GB2376744A - Air leak detection in a vacuum system - Google Patents

Abstract

A vacuum system is provided with an oxygen sensor 3 which is located in a wall 1 of a pipe, vacuum chamber or vacuum pump. The sensor 3 is provided with a heating means 20 and detects oxygen in a gas flow 2. The gas flow 2 may be nitrogen purge gas, and the presence of oxygen indicates an air leak somewhere in the vacuum system. The sensor may be mounted behind valves to protect the sensor during normal operation of the vacuum system and advantageously, the sensor may be located in the exhaust stream of the vacuum pump. The sensor may be used in vacuum systems used in semiconductor processing.

Description

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Air leak detection Background Improvements in sensors and methods for the rapid detection and elimination of air leakage into the vacuum systems that are utilised within semiconductor and optical processing facilities and similar high volume manufacturing plant are of great commercial significance to a reduction of total cost of production.

In addition to these there are some very long lengths of evacuated pipeline used in physics research establishments and ultra high purity gas producers.

In many cases the vacuum pumps and/or their vacuum gauges are operated as part of a communications network for effective supervisory control and pre- failure maintenance predictors, with the benefits of scheduled maintenance.

However once maintenance or other activity is required, there are additional risks for the vacuum systems: the mechanical changes may introduce leaks.

As these facilities usually have many vacuum pumps and vacuum valves and gauges, service engineers may well need to use them as part of risk reduction methods following commissioning, cleaning or service work on these systems.

A typical procedure may involve some or even all of the following activities: (a) Visual inspection by the maintenance/service engineers of all changed components; (b) checking that the initial system pump down rates from are reasonably consistent with normal observations; (c) normal base vacuum at very low pressure is achieved within expected variations; (d) analysis of the residual gas composition at or near the base pressure using quadruple mass spectrometers or similar device types; (e) by spraying or applying helium to vacuum components connected to helium mass spectrometer leak detectors.

Clearly, where special gases are present for the processing, the analysis of residual gases may readily show when the system handling them has failed.

However the most common failure mode will be due to the ingress of air from the atmosphere surrounding the vacuum components or through pump seals.

In some applications the vacuum system should contain a relatively pure gas such as nitrogen or argon, rather than air, as a residual gas. This is especially true within and in the exhaust ports of nitrogen purged dry vacuum pumps, but mass spectrometer analysers have significant problems with traces of oxygen.

It would be an advantage to use ambient air as a tracer not helium gas spray.

Ambient air contains a significant fraction ( > 10%) of only two gases: nitrogen and oxygen; all other gases present are typically well below ( < 1 %) except for argon (about 0.9%) and water vapour. Leak free vacuum systems with dry nitrogen purged pumps should not therefore contain residual oxygen gases, but sensitive mass spectrometer analysers cannot be used for small air leaks.

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Essential features The present invention comprises air leak detection by heated sensor means deemed appropriate to detection of low levels of oxygen in nitrogen gas flows, and most aspects ! ! y associated with dry vacuum pumps and vacuum systems.

Many such dry pumps actually run with exhaust gas temperatures that are far too high for many sensor types, but heated sensor types have an even higher characteristic running temperature enabling them to provide a suitable output.

Heated sensors having time constants suitable for applications where vacuum conditions improve, which are characteristic of system vacuum pumping after commissioning, cleaning or servicing of vacuum pumps and vacuum systems.

Low levels of oxygen arising from air leakage into the vacuum system, can get detected within dry nitrogen purge flows at typical dry pump seal flow rates, usually lower than when vacuum pumps handle process gas rates and flows.

The sensor types are mechanically robust and only standard metallic surfaces are exposed to vacuum, to nitrogen gas purge flows, or to process gas flows.

The high operating temperature of the sensor means tends to ensure that the sensor means will be self-cleaning in most process gas flows. As the exhaust port of a dry pump can be used, installation of retrofit kits becomes feasible.

Manufacturing facilities do not want additional sensors within chambers, or even the vacuum pumping inlets, if there is any risk of added particulates that might eventually reduce the production yield or the quality of product batches.

The present invention is isolated by the pump if installed at the pump exhaust, and simply gets swept by the normal flows used in the vacuum pump system.

The heated sensor has a very well known output voltage characteristic, which is a function of temperature and the concentration of the oxygen gas present.

This characteristic makes it ideal for use in distributed arrays of such sensors, as only their temperatures are required to determine leak sensitivity functions.

Important features The present device may take heater power from the vacuum pump system, to provide a more integrated system means of self-diagnostic vacuum pumping, or it may have independent power supplied by an external instrument means.

It may have a protection valve or control of a protection valve associated with the sensing means to avoid contamination of the sensor by excessive oxygen.

It may be part of a network of similar sensors suitably arranged for more rapid identification and/or isolation of the defective vacuum component by means of the vacuum system supervisory control system or other software means.

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Introduction to the drawings Figure 1 shows the essentials of the present invention. The following brief introduction is referenced to the numbering system shown in the drawing.

A leak-tight wall (1) forming part of a vacuum chamber, or a vacuum pump or connecting pipe-work as used in vacuum technology has a gas flow (2) past the sensor (3) which is introduced through the wall by a leak-tight seal (30).

The sensor (10) determines the level of oxygen within the gas flow (2) that is present within the wall (1). This is used to infer the presence of any leaks in the system. The sensor means (3) has a heating means (20) for operation.

Particular examples Where the present invention is incorporated into a dry pump or it's exhaust, for example BOC Edwards High Vacuum type QDP80 or similar, the gas flow (2) may contain both nitrogen purge gas and oxygen from water and air leaks.

To be especially useful the dry pump nitrogen purge gas flows should be below about 10 litres/minute and the sensor means (3) should be kept isolated from high levels of oxygen in the gas flow (2) whilst being heated.

This may be achieved by suitably positioning the sensor means (3) and flange (30) behind valves (not shown) within the normal vacuum system.

The sensor means (3) is a rugged Bosch type lambda sensor or similar, which operates at a high temperature due to the electrical heating means (20) for optimum performance and sensitivity. There is a particular advantage In that all the normal semiconductor process gases are likely to be cleaned off the sensor means (3), rather than prevent operation or cause corrosion, while the surface cleaning process is decoupled from the actual process by the pump.

The sensor means (3) may have an additional local processing means (10), which forms part of a network of similar or complementary (e. g. vacuum level) sensor means on the scale of a semi-conductor fabrication plant installation.

The sensor heating means (20) may be conveniently derived from the power supplies or the controllers for dry vacuum pumps. There may be additional heat control sequencing for sensor means (3) when linked to a larger system.

The sensor means (3), the output processing means (10) and the sensor power supply means (20) may all or separately form part of a special adapter flange (30) for a standard wall (1) providing a modular retrofit kit for vacuum.

The design of flange means (30) can conveniently accommodate the physical length of a standard sensor means (3) without compromising on performance.

Claims (10)

1. CLAIMS 1. A device for the detection of air leaks into vacuum systems by a heated sensor means capable of withstanding the gas temperatures within a dry vacuum pump and detecting low oxygen content of the dry purge gases used.
2. 2. A device as in claim 1 mounted behind valve flanges for the protection of the sensor means from the effects of moisture or processing gases passing through the dry vacuum pump during normal use.
3. 3. A device as in claim 1 or 2 that is located in the exhaust stream of the dry vacuum pump.
4. 4. A device as in claim 3 for which any materials generated by the surface cleaning of the heated sensor means being within the exhaust stream are isolated from the vacuum system by the dry vacuum pump itself.
5. 5. A device as in claims 1 to 3 with a sensor means that has additional local processing means compatible with the standard signal networks for vacuum gauges.
6. 6. A device as in claims 1 to 3 for which the sensor heating means is derived from the power supplies or controllers of dry vacuum pumps.
7. 7. A device as in claims 1 to 3 for which the sensor heating means is controlled by means of a larger system communications network connected to it.
8. 8. A device as in claims 1 to 7 that is in the form of a modular retrofit kit for substitution of vacuum pipe work or dry pump exhaust systems.
9. 9. A device according to any or all the claims above that is specifically designed for use within semiconductor fabrication facilities.
10. 10. A device according to any or all the claims 1 to 8 above that is specifically designed for use within optical processing facilities.