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SEMI MF1535-1104 © SEMI 2004 5 7.4 Wafer Mounting Stage — For holding the waf er (with vacuum hold do wn) in the desire d position under the pulsed light source. Th e stage may contain a heater for controlling its temper…
SEMI MF1535-1104 © SEMI 2004 4
minority and majority carrier capture time constants
(see Related Information 1).
5.1.3 surface recombination velocity — a measure of
the recombination of excess minority carriers at the
surface of a semiconductor crystal or wafer given by
the ratio of the surface-directed hole or electron current
to the product of the hole or electron charge and hole or
electron density at the surface.
5.2 Definitions of other terms used in silicon
technology may be found in SEMI M1 and SEMI
MF1241.
6 Summary of Test Method
6.1 Excess hole-electron pairs are locally created in the
wafer for a very brief time by a short pulse (width 200
ns, rise and fall times 25 ns) of light with energy
slightly greater than the width of the forbidden energy
gap at a specified power density (injection level). The
specimen surface is conditioned in such a way that
surface recombination has a negligible effect on the
decay of the conductivity following cessation of the
light pulse. This decay is monitored by means of
microwave reflectance, and the carrier recombination
lifetime is determined as the time constant of the
appropriate portion of the exponential conductivity
decay.
6.2 A narrow-beam light source may be used so that
measurements may be made repeatedly at different
localized points on the wafer to obtain a map of the
distribution of carrier recombination lifetime.
6.3 The measurement may be repeated at several
different values of specific parameters, such as injection
level (light source intensity) or temperature in order to
obtain more detailed information about the nature of the
recombination centers.
6.4 A process step that acts as a contamination source
can sometimes be identified by comparing
measurements of carrier recombination lifetime made
before and after the step.
7 Apparatus
7.1 Pulsed Light Source — A laser diode with
wavelength between 0.9 and 1.1 m. Pulse length is
nominally 200 ns, and the rise and fall times are 25
ns (Note 2). It is preferred that the output power of the
light source be variable such that photon densities
between 2.5 10
10
and 2.5 10
15
photon/cm
2
are
generated at the wafer surface during the pulse.
NOTE 2: The rise and fall times of the pulsed light source
and the sampling time of the signal conditioner (see Section
7.5) should be 0.1 of the shortest lifetime to be measured.
7.2 Photon Detector — Suitable means, such as a
semitransparent mirror in the light path at an angle of
45° and a silicon photodetector, to provide feedback
control to maintain the laser power at a constant level
appropriate to the specified injection level.
7.3 Microwave Pick-Up System — Including a
microwave source operating at a nominal frequency of
10 ± 0.5 GHz and an apparatus for measuring reflected
power, such as a circulator, an antenna, and a detector
(see Figure 1). The sensitivity of the detection system
shall be as great as possible to permit measurement of
photoconductivity decay at low injection levels.
Figure 1
Example Block Diagram of Pulsed Light and Microwave Systems
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.