WO2006029824A2

WO2006029824A2 - Monitoring element for lithographic projection systems

Info

Publication number: WO2006029824A2
Application number: PCT/EP2005/009841
Authority: WO
Inventors: Stephan Back; Guido Soyez; Joachim Büchele
Original assignee: Carl Zeiss Smt Ag
Priority date: 2004-09-16
Filing date: 2005-09-14
Publication date: 2006-03-23
Also published as: WO2006029824A3

Abstract

A monitoring element for use in a lithographic projection system is provided, comprising a sensor element for detecting at least one chemical compound present in a fluid present in said projection system and a data collection element collecting data from said sensor element.

Description

MONITORING ELEMENT FOR LITHOGRAPHIC PROJECTION SYSTEMS

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0001] The present invention relates to a lithographic projection system comprising a radiation source, a mask support element to support a mask comprising a pattern to be trans¬ ferred to a substrate, an optical element to project said pat¬ tern onto said substrate and a substrate support element to support said substrate.

[0002] The invention further relates to a monitoring element for use in lithographic projection systems and a method of monitoring at least one chemical compound present in a fluid present in lithographic projection systems. RELATED PRIOR ART

[0003] Lithographic projection systems are widely used in the manufacture of semiconductor devices such as integrated circuits (ICs) .

[0004] They are used to project a pattern onto a sub¬ strate such as a silicon wafer which has previously been coated with a radiation sensitive substance (resist). After the pat¬ tern has been projected onto the substrate, the substrate can then be developed. This can either result in the resist being removed in the places where it has been in contact with radia¬ tion (positive resist) or in the resist being hardened by the contact with radiation (negative resist). The resulting pat¬ terned silicon wafer can then be further processed by various techniques such as doping or etching.

[0005] The pattern projected onto the substrate by the lithographic projection system forms a structure on the silicon wafer such as a circuit pattern corresponding to an individual layer of an IC. In recent years, there has been a demand for ICs with a higher degree of integration, generating the need for ICs with finer and finer structures. This, in turn, has led to a demand for lithographic projection systems that are able to project finer and more complex patterns onto the substrate, demanding a higher and higher degree of accuracy from the pro¬ jection system.

[0006] Although the loss of accuracy in present projec¬ tion systems caused by particulate material such as dust has been virtually eliminated through the use of cleanroom technol- ogy, it has been shown that chemical impurities on a molecular level can also cause a loss of accuracy within the projection system.

[0007] Two sources for such chemically caused loss of accuracy have been identified.

[0008] The first source is the presence of contaminants such as NH₃, SO_x (x = 1.5, 2, 3), H₂O, O₂ or e.g. amines, sul- fanes, phosphines, phosphates in the atmosphere surrounding, for example, the radiation source or the optical system. Al¬ though in general these components are kept under an inert gas atmosphere, it has been shown that even trace quantities of the above-mentioned contaminants can lead to a degradation of the accuracy of the projection system if they are present over a longer period of time.

[0009] NH₃ and SO₃ can, for example, in the presence of H₂O combine to (NHJ₂SO₄ or NH₄HSO₄ which are crystalline sub¬ stances that can precipitate onto optical components within the optical element or the radiation source. Even smallest quanti¬ ties of such substances can lead to local scattering of the radiation, thereby severely degrading the accuracy of the pro¬ jection system. This is made worse by the fact that in the long run these trace quantities of crystalline material can act as crystallization nuclei and will in turn lead to an increase in the crystallization of contaminants.

[0010] O₂ can under the high energy conditions of a beam of electromagnetic radiation supplied by the radiation source, such as a beam of deep ultraviolet (DUV) or very deep ultraviolet (VXJV) radiation, form highly reactive oxygen radi¬ cals, which can either react with other contaminants to form higher oxides or attack parts of the optical element such as seals or glues.

[0011] Further, it has been shown that mishandling of the inert gas system or accidents, for example while changing the inert gas supply, can lead to a sharp increase in the pres¬ ence of contaminants in the inert gas atmosphere over a short period of time. This is especially important to producers of such lithographic projection systems, since they often have to accept the loss of accuracy in the projection system as a case of warranty, when in fact it was caused by operator error.

[0012] A number of lithographic projection systems used today employ a so-called immersion liquid in order to increase their performance. An immersion liquid is a liquid that is disposed between the last optical component (LOC) of the opti¬ cal element and the substrate. This immersion liquid usually has a refractive index similar to the LOC and it can be used to significantly increase the aperture of the projection system and thereby its resolution.

[0013] The use of an immersion liquid has greatly in¬ creased the performance of lithographic projection systems but has in turn led to a new set of problems. The material of the LOC is usually a silica-based inorganic glass such as quartz but in projection systems employing radiation of a very short wavelength CaF₂ based glasses can also be used. At least some of the constituents of such glasses are soluble in water, and even though this solubility is not large enough to damage the LOC on a macroscopic scale, small amounts of material dissolved out of the LOC can increase the surface roughness of the LOC and can in turn lead to an increase in light scattering at the LOC and thereby a loss of accuracy of the lithographic projec¬ tion system.

[0014] A second problem is that some compounds con¬ tained in the substrate, such as degradation products of the resist or small amounts of solvent still present in the resist, can diffuse into the water and through the water onto the sur¬ face of the LOC. These compounds can also attack the LOC, espe¬ cially in the areas where a high energy radiation beam passes through the LOC. This in turn will again lead to an increase in the surface roughness or the formation of nano-scale opaque spots on the LOC resulting in a loss of accuracy, especially over longer stretches of time.

[0015] It has also been found that additives can be used to adjust various properties of the immersion liquids, such as its refractive index or its wetting ability. In order to guarantee continuous performance over a long period of time, the concentration of these additives must be strictly monitored and changes to their concentrations balanced as quickly as possible.

[0016] It is therefore object of the present invention to allow a monitoring of the conditions of the fluids used in a lithographic projection system. SUMMARY OF THE INVENTION

[0017] The object can be solved by providing a monitor¬ ing element in a lithographic projection system which comprises a sensor element for detecting at least one chemical compound present in a fluid present in said projection system and a data collection element collecting data concerning said chemical compound from said sensor element.

[0018] In a lithographic projection system in which at least one of the following, the radiation source, the mask support element, the optical element, and the substrate support element are provided with an inert gas atmosphere, at least one monitoring element is provided in contact with said inert gas atmosphere, wherein said monitoring element comprises a sensor element for detecting at least one chemical compound present in said gas and a data collection element collecting data from said sensor element.

[0019] In a lithographic projection system in which the substrate support element further comprises a holding element holding an immersion liquid between said optical element and said substrate, there is provided at least one monitoring ele¬ ment in contact with said immersion liquid, said monitoring element comprising a sensor element for detecting at least one chemical compound present in said liquid and a data collection element collecting data from said sensor element.

[0020] Further, the object is solved by a method of monitoring at least one chemical compound present in a fluid used in lithographic projection systems, comprising providing a monitoring element which comprises a sensor element for detect¬ ing said at least one chemical compound present in said fluid and a data collection element collecting data from said sensor element, whereby said monitoring element is in contact with said fluid to be monitored, and reading said data collected by said collection element.

[0021] In the context of this application, the expres¬ sion "radiation" refers to all types of electromagnetic radia¬ tion capable of interacting with a substrate, including infra¬ red, visible light, UV, and X-ray radiation. For the production of semiconductors it is particularly preferred to use UV-radia- tion with wavelengths shorter than 370 nm, especially DUV and VUV-radiation. The most commonly used wavelengths are 365 nm, 248 nm, 193 nm and 157 nm.

[0022] The expression "immersion liquid" refers to any chemical compound which is translucent to the radiation used and is a liquid under the conditions used. Favorably, this compound has a refractive index similar to that or higher than that of the LOC. This immersion liquid can consist of water or water containing additives, the additives being e.g. dissolved chemical compounds of which the LOC and/or the anti reflective coating of the LOC consists of. The immersion liquid can also consist of linear or cyclic aliphatic or aromatic organic com¬ pounds which can be partially or fully substituted by hetero- atoms other than hydrogen. Substituting heteroatoms can e.g. be selected from group 3, 14, 15, 16 or 17 of the periodic table, e.g. fluorine. The immersion liquid can also be any type of inorganic or organic salt above its melting point or an eutec- tic mixture of inorganic or organic salts. The immersion liquid can further be any type of polar or nonpolar inorganic fluid, especially halongenated or partially halongenated compounds of elements of groups 14 — 18 of the periodic table, e.g. SbF₅.

[0023] In the context of this application, the expres¬ sion "mask." refers to any element that can be used to endow an incoming radiation beam with a patterned cross-section, corre¬ sponding to a pattern that is to be created in a target portion of the substrate. This includes all mask types known in lithog¬ raphy, as well as programmable mirror arrays, programmable LCD arrays, or other techniques.

[0024] The monitoring element comprises a sensor ele¬ ment for detecting at least one chemical compound present in a fluid present in said projection system and a data collection element collecting data concerning said at least one chemical compound from said sensor element.

[0025] The sensor element measures, for example, the presence and concentration of a contaminant and the generated data is collected by the data collection element over a longer period of time. Alternatively, the sensor element can be used to measure the presence and concentration of an additive. This enables a user, be it an operator or a maintenance technician, to follow the development of the concentration of the at least one chemical compound in a fluid present in said projection system over a period of time. The length of this period is only limited by the capacity of the data collection element.

[0026] The sensor element can be any type of sensor be¬ ing able to detect the chemical compound(s) to be monitored known to a man of the art. The only demands made on the sensor is that it shows a reasonably quick response time and that it can detect components in sufficiently low quantities. Preferred detection limits would be lower than 10 ppb, preferably lower than 1 ppb. Sensor elements that could be used in the present invention include optical transducers, whether they are extrin¬ sic or intrinsic optical sensors or mass sensitive sensors, such as crystal resonators. The sensor element can further be furnished with elements to increase its sensitivity or life¬ span, like coatings or membranes.

[0027] The data collection element can be any type of information storage device or interface for an information storage or transfer device. Examples include flash memory, removable memory keys, hard drives, writeable optical discs, floppy discs or even printed matter. The data collection ele¬ ment can be a single unit or multiple, possibly differing units.

[0028] Monitoring the concentration of a chemical com¬ pound in such fluids enables the user to judge for example when maintenance to any optical components in any of the above ele¬ ments of the lithographic projection system is needed. It also enables the user to recognize the presence of long-term prob¬ lems, such as changes in the composition of the immersion liq¬ uid or a slow leak of inert gas.

[0029] It can also be used to identify short-term changes in the concentration of such a chemical compound (spikes) which can be caused by operator error. Therefore, these monitoring elements can help the producer of lithographic projection systems to defend himself against unwarranted war¬ ranty claims.

[0030] By measuring the presence of certain gases dis¬ solved in the immersion liquid, such as molecular oxygen or CO₂ one can further detect if the immersion liquid has come into contact with air.

[0031] These monitoring elements for chemical compounds can be complemented by other sensor or monitoring elements, such as temperature, pressure, flow or pH sensors. These other sensors can either output their data onto the data collection elements of a present monitoring element, or can be furnished with a separate data collection element. It can further be advantageous to equip the data collection elements with timing elements, such as clocks or calendars, in order to obtain time- correlated data.

[0032] In an embodiment of the invention, the sensor element comprises a gas sensor, preferably one that detects at least one chemical compound selected from group consisting of NH₃, SO_x (x = 1.5, 2, 3), H₂O, O₂, amines, sulphates, phosphines and phosphates in general and from the group consisting of NH₃, SO_x (x = 1.5, 2, 3), H₂O and O₂ in particular.

[0033] As described above, the quality of the inert gas atmosphere provided around at least some of the elements of a lithographic projection system can influence the accuracy of the projection system, especially over long periods of time. It is, therefore, necessary to closely monitor these gases using a gas sensor. [0034] The inert gas atmosphere is especially prone to operator errors and is, therefore, of particular interest to the producers of lithographic projection systems.

[0035] In another embodiment of the invention, said sensor element comprises a liquid sensor, preferably one that detects at least one chemical compound selected from the group consisting of Ca²⁺, F^", silicates, organic compounds, molecular oxygen and CO₂

[0036] As described above, a liquid sensor can serve three purposes in a lithographic projection system. It can be used to monitor the washout of small quantities of LOC material or the presence of materials potentially damaging to the LOC. It can also be used to monitor the quantity of additives being present in the immersion liquid, whereby this data can then in turn be used to adjust these concentrations, in order to guar¬ antee a constant concentration of said additives. It can also be used to monitor the level of impurities of the immersion liquid before usage. By measuring the presence of certain gases dissolved in the immersion liquid, such as molecular oxygen or CO₂ one can further detect if the immersion liquid has come into contact with air.

[0037] In an embodiment of the invention, said liquid sensor measures a luminescence signal of a liquid after said liquid has been irradiated. Alternatively said liquid sensor measures a luminescence signal of a sensor material in contact with a liquid after said sensor material has been irradiated. [0038] In another embodiment of the invention, said sensor element comprises a crystal resonator and a layer of a material capable of interacting with said at least one chemical compound to be monitored, said material being provided on said crystal resonator.

[0039] The use of a crystal resonator in order to moni¬ tor small quantities of material is known in the art and is also sometimes referred to as a quartz crystal microbalance. It works on the principle that an alternating voltage is applied to a piezoelectric crystal, in order to induce oscillation in the crystal. By using alternating voltages of certain frequen¬ cies, it is possible to create standing waves within the crys¬ tal. The resonance frequency of the crystal resonator is in¬ verse proportional to its thickness (mass), i.e. the resonance frequency decreases with the increase of the thickness of the crystal resonator. If, for example, a contaminant to be meas¬ ured is deposited on the surface of a crystal resonator, the resonator's mass increases and its resonance frequency de¬ creases . This phenomenon can be used to measure changes in weight down to nanogram level.

[0040] The material capable of interacting with said at least one chemical compound can be any material known to a man of the art which is able to specifically interact with either one single chemical compound or a specific class of chemical compounds. Possible materials include imprinted polymers, natu¬ ral or artificial antibodies, host guest-type compounds, com- plexing compounds, or other molecularly imprinted compounds. [0041] The type of interaction can again be any type of interaction known to a man of the art as long as it is selec¬ tive, including chemical, physical or biological interactions. These interactions are not limited to simple binding phenomena but can also include chemical or biological processes as long as they change the mass of the crystal resonator and thereby its resonance frequency.

[0042] Since the layer of material specifically inter¬ acts with the at least one chemical compound and the crystal resonator can be used to detect even very small quantities, the combination of the two results in a sensor element with a high degree of specifity and sensitivity.

[0043] In an embodiment of the above-named feature, said material is capable of chemically interacting with said at least one chemical compound.

[0044] Chemical interaction includes van der Waal's forces; electrostatic interactions, i.e. interactions between permanent electrical moments, such as dipole moments; induction interactions caused by the permanent electric moment in one molecule inducing a dipole moment in another one; dispersion interactions; hydrogen bridges and coordinative or covalent bonds.

[0045] Chemical interactions have the advantage that they can be designed to detect a whole species of compounds, such as for example carboxylic acids rather than individual compounds, making it possible to detect a group of molecules which might be potentially harmful with a single sensor. [0046] In another embodiment of the invention, said sensor element comprises a crystal resonator and a layer of a material capable of physically interacting with said at least chemical compound to be monitored, said material layer being provided on said crystal resonator.

[0047] Physical interaction describes processes in which no strong bond between the material capable of interact¬ ing and the compound to be monitored is formed. These are, for example, absorption processes. An illustrative example being the absorption of compounds to silica during column chromatog¬ raphy. An example of a material capable of physical interaction with at least one chemical compound would be molecularly im¬ printed material, especially a polymer, such as a polyurethane. In this case, materials, such as polymers, are formed in the presence of the compound to be detected. This compound is then removed from the polymer, leaving behind specific openings in the polymer which allow a selective reuptake of this compound to be detected. This leads to an increase in the mass of the polymer which can be detected by the crystal resonator.

[0048] The advantage of physical interaction is on the one hand the weaker bond which makes the binding of the analyte to the material capable of interacting with it reversible, leading to a longer lifetime of the sensor element.

[0049] The other advantage of such physical interac¬ tions, especially if a molecularly imprinted material is being used, is that such a sensor can be tuned to measure only a single compound making it extremely accurate. [0050] In another embodiment of the invention, said monitoring element further comprises a data transfer element transferring said data collected by said data collection ele¬ ment to a data output. This data output can be a data storage device or a data transmitter. Preferably the data storage de¬ vice is selected from the group consisting of flash memory, memory keys, hard drives, writeable optical disks, floppy disks and printed matter. Preferably the data transmitter is selected from the group consisting of RS232 interfaces, USB interfaces, infrared transmitters, bluetooth transmitters, ethernet connec¬ tions, telephone connections, mobile telephone connections and wireless network connections.

[0051] Although it is possible to use a data collection element which uses removable/replaceable media to store its data, it is sometimes preferable to directly transfer the data collected by the data collection element to another system. Such a data transfer can occur via standard interfaces, such as RS232 or USB interfaces, via infrared or blue tooth links, via ethernet or telephone connections, or even via mobile telephony or wireless networks. This enables, for example, the maker of a lithographic projection system to online monitor the health and performance of the system, thereby enabling him to send out the maintenance technician precisely when maintenance is needed, instead of relying on prescheduled maintenance.

[0052] In another embodiment of the invention, the lithographic projection system comprises a dispensing element, dispensing at least one additive into said immersion liquid. Preferably, said dispensing element is connected to said data collection element, with said data from said data collection element controlling said dispensing element.

[0053] Additives, such as wetting agents or compounds with a different refractive index can be added to an immersion liquid to change some of its characteristics, such as its re¬ fractive index or its wetting ability. Over time, the concen¬ tration of these additives changes due to evaporation, degrada¬ tion or other processes. It is, therefore, advantageous to provide the lithographic projection system with a dispensing element, dispensing additives into the immersion liquid in a controlled manner.

[0054] The dispensing element can work by predetermined parameters, such as a given time interval, but it is particu¬ larly preferred if the dispensing element is controlled in a way depending directly on the concentration of the at least one additive already present in the immersion liquid. In this way, a stable concentration of the at least one additive can be secured. This is best achieved by connecting the dispensing element with a data collection element of a monitoring element for said additive and using the data collected by said data collection element to control the dispensing element.

[0055] In an embodiment of the invention said radiation source comprises an excimer laser, especially one emitting radiation at a wavelength selected from the group consisting of 248, 193 and 157 nm. [0056] Excimer lasers are particularly useful in gener¬ ating monochromatic radiation of a short wavelength, especially at the above named wavelengths.

[0057] In an embodiment of the invention said litho¬ graphic projection system comprises a control element receiving data from said data collection element and controlling said radiation source in relation to said data. Said control element can thereby interrupt radiation being emitted from said radia¬ tion source. Preferably said interruption is achieved by either switching said radiation source off or by introducing a shutter element in a path of said radiation.

[0058] By interrupting the radiation emitted from said radiation source if a chemical compound detected by one of the sensor elements exceeds a given threshold, damage to the litho¬ graphic projection system can be avoided. The control system can thereby act by immediately interrupting the light path if the sensor signal exceeds a set threshold limit or it is possi¬ ble to use a fixed time delay or a more sophisticated software control which allows the completion of the illumination of the current wafer or a number of wafers prior to interrupting the illumination. The last option thereby is particularly preferred since it minimizes the number of rejects generated by a sudden interruption of the radiation source.

[0059] The interruption of said radiation being emitted from said radiation source can be achieved by either switching the radiation source off or by introducing a shutter element in a path of the radiation. Switching the light source off is thereby the technically simpler option. Introducing a shutter element on the other hand is often preferred since a sudden switching off of the radiation source can damage the radiation source. Introducing a shutter element hereby refers to the introduction of a shutter element from the outside as well as to the closure of an already present shutter element. This shutter element can thereby be arranged between the radiation source and the optical element or within the optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Embodiments of the invention will now be de¬ scribed by way of example only with reference to the accompany¬ ing schematic drawings in which

[0061] Fig. 1 shows a sensor element according to the invention being disposed within a duct containing an immersion liquid;

[0062] Fig. 2 shows a first embodiment of a litho¬ graphic projection system according to the present invention;

[0063] Fig. 3 shows a second embodiment of a litho¬ graphic projection system according to the present invention;

[0064] Fig. 4 shows a third embodiment of a litho¬ graphic projection system according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS [0065] In Fig. 1 a sensor element in its entirety is assigned the reference numeral 10.

[0066] This sensor element 10 comprises a crystal reso¬ nator 12. This crystal resonator 12 is furnished with two elec¬ trical contacts 14 and 16 which are used to apply an alternat¬ ing voltage to the crystal resonator. Contacts 14 and 16 are connected to control unit 17, which supplies the alternating voltage and at the same time measures the changes in the reso¬ nance frequency of the crystal resonator 12 and acts as data collection element. An example for such a system would be a quartz crystal microbalance as being available from Stanford Research Systems as QCM 200.

[0067] One surface of the crystal resonator 12 is cov¬ ered with a layer 18 of an imprinted polymer. This polymer is a polyurethane that has been polymerized in the presence of a contaminant present in the resist on a substrate. This contami¬ nant was then washed out of the polymer layer 18, leaving open¬ ings 20.

[0068] The sensor element 10 of the present invention is disposed in the wall of a duct 22 carrying an immersion liquid 24. The imprinted polymer layer 18 is hereby in direct contact with the immersion liquid 24. In order to increase sensitivity of the sensor element 10 it is also possible to provide a membrane between the immersion liquid 24 and the imprinted polymer layer 18.

[0069] Dissolved in the immersion liquid 24 are con¬ taminants 26, here simply shown as triangles. These contami- nants 26 are the same type of molecules with which the im¬ printed polymer layer 18 has been imprinted. When the immersion liquid 24 with the contaminants 26 dissolved therein comes into contact with the imprinted polymer layer 18, the contaminants 26 are taken up into the openings 20 of the imprinted polymer layer 18. This is indicated by arrow 28. By taking up the con¬ taminants 26, the mass of the imprinted polymer layer 18 in¬ creases and this can be measured by a change in the resonance frequency of the crystal resonator 12. This change in the reso¬ nance frequency is recorded by the control unit 17.

[0070] In Fig. 2 a lithographic projection system in its entirety is assigned the reference numeral 30.

[0071] The lithographic projection system 30 consists of a radiation source 32, a mask support element 34, an optical element 36, and a substrate support element 38.

[0072] The radiation source consists of a housing 40 in which there is disposed a KrF excimer laser 42 generating mono¬ chromatic UV radiation with a wave length of 248 nm. Also dis¬ posed in the housing 40 is a lens or an arrangement of lens elements 44 which is arranged in the path of a radiation beam 45 generated by the laser 42, which is represented here by a dotdashed line. Inside the housing 40 there is provided a pro¬ tective gas atmosphere such as a noble gas, nitrogen or clean air atmosphere. The radiation beam 45 leaves the housing 40 by a radiation-transparent optical part towards the mask support element 34. [0073] The mask support element 34 consists of a mask support structure 46 into which a mask 48 is inserted. This mask 48 contains a pattern that is to be transferred on the substrate by means of the radiation beam 45. The mask support structure 46 is movable in up to two dimensions, i.e. in the two planes perpendicular to the plane of the drawing, so that different parts of the mask can be used to illuminate the sub¬ strate.

[0074] After passing through the mask 48, the radiation beam 45 passes through an optically transparent part into the optical element 36.

[0075] This optical element 36 consists of a housing 50 in which various optical components, such as lens elements 52, 54, and 56 are disposed. For simplicity sake, only three lens elements 52, 54, 56 have been depicted but it is clear to the man of the art that the optical element 36 usually contains more than the three depicted lens elements and can also contain other optical components, such as filters or prisms. Lens ele¬ ment 56 thereby forms the last optical component (LOC) of the optical element 36. Radiation beam 45 leaves the optical ele¬ ment 36 through lens element 56 in direction of the substrate support element 38.

[0076] The substrate support element 38 consists of a base support block 58 on which an immersion liquid tray 60 is disposed. This immersion liquid tray 60 is filled with an im¬ mersion liquid 62 and a substrate 64 is provided within this immersion liquid 62. The substrate 64 can be a resist covered silicon wafer. [0077] The optical element 36 is disposed in such a way that at least the outer surface of lens element 56 is in con¬ tact with the immersion liquid 62, and the refractive indices of lens element 56 and immersion liquid 62 are as similar as possible, preferably equal. Thereby, the radiation beam is virtually unrefracted when it passes from lens element 56 into the immersion liquid 62 and hits the substrate 64 with a high degree of accuracy.

[0078] The immersion liquid tray 60 is further provided with a duct loop 66. This duct loop 66 carries the immersion liquid 62 from the immersion liquid tray 60 towards a pump 68 which circulates the immersion liquid 62 through the duct loop 66 back into the immersion liquid tray 60. The pump 68 can optionally contain filters, in order to filter out particulate contaminations, such as small particles of resist from the substrate 64. The duct loop 66 is further connected to an addi¬ tive reservoir 70 by a valve 72. This reservoir 70 and the valve 72 are used to supply an additive to the immersion liquid 62 in order to adjust its refractive index to a value equal that of lens element 56.

[0079] Disposed within the duct loop 66 further are sensor elements 74, 76, and 78. These sensor elements are of the same type as sensor element 10 of Fig. 1. Sensor element 74 is disposed in the flow direction of the immersion liquid 62 upstream of the pump 68, sensor 76 is disposed between pump 68 and additive reservoir 70, and sensor 78 is disposed downstream of the additive reservoir 70. [0080] Sensor 74 is a sensor for silicic acid but could also be a sensor for Ca²⁺ or F^" ions or combinations thereof. Silicic acid is a compound that is formed by the dissolution of the quartz of lens element 56. Its presence indicates that material of lens element 56 is being dissolved by the immersion liquid 62. A high accumulated level of silicic acid can indi¬ cate that maintenance to lens element 56 is necessary.

[0081] Sensor 76 is designed to measure the presence of several organic compounds which can either be released by the substrate 64 or the pump 68. Sensor 76 can also serve as a detector of organic or inorganic impurities in an organic im¬ mersion liquid.

[0082] Sensor 78 is designed to measure concentration of the additive in the immersion liquid 62.

[0083] The sensors 74, 76, 78 are connected with con¬ trol unit 80 in form of a computer which acts as data collec¬ tion element for all three sensors 74, 74, 78, thereby forming the monitoring elements 82, 84, and 86.

[0084] The control unit 80 is further connected to the valve 72 which can be opened and closed by the central unit 80 depending on the additive concentration measured by sensor 78, thereby guaranteeing a constant concentration of additives in the immersion liquid 62.

[0085] Through this setup in the lithographic projec¬ tion system 30, the time for maintenance can be judged by the amount of silic acid being dissolved from lens element 56. The amount of organic components present in the immersion liquid 62 which might be damaging to the lens element 56 can be moni¬ tored, and if necessary, countermeasures to a too high concen¬ tration can be taken. Further, through sensor 78 combined with control unit 80 and valve 72, a constant concentration of addi¬ tive in the immersion liquid 62 can be guaranteed.

[0086] In Fig. 3 a lithographic projection system in its entirety is assigned reference numeral 90.

[0087] This lithographic projection system consists of a radiation source 92, a mask support element 94, an optical element 96, and a support element 98.

[0088] The optical element 92 comprises a housing 100 within which e.g. an ArF excimer laser 102 is disposed. This excimer laser creates monochromatic UV-radiation with a wave¬ length of 193 nm. The radiation source further comprises a lens

104 which is disposed in the traveling path of radiation beam

105 created by the laser 102. The radiation beam 105 is again represented by dotdashed line.

[0089] The radiation beam 105 leaves the housing 100 of the radiation source 92 via a radiation-transparent window towards the mask support element 94. The mask support element 94 is identical to the one shown in Fig. 2 and comprises a movable mask support structure 106 and a mask 108.

[0090] The radiation beam 105 then travels towards the optical element 96. The optical element 96 comprises a housing 110 in which three lenses 112, 114, 116 are depicted. Again, for simplicity sake only, three lenses have been depicted, but it is clear to the man of the art that more lenses or optical components can be present in the optical element 96. Lens 116 forms the last optical component of the optical element 96.

[0091] The housing 110 is further furnished with an inlet duct 118 which is connected by a valve 120 to an inert gas container 122. Inert gas can be transferred from container 122 by a valve 120 through the inlet duct 118 into the housing 110 of the optical element 96, thereby creating an inert atmos¬ phere inside housing 110 of optical element 96. The inert gas in this instance is argon.

[0092] The housing 110 of optical element 96 is further furnished with an outlet duct 124 which is used to withdraw inert gas from the housing 110 of the optical element 96, in order to avoid an overpressure of inert gas within the housing 110 of the optical element 96. The outlet duct 124 is connected to a monitoring section 126 in which sensor elements 128, 130, and 132 are disposed.

[0093] The sensor elements 128, 130, 132 are gas sen¬ sors, especially designed to detect the following, sensor ele¬ ment 128 detects NH₃/NH₄ ⁺, sensor element 130 detects SO_x and NH₃/NH₄ ⁺ sensor element 132 detects H₂O. These sensor elements are of the same type as sensor 10 of Fig. 1 but are designed as gas sensors.

[0094] The detection of the presence of these contami¬ nants is highly important since together they can form crystal¬ line deposits of ammonium sulfate and related compounds which degrade the performance of the optical components of the opti¬ cal element 96.

[0095] The sensors 128, 130, and 132 are connected to a data collection element 134, in this case a simple data reader, which acts as data collection element for all three sensors by forming the monitoring elements 136, 138, and 140. The data collecting element 134 collects the data from the sensor ele¬ ments 128, 130, and 132 and passes then on to the transferring element 142 which in this case is a mobile telephony transmit¬ ter which transmits the data received from the data collection element 134 to a here not depicted remote station enabling online monitoring of the data collected by the data collection element 134.

[0096] After leaving the optical element 96 via lens 116, the radiation beam 105 is directed towards the substrate support element 98.

[0097] The substrate support element 98 consists of a base support plate 144 on which substrate 146 again a resist- covered silicon wafer is provided. In this case, no immersion liquid is used.

[0098] In Fig. 4 a lithographic projection system in its entirety is assigned the reference numeral 150.

[0099] The lithographic projection system 150 consists of a radiation source 152, a mask support element 154, an opti¬ cal element 156, and a substrate support element 158. [00100] The radiation source 152 comprises a housing 160 within which a high pressure Hg-vapor lamp 162 with e.g. an interference filter 163 is disposed creating monochromatic UV- radiation with a wavelength of 365 nm. Also disposed within the housing 160 is a lens 164 which is arranged in the traveling path of a radiation beam 165 which has been produced by the lamp 162 and is represented here by dotdashed lines.

[00101] Housing 160 of the radiation source 152 is also furnished with an inlet duct 166 and an outlet duct 168. The inlet duct 166 and the outlet duct 168 are designed to create an inert gas atmosphere in the housing 160. Inert gas, in this case nitrogen, can be moved through the inlet duct 166 into the housing 168 and out of the housing 160 by the outlet duct 168, thereby creating a protective gas atmosphere within the housing 160.

[00102] The outlet duct 168 is further furnished with a sensor element 170 which is connected to a data collection element 172 which in this case is a memory key onto which the data generated by sensor element 170 are collected. This memory key can be collected and replaced by a maintenance technician and later read out in order to access the data. Sensor element 170 and data collection element 162 form the monitoring element 174.

[00103] The sensor element 170 is an oxygen sensor. Oxy¬ gen can, especially in the presence of a high energy radiation beam, react with the material of lens 164, degrading its per¬ formance. Therefore, the oxygen levels present in the housing 160 must be carefully monitored. The sensor element 170 is of the same type as sensor element 10 of Fig. 1.

[00104] The difference between the lithographic projec¬ tion system 150 depicted here and the lithographic projection systems 30 and 90 depicted in Figures 2 and 3 is that in this case the radiation source 152 is not in line with the mask support element 154 and the optical element 156, therefore the radiation beam 165 has to be deflected by the mirrors 176 and 178 towards the mask support element 154.

[00105] Such a deflection of the radiation beam is not limited to projection systems as depicted in this drawing. Projections systems in which the radiation source is in line with e.g. the optical element as depicted in Figures 2 and 3 can also comprise mirror systems for the deflection of the radiation beam along its path.

[00106] The optical element 156 consists of a housing 182 in which a set of lens elements represented by lens ele¬ ments 184, 186, and 188 are disposed. Again, for simplicity sake only, three lens elements are depicted but more and dif¬ ferent optical components are immediately apparent to the man of the art.

[00107] The housing 182 is further furnished with an inlet duct 190 and an outlet duct 192. The inlet duct 190 is designed to move an inert gas, such as nitrogen, noble gases or mixtures thereof, into the housing 182, and the outlet duct is designed to remove the inert gas from the housing 182, thereby creating an inert gas atmosphere inside the housing 182. The outlet duct 192 is furnished with sensor element 194 which in this case is an oxygen sensor identical to the sensor element 130 of the radiation source 152.

[00108] This sensor element 194 is connected to the data collection element 196, thereby forming the monitoring element 198. The data collection element 196 is also a memory key like the one used in the data collection element 172.

[00109] The radiation beam 165 travels through the opti¬ cal element 156 and leaves it via lens element 188 which forms the last optical component of the optical element 156 and trav¬ els towards the substrate support element 158.

[00110] The substrate support element 156 consists of base support plate 200 on which an immersion liquid tray 202 is disposed. The immersion liquid tray is filled with an immersion liquid 204 to such a level that at least the outer surface of lens element 188 of the optical element 156 is immersed in the immersion liquid 204. Similar to the system depicted in Fig. 2, the immersion liquid 204 is designed to have a refractive index as close as possible to the refractive index of the material of lens element 188.

[00111] Disposed in the immersion liquid and in the path of the radiation beam 156 is a substrate 206, a resist-covered silicon wafer.

[00112] Disposed in the wall of the immersion liquid tray 202, there is a sensor element 208, being a sensor for e.g. silicic acid, Ca²⁺ or F^". This sensor element is connected to the data collection element 210, thereby forming the moni¬ toring element 212. The data collection element 210 is a memory key mirroring the data collection elements 196 and 172. The sensor element 208 is of the same type as the sensor element 10 of Fig. 1.

[00113] By measuring the amount of silicic acid, Ca²⁺ or F' in the immersion liquid 202, sensor element 208 and thereby monitoring element 212 can detect the amount of those ions or compounds that has been dissolved out of the surface of lens element 188 of optical element 156 and thereby allow a mainte¬ nance technician to judge the state of the surface of lens element 188 and whether maintenance of the optical system 156 is necessary.

Claims

1. A monitoring element for use in a lithographic pro¬ jection system, comprising a sensor element for detecting at least one chemical compound present in a fluid present in said projection system and a data collection element collecting data from said sensor element.

2. The monitoring element of claim 1, wherein said sen¬ sor element comprises a gas sensor.

3. The monitoring element of claim 2, wherein said at least one chemical compound is selected from the group consist¬ ing of NH₃, SO_x (x = 1.5, 2, 3), H₂O, O₂, amines, sulphates, phosphines and phosphates .

4. The monitoring element of claim 3, wherein said at least one chemical compound is selected from the group consist¬ ing of NH₃, SO_x (x = 1.5, 2, 3), H₂O and O₂.

5. The monitoring element of claim 1, wherein said sen¬ sor element comprises a liquid sensor.

6. The monitoring element of claim 5, wherein said at least one chemical compound is selected from the group consist¬ ing of Ca²⁺, F^", silicates, organic compounds, molecular oxygen and CO₂.

7. The monitoring element of claim 5 or 6, wherein said liquid sensor measures a luminescence signal of a liquid after said liquid has been irradiated.

8. The monitoring element of claim 5 or 6, wherein said liquid sensor measures a luminescence signal of a sensor mate¬ rial in contact with a liquid after said sensor material has been irradiated.

9. The monitoring element of anyone of claims 1 to 6, wherein said sensor element comprises a crystal resonator and a layer of a material capable of interacting with said at least one chemical compound to be monitored, said material layer being provided on said crystal resonator.

10. The monitoring element of claim 9, wherein said mate¬ rial is capable of chemically interacting with said at least one chemical compound.

11. The monitoring element of claim 9, wherein said mate¬ rial is capable of physically interacting with said at least one chemical compound.

12. The monitoring element according to anyone of claims 1 to 11 further comprising a data transfer element transferring data collected by said data collection element to a data out¬ put.

13. The monitoring element of claim 12, wherein said data output comprises a data storage device.

14. The monitoring element of claim 13, wherein said data storage device is selected from the group consisting of flash memory, memory keys, hard drives, writeable optical disks, floppy disks and printed matter.

15. The monitoring element of claim 12, wherein said data output comprises a data transmitter.

16. The monitoring element of claim 15, wherein said data transmitter is selected from the group consisting of RS232 interfaces, USB interfaces, infrared transmitters, bluetooth transmitters, ethernet connections, telephone connections, mobile telephone connections and wireless network connections.

17. A lithographic projection system comprising a radia¬ tion source, a mask support element to support a mask compris¬ ing a pattern to be transferred to a substrate, an optical element to project said pattern onto said substrate and a sub¬ strate support element to support said substrate, wherein at least one of the above is provided with an inert gas atmos¬ phere, and wherein there is provided at least one monitoring element in contact with said inert gas atmosphere, said moni¬ toring element comprising a sensor element for detecting at least one chemical compound present in said gas and a data collection element collecting data from said sensor element.

18. The lithographic projection system of claim 17, wherein said at least one chemical compound is selected from the group consisting of NH₃, SO_x (x = 1.5, 2, 3), H₂O, O₂, amines, sulphates, phosphines and phosphates.

19. The lithographic projection system of claim 18, wherein said at least chemical compound is selected from the group consisting of NH₃, SO_x (x = 1.5, 2, 3), H₂O and O₂.

20. The lithographic projection system of anyone of claims 17 to 19, wherein said monitoring element further com¬ prises a data transfer element transferring data from said data collection element to a data output.

21. The lithographic projection system of claim 20, wherein said data output comprises a data storage device.

22. The lithographic projection system of claim 21, wherein said data storage device is selected from the group consisting of flash memory, memory keys, hard drives, writeable optical disks, floppy disks and printed matter.

23. The lithographic projection system of claim 20, wherein said data output comprises a data transmitter.

24. The lithographic projection system of claim 23, wherein said data transmitter is selected from the group con¬ sisting of RS232 interfaces, USB interfaces, infrared transmit¬ ters, bluetooth transmitters, ethernet connections, telephone connections, mobile telephone connections and wireless network connections.

25. The lithographic projection system of anyone of claims 17 to 24, wherein said sensor element comprises a crys¬ tal resonator and a layer of a material capable of interacting with said at least one chemical compound to be monitored, said material layer being provided on said crystal resonator.

26. The lithographic projection system of claim 25, wherein said material is capable of chemically interacting with said at least one chemical compound.

27. The lithographic projection system of claim 25, wherein said material is capable of physically interacting with said at least one chemical compound.

28. The lithographic projection system of anyone of claims 17 to 27, wherein said radiation source comprises an excimer laser.

29. The lithographic projection system of claim 28, wherein said excimer laser emits radiation at a wavelength selected from the group consisting of 248, 193 and 157 nm.

30. The lithographic projection system of any of claims 17 to 29, wherein said projection system further comprises a control element receiving said data from said data collection element and operating said radiation source in relation to said data.

31. The lithographic projection system of claim 30, wherein said control element interrupts radiation being emitted from said radiation source.

32. The lithographic projection system of claim 31, wherein said control element interrupts said radiation being emitted from said radiation source by switching said radiation source off.

33. The lithographic projection system of claim 31, wherein said control element interrupts said radiation being emitted from said radiation source by introducing a shutter element into a path of said radiation.

34. A lithographic projection system comprising a radia¬ tion source, a mask support element to support a mask compris¬ ing a pattern to be transferred to a substrate, an optical element to project said pattern onto said substrate and a sub¬ strate support element to support said substrate, wherein said substrate support element comprises a holding element to hold an immersion liquid between said optical element and said sub¬ strate wherein there is provided at least one monitoring ele¬ ment in contact with said immersion liquid, said monitoring element comprising a sensor element for detecting at least one chemical compound present in said liquid and a data collection element collecting data from said sensor element.

35. The lithographic projection system of claim 34, wherein said at least one chemical compound is selected from the group consisting of Ca²⁺, F", silicates, organic compounds, molecular oxygen and CO₂.

36. The lithographic projection system of claim 34 or 35, wherein said monitoring element further comprises a data trans¬ fer element transferring data from said data collection element to a data output.

37. The lithographic projection system of claim 36, wherein said data output comprises a data storage device.

38. The lithographic projection system of claim 37, wherein said data storage device is selected from the group consisting of flash memory, memory keys, hard drives, writeable optical disks, floppy disks and printed matter.

39. The lithographic projection system of claim 36, wherein said data output comprises a data transmitter.

40. The lithographic projection system of claim 39, wherein said data transmitter is selected from the group con¬ sisting of RS232 interfaces, USB interfaces, infrared transmit¬ ters, bluetooth transmitters, ethernet connections, telephone connections, mobile telephone connections and wireless network connections .

41. The lithographic projection system of anyone of claims 34 to 40, wherein said projection system further com¬ prises a dispensing element dispensing at least one additive into said immersion liquid.

42. The lithographic projection system of claim 41, wherein said dispensing element is connected to said data col¬ lection element, said data from said data collection element controlling said dispensing element.

43. The lithographic projection system of anyone of claims 34 to 42, wherein said sensor element measures a lumi- nescence signal of said immersion liquid after said liquid has been irradiated.

44. The lithographic projection system of anyone of claims 34 to 42, wherein said sensor element measures a lumi¬ nescence signal of a sensor material in contact with said im¬ mersion liquid after said sensor material has been irradiated.

45. The lithographic projection system of anyone of claims 34 to 42, wherein said sensor element comprises a crys¬ tal resonator and a layer of a material capable of interacting with said at least one chemical compound to be monitored, said material layer being provided on said crystal resonator.

46. The lithographic projection system of claim 45, wherein said material is capable of chemically interacting with said at least one chemical compound.

47. The lithographic projection system of claim 45, wherein said material is capable of physically interacting with said at least one chemical compound.

48. The lithographic projection system of anyone of claims 34 to 47, wherein said radiation source comprises an excimer laser.

49. The lithographic projection system of claim 48, wherein said excimer laser emits radiation at a wavelength selected from the group consisting of 248, 193 and 157 nm.

50. The lithographic projection system of any of claims 34 to 49, wherein said projection system further comprises a control element receiving said data from said data collection unit and operating said radiation source in relation to said data.

51. The lithographic projection system of claim 50, wherein said control element interrupts radiation being emitted from said radiation source.

52. The lithographic projection system of claim 51, wherein said control element interrupts said radiation being emitted from said radiation source by switching said radiation source off.

53. The lithographic projection system of claim 51, wherein said control element interrupts said radiation being emitted from said radiation source by introducing a shutter element into a path of said radiation.

54. A method of monitoring at least one chemical compound present in a fluid present in a lithographic projection system, comprising providing a monitoring element which comprises a sensor element for detecting at least one chemical compound present in said fluid and a data collection element collecting data from said sensor element, whereby said monitoring element is in contact with said fluid to be monitored and reading said data collected from by said data collection element.

55. The method of claim 54, wherein said fluid present in said lithographic projection system is a gas.

56. The method of claim 55, wherein said at least one chemical compound is selected from the group consisting of NH₃, SO_x (x = 1.5, 2, 3), H₂O, O₂, amines, sulphates, phosphines and phosphates.

57. The method of claim 56, wherein said at least one chemical compound is selected from the group consisting of NH₃, SO_x (x = 1.5, 2, 3) , H₂O and O₂.

58. The method of claim 54, wherein said fluid present in said lithographic projection system is a liquid.

59. The method of claim 58, wherein said at least one chemical compound is selected from the group consisting of Ca²⁺, F^", silicates, organic compounds, molecular oxygen and CO₂

60. The method according to claim 58 or 59, wherein said data read from said data collection element is used to control an addition of additives to said liquid.

61. The method of anyone of claims 54 to 60, wherein said data read from said data collection element is used to control the chemical purity of said fluid prior to use in said litho¬ graphic projection system.

62. The method of anyone of claims 54 to 61, wherein said sensor element is capable of chemically interacting with said at least one chemical compound.

63. The method of anyone of claims 54 to 61, wherein said sensor element is capable of physically interacting with said at least one chemical compound.

64. The method of anyone of claims 54 to 63, wherein said data read from said data collection element is used to control a radiation source.

65. The method of claim 64, wherein controlling said ra¬ diation source comprises interrupting radiation being emitted from said radiation source.

66. The method of claim 65, wherein interrupting said ra¬ diation being emitted from said radiation source comprises switching said radiation source off.

67. The method of claim 65, wherein interrupting said ra¬ diation being emitted from said radiation source comprises introducing a shutter element into a path of said radiation.