40 times the counts for thin targets. In conclusion, when counting statistics permit the 19F(p,~0)160 reaction around 1260 keV at 150 ° is preferable for the F / c m 2 determination as it does not require a precise bombarding energy and is insensitive to the position of the fluorine atoms within the first few microns. Moreover the result is obtained in a single measurement, whereas with resonances, the recording of a section of the excitation curve is essential. For depth profiles, ~z-particle spectra may be interpreted (see ref. 17), with poor depth resolution, using the same experimental conditions as above. With a - 3 0 / z m thick Mylar absorber the energy spread of the detected ~z particles is - 100 keV; this yields typically a surface depth resolution - 6 5 0 0 A in silicon. This method is thus useful for thick films, rich in F, when rapid measurement is required, and good depth resolution is not essential.

101

If no Mylar absorber is used, the ~z particles may still be detected for low-Z samples provided the beam current is reduced so that the backscattered particles do not block the amplifier; 6-fold pile-up of the proton pulses is unlikely, so that the 7 MeV ~z peak may be background free. A - 6 fold improvement in depth resolution may thus be obtained in favourable cases, but the solid angle must be decreased to reduce the kinematic spread. These resolutions cannot be improved by glancing incidence, as the overall energy loss is dominated by the outgoing c~ particle; glancing detection requires very small solid angles, precluded here by the low cross section.

19F(p, ~)))160 REACTION This is mostly useful for depth profiling using the interpretation of the yield curves as described in ref. 17 and used by numerous workers as for instance in ref. 1, 5, 7, 8, 9, 14 and 15. We shall not discuss this technique here since it will be studied theoretically in detail in ref. 22. We shall discuss only the relative merits of the various resonances. The very detailed cross-section curves reported here for these resonances allow one to carry out the computer calculations required to convert the results into depth profiles. The best sensitivity is obtained with the widely used 872 keV, F = 4.2 keV resonance. Typical surface depth resolutions are ~ 1000 A in silicon or - 6 5 0 A in zirconium. This resolution deteriorates with depth x, the energy straggling being equal to F at a depth of - 1 7 0 0 A in silicon and - 6 0 0 A in zirconium. Glancing incidence may be used to improve depth resolutions for flat smooth surfaces; a 4-fold improvement is easily achieved. It should be noted however, that, as shown in fig. 8, this resonance is not isolated. The detailed shape of cr(E) in its vicinity must be included in the computer calculations. Moreover for deep profiles the excitation curve runs rapidly into the neighbouring 934keV strong resonance ( - 1 , 4 / z m in silicon), which must then also be taken into account for the calculations. The best depth resolution, with a routinely useful sensitivity, is obtainable using the 484 keV, F = 800eV resonance. Surface depth resolution is 130 A in silicon, - 100 A in zirconium, since F is 5 times smaller than for the 872 keV resonance, while dE/dx is higher. Moreover this resonance is well separated from its neighbours. Thus, although much less sensitive than the 872 keV resonance, it 4.2.

I1. CROSS S E C T I O N S

102

D.

DIEUMEGARD

might be very useful. One should remember however that the depth resolution deteriorates with depth; a resolution equivalent to that of the 872 keV resonance is reached in silicon at 3000 A. So this resonance is only useful if very good depth resolution near the surface is aimed at. It should be noted that for such narrow resonances sophisticated computer calculations are required to account correctly for the energy loss fluctuations of the protons22). The 340 keV resonance has marked advantages for depth profiling: i) It is the first strong resonance of the reaction; thus the counting rate below the resonance is negligible even for very thick targets and the details of a profile near the surface may be studied with high sensitivity even if underlaying concentrations of fluorine are high; ii) it is well isolated, the next resonance being 140 keV above; iii) at this low energy most of the low Z nuclei do not react so that the background from low Z matrices is considerably reduced; iv) this low energy may be reached by most of the modern implantation machines, allowing one implant and analyse 19F with the same equipment. This was achieved with the 400 keV HVEE implantation machine of Thomson-CSF, as reported by Croset et al.24); v) the width is relatively small, while the intensity is still quite favourable. Surface depth resolution is ~ 3 5 0 A in silicon and ~ 2 8 0 A in zirconium. This is larger than for the 484 keV resonance but it is degraded less rapidly with depth. This resonance is recommended when sensitivity is not of prime importance. It should be emphasised that this resonance is especially useful for 1H profiling using the reverse reaction, 1H(19F,~7)160 as it requires much smaller 19F energies than the 872 keV resonance; - 7 MeV instead of - 1 7 MeV. It is thus in the reach of a 3 MeV Van de Graaff with 19F3+ ions. Finally it should be noted that the weak but extremely narrow resonance at 1088 keV may be useful for very high resolution 19F depth profiling in fluorine-rich targets. In this case the exact theory of energy loss fluctuations must be used for interpreting the yield curves, as mentioned for a similar resonance in the 180(p, ot)~SN reaction in ref. 17 and as will be described in ref. 22. The surface depth resolution is - 4 0 A in silicon, - 2 5 A in zirconium. In fact, fig. 5(b) may be considered as a

et al.

depth profile, as it demonstrates that the CaF2 target of mean thickness N 70 A has in fact a thickness fluctuating up to N 150 A. A precise calculation could yield more detailed information on these fluctuations. This resonance might be useful in 1H profiling. In fig. 1 l(a) 2 typical application of these methods is shown. Tantalum was polished for 15 s in a H2SO4, HNO3, HF bath and rinsed for 5 min in boiling water. The metal surface was then contaminated with N 1.6 × 1015 ]gF/cm2. These samples were anodically oxidized at 45 V and 90V in a citric acid solution at a current of 1 mA/cm 2, yielding Ta205 oxide thicknesses of 760 A and 1560 A. Oxidized and unoxidized samples were subjected to depth profiling with 19F(p, 0t7)160 using the above

,

,

0

F19(p,~.i) 016 ER =872 KeV

o

768 eV/channel o

o o o

40

20

6O

CHANNELS i

15 oo C:~ 10

o°

\

°°

i F19 (p,~.~) 016

ER = 3 4 0 K W 384 e V / c h i n n e l

b

z

0

S0 -

100

CHANNELS Fig. ] ] . Depth profiting of fluorine in a tantalum sample contaminated during polishing by HF and anodical]y oxidized. (a) 872keV resonance, before and after anodization (oxide 1560 A); 50 pC/channel; (b) 340 keV resonance, before and after anodization (oxide thicknesses 760A, and 1560A,); 2 0 0 p C l channel. Solid fines are calculated assuming all F to be positioned near the oxide-metal interface. Total amount of F was 1.6 × 10 ] 5 atomslcm 2.

MICROANALYSIS OF FLUORINE

experimental conditions, to establish the position of the surface fluorine after anodization. Figure 1 l(a) shows the results for the 872keV resonance, figure l l(b) for the 340 keV resonance. The solid lines correspond to calculations 22) assuming that the fluorine is contained in a 50 A thin Ta205 layer first on the surface, then at the metal oxide interface. This assumption is proved to be correct. Fig. 11 (b) illustrates the better depth resolution of the 340 keV resonance, while the counting rate is still acceptable even for a target with a content equivalent to only - 2 monolayers of fluorine. The broadening of the yield curve with increasing depth is well illustrated. Details of this experiment were given in ref. 9. The results we have obtained are clearly of great value in the interpretation of 19F excitation curves as depth profiles. However, accurate communication of the detailed data is impossible through figures and is excessive numerically. This problem is general in nuclear microanalysis and requires a solution. The present results are also most useful for hydrogen profiling using the 1 H ( 1 9 F , ~ y ) ~ 6 0 reaction as shown by Ziegler et al. 25) and Clark et al. 26) who used some of our data which were reported in ref. 16. Finally to obtain higher quantitative accuracy, a method must be devised for the manufacture of highly stable thin fluorine reference standards; this seems to be an open problem.

The authors are indebted to M. Croset from Thomson-CSF for help and advice in the practical applications of 19F microanalysis. They wish to extend their warmest thanks to E. d'Artemare and E. Girard for their constant assistance and cooperation in the automatic energy scanning experiments and to J. Moulin for his help in problems related to the scattering chamber.

103

References l) E. M611er and N. Starfelt, Nucl. Instr. and Meth. 50 (1967)

225. 2) F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1; F. Ajzenberg-Selove, Nucl. Phys. AI90 (1972) 1. 3) G.M. Lerner and J.B. Marion, Nucl. Instr. and Meth. 69 (1969) 115. 4) I. Golicheff, M. Loeuillet and Ch. Engelmann, J. Radioanalyt. Chem. 22 (1974) 113. 5) j. Stroobants, F. Bodart, G. Deconninck, G. Demortier and G. Nicolas, in: Ion beam surface layer analysis, ed. O. Meyer (Plenum Press, New York, 1976) p. 933. 6) M. A. Chaudri, G. Bums, J. L. Rouse and B. M. Spicer, ibid., p. 873. 7) I. Golicheff and Ch. Engelmann, J. Radioanalyt. Chem. 16 (1973) 503. 8) R.A. Jarjis, Nucl. Instr. and Meth. 154 (1978) 383. 9) B. Maurel, D. Dieumegard and G. Amsel, J. Electrochem. Soc. 119 (1972) 1715. 10) M. Croset and D. Dieumegard, J. Electrochem. Soc. 120 (1973) 526. ll) M. Croset and D. Dieumegard, Corros. Sci. 16 (1976) 703. 12) F. Bodart, G. Deconninck and J. Vreven, Health Phys. 36 (1979). 13) L. Porte, J.-P. Sandino, J. Gr6a, J.-P. Thomas and J. Tousset, J. Radioanalyt. Chem, 16 (1973) 493. 14) R. E. Taylor, World Archeology 7 (1975) 125. 15) D. Dieumegard, Th6se de 3e cycle, Orsay (1971). 16) Ion Beam Handbook for Material Analysis, eds. J. W. Mayer and E. Rimini tAcademic Press, New York, 1977) p. 166. 17) G. Amsel, J.-P. Nadai, E. d'Anemare, D. David, E. Girard and J. Moulin, Nucl. Instr. and Meth. 92 (1971) 481. 18) G. Amsel, E. d'Artemare and E. Girard, to be published. 19) j . W . Butler, Phys. Rev, 123 (1961) 873. 2o) C. F. Williamson, J.-P. Boujot and J. Picard, Tables of range and stopping power of chemical elements for charged panicles, C.E.A.Report R 3042 (1966). 21) p. M. Endt and C. Van der Leun, Nucl. Phys. AI05 (1967) 1. 22) G. Amsel, B. Maurel and J.-P. Nadai, to be published. 23) L. Keszthelyi, 1. Berkes, 1. Demeter and I. Fodor, Nucl. Phys. 29 (1962) 241. 24) M. Croset, D. Dieumegard, A. Grouillet and G. Amsel, Low energy ion beams 1977, Conference Series No. 38, (Institute of Physics, London, 1978) p. 109. 25) j. F. Ziegler et al., Nucl. Instr. and Meth. 149 (1978) 19. 26) G. J. Clark, C. W. White, D. D. Allred, B. R. Appleton, F. B. Kock and C.W. Magee, Nucl. Instr. and Meth. 149 (1978) 9.

11. CROSS SECTIONS