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SEMI MF1535-1104 © SEMI 2004 6 9.3 The chem icals used for etching and for some surface passivating solutions are potentially harmful and must be handled in a n acid exha ust fume hood, with proper protecti v e gear incl…
SEMI MF1535-1104 © SEMI 2004 5
7.4 Wafer Mounting Stage — For holding the wafer
(with vacuum hold down) in the desired position under
the pulsed light source. The stage may contain a heater
for controlling its temperature over a small temperature
range above room temperature. It may be driven by
computer controlled motors to provide x-y or r-
motion for mapping capability over the wafer surface
and may have automatic wafer loader and transport to
facilitate automatic sequential measurement of a group
of wafers.
7.5 System for Analysis of the Decay Signal —
Appropriate signal conditioners and display unit (real or
virtual oscilloscope with suitable time sweep and signal
sensitivity). The signal conditioner shall have a
bandwidth 40 MHz, or a minimum sampling time 25
ns (Note 2). The display unit shall have a continuously
calibrated time base with accuracy and linearity better
than 3%. The system shall be such that the time
constant of user-specified portions of the decay signal
can be established independently.
7.6 Computer System — Although the measurement
can be made manually, it is recommended that it be
carried out using a suitable computer system that
controls the wafer loading, stage motion, the pulse and
detector operation, decay signal analysis, statistical
analysis of the data, data logging and storage, and
printing or plotting of results.
7.7 Facilities for Etching or Passivating Wafer
Surfaces — If required.
7.7.1 For Chemical Passivation — A fume hood
equipped with an acid-proof sink and suitable beakers
or other containers for holding wet chemicals, including
hydrofluoric acid at room temperature and protective
gear appropriate to the chemicals used.
7.7.2 For Oxidation — A clean furnace capable of
high quality dry oxidations at temperatures of 950° to
1050°C and associated cleaning, drying, and wafer
handling facilities.
7.8 Wafer Holder — If required. In some cases, it may
be necessary to measure the wafer while it is immersed
in a passivating solution (see Section 11). In this case,
a flat chemically inert, optically transparent holder is
required to contain both the wafer and the passivating
solution.
8 Reagents
8.1 Purity of Reagents — All chemicals for which such
specifications exist shall conform to Grade 1 SEMI
specifications for those specific chemicals. Other
chemicals shall conform to reagent grade, as specified
in Reagent Chemicals.
9
Other grades may be used,
provided it is first determined that the chemical is of
sufficiently high purity to permit its use without
lessening the accuracy of the test.
8.2 Purity of Water — Reference to water shall be
understood to mean Type E-3 or better water as
described in ASTM Guide D 5127.
8.3 The recommended chemicals shall have the
following nominal assays:
8.3.1 Ethanol (CH
3
CH
2
OH) — Absolute, 99.9%.
8.3.2 Iodine (I
2
) — >99.8%.
8.3.3 Hydrofluoric Acid (HF) — Concentrated, 49.00 ±
0.25%. Warning: see Section 9.3 for warning
statement.
8.3.4 Nitric Acid (HNO
3
) — Concentrated, 70.0–
71.0%.
8.4 Iodine-Ethanol Passivating Solution — Mix 1 g
iodine with 100 mL ethanol.
NOTE 3: Other passivating solutions may be utilized
provided that they (1) reduce the surface recombination
velocity to a value at which surface recombination no longer
interferes with the determination of the bulk recombination
lifetime (see Section 11), and (2) result in stable surfaces (see
Section 6.2.1).
8.5 Bright Etching Solution, for Etching Non-Polished
Surfaces — Mix 95 mL concentrated HNO
3
with 5 mL
concentrated HF. Warning: See Section 9.3 for
warning statement.
8.6 Dilute HF Solution, for Etching Surface Oxide
Films — To obtain 100 mL of a 2 % solution of HF,
mix 4 mL concentrated HF with 96 mL of water.
Warning: See Section 9.3 for warning statement.
9 Hazards
9.1 The laser illumination system should be interlocked
so that direct observation of the laser beam is
prevented. Warning: Do not operate the laser
illumination system with the interlock disabled.
9.2 The microwave system should be shielded and
interlocked so that personnel cannot come into contact
with the beam. Warning: Do not operate the
microwave system with the interlock disabled.
9 Reagent Chemicals, American Chemical Society Specifications,
American Chemical Society, Washington, DC. For suggestions on
the testing of reagents not listed by the American Chemical Society,
see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole,
Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC),
Rockville, MD.
SEMI MF1535-1104 © SEMI 2004 6
9.3 The chemicals used for etching and for some
surface passivating solutions are potentially harmful
and must be handled in an acid exhaust fume hood,
with proper protective gear including safety goggles,
and with utmost care at all times. Warning:
Hydrofluoric acid solutions are particularly hazardous.
HF solutions should not be used by anyone who is not
familiar with the specific preventive measures and first
aid treatments given in the appropriate Material Safety
Data Sheet.
10 Sampling
10.1 If the test method is not used on a 100%
inspection basis, sampling procedures shall be agreed
upon by the parties to the test.
10.2 If sampling by lot is required, the determination
of what constitutes a lot and the procedures for
sampling and the procedures for sampling by lot shall
be agreed upon by the parties to the test.
10.3 Because the concentration of recombination
centers in a wafer may be non-uniform, it is desirable to
determine the recombination lifetime at various points
across the wafer surface. The point density and
location of points measured shall be agreed upon by the
parties to the test.
11 Test Specimen Preparation
11.1 The required test specimen preparation depends
on both the surface condition of the test specimen and
the expected magnitude of the bulk recombination
lifetime,
b
, to be measured.
11.2 No test specimen preparation is required if the
value of
b
is no greater than one-tenth of the surface
recombination lifetime,
s
. The surface recombination
lifetime is composed of two terms, a diffusion term,
diff
, which accounts for the diffusion of carriers to the
surface, and a surface recombination term,
sr
, which
accounts for the recombination at the surface. The
surface recombination lifetime may be computed from
the following approximate relation:
10
S
L
D
L
srdiffs
2
2
2
(1)
where:
D = minority carrier diffusion coefficient, in cm
2
/s,
L = wafer thickness, in cm, and
10 Horányi, T. S., Pavelka, T., and Tüttô, P., “In Situ Bulk Lifetime
Measurement on Silicon with Chemically Passivated Surface,”
Applied Surface Science, Vol 63, 1993, pp. 306–311.
S = surface recombination velocity, in cm/s, assumed
equal on both surfaces.
Electron and hole surface recombination lifetimes are
shown in Figure 2 as a function of surface
recombination for wafers with different thickness
11
(Note 4).
NOTE 4: If S is very large (>10
4
cm/s) excess carriers
recombine immediately on striking the surface so the surface
recombination lifetime is dominated by
diff
. A well polished
surface has a surface recombination velocity of ~10
4
cm/s
10
while for an abraded (lapped) surface the surface
recombination velocity is even larger (~10
7
cm/s, the carrier
saturation velocity). In such cases, the maximum bulk
recombination lifetime that can be measured to 10% accuracy
in wafers of standard thickness is about 1 s for p-type wafers
and about 2 s for n-type wafers. In spite of this limitation of
accurate determination of the bulk recombination lifetime, it
is possible to detect relative variations of bulk recombination
lifetime on unpassivated polished wafers that have bulk
recombination lifetime as large as 0.5 to 1 ms provided that
the following conditions are met:
(1) the diffusion coefficient and surface recombination
velocity are uniform over the wafer, and
(2) the microwave system is sensitive enough to resolve
measured lifetimes which differ by 1%.
Under these same conditions, relative measurements can be
made on lapped wafers with bulk recombination lifetimes up
to about 100 s. In this case, it may be necessary to etch the
surfaces in bright etching solution (see Section 8.5) for about
1 min in order to obtain sufficient uniformity of the surface
recombination velocity.
11.3 If bulk recombination lifetimes larger than about
0.1
s
are to be measured, the wafer surfaces must be
passivated by one of the following methods (Note 5) to
obtain accurate measurements.
11.3.1 Oxidation — Bulk recombination lifetimes up
to about 1 ms can be measured on wafers 0.5 mm
thick that have a very high quality (dry) thermal oxide
(D
it
< 10
10
/cm
2
·eV) (Note 6). Ensure that the oxidation
conditions are such that significant numbers of oxide
precipitates do not form during the oxidation cycle (see
Section 3.2.2). For measurement of lifetimes between
about 1 ms and 10 ms, strip the oxide in dilute HF (see
Section 8.6) and make the measurement within 15 min.
11 For these estimates, the diffusion coefficients were assumed to be
constant at the following limiting values: D
n
= 33.5 cm
2
/s and
D
p
= 12.4 cm
2
/s. These values are somewhat smaller than the limiting
values given in the 1993 edition of DIN 50 440, Part 1.
SEMI MF1535-1104 © SEMI 2004 7
Note: The maximum bulk recombination lifetime that can be accurately measured is about 1/10 of the surface lifetime.
Figure 2
Surface Recombination Lifetime as a Function of Surface Recombination Velocity for Constant Diffusion
Coefficient and Selected Values of Wafer Thickness
11.3.2 Immersion in Passivating Solution — To
measure bulk recombination lifetimes up to ~1 ms on a
bare polished wafer 0.5 mm thick, first pretreat the
wafer in iodine-ethanol passivating solution (see
Section 8.4) or an alternative passivating solution.
Then enclose the wafer in a small plastic bag or other
fixture containing enough of the passivating solution to
coat the surface with a thin film while the measurement
is being made. Ensure that the passivation technique
results in stable and repeatable measurements before
proceeding with the test (see Section 3.2.1).
11.3.2.1 If the wafer is oxidized, passivate it with
iodine-ethanol, an alternative passivating solution, after
first removing the oxide by etching in dilute HF
solution for a time that depends on oxide thickness;
etch times range from about 30 s for thin oxide (<5 nm)
to about 10 min for thick oxide (~200 nm).
11.3.2.2 Again, ensure that the passivation technique
results in stable and repeatable measurements before
proceeding with the test (see Section 3.2.1).
NOTE 5: Polished surfaces that have been oxidized or
passivated with certain chemical solutions have much reduced
surface recombination velocity. For example, carefully
prepared thermally oxidized silicon surfaces have surface
recombination velocity as low as 1.5 to 2.5 cm/s while the
surface recombination velocity can be as low as 0.25 cm/s
following stripping of the oxide in hydrofluoric acid.
12
This
reference also outlines a procedure for determining surface
recombination velocity. Immersion in the iodine-ethanol
passivating solution (Section 8.4) has been shown to reduce
surface recombination velocity of a chemically polished,
oxide-free silicon wafer to 10 cm/s.
10
12 Yablonovitch, E., Allara, D. L., Chang, C. C., Gmitter, T., and
Bright, T. B., “Unusually Low Surface-Recombination Velocity on
Silicon and Germanium Surfaces,” Phys. Rev. Lett. 57, 249–252
(1986).
NOTE 6: The density of interface trapped charge (D
it
) can be
measured by a variety of techniques described in the
literature;
13
however, none of these techniques has been
standardized.
12 Procedure
NOTE 7: The following procedures are given in sufficient
detail for manual data collection and analysis. However it is
strongly recommended that instrument setup, data collection,
and analysis be carried out using computer-controlled
equipment, with data storage and display capabilities. In such
cases, the procedures and algorithms employed must be
equivalent to those given in this section.
12.1 If they are not known, determine the conductivity
type in accordance with SEMI MF42, the center-point
wafer thickness in accordance with SEMI MF533 or
SEMI MF1530, and the center-point resistivity in
accordance with SEMI MF84 or SEMI MF673.
Convert the resistivity to the density of the majority
carriers (n
maj
, in carriers/cm
3
) in accordance with SEMI
MF723. Record these data together with the nominal
diameter and the condition (polished, etched, lapped,
as-cut, etc.) of the front and back surfaces.
12.2 Record the temperature of the room, or if the
stage is temperature-controlled, the temperature of the
stage surface.
12.3 Load the wafer onto the stage so that the light
pulse will strike the desired region.
12.4 Switch on the pulsed laser light source (see
Section 7.1).
12.5 Adjust the intensity so that the injection level,
,
is at the specified value. If an injection level has not
been specified, set it to 100. If it is not adjusted
13 See, for example, Schroder, D. K., Semiconductor Material and
Device Characterization (John Wiley & Sons, New York, 1990) pp.
267–286.