Determination of Uranium and Thorium Concentration in. 16 Megabit DRAM Metallizations by Neutron Activation. Analysis. by. Robert C. Baumann (0.5 µm ...
Determination of Uranium and Thorium Concentration in 16 Megabit DRAM Metallizations by Neutron Activation Analysis. by Robert C. Baumann (0.5 µm Reliability Development, MOS Memory QRA), and Tim Z. Hossain (Central Research Laboratory) I. Introduction
Alpha decay is a naturally occurring radiation emitted from most radioactive elements and is characterized by the emission of a doubly-ionized helium atom(two protons and two neutrons) from an unstable nucleus.
The energy of an alpha particle is determined by
the quantum mechanical "state" of the nucleus from which it is emitted, and is typically in the 1-10 MeV range.
When an alpha
particle travels through a material, it loses its kinetic energy predominantly through interaction with the electrons of that material.
Thus, as an alpha passes through a material, it leaves a
trail of ionized atoms and/or molecules in its wake.
Obviously,
the higher the energy of the alpha particle, the farther it travels before being "stopped" by the material, and the larger the number of electron-hole pairs it produces.
The distance required to
"stop" an alpha particle (the range) is both a function of its initial energy and the material in which it is traveling. Generally, the denser the material, the shorter the range of an alpha particle.
Typical ranges for alphas are 5-100 µm in solids
and 1-20 cm in gases at atmospheric pressure. In silicon devices the creation of electron-hole pairs can be detrimental to device performance, especially in dynamic memory devices (ie. DRAMS) where information is stored as a transient
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charge on a capacitor.
Alpha-induced electron-hole pairs which are
collected by such a storage node can change the amount of stored charge enough to cause an error in that bit.
While destroying the
information stored, these events are known as "soft errors" since they cause no permanent damage to the cell.
The soft error rate
(SER) performance of memory devices has become more critical as the circuit density has increased from thousands of bits to tens of millions of bits per device.
The use of three-dimensional cell
designs (trenches in particular) can also increase the cell's sensitivity since the collection volume is increased.
Minimizing
the alpha emission from semiconductor processing and assembly materials is tantamount to improving the SER performance of megabit memory devices. The two impurities of primary concern are the radioactive elements uranium and thorium. Uranium emits eight alpha particles in decaying from the radioisotope U-238 to the stable isotope Pb-206 with energies between 4.1-7.7 MeV.
Thorium emits six alpha particles in decaying
from the radioisotope Th-232 to the stable isotope Pb-208 with energies between 3.9-8.8 MeV.
The decay chains for U-238 and Th-
232 are shown in tables 1 and 2.
Uranium and thorium are found
in varying concentrations in the gold used for wire bonding and plating package lids, the silica filler of the molding compound, the alumina of ceramic packages, the lead-frame alloy, and the interconnect metallizations.
Uranium and thorium may also be
present during the various fabrication steps from processing materials and the equipment itself.
An accurate assessment of the
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uranium and thorium content is necessary to define conservative limits which will ensure that alpha-induced soft errors are minimized.
Table 1. Species
Uranium Series. half-life decay mode
U-238
4.47x109 yrs
alpha
Th-234 Pa-234 U-234
24.1 days 6.69 hrs 2.45x105 yrs
beta beta alpha
Th-230
7.54x104 yrs
alpha
Ra-226
1.60x103 yrs
alpha
Rn-222 Po-218 Pb-214 Bi-214 Po-214 Pb-210 Bi-210 Po-210 Pb-206
3.82 days 3.05 min 26.8 min 19.7 min 164 usec 22.3 yrs 5.01 days 138.4 days stable
alpha alpha beta beta alpha beta beta alpha
Table 2.
Energy (MeV) 4.196(77), 4.149(23) 0.198 0.650 4.774(72), 4.723(28) 4.688(74), 4.621(26) 4.785(95), 4.602(5) 5.490 6.002 0.730 3.270 7.687 0.061 1.162 5.305
Thorium series.
Species Th-232
half-life 1.41x1010 yrs
decay mode alpha
Ra-228 Ac-228 Th-228
5.76 yrs 6.13 hrs 1.91 yrs
beta beta alpha
Ra-224
3.66 days
alpha
Rn-220 Po-216 Pb-212 Bi-212
55.6 sec 0.15 sec 10.64 hrs 60.60 min
alpha alpha beta beta(0.64) alpha(0.36) β
Energy (MeV) 4.016(77), 3.957(23) 0.039 2.100 5.426(71), 5.343(29) 5.686(94), 5.449(6) 6.288 6.779 0.569 2.251 6.336(57), 6.297(43)
α
Po-212 Pb-208
0.30 usec stable
alpha
8.785
Tl-208 Pb-208
3.05 min stable
beta
1.796
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This study has targeted the interconnect metallizations, since these sources of uranium and thorium are closest to the active devices and cannot be shielded.
The uranium and thorium content of
Ti:W, CVD-W, and AlSiCu films was determined with neutron activation analysis (NAA).
II. Neutron Activation Analysis Procedure
Very accurate (< 0.1 ppba) impurity concentration determinations are possible with NAA. step process.
The procedure is a multi-
First, the sample to be analyzed is "activated" by
exposure to the neutron flux of a nuclear reactor.
Then, if
required, all unwanted radioisotopes are chemically separated from the sample.
Finally, a detector is used to measure the output
energies and intensities of the gamma radiation emitted by the sample - information which can then be accurately related to specific impurity concentrations. Activation of the sample occurs when an incident neutron collides and binds with the nucleus of a target atom, thereby creating an unstable (radioactive) isotope, or radioisotope.
These
radioisotopes have much shorter half-lives than their parents (seconds-days vs. millions of years), so a small volume of activated elements can emit a large flux which is easily detected. The radioisotopes emit various radiations (typically alpha and beta particles, and almost always, gamma rays) whose energies are unique to the particular radioisotope emitting it.
Since all elements
present in the sample are exposed to the neutron flux, a
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corresponding fraction of every element present will be activated. The intensity of the radiation emitted will be proportional to the concentration and neutron capture cross-section(a unique property of every element based on the effective cross-sectional area of its nucleus.
This is typically 10-23-10-19 cm2) of the materials
present in the sample.
Thus by monitoring the specific energy and
intensity of radiation emitted from a sample, the types and concentrations of virtually all impurities present can be accurately determined.
Figure 1.
Various steps in the NAA procedure.
The focus of the NAA procedure was the determination of the uranium and thorium content in the 16 megabit DRAM metallizations. For each metallization, two 6" oxide pilot wafers with 5000 kÅ of deposited metal were irradiated with a neutron flux of 2x1013 /cm2sec for 14 hours (small samples of the Ti:W and AlSiCu sputtering
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targets were also irradiated) at the Texas A & M nuclear facility. The nuclear reactions which are induced in the activated uranium and thorium impurities are illustrated in figure 1.
The uranium-
238 is converted into uranium-239, which quickly decays into neptunium-239.
The resulting concentration of neptunium-239 beta-
decays into plutonium-239.
It is the gamma photons (227 and 228
KeV) emitted during this decay which are used to calculate the uranium concentration.
Beyond this point the decay chain
essentially stops due to the long-lived plutonium radioisotope. The thorium decay chain is similar, starting with the conversion of thorium-232 into thorium-233.
This is followed by the beta-decay
of the thorium-233 into protactinium-233.
The resulting
protactinium-233 then beta-decays into uranium-233, simultaneously emitting gamma photons (312 KeV) which are detected and counted to determine the thorium concentration.
Again, the decay process is
restrained by the long-lived uranium radioisotope. Unfortunately the NAA procedure does have some drawbacks. Typically, since the entire sample is exposed to the neutron flux, the signal from the radioisotopes created from the host material will initially "drown out" the much weaker impurity signal. Impurities with half-lives which are longer than the host will eventually be detected when the strength of the host signal, which is diminishing at a faster rate, drops below the impurity signal. The problem arises when the radioisotopes of the host are longer lived than the radioisotopes of the impurities, since in this case, the signal from the host will always mask the impurity signal.
One
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solution to this problem is to chemically separate the host radioisotopes from the impurity radioisotopes. Use of chemical separation techniques was necessitated for the 16 megabit DRAM metallizations to remove the host radioisotopes of copper and tungsten which would have masked the uranium and thorium signals.
After the neutron activation the metallizations were
etched from their silicon substrates.
For Ti:W and CVD-W a heated
solution of hydrogen peroxide was used, while for the AlSiCu, nitric acid was used.
The resulting solutions were placed in
beakers with zinc foils for approximately twelve hours.
In the
presence of zinc, an ion exchange occurred; the radioactive copper ions and tungsten oxides were removed from the solution and accumulated in the zinc foil (by a substitutional process), while the non-radioactive zinc ions were dissolved into the solution.
By
this process, enough of the radioactive copper and tungsten were removed to enable sensitive gamma counting of the uranium and thorium impurities.
The gamma counts were then correlated to
calculations of the volume of material obtained from the metallization samples.
In this way, accurate uranium and thorium
concentrations were obtained.
III. Results
The results of all the NAA experiments have been tabulated and are shown below. The values obtained from the actual target materials are more accurate than the film values because the mass of material, and therefore, the gamma signal level, was substantially greater than that of the films.
The impurity
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concentrations are expressed as inequalities, indicating that the numbers were the minimum detection limits of the NAA procedure for those particular samples. Table 3.
U and Th concentrations in TI 16 mb DRAM metals. Ti:W Ti:W AlSiCu AlSiCu CVD-W Target Film Target Film Film
Thorium
< 0.1 ppb < 0.5 ppb
< 0.1 ppb
< 0.1 ppb
< 0.5 ppb
Uranium
< 0.1 ppb < 0.5 ppb
< 0.1 ppb
< 0.1 ppb
< 0.5 ppb
All impurity concentrations are expressed as atomic percent.
To
convert to weight percent, simply find the ratio of the mass of the host material to that of the mass of the impurity, and multiply by the atomic concentration. A similar experiment was performed with 16 megabit Ti:W and CVD-W metallization samples obtained from Hitachi Ltd.
As with the
previous experiment, the samples consisted of 5 kÅ of metal on 6" oxide pilots.
The results obtained with NAA are shown below: Table 4. Uranium and thorium concentrations in Hitachi 16 mb DRAM metals. Ti:W CVD-W Film Film Thorium
< 0.5 ppb
< 0.5 ppb
Uranium
< 0.5 ppb
< 0.5 ppb
The TI and Hitachi maximum uranium and thorium impurity concentration specification
is shown below.
Although Hitachi has
more stringent specifications, there is no measurable difference
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between the uranium and thorium concentrations in Ti and Hitachi metallizations. Table 5. TI and Hitachi uranium and thorium impurity specifications. Material
Uranium Thorium Limits TI Hitachi
AlSiCu
< 0.5 ppb
< 0.1 ppb
Ti:W
< 2.0 ppb
< 0.1 ppb
CVD-W
< 2.0 ppb
< 0.1 ppb
As mentioned earlier, NAA impurity levels obtained from the target materials were thought to be more accurate since the greater mass of target material produced a larger gamma signal.
However,
uranium and thorium impurities which might be introduced during processing(especially from process equipment in plasma or hightemperature environments) were not present in the target materials. Thus while the film impurity levels obtained with NAA may not be as accurate, the films may actually have higher impurity levels than those of the targets. Although the focus of this study was the determination of the uranium and thorium impurity levels in the 16 megabit metallizations, NAA can also be used for general impurity analysis. The technique is an excellent means of monitoring virtually any metallic impurity(depending on the composition of the host material) down to, and often, below the ppb level.
While not
specifically targeted, several other impurities were observed in the metallizations.
The detection limits of NAA for a number of
metallic impurities in silicon are shown in table 7.
These
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detection limits could also be achieved in polysilicon, silicon oxide, and silicon nitride samples.
As mentioned previously,
since the sensitivity of NAA is limited by the half-lives of the host radioisotopes relative to the half-lives of impurity radioisotopes.
In metal samples, which often have radioisotopes
with relatively Table 6. Metallic impurity concentrations detected by NAA in AlSiCu metallizations Metallic Impurity W Au Fe Sc
Concentration (ppma) 4.1 3.5x10-3 27.4 1.0
long half-lives, the detection limits are often significantly higher than those shown in table 7.
In some cases certain
impurities are masked completely, thus necessitating the use of separation chemistry. Table 7. NAA detection limits for metallic impurities in Silicon. Metallic Detection Impurity Limit (ppba) Na 0.08 Cr 0.01 Mn 0.02 Fe 2.00 Co 0.002 Ni 4.00 Cu 0.002 Zn 0.01 As 0.0004 Mo 0.002 Sb 0.0016 W 0.002 Au 0.00001 Ag 0.002
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IV. Discussion and Conclusion
NAA is an extremely useful technique for the determination of impurity concentrations down to, and often below, ppb levels.
In
this study, NAA has been utilized to determine the uranium and thorium concentrations in the various target metals and the resulting metallizations used in the 16 Meg DRAM.
The results
indicated that the uranium and thorium content of both Texas Instruments' and Hitachi's metallizations was below 0.5 ppba. Samples of the Ti:W and AlSiCu target materials used in the deposition of the Texas Instruments' metallizations had uranium and thorium impurity levels of less than 0.1 ppba.
This study revealed
that all target and metallization samples were below Texas Instrument's maximum uranium and thorium content specifications, and that even though Hitachi's specifications are significantly lower, their metallizations had similar uranium and thorium content. Currently, NAA and alpha counting studies are being performed to determine the uranium and thorium content of the various materials (molding compound, PIX, tape, bond wire, and lead frame alloy) used in the manufacturing process so that their impact on the SER performance of the 16 Meg DRAM can be ascertained.
V.
References.
(1) E. W. Haas and R. Hofman, The Application of Radioanalytical Methods in Semiconductor
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Technology, Solid-State Electronics, Vol. 30, no. 3, p. 329 (1987). (2) L. F. Bellido and B. de C. Arezzo, Uranium and Thorium Determination in Brazilian Coals by Epithermal Neutron Activation Analysis, J. Radioanal. Nucl. Chem., Vol. 92, p. 151 (1985). (3) M. Pimpl and H. Scheuttelkopf, A fast radiochemical procedure to measure Neptunium, Plutonium, Americium, and Cerium in environmental monitoring and in radioecology research, Proc. 5th Int. Conf. Nucl. Methods in Environ. and Energy Res., Vogt., J.R., ed., NTIS, Springfield, Va., p. 216 (1984). (4) F. Girardi, R. Pietra, and E. Sabbioni, Radiochemical Separations by Retention on Ionic Precipitate Adsorption Tests on 11 Materials, J. of Radioanal. Chem., Vol. 5, p. 141 (1970). (5) L. A. Currie, Limits for Qualitative Detection and Quantitative Determination, Anal. Chem., 40(3), p. 586 (1968). (6) G. Friedlander, J. W. Kennedy, E. S. Maclas, and J. M. Miller, Nuclear and Radiochemistry, Third Edition, John Wiley & Sons, New York, N.Y. (1981) VI.
Acknowledgements
We would like to thank the following people for the metal samples used in this study; Tom Bonifield, Rock Blumenthal, Cary Pico, and Dennis Yost.
We would also like to thank Kadota-san of Hitachi, and
the TI/Hitachi GT Working Committee for obtaining the Hitachi metallization samples.
In addition we would also like to thank
Cheryl Blackburn for doing the gamma counting experiments.
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Appendix A - Natural Abundance of Uranium and Thorium.
In trying to ascertain the amount of uranium and thorium impurities found in various materials, particularly metals and silicates (such as those found in the plastic molding compound), knowledge of the natural abundance of these impurities in terrestrial crust and various rocks can be of value.
While some
refinement processes may reduce uranium and thorium impurities to immeasurable quantities, other processes might not affect the quantity of these impurities at all.
Estimates of the uranium and
thorium content of terrestrial crust and rocks are shown below (all data obtained from the Geochemical Table, H. J. Rosler and H. Lange, Elsevir, Amsterdam, 1972.):
I.
Uranium and Thorium concentration estimates for terrestrial
crust - lithosphere Quarke 1924
Friesman 1939
Goldschmi dt 1947
(?) 1962
Taylor 1964
Uranium (ppm)
80
4
4
2.5
2.7
Thorium (ppm)
20
10
11.5
13
9.6
II.
Uranium and Thorium concentrations in rock Acid Acid Basic Granite Rock Rock (high (low Ca) Rock Ca)
Sandston e
Uranium (ppm)
3.0
3.0
1.0
3.5
0.45
Thorium (ppm)
8.5
17
4.0
18
1.7
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By the latest accounts, uranium to thorium ratios in terrestrial crust and various rocks seem to range from 1:3 to almost 1:6, thus it may be reasonable to assume at least a 1:3 uranium - thorium ratio exists in most materials before refining has taken place.
The naturally occurring
concentrations of these two impurities is 1-20 ppm.
If such levels were
allowed in processing materials, they would have disasterous impact on SER performance of megabit memory systems.
