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SEMI C15-95 © SEMI 1995, 2002 5 detail, because such a calculation can be m ade by applying proce dures described elsewhere. The accuracy required in validation will vary with the proposed application of the moisture sta…

SEMI C15-95 © SEMI 1995, 2002 4

of the moisture generator, and to its operating

temperature.

If the background moisture concentration is B (ppb),

the concentration delivered by the moisture standard is

C (ppb) and the dilution factor x is given by

x =

flow of moisture standard

total flow

(1)

then c, the delivered moisture concentration is given (in

ppb) by

c = (1 −

)B +

(2)

The accuracy of calibration will reflect the combined

accuracy of the standard and the dilution system,

together with the accuracy of the background

determination; all must be taken into account in any

statement of analytical uncertainty.

7.3 Changing the Range of Performance of a Moisture

Standard — If a moisture standard is based on some

device which is believed to deliver moisture at a

constant rate, such as a permeation or effusion tube, the

concentration range delivered by the moisture standard

may be changed by delivering a substantially different

flow of carrier gas. This usually requires changing a

mass flow controller or other flow control device inside

the standard for a flow controller of a different range.

This may be done provided the new flow controller' s

performance is verified by comparison with a reliable

flow standard.

The definition of dilution factor is revised to

x =

flow generated by original flow controller

flow generated by new flow controller

(3)

The background, B, is determined by removing the

permeation or effusion tube from the system and

capping off its point of connection, or by preventing the

source from delivering moisture by some other means.

The same equation for c applies as does the statement

regarding analytical uncertainty.

7.4 Validation at Intermediate Concentrations — It

may be desirable to validate a moisture standard at a

concentration below the detection limit of the primary

hygrometer, if that standard is to be used for very low-

level generation. For example, if a standard based on a

permeation device is intended to generate levels

between 0.5 and 5 ppb at flows of 1 to 10 slm, it could

be validated in the 0.5 to 5 ppm range using a frost-

point hygrometer by replacing the flow controller with

one operating between 1 and 10 sccm

. However, such

a flow controller may not be readily available and is

difficult to calibrate in any case. Instead, it is preferable

to use a flow controller operating between 50 and 500

sccm to generate concentrations between 0.1 and 1.0

ppm together with a secondary hygrometer having a

detection limit no higher than 50 ppb. This procedure is

acceptable, provided that

• the secondary hygrometer has been validated with

a primary hygrometer at least at one point in the

range to be used for validation of the moisture

standard,

• the moisture concentrations generated by the

standard and that observed by the secondary

hygrometer agree throughout the range of

validation of the standard.

This approach could lead to errors if the hygrometer

and the standard each exhibited a deviation from

predicted performance, but these deviations would have

to be the same and cancel, which is extremely unlikely.

In the case of operation in this mode, it will be

necessary to know the background level of the system

and use this as an input to the calculation of generated

moisture levels. To continue the example of a

permeation based system, if the permeation rate

determined by weight loss is P (ng/min), the flow rate

of carrier gas is F (sccm), K is a factor to convert

ng/min to sccm moisture, and B is the experimentally

determined background, then the generated moisture

level, c

gen

, is calculated according to

gen

+ B (4)

For other moisture generation systems, it is similarly

possible to calculate the generated moisture level while

taking into account the background level of the system.

This assumes that the background moisture

concentration is constant, which is only approximately

true. Thus, in applying this approach, in addition to the

sources of error mentioned above, the estimate of

analytical uncertainty must also include the variation in

background moisture.

8 Validation Precision and Accuracy

This procedure does not explicitly consider the

estimation of the accuracy of a moisture standard in

1 It is assumed that, although flow controllers are calibrated using a

standard, no unusual measures are used to enable the full accuracy of

calibration to be achieved over the entire range of the flow controller.

In practice this means that a flow controller has a usable dynamic

range of 10-100% of its flow rating.

SEMI C15-95 © SEMI 1995, 2002 5

detail, because such a calculation can be made by

applying procedures described elsewhere. The accuracy

required in validation will vary with the proposed

application of the moisture standard and may be left to

the discretion of the user of the procedure. It is

recommended that a propagation of errors calculation

be carried out to estimate the accuracy of the validation.

If no such calculation is made, and the minimum

criteria of the procedure are followed, it is estimated

that a validated moisture generator may output moisture

levels which differ from the true values by up to 20%.

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 mentioned herein. These

standards are subject to change without notice.

The user’s attention is called to the possibility that

compliance with this standard may require use of

copyrighted material or of an invention covered by

patent rights. By publication of this standard, SEMI

takes no position respecting the validity of any patent

rights or copyrights asserted in connection with any

item mentioned in this standard. Users of this standard

are expressly advised that determination of any such

patent rights or copyrights, and the risk of infringement

of such rights, are entirely their own responsibility.

International), 3081 Zanker Road, San Jose, CA 95134. Reproduction o

the contents in whole or in part is forbidden without express written

consent of SEMI.

SEMI C52-0301 © SEMI 20011

SEMI C52-0301

SPECIFICATION FOR THE SHELF LIFE OF A SPECIALTY GAS

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

the European Gases Committee. Current edition approved by the European Regional Standards Committee on

December 20, 2000. Initially available at www.semi.org January 2001; to be published March 2001.

1 Purpose

1.1 The purpose of this document is to define

terminology and to recommend minimum periods for

the shelf lives of specialty gases. It is meant to provide

consistency in terminology among gas suppliers and to

provide a general guideline for users of these gases.

2 Scope

2.1 This document applies to the shelf life of properly

packaged, filled and analyzed specialty gases as stored

or supplied by a specialty gas manufacturer or supplier.

In this document, shelf life is viewed from a quality

point of view. The document does not address safety

aspects associated with the prolonged storage of gases.

2.2 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.

3 Referenced Standards

None.

4 Terminology

4.1 container — a lecture bottle, cylinder, cylinder

pack, drum or any other vessel which is used to supply

a specialty gas.

4.2 residual level — The amount of product, as a

fraction of the fill weight or fill pressure, which should

be left in a container in order to minimize the impact on

manufacturing processes of the higher level of

impurities in the last gas fraction from a container.

4.3 shelf life — the period of time for which the

specification of a gas is guaranteed by the supplier,

starting from the time of analysis. It defines the period

for which the supplier guarantees the actual level of

impurities, as analyzed, to remain at or below the

specification limit for a particular gas grade.

5 Requirements

5.1 Factors Affecting the Shelf Life of a Specialty Gas

— Shelf life is a function of the following factors:

a. Thermal stability of the gas

b. Quality of the container and valve

• material selection

• leak tightness

c. Time-dependent reactions between

• gas and impurities

• gas and packaging materials

d. Filling and analytical operations

e. Transport and storage conditions

f. Amount of product in the container

5.1.1 The integrity of the valve is a key factor

determining the shelf life of a gas, in particular for low

vapor pressure gases. Time dependent reactions can be

significantly reduced by purification of the gas to a

level where impurities no longer have a significant

effect over time, and by selection of proper packaging

materials. Chemical and physical treatment of the

internal container surface and/or passivation of this

surface before filling also reduces the effect of time-

dependent reactions. For material and gas compatibility

information, please refer to the individual SEMI gas

standards.

5.2 Classification of Specialty Gases on the Basis of

Shelf Life — While there are a large number of

specialty gases used in semiconductor manufacturing,

for the purpose of defining shelf lives, they can be

divided into four groups. It should be noted that for

example chemically reactive gases are sometimes

classified as non-reactive in the table below as the

classification is done from a shelf-life point of view

only.

5.2.1 Non-reactive — Under normal storage

conditions, these gases do not react with ambient air,

the packaging materials or the impurities commonly

found in the specialty gas.

5.2.2 Reactive — These gases react either with

ambient air or standard packaging materials.

5.2.3 Corrosive — These gases react with packaging

materials usually in the presence of traces of moisture.

5.2.4 Unstable — These gases are thermally unstable

and spontaneously decompose over time.