Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits (ICs), and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art will recognize that the description herein also applies to other types of substrates. In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g., a silicon wafer) that has been coated with a layer of radiation-sensitive material (e.g., resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (with M<1) the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (e.g., resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are desired, then the whole procedure, or a variant thereof, should be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The position of a second element traversed by the projection beam relative to a first element traversed by the projection beam will for simplicity hereinafter be referred to as “downstream” of or “upstream” of said first element. In this context, the expression “downstream” indicates that a displacement from the first element to the second element is a displacement along the direction of propagation of the projection beam; similarly, “upstream” indicates that a displacement from the first element to the second element is a displacement opposite to the direction of propagation of the projection beam. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices, the additional 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 exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and International Patent Application Publication No. WO 98/40791, incorporated herein by reference.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this it is desirable to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. For the projection system this means that the projection system and the lens elements used in the projection system should comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system, they both may still suffer from wave front aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration across an image field projected with the projection system onto a target portion of the substrate. The aberrations are sources of variations of the imaged line widths occurring across the image field. The imaged line widths at different points within the image field should be constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting masks, or off-axis illumination, the influence of wave front aberrations on the imaged line widths may further increase.
During manufacture of a lens element, it may be advantageous to measure the wave front aberrations of the lens element and to use the measured results to tune the aberrations in this element or even to reject this element if the quality is not sufficient. When lens elements are put together to form the projection system it may again be desirable to measure the wave front aberrations of the projection system. These measurements may be used to adjust the position of certain lens elements in the projection system in order to minimize wave front aberrations of the projection system.
After the projection system has been built into a lithographic projection apparatus, the wave front aberrations may be measured again. Moreover, since wave front aberrations are variable in time in a projection system, for instance, due to deterioration of the lens material or lens heating effects from local heating of the lens material, it may be desirable to measure the aberrations at certain instants in time during operation of the apparatus and to adjust certain movable lens elements accordingly to minimize wave front aberrations. It may be desirable to measure the wave front aberrations frequently due to the short time scale on which lens-heating effects may occur.
United States Patent Application Publication No. 2002/0145717 describes a wavefront measurement method that uses within the lithographic apparatus a grating, a pinhole and a detector, e.g. CCD detector. The detector may have a detector surface substantially coincident with a detection plane that is located downstream of the pinhole at a location where a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier Transformation of a spatial distribution of the electric field amplitude of the projection beam in the pinhole plane. With this measurement system built into the lithographic projection apparatus it is possible to measure in situ the wave front aberration of the projection system.
As illustrated in FIG. 1A, in this measurement system a component, plane wave PW10 of wave W1 is diffracted by the grating as an emanating wave WD. The wave WD emanating from the grating can be considered as a sum of diffracted plane waves PW2i, [i=0, 1, 2 . . . ]. The plane waves PW22, PW20 and PW21 are, respectively, the +1st, 0th and −1st diffracted order of the incoming wave PW10. In the projection system schematically shown in FIG. 1B the plane waves PW2i, [i=0, 1, 2 . . . ] will focus near or at the pupil plane PU, and sample the pupil plane in three points. The aberrations of the projection system PL can be thought of as phase errors that are endowed on the focused plane waves PW2i, [i=0, 1, 2 . . . ] in the pupil plane PU. These focused plane waves will exit the lens as plane waves PW3i, [i=0, 1, 2 . . . ], respectively. As shown in FIG. 1C, to measure the phase errors representative for the lens aberrations, the plane waves PW3i, [i=0, 1, 2 . . . ] are directionally recombined by diffraction at the pinhole 17 in a pinhole plate 11. For instance, PW400 is the 0th order diffracted wave originating from PW30, PW411 is the +1st order diffracted wave from PW31 and PW422 is the −1st order diffracted wave originating from PW32 and these directionally recombined waves can interfere. Their interference intensity is harmonic with the phase stepping of the grating. Other recombination's of diffracted waves originating from the PW3i [i=0.1.2 . . . ], are possible as well. However, the intensity resulting from the interference of such recombination varies as a higher harmonic of the phase stepping movement of the grating. Such higher order harmonic signals can be filtered out from each CCD-pixel signal.
It is desirable to further reduce the size of the structures obtained by lithography. Therein the wavelength of the radiation may play an essential role. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors may result in a more powerful, faster microprocessor. In order to enable processing with light of a shorter wave length, chip manufacturers developed a lithography process known as Extreme Ultraviolet Lithography (EUVL). In this process, transparent lenses are replaced by mirrors. U.S. Pat. No. 6,867,846 describes a phase shift mask for measuring wave front aberrations in such a process.
According to the method described therein, schematically shown in FIG. 3, the projection optical system PO projects an image of the first grating 203 at a second grating 201 that is positioned in its focal plane. Analogous to the method described in United States Patent Application Publication No. 2002/0145717, the second grating recombines diffracted waves. Wave front aberrations caused by the optical system PO become visible as an interference pattern, which can be inspected by a wave front sensor 106, such as a CCD-camera.
A particular problem that frequently exists in many EUV photolithographic systems is that the EUV source does not provide uniform information, but instead has a number of facets, or hot spots in its exit pupil that result from use of fly's eye lenses in the optics of the EUV source. This results in a non-uniform wavefront at the input numerical aperture of the pupil of the projection optics, or sometimes, in underfilled numerical aperture of the projection optics. These problems may affect the measurement of the wavefront by the wavefront sensor discussed above.
Thus, it is desirable to be able to eliminate the underfilling and intensity nonuniformity at the input numerical aperture of the projection optics. Accordingly, the phase shift mask additionally serves to condition the illumination reaching the input numerical aperture pupil plane of the projection optics PO. The phase shift mask washes out spatial variations introduced by the source, so that the pupil plane is substantially fully and homogeneously illuminated.
This may be achieved in that the first grating 203 includes a plurality of reflecting lines each formed by a plurality of reflecting dots.
The reflecting dots generate a diffraction pattern within a diffraction pattern. Thus, each reflecting dot becomes a wavefront source, as viewed from the focal plane. Therefore, irregularities in intensity, particularly due to fly's eye facets of the source, will disappear, presenting a clean, regular image of the source at the focal plane. According to United States Patent Application Publication No. 2002/0145717, the reflecting dots have a random height with a standard deviation of many times a wavelength.
One possible disadvantage of this known grating that it may be difficult to manufacture, as it is difficult to obtain a pattern of reflective dots having a large difference in height while having a predetermined in-plane size.