semi合集-English.pdf - 第7313页
SEMI MF1188-0305 © SEMI 2003, 2005 7 12.2 Instrume ntal Checks ( FT -IR Spectrophot ometers ) NOTE 4: If a DIR spectrophotometer is being us ed, substitute the procedure in ¶A1-2.1.2 for the procedure in ¶12.2.2. 12.2.1 …
SEMI MF1188 © SEMI 2003, 2005 6
9.1.3 The stated oxygen content is referenced to the IOC-88 calibration factor,
1
and
9.1.4 The set includes an additional oxygen-free reference specimen.
9.2 To extend the test method to measurement of test specimens with thickness other than 2 mm or with only a
single polished surface, suitable sets of reference materials must be used to calibrate the instrument for use with
specimens of similar thickness and surface conditions.
9.2.1 Sufficient sets of reference materials must be utilized to provide traceability from the reference materials with
thickness and surface conditions of the specimens to be tested to the set of CRMs.
NOTE 2: A practice for calibrating infrared spectrophotometers for measuring oxygen in silicon using reference materials of
various thickness and surface condition is under development. In the interim, some, but not complete, guidance for this
calibration can be found in the NIST SRM 2551 Report.
5
10 Test Specimen
10.1 Choose test specimens that are as homogeneous as possible, so that the oxygen content measured is a fair
representation of the oxygen content of the entire specimen. This is particularly important for dispersive
spectrophotometers, since in many such instruments the illuminated area of the specimen varies during the scan. For
specimens with a large lateral inhomogeneity, this area variation may result in the appearance of undesirable
instrumental artifacts in the spectra.
10.1.1 Prepare a slice of the crystal to be tested so as to obtain two optically flat surfaces parallel to 5 min of arc or
less, as measured with a micrometer caliper or other suitable instrument. The surfaces of the specimen must be as
free as possible of surface films.
NOTE 3: When the specimen faces are parallel and well-polished, and the data are being obtained at a sufficiently high
resolution, interference may occur between light rays reflecting from the front and back surfaces of the specimen. The contrast of
the interference fringes depends upon the parallelism of the specimen surfaces, and the fringe spacing depends on the optical
thickness of the specimen. These fringes can obscure a weak spectral line and prevent accurate measurement of the baseline. To
prevent obscuration by these interference fringes, nonparallel specimen surfaces may be a necessity. However, the use of
specimens with nonparallel surfaces can also result in photometric errors. Because silicon has a high refractive index, any
nonparallelism of the specimen can displace the spectrometer beam relative to the active area of the detector. Thus, an apparent
lowering of the transmission occurs. Improper positioning or nonparallelism of the specimen can be checked by rotating the
specimen to determine whether the transmission level stays constant. Any variation is a possible indication of problems with the
specimen positioning or preparation.
11 Reference Specimens
11.1 Because a difference technique is used in this test method, prepare an oxygen-free reference specimen of the
same type of material as the sample.
11.1.1 Choose the oxygen-free reference specimen from slices taken from five to ten different silicon crystals that
are thought to be free of oxygen. Compare these slices with one another, and choose the specimen with the lowest
absorption as the oxygen-free reference specimen. If no absorption is seen for any of the specimens, then the
assumption can be made that all specimens contain less than the limit of detection of oxygen and any of the
specimens can be used as the oxygen-free reference specimen.
11.1.2 Prepare the oxygen-free reference specimen to the same tolerances as the test specimen with a thickness of
equal to that of the test specimen to within ±0.5%.
11.2 Prepare separate oxygen-free reference specimens for each type of material to be tested.
12 Procedure
12.1 Calibration of Spectrophotometer
12.1.1 Obtain a set of certified reference materials for oxygen in silicon (see §9) and, if necessary, suitable sets of
traceable reference materials with different thickness or surface condition, or both.
12.1.2 Use these reference materials to calibrate the spectrophotometer in accordance with manufacturer’s
instructions. The calibration results in a linear regression curve that relates the measured absorption coefficient to
the interstitial oxygen content of the measured specimen (Note 1).
SEMI MF1188-0305 © SEMI 2003, 2005 7
12.2 Instrumental Checks (FT-IR Spectrophotometers)
NOTE 4: If a DIR spectrophotometer is being used, substitute the procedure in ¶A1-2.1.2 for the procedure in ¶12.2.2.
12.2.1 Establish the 100% baseline to measure the noise level: On double beam instruments, record the
transmittance spectrum with both the sample and reference beams empty. On single-beam instruments, obtain the
transmittance spectrum as the ratio of two spectra taken with the sample beam empty. Plot the 100% baseline over a
wavenumber range covering 900 cm
1
to 1300 cm
1
. If the baseline is not 100% ± 0.5% over the entire range,
increase the measurement time until it does. If the problem persists, have the instrument repaired.
12.2.2 Record the throughput characteristics of the spectrophotometer by plotting a single-beam spectrum, obtained
with the sample beam empty, over the wavenumber range from 450 cm
1
to 4000 cm
1
. Use such a spectrum,
recorded after the instrument has been properly aligned according to factory specifications as a reference to evaluate
the instrument’s performance. Whenever the spectrum obtained deviates significantly from the instrument's
reference spectrum, realign the instrument (Note 3).
12.2.3 Determine mid-scale linearity of the instrument by obtaining an air reference spectrum of the silicon
reference specimen over the wavenumber range from 1600 cm
1
to 2000 cm
1
. If the value of the transmittance is
not 53.8% ± 2% over this wavenumber range, align the sample with the spectrometer in accordance with ¶8.3.
12.3 Immediately prior to the initial measurement in any laboratory, etch all specimens, including the reference
specimen, in dilute hydrofluoric acid to remove any surface oxide, and rinse.
12.4 Measure the thicknesses of the test and reference specimens to within ±0.2%, at their centers. If thickness of
the reference specimen does not match the thickness of the test specimen to within ±0.5%, obtain a reference
specimen with the proper thickness.
12.5 Measure and record the temperature of the spectrophotometer chamber.
12.6 Determine the measurement time for the spectra by obtaining the transmittance spectrum of a high resistivity
(greater than 5 ·cm), 0.04 cm to 0.085 cm thick double-side polished silicon slice containing between 12 ppma and
18 ppma oxygen (IOC-88) using a minimum of 64 scans. If the ratio of the net amplitude of the oxygen band, T
b
T
p
, to the standard deviation in the transmittance spectrum is not greater than 100, increase the number of scans until
that criterion is met.
12.7 Obtain the infrared transmittance spectrum with a resolution of 4 cm
1
, or better, at 1107 cm
1
, for FT-IR
instruments or, for dispersive instruments, 5 cm
1
, or better, over (at least) the range from 900 cm
1
to 1300 cm
1
.
The test and reference specimens must be positioned so that the IR beam is centered on them. On double beam
instruments, obtain the transmittance spectrum with the oxygen-free reference specimen in the reference beam, and
the test specimen in the sample beam. On single beam instruments, compute the transmittance spectrum as the ratio
of the emission spectrum of the test specimen to the emission spectrum of the reference specimen.
12.8 Plot the transmittance spectrum over the range from 900 cm
1
to 1300 cm
1
.
12.9 Define the baseline of the oxygen-only transmittance spectrum by drawing a straight line from the
transmittance at 1040 cm
1
to the transmittance at 1160 cm
1
.
12.10 Locate the wavenumber corresponding to the minimum transmittance in the region from 1102 cm
1
to
1112 cm
1
. Record the value of that wavenumber, to five significant figures, as W
p
. Record the minimum
transmittance as T
p
, the transmittance at the absorption peak. Record the baseline transmittance, T
b
, as the value of
the baseline defined in ¶12.9 at W
p
. Record both T
p
and T
b
to three significant figures.
12.11 Determine and record the full width at half maximum (FWHM) of the peak.
13 Calculations
13.1 Calculate the peak and baseline absorption coefficients using the following equations:
p
x70.12
p
2x70.1x70.1
p
T18.0
eT36.0)e09.0()e09.0(
ln
x
1
(1)
SEMI MF1188 © SEMI 2003, 2005 8
b
x70.12
b
2x70.1x70.1
b
T18.0
eT36.0)e09.0()e09.0(
ln
x
1
(2)
where:
p
=
peak absorption coefficient, cm
1
,
b
=
baseline absorption coefficient, cm
1
,
x = thickness, cm,
T
p
= peak transmittance, and
T
b
= baseline transmittance.
13.2 Calculate the net absorption coefficient,
O
, due to interstitial oxygen:
bpO
(3)
13.3 Determine the interstitial oxygen content of the silicon slice from the linear regression curve (see ¶12.1.2) that
relates the measured absorption coefficient to the interstitial oxygen concentration in ppma or the interstitial oxygen
volume density in cm
3
, as desired. This value of oxygen content is based on the IOC-88 calibration factor.
13.3.1 If reference materials are not used to calibrate the spectrophotometer, calculate the oxygen content from
O
in accordance with ¶A1-3.3.
13.4 If desired, calculate and record the interstitial oxygen content in terms of any other standardized calibration
factor using the appropriate conversion factor in SEMI M44.
14 Report
14.1 Report the following information:
14.1.1 The instrument used, the operator and the date of the measurements.
14.1.2 Test method and baseline used (SEMI MF1188, Short Baseline).
14.1.3 Identification of test and reference specimens.
14.1.4 Temperature of the spectrophotometer chamber.
14.1.5 Apodization function used (FT-IR instruments).
14.1.6 Identification of all certified and working reference materials used to calibrate the spectrophotometer.
14.2 For each test and reference specimen used, report the following information:
14.2.1 Specimen thickness.
14.2.2 Location and size of the illuminated area on the specimen.
14.2.3 W
p
, wavenumber of the absorption peak, in cm
1
.
14.2.4 Spectral full width at half maximum of the absorption peak.
14.2.5 Absorption coefficient due to interstitial oxygen,
O
, in cm
1
.
14.2.6 Oxygen concentration, in ppm or in atoms/cm
3
.
14.2.7 The calibration factor used (IOC-88 recommended).
15 Precision
15.1 The uncertainty due to measurement variations in the certification of the NIST SRM 2551 set was found to be
equal to or less than 0.17%.
5