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SEMI F55-0600 © SEMI 2000 2 control valve. This output is so metimes called valve volta ge. 6 Summary of Method 6.1 C orros ive g as and m oist ure- co n t aining N 2 are alternately flowed through t he DUT with interven…

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SEMI F55-0600 © SEMI 20001
SEMI F55-0600
TEST METHOD FOR DETERMINING THE CORROSION RESISTANCE
OF MASS FLOW CONTROLLERS
This test method was technically approved by the Global Facilities Committee and is the direct responsibility
of the North American Facilities Committee. Current edition approved by the North American Regional
Standards Committee on April 10, 2000. Initially available on www.semi.org May 2000; to be published
June 2000.
1 Purpose
1.1 A mass flow controller (MFC) is often used to
control corrosive gases under unfavorable conditions.
This test method is intended to help differentiate
between MFC designs on the basis of relative resistance
to corrosion-induced failure.
2 Scope
2.1 This test is intended to show the effect of
corrosion caused when a corrosive gas such as HCl is
contaminated by an oxidizer such as atmospheric
moisture. For the purpose of this test HCl is the
preferred test gas, however this test can also be
performed with other gasses. This test method
describes a corrosive gas exposure test for mass flow
controllers. The test is intended to accelerate the
corrosion while simulating conditions that may be
found within process equipment and gas systems in the
semiconductor industry. As the relationship between
corrosion and performance may differ with MFC
design, corrosion is not measured directly. The effects
of corrosion are detected by observing changes in MFC
calibration and other operating parameters.
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 Limitations
3.1 This test method should not be expected to yield
comparable quantitative results from one test facility to
another.
3.2 Results may be compromised by the methods used
to construct the apparatus.
3.3 This test is designed to be a destructive test. The
MFC tested will be destroyed. This test is intended
only for MFCs manufactured for use in HCl or a similar
highly corrosive gaseous environment. MFCs
manufactured for non-corrosive service may develop
leaks or other catastrophic failures if tested by this
method. For practical reasons in constructing and
operating the test bed, test samples may be limited to
100 sccm N
2
equivalent full scale (FS) flow.
3.4 This method does not measure corrosion directly
by analyzing or inspecting the gas-wetted surfaces
following exposure to a corrosive. No attempt is made
to detect particles in the exit gas stream resulting from
the corrosion process.
4 Referenced Documents
SEMI F1 — Specification for Leak Integrity of High-
Purity Gas Piping Systems and Components
NOTE 1: As listed or revised, all documents cited shall be the
latest publications of adopted standards.
5 Terminology
5.1 Acronyms and Abbreviations
5.1.1 DUT — device under test
5.1.2 FS — full scale
5.1.3 kPa — kiloPascal
5.1.4 MFC — mass flow controller
5.1.5 MFM — mass flow meter
5.1.6 ppbv — parts per billion by volume
5.1.7 ppmv — parts per million by volume
5.1.8 sccm — standard cubic centimeters per minute
using a standard temperature of 0° C and a standard
pressure of 101.32 kPa.
5.2 Definitions
5.2.1 actual flow — for the purpose of this standard,
the output value of the reference flowmeter.
5.2.2 ambient temperature — the temperature of the
medium surrounding the device. Under ordinary
laboratory benchtop conditions, ambient temperature is
the temperature of the room.
5.2.3 indicated flow — the electrical output of the
device under test (DUT).
5.2.4 valve drive — electrical output from the DUT
which is analogous to the level of power supplied to the
SEMI F55-0600 © SEMI 2000 2
control valve. This output is sometimes called valve
voltage.
6 Summary of Method
6.1 Corrosive gas and moisture-containing N
2
are
alternately flowed through the DUT with intervening
dry N
2
purges. The test MFC is monitored over time
for changes in performance resulting from corrosion.
6.2 This test is intended to be an accelerated
simulation of the effects of alternating corrosive and
moisture-exposure conditions. The test records how
long it takes for corrosion to affect calibration. MFCs
that are more resistant to corrosion failures should last
longer under the test conditions. A dry N
2
purge
separates each MFC's exposure to the corrosive gas and
to the moisture-containing gas. This purging favors
MFC designs that perform well in corrosive service
because those MFCs can be dried out quickly with
limited purging.
6.3 This method is expected to yield data that can be
used to compare the relative performance of
components tested for the purpose of qualification.
7 Interference’s
7.1 Portions of the gas system exposed to HCl may
require replacement or cleaning and refurbishment
between test sequences to avoid invalidating subsequent
tests by prematurely contaminating DUTs. Take care in
the design and maintenance of the system so that
particles shed from upstream piping components as a
byproduct of corrosion are minimized so that they do
not affect the DUT.
8 Apparatus
8.1 Containment System to contain corrosive gas
leaks that may develop in the DUT or gas system. The
portion of the system that contains the corrosive gas
should be operated in a suitable secondary containment
environment.
8.2 Power Supply, Controls, and Data Collection
Devices capable of recording data 20 times per
second. Signals should be measured with a resolution
of one mV or better. Data may be recorded in
convenient units such as volts, sccm, or percent of FS.
For the purpose of analysis and reporting, data should
be converted to percent of FS except for valve drive,
which may be reported in either percent of maximum
drive or volts.
8.3 DUT
8.4 Data Acquisition System — Recommended as a
data collection system to facilitate test sequencing and
data collection.
8.5 Plumbing — with tubing and gas-switching valves
constructed from 316L electropolished stainless steel or
other material that exhibits superior corrosion resistance
compared to 316L. The use of materials such as
plastics, which retain or are permeable to water vapor,
are not appropriate. All-metal sealed valves (no
elastomers) are readily available and must be used.
8.6 The test plumbing is intended to represent the
technology level found in the semiconductor process
gas piping in which a typical DUT will operate.
8.7 A filter has not been specified immediately
upstream of the DUT because it would harbor
contaminates and make it difficult to alternate gas
types.
8.8 Dry N
2
Source — pressure regulated source with a
moisture level no greater than 50 ppbv.
8.9 Dry HCl Source — pressure regulated source with
a moisture level no greater than 500 ppbv.
8.10 Reference Flowmeter — traceable with
calibration certificate, calibrated for N
2
, FS flow 100%
to 125% of the FS N
2
equivalent flow range of the DUT
Accuracy better than 5%, linear to 0.5% FS repeatable
to 0.15% over a three-hour period, and reproducible to
0.3% of the test flow for the duration of the test .
NOTE 2: As this test is concerned with changes in DUT
calibration and not the value of DUT calibration itself, the
accuracy of the reference flowmeter is not paramount.
8.11 Moisture Generator to introduce water vapor
in a concentration of 100 ppmv (µl/l) ± 20% in N
2
.
8.12 Moisture Analyzer to periodically verify the
output of moisture generator.
9 Reagents and Materials
9.1 HCl
9.2 N
2
10 Safety Precautions
10.1 This standard may involve hazardous materials,
operations, and equipment. This standard does not
purport to address all of the safety problems associated
with its use. It is the responsibility of the user of this
standard to establish appropriate safety and health
practices and to determine the applicability of
regulatory limitations prior to use.
NOTE 3: The DUT will be contaminated with corrosive
material at the conclusion of this test. Caution should be
exercised in handling. A contaminated DUT should be
properly decontaminated.
SEMI F55-0600 © SEMI 20003
10.2 Become familiar with the safe handling practices
associated with the corrosive gas before undertaking
this test.
11 Preparation of Apparatus
11.1 The test apparatus is the source of three gases—
dry N
2
, dry HCl, and N
2
containing a known level of
water vapor. Each gas can be valved through the DUT
individually with minimal cross contamination. The
DUT actively controls the gas flow in each case. The
gas then flows into either a scrubbed exhaust system for
safe disposal or a reclamation system. A reference
flowmeter in the dry N
2
supply line is used to detect
changes in calibration of the DUT when dry N
2
is
flowing in series through both devices.
11.2 Setup and Schematic — See Figure 1 for a
schematic of the test apparatus. This schematic may
not show in detail all the components required in an
actual system.
11.3 Supply the manufacturer's reco mmended power
to the DUT. Provide MFC setpoint and purge
command (if available). Monitor the following:
11.3.1 DUT setpoint input.
11.3.2 DUT indicated flow output.
11.3.3 DUT valve drive output (if available)
11.3.4 actual flow (output of reference flowmeter)
11.4 Verify that the proper moisture levels exist in
each of the test gases.
11.5 Provide a purge flow loop that maintains a
constant flow of dry N
2
in the line from the DUT to the
exhaust system. Flow in this loop must be sufficient to
prevent back diffusion into the DUT of moisture
generated in the exhaust system, particularly when flow
into the DUT is shut off for measuring zero.
11.6 If the moisture generator is sup plied from the
same N
2
source that supplies the dry N
2
, take
precautions to avoid the diffusion of moisture from the
moisture generator into the dry N
2
line. Such
precautions may include the use of separate pressure
regulators and other valving.
11.7 Moisture generators typically take time to
stabilize. A continuous flow should be maintained
through the moisture generator during the test to
maintain a stable moisture flow. This may require
diverting the output flow to a vent when not flowing
through the DUT.
11.8 Periodically verify the moisture concentration of
wet N
2
with a suitable moisture analyzer.
11.9 Follow manufacturer set-up procedures for all
equipment.
11.10 Conduct the test at an ambient temperature of 22
± 2°C.
11.11 The DUT moisture generator, flowmeters, and
other instrumentation should have been stabilized at
room temperature for 24 hours before beginning the
tests. It is not necessary to flow gas during this
stabilization time.
11.12 All portions of the plumbing system that could
contain HCl must be helium leak tested per SEMI F1
before introducing HCl into the system. When
changing the DUT or other components, at least those
portions of the system that were changed must be re-
tested.
NOTE 4: Small leaks in a corrosive gas system can become
much larger without warning.
12 Calibration and Standardization
12.1 Calibrate all instrumentation using current
standards that are traceable to an appropriate standards
laboratory. In the absence of appropriate standards, use
manufacturer recommendations for calibration.
13 Procedure
NOTE 5: Refer to Figure 2 for a flowchart of the following
test.
13.1 Set the pressure of each of the HCl and Dry N
2
gas sources at 140 kPa ± 5%. Set the pressure of the
N2 to the moisture generator to the appropriate value
recommended by the manufacturer. If the test must be
run at some other pressure, because of a limitation of
the DUT, report this fact in the test results.
13.2 Start the wet N
2
flow to vent or through the
moisture analyzer. Establish a stable moisture
concentration at 100 ppmv ± 20% before beginning the
test sequence. Set the wet N
2
to flow within ± 5% of
the DUT flow that will be run during the test.
13.3 Place the DUT control valve in the purge or fully
open position by supplying a purge command or a
200% setpoint. Purge with dry N
2
at a rate of one to
two times FS flow for one hour to establish initial
conditions. The purge rate is controlled by adjusting
the supply pressure and is monitored by a purge-rate
flowmeter located in the gas-supply path. Follow the
manufacturer's procedure for fully opening the DUT
control valve.
13.4 Provide a setpoint to the DUT for 80 ± 5% of FS
flow.
13.5 Determining the Test Baseline and Measurement
Repeatability.