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SIA Revised Proposal to Amend the Apportionment Model Validation Criteria in 40 CFR 98.94(c)

SIA has several concerns with the current language. 

1. Most U.S. fabs have no way to measure the use of fluorinated gas exclusively for etch or 

chamber clean as required in 98.94(c)(2) unless it is only used in etch or clean, in which case 

apportionment may not be relevant. SIA and its members requested the ability to model gas 

use to eliminate the need to perform direct measurements in a complex system.

2. Some fabs may be configured such that they can apportion gas use based on gas delivery system 

connections and flow values rather than engineering models and, therefore, we would like to 

create this option under the rule. Where supply systems serve only one process type, gas use 

can be applied directly to a process type. Where supply systems serve multiple processes, gas 

use might be apportioned based on flowmeter readings rather than an apportioning model. This 

is not allowed for under the current language in 98.94(c). 

3. The criteria used to determine which time period to assess use against the model are

unnecessarily complex and uncertain. As specified in 98.94(c)(2)(i), data to assess the maximum 

30 day period will not be available until sometime in January of the next year. Then the facility 

has to determine the assessment period, complete the modeling and gas use measurement 

analysis, perform the comparison and make corrections, and complete the emissions estimates 

prior to the March deadline making it difficult to complete the validation in a timely manner so 

that any issue with the model can be addressed. 

4. Lastly, the closure standard in the rule (between modeled use and actual gas use) is 

unrealistically tight and not necessary for several reasons. 

a. First, with the use of a 2-factor approach to etch and the process of combining wafer 

cleaning with etch as an emissions factor category, the level of apportioning is greatly 

reduced. 

b. Second, SIA has demonstrated, in its recent analysis of the Etch Factor Proposal, that 

even a relatively high level of error in apportionment does not compromise the 

emissions method precisions because all actual gas use is accounted for, and any error 

that is introduced is due only to any difference in the emission factor for the different 

uses of the gas. 

c. There are a limited number of gases that require apportioning. Generally, for 300 mm 

facilities, only NF3 requires apportioning (where the etch and wafer clean categories are 

combined) and for 200 mm fabs 3 to 5 gases have to be apportioned over the 4 

categories. If wafer cleans and etch are combined, greater accuracy can be achieved.

With this background, SIA is proposing the following approach to validation of the apportioning model:

Allow fabs to apportion F-gas either through a modeling method or direct measurements, such 

as flow and scale values, to create the apportioning factors.

The comparison of actual gas use to modeled/measured gas use will be for use in the entire fab 

and it will be done for the F-gas with the greatest use that will need to be apportioned.

The time period for the comparison will be selected by the reporting party as representative of 

normal operation during the reporting period and can be no smaller than a month and no 

greater than a year.

The model will be deemed acceptable for apportioning actual gas use if the modeled/measured 

use and the actual use in the selected time period match within 20% of actual use. 

If the match is not within 20%, the reporting party is required to review and correct the model

or measurement method until the 20% criterion is met. Once the 20% criterion is met, the 
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modeled or measured apportionment fractions can be used to apportion gas use for purposes of 

determining and reporting emissions.

Here are suggested amendments to the language of 40 CFR 98.94(c)

(c) You must develop apportioning factors for fluorinated GHG and N2O consumption to use in Equation I–

13 of this subpart for each input gas i, as appropriate, using a facility-specific engineering model that is 

documented in your site GHG Monitoring Plan as required under §98.3(g)(5). This model must be based 

on a quantifiable metric, such as wafer passes, or wafer starts, or measured gas use data. To verify your 

model, you must demonstrate its precision and accuracy by adhering to the requirements in paragraphs 

(c)(1) and (c)(2) of this section.

(1) You must demonstrate that the fluorinated GHG and N2O apportioning factors are developed using 

calculations that are repeatable, as defined in §98.98.

(2) You must demonstrate the accuracy of your facility-specific model by comparing the actual amount of 

input gas i consumed and the modeled amount of input gas i consumed in the facilityfor the plasma 

etching and chamber cleaning process types, as follows:

(i) You must analyze at least a 30-day period of operation but no more than one year of operation that is 

representative of normal operation. during which the capacity utilization equals or exceeds 60 percent of 

its design capacity. In the event your facility operates below 60 percent of its design capacity during the 

reporting year, you must use the period during which the facility experiences its highest 30-day average 

utilization for model verification.

(ii) You must compare the actual gas consumed of input gas i to the modeled gas consumed of input gas i 

for one fluorinated GHG reported under this subpart under the plasma etching process type and the 

chamber cleaning process type. You must certify that the fluorinated GHGs selected for comparison 

corresponds to the largest quantitiesy, on a mass basis, of fluorinated GHGs used at your facility during 

the reporting year of the fluorinated GHGs that will be apportioned in the determination of emissions 

under this subpart. If there are two gases which may meet these criteria, that is their use is similar in 

quantity and both are apportioned, you may do this comparison for both and demonstrate conformance

on aggregate use basis for both fluorinated GHGs.for the plasma etching process type and the chamber 

cleaning process type.

(iii) You must demonstrate that the comparison performed for the largest quantity of gas, on a mass basis, 

that is apportioned for reporting under this subpartconsumed under the plasma etching process type in 

paragraph (c)(2)(ii) of this section, does not result in a difference between the actual and 

modeled/measured gas consumption that exceeds five20 percent relative to actual gas consumption, 

reported to onetwo significant figures using standard rounding conventions. If the comparison shows a 

difference greater than 20% , you are required to review and correct the facility specific engineering 

model or measurements until the 20% criterion is met. Once the 20% criterion is met, the 

modeled/measured fractions are to be used to apportion gas use for purposes of determining and 

reporting emissions under this subpart.