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SEMI M33-0998 © SE MI 1998 1 SEMI M33-0998 TEST METHOD FOR THE DETERMINAT ION OF RESIDUAL SURF ACE CONTAMINATION ON SILICON WAFERS BY MEANS OF TOTA L REFLECTION X-RA Y FLUORESCENCE SPECTROSCOPY (TXRF) 1 Purpose 1.1 The t…

SEMI M32-0998 © SEMI 1998, 2004 12

A4-4.1 First, please note that the term “sigma” is

intentionally used, rather than “standard deviation”.

This does not in any way imply 100% sampling.

Instead, the implication is that the process would need

to be monitored long enough to understand the “process

sigma” (i.e., the state at which the process can be

controlled).

A4-4.2 Mean and sigma specifications intuitively seem

very promising, but they have many of the same

problems as multiple distributional percentile

specifications. Here, the issue of statistical sensitivity is

particularly important. Table A4-1, and the discussion

in Section A4-3.4 concerning sampling requirements,

explain the central issue. The sample sizes required to

detect smaller mean shifts are very large, so they

usually are not feasible at the process control point. As

a result, mean and sigma specifications could lead to

much more sampling in an inspection area or in the

warehouse, where larger quantities are available. To

keep the sample sizes small, it is necessary to allow a

specified amount of process drift. Therefore, some form

of tolerance is necessary.

A4-4.3 Another factor that needs careful consideration

is the movement toward controlling surrogate variables

instead of outgoing parameters. Many types of variation

can be controlled by carefully applying the appropriate

samplings (within batch samples for batch-to-batch

variation, within lot samples for lot-to-lot variation,

etc.). However, there are practical limitations on how

many types of variation can be continuously monitored.

There is a large amount of work involved, and there are

hundreds of potential sources of variation. Resource

restrictions regularly force suppliers to concentrate on

the known key variables and leave the rest as error in

the system (at least for now). This practice, called

homogeneous sampling, is designed to continuously

home in on the variables that have the greatest impact

on the distribution. Strictly speaking, for characterizing

process capability, a random and over-time sampling

which covers all sources of variation should be used. In

practice, this is difficult to do, so engineering judgment

is needed to select the most appropriate homogeneous

sampling. This is yet another issue that could generate

significant errors if it is not fully understood.

A4-4.4 From an ease-of-use standpoint, mean and

sigma specifications have many of the same issues as

multiple distributional percentile specifications. Mixing

and matching of lots would be especially tempting with

this approach because it might be the only practical

alternative.

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forth herein for any particular application. The

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respecting any materials mentioned herein. These

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SEMI M33-0998 © SEMI 19981

SEMI M33-0998

TEST METHOD FOR THE DETERMINATION OF RESIDUAL SURFACE

CONTAMINATION ON SILICON WAFERS BY MEANS OF TOTAL

REFLECTION X-RAY FLUORESCENCE SPECTROSCOPY (TXRF)

1 Purpose

1.1 The test provides the analytica l procedure to

determine the trace level of contaminating elements of

an atomic number higher than 15 on polished or

epitaxial silicon wafer surfaces in native or thermally

grown or tetraethylorthosilicate (TEOS) oxide or in

residues of microdroplets of process chemicals or

media as analyzed with TXRF on silicon wafer surfaces

as described in Sections 15.1 and 15.2.

2 Scope

2.1 This document specifies a VPD -TXRF (Vapor

Phase Decomposition Total Reflection X-Ray

Fluorescence Spectroscopy) method to analyze the

elemental composition and areal density of impurities,

that include cations and anions with atomic numbers

between 16 (S) and 92 (U) independent of their

chemical state, with the exception of the X-ray source

material, on polished or epitaxial silicon wafer surfaces

in native or thermally grown oxide or in residues of

microdroplets of process chemicals or media as

analyzed with TXRF on silicon wafer surfaces.

2.2 This test is especially useful fo r analyzing metallic

elements such as K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,

Zn, (Mo), Pd, Ag, Sn, Sb, Ta, (W), Pt, (Au), Hg, and Pb

and non-metallic elements such as S, Cl, As, Br, and I

through their characteristic K and L lines. (Elements in

brackets are usual X-ray sources.) For limitations in the

nature of analytes refer to the note in Section 14.7.

2.3 This test method can be used t o analyze areal

surface contamination that can be collected in a

microdroplet during the specified VPD preparation and

the collection of the digested surface contamination in

the range of 5 × 10

through 5 × 10

atoms/cm

2.4 Theoretically, the detection limit (LOD) of each

analyte depends upon its atomic number. As defined by

DIN 32645 “Limit of detection, determination and

quantification” the LOD of TXRF is also depending

upon many parameters, such as:

— excitation energy,

— intensity of incident X-ray,

— instrumental background,

— crystallographic interferences, such as Bragg

diffraction conditions,

— impurities in the beam path,

— contamination of the blank scanning solution

(see Section 4.5),

— contamination level in the analytical ambient,

— surface microroughness of wafer at the

microdroplet (see Section 6.7), and

— integration time.

2.5 Concerning the surface conditions to be analyzed,

the VPD-TXRF method is invasive. Nevertheless, the

TXRF analysis of the microdroplet residue can be

repeated many times provided that the prepared

specimen is stored in a clean environment. The

substrate and/or surrogate wafers can be recycled for

monitoring purposes.

2.6 The user of this test method must assure that the

metrology equipment is under control by the procedures

commonly utilized in the performing laboratory. In the

absence of established control procedures the use of

4.11.2 EN-ISO 9001 is recommended.

NOTICE SAFETY PRECAUTIONS — This standard

does not purport to address the safety concerns,

associated with its use. It is the responsibility of the

user of this standard to establish and maintain

appropriate safety and health practices and comply with

the local regulatory ordinance. X-ray irradiation and

handling of HNO

, HF and H

are dangerous.

Operators must comply with X-ray safety regulations

and be trained to wear protective garments and glasses

when handling HNO

, HF and H

. These chemicals

should be handled in a ventilated area (under exhaust.)

3 Referenced Documents

3.1 SEMI Standards

SEMI C7.3 — Standard for Hydrofluoric Acid, Grade 2

SEMI C7.5 — Standard for Hydrogen Peroxide, Grade

SEMI C7.6 — Standard for Nitric Acid, Grade 2

SEMI C10.1 — Guide for Determination of Method

Detection Limits for Trace Metal Analysis by Plasma

Spectroscopy

SEMI E45 — Test Method for the Determination of

Inorganic Contamination from Minienvironments

SEMI M33-0998 © SEMI 1998 2

SEMI M1 — Specifications for Polished

Monocrystalline Silicon Wafers

SEMI M20 — Specification for Establishing a Wafer

Coordinate System

3.2 ASTM Specifications

ASTM D 5127 — Standard Guide for Electronic Grade

Water (Type E-1)

ASTM E 691 — Practice for Conducting an

Interlaboratory Study to Determine the Precision of a

Test Method

ASTM F 1526 — Test Method for Surface

Metals/TXRF

3.3 DIN Specifications

DIN-ISO 5725 — Precision of Test Methods,

Evaluation of Round-robins

DIN 12650 Part 6 — Gravimetrical Test for Piston

Operated Volumetric Apparatus

DIN 12650 Part 6 (Apr. 1983) — Gravimetrical Test

for Piston Operated Volumetric Apparatus

DIN 32645 — Limit of Detection and of Quantification

3.4 ISO Specifications

EN-ISO-DIN 9001 — Quality Systems; Quality

Assurance

3.5 Other Specification

U.S. Federal Standard 209 — Airborne Particulate

Cleanliness Classes in Cleanrooms and Clean Zones

4 Terminology

4.1 anglescan — A measurement of the emitted

fluorescene signal as a function of the glancing angle of

incident X-ray beam.

4.2 areal density — Amount of im purities in a unit

area of native or thermally grown silicon oxide as

converted from the detected amount of analytes into the

whole analyzed (i.e., scanned surface area).

1 American Society for Testing and Materials, 100 Barr Harbor

Drive, West Conshohoken, PA 19428-2959

2 Deutches Institut fur Normung e.V., Beuth Verlag GmbH,

Burggrafenstrasse 4-10, D-10787 Berlin 30, Germany

3 ISO Central Secretariat, C2.P2 56 Clf-1211 Geneve 20,

Switzerland, available in the U.S. from American National Standards

Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036

4 Available from Standardization Documents Order Desk, Bldg. 4,

Section D, 700 Robbins Avenue, Philadelphia, PA 19111-5094, Attn:

NPODS

4.3 as-polished wafer — Mirror-fi nished wafer

planarized by chemi-mechanical polishing.

4.4 azimuthal position — Orientat ion around the z-

crystal/ingot axis as specified in SEMI M20.

4.5 contamination collection — M icroanalytical

method to collect the VPD decomposition products

from the silicon surface by rolling a scanning

microdroplet on the hydrophobic silicon wafer surface

after VPD preparation as originally described in

Sections 15.2, 15.4, and 15.5 (also see Section 8.3 of

this document).

4.6 critical angle — The incident X-ray glancing

angle below which total reflection of the incident X-ray

occurs. At the critical angle the X-ray reflection equals

0.5.

4.7 detection spot area — The sur face area where

above the fluorescence counts are integrated.

4.8 epitaxial wafer — As-polished wafer covered with

a layer of monocrystalline silicon deposited from a

heterogeneous phase.

4.9 glancing angle — Incidence a ngle of X-ray

excitation.

4.10 hydrophobic surface — Contact angle of wafer >

60° (e.g., virgin epitaxial or HF- or HMDS

(hexamethyldisilazane) -treated surface as described in

Section 15.3).

4.11 impurities — Elements in/on t he specimen other

than silicon or elements in ultra pure process media as

listed in Section 2.2.

4.12 native oxide — Compound of silicon, oxygen,

and water on as-polished or epitaxial wafer, grown in

air or in cleaning solutions.

4.13 recovery rate — The ratio of a nalytes found after

the first VPD and contamination collection procedure to

the sum of the analytes found after two or more

repeated scanning with unused scanning droplet of

unchanged chemical composition.

NOTE: Recent efforts of the Statistical Task Force of the

SEMI Chemicals and Gases Committee may result in a new

definition (see Section 14.2 of this document).

4.14 spurious peaks — Peaks that are detected but not

originated from impurities of the silicon wafer (c.f.,

6.1).

4.15 thermally grown oxide — SiO

up to 1000 nm

thickness deposited or grown in thermal processing in

oxygen containing atmosphere.

4.16 vapor phase decomposition (VPD) — Vapor

phase decomposition of silicon oxides using HF vapor

at room temperature as a surface preparation method for