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SEMI F77-0703 © SEMI 2003 5 APPENDIX 1 EXAMPLE OF A CPT TEST PROC EDURE NOTICE : The material in this appendix is an official pa rt of SEM I F77 and was approved by full letter ball ot procedures. A1-1 Principle A1-1.1 C…

SEMI F77-0703 © SEMI 2003 4

Alternately, plater’s tape can also be used, leaving 0.25

in. uncovered at the end for electrical contact.

9.4 Finish — In general, refer to ASTM G150 for

details on the typical finish of the test specimens. Since

surface finish can effect the CPT results, surface

finished should be done as consistently as possible

within a sample set. Details of the surface finishing

process, parameters and characterization should be

provided in the test report as outlined in Section 15.

9.5 Sampling — It is recommended that a minimum of

3 specimens, and preferably 5 or more, should be used

to obtain statistical significance in the testing results.

9.6 Test Area — In general, refer to ASTM G150,

which states a minimum test area of 1 cm

. This

minimum test area is required for both flat specimens

and tubing specimens.

10 Reagents And Materials

10.1 In general, refer to Section 8 of ASTM G150

describing the purity of the reagents and water needed

for the test solution as well as the purity of the N

purging gas.

10.2 Standard Test Solution — Comparison of the CPT

results requires consistency in the test solution, so it is

recommended that a 1 M (mole/liter) sodium chloride

(NaCl) electrolyte be used as electrolyte. However,

because of the different processes gases used by the

semiconductor industry (HCl, HBr, Cl

, BCl

, HF,

), it is possible to substitute another test solution to

represent the particular anion of the process gas which

may impart different pitting characteristics (e.g., 1 M

NaBr, or 1M NaF).

11 Applied Potential

11.1 Stainless Steel — In general, refer to section 9 of

ASTM G150, which provides the standard anodic

potential commonly used for most stainless steels (an

anodic potential of 700 mV versus SCE at 25°C).

Section 9 also provides an alternative potential if there

is uncertainty as to whether this potential is sufficiently

high to obtain a potential independent CPT.

11.2 Alternative Alloys — Alloys other than 316L

stainless steel can be tested using the applied potential

given in Section 11.1.

12 Procedure

12.1 In general refer to Section 10 of ASTM G150 for

details on specimen mounting, cleaning, and placement,

test solution preparation, test procedure, and completion

of the test.

12.2 Refer to Annexes A1 and A2 in ASTM G150 for

guidelines to calibrate the specimen temperature.

12.3 Tubing specimens — Tubing testing requires

different specimen mounting and preparation than given

in Section 9 of ASTM G150. Tubing specimens should

be placed vertically in solution so that approximately

one inch is submerged.

13 Visual Examination Of Test Specimen

13.1 In general, refer to Section 11 of ASTM G150 for

a description of the visual examination of the test

specimen.

14 Data Analysis

14.1 In general, refer to Section 12 of ASTM G150 for

details on evaluating the critical pitting temperature.

15 Reporting Results

15.1 Section 13 of ASTM G150 provides a list of

mandatory information required in the test report.

15.2 Other mandatory information that should be

provided are the starting test electrolyte composition,

the diameter of the tubing for tubing specimens,

identification of the surface finish process (e.g.,

electropolish, passivation), and any other specific

parameters or characterization related to the surface

finish process. Welded and non-welded specimens

should also be clearly identified, as well the welding

procedure used to weld the specimen.

16 Precision and Bias

16.1 The precision of this test method is yet to be

determined

16.2 The bias of this test method is yet to be

determined.

17 References

17.1 Blum, Michael. M.S. Thesis. University of

London, England, December 1970.

17.2 Philip A. Schweitzer, Ed. “Corrosion and

Corrosion Protection Handbook.” Marcel Dekker, New

York, p. 11, 1983.

SEMI F77-0703 © SEMI 2003 5

APPENDIX 1

EXAMPLE OF A CPT TEST PROCEDURE

NOTICE: The material in this appendix is an official part of SEMI F77 and was approved by full letter ballot

procedures.

A1-1 Principle

A1-1.1 Critical pitting temperature (CPT) is an

electrochemical test that determines the localized

corrosion resistance of stainless steel and other alloys.

The test is performed in a solution of sodium chloride

(NaCl). The sample is polarized at 700 mV

SCE

while

the temperature is gradually increased from 0°C at a

rate of 1°C/min. Above some critical temperature,

pitting corrosion will occur signified by a rapid increase

in measured current above 100 µ A/cm

for 60 seconds.

The experiment is monitored and controlled using

special software.

A1-2 Precautions

A1-2.1 Use caution with heating mantles, as they can

become very hot.

A1-3 Equipment and Supplies

A1-3.1 Corrosion Test System (potentiostat and

software)

A1-3.2 Chiller (capable of –10°C to 100° C)

A1-3.3 Temperature controller

A1-3.4 Heating Mantle

A1-3.5 Stir Plate/Bar (Immersible)

A1-3.6 Glass cell (adapted to contain electrodes,

probes, and cooling coil)

A1-3.7 Counter electrodes (graphite rods)

A1-3.8 Reference electrode (saturated calomel) and

luggin probe

A1-3.9 Cooling coils

A1-4 Chemicals and Reagents

A1-4.1 Sodium Chloride (ACS Reagent Grade)

A1-4.2 18 MΩ deionized water

A1-5 Procedure

A1-5.1 Turn chiller on and set at -2°C.

A1-5.2 Fill Beaker with 800 ml of 1 M (mole/liter)

NaCl electrolyte (Preferably chilled in refrigerator).

Turn stir plate on and ensure adequate stirring with stir

bar.

A1-5.3 Turn on temperature controllers. Adjust set

point to –1°C. Place lid on beaker and ensure

temperature probe is in solution.

A1-5.4 Place two counter electrodes and one reference

electrode in solution. The reference electrode should be

placed in a luggin probe. The luggin probe tip should

be placed as close to the sample as possible without

interfering with the stir bar. Connect electrical wires to

counter and reference electrodes.

A1-5.5 Turn on computer and potentiostat and start

software.

A1-5.6 After temperature has dropped to 0°C, insert

specimen into test cell. One inch of the sample for tube

stubs should be submerged. Temperature probe should

be touching sample (masked section only).

A1-5.7 Connect electrical wires to specimen and start

the test.

A1-5.8 After approximately 500 seconds during the

initial delay, decrease stir bar rotation speed by one

half.

A1-5.9 After temperature has increased to 10°C,

periodically monitor temperature to ensure proper

heating rate.

A1-5.10 When test is completed, remove the specimen

and rinse with DI water.

A1-5.11 Examine specimen for pit location. If pit is

touching the masked-off or lacquered area, the test is

not valid. See Figure A-1.1 for a SEM micrograph of a

typical pit from a valid CPT test.

A1-5.12 Determine critical pitting temperature. The

critical pitting temperature is the temperature at which

the current density exceeds 100 µA/cm

for at least 60

seconds. Figure A-1.2 shows an example of a

temperature vs. current density scan. In this particular

example, the critical pitting temperature was

determined to be 18°C.

Figure A-1.1

SEM Micrograph of Pit Formed During CPT

Testing. The Pit is Approximately 100 Microns

Across.

Figure A-1.2

Current Density as a Function of Temperature. The

Critical Pitting Temperature is the Temperature at

Which the Current Density Exceeds 100

µA/cm

for

At Least 60 Seconds.

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