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SEMI E45-1101 © SEMI 1995 , 2001 2 3.3 DIN Standards 2 DIN 1265 0 Part 6 — Mechan ical, physical and electrical la boratory apparatus; Piston operated volumetric apparatus; Gravimetric assess ment of metrological reliabi…

SEMI E45-1101 © SEMI 1995, 20011

SEMI E45-1101

TEST METHOD FOR THE DETERMINATION OF INORGANIC

CONTAMINATION FROM MINIENVIRONMENTS USING VAPOR PHASE

DECOMPOSITION-TOTAL REFLECTION X-RAY SPECTROSCOPY

(VPD/TXRF), VPD-ATOMIC ABSORPTION SPECTROSCOPY

(VPD/AAS), OR VPD/INDUCTIVELY COUPLED PLASMA-MASS

SPECTROMETRY (VPD/ICP-MS)

This test method was technically approved by the Global Metrics Committee and is the direct responsibility

of the European Equipment Automation Committee. Current edition approved by the North American

Regional Standards Committee on August 27, 2001. Initially available at www.semi.org September 2001; to

be published November 2001. Originally published in 1995; previously published March 2001.

1 Purpose

1.1 This test method provides the analytical procedures

to determine the level of inorganic contamination from

a minienvironment.

2 Scope

2.1 This document relates to inorganic impurities,

which includes metallic contaminants, whether they

occur as atoms, molecules, or particles. The number of

metals to be analyzed is restricted to the four elements

sodium (Na), calcium (Ca), iron (Fe), and copper (Cu)

in order to rapidly characterize minienvironments from

a practicable point of view. While Na, Ca, and Fe

represent one ensemble of highly detrimental impurities

with respect to contamination from human sources

(Na), the environment (Ca), or from equipment and

corrosive effects (Fe), Cu is analyzed due to its

increasing importance in semiconductor manufacturing.

Additionally, they are easily analyzed with sufficiently

low detection limits. It is up to the user of this test

method to quantify additional elements. A list of

suggested polished wafer surface metal contamination

inappropriate to circuits and devices is shown in Table

1 (based on SEMI M1). The inorganic contamination

on silicon wafer surfaces is collected by VPD.

2.2 To quantify Ca and Fe, VPD/TXRF is used due to

its sufficiently low detection limits. Na and Cu are

quantified by VPD/GFAAS or VPD/ICP-MS. All

analytical methods are widely used for the

characterization of surface cleanliness.

2.3 This measurement technique can also be used to

check the influence of certain process steps on

minienvironments.

2.4 This standard does not purport to address safety

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

responsibility of the users of this standard to establish

appropriate safety and health practices and determine

the applicability of regulatory limitations prior to use.

Table 1 Suggested Polished Wafer Surface Metal

Contamination Inappropriate to Circuits and

Devices

Element Test Method

Na VPD/(AAS or ICP-MS)

Al VPD/(AAS or ICP-MS)

K VPD/(AAS or ICP-MS or TXRF)

Cr VPD/(AAS or ICP-MS or TXRF)

Fe VPD/(AAS or ICP-MS or TXRF)

Ni VPD/(AAS or ICP-MS or TXRF)

Cu VPD/(AAS or ICP-MS or TXRF)

Zn VPD/(AAS or ICP-MS or TXRF)

Ca VPD/(AAS or ICP-MS or TXRF)

3 Referenced Standards

3.1 SEMI Standards

SEMI C28 — Specifications and Guidelines for

Hydrofluoric Acid

SEMI C35 — Specifications and Guidelines for Nitric

Acid

SEMI E19 — Standard Mechanical Interface (SMIF)

SEMI M1 — Specification for Polished

Monocrystalline Silicon Wafers

3.2 ISO Standards

ISO 9001 — Quality Systems—Model for Quality

Assurance in Design, Development, Production,

Installation, and Servicing

ISO 14644-1 — Cleanrooms and associated

environments – Classification of air cleanliness

1 International Organization for Standardization, ISO Central

Secretariat, 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20,

Switzerland. Website: http://www.iso.ch

SEMI E45-1101 © SEMI 1995, 2001 2

3.3 DIN Standards

DIN 12650 Part 6 — Mechanical, physical and

electrical laboratory apparatus; Piston operated

volumetric apparatus; Gravimetric assessment of

metrological reliability

NOTE 1: Unless otherwise indicated, all documents cited

shall be the latest published versions.

4 Terminology

4.1 Abbreviations and Acronyms

4.1.1 GFAAS — Graphite Furnace Atomic Absorption

Spectroscopy

4.1.2 ICP-MS — Inductively Coupled Plasma – Mass

Spectroscopy

4.1.3 PFA — Perfluoroalkoxy

4.1.4 PTFE — Polytetrafluoroethylene

4.1.5 PVDF — Polyvinylidene fluoride

4.1.6 TXRF — Total Reflection X-Ray Fluorescence

Spectroscopy

4.1.7 ULSI — Ultra Large Scale Integration

4.1.8 VPD — Vapor Phase Decomposition

4.2 Definitions

4.2.1 box — a protective portable container for a

cassette and/or substrates.

4.2.2 cassette — an open structure that holds one or

more substrates (e.g., wafer, masks).

4.2.3 DI water — deionized water (specified with

specific resistivity ≥ 18 MΩcm, cations: Na, Cu, Fe, Ca

≤ 0.2 µg/L).

4.2.4 minienvironment — a localized environment

created by an enclosure to isolate the product from

contamination and people.

4.2.5 pod — a box having a Standard Mechanical

Interface (SMIF) (See SEMI E19).

4.2.6 reference wafer — a cleaned wafer (see Section

8.2).

4.2.7 sampling wafer — a cleaned wafer (see Section

8.2), which will be or was exposed to the

minienvironment for a certain time.

4.2.8 standard mechanical interface (SMIF) — the

interface plane between a pod and another

minienvironment (see SEMI E19).

2 Available from Deutches Institut für Normung e.V., Beuth Verlag

GmbH, Burggrafenstrasse 4-10, D-10787 Berlin, Germany. Website:

http://www.din.de

4.2.9 vapor phase decomposition — a method in which

impurities on the surface are collected by the so-called

VPD procedure, i.e., the non-volatile products formed

by acid decomposition of the oxide at the wafer surface

are collected by a droplet of collecting agent, usually

ultra-pure hydrofluoric acid or other reagent or

combination of reagents, and the droplet subsequently

being analyzed by AAS or ICP-MS, or dried in a

manner which gives the least environmental

contamination, the residue from the droplet

subsequently being analyzed by TXRF.

5 Interferences

5.1 For worst cases, preconditioning of wafers can

result in different surface properties indicated by

different sensitivities for contamination absorption.

5.2 Non-linearity effects of the TXRF detector are

significant at higher concentration levels (> 10

atoms/cm

under the detector area).

5.3 The collection efficiency of VPD depends on:

• the chemistry of the collecting solution

• the bonding of the metal impurities to the silicon

surface

• the speed of the droplet, which is rolled over the

wafer surface

5.3.1 Careful control of contamination and all other

factors affecting the results such as solution

concentrations, scanning methods and other procedures

are necessary to obtain reproducible analytical results.

5.4 The measured TXRF intensity depends on the

accuracy of the procedure to localize and to adjust to

the sampled residue. It also depends on the distribution

of different elements in and around the residue.

5.5 The detection of Fe or Ca using ICP-MS can be

interfered with by background ions originated from the

plasma unless some controlled measures are taken to

minimize these interferences to acceptable levels.

6 Safety Precautions

6.1 Handling hydrofluoric acid is dangerous and shall

be performed according to local regulations for

laboratories. Operators shall be trained to deal with

dangerous chemicals and vapors, especially

hydrofluoric acid and HF vapor. Protective clothes and

glasses must be worn when handling hydrofluoric acid.

7 Apparatus

7.1 The VPD treatment and contamination collection

particularly, but also the handling and measurement of

the specimen wafer is to be carried out in a specified

SEMI E45-1101 © SEMI 1995, 20013

and controlled ambient (e.g., ISO Class 4 (as defined in

ISO 14644-1)).

7.2 The VPD and the advisable drying chamber(s) shall

have opening(s) made of PVDF, PFA, PTFE or similar

resistant and pure polymer materials that are not

attacked by HF. The chamber(s) may contain one or

more wafer stacks. After evacuation, the chamber shall

be flushed with filtered N

until the complete drying of

the microdroplet residue is achieved.

7.3 For the aliquots of standard stock and scanning

solutions, validated micropipettes shall be used. DIN

12650 Part 6 provides an applicable validation

procedure.

8 Procedure

These procedures show the determination of Na, Ca,

Cu, and Fe on a silicon wafer surface with

VPD/GFAAS, VPD/ICP-MS and VPD/TXRF. For

achieving best detection limits, VPD/GFAAS and

VPD/ICP-MS shall be used for NA and Cu whereas

VPD/TXRF shall be used for Ca and Fe.

8.1 Test Requirements — The evaluation of the

minienvironment and the analysis shall be carried out

under appropriate clean conditions. Any potential for

cross contamination shall be checked in advance.

Possible contamination sources are:

• VPD preparation

• storage

• contaminated GFAAS or ICP-MS vessels

• environment

• measurement methods and collection efficiency of

VPD

• handling

8.1.1 The capability of the analytical lab has to be

checked carefully for compliance with Sections 8.2,

8.6, and 9.

8.2 Surface Conditions and Cleaning Procedure —

Polished silicon wafers with the following

specifications must be used:

• Specific resistivity = 1–100 Ωcm

• CZ crystal growth method

• Cleaned to leave a native oxide with hydrophilic

surface conditions and with Na, Ca, Cu, and Fe

concentrations lower than 1 × 10

atoms/cm

8.2.1 Wafer cleaning must be done less than ten

minutes before any further processing. This restricted

time limitation is necessary to ensure cross

contamination avoidance.

8.3 VPD Preparation — The vessel for (opening > 25

) inside the VPD box is filled with 25 vol-% HF by

mixing DI water with 50 vol-% VLSI-grade

hydrofluoric acid (see SEMI C28) allowing

hydrofluoric acid of between 25 to 50 wt % to be used.

The wafers are then exposed to the hydrofluoric acid

vapor. Allow wafers exposure to hydrofluoric acid

vapor for 15 to 30 minutes, at which time the wafer

should become hydrophobic due to oxide removal. The

liquid reaction products are collected by rolling a DI

water droplet over the whole wafer surface using up to

100 µL for advisable machine operation or an

appropriate volume for manual operation. Any cross

contamination is minimized by using DI water as

solvent.

8.4 Collection Procedure — An automatic scanning

procedure is preferable, but if the collection procedure

is manual the following procedure should be used.

8.4.1 Use appropriate method to exclude the wafer

edge.

8.4.2 A droplet of collecting agent, usually ultra-pure

hydrofluoric acid or another reagent or a combination

of reagents, is rolled over the whole surface of the

wafer in a parallel pattern.

8.4.3 The same droplet is then moved over the whole

surface, this time in a pattern orthogonal to the first.

8.4.4 Finally, the droplet is rolled in a spiral pattern

from the wafer periphery to its center.

8.5 Pre-analysis Procedure

8.5.1 GFAAS Analysis — The wafer droplet is diluted

to 500 µL.

8.5.2 TXRF Analysis — The droplet is evaporated on

the wafer surface in a clean environment at room

temperature under a nitrogen purge.

8.5.3 ICP-MS Analysis — The wafer droplet is diluted

to 500 µL.

8.6 Sodium and Copper Analysis by GFAAS

8.6.1 Calibration Standards — 1 µg/L for Na.

8.6.2 The calibration frequency and procedures shall be

in accordance with the requirements of the ISO 9001

quality system. The temperature program for the

graphite furnace (dry, ash, atomize) is optimized for

maximum sensitivity. Volatility (e.g., NaF) should be

avoided by spiking the liquid samples with nitric acid

(VLSI grade—see SEMI C35). Prior to analysis of the

liquid sample, a three-point calibration of the element