Patent Document

CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 13/964,931, filed on Aug. 12, 2013, which is a continuation of U.S. Pat. No. 8,520,189, filed on May 3, 2010, the entirety of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each new generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of integrated circuit evolution, there has been an increase in functional density (the number of interconnected devices per chip area) and a decrease in geometric size (the smallest component or line that can be created using a fabrication process). This scaling-down process generally provides benefits by increasing production efficiency and lowering associated costs. 
     But scaling down presents challenges as well. For instance, as geometric size and critical dimension (CD) have decreased, it has become more difficult to obtain optimal photolithography results. Lithography tools have generally kept up with the scaling down trend by moving to light sources in the deep ultraviolet spectrum, but problems arise when smaller wavelength light is used to expose current photoresist materials. For example, when a lithography tool&#39;s depth of focus is smaller than the thickness of the layer of photoresist to be exposed, only a portion of the layer is actually exposed. Non-uniform exposure of a layer of photoresist may lead to non-uniform etch patterns and etch bias, ultimately degrading device performance. 
     Techniques to compensate for inadequate depth of focus have been devised and are generally adequate for their intended purpose, but they are not entirely satisfactory. For example, some suffer from problems such as unacceptable critical dimension variation (also known as “tiger skin”). 
     SUMMARY 
     According to one of the broader forms of the invention, a method includes directing a beam of radiation along an optical axis toward a workpiece support, measuring a spectrum of the beam at a first time to obtain a first spectral profile, measuring the spectrum of the beam at a second time subsequent to the first time to obtain a second spectral profile, determining a spectral difference between the first spectral profile and the second spectral profile, and adjusting a position of the workpiece support along the optical axis based on the spectral difference. 
     According to another of the broader forms of the invention, an apparatus includes a workpiece support, beam directing structure that directs a beam of radiation along an optical axis toward the workpiece support, spectrum measuring structure that measures a spectrum of the beam at first and second times to obtain respective first and second spectral profiles, the second time being after the first time, processing structure that determines a spectral difference between the first spectral profile and the second spectral profile, and support adjusting structure that adjusts a position of the workpiece support along the optical axis based on the spectral difference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features may not be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a diagrammatic sectional side view of a semiconductor wafer and shows three photolithography exposures. 
         FIG. 2  is a diagrammatic side view of an apparatus that is a lithography system. 
         FIG. 3  is a graph depicting spectrum data generated by the lithography system in  FIG. 2 . 
         FIG. 4  is high-level flowchart showing a process carried out by the lithography system in  FIG. 2 . 
         FIG. 5  is a graph depicting critical dimension data for the lithography system in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
       FIG. 1  is a diagrammatic sectional side view of a semiconductor wafer  10  showing three different photolithography exposures. For the sake of simplicity, wafer  10  is depicted with only a substrate  12  and a photoresist layer  14 , but wafer  10  may include more layers such as an etch stop layer and a dielectric layer. Photoresist layer  14  has a thickness  16  and facilitates the transfer of a reticle pattern to wafer  10 . 
     During an exposure  18 , a narrowband beam of radiation  20  emitted from a not-illustrated lithography light source exposes a portion of the photoresist layer  14  of wafer  10 . Beam  20  is a spectrum of light in the deep ultraviolet (DUV) range with a spectral width of approximately 0.3 picometers (pm) and a depth of focus  22 . As evident from  FIG. 1 , depth of focus  22  is substantially smaller than thickness  16  of photoresist layer  14 . Consequently, only a portion of the layer  14  is properly exposed. That is, the layer  14  does not receive uniform exposure from top to bottom. Non-uniform exposure of the photoresist layer  14  may lead to non-uniform photoresist development, etch bias, and ultimately, degradation in device performance. 
     During a different exposure  24 , a broadband beam of radiation  26  emitted by a different not-illustrated lithography light source exposes the photoresist layer  14 . Beam  26  is also a spectrum of light in the DUV range but has a spectral width of approximately 1.2 pm. A larger spectral width means beam  26  contains a plurality of wavelengths, and each has a different focus plane. In particular, beam  26  passes through not-illustrated optics that provide intentional chromatic aberration in order to increase the depth of focus. Beam  26  has an effective depth of focus  28  that is much larger than the depth of focus  22  of narrowband beam  20 , and is also larger than thickness  16  of the photoresist layer  14 . Thus, during exposure  24 , the entire thickness  16  of photoresist layer  14  is exposed, eliminating the possibility of non-uniformity that can lead to etch bias during subsequent development of layer  14 . 
     Light sources that emit broadband beams may exhibit spectral shift over the course of multiple exposures, even if spectral width is maintained. This phenomenon is depicted by an exposure  30  after exposure  24 . During exposure  30 , the same broadband beam  26  exposes the photoresist layer  14 . Beam  26  has a depth of focus  32  that is approximately the same size as depth of focus  28 . However, depth of focus  32  is offset vertically with respect to depth of focus  28  such that it does not align with photoresist layer  14 . Consequently, the entire thickness  16  of layer  14  is not uniformly exposed, leading to a possibility of etch bias after development. Etch bias may ultimately lead to unwanted variation in critical dimension (CD) at different pitches on wafer  10 . 
     The specific values presented in association  FIG. 1  may be larger or smaller in practice. Further, spectral shift in broadband beams of radiation may also produce changes in exposure intensity or the size of the depth of focus. 
       FIG. 2  is a diagrammatic side view of an apparatus that is a lithography system  110  for the exposure of semiconductor wafers. The lithography system  110  includes a broadband light source  112  that produces a beam of radiation  114  similar to the broadband beam of radiation  26  in  FIG. 1 . The light source  112  is a excimer laser capable of producing light in the DUV range. Alternatively, the light source  112  may produce a beam of radiation in other ranges such as vacuum ultraviolet (VUV) or extreme ultraviolet (EUV). The beam  114  includes a broadband range of wavelengths in the DUV spectrum and has a spectral width of approximately 1.2 pm. However, the beam&#39;s spectral width may alternatively be larger or smaller depending on the exposure application. Here, spectral width is defined as the spectral distance between the two wavelengths that encompass ninety-five percent of spectral energy of the beam. This method of measuring spectral width is commonly known as E95. 
     The beam of radiation  114  is directed along an optical axis  115  through the lithography system  110  by optics that include a standard condenser lens  116 . The condenser lens  116  is configured to collimate and direct the beam  114  along the optical axis  115  toward a reticle  118 . The reticle  118  is held by a reticle stage  120  at a location along the optical axis  115  and includes a pattern image to be transferred along the optical axis. The reticle stage  120  is configured to adjust the position of the reticle  118  in directions transverse to the optical axis  115  for stepping between exposure fields on a wafer and for aligning the reticle  118  with the optical axis. After passing through the reticle  118 , the beam  114  passes through a standard projection lens  122  that is part of the optics. The projection lens  122  is configured to focus the pattern image carried by the beam  114  along the optical axis. Both condenser lens  116  and projection lens  122  are exemplary and alternatively may each be a lens group. Once through the projection lens  122 , a small portion of beam  114  is reflected away from the optical axis  115  by a beam splitter  124  that is disposed along the optical axis. The beam splitter  124  has a reflection coefficient of less than 5% so as to divert only an insignificant portion of the beam  114  away from the optical axis. Alternatively, the beam splitter  124  may have a larger reflection coefficient or may intercept the beam  114  at a different location along the optical axis  115 , for example between the condenser lens  116  and the reticle  118 . 
     A semiconductor wafer  126  is disposed along the optical axis  115  below the beam splitter  124 . The wafer  126  includes a plurality of exposure fields that may be successively aligned with the reticle  118  so that the beam  114  individually exposes each exposure field with the pattern contained on the reticle. A transmission image sensor (TIS)  128  is disposed immediately next to the wafer  126 . The TIS  128  is configured to detect light from the broadband light source  112  and gather alignment data for alignment of the wafer  126 . Alternatively, the TIS  128  may gather alignment data based upon light from an independent alignment light source. 
     A wafer stage  130  supports the wafer  126  and is configured to movably position it for proper alignment along the optical axis. TIS  128  is also disposed on the wafer stage  130 , immediately adjacent to the wafer  126 . The wafer stage  130  can be adjusted in three orthogonal directions, x, y, and z, where z is parallel to the optical axis  115 , and x and y lie in a plane substantially perpendicular to the optical axis  115 . A wafer stage drive  132  is included in the wafer stage  130  and contains hardware to make the adjustments to the position of the wafer stage  130 . A wafer stage control  134  is electronically coupled to the wafer stage drive  132  and is configured to transmit control data for controlling the position of the wafer stage  130  and thus the wafer  126 . The wafer stage control  134  may also be configured to electronically receive wafer alignment data from TIS  128  to initially align wafer stage  130  before exposure of wafer  126 . The wafer stage control  134  includes a not-illustrated digital processor, and a memory storing a computer program executed by the processor, but could alternatively be implemented in some other manner. 
     Lithography system  110  includes a spectrometer  136  that receives the portion of the beam  114  reflected by the beam splitter  124 . The spectrometer  136  measures the spectrum of the beam  114  and electronically transmits spectrum data to the wafer stage control  134 . The wafer stage control  134  is configured to receive and process spectrum data from the spectrometer  136 . Alternatively, the spectrometer or some other processing structure may process the raw spectrum data instead of the wafer stage control  124 . 
     As noted in the discussion of  FIG. 1 , broadband light sources such as broadband light source  112  are generally advantageous for improved depth of focus, but they may exhibit spectral shift over the course of multiple exposures (such as that from exposure  24  to exposure  28  in  FIG. 1 ), even if spectral width (as measured by E95) is maintained. Spectral shift may cause a shift in depth of focus at the point of wafer exposure which, if not compensated for, may result in uneven photoresist exposure in subsequent exposures. Uneven exposure may in turn lead to unwanted variation in critical dimension (CD) at different pitches. In operation, the lithography system  110  dynamically compensates for spectral shift in the beam  114  to improve wafer exposure results. More specifically, to compensate for this shift and maintain through-pitch CD uniformity, lithography system  110  uses inline metrology to measure the spectrum of beam  114  during wafer exposure, and then may adjust the position of wafer stage  130  based on the measured spectrum data. 
     In more detail, broadband light source  112  outputs the beam  114  along the optical axis  115  through the condenser lens  116 , the reticle  118 , the projection lens  122 , and the beam splitter  124 , and onto the wafer  126 . The light source  112  periodically emits the beam  114  in a series of exposures, as needed to expose all exposure fields on wafer  114  and subsequent wafers in a production run. During each exposure, a portion of the beam  114  is diverted by the beam splitter  124  to the spectrometer  136  before beam  114  reaches the wafer  126 . The spectrometer measures the spectrum of the beam  114  and sends spectrum data to the wafer stage control  134 . The wafer stage control  134  analyzes the spectrum output by the light source  112  during the current exposure and compares it to a spectrum output during a past exposure, in a manner described in more detail later. The wafer stage control  134  then determines a spectral shift from the comparison and calculates a distance and direction along the optical axis  115  that the wafer stage  130  needs to be moved to compensate for the spectral shift. Next, based on the result of the calculation, the wafer stage control  134  sends an adjustment signal to the wafer stage drive  132 , which in turn moves the wafer stage  130  along the optical axis  115 . Adjustment to the wafer stage position occurs after the current exposure is finished and before another exposure occurs. Alternatively, the adjustment may be made after a subsequent exposure and before the next wafer is positioned on the wafer stage  130 . 
       FIG. 3  is a graph depicting two different spectral profiles  137  and  138  of the broadband beam of radiation  114  emitted by lithography tool  110 , as measured during two different exposures. The spectral profiles  137  and  138  represent the intensity of each exposure as a function of wavelength. The y-axis of the graph represents wavelength intensity and the x-axis represents wavelength. However, for the sake of clarity, specific wavelength values on the x-axis have been replaced with arbitrary spectrum offset values to aid in measurement of spectral offset. 
     Spectral profile  137 , depicted with a solid line curve, represents the intensity of each wavelength in the beam&#39;s spectrum during a first exposure. Likewise, spectral profile  138 , depicted with a broken line curve, represents the intensity of each wavelength in the beam&#39;s spectrum during a subsequent exposure. For the purposes of clarity, the curves are shown as smooth but in actuality are defined by a plurality of raw data points gathered by inline metrology. Although not explicitly indicated, the spectral widths of the two profiles are equal, where spectral width is the distance in the x-axis direction between symmetrical points on the curve that encompass ninety-five percent of the spectral energy (E95). Further, each spectral profile has an E50 intensity center,  139  or  141 , which is defined as the wavelength in the spectral profile where the spectral energy on one side of the wavelength is equal to the spectral energy on the other side of the wavelength. In other words, E50 is the wavelength at 50% integral spectrum energy. The E50 wavelength of spectral profile  137  is represented by a solid vertical line  139 , and the E50 wavelength of spectral profile  138  is represented by a broken vertical line  141 . It should be noted that the value of zero along the x-axis is arbitrarily set to the value of the E50 wavelength  139  for spectral profile  137 . 
     As evident from the graph, the spectral profiles are offset, which represents a spectral shift in the beam  114  between the first exposure and the subsequent exposure. The amount of the spectral offset (Δλ) is the distance along the x-axis between the E50 wavelength  139  of profile  137  and the E50 wavelength  141  of profile  138 . As mentioned above, to compensate for this spectral shift, the wafer stage  130  of the lithography tool  110  is adjusted along the optical axis  115  based on this spectral shift. Specifically, the amount and direction of adjustment along the optical axis  115  is calculated as a function of the spectral offset (Δλ) and a longitudinal aberration (constant C), using the equation: Δz=C*Δλ. 
     The data represented in the graph in  FIG. 3  is a hypothetical example for explanation and illustration purposes only, but is representative of what will actually be generated by the lithography system  110  of  FIG. 2 . 
       FIG. 4  is a high-level flowchart showing a process  140  for dynamically adjusting wafer stage  130  of lithography system  110  to compensate for spectral shift during wafer exposure. Process  140  is carried out by the lithography system  110  of  FIG. 2  and implements the concepts discussed in association with the graph of  FIG. 3 . Process  140  begins at block  142 , and proceeds to block  144  where the constant C representing longitudinal aberration of the optics of the lithography system  110  is determined. In this regard, every lithography tool has a distinct longitudinal aberration constant, which is the distance along a tool&#39;s optical axis from the focus of paraxial rays to the point where rays coming from the outer edges of its lens or reflecting surface intersect this axis. Longitudinal aberration values are typically in the range of 0.2 to 0.5 μm/pm, and remain static for each lithography tool. Constant C may be determined by configuring the light source  112  to output a light spectrum with a pre-determined center wavelength, and by then using the transmission image sensor (TIS)  128  to determine where along the optical axis  115  the beam  114  has its maximum intensity. Alternatively, C may be determined by any other suitable method. 
     Next, in block  146 , a first wafer  126  is loaded onto the wafer stage  130  and the lithography system  110  is properly focused and aligned for exposure with reticle  118 . Calibration of the reticle stage  120  and wafer stage  130  in the x, y, and z directions may be performed with TIS  128  or other suitable equipment, in a manner known in the art. Upon completion of appropriate calibration steps, a first exposure of the wafer  126  is initiated. 
     Process  140  proceeds to block  148 , where, during the first exposure, the spectrum of the beam  114  is measured using inline metrology. Specifically, the beam splitter  124 , disposed along the optical axis  115 , diverts a small portion of the beam  114  to the spectrometer  136 . A detailed spectral profile (or curve) is obtained from the raw spectrometer data by plotting measured wavelength intensity against wavelength for each wavelength in the spectrum. This might, for example, be the spectral profile  137  of  FIG. 3 . Next, in block  150 , the intensity center of the spectral profile obtained in block  148  is determined (and is the wavelength at 50% integral spectral energy). In  FIG. 3 , this is the E50 wavelength  139 . The E50 wavelength of the initial spectrum reading is stored and saved for later calculations. 
     Then, process  140  proceeds to block  152  where the lithography system  110  initiates a subsequent exposure, which is the exposure immediately after the first exposure. Then, in block  154 , during the subsequent exposure, the beam splitter  124  again deflects a portion of the beam  114  to the spectrometer  136  to measure the spectrum a second time. A second spectral profile is obtained from the second spectrum measurement and the E50 wavelength for the second spectral profile is determined and is stored. In  FIG. 3 , this would be the spectral profile  138  and E50 wavelength  141 . 
     Process  140  proceeds to block  156  where a spectral shift of the beam, if any, is determined. The spectral shift is determined by measuring the offset between the intensity center of the first spectral profile ( 139  and  137  in  FIG. 3 ) and the intensity center of the second spectral profile ( 141  and  138  in  FIG. 3 ). Specifically, the difference between the first E50 wavelength and the second E50 wavelength is calculated to determine an E50 offset (Δλ). E50 offsets may be in the range of 0.2 pm, but larger or smaller offsets may occur depending on various factors, including the number of exposures between spectrum measurements. 
     Next, in block  158 , the amount of wafer stage adjustment (Δz) needed to compensate for spectral shift is determined. Specifically, the amount and direction of adjustment along the optical axis  115  is calculated as a function of E50 offset (Δλ) and longitudinal aberration (C), using the equation: Δz=C*Δλ. Then, in block  160 , the wafer stage  130  is adjusted along the optical axis  115  by an amount equal to the Δz value calculated at block  158 . The adjustment is made after the most recent exposure but before the next exposure is initiated. More generally, the adjustment may be made between any two exposures, for instance, while the lithography tool is aligning the optical axis  115  with another exposure field on the wafer. 
     Then, in block  162 , process  140  continues on to either block  163  or block  164  depending on whether processing of the current wafer is finished. If every exposure field on the wafer  114  has not been exposed, then process  140  proceeds to block  163  where the wafer stage  130  moves the wafer  126  to align the next unexposed field with the optical axis  115 . If it is instead determined at decision block  162  that every exposure field on the wafer  114  has been exposed, then process  140  proceeds to block  164 , where the next wafer is loaded onto the wafer stage and aligned. From each of blocks  163  and  164 , process  140  returns to block  152  and another exposure is initiated. From there, process  140  repeats blocks  154  through  162  to determine a new E50 offset, and to again adjust the wafer stage  130  along the optical axis  115  to compensate for any additional spectral shift. The iterative feedback loop of blocks  152  through  164  continues until processing of every wafer in the production run has been completed. As process  140  repeatedly proceeds through this loop, the wafer stage  130  is dynamically adjusted along the optical axis  115  to compensate for any measured spectral shift. Thus, depth of focus is maintained through successive exposures and through-pitch critical dimension is substantially uniform. 
     The lithography system  110  may alternatively compensate for spectral shift with a combination of inline metrology data and alignment data from TIS  128 . For example, during an exposure, TIS alignment data indicating the intensity of the beam  114  at points along the z-axis may be transmitted to the wafer stage control  134  or other processing structure for analysis. The peak intensity of the beam along the z-axis may be calculated and compared to a previous TIS peak intensity measurement to produce a TIS offset value. This TIS offset value may be used in conjunction with the spectral offset value to adjust wafer stage  130 . For example, the amount of adjustment needed to wafer stage  130  along the z-axis may be calculated by averaging the TIS offset value and the Δz value corresponding to the spectral offset value (as calculated in block  158  of  FIG. 4 ). Other equations that are a function of both TIS offset and spectral offset may alternatively be utilized. Further, the lithography system  110  may additionally perform other adjustments to its various components upon detection of spectral shift in beam  114 . 
       FIG. 5  is a graph depicting the difference in critical dimension (CD) deviation at different pitch values between two alternative broadband exposures. Each exposure was initiated after a common prior exposure. However, before one of the alternative exposures, the position of wafer stage  130  was adjusted along the optical axis to compensate for spectral shift, whereas before the other alternative exposure, no such adjustment was made. In the graph, each data point represents the difference in CD at a specific pitch between the prior exposure and one alternative subsequent exposure. For example, a data point  170  indicates that, at a pitch of 120 nm, the CD of the second exposure (with no adjustment to the wafer stage position) was approximately 1 nm greater than the CD of the common prior exposure. And a data point  172  indicates that, at a pitch of 120 nm, the CD of an alternate second exposure (with adjustment to the wafer stage position) was approximately 0.3 nm less than the CD of the prior exposure. As such, a curve  174  (defined by square data points) represents data collected after positional adjustment of wafer stage  130  along the optical axis  115  in order to compensate for spectral shift. That is, an iteration of process  140  described in conjunction with  FIG. 3  was carried out between two exposures. And a curve  176  (defined by the diamond-shaped data points) represents data collected during an alternative subsequent exposure, where no positional adjustment of the wafer stage is made between the successive exposures. 
     As evident from the graph, when a wafer is exposed to a broadband beam without any compensation for spectral shift, the critical dimension variation from a previous exposure may increase substantially at larger pitches. But when wafer stage adjustment is made before a subsequent exposure, critical dimension variation from a previous exposure may be less than or equal to 0.5 nm, even as pitch increases. The data in  FIG. 5  is representative of the improvement in critical dimension uniformity achieved by the lithography system  110  of  FIG. 2  and the process  140  of  FIG. 4 . However, data collected in practice may vary somewhat from the data shown in the graph. 
     The foregoing discussion outlines one embodiment in detail so that those skilled in the art may better understand aspects of the present disclosure. Specifically, the lithography system  110  and process  140  as described in conjunction with  FIGS. 2 and 4  are solely to facilitate the understanding of the present disclosure and not to limit it. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiment introduced herein. Accordingly, structure and procedures disclosed in the above embodiment may be modified or added to, or removed without departing from the spirit and scope of the present disclosure. For example, the disclosure above generally relates to a step-and-scan reduction projection type lithography system, but the lithography system  110  may include a step-and-repeat reduction projection scanner or other type of scanner without departure from the scope of the present disclosure. Additionally, the present disclosure may be applied not only to semiconductor devices but also to image pickup devices, liquid crystal displays, and other applications that benefit from exposure with a broadband light source.

Technology Category: 3