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Factors Affecting the Quantification of Boron in SiO2 and Si by Sputtered Neutral Mass Spectrometry

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SURFACE AND INTERFACE ANALYSIS,
VOL. 24,
371-374 (1996)
Factors Affecting the Quantification of Boron in
SiOz and Si by Sputtered Neutral Mass
Spectrometry
Michael L. Wise, Netzer Moriya and Stephen W. Downey*
Lucent Technologies,Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974, USA
Boron implanted in SiOz and Si is characterized using sputtered neutral mass spectrometry (SNMS). Nonresonant, ultrahigh-intensity postionization finds that a fraction of boron not sputtered as neutral atoms from SiO,
and Si is partially present in the form of BO and BSi molecules. Total boron detection is matrix sensitive. A deficit
in the boron measured in SiO, suggests that boron may go partially undetected due to the substantial production of
secondary ions during sputtering.
INTRODUCTION
EXPERIMENTAL
The measurement of the distribution of boron in thin
layers of Si and SiO, continues to be of great importance to microelectronic technologies. As device sizes
shrink below 0.5 pn, the determination of boron concentrations and gradients by sputter-based techniques
like secondar ion mass spectrometry (SIMS) in and
through 100 f l sub-layer of SiO, is exceedingly difficult
due to stringent depth resolution requirements and
strong matrix effects. The accurate determination of
boron in the thin SiO, gate oxides is effectively precluded.
Postionization detection of neutrals often reduces
matrix effects because the atomization via sputtering
and laser ionization are separate processes. As a result,
many times only changes in relative sputter rates have
any substantial effect on measured signals during depth
profiling through thin layers. The application of
ultrahigh-intensity (> loi4 W ern-,) laser pulses allows
non-resonant ionization of all sputtered species.'*' Consequently, in many cases, the sputtered species are
detected with nearly uniform sensitivity. Constant
detection efficiency of all sputtered species, including
many molecules, can result in more accurate and standardless determination of dopant profiles in thin layers
of differing materials. However, if the assumption that
the majority of sputtered species are neutral is not valid,
then the manner in which postionization is implemented is critical for obtaining satisfactory results in thinlayer depth profiling.
In this work, ultrahigh-intensity postionization mass
spectrometry is used to profile ion-implanted "B in
SiO, and Si. Three different sputtering ions are utilized.
Perhaps surprisingly, total boron detection is found to
be matrix sensitive. Those factors creating this postionization 'matrix effect' and possible solutions to this circumstance are discussed.
The postionization experiments are performed with a
modified magnetic-sector SIMS instrument in which the
output of the laser system is directed into the ionization
region of the mass spectrometer.' The laser is a regeneratively amplified Ti :sapphire system based on an argonion pumped, self-mode-locked oscillator and a
frequency-doubled YAG pumped amplifier. Output
pulses at 800 nm contain up to 1 mJ of energy in a
pulse width of 100-150 fs with a focused diameter of
-50 pm. The repetition rate is -1 kHz. The pulse
width is measured by a scanning Michelson
interferometer-type:autocorrelator with 10 fs resolution.
The average intensity present in each pulse can be in
excess of loi4 W cm-,.
The ion source is a mass-filtered duoplasmatron
capable of producing O,', Xe' and Ar'. The ion
energy on target is adjustable from 2 to 10 keV, but for
these experiments it is 3 keV. At this energy, the incident angle of the primary ion beam on the sample is
estimated to be 60" off-normal. The operating current is
1-2.5 pA and the beam is -100 pm in diameter. The
primary ion beam is pulsed for postionization. Laser
pulses entering the chamber are synchronized with a
pulsed, primary ion beam for sample sputtering. An ion
pulse precedes a laser pulse by -2 ps and is -0.5 ps in
duration. All the timing delays are set to maximize the
laser beam/sample atom cloud overlap in space and
time for the greatest detected signal while minimizing
sample erosion during data acquisition. Depth profiles
consist of alternating cycles of continuous, rastered
primary ion patterns for material removal and pulsed
sputtering for static analysis of the center of the crater
bottom. The SIMS data are obtained in the same
fashion while the laser is blocked, with the sample bias
lowered 300 V to select those ions produced at the
sample surface to pass through the energy analyzer.
The sample is loo0 A of thermally grown SiO, on
Si(l00) which is "B-implanted (26 keV, 1 x 10'' cm-')
such that the total area density in each material is the
* Author to whom correspondence should be addressed.
CCC 0142-2421/96/06037
1-04
0 1996 by John Wiley & Sons, Ltd
Received 9 August 1995
Accepted 22 January 1996
M. L. WISE, N. MORIYA AND S. W. DOWNEY
372
same, 5 x 1014 anT2.
This was confirmed by the "B[p,
a] nuclear reaction analytical technique3 on the layered
sample and on a portion of the sample with the oxide
film removed by HF e t ~ h i n gThe
. ~ peak of the implant,
located at the interface, has a boron concentration of
9 x 1019~ m - ~ .
can be observed while sputtering with O,+ across the
interface, while a three orders of magnitude drop in
signal can be seen on the Xe+- and Ar+-sputtered
samples. These changes are the classic SIMS matrix
effect. Predictably, the presence of oxygen in the matrix
considerably enhances positive secondary ion yield^.^
The matrix effect for O2 sputtering while transitioning
from SiO, to Si is far less severe than that for Xe' and
Ar+ sputtering due to the greater oxygen content of the
Si substrate.
A postionization depth profile of the same sample
using Xe+ sputtering is shown in Fig. 2. A depth profile
using Ar + sputtering (not shown) is almost identical.
Because of the non-selective, universal detection of sputtered neutrals, a variety of species are tracked with one
laser wavelength (800 nm). A survey of the possible
sputtered species from the two substrates indicates that,
in addition to IIB+ and "Si',
2eSi160+(mIe = 441,
+
RESULTS A N D DISCUSSION
Three conventional SIMS profiles of the "B-implanted
1O00 A SiO, on Si sample using Xe+, Ar' and 02+
sputtering are shown in Fig. 1. We note that no significant sputtering rate changes between these two
materials are observed when using these primary ions.
An order of magnitude drop in the secondary ion signal
Depth
(A)
Figure 1. Normalized "B SlMS depth profiles of O,+-,Ar+- and Xe+-sputtered 1000 A SiO, on Si implanted with B (26 keV, 1 x I O l 6
cm-*). The peak of the implant is at the interface where a sharp decrease in SlMS signal is found.
1 o6
t
o5
-
6
._
10'
(I)
c
'a
s
..-E
1
o3
L
B
1 o2
10
'
0
1000
2000
Depth
3000
(A)
Figure 2. Postionization depth profile of the l1
B-implanted I O O O A SiO, on Si sample obtained using Xe+ sputtering. The average intensity
of the focused laser beam is 4 x 1O'* W cm-*.
QUANTIFICATION OF BORON IN SiO, AND Si BY SNMS
11B160' (m/e = 27) and 11B28Si' (m/e = 39) are
detectable. Interestingly, the B+ profle displays a
sudden increase at the SiO,/Si interface, opposite to the
matrix effect encountered in the SIMS results. Integration of the prolile indicates that 30% of the atomic
boron is detected in the SiO, with 70% detected in the
Si. Molecular BO' and BSi' also represent a large
fraction of the total detected neutral boron under these
sputtering conditions. We cannot prove that BSi and
BO are detected with the same efficiency as atomic
boron. It is suspected that the molecules are detected
somewhat more efficiently because of their expected
lower velocities. The molecular percentages stated are
probably an upper limit to their sputtered fractions.
Ignoring any surface peaks, which contain hydrocarbon
isobars and environmental boron, BO ' accounts for
40% of the total detected boron in the oxide while BSi'
accounts for 30% of the total detected boron in the Si.
Nonetheless, adding the signals from BO' and BSi' to
the B' signal does not significantly change the disproportionality in the measured boron in the two materials.
Less total neutral boron is sputtered from the SiO, than
Si.
,' sputtering
Postionization of the sample with 0
detects the same sputtered species as those obtained
with noble gas ion sputtering (Fig. 3). Likewise, a disproportionality in measured boron in the two materials
is observed but is less severe. Integrating the atomic
signal indicates that 40% of the atomic boron is
detected in the SiO,, as compared to 60% detected in
the Si substrate. Molecular contributions are different
with O2 sputtering than with the noble gas sputtering;
BO' accounts for 30% of the total boron detected in
the oxide, while BSi+ and BO+ account for only 20%
of the total boron detected in the Si. Again, accounting
for the boron detected as BO' and BSi+ does not significantly alter the deficit in detected boron in the SO,.
The magnitude of the sputtered neutral signals from Si
are noted to be lower when using 0 2 +
as compared to
Ar+ or Xe'.
Many factors can affect the signals obtained with a
postionization technique. Ionization efficiency, sputter
+
373
yield, angular and velocity distributions of sputtered
neutrals, molecular contributions, instrumental biases,
as well as the production of secondary ions during sputtering can all affect the measured signals. Unfortunately,
these different effects are extremely difficult to distinguish from one another when looking only at relative
signals. In these experiments, a difference in the postionization detection of boron in SiO, and Si is apparent.
Assuming that the molecular species in the two
materials are being detected uniformly and with similar
efficiency to the atomic boron, molecular contributions
are not able to compensate for the disproportionality.
Likewise, differences in ionization efficiency, instrumental biases and angular/velocity distributions are unlikely
to significantly affect the measurement of the same
species from two different matrices.
One factor which may be especially important in this
specific chemical system is the production of secondary
ions. During the postionization experiments the sample
is biased such that secondary ions produced by the
sputtering are not allowed through the energy spectrum
analyzer, nor are they subjected to the laser light,
because the high extraction field sweeps them out of the
laser focal spot during the delay optimized for the neutrals. For this reason, no secondary ions are counted
during the postionization experiments. Correspondingly, significant production of secondary ions detracts
from the sputtered neutral species which are probed by
the laser. Figure 1 indicates that the "B' SIMS signal
has a large matrix effect at the interface of the two substrates, especially when using the noble gas ions for
sputtering. This large decrease in secondary ion yield
when going from the SiO, to the Si may explain the
inverse trend in the postionization signals due to the
suppressionn of sputtered neutrals by secondary ion
production.
The effect of secondary ion matrix effects on postionization signals may be surprising. Nonetheless, under
certain sputtering conditions, a large fraction of the
total sputtered species may be secondary ions. In these
experiments, SIMS signals from the SiO, are always
greater than the postionization signals, even with the
1 o2
1 0'
1 1 28
+
8 8
0
500
1000
2000
1500
Depth
2500
3000
(A)
Figure 3. Postionization depth profile of the "B-implanted 1OOOA SiO, on Si sample obtained usingo,' sputtering.
M. L. WISE, N. MORIYA AND S. W. DOWNEY
314
short (500 ns) sputtering pulses. The extraction parameters of the SIMS and postionization are different
enough to prevent an effective comparison of absolute
ion us. neutral yields, but the observation supports the
contention that ions are a large fraction of the sputtered
material in the SiO, . Absolute secondary ion yields
(secondary ion/incident ion) have been measured for a
few metallic species using secondary ion mass spectrometers with known secondary ion transmissions6-' and
using other techniques.' The absolute secondary ion
yield for Si+ from oxygen-covered silicon using 3 keV
Ar sputtering was determined by Benninghoven to be
O X 6 , ' Without the adsorbed oxygen, this yield drops
to 8.4 x 10-3.
The current Si+ postionization results mirror these
absolute secondary ion yields. Figure 2 indicates that
the Si+ postionization signal changes by a factor of 4.1
when going from SiO, to Si. Because SiO, and Si have
the same sputter rates with Xe+ sputtering, a factor of
2.1 increase in Si' when transitioning to the Si substrate is predicted based on the greater density of elemental Si in the Si matrix. The additional factor of two
observed in the postionization signal can be explained
by a secondary ion yield in the SiO, approximately
equal to the yield of neutrals. Upon transitioning to the
Si substrate, this secondary ion yield is reduced by a
factor of 1000 and the corresponding yield of neutrals is
doubled.
While the absolute secondary ion yield of B+ has not
been measured, boron and silicon in general display
similar relative sensitivity factors (RSFs) when sputtered
from SiO, and Si with O,'.' For this reason, it may be
reasonable to assume that the secondary ion fraction of
B + from SiO, is high. A secondary ion yield of 30-50%
in SiO, (B' plus BO') would explain the undetected
neutral boron in the SiO,. As indicated by the SIMS
results in Fig. 1, this secondary ion fraction would drop
to 5% or less in the Si substrate with the corresponding
increase of the boron postionization signal. The matrix
effect observed here would not be too much different for
+
any other type of SNMS technique because of the inherent secondary ion production in this chemical system. It
is noteworthy that this matrix pair may be the most
severe with respect to this effect. Sputtered neutral mass
spectroscopy can be successfully applied to the analysis
of many other material systems where secondary ion
emission is low (<10%).The importance of SiO, and Si
to the microelectronics industry warrants this special
investigation.
To alleviate this matrix effect caused by secondary
ions, an alternative ion extraction scheme, like that
commonly used in time-of-flight mass spectrometry,
would be beneficial. If sputtering is performed in a fieldfree region, then the neutrals and ions move roughly
with the same velocities. A pulsed extraction field coincident with the laser pulse would accept all ions from
the ionizing volume, regardless of their origin, thus
summing the secondary ions with the postionized neutrals. In this scenario, secondary ions as well as neutrals
are subjected to the intense laser field, increasing the
possible contribution from multiply charged ions to the
mass spectrum. Time-of-fight detector with adequate
mass resolution to separate isobars would be ideal for
this type of measurement. An intense peak anticipated
from the sputtered matrix constituents would have to be
handled by rapid mass blanking or detector gain adjustment.
CONCLUSIONS
Postionization techniques have determined how boron
sputtered from SiO, and Si fractionates into atoms, ions
and molecules. Using Xe', Ar' and 0,' sputtering, a
significant amount of boron was sputtered from SiO,
and Si as BO and BSi molecules. The shape of the
boron profiles is affected by secondary ion emission,
which in the present experimental geometry subtracts
from the sputtered neutrals available for postionization.
REFERENCES
1. C. H. Becker and J. S. Hovis, J . Vac. Sci. Technol.A 12, 2352
(1994).
2. M. L. Wise, A. B. Emerson and S. W. Downey, Anal. Chem. 67,
4033 (1995).
3. L. C. Feldman and J. W. Mayer, Fundamentals of Surface and
Thin Film Analysis. North-Holland, New York (1 986).
4. A. 6. Emerson, Y. Ma, M. L. Wise, M . L. Green and S. W.
Downey, J. Vac. Sci. Technol. J. Vac. Sci. Technol., B14, 301
(1996).
5. R. G. Wilson, F. A. Stevie and C. W. Magee, Secondary Ion
Mass Spectrometry: a practical Handbook for Depth Profiling
and Bulk IrnpurityAnalysis.Wiley, New York (1989).
6. A. Benninghoven, Surf. Sci. 53,596 (1975).
7. A. Benninghoven, F. G. Riidenauer and H. W. Werner, Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental
Aspects, Applications and Trends. Wiley, New York (1 987).
8. P. Williams, I. S. T. Tsong and S. Tsuji, Nucl. Instrum. Methods
170,591 (1980).
9. W. Husinsky, J. Vac. Sci. Technol. B 3,1546 (1985).
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