WO2011007280A2

WO2011007280A2 - Measuring molecular contamination in vacuum environments

Info

Publication number: WO2011007280A2
Application number: PCT/IB2010/052982
Authority: WO
Inventors: Ruud Johannes Theodorus Soers; Hugo Hubertus Knobel
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-07-13
Filing date: 2010-06-30
Publication date: 2011-01-20
Also published as: TW201132978A; WO2011007280A3

Abstract

A gas analysis system (10) is provided for detecting molecular contamination in a vacuum environment (11). The system (10) comprises a pump system (12), a pre-concentration device (13) and a cooling means (16). The pump system (12) transports gas from the vacuum environment (11) through the pre-concentration device (13). The pre-concentration device (13) comprises a hollow element (14) with a gas inlet for receiving a gas sample from the vacuum environment (11) and a gas outlet for being coupled to the pump system (12), an inner wall of the hollow element (14) comprising a coating of adsorbent particles (15). The cooling means (16) are provided for cooling the pre-concentration device (13). The invention also relates to a method for using the gas analysis system (10) and a method for producing such a system (10).

Description

Measuring molecular contamination in vacuum environments

FIELD OF THE INVENTION

This invention relates to a gas analysis system for detecting molecular contamination in a vacuum environment, the system comprising a pump system for transport of gas from the vacuum environment, a pre-concentration device comprising a hollow element with a gas inlet for receiving a gas sample and a gas outlet for being coupled to a pump system, the hollow element comprising an adsorbent material.

This invention further relates to a method of analyzing gas for detecting molecular contamination in a vacuum environment and to a method for producing a pre- concentration device.

BACKGROUND OF THE INVENTION

The control of molecular contamination in advanced semiconductor processing is critical to successful manufacturing. The use of very short wavelengths (Extreme Ultraviolet, EUV; 13.5nm) increases photo-chemical decomposition and subsequent deposition of contaminants on optical parts. The results are yield loss, shortened tool life and reduced long-term device reliability. Therefore, very strict specifications on the out-gassing of organics are imposed on sub-assemblies and parts used in semiconductor manufacturing equipment.

In vacuum or low pressure circumstances, it is difficult to pump a sufficient amount of gas through a detection system. In such circumstances it is known to use a pre- concentration device. In a pre-concentration device, the molecular contamination is gathered during a relatively long time span. Then, the gathered molecules are released in a short time interval. From an analysis of the released molecules, the molecular contamination can be determined.

An example of a known pre-concentration device is shown in figure 2. The pre-concentration device is a hollow tube packed with sorbent material. The interconnecting open spaces between the particles in the packed bed can be thought of as a large number of capillaries with a much smaller effective diameter than the device they are packed into. Reduced effective diameter leads to reduced gas flow. In normal circumstances that would not be a problem. However, when working in the molecular or transition flow range (vacuum or very low pressure), a reduced effective diameter may cause bigger problems. Because the conductivity of the device and the gas flow in the molecular and transition flow range are highly dependent on the cross sectional area of the device it must flow trough, the gas flow decreases drastically. The inset shows the particles of the sorbent and the path which interconnecting open spaces between the particles in the packed bed can be thought of as a large number of capillaries with a much smaller effective diameter than the device they are packed into. When working in the molecular or transition flow range, the above described pre-concentration device thus has a serious problem with the gas flow.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a gas analysis system according to the opening paragraph, which system is capable of effectively determining molecular contamination in a vacuum or low pressure environment.

According to a first aspect of the invention, this object is achieved by providing a gas analysis system for detecting molecular contamination in a vacuum environment, the system comprising a pump system, a pre-concentration device and a cooling means. The pump system is provided for transport of gas from the vacuum environment. The pre-concentration device comprises a hollow element with a gas inlet for receiving a gas sample from the vacuum environment and a gas outlet for being coupled to the pump system, an inner wall of the hollow element comprising a coating of adsorbent particles. The cooling means are provided for cooling the pre-concentration device.

The effectiveness of the gas analysis system according to the invention comes from a combination of two important new features. To maximize the conductivity of the system and to maintain a sufficient flow from the reduced pressure/vacuum system through the pre-concentration device, the large obstruction formed by particles of an adsorbent which adsorbs organic compounds is overcome by attaching particles of this material only to the inner wall of the device. The increased conductivity of the device enables sufficient transport of gas molecules from the reduced pressure/vacuum environment through the pre- concentration device. However, just increasing the effective diameter of the hollow element by only applying adsorbent particles to the inner wall of the pre-concentration device may not result in the desired improvement of the gas analysis system. Many molecules that should be collected by the concentration device would pass the hollow element without ever adsorbing onto an adsorbent particle. Therefore, the gas analysis system according to the invention further comprises cooling means. The cooling means cause the molecules colliding with the adsorbent particles on the wall of the hollow element to lose adsorption heat and thereby increase the chance of staying adsorbed by the coating on the inner wall. The object of the invention thus is achieved because of the combination of the hollow element with adsorbent coating and the cooling means.

Preferably, the pre-concentration device comprises a coolant inlet being coupled to the cooling means and wherein the cooling means is arranged for providing a coolant to the pre-concentration device. For example, liquid nitrogen may be sent through cooling channels in the hollow element in order to cool the pre-concentration device from within. Alternatively, the pre-concentration device may be cooled from the outside of the hollow tube.

The pre-concentration device may further comprise a purge flow inlet for enabling rinsing of the pre-concentration device. Rinsing with a preferably pure gas is important for the transport of all collected contaminants to the analysis system, after desorption and/or for cleaning the pre-concentration device before re-using it for a further measurement.

Preferably, the gas analysis system further comprises gas insertion means for inserting an amount of a known gas into the vacuum environment in order to promote the transport of gas molecules by the pumping system. In a vacuum, it may be very difficult for the pumping system to pump the few contaminating molecules through the hollow element. If, before or during the pumping, a small amount of known gas is brought into the vacuum environment, the pumping will be improved and the chance of all contaminating molecules being collected increases. The 'known gas' preferably is an ultra pure gas, but may be a gas mixture with a predetermined composition.

According to a further aspect of the invention, a method is provided for analyzing gas for detecting molecular contamination in a vacuum environment, using the system according to the invention.

According to a further aspect of the invention, a method is provided for producing a pre-concentration device for use in a gas analysis system according to the invention. The method comprises the steps of (i) providing a hollow element, (ii) cooling at least a part of the hollow element, (iii) condensing a solvent on an inner wall of the cooled part of the hollow element and letting the solvent solidify, and (iv) applying adsorbent particles to the inner wall of the cooled part of the hollow element. Preferably, this method further comprises a step of stopping the cooling and evaporating excessive solvent and optionally also a step of removing adsorbent particles that are not attached to the inner wall.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. 1 schematically shows a gas analysis system according to the invention, Fig. 2 shows a cross section of a pre-concentration device as used in the prior art,

Fig. 3 shows a cross section of a pre-concentration device for use in the system according to the invention,

Fig. 4 shows another cross section of the pre-concentration device of figure 3, Fig. 5 shows a flow diagram of a method of analyzing gas according to the invention, and

Fig. 6 shows a flow diagram of a method of producing a pre-concentration device as shown in figures 3 and 4. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically shows a gas analysis system 10 according to the invention. The system 10 is used for detecting molecules which are out-gassed by a test object 18 which has been placed inside a vacuum environment 11. Throughout the description, the word vacuum will be used for (near) vacuum circumstances as well as for (very) low pressure environments. A pre-concentration device 13 is coupled to the vacuum environment 11. The pre-concentration device 13 comprises a hollow tube 14 made from a material that can withstand low as well as high temperatures. An open end of the hollow tube 14 is coupled to the vacuum environment 11 to receive the gas to be analyzed. The hollow tube 14 may, e.g., be made of glass. The inner wall of the hollow tube 14 is at least partly covered with a coating 15 of adsorbent particles. The coating 15 will be described in more detail below with reference to figures 3 and 4. A pump 12 is coupled to the other open end of the hollow tube 14 to transport the gas to be analyzed through the hollow tube 14. Pumping molecules from a vacuum or very low pressure environment 11 is very difficult. The transport may be made easier by providing a controlled leak 17 to the vacuum environment 11. When adding an amount of a known gas to the vacuum environment 11 via the controlled leak, the transport of gas species, through the device will be promoted. When the amount and/or type of leaked- in gas is known, the measurement results may be corrected for the influence of this gas on the measurement results.

Additionally, the system 11 comprises a cooling means 16 for cooling the pre- concentration device 13 or at least the coated part 15 of the hollow tube 14. Cooling may be done using, e.g., a cooling liquid or a cold gas. For example, liquid nitrogen may be used. The cooling may be performed from the outside of the hollow tube 14 and/or from the inside. The cooling of the coating 15 significantly enhances the adsorbing properties of the coating 15 and the capacity of the pre-concentration device 13 to, temporarily, store the molecules to be analysed.

After collecting the molecular contamination in the pre-concentration device 13 for some time, the coating with the adsorbed molecules is analyzed. For best performance, the pre-concentration device 13 should make use of long sampling times. For that purpose, the pre-concentration device 13 is preferably brought back to ambient pressure by inserting a known gas into the hollow tube 14. The cooling is stopped and the pre-concentration device 13 is brought back to ambient temperature. The pre-concentration device 13 may then be analyzed in-line or removed from the vacuum environment 11 and transferred to, e.g., a Thermal-Desorption-Gas-Chromatography system (TD-GC) equipped with a suited detector (for example FID (flame ionisation detector)/ AED (atomic emission detector)/FT-IR (Fourier Transform Infrared Detector)/MSD (mass spectrometric detector) or a combination of detectors, to quantify and qualify the organic contamination on (sub-)ppt level. For the in-line analysis, an analysis unit 19 may be provided for collecting and detecting the molecules that are desorbed from the adsorbent when the coating 15 is heated above a certain temperature.

Figure 2 shows a cross section of a pre-concentration device 23 as used in the prior art. The pre-concentration device 23 comprises a hollow tube 24 filled with a porous adsorbent 25. The main problem of the use of this prior art pre-concentration device 23 is that the closely packed adsorbent particles 25 cause a drastic decrease of the gas flow in the molecular and transitional phases (vacuum or (very) low pressure).

Figure 3 shows a cross section of a pre-concentration device 13 for use in the system 10 according to the invention. Here, the adsorbent particles 15 are applied to the inner wall of the hollow tube 14, only. Thereby the transport of gas molecules through the tube 14 is significantly improved. Preferably the coated section 15 of the inner wall is long enough to prevent the molecules to be adsorbed to cross the tube 14 without hitting the coated part 15 of the wall (e.g. by following path 33). The adsorbing function of the coating 15 is significantly enhanced by cooling the coating 15.

Figure 4 shows another cross section of the pre-concentration device 13 of figure 3. It also shows the hollow tube 14 and the adsorbing coating 15.

Figure 5 shows a flow diagram of a method of analyzing gas according to the invention. The method comprises a step of cooling 51 the pre-concentration device 13 in order to enhance the adsorbing characteristics of the coating 15. Cooling 51 may, e.g., be done using a gaseous or liquid cooling agent which flows through or along the walls of the pre-concentration device 13.

When the adsorbing coating 15 is sufficiently cooled, the next step is pumping

52 gas from the vacuum environment 11 through the hollow tube 14 of the pre-concentration device 13. The pumping 52 may be enhanced by providing a controlled leak for letting additional gas enter the vacuum or low pressure environment 11.

Then the step of collecting 53 the molecular contamination on the adsorbent particle starts. While the gas is pumped through the tube 14, the molecules are adsorbed by the particles 15 on the inner wall of the cooled tube 14.

When the total sampling time has passed, a step of bringing 54 the pre- concentration device 13 back to ambient pressure, using a known gas, follows. The cooling may now be stopped, because no more molecules have to be adsorbed. Molecules adsorbed earlier stay adsorbed due to the adsorption characteristics of the adsorbent particles. The pre- concentration device 13 may then be taken from the system for further analysis by another system. Alternatively, the analysis is performed by an analysis unit 19 comprised in the system 10 itself.

In a subsequent heating step 55 the pre-concentration device 13 is heated to release the molecular contamination from the adsorbent particles. Then, in the analysis step 56, gas chromatography may be used for separation and detection of the released molecular contamination.

Figure 6 shows a flow diagram of a method of producing a pre-concentration device 13 as shown in figures 3 and 4. The method starts with a step of providing 61 a hollow element 14. The hollow element is made from a material that can withstand both very low and very high temperatures. For example, glass is a suitable material which can withstand the low temperatures needed for collecting the molecules as well as the high temperatures needed for the release of those molecules from the adsorbent. The step of providing 61 the hollow element 14 is followed by a step of cooling 62 at least a part of the hollow element 14. That step 62 is then followed by a step of condensing 63 a suited solvent on an inner wall of the cooled part of the hollow element 14. The solvent will then solidify.

In coating step 64, adsorbent particles are applied to the inner wall of the cooled part of the hollow element 14. Many types of adsorbent particles may be suitable for making a pre-concentration device 13 according to the invention. The type of adsorbent particles used may depend on the type of molecule to be collected by the pre-concentration device 13. Some examples of suitable particles are:

- polymeric materials (solid), e.g. Tenax (diphenylene oxide polymer),

polymeric materials (liquid), e.g. Silicone polymers, divinylbenzene-styrene copolymers,

solid materials, e.g. (carbon), alumina (A12O3),

molecular sieves,

- ionic liquids coated on substrate particles, e.g. [l,9-di(3 -vinyl- imidazo Hum) nonane bis(trifluoromethyl) sulfonium imidate

other materials, including active coal, porous functionalized sol-gel materials.

From the above list, especially Tenax seems to be a very suitable material. Upon cooling and dissolving Tenax with a suitable solvent, adhesion of the polymer with glass can be achieved. To prepare the device using Tenax, a part of the inner wall of a suited sampling device has to be covered with a thin layer of mainly non- interconnecting Tenax beads. The layer of Tenax has to be applied in such a way that the layer can withstand large temperature differences (e.g. 50 to 600 K) while maintaining a low background when desorbed using thermal desorption. The achieve this, the part of the device where the Tenax beads need to be placed is cooled, and a suited solvent is condensed on the cooled area. After the solvent is solidified, Tenax beads are applied to the device and the cooling is stopped (step 65). After the device is brought to ambient temperature, the excess solvent is evaporated using a gas flow (eg. nitrogen). When the solvent is evaporated completely, the Tenax which is not attached to the wall of the device is removed (step 66). After proper conditioning, the device is ready to be used.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A gas analysis system (10) for detecting molecular contamination in a vacuum environment (11), the system (10) comprising:

a pump system (12) for transport of gas from the vacuum environment (11), a pre-concentration device (13) comprising a hollow element (14) with a gas inlet for receiving a gas sample from the vacuum environment (11) and a gas outlet for being coupled to the pump system (12), an inner wall of the hollow element (14) comprising a coating of adsorbent particles (15), and

a cooling means (16) for cooling the pre-concentration device (13).

2. A gas analysis system (10) as claimed in claim 1, wherein the pre- concentration (13) device comprises a coolant inlet being coupled to the cooling means (16) and wherein the cooling means (16) is arranged for providing a coolant to the pre- concentration device (13).

3. A gas analysis system (10) as claimed in claim 1, wherein the pre- concentration device (13) comprises a purge flow inlet for enabling rinsing of the pre- concentration device (13).

4. A gas analysis system (10) as claimed in claim 1, further comprising gas insertion means (17) for inserting an amount of a known gas into the vacuum environment

(11) in order to promote the transport of gas molecules by the pumping system (12).

5. A gas analysis system (10) as claimed in claim 1, wherein the coating (15) is arranged to withstand temperatures between 50 K and 600 K.

6. A gas analysis system (10) as claimed in claim 1, wherein the adsorbent particles (15) comprise a polymeric material, such as Tenax, Silicone polymers or divinylbenzene-styrene copolymers .

7. A gas analysis system (10) as claimed in claim 1, wherein the adsorbent particles (15) comprise a solid material, e.g. AI2O3.

8. A gas analysis system (10) as claimed in claim 1, wherein the adsorbent particles (15) comprise an ionic liquid, e.g. l,9-di(3 -vinyl- imidazo Hum) nonane

bis(trifluoromethyl) sulfonium imidate.

9. A method of analyzing gas for detecting molecular contamination in a vacuum environment (11), the method comprises steps of:

- cooling (51) a pre-concentration device (13), the pre-concentration device (13) comprising a hollow element (14) with a gas inlet for receiving a gas sample from the vacuum environment (11) and a gas outlet for being coupled to a pump system (12), an inner wall of the hollow element (14) comprising a coating of adsorbent particles (15),

using the pump system (12) for pumping (52) gas molecules from the vacuum environment (11) through the pre-concentration device (13),

collecting (53) the molecular contamination on the adsorbent particles (15), bringing (54) the pre-concentration (13) device to ambient pressure, heating (55) the pre-concentration device (13) to release the molecular contamination from the adsorbent particles (15), and

- using (56) gas chromatography for analysis of the released molecular contamination.

10. A method of producing a pre-concentration device for use in a gas analysis system as claimed in claim 1 , the method comprising the steps of:

- providing (61) a hollow element,

cooling (62) at least a part of the hollow element,

condensing (63) a solvent on an inner wall of the cooled part of the hollow element and letting the solvent solidify, and

applying (64) adsorbent particles to the inner wall of the cooled part of the hollow element.

11. A method of producing a pre-concentration device as claimed in claim 10, further comprising a step of stopping (65) the cooling and evaporating excessive solvent.

12. A method of producing a pre-concentration device as claimed in claim 11, further comprising a step of removing (66) adsorbent particles that are not attached to the inner wall.