Dual mode leak detector

Details Dual mode leak detector Dual mode leak detector

1999-03-31

2001-09-11

Larkin, Daniel S. (Department: 2856)

Measuring and testing

With fluid pressure

Leakage

Reexamination Certificate

active

06286362

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for the detection of leaks and qualification of vacuum systems. More specifically, an apparatus and method is provided for detecting gaseous components of air, as well as a trace gas.
2. Background of the Related Art
Vacuum system qualification against leaks is necessary to ensure proper operation of the system. Various methods and apparatus are currently used for leak detection. Typically, vacuum systems and individual components are qualified either by trace gas leak detection or by Rate of Rise (ROR) and/or base pressure (P
B
) testing or a combination of these methods.
A trace gas leak detector utilizes a Mass Spectrometer Leak Detector (MSLD) capable of detecting a gas, such as helium, in a test object, such as a vacuum system. The MSLD comprises a spectrometer tube to measure the partial pressure of the trace gas in a vacuum system. Electrons produced by a hot filament in the spectrometer tube travel toward a positive grid. In transit, some of the electrons collide with the gas molecules, thereby creating ions. A magnetic field is employed to deflect the various ions according to their mass-to-charge ratio allowing only the desired trace gas ions to pass through the field and arrive at a collector. The trace gas ions then strike the collector and raise the potential of the collector in proportion to their arrival rate. This potential increase is measured by an electrometer amplifier and displayed on the output meter. The readout is proportional to the total pressure in the spectrometer tube. The higher the pressure in the tube, the more trace gas molecules are present, and the more trace gas ions are created.
An exemplary MSLD
10
is shown in
FIG. 1
coupled to a test object
11
, such as a vacuum chamber. In general, the MSLD
10
consists of a manifold
12
having a series of valves and a manometer
32
disposed therein, a magnetic sector spectrometer tube
14
sensitive to helium (or other trace gas), a high vacuum pump
16
, such as a turbomolecular pump, to maintain an adequately low operating pressure in the spectrometer tube
14
, a mechanical pump
18
such as a roughing pump, to evacuate the component or system to be tested, a power supply
20
, and an output unit
22
comprising an amplifier and readout instrumentation, such as a meter, to monitor an output signal from the spectrometer tube
14
. The test object
11
is coupled to an inlet end
24
of the manifold
12
and selectively communicates with either the mechanical pump
18
or the high vacuum pump
16
according to the valve sequencing during operation. A manual shut-off valve
30
is disposed between the test object
11
and the MSLD
10
to selectively isolate the two components from one another.
Initially, the roughing valve
26
is opened and the test object
11
is pumped down by the roughing pump
18
. The pressure in the test object
11
is measured by the manometer
32
. Once a base pressure is reached in the test object
11
, the test valve
28
is opened to provide fluid communication with the high vacuum pump
16
, thus allowing gas molecules from the test object
11
to flow into the spectrometer tube
14
. A trace gas, such as helium, is then sprayed around the exterior of the test object
11
. If vacuum leaks are present, the helium is drawn into the test object
11
at the location of the leaks. The spectrometer tube
14
then measures the partial pressure of the trace gas and generates a signal which is received by the amplifier and displayed on the output meter.
The foregoing apparatus and method is currently accepted in the industry as an excellent leak check tool for isolating leaks. Further, helium leak checks are capable of detecting very small leaks on the order of 10
−8
to 10
−10
sccs. However, helium leak detection is an extremely sensitive technique requiring the technician to apply a calibrated amount of helium, at a specified distance, moving at a specified rate across the unit being tested. A complex test object, such as a typical semiconductor processing cluster tool for example, comprises thousands of sealing surfaces and welds of varying types. Successful helium testing requires uniform testing methods at each location on the cluster tool. Consequently, the accuracy may vary by a factor of ten for a single operator and a factor of twenty to one hundred between different operators. In some cases, the leaks may be missed altogether if the appropriate location is not sprayed with helium.
In order to avoid the inaccuracy of helium leak detection and in an attempt to further automate leak detection, the industry has adopted various very rough methods of gross leak detection. Gross leak detection implies testing techniques adapted merely to indicate the presence of a leak in the device under test without locating the precise location of the leak. Such methods include the use of trace gas environments testing, the Rate of Rise (ROR) method and the P
B
(base pressure) method.
A trace gas environment test involves establishing an enclosure around a test object and subjecting the enclosure to a trace gas. Typically, the enclosure is provided using an inflatable bag that is disposed around the test object to seal the object from ambient conditions. A trace gas, such as helium, is then introduced into the bag to create a helium-rich environment around the test object. The test object is then pumped to a sub-atmospheric condition and a trace gas detector is used to monitor the presence of the trace gas in the test object. If the trace gas is detected in the device being tested, a leak is present.
The P
B
and ROR methods both use conventional manometers available on a vacuum chamber, such as capacitance manometers and ion gauges, to determine the existence of a leak. Both methods are total pressure tests, i.e., the methods observe the total pressure of the system rather than characterizing the component partial pressures which make up the total pressure.
The P
B
method involves pumping a chamber down to determine the lowest achievable pressure which is then checked against an acceptable pass/fail P
B
value. If the lowest achievable pressure is less than or equal to the predetermined pass/fail P
B
value, the chamber is considered qualified and sufficiently leak free. Conversely, if the lowest achievable pressure is greater than a predetermined pass/fail P
B
value, the chamber fails the test and must be reworked to eliminate any leaks.
The ROR method involves pumping a chamber down to a desired base pressure, P
B
, and then isolating the chamber from the associated pumping system. The internal chamber pressure change is then observed and checked against an acceptable pass/fail rate. If the rate of rise of the chamber is less than the acceptable rate, the chamber is considered qualified. Conversely, if the rate of rise is greater than the acceptable rate, the chamber fails the test and the chamber must be reworked to eliminate leaks.
The ROR method and the P
B
method may be used independently or in combination. Which test is most appropriate is dependent on the system under test. For baked systems, either the P
B
or the ROR methods are appropriate. For unbaked medium vacuum systems, e.g., in the millitorr regime, the ROR method is most appropriate. A baked system refers to a system which has been outgassed for a period of time to remove contaminants (e.g., water vapor and oxygen) from the internal chamber surfaces. Typically, baking a system involves pumping the system down to a pressure below the vapor pressures of the contaminants and may also involve heating the system to an elevated temperature to enhance the outgassing.
Each of the foregoing gross leak tests are somewhat limited. The trace gas environment test, for example, requires establishing an artificial environment by means of an inflatable bag. The bag is cumbersome and requires time-consuming efforts to ensure that a sufficiently leak-free enclosure has been established. Further, large quant

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