Patent Number: 
Section: description

In the drawings, like references indicate like parts. FIG. 1 schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises: a radiation system LA, Ex, IN, CO for supplying a projection beam PB of 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 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 for imaging an irradiated portion of the mask MA onto a target portion C (die) of the substrate W. As here depicted, the apparatus is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type, for example. The radiation system comprises a source LA that produces a beam of radiation. This beam is passed along various optical components comprised in an illumination system,xe2x80x94e.g. beam shaping optics Ex, an integrator IN and a condenser COxe2x80x94so that the resultant beam PB is of a desired cross-section and uniformly intense throughout its area. The beam PB subsequently intercepts the mask MA which is held on a mask table MT. Having passed through the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the interferometric displacement and measuring means IF, the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning means can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT 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 to a short stroke positioning module only, or it may just be fixed. The depicted apparatus can be used in two different modes: 1. In 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 a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; 2. In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single xe2x80x9cflashxe2x80x9d. Instead, the mask table MT is movable in a given reference 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 simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=1/4 or 1/5). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution. In what follows, it will be assumed that the radiation system produces UV light with a wavelength of 157 nm. According to the invention, the spaces in the apparatus that are traversed by the illumination beam, both before and after it has passed through the mask, are flushed with a laminar flow of flushing gas. The flushing gas may be ultra-pure nitrogen (N2) or other gas or gas mixture sufficiently transparent to the illumination radiation used in the apparatus. N2 has an extinction coefficient, k, at 1 standard atmosphere of less than about 0.0001 per cm traversed, as compared to air at 1 standard atmosphere for which k is approximately 46 per cm traversed. The actual gas pressure in the beam path may be above atmospheric pressure, so that any leak results in an outflow of flushing gas rather than contamination by air, or below atmospheric to reduce beam absorption further. In critical areas, for example the beam delivery and illumination optics, the flushing nitrogen is provided at high purity, i.e. with an air contamination of less than 1 ppm. In less critical areas, such as the projection lens, a contamination of up to 10 ppm can be tolerated whilst in the reticle and wafer stages contamination levels of up to 100 ppm and up to 500 ppm respectively may be tolerable. FIG. 2 shows the mask stage of the lithographic apparatus according to the invention in greater detail than FIG. 1. It will be seen that the mask M is held in a recess in mask table MT, which can be manufactured from a ceramic material such as Zerodur (RTM) and is positioned by a drive system (not shown) during operation of the lithography apparatus. The mask table MT is closely sandwiched between the last element of the collimating optics CO, which generate the projection beam PB, and the first element of the projection lens system PL, which projects the projection beam PB, having traversed the mask M, onto the wafer W (shown in FIGS. 1 and 3). The mask stage is divided into zones or spaces 2 to 6 as follows: space 2 is between the final illuminator optics CO and mask table MT; space 3 is within the mask table MT above the mask M; space 4 is within the mask table MT, between the mask M and pellicle 13; space 5 is within the mask table MT below the pellicle 13; and space 6 is between the mask table MT and projection lens system PL. Each of the spaces is flushed with a substantially laminar flow of flushing gas provided from flushing gas supply 11 via respective flow regulators 112 to 116. At the other side of each space the flushing gas is removed to reservoir 12 via respective vacuum pumps 122 to 126. Reservoir 12 may be partitioned to allow controlled re-use of the gas in selected spaces and may include devices 12a to clean or scrub the recovered gas. To ensure laminar flow, the various spaces in the mask stage can be separated from one another. In particular, a thin sheet 14, e.g. of a material substantially transmissive to the employed radiation, such as CaF2 or fused SiO2, for example, is provided to cover the recess in the mask table MT and separate space 2 from space 3. Flow regulators 112, 113 and vacuum pumps 122, 123 are controlled to ensure that there is no, or only minimal, pressure differential between these two spaces to avoid loading sheet 14. Sheet 14 is arranged to be easily dismountable and replaceable during reticle exchange. Similarly, space 5, below pellicle 13, is closed off from space 6, between the mask table MT and projection lens PL, by a second thin sheet 15 which may be made of CaF2 or fused SiO2. Sheets 14 and 15 may also be made of MgF2, BaF2 or any other suitable material that transmits radiation at the wavelength used in the apparatus. Similar sheets may also be used to cover any irregularities or dead spaces in the system. For example, a third sheet 16 may be used to cover the non-flat surface of the first element of the projection lens system PL. Sheets 14 and 15 and their like form partitions to isolate parts of the beam path in which the laminar flushing gas flow is provided. It should be noted that sheets 14, 15 and 16 are provided to smooth the laminar flow of flushing gas and need not be gas-tight, nor necessarily form a gas-tight seal to the parts to which they are connected. To supply and remove the gas flow to spaces 3, 4 and 5, within the mask table MT, appropriate conduits are provided in the body of the mask table. When the mask table has been exposed to air, e.g. after a period of non-operation of the apparatus or after mask exchange, flushing gas is supplied for a short period before an exposure is taken to flush out any air that may have accumulated in non-flat parts of the mask table, e.g fiducials. In this embodiment a pellicle is provided and may be made of solid SiO2 or CaF2. Fused silica (SiO2) which has an improved transmission at 157 nm may also be used. Polymer pellicles are preferably avoided to avoid diffusion across them. In some embodiments of the invention the pellicle may be omitted altogether in which case the flushing gas supply is simplified. In any of the spaces, aerodynamic features such as small strips or fins may be provided as desired to smooth or guide the flushing gas flow and eliminate or control vortex production. The gas supply and evacuation conduits, particularly in spaces 2 and 6, are positioned to minimize the length of the gas flow to reduce the opportunity for mixing with air. FIG. 3 shows the wafer stage of the lithographic apparatus of FIG. 1. In the wafer stage there is only a single space to be flushedxe2x80x94between the last element of the projection lens system PL and the wafer W. To avoid having to provide a flushing gas path covering the entire range of movement of the wafer stage, the flushing gas supply outlets 17 and evacuation inlets 18 are mounted on the lower end of the projection lens system PL, either side of the final element. Outlets 17 and inlets 18 are respectively connected to the flushing gas supply 11 and reservoir 12 via flow regulator 117 and vacuum pump 127 respectively. The outlets 17 in particular, but also the inlets 18, may be provided with vanes to guide the flow of flushing gas. If not already flat, the final element of the projection lens system PL may be covered with a thin sheet as discussed above. The flow regulators 112 to 117 mentioned above may comprise static or controllable pressure or flow reducers and/or blowers as required to provide the necessary gas flow rates for the particular embodiment and the available gas supply. As described above, a laminar flow of flushing gas can be used to reduce absorption of the illumination beam in the moving parts of the lithography device without leading to excessive consumption of the flushing gas. Similar arrangements may also be used in the static components, such as the illumination beam generator and shaper and the projection lens system. However, it is simpler to seal static components than it is to seal moving components and it so may be more convenient to do so and maintain the static components under vacuum or with a static fill of transparent gas such as N2. While we have described above a specific embodiment 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. In particular, it will be appreciated that the invention may be used in either or both the mask or substrate stage of a lithographic apparatus, and in any other type of apparatus employing a short wavelength radiation beam.