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SEMI F79-0703 © SEMI 2003 1 SEMI F79-0703 GUIDELINE FOR GAS COMPATIBILIT Y WITH SILI CON USED IN GAS DISTRIBUTION COMPONENTS This guideline was technically approved by the Global Gases Com mittee and is the direct respon…

SEMI F78-0304 © SEMI 2003, 2004 16

APPENDIX 1

STAINLESS STEEL AND WELDING FUME

NOTICE: The material in this appendix is an official part of SEMI F78. It has been derived from the cited

documents. Determination of the suitability of the material is solely the responsibility of the user.

A1-1 General

A1-1.1 The fume generated when welding stainless

steels includes respirable particles, the composition of

which—particularly with the flux-shielded welding

processes—suggests a risk to cause cancers. However,

epidemiological analyses have not identified any actual

risk specific to stainless steels but have shown a slight

excess of lung cancers among welders as a whole, i.e.

both welders of non-alloyed steels and welders of

stainless steels, compared with the general population.

The cause of this excess has not been identified but may

be connected with factors incidental to welding.

Nevertheless, appropriate precautions to avoid exposure

to welding fume of all kinds are advisable and indeed

necessary if regulatory limits are to be observed.

A1-1.2 There is an important difference between the

chemical forms of chromium in fume from the flux

processes and from the gas-shielded processes. In the

former group most of the chromium is present in

hexavalent form (chromates), while almost all

chromium in fume produced by the gas-shielded

processes is in the trivalent form and hexavalent

compounds are only present in very small proportions.

The relevance of this difference is that, without

reference to welding, hexavalent chromium compounds

are classified as carcinogenic to humans (Group 1) by

the IARC

particularly lung cancer. This fear is based

on the chemical composition of the fume, especially

that produced by the flux processes, and the very small

size of the particles, which puts them in the respirable

range, i.e., capable of penetration down to the level of

the lung alveoli. Trivalent chromium compounds are

unclassifiable to carcinogenicity to humans (Group 3).

Nickel compounds are also classified in Group 1 by the

IARC.

A1-1.3 Further information on the above topic can be

found in Status Report: Stainless Steel and Welding

Fume, SR-0008, March 2001, Nickel Development

Institute, 214 King Street West, Suite 510, Toronto,

Ontario, Canada M5H 3S6.

A1-1.4 For information about hexavalent chromium,

refer to HESIS Hazard Alert, June 1992, Hazard

Evaluation System & Information Service, California

Occupational Health Program, 2151 Berkeley Way,

Annex 11, Third Floor, Berkeley, CA 94704.

5 International Agency for Research on Cancer

NOTICE: SEMI makes no warranties or

representations as to the suitability of the standards set

forth herein for any particular application. The

determination of the suitability of the standard is solely

the responsibility of the user. Users are cautioned to

refer to manufacturer's instructions, product labels,

product data sheets, and other relevant literature,

respecting any materials or equipment mentioned

herein. These standards are subject to change without

notice.

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Equipment and Materials International (SEMI) takes no

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mentioned in this standard. Users of this standard are

expressly advised that determination of any such patent

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SEMI F79-0703 © SEMI 2003 1

SEMI F79-0703

GUIDELINE FOR GAS COMPATIBILITY WITH SILICON USED IN GAS

DISTRIBUTION COMPONENTS

This guideline was technically approved by the Global Gases Committee and is the direct responsibility of

the North American Gases Committee. Current edition approved by the North American Regional Standards

Committee on April 11, 2003. Initially available at www.semi.org June 2003; to be published July 2003.

1 Purpose

1.1 The purpose of this guideline is to identify resource

information on compatibility of gases in contact with

silicon in the wetted path of a gas delivery system

operating at typical gas stick conditions.

2 Scope

2.1 The information and conclusions provided are

taken from published literature. References are cited.

No opinion is made as to the validity of the published

conclusions. The suggestions are specific to high purity

silicon, single or poly crystal, covered by native oxide.

NOTICE: 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 or other limitations prior

to use.

3 Limitations (Prescribed Conditions)

3.1 The guideline applies to the gas distribution system

for a typical semiconductor process tool. It is assumed

that silicon is in the wetted path as a coating or a

silicon-based component (eg. a valve, orifice, sensor,

mass flow controller) in the gas distribution system.

The ambient temperature and operating temperature

limits are typical for the industry. No additional energy

source, thermal, radiative or ionizing, is present. The

moisture content of the gas is not specified for the

purpose of this guideline; the recommendations apply

whether or not the conditions are anhydrous.

4 Referenced Standards

4.1 SEMI Standard

SEMI E52 — Practice for Referencing Gases Used in

Digital Mass Flow Controllers

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

5 Classification of Gases

5.1 The gases included are taken from SEMI E52; the

number assigned to a gas is the same as in that

document.

5.1.1 Inerts

5.1.2 Hydrogen

5.1.3 Hydrocarbons

5.1.4 Halogenated Hydrocarbons

5.1.5 Hydrides

5.1.6 Halogens, other than Fluorine

5.1.7 Halides, other than Fluorides

5.1.8 Fluorine and Fluorides

5.1.9 Organo-metallic and Siloxanes

5.1.10 Oxygen and Oxides and Sulfides

5.1.11 Nitrogen and Nitrogen Compounds

5.1.12 Acids

5.1.13 Other

6 Review of the literature

6.1 Inerts

6.1.1 References (1, 2, 3, 4, 5) are cited which indicate

these gases do not react with silicon under the

prescribed conditions.

6.2 Hydrogen

6.2.1 References (1, 2, 3, 4, 5) are cited which indicate

these gases do not react with silicon under the

prescribed conditions. Hydrogen has been reported to

react with silicon (75) and silicon carbide (76) at

temperatures above 1100° C.

6.3 Hydrocarbons

6.3.1 References (1, 2, 3, 4, 5, 55, 56) are cited which

indicate these gases do not react with silicon under the

prescribed conditions.

6.4 Halogenated Hydrocarbons

6.4.1 References (1, 2, 3, 4, 5, 35, 51, 54, 55, 56, 57,

58, 59) are cited which indicate these gases do not react

with silicon under the prescribed conditions. Extensive

review papers by Flamm (55) and Coburn (56) cite

numerous references on the etching of silicon and other

materials using halogenated hydrocarbons. The critical

requirement for etching of silicon, or any material using

SEMI F79-0703 © SEMI 2003

a halogenated hydrocarbon, is a radical specie of

fluorine, chlorine or other halogen. A low pressure

plasma is the typical method of creating this radical. In

addition, Winters (78) points out that for an

“unassisted’ etching reaction to take place adsorption

onto the silicon surface must be followed by four well

defined steps. Should any of the steps not occur, or be

interrupted, etching will not proceed.

6.4.2 Two of the most widely used halogenated

hydrocarbons are CF

and C

. The Matheson Gas

Book (79) says this about the chemical properties of

, “...Carbon tetrafluoride is extremely stable,

reacting only slightly even at the temperature of a

carbon arc...”. About C

, “...Octafluorocyclobutane

is extremely stable. It is unreactive with other materials

under ordinary conditions. At high temperatures

(600°C), it dissociates to form carbon and carbon

tetrafluoride and some toxic compounds...”

6.5 Hydrides

6.5.1 References (1, 2, 3, 4, 5, 64, 65, 66) are cited

which indicate these gases do not react with silicon

under the prescribed conditions.

6.6 Halogens, other than fluorine

6.6.1 The halogens represent a special class of

materials. Molecular or atomic chlorine, iodine or

bromine have been reported not to etch silicon at

temperatures below 200° C or without the assistance of

a plasma (19, 26, 50, 55, 56, 63). Schwartz (74) makes

the comment. “Although Cl

chemisorbs spontaneously

on silicon, (in a plasma) at low temperatures the

reaction between Si and Cl does not proceed to the

formation of SiCl

. That is, silicon does not etch in Cl

except at elevated temperature.” He goes on to note:

“...except in the case of highly doped silicon, etching

occurs at low temperatures in the reactive ion etching

system not because of the ease of Cl attachment but

because partially reacted silicon can be sputtered

readily.” Schwartz then reports the observation that

there was no perceptible etching after thirty minutes in

a Cl

plasma when there was no cathode bias on the

wafer; in contrast to 4.5 microns of removal when the

bias was applied.

6.6.2 Flamm (55) published in 1981 and titled “The

Design of Plasma Etchants”, states, ”...Chlorine

containing halocarbon feed gases are frequently used

instead of Cl

because they are not hazardous (before

reaction), are noncorrosive, and they decompose in the

plasma to generate radicals (in addition to chlorine

atoms) which induce some desired effects.” This

statement applies to a plasma based system; Flamm is

pointing out the difficulty of etching silicon with just

or Br

in a plasma. He points out that Wang and

Maydan (from a private communication in 1979) have

etched silicon in a pure Cl

plasma at pressures below

0.05 torr.

6.6.3 Flamm goes on to say that, “...It seems that

chlorine and bromine atoms do not spontaneously etch

SiO

or undoped single crystal silicon but do etch some

forms of polycrystalline silicon and heavily n-doped

single crystal and polycrystalline silicon...”

6.6.4 A conclusion here is that even with a plasma Cl

and Br

require very specialized conditions to etch

silicon.

6.7 Halides, Other Than Fluorides

6.7.1 Gaseous mixtures of HCl, HBr or HI will not etch

silicon at the prescribed conditions (1, 2, 3, 4, 5, 62).

However, mild pitting of the silicon may be observed in

HBr or HI under aqueous conditions (5). In laboratory

testing (10) no reaction of the silicon was noted with

50% aqueous HBr after 45 days immersion.

6.7.2 One reason for the lack of reactivity of silicon

with chlorine, bromine or iodine halides is the low

vapor pressure of the reaction product. SiCl

has the

lowest boiling point at 57° C; SiBr

boils at 154° C and

SiI

at 286° C. On the other hand, the boiling point of

SiF

is –86° C. Practical advantages of this low vapor

pressure and non-reactivity were demonstrated by

Texas Instruments in their patents (24, 25) on RIE

etching; controlled additions of HBr, HI, BCl

, among

other gases, led to passivating films of SiBr

, etc. on the

silicon side walls and allowed vertical wall etching to

be achieved.

6.7.3 The moisture content of the gas stream can be a

critical factor in the corrosion resistance of most

materials. Typical stainless steel passivity is strongly

dependent upon the amount of H

O present, declining

sharply as water content increases above 1 ppm in the

presence of halogens or halides (6, 9, 10, 72). Paciej

(72) reported that 0.1 ppm moisture in HCl does not

attack 316L stainless, while 200 ppm moisture in HCl

can significantly corrode the surface of this material.

Fine (9) points out similar behavior for 316L in HBr at

0.5 ppm and 100 ppm moisture over the course of 10

days.

6.7.4 However, silicon corrosion resistance is virtually

unaffected by moisture content up to and including

aqueous solutions. The “RCA clean” (67) and

conventional wet etches (60, 68, 69) are obvious

examples of this.

6.7.5 Silicon based halides, such as SiCl

, SiH

, etc.

have not been reported to react with silicon under the

prescribed conditions. (55)