SEMI E16-90 (Reapproved 1104)

SEMI E16-90 (Reapproved 1104)

GUIDELINE GUIDE FOR DETERMINING AND DESCRIBING MASS FLOW CONTROLLER LEAK RATES

This guideline 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 July 11, 2004. Initially available at www.semi.org September 2004; to be published November 2004. Originally published in 1990, last published June 1999.

1 Purpose

1.1 The purpose of this guideline is to establish a uniform, worldwide means for describing and measuring leak rates of mass flow controllers. The leak integrity of a gas delivery system is important to maintaining product quality and performance. This guideline is intended to prevent confusion and misunderstanding between manufacturers and users. In particular, it distinguishes between mechanical and diffusion leak rates.

2 Scope

2.1 This guideline contains definitions of terms and procedures for determining the Leak Rates of mass flow controllers as used in the semiconductor industry.

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 Terminology

3.1 Definitions

3.1.1 leak — a path or paths in a sealed system which will pass helium when a partial pressure differential exists. A partial pressure differential can exist for helium even though a total gas pressure differential may not exist. There are two major leak mechanisms, a mechanical passage or a material through which gas can diffuse or permeate. In a real system, a leak may have both mechanisms operating in parallel.

3.1.1.1 A mechanical leak may be a physical crack, pit, scratch or other imperfection in a sealing surface, or contamination or debris on the seals. A diffusion or permeation leak is caused by the movement of helium through gaskets, O-rings, polymers, or other materials through which helium can diffuse.

3.1.2 measured leak rate — the leak rate of a given system measured under specified conditions and employing a specified test gas (helium). For the purposes of comparison with rates determined by other methods of testing, measured leak rates must be converted to equivalent standard leak rates.

3.1.3 sensitivity (minimum detectable leak rate) — the smallest standard leak rate that an instrument, method or system is capable of measuring under specified conditions.

3.1.4 standard leak rate — the quantity rate at which of helium flows at 25°C and 101.3 kPa (760 Torr) flowing through a leak when the high pressure side is at 101.32 kPa and the low pressure side is below 100 Pa (approximately 1 Torr). Standard Leak Rate shall be expressed in the following units:

Pa-m3/s (He) = “Pascal cubic meters per second, helium”

or, alternatively, atm-cc/s (He) = “atmospheric cubic centimeters per second, helium”

For the purpose of this document, the Measured Leak Rate shall be corrected to standard Leak Rate by multiplying by the ratio of 101.32 kPa to the absolute value of the pressurizing helium. unless otherwise called for by the MFC specifications.

3.1.4.1 NOTE 1 The “mass spectrometer helium leak detector” is generally used for leak rate testing of high and medium level vacuum apparatus. Units of sccs, Torr-L/s, and m bar-L/s, have been used in the past but are not encouraged. Reference materials include MIL STD-202E, C-1.

NOTE 1:NOTE 2 The Pascal (1Pa = 1 N/m2) is defined as the pressure unit of the international unit system SI. Therefore, the SI units above are preferred. Atm-cc/s is acceptable, as it is widely used in the semiconductor industry.

4 Testing

4.1 General Requirements

4.1.1 Leak Detector — The leak detector shall be of the helium mass spectrometer type. It shall have sensitivity at least equal to or smaller than the specified leak rating of the mass flow controller to be tested. If the actual leak rate is to be reported, the sensitivity shall be five times smaller than the leak to be measured. If the sensitivity is not five times smaller, the actual leak rate may be reported if the sensitivity of the detector is also reported.

4.1.2 Helium must have access to all primary seals.

4.1.3 Connections between the MFC and the leak detector must be leak-tight.

4.1.4 The ambient temperature of the MFC should be 25° ± 5°C unless otherwise specified. If another test temperature is used, it must be recorded during the test.

4.2 Test Procedures — There are two basic setups which may be used to measure the leak rate from the external environment to the internal gas passages of the MFC or from the internal passages to the external environment. Results for either test method may be reported. The method used must be reported as well. A third test, the through-the-valve setup, is intended to measure the quality of the valve seat shutoff.

4.2.1 Internally-Pressurized Leak Test — The purpose of this test set-up is to simulate operation of the MFC under conditions where the internal pressure is above ambient. The recommended internal pressure is 300 kPa absolute
(30 psig) of helium (see Figure 1).

Figure 1
Internally-Pressurized Leak Test

4.2.2 Externally-Pressurized Leak Test — The purpose of this test is to simulate operation of the MFC under conditions where the internal pressure is at vacuum. The external pressure should be equal to atmospheric pressure. The internal pressure should be less than 100 kPa (see Figure 2).

Figure 2
Externally-Pressurized Leak Test

4.2.3 Control Valve Seat Leak Test — The purpose of this test is to determine the leakage through the control valve under simulated operation in the closed control mode. The MFC should be electrically energized for normal operation and placed in the closed position as specified for the operation of the MFC. The input pressure to the MFC should be 100 kPa ± 20%. The outlet should be connected directly to the helium leak detector, and pressure should be as low as possible using good leak detector practice (see Figure 3).

Figure 3
Control Valve Seat Leak Test

4.2.3.1 In the case of MFCs which are not designed for positive shutoff at the control valve, alternative methods may be employed if documented and reported.

4.3 Reporting Results — The example shown in Figure 4 is a plot of leak detector output value vs. time for a representative elastomer-sealed MFC. This curve is the sum of mechanical and permeation leak components.

NOTE 2: All times are from application of helium, starting with a leak detection system pumped down to base reading.

Interval / Rate / Example
t1 Initial System Response / Less than 10 seconds
t2 Leak Prior to Onset of Permeation / w1 / 10 seconds to 1 minute
t3 Increasing Permeation / 1 minute to 30 minutes
t4 Total Saturation / w2 / Beyond 30 minutes

Figure 4
Leak Detector Output Value vs. Time

4.3.1 The actual shape of these curves and time intervals is dependent on the design of the MFC under test, the elastomer used, if any, and the characteristics of the leak detection system. These time intervals must be determined using sound engineering judgment following qualification testing of the specific MFC model and test set-up. Once determined, it is recommended that receiving inspection consist of measuring for leak rate value w1 at the end of interval t2.

4.3.2 Following qualification testing, report typical values for t1 through t4 and w1 and w2. w1 is primarily the mechanical portion of the leak, and w2 is mechanical plus permeation. In the case where w2 is significantly greater than w1, w2 is primarily permeation. In the case of a gross mechanical leak, w1 could greatly exceed, and thereby mask, w2.

NOTE 3: This test must be performed with elastomers that are devoid of heliumTo prevent false leak rate readings that could arise from helium permeation, this test should be performed with elastomers that were not previously exposed to helium. Such elastomers have either not been previously exposed to helium or have been degassed following exposure. If exposed to helium,,Once this test has been performed, the elastomers the MFC must be purged of helium by the passage of time and/or baking.

4.3.3 In good leak testing practice, the background level should be verified before the application of helium to ensure that the elastomers are in a helium degassed state and that the leak detecting system is in proper operation.

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 or equipment mentioned herein. These standards are subject to change without notice.

By publication of this standard, Semiconductor Equipment and Materials International (SEMI) takes no position respecting the validity of any patent rights or copyrights asserted in connection with any items 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.