semi合集-English.pdf - 第7450页

SEMI MF1535-1104 © SEMI 2004 8 automatically by the app aratus, set the injection level as follows: 12.5.1 If the test specimen is oxidized and the thickness of th e oxide layer is not known, m easure or estimate it, usi…

100%1 / 7923
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.
SEMI MF1535-1104 © SEMI 2004 8
automatically by the apparatus, set the injection level as
follows:
12.5.1 If the test specimen is oxidized and the
thickness of the oxide layer is not known, measure or
estimate it, using a method acceptable to the parties to
the test. Record the thickness.
12.5.2 Determine and record the fraction of the
incident light that penetrates the oxide and is absorbed
by the specimen from the dashed curve in Figure 3.
Note: For these calculations the wavelength of the incident
radiation, , is assumed to be 905 nm, the index of refraction
of silicon is taken as 3.610, and the index of refraction of SiO
2
is taken as 1.462. Maximum absorption occurs at an oxide
thickness d = (2n + 1)/4 while minimum absorption occurs
at an oxide thickness d = n/2, when n = 0, 1, 2, etc.
Therefore, if the wavelength of the incident radiation,
1
,
differs from 905 nm, these curves can be used by determining
the relative intensity for an effective oxide thickness d
0
= 905
d
1
/
1
where d
1
is the actual thickness of the oxide.
Figure 3
Fraction of Incident Radiation Reflected from (solid
line) or Absorbed in (dashed line) a Silicon Wafer
Covered with a Silicon Dioxide (SiO
2
) Layer
between 0 and 1
m Thick
12.5.3 Adjust the light source intensity so that the
photon density absorbed in the silicon during the pulse,
, is equal to

n
maj
, where
is the desired injection
level and n
maj
is the density of majority carriers in the
wafer as determined in Section 12.1. The photon
density,
, in photons/cm
3
, is given by:
L
tf
p
t
I
0
d
(2)
where:
f
=
the fraction absorbed found from Figure 3 (see
Section 12.5.2),
I
=
the intensity of the incident light, in photons/cm
2
·s,
t
p
=
the length of the light pulse, in s, and
L
=
wafer thickness, in cm.
12.6 Turn on the microwave power source and view
the photoconductivity decay on the display unit. Adjust
the time and voltage scales so as to display the desired
portion of the decay signal. In the absence of
indications to the contrary, observe the decay signal
from 45 to 5% of the peak voltage.
12.7 Determine that the decay is exponential over the
desired range. Determine the time constant by fitting
an exponential curve to the voltage, V, as a function of
time, t, or (for manual data collection) a straight line to
the curve of lnV as a function of t.
12.8 Record this time constant as the recombination
lifetime.
12.9 If desired, move the wafer position and repeat
Sections 12.6 through 12.8 as required to obtain a wafer
map, noting the point spacing and pattern together with
the radius of the mapped area.
12.10 Alternatively, if desired, repeat Sections 12.2
and 12.6 through 12.8 at the same location for different
temperatures or repeat Sections 12.5 through 12.8 at the
same location for different values of injection level.
13 Report
13.1 Report the following information:
13.1.1 Date and location of the test,
13.1.2 Operator,
13.1.3 Instrument type, model number, and, if
computer controlled, software version,
13.1.4 Wafer description including any identification
markings, center-point resistivity, center-point
thickness, conductivity type, surface condition (front
and back), and nominal diameter,
13.1.5 Portion of the decay signal from which the time
constant was determined,
13.1.6 Injection level,
, as established in Section 12.5,
13.1.7 Surface passivation procedure used (see Section
11), and
13.1.8 Carrier recombination lifetime,
, in s.
13.2 If measurements were made at several injection
levels, report
for each value of
.
SEMI MF1535-1104 © SEMI 2004 9
13.3 If a wafer map was made, report the following
information in addition to a density plot of the carrier
recombination lifetime:
13.3.1 Point spacing and pattern, and
13.3.2 Radius of the mapped area.
13.4 If measurements were made at a specific or
several temperatures, report the temperature of each
measurement.
14 Precision and Bias
14.1 Precision — Neither the intralaboratory nor the
interlaboratory precision of this test method has yet
been established. In spite of this limitation, the
technique is widely utilized in the industry without the
benefit of standardization of the test conditions.
Because of the lack of a precision determination, this
test method should be used for materials specification
and acceptance only after the parties to the test have
established reproducibility and correlation.
14.2 Bias — No information can be presented on the
bias of this test method because no material having an
accepted reference value of carrier recombination
lifetime is available.
15 Keywords
15.1 contactless measurement; microwave reflection;
photoconductivity decay; recombination lifetime;
silicon wafers