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Properties of anodic oxide layers formed on nitrogen-containing tantalum films
Thin Solid Films, 23 (1974) 75-87 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland
75
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON NITROGEN-CONTAINING TANTALUM FILMS* R. T. SIMMONS**
Western Electric Company, Inc., Winston-Salem, N. C. 27102 (U.S.A.) P. T. MORZENTI**
Western Electric Company, Inc., North Andover, Mass. 01845 (U.S.A.) D. M. SMYTH Materials Research Center, Lehigh University, Bethlehem, Penn. 18015 (U.S.A.)
D. GERSTENBERG Bell Telephone Laboratories, Inc., Allentown, Penn. 18103 (U.S.A.) (Received March 20, 1974; accepted March 20, 1974)
The growth mechanism, dielectric properties and composition of anodic films on sputtered tantalum-nitrogen alloy films have been studied as a function of nitrogen content from 0 to 27 at. ~. For the anodization conditions used in this study both tantalum and oxygen ions were mobile during oxide growth while the nitrogen remained essentially stationary. The result was a nitrogen-free region in the oxide adjacent to the oxide--electrolyte interface. Incorporation of nitrogen into the anodic film caused a reduction of its density and required higher fields to produce oxide growth at a given ionic current density as compared with films grown from pure 13-tantalum. For nitrogen contents up to 21 at. ~o in tantalum the dielectric constant of the anodic oxide decreased linearly as a function of density. Structural changes in the as-sputtered tantalum films observed while the nitrogen content was increased from 0 to 27 at. ~o did not have an effect on the anodization mechanism.
1. INTRODUCTION Anodic oxidation is one of the processes used in the fabrication of tantalum film integrated circuits. It serves to form the capacitor dielectric on 13-tantalum films and as a method for trimming Ta2N resistors to a desired value 1. 13-Ta, which forms the basis of tantalum film capacitors, is a metastable allotrope of tantalum 2. Recent studies have shown that it is formed during sputtering (or * Based in part on a thesis by R. T. Simmons in partial fulfillment of the requirements for the degree of Master of Science in Metallurgy and Materials Science from Lehigh University. ** Work performed by the authors at Western Electric Co., Inc., Engineering Research Center, Princeton, N. J. 08540, U.S.A.
76
R.T. SIMMONSet al.
evaporation) of tantalum films onto the insulating substrates normally used for electronic circuits 3, 4. The anodization behavior of [3-Ta has been compared with that of bulk tantalum and b.c.c, tantalum films deposited by sputtering 5. The field E in the oxide required to produce a given ionic current and the dielectric constant of the oxide both differed by less than 4 ~ for all three types of tantalum, and the current efficiency for oxide formation on both [3- and b.c.c. Ta was 99 + 1 ~ 5, 6. Several recent studies dealing with the growth mechanism of anodic films on pure bulk tantalum have revealed that both tantalum and oxygen ions are mobile and contribute to the growth of the oxide v' s. It has also been found that the presence of nitrogen in sputtered tantalum films does not interfere with the formation of adherent anodic films 9. For example, anodic films on Ta2N display the same sequence of interference colors with increasing formation voltage, as well as equivalent dielectric strength, compared with anodic films formed on pure tantalum 9. Both the mass density and dielectric constant of anodic films on sputtered Ta2N, however, are significantly lower than the values reported for anodic films on pure tantalum. In addition, the dielectric constant was found to be dependent on anodic film thickness9. The stoichiometric composition of anodic films grown on Ta2N as determined from weight gain measurements and the growth constant was tentatively identified as that of Ta205 Nl°. Nothing is known about the dependence of the dielectric properties on the amount of nitrogen present in the sputtered films, nor about the distribution of the nitrogen in the subsequently grown anodic films. The purpose of this investigation was to establish a quantitative correlation between the properties of anodic films and the composition of the as-sputtered tantalum films. The composition of the sputtered films and the distribution of the nitrogen throughout the subsequently formed anodic films were determined by Auger electron spectroscopy (AES) in combination with in situ ion sputtering TM 12. The result found in this study, that the nitrogen is not homogeneously distributed throughout the anodic film, allows speculation about the role nitrogen plays in the oxidation of nitrogendoped tantalum films. 2.
EXPERIMENTAL PROCEDURE
Tantalum films between 1700 and 5200 A thick were prepared by d.c. diode sputtering of tantalum in argon-nitrogen mixtures in liquid nitrogen trapped, oil diffusion pumped, vacuum systems. The sputtering was carried out at a potential of 5 kV, a current density of 0.2 to 0.4 mA/cm 2, an argon gas pressure of 30 mtorr, and a substrate heater temperature of 250 °C. The interelectrode spacing was 6.35 cm. Nitrogen flow was monitored at levels between 0 and 0.6 cm 3 (standard conditions) of nitrogen per minute with a thermocouple type Hastings mass flowmeter. The substrates were 7059 Coming glass slides. The properties of the tantalum films sputtered at six different nitrogen flow rates are given in Table I. Structure and composition of selected samples from each of the six depositions were determined by X-ray techniques and Auger electron spectroscopy. The quantitative analysis of light elements (N, C, O) in sputtered tantalum by ion sputtering-Auger electron analysis via calibration with standards and
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N - C O N T A I N I N G
Ta
FILMS
77
TABLE I COMPOSITION AND PROPERTIES OF Ta FILMS SPUTTERED IN ARGON-NITROGENMIXTURES
N content (at. ~o)
C content (at. ~o)
Crystal structure
Density* (g/em a)
0 2 13 21
0 < 1 1 3
23 27
< 1 5
tetragonal (13-Ta) tetragonal (13-Ta) b.c.c. Ta b.c.c. (tetragonally distorted) h.c.p. (Ta2N) h.c.p. (Ta2 N)
Resistivity (11.0 cm)
Film thickness (/~)
16.0 15.2 14.6
183 179 126 285
4900 2060 1950 1780
14.3
355 426
5200 1720
* From Brown13.
electron microprobe analysis has been described by Morabito 12, The nitrogen concentration in the Ta films was varied from 0 to 27 at. 9/0.The error in the analysis based on the ratio of the nitrogen to tantalum Auger peak heights was _ 10 ~ of the listed value, and within the limits of error the nitrogen was homogeneously distributed throughout the thickness of the film. A number of films contained small amounts of carbon as an impurity. The values of the density of the Ta and nitrogen alloy films listed in Table I were taken from data published by Brown 13. Anodic oxide layers were formed at 24 °C by anodizing the Ta films at a constant current density of 1 mA/cm 2 in dilute aqueous citric acid solution to 50, 100, 150, 200 and 250 V. In a number of cases anodization was continued at the final voltage level for another 30 min. Plating tape* was used to delineate capacitor areas of 0.5 cm 2 for capacitance measurements at 1 kHz in a sulfuric acid cell with a Ta counterelectrode. For Ta films with 0 and 23 at. ~ nitrogen, the oxide thickness tox was determined as follows: (a) by using a Talysurf~* instrument to measure the step height of the oxide formed above the metal surface, to,,; and (b) by measuring the increase in the electrical resistance of 2000 A thick metal samples and calculating the reduction of the conducting film thickness due to anodic oxidation, to, 2 (see Fig. 1 for definition of thickness symbols). The resistor films with 0 and 23 at. ~ nitrogen used for procedure (b) were deposited
/,L
,!,
/
Fig. 1. Cross section of anodic oxide on Ta film to illustrate anodic oxide formation.
# 470 plating tape, 3M C o m p a n y . ** T a y l o r - H o b s o n , Ltd.
R . T . SIMMONS et al.
78
on sapphire substrates because sapphire is not attacked by the chemical etchant used for the delineation of the resistor pattern and permits, therefore, accurate measurement of the tantalum-nitrogen alloy film thickness, /metab after pattern delineation. The accuracy of the thickness measurements was _+5 ~. The thickness of the anodic film then is the sum of tox 1 and fox2, and the resulting accuracy of the oxide thicknesses determined for formation voltages of 50, 100, 150, 200 and 250 V was + 10~o. A number of oxide layers formed on Ta films with 13, 21 and 27 at. ~ nitrogen were subjected to Auger electron spectroscopy analysis in combination with in situ ion sputtering to determine the distribution of the nitrogen in the anodic films 11. The analysis was performed by Physical Electronics Industries, Inc., Edina, Minn. Dual Xe + beams were used to remove material at a constant rate over an area 5 mm in diameter. Each gun was operated at an emission current of 40+ 1 mA and an accelerating potential of 1.00_+ 0.03 kV. The angle of incidence was 20 ° from the specimen normal. During the sputtering operation, a 3.0 kV, 30__+ 1 gA electron beam was used to excite Auger electrons of the elements of interest. The incidence angle was 60 ° from the sample normal, and the 100/am beam was positioned in the center of the sputtered crater. A cylindrical mirror analyzer was used to analyze the secondary electron energies. The Auger electron spectrum from 0 to 600 eV was scanned at a rate of one scan per minute using the following transitions to monitor the O, N and Ta profiles in the oxide: O: K L L transition atS10 eV N: K L L transition at 380 eV Ta: N4N6N 6 transition at 179 eV From the thickness of the oxide layer and the sputtering time needed to go through the oxide, it is possible to obtain information concerning the profiles of the three elements as a function of oxide thickness. 3. RESULTS In Fig. 2 the results of the capacitance measurements of the 0.5 cm 2 test capacitors formed under constant current conditions at five anodization voltages ranging from 50 to 250 V have been used to plot the normalized capacitance, C / C o, as a function of the nitrogen content in the as-sputtered tantalum for three of the five formation voltages. Co is the capacitance of capacitors formed on pure [3-tantalum films sputtered without deliberate addition of nitrogen. Up to 11 at. ~ , the nitrogen content causes a linear decrease in the capacitance of about 1 for each at. ~ of nitrogen. From 0 to 11 at. ~ the normalized capacitance appears to be independent of the formation voltage. A more rapid decrease in C/Co is observed at higher nitrogen concentrations, and above 20 at. 5/o the normalized capacitance also becomes a strong function of the formation voltage. The dependence of the capacitance on formation voltage is illustrated in more detail in Fig. 3 for various nitrogen levels. In Fig. 3 the inverse of the capacitance per unit area (capacitance density) has been plotted as a function of formation voltage.
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
o
Ta FILMS
79
i.o
¢j
° 0.8 w
~
0.6
o
N 0.4
o 5 0 VOLTS [] 1 5 0 VOLTS ZX 2 5 0 VOLTS
.J
=E n,. z0 0.2
0 0
I I I I I 5 tO 15 20 25 NITROGEN CONCENTRATION IN SPUTTERED To, a t . %
.~
Fig. 2. Normalized capacitance C/C o as a function of nitrogen concentration in sputtered tantalum. Co is the capacitance of anodic film formed on pure 13-Ta to 50, 150 and 250 V without the 30 min soak at voltage.
u. 5 0 ~
CONTENT OF NITROGEN IN To:
_z
2 7 ot. %
40 p-
23at. %
~ 3o
21 at. %
hJ Z
13 at.% Oat.% (J
o
0
50
100
150
200
250
FORMATION VOLTAGE (VOLTS)
Fig. 3. Inverse capacitance per unit area of anodic films formed on T a - N alloy films as a function of anodization voltage (no 30 min soak at voltage).
Only the anodic films formed on pure I)-tantalum show the linear dependence expected for a dielectric whose thickness is a linear function of the formation voltage and whose dielectric constant is independent of anodic film thickness. The inverse capacitance density for the T a N alloy films deviates more and more from straight line behavior as the nitrogen content increases. It is especially
80
R . T . SIMMONS e t
al.
pronounced for the Ta films with 21, 23 and 27 at. ~. In an earlier study of the anodization of Ta2N films it was found that the anodization constant was independent of the formation voltage and that the dielectric constant of the anodic films formed on Ta2N decreases steadily as a function of dielectric thickness 9. The exact composition of the Ta2 N used by Gerstenberg, however, was not known. For both the pure [3-Ta and the 23 at. % nitrogen alloy, toxl and tox2 were a linear function of the formation voltage from 50 to 250 V. The resulting values for the anodization constants, Kox, Kox, and Kox2 are, therefore, independent of the anodization voltage. As shown in Fig. 4 a difference of ~<10% in anodization constants was found between anodic films grown at constant current and those which had been kept at voltage for an additional 30 min, independent of composition. Data of this work not shown in Fig. 4 indicate that the anodization constant Kox is a slowly decreasing function of nitrogen. The anodization constant Kox for the 23 at. % nitrogen alloy is about 11 ~ lower than that obtained for [3-Ta. The anodization constant of 16.6 A/V found for [3-Ta anodized under constant current conditions without the 30 min soak is 7% higher than the value of 15.6 A/V reported by Muth 6, but within the limits of error, which are __+10~ for the present procedure involving Talysurf step height measurements. In calculating the dielectric constants it was assumed that the anodization constant decreases linearly with increasing nitrogen concentration--an assumption that might not be entirely correct but seems justified in view of the limits of error in determining the oxide film thicknesses. In the equation e = Kox VA C/eo A
C is the measured capacitance, A the capacitor area, VAthe anodization voltage, /Cox the anodization constant in A/V and e0 = 8.86 × 10-12 F/m. The dielectric constant has been plotted in Fig. 5 as a function of nitrogen concentration for anodic films formed at 100, 200 and 250 V. The values shown
20 > •= 0 , - ~
KOX
¢R t5 IZ
Z 0
tu X 0
.
.
.
.
. . . . . .
5
""O'"
KOXI
~
~ ' ~ ~-~ ~ ~'-~,~ ~ ' ~ _ ~ 0 50 MINUTESOAK L~NOSOAK I
I
I
I
KOX2
t
5 tO t5 20 25 NITROGEN CONTENT IN SPUTTERED T0, of.%
Fig. 4. Oxide growth constants of 0 and 23 at. % N alloy films with and without 30 min soak at voltage (growth constants were determined from the measured thickness indicated in Fig. 1).
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
Ta
FILMS
8|
30
~
w z
25
tO0 v
0
F0 ul ._1
--- 15
tO
0
I I 5 to 15 20 25 NITROGEN CONTENT OF T(I FILM IN ut.%
Fig. 5. Dielectric constant of I00, 200 and 250 V anodic films as a function of nitrogen concentration in sputtered T a - N alloy films. (Anodie films did not receive 30 min soak at voltage.)
are for anodic films prepared at constant current anodization without the subsequent 30 min soak at voltage. The values for anodic films on ~-Ta and the 23 at. nitrogen alloy prepared with the 30 min soak (not shown) were within +__5 ~ of the values shown in the graph. Comparison of the present value for pure 13-Ta, 26.5 + 10 ~o, with values published in the literature for anodic f i l l on ~-Ta shows good agreement 6' 14. A steady decrease is observed in the dielectric constant with increasing nitrogen concentration in the tantalum, as found earlier 9. Above 15 at. ~o nitrogen there is also a shift of the dielectric constant toward lower values with increasing formation voltage, suggesting that the average amount of nitrogen in the oxide becomes a function of formation voltage. An estimate of the amount of nitrogen which could cause the observed decrease in the dielectric constant as the formation voltage is increased from 100 to 250 V shows that for the 27 at. ~o nitrogen alloy the difference would have to be about 3 at. nitrogen between the two oxide thicknesses. This estimate is based on the results shown in Fig. 5 for the dielectric constant of the 100 V anodic films versus nitrogen content, assuming that the shift in the dielectric constant for different anodization voltages is caused by a variation in the average concentration incorporated in the anodic film. In order to determine the nitrogen distribution in the anodic films, several oxide fills have been subjected to analysis by Auger electron spectroscopy combined with in situ ion sputtering 11. The low escape depth of the emitted Auger electrons (<20 A), in combination with the closely controlled removal rate by ion sputtering, make the techniques suitable for analyzing the elemental distribution of oxygen, nitrogen and tantalum in the anodic oxide and the underlying metal with a depth resolution of about 5 ~ of oxide thickness. The results of this profiling method are shown in Fig. 6 for a pure 13-Ta f i l l and in Fig. 7 for a 13 at. ~o nitrogen alloy fill, both covered with an anodic oxide layer formed to 150 V. Shown in the two figures are the peak heights of the oxygen (510 eV),
82
R . T . SIMMONS et
6E ~
DIELECTRIC
~ .....
T
,'," 4
.....
IX
B: I--
3
METAI-~SUBSTRATE
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I 20
i
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40 60 80 tO0 SPUTTERING TIME (MINUTES)
120
I
140
Fig. 6. Peak height of Ta and O Auger signals as a function of sputtering time to illustrate distribution o f these elements in anodic films formed to 150 V on pure fI-Ta.
~
DIELECTRIC
METAL~.~SUBSTRATE
+
5 f-'.~
IX
>,-
=:4
~/,
•
!
m
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5
z
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N'r"----I----'
40 60 80 100 SPUTTERING TIME (MINUTES)
t20
140
Fig. 7. Peak height of Ta, O and N Auger signals as a function of sputtering time to illustrate distribution of these elements in anodic films formed to 150 V on Ta-13 at. ~ N alloy film.
nitrogen (380 eV) and tantalum (179 eV) Auger transitions as a function of sputtering time. There is a surface compositional change due to differences in sputtering yields of the different species. The magnitude of the compositional change for binary (Ta-O) and ternary (Ta-N-O) systems bombarded with 1 kV xenon ions is not known, but the region of this compositional change is small (50 A) and steady state conditions are established within a few minutes (see Figs. 6 and 7). Once steady state conditions are reached, the magnitudes of the Auger peak heights monitored during ion sputtering are a relative measure of the actual bulk composition of the film 12. For the anodic oxide on 13-Ta, the tantalum and oxygen peak heights are constant throughout the bulk of the oxide.
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
Ta
83
FILMS
The oxygen peak drops off sharply at the oxide-metal interface, while the Ta peak reveals a sharp increase at this interface. Such an increase is to be expected because of the higher density of the metal. The results plotted in Fig. 7 for the oxidecovered 13 at. % nitrogen alloy film suggest that there is a nitrogen-free region in the oxide. About 15{ rain of sputtering are required before the nitrogen peak appears. This peak rises rapidly and remains constant throughout the remainder of the oxide. The nitrogen signal is also constant throughout the metal. The reason for the peak of the nitrogen signal at the oxide-metal interface and its reduction in the metal are not understood. Both the oxygen and the tantalum peaks are somewhat higher in the nitrogen-free region than throughout the bulk of the oxide. Similar profiles were found for 21 and 27 at. ~o alloy films which all showed a nitrogen-free region adjacent to the top surface of the anodic film. The results of the analyses listed in Table II indicate that the sputter time needed to remove the nitrogen-free layer of the oxide is approximately proportional to the anodization voltage (oxide thickness). Table II also contains information on the total oxide thickness determined earlier, and the thicknesses of the nitrogen-free regions, assuming that the removal rate of the nitrogen-free regions is the same as that of the anodic oxide on pure ~-Ta. TABLE II NITROGEN DISTRIBUTIONIN ANODICFILMS
Film composition Anodization voltage 13-Ta T a + 13 at. ~ Ta+21 a t . ~ Ta+21 a t . ~ Ta+21 a t . ~ Ta+27 at.~
N N N N N
Oxide thickness tox
Sputtertime of N-free region
Thicknessof N-freeregion
(v)
(h)
(s~)
(h)
150 150 50 150 250 150
2460 2340 750 2250 3750 2180
4362 939 258 810 1206 654
2460 530 150 460 680 370
tm
0.23 0.20 0.20 0.18 0.17
The ratio of the nitrogen-free oxide thickness to total oxide thickness, tin, was calculated for the nitrogen alloy films and the results are listed in Table II. The ratio becomes smaller with increasing nitrogen concentration, dropping from 0.23 for the 13 at. % nitrogen alloy film to 0.17 for the 27 at. ~ nitrogen film. As the nitrogen content approaches 0 at. 9/0, tm extrapolates to the value of 0.243 found by Pringle 8 as the Ta transport number for bulk tantalum anodized at 1 mA/cm 2 in 1 M H2SO4 solution using marker techniques. This result suggests that the anodic oxidation mechanism is not greatly affected by the presence of nitrogen, which acts as an immobile marker. As in the case of pure tantalum, the anodic film grows by the simultaneous movement of both tantalum and oxygen ions through the growing oxide, while the nitrogen atoms appear to be immobile. From the present results it appears that nitrogen in tantalum behaves qualitatively like a noble gas during anodic oxidation. There is a large reduction in density in the portion of the oxide in which it is incorporated. For example, the density of the anodic films formed on pure 13-Ta is (8.3_+ 10%) g/cm a, while
84
R.T. SIMMONSet al.
the average density of the anodic film formed on 23 at. ~o nitrogen alloys is (6.0_+ 10~) g/cm3--a value that is in reasonable agreement with values found for anodic films on Ta2N in earlier work. The densities have been calculated using the following equation9:
MoxideKox2 Poxlde = Mmetal K o x Pmetai
where p is the mass density, Kox2 and Ko, are the anodization constants defined earlier (see Fig. 4) and Mo.ide and Mmcta I a r e the molecular weights of the oxide and the metal, respectively. 4. DISCUSSION In a recent investigation of the behavior of ion-implanted atoms during anodic oxidation of aluminum using Rutherford backscattering, Brown and Mackintosh 15 found that halogens (CI, Br, I) and alkali metals (Cs, Rb, K) implanted into the aluminum prior to anodization are mobile during oxide film formation. The direction of their motion indicated that the halogens acted as anions and the alkali metals as cations. Examination of anodic films formed on aluminum implanted with noble gas atoms (Ar, Kr, Xe), on the other hand, indicated that the noble gas atoms remained immobile, and their location in the oxide was controlled by the transport number of aluminum, 0.38. There are also several recent studies of the anodization behavior of Ta-based alloy systems where metals which form anodic oxides (Si 16, Nb 17, AP 8) were added as alloying agents. The distribution of the alloying agent in the oxide was found to be dependent on the transport number of the alloying agent in the oxide in comparison with that of tantalum. For example, mixed oxides formed on 75 % Ta + 25 % Si alloy films were deficient in SiO2 at the oxide-electrolyte interface 16, oxides formed on Ta-Nb alloys were rich in Nb20 5 at the outer oxide surface 17, while oxides formed on Ta films with 23 and 45 at. ~ of aluminum appeared to be homogeneous 18. Thus, the mobility of Si seems to be lower, that of Nb higher, and that of A1 about equal to the mobility of Ta. Their respective transport numbers in those alloy systems have not been determined. Considerable attention has been given to the properties of anodic oxide films formed on pure tantalum in phosphate-containing electrolytes, e.g. aqueous HaPO419-21. In this case substantial amounts of phosphorus are incorporated into that portion of the oxide which is formed adjacent to the electrolyte, and the incorporated phosphorus appears to be only slightly mobile in the film during subsequent growth of oxide 19. For the case of anodization in H3PO4, the oxide is free of incorporated phosphorus for a characteristic thickness adjacent to the metal-oxide interface, in contrast to the situation described here. This can be considered to be the converse of the present case where an immobile species is incorporated into the oxide film from the metal phase. Assuming a linear decrease in Kox2 and Ko~ between 0 and 27 at. ~ N, the average densities of the duplex films formed on the 13, 21 and 27 at. ~ nitrogen
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N - C O N T A I N I N G
Ta
FILMS
85
alloys can be calculated. Of greater interest, however, are the density, dielectric constant and electrical field during oxide growth in the nitrogen-containing portion of the oxide and their dependence on the nitrogen content in the alloy films. All three quantities have been calculated for 150 V films formed on these three alloy compositions from the total thickness of the duplex dielectric and the difference between the total thickness and that of the nitrogen-free layer (Table II). Density, dielectric constant, electric field of the nitrogen-free layer and those of the duplex dielectric are all available from the measurements. The nitrogen-free oxide region was assumed to have the same properties as the oxide formed on the pure f3-tantalum. The fields present during anodization at constant current (without the 30 min soak) are the inverse of the anodization constant (Eox = 1/Kox). Using the same field as that found for 13-Ta, 6.0 x 106 V/cm at 1 mA/cm 2, and determining the voltage across the nitrogen-free portion of the oxide, the voltage and field across the nitrogen-bearing portions of the duplex dielectric have been determined. The dielectric constants of the nitrogen-bearing layer have been calculated from the total capacitance and that of the nitrogen-free region assuming that both capacitances are in series. The dielectric constants and electric fields of the nitrogen-bearing portions of 150 V anodic films have been plotted in Fig. 8 as a function of their average mass density. The respective Ta film compositions are also shown. The results reveal a nearly linear decrease in dielectric constant (between 0 and 21 at. 9/0 N) as a function of density. This drop in dielectric constant is accompanied by a linear increase in the field needed to sustain the current during oxide growth. From the results plotted in Fig. 8 it can be shown that there is a near inverse proportionality between the dielectric constant and the anodization field with increasing nitrogen content. Such a relationship has been previously noted for anodic tantalum oxide films containing phosphorus 19 and a similar trend is evident when comparison is made between the properties of oxides formed on different valve metals TM 22. This is evidence of a general relationship between 30
7.5 27ot. %N
~
_
0 at.% N ,~. 7.0
z~ 25
t.-3t o fit:
(n z o o 20 (J m rv.
I(.1 LM
es
6.0-~
t5
/ o 27 at, % N
0 ELECTRIC FIELD 0 DIELECTRIC CONSTANT
LU
I.¢.) tu ..I ILl
I I I I I I 5.5 5.2 5.5 6.0 6.5 7.0 7.5 8.0 8.5 DENSITY OF NITROGEN CONTAINING PORTION OF ANODIC OXIDE, G/cm 3
lO
Fig. 8. Dielectric constant and electric field at 1 m A / c m 2 of the nitrogen-bearing portion of 150 V anodic films as a function of anodic film density. Also indicated are the nitrogen concentrations in the as-sputtered Ta film.
86
R.T. SIMMONSet al.
the dielectric constant of a dielectric film and the applied field necessary to pass an ionic current of given magnitude. In apparent contrast with the reported increase of the transport factor for tantalum with increasing concentration of incorporated phosphorus 19, Table II indicates that the thickness ratio of the nitrogen-free and nitrogen-bearing portion of the oxide decreases with increasing nitrogen content in the film. The values in Table II for the 150 V anodic films do not take into account any differences in density between the nitrogen-free and the nitrogen-bearing layer. If the densities are taken into account, and assuming that the stoichiometry of the nitrogen-bearing layer is governed by the amount of nitrogen present in the alloy film, the number of Ta atoms in each layer can be determined. The results of such an estimate yield 0.260+0.015 as a value for the transport factor of the Ta atoms used to form the nitrogen-free region. This value is independent of the composition of the alloy film over the whole concentration range, from 0 to 27 at. ~ nitrogen. The value of the dielectric constant may be related to the product of the number of polarizable molecules, n per cm 3, and the molecular polarizability c~. A comparison of the change in the dielectric constant of the nitrogen-bearing portion of the anodic film with increasing nitrogen concentration in the alloy films (see Fig. 8) with the decrease in the number of Ta205 molecules per cm 3 of this portion of the oxide shows the following: (i) the decrease in dielectric constant up to 10 at. ~o nitrogen is directly proportional to the decrease in the number of Ta205 molecules in the nitrogen-bearing portion of the oxide, suggesting that the polarizability of the nitrogen atoms is small compared with that of the Ta205 molecules and that up to 10 at. ~o nitrogen the molecular polarizability of the Ta205 molecules remains unaffected by the presence of nitrogen in the anodic film; (ii) above 10 at. ~ the dielectric constant drops off more and more rapidly as the number of Ta205 molecules continues to decrease. This deviation from linearity could be due to a decrease of the polarizability of the Ta205 molecules at the higher nitrogen concentrations. 5. CONCLUSIONS
The anodization mechanism of Ta-N alloy films with nitrogen concentrations as high as 27 at. ~o at 1 mA/cm2 in dilute aqueous citric acid is the same as that of pure tantalum. Both tantalum and oxygen ions are mobile, while the nitrogen appears to remain stationary during oxide growth, resulting in a nitrogen-free region of Ta205 at the oxide-electrolyte interface. Over the whole range of nitrogen concentrations the transport number for tantalum during anodization is very close to that of pure tantalum. Incorporation of nitrogen into the anodic oxide causes a steady decrease in the average anodic film density from 8.30 g/cm 3 for the oxide on pure tantalum to less than 6.0 g/cm 3 for the oxide on the 27 at. nitrogen alloy film. The electric field needed to sustain growth of the anodic oxide at a given ionic current increases over this concentration range from 6.0 x 106 V/cm to close to 7.0 x 10 6 V/cm, respectively. The decrease in anodic film density with increasing nitrogen concentration is accompanied by a steady decrease in dielectric constant from 26.5 for the oxide formed on pure [3-tantalum to less
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING Ta FILMS
87
than half for the anodic film on the 73 ~ Ta + 27 ~ N alloy film. The anodization mechanism is not affected by structural changes in the as-sputtered films which take place with increasing nitrogen content. ACKNOWLEDGEMENTS
The authors wish to express their appreciation to C. T. Hartwig (Bell Laboratories, Allentown) for the preparation and anodization of several Ta films, to J. M. Morabito (Bell Laboratories, Allentown) for the nitrogen analysis of the as-sputtered films, and to Physical Electronics Industries, Inc., Edina, Minnesota, for the elemental profiles of the anodic films. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
H. Basseches and D. Gerstenberg, Thin Solid Films, 12 (1972) 295. M . H . Read and C. Altman, Appl. Phys. Letters, 7 (1965) 51. L.G. Feinstein and R. D. Huttemann, Thin Solid Films, 16 (1973) 129. N . N . Axelrod and R. B. Marcus, Thin Solid Films, 15 (1973) $21. D. Mills, L. Young and F. G. R. Zobel, J. Appl. Phys., 37 (1966) 1821. D . G . Muth, J. Vacuum Sci. Technol., 6 (1969) 749. J . P . S . Pringle, J. Electrochem. Soc., 119 (1972) 482. J . P . S . Pringle, J. Electrochem. Soc., 120 (1973) 398. D. Gerstenberg, J. Electrochem. Soc., 113 (1966) 542. W. Juergens, Thin Solid Films, 16 (1973) 359. P.W. Palmberg, J. Vacuum Sci. Technol., 9 (1971) 160; 10 (1973) 274. J.M. Morabito, Anal. Chem., 46 (1974) 189. R. Brown in E. M. Murt and W. A. Guldner (eds.), Physical Measurements and Analysis of Thin Films, Plenum Press, New York, 1969, p. 100. P.S. Wilcox and W. D. Westwood, Can. J. Phys., 49 (1971) 1543. F. Brown and W. D. MacKintosh, J. Electrochem. Soc., 120 (1973) 1096. P.J. Silverman and N. Schwartz, J. Electrochem. Soc., 121 (1974) 550. G.C. Wood and S. W. Khoo, J. Appl. Electrochem., 1 (1971) 189. D . G . Muth and J. P. Sitarik, J. Appl. Phys., 42 (1971) 4941. J.J. Randall, Jr., W. J. Bernard and R. R. Wilkinson, Electrochem. Acta, 10 (1965) 183. C.J. Dell'Oca and L. Young, J. Electrochem. Soc., 117 (1970) 1545, 1548. D . M . Smyth, T. B. Tripp and G. A. Shim, J. Electrochem. Soc., 113 (1966) 100. L. Young, Can. J. Chem., 38 (1960) 1141.
75
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON NITROGEN-CONTAINING TANTALUM FILMS* R. T. SIMMONS**
Western Electric Company, Inc., Winston-Salem, N. C. 27102 (U.S.A.) P. T. MORZENTI**
Western Electric Company, Inc., North Andover, Mass. 01845 (U.S.A.) D. M. SMYTH Materials Research Center, Lehigh University, Bethlehem, Penn. 18015 (U.S.A.)
D. GERSTENBERG Bell Telephone Laboratories, Inc., Allentown, Penn. 18103 (U.S.A.) (Received March 20, 1974; accepted March 20, 1974)
The growth mechanism, dielectric properties and composition of anodic films on sputtered tantalum-nitrogen alloy films have been studied as a function of nitrogen content from 0 to 27 at. ~. For the anodization conditions used in this study both tantalum and oxygen ions were mobile during oxide growth while the nitrogen remained essentially stationary. The result was a nitrogen-free region in the oxide adjacent to the oxide--electrolyte interface. Incorporation of nitrogen into the anodic film caused a reduction of its density and required higher fields to produce oxide growth at a given ionic current density as compared with films grown from pure 13-tantalum. For nitrogen contents up to 21 at. ~o in tantalum the dielectric constant of the anodic oxide decreased linearly as a function of density. Structural changes in the as-sputtered tantalum films observed while the nitrogen content was increased from 0 to 27 at. ~o did not have an effect on the anodization mechanism.
1. INTRODUCTION Anodic oxidation is one of the processes used in the fabrication of tantalum film integrated circuits. It serves to form the capacitor dielectric on 13-tantalum films and as a method for trimming Ta2N resistors to a desired value 1. 13-Ta, which forms the basis of tantalum film capacitors, is a metastable allotrope of tantalum 2. Recent studies have shown that it is formed during sputtering (or * Based in part on a thesis by R. T. Simmons in partial fulfillment of the requirements for the degree of Master of Science in Metallurgy and Materials Science from Lehigh University. ** Work performed by the authors at Western Electric Co., Inc., Engineering Research Center, Princeton, N. J. 08540, U.S.A.
76
R.T. SIMMONSet al.
evaporation) of tantalum films onto the insulating substrates normally used for electronic circuits 3, 4. The anodization behavior of [3-Ta has been compared with that of bulk tantalum and b.c.c, tantalum films deposited by sputtering 5. The field E in the oxide required to produce a given ionic current and the dielectric constant of the oxide both differed by less than 4 ~ for all three types of tantalum, and the current efficiency for oxide formation on both [3- and b.c.c. Ta was 99 + 1 ~ 5, 6. Several recent studies dealing with the growth mechanism of anodic films on pure bulk tantalum have revealed that both tantalum and oxygen ions are mobile and contribute to the growth of the oxide v' s. It has also been found that the presence of nitrogen in sputtered tantalum films does not interfere with the formation of adherent anodic films 9. For example, anodic films on Ta2N display the same sequence of interference colors with increasing formation voltage, as well as equivalent dielectric strength, compared with anodic films formed on pure tantalum 9. Both the mass density and dielectric constant of anodic films on sputtered Ta2N, however, are significantly lower than the values reported for anodic films on pure tantalum. In addition, the dielectric constant was found to be dependent on anodic film thickness9. The stoichiometric composition of anodic films grown on Ta2N as determined from weight gain measurements and the growth constant was tentatively identified as that of Ta205 Nl°. Nothing is known about the dependence of the dielectric properties on the amount of nitrogen present in the sputtered films, nor about the distribution of the nitrogen in the subsequently grown anodic films. The purpose of this investigation was to establish a quantitative correlation between the properties of anodic films and the composition of the as-sputtered tantalum films. The composition of the sputtered films and the distribution of the nitrogen throughout the subsequently formed anodic films were determined by Auger electron spectroscopy (AES) in combination with in situ ion sputtering TM 12. The result found in this study, that the nitrogen is not homogeneously distributed throughout the anodic film, allows speculation about the role nitrogen plays in the oxidation of nitrogendoped tantalum films. 2.
EXPERIMENTAL PROCEDURE
Tantalum films between 1700 and 5200 A thick were prepared by d.c. diode sputtering of tantalum in argon-nitrogen mixtures in liquid nitrogen trapped, oil diffusion pumped, vacuum systems. The sputtering was carried out at a potential of 5 kV, a current density of 0.2 to 0.4 mA/cm 2, an argon gas pressure of 30 mtorr, and a substrate heater temperature of 250 °C. The interelectrode spacing was 6.35 cm. Nitrogen flow was monitored at levels between 0 and 0.6 cm 3 (standard conditions) of nitrogen per minute with a thermocouple type Hastings mass flowmeter. The substrates were 7059 Coming glass slides. The properties of the tantalum films sputtered at six different nitrogen flow rates are given in Table I. Structure and composition of selected samples from each of the six depositions were determined by X-ray techniques and Auger electron spectroscopy. The quantitative analysis of light elements (N, C, O) in sputtered tantalum by ion sputtering-Auger electron analysis via calibration with standards and
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N - C O N T A I N I N G
Ta
FILMS
77
TABLE I COMPOSITION AND PROPERTIES OF Ta FILMS SPUTTERED IN ARGON-NITROGENMIXTURES
N content (at. ~o)
C content (at. ~o)
Crystal structure
Density* (g/em a)
0 2 13 21
0 < 1 1 3
23 27
< 1 5
tetragonal (13-Ta) tetragonal (13-Ta) b.c.c. Ta b.c.c. (tetragonally distorted) h.c.p. (Ta2N) h.c.p. (Ta2 N)
Resistivity (11.0 cm)
Film thickness (/~)
16.0 15.2 14.6
183 179 126 285
4900 2060 1950 1780
14.3
355 426
5200 1720
* From Brown13.
electron microprobe analysis has been described by Morabito 12, The nitrogen concentration in the Ta films was varied from 0 to 27 at. 9/0.The error in the analysis based on the ratio of the nitrogen to tantalum Auger peak heights was _ 10 ~ of the listed value, and within the limits of error the nitrogen was homogeneously distributed throughout the thickness of the film. A number of films contained small amounts of carbon as an impurity. The values of the density of the Ta and nitrogen alloy films listed in Table I were taken from data published by Brown 13. Anodic oxide layers were formed at 24 °C by anodizing the Ta films at a constant current density of 1 mA/cm 2 in dilute aqueous citric acid solution to 50, 100, 150, 200 and 250 V. In a number of cases anodization was continued at the final voltage level for another 30 min. Plating tape* was used to delineate capacitor areas of 0.5 cm 2 for capacitance measurements at 1 kHz in a sulfuric acid cell with a Ta counterelectrode. For Ta films with 0 and 23 at. ~ nitrogen, the oxide thickness tox was determined as follows: (a) by using a Talysurf~* instrument to measure the step height of the oxide formed above the metal surface, to,,; and (b) by measuring the increase in the electrical resistance of 2000 A thick metal samples and calculating the reduction of the conducting film thickness due to anodic oxidation, to, 2 (see Fig. 1 for definition of thickness symbols). The resistor films with 0 and 23 at. ~ nitrogen used for procedure (b) were deposited
/,L
,!,
/
Fig. 1. Cross section of anodic oxide on Ta film to illustrate anodic oxide formation.
# 470 plating tape, 3M C o m p a n y . ** T a y l o r - H o b s o n , Ltd.
R . T . SIMMONS et al.
78
on sapphire substrates because sapphire is not attacked by the chemical etchant used for the delineation of the resistor pattern and permits, therefore, accurate measurement of the tantalum-nitrogen alloy film thickness, /metab after pattern delineation. The accuracy of the thickness measurements was _+5 ~. The thickness of the anodic film then is the sum of tox 1 and fox2, and the resulting accuracy of the oxide thicknesses determined for formation voltages of 50, 100, 150, 200 and 250 V was + 10~o. A number of oxide layers formed on Ta films with 13, 21 and 27 at. ~ nitrogen were subjected to Auger electron spectroscopy analysis in combination with in situ ion sputtering to determine the distribution of the nitrogen in the anodic films 11. The analysis was performed by Physical Electronics Industries, Inc., Edina, Minn. Dual Xe + beams were used to remove material at a constant rate over an area 5 mm in diameter. Each gun was operated at an emission current of 40+ 1 mA and an accelerating potential of 1.00_+ 0.03 kV. The angle of incidence was 20 ° from the specimen normal. During the sputtering operation, a 3.0 kV, 30__+ 1 gA electron beam was used to excite Auger electrons of the elements of interest. The incidence angle was 60 ° from the sample normal, and the 100/am beam was positioned in the center of the sputtered crater. A cylindrical mirror analyzer was used to analyze the secondary electron energies. The Auger electron spectrum from 0 to 600 eV was scanned at a rate of one scan per minute using the following transitions to monitor the O, N and Ta profiles in the oxide: O: K L L transition atS10 eV N: K L L transition at 380 eV Ta: N4N6N 6 transition at 179 eV From the thickness of the oxide layer and the sputtering time needed to go through the oxide, it is possible to obtain information concerning the profiles of the three elements as a function of oxide thickness. 3. RESULTS In Fig. 2 the results of the capacitance measurements of the 0.5 cm 2 test capacitors formed under constant current conditions at five anodization voltages ranging from 50 to 250 V have been used to plot the normalized capacitance, C / C o, as a function of the nitrogen content in the as-sputtered tantalum for three of the five formation voltages. Co is the capacitance of capacitors formed on pure [3-tantalum films sputtered without deliberate addition of nitrogen. Up to 11 at. ~ , the nitrogen content causes a linear decrease in the capacitance of about 1 for each at. ~ of nitrogen. From 0 to 11 at. ~ the normalized capacitance appears to be independent of the formation voltage. A more rapid decrease in C/Co is observed at higher nitrogen concentrations, and above 20 at. 5/o the normalized capacitance also becomes a strong function of the formation voltage. The dependence of the capacitance on formation voltage is illustrated in more detail in Fig. 3 for various nitrogen levels. In Fig. 3 the inverse of the capacitance per unit area (capacitance density) has been plotted as a function of formation voltage.
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
o
Ta FILMS
79
i.o
¢j
° 0.8 w
~
0.6
o
N 0.4
o 5 0 VOLTS [] 1 5 0 VOLTS ZX 2 5 0 VOLTS
.J
=E n,. z0 0.2
0 0
I I I I I 5 tO 15 20 25 NITROGEN CONCENTRATION IN SPUTTERED To, a t . %
.~
Fig. 2. Normalized capacitance C/C o as a function of nitrogen concentration in sputtered tantalum. Co is the capacitance of anodic film formed on pure 13-Ta to 50, 150 and 250 V without the 30 min soak at voltage.
u. 5 0 ~
CONTENT OF NITROGEN IN To:
_z
2 7 ot. %
40 p-
23at. %
~ 3o
21 at. %
hJ Z
13 at.% Oat.% (J
o
0
50
100
150
200
250
FORMATION VOLTAGE (VOLTS)
Fig. 3. Inverse capacitance per unit area of anodic films formed on T a - N alloy films as a function of anodization voltage (no 30 min soak at voltage).
Only the anodic films formed on pure I)-tantalum show the linear dependence expected for a dielectric whose thickness is a linear function of the formation voltage and whose dielectric constant is independent of anodic film thickness. The inverse capacitance density for the T a N alloy films deviates more and more from straight line behavior as the nitrogen content increases. It is especially
80
R . T . SIMMONS e t
al.
pronounced for the Ta films with 21, 23 and 27 at. ~. In an earlier study of the anodization of Ta2N films it was found that the anodization constant was independent of the formation voltage and that the dielectric constant of the anodic films formed on Ta2N decreases steadily as a function of dielectric thickness 9. The exact composition of the Ta2 N used by Gerstenberg, however, was not known. For both the pure [3-Ta and the 23 at. % nitrogen alloy, toxl and tox2 were a linear function of the formation voltage from 50 to 250 V. The resulting values for the anodization constants, Kox, Kox, and Kox2 are, therefore, independent of the anodization voltage. As shown in Fig. 4 a difference of ~<10% in anodization constants was found between anodic films grown at constant current and those which had been kept at voltage for an additional 30 min, independent of composition. Data of this work not shown in Fig. 4 indicate that the anodization constant Kox is a slowly decreasing function of nitrogen. The anodization constant Kox for the 23 at. % nitrogen alloy is about 11 ~ lower than that obtained for [3-Ta. The anodization constant of 16.6 A/V found for [3-Ta anodized under constant current conditions without the 30 min soak is 7% higher than the value of 15.6 A/V reported by Muth 6, but within the limits of error, which are __+10~ for the present procedure involving Talysurf step height measurements. In calculating the dielectric constants it was assumed that the anodization constant decreases linearly with increasing nitrogen concentration--an assumption that might not be entirely correct but seems justified in view of the limits of error in determining the oxide film thicknesses. In the equation e = Kox VA C/eo A
C is the measured capacitance, A the capacitor area, VAthe anodization voltage, /Cox the anodization constant in A/V and e0 = 8.86 × 10-12 F/m. The dielectric constant has been plotted in Fig. 5 as a function of nitrogen concentration for anodic films formed at 100, 200 and 250 V. The values shown
20 > •= 0 , - ~
KOX
¢R t5 IZ
Z 0
tu X 0
.
.
.
.
. . . . . .
5
""O'"
KOXI
~
~ ' ~ ~-~ ~ ~'-~,~ ~ ' ~ _ ~ 0 50 MINUTESOAK L~NOSOAK I
I
I
I
KOX2
t
5 tO t5 20 25 NITROGEN CONTENT IN SPUTTERED T0, of.%
Fig. 4. Oxide growth constants of 0 and 23 at. % N alloy films with and without 30 min soak at voltage (growth constants were determined from the measured thickness indicated in Fig. 1).
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
Ta
FILMS
8|
30
~
w z
25
tO0 v
0
F0 ul ._1
--- 15
tO
0
I I 5 to 15 20 25 NITROGEN CONTENT OF T(I FILM IN ut.%
Fig. 5. Dielectric constant of I00, 200 and 250 V anodic films as a function of nitrogen concentration in sputtered T a - N alloy films. (Anodie films did not receive 30 min soak at voltage.)
are for anodic films prepared at constant current anodization without the subsequent 30 min soak at voltage. The values for anodic films on ~-Ta and the 23 at. nitrogen alloy prepared with the 30 min soak (not shown) were within +__5 ~ of the values shown in the graph. Comparison of the present value for pure 13-Ta, 26.5 + 10 ~o, with values published in the literature for anodic f i l l on ~-Ta shows good agreement 6' 14. A steady decrease is observed in the dielectric constant with increasing nitrogen concentration in the tantalum, as found earlier 9. Above 15 at. ~o nitrogen there is also a shift of the dielectric constant toward lower values with increasing formation voltage, suggesting that the average amount of nitrogen in the oxide becomes a function of formation voltage. An estimate of the amount of nitrogen which could cause the observed decrease in the dielectric constant as the formation voltage is increased from 100 to 250 V shows that for the 27 at. ~o nitrogen alloy the difference would have to be about 3 at. nitrogen between the two oxide thicknesses. This estimate is based on the results shown in Fig. 5 for the dielectric constant of the 100 V anodic films versus nitrogen content, assuming that the shift in the dielectric constant for different anodization voltages is caused by a variation in the average concentration incorporated in the anodic film. In order to determine the nitrogen distribution in the anodic films, several oxide fills have been subjected to analysis by Auger electron spectroscopy combined with in situ ion sputtering 11. The low escape depth of the emitted Auger electrons (<20 A), in combination with the closely controlled removal rate by ion sputtering, make the techniques suitable for analyzing the elemental distribution of oxygen, nitrogen and tantalum in the anodic oxide and the underlying metal with a depth resolution of about 5 ~ of oxide thickness. The results of this profiling method are shown in Fig. 6 for a pure 13-Ta f i l l and in Fig. 7 for a 13 at. ~o nitrogen alloy fill, both covered with an anodic oxide layer formed to 150 V. Shown in the two figures are the peak heights of the oxygen (510 eV),
82
R . T . SIMMONS et
6E ~
DIELECTRIC
~ .....
T
,'," 4
.....
IX
B: I--
3
METAI-~SUBSTRATE
0
>-
m
) xlr
To
f
~-
,
2x
-I0
'0
i
[] 2 -r
o
,
I 20
i
I! A It , i .... .._j,\
~t o
al.
40 60 80 tO0 SPUTTERING TIME (MINUTES)
120
I
140
Fig. 6. Peak height of Ta and O Auger signals as a function of sputtering time to illustrate distribution o f these elements in anodic films formed to 150 V on pure fI-Ta.
~
DIELECTRIC
METAL~.~SUBSTRATE
+
5 f-'.~
IX
>,-
=:4
~/,
•
!
m
~
5
z
2X
;':;2 ,1v.
#
~t
I N
I
0
20
'
21< I
~
N'r"----I----'
40 60 80 100 SPUTTERING TIME (MINUTES)
t20
140
Fig. 7. Peak height of Ta, O and N Auger signals as a function of sputtering time to illustrate distribution of these elements in anodic films formed to 150 V on Ta-13 at. ~ N alloy film.
nitrogen (380 eV) and tantalum (179 eV) Auger transitions as a function of sputtering time. There is a surface compositional change due to differences in sputtering yields of the different species. The magnitude of the compositional change for binary (Ta-O) and ternary (Ta-N-O) systems bombarded with 1 kV xenon ions is not known, but the region of this compositional change is small (50 A) and steady state conditions are established within a few minutes (see Figs. 6 and 7). Once steady state conditions are reached, the magnitudes of the Auger peak heights monitored during ion sputtering are a relative measure of the actual bulk composition of the film 12. For the anodic oxide on 13-Ta, the tantalum and oxygen peak heights are constant throughout the bulk of the oxide.
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING
Ta
83
FILMS
The oxygen peak drops off sharply at the oxide-metal interface, while the Ta peak reveals a sharp increase at this interface. Such an increase is to be expected because of the higher density of the metal. The results plotted in Fig. 7 for the oxidecovered 13 at. % nitrogen alloy film suggest that there is a nitrogen-free region in the oxide. About 15{ rain of sputtering are required before the nitrogen peak appears. This peak rises rapidly and remains constant throughout the remainder of the oxide. The nitrogen signal is also constant throughout the metal. The reason for the peak of the nitrogen signal at the oxide-metal interface and its reduction in the metal are not understood. Both the oxygen and the tantalum peaks are somewhat higher in the nitrogen-free region than throughout the bulk of the oxide. Similar profiles were found for 21 and 27 at. ~o alloy films which all showed a nitrogen-free region adjacent to the top surface of the anodic film. The results of the analyses listed in Table II indicate that the sputter time needed to remove the nitrogen-free layer of the oxide is approximately proportional to the anodization voltage (oxide thickness). Table II also contains information on the total oxide thickness determined earlier, and the thicknesses of the nitrogen-free regions, assuming that the removal rate of the nitrogen-free regions is the same as that of the anodic oxide on pure ~-Ta. TABLE II NITROGEN DISTRIBUTIONIN ANODICFILMS
Film composition Anodization voltage 13-Ta T a + 13 at. ~ Ta+21 a t . ~ Ta+21 a t . ~ Ta+21 a t . ~ Ta+27 at.~
N N N N N
Oxide thickness tox
Sputtertime of N-free region
Thicknessof N-freeregion
(v)
(h)
(s~)
(h)
150 150 50 150 250 150
2460 2340 750 2250 3750 2180
4362 939 258 810 1206 654
2460 530 150 460 680 370
tm
0.23 0.20 0.20 0.18 0.17
The ratio of the nitrogen-free oxide thickness to total oxide thickness, tin, was calculated for the nitrogen alloy films and the results are listed in Table II. The ratio becomes smaller with increasing nitrogen concentration, dropping from 0.23 for the 13 at. % nitrogen alloy film to 0.17 for the 27 at. ~ nitrogen film. As the nitrogen content approaches 0 at. 9/0, tm extrapolates to the value of 0.243 found by Pringle 8 as the Ta transport number for bulk tantalum anodized at 1 mA/cm 2 in 1 M H2SO4 solution using marker techniques. This result suggests that the anodic oxidation mechanism is not greatly affected by the presence of nitrogen, which acts as an immobile marker. As in the case of pure tantalum, the anodic film grows by the simultaneous movement of both tantalum and oxygen ions through the growing oxide, while the nitrogen atoms appear to be immobile. From the present results it appears that nitrogen in tantalum behaves qualitatively like a noble gas during anodic oxidation. There is a large reduction in density in the portion of the oxide in which it is incorporated. For example, the density of the anodic films formed on pure 13-Ta is (8.3_+ 10%) g/cm a, while
84
R.T. SIMMONSet al.
the average density of the anodic film formed on 23 at. ~o nitrogen alloys is (6.0_+ 10~) g/cm3--a value that is in reasonable agreement with values found for anodic films on Ta2N in earlier work. The densities have been calculated using the following equation9:
MoxideKox2 Poxlde = Mmetal K o x Pmetai
where p is the mass density, Kox2 and Ko, are the anodization constants defined earlier (see Fig. 4) and Mo.ide and Mmcta I a r e the molecular weights of the oxide and the metal, respectively. 4. DISCUSSION In a recent investigation of the behavior of ion-implanted atoms during anodic oxidation of aluminum using Rutherford backscattering, Brown and Mackintosh 15 found that halogens (CI, Br, I) and alkali metals (Cs, Rb, K) implanted into the aluminum prior to anodization are mobile during oxide film formation. The direction of their motion indicated that the halogens acted as anions and the alkali metals as cations. Examination of anodic films formed on aluminum implanted with noble gas atoms (Ar, Kr, Xe), on the other hand, indicated that the noble gas atoms remained immobile, and their location in the oxide was controlled by the transport number of aluminum, 0.38. There are also several recent studies of the anodization behavior of Ta-based alloy systems where metals which form anodic oxides (Si 16, Nb 17, AP 8) were added as alloying agents. The distribution of the alloying agent in the oxide was found to be dependent on the transport number of the alloying agent in the oxide in comparison with that of tantalum. For example, mixed oxides formed on 75 % Ta + 25 % Si alloy films were deficient in SiO2 at the oxide-electrolyte interface 16, oxides formed on Ta-Nb alloys were rich in Nb20 5 at the outer oxide surface 17, while oxides formed on Ta films with 23 and 45 at. ~ of aluminum appeared to be homogeneous 18. Thus, the mobility of Si seems to be lower, that of Nb higher, and that of A1 about equal to the mobility of Ta. Their respective transport numbers in those alloy systems have not been determined. Considerable attention has been given to the properties of anodic oxide films formed on pure tantalum in phosphate-containing electrolytes, e.g. aqueous HaPO419-21. In this case substantial amounts of phosphorus are incorporated into that portion of the oxide which is formed adjacent to the electrolyte, and the incorporated phosphorus appears to be only slightly mobile in the film during subsequent growth of oxide 19. For the case of anodization in H3PO4, the oxide is free of incorporated phosphorus for a characteristic thickness adjacent to the metal-oxide interface, in contrast to the situation described here. This can be considered to be the converse of the present case where an immobile species is incorporated into the oxide film from the metal phase. Assuming a linear decrease in Kox2 and Ko~ between 0 and 27 at. ~ N, the average densities of the duplex films formed on the 13, 21 and 27 at. ~ nitrogen
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N - C O N T A I N I N G
Ta
FILMS
85
alloys can be calculated. Of greater interest, however, are the density, dielectric constant and electrical field during oxide growth in the nitrogen-containing portion of the oxide and their dependence on the nitrogen content in the alloy films. All three quantities have been calculated for 150 V films formed on these three alloy compositions from the total thickness of the duplex dielectric and the difference between the total thickness and that of the nitrogen-free layer (Table II). Density, dielectric constant, electric field of the nitrogen-free layer and those of the duplex dielectric are all available from the measurements. The nitrogen-free oxide region was assumed to have the same properties as the oxide formed on the pure f3-tantalum. The fields present during anodization at constant current (without the 30 min soak) are the inverse of the anodization constant (Eox = 1/Kox). Using the same field as that found for 13-Ta, 6.0 x 106 V/cm at 1 mA/cm 2, and determining the voltage across the nitrogen-free portion of the oxide, the voltage and field across the nitrogen-bearing portions of the duplex dielectric have been determined. The dielectric constants of the nitrogen-bearing layer have been calculated from the total capacitance and that of the nitrogen-free region assuming that both capacitances are in series. The dielectric constants and electric fields of the nitrogen-bearing portions of 150 V anodic films have been plotted in Fig. 8 as a function of their average mass density. The respective Ta film compositions are also shown. The results reveal a nearly linear decrease in dielectric constant (between 0 and 21 at. 9/0 N) as a function of density. This drop in dielectric constant is accompanied by a linear increase in the field needed to sustain the current during oxide growth. From the results plotted in Fig. 8 it can be shown that there is a near inverse proportionality between the dielectric constant and the anodization field with increasing nitrogen content. Such a relationship has been previously noted for anodic tantalum oxide films containing phosphorus 19 and a similar trend is evident when comparison is made between the properties of oxides formed on different valve metals TM 22. This is evidence of a general relationship between 30
7.5 27ot. %N
~
_
0 at.% N ,~. 7.0
z~ 25
t.-3t o fit:
(n z o o 20 (J m rv.
I(.1 LM
es
6.0-~
t5
/ o 27 at, % N
0 ELECTRIC FIELD 0 DIELECTRIC CONSTANT
LU
I.¢.) tu ..I ILl
I I I I I I 5.5 5.2 5.5 6.0 6.5 7.0 7.5 8.0 8.5 DENSITY OF NITROGEN CONTAINING PORTION OF ANODIC OXIDE, G/cm 3
lO
Fig. 8. Dielectric constant and electric field at 1 m A / c m 2 of the nitrogen-bearing portion of 150 V anodic films as a function of anodic film density. Also indicated are the nitrogen concentrations in the as-sputtered Ta film.
86
R.T. SIMMONSet al.
the dielectric constant of a dielectric film and the applied field necessary to pass an ionic current of given magnitude. In apparent contrast with the reported increase of the transport factor for tantalum with increasing concentration of incorporated phosphorus 19, Table II indicates that the thickness ratio of the nitrogen-free and nitrogen-bearing portion of the oxide decreases with increasing nitrogen content in the film. The values in Table II for the 150 V anodic films do not take into account any differences in density between the nitrogen-free and the nitrogen-bearing layer. If the densities are taken into account, and assuming that the stoichiometry of the nitrogen-bearing layer is governed by the amount of nitrogen present in the alloy film, the number of Ta atoms in each layer can be determined. The results of such an estimate yield 0.260+0.015 as a value for the transport factor of the Ta atoms used to form the nitrogen-free region. This value is independent of the composition of the alloy film over the whole concentration range, from 0 to 27 at. ~ nitrogen. The value of the dielectric constant may be related to the product of the number of polarizable molecules, n per cm 3, and the molecular polarizability c~. A comparison of the change in the dielectric constant of the nitrogen-bearing portion of the anodic film with increasing nitrogen concentration in the alloy films (see Fig. 8) with the decrease in the number of Ta205 molecules per cm 3 of this portion of the oxide shows the following: (i) the decrease in dielectric constant up to 10 at. ~o nitrogen is directly proportional to the decrease in the number of Ta205 molecules in the nitrogen-bearing portion of the oxide, suggesting that the polarizability of the nitrogen atoms is small compared with that of the Ta205 molecules and that up to 10 at. ~o nitrogen the molecular polarizability of the Ta205 molecules remains unaffected by the presence of nitrogen in the anodic film; (ii) above 10 at. ~ the dielectric constant drops off more and more rapidly as the number of Ta205 molecules continues to decrease. This deviation from linearity could be due to a decrease of the polarizability of the Ta205 molecules at the higher nitrogen concentrations. 5. CONCLUSIONS
The anodization mechanism of Ta-N alloy films with nitrogen concentrations as high as 27 at. ~o at 1 mA/cm2 in dilute aqueous citric acid is the same as that of pure tantalum. Both tantalum and oxygen ions are mobile, while the nitrogen appears to remain stationary during oxide growth, resulting in a nitrogen-free region of Ta205 at the oxide-electrolyte interface. Over the whole range of nitrogen concentrations the transport number for tantalum during anodization is very close to that of pure tantalum. Incorporation of nitrogen into the anodic oxide causes a steady decrease in the average anodic film density from 8.30 g/cm 3 for the oxide on pure tantalum to less than 6.0 g/cm 3 for the oxide on the 27 at. nitrogen alloy film. The electric field needed to sustain growth of the anodic oxide at a given ionic current increases over this concentration range from 6.0 x 106 V/cm to close to 7.0 x 10 6 V/cm, respectively. The decrease in anodic film density with increasing nitrogen concentration is accompanied by a steady decrease in dielectric constant from 26.5 for the oxide formed on pure [3-tantalum to less
PROPERTIES OF ANODIC OXIDE LAYERS FORMED ON N-CONTAINING Ta FILMS
87
than half for the anodic film on the 73 ~ Ta + 27 ~ N alloy film. The anodization mechanism is not affected by structural changes in the as-sputtered films which take place with increasing nitrogen content. ACKNOWLEDGEMENTS
The authors wish to express their appreciation to C. T. Hartwig (Bell Laboratories, Allentown) for the preparation and anodization of several Ta films, to J. M. Morabito (Bell Laboratories, Allentown) for the nitrogen analysis of the as-sputtered films, and to Physical Electronics Industries, Inc., Edina, Minnesota, for the elemental profiles of the anodic films. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
H. Basseches and D. Gerstenberg, Thin Solid Films, 12 (1972) 295. M . H . Read and C. Altman, Appl. Phys. Letters, 7 (1965) 51. L.G. Feinstein and R. D. Huttemann, Thin Solid Films, 16 (1973) 129. N . N . Axelrod and R. B. Marcus, Thin Solid Films, 15 (1973) $21. D. Mills, L. Young and F. G. R. Zobel, J. Appl. Phys., 37 (1966) 1821. D . G . Muth, J. Vacuum Sci. Technol., 6 (1969) 749. J . P . S . Pringle, J. Electrochem. Soc., 119 (1972) 482. J . P . S . Pringle, J. Electrochem. Soc., 120 (1973) 398. D. Gerstenberg, J. Electrochem. Soc., 113 (1966) 542. W. Juergens, Thin Solid Films, 16 (1973) 359. P.W. Palmberg, J. Vacuum Sci. Technol., 9 (1971) 160; 10 (1973) 274. J.M. Morabito, Anal. Chem., 46 (1974) 189. R. Brown in E. M. Murt and W. A. Guldner (eds.), Physical Measurements and Analysis of Thin Films, Plenum Press, New York, 1969, p. 100. P.S. Wilcox and W. D. Westwood, Can. J. Phys., 49 (1971) 1543. F. Brown and W. D. MacKintosh, J. Electrochem. Soc., 120 (1973) 1096. P.J. Silverman and N. Schwartz, J. Electrochem. Soc., 121 (1974) 550. G.C. Wood and S. W. Khoo, J. Appl. Electrochem., 1 (1971) 189. D . G . Muth and J. P. Sitarik, J. Appl. Phys., 42 (1971) 4941. J.J. Randall, Jr., W. J. Bernard and R. R. Wilkinson, Electrochem. Acta, 10 (1965) 183. C.J. Dell'Oca and L. Young, J. Electrochem. Soc., 117 (1970) 1545, 1548. D . M . Smyth, T. B. Tripp and G. A. Shim, J. Electrochem. Soc., 113 (1966) 100. L. Young, Can. J. Chem., 38 (1960) 1141.