Abstract:
An inspection tool or inspection system can be utilized to determine whether the appropriate pattern is on a reticle. The reticle can be associated with EUV lithographic tools. The system utilizes an at least two wavelengths of light. The light is directed to the reticle at the at least two wavelengths of light.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an inspection system and an inspection method. More particularly, the present invention relates to an inspection system for and a method of determining errors introduced by a photolithographic camera or stepper unit. 
     BACKGROUND OF THE INVENTION 
     The semiconductor or IC industry desires to manufacture integrated circuits (ICs) with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features. 
     The ability to reduce the size of structures, such as, gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance. In conventional commercial fabrication processes, lithographic systems, such as, photolithographic cameras or stepper units, expose a photoresist material to a pattern of radiation. The photoresist material is developed in accordance with the pattern of radiation to form a pattern of the photoresist material on a wafer. The wafer is processed in accordance with the pattern of photoresist material. 
     A conventional lithographic system or photolithographic machine can be a projection printing machine using refractive optics in a step-and-repeat projection method. 
     Conventional lithographic systems generally include a light source configured to provide radiation or light at one or more wavelengths. For example, the light source may include an excimer laser producing radiation at a wavelength of 248 nm, 193 nm, and/or 157 nm. The excimer laser can use a KrF source, a ArF source, a F 2  source, etc. The lithographic systems can further include a first lens assembly, a Chromium (Cr) mask, and a second lens assembly. The radiation is provided from the light source through the first lens assembly, through the mask, through the second lens assembly to a semiconductor wafer having a layer of photoresist material. 
     The first lens assembly can be a condenser lens, and the second assembly can be an objective lens. The radiation can be light, such as ultraviolet light, vacuum ultraviolet (VUV) light, and deep ultraviolet light. In alternative systems, the radiation can be x-ray radiation, e-beam radiation, extreme ultraviolet (EUV) light, etc. 
     As described above, conventional lithographic systems can utilize multiple optical elements to focus and direct light to the semiconductor wafer. Generally, the multiple optical elements can be considered as a single equivalent lens. The pupil of the lithographic system refers to the equivalent lens. The size of the pupil is the diameter of the equivalent lens and the location of the pupil is the location of the plane of the equivalent lens. The pupil is utilized to mathematically model image formation by the optical elements of the lithographic system. 
     Conventional lithographic systems include lens assemblies which are susceptible to lens aberrations or errors. These errors result in errors in the wavefront that is used by the lithographic stepper unit to produce the image on the wafer. As light passes through the objective lens assembly, an imperfection can locally increase or decrease the finite optical path. These imperfections can result in placement errors or critical dimension (CD) errors in the lithographic pattern. These errors are particularly problematic as sizes of lithographic features become smaller. 
     Accordingly, the pupil of the conventional lithographic system is often tested to determine at which locations errors are introduced into the pupil plane. Heretofore, the pupil of the conventional lithographic systems are probed or tested before installation (e.g., off-line) of the lithographic system by a laser interferometer. The use of a laser interferometer is not practicable after the lithographic system is installed. Other conventional techniques probe particular aberrations and require overlay measurement tools. 
     Thus, there is a need for a highly accurate inspection system that can be utilized to detect defects and patterns on a pupil. Further, there is a need for a semiconductor fabrication inspection tool for measuring and locating wavefront errors associated with a lithographic system. Even further still, there is a need for a process or method of detecting pupil errors or lens aberrations in situ (e.g., in-line). Even further still, there is a need for an inspection tool and inspection method that is capable of reliably detecting errors on the entire pupil. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment relates to a method of inspecting a lens assembly for a lithographic stepper. The method includes providing radiation at a first coherence through a mask or reticle to a photoresist material, and providing radiation at a second coherence through the mask or reticle to the photoresist material. The method can also include developing the photoresist material, and observing the photoresist material. 
     Another exemplary embodiment relates to a method of inspecting a pupil associated with manufacture of an integrated circuit. The method includes providing a pattern of low coherence radiation to a photoresist material, providing a pattern of high coherence radiation to the photoresist material, and developing the photoresist material. The method also includes observing the photoresist material. 
     Still another exemplary embodiment relates to an inspection system for an optical system. The optical system is for use in an integrated circuit fabrication system. The inspection system includes means for providing radiation at a first coherence to a photoresist material, means for providing radiation at a second coherence to the photoresist material. The inspection system also includes means for developing the photoresist material and means for observing the photoresist material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and: 
     FIG. 1 is a general schematic block diagram of a lithographic system including condenser and imaging lens assemblies and a reticle or mask, the imaging lens assembly of the lithographic system can be inspected in accordance with an exemplary embodiment; 
     FIG. 2 is a general schematic block diagram of the lithographic system illustrated in FIG. 1, the block diagram of FIG. 2 includes a pupil representing the imaging lens assembly shown in FIG. 1; 
     FIG. 3 is a general schematic representation of errors or aberrations associated with the lens assemblies of the lithographic system illustrated in FIG. 1 or the pupil illustrated in FIG. 2; 
     FIG. 4 is a more detailed schematic cross-sectional view about line  4 — 4  of FIG. 5 of the mask or reticle illustrated in FIGS. 1,  2  and  3 , the mask or reticle includes a pattern in accordance with another exemplary embodiment; 
     FIG. 5 is a top view of the mask or reticle illustrated in FIG. 4, the mask or reticle is utilized to inspect the lithographic system illustrated in FIG. 1; 
     FIGS. 6A and 6B are schematic general drawings of an exemplary pattern on a photoresist layer formed by the process of FIG. 7 in accordance with still another exemplary embodiment; 
     FIG. 7 is a flow diagram showing the process of the inspection for the photolithographic system illustrated in FIG. 1 in accordance with yet another exemplary embodiment; 
     FIG. 8 is a graph showing the intensity of light transmitted through the mask illustrated in FIG. 4 for both coherence settings (σ 1  and σ 2 ), for a lens that is free of aberrations; and 
     FIG. 9 is a graph showing the intensity of light transmitted through the mask illustrated in FIG. 4 for both coherence settings (σ 1  and σ 2 ), for a lens having some degree of aberrations. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a camera stepper unit or lithographic system  10  can be analyzed in situ, or in-line, according to an advantageous process. System  10  includes reticle or mask  18  optimized to test system  10  for lens aberration errors. Lithographic system  10  can be any tool for use in integrated circuit (IC) fabrication which includes at least one lens assembly. 
     Lithographic system  10  includes a light source  12 , a lens assembly  14 , and a lens assembly  20 . Mask  18  is placed between lens assembly  14  and lens assembly  20 . System  10  is configured to provide radiation from light source  12  through lens assembly  14 , through mask  10 , and lens assembly  20  to a photoresist layer  22 . 
     Photoresist layer  22  can be provided on a substrate  24 . Substrate  24  can be an integrated circuit (IC) wafer, a semiconductive material, an insulative material, a conductive material, layers above any of the listed materials, or a base layer. Substrate  24  can be an industry standard silicon wafer. Substrate  24  is not described in a limiting fashion. 
     Light source  12  can be any number of sources of electromagnetic radiation. System  10  is designed to utilize radiation at a wavelength (λ) of 10-1000 nm at a coherence level between 0 and 1 (pupil fill factor (σ) (PFF)). Light source  12  preferably provides light at a wavelength of 450 nm. 
     Light provided from light source  12  is provided through mask  18  in accordance with the pattern on mask  18  to lens assembly  20 . Lens assembly  20  provides the patterned light to photoresist layer  22 . Photoresist layer  22  is selected to have photochemical reactions in response to the light from source  12 . Photoresist layer  22  can be any conventional positive or negative resist material. 
     Optics  14  preferably allows the coherence level to be adjusted from a pupil fill factor of σ=0.25 or less (high coherence) to a pupil fill factor of σ=0.85 or more (low coherence). 
     When a low coherence level (σ 1 ) of light is provided from light source  12  through mask  18  to photoresist layer  22 , a low coherence image is provided on photoresist layer  22  that is not as sensitive to local errors in the pupil (wavefront errors) as a high coherence image would be. The low coherence image can thus be used as a reference image because it is not as sensitive to local pupil errors. 
     When the high coherence level (σ 2 ) of light is provided from light source  22  through mask  18  to photoresist layer  22 , a high coherence image is provided on photoresist layer  22  that is more sensitive to local pupil errors. The low coherence image and the high coherence image are both recorded in the same photoresist layer  22 . 
     The low coherence image and the high coherence image are superimposed upon each other. The presence of errors will distort the patterns provided in photoresist layer  22  and provide a moiré effect that can be observed with an optical microscope. If errors are not present in lens assembly  20 , the low coherence image and the high coherence image are provided without significant distortion and the high coherence image matches the location of the reference image (e.g., the low coherence image). Thus, the pupil assembly  20  can be probed by utilizing high and low coherence levels (σ 1  and σ 2 ) light and imaging photoresist pattern accordingly. 
     With reference to FIG. 2, system  10  is shown including an equivalent lens  30  representative of lens assembly  20 . The equivalent lens assembly  30  includes a pupil  32 . Errors can be ascertained with respect to pupil  32  via the process discussed below with reference to FIG.  7 . Errors can be localized and characterized on pupil  32  so that appropriate corrective action may be taken. 
     With reference to FIG. 3, aberration errors associated with pupil  32  are demonstrated. A dashed line  34  represents the wavefront through pupil  32 . The errors (phase delays, etc.) on the wavefront associated with pupil  32  are manifested as the non-spherical portions of dashed line  34 . The errors result in an increase or decrease of the optical path from the reticle to layer  22 . 
     A solid line  36  represents an ideal wavefront provided in a pupil  32  without errors or aberrations. The shape of line  36  is spherical. 
     With reference to FIGS. 4 and 5, an exemplary pattern for mask  18  is shown. Mask  18  includes an absorbing pattern  21  provided on a substrate  25 . Mask  18  can include a fused silica substrate  25  and opaque material  23  provided in pattern  21 . Mask  18  can also include anti-reflective coatings for reducing loss of light during lithography due to reflections. Mask  18  can also include phase shift regions for increasing resolution. 
     Substrate  25  can be a 6.35 millimeter, a 9 millimeter, or other standard thickness glass or fused silica material. Opaque material  23  is an absorbing material layer which can have a thickness of 200-1000 Angstroms (preferably 500 Angstroms or less). Optical material  23  can include chromium, chromium oxide, or other absorbing materials. The materials associated with mask  18  are not discussed in a limiting fashion. 
     Pattern  21  is shown in FIGS. 4 and 5 in exemplary fashion only. Pattern  21  is not drawn to scale. Preferably, pattern  21  can be embodied as a circular chirped grating. One type of circular chirped grating can be a fresnel zone plate pattern. 
     Pattern  21  can include a solid circle  40  surrounded by rings  42  having decreased widths as the perimeter of mask  18  is reached. Preferably, the final ring at the perimeter has a dimension on the order of the resolution of lens assemblies  14  and  20  (e.g., 50 nm to 150 nm). 
     The relationship between the sizes of rings  42  can vary. For example, a linear decrease in size can occur as rings  42  approach the perimeter of mask  18 . Alternatively, inverse square or other relationships can be utilized. Preferably, circle  40  and rings  42  are concentric to each other. The pattern discussed with reference to FIGS. 4 and 5 is only one exemplary embodiment. Other patterns can replace pattern  21 . 
     With reference to FIGS. 6A and 6B, photoresist layer  22  includes moiré pattern  60  as a result of the development of layer  22  after exposure to light at the first and second coherence levels (σ 1  and σ 2 ). The pattern  60  in FIG. 6A is representative of a moiré pattern that would result from a perfect lens, whereas the pattern  60  in FIG. 6B represent a moiré pattern that would result from an aberrated lens. Pattern  60  is formed after the provision of high and low coherence light through mask  18 . 
     With reference to FIG. 7, the process of testing lithographic system  10  is described with reference to flow diagram  100 . Flow diagram  100  advantageously utilizes a double exposure at different coherence levels (σ 1  and σ 2 ) to accentuate local errors of pupil  32  (FIG. 2) in photoresist layer  22  (FIG.  1 ). 
     At a step  102 , substrate  24 , including a photoresist layer  22 , is provided in system  10  to receive radiation provided through lens assemblies  14  and  20  (FIG.  1 ). A mask  18  is provided between lens assemblies  14  and  20  at a step  104 . 
     At a step  106 , photoresist layer  22  is exposed to radiation through mask  18 . Condenser optics  14  (FIG. 1) can be set to provide high coherence level (σ 1 ) radiation. Preferably, the high coherence radiation has a pupil fill factor of σ=0.25 or less. After exposure to high coherence level radiation, layer  22  can be moved laterally with respect to source  12 . System  10  can include a stage for moving substrate  24  slightly (e.g., 20 nm). The movement of layer  22  can help establish moiré patterns in the presence of errors on pupil  32 . 
     At a step  108 , photoresist layer  22  is exposed to radiation through mask  18  at a low coherence level (σ 2 ). Light source  12  can be set to provide low coherence level (σ 2 ) at a pupil fill factor of 0.85 or more. 
     At a step  112 , substrate  24 , including layer  22 , can be removed from system  12 , and photoresist layer  22  developed. 
     At a step  114 , photoresist layer  22  can be viewed via a microscope or a camera. The viewing of patterns on developed layer  22  reveals whether defects or aberration errors are present on lens assembly  20 . For example, moiré pattern  60  (FIG. 6) can reveal the presence of lens aberrations. 
     With reference to FIGS. 8 and 9, high and low coherence image received in photoresist layer  22  (steps  106  and  108 , respectively, in FIG. 7) are represented on graphs  120  and  130 . The X-axes of graphs  120  and  130  represent the position on layer  22 , and the Y-axes of graphs  120  and  130  represent light received at the first and second coherence levels (σ 1  and σ 2 ). 
     Graph  120  includes a solid line  122  representative of the intensity of light at the first coherence level (all received through mask  18  (the high coherence image) (step  106 ). Graph  120  also includes a solid line  124  representative of the intensity of light received at the second coherence level (σ 2 ) (the low coherence image)(step  108 ). Graph  120  further includes a dashed line  126  representative of the combination of lines  122  and  124 . Dashed line  126  represents the intensity of light received by layer  22  at the first and second coherence levels (σ 1  and σ 2 ) (a combined image of the low coherence image and the high coherence image). 
     The image represented by dashed line  126  indicates that errors are not present in pupil  32 . The image corresponds to the circular chirped grating pattern associated with mask  18 . 
     With reference to FIG. 8, graph  130  includes a solid line  132  representative of the intensity of light at the first coherence level (σ 1 ) received through mask  18  (the high coherence image) (step  106 ). Graph  130  also includes a solid  132  representative of the intensity of light received at the second coherence level (σ 2 ) (the low coherence image) (step  108 ). Graph  130  further includes a dashed line  136  representative of the combination of lines  132  and  134 . Dashed line  136  represents the intensity of light received by layer  22  at the first and second coherence levels (σ 1  and σ 2 ) (a combined image of the low coherence image and the high coherence image). 
     Dashed line  136  indicates that local errors are present in pupil  32 . Line  136  includes a beat frequency associated with the combination of lines  132  and  134 . The beat frequency is manifested as a moiré effect. 
     It is understood that while preferred embodiment and specific examples are given, they are for the purpose of illustration only and is not limited to the precise details disclosed. For example, although specific wavelengths of light are described, other types of light can be utilized. Further, although two coherence levels are discussed, different coherence levels can be utilized. Various modifications may be made in the details within the scope and range of the equivalence of the claims without departing from what is claimed.