semi合集-English.pdf - 第7585页
SEMI MF2139-1103 © SEMI 2003 5 7 Apparatus 7.1 SIMS Instr ument — Equipped with a cesium primary ion source, electron multiplier detector , and Faraday cup det ector, capable of measuring ne gative secondary ions. The S …
SEMI MF2139-1103 © SEMI 2003 4
[N
b
] = contribution of background nitrogen to the
observed nitrogen concentration under the
higher sputter rate conditions,
[N
B
] = contribution of background nitrogen to the
observed nitrogen concentration under the
lower sputter rate conditions,
I
B
= secondary ion intensity from the adsorbe
d
b
ackground nitrogen under both sputter rate
conditions,
I
si
= secondary ion intensity of the silicon matrix
(e.g.,
30
Si) under the higher sputter rate
conditions,
I
SI
= secondary ion intensity of the silicon matrix
(e.g.,
30
Si) under the lower sputter rate
conditions,
RSF = relative sensitivity factor for converting the ion
intensity ratio to concentration, and
[N] =
b
ulk nitrogen concentration in the test
specimen.
6.8 Equations (1) through (4) can be used to determine
[N] and [N
b
] from measured data as follows:
)/(1
)/(1
][
siSI
nN
si
n
II
II
I
I
RSFN
−
−
××= (5)
][][ N
I
I
RSFN
si
n
b
−×= (6)
In these equations, the secondary ion intensities used
are averages of these signal intensities. In the example
shown in Figure 1, the lower sputter rate was used at
the beginning of the profile, and the higher sputter rate
in the middle of the profile. For this example, [N] was
determined to be 8 × 10
14
/cm
3
and [N
b
] was 2 ×
10
14
/cm
3
. The use of averages for I
n
and I
N
is
particularly critical for detecting low levels of nitrogen
where the signal intensity count rates can be low and
therefore noisy. By using averages, it is possible to
detect an [N] level that is less than [N
b
] in the analysis.
6.9 The SIMS measurement of nitrogen in bulk-doped
silicon sometimes reveals anomalous intensity spikes
that are greater than a random fluctuation of signal
intensity, even in silicon that has not been thermally
processed beyond crystal growth.
6,7
These nitrogen
intensity spikes are not well understood, but are
suspected to be associated with oxygen defects,
possibly a co-aggregation of nitrogen and oxygen in a
precipitate form. Similar SIMS effects have been well
known for oxygen and even carbon in thermally-
processed silicon.
11
Figure 2 shows an expanded view
of the carbon and nitrogen data taken from Figure 1.
11 Hockett, R. S., Fraundorf, P. B., Reed, D. A., and Wayne, D. R.,
“Oxygen and Carbon Defect Characterization in Silicon by SIMS,” in
Oxygen, Carbon, Hydrogen and Nitrogen in Crystalline Silicon,
edited by J. C. Mikkelsen, Jr., S. J. Pearton, J. W. Corbett, and S. J.
Pennycook, Materials Research Society Symposia Proceedings Vol
59 (Materials Research Society, Pittsburgh, PA, 1986) pp. 433-438.
The vertical scale is linear. The unusually high
nitrogen signal at about 325 s may be one of these
anomalous intensity spikes. The presence of such non-
random signals, if significant, can make the averaging
method used here questionable.
NOTE: Top curve (
!) Mass 42 (
14
N
28
Si), bottom curve
(
") Mass 40 (
12
C
28
Si)
Figure 2
SIMS Signals from Nitrogen-Doped Silicon, Linear
Scale
6.10 Depending upon the carbon concentration in the
test specimen and the level of nitrogen, it may be
necessary to correct the
14
N
28
Si signal for an interfer-
ence from
12
C
30
Si. This correction is done by co-
detecting
12
C
28
Si at mass 40, and calculating the
contribution of
12
C
30
Si from isotopic ratios. The
contribution to mass 42 from a carbon-related signal at
mass 40 is 0.0341 times the signal at mass 40. In the
example in Figure 1, where the actual carbon concen-
tration is about 1.5 × 10
15
/cm
3
by SIMS calibration, the
carbon interference is insignificant. A carbon concen-
tration of 5 × 10
15
/cm
3
(the detection limit of most
Fourier Transform Infrared (FITR) measurements of
carbon in silicon) corresponds to an interference equi-
valent to about 8 × 10
13
/cm
3
of nitrogen. Thus, in
detecting nitrogen in the low 10
14
/cm
3
level, a co-
measurement of the carbon interference is critical.
6.11 Depending upon the boron concentration in the
test specimen, the procedure to correct for the carbon
interference may need to be modified by detecting
11
B
28
Si at mass 39, and calculating the contribution of
11
B
29
Si and
10
B
30
Si at mass 40. This procedure
becomes important for test specimens with boron
concentrations at 1 × 10
17
/cm
3
and greater. In the
example in Figure 1, the boron concentration is on the
order of 10
15
/cm
3
and does not interfere.
SEMI MF2139-1103 © SEMI 2003 5
7 Apparatus
7.1 SIMS Instrument — Equipped with a cesium
primary ion source, electron multiplier detector, and
Faraday cup detector, capable of measuring negative
secondary ions. The SIMS instrument should be
adequately prepared (that is, baked) so as to provide the
lowest possible instrumental background, since the
instrumental background can affect the detection
capability. High quality vacuum is desired. A liquid
nitrogen- or liquid helium-cooled cryopanel, which
surrounds the test specimen holder in the analysis
chamber, may be helpful.
7.2 Test Specimen Holder
7.3 Oven — For baking the test specimen holder.
7.4 Stylus Profilometer — Or equivalent device to
measure SIMS crater depths. This device is required to
calibrate depth scale for concentration profiles of
calibration standard samples.
8 Reagents and Materials
8.1 No reagents or materials are required for this test
method.
9 Safety Precautions
9.1 The preparation of silicon test specimens from
silicon substrates requires the crystal to be cleaved or
broken. This procedure may generate very sharp
silicon shards. Care must be taken to protect the body
and particularly the eyes from these shards.
10 Sampling, Test Specimens, and Test Units
10.1 Since this procedure is destructive in nature, a
sampling procedure must be used to evaluate the
characteristics of a group of silicon wafers. No general
sampling procedure is included as part of this test
method, because the most suitable sampling plan
depends upon individual conditions. For referee
purposes, a sampling plan shall be agreed upon before
conducting the test. See ASTM Practice E 122 for
suggested choices of sampling plans.
10.2 The calibration or reference material shall be
silicon crystal with boron concentration below 2 × 10
17
atoms/cm
3
and implanted with ions of
14
N or
15
N; an
ion implant of energy 200 keV and dose of about 1 ×
10
14
atoms/cm
2
is preferred.
10.3 Sample specimens must have a chemo-mechanical
polished surface on the side used for analysis.
11 Calibration and Standardization
11.1 Calculate the RSF for an element in a matrix
experimentally from the SIMS profile of a reference
material containing a known areal density of the
impurity of interest as follows:
)( nISId
tInD
RSF
bi
m
−
= (7)
where:
D = a real density of the impurity, atoms/cm
2
,
n = number of data cycles in the profile,
I
m
= matrix isotope secondary ion intensity, counts/s,
d = sputtered depth, cm,
SI
i
= sum of the impurity isotope secondary ion
(counts over the depth of the profile),
I
b
= constant background intensity of the impurity
isotope, counts/cycle and
t = analysis time for the species of interest, s/cycle.
11.2 Calculate the calibration of concentration in the
SIMS profile as follows:
EM
FC
RSF
I
I
C
m
i
i
××= (8)
where:
C
i
= impurity atomic concentration, atoms/cm
3
,
I
i
= isotope secondary ion intensity, counts/s,
I
m
= matrix isotope secondary ion-intensity,
counts/s,
RSF = relative sensitivity factor, atoms/cm
3
, and
EM
FC
= ratio of matrix intensities on the Faraday
cup (FC) to the electron multiplier (EM),
when two detectors are used.
11.3 Complete the conversion of data cycles or time
into depth by measuring the crater depth and the total
time (or data cycles) of the sputtering that formed the
crater.
NOTE 1: The depth scale is assumed to be linear with time or
data cycles.
12 Procedure
12.1 Cleave or break specimens from the silicon
substrates in order to fit within the SIMS sample holder.
Specimens must be handled under normal analytical
laboratory practices, but no special ambient is required.
12.2 Load the specimens into the SIMS sample holder,
checking to see that the specimens are flat against the
backs of the windows and cover the windows as much
as is possible. A specimen load includes one
calibration specimen, and one or more test specimens.
12.3 Bake the loaded sample holder at 100 ± 10° C for
a minimum of 1 h in air.
12.4 Turn on the instrument in accordance with the
manufacturer' s instructions.
SEMI MF2139-1103 © SEMI 2003 6
12.5 If a cryopanel is used, fill the liquid nitrogen or
helium cold trap.
12.6 For the calibration specimen use a cesium primary
ion beam and detect the negative ion of
14
N
28
Si or
15
N
28
Si, depending upon the nitrogen isotope used in
the ion implantation; detect during or at the end of the
profile a matrix negative ion (
28
Si,
29
Si, or
30
Si). After
the entire sample load is analyzed, measure the SIMS
crater depth in this sample using a profilometer.
12.7 Position the specimen holder to sputter a crater in
a test specimen near the center of the window. Without
detecting secondary ion intensities, begin a SIMS
profile.
12.8 Repeat Section 12.7 for each test specimen.
12.9 If the boron level in the test specimen is below 1
× 10
17
/cm
3
, detect the negative ions of
12
C
28
Si,
14
N
28
Si,
and a matrix signal (
28
Si,
29
Si,
30
Si, or
30
Si
2
). If the
boron level is at or above 1 × 10
17
/cm
3
, include the
detection of the negative ion of
11
B
28
Si.
12.10 For the analysis of the test specimens, select two
raster conditions, one to maximize the sputter rate, and
one that provides a slower sputter rate. Choose
apertures or other means to maintain the analysis area
under raster change and to keep the negative ion count
rates on the electron multiplier detector below 1 × 10
5
counts/s for the test specimen.
12.11 Position the specimen holder to sputter a new
crater near the crater created in Section 12.7 or Section
12.8 . Do not make the second crater in the same crater
created in Section 12.7 or Section 12.8 .
12.12 Center the primary ion beam and begin a SIMS
profile with the raster area that produces the lower
sputter rate. Continue the profile until all signals
provide relatively constant intensity for at least 20 data
points.
12.13 Change the raster area to maximize the signal
intensity. Continue the profile under this raster
condition for at least 20 data points.
12.14 Reduce the raster area back to the original
condition.
12.15 If the boron content is less than 1 × 10
17
/cm
3
,
subtract 0.0341 times the measured mass 40 signal
(counts/s) from the measured mass 42 signal (counts/s)
to obtain the corrected mass 42 signal corresponding to
real nitrogen (
14
N
28
Si).
NOTE 2: If mass 40 carbon (
12
C
28
Si) is defined = 1, mass 42
(
12
C
30
Si +
13
C
29
Si) = 0.0335 + 0.0006 = 0.0341.
NOTE 3: A hydride correction should be negligible for H on
the order of 10
19
atoms/cm
3
or less.
12.16 If the boron concentration is at or greater than 1
× 10
17
/cm
3
, subtract 0.0583 times the mass 39 signal
from the carbon correction to correct for the boron
interference.
NOTE 4: The mass 39 boron (
11
B
28
Si +
10
B
29
Si) is defined =
1, and mass 40 (
11
B
29
Si +
10
B
30
Si) = 0.0583.
12.17 Calculate the bulk nitrogen concentration [N]
and the background nitrogen concentration [N
b
] for
each test specimen using Equations 5 and 6.
12.18 Record the specimen identification.
13 Report
13.1 Report the following information:
13.2 The instrument used, the operator, and the date of
the measurements,
13.3 Identification of the test and calibration
specimens,
13.4 The bulk nitrogen concentration values for the test
specimens,
13.5 The nitrogen instrumental background concentra-
tion during the faster sputter rate, and
13.6 Whether anomalous nitrogen spikes were present,
and if so, whether the averaging included or excluded
these spikes.
14 Precision and Bias
14.1 The precision of the nitrogen determination has
not yet been determined either by pilot study or by
interlaboratory test.
14.2 There is no accepted reference material for
determining the bias for the procedure in this test
method. Therefore, no statement on bias can be made.
15 Keywords
15.1 nitrogen concentration; secondary ion mass
spectrometry; silicon; SIMS