Abstract:
Disclosed is a pupil aperture, and method for making the pupil aperture for use in a photolithography scanner system. The pupil aperture includes a plate having a set of pole apertures that are radially offset from a reference center point of the plate. The plate further includes a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. In a specific aspect of this invention, a set ranging between 3 to 9 pole apertures can be defined in the plate, and their offset from the center point can be selected to be between about 0.3 inches and about 0.9 inches.

Description:
This application claims benefit of Provisional Appln. No. 60/097,750 filed Aug. 24, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to photolithography systems and more particularly, to techniques for designing features of photolithography systems to improve the resolution and focus of an image that is projected onto a semiconductor wafer. 
     2. Description of the Related Art 
     Photolithography is an important part of semiconductor technology. Devices made from semiconductor wafers depend greatly on the resolution and focus of images directed onto selected regions of the wafers. Although much improvement has occurred in the development of photolithography systems that enable the fabrication of smaller and smaller features sizes, photolithography engineers continue to battle defects in resolution as geometries continue to decrease. 
     For example, many of today&#39;s photolithography systems are now using deep UV wavelengths (i.e., 248 nm) and deep UV photoresists in efforts to better define the image resolution of patterned photoresist. Unfortunately, it has been observed that many resolution defects occur when feature geometries have angled profiles, with respect to horizontal and vertical features. Consequently, when photoresists are developed after being exposed, only horizontal and vertical feature geometries exhibit good resolution, while angled features are substantially distorted. 
     An example of a photolithography system  100  is shown in FIG. 1A, which includes a scanner system  102 . The scanner system  102  is also known as a stepper apparatus. A light source  104  is commonly positioned near a top region of the scanner system  102  in order to allow produced light waves to be directed toward a first lens system  106 . From the first lens system  106 , the light is projected through a pupil aperture  108  that is used to better direct light onto a second lens system  110 . As is well known, the pupil aperture  108  assists in precisely directing the light source onto the desired location of a reticle  112 . 
     The reticle  112  being a glass plate, is patterned with exemplary feature geometries typically defined by a chromium material, which blocks light from propagating through the reticle  112 . After the desired light passes through the reticle  112 , it leaves the scanner system  102  and comes into contact with a die region  114   a  of a semiconductor wafer  114  having a photoresist covered surface. 
     The light then changes the chemical composition of the photoresist so that a developer will allow removal of the exposed regions of photoresist material (i.e., for positive photoresists). In this manner, the feature geometries of the reticle  112  are transferred to the die region  114   a.  For ease of illustration, only one die region  114   a  is shown, but as is well known in the art, many more die regions  114   a  are formed throughout the semiconductor wafer  114  during normal fabrication. 
     FIG. 1B is a top view of one example of a conventional pupil aperture  108   a.  The pupil aperture  108   a  (also known as a clean-up aperture) includes an aperture  116   a  with a σ value of about 0.6. The pupil aperture  108   a  is used to more precisely direct light received from the light source  104  onto the reticle  112 . Generally, the aperture  116   a  will define a cone of light that is directed toward the second lens system  110  and then illuminates the reticle  112 . Although this pupil aperture  108   a  assists in more precisely controlling the direction of the light from the light source  104 , as demands for smaller and more defined feature resolution continues to increase, the precision provided by the pupil aperture  108   a  has failed to produce adequate results. 
     In order to increase resolution of the pattern printed on the die region  114   a,  several different pupil aperture designs have been devised. FIG. 1C shows an example of an off-axis pupil aperture  108   b.  The pupil aperture  108   b  includes a number of off-axis apertures  116   b.  For purposes of explanation, a zero order region  118  is shown defined around a center point  119  from which an offset  120  measurement is made to the off axis apertures  116   b.    
     In the pupil aperture  108   b,  most of the center portion actually blocks the passage of light, thus enabling a focusing of the light that passes through the off-axis apertures  116   b.  Although the added level of focus precision provided by off-axis apertures is well known, many defects in resolution have still been detected when an off-axis pupil aperture, such as the pupil aperture  108   b  is used. 
     FIG. 1D shows an example of a quadrupolar off-axis pupil aperture  108   c.  The pupil aperture  108   c  includes a set of four pole apertures  116   c,  each with a σ value of about 0.1. A horizontal axis  117  is defined through the center point  119  of the pupil aperture  108   c.  The distance between the center point  119  and the pole apertures is defined by an offset  120 . The angle between the pole apertures  116   c  and the horizontal axis  117  is defined by φ, which is strongly suggested by photolithography scanner equipment manufactures to be exactly 45° from the horizontal axis  117 . 
     In fact, scanner equipment manufacturers recommend that when very small feature geometries are being patterned, standard 45° quadrupole pupil apertures be used because light received from the first lens system  106  will be more accurately directed to the second lens system  110  and then to the surface of the reticle  112  (as shown in FIG.  1 A). Consequently, the scanner equipment manufacturers provide users of their photolithography equipment with standard machined pupil apertures having the aforementioned 45° quadrupole design. 
     In addition, some scanner equipment manufacturers, such as Silicon Valley Group, Inc. (SVG) of Wilton, Conn. provide users of their equipment with guidelines for using the standard 45° quadrupole design pupil apertures. Unfortunately, none of the prior art pupil apertures have been able to supply an adequate level of resolution for very small features having angled geometries. 
     FIG. 1E shows an example of a reticle  112  with a number of feature lines  112   b  and a corresponding number of angled feature lines  112   b ′ patterned on the reticle&#39;s glass surface. Also shown are a number of inter-feature spaces  112   c  defined between any two of the feature lines  112   b  and its corresponding angled feature lines  112   b′.  For exemplary purposes, the feature lines  112   b / 112   b ′ are patterned such that line widths and spaces as small as 160 nm are transferred onto a resist covered die region  114   a  as shown in FIG.  1 F. 
     As shown, the die region  114   a  includes a number of photoresist lines  114   b  and angled photoresist lines  114   b ′ that result after development of the exposed photoresist. As evidenced from numerous experimental trials, the photoresist lines  114   b ′, which have an angled geometric orientation (with respect to a vertical axis), will not produce the ideal pattern shown in the reticle  112 . 
     In fact, because none of the above-described pupil apertures are able to accurately and precisely direct light onto the surface of the reticle  112  when small geometries are being fabricated, major distortion in the developed photoresist will occur as shown in FIG.  1 F. It should also be noted that when such distortion occurs, the feature geometries will not produce the desired electrical interconnections, thereby producing a malfunctioning integrated circuit device. Of course, when such malfunctions occur, semiconductor devices are scrapped, and corresponding fabrication yield will suffer. 
     In view of the foregoing, there is a need for photolithography scanner pupil apertures that assist in more accurately directing light onto a reticle when features having very small critical dimensions are being patterned. There is also a need for methods for manufacturing custom pupil apertures to correct resolution distortions when features having small angled geometries are patterned over photoresist covered wafers. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by providing an apparatus, and method for making an off-axis pupil aperture for use in a photolithography system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below. 
     In one embodiment, a pupil aperture for use in a photolithography scanner system is disclosed. The pupil aperture includes a plate having a set of pole apertures that are radially offset from a reference center point of the plate. The plate further includes a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. In a specific aspect of this embodiment, a set of 4 or 8 pole apertures can be defined in the plate, and their offset from the center point can be selected to be between about 0.3 inches and about 0.9 inches. 
     In another embodiment, a pupil aperture for use in a photolithography scanner system is disclosed. The pupil aperture includes a plate having a center region that is semi-transparent. The center region has a set of pole apertures that are radially offset from a reference center point of the center region. The plate further includes a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. In a specific aspect of this embodiment, the sigma value for each of the pole apertures can be selected to be between about 0.05 and about 0.15. In a more preferred aspect of this embodiment, a set of 4 or 8 pole apertures can be defined in the plate. 
     In yet another embodiment, a method for making a pupil aperture for use in a photolithography scanner system is disclosed. The method includes machining a plate having a set of pole apertures that are radially offset from a reference center point of the plate, where the plate has a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. 
     In yet another embodiment, a pupil aperture is disclosed. The pupil aperture includes a disc means having a set of pole aperture means that are radially offset from a reference center point of the disc means. The disc means has a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defining an off-axis location for each of the set of pole aperture means. Most preferably, the target angle is selected to be about 22.5 degrees from the horizontal reference line. 
     One advantage of the present invention is that it allows light to be more precisely directed onto a photoresist covered semiconductor wafer. Thus, any major distortions resulting from stray light is eliminated and the pupil aperture is able to improve the resolution of the features being patterned from the reticle to the photoresist. This is particularly powerful when features having very small critical dimensions and angled geometries are being patterned. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. 
     FIG. 1A shows an example of a photolithography system which includes a scanner system. 
     FIG. 1B is a top view of one example of a conventional pupil aperture. 
     FIG. 1C is a top view of one example of an off-axis pupil aperture. 
     FIG. 1D shows an example of a quadrupolar off-axis pupil aperture. 
     FIG. 1E shows an example of a reticle with a number of feature lines and a corresponding number of angled feature lines patterned on the reticle&#39;s glass surface. 
     FIG. 1F shows an example of feature lines that are transferred onto a resist covered die region by directing light through any of the prior art pupil apertures and the reticle in FIG.  1 E. 
     FIG. 2 shows a cross sectional view of an exemplary photolithography scanner system that is configured to receive a pupil aperture in accordance with one embodiment of the present invention. 
     FIG. 3 shows a top view of a quadrupolar rotated off-axis pupil aperture in accordance with one embodiment of the present invention. 
     FIG. 4 shows a top view of a eight pole rotated off-axis pupil aperture in accordance with another embodiment of the present invention. 
     FIG. 5A shows a top view of a quadrupolar rotated off-axis pupil aperture in accordance with another alternative embodiment of the present invention. 
     FIG. 5B shows a top view of an eight pole rotated off-axis pupil aperture in accordance with yet another alternative embodiment of the present invention. 
     FIG. 6A shows an example of a reticle with a number of feature lines and a corresponding number of angled feature lines patterned on the reticle&#39;s glass surface. 
     FIG. 6B shows an example of the feature lines that are transferred onto a resist covered die region by directing light through any of the pupil aperture embodiments of the present invention and the reticle in FIG.  6 A. 
     FIG. 7 shows a flow chart encompassing a method for manufacturing custom pupil apertures in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides an apparatus and method for designing photolithography scanner pupil apertures that assist in more accurately directing light onto a reticle when features having very small critical dimensions are being patterned. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIG. 2 shows a cross sectional view of an exemplary photolithography scanner system  200  that is configured to receive a pupil aperture  206  in accordance with one embodiment of the present invention. The scanner system  200  includes a first lens system  202  having components such as a beamsplitter, a condenser lens and a vertical delimiter among other optical components. From the first lens system  202 , light is projected through the pupil aperture  206  that is inserted into the scanner system  200  from a side slit  207  receiving region. As is well known, the pupil aperture  206  is used to better direct light onto a second lens system  204 . The second lens system  204  then focuses and delivers the light onto a reticle  208 . 
     This scanner system  202  is a simplified representation of a Micrascan III projection optics assembly, which may be obtained from Silicon Valley Group, Inc. (SVG) of Wilton, Conn. Of course, there are many types of scanner systems which also take advantage of pupil apertures to precisely control the projected light waves onto a photoresist covered wafer to improve patterning of small critical dimensions. 
     FIG. 3 shows a top view of a quadrupolar rotated off-axis aperture  206   a  in accordance with one embodiment of the present invention. A horizontal axis  212  is defined through a center point  210  of the pupil aperture  206   a  to assist in defining the preferred location and angles of a set of pole apertures  214 . As shown, the pupil aperture  206  includes four pole apertures  214   a - 214   d  which are located at a distance D offset  from the center point  210 . In this embodiment, D offset  may be in the range from about 0.3 inches to about 1.2 inches, and more preferably in the range from about 0.3 inches to about 0.9 inches, and most preferably about 0.6 to about 0.8 inches. 
     To achieve an optimum resolution on the developed photoresist when very small critical dimensions are being patterned, an angle between the pole apertures  214   a  and  214   c  from the horizontal axis  212  is defined by θ, and an angle between the pole apertures  214   b  and  214   d  from the horizontal axis  212  is defined by −θ. The preferred angle θ is between about 15° and about 35°, and more preferably between about 20° and about 25°, and most preferably about 22.5°. 
     Also, for the pole apertures  214   b  and  214   d,  the preferred angle −θ is between about −15° and about −35°, and more preferably between about −20° and about −25°, and most preferably about −22.5°. The preferred σ value of each of the pole apertures  214   a - 214   d  is between about 0.025 and about 0.2, more preferably between about 0.05 and about 0.15, and most preferably about 0.1. 
     As will be illustrated in greater detail below with reference to FIGS. 6A and 6B, this preferred range of angle ±θ and preferred offset value will generate substantial improvements in the resolution of developed photoresists. Specifically, this improved resolution is maintained even when angled geometries are patterned in complex integrated circuit devices. 
     In addition, this improvement in resolution is even more important when it is considered that integrated circuit lithography systems continue to use wavelengths in the deep UV range (i.e., 248 nm) and smaller. Of course, the improved resolution of the present invention is also beneficial for systems using I-line (i.e., 365 nm) wavelengths or larger. Consequently, it should be appreciated that this improvement in resolution is a powerful advancement which will continue to enable the fabrication of faster and more dense circuit designs. 
     FIG. 4 shows a top view of an eight pole rotated off-axis pupil aperture  206   b  in accordance with another embodiment of the present invention. A horizontal axis  212  and a vertical axis  216  are defined through a center point  210  of the pupil aperture  206   b  to assist in defining the preferred locations and angles the pole apertures  214 . Quadrants I-IV have been identified to further clarify the relationships between the pole apertures  214  and both axes  212  and  216 . 
     As shown, the pupil aperture  206   b  of this embodiment has eight pole apertures  214   a  - 214   h  which are oriented around the center point  210 . In other embodiments, more or less poles may be defined, depending on the resolution requirements. An angle between pole apertures  214   a  and  214   c  from the horizontal axis  212 , and the pole apertures  214   e  and  214   g  from the vertical axis  216  is defined by θ. The preferred angle θ for this embodiment is between about 15° and about 35°, and more preferably between about 20° and about 25°, and most preferably about 22.5°. 
     An angle between the pole apertures  214   b  and  214   d  from the horizontal axis  212 , and the pole apertures  214   f  and  214   h  from the vertical axis  216  is defined by −θ. The preferred angle −θ is between about −15° and about −35°, and more preferably between about −20° and about −25°, and most preferably about −22.5°. The preferred σ value of each of the pole apertures  214   a  - 214   h  is between about 0.025 and about 0.2, more preferably between about 0.05 and about 0.25, and most preferably about 0.1. 
     By arranging the pole apertures  214  in the above described angled orientations, each of the quadrants I-IV will contain two pole apertures  214 , and a total of eight pole apertures  214  will surround the center point  210 . In addition, a preferred D offset  to each pole aperture  214  from the center point  210  will be between about 0.5 inches and about 1.2 inches, and more preferably between about 0.6 inches to about 0.9 inches, and most preferably about 0.7 inches. 
     Again, as will be illustrated in greater detail below with reference to FIGS. 6A and 6B, this preferred range of angle ±θ will generate substantial improvements in the resolution of developed photoresists, particularly with respect to angled geometric patterns in small and complex integrated circuit devices. In some experimental data, it has been observed that the use of the eight pole rotated off-axis pupil aperture  206   b  can bring additional control to the directing of light onto the photoresists covered wafer, which may better define the resolution of the aforementioned angled geometric patterns. 
     FIG. 5A shows a top view of a quadrupolar rotated off-axis pupil aperture  206   c  in accordance with an alternative embodiment of the present invention. As described above with reference to FIG. 3, the pupil aperture  206   c  has four pole apertures  214   a  - 214   d  located at angles θ and −θ from the horizontal axis  212 . In this alternative embodiment, a semi-transparent surface  220 , which is defined within a stainless steel border  218  of the pupil aperture  206   c  allows a regulated amount of light to pass through to the photoresist covered wafer in addition to the light that is precisely directed through the pole apertures  214 . Of course, other materials besides stainless steel may be used. 
     FIG. 5B shows a top view of an eight pole rotated off-axis pupil aperture  206   d  in accordance with yet another alternative embodiment of the present invention. As described above with reference to FIG. 4, the pupil aperture  206   d  has eight pole apertures  214   a  - 214   h  located at angles θ and −θ from both the horizontal and the vertical axes  212  and  216 . As with the embodiment described above with reference to FIG. 5A, a semi-transparent surface  220 , which is defined within the stainless steel border  218  of the pupil aperture  206   d  allows a regulated amount of light to pass to the photoresist covered wafer in addition to light directed through the pole apertures  214 . 
     With reference to FIGS. 5A and 5B, the semi-transparent surface  220  is yet another advancement which will continue to enable the precise fabrication of smaller, faster, and more dense circuit designs. It is believed that the semi-transparent surface  220  will further enhance the resolution of developed photoresists by presenting additional techniques to better control to the directing of light onto the photoresists covered wafer. Furthermore, it is believed that the alternative embodiments of the present invention may further improve the resolution of complex geometric patterns, including traditionally hard to pattern angled geometries . 
     FIG. 6A shows an example of a reticle  112  with a number of feature lines  112   b  and a corresponding number of angled feature lines  112   b ′ patterned on the reticle&#39;s glass surface. Also shown are a number of inter-feature spaces  112   c  defined between any two of the feature lines  112   b  and its corresponding angled feature lines  112   b ′. For exemplary purposes, the feature lines  112   b / 112   b ′ are transferred onto a resist covered die region  314   a  as shown in FIG. 6B by directing light through any of the pupil aperture embodiments of the present invention. 
     As shown, the die region  314   a  includes a number of photoresist lines  314   b  and angled photoresist lines  314   b ′ that result after a development of the exposed photoresist. As evidenced from numerous experimental trials, the photoresist lines  314   b ′, which have an angled geometric orientation (with respect to a vertical axis), will produce a nearly identical pattern to the ideal geometric pattern defined on the reticle  112  of FIG.  6 A. Because all of the embodiments of the present invention are able to accurately and precisely direct light onto the surface of the reticle  112 , all of the major distortions in the developed photoresist of the prior art (shown in FIG. 1F) are essentially eliminated, even when very small geometries are being fabricated. 
     In one embodiment, the pupil apertures of the present invention have been proven to precisely define photoresist lines  314   b / 314   b ′ and corresponding spaces having widths that may be as small as about 160 nm or less, without experiencing the aforementioned distortion. Thus, the feature geometries will produce the desired electrical interconnections intended by the manufacturer, and enable the production of a functioning integrated circuit device. 
     The above described inventions may be further understood with reference to a flow chart presented in FIG.  7 . The flow chart encompasses a basic method  350  for manufacturing custom pupil apertures to correct resolution distortions when features having small simple and/or complex geometries (including angled geometries) are being patterned over photoresist covered wafers. The method  350  begins at an operation  352  where a photolithography scanner system is provided. The scanner system, such as SVG&#39;s Micrascan III projection optics assembly, typically includes a light source, one or more lens systems, and a reticle. 
     In operation  354 , a pupil aperture having a set of apertures that are optimized at an angle from a horizontal and a vertical line is generated. In one embodiment, the apertures include four pole apertures located at an angle ±θ from the horizontal line. In another embodiment, the apertures include eight pole apertures located at an angle ±θ from either the horizontal or the vertical line. In yet another embodiment, the pupil aperture includes a semi-transparent surface in addition to the set of apertures. 
     However, it should be noted that any design that includes apertures located at an angle ±θ from either the horizontal or vertical lines will increase the resolution of a pattern printed on a photoresist layer of a semiconductor wafer. As illustrated above with reference to FIGS. 6A and 6B, this preferred range of angle ±θ generates substantial improvements in the resolution of developed photoresists, particularly with respect to angled geometric patterns in small and complex integrated circuit devices. 
     In operation  356 , the pupil aperture is placed in the photolithography scanner system to perform an off-axis photolithography operation. In operation  358 , the scanner system is powered up, and a light source is activated to begin exposure of a photoresist layer that is over a semiconductor wafer. The light is directed to the photoresist layer through the lens systems, the pupil aperture and then the reticle. The pupil aperture enables the achievement of an optimum resolution on the developed photoresist when very small critical dimensions are being patterned. 
     In operation  360 , the photoresist is exposed to the light produced by the photolithography scanner system. In operation  362 , the photoresist is developed. The pattern that results after development precisely defines photoresist lines and corresponding spaces having widths that may be as small as about 160 nm or less as intended from the feature lines patterned on the reticle. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Of course, it should be understood that the pupil aperture embodiments of the present invention can be used to pattern any type of photoresist, such as, I-line photoresist, deep UV photoresist, etc., and any type of scanner system can be used, irrespective of manufacturer. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.