1. Field of the Invention
The present invention relates to the manufacture of integrated circuits and, in particular, to a method and system for determining overlay error between circuit layers made by a lithographic process.
2. Description of Related Art
Semiconductor manufacturing involves the printing of multiple integrated circuit patterns using lithographic methods on successive levels of exposure tools. A requirement of semiconductor manufacturing is to keep the alignment of each level to previous levels below product tolerance. Currently this is done using the optical microscope based tool that measures structures printed in the field kerf outside the product cell that comprises the printed circuit pattern. The field kerf is the area which separates the individual cells or patterns and which is unusable due to the width of the blade used to cut apart the cells or patterns upon completion of the printing. These structural features printed in the field kerf must be larger than the printed circuit pattern to enable the low resolution to image and make measurements of the current to prior level alignment.
Kerf to device overlay error prediction is an industry wide issue. A problem of conventional overlay metrology technique is that the printed structure used in the measurement is printed at a much larger size and different shape than that of the printed circuit. Due to the physics of optical lithography, mask making and the like, this can lead to errors in the measured structure overlay to that of the printed circuit overlay. In addition, typical high resolution methods of measuring in-chip overlay such as scanning electron microscopy (SEM) are complicated by the required direct placement of subsequent patterns on top of each other. This leads to difficulty or even impossibility of measuring the overlay directly in the product chip device since the structures typically sit on top of each other and it may be difficult to discern an edge of a device feature on one level from an edge of a device feature on another level. At sub 0.3 μm ground rules, the magnitude of the problem starts to become a potentially significant contribution to yield loss due to overlay error.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved system and method for determining overlay error between different lithographically produced layers of an integrated circuit chip.
It is another object of the present invention to provide a system and method for determining overlay error between superimposed active circuit features on different lithographically produced layers of an integrated circuit chip.
A further object of the invention is to provide a system and method for determining overlay error that avoids the problem of discerning different superimposed active circuit features on different lithographically produced layers of an integrated circuit chip.
It is yet another object of the present invention to provide such a system and method for determining overlay error that does not reduce the amount of active circuit area on a semiconductor wafer.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of determining overlay error in a desired direction in an integrated circuit made by a lithographic process. The method includes creating a first layer of the integrated circuit having at least one circuit area, the first layer circuit area including a first active circuit feature and a kerf area adjacent to the circuit area substantially free of active circuit features. The first layer kerf area includes a first measurement feature corresponding substantially to the first layer active circuit feature and which is separated from the first layer active circuit feature by a distance. The method also includes creating a second layer of the integrated circuit having at least one circuit area, the second layer circuit area including a second active circuit feature and a kerf area adjacent to the circuit area substantially free of active circuit features. The circuit and kerf areas of the first and second layers are substantially superimposed. The second layer kerf area includes a second measurement feature corresponding substantially to the second layer active circuit feature and which is separated therefrom by a distance. The distance of separation between the separated second layer active circuit feature and the second layer kerf measurement feature in the direction that the overlay error is to be determined is the same as the distance of separation between the separated first layer active circuit feature and the first layer kerf measurement feature in such direction. The second layer kerf measurement feature is displaced from the first layer kerf measurement feature compared to the first and second active circuit features in a direction perpendicular to the direction that the overlay error is to be determined. The method then includes determining a common point of reference of each of the first and second layer kerf measurement features, and measuring distance of separation in the direction of overlay error between the common points of reference of each of the first and second layer kerf measurement features to determine overlay error of the first and second active circuit features.
In another aspect, the present invention is directed to an integrated circuit wafer adapted to measure overlay error between layers made by a lithographic process. The wafer includes a first layer of the integrated circuit having at least one circuit area. The first layer circuit area includes a first active circuit feature and a kerf area adjacent to the circuit area substantially free of active circuit features. The first layer kerf area includes a first measurement feature corresponding substantially to the first layer active circuit feature and which is separated from the first layer active circuit feature by a distance. The wafer also includes a second layer of the integrated circuit having at least one circuit area. The second layer circuit area includes a second active circuit feature and a kerf area adjacent to the circuit area substantially free of active circuit features. The circuit and kerf areas of the first and second layers are substantially superimposed. The second layer kerf area includes a second measurement feature corresponding substantially to the second layer active circuit feature and separated therefrom by a distance. The distance of separation between the separated second layer active circuit feature and the second layer kerf measurement feature in the direction that the overlay error is to be determined is the same as the distance of separation between the separated first layer active circuit feature and the first layer kerf measurement feature in such direction. The second layer kerf measurement feature is displaced from the first layer kerf measurement feature compared to the first and second active circuit features in a direction perpendicular to the direction that the overlay error is to be determined. Common points of reference of each of the first and second layer kerf measurement features are determinable to permit measurement of any separation between the common points of reference of each of the first and second layer kerf measurement features to determine overlay error.
In the method and wafer of the present invention, preferably the first and second active circuit features corresponding to the first and second layer kerf measurement features are in contact with each other. The first and second layers of the integrated circuit each may have a plurality of circuit areas separated by kerf areas. Preferably, the second layer kerf measurement feature is displaced from the first layer kerf measurement feature by a distance sufficient to distinguish the corresponding active features in the circuit area so that the first and second layer kerf measurement features are more easily discerned. The common points of reference of the first and second layer kerf measurement features may comprise centerlines or edges of the features. The measurement features in the kerf areas are adapted to be destroyed when the plurality of circuit areas are cut apart.
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
In describing the preferred embodiment of the present invention, reference will be made herein to
In order to easily correlate the conventional overlay measurement to device overlay, the present invention provides a system and method of representing a measurable device structure in the kerf area, between the printed functional circuit areas, to aid in direct device overlay measurement. The present invention is based on a method that allows the overlay to be determined on circuit size and shaped device patterns without the difficulty related to pattern-on-pattern placement. To accomplish this, in the preferred embodiment the device design itself is printed in the kerf, with two patterns being printed not on top of each other, but physically separated in at least one dimension. This permits the use of high resolution microscopy to measure edge or centerline overlay error without the confluence of edges. The preferred kerf measurement structures use the actual device design, displaced relative to each other allowing easily discernible edges of each structure. They are still located symmetrically about each other such that conventional edge or centerline overlay measurements may still be made.
A typical pattern made in a dynamic random access memory (DRAM) chip is made up of a plurality of active circuit areas separated by the kerf areas, also known as streets. A typical memory device cell has many active areas that need to be aligned between different lithographically produced levels. In practice, however, the active circuit areas on one level may misalign with respect to active circuit areas on another level and cause short and/or open circuits. To obtain an error measurement of how far the different levels are displaced, the present invention takes the same structure or design features as in the active circuit area, or structures that are substantially corresponding to them, and actually places them in the kerf region and separates them in one dimension. The separation need not be a known amount, so long as the two edges on the two different layers can be clearly discerned and one can determine a centerline estimate of overlay misalignment.
By a substantially corresponding structure or design feature is meant that the structure or design feature is substantially replicated with respect to the features in issue, for example edges, and under the same design rule. Preferably, the kerf measurement feature is reproduced as closely as practical, and more preferably the kerf measurement feature replicates the size and pitch of the corresponding active feature in the circuit pattern, as well as its shape and its proximity to other structures, as closely as possible. Thus, the kerf measurement feature corresponds substantially to the corresponding active circuit feature.
In accordance with the present invention, instead of overlaying the measurement features from different layers on top of each other in the kerf area, they are displaced or separated by some amount, in a direction normal to the direction of overlay error measurement, such that one can easily more obtain a measurement of what the overlay misalignment is between them. The location in the kerf measurement structure is not normally important in practicing the present invention, and can be at random in various places, or it can be at one set reference location. A kerf measurement structure is separated from its corresponding similar active structure in the circuit area on each level, and the degree of separation in one of the x- or y-dimension is by the same distance. Further, each layer's kerf measurement structure should be displaced or separated in the dimension 90° or perpendicular to the direction of overlay error measurement so that the edges can be clearly identified for each layer, and the measurement structures of the two layers are not superimposed upon each other. The two kerf measurement features on the different layers need not be completely separated, but should be displaced by some amount so that features on each can be distinguished. For example, where the overlay error measurement is to be made in the y-dimension, the amount of the x-dimension separation is immaterial, as long as the kerf measurement features of the two layers which are difficult to discern in the active structure are physically displaced or separated in the kerf by a sufficient distance so that the separate layer features are easier to discern. However, in each layer the distance of separation or offset between the active feature in the circuit pattern and the measurement feature in the kerf area, in the direction of the offset error measurement (e.g., the y-direction), has to be identical. This distance of separation in the direction of overlay error measurement may range from zero to any maximum that enables the active circuit structures and the corresponding kerf measurement structures to fit on the wafer.
A first example of the present invention is depicted in
However, in
In accordance with the present invention, kerf measurement features 20′, 22′ are formed in the kerf area adjacent to the circuit area which substantially correspond to and are representative of active features 20, 22, respectively, in the active circuit area. As shown in
Another example of the corresponding active circuit features and kerf measurement features is shown in
A third example of the present invention is shown in
An example of the placements of the corresponding active circuit features and kerf measurement features on an integrated circuit wafer is shown in
Accordingly, the present invention provides an improved system and method for determining overlay error between different lithographically produced layers of an integrated circuit chip, particularly for determining overlay error between superimposed active circuit features on different layers of an integrated circuit chip. It overcomes the problem of discerning different superimposed active circuit features on different lithographically produced layers of an integrated circuit chip, yet does not reduce the amount of active circuit area on a semiconductor wafer. By the present invention, edges of features on different layers which are difficult to discern in active circuit areas are readily discerned in the kerf measurement structures used for overlay error measurement.
While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations wilt be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.
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