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SEMI MF1391-0704 © 2004 2 2 3.3 Spectrophotometer tec hnique is critical to a successful carbon determination. The m a nufacturer's instrument instructi on manu als should be studied t o familiarize the operator wit…

SEMI MF1391-0704

TEST METHOD FOR SUBSTITUTIONAL ATOMIC CARBON CONTENT

OF SILICON BY INFRARED ABSORPTION

This test method was technically approved by the Global Silicon Wafer Committee and is the direct

responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by

the North American Regional Standards Committee on March 14, 2004. Initially available at

www.semi.org

May 2004; to be published July 2004. Original edition published by ASTM International as ASTM F 1389-

92. Last previous edition SEMI MF1389-00.

1 Purpose

1.1 Carbon may have an important role in defect

formation processes. Some laboratories have attributed

carbon as being involved in the formation of swirl.

Carbon has also been shown to serve as a nucleation

center for the precipitation of oxygen.

1.2 Although electrically inactive, substitutional carbon

causes stress that can be observed by X-ray topography.

1.3 Direct effects on the reverse bias characteristics of

power devices and annealing problems in neutron

transmutation doped silicon have been associated with

carbon.

1.4 This test method has applicability in production

control, materials research, quality assurance, and

materials acceptance.

2 Scope

2.1 This referee test method

covers the determination

of substitutional carbon concentration in single crystal

silicon. Because carbon may also reside in interstitial

lattice positions when in concentrations near the solid

solubility limit, the results of this test method may not

be a measure of the total carbon concentration at such

concentrations.

2.2 The useful range of carbon concentration

measurable by this test method is from the maximum

amount of substitutional carbon soluble in silicon down

to about 0.1 parts per million atomic (ppma), that is,

5 × 10

−3

for measurements at room temperature,

1 This test method was developed in cooperation with the Silicon

Technologies Committee of the Japan Electronics and Information

Technology Industries Association (JEITA). It is essentially

equivalent to JEITA EM-9503, Standard Test Method for

Substitutional Atomic Carbon Content of Silicon by Infrared

Absorption, which is available from JEITA, 3rd floor, Mitsui

Sumitomo Kaijo Bldg. Annex, 11, Kanda-Surugadai 3-chome,

Chiyoda-ku, Tokyo 101-0062, Japan, Web site:

www.jeita.or.jp

. DIN

50438/2, Testing of Inorganic Semiconductor Materials:

Determination of the Impurity Content in Silicon by Means of

Infrared Absorption; Carbon, is also a method for measuring the

substitutional carbon content of silicon. It differs in some aspects,

including different conversion coefficients, from this test method. It

is available from Beuth Verlag GmbH, Burggrafenstrasse 4-10, D-

1000 Berlin 30, Germany.

and down to about 0.01 ppma, that is, 0.5 × 10

−3

at cryogenic temperatures (below 80 K).

2.3 This test method utilizes the relationship between

carbon concentration and the absorption coefficient of

the infrared absorption band associated with

substitutional carbon in silicon. At room temperatures

(about 300 K), the absorption band peak is at 605 cm

−1

or 16.53 µm. At cryogenic temperatures (below 80 K),

the absorption band peak is at 607.5 cm

−1

or 16.46 µm.

2.4 This test method is applicable to slices of silicon

with resistivity higher than 3 Ω·cm for p-type and

higher than 1 Ω·cm for n-type. Slices can be any

crystallographic orientation and should be polished on

both surfaces.

2.5 This test method is intended to be used with

infrared spectrophotometers that are equipped to

operate in the region from 2000 to 500 cm

−1

(5 to 20

µm).

2.6 This test method provides procedure and calculation

sections for the cases where thickness values of test and

reference specimens are both closely and not closely

matched.

NOTICE: This standard does not purport to address

safety issues, if any, associated with its use. It is the

responsibility of the user of this standard to establish

appropriate safety and health guides and determine the

applicability of regulatory or other limitations prior to

use.

3 Limitations

3.1 Stray light that reaches the detector tends to reduce

the calculated absorbance value and thereby reduces the

reported carbon concentration.

3.2 The carbon level of the reference slice should be

less than 2 × 10

atoms/cm

(0.04 ppma) to minimize

the comparative error at room temperature. The

detection limit at cryogenic temperatures (below 80 K)

is about 0.01 ppma. Obtaining reference samples much

below 0.01 ppma in carbon content may prove to be

difficult. Therefore the measurement of very low

carbon content silicon near the 0.01 ppma detection

limit is necessarily a comparative measurement only.

3.3 Spectrophotometer technique is critical to a

successful carbon determination. The manufacturer's

instrument instruction manuals should be studied to

familiarize the operator with the proper use of the

spectrometer. Since the transmittance at the carbon

peak can be very low, while the transmittance at the

baseline regions is about 40%, extremely good

photometric linearity is critical. Wavenumber precision

is also critical because the carbon peak lies on the

shoulder of a very intense lattice absorption band.

3.4 The FWHM of the carbon absorption band at room

temperature must be less than 6 cm

−1

for acceptable

measurements. At cryogenic temperatures (below 80

K), it must be less than 3 cm

−1

for acceptable

measurements. Excessive width may be due to

improper thickness matching, to stress, or to the use of

a low resolution setting of the instrument. In dispersive

instruments, excessive widths can also result from

incorrect instrument balance setting or too fast a scan

speed. In Fourier transform instruments, excessive

widths can result from the use of a source-defining

aperture that is too wide.

3.5 Specimens that do not exceed the instrument beam

size cause error. Use of apertures at the sample, or

preferably beam condensers to reduce the beam size at

the sample, can correct this problem.

3.6 The main two-phonon lattice band of silicon, at

about 610 cm

−1

(16 µm), is very intense; the absorption

coefficient for this band is about 9 cm

−1

at room

temperature, and about 5 cm

−1

at 78 K.

This broad

band peak is close to the wavelength of the carbon-in-

silicon band and so presents a problem in measuring the

intensity of the carbon band.

3.7 Reference and test slices must be as close as

practically possible to the same temperature to avoid

the effects of temperature on the intensity of the lattice

band.

3.8 The minimum detection level of this test method is

limited by the signal-to-noise ratio of the spectrum.

Thus attaining the highest possible sensitivity by this

test method requires long measurement times and stable

spectrophotometers.

3.9 Free carrier absorption in silicon specimens with

resistivities less than 3 Ω·cm for p-type, or 1 Ω·cm for

n-type, reduces the available energy below the level

required for satisfactory operation of most

spectrophotometers.

3.10 For samples at cryogenic temperatures (below 80

K), plane parallel, polished surfaces may cause

interference fringes on the spectrum. Increasing the

2 Johnson, F. A., Proc. Physics Society 73, 265–272 (1959).

sample thickness or inducing a wedge (non-flatness) in

the sample reduces these interference fringes.

4 Referenced Standards

4.1 SEMI Standard

SEMI MF1241 — Terminology of Silicon Technology

4.2 ASTM Standard

E 131 — Terminology Relating to Molecular

Spectroscopy

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

5 Terminology

5.1 General definitions for terms related to infrared ab-

sorption spectroscopy are found in ASTM Terminology

E 131.

5.2 Definitions for terms related to silicon materials

technology are found in SEMI MF1241.

5.3 Definitions

5.3.1 background spectrum, n — in FT-IR instruments,

the single-beam spectrum obtained without a specimen

in the infrared light path that is usually obtained with

only nitrogen, dry air, or a vacuum in the beam.

5.3.2 baseline, n — a straight line interpolation

between points on either side of the carbon peak of the

absorbance spectrum, drawn to represent the spectrum

that would have been obtained in the absence of the

impurity.

5.3.3 baseline absorbance, n — the value of the

baseline at the wavenumber corresponding to the

carbon peak that is used for evaluating the absorbance

peak height.

5.3.4 Fourier transform infrared (FT-IR) spectrometer,

n — type of infrared spectrometer in which the data are

obtained as an interferogram.

5.3.4.1 Discussion — An interferogram is a record of

the modulated component of the interference signal

measured by the detector as a function of retardation in

the interferometer. This interferogram is then subjected

to a Fourier transformation to obtain an amplitude-

wavenumber (or wavelength) spectrum. FT-IR

instruments are always used in conjunction with a

computer to control the interferometer, collect and

manipulate the data, and for spectral output.

3 Annual Book of ASTM Standards, Vol 03.06, ASTM International,

100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:

610-832-9500, Fax: 610-832-9555, Website:

www.astm.org

5.3.5 FWHM — acronym for full width at half

maximum, the width of the absorbance band at half its

magnitude as measured from the baseline.

5.3.6 reference spectrum, n — the spectrum of the

reference specimen.

5.3.6.1 Discussion — For true double-beam

instruments, it is obtained directly with the reference

specimen in the sample beam and the reference

(second) beam empty. For FT-IR and other single-

beam instruments, it is the result of ratioing the single-

beam spectrum of the reference specimen to the

background spectrum (see Section 5.3.1).

5.3.7 sample spectrum, n — the spectrum obtained

when the sample specimen is placed in the infrared

beam.

5.3.7.1 Discussion — For true double-beam

instruments, it is obtained directly with the sample in

the sample beam and the reference (second) beam

empty. For FT-IR and other single-beam instruments, it

is the result of ratioing the single-beam spectrum of the

sample to the background spectrum (see Section 5.3.1).

6 Summary of Test Method

6.1 At room temperature, test slices are prepared that

are polished on both sides to a nominal thickness of 2.0

mm. At cryogenic temperatures, the thickness of the

test slices can range from 2.0 to 4.0 mm.

6.2 A reference slice of known low carbon content (see

Section 3.2) is prepared in the same manner.

6.3 After verifying that the instrument is suitably set up,

transmittance spectra of the sample and reference are

obtained over the range from 700 to 500 cm

−1

(14.3 to

20.0 µm) on a double-beam dispersive or single-beam

Fourier Transform Infrared (FT-IR) spectrophotometer

in accordance with manufacturer's instructions.

6.4 Absorbance spectra are derived from these

transmittance spectra and a carbon-only spectrum is

obtained as the difference between the two absorbance

spectra.

6.5 A baseline is drawn between the regions on both

sides of the carbon peak on this difference absorbance

spectrum, and absorbance values of both the peak and

baseline are recorded.

6.6 The absorbance peak height is taken as the

difference between these two values. This peak height,

corrected for sample thickness, is multiplied by a

constant to calculate the substitutional carbon concen-

tration. Two constants are used, one for measurements

at room temperature (300 K), and one for measure-

ments at cryogenic temperatures (below 80 K).

7 Apparatus

7.1 Infrared Spectrophotometer — Either a dispersive

(computerized or non-computerized) or a Fourier trans-

form (FT-IR) spectrophotometer may be used. The

resolution of the spectrophotometer must be at least 2

−1

at room temperature, or 1 cm

−1

at cryogenic

temperatures, over the range from 500 to 700 cm

−1

for

either dispersive or Fourier transform infrared spectro-

photometers. The total operating range of the

spectrophotometer shall include the range from 500 to

2000 cm

−1

7.2 Micrometer Caliper — or other instrument, capable

of measuring the thickness of the specimens to an

accuracy of 0.005 mm.

7.3 Equipment and Materials for Slicing and Polishing

Silicon — to a final thickness tolerance of 0.005 mm or

less, and a total thickness variation of 0.01 mm or less.

7.4 Thermometer — or other instrument capable of

measuring the temperature of the sample chamber (for

room temperature measurements) or the sample holder

(for cryogenic temperature [below 80 K]

measurements) to within ± 2°C.

7.5 Calcium Fluoride Crystal (CaF

), cut to nominal

thickness of 5 mm.

8 Sampling

8.1 Unless otherwise specified, a silicon slice used for

the carbon test is to be measured at the nominal slice

center.

9 Test and Reference Specimens

9.1 A single crystal slice of about 2-mm thickness must

be used at room temperature, and a slice of 2.4 to 3.5-

mm thickness is preferable at cryogenic temperatures.

9.2 Both the test and reference specimens must be

carefully shaped to the following criteria:

9.2.1 Thickness variation over the measurement area

shall be 0.005 mm or less.

9.2.2 Surface preparation shall be identical,

9.2.3 Thickness equality shall meet one of the

following:

9.2.3.1 When the measurement is made with double

beam simple dispersive infrared spectrophotometers,

the final thickness of test and reference slices shall be

equal to within ± 0.01 mm (see Section 10.4).

9.2.3.2 When the measurement is made with computer

assisted double-beam dispersive or single-beam FT-IR

spectrophotometers, the final thickness of test and