Patent Publication Number: US-2003235682-A1

Title: Method and device for controlling thermal distortion in elements of a lithography system

Description:
RELATED APPLICATION  
     [0001] This application is a continuation of Provisional Application Serial No. 60/390,661 filed on Jun. 21, 2002, entitled “METHOD AND DEVICE FOR CONTROLLING THERMAL DISTORTION IN ELEMENTS OF A LITHOGRAPHY STSTEM” which is currently pending. As far as is permitted, the contents of provisional Application Serial No. 60/390,661 is incorporated herein by reference. 
    
    
     
       BACKGROUND  
       [0002] Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly and a wafer stage assembly that positions a semiconductor wafer.  
       [0003] The size of the features within the images transferred from the reticle onto the wafer is extremely small. In order to increase the resolution of the features and decrease the size of the features within the images, there is a need to use an illumination source that generates smaller or shorter wavelengths of light to transfer the images from the reticle onto the wafer. For example, extreme ultraviolet (EUV) radiation, including wavelengths in the 11 to 13 nm range, is being evaluated for use in lithography systems.  
       [0004] For extreme ultraviolet lithography systems, the optical assembly typically includes one or more reflective, optical elements, e.g. mirrors. Each optical element can include multiple layers of two different indices to reflect the extreme ultraviolet radiation. With the layers, these optical elements have a reflectivity of no greater than approximately 0.65. As a result thereof, a portion of the extreme ultraviolet radiation is absorbed by the optical element. The absorbed ultraviolet radiation heats the illuminated regions of the optical element and causes the temperature in the illuminated regions to rise to a greater extent than the temperature in non-illuminated regions of the optical element.  
       [0005] Unfortunately, the increase in temperature in the illuminated regions causes the optical element, including the figure of the optical element, to distort. For extreme ultraviolet radiation lithography, distortions of the figure that are as small as approximately 1 nm RMS can cause image degrading optical aberrations. For example, this can blur the image that is transferred onto the wafer.  
       [0006] Further, in order to achieve relatively high throughputs for the lithography system, the illumination source will be required to generate significant levels of power. This can lead to significant heating of the optical elements and thus significant optical aberrations in the optical elements.  
       [0007] One attempt to reduce thermally induced optical aberrations in an optical element includes directing a cooling fluid through one or more fluid channels in the optical element. However, even with cooling, thermal stresses in the optical element may still cause significant thermal deformation. Therefore, some residual surface deformation may remain from thermal stresses, even with active cooling. In addition, there may be a practical limitation on active cooling performance, because vibrations of the optical element from the cooling fluid must be severely constrained to maintain the optical figure and alignment tolerances.  
       [0008] Another attempt to reduce optical aberrations includes pre-distorting the optical element with one or more of the optical mounts that retain the optical element. In this embodiment, the optical mounts pre-distort the optical element to approximately cancel out the thermal distortion produced by the illumination beam. However, pre-distorting with the optical mounts may only be successful in correcting relatively simple form distortions. For example, pre-distorting the optical element is not expected to be very successful when the heat load is restricted to a small asymmetric location on the optical element, and/or if the heat load varies with time.  
       [0009] In light of the above, there is a need for device and method for reducing and controlling thermal distortion, and reducing optical aberrations in optical elements. Additionally, there is a need for a device and method for providing a controlled and constant temperature distribution in optical elements. Moreover, there is a need for a lithography system capable of manufacturing precision devices such as high density, semiconductor wafers.  
       SUMMARY  
       [0010] The present invention is directed to an optical element control system that reduces thermal distortions in an optical element. The optical element includes an illuminated region and a non-illuminated region. The element control system includes a heat source that primarily heats at least a portion of the non-illuminated region of the optical element. As provided herein, the heat source can direct heat to substantially the entire non-illuminated region of the optical element or at least a portion of the non-illuminated region. The heat is absorbed by the optical element thereby heating the element. In one embodiment, the heat source is a beam of electromagnetic radiation. Alternatively, in another embodiment, the heat source can be a heated surface in proximity to the non-illuminated region, with the heating of the non-illuminated region produced by radiant heat transfer. In one embodiment, the element control system alters the shape of the thermal distortions.  
       [0011] With this design radiation from the heat source heats a portion of the optical element to control the shape of the optical element, and/or provide a controlled and uniform temperature distribution in a region of the optical element. Stated another way, the heat source transfers radiation to selective regions of the optical element to alter the shape of the optical element. In alternative embodiments, the amount of radiation from the heat source collected by the non-illuminated region is at least approximately 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% greater than the amount of radiation collected by the illuminated region. This should simplify and stabilize thermal distortions, and reduce some optical aberrations in the optical element.  
       [0012] The distortion arises from the physical expansion most materials experience when their temperature is increased. The expansion causes the surface of the optical element to be displaced. However, if the temperature rise of the optical element is substantially uniform, the resulting displacement of the surface may be of a relatively simple form, and simple mechanical realignment of the optical element may eliminate the effects of the heating on the optical aberrations. For example, if thermal expansion causes the surface to be displaced uniformly along the optical axis, it can be compensated by a simple adjustment of the position of the optical element, so that the surface is returned to its original position. If the expansion causes a surface of spherical shape to change its radius of curvature slightly, again a simple position adjustment along the optical axis may eliminate the effects of expansion. If the expansion causes the surface to be uniformly tilted relative to the optical axis, it can again be corrected for by a simple compensating tilt to the optical element. The advantage of correcting the thermal distortion to a relatively simple form is maintained, even if the total distortion is actually increased in magnitude by the additional sources of heat.  
       [0013] Aberrations more complicated than those discussed above are here described as higher order. Higher order aberrations include, for example, more complicated shapes such as astigmatism, spherical aberrations, coma, and sealed curvature to all orders. When the higher order aberrations are reduced by the present invention, the aberrations are here described as being simplified.  
       [0014] In certain situations, the temperature distribution of the heated optical element cannot be controlled to be completely uniform. For example, the temperature at and/or near the surface of the optical element is uniform over the illuminated region, but the temperature varies through the thickness of the optical element, much of the above argument still holds. The effect of a temperature gradient through the thickness of the optical element contributes a thermal distortion that is mainly non-local in effect, leading to relatively simple changes in the shape of the surface of the optical element. Such changes may be correctable by the means described above. Thus, higher order aberrations may still be reduced or avoided.  
       [0015] Further, if the temperature distribution in the heated optical element is non-uniform near its surface and in the region including and adjacent to the illuminated region, local deformation of the surface within the illuminated region will in general occur, and the aberrations associated with this distortion are unlikely to be compensated by mechanical adjustment of the optical element. The heat source provided herein can be used to simplify these types of aberrations.  
       [0016] In one embodiment, the shape and/or intensity of the heating radiation from the heat source is controlled so that a temperature of a portion of the non-illuminated region is substantially equal to a temperature of the illuminated region. Moreover, the shape and/or intensity of the heating radiation from the heat source can be varied with time.  
       [0017] In one embodiment, the heating radiation is a beam of light having a wavelength, or wavelengths, that does not influence a photoresist on a wafer. With this design, stray light from the heat source that reaches the wafer will not influence the photoresist on the wafer. However, in one embodiment, stray radiation is limited in order not to heat the wafer and its photoresist appreciably, or other critical parts of the lithography system.  
       [0018] Additionally, the element control system can include an element measurement system that monitors at least a portion of the optical element for thermal distortions. With this design, the shape and/or intensity of the heating radiation from the heat source can be varied according to the measurements take by the element measurement system.  
       [0019] In another embodiment, a portion of the non-illuminated region of the optical element includes an absorbing layer that enhances the absorption of energy from the heat source.  
       [0020] The present invention is also directed to an optical assembly, an exposure apparatus, a device made with the exposure apparatus, a wafer made with the exposure apparatus, a method for controlling thermal distortion of an optical element, a method for making an exposure apparatus, a method for making a device and a method for manufacturing a wafer. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021] The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
     [0022]FIG. 1 is a schematic view of a lithography system having features of the present invention;  
     [0023]FIG. 2A is a front plan view of an optical element illustrating an illuminated region and a non-illuminated region;  
     [0024]FIG. 2B is a front plan view of an optical element illustrating the illuminated region, the non-illuminated region, and a heated region;  
     [0025]FIG. 2C is a plot of the temperature distribution along the azimuthal direction in and near the illuminated region for several embodiments of the non-illuminated region;  
     [0026]FIG. 3A is a side illustration of another optical element, and a portion of an element control system having features of the present invention;  
     [0027]FIG. 3B is a front plan view of the optical element of FIG. 3A illustrating the illuminated region, the non-illuminated region, and the heated region;  
     [0028]FIG. 4A is a front plan view of another optical element, illustrating the illuminated region, the non-illuminated region, and the heated region;  
     [0029]FIG. 4B is a plot of the temperature distribution along the radial direction in and near the illuminated region for several embodiments of the non-illuminated region;  
     [0030]FIG. 4C is a front plan view of yet another optical element illustrating the illuminated region, the non-illuminated region, and the heated region;  
     [0031]FIG. 5A is a side illustration of another optical element and a portion of an element control system having features of the present invention;  
     [0032]FIG. 5B is a front plan view of the optical element of FIG. 5A illustrating the illuminated region, the non-illuminated region, and the heated region;  
     [0033]FIGS. 6A and 6B illustrate alternate embodiments of optical elements, including the illuminated region, the non-illuminated region, and the heated region; and  
     [0034]FIG. 7 is a front plan view of an optical element illustrating the illuminated region, the non-illuminated region, and an absorbing layer. 
    
    
     DESCRIPTION  
     [0035]FIG. 1 is a schematic view that illustrates a precision assembly, namely a lithography system  10 . The lithography system  10  can be used to transfer a pattern (not shown) of an integrated circuit from a reticle  12  onto a device, such as a semiconductor wafer  14 . In FIG. 1, the lithography system  10  includes an illumination system  16  (irradiation apparatus), a reticle stage assembly  18  (illustrated as a box), a wafer stage assembly  20  (illustrated as a box), a control system  22  (illustrated as a box), and an optical assembly  24 . The design of the components of the lithography system  10  including the components of the optical assembly  24  can be varied to suit the design requirements of the lithography system  10 .  
     [0036] The optical assembly  24  includes one or more optical elements  28 , one or more element mounts  30 , and an element control system  32 . As an overview, the element control system  32  reduces thermal distortions and/or optical aberrations in one or more of the optical elements  28 . For example, the element control system  32  can provide a controlled and constant temperature distribution in a region of one or more of the optical elements  28 . This reduces and stabilizes thermal distortions and reduces optical aberrations. As a result thereof, the lithography system  10  is capable of manufacturing precision devices such as high density, semiconductor wafers. Moreover, this can also simplify additional mechanical adjustments to the surface of the one or more optical elements  28 .  
     [0037] There are a number of different types of lithographic systems  10 . For example, the lithography system  10  can be used as a scanning type photolithography system that transfers the pattern from the reticle  12  onto the wafer  14  with the reticle  12  and the wafer  14  moving synchronously. In a scanning type lithographic device, the reticle  12  is moved perpendicular to an optical axis of the optical assembly  24  by the reticle stage assembly  18  and the wafer  14  is moved perpendicular to an optical axis of the optical assembly  24  by the wafer stage assembly  20 . Scanning of the reticle  12  and the wafer  14  occurs while the reticle  12  and the wafer  14  are moving synchronously.  
     [0038] Alternatively, the lithography system  10  can be a step-and-repeat type photolithography system that exposes the reticle  12  while the reticle  12  and the wafer  14  are stationary. In the step and repeat process, the wafer  14  is in a constant position relative to the reticle  12  and the optical assembly  24  during the exposure of an individual field. Subsequently, between consecutive exposure steps, the wafer  14  is consecutively moved with the wafer stage assembly  20  perpendicular to the optical axis of the optical assembly  24  so that the next field of the wafer  14  is brought into position relative to the optical assembly  24  and the reticle  12  for exposure. Following this process, the images on the reticle  12  are sequentially exposed onto the fields of the wafer  14  so that the next field of the wafer  14  is brought into position relative to the optical assembly  24  and the reticle  12 .  
     [0039] The reticle  12  can be a reflective type as illustrated in FIG. 1 or a transmissive type. However, in the following description the reticle  12  is reflective. The pattern in the reticle that is to be transferred to the wafer is defined by the local regions of the reticle where the reflectivity at the illumination radiation wavelengths of the reticle surface has been reduced to a very small value, thereby providing maximum image contrast at the wafer. The wafer  14  includes a substrate that is covered with a photoresist. The photoresist can be photosensitive to some wavelengths of radiation and not sensitive to other wavelengths of radiation. For example, the photoresist can be sensitive to extreme electromagnetic ultraviolet radiation including wavelengths in the 10 to 15 nm range and not sensitive to radiation having wavelengths that are greater than approximately 50 nm, 75 nm, 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm or 500 nm.  
     [0040] The illumination system  16  includes an illumination source  34  and an illumination optical assembly  36 . The illumination source  34  and the illumination optical assembly  36  create and guide the illumination beam  40  from the illumination source  34  to the reticle  12 . The illumination beam  40  selectively illuminates a portion of the reticle  12 . Radiation reflected from the reticle is collected by the optical assembly  24  and focused on the semiconductor wafer  14  to expose the photosensitive resist. The fraction of radiation reflected depends on both the intrinsic reflectivity of the reticle surface and the fraction of the surface occupied by the pattern.  
     [0041] In one embodiment, the illumination source  34  generates an illumination beam  40  that provides extreme ultraviolet (EUV) electromagnetic radiation, including illumination wavelengths of between approximately 10-15 nm and typically illumination wavelengths in the 11 to 13 nm range, also referred to as the soft X-ray region. In this design, the illumination source  34  can be a synchrotron radiation source or laser plasma source. Alternatively, for example, the illumination source  34  can be a gas discharge source.  
     [0042] In one embodiment, the illumination source  34  is pulsed in time, with repetition rates of between approximately 0.1-10 kHz. Alternatively, in another embodiment, the illumination source  34  is continuous. Additionally, the total radiation from the illumination source  34  incident on the optical elements  28  will vary with the average reflectance of the illuminated region of the reticle  12 .  
     [0043] The reticle stage assembly  18  holds and positions the reticle  12  relative to the optical assembly  24  and the wafer  14 . Similarly, the wafer stage assembly  20  holds and positions the wafer  14  with respect to the projected image of the illuminated portion of the reticle  12 . Each stage assembly  18 ,  20  can include one or more actuators or motors.  
     [0044] The optical assembly  24  collects and focuses the illumination beam  40  of radiation that is reflected from the reticle  12  to the wafer  14 . Depending upon the design of the lithography system  10 , the optical assembly  24  can magnify or reduce the size of the image at the wafer of the illuminated region of the reticle  12 . Alternately, the optical assembly  24  could also be a lx magnification system.  
     [0045] As provided above, the optical assembly  24  includes one or more optical elements  28 , one or more element mounts  30 , and the element control system  32 . The number of optical elements  28  utilized and the design of each optical element  28  can be varied to suit the requirements of the optical assembly  28 . In the embodiment illustrated in FIG. 1, the optical assembly  24  is an all reflective system that includes four optical elements  28 , namely a convex, first optical element  28 A, a concave, second optical element  28 B, a convex, third optical element  28 C, and a concave, fourth optical element  28 D. Further, each optical element  28  is a reflecting mirror. Alternatively, the optical assembly  24  can include more than four or less than four optical elements  28 .  
     [0046] In FIG. 1, each optical element  28  includes a front surface  38 A and an opposed rear surface  38 B. The front surface  38 A defines a figure that is curved so that the light rays that strike the front surface  38 A converge or diverge. Each optical element  28  includes an element body that is coated with multiple thin layers of material that collectively create a fairly reflective surface at the wavelength of the illumination beam  40 . The element body can be made of a glass or ceramic material having a relatively low coefficient of thermal expansion. The type of material utilized for the layers of reflective material will depend upon the wavelength of the radiation generated by the illumination source  34 . For example, suitable layers include molybdenum/silicon for wavelengths of approximately 13 nm and molybdenum/beryllium for wavelengths of approximately 11 nm. However, other materials may be utilized.  
     [0047] At the short wavelengths of EUV radiation, materials are currently not available for the reflective thin layers which will provide very high reflectivities typical of optical reflective coatings at visible and near visible wavelengths. Achievable reflectivities may not exceed much more than r=0.65, as compared to greater than 0.99 at longer wavelengths. As a result, significant amounts of optical power are absorbed in the surfaces of the optical elements  28 . For a mirror reflectivity of r, a fraction r of the optical power incident on the optical element  28  is reflected, and a fraction (1−r) is absorbed. For the case of an optical assembly  24  that includes four optical element  28 , as illustrated in FIG. 1, if the optical power emerging from the reticle  12  and collected by the optical element  28  is P R , and the optical power incident on the wafer  14  is P W , then it follows that P W =r 4 P R  (assuming the reflectivities of the optical elements  28  to be the same, and no apertures between the optical elements  28 A- 28 D intercept part of the illumination beam  40 ). Furthermore, the power absorbed in optical elements  28 A- 28 D is given by the expressions:  
       P   28A =(1 −r ) P   R    
       P   28B   =r (1 −r ) P   R    
       P   28C   =r   2 (1 −r ) P   R    
       P   28D   =r   3 (1 −r ) P   R .  
     [0048] For example, if the power incident on the wafer  14  is 0.1 W, the power emerging from the reticle  12  and collected by the optical element  28  must be equal to 0.1/(0.65) 4 =0.56 W, and the power absorbed in the mirrors is then P 28A =0.196 W, P 28B =0.127 W, P 28C =0.083 W, and P 28D =0.054 W. Thus significant amounts of power may be absorbed by the optical elements  28 , with the largest amounts being in the optical elements  28  closest to the reticle.  
     [0049] Each optical element  28  can include one or more circulating channels (not shown) that extend through the element body for cooling the optical elements  28 . The circulating channels can be positioned in the element body so that a circulating fluid can be circulated relatively evenly throughout the optical element  28 .  
     [0050] In FIG. 1, the illumination system  16  directs illumination beam  40 A at the reticle  12 . An illumination beam  40 B is reflected off the reticle  12 . An illumination beam  40 C is reflected off the first optical element  28 A. An illumination beam  40 D is reflected off the second optical element  28 B. An illumination beam  40 E is reflected off the third optical element  28 C. An illumination beam  40 F is reflected off the fourth optical element  28 D.  
     [0051] It should be noted that for each optical element  28 , the illumination beam  40  may not be directed to the entire front surface  38 A. For example, during an exposure procedure, the illumination beam  40 C is reflected off only the bottom portion of the second optical element  28 B. At this time, the second optical element  28 B includes an illuminated region  42  that reflects the illumination beam  40 C and a non-illuminated region  44 . As used herein, the term “illuminated region”  42  shall mean and represent the area on the front surface  38 A that is illuminated by and collects light from the illumination beam  40 C. Further, the term “non-illuminated region”  44  shall mean and represent the area on the front surface  38 A that does not collect and is not illuminated by the illumination beam  40 C.  
     [0052] The size and location of the illuminated region  42  can vary with time according to the illumination beam  40 B that is being reflected off the reticle  12 . Stated another way, the total radiation incident on the optical element  28  will vary with the average reflectance of the illuminated reticle  12 . In addition, the illuminated region  42  can be somewhat arc shaped, annular shaped, or semicircular shaped. As provided herein, one or more of the first optical element  28 A, the third optical element  28 C and/or the fourth optical element  28 D can also include an illuminated region and a non-illuminated region.  
     [0053] Since the illuminated region  42  may represent only a fraction of the surface of the optical element  28 , and since significant amounts of power may be absorbed by the optical element  28 , non-uniform heating of the surface of the optical element  28  will occur, resulting typically in a non-uniform temperature distribution, and thermal distortions which are non-uniform. Typically the non-uniformities will be largest near the boundary between the illuminated region  42  and non-illuminated region  44  of the optical element  28 . Moreover, the illuminated region  42  may not be illuminated uniformly by the illumination radiation, because of the optical design, thereby further exacerbating the situation.  
     [0054] The optical mounts  30  retain the optical elements  28 . The optical mounts  30  can hold the optical elements  28  in a quasi-kinematic mode, such as described in U.S. Pat. No. 6,239,924. As far as is permitted, the contents of U.S. Pat. No. 6,239,924 are incorporated herein by reference. As provided herein, the optical mounts  30  can be used in conjunction with the element control system  32  to reduce optical aberrations. More specifically, the element control system  32  can be used to simplify the form of the thermal distortion of the optical element  28 . If residual distortions in the optical element  28  are of a relatively simple form, it may be possible to adjust the position and orientation of the optical elements  28  with the element mounts  30  to cancel out the thermal distortion produced by the illumination beam  40 .  
     [0055] The element control system  32  can provide a controlled and constant temperature distribution, reducing the complexity of thermal distortions and thereby reduce, alter, or simplify optical aberrations in one or more of the optical elements  28 . For example, in FIG. 1, the element control system  32  can be used to reduce thermal distortions in the first optical element  28 A, the second optical element  28 B, the third optical element  28 C, and/or the fourth optical element  28 D. Alternately, the element control system  32  could be used to reduce thermal distortions in more than four or less than four optical elements  28 . For example, the third or the fourth optical elements  28 C and  28 D may not require correction of thermal deformations, because the optical power absorbed by them is relatively small.  
     [0056] In FIG. 1, the element control system  32  includes a circulating system  46  (illustrated as a box), and a thermal adjuster  48 . The circulating system  46  directs a circulation fluid  49  through circulating channels in one or more of the optical elements  28  to cool one or more of the optical elements  28 . The design of the circulating system  46  can be varied to suit the cooling requirements of the optical elements  28 . For example, the circulating system  46  can direct the circulation fluid  49  to each of the optical elements  28  and can include a reservoir for receiving the circulation fluid  49 , a heat exchanger, i.e. a chiller unit, for cooling the circulation fluid  49 , and a fluid pump. The temperature, flow rate, and type of the circulation fluid  49  are selected and adjusted to precisely control the temperature of the one or more of the optical elements  28 . Alternatively, a heat pipe could be used.  
     [0057] The thermal adjuster  48  selectively heats portions of one or more of the optical elements  28 . In one embodiment, as illustrated in FIG. 1, the thermal adjuster  48  heats the front surface  38 A of one or more of the optical elements  28 . As an example, the thermal adjuster  48  can provide a controlled and constant temperature distribution in a portion of one or more of the optical elements  28 .  
     [0058] In FIG. 1, the thermal adjuster  48  may include a heat source  50  for each optical element  28 . Additional heat sources  50  can be used so that the temperature distribution in the optical elements  28  near the front surface  38 A can be fine tuned. In this embodiment, each heat source  50  projects a beam of radiation  52  at one of the optical elements  28 , a portion of which is absorbed by the optical element  28 . The radiation  52  selectively heats portions of one of the optical elements  28 . The area of the optical element  28  directly heated by the radiation  52  is referred to herein as the heated region  54 . For example, the radiation  52  can be used to heat a portion or all of the non-illuminated region  44 , and/or the radiation  52  can be used to heat a portion or all of the illuminated region  42 .  
     [0059] The shape and intensity of the radiation  52  can be varied. For example, the heat source  50  could be pulsed at the same rate as the illumination source  34 , to maintain instantaneous temperature uniformity. Alternatively, the heat source  50  output could be constant, and an auxiliary heat source could be directed at the illuminated region  42  and pulsed on at times between the pulses of the illumination source  34 . Thus, the temperature distribution on the optical element  28  would have no time dependent component, once the optical element  28  had reached its temperature equilibrium after turn on. In addition, the auxiliary heat source could be directed at the illuminated region  42  and turned on at times when the illumination source  34  is turned off, for example when a wafer  14  or reticle  12  is being exchanged.  
     [0060] As provided herein, the intensity from the heat source  50  can be adjusted so that the temperature in the heated region  54  is approximately equal to the temperature in the illuminated region  42 . Alternatively, the heat source  50  can be controlled so that the temperature in the heated region  54  is greater than or less than the temperature in the illuminated region  42 .  
     [0061] Examples of suitable heat sources  50  include light from a mercury arc lamp or other incandescent light source, possibly including a filter to eliminate spectral components of the radiation that could expose the photoresist, or a laser. In one embodiment, the spectrum of the radiation  52  from the heat source  50  does not include spectral components that could expose the photoresist on the wafer  14 , should scattered light reach the wafer  14 . For example, heat source  50  can be a laser or an incoherent source of much longer wavelength than 15 nm. For example, the heat source  50  could generate radiation  52  having a radiation wavelength that is greater than approximately 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. In one embodiment, the wavelength of the radiation is greater than the illumination wavelength of the illumination beam  40 . For example, the radiation wavelength can be at least approximately 2, 3, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 times greater than the illumination wavelength. Additionally, the radiation spectrum of the radiation  52  can be chosen such that absorbance in the optical element  28  is maximized.  
     [0062] The radiation  52  from the heat source  50  may be projected onto a predetermined area of the heated region  54 , or it may be scanned over the area of the heated region  54 .  
     [0063] Alternatively, the heat source  50  can be a surface in close proximity to and overlying the non-illuminated region  44 , which surface is maintained at an elevated temperature. Radiant heat transfer between the source  50  and the non-illuminated region  44  will raise the temperature of the non-illuminated region  44 . Proper adjustment of the temperature of the heat source  50  will allow adjustment of the temperature of the non-illuminated region  44  to the desired value. The radiation spectrum of the heat source  50  would then be a broad, approximately black body spectrum. At temperatures near room temperature, the spectrum would peak in the infrared at a wavelength of approximately 10 micron. The fraction of this spectrum which overlaps the range of wavelengths where the photosensitive resist is sensitive is negligible.  
     [0064]FIG. 2A is a view of a front surface  238 A of one optical element  228  that illustrates an example of an illuminated region  242  (in diagonal line shading) and an example of a non-illuminated region  244  (without shading). In this embodiment the optical element  228  possesses axial symmetry. The illuminated region  242  represents the shape of the area on the front surface  238 A that is illuminated by the illumination beam  40  (illuminated in FIG. 1) from the illumination system  16  (illustrated in FIG. 1). The non-illuminated region  244  represents the area on the front surface  238 A that is not illuminated by the illumination beam  40  from the illumination system  16 .  
     [0065]FIG. 2B is a view of the front surface  238 A of the optical element  228  from FIG. 2A with the heat source  50  (illustrated in FIG. 1) heating a portion of the optical element  228 . More specifically, FIG. 2B illustrates the illuminated region  242  (in diagonal line shading), a heated region  254  (in cross-shading), and the non-illuminated region  244  (no shading and cross-shading). The heated region  254  represents the area on the front surface  238 A that is heated by the radiation  52  (illustrated in FIG. 1) from the heat source  50 .  
     [0066] In this embodiment, the illuminated region  242  is somewhat arc shaped and the heated region  254  is also somewhat arc shaped. With this design, the illumination region  242  and the heated region  254  combine to form an annular shaped area of elevated temperature that is axially symmetric. Stated another way, FIG. 2B illustrates that the temperature distribution in the illuminated region  242  is made axially symmetric by heating the heated region  254 . Because the azimuthal temperature distribution is now substantially uniform, the azimuthal components of any thermal distortion in the illuminated region  242  should be eliminated, and therefore optical aberrations associated with azimuthal distortions should be eliminated as well. Thus, some higher order geometric aberrations might be reduced to a tolerable level, and some lower order aberrations dealt with by adjustment of the optical element  228  with the element mounts  30  (illustrated in FIG. 1).  
     [0067] In one embodiment, the intensity of the radiation  52  from the heat source  50  is adjusted so that the absorbed intensity from the heat source  50  is approximately equal to the absorbed intensity from the illumination source  34 . Stated another way, the heat source  50  is controlled so that the temperature in the heated region  254  is approximately equal to the temperature in the illuminated region  242 . In this embodiment, the average intensity of the radiation from the illumination source  34  is relatively constant and the average intensity of the radiation  52  from the heat source  50  is relatively constant.  
     [0068]FIG. 2C illustrates the application of this embodiment with some modeling results. The illuminated region  242  of the optical element  228  (illustrated in FIG. 2B) is exposed uniformly to an illumination source  34  (illustrated in FIG. 1), and the temperature in the optical element  228  is calculated using the heat equation. The illuminated region  242  is an annular segment of 120 degrees extent. Half of the segment, extending between 30 and 90 degrees, is illustrates in FIG. 2C. Despite the uniform illumination, FIG. 2C illustrates that the temperature at the surface of the optical element  228  is far from uniform in the region near the azimuthal edge of the illuminated region  242 , between the angles of 30 and approximately 50 degrees. Within this region, local distortion may be expected to be non-uniform, leading to the presence of higher order aberrations with an azimuthal dependence. Additional heating from heat sources  50 , adjusted to provide the same absorbed power density as the illumination source  34 , eliminates this temperature variation and should substantially reduce or eliminate the distortion non-uniformity. A heated region  254  covering an angular range of 30 degrees on each side of the illuminated region  242  is adequate to completely remove the temperature variations in the illuminated region  242 . However, if the optical element  228  possesses axial symmetry, extending the heated region  254  to the full azimuthal range may simplify the non-local distortions, thereby reducing lower order aberrations and making mechanical adjustment of the optical element  228  easier.  
     [0069] In this embodiment, the heated region  254  is entirely part of the non-illuminated region  244 . However, the heated region  254  can be part of the illuminated region  242 , if necessary, to minimize thermal distortion. For example, in the embodiment shown in FIG. 2A, the illuminated region  242  is assumed to be illuminated uniformly by the illumination beam  40 . For that reason, the heated region  254  is contiguous with the azimuthal boundaries of the illuminated region  242 , and it does not extend into the illuminated region  242 . However, if the intensity of the illumination beam  40  varies within the illuminated region  242 , the heated region  254  could be expanded to include the regions of the illuminated region  242  which are non-uniformly illuminated, so that the absorbed power density in the optical element  228  from the sum of the incident radiations is approximately uniform over the illuminated region  242 .  
     [0070]FIG. 3A is a side view of an optical element  328 , an illumination beam  340  and another embodiment of an element control system  332 . In this embodiment, the element control system  332  includes a first heat source  350 A and a second heat source  350 B. This embodiment can be used if the intensity of the illumination beam  340  collected by the optical element  328  is spatially dependant. In FIG. 3A, the first heat source  350 A directs radiation  352 A at a non-illuminated region  344  and the second heat source  350 B directs radiation  352 B at an illuminated region  342 . FIG. 3A also illustrates that the illumination beam  340  is directed at the illuminated region  342 .  
     [0071] In this embodiment, the second heat source  350 A is independent from the first heat source  350 B. The second heat source  350 B is time dependant and can be utilized for example if the average reflectance from the reticle varies as the illumination beam  340  scans over it, causing variations in the intensity of the illumination beam  40 B. As an example, the first heat source  350 A provides an absorbed constant intensity of Ib on the optical element  328 , the illumination beam  340  provides a time dependant, average absorbed intensity I(t), and the second heat source  350 B provides a time dependant, average absorbed intensity Ic(t). For this example, absorbed intensity from the heat sources  350 A,  350 B is adjusted to instantaneously satisfy the relation Ib=I(t)+Ic(t). Consequently the temperature distribution in the illuminated region  342  should be relatively constant in time, and proper adjustment of the heat sources  350 A and  350 B can eliminate any azimuthal dependence in distortion within the illuminated region  342 .  
     [0072] In this embodiment, the first and second heat sources  350 A,  350 B can be somewhat similar to the heat source  50  described above. FIG. 3A illustrates that the first heat source  350 A can be a proximity heater, which heats by radiant heat transfer, mounted above the optical element  328 , while the second heat source  350 B generates a beam of electromagnetic radiation and is focused on the illumination region  342  from a source off-axis, but other configurations are possible.  
     [0073]FIG. 3B is a view of the front surface  338 A of the optical element  328  that illustrates the illuminated region  342  (in diagonal line shading), the non-illuminated region  344  (no shading and cross-shading), and the heated region  354  (in cross-shading). Again the element  328  is axially symmetric. The heated region  354  represents the area that is heated by the radiation  352 A (illustrated in FIG. 3A). In this embodiment, the illuminated region  342  represents the area that is illuminated by the illumination beam  340  (illustrated in FIG. 3A) and the radiation  352 B (illustrated in FIG. 3A). With this design, the irradiation pattern from the illumination beam  340  and the temperature distribution is made axially symmetric by irradiating the optical element  328  with radiation  352 A,  352 B. Therefore, distortions with an azimuthal dependence should be eliminated, and optical aberrations associated with azimuthal distortions should be eliminated as well. Thus, some higher order geometric aberrations might be reduced to a tolerable level, and some lower order aberrations dealt with by realignment of the optical element  228  with the element mounts  30  (illustrated in FIG. 1).  
     [0074]FIG. 4A is a view of a front surface  438 A of another embodiment of an axially symmetric optical element  428  that illustrates the illuminated region  442  (in diagonal line shading), the non-illuminated region  444  (without shading and cross-shading), and the heated region  454  (in cross-shading). In this embodiment, the illuminated region  442  is somewhat arc shaped and the heated region  454  is also somewhat annular shaped and includes an arc shaped opening. With this design, the heated region  454  provides axial symmetry, and the heated region  454  encircles and surrounds the illuminated region  442 . As before, the axial symmetry of the temperature distribution should eliminate optical aberrations possessing an azimuthal dependence. Providing heated regions  454  at the inner and outer radii of the illuminated region  442  should reduce or eliminate the radial dependence of the thermal distortions within the illuminated region  442 . This should reduce or eliminate a class of optical aberrations that would be associated with the radially dependent distortion.  
     [0075] In this embodiment, the illuminated region  442  is somewhat arc shaped and the non-illuminated region  444  is somewhat circular shaped without an arc shaped area that represents the illuminated region  442 . However, the illuminated region  442  and the non-illuminated region  444  can have other shapes. For example, the illuminated region  442  can be a semicircular shape or a rectangular shape. However, in order to preserve any advantages of heating a total area which possesses azimuthal symmetry, the non-illuminated region  444  should be designed, so that the total illuminated and heated regions possess azimuthal symmetry.  
     [0076]FIG. 4B illustrates the application of heat sources to the radial edges of the illuminated region, in order to reduce radial temperature variations in the illuminated region. Again, the heat equation was used to explore the effects of adding heat sources. Although the illumination beam is assumed to have uniform intensity, the resulting temperature distribution  465  is significantly non-uniform within the illuminated region  460 , which in this model lies within the radii 0.09 m and 0.12 m. The peak temperature within the illuminated region  460  is approximately 0.87 degree, and the variation is 0.35 degree. By adding uniform heat sources  470  and  472  of radial extent 0.01 m and adjacent to the radial edges of the illuminated region  460 , the variation of the temperature  475  within the illuminated region  460  can be reduced to 0.017 degree, an improvement of a factor of 20. In this case the absorbed intensity of the added heat sources  470 ,  472  is different from that of the illumination radiation: the absorbed intensity of heat source  470  is 50% higher and that of heat source  472  is 40% higher. The temperature uniformity can be improved further if desired. For example, heat source  480 , lying between radii 0.12 m and 0.14 m and with absorbed intensity 15% higher than the illumination radiation, and heat source  482 , lying between radii 0.07 m and 0.09 m and with absorbed intensity 20% higher than the illumination radiation, create a temperature distribution  485  with a variation of only 0.01 degree within the illuminated region  460 .  
     [0077] Note that this improvement in temperature uniformity results in an increase in the temperature within the illuminated region  460  from 0.87 to over 1.1 degree. Therefore, the absolute value of the thermal distortion is actually increased. However, since the local shape of the distortion is expected to be of a uniform shape now, the optical aberrations associated with the distortion should be simplified and more easily correctable.  
     [0078]FIG. 4C is a view of the front surface  438 A of the optical element  428  that illustrates the illuminated region  442  (in diagonal line shading), the non-illuminated region  444  (in cross-shading), and the heated region  454  (in cross-shading). In this embodiment, the illuminated region  442  is somewhat arc shaped and the heated region  454  is somewhat circular shaped and includes an arc shaped opening. With this design, the heated region  454  provides axial symmetry, and the heated region  454  encircles and surrounds the illuminated region  442 . As before, providing heated regions  454  at the inner and outer radii of the illuminated region  442  should reduce or eliminate the radial dependence of the thermal distortions within the illuminated region  442 . This should reduce or eliminate a class of optical aberrations which would be associated with the radially dependent distortion.  
     [0079] It is believed that the heating schemes illustrated in FIGS. 4A and 4C will reduce the radial temperature distribution in the optical element  428 , and reduce some higher order geometric aberrations. Adding a radial dependence to the heat sources may reduce the radial temperature distribution in the illuminated region  442  further.  
     [0080] With the addition of one or more heat sources, thermal distortion of the optical element  428  may be reduced in complexity such that higher order optical aberrations are reduced to a tolerable level and relatively simple mechanical distortion or realignment of the optical element can eliminate the remaining lower order aberrations. The deliberate mechanical distortion could be done either by means of the mechanical element mount or perhaps by the addition of more heat sources at other locations.  
     [0081]FIG. 5A illustrates another embodiment of an optical element  528 , a plurality of element mounts  530 , an illumination beam  540 , and another embodiment of an element control system  532 . In this embodiment, the element control system  532  includes a first heat source  550 A generating radiation  552 A, a second heat source  550 B generating radiation  552 B and an element measurement system  560 . The illumination beam  540 , the first heat source  550 A and the second heat source  550 B are somewhat similar to the corresponding components described above and illustrated in FIG. 3A.  
     [0082] The element measurement system  560  monitors the optical element  528  for thermal distortions. For example, the measurement system  560  could monitor the temperature of the optical element  528  at one or more spaced apart locations, and infer the resulting distortion from a model and/or previously calibrations, or monitor the shape of the optical element  528  directly at one or more locations. For example, the element measurement system  560  can monitor temperature or shape of the front surface  538 A or the back surface  538 B of the optical element  528 . In FIG. 5A, the element measurement system  560  monitors a portion of the front surface  538 A of the optical element  528 . The element measurement system  560  can utilize laser interferometers, or other optical or non-optical sensors to monitor the optical element shape, and/or it can use thermistors, bolometers, or other temperature sensing means to monitor the element temperature.  
     [0083] With the information from the element measurement system  560 , the intensity and shape of the radiation  552 A,  552 B can be adjusted. Stated another way, the element control system  532  can make real time changes to the shape and intensity of the radiation  552 A,  552 B based upon the information from the element measurement system  560  to reduce and/or simplify distortion caused by the illumination beam  540 .  
     [0084]FIG. 5B is a view of the front surface  538 A of the optical element  528  that illustrates the element mounts  530 , the illuminated region  542  (in diagonal line shading), the non-illuminated region  544  (without shading and with cross-shading), and the heated region  554  (in triangular cross-shading) and the monitored area  562  (square cross-shading) that is monitored by the element measurement system. In this embodiment, the element measurement system includes a plurality of sensors positioned in the monitored area  562  that monitor a region of the front surface  538 A, which is identical in shape to the illuminated region  542  and located relative to the attachment points of the element mounts  530  identically to that of the illuminated region  542 . Further, the sensors are located in the non-illuminated region  544  and away from the illuminated region  542 . Thus, the sensors will not adversely influence the illumination beam and will not interfere with normal lithography operation.  
     [0085] In this embodiment, the axial symmetry of the mirror and the temperature distribution can allow the sensors to be located away from the illuminated region  544  but at radial and azimuthal locations which are as environmentally comparable to the illuminated region  544  as possible. In this example, the optical element  528  is mounted at four symmetrical locations with four identical element mounts  530 . Other mounting schemes are possible, such as a three point kinematic mounting scheme. The monitored area  562  of the optical measurement system is substantially identical in shape and radial location to the illumination region  542 . In addition, its position relative to the element mounts  530  is the same as the illumination region  542 . Therefore, identical adjustments to the element mounts  530  adjacent to the illumination region  542  and the monitored area  562  are likely to produce similar changes to the two surface areas. After initial calibration, changes to the surface in the monitored area  562  may track the changes in the illumination region  542  to within tolerable accuracy.  
     [0086] Alternatively, the sensors could be positioned to monitor the illuminated region  542  directly, provided they do not interfere with the illumination beam  40  incident on the optical elements  528 .  
     [0087] In some optical designs, one or more of the optical elements may not possess axial symmetry, either to reduce the size of the optics or to permit passage of a reflected illumination beam in a folded optical configuration. For example, FIG. 6A illustrates a view of a front surface  638 A of an optical element  628  that illustrates an illuminated region  642  (in diagonal line shading), a non-illuminated region  644  (in cross-shading), and a heated region  654  (in cross-shading). In this embodiment, the optical element  628  is truncated to a sector, the illuminated region  642  is arc shaped and the heated region  654  encompasses the rest of the front surface  638 A.  
     [0088] Further, FIG. 6B illustrates a view of the front surface  638 A of the optical element  628  that illustrates the illuminated region  642  (in diagonal line shading), the non-illuminated region  644  (in cross-shading), and the heated region  654  (no shading and in cross-shading). In this embodiment, the optical element  642  is a sector of an annulus, the illuminated region  642  is arc shaped and the heated region  654  encompasses the rest of the front surface  638 A.  
     [0089] In these cases, axial symmetry of the optical element and the heat from the heat source (not shown) plus the illumination beam  40  cannot be invoked to ensure the elimination of azimuthal thermal distortion within the illuminated region  642 . However, it is still possible to apply additional heat from the heat source to the non-illuminated region  644  around the illuminated region  642  and within the illumination region  642  as described above, so that the temperature distribution within the illumination region  642  is uniform, and any residual thermal distortions in the illuminated region  642  of the optical element  628  are constant in time and of relatively simple form. Choosing the appropriate additional heat sources is more complicated now and is likely to require finite element modeling and calibration with real optical elements  628 .  
     [0090]FIG. 7 illustrates a view of a front surface  738 A of an optical element  728  including an illuminated region  742  (in diagonal shading) and a non-illuminated region  744  (in cross-shading). In this embodiment, the entire non-illuminated region  744  is coated with an absorbing layer  770 . Alternatively, only a portion of the non-illuminated region  744  is coated with the absorbing layer  770 .  
     [0091] The absorbing layer  770  enhances the absorption of radiation from the heat source (not shown). For example, the non-illuminated region  744  could be coated with a black layer that absorbs more radiation than the illuminated region  742 . As a result thereof, the intensity of the radiation from the heat source can be reduced while still maintaining a uniform temperature.  
     [0092] While the method and system as shown and disclosed herein is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.