Determination of Oxide Film Thickness by Proton Activation BARBARA A. THOMPSON General Engineering laboratory, General Electric Co., Schenectady, N. Y.
b A method has been developed using a nuclear reaction for the determination of oxide film thickness on metals. The method is based on the activation of naturally occurring oxygen-1 8 with protons according to the reaction 0l8(p, n)F1*. A measure of the positron radiation emitted b y the fluorine-1 8 yields a measure of the amount of oxygen present in the film. The Brookhaven cyclotron was used as a proton source. With a proton energy of 4 m.e.v. and a beam current of 10 pa., the lower limit of detection of the method corresponds to a thickness of the order of 1 A. The upper limit is approximately 105 A. Elements interfering with the measurement are copper, nickel, zinc, and titanium. Titanium can b e eliminated b y choice of a proton energy below 3.8 m.e.v.
T
of oxide film thickness on metals is a problem often encountered in the study of such phenomena as oxidation, corrosion, and passivation. -4 number of specialized techniques are available for this measurement, including ellipsometry, interferometry, and electron diffraction; however, all of these suffer from limitations in range and from a lack of general applicability. Thus, a method free from these limitations is needed. Since very small amounts of material are present in the films to be measured and since methods utilizing nuclear reactions have been applied with great success to the measurement of such small amounts of material, it seemed appropriate to consider the use of such mc%hodsfor the determination of oxide film thickness. The measurement of oxygen by an ordinary (n, y) reaction with thermal neutrons is quite difficult because of the short half life of the oxygen isotope produced (29 seconds), because of the low cross section for its production, and because of many interferences from other elements present. This general method has been modified by Coleman and Perkin (e),who use fast neutrons to produce 7-second nitrogen-16 by an (n,p) reaction and by Osmund and Smales ( 8 ) , who form 1.8-hour fluorine18 by a (f,n)reaction, the tritons being produced by an ( n , y ) reaction on HE DETERMINATION
lithium. Both of these methods are subject to many interferences from ( n , y ) reactions with other elements present, and, in addition, the (t, n) reaction is limited in sensitivity. A method involving a ( y , n ) reaction to form 2-minute oxygen-15 has been described by Basile ( 1 ) and by Beard and coworkers ( 2 ) . This method appears to have limited sensitivity, and the extent to -mhich other elements present will interfere is unknown. Fogelstrom-Fineman and cotvorkers ( 7 ) have used proton activation to detect oxygen-18 in the course of photosynthesis studies using enriched oxygen. The folloning nuclear reaction was employed: O1*(p,n) F18
The radiation from fluorine-18, a positron emitter with a half life of 1.8 hours, was used as a measure of the oxygen-18 originally present. The method showed a very high sensitivity and also appeared to be relatively free from interferences. It therefore seemed appropriate to consider its applicability to the detection of oxygen-18 in its naturally occurring isotopic abundance with particular reference to the measurement of oxide film thickness. THEORY
Activation of Oxygen. The principles governing the production of radionuclides by activation are well known and are discussed by Overman and Clark (9). The activity, A (in disintegrations per second), of a product isotope present after an irradiation time, t , is given by the following equation:
W
number of atoms of parent nuclide present in sample 6 = flux of activating particles expressed as particles per sq. cm.-sec. u = isotopic cross section in sq. em. for reaction in question t = time of irradiation TilS = half life of product nuclide =
The induced activity is directly proportional to the number of parent atoms present and thus should be expected to
vary linearly with oxide film thickness provided that u is constant. Where charged particles such as protons are used for activation the situation becomes more complicated than with neutrons because of the loss in energy of the charged particle as it travels through the sample. As the particle loses energy, the cross section for the reaction may change. In the case of oxygen this variation has been measured by Blaser and coworkers ( 5 ) , who found it to be a complex function with several resonance peaks. They also found the threshold energy required to initiate the reaction to be 2.56 m.e.v. Thus, the effective range of the proton for producing a ( p , n) reaction with oxygen-18 is less than the total range since no reaction occurs after the energy has been reduced below the threshold energy. Although it is possible in principle to calculate the amount of actiIity produced in any thickness oxide film by protons of a given initial energy, this becomes extremely difficult in practice because of the nonlinearity of the variation in cross section with energy. Thus, it is simpler and much more accurate to compare unknown films to those of known thickness. When thin films, 1000 A. or less, are to be measured, the energy loss in the film becomes negligible and a linear variation of activity with thickness is to be expected. The protons will not distinguish between oxygen in an oxide film and oxygen dissolved in the metal substrate. Thus, the lower limit of film thickness which can be measured may be limited by the background rather than by the available flux, cross section, etc. This interference will, of course, become less important as the proton penetrates into the metal and its energy decreases. Interference from Other Elements. The excitation functions, or variations in reaction cross section with proton energy, for a great many ( p , n ) reactions have been measured by Blaser and coworkers (3, 6, 5 ) and by Tanaka and Furukawa (11). A comparison of these data shows that the amount of interference from the activation of other elements which may be present depends on the proton energy used. The oxygen cross section VOL. 33, NO.
4, APRIL 1961
583
Table I.
Summary of Significant Interferences in Proton Activation of Oxygen
Millibarns, 4 M.E.V. u,
Element Ti
Ni
cu Zn
Nuclear Reaction Ti47(pln)V47
Ti48(p,n)V48 Ni60 i~ .nl CUB0 Xiel Tp:nj N P ( p , n )Cue2 NP4( p ,n)Cu64 Cu63(p,n)Zn63 Cu66 (p,n)Zn65 Zn66 (p,n)Ga66 Zn67 (p,n)Ga67 Znss ( p , n )Ga68
0 18 (p,n)F E.C. = electron capture.
0 8
(3, 4,
6, 11) 20
... ...
50
'SO 80
..
'60 100 150
is a maximum (500 millibarns) a t 5 m.e.v.; however, many other reactions have appreciable cross sections a t this energy. By choosing an energy in the neighborhood of 4 m.e.v. the *oxygen cross section is reduced by about a factor of 3, while those for most other reactions are reduced 1 or more orders of magnitude. Using this energy and considering radiation characteristics of the product nuclides as well as the reaction cross sections, the only elements which will interfere seriously with the oxygen measurement are copper, nickel, zinc, and titanium. Table I lists the nuclear reactions taking place with these interfering elements and also gives the radioactive decay characteristics of the nuclides formed. The data for the oxygen reaction are listed for comparison. Clearly, titanium interference can be eliminated by choice of a proton energy below 3.8 m.e.v. Some longer-lived products can be discriminated against by choice of an irradiation time which is short with respect to one half life, since the oxygen activation will approach saturation much more rapidly than these longer-lived nuclides. However, many of the products formed by the interfering reactions have half lives of the same order of magnitude as the fluorine-18 which is to be measured (Table I). In these cases there is little or no possibility of discrimination by choice of irradiation time, and chemical separations would be required. Any of these interferences will be significant only if the particular element is a major constituent of the material being measured. Low or trace concentrations can in general be corrected for by resolving the radioactive decay curve and subtracting the interference. Standard Samples. The considerations discussed above showed that oxide film thickness could be measured 584
ANALYTICAL CHEMISTRY
Threshold Energy, M.E.V.
Product Half Life (10) 31 min. 16 days
P+ P f , E.C.5
25 min. 3 . 3 hr. 9 . 9 min. 12.8 hr. 38 min. 250 days 9 . 2 hr. 3 . 3 days 68 min. 1 . 8 hr.
R-C
b+'
RC)"
3.8 5 7.0
2.0 4.7 2.5 2.7 4.2 6.1 2.0 3.5
P+ P+ P f B f
%+
P t P+
2.6
most accurately by comparison t o standards, and i t therefore became necessary to prepare standard samples having oxide films of known thickness. Vermilyea (IS) has demonstrated that the formation of anodic films on tantalum metal a t a constant voltage is governed by a definite relationship which is as follows: i
=
e27.22 ~
5.23
e-
11.47
,;I
69~1
where
i
= current density in ma. per sq.
cm. anodizing potential in volts per A. k = Boltzmann constant, 8.62 X 10-5 e.v. per O K. T = absolute temperature
E
=
The value of E for any temperature is found by arbitrarily setting i equal to 0.1 ma. per sq. em. and solving for E. This is clarified somewhat in the experimental procedure below. At 25' C., E is equal to 0.0645 volt per A. Vermilyea has verified this equation for several temperatures and over a wide range of oxide thicknesses. Electrochemical measurements combined with electron microscope determinations of the film thicknesses showed that the stoichiometric composition and the density corresponding to a film of pure Ta20S are obtained ( I S ) . The procedure is relatively simple, and, in addition, tantalum is a very satisfactory substrate from a nuclear point of view, since its activation has been reported to be very slight ( 7 ) . EXPERIMENTAL
Preparation of Standard Samples. The general experimental procedure for the preparation of anodized specimens was adapted from that of Vermilyea ( I d ) . Rectangular samples of tantalum metal, 2.5 X 2 cm., were cut from a 5-mil sheet obtained from
Fansteel Metallurgical Corp. A small tab approximately 0.8 x 0.1 em. was left projecting from one corner to 15-hich a clip lead was attached during the subsequent anodization. Before anodizing, all the samples were degreased with alcohol and then chemically polished by immersing for a few seconds in a freshly prepared mixture of a 5 : 2 : 1 . 5 ratio of volumes of concentrated HzS04, HN03, and HF, respectively. The samples were rinsed in distilled water and kept under distilled water until anodized. The anodization was carried out using a 1% solution of sodium sulfate as an electrolyte and a stainless steel cathode approximately 1 X 2 inches. The electrolyte, in a beaker, was set in a constant temperature bath which was maintained at 27" C. The electrodes were connected, being careful to have the tantalum sample just barely submerged so that the area being anodized was always the same. Power was obtained from a Matthew Laboratories constant voltage constant current power supply. Initially a constant current of 5 ma. per sq. em. was passed through the solution and was maintained until the voltage rose to the predetermined level corresponding to the film thickness desired. This operation required less than 1 minute in general, evcn for the higher voltages. When the desired voltage was attained, the power supply was switched to constant voltage and this voltage maintained until the current density dropped to 0.1 ma. per sq. em. At this point the power was shut off and the sample was removed from the bath, washed in distilled water and absolute alcohol, and dried. Because of the wide variation in color with film thickness, it mas possible to make a rough check for uniformity and reproducibility a t a given voltage by visual inspection. Although some difficulties were encountered initially in preparing reproducible specimens, all could be traced to inadequate preliminary cleaning. Some improvements were noted when 4 to 1 HSOa to HF was used as an etchant; however, this must be used Kith caution to prevent complete dissolution of the metal. Specimens were prepared having thicknesses of 500, 1000, 1800, and 2000 A. The lower limit was determined by the speed with which the requiied voltage was reached (1 or 2 seconds for the 30 volts used for the 500-A. films) and by the short time required for the current density to fall to 0.1 ma. per sq. em. (again 1 or 3 seconds for the 500-rl. films). Thus, the time required for instrumental adjustment could introduce appreciable error. This error could have been reduced or eliminated by the use of more elaborate electronics or by an independent measurement of the film thickness. Irradiation of Samples. All the irradiations were carried out nsing the cyclotron a t the Brookhaven S a tional Laboratory as a source of protons. This instrument accelerates protons to an energy of 10 m.e.v. An aluminum absorber 20.5 mils
IO0
4000
4
0
I 0
0
1
-
I
0 0
I3
I ul 3000
80
LL W
-1 W
!LL
a
z z a
v,
n
I
I N 2000
V
LL W
6C
a
W
>
3
L
c
w
a
I
I
I
W
zE a
I
!-
n
I
a
0 1030
r'
4c
/
a'
ul
/
!-
z
/
3
I
/
0 V
I
I
2000
2500
I
500
1000
l50C
FILM THICKNESS, A.
20
Figure 2. Activity of irradiated films as a function of film thickness
0
Figure 1.
I 50
60
I 70
CHANNEL NUMBER 0.3 0.4 0.5 GAMMA ENERGY, M.E.V.
0.6
0.7
1
I
IO
20
30
0.1
0.2
1 -
40
film thicknesses. A linear relationship is evident except for the thinnest films to a precision of about *lo%. The limits marked around each point r e p resent the counting precision.
Gamma spectrum of irradiated Ta2O6 DISCUSSION
film
thick vias therefore placed in front of each sample to reduce the proton energy to 4 m.e.v. The samples were mounted in a standard holder which provided for an exactly defined circular beam with a diameter of 0.7 em. by a collimator. The samples were positioned so that the entire beam area was cut off by the sample. A beam current of 1 pa. was chosen for the initial irradiations. Higher currents can be produced, but their use requires water cooling of the samples. This value of 1 pa. refers to the total beam and not to the small area passing through the collimator. The beam is not of uniform intensity over its entire area of some 5 x inches, and thus it is not possible to determine the exact flux of protons over a given area nithout some additional measurement. This is an additional argument in favor of comparison to standards rather than absolute calculation as discussed previously. The irradiation time mas nominally 10 minutes. Fluctuations in the beam current occur during irradiation and all the samples were irradiated to the same total dose rather than the same time. Using this procedure the irradiation times varied betn-een 8 and 11 minutes. Factors for normalizing all the results to a time of 10 minutes were calculated, but were found to be a t most a 1% correction. Since this was well within the precision of the measurements, the correction was omitted in the calculations. Radiation Measurements. The radiation induced in the samples was measured using a Penco 100-channel
analyzer coupled with a sodium iodide scintillation crystal. The characteristic positron annihilation radiation of 0.511-m.e.v. energy was used as a measure of the amount of fluorine-18 present. Counts were repeated a t intervals for several half lives as a check on the purity of the radiation, since any other positron emitter present would give a peak a t the same energy. The decay rate was determined by summing the counts in the photopeak, dividing to obtain counts per minute, and plotting these numbers as a function of the time since the end of irradiation. Any contribution from side reactions was subtracted out and the samples were then compared by reading from the curve the activity level a t 2 hours after shutdown. RESULTS
I n every case the gamma spectrum showed only the 0.511-m.e.v. positron peak with no extraneous gamma peaks from other nuclides. A typical spectrum is shown in Figure 1. All the decay curves gave linear plots on semilog paper of activity us. time. The slope corresponded to a half life of 1.8 hours. I n one or two cases there was some indication of a very shortlived activity which had nearly died out by the time the measurements were made. Figure 2 is a plot of the activity levels at 2 hours after shutdown for eight samples representing four different oxide
In Figure 2 a line has been drawn connecting the upper three points and passing through the origin but bypassing the 500-A. point. This can be justified by consideration of the previously described difficulties involved in preparing such thin films. The source of error is the time lag in switching from constant current to constant voltage and the direction of the error should be toward a thicker film which is consistent with the results. That this is the source of error is further indicated by the fact that the extra activity cannot be attributed to dissolved oxygen in the substrate metal since this leads to an excessively high concentration. A very rough calculation, using a weighted cross section to take the reduction in proton energy into account, showed that the dissolved oxygen concentration in the metal would have to be of the order of 1 or 2% to account for the observed deviation from linearity. This is two orders of magnitude higher than the normal concentration in tantalum and would have caused the metal to be much more brittle than was the actual case. Of course, for any study in which the films to be measured were only a few angstroms thick, the exact determination of the oxygen blank in the metal would be imperative. In the present case such a determination could not be made by activation since an oxide film forms on tantalum upon exposure t o air. VOL 33, NO. 4, APWL 1961
0
585
T
I n addition to the oxygen blank, factors limiting the sensitivity of the method are the available beam current, the beam area, and the irradiation time. The beam current used in these runs, 1 pa., could be increased to as high a value as 50 pa. provided that arrangements were made for water cooling the samples during irradiation. The beam area can be increased somewhat, although special instrumentation must be provided to eliminate errors due to nonuniforniity of the beam. The irradiation time can of course be increased to any desired value; homever, irradiation for one half-life will give half the activity that would be obtained a t saturation. Because of the activation of longer-lived impurities and because of the high cost of cyclotron time, irradiation for more than one half-life is not recommended. Therefore, measurement of films as thin as 1 A. is entirely feasible; however, in such a situation the blank would probably prove to be the limiting factor. The total amount of oxygen measured in such a situation would be 6 x pg. (1.2 x IO-Spg. of 0’s). The upper limit on film thickness is determined by the proton range and is about 1 mil or 250,000 A. for 4-m.e.v. protons. For samples as thick as this, the activity will not vary linearly with thickness and careful calibration with standards will be required. From Figure 2 a precision of approximately ilOyo is indicated. The various sources of error which could contribute to this include variations in film thickness, fluctuations in beam current, and counting precision. Of these the most significant is probably variations in film thickness introduced during anodizing. The anodizing voltage was accurate only to =1=1volt a t best, and variations of the order of magnitude observed could easily have been in-
troduced. The other factors were controlled to approximately =tl%as previously noted. In applying this method to other materials, it should be noted the quantity measured is atoms of oxygen per unit of area. Thus, some assumption must be made about the density of the film in order to obtain a value for the thickness. CONCLUSIONS
Proton activation of naturally occurring oxygen-18 according to the reaction 01* (p,n) FI8 has been successfully applied to the measurement of oxide film thickness on tantalum. The films were prepared by anodization and ranged from 500 to 2000 A. in thickness. With a proton energy of 4 m.e.v., the lower limit of the method is about pg. of oxygen-18 in an area of 0.38 sq. cm. This corresponds to a thickness of about 1 A. of TazOs. This limit can be attained only in the absence of dissolved oxygen in the metal substrate. The upper limit of the method is determined by the proton range and is about IO6 A. for 4-m.e.v. protons. For 4-m.e.v. protons, the elements causing interferences in the oxygen measurement are copper, nickel, zinc, and titanium. Titanium can be eliminated by choice of a proton energy below 3.8 m.e.v. While this work has been confined to a study of oxide film thickness, the method is equally applicable to determination of the oxygen content of almost any type of thin film and, with proper standardization, to the determination of trace amounts of oxygen in metals. ACKNOWLEDGMENT
The author thanks W. H. Bauer of Rensselaer Polytechnic Institute for
helpful advice and criticism. The assistance of Charles Baker and the operations crew of the Brookhaven cyclotron in performing the irradiations is also acknowledged with thanks. Thanks are also due to H. L. Finston, of Brookhaven National Laboratory, who kindly supplied the 100-channel analyzer used for the radiation measurements, and to V. IT. Perry, of the General Electric Co., who assisted with the radiation measurements. LITERATURE CITED
(1) Basile, R., Compt. rend. 239, 422 (1954). (2) Beard, D. B., Johnson, R. G., Brad-
shaw, W. G., Nucleonics 17, KO. 7 ,
90 (1959). (3) Blaser, J. P., Boehm, F., Narmier, P., Peaslee, D. C., Helv. Phys. Acta 24, 3 (1951). (4) Ibid., p. 441. (5) Ibid., p. 465. (6) Coleman, R. F., Perkin, J. L., Analyst 84,233 (1959). (7) Fogelstrom-Fineman, I., HolmHansen, O., Tolbert, B. M.,Calvin, P.I., Intern. J. Appl. Radiation and Isotopes 2, 280 (1957). (8) Osmund, R. G., Smales, A. B . , Anal. Chim. Acta 10, 117 (1954). (9) Overman, R. T., Clark, H. RI., “Radioisotope Techniques,” pp. 392-5, McGrav-Hill, New York, 1960. (10) Strominger, D., Hollander, H. M., Seaborg, G. T., Revs. Modern Phys. 30, 585 (1958). (11) Tanaka, S., Furukawa, M., J . Phys. SOC.Japan 14, 1269 (1959). (12) Vermilyea, D. A., General Electric Co., Schenectady, N. Y., private communication, 1960. (13) Vermilyea, D. A,, J . Electrochem. SOC.102, 655 (1955).
RECEIVED for review September 28, 1960. Accepted December 8, 1960. Based on a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, Rensselaer Polytechnic Institute. Division of Physical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
Quantitative Determination of Metallic Iron in the Presence of Iron Oxides in Treated Ores and Slags M. G. HABASHY 25 Sulfan Hussein Sfreef, Alexandria, Egypt, U. A. R.
b Metallic iron in the presence of iron oxides is determined in uncrushable Samples such as sponge iron, Slags,
copper obtained is multiplied by atomic weight of iron atomic weight of copper to Obtain
treated Ores, The is shaken with copper sulfate and water and also with mercury which catalyzes the reaction. After complete displacement of the iron by copper, the copper equivalent is filtered, redissolved in nitric acid, and determined electrolytically. The per cent of
the per cent of iron. Results show a mean toleranceof &0.1%. The inter-
586
ANALYTICAL CHEMISTRY
ference Of e’ements which have a higher oxidation Potential than copper can be eliminated. Data are given which show the effect of particle size of the sample on the determination.
T
done so far on determining metallic iron in presence of iron oxides does not completely solve the problem for this determination, especially in samples of coarse particles which cannot be crushed. The principal methods for the determination of metallic iron in samples containing the metal in the presence of its oxides are the mercuric chloride method of Wilner and Merck (8), the HE WORK
