NL2017342A - A Gas Leak Detector and a Method of Detecting a Leak of Gas - Google Patents

Abstract

A gas leak detector for detecting gas leakage from a flow path between a first flow restriction and a second flow restriction to gas flowing in the flow path, the first flow restriction upstream and spaced apart from the second flow restriction, the gas leak detector comprising: a first sensor device for sensing a first difference between a first pressure of gas upstream of the first flow restriction and a second pressure of gas inside or downstream of the first flow restriction in the flow path and to generate on the basis of the first difference a first electrical signal indicative of a first mass flow rate of gas through the first flow restriction; a second sensor device for sensing a second difference between a third pressure of gas upstream of the second flow restriction and a fourth pressure of gas inside or downstream of the second flow restriction in the flow path to generate on the basis of the second difference a second electrical signal indicative of a second mass flow rate through the second flow restriction; and a comparator for comparing a first magnitude of the first electrical signal and a second magnitude of the second electrical signal and generating an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value and/or generating a safe signal if the difference between the first magnitude and the second magnitude is lower than the threshold value.

Description

A Gas Leak Detector and a Method of Detecting a Leak of Gas

Field [0001] The present invention relates to a gas leak detector and a method of detecting a leak of gas, particularly a gas leak detector in a lithographic apparatus and a method of detecting a leak of gas in a device manufacturing method.

Background [0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging of exposure radiation through a projection system onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0003] It has been proposed to immerse the substrate in the lithographic projection apparatus in an immersion liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of a projection system and the substrate. In an embodiment, the immersion liquid is distilled water, although another liquid can be used. An embodiment of the invention will be described with reference to an immersion liquid. The point of this is to enable imaging of smaller features because exposure radiation used for transferring the pattern will have a shorter wavelength in the immersion liquid. (The effect of the immersion liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) [0004] In a lithographic apparatus it may be desirable to provide a hazardous gas to one or more components. If a hazardous gas is used in a lithographic apparatus, it is necessary to incorporate a gas leak detector. A gas leak detector may raise an alarm signal if a leak of hazardous gas is detected and/or may shut down the hazardous gas supply.

[0005] One example of a hazardous gas which might be used in a lithographic apparatus is CO2. In particular the use of CO2 in an immersion apparatus may be desirable. This is because in an immersion apparatus gas flows may be used for controlling the position of the immersion liquid. Because carbon dioxide gas typically dissolves in immersion liquid faster than other gases such as air, the use of carbon dioxide for such a purpose is preferred as any gas entrained in the liquid will dissolve faster and so result in fewer imaging errors.

[0006] In an immersion apparatus liquid confinement structure it may be desirable to vary the mass flow rate of CO2 over time to suit specific applications or processing states. Thus, it is desirable that any gas leak detector is capable of operation at different gas mass flow rates.

SUMMARY

[0007] It is desirable, for example, to provide a gas leak detector which can easily operate at different gas mass flow rates.

[0008] According to an aspect, there is provided a gas leak detector for detecting leaking of gas from a flow path between a first flow restriction to gas flowing in the flow path and a second flow restriction to gas flowing in the flow path, the first flow restriction being upstream and spaced apart from the second flow restriction, the gas leak detector comprising: a first sensor device for sensing a first difference between a first pressure of gas in the flow path upstream of the first flow restriction and a second pressure of gas in the flow path inside or downstream of the first flow restriction and to generate on the basis of the first difference a first electrical signal indicative of a first mass flow rate of gas in the flow path through the first flow restriction; a second sensor device for sensing a second difference between a third pressure of gas in the flow path upstream of the second flow restriction and a fourth pressure of gas in the flow path inside or downstream of the second flow restriction to generate on the basis of the second difference a second electrical signal indicative of a second mass flow rate in the flow path through the second flow restriction; and a comparator for comparing a first magnitude of the first electrical signal and a second magnitude of the second electrical signal and generating an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value and/or generating a safe signal if the difference between the first magnitude and the second magnitude is lower than the threshold value.

[0009] According to an aspect, there is provided a method of detecting a leak of gas in a flow path between a first flow restriction to gas flowing in the flow path and a second flow restriction to gas flowing in the flow path, the first flow restriction being upstream and spaced apart from the second flow restriction, the method comprising: sensing a first difference between a first pressure of gas in the flow path upstream of the first flow restriction and a second pressure of gas in the flow path downstream or inside of the first flow restriction and, on the basis of the first difference, generating a first electrical signal indicative of a first mass flow rate of gas in the flow path through the first flow restriction; sensing a second difference between a third pressure of gas in the flow path upstream of the second flow restriction and a fourth pressure of gas in the flow path downstream or inside of the second flow restriction, and on the basis of the second difference, generating a second electrical signal indicative of the mass flow rate of gas in the flow path through the second flow restriction; and comparing a first magnitude of the first electrical signal and a second magnitude of the second electrical signal and generating an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value and/or generating a safe signal if the difference between the first magnitude and the second magnitude is lower than the threshold value.

Brief Description of the Drawings [0010] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: [0011] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention; [0012] Figure 2 depicts a liquid supply system for use in a lithographic projection apparatus; [0013] Figure 3 depicts schematically a gas leak detector; and [0014] Figure 4 depicts schematically an embodiment of the gas leak detector of Figure 3.

Detailed Description [0015] Figure 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters; a support table, e.g. a sensor table to support one or more sensors or a substrate table WT constructed to hold a substrate (e.g. a resist-coated substrate) W, connected to a second positioner PW configured to accurately position the surface of the table, for example of a substrate W, in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising part of, one, or more dies) of the substrate W.

[0016] The illuminator IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0017] The support structure MT holds the patterning device MA. It holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” [0018] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0019] The patterning device MA may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0020] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0021] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0022] The lithographic apparatus may be of a type having two or more tables (or stage(s) or support apparatus), e.g., two or more substrate tables or a combination of one or more substrate tables and one or more sensor or measurement tables. In such “multiple stage” machines the multiple tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. The lithographic apparatus may have two or more patterning device tables (or stage(s) or support(s)) which may be used in parallel in a similar manner to substrate, sensor and measurement tables.

[0023] The lithography apparatus is of a type wherein at least a portion of the substrate W may be covered by an immersion liquid 11 having a relatively high refractive index, e.g. water such as ultra pure water (UPW), so as to fill an immersion space 10 between the projection system PS and the substrate W. An immersion liquid 11 may also be applied to other spaces in the lithography apparatus, for example, between the patterning device MA and the projection system PS. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate W, must be submerged in immersion liquid 11; rather “immersion” only means that an immersion liquid 11 is located between the projection system PS and the substrate W during exposure. The path of the patterned radiation beam from the projection system PS to the substrate W is entirely through immersion liquid 11.

[0024] Referring to Figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus may be separate entities, for example when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source SO may be an integral part of the lithographic apparatus, for example when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0025] The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator IL can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO.

The illuminator IL may be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section. Similar to the source SO, the illuminator IL may or may not be considered to form part of the lithographic apparatus. For example, the illuminator IL may be an integral part of the lithographic apparatus or may be a separate entity from the lithographic apparatus. In the latter case, the lithographic apparatus may be configured to allow the illuminator IL to be mounted thereon. Optionally, the illuminator IL is detachable and may be separately provided (for example, by the lithographic apparatus manufacturer or another supplier).

[0026] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B.

[0027] Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW.

[0028] In the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions C (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device MA, the patterning device alignment marks may be located between the dies.

[0029] The depicted apparatus could be used in at least one of the following modes: [0030] 1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0031] 2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion C in a single dynamic exposure, whereas the length of the scanning motion (and size of the exposure field) determines the height (in the scanning direction) of the target portion C.

[0032] 3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0033] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0034] A controller 500 controls the overall operations of the lithographic apparatus and in particular performs an operation process described further below. Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses. In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus. The controller 500 may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus forms a part. The controller 500 can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab. In an embodiment the controller operates the apparatus to perform an embodiment of the present invention. In an embodiment the controller 500 has a memory to store the results of a step one described herein for later use in a step two.

[0035] Arrangements for providing immersion liquid between a final optical element 20 of the projection system PS and the substrate can be classed into three general categories. These are the bath type arrangement, the so-called localized immersion systems and the allwet immersion systems. An embodiment of the present invention relates particularly to the localized immersion systems.

[0036] In an arrangement which has been proposed for a localized immersion system a liquid confinement structure 12 extends along at least a part of a boundary of an immersion space 10 between the final optical element 20 of the projection system PS and a facing surface facing the projection system PS. The facing surface is referred to as such because the substrate table WT is moved during use and is rarely stationary. Generally, the facing surface is a surface of a substrate W and/or a surface of the substrate table WT which surrounds the substrate W or both. Such an arrangement is illustrated in Figure 2. The arrangement illustrated in Figure 2 and described below may be applied to the lithography apparatus described above and illustrated in Figure 1.

[0037] Figure 2 schematically depicts the liquid confinement structure 12. The liquid confinement structure 12 extends along at least a part of a boundary of the immersion space 10 between the final optical element 20 of the projection system PS and the substrate table WT or substrate W. In an embodiment, a seal is formed between the liquid confinement structure 12 and the surface of the substrate W/substrate table WT. The seal may be a contactless seal such as a gas seal 16 (such a system with a gas seal is disclosed in European patent application publication no. EP-A-1,420,298).

[0038] The liquid confinement structure 12 is configured to supply and confine immersion liquid 11 to the immersion space 10. Immersion liquid 11 is brought into the immersion space 10 through one of liquid openings 13. The immersion liquid 11 may be removed through another of liquid openings 13. The immersion liquid 11 may be brought into the immersion space 10 through at least two liquid openings 13. Which of liquid openings 13 is used to supply immersion liquid 11 and optionally which is used to remove immersion liquid 11 may depend on the direction of motion of the substrate table WT.

[0039] Immersion liquid 11 may be contained in the immersion space 10 by the gas seal 16 which, during use, is formed between the bottom of the liquid confinement structure 12 and the facing surface (i.e. the surface of the substrate W and/or the surface of the substrate table WT). The gas in the gas seal 16 is provided under pressure via gas inlet 15 to a gap between the liquid confinement structure 12 and substrate W and/or substrate table WT. The gas is extracted via a channel associated with gas outlet 14. The overpressure on the gas inlet 15, vacuum level on the gas outlet 14 and geometry of the gap are arranged so that there is a high-velocity gas flow inwardly that confines the immersion liquid 11. The force of the gas on the immersion liquid 11 between the liquid confinement structure 12 and the substrate W and/or substrate table WT contains the immersion liquid 11 in the immersion space 10. A meniscus forms at a boundary of the immersion liquid 11. Such a system is disclosed in United States patent application publication no. US 2004-0207824. Other immersion liquid confinement structures 12 can be used with embodiments of the present invention.

[0040] It may desirable to use carbon dioxide in the gas seal 16. Carbon dioxide is provided under pressure via gas inlet 15 to the gap between the liquid confinement structure 12 and substrate W and/or substrate table WT. The reason for using carbon dioxide is that carbon dioxide has a high solubility in the immersion liquid (ultra pure water). As a result, any bubbles of carbon dioxide which get entrapped into the immersion liquid will dissolve faster than if a less soluble gas were entrained as a bubble in the immersion liquid. A bubble of carbon dioxide gas typically dissolves faster than a bubble of air. A bubble of CO2, which has a solubility of 55 times larger than that of nitrogen. A bubble of CO2 has a diffusivity of 0.86 times that of nitrogen. Therefore, bubble of CO2 will typically dissolve in a time 37 times shorter than the time for a bubble of the same size of nitrogen to dissolve. Supplying CO2 adjacent to the immersion liquid 11 means that a bubble of CO2 gas will dissolve into the immersion liquid 11 much faster than if other gases with lower diffusivity were used. Therefore, using CO2 will reduce the number of imaging defects due to bubbles in the immersion liquid 11 at a given speed of relative movement between the projection system PS and substrate W. Thus, a higher throughput (e.g. higher speed of the substrate W relative to the liquid confinement structure 12) and/or lower defectivity can be achieved.

[0041] A disadvantage of using CO2 is that CO2 is a gas which is hazardous to humans in high concentrations. Use of CO2 requires a safety system classified with a safety integrity level of 3.

[0042] The gas leak detector 100 of the present invention will be described in relation to a safety system for detecting leaking of CO2 from a flow path. In an embodiment the CO2 is supplied to a gas seal 16 of a liquid confinement structure 12. However, the gas leak detector may be used to detect a leak of gas of any type. The gas leak detector may be used to detect a leak of gas from a flow path in any type of apparatus. The invention is not limited to an immersion lithographic apparatus or even to a lithographic apparatus.

[0043] One type of gas leak detector is one which uses two pressure switches at two locations spaced apart from one another in a flow path. If the detected pressures are abnormal, a leak signal is generated. Such a system only works for a fixed mass gas flow, fixed gas supply system and fixed conditions downstream of the gas supply (e.g. fixed working parameters such as distance from substrate W/substrate table WT of a fluid confinement structure 12). If one of those fixed parameters is changed, the gas leak detector will not function properly. For example different pressure switches which switch at different pressures may be required to make the gas leak detector work properly.

[0044] The present invention is illustrated schematically in Figure 3. The gas leak detector 100 detects a leak of gas from a flow path 110. The flow path 110 has a first flow restriction 121 to gas flowing in the flow path 110. The flow path 110 has a second flow restriction 122 to gas flowing in the flow path 110. The first flow restriction 121 and the second flow restriction 122 are spaced from one another. The gas leak detector 100 detects a leak of gas from the flow path 110 between the first flow restriction 121 and the second flow restriction 122.

[0045] From transport phenomena, the pressure drop-flow relation for gas flow through a pipe is given by:

where AP is the pressure drop,/the friction factor, L the length of the pipe, D the hydraulic diameter, K the flow losses of non-pipe parts, p the density of the gas and v the superficial velocity. The superficial velocity v is the volume flow rate divided by the cross sectional area of the pipe. The friction factor ƒ depends on the flow regime. The flow regime is characterized by the Reynolds number Re:

where Re is the Reynolds number and μ is the viscosity.

[0046] For turbulent flow (Re>4000), ƒ can be estimated depending on pipe roughness and Reynolds number. In such a case, ƒ is largely independent of v. For laminar flow (Re<2000), the friction factor ƒ is 64/Re. This means that ƒ scales with 1/v because Re is proportional to v.

[0047] Except for ΣΚ and v, all variables in the equation are fluid properties. Usually the pressure losses in ΣΚ scale in the same way as pipe flow or are very small compared to fL/D. Therefore, the entire pressure drop-flow relation can be simplified to:

Ap = ki *v for laminar flow Ap = ki ·ν2 for turbulent flow.

[0048] The relationship between pressure drop over a venturi and superficial velocity of gas v flowing through the venturi also varies quadratically.

[0049] Therefore, the pressure drop over a flow restriction (for example specifically a venturi) is indicative of the superficial velocity v of gas through the restriction. The gas leak detector 100 uses this theory to detect any leak of gas between the first flow restriction 121 and second flow restriction 122.

[0050] As illustrated in Figure 3 the first flow restriction 121 and second flow restriction 122 are positioned in the flow path 110 between a gas supply 130 and the liquid confinement structure 12.

[0051] A first sensor device 141 senses a pressure drop in gas in the flow path 110 over the first flow restriction 121. That is, the first sensor device 141 senses a first difference between a first pressure of gas in the flow path 110 upstream of the first flow restriction 121 and a second pressure of gas in the flow path 110 inside or downstream of the first flow restriction 121. On the basis of the first difference the first sensor device 141 generates a first electrical signal indicative of a first mass flow rate of gas in the flow path 110 through the first flow restriction 121.

[0052] As illustrated in Figure 4 in an embodiment the first sensor device 141 comprises a first pressure sensor PI. The first pressure sensor PI measures the first pressure of gas in the flow path 110 upstream of the first flow restriction 121. A first pressure signal is output from the first pressure sensor PI indicative of the first pressure. A second pressure sensor P2 is used to measure the second pressure of gas in the flow path 110 inside or downstream of the first flow restriction 121. A second pressure signal is output from the second pressure sensor PI indicative of the second pressure.

[0053] In an embodiment a first amplifier 1411 amplifies the first pressure signal from the first pressure sensor PI to output an amplified first pressure signal. In an embodiment a second amplifier 1412 amplifies the second pressure signal from the second sensor P2 to output an amplified second pressure signal. A first subtractor 1413 subtracts the first amplified pressure signal from the second amplified pressure signal to output a first difference signal. The first difference signal is an electrical signal indicative of the first difference. The first difference signal is provided as the first electrical signal or is converted into the first electrical signal.

[0054] The second sensor device 142 operates in the same way as the first sensor device 141 to generate a second electrical signal indicative of a second mass flow rate in the flow path 110 through the second flow restriction 122. The second electrical signal is generated on the basis of a second difference. The second difference is the difference between a third pressure of gas in the flow path 110 upstream of the second flow restriction 122 and a fourth pressure of gas in the flow path 110 inside or downstream of the second flow restriction 122.

[0055] The third pressure of gas in the flow path 110 upstream of the second flow restriction 122 is measured by a third pressure sensor P3. The fourth pressure of gas in the flow path 110 inside or downstream of the second flow restriction 122 is measured by the fourth pressure sensor P4.

[0056] The third pressure sensor P3 generates a third pressure signal indicative of the third pressure. The third pressure signal is optionally amplified by a third amplifier 1421 to generate an amplified third pressure signal. The amplified third pressure signal is supplied to a second subtractor 1423. The fourth pressure sensor P4 outputs a fourth pressure signal indicative of the fourth pressure. The fourth pressure signal is amplified by a fourth amplifier 1422 to generate an amplified fourth pressure signal. The amplified fourth pressure signal is supplied to the second subtractor 1423.

[0057] The second subtractor 1423 subtracts the third amplified pressure signal from the fourth amplified pressure signal to output a second difference signal. The second difference signal is an electrical signal indicative of the second difference. The second difference signal is provided as the second electrical signal or is converted into the second electrical signal.

[0058] In an embodiment, the first pressure sensor PI, second pressure sensor P2, first amplifier 1411, second amplifier 1412 and first subtractor 1413 can be replaced by a single pressure differential sensor and an amplifier. The output of the single pressure differential sensor is provided to the amplifier where it is amplified. The output of the amplifier is the first difference signal which is provided as the first electrical signal or is converted into the first electrical signal. The third pressure sensor P3, fourth pressure sensor P4, third amplifier 1421, fourth amplifier 1422 and second subtractor 1423 can be similarly replaced by a single pressure differential sensor and an amplifier.

[0059] In an embodiment the first electrical signal has a first magnitude. The first magnitude is indicative of the first mass flow rate.

[0060] In an embodiment the second electrical signal has a second magnitude. The second magnitude is indicative of the second mass flow rate.

[0061] If there is no leak of gas between the first flow restriction 121 and the second flow restriction 122, the mass flow rate through the first flow restriction 121 will be equal to the mass flow rate through the second flow restriction 122. Therefore, a difference between the first magnitude and second magnitude will be indicative of a gas leak between the first flow restriction and the second flow restriction.

[0062] The first electrical signal and second electrical signal are provided to a comparator 150. The comparator 150 compares the first magnitude and the second magnitude. This is done using a subtractor 1510. The comparator 150 generates an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value. This is achieved by providing the output of the subtractor 1510 to a first comparing circuit 1520. The first comparing circuit 1520 compares the difference between the first magnitude and the second magnitude (the output of the subtractor 1510) to a threshold value 1530. If the first comparing circuit 1520 detects that the difference between the first magnitude and the second magnitude is greater and the threshold value indicates a leak, it generates an appropriate signal. For example if the difference between the first magnitude and the second magnitude is greater than the threshold value 1530 the first comparing circuit 1520 generates an alarm signal. The alarm signal is provided to a leak signal generator 170. The use of the threshold value 1530 allows for some small fluctuations in mass flow rate through the flow path 110 which will inevitably be present in practice. The threshold value 1530 is set at a level so that any dangerous level of gas leak between the first flow restriction 121 and second flow restriction 122 will result in the alarm signal being generated. In an alternative embodiment, the first comparing circuit 1520 determines whether the difference between the first magnitude and the second magnitude is lower than the threshold value 1530 and if so, generates a safe signal. If the leak signal generator 170 does not receive a safe signal, an alarm is generated.

[0063] A second input is provided to the leak signal generator 170 from a flow regime analyser 160. The basis of the ability to convert pressure drop over a flow restriction to a mass flow rate used in the gas leak detector 100 is that the flow of gas is in the turbulent flow regime (the Reynolds No. Re is greater than about 4000). The flow regime analyser 160 confirms whether or not the gas flow rate in the flow path 110 is in the turbulent regime. If the flow regime analyser 160 confirms that the gas flow is turbulent, the generation of an alarm signal by the comparator 150 can be trusted.

[0064] The first electrical signal and/or the second electrical signal is/are provided to the flow regime analyser 160. Preferably the first electrical signal is used. The flow regime analyser 160 comprises a second comparing circuit 1610 which compares the first magnitude (optionally alternatively or additionally the second magnitude) to a turbulent flow value 1620. If the first magnitude and/or the second magnitude is/are greater than the turbulent flow value 1620, a calculation acceptable signal is output by the second comparing circuit 1610 and so by flow regime analyser 160. The calculation acceptable signal is provided to the leak signal generator 170. An AND gate 1710 of the leak signal generator 170 receives the output of the comparator 150 and the output of the flow regime analyser 160. If the AND gate 1710 receives both an alarm signal and a calculation acceptable signal, a leak signal is generated. If the AND gate 1710 does not receive an alarm signal from the comparator 150 or a calculation acceptable signal from the flow regime analyser 160, no leak signal is generated by the leak signal generator 170. In an alternative preferred embodiment in which the first comparing circuit 1520 outputs a safe signal, the AND gate 1710 outputs a system safe signal if the AND gate 1710 receives a calculation acceptable signal and a safe signal.

Additionally or alternatively if the AND gate 1710 does not receive both a calculation acceptable signal and a safe signal, a leak signal is generated.

[0065] In the turbulent flow regime, the first difference (i.e. ΔΡ) is proportional to the square of the superficial velocity v. Therefore, in a preferred embodiment, as illustrated in Figure 4, the first sensor device 141 is adapted to take the square root of the first difference. This is implemented by a first square root calculator 1414 which receives the first difference signal directly from the first subtractor 1413. The first square root calculator 1414 generates a first root signal.

[0066] The second sensor device 142 comprises a second square root calculator 1424 which outputs a second root signal which is the square root of the second difference signal. Calculating the square roots of the first difference signal and second difference signal is advantageous. This is because the gas leak detector 100 then has a wider dynamic flow range.

[0067] Because the gas leak detector 100 is adapted for detecting leaking of a gas, even if the mass flow rate of gas through the first restriction 121 is the same as the mass flow rate of gas through the second flow restriction 122, the superficial velocity v of gas through the first restriction 121 may not be the same as the superficial velocity v of gas through the second flow restriction 122. This is because there is likely to be a pressure drop in gas between the first flow restriction 121 and the second flow restriction 122. The pressure drop may be due to friction of the gas with the sidewalls defining the flow path 110. Asa result of the pressure drop, molecules of gas at the first flow restriction 121 will be closer together than molecules of gas at the second flow restriction 122 (i.e. the pressure of gas at the second flow restriction 122 will be lower). Therefore, the volume flow rate of gas will be different through the first flow restriction 121 compared to the volume flow rate of gas through the second flow restriction 122. Therefore, it can be seen that the first difference signal depends upon the level of a first absolute pressure in the flow path 110 at the first flow restriction 121 as well as the first difference. The second difference signal depends upon the level of a second absolute pressure in the flow path 110 at the second flow restriction 122 as well as the second difference. To compensate for this in an embodiment the first sensor device 141 includes a first scaler 1415 (for example an operational amplifier). A first scaling factor 1416 is provided to the first scaler 1415 along with the first difference signal or first root signal. The first difference signal or first root signal is multiplied by the first scaling factor 1416.

The first scaler 1415 outputs the first electrical signal. Scaling compensates for a pressure difference in the first absolute pressure (for example measured by the first sensor PI or second sensor P2) compared to the second absolute pressure (for example measured by the third pressure sensor P3 or the fourth pressure sensor P4).

[0068] In an embodiment, the leak signal generator 170 only generates an alarm signal or discontinues a safe signal if the alarm signal persists for a predetermined length of time or the calculation acceptable signal or safe signal are discontinued for the predetermined length of time. In an embodiment, the gas leak detector 100 only triggers a leak signal if the AND gate 1710 receives an alarm signal from the comparator 150 and a calculation acceptable signal from the flow regime analyser 160 for more than the predetermined length of time (e.g. two seconds). In an embodiment in which the AND gate 1710 generates a system safe signal, optionally the system safe signal is only discontinued if the safe signal generated by the comparator 150 or the calculation acceptable signal output by the flow regime analyser 160 is interrupted for longer than the predetermined length of time. For this purpose a timer 1720 may be provided downstream of the AND gate 1710. In this way sudden pressure peaks can be filtered out.

[0069] In an embodiment the second sensor device 142 is provided with a second scaler 1425 and a second scaling value 1426. The second scaler 1425 is the same as the first scaler 1415. The second scaling value 1426 is different to the first scaling value 1416 because the magnitude of the second absolute pressure is different to the first absolute pressure. In an embodiment both the first sensor device 141 and second sensor device 142 include a scaler 1415, 1425. In another embodiment only one of the first sensor device 141 and second sensor device 142 have a scaler 1415, 1425.

[0070] In an embodiment, rather than measuring the first absolute pressure and/or second absolute pressure, for a given set of flow characteristics the scaling factor 1416, 1426 is/are adjusted until the magnitude of the first electrical signal and the magnitude of the second electrical signal are equal. When there is no leak it is known that the mass flow rate through the first restriction 121 will be the same as the mass flow rate of gas through the second flow restriction 122.

[0071] Thus, the gas leak detector checks if mass flow rate is the same in two points in series in the flow path 110. It does this by exploiting predictable pressure-flow relationships, thereby verifying that no leakage occurs between the two points in series.

[0072] In an embodiment the functions of the first sensor device 141 and/or second sensor device 142 and/or comparator 150 and/or flow regime analyser 160 and/or leak signal generator 170 are performed without an analogue hardware-software interface. That is, those components are implemented in hardware logic. Such a system is advantageous for safety critical features such as a gas leak detector 100 as there is no need for safety level 3 analogue hardware to software interfaces and a concise dedicated system performs all safety critical signal processing.

[0073] In an embodiment the first electrical signal and second electrical signal are analogue signals. This avoids the need to convert the outputs from the first pressure sensor PI, second pressure sensor P2, third pressure sensor P3 and fourth pressure sensor P4 into digital signals. Additionally, this makes it easier to implement the first sensor device 141 and second sensor device 142 in hardware logic.

[0074] An advantage of the system illustrated in Figure 4 is that if the flow rate of gas through the flow path 110 is deliberately varied, the gas leak detector 100 should still function. This is because the pressures measured at the first sensor PI and second sensor P2 will be effected in a similar way to the pressures measured by the third sensor P3 and fourth pressure sensor P4. If very significant changes are made to the flow of gas through the flow path 110, simple adjustment by varying the scaling factor 1416, 1412 can recalibrate the system. Compared to previous systems which used pressure switches, this is advantageous as no change in hardware is necessary to calibrate the system to the changes.

[0075] In an embodiment, the functions of one or more of the first subtractor 1413, second subtractor 1423, first square root calculator 1414, second square root calculator 1424, first scaler 1415, second scaler 1425, subtractor 1510, first comparing circuit 1520, second comparing circuit 1610, AND gate 1710 and timer 1720 may be implemented in software. Analogue to digital converters to convert the outputs of the first-fourth pressure sensors Pl-P4 from analogue to digital might be necessary in that case.

[0076] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers.

[0077] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 436, 405, 365, 248, 193, 157 or 126 nm). The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.

[0078] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the embodiments of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Further, the machine readable instruction may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and/or data storage media.

[0079] Any controllers described herein may each or in combination be operable when the one or more computer programs are read by one or more computer processors located within at least one component of the lithographic apparatus. The controllers may each or in combination have any suitable configuration for receiving, processing, and sending signals. One or more processors are configured to communicate with the at least one of the controllers. For example, each controller may include one or more processors for executing the computer programs that include machine-readable instructions for the methods described above. The controllers may include data storage media for storing such computer programs, and/or hardware to receive such media. So the controller(s) may operate according the machine readable instructions of one or more computer programs.

[0080] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A gas leak detector for detecting leaking of gas from a flow path between a first flow restriction to gas flowing in the flow path and a second flow restriction to gas flowing in the flow path, the first flow restriction being upstream and spaced apart from the second flow restriction, the gas leak detector comprising: a first sensor device for sensing a first difference between a first pressure of gas in the flow path upstream of the first flow restriction and a second pressure of gas in the flow path inside or downstream of the first flow restriction and to generate on the basis of the first difference a first electrical signal indicative of a first mass flow rate of gas in the flow path through the first flow restriction; a second sensor device for sensing a second difference between a third pressure of gas in the flow path upstream of the second flow restriction and a fourth pressure of gas in the flow path inside or downstream of the second flow restriction to generate on the basis of the second difference a second electrical signal indicative of a second mass flow rate in the flow path through the second flow restriction; and a comparator for comparing a first magnitude of the first electrical signal and a second magnitude of the second electrical signal and generating an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value and/or generating a safe signal if the difference between the first magnitude and the second magnitude is lower than the threshold value. 2. The gas leak detector of clause 1, wherein the mass flow comparator is implemented in hardware logic. 3. The gas leak detector of clause 1 or 2, wherein the first sensor device is adapted to generate the first electrical signal in hardware logic. 4. The gas leak detector of clause 1, 2 or 3, wherein the second sensor device is adapted to generate the second electrical signal in hardware logic. 5. The gas leak detector of clause 1, 2, 3 or 4, wherein the first sensor device is adapted during generation of the first electrical signal to take the square root of the first difference. 6. The gas leak detector of any of clauses 1-5, wherein the second sensor device is adapted during generation of the second electrical signal to take the square root of the second difference. 7. The gas leak detector of any of clauses 1-6, wherein the first sensor device is adapted during generation of the first electrical signal to scale the first difference to compensate for a pressure difference in a first absolute pressure of gas in the flow path at the first flow restriction compared to a second absolute pressure of gas in the flow path at the second flow restriction; and/or the second sensor device is adapted during generation of the second electrical signal to scale the second difference to compensate for the pressure difference. 8. The gas leak detector of any of clauses 1-7, further comprising: a flow regime analyser for determining whether the first magnitude and/or the second magnitude is/are greater than a turbulent flow value and outputting a calculation acceptable signal if the first magnitude and/or the second magnitude is/are greater than the turbulent flow value; and a leak signal generator for generating a leak signal when an alarm signal is generated by the mass flow comparator and a calculation acceptable signal is output by the flow regime analyser and/or for generating a system safe signal when a safe signal is generated by the mass flow comparator and a calculation acceptable signal is output by the flow regime analyser. 9. The gas leak detector of clause 8, wherein the flow regime analyser and/or leak signal generator is/are implemented in hardware logic. 10. The gas leak detector of clause 8 or 9, wherein the leak signal generator is adapted to generate the leak signal or discontinue the safe signal if the alarm signal persists for a predetermined length of time or the calculation acceptable signal or safe signal are discontinued for the predetermined length of time. 11. The gas leak detector of any of clauses 1-10, wherein the first electrical signal and second electrical signal are analogue signals. 12. A method of detecting a leak of gas in a flow path between a first flow restriction to gas flowing in the flow path and a second flow restriction to gas flowing in the flow path, the first flow restriction being upstream and spaced apart from the second flow restriction, the method comprising: sensing a first difference between a first pressure of gas in the flow path upstream of the first flow restriction and a second pressure of gas in the flow path downstream or inside of the first flow restriction and, on the basis of the first difference, generating a first electrical signal indicative of a first mass flow rate of gas in the flow path through the first flow restriction; sensing a second difference between a third pressure of gas in the flow path upstream of the second flow restriction and a fourth pressure of gas in the flow path downstream or inside of the second flow restriction, and on the basis of the second difference, generating a second electrical signal indicative of the mass flow rate of gas in the flow path through the second flow restriction; and comparing a first magnitude of the first electrical signal and a second magnitude of the second electrical signal and generating an alarm signal if a difference between the first magnitude and the second magnitude is greater than a threshold value and/or generating a safe signal if the difference between the first magnitude and the second magnitude is lower than the threshold value. 13. The method of clause 12, wherein the generating a first electrical signal and/or generating a second electrical signal and/or comparing is/are carried out in hardware logic. 14. The method of clause 12 or 13, wherein generating the first electrical signal comprises taking the square root of the first difference. 15. The method of clause 12, 13 or 14, wherein generating the second electrical signal comprises taking the square root of the second difference. 16. The method of any of clauses 12-15, wherein generating the first electrical signal comprises scaling the first difference to compensate for a pressure difference in a first absolute pressure of gas in the flow path at the first flow restriction compared to a second absolute pressure of gas in the flow path at the second flow restriction; and/or generating the second electrical signal comprises scaling the second difference to compensate for the pressure difference. 17. The method of any of clauses 12-16, further comprising: determining whether the first magnitude and/or the second magnitude is/are greater than a turbulent flow value and outputting a calculation acceptable signal if the first magnitude and/or the second magnitude is/are greater than the turbulent flow value; and generating a leak signal when an alarm signal is generated in the comparing step and a calculation acceptable signal is output in the determining step and/or for generating a system safe signal when a safe signal is generated by the mass flow comparator and a calculation acceptable signal is output by the flow regime analyser. 18. The method of clause 17, wherein the determining and generating a leak signal is/are implemented in hardware logic. 19. The method of clause 17 or 18, wherein the leak signal is generated or the safe signal discontinued if the alarm signal persists for a predetermined length of time or the calculation acceptable signal or safe signal are discontinued for the predetermined length of time. 20. The method of any of clauses 12-19, wherein the first electrical signal and the second electrical signal are analogue signals.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.