Patent Number: 
Section: description

In the drawings, like parts are identified by like references. FIG. 1 schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises: a radiation system LA, IL for supplying a projection beam PB of UV or EUV radiation; a first object table (mask table) MT for holding a mask MA (e.g. a reticle), and connected to first positioning means for accurately positioning the mask with respect to item PL; a second object table (substrate or wafer table) WT for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means for accurately positioning the substrate with respect to item PL; a projection system (xe2x80x9clensxe2x80x9d) PL (e.g. a mirror group) for imaging an irradiated portion of the mask MA onto an exposure area C of a substrate W held on the substrate table WT. As here depicted, the apparatus is of a reflective type (i.e. has a reflective mask). However, in general, it may also be of a transmissive type, for example. The radiation system may include a source LA (e.g. an Hg lamp, an excimer laser, a laser-produced plasma source, a discharge plasma source or an undulator or wiggler provided around the path of an electron beam in a storage ring or synchrotron) which produces a beam of UV or EUV radiation. This beam is caused to traverse either directly or after being passed through conditioning means, such as a beam expander Ex, various optical components comprised in the illumination system ILxe2x80x94e.g. beam shaping optics, adjusting means AM, an integrator IN and a condenser COxe2x80x94also included in the radiation system so that the resultant beam PB has a desired shape and intensity distribution in its cross-section. The beam PB subsequently intercepts the mask MA which is held on a mask table MT. Having been selectively reflected by the mask MA, the beam PB traverses the lens PL, which focuses the beam PB onto an exposure area C of the substrate W. With the aid of the interferometric displacement measuring means IF, the substrate table WT can be moved accurately by the second positioning means PW, e.g. so as to position different exposure areas C in the path of the beam PB using wafer alignment marks P1, P2. Similarly, the first positioning means PM can be used to accurately position the mask MA using mask alignment marks M1, M2 with respect to the path of the beam PB. In general, movement of the object tables MT, WT relative to a base plate BP will be realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 1. In the case of a waferstepper (as opposed to a step-and-scan apparatus) the mask table may be connected only to a short-stroke positioning device, to make fine adjustments in mask orientation and position, or it may simply be fixed. The depicted apparatus can be used in two different modes: In step-and-repeat (step) mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at once (i.e. a single xe2x80x9cflashxe2x80x9d) onto an exposure area C. The substrate table WT is then shifted in the X and/or Y directions so that a different exposure area C can be irradiated by the beam PB; In step-and-scan (scan) mode, essentially the same scenario applies, except that a given exposure area C is not exposed in a single xe2x80x9cflashxe2x80x9d. Instead, the mask table MT is movable in a given direction (the so-called xe2x80x9cscan directionxe2x80x9d, e.g. the Y direction) with a speed v, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table WT is moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=xc2xc or ⅕). In this manner, a relatively large exposure area C can be exposed, without having to compromise on resolution. In an embodiment of the present invention, the optical component to be cleaned is an optical component within the illumination system. However, the present invention may be used to remove contaminants from any optical component in the system, for example the mask or the optical components contained within the projection system. The present invention can be applied to one or several optical components either simultaneously or separately. FIG. 2 shows a part of the illumination system of a specific embodiment of the invention in more detail. A space 2 within the illumination system, and containing an optical component 3, is supplied with a purge gas from purge gas supply 4, which may be a pressurized container containing the purge gas in gaseous or liquid form. The purge gas comprising molecular oxygen is supplied to the space 2 via inlet 5, which may comprise a valve. Space 2 now containing oxygen-containing species is then irradiated with UV or EUV radiation, which is produced by the source LA. In this embodiment, the irradiation step is carried out at the same time as exposure, i.e. the projection beam PB is used to crack the oxygen-containing species. The oxygen-containing species within the space, when irradiated with UV or EUV radiation having a wavelength of about 250 nm or less, are cracked, forming oxygen radicals and other radicals which may be OH-radiacals or other hydrocarbon radicals depending on the nature of the oxygen-containing species. The oxygen radicals formed act as highly effective cleaning agents and remove hydrocarbons and other contaminant particles from the surface of the optical component. The purge gas may contain one or a mixture of oxygen-containing species selected from water, nitrogen oxide (NOx) and oxygen-containing hydrocarbons. Suitable oxygen-containing hydrocarbons include alcohols, alkanones and ethers. Other useful oxygen-containing hydrocarbons are those with a high O:C ratio such as C1-6 alcohols (comprising 1(C1) to 6(C6) carbon atoms) including methanol, ethanol and propanol, C1-6 alkanones such as formaldehyde, ethanal, propanal and acetone and C1-6 ethers such as methoxy methane, ethoxy ethane. Likewise, the inventors contemplate the use of the oxygen-containing species for use in the present invention including water, nitrogen oxide, methanol and ethanol. These compounds may be used alone or as a mixture of 2 or more compounds. The inventors have compared the cleaning rates of molecular oxygen and water, while exposing pre-contaminated reticles with 172 nm radiation. They found that water is able to clean the said reticles, thereby restoring their transmission, much faster than molecular oxygen. Addition of molecular oxygen and water to the purge gas does not significantly influence the cleaning rate compared to mere water addition. At the employed wavelength the dissociation rate of water is significantly higher than the rate of oxygen dissociation. Moreover, it appears that water is dissociated by ultraviolet radiation in a different manner than molecular oxygen. From experiments it is deduced that water predominantly dissociates at the surface of the reticle, thereby forming highly reactive OH-radicals. As these OH-radicals are located near to or on the hydrocarbon contaminant residing on said surface, it reacts readily therewith. Molecular oxygen is believed not to dissociate at the surface but in the space surrounding the reticle. Apparently, such a difference in dissociation behavior between water and oxygen may explain the discrepancy in cleaning rates. In one embodiment of the present invention the space containing the optical component to be cleaned is purged with a substantially inert gas. In this case molecular oxygen is present in a small amount in the purge gas. The purge gas may comprise any gaseous composition which is suitable for use in a lithographic apparatus, together with one or more oxygen-containing species as defined above. Typical purge gases comprise one or a mixture of inert gases such as noble gases or nitrogen, together with one or molecular oxygen-containing species as defined above. Additionally, useful inert gases include argon, helium and nitrogen, for example ultra-pure nitrogen. The inventors have determined that there may be an advantage to the use of purge gas compositions of the inventions consisting only of one or more inert gases and one or more oxygen-containing species as defined above. As a result, there may be an advantage if other contaminants are removed from the gas. It is possible to use a purifier in the present invention which removes most hydrocarbons but does not affect the presence of the relevant oxygen-containing species. The total amount of molecular oxygen present in the purge gas is typically from about 1 ppb to about 10 ppm by volume. If the amount of oxygen-containing species is less than about 1 ppb by volume, the amount of contaminant which is removed from the optical component may be insufficient, unless cleaning is carried out for a period of several hours, which is in itself undesirable. Further, concentrations of below about 1 ppb may be very difficult to detect. Alternatively, if the concentration of oxygen-containing species is above about 1 ppm by volume, the absorption of the projection beam by the oxygen-containing species is generally so high that the transmission is decreased below an acceptable level. The level of transmission loss due to this absorption of the projection beam depends on the path length of the optical system to be cleaned. For example, the beam delivery system in general has a much longer path length than the illumination system and a decrease in transmission of 10% due to UV-absorption in the beam delivery system may equate to a decrease of around only 1% in the illumination system, given the same concentration of oxygen-containing species. Therefore, while a concentration of around 1 ppm may be acceptable in an illumination system, systems with a longer path length may require lower concentrations such as 300 or 400 ppb. In a variation of the first embodiment of the present invention, the space containing the optical component to be cleaned is evacuated. In this embodiment, the oxygen-containing species or mixture of oxygen is preferably substantially the only component(s) of the purge gas. The purge gas is introduced into the space at a low partial pressure. The pressure of molecular oxygen in the space must be sufficiently high that contaminants can be effectively cleaned from the optical component within a reasonable time, but sufficiently low that the transmission of the projection beam is not reduced below an acceptable level. Typically, the total partial pressure of all oxygen-containing species present is from about 1xc3x9710xe2x88x924 Pa to about 1 Pa. If the pressure is below about 1xc3x9710xe2x88x924 Pa, cleaning must be carried out for several hours in order to remove a sufficient amount of contaminant. Conversely, if the pressure is above about 1 Pa, absorption of the (E)UV radiation by the oxygen-containing species is high, causing an unacceptable loss in transmission. As described above, the maximum acceptable amount of oxygen-containing species used may vary depending on the path length of the system to be cleaned. If desired, the degree of contamination may be monitored using sensor 6. Sensor 6 acts by measuring the reflectance or transmission of (E)UV radiation by the optical component to be cleaned. As is depicted in FIG. 2, the optical component may be reflective, and the sensor will therefore measure the reflectance of the (E)UV radiation. However, if the optical component is of a transmissive type, the sensor will be positioned such that it measures the degree of transmission through the optical component. The degree of absorption of (E)UV radiation can be used to indicate the degree of coverage of the optical component with contaminants. In this embodiment, the system will generally be purged of all (E)UV absorbing agents except the oxygen-containing species, whose concentration is known and is preferably kept constant. Therefore, any (E)UV absorption observed, aside from that which can be attributed to oxygen-containing species present, is due to the presence of contaminants. The sensor can, in this way, be used to monitor the level of contamination, and any changes to the level of contamination, of the optical system. The sensor may be employed before and/or after cleaning to indicate whether the optical component in question is sufficiently clean for exposure to take place, or whether further cleaning is required. Regular use of this detection process may be desirable so that it can be determined when an optical component requires cleaning. The sensor may also be used during the cleaning process. Cleaning is carried out as described above, and while irradiation is taking place, the absorption of said radiation is monitored using sensor 6. When the sensor indicates that the absorption level has dropped below a sufficient level, and thus the contamination level of the optical component is acceptable, the cleaning process may be stopped. In another variation of the first embodiment, the optical components and/or mask is cleaned with one or more of the cleaning agents in between exposing wafers. For example, between two batches or on a regular basis e.g. as part of a general maintenance program. In this way, higher concentrations of the cleaning agent can be used as wafers are not exposed and transmission loss during cleaning is not detrimental. In a second embodiment of the invention, which is the same as the first embodiment except as described below, cleaning is carried out as a separate process and/or at a different time for exposure, e.g. cleaning the mask in a separate unit within the lithographic apparatus. In this embodiment the amount of oxygen-containing species is not limited by an acceptable loss in transmission of the projection beam and concentrations of greater than 10 ppm (or higher partial pressures than 1 Pa) may be used. Thus, for example concentrations of up to 20% of oxygen-containing species in an inert gas are suitable. In particular, water concentrations of, for instance, 1000 to 15000 ppm can be used. Typically, the apparatus is not evacuated since illumination is carried out at the same time as cleaning and evacuation is unnecessary. However, if the apparatus is evacuated, the total partial pressure of oxygen-containing species may be as high as approximately 2xc3x97104 Pa. In particular, water concentrations of, for instance, 100 to 1500 Pa can be used. In general, increasing the amount of oxygen-containing species present is advantageous since it reduces the cleaning time and thereby reduces the down-time of the apparatus. This embodiment, when employed with concentrations of oxygen-containing material of greater than 10 ppm (or 1 Pa in vacuum) provides an increased cleaning effect which can be used when it is necessary to remove large amounts, or particularly strongly adsorbed contaminants. This technique may be used, for example, when it is known that contaminant levels are particularly high, on starting-up the apparatus, or on a regular basis e.g. as a part of a general maintenance program. It must be noted that the above-described cleaning unit may also be employed outside the lithographic projection apparatus as a separate cleaning unit. With such an external cleaning unit all kinds of contaminated objects can be cleaned with for example water, nitrogen oxides ore oxygen-containing hydrocarbons, while exposing the object with ultraviolet radiation. The objects to be cleaned are not limited to optical components or patterning structure as described above, but do also encompass e.g. metal sheets, (resist-coated) wafers, solar panels or any other kind of contaminated article. FIG. 3 depicts a third embodiment of the invention, which is the same as the second embodiment except as described below. In this embodiment a further source of UV or EUV radiation 7 is provided. Source 7 provides radiation having a wavelength of 250 nm or less. Suitable sources of such radiation are the same as those described above with reference to source LA. In this embodiment, the optical component 3 is irradiated by either EUV or UV radiation having wavelengths shorter than 250 nm, while simultaneously projecting the patterned beam of EUV radiation. Preferably, UV radiation is used, which is capable of selectively dissociating molecular oxygen more profoundly than EUV radiation. For example in the case of oxygen, UV radiation having wavelength of about 157 nm may be used. In this way, relatively low concentrations of oxygen-containing species in the purge gas can be employed to ensure relatively low absorption of EUV radiation by the cleaning agent. Consequently, the optical component 3 can be cleaned, while exposing a wafer, with acceptable transmission loss. It is further contemplated to irradiate the optical component 3 located in space 2 using the UV or EUV radiation supplied from source 7, either before or after exposure by the projection beam PB. Preferably, irradiation is carried out before exposure, thus providing a cleaned optical component which will improve the transmission and uniformity levels during exposure. In this embodiment, the radiation provided by source 7 is depicted as being directed at optical component 3. However, it is also possible to direct the radiation other than directly at the optical component, for example across the optical component. If desired, sensor 6 may be used to monitor the level of contamination as described above. In the embodiments described above, a mask or reticle is described, which may also comprise a pellicle. In a space between the mask and the pellicle, a purge gas comprising molecular oxygen can be supplied in order to remove contaminants from said space according to the above-described cleaning process. While we have described above specific embodiments of the invention it will be appreciated that the invention may be practiced otherwise than described. The description is not intended to limit the invention.