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SEMI MF1389-0704 © 2004 3 4.2 ASTM St andard D 5127 — Guide for Ultra Pure Water Used in the Electronics and Sem iconductor Industry 7 NOTICE: Unless ot herwise indi cated, all documents cited shall be the latest publish…
SEMI MF1389-0704 © 2004 2
3 Limitations
3.1 Variations in Excitation Intensity — The extrinsic
bound exciton (BE) and intrinsic free exciton (FE)
luminescence features do not vary at the same rate with
excitation intensity.
4
The FE features increase
proportionally with excitation intensity, while the BE
features increase proportionally at low excitation levels
but become more slowly increasing at higher excitation
levels generally above the electron hole droplet (EHD)
onset point. Since the calculated concentrations are
derived from the ratio of extrinsic features to the
intrinsic feature, the ratio will be decreasing as the
excitation intensity is increasing. Thus, if a sample is
measured at a higher excitation level than the
instrument is calibrated for, the calculated
concentration will be artificially low, and vice versa.
3.2 Sample surface damage, bulk defects, or other
lifetime reducers also affect the line-intensity ratios and
overall luminescence intensity owing to their tendency
to reduce the steady state population of excitons, thus
mimicking the effects of a lower excitation intensity.
Other mechanisms related to the presence of these
defects may also introduce decalibrating effects. Some
luminescence features are generated by defects; for
example, those at 6510 and 7050 cm
−1
are typical of
thermally stressed samples. Such features can provide
qualitative information about the presence of defects.
5,6
3.3 The ratios and line widths of the silicon
luminescence features vary strongly with temperature,
hence variations in sample temperature must be
avoided. Corrections for the effects of temperature
variations are included in these test methods (see
Section 7.1).
3.4 Overlapping Spectral Features
3.4.1 The boron B
TO
(BE) feature at 8812.6 cm
−1
overlaps the P
TO
(b′
1
) feature at 8812.7 cm
−1
causing a
direct error when calculating boron concentration. The
beta-series P
TO
(b′
1
) line is approximately one-tenth the
intensity of the alpha series P
TO
(BE) line at 8806.6
cm
−1
, so a subtraction based on the amount of
phosphorus present can be made in the boron feature
measurement. An alternative approach is to use the
B
TO
(b
1
) boron line, which is free from a coincident
4 Nakayama, H., Nishino, T., and Hamakawa, Y.,“Analysis of the
Excitation Luminescence of Silicon for Characterization of the
Content of Impurities,” Jap. J. Appl. Phys. 19(3), 501–511, (1980).
5 Drozdov, N. A., Patrin, A. A., and Tkachev, V. D.,
“Recombination Radiation on Dislocations in Silicon,” Soviet
Physics, J.Experimental and Theoretical Physics 23(11), 597 1976).
6 Suezawa, M., Sasaki, Y., Nishina, Y., and Sumino, K., “Radiative
Recombination on Dislocations in Silicon Crystals,” Jap. J. Appl.
Phys. 20(7), L537 (1981).
phosphorus line, provided the instrument has been
calibrated for this feature.
3.4.2 The antimony Sb
TO
(BE) line falls between the
boron and phosphorus transverse optical (TO) features.
Since the line widths are broad relative to the line
positions of these features, the presence of antimony
affects the apparent intensities of boron and phosphorus
TO features. The no-phonon, (NP) features for these
elements could be used providing the instrument is
calibrated for them.
3.4.3 In general, the broad TO phonon region features
can interfere, particularly when a sample contains
widely differing levels of impurities. For example,
when boron greatly exceeds phosphorus, the P
NP
(BE)
line must be used, and when phosphorus greatly
exceeds boron, one of the approaches described in
Section 3.4.1 must be used.
3.5 Stress in samples can cause NP region feature
splitting and thus affect the established TO/NP line
ratio for a given impurity. The concentrations
calculated from such spectra would be artificially low.
Peak area calculation methods reduce the
measurement' s sensitivity to line splitting effects.
3.6 The calibration curves presented in these test
methods are traceable to resistivity measurements (see
SEMI MF723), neutron transmutation doped, (NTD)
reference material, and low-temperature infrared
absorption spectroscopy (FT-IR) measurements (as a
secondary standard) (see SEMI MF1630). Hence
inaccuracies in the work that produced the calibration
curves impact the accuracy of the PL results.
4 Referenced Standards
4.1 SEMI Standards
SEMI C28 — Specifications and Guidelines for
Hydrofluoric Acid
SEMI C30 — Specifications and Guidelines for
Hydrogen Peroxide
SEMI C35 — Specifications and Guideline for Nitric
Acid
SEMI MF723 — Practice for Conversion Between
Resistivity and Dopant Density for Boron-Doped,
Phosphorus-Doped, and Arsenic-Doped Silicon
SEMI MF1630 — Test Method for Low Temperature
FT-IR Analysis of Single Crystal Silicon for III-V
Impurities
SEMI MF1723 — Practice for Evaluation of
Polycrystalline Silicon Rods by Float-Zone Crystal
Growth and Spectroscopy
SEMI MF1389-0704 © 2004 3
4.2 ASTM Standard
D 5127 — Guide for Ultra Pure Water Used in the
Electronics and Semiconductor Industry
7
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Definitions
5.1.1 defect luminescence lines — those features arising
from defect structures in the silicon.
5.1.2 electron hole droplet (EHD) — the condensed
phase (liquid) of the excitonic gas generated by
photoexcitation.
5.1.2.1 Discussion — The exciton population is
dependent upon excitation intensity and can be raised to
the point where the exciton density is sufficient to allow
condensation of the exciton gas into a liquid like
exciton state.
8
The existence of EHD luminescence can
be an indirect measurement of excitation power density
on the sample.
5.1.2.2 EHD onset occurs at the point in the sample
excitation intensity curve where the electron-hole-
droplet begins to form.
9
The EHD luminescence
includes a very broad feature underlying the TO region
impurity lines which, with increasing excitation
intensity, both intensifies and shifts to lower energies
relative to the other silicon luminescence features.
5.1.3 excitons — the electron-hole pairs that give rise to
the luminescence of interest upon recombination at
either a free lattice site (free exciton) or an impurity
atom site (bound exciton).
5.1.4 extrinsic line (X
TO
(BE) or X
NP
(BE)) — the
luminescence that arises from an exciton captured by an
impurity site in the crystal lattice (a bound exciton).
5.1.4.1 Discussion — Its energy is lower than the
intrinsic emission by an amount related to the exciton
binding energy of the impurity at 4.2 K. “X” is the
impurity element symbol and “BE” indicates bound
exciton luminescence line. Extrinsic luminescence also
includes features attributed to bound multi-exciton
complexes (b
1
, b
2
, or b
3
would indicate the first, second
and third bound multi-exciton complex lines,
respectively). In donor luminescence, these complexes
7 Annual Book of ASTM Standards, Vol 11.01, ASTM International,
100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:
610-832-9500, Fax: 610-832-9555, Website:
www.astm.org.
8 Nakashima, H., and Shiraki, Y., “Role of Shallow Impurities and
Lattice Defects in Nucleation of Electron-Hole Droplets in Si,” Solid
State Comm. 40, 195–197 (1981).
9 Hammond, R. B., and Silver, R. N., “Onsets of the Electron-Hole
Droplet Luminescence in Si,” Phys. Rev. Lett. 42(8), 523–526 (1979).
give rise to two series of lines in the TO region, called
the alpha and beta series. The weaker beta series
features are denoted by an apostrophe after the line
notation (that is, P
TO
(b′
1
)). See Table 1 and Table 2 for
line locations.
5.1.5 intrinsic line (I
TO
(FE)) — the luminescence that
arises from the silicon itself, with no impurity species
affecting the exciton recombination.
10,11
Table 1 Photoluminescence Line Locations
(Vacuum Wavenumbers)
Silicon Free Exciton (FE) Lines:
I
TO
(FE) at 8848 cm
−1
I
LO
(FE) at 8860 cm
−1
I
TA
(FE) at 9166 cm
−1
Major Shallow Impurity Bound Exciton (BE) Lines:
Element
TO region,
cm
−
1
NP region,
cm
−
1
NP/TO region BE
line intensity ratio
A
Boron 8812.6 9281.3 0.017
(Antimony) (8810.5) (9280.0) (0.010)
Phosphorus 8806.8 9275.4 1.4
Aluminum 8803.4 9271.8 0.7
Arsenic 8801.0 9269.4 2.0
NOTE 1: To convert vacuum wavenumbers to air wavenumbers:
air wavenumber = 1.00030025 × vacuum wavenumber
NOTE 2: To convert wavenumbers to electronvolts:
eV = 1.23985 × 10
−4
× wavenumber
A
Instrument resolution = 0.5 cm
−1
, sample = FZ silicon.
Table 2 Photoluminescence Line Locations
(Vacuum Wavenumbers) Detailed Listing for Boron
and Phosphorus
Boron Features, cm
−
1
Phosphorus Features, cm
−
1
B
NP
(BE) = 9281.3 P
NP
(BE) = 9275.4
B
NP
(b
1
) = 9263.6 P
NP
(b
1
) = 9246.4
B
NP
(b
2
) = 9245.9 P
NP
(b
2
) = 9223.9
P
NP
(b
3
) = 9208.4
P
NP
(b
4
) = 9197.8
B
TA
(BE) = 9130.1 P
TA
(BE) = 9124.4
B
TA
(b
1
) = 9112.4 P
NP
(BE-2e) = 8992.8
A
B
TA
(b
2
) = 9095.2
B
TO
(BE)= 8812.6
P
TO
(b
1
′) = 8812.7
B
B
TO
(b
1
) = 8795.0 P
TO
(BE) = 8806.8
C
10 Tajima, M., “Determination of Boron and Phosphorus Concen-
tration in Silicon by Photoluminescence Analysis,” Appl. Phys. Lett.
32(11), 719 (1978).
11 Tajima, M., “Quantitative Impurity Analysis in Si by the Photo-
luminescence Technique,” edited by J. Nishizawa, Japan Annual
Reviews in Electronics, Computers, and Telecommunications—
Semiconductor Technology (OHM—North Holland, 1982).
SEMI MF1389-0704 © 2004 4
Boron Features, cm
−
1
Phosphorus Features, cm
−
1
B
TO
(b
2
) = 8777.5
P
TO
(b
2
′) = 8790.4
B
B
TO
(b
3
) = 8763.2 P
TO
(b
1
) = 8778.0
C
B
TO
(b
4
) = 8752.1
P
TO
(b
3
′) = 8771.3
B
B
TO
(b
5
)= 8742.6 P
TO
(b
2
) = 8756.0
C
P
TO
(b
4
′) = 8756.2
B
P
TO
(b
5
′) = 8745.4
B
P
TO
(b
3
) = 8740.2
C
A
The two-electron transition is represented by 2e (see Ref (3)).
B
Beta series transition.
C
Alpha series transition.
5.1.5.1 Discussion — “I” indicates intrinsic silicon
emission, “TO” indicates the transverse optical phonon
associated with the transition, and “FE” refers to the
free exciton recombination responsible for the emission
(see Table 1 for line locations).
5.1.6 phonon — a quantum of lattice vibrational
energy, as a photon is a quantum of electromagnetic
energy.
5.1.6.1 Discussion — The recombination of excitons in
silicon requires a momentum conserving mechanism
owing to the indirect band gap of the crystal. Phonons
provide such a mechanism. The principal phonon types
of interest in silicon luminescence are the transverse
acoustic (TA), transverse optical (TO), and longitudinal
optical (LO) phonons. These test methods address the
use of features associated with the TO phonon as well
as those not including phonon emission in their
momentum-conserving processes. These latter features
are designated “NP” or no-phonon features.
6 Summary of Test Method
6.1 A sample of monocrystalline silicon is cooled to 4.2
K and photoexcited with greater-than-bandgap energy
light at one of two intensities listed, depending upon the
type of instrument used. The resulting luminescence is
collected and recorded. Spectral features corresponding
to intrinsic silicon and extrinsic impurity emissions are
measured and related to calibration curves to yield
dopant density.
7 Apparatus
7.1 Cryostat — To maintain sample temperature at 4.2
K. Either open-cycle liquid helium immersion or
exchange gas cryostats, or closed-cycle refrigeration
systems may be used. The bath immersion type
cryostat is recommended for higher confidence in the
temperature stability of the sample. In both the
exchange gas and closed-cycle systems, careful
attention must be paid to thermal sinking and accurate
sample temperature measurement (see Section 3.3).
7.2 Sample Holder — Which does not cause excessive
strain on the ample through spring forces or other
means, so as to avoid line splitting associated with
crystal stresses.
7.3 Laser Excitation Source — Capable of generating
electron hole pairs in the silicon crystal. An argon-ion
laser operated at 514.5 nm is used. To obtain accurate
measurements, laser light intensity must be controllable
and stable.
7.4 Infrared Spectrophotometer — Equipped with a
detector and optics suitable for use between 8750 and
9300 cm
−1
, and capable of at least 0.5 cm
−1
resolution at
9300 cm
−1
.
8 Reagents
8.1 Purity of Water — Reference to water shall be
understood to mean Type E-1 or better water as
described in ASTM Guide D 5127.
8.2 Nitric Acid (HNO
3
), 65%, in accordance with Grade
2 of SEMI C35.
8.3 Hydrofluoric Acid (HF), 48%, in accordance with
Grade 2 of SEMI C28.
8.4 Hydrogen Peroxide (H
2
O
2
), 30%, in accordance
with Grade 2 of SEMI C30.
8.5 Acid Cleaning Mixture, (1:1:1:25) HNO
3
:HF:H
2
O
2
:
H
2
O.
8.6 Hazards — Chemicals used in this procedure are
potentially harmful and must be handled with the
utmost care at all times.
9 Sample Preparation
9.1 Perform either the procedure in Section 9.1.1 or the
procedure in Section 9.1.2 to remove all work damage
and surface contamination on all samples except as-
received chemical-mechanically polished wafers, which
do not need further preparation.
9.1.1 Etch sample using a suitable etchant (for example,
the acid cleaning mixture of Section 8.5 or CP-5
hydrofluoric-nitric-acetic acid etchant).
9.1.2 Polish the surface of the sample with a suitable
chemical-mechanical polishing compound.
9.2 Luminescence efficiency has been observed to
decrease after etching, thus introduction of the sample
into the cryostat within a few hours of etching is
recommended. Chemical-mechanically polished
samples appear to be more stable.
2