Patent ID: 12216031

In these figures, the elements that are identical bear the same reference numbers.

The following embodiments are examples. Although the description refers to one or more embodiments, that does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Simple features of different embodiments can also be combined or swapped to provide other embodiments.

“Upstream” is understood to mean an element which is placed before another with respect to the direction of circulation of the gas to be pumped. By contrast, “downstream” is understood to mean an element placed after another with respect to the direction of circulation of the gas to be pumped.

FIG.1shows an exemplary measurement station1for measuring airborne molecular contamination, intended in particular to monitor molecular contamination concentrations in the atmosphere of cleanrooms, such as the cleanrooms of semiconductor fabrication plants.

As can be seen better inFIG.2, the measurement station1comprises at least one gas analyser2, at least two controllable isolation valves V1-V64connected in parallel to the input of the at least one gas analyser2, a conditioning pump3, at least two calibrated orifices C1-C64connected in parallel to the input of the conditioning pump3and at least one distributor4configured to connect each controllable isolation valve V1-V64with, on one side, a sampling line L1-L64and, on the other side, a calibrated orifice C1-C64.

The gas analyser2makes it possible to measure the concentration of at least one gaseous species in real time, that is to say with a measurement duration less than a few seconds, even a few minutes, with low concentrations lower than ppm or ppb. The gaseous species measured is, for example, an acid, such as hydrofluoric acid HF or hydrochloric acid HCl or a solvent, such as PGM EA (propylene glycol methyl ether). According to another example, the gaseous species is ammonia NH3. The at least one gas analyser2also comprises an internal pump for taking gaseous samples. A gas analyser2can be adapted to measure a distinct gaseous species or a group of distinct gaseous species. There are seven gas analysers in the illustrative example ofFIG.1.

The end of each sampling line L1-L64is intended to emerge in a test zone at ambient pressure, that is to say atmospheric pressure. The sampling lines L1-L4link the measurement station1to distinct test zones, for example in a place distinct from a cleanroom. There are at least two sampling lines L1-L64emerging in two distinct places. The measurement station1can comprise a number of sampling lines L1-L64greater than or equal to 16, such as greater than or equal to 128. There are 64 sampling lines L1-L64in the illustrative example of the measurement station1ofFIG.1. The length of the sampling lines L1-L4can vary between the different test zones to be joined and can be a few metres or several tens of metres, such as more than 200 metres.

As can be seen inFIG.2, the measurement station1also comprises a control unit6linked to the controllable isolation valves V1-V64and configured to command the opening or the closing of the controllable isolation valves V1-V64to be able to connect the at least one gas analyser2with at least one sampling line L1-L64. The controllable isolation valves V1-V64are, for example, solenoid valves or pneumatic valves. They are controllable in on or off mode (open or closed) by the control unit6.

The sampling lines L1-L64and the controllable isolation valves V1-V64can have internal surfaces in contact with the gases, produced in materials limiting the adhesion of the gaseous species, such as one or more fluoropolymer materials, such as perfluoroalkoxy (also called PFA) or polytetrafluoroethylene (also called PTFE).

The at least one gas analyser2, the conditioning pump3and the at least one distributor4are, for example, mounted in a rack10of the measurement station1, that can be connected to an electrical enclosure (not represented) supporting the control unit6for example. The rack10can also receive one or more gas cylinders11for the calibration of the at least one gas analyser2.

According to an exemplary embodiment that can be seen better inFIG.3, the distributor4comprises a collector5having a common section7, at least two main branches8connected to the common section7, here eight of them, at least two secondary branches9connected to the main branches8, here eight of them, the secondary branches9having a respective calibrated orifice C1-C64. The distributor4is, for example, of rigid structure, such as of stainless steel, each branch8,9and the section7being formed by straight pipes, the cross section of the section7being greater than that of the main branches8, which themselves have a cross section greater than that of the secondary branches9. There is no need to provide special coatings limiting the adhesion of the gaseous species in the collector4because of the continual pumping by the conditioning pump3.

The calibrated orifices C1-C64are, for example, fixed to the secondary branches9by screwing or welding. The calibrated orifices C1-C64are connected on the upstream side, in parallel to the sampling lines L1-L64and to the valves V1-V64, by pneumatic “T” connectors12.

The measurement station1can also comprise a pressure sensor13interposed between the conditioning pump3and the at least two calibrated orifices C1-C64, configured to measure the pressure downstream of the calibrated orifices C1-C64. This pressure sensor13is, for example, arranged on the common section7of the collector5(FIG.2). The pressure sensor13makes it possible to check the correct operation of the measurement station1, and notably that the downstream pressure is sufficiently low to allow a flow at critical regime through the calibrated orifices C1-C64.

In fact, the measurement station1can be configured for the flow of the gases to be at critical or “sonic” flow (“choked flow” in the literature) in the narrowest section of the calibrated orifice C1-C64. The critical regime is reached when the pressure downstream of the calibrated orifice C1-C64is such that the ratio of the downstream pressure to the upstream pressure is less than or equal to a critical value, the speed of the gas (at the downstream pressure) in the narrowest section of the calibrated orifice being then equal to the speed of sound. This critical value is 0.53 in air, it being assumed that most of the gas circulating in the sampling lines is air.

The throughput through the calibrated orifices C1-C64is then sonic, which makes the backscattering of gas through the calibrated orifices C1-C64virtually impossible, and therefore makes interferences between the different sampling lines L1-L64impossible. Although there is no mechanical barrier (there are no valves) between the sampling lines, L1-L64, the flow dimensions of the calibrated orifices C1-C64allowing the flow at critical regime in the calibrated orifices C1-C64thus form a “fluidic barrier” preventing cross-contamination between the sampling lines L1-L64and allowing a pumping that is evenly distributed in all the sampling lines L1-L64.

The throughput through the calibrated orifices C1-C64at critical regime is, for example, less than 1.69 Pa·m3/s (1 slm), such as less than 1.352 Pa·m3/s (0.8 slm). This flow is for example 1.1323 Pa·m3/s (0.67 slm).

The at least two calibrated orifices C1-C64have, for example, a dimension (the narrowest section) less than 6/10 mm, such as less than 4/10 mm. The calibrated orifices C1-C64have, for example, a dimension equal to 0.28 mm.

The calibrated orifices C1-C64of the measurement station1may or may not have the same cross section. They are for example pierced crystals (or other parts). The pumping capacity of the conditioning pump3is defined to be greater than the product of the throughput of the calibrated orifices C1-C64by the number of calibrated orifices (or of sampling lines L1-L64). It is also chosen as a function of the desired rate of renewal in the sampling lines L1-L64. The throughput of the conditioning pump3is for example greater than 25.35 Pa·m3/s (15 slm). The throughput of the conditioning pump3is greater than 64*0.67 slm in the illustrative example.

It is possible to monitor the pressure to ensure that it is sufficiently low at the conditioning pump3to guarantee the critical regime at the calibrated orifices C1-C64. For that, for example, the control unit6is configured to generate a warning when the pressure measured by the pressure sensor13exceeds 50 000 Pa (500 mbar), even 40 000 Pa.

These gas stream values at critical regime determined by the dimension of the narrowest section of the calibrated orifices C1-C64, from the pumping capacity of the conditioning pump3and from the number of sampling lines L1-L64to be conditioned, make it possible to obtain a minimum acceptable rate of renewal in the sampling lines and to render the differences between the sampling lines L1-L64negligible so that all the sampling lines L1-L64can evenly distribute the pumping flow and no line is less well degassed than any other even if it is much shorter.

In operation, the gas analyser2is connected with one line at a time, the control unit5commanding a sequencing of the measurements by opening just one controllable isolation valve V1-V64at a time and in turn.

The conditioning pump3pumps continually in all the sampling lines L1-L64simultaneously, through the calibrated orifices C1-C64, including the sampling line L1-L64in which a measurement is performed. All the sampling lines L1-L64can thus be subjected to a continuous pumping, which ensures, on the one hand, effective conditioning and, on the other hand, that the lines are always ready for a measurement, which makes it possible to optimize the rate of the measurement station1.

The calibrated orifices C1-C64are simple mechanical parts, they are inexpensive compared to valves. Furthermore, they do not require preliminary adjustments or settings, or any particular maintenance, which makes it possible to limit the labour costs and the installation time.

The measurement station1is therefore simple to implement and there are no risks of drifts of the settings in time. It is also possible to change a sampling line, notably its length, without needing to carry out new settings, and without that being detrimental to the measurements in the other sampling lines L1-L64, given the impossibility of backscattering in the calibrated orifices C1-C64at critical regime.