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SEMI MF1809-0704 © SEMI 2003, 2004 2 3.1.7 The samples m ust be free of wor k damage, contaminati on, and other c omplicating resi dues. Clean, specular surfaces are suit able for metallographic examination and provi de …

SEMI MF1809-0704 © SEMI 2003, 2004 1

SEMI MF1809-0704

GUIDE FOR SELECTION AND USE OF ETCHING SOLUTIONS TO

DELINEATE STRUCTURAL DEFECTS IN SILICON

This guide 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 April 22, 2004. Initially available at www.semi.org May 2004;

to be published July 2004. Original edition published by ASTM International as ASTM F 1809-97. Last

previous edition SEMI MF1809-02.

1 Purpose

1.1 Structural defects formed in the bulk of a silicon

wafer during its growth or induced by electronic device

processing can affect the performance of the circuitry

fabricated on that wafer. These defects take the form of

dislocations, slip, stacking faults, shallow pits, or

precipitates.

1.2 The exposure of the various defects found on or in

a silicon wafer is often the first critical step in

evaluating wafer quality or initiating failure analysis of

an errant device structure. Etching often accomplishes

this task. This guide provides information on the

selection and application of appropriate etching

solutions.

2 Scope

2.1 This guide covers the formulation, selection, and

use of chemical solutions developed to reveal structural

defects in silicon wafers.

2.2 Sample preparation, temperature control, etching

technique, and choice of etchant are all key factors in

the successful use of an etching method. This guide

provides information for several etching solutions and

provides guidance for the user to select according to the

need. Illustrations of results obtained with these

etching solutions are provided in Figures 1–32.

2.3 This guide is intended for use with other practices

and test methods. SEMI MF1725 and SEMI MF1726

are practices for analysis of crystallographic perfection

of silicon ingots and wafers, respectively, utilizing

etching solutions found in this guide, and followed by

counting in accordance with the test method SEMI

MF1810. SEMI MF1727 defines the procedures for

oxidation of the wafer prior to etching, if that is

required to expose the defect structures of interest. JIS

H 0609 is a test method that covers the entire process of

determining crystalline defects in silicon wafers.

2.4 Both this guide and JIS H 0609 emphasize the use

of etching solutions that do not contain chromic acid,

which is biologically and ecologically very hazardous

(see Section 7.6). Because of the hazardous nature of

chrome containing etchants, the use of chromic acid

containing etchants is prohibited in some jurisdictions

located in various parts of the world.

NOTE 1: See Related Information 1 for a discussion of the

relationships between this guide and JIS H-0609.

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 practices and determine

the applicability of regulatory or other limitations prior

to use.

3 Limitations

3.1 Complicating factors are different for each etchant.

Research the choice of etchants in advance to ensure

the method and solution are compatible with the sample

and objectives. Commonly encountered problems are:

3.1.1 Inadvertent etching through the denuded zone of

an oxidized sample delineates irrelevant bulk defects

instead of the surface oxidation induced stacking faults

(OISF) expected.

3.1.2 Accelerated etching and etching artifacts can

result from excessive solution heating during the

etching process.

3.1.3 Insufficient agitation, bubble formation or

particles in the etching solution can generate artifacts

on the silicon surface that mimic actual defects.

Insufficient agitation can alter the etching rate,

increasing or decreasing it depending upon the

formulation.

3.1.4 Any solution in which the oxidation rate is

greater than the oxide dissolution rate may form oxide

layers that slow or even quench the etching process.

The presence of these oxide layers (especially for n

and p

material) obstructs the interpretation of etched

defects. Before evaluation, remove any surface oxides.

3.1.5 The wafer surface becomes rougher with longer

etch time. This rougher surface does not prevent evalu-

ation under the microscope, but it greatly reduces the

effectiveness of visual inspection under bright light.

3.1.6 Etching solutions can generate false pits that are

not associated with defects.

SEMI MF1809-0704 © SEMI 2003, 2004 2

3.1.7 The samples must be free of work damage,

contamination, and other complicating residues. Clean,

specular surfaces are suitable for metallographic

examination and provide the best results. Surfaces

examined should be flat with parallel faces, to simplify

microscope inspection.

4 Referenced Standards

4.1 SEMI Standards

SEMI C18 — Specification for Acetic Acid

SEMI C28 — Specifications and Guidelines for

Hydrofluoric Acid

SEMI C35 — Specifications and Guidelines for Nitric

Acid

SEMI MF523 — Practice for Unaided Visual

Inspection of Polished Silicon Wafer Surfaces

SEMI MF1241 — Terminology of Silicon Technology

SEMI MF1725 — Practice for Analysis of

Crystallographic Perfection of Silicon Ingots

SEMI MF1726 — Practice for Analysis of

Crystallographic Perfection of Silicon Wafers

SEMI MF1727 — Practice for Detection of Oxidation

Induced Defects in Polished Silicon Wafers

SEMI MF1810 — Test Method for Counting

Preferentially Etched or Decorated Surface Defects in

Silicon Wafers

4.2 ASTM Standard

D 5127 — Guide for Ultra Pure Water Used in the

Electronics and Semiconductor Industry

4.3 Japan Industrial Standard

JIS H 0609 — Test Methods Of Crystalline Defects In

Silicon By Preferential Etch Techniques

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

5 Terminology

5.1 Defect-related terminology may be found in SEMI

MF1241.

1 Annual Book of ASTM Standards, Vol. 11.01. ASTM International,

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

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

www.astm.org.

2 Available, in Japanese language edition only, through the Japanese

Standards Association, 1-24, Akasaka 4-Chome, Minato-ku, Tokyo

107-8440, Japan. Telephone: 81.3.3583.8005; Fax: 81.3.3586.2014

Website: www.jsa.or.jp

6 Apparatus

6.1 No standard apparatus or facility satisfies the

universal needs for the various applications of these

etching solutions. Systems range from a simple HF-

resistant beaker to large etching tanks complete with

nitrogen bubblers, temperature control and nitrous

oxide and hydrofluoric acid (HF) scrubbers.

6.1.1 For larger samples (wafers or slugs), use large

etching tanks with nitrogen bubble agitation or

ultrasonic agitation. Most of the etchant solutions listed

work more effectively with the aid of agitation. Heat

exchangers or just the thermal mass of the solution can

control temperature. Large volumes of acid heat more

slowly and allow an intrinsic form of temperature

control. To reduce heating effects, maintain 1 L of

solution for each 1,000 cm

of sample surface area.

6.1.2 Maintain proper environmental controls. Make

provisions to dispose of nitrous oxides, HF fumes, and

any solid wastes evolved whatever system is chosen.

Chromium and copper-based etching solutions produce

solid waste and gaseous byproducts. Chromium-free

etching solutions produce no measurable solid waste

but do generate nitrous oxides and HF fumes.

7 Reagents and Materials

7.1 Purity of Reagents — Chemicals shall conform to

the assay and impurity levels of Grade 1 SEMI

Specifications where they exist. Reagents for which

SEMI specifications have not been developed shall

conform to the specifications of the Committee on

Analytical Reagents of the American Chemical

Society,

where such specifications are available. Other

grades may be used provided it is first ascertained that

the reagent is of sufficiently high purity to permit its

use without lessening the accuracy of the determination.

7.2 Purity of Water — Reference to water shall be

understood to mean Type E-3 or better water as

described in ASTM Guide D 5127.

7.3 Volume of components describes all solutions in

parts of a standard assay. The formulas give solid or

dissolved components in grams per 100 mm of total

solution.

7.4 All formulations employ a Standard Solution

Convention (SSC) that specifies each solution

component as an acceptable assay ± some tolerance.

3 Reagent Chemicals, American Chemical Society Specifications,

American Chemical Society, Washington, DC. For suggestions on the

testing of reagents not listed by the American Chemical Society, see

Analar Standards for Laboratory Chemicals, BDH Ltd., Poole,

Dorset, U.K., and the United States Pharmacopeia and National

Formulary, U.S. Pharmacopeial Convention, Inc., (USPC), Rockville,

MD.

SEMI MF1809-0704 © SEMI 2003, 2004 3

Formulations of the standard assay follow this example:

A HF/HNO

/Acetic solution, in the 1:1:2 ratio is the

same as 25% (49% HF) + 25% (70% HNO

+ 50%

(glacial acetic) by volume. The specified chemicals

shall have the following nominal assay:

• Acetic acid, glacial >99.7%

• Chromium trioxide >98%

• Copper nitrate >98%

• Hydrofluoric acid 49± 0.25%

• Nitric acid 70 to 71%

7.5 The chemicals used in these etching solutions are

potentially harmful. Handle and use them in a chemical

exhaust fume hood, with the utmost care. Hydrofluoric

acid solutions are particularly hazardous.

7.6 Release of chromic acid or solutions of chromic

acid into domestic sewer systems is usually not

allowed. Chromates are extreme biological and

ecological hazards. Chromic acid is a strong oxidizing

agent and should not contact organic solvents or other

easily oxidized materials.

7.7 Safety or protective gear should be worn while

handling these acid solutions or their components.

Safety requirements vary, but essential items are plastic

gloves, safety glasses, face shield, acid gown, and shoe

covers. The handling of large quantities of powdered

chromic acid may require a respirator or other breathing

apparatus.

8 Procedure

8.1 Selection of Etching Solutions

8.1.1 The following tables show two broad categories

of solutions. The first group in Tables 1 and 3 includes

only chromium-free etching solutions. Although uses

may be limited, these solutions should be considered

whenever appropriate. The second group in Tables 2

and 4 includes only solutions that contain chromium

compounds. These do well for their intended

applications, but they can be harmful to the

environment. Use these highly contaminated etching

solutions with caution. Release into the environment of

hexavalent chromium waste is a recognized hazard (see

Section 7.6). Exposure can cause cancer.

NOTE 2: Copper-3 solution is also identified as “MEMC

etch” in referenced publications.

NOTE 3: Sopori and Sato etches have been suggested for

inclusion in Tables 2 and 4, and these solutions may be added

when solution information and pictures become available

from sponsors (see Related Information 2 for the

methodology by which new solutions can be added to the

guide).

8.1.2 Tables 1 and 2 list the figure numbers of the

etched defect examples for these etching solutions

together with their etching rates.

8.1.3 Tables 3 and 4 classify the suitability of each

listed etching solution for the various applications. In

the tables,

• A = Excellent,

• B = Good,

• C = Acceptable, and

• D = Unacceptable.

Each solution has advantages and disadvantages and

this guide does not endorse one in favor of another.

Selection of an etchant solution should be based upon

etch rate, etchant life, solution heating, environmental

harm, ease of interpretation, and range of use.

8.1.4 Investigate local environmental and safety

requirements before selecting an etchant solution.

Identify the sample orientation, type, resistivity level,

and primary defects of interest; this information helps

the user rank the various possible solutions and then

select the most appropriate choice.

8.1.4.1 Although this guide does not require a specific

solution, it is highly recommended that chromium-free

etching solutions should be used whenever possible, for

environmental and biological reasons.

8.2 Sample Preparation

8.2.1 Most silicon samples have residual oxide on the

surface, either thermally grown as part of the

fabrication process or occurring naturally due to

exposure to air.

8.2.2 Immediately before defect etching, submerge the

sample in concentrated HF solution for 1 min or until

the surface becomes hydrophobic to remove surface

oxide layers.

8.2.3 Rinse and hold the samples in deionized water

until transferred to the etching solution.

8.3 Defect Etching

8.3.1 No standard design for etching systems exists

(see Section 6.1). In general, the procedures defined for

the etching system available to the user of this guide

must be followed. A simple system for manual use is

as follows:

8.3.1.1 Place the specimen in the bottom of a HF-proof

beaker with the surface to be inspected facing upward.

8.3.1.2 Pour in sufficient etchant to cover the specimen

with about 2 cm of solution.