BRIEF DESCRIPTION OF THE DRAWINGS
In the following, embodiments of the invention will be described in greater detail with reference to the accompanying drawings.
FIG. 1 is a diagrammatic, exploded perspective view of a photolithography apparatus;
FIG. 2 is a diagrammatic illustration of an exemplary conventional imaging with off-axis illumination;
FIG. 3 is a diagrammatic illustration of an exemplary imaging of a pattern with an AltPSM mask;
FIG. 4 is a diagrammatic illustration of an exemplary conventional aperture element for implementing on-axis illumination;
FIG. 5 is a diagrammatic block diagram of a photolithography apparatus according to an exemplary embodiment of the present invention;
FIG. 6 is a plan view of an illumination distribution generated by the illumination system of an exemplary embodiment of the present invention;
FIG. 7 is a plan view of another illumination distribution generated by the illumination system of an exemplary embodiment of the present invention;
FIG. 8 is a plan view of a further illumination distribution generated by the illumination system of an exemplary embodiment of the present invention;
FIG. 9A is a plan view of yet another illumination distribution generated by the illumination system of an exemplary embodiment of the present invention;
FIG. 9B is a plan view of still a further illumination distribution generated by the illumination system of an exemplary embodiment of the present invention;
FIG. 10 is a plan view of exemplary positions of diffracted light beams in the plane of the entrance pupil of an optical projection system according to the present invention;
FIG. 11 is a plan view of yet a further illumination distribution generated by the illumination system of an exemplary embodiment of the present invention; and
FIG. 12 is a plan view of further exemplary positions of diffracted light beams in the plane of the entrance pupil of the optical projection system according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following, the invention will be described in more detail by exemplary embodiments and the corresponding figures. By schematic illustrations that are not true to scale, the figures show different exemplary embodiments of the invention.
Referring now to the figures of the drawings in detail and first, particularly to FIG. 5 thereof, there is shown an exemplary photolithographic apparatus according to an embodiment of the present invention. The photolithographic apparatus includes an illumination system 24 that has an illumination source 21. The illumination source can be any light source or other device or combination of devices that are capable of generating light used to create a photolithographic image. As used throughout this disclosure, the term “light” refers to electromagnetic radiation in the visible light spectrum as well as the invisible spectrum, including without limitation visible light, ultraviolet light, and X-rays. For example, the illumination source 21 may include a laser such as an argon fluoride laser, a fluorine excimer laser, or a helium neon laser. The illumination system 24 further includes a polarization device 23 and a device for generating an illumination distribution 3. The polarization device may be disposed between the illumination source 21 and the illumination distribution generating device 3. The polarization device may be adapted to generate linearly polarized electromagnetic radiation having a locally varying polarization direction as will be described hereinafter. In particular, an exemplary structure of the polarization device 23 will be described after the description of an illumination distribution generated by the device for generating an illumination distribution 3.
The pattern 41 of the reticle 4 may be transferred from the reticle 4 to a wafer 5 by irradiating the reticle with the illumination distribution generated by the illumination system 24. For example, the pattern 4 may be imaged onto the wafer by the projection system 11. The reticle 4 usually may be held by a stage (not shown). Moreover, the wafer 5 may be held by a wafer stage 51.
As is shown in FIG. 6 illustrating an illumination distribution generated by the illumination system according to an embodiment of the present invention, the illumination distribution has a center point 32 at the center thereof. The first opaque region is defined about the center point 32, and a second opaque portion is defined adjacent to the outer edge. Each point located in the first opaque portion 34 has a distance from the center point 32 that is smaller than rin. Each point in the second opaque portion 35 has a distance from the center point 32 that is larger than rout. As is shown in FIG. 6, the first opaque portion 34 may have a circular shape around the center point 32. Nevertheless, as is to be understood, the first opaque portion can, as well, have a shape that deviates from circular. In particular, the first opaque portion may have an elliptical shape.
The illumination distribution 6 further includes a light transmittant portion 36, which is located between the first and the second opaque portions 34, 35. The light transmittant portion 36 is a portion having regions with a high light intensity. For example, the transmittant portion 36 may be entirely illuminated or it may include a predetermined number of, for example, four illumination poles 31a, 31b, 31c and 31d. Nevertheless, as is clearly to be understood any other number of poles may be used. For example, 6 or 8 poles may be used as well. The polarization device is adapted to provide a locally varying polarization direction of the light. For example, as is indicated by the arrows in the poles 31a to 31d, the polarization direction in each of the poles may be parallel to a direction connecting the center point of the poles with the center point 32 of the illumination distribution 6. For example, as is shown in FIG. 6, the polarization direction of the poles 31a, 31c may be along the y-direction, whereas the polarization direction of the poles 31b, 31d may be along the x-direction.
Likewise, the illumination distribution 6 shown in FIG. 6 could as well have 8 poles, where the polarization direction of light transmitted by the pole that is disposed between the poles 31a, 31b is rotated by 45° with respect to the y-direction and the light transmitted by the pole that is disposed between the poles 31b, 31c is rotated by 45° with respect to the x-direction. Alternatively, the illumination distribution 6 may have 8 poles, where the polarization direction of light transmitted by the pole which is disposed between the poles 31a, 31b is parallel to the y- or x-direction.
FIGS. 7 and 8 illustrate a further embodiment of the present invention, where the light transmittant portion 36 of the illumination distribution 6 has an annular shape. As can be seen, the polarization direction of the light transmittant portion 36 can have a radial direction, i.e., the polarization direction at each point of the annular ring around the center point 32 has a direction parallel to a direction of this point connected with the center point 32. Such a distribution of the polarization directions can be simplified, as is shown in FIG. 8. For example, in a specific range of angles between the outer edge 33 of the illumination distribution 6 and the center point 32, the polarization direction is along the y-direction, whereas, in another angular range between the outer edge 33 and the center point 32 with respect to the x-direction, the polarization direction is along the x-direction. The angular ranges for the polarization direction along the y-direction and the x-direction can be arbitrarily chosen dependent upon the system requirements.
According to a further embodiment of the present invention, as is shown in FIGS. 9A and 9B, the light transmittant portion 36 of the illumination distribution 6 may include segments of a ring. In particular, the four illumination poles 31a, 31b, 31c, and 31d are not circular or elliptical but have the shape of a segment of a ring. As is shown in FIG. 9A, the polarization direction of each of the poles may have a radial direction, i.e., the polarization direction at each point of a pole has a direction parallel to a direction of this point connected with the center point 32. This distribution of the polarization directions may be simplified, as is shown in FIG. 9B. For example, in a similar manner as in FIG. 6, the poles 31a, 31c may have a polarization direction that is parallel to the y-direction, whereas the poles 31b, 31d have a polarization direction that is parallel to the x-direction. The angles σ1 and σ2 may be arbitrarily chosen in accordance with the system requirements.
The locally varying polarization direction of the incident light beam may be accomplished in different manners. For example, as is indicated in FIG. 5, the polarization device may include a polarizer 231 for generating linearly polarized light 133. As is clearly to be understood, the polarizer 231 can be formed integrally with the light source 21, although it is illustrated as a distinct device in FIG. 5. The polarization device 23 may further include a prism system 232. The prism system 232 may be adapted to divide the linearly polarized light beam 133 into one or more light beams that are locally separated from one another. The polarization device 23 may further include two polarization rotating elements 233a, 233b, which may rotate the polarization direction of the incident light beams by 90°. In a similar manner, further prisms may be provided to obtain more divided light beams. In addition, the polarization direction may be rotated by any desired angle by providing a suitable polarization rotating elements 233a, 233b. As a further alternative, the polarization device 23 may be formed integrally with the aperture element 3, for example, as a so-called wire grid polarizer, including a grid in a predetermined direction.
The orientation of the wire grid polarizer is selected so that the polarization direction is achieved for the transmittant portion of the aperture element 3 as has been described above.
The central portion of FIG. 10 shows the positions of the +1st and −1st diffracted beams in the entrance pupil of the projection system 11 when using the illumination distribution shown in FIG. 7 and the pattern 41 on the reticle 4 as indicated in the lower portion of FIG. 10. In particular, FIG. 10 illustrates the positions of the poles 31a, 31b, 31c, 31d when being diffracted by the pattern 41. According to an embodiment of the present invention, a special effect is obtained if the geometrical dimensions of the device 3 for generating an illumination distribution are determined dependent upon the pitch of the pattern 41 as well as the numerical aperture (NA) of the projection system 11 for imaging the pattern onto the substrate 5. As can be seen in FIG. 10, the size of the light transmittant portion of the illumination distribution 6 may be selected, so that the pole 31b is cut off the +1st diffraction order and the pole 31d is cut off the −1st diffraction order. As a consequence, when imaging the pattern 41 from the photomask onto the substrate, the TM polarized radiation may be decreased, resulting in a decreased amount of interference. Because the poles 31a, 31c are maintained within the entrance pupil 111, these portions may interfere with each other, resulting in an increased contrast.
In FIG. 10, the position of the +1st and −1st diffraction order depends on the pattern size dx of the pattern to be transferred. Accordingly, the smaller the pattern, the smaller the value of rin.
For imaging a horizontal pattern 41, which is rotated by 90° with respect to the pattern shown in FIG. 10, the same applies. More specifically, if the diffraction grating is rotated by 90°, the +1st and −1st diffraction orders are located above and below the center point shown in FIG. 10. In such a case, the pole 31a may be cut off the +1st diffraction order, whereas the pole 31c may be cut off the −1st diffraction order. In any case, the rim of the pupil is such that one pole including light having a polarization direction that will cause destructive interference with respect to the pattern to the image is cut off from the entrance pupil 111 of the projection system 11. Differently stated, the diffracted beam of the +1st order due to illumination with pole 31b is not captured by the pupil. Likewise, the diffracted beam of the −1st order due to illumination with pole 31d may not be captured by the pupil. As a consequence, the contrast of the image may be remarkably increased. Hence, the image quality may be improved.
When imaging a pattern including a horizontal lines/spaces pattern as well as a vertical lines/spaces pattern, the +1st and −1st diffraction orders are located on the x-axis as well as on the y-axis of the system. If, in addition, the vertical pattern size dx is different from the horizontal pattern size dy, the poles on the x-axis have a size that may be different from the size of the poles on the y-axis. In other words, in such a case, the first and the second poles have a diameter that may be different from the diameter of the third and fourth poles.
FIG. 11 shows an exemplary illumination distribution in a case in which a first pattern extending in the x-direction (having a pattern size dx) and a second pattern extending in the y-direction (the second pattern having pattern size dy) are to be images. As is shown in the lower portion of FIG. 11, the pattern size dy of the pattern extending in the y-direction is smaller than the pattern size dx of the pattern extending in the x-direction. In this embodiment, the light transmittant portion 36 has an annular elliptical shape, wherein the outer radius rout,x which is measured in the x-direction may be larger than the outer radius rout,y which is measured in the y-direction. Moreover, the inner radius rin,x which is measured in the x-direction is larger than the inner radius rin,y which is measured in the y-direction. As is clearly to be understood, in this case, the light transmittant portion 36 may include illumination poles having an arbitrary shape. In particular, the poles may have a circular or an elliptical shape, or they may form segments of a ring. Alternatively, the light transmittant portion 36 may have an annular shape.
The photolithography apparatus is not only restricted to a lines/spaces pattern. It can be similarly applied to any other kind of patterns. For example, if a contact hole pattern is to be transferred, a similar illumination scheme may be used. FIG. 12 shows the ±1st diffraction orders in this case. As is shown in FIG. 12, in this case, the diffracted beams due to an illumination distribution 6 that is shown in FIG. 6 may be disposed on the diagonals of the system. In this case, again, the dimensions of the illumination distribution are selected so that one pole of the diffraction image will be removed from the pupil 111. In the case of a contact hole pattern illustrated in the lower portion of FIG. 12, light transmitted by the outermost poles of the illumination distribution, the light having a polarization direction that is rotated by 45° with respect to the X— or Y-direction, would be destructive. Accordingly, the device for generating an illumination distribution that forms part of the illumination system of the present invention can be used in this case as well. As a consequence, the light being transmitted by the outermost poles is cut off the pupil of the projection system 11 so that finally the pattern having an improved contrast is transferred onto the wafer.
Patent Application
20070263192
