The patent's assignee for patent number 8797507 is
News editors obtained the following quote from the background information supplied by the inventors: "Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light or soft X-ray radiation (EUV). Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a transmissive or reflective mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
"A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
"As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
"The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
"In EUV projection exposure apparatus the illumination system usually includes a mirror array (sometimes also referred to as faceted mirror) which directs the projection light produced by the EUV light source towards the system pupil surface so that a desired intensity distribution is obtained in the system pupil surface.
"WO 2005/026843 A2 proposes for a DUV illumination system to use a mirror array that illuminates the pupil surface. For increasing the flexibility in producing different angular distribution in the mask plane, each of the mirrors can be tilted about two perpendicular tilt axes. A condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This known illumination system makes it possible to produce on the pupil surface a plurality of light spots, wherein each light spot is associated with one particular microscopic mirror and is freely movable across the pupil surface by tilting this mirror. It is also proposed to vary the size of the spots by using adaptive mirrors having a mirror surface whose shape can be varied to a limited extent using suitable actuators, for example piezoelectric actuators.
"US 2005/0018269 A1 discloses a correction device which makes it possible to heat up certain portions of selected mirrors contained in a projection objective of a microlithographic exposure apparatus. To this end a light ray scans over the portions of the mirrors to be heated up. The device makes it possible to increase the temperature very selectively so that a desired, in particular a rotationally symmetric, temperature distribution can be achieved. In one embodiment the desired temperature distribution is determined such that the heated mirror changes its shape in a predetermined manner, thereby correcting aberrations produced in other optical elements of the objective.
"WO 2004/092843 A2 discloses a correction device for a EUV projection objective of a microlithographic exposure apparatus that directs correction light to one of the large mirrors of the objective. The correction light is controlled such that the temperature in the vicinity of the reflective surface comes close to the temperature where the coefficient of thermal expansion of the mirror substrate is zero.
"EP 0 532 236 A1 discloses another correction device for a EUV projection objective of a microlithographic exposure apparatus. In one embodiment infrared radiation is directed on one of the large mirrors of the objective. The infrared light is controlled such that the shape of the mirror does not substantially alter even under the impact of the high energy EUV projection light. In other embodiments heating or cooling devices are integrated into the mirror support for the same purpose.
"The mirror array including adaptive mirrors as disclosed in the aforementioned WO 2005/026843 A2 is particularly advantageous because additional reflective power may be added to correct for non-ideal optical properties of a subsequent condenser, or for aberrations caused by material defects and manufacturing tolerances. However, the use of piezoelectric actuators proposed in this document has some significant drawbacks. In order to achieve a desired curvature of the mirror surface, it is desirable to provide a large number of such actuators which adds to the system complexity. For example, a very large number of electrical leads have to be provided for individually controlling the piezoelectric actuators. In a mirror array including several thousand mirror elements on a total area of less than 100 cm.sup.2 the electrical wire density becomes critical. Apart from that it is difficult to obtain the desired surface shape of the mirror elements under varying temperature conditions."
As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "The present disclosure provides an illumination system which makes it possible to vary the spot shape of the light bundles produced by the mirror elements in a system pupil surface in a very accurate and variable fashion, while involving limited system complexity.
"An illumination system can include a primary light source, a system pupil surface and a mirror array. The mirror array is arranged between the primary light source and the system pupil surface. The array includes a plurality of adaptive mirror elements, wherein each mirror element may be tiltably mounted with respect to a support structure. Each mirror element includes a mirror support and a reflective coating and is configured to direct light produced by the primary light source towards the system pupil surface. According to the disclosure the mirror elements include structures having a different coefficient of thermal expansion and being fixedly attached to one another. The illumination system further includes a temperature control device that is configured to variably modify the temperature distribution within the structures so as to change the shape of the mirror elements.
"The disclosure thus exploits the effect that plates including materials having different coefficients of thermal expansion bend when the temperature changes, similar to bimetallic strips used for temperature controllers. The disclosure is furthermore based on the consideration that it is meanwhile possible to compute very accurately not only the temperature profile of mirror elements when heated or cooled at certain target areas, but also to predict the deformations occurring as a result of this temperature profile. In the context of the present disclosure this prediction has to take into account bending forces produced by the different coefficients of thermal expansion. However, bending forces produced by a non-homogeneous temperature profiles in the mirror elements may be taken into account, too.
"In sophisticated mirror arrays such computations should be carried out anyway in order to prevent optical aberrations due to mirror deformations induced by the absorption of projection light. Thus, from a computational point of view, the temperature induced mirror adaptation according to the present disclosure does not substantially add to the system complexity.
"From a hardware point of view, it has become apparent that it suffices to carefully heat or cool very few and/or small areas on the mirrors in order to produce a very large variety of different deformations with high accuracy. Significantly less wiring etc. is involved to control very few, for example 2 or 4, heater or cooler members as compared to the control of a large number of piezoelectric elements.
"The disclosure relies only the mirror elements including structures having a different coefficient of thermal expansion and being fixedly attached to one another. Preferably the structures are planar or curved structures having a pair of parallel surfaces. Usually such structures exist anyway, because mirror elements typically include a mirror support and a reflective coating applied thereon, wherein both structures have different coefficients of thermal expansion. Since the effect of bending as a result of temperature changes becomes larger the greater the difference between the coefficients of thermal expansion is, the difference between the coefficients of thermal expansion should be substantial if a high sensitivity of the mirror elements to temperature changes is desired. Metals are a material class in which a wide variety of large coefficients of thermal expansion is available, and therefore the structures are made of metals in some embodiments.
"The structures that produce the bending effect do not necessarily have to be the reflective coating and the mirror support, however. Since reflective coatings are usually formed by a stack of thin layers having alternate refractive indices and also different coefficients of thermal expansion, the bending effect produced by these layers after a change of the temperature profile may suffice to obtain the desired surface shape.
"On the other hand, if a larger bending effect is desired, the contribution of the reflective coating may be insufficient. Then it may be considered to have a mirror support formed by a layer structure including at least two layers having different coefficients of thermal expansion.
"In one embodiment the temperature control device includes heating or cooling members applied to the mirror support. With very few such heating or cooling members it is possible to produce complex deformations of the mirror element. Cooling and/or heating members may be formed by Peltier elements; heating members may include patterns of electrically conductive resistance wires which are directly applied to an underside of the mirror support.
"The use of cooling members is particularly advantageous in EUV illumination systems in which the primary light source is configured to produce projection light having a wavelength below 50 nm, preferably below 25 nm, and most preferably between 13 and 14 nm. Since the mirror elements often have to be cooled anyway because a considerable portion of the impinging high energy EUV projection light is absorbed by the mirror elements and heats them up, the cooling may be performed in a locally resolved manner, i.e. different portions of the mirror elements are cooled to different extents. For example, the mirror elements may be provided with an array of Peltier elements which are controlled such that only those Peltier elements that are arranged on a certain area, which may have the contour of a stripe or an ellipse, for example, are operated and cool the adjacent portions of the mirror elements.
"In a preferred embodiment the temperature control device includes a radiation system configured to selectively direct radiation to target areas on the reflective coating of the mirror elements. This makes it possible to change the shape of the mirror elements and thus to modify their reflective power without the need to accommodate any additional electrical components in the restricted space available for each mirror element. The radiation system thus provides for a kind of remote control for the shape of the mirror elements.
"The reflective surface of the mirror elements on the one hand and the wavelength of the radiation produced by the radiation system should be determined such that the reflective surface is absorbent for the radiation. This ensures a maximum heating effect and simultaneously reduces undesired effects produced by reflected radiation.
"The radiation system may include a diaphragm including an arrangement of apertures corresponding to the desired target areas on the mirror support. The diaphragm is illuminated by a radiation source and imaged onto the mirror array such that only the target areas are illuminated by the radiation. By inserting different diaphragms it is even possible to vary the target areas which are exposed to the radiation. It is also possible to have different target areas for different mirror elements by suitably determining the apertures in the diaphragm.
"Target areas that can be varied individually for each mirror element may be more easily provided for if the radiation system includes a secondary light source producing a radiation beam and a spatial light modulator that is configured to move the radiation beam over the target areas. In this case the target areas are 'written' by a moving radiation beam. If desired, any arbitrary target area may be heated on any of the mirror elements by suitably controlling the spatial light modulator. Such a radiation system is particularly suitable in embodiments in which the target areas are line patterns. It may be desirable to provide more than one secondary light source and spatial light modulator in order to ensure that each mirror element is heated up with a sufficiently high refresh rate.
"In one embodiment the mirror elements have concave mirror surfaces, and the temperature control device is configured to modify the temperature distribution within the structures such that the mirror elements have different focal lengths in two orthogonal planes. Such astigmatic mirror elements are particularly advantageous because this suppresses aberrations that would otherwise be present if spherical mirrors reflect light impinging at large angles of incidence.
"Astigmatically deformed mirror elements are particularly useful in EUV illumination systems. Then, without causing significant aberrations, the optical axes of the mirror elements are allowed (irrespective of their tilting angles) to form an angle of more than 20.degree., preferably of more than 30.degree., with an optical axis of a mirror which immediately precedes the mirror array in a light propagation direction. A larger deviation of the projection light beam makes it possible to arrange the optical elements of the illumination system in more advantageous and convenient manner, in particular with regard to mounting technology and space issues.
"For example, an axis of symmetry, which is associated with the light source, may then form an angle of less than 45.degree., preferably of less than 20.degree., with respect to a horizontal plane. In such a configuration the bulky light source does not have to be arranged in a basement of the semiconductor facility or very high above the remaining parts of the illumination system, but can be conveniently arranged side by side to the mirrors of the EUV illumination system.
"Producing the astigmatic shape of the mirror elements with the help of the temperature control system is advantageous because it is difficult and costly to produce a large number of astigmatic mirror elements for the array. If a temperature control system is provided anyway for additionally varying the spot shapes in the system pupil surface, it is simpler and cheaper to use spherical or at least rotationally symmetrical mirrors and use the temperature control system also for producing the astigmatic shape for allowing large deviation angles. However, it is to be understood that the mirror elements could have the desired astigmatic effect also initially, i.e. not as a result of a deformation achieved with the help of the temperature control system.
"In this case the illumination system includes a mirror array which is arranged between a light source and a system pupil surface. The array includes a plurality of mirror elements, wherein each mirror element is tiltably mounted with respect to a support structure and is configured to direct light produced by the primary light source towards the system pupil surface. The mirror elements have concave astigmatic mirror surfaces, i.e. the mirror elements have different focal lengths in two orthogonal planes. It is not mandatory to provide also a temperature control device.
"In one embodiment the mirror elements include heat barriers which have a lower coefficient of thermal conduction than the materials which are arranged on either side of the heat barriers. Such heat barriers ensure that the heat or the cold produced at the target areas and the adjacent material remains confined to this portion of the mirror elements over a longer period of time. In other words, the temperature difference between the target areas and the adjacent material on the one hand and the surrounding material on the other hand does not decrease too quickly. This makes it possible to reduce the refresh rate at which the target areas have to be heated or cooled by the temperature control device in order to ensure stable optical properties of the mirror elements."
For additional information on this patent, see: Bach, Florian; Benz, Daniel; Waldis, Severin; Werber, Armin; Warm, Berndt. Illumination System of a Microlithographic Projection Exposure Apparatus Having a Temperature Control Device. U.S. Patent Number 8797507, filed
Keywords for this news article include: Nanotechnology, Medical Devices, Microlithography, Radiation System,
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