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SEMI C15-95 © SEMI 1995, 2002 4 of the moisture generator, and to its op erating temperature. If the background moisture concentration is B (ppb), the concentration d elivered by the moisture standard is C (ppb) and the …

SEMI C15-95 © SEMI 1995, 2002 3

order to improve agreement with the primary

hygrometer.

6.2.3 Frequency of Hygrometer Validation — The

frequency of validation required for a secondary

hygrometer will vary dependent upon the performance

established for hygrometers of the same general type.

If three successive monthly validations show continuing

agreement (within 10%) with the primary hygrometer

without calibration adjustment, the validation interval

may be extended to three months. Similar extensions of

validation interval to six months and one year may be

made if no drift is apparent and no calibration

adjustments are necessary to bring the secondary within

10% of the primary at all points.

6.3 Validation Procedure for Moisture Standards

6.3.1 Validation Within the Range of an Available

Primary Hygrometer — If a primary hygrometer is

available, and its detection limit is no higher than the

lowest concentration at which it is desired to use the

moisture standard, this is the simplest case. Use the

standard to generate four concentration levels or two

per decade (whichever is less) in which the standard

will be used, half of the concentrations generated while

increasing concentration, half while decreasing,

including at least one comparison at the highest point of

the concentration range. The first and last comparisons

should be at the lowest point of the concentration range.

If the generated levels, calculated according to the basic

properties of the generator (e.g., weight loss of a

permeation tube) agree with the measured levels to

within 10%, the standard may be considered validated

with an accuracy of 10%. Higher validation accuracy

requires better agreement. It has been assumed that the

precision of the primary hygrometer, as determined by

a national standards laboratory, is much better than

10% and may be neglected for validation at the 10%

level. Validation at higher accuracy requires explicit

consideration of the certification precision.

If a standard only generates one level, connect it to the

hygrometer and allow to come to equilibrium.

Assuming the equilibrium level agrees with the

calculated level of the generator, disconnect the

hygrometer and bring it to equilibrium with either dry

nitrogen or ambient air. Then reconnect the moisture

standard and reestablish equilibrium between the

standard and the hygrometer. Disconnect again and

repeat a third time. If the three measurements agree

within 10% among themselves and with the generated

level, the standard is validated.

It is implicit in the above that the background moisture

level of the system be less than 50 ppb so that it can be

neglected for all measurements above 1 ppm with an

error no larger than 5%. We can usually neglect

background moisture in validating moisture standards

in this concentration range.

6.3.2 Validation Using Secondary Hygrometers — If

no primary hygrometer is available, then validation can

be performed using a secondary hygrometer. If the

lowest concentration to be generated using the standard

is no higher than the detection limit of the primary

hygrometer used for validation of the secondary

hygrometer, then the procedure is as above, except that

the secondary must first have been validated over the

entire concentration range which the standard will be

used to generate. (A secondary hygrometer and a

moisture standard can be validated simultaneously if

the primary and secondary hygrometers are connected

in parallel to the moisture standard.)

When assessing the accuracy of validation using a

secondary hygrometer, it is necessary to take into

account the precision of the comparison of the primary

hygrometer with the secondary hygrometer as well as

the precision of the comparison of the generator with

the secondary hygrometer.

7 Practice for Generating ppb Moisture

Standards

The detection limit of the primary hygrometer will

generally be on the order of 1 ppm, whereas it will

frequently be necessary to use the moisture standard at

concentrations below this level, by a factor of as much

as 10,000. Therefore, valid methods for reducing

concentration are necessary.

7.1 Dilution of Validated Moisture Standards — If a

moisture standard validated in accordance with Section

6.2 or Section 6.2.1 is diluted with a zero gas stream

satisfying the criteria of Section 5.3, then the resultant

diluted standard may be considered valid and may be

used for calibration of analyzers, such as APIMS.

7.2 Consideration of Background Moisture — In any

ppb or lower moisture measurement, it is necessary to

consider the background moisture level due to

outgassing in the sampling system and residual

moisture in the generator + analyzer. Often the

background will be sufficiently large that it must be

taken into account explicitly. The background moisture

level is the moisture concentration measured in the

absence of a moisture source (i.e., when the driest

carrier gas available is fed to the analyzer using the

same flow rate and flow path as is used for the sample

gas during analysis).

It should be noted that the importance of the

background level increases for measurements at low

concentrations, and that the background level is

sensitive to the materials of construction, to the design

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.

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.

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