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SEMI F35-0304 © SEMI 1998, 2004 2 NOTICE: Unless ot herwise i ndicated, all docum ents cited shall be the latest published versions. 5 Terminology 5.1 Abbreviations an d Acronyms 5.1.1 ppbv — parts per billion by volume.…
SEMI F35-0304 © SEMI 1998, 2004 1
SEMI F35-0304
TEST METHOD FOR ULTRA-HIGH PURITY GAS DISTRIBUTION
SYSTEM INTEGRATION VERIFICATION USING NON-INVASIVE
OXYGEN MEASUREMENT
This test method 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 December 4, 2003. Initially available at www.semi.org February 2004; to be published March
2004. Originally published September 1998.
1 Purpose
1.1 This test method defines a procedure to monitor the
integrity of ultra-high purity (UHP) gas distribution
systems by detecting the ingress of atmospheric
oxygen. This test method would be used to evaluate an
“active” UHP gas distribution system on a continuous
basis by using non-invasive O
2
measurement, without
requiring an interruption of the process tools using the
UHP gases of interest.
1.2 This test method should be used to protect
semiconductor fabrication processes using the UHP
gases, which may be sensitive to contamination by any
of the common atmospheric impurities, such as N
2
, O
2
,
H
2
O, CO
2
, before product yield problems develop.
1.3 This is the first such test method that describes a
noninvasive leak detection and locating procedure. It
differs from SEMI F1, which is an invasive technique
for identifying leak sources using a mass spectrometer
and a helium tracer gas.
2 Scope
2.1 This test method applies to UHP gas distribution
systems used in semiconductor manufacturing facilities
and comparable research and development areas.
2.2 This test method applies to bulk gas distribution
systems carrying UHP gases such as N
2
, Ar, He, H
2
,
N
2
O, SF
6
, and many halocarbons. In most cases, O
2
is
present only in ultra-low trace levels (typically less than
1.0 ppb).
2.3 This test method will provide real-time monitoring
of UHP gas distribution systems, resulting in
meaningful system integrity verification, atmospheric
contaminant trending analysis, and leak locating.
2.4 This test method will provide the user with
sufficient information to identify and troubleshoot
sources of atmospheric leakage into the UHP gas
distribution system.
2.5 This test method includes the specification of the
required O
2
analytical equipment, standard methods for
proper use of the O
2
analytical equipment, and
manipulation of the O
2
data in identifying atmospheric
leak sources.
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
3.1 This test method will only be successful in
identifying the presence and location of leaks from
atmospheric air using O
2
as the tracer gas representative
of all atmospheric air contaminants (i.e., N
2
, O
2
, H
2
O,
CO
2
). There is no capability to identify cross-
contaminant species between different UHP gases
which only contain ultra-low trace levels of O
2
or
outward leaks to atmosphere.
3.2 This test method will not focus specifically on the
process of detecting the exact location of every possible
atmospheric air leakage source. Rather, it will describe
techniques and examples demonstrating how
atmospheric air leaks can be reliably identified and
located, in general.
3.3 Results from this test method may not ensure that
other common atmospheric impurities (e.g., H
2
O) are
below their required specification limits after a leak has
been identified, corrected, and O
2
levels have returned
to normal. For example, since O
2
diffuses more quickly
and has a weaker surface adsorption than H
2
O, O
2
will
be the first atmospheric impurity detected. However,
H
2
O will likely be the last atmospheric impurity to be
purged out once a leak has been identified and repaired.
3.4 This test method is not a direct substitution for
SEMI F1.
4 Referenced Standard
4.1 SEMI Standard
SEMI F1 — Specification for Leak Integrity of High
Purity Gas Piping Systems and Components
SEMI F35-0304 © SEMI 1998, 2004 2
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Abbreviations and Acronyms
5.1.1 ppbv — parts per billion by volume.
5.2 Defintions
5.2.1 atmospheric impurities — the common
impurities to UHP gases that are found in atmospheric
air.
NOTE 1: Impurities such as N
2
, O
2
, H
2
O, CO
2
.
5.2.2 contaminant signature — typical baseline
performance of a specific sample point within a large
distribution system with respect to its O
2
impurity
concentration. This is considered to be a normal and
acceptable impurity level.
5.2.3 critical sample point — gas sample point which
is monitored continuously because it is deemed in a
critical location in the distribution system, or that which
is most sensitive to causing product/process quality
problems from the ingress of atmospheric air
impurities.
5.2.4 investigative sample point — gas sample point
which is selected for spot checking analysis as part of
an investigation to locate an atmospheric air leak
source.
5.2.5 low detectable limit — defined as the smallest
level of O
2
measurement which can be quantified after
performing a blank test on zero gas. For our purposes,
this is determined to be 2 times the total peak-to-peak
noise over 8 hours while the O
2
analyzer is sampling on
zero gas.
5.2.6 point of use — the connection point where the
gas distribution system connects to the process tool
which uses the UHP gas.
5.2.7 sensitivity — Defined as the smallest detectable
step change in O
2
that the analyzer is capable of
sensing.
5.2.8 ultra-high purity (UHP) gas distribution system
— semiconductor fab gas delivery system which
typically contains impurities from atmospheric air of <
0.2 ppb at the input source, and atmospheric impurities
at the point-of-use of typically < 1 ppb.
5.2.9 weekly zero drift — total analyzer drift while
measuring zero gas over a 1-week time period. Includes
all components of analyzer drift.
5.2.10 zero gas — test gas which is known to contain
less than 0.1 ppb of O
2
as an impurity.
6 Summary of Method
6.1 The user must identify critical sampling locations
in the UHP gas distribution system. These points are
either specific process tool points-of-use or key lateral
branch lines, which must avoid atmospheric
contamination, or representative of a critical purity zone
containing process tools that are particularly sensitive
to atmospheric contaminants.
6.2 Connect O
2
analysis equipment to these critical
sample points for continuous monitoring.
6.3 Qualify the performance of the O
2
analysis
equipment before collecting data.
6.4 Begin continuous monitoring of critical sample
points. Identify typical or baseline “contaminant
signature” performance of the UHP gas distribution
system.
6.5 Use continuous trend data to identify problematic
events (contaminant spikes) and/or significant changes
from the “contaminant signature” performance level.
6.6 Use additional investigative sample point
measurements (spot checks) along with observable
correlations to gas distribution system usage to locate
and correct atmospheric leak sources.
7 Interferences
7.1 While sampling from any point in the gas
distribution system, it is critical that the sample be
delivered to the O
2
analytical equipment without
introducing additional atmospheric O
2
. This could
create significant errors in the analysis. Observing a
higher O
2
reading after reducing the sample flow to the
analyzer may be an indication that there is a leak
between the sample point and the O
2
analysis
equipment.
7.2 An O
2
analyzer may become inaccurate due to
improper calibration or simply due to long-term
analyzer drift. This will cause inaccurate O
2
readings
which do not represent the true performance of the
UHP gas distribution system. The O
2
analyzer must be
operated per the manufacturer’s recommendation for
calibration technique, calibration frequency, and other
routine maintenance.
7.3 Some O
2
analytical methods have cross-sensitivity
to other gaseous components which may be found in
UHP gases (i.e., H
2
, CO, CH
4
, and other hydrocarbons).
8 Requirements
8.1 Personnel Qualification — Personnel performing
these tests in accordance with this test method shall
have suitable training and experience. Such personnel
shall, as a minimum, be knowledgeable of:
SEMI F35-0304 © SEMI 1998, 2004 3
8.1.1 The operation and calibration of the specific
equipment used in performing this test.
8.1.2 The proper procedures in handling UHP gases as
agreed upon by supplier and user.
8.1.3 The proper safety procedure in handling
combustible or toxic gases as agreed upon by supplier
and user.
8.1.4 The behavior of atmospheric leak sources relative
to position, pressure/flow, and gas usage.
9 Apparatus
9.1 An analyzer must be selected which is capable of
continuous ultra-low trace O
2
measurements, with the
ability to report measurements with at least once per
minute frequency.
9.2 The ultra-low trace O
2
analyzer shall meet or
exceed the following specifications. These
specifications are appropriately defined in Section 5:
Sensitivity < 0.1 ppb
Low Detectable Limit < 0.3 ppb
Weekly Zero Drift < 0.5 ppb
Response Time < 10 minutes for 90% of a
0–1 ppb O
2
step change
NOTE 2: The O
2
analysis must be conducted using an
analytical method which meets or exceeds the specifications
required by this test method. If not, the user will not be able to
confidently distinguish true gas distribution system impurity
trends from false analyzer trends.
10 Safety Precautions
10.1 This test method is not a replacement for safety
regulations. It is the responsibility of the user to ensure
that the UHP gas distribution systems under analysis
comply with applicable safety regulations, as agreed to
between gas supplier and user.
10.2 It is also the responsibility of the user to comply
with applicable safety regulations governing the
operation of the required O
2
analytical equipment, as
specified by the analyzer manufacturer.
11 Identify Sample Point Locations
11.1 Identify the critical sample point locations in the
UHP gas distribution system which allow for a
continuous gauging of overall system integrity. These
shall include, at a minimum:
11.1.1 An exit purity O
2
measurement of the UHP gas
at the furthest point of the gas distribution system
within the fab.
11.1.2 Sample point locations representing sub-
sections of the UHP gas distribution system, which are
deemed critical for the avoidance of atmospheric
impurities.
11.1.3 Point-of-use process tool locations which are
deemed critical for the avoidance of atmospheric
impurities.
11.2 A source purity O
2
measurement of the UHP gas
entering the gas distribution system is suggested to be
used as a reference comparison against other points
within the fab.
11.3 Additional investigative (spot check) O
2
measurements may be required at sample point
locations which aid in deducing the location of
suspected atmospheric leak sources. This will vary from
situation to situation. (See Section 16.)
11.4 The exact sample tap location must be
representative of the measurement point of interest.
Avoid measurement of dead-leg locations.
12 Preparation of Sample Point Locations
12.1 Thoroughly pre-purge the sample point prior to
connecting the O
2
analyzer.
12.2 Thoroughly cycle-purge any regulators or other
components in the sample system. Cycle the pressure at
least 20 times with a high/low pressure ratio of at least
5 times.
12.3 Connect the O
2
analyzer to the sample point using
appropriate UHP gas lines and fittings. Avoid using
long lengths of bellows-type tubing because of long
purge-down requirements. Avoid using Teflon or
plastic tubing of any kind. Teflon and most other
plastics are extremely permeable to O
2
and will cause
atmospheric O
2
leakage. Only a few feet of plastic
tubing can introduce ppm levels of O
2
.
12.4 Establish flow from the sample point to the O
2
analyzer per the manufacturer’s recommendations.
13 Calibration and Qualification
13.1 Follow the manufacturer’s recommendations for
initial start-up and calibration of the O
2
analyzer.
13.2 After initial start-up and calibration, adhere to the
manufacturer’s recommendations for routine calibration
and maintenance needed to achieve the required
performance specification as described in Section 9.2.