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SEMI MF1526-95 © SEMI 2004 2 under the same condition s is 18.75 R (4.85 × 10  3 C/kg) per calendar quarter (e quiv alent to 9.3 mR /h (2.4 × 10  6 C/kg-h)). Besides the above stat ed regulati ons, vario u s other go v…

SEMI MF1526-95 © SEMI 2004 1

SEMI MF1526-95 (Withdrawn 1104)

Test Method for Measuring Surface Metal Contamination on Silicon Wafers

by Total Reflection X-Ray Fluorescence Spectroscopy

This standard was originally published by ASTM International as ASTM F 1526-94. It was formally approved by ASTM

balloting procedures and adhered to ASTM patent requirements. Though ownership of this standard has been transferred

to SEMI, it has not been formally approved by SEMI balloting procedures and does not adhere either to SEMI

Regulations dealing with patents or to SEMI Editorial Guidelines. Available at www.semi.org September 2004, to be

published November 2004. Last published by ASTM International as ASTM F 1526-95 (Reapproved 2000).

NOTICE: This document was balloted and approved for withdrawal in 2004.

1. Scope

1.1 This test method covers the quantitative determination of elemental areal density on the surface of polished single

crystal silicon substrates using total reflection X-ray fluorescence spectroscopy (TXRF

) with a monochromatic X-ray

source.

1.2 This test method can be used for both n-type and p-type silicon.

1.3 This test method can be used to detect surface elemental contamination that is within the analyte depth of approximately

5 nm for highly mirror-polished silicon wafers. The analytic depth increases with surface roughness.

1.4 This test method is especially useful for determining the surface elemental areal densities in the native oxide or in

chemically grown oxide of polished silicon wafers after cleaning.

1.5 This test method is useful for elemental areal densities between 10

and 10

atoms/cm

within the measurement area.

See Annex A1 for a discussion of the relationship between repeatability and detection limit.

1.6 This test method is useful for detecting elements with atomic number between 16 (S) and 92 (U), depending upon the

X-ray source provided in the instrument. This test is especially useful for detecting the following metals or elements:

potassium, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, arsenic, molybdenum,

palladium, silver, tin, tantalum, tungsten, platinum, gold, mercury, and lead.

1.7 The detection limit depends upon atomic number, excitation energy, photon flux of excitation X-rays, instrumental

background, integration time, and blank value. For constant instrumental parameters, the interference-free detection limits

vary over two orders of magnitude as a function of atomic number of the element.

1.8 This test method is nondestructive.

1.9 This test method is complementary to a variety of other test methods:

1.9.1 Electron spectroscopy for chemical analysis that can detect elemental surface areal densities down to the order of 10

atoms/cm

1.9.2 Auger electron spectroscopy that can detect elemental surface areal densities down to the order of 10

atoms/cm

1.9.3 Nitrogen-beam Rutherford backscattering spectrometry that can detect down to 10

atoms/cm

for some elements

but cannot mass resolve heavy elements of nearby atomic number.

1.9.4 Secondary ion mass spectrometry that can detect low-atomic-number elemental areal densities in the range of 10

atoms/cm

but cannot provide adequate detection limits for transition metals with atomic number between 22 titanium and

30 zinc. This method is destructive.

1.9.5 Vapor phase decomposition (VPD) of surface metals followed by atomic absorption spectroscopy (AAS), where the

metal detection limits are from 10

to 10

atoms/cm

, but there is no spatial information available and the analysis time is

longer than TXRF. This method is destructive.

1.10 This test method uses X-radiation; it is absolutely necessary to avoid personal exposure to X-rays. It is especially

important to keep hands or fingers out of the path of the X rays and to protect the eyes from scattered secondary radiation.

The use of commercial film badge or dosimeter service is recommended, together with periodic checks of the radiation level

at the hand and body positions with a Geiger-Muller counter calibrated with a standard nuclear source. The present maximum

permissible dose for total body exposure of an individual to external X-radiation of quantum energy less than 3 MeV over an

indefinite period is 1.25 R (3.22 × 10

4

C/kg) per calendar quarter (equivalent to 0.6 mR/h (1.5 × 10

7

C/kg-h) as established

in the Code of Federal Regulations, Title 10, Part 20. The present maximum permissible dose of hand and forearm exposure

1 There are several acronyms in use: TXRF, TRFA, and TRXRF; however, TXRF is the most common in the technical literature.

2 There are some non-monochromatic TXRF instruments that are no longer commercially available and that do not provide the detection limits described

herein.

3 The extreme case of roughness on the backside of wafers is addressed by Hockett, R. S., “TXRF Measurement of Substrate Backside Contamination,”

Cleaning Technology in Semiconductor Device Manufacturing, ECS Proceedings, Vol 92-12, The Electrochemical Society, Inc., Pennington, NJ, 1992, p.

350.

SEMI MF1526-95 © SEMI 2004 2

under the same conditions is 18.75 R (4.85 × 10

3

C/kg) per calendar quarter (equivalent to 9.3 mR/h (2.4 × 10

6

C/kg-h)).

Besides the above stated regulations, various other government and regulatory organizations have their own safety

requirements. It is the responsibility of the user to make sure that the equipment and the conditions under which it is used

meet these regulations (see 1.11).

1.11 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the

responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability

of regulatory limitations prior to use.

2. Referenced Documents

2.1 ASTM Standards:

E 122 Practice for Choice of Sample Size to Estimate a Measure of Quality of a Lot or Process

E 135 Terminology Relating to Analytical Chemistry for Metals, Ores, and Related Materials

2.2 Federal Standard:

CFR Title 10, Part 20

3. Terminology

3.1 Most terms used in this test method are defined in Terminology E 135.

3.2 Definitions of Terms Specific to This Standard:

3.2.1 anglescan—a measurement of the emitted fluorescence signal as a function of glancing angle.

3.2.2 critical angle—the incident X-ray glancing angle below which total external reflection of the incident X-ray occurs.

4. Summary of Test Method

4.1 Fig. 1 shows a block diagram of the technique. Monochromatic X rays from an X-ray source impinge the surface of a

polished silicon substrate at a glancing angle that is below the angle for total external reflection of the X rays. The evanescent

wave of the X rays penetrates the polished silicon surface with an exponential decay of intensity versus depth dependent upon

the total electron density of the native oxide and the silicon substrate. One exponential decay length is approximately 5 nm for

silicon of all resistivity.

4.2 The evanescent wave excites the fluorescence energy levels of the surface atoms which then emit fluorescence X-rays

characteristic of their atomic number. Emitted fluorescence X-rays are detected by a lithium-drifted silicon detector, or other

solid state detector, which is an energy dispersive spectrometer. Experience indicates that for measurement of samples with

high levels (>10

atoms/cm

) of specific elements that have been measured with other methods, such as those listed in 1.9,

the integrated counts per second under the fluorescence peaks are linearly proportional to the elemental areal density.

4.3 Reproducible, rapid analysis can be accomplished using a calibration specimen, supplied by the TXRF instrument

manufacturer or developed by another company, with at least one known elemental areal density in the measurement area.

This calibration specimen is analyzed by the TXRF instrument to provide a measured number of integrated fluorescence

counts per second corresponding to the known elemental areal density. Then one or more test specimens are analyzed under

the same instrumental conditions. The integrated fluorescence counts per second for the elements detected on the test

specimen are quantified using relative sensitivity factors (RSFs) with respect to the calibration element count rate per known

areal density, where the RSFs are contained within the instrument software. The lack of true standards precludes

determination of the accuracy of this test method.

5. Significance and Use

5.1 TXRF can measure the elemental, particularly metal, areal densities on polished silicon wafer product.

5.2 The TXRF measurement facilitates the production of silicon wafers with controlled upper limits on metal areal

densities.

5.3 This test method can be used for monitoring a mirror-polished wafer cleaning process, research and development, and

materials acceptance purposes.

4 Annual Book of ASTM Standards, Vol 14.02, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone: 610-832-9500,

Fax: 610-832-9555, Website: www.astm.org

5 Annual Book of ASTM Standards, Vol 03.05.

6 Available from Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.

6. Interferences

6.1 The interferences in conventional X-ray fluorescence spectroscopy are common to TXRF also. These include, but are

not limited to: overlap of fluorescence lines, escape peak and sum peak overlap, energy gain calibration drift, X-ray source

stability, and instrumental background peaks. However, no X-ray fluorescence corrections for secondary fluorescence or for

matrix absorption are required for TXRF. Interferences common to software procedures and calculations can be evaluated by

comparing data sets; see Annex A1.

6.2 In addition to conventional interferences, there are interferences that are unique to TXRF as follows:

6.2.1 If the glancing angle calibration is not reproducible, variability is introduced to the measurement,

6.2.2 If the glancing angle calibration is inaccurate, bias is introduced to the measurement,

6.2.3 If the anglescan of the known elemental impurity on the calibration specimen is different from the elemental impurity

anglescan on the test specimen, this may introduce a bias to the quantification. An example can be the measurement of

particulate metal contamination on a test specimen while using a calibration specimen which has the calibration metal

localized within the native oxide,

6.2.4 Mechanical vibration may degrade the detector energy resolution, and this may degrade detection limits,

6.2.5 If the specimen is not chem-mechanically polished, there will be a loss in detection capability, a bias in quantification,

and an increase in variability of the measurement. There is some possibility that the surface roughness and waviness

differences from different cleaning processes may cause these interferences also. The semiquantitative effect of surface

roughness upon the TXRF measurement is under study.

6.2.6 Bias in the assigned elemental areal density of the calibration specimen can introduce bias into the TXRF measured

areal densities,

6.2.7 Surface contamination introduced during handling of the test specimens or during the measurement itself will

introduce a bias to the measurement, if this surface contamination contains the element(s) to be measured,

6.2.8 Bias in the RSF of a fluorescence line can introduce bias to the measurement,

6.2.9 Nonlinearity of detected fluorescence signal versus impurity areal density may occur due to high deadtime of some

detectors under the condition of high-total signal count rates, and

6.2.10 Fluorescence curve smoothing may affect the quantification accuracy.

6.2.11 Instrumental peaks may be generated by the incident X-ray beam diffracting from the silicon crystal and the

diffracted beam entering the detector to excite metals in the detector window or detector housing. This effect can be tested for

by appropriate experiments.

7. Apparatus

7.1 TXRF Instrument, equipped with a monochromatic X-ray source, test specimen handling equipment, a method for

glancing angle calibration, an energy-dispersive spectrometer X-ray detector, software for background subtraction, peak

integration, analysis and RSFs, and an analysis ambient without argon (for example, vacuum of 10

2

torr, or helium gas). The

methods for glancing angle calibration are presently proprietary for each TXRF instrument manufacturer. A TXRF

manufacturer may include a subtraction routine for escape peaks and these signals may already be removed.

7.2 Reference Wafer— The suitability of the apparatus shall be determined with the use of a reference wafer and its

associated data set in accordance with the procedures of Annex A2, or by performance of a statistically-based instrument

repeatability study to ascertain whether the equipment is operating within the manufacturer's stated specification for

repeatability.

7.3 Class 100 Air Environment—The area for sample transfer to the instrument measurement stage must be enclosed in

this.

8. Sampling

8.1 A sampling procedure must be used to evaluate the characteristics of a group of silicon wafers. No general sampling

procedure is included as part of this test method, because the most suitable sampling plan will vary considerably depending

upon individual conditions. For referee purposes, a sampling plan shall be agreed upon before conducting the test. See

Practice E 122 for suggested choices of sampling plans.

9. Specimen Requirements

9.1 Test specimens must be chem-mechanical polished on the side used for analysis.

7 Yakushiji, K., Ohkawa, S., Yoshinga, A., and Harada, J.,“ Origins of Spurious Peaks of Total Reflection X-Ray Fluorescence Analysis of Si Wafers

Excited by Monochromatic X-Ray Beam W1-beta,” Japanese Journal of Applied Physics, Vol 33, 1994, pp. 1130—1135.