Abstract:
A verification photomask disclosed. The mask may be for process window verification purposes when switching between fabrication equipment, and/or for optical proximity correction (OPC) verification purposes. The mask includes device areas that are separated by scribe lines. One or more verification patterns are integrated into the scribe lines for verification purposes. These patterns can include: proximity patterns, photoresist-spacing patterns, polysilicon end cap patterns, as well as other patterns. A method for making the mask, and a semiconductor device created at least in part by a method including use of the mask, are also disclosed.

Description:
RELATED APPLICATIONS  
       [0001]    This is a continuation-in-part (CIP) application of U.S. patent application Ser. No. 09/941,538, filed Aug. 29, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to semiconductor device fabrication, and more particularly to photolithographic process windows and optical proximity correction (OPC) as used in such fabrication.  
         BACKGROUND OF THE INVENTION  
         [0003]    Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC&#39;s with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC&#39;s have begun to be manufactured that have features smaller than the lithographic wavelength.  
           [0004]    Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. The resulting semiconductor features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. These distortions include line-width variations dependent on pattern density, which affect a device&#39;s speed of operation, and line-end shortening, which can break connections to contacts. To avoid these and other optical proximity effects, the semiconductor industry has attempted to compensate for them in the photomasks themselves.  
           [0005]    This compensation is generally referred to as optical proximity correction (OPC). The goal of OPC is to produce smaller features in an IC using a given equipment set by enhancing the printability of a wafer pattern. OPC applies systematic changes to mask geometries to compensate for the nonlinear distortions caused by optical diffraction and resist process effects. A mask incorporating OPC is thus a system that negates undesirable distortion effects during pattern transfer. OPC works by making small changes to the IC layout that anticipate the distortions. OPC offers basic corrections and a useful amount of device yield improvement, and enables significant savings by extending the lifetime of existing lithography equipment. Distortions that can be corrected by OPC include line-end shortening, corner rounding, isolated-dense proximity effect, and isolated-line depth of focus reduction.  
           [0006]    Line-end shortening (LES) is the shortening of the end of a metal line end in the actual fabricated semiconductor device as compared to the circuit designer&#39;s originally contemplated ideal device. An example of LES is shown in FIG. 1A. The line  102  should extend to the originally designed end  104 . However, in actuality, the line  102  may only extend to the actually fabricated end  106 . OPC can be used to correct LES by adding serifs or hammerheads to the originally designed end in the photomask, such that during photolithography, the actually fabricated end more closely approximates the location of the originally designed end. The addition of serifs is shown in FIG. 1B, in which the serifs  110  and  112  have been added to the line  102  at its end  104 . The addition of a hammerhead is shown in FIG. 1C, in which the hammerhead  120  has been added to the line  102  at its end  104 .  
           [0007]    Corner rounding is the degree to which feature comers that should be at sharp angles are instead rounded by the lithography process. An example of comer rounding is shown in FIG. 2A. The feature  202  should include the outside sharp corner  204  and the inside sharp corner  206 . However, in actuality, the feature  202  may only include the outside rounded corner  208  and the inside rounded corner  210 . OPC can be used to correct corner rounding by adding serifs to outside corners, which are called positive serifs, and subtracting serifs from the inside corners, which are called negative serifs, to the feature in the photomask. This is shown in FIG. 2B, in which the positive serif  220  has been added to the outside corner  204  of the feature  202 , and the negative serif  222  has been removed from the inside corner  210  of the feature  202 .  
           [0008]    Isolated-dense proximity effect, or bias, refers to the degree to which the mean of measured dense features differs from the mean of like-sized measured isolated features. Isolated-dense bias is especially important in the context of critical dimensions (CD&#39;s), which are the geometries and spacings used to monitor the pattern size and ensure that it is within the customer&#39;s specification. CD bias, therefore, refers to when the designed and actual values do not match. Ideally, bias approaches zero, but in actuality can measurably affect the resulting semiconductor device&#39;s performance and operation. Isolated features, such as lines, can also negatively affect depth of focus, such that they cannot be focused as well with the lithography equipment as can dense features.  
           [0009]    OPC can be used to correct the isolated-dense proximity effect and the isolated-feature depth of focus reduction by adding scattering bars (SB&#39;s) and anti-scattering bars (ASB&#39;s) near the edges of opaque and clear features, respectively, on a photomask. SB&#39;s are sub-resolution opaque-like features, whereas ASB&#39;s are sub-resolution clear-like features. Both serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve depth of focus so that isolated lines can be focused as well as dense lines can with the lithography equipment. For example, FIG. 3A shows a set of SB&#39;s  300 , whereas FIG. 3 b  shows the placement of such sets of SB&#39;s  300  near an isolated line  302 , in contradistinction to the dense sets of lines  304  and  306 .  
           [0010]    Unfortunately, OPC is a difficult process, because determining the optimal type, size, and symmetry of the compensations to be made on the mask can be very complex, and depends on neighboring geometries and process parameters. Usually, a sophistical computer program is used to properly implement OPC. Using empirical data, OPC software creates a mathematical description of the process distortions, which can be in the form of simple shape manipulation rules, or a more detailed and intricate process model. Once this description is generated, automated software changes the shapes of the polygons in the pattern layout files, moving segments of line edges and adding features that compensate the layout for the distortions that will result. The critical levels of the photomask set can then be made using these modified, predistorted layout designs.  
           [0011]    Like other semiconductor processes, OPC is desirably continually monitored and verified to ensure mask quality. Usually, OPC is inserted as part of a verification/tape-out activity. While OPC can more efficiently be included as part of mask data preparation, enough errors have been detected on wafers processed in this manner that many users are hesitant to make such significant changes to their pattern date without the insurance providing by repeating other verification steps after OPC has been applied. Mask inspection is also negatively impacted by OPC, since the addition of the small geometries may appear identical to features that mask inspection machines have been trained to recognize as defects. Masks and reticles with these features will appear to contain thousands of such defects, and be rejected. Manual inspection is also slow, because the technician must examine many different parts of each mask to ensure that the mask has been produced correctly. Since masks inherently differ based on the semiconductor device being fabricated, manual inspection can become a very laborious and non-standardized process. Production yield of new semiconductor devices as a result is usually reduced when using OPC.  
           [0012]    Another issue that impacts the quality of lithography is focus variation, which is nearly ubiquitous in IC manufacturing because of the combined effects of many problems, such as wafer non-flatness, auto-focus errors, leveling errors, lens heating, and so on. A useful lithographic process should be able to print acceptable patterns in the presence of focus variation. Similarly, a useful lithographic process should be able to print acceptable patterns in the presence of variation in the exposure dose, or energy, of the light source being used. To account for these simultaneous variations of exposure dose and/focus, it is useful to map out the process window, such as an exposure-defocus (ED) window, within which acceptable lithographic quality occurs. The process window for a given feature, with or without OPC to compensate for distortions, shows the ranges of exposure dose and DOF that permit acceptable lithographic quality.  
           [0013]    For example, FIG. 4 shows a graph  400  of a typical ED process window for a given semiconductor pattern feature. The y-axis  402  indicates exposure dose of the light source being used, whereas the x-axis  404  indicates DOF. The line  406  maps exposure dose versus DOF at one end of the tolerance range for the CD of the pattern feature, whereas the line  408  maps exposure dose versus DOF at the other end of the tolerance range for the CD of the feature. The area  410  enclosed by the lines  406  and  408  is the ED process window for the pattern feature, indicating the ranges of both DOF and exposure dose that permit acceptable lithographic quality of the feature. Any DOF-exposure dose pair that maps within the area  410  permits acceptable lithographic quality of the pattern feature. As indicated by the area  410 , the process window is typically represented as a rectangle, but this is not always the case, nor is it necessary.  
           [0014]    Unfortunately, the photolithographic process window can also vary, from fabrication equipment to fabrication equipment. For example, a semiconductor device designer may use a given type of research and development fabrication equipment when designing a device. The designer may use OPC in generating the photomasks for the device to ensure that proper fabrication of the device occurs. The process windows for the photomasks may be dependent on the equipment used by the designer.  
           [0015]    However, when the designer finalizes the design, and sends it to a manufacturer for mass production, the manufacturer may use different fabrication equipment. This means that the semiconductor photomasks provided by the designer may not be able to used for proper fabrication of the device on the fabrication equipment of the manufacturer. The process windows for the photomasks may be different for the fabrication equipment used by the manufacturer, as compared to that used by the designer. Similar problems occur with different fabrication equipment at the developer product and transfer production stages of fabrication.  
           [0016]    Currently, time-intensive and costly manual verification of photomasks are performed, to ensure that they yield proper semiconductor device fabrication with the manufacturer&#39;s fabrication equipment, as they did with the designer&#39;s fabrication equipment. Because semiconductor designs are very complex, significant effort must be expended to ensure that all critical dimensions (CD&#39;s) of the device are properly fabricated with the manufacturer&#39;s fabrication equipment. That is, significant effort must be expended to ensure that the process window for fabricating the device on the manufacturer&#39;s fabrication equipment is acceptable. Where necessary, OPC and other modifications may be made to the semiconductor design, to ensure that properly fabricated devices are yielded by the manufacturer&#39;s fabrication equipment.  
           [0017]    Therefore, there is a need for an improved verification process that alleviates these shortcomings. In particular, there is a need for such a verification process that can be accomplished relatively quickly and without incurring additional cost. There is a need to easily verify mask quality for each critical pattern, and for manual inspection to be as standardized as possible. There is also a need for such easy verification to be performed when a photomask is transferred from development to production, from design to manufacturing, and so on. For these and other reasons, there is a need for the present invention.  
         SUMMARY OF THE INVENTION  
         [0018]    The invention relates to a verification mask, for process window verification purposes when switching between fabrication equipment, and/or for optical proximity correction (OPC) verification purposes. The mask includes device areas that are separated by scribe lines. One or more process window verification patterns are integrated into the scribe lines for verification purposes.  
           [0019]    The invention provides for advantages not found within the prior art. The integration of verification patterns into the scribe lines of a photomask allows for easy and standardized monitoring and verification of the process window when switching between semiconductor fabrication equipment, as well as for the verification of mask quality when performing OPC. For example, in the former case, the new fabrication equipment, such as the manufacturer&#39;s equipment, should be able to yield clear imprints of the patterns on semiconductor wafers, just as the old fabrication equipment, such as the designer&#39;s equipment, did. The yielding of a clear imprint indicates to the technician that the process window for fabricating semiconductor devices using the mask on the new equipment is adequate.  
           [0020]    The technician, therefore, does not have to examine the entire mask, but rather only has to examine the standard verification patterns integrated into the scribe lines of the mask. The verification patterns are desirably standardized as integrated into the scribe lines of photomasks. This standardizes approach to mask verification allows for faster checking of the masks insofar as transfer between semiconductor equipment is concerned. Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIGS. 1A, 1B, and  1 C are diagrams showing the line-end shortening (LES) distortion, and how optical proximity correction (OPC) can be used to compensate for LES.  
         [0022]    [0022]FIGS. 2A and 2B are diagrams showing the corner rounding distortion, and how OPC can be used to compensate for corner rounding.  
         [0023]    [0023]FIGS. 3A and 3B are diagrams showing a scattering bar (SB) and its placement near an isolated line, to compensate for isolated-dense proximity effect.  
         [0024]    [0024]FIG. 4 is a diagram showing an example exposure-defocus (ED) process window for a given pattern feature showing the ranges of exposure dose and focus that permit acceptable lithographic quality.  
         [0025]    [0025]FIG. 5 is a diagram showing where verification patterns according to embodiments of the invention can be placed on a wafer, specifically on the scribe lines separating the dies of the wafer. The verification patterns themselves are imprinted on the wafer via lithography, using a photomask according to an embodiment of the invention.  
         [0026]    [0026]FIG. 6 is a diagram showing the side profile of an example photomask, such as that  20  which can include verification patterns according to an embodiment of the invention.  
         [0027]    [0027]FIG. 7 is a flowchart of a method to create a photomask having verification patterns, according to an embodiment of the invention.  
         [0028]    [0028]FIG. 8 is a flowchart of a typical method to create one or more semiconductor devices on a wafer using a photolithographic process, and preferably employing a photomask including verification patterns according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0030]    [0030]FIG. 4 shows a semiconductor wafer  402  on which a verification pattern can be created, according to an embodiment of the invention. The verification pattern can be, for instance, a process window verification pattern, or an OPC verification pattern. The wafer  402  has a number of semiconductor device areas, such as the areas  404  and  406 . Each device area of the wafer  402  is for a separate semiconductor device to be fabricated. The device areas themselves may also be referred to as chips, dies, devices, circuits, microchips, and bars. They are used to identify the microchip patterns covering the majority of the surface of the wafer  402 . The device areas are separated by scribe lines, such as the scribe lines  408  and  408 . The scribe lines are also referred to as saw lines, streets, and avenues, and are the spaces between the chips that allow separation of the chip from the wafer.  
         [0031]    The verification patterns are created on one or more of the scribe lines. For example, as shown in FIG. 5, the verification patterns can be created in the areas on the scribe lines identified by the circles  512 ,  514 , and  516 . The process window verification patterns are created on the scribe lines by using a corresponding photomask, or mask or reticle, in conjunction with a photolithographic process. A side profile of an example mask is shown in FIG. 6, and includes a glass surface  602  on which chrome  604  is added. The chrome  604  corresponds to the pattern, such as the OPC or process window verification pattern, to be imprinted on the semiconductor wafer. Areas of the mask that do not have chrome added to the glass surface are referred to as clear regions, whereas areas that do have chrome added to the glass surface are referred to as opaque regions.  
         [0032]    The types of process window verification patterns that can be included on the scribe lines of a photomask for transfer to a semiconductor wafer during semiconductor photolithography processing is not limited by the invention. As examples only, these patterns can include proximity patterns, wide photoresist-spacing patterns, polysilicon end cap patterns, island and line patterns, line-end shortening patterns, corner rounding patterns, isolated-dense proximity patterns, isolated-feature depth of focus reduction patterns, and so on. Other patterns include representative memory cells, such as representative static and dynamic random-access memory (SRAM and DRAM, respectively) cells. Other critical photo effect and/or electrical test patterns can also be included. Such patterns can include OD patterns, polysilicon patterns, contact patterns, metallization and inter-metal patterns, and so on.  
         [0033]    OD patterns can include thermal conductivity detection (TCD) patterns, line-end shortening patterns, wide OD patterns, island patterns, proximity patterns, OD corner-rounding patterns, and so on. Polysilicon patterns can include poly end cap bridge and pull back patterns, poly OPC verification patterns, proximity patterns, wide polysilicon patterns, SRAM cell patterns, and so on. Contact patterns can include photoresist linearity patterns, sub-ground rule patterns, phase-shift mask (PSM) side lobe window patterns, and so on. Metal patterns can include TCD patterns, line-end shortening patterns, wide metal patterns, island patterns, proximity patterns, and so on.  
         [0034]    The process window verification patterns included on the scribe lines of the photomask and the resulting processed semiconductor wafer in conjunction with the invention are preferably standardized, both in terms of pattern and location. This enables the technician to more easily monitor and verify the mask and the patterned wafers for purposes of process window verification, when switching between fabrication equipment. For example, the mask may be within the process window of a designer&#39;s development fabrication equipment. However, then the mask is given to a manufacturer, and the patterns within the scribe lines can be used by the technician to verify that the mask is within the process window of the manufacturer&#39;s production fabrication equipment.  
         [0035]    Furthermore, the types of OPC verification patterns that can be included on the scribe lines of a photomask for transfer to a semiconductor wafer during semiconductor photolithography processing is not limited by the invention. As examples, the line-end shortening (LES) OPC verification patterns of FIGS. 1B and 1C may be included, specifically the serifs of FIG. 1B and the hammerhead of FIG. 1C. Furthermore, the corner rounding OPC verification pattern of FIG. 2B may be included, specifically the positive and negative serifs of FIG. 2B. As another example, scattering bars (SB&#39;s), such as those shown in FIG. 3B, and anti-scattering bars (ASB&#39;s) may be included. The OPC verification patterns included on the scribe lines of the photomask and the resulting processed semiconductor wafer in conjunction with the invention are preferably standardized, both in terms of pattern and location. This enables the technician to more easily monitor and verify the mask and the patterned wafers, as compared to the prior art.  
         [0036]    [0036]FIG. 7 shows a method  700  for producing a photomask including verification patterns in accordance with the invention. First, the scribe lines are created on the mask ( 702 ), to define device areas on the mask. Next, verification patterns are created on the mask in various locations in the scribe lines ( 704 ). The actual device masks on the device areas may then be created on the mask ( 706 ), where the device masks correspond to the dies on the semiconductor wafer that will be patterned using the mask created according to the method  700 . The mask surface is typically borosilicate glass or quartz, and the patterns on the mask added in  702 ,  704 , and/or  706  are typically added by sputtering and then patterning layers of chrome, chromium, chromium oxide, and/or chromium nitride.  
         [0037]    [0037]FIG. 8 outlines a typical method  800  for fabricating semiconductor devices on a semiconductor wafer, and in conjunction with which a photomask having a process window verification and/or an OCR verification pattern according to the invention may be utilized. Other steps and acts may be performed in addition to or in lieu of those shown in FIG. 8, as can be appreciated by those of ordinary skill within the art. The mask is first positioned over the wafer, where the wafer has a top layer of photoresist ( 802 ). The wafer is exposed through the mask ( 804 ), such that the photoresist has unexposed and exposed regions. The exposed regions correspond to the photoresist that is beneath clear regions of the mask, and the unexposed regions correspond to the photoresist that is beneath opaque regions. The wafer is developed to remove the exposed photoresist ( 806 ), and then etched where the wafer has been revealed through the exposed photoresist that has been removed ( 808 ). The unexposed photoresist is then also removed ( 810 ).  
         [0038]    It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.