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SEMI MF1526-95 © SEMI 2004 3 6. Interferences 6.1 The interferences in conventional X -ray fluorescence sp ectroscopy are common to TXRF also. These include, but are not limited to: ov erlap of fluorescence lines , escap…

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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.

SEMI MF1526-95 © SEMI 2004 3

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.

SEMI MF1526-95 © SEMI 2004 4

10. Test Specimen Measurement

10.1 Load the test specimen into the TXRF instrument in a contamination free environment of Class 100 or better.

10.2 Measure the TXRF spectrum for the test specimen.

10.3 Calculate the net integrated counts/second for detected elemental peaks.

10.4 Using the calibration wafer data for the calibration element and the RSFs for the other elements, calculate the

elemental areal densities for each detected element on the test specimen according to Eq. 1.

um

()

1/F

sm

()

CPS

um

()

CPS

1

()

(1)

where:

su,m

= the areal density in atoms/cm

of element “m” on the

test specimen “u”,

s,m

= the RSF of element “m” with respect to the

calibration element “s”,

CPS

u,m

= the integrated counts/second of element “m”

detected on test specimen “u”,

CPS

= the integrated counts/second of the calibration

element on the calibration standard, and

= the assigned areal density in atoms/cm

of the

calibration element.

11. Calibration Standard

11.1 The calibration standard and the method of its preparation must be agreed upon between the parties.

NOTE 1—Several methods for preparing calibration standards are in practice.

The present state-of-the-art calibration standard is a chem-mechanical polished silicon substrate with a surface metal areal density of between

1 by 10

and 1 by 10

atoms/cm

located in the analysis area. The k-alpha fluorescence signal from the calibration standard metal is free

of interferences, that is, escape peaks, sum peaks, and fluorescence peaks from other contamination, and free from external contamination

sources. The preferred element should not be easily added as a contaminated element (for example, iron) or diffuse away from the analyte

depth over time (for example, gold or copper). The preferred element is nickel or vanadium. Calibration can also be done using a set of

wafers with different known areal densities of a particular element.

The calibration standard should have been measured by an appropriate analytical method to determine the metal areal density of the

calibration metal. Some analytical methods which have been used for assigning quantitative values to calibration standards include:

(a) Nitrogen-Beam Rutherford Backscattering Spectrometry (N-RBS)—If this method is used, there should not be any other metals present at

levels greater than 1 % of the calibration metal areal density within a mass range of ±5 atomic mass unit. The N-RBS measurement is

absolute and must be made within the analysis area of the TXRF. Other backscattering methods are also used to calibrate elemental areal

densities. These include forward scattering Rutherford backscattering (F-RBS) and heavy ion backscattering spectrometry (HIBS).

(b) Vapor Phase Decomposition—This followed by atomic absorption spectrometry (VPD/AAS) is used to calibrate a spin coating

contamination process for making calibration standards. The VPD/AAS measurement is destructive. The contamination process should also

be shown to be uniform across the wafer by using TXRF or SIMS mapping. The VPD/AAS absolute assignment is based upon Atomic

Absorption Spectroscopy standards. An error in the elemental recovery rate of VPD is a source of error in accuracy.

(c) Implant—Ion implant the reference element into a pre-amorphized silicon surface and using solid phase epitaxy to regrow the amorphous

silicon into crystal silicon.

This procedure sweeps the ion implanted reference element to the sample surface if the reference element is

much more soluble in the amorphous silicon than in the crystal silicon. The assignment of the quantitative level in atoms/cm

is done using

the ion implant current to determine the implant dose.

(d) Diluted Atomic Absorption Standard Solution—Deposit onto a polished silicon substrate a diluted atomic absorption standard solution of

a metal onto a localized spot much less than the size of the TXRF analysis area. This approach assumes none of the calibration metal is lost

8 Hockett, R. S., “TXRF Reference Standards: A Discussion,” Contamination Control and Defect Reduction in Semiconductor Manufacturing III, Vol 94-9,

edited by Dennis N. Schmidt, The Electrochemical Society, Pennington, NJ, 1994, pp. 323–334.

9 Hockett, R. S., and Jacobson, D. C., “A New Approach to TXRF Standards,” Extended Abstracts, Vol 93-1, The Electrochemical Society, Inc., Pennington,

NJ, 1993, p. 1289; Jacobson, D. C., Poate, J. M., Higashi, G. S., Boone, T., Eaglesham, D. J., and Hockett, R. S., “Ion Implanted Calibration Standards for Si

Surface Contamination Detection by TXRF,” Materials Research Society, Proceedings of the Spring '93 MRS Meeting, San Francisco, April 26–29, 1993.