Patent Application
20100017778
Embodiments of the present invention relate to the field of semiconductor fabrication. More particularly, embodiments of the present invention relate to methods of defining evaluation points for optical proximity correction of a pattern in a radiation-patterning process.
Photolithography is commonly used during the fabrication of integrated circuits on semiconductor wafers and other bulk substrates comprising a layer of semiconductor material. During photolithography, a form of radiant energy is passed through a radiation-patterning tool onto a radiation-sensitive material, commonly referred to as photoresist, which is placed upon a surface of a semiconductor wafer. The radiation-patterning tool is commonly known as a photomask or reticle.
Radiation-patterning tools contain light-restrictive regions and light-transmissive regions. Light-restrictive regions may be, for example, opaque or partially light transmissive. The light-transmissive regions or portions of a radiation-patterning tool, in conjunction with the light-restrictive regions, cooperatively facilitate the formation of a desired pattern on a semiconductor wafer. For the formation of patterns on a semiconductor wafer, the wafer is coated with a layer of radiation-sensitive material (e.g., photoresist). Subsequently, radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers onto the photoresist a pattern defined by the radiation-patterning tool. Using a form of a photographic process, the photoresist is then developed to remove either the portions exposed to the radiant energy in the case of a “positive” photoresist or the unexposed portions when a “negative” photoresist is utilized. The residual photoresist pattern thereafter serves as a mask for a subsequent semiconductor fabrication process.
With advances in semiconductor integrated circuit processes, the dimensions associated with integrated circuit device features have decreased. Furthermore, the demand for smaller and faster-performing semiconductor devices requires increasing precision and accuracy in photolithographic processes.
Radiation-patterning tool 120 typically includes an obscuring material that may either be an opaque (e.g., chrome) or a semi-opaque material placed over a transparent material (e.g., glass). Radiation-patterning tool 120 is illustrated in
Following creation and verification of the design, an optical proximity correction process 112 is used to account for various interference factors that influence radiation passing through the radiation-patterning tool. Such interference factors may include constructive and destructive interference effects resulting as the radiation wavelength approximates the dimensions of portions of the profiles or elements of the radiation-patterning tool. In an optical proximity correction process, a geometry, such as a square or a rectangle, within a pattern layout may be adjusted to compensate for optical effects (e.g., optical, micro-loading, etch, resist, etc.) such that a resultant pattern more closely approximates the desired pattern. These adjustments are made based upon results of model-based simulations of the pattern layout at so-called “evaluation points,” which are defined on the geometry to be adjusted.
As a result of the optical proximity correction process 112, a data set that corresponds to a pattern capable of generation by a radiation-patterning tool is typically generated. The data set is subsequently “taped out” to a radiation-patterning tool through the use of, for example, laser writing and/or electron-beam writing methodologies. Following the formation of the pattern on the radiation-patterning tool, the tool is capable of being utilized for semiconductor fabrication.
Conventionally, in an optical proximity correction process, the locations of the evaluation points are pre-determined through simple rules, such as by taking the mid-point of an edge of a geometry, or by considering other factors, such as the shape of the geometry. Unfortunately, these simple rules frequently do not select the optimal evaluation points for a given geometry.
There is a need for methods of enhancing a radiation-patterning process Specifically, there is a need for methods of defining evaluation points for increased accuracy of optical proximity correction of a pattern in a radiation-patterning process.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof and, in which is shown by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical, and electrical changes may be made within the scope of the disclosure.
Specific implementations shown and described in this description are only examples and should not be construed as the only way to implement the present invention unless specified otherwise herein. For the most part, details concerning timing considerations, and the like, have been omitted where such details are not necessary to obtain a complete understanding of the present invention in its various embodiments and are within the abilities of persons of ordinary skill in the relevant art.
Referring in general to the following description and accompanying drawings, various aspects of the present invention are illustrated to show structure and method of operation of embodiments thereof. Common elements of the illustrated embodiments are designated with like numerals. It should be understood the figures presented are not meant to be illustrative of actual views of any particular portion of the actual structure or method, but are merely idealized representations which are employed to more clearly and fully depict the present invention.
The term “photomask” traditionally is understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized with their conventional meanings.
When executed as firmware or software, the instructions for performing the methods and processes described herein may be stored on a computer readable medium. A computer readable medium includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), and semiconductor devices such as RAM, DRAM, ROM, EPROM, and Flash memory.
In the following description, the term “target geometry” is defined as a representative data set of a desired geometry within a target pattern layout that is to be formed on a semiconductor wafer. The term “simulated image” is an image produced during an optical proximity correction process simulation of a target geometry. The term “reticle geometry” is defined as a representative data set of a geometry within a reticle pattern layout to be formed on a radiation-patterning tool (i.e., radiation-patterning tool 120 illustrated in
For explanation purposes, an example of a conventional optical proximity correction process implementing evaluation points defined on a target geometry will first be described with reference to
As mentioned above, an optical proximity correction process may be implemented to compensate for interference effects that may occur when radiation passes through a radiation tool and exits out onto a radiation-sensitive material. More specifically, an optical proximity correction process may iteratively adjust a geometry within a target pattern layout in order to minimize some measure of the difference between the target geometry and a simulated geometry predicted by a model. Adjustment of a geometry is usually done by moving one or more segments of the geometry. Therefore, in order to adjust a geometry, dissection points are initially defined for the geometry. Between adjacent dissection points exists a line segment, wherein each line segment includes an evaluation point. After defining dissection points and evaluation points, the optical proximity correction process may calculate an error for each segment at a corresponding evaluation point. Each segment may then be moved, or biased independently by an amount based upon the deviation of a simulated image from the target geometry.
As known by one having ordinary skill in the art, producing a corner within a geometry, such as a rectangle, on a photoresist by photolithography has proven to be difficult due to interferences in small scale printing. Commonly, the corners of a rectangular geometry print with a rounded pattern, and therefore, a resultant image formed on a semiconductor wafer may resemble a circle or an ellipse. To partially counteract these interferences, an optical proximity correction process may involve the use of modified geometries or adjacent sub-resolution geometries to improve imaging. An example of this, as described more fully below, is the use of a “serif” on one or more corners of a geometry.
Target geometry 302 also includes evaluation points 304-315, wherein each evaluation point is positioned in a corresponding line segment. Evaluation points 304-307 each lie in the middle of a corresponding mid-line segment. Therefore, for ease of description, evaluation points 304-307 will be hereinafter referred to as mid-points 304-307. Furthermore, evaluation points 308-315 each lie in a corresponding corner line segment. Therefore, for ease of description, evaluation points 308-315 will be hereinafter referred to as corner points 308-315. Subsequent to defining dissection points and evaluation points, simulations are performed at each mid-point 304-307 and at each corner point 308-315, and the corresponding line segments may be adjusted according to the difference between a simulated image and the target geometry. Conventionally, as mentioned above, evaluation points may be pre-determined by simple rules, such as by taking the mid-point of a corresponding line segment, or by considering other factors, such as the shape of the geometry to which the segment belongs.
In accordance with one or more embodiments of the present disclosure, a method of defining evaluation points and use of the evaluation points within an optical proximity correction method will now be described with reference to
FIG. SA illustrates a target geometry 402 wherein dissection points N-Y and line segments (NO, OP, PQ, QR, RS, ST, TU, UV, VW, WX, XY, YN) have been defined. Although target geometry 402 is illustrated as an oblong geometry, target geometry 402 may comprise any rectangular geometry (e.g., a square). Furthermore, although target geometry 402 is shown as having dissection points N-Y, a target geometry having any number of dissection points, and thus, any number of line segments is within the scope of an embodiment of the present disclosure. Similar to target geometry 302 illustrated in
At act 512, a contour of a predicted image (i.e., an image to be produced during an optical proximity correction simulation) may be predicted. A contour of an image may be predicted at least partially based on an assumption that the contour should pass through each mid-point 404-407 and by knowing the radius of curvature of the image at each mid-point 404-407. More specifically, because it may be desirable for a predicted image to have a contour which passes through each mid-point 404-407, the radius of curvature of the predicted image at each mid-point 404-407 may first be determined. Thereafter, a contour of the predicted image may be determined using the radius of curvature of the predicted image at each mid-point 404-407. Conventionally, lithography of large shapes (i.e., L>>r, where L is a length of a side of a square, or the shorter side of a rectangle and r is the radius of a simulated image) requires minimal or no optical proximity corrections, and, therefore, it should be noted that this disclosure focuses on target geometries that are small compared to the wavelength of the light in the illumination system (e.g., r˜L/2).
For example, in an embodiment wherein target geometry 402 comprises a rectangle having a width equal to its length (i.e., a square), a predicted image should comprise a circle and, therefore, the radius of curvature of the predicted image should be consistent throughout. Initially, a minimum radius of curvature may be predicted based on a measure of resolution of an illumination system. For example, a minimum radius of curvature may be approximated by an imaging theory (e.g., Hopkins Equation) or by the following equation:
r=λ/[2*NA*(1+σ)]; (1)
wherein r is the minimum radius of curvature of the predicted image, σ is the partial coherence, λ is the wavelength of the illumination light, and NA is the numerical aperture of the illumination system utilized in the photolithographic process. It should be noted that in the derivation of equation (1), it is assumed that a pupil shape of each lens 801 within the illumination system utilized is circular with an acceptance angle of NA, as illustrated in
theta1=arctan[(r−x1)/r]; (2)
x1=r*(1−sine(theta1)); (3)
y1=r*(1−cosine(theta1)); (4)
wherein r is the minimum radius of curvature of the predicted image solved for above in equation (1), theta1 is an angle of displacement from the vertical axis, and x1′ is the distance from corner point 615 to a corner 640 of the target geometry 402.
Furthermore, in an embodiment wherein target geometry 402 comprises an oblong (e.g., a rectangle having a width that is unequal to its length), the location of one or more points along a contour of a predicted image and adjacent to a corner of target geometry 402 may be determined based on the radii of curvature of the two mid-points adjacent to the corner. For example, in reference to
theta2=arctan[(r1−x2′)/r2=arctan[(r1−x2)/(r2−y2)]; (5)
[(r1−x2′)/r2=[(r1−x2)/(r2−y2)]); (6)
[(r1−x2)/r1]̂2+[(r2−y2)/r2]̂2=1; (7)
wherein r1 is half the distance of the larger rectangle side of the target geometry 402, r2 is half the distance of the smaller rectangle side of the target geometry, x2′ is the distance from corner point 415 to a corner 740 of the target geometry, and theta2 is an angle of displacement from the vertical axis. Specifically, as known by a person of ordinary skill in the art, variables x2 and y2 may then be solved for from equations (6) and (7) to determine a location of associated location C2.
Subsequently, at act 516, an evaluation point overlying predicted image 432 may be selected and an optical proximity correction process may then be performed on the selected evaluation point at act 517. If the image simulated during optical proximity correction does not overlie the selected evaluation point, then, at act 518, the optical proximity correction process may determine an edge placement error, which is the distance between the selected evaluation point and a simulated image (an image produced during simulation in an optical proximity correction process). Thereafter, also at act 518, the line segment corresponding to the selected evaluation point may be displaced by an amount relative to the edge placement error. As an example, with reference to
It may be desirable that after an optical proximity correction process is performed at each mid-point 404-407 and each corner point 408′-415′/450-453, the contour of the simulated image will overlie each mid-point 404-407 and each corner point 408′-415′/450-453. Because it may not always be possible to produce a simulated image having a contour overlying each mid-point 404-407 and each corner point 408′-415′/450-453, each mid-point 404-407 and each corner point 408′-415′/450-453 may be scaled, or in other words, a specific weight may be assigned to each mid-point 404-407 and each corner point 408′-415′/450-453. As a result, it may be possible to assign each mid-point 404-407 a higher weight than each corner point 408′-415′/450-453 and, thus, ensure that a contour of a simulated image overlies each mid-point 404-407. For example only, and not by way of limitation, each mid-point 404-407 may be scaled to 100% and each corner point 408′-415′/450-453 may be scaled to a value less than 100% (i.e., 50%).
Specific embodiments have been shown by way of non-limiting example in the drawings and have been described in detail herein; however, the various embodiments may be susceptible to various modifications and alternative forms. It should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention encompasses all modifications, equivalents, and alternatives falling within the scope of the following appended claims and their legal equivalents.
