Photolithography is a basic technique for forming patterns in semiconductor manufacturing processes. Photolithography generally involves: (1) coating a wafer with a photoresist material; (2) placing a mask having desired patterns above the wafer; and (3) exposing the mask and wafer to light. Light exposure causes a chemical reaction in the photoresist which enables the transfer (or printing) of the mask patterns. The wafer is then generally subject to a development process to remove portions of the photoresist (either the exposed portions or the unexposed portions, depending on the type of photoresist used) while retaining the desired printed patterns. There are generally two types of photoresists: positive photoresist and negative photoresist. When a positive photoresist is used, the exposed portions are removed during development. When a negative photoresist is used, the unexposed portions are removed during development. For ease of explanation only, throughout this application, various exemplary implementations are described as using the negative photoresist. One skilled in the art will readily recognize that the positive photoresist may be used instead in accordance with design requirements or preferences.
In semiconductor manufacturing, device miniaturization has always been one of the most important research and development goals. One way to achieve this goal is to print (and develop) smaller contact holes in the semiconductor devices, for example, by using masks having smaller contact hole dimensions.
However, resolution of the printed patterns worsens as contact hole dimensions become smaller. This is the result of optical diffraction. As pattern dimensions shrink, exposure light passing through the openings on the mask will also expose unintended areas around the openings. The exposure of unintended areas will cause a reduction in light contrast, resulting in degraded pattern resolution.
One technique to improve pattern resolution is to use the so-called phase shift masks. Conventional masks used in photolithography are generally referred to as binary masks. Binary masks are typically made by forming patterned opaque materials (such as chromium) on a transparent substrate (such as glass). In a phase shift mask, however, one or more phase shifting materials (e.g., phase shifters) are formed on a transparent substrate (with or without the opaque materials) to change the phase of light passing through such phase shifters by a predetermined amount (e.g., 90°, 180°, 270°, etc.). It is generally understood that the tranmission coefficients of light through both the transparent substrate and phase shifters of different phases are substantially the same. A phase shift mask typically has multiple regions having different (e.g., alternating) phase shifters.
For example, a phase shift mask may have a first region having 0° phase and a second region having 180° phase. In this example, when the phase shift mask (placed above a wafer coated with photoresist) is exposed to light, light passing through the first region having 0° phase and the second region having 180° phase will cancel each other in any overlapping areas, thereby printing a thin, well defined line(s) at the edge(s) separating the two regions. Due to optical interference, light intensity is reduced at the overlapping regions and greater difference in light intensity between the two exposed regions on the wafer can be achieved. As a result, better defined patterns can be formed.
One technique to control the size of these regularly-spaced contact holes is to form an opaque material (e.g., a thin layer of chromium) or the so-called regulators along the edges of the vertical and horizontal rectangular phase shifters described in the example above. Exemplary phase shift masks implementing chromium regulators are illustrated in
Existing techniques implementing phase shift masks to print contact holes generally result in regularly-spaced contact holes. In practice, however, most semiconductor devices require irregularly-spaced contact holes.
Three-phase phase shift masks may be implemented to print single contact holes at desired locations of a semiconductor device.
Although the three-phase phase shift masks may be used to print single contact holes which may be irregularly spaced, this technique typically requires multiple layers of phase-shifters; thus, it's too costly to be widely implemented in practice.
Thus, a market exists for techniques to use phase shift masks to print irregularly-spaced contact holes.
An exemplary method for printing irregularly-spaced contact holes of a semiconductor device comprises printing a semiconductor wafer using at least one multi-phase phase shift mask. The mask has a plurality of polygons on a substrate, where at least one of the polygons is irregularly-spaced with respect to another polygon, and one or more of polygons provide sufficient phase shift relative to an exterior region of the polygons to effect destructive light interference for printing irregularly-spaced contact holes along one or more phase shift edges.
An exemplary method for forming phase shift masks for printing irregularly-spaced contact holes on a semiconductor wafer comprises classifying contact holes into groups, each group comprising at least two contact holes, designing a first set of polygons, each of the first set of polygons intersecting all contact holes of one group, and at least one polygon of the first set being irregularly-spaced with respect to one or more other polygons of the first set, designing a second set of polygons, each of the second set of polygons intersecting at least one polygon of the first set at the locations of the contact holes being intersected by the at least one polygon of the first set, forming a phase shifter in an interior region of each polygon such that the interior region provides sufficient phase shift relative to an exterior region of the polygon to effect destructive light interference during a light exposure for printing irregularly-spaced contact holes along one or more phase shift edges, and forming one or more phase shift masks based on the first and second sets of polygons.
These and other exemplary embodiments and implementations are disclosed herein.
I. Overview
Techniques for using phase shift masks to print irregularly-spaced contact holes on a wafer are disclosed herein. For ease of explanation, throughout this application, the term contact holes will be used. However, this term shall include, without limitation, vias, posts (e.g., if positive photoresist is used), and/or other similar features on a device.
Section II describes exemplary implementations for aligning two sets of polygons (e.g., of two phase shift masks) for printing irregularly-spaced contact holes.
Section III describes exemplary implementations for printing redundant and/or single contact holes to complement the implementations described in Section II.
Section IV describes exemplary implementations for enabling printing of contacts holes of different sizes and shapes.
Section V describes potential errors and exemplary error correction techniques.
Section VI describes exemplary embodiments for implementing other phase shift masks (e.g., three-phase or four-phase masks) to print irregularly-spaced contact holes.
Section VII describes an exemplary process for designing polygons of phase shift mask(s) in accordance with the exemplary implementations described herein.
Section VIII describes exemplary techniques for resolving a phase conflict.
Section IX describes an exemplary operating environment.
II. Exemplary Polygons of Phase Shift Masks for Printing Irregularly-Spaced Contact Holes
In general, when two phase shift masks are used to print irregularly-spaced contact holes, polygons of the first phase shift mask should intersect with polygons of the second phase shift mask only at the locations of the desired contact holes (including intentionally inserted redundant holes). These intersecting polygon pairs can be called companion polygons. However, polygons of the same phase shift mask may intersect other polygons of the same mask at any location.
Similar to
III. Enabling Printing of Redundant or Single Contact Holes
In general, when two polygons intersect, there are typically two locations on each polygon where intersections occur. Consequently, when designing two sets of polygons (e.g., of two phase shift masks), a majority of contact holes will be designed in pairs. While these implementations effectively cover the printing of most desired contact holes, from time to time, a redundant hole or a single contact hole may have to be printed. For example, if the desired contact hole count is odd, a redundant (or dummy) hole may be designed to enable efficient application of the exemplary implementations described herein. A single contact hole may be needed sometimes, for example, in a location that does not have enough space for a redundant hole or if placing a redundant hole is inappropriate according to design rules. In this case, a single contact hole may be printed to complement the various exemplary implementations described herein.
A. Redundant Holes
It is important to design redundant holes at locations that will not interfere with the proper function of the device. For example, a redundant hole should not form unintended contact or connection with any other structure(s) (e.g., metal lines) on the device.
B. Single Contact Holes
In the top series of figures, the resist image of a polygon (having a 180° phase shift relative to its surrounding substrate) printed on a wafer is a rectangle having an exposed interior and an unexposed border (indicating by the darkened border). As the polygon becomes smaller, the printed resist image of the polygon will have a smaller exposed interior, which can eventually disappear and result in a single unexposed bar (see bottom figures). If two such small polygons of two masks are aligned orthogonally to each other, the resulting resist image will be a single unexposed dot (which can be developed into a single contact hole).
The technique described above is merely exemplary. A person skilled in the art will recognize that other techniques known in the art may also be implemented to from single contact holes. For example, certain three-phase phase shift mask technique known in the art may be implemented instead to print single contact holes at certain desired locations to complement the exemplary implementations described herein.
IV. Controlling the Sizes and Shapes of Contact Holes
In general, larger contact holes are more robust than smaller contact holes. Also, square or rectangular contact holes are generally more robust than non-square and non-rectangular (e.g., round, etc.) contact holes of the same design rule. A square or rectangular contact hole typically has up to 36% more contact area than a circular contact hole of the same design rule. Thus, it is desirable to have more control over the sizes and shapes of the contact holes formed on a device.
A. Add Regulators
In an exemplary implementation, the size and shape of contact holes may be controlled by forming regulators of varying thicknesses around the polygons of each mask.
B. Adjust the Angle of Intersection
In an exemplary implementation, the regulators of polygons of phase shift masks may be placed at different intersecting angles to control the size and/or shape of the resulting contact holes.
C. Alignment with Metal Lines
Contact holes are often used to electrically connect multiple metal lines. Thus, alignment and lateral widths of the metal lines to be connected can affect the size and shape of the connecting contact hole. For example,
V. Errors and Error Corrections
A. Misalignment Errors
In any photolithography designs, mask misalignment error must be considered. In general, misalignment error increases as more masks are involved in a manufacturing process. However, the exemplary implementations described herein generally incur the same misalignment error(s) as single mask processes. This is illustrated in
Similarly, hole 2 is not sensitive to a misalignment in the x direction of the first exposure and y direction of the second exposure. Hole 2 may incur a misalignment error in the y direction if a misalignment occurs in the first exposure but will be unaffected if a misalignment (in the y direction) occurs in the second exposure. Further, hole 2 may incur a misalignment error in the x direction if a misalignment occurs in the second exposure but will be unaffected if a misalignment (in the x direction) occurs in the first exposure. Thus, hole 2 is not subject to cumulative misalignment errors resulting from the two exposures and has the same misalignment tolerance as if it were printed by a single mask process.
Techniques for correcting and/or compensating for misalignment errors in single exposure processes are well known in the art and need not be described in detail herein.
B. Image Shift Due to Asymmetry
Due to geometrical asymmetry between interior and exterior regions of polygons, the resist image of lines printed at the borders of the polygons may shift in different directions (e.g., toward the inside or outside of the polygons).
For example, the top figure in
Given the intensity of light used in a photolithography exposure, a person skilled in the art can calculate the amount and direction of the slight shift in the resist image. Thus, in an exemplary implementation, one or more polygons may be intentionally shifted in the opposite direction by a pre-calculated amount. The top figure in
Of course, the example provided above is merely exemplary. Depending on the specific design of the polygons and a given exposure environment, the printed resist images of one or more polygons may be shifted in one or more other directions (e.g., in opposite directions of the example provided above).
VI. Other Phase Shift Masks
The various implementations described above generally involve two-phase phase shift masks. However, a person skilled in the art will recognize that masks having more than two phases may also be used in accordance with embodiments described herein. For example, three-phase or four-phase phase shift masks may be implemented.
A. Three-Phase Phase Shift Masks
When the two three-phase phase shift masks are successively exposed, contact holes are printed at the intersections of the 180° phase shift edges of polygons of the first mask and the 180° phase shift edges of polygons of the second mask.
In another exemplary implementation, the three-phase phase shift masks may be designed directly (without first determining the polygons for the two-phase phase shift masks) by determining a first set of polygons each intersecting desired irregularly-spaced contact holes at their 180° phase shift edges and determining a second set of polygons each intersecting the first set of polygons at the locations of the desired irregularly-spaced contact holes.
The exemplary implementation described above is merely illustrative. A person skilled in the art will recognize that other techniques for implementing three-phase phase shift masks can also be implemented in accordance with design choice and/or requirements.
B. Four-Phase Phase Shift Masks (Vortex Masks)
In another exemplary embodiment, the substrate of the mask may have one phase shift value (0°) and three polygons having three other different phase shift areas can be used to form a four-phase phase shift mask. This mask may be used to print even-numbers of irregularly-spaced contact holes. In an exemplary implementation, a four-phase phase shift mask may be formed using the two-phase phase shift masks described in exemplary implementations herein (e.g., see
The exemplary implementation described above is merely illustrative. A person skilled in the art will recognize that other techniques for forming and/or using four-phase phase shift masks can also be implemented in accordance with design choice and/or requirements. For example, a four-phase phase shift mask may be formed by first determining two sets of polygons in accordance with implementations described herein without explicitly assigning the polygons to two two-phase phase shift masks, then applying the above transformation to form the four-phase phase shift mask. Further, one may also design a four-phase phase shift mask by determining a first set of polygons each having a 90° phase shift in its interior region and a second set of polygons each having a 270° phase shift in its interior region, then determine the 180° and 0° regions by derivation. For example, typically, the 180° polygons on a vortex mask are surrounded by one or more 90° phase shift polygons and one or more 270° phase shift polygons. Therefore, one can derive the locations of the 180° phase shift polygons when one has knowledge of the locations of the 90° and 270° phase shift polygons. Also, the substrate outside of all polygons on the mask generally has 0° phase shift. Therefore, one can easily derive the phase shift of the remaining areas outside of the 90° and 270° phase shift polygons (i.e., 0° phase shift).
The transformation described above is merely exemplary. A person skilled in the art will recognize that other transformations may also be applied to transform one or more multi-phase phase shift masks to another one or more multi-phase phase shift masks. For example, a set of two-phase phase shift masks can be obtained from a four-phase phase shift mask by reversing the transformation described above.
VII. An Exemplary Process for Designing Phase Shift Masks
In general, when designing phase shift masks in accordance with exemplary implementations described herein, the masks should print contact holes where desired and not where undesired.
At step 2410, desired contact holes are classified into even-numbered groups. For example, each group may include two contact holes. Redundant (or dummy) holes may be added into any group whenever necessary in accordance with design rules. For any desired contact hole that cannot be paired with other contact holes into an even-number group and if a redundant hole cannot be added nearby, a single contact hole may be designed and implemented in accordance with exemplary implementations described above in Section III and/or other techniques known in the art.
At step 2420, determine a first polygon of a first set of polygons for a first group of contact holes. The first polygon should intersect all the contact holes within the first group. The size and shape of each polygon may be determined based on available space and locations of nearby groups.
At step 2430, determine a first polygon of a second set of polygons for the first group of contact holes. The first polygon of the second set should intersect the first polygon of the first set at only the locations of the desired contact holes of the first group.
At step 2440, whether another group exists is determined.
If not, the process ends at step 2470.
If another group exists, at step 2450, a second polygon of the first set for the next group of contact holes is determined. This polygon should intersect all the contact holes within the next group.
At step 2460, a second polygon of the second set for the next group of contact hole is determined. This polygon should intersect the second polygon of the first set determined at step 2450 at only the locations of the desired contact holes of the next group. Steps 2450 and 2460 are repeated for the next group until all groups have been processed and the process ends at step 2470.
In an exemplary implementation, for ease of design, polygons of a first set may be distinguished by a different color than polygons of a second set. In general, polygons that overlap or contact each other (other than at the locations of the desired contact holes) must be of the same set (or color). For improved efficiency, overlapping polygons of the same set (or color) can be merged into one polygon. In addition, for improved robustness, polygons that are close to each other should also be of the same set (or color).
The two sets of polygons formed by the process described above then can be implemented to form one or more two-phase, three-phase, or four-phase (or other multi-phase) phase shift masks. For example, the first set of polygons may be assigned to a first mask and the second set of polygons may be assigned to a second mask to form two two-phase phase shift masks or two three-phase phase shift masks which can be used to print irregularly-spaced contact holes as described herein. Alternatively, a four-phase phase shift mask can be formed by applying the transformation set forth in Section VI.B. above to the two sets of polygons. The single four-phase phase shift mask can be used to print irregularly-spaced contact holes.
The various exemplary implementations described herein are merely illustrative. One skilled in the art will recognize that other design techniques may be used to print irregularly-spaced contact holes with one or more phase shift masks. Further, one skilled in the art will recognize that the described implementations are not precluded from being used to print regularly-spaced contact holes exclusively, or in combination with printing irregularly-spaced contact holes.
VIII. Resolving Phase Conflict
From time to time, a phase conflict may occur when two polygons of different sets overlap or contact each other at unintended locations. For ease of explanation, polygons causing a phase conflict will be referred to as offending polygons. When phase conflict occurs, one or more of the following techniques may be used to resolve the issue.
A. Reassign One or More Offending Polygons to a Different Set
At step 2510, change the assignment of an offending polygon from belonging to a first set to a second set. For ease of explanation, we will refer to polygons of a first set as being red and polygons of a second set as being blue. If red and blue polygons overlap (or intersect) each other at unintended locations, a phase conflict occurs. Thus, in this implementation, an offending red polygon is changed to be a blue polygon (“first new blue polygon”).
Typically, each polygon on a first set intersects a corresponding polygon of a second set to enable printing of at least two contact holes. Thus, the offending red polygon in this example also legitimately intersects another blue polygon (i.e., a companion polygon of the second set of polygons). Therefore, when the offending red polygon is changed to be the first new blue polygon, one will have to change the color of the companion polygon to red (“companion new red polygon”). In other words, the companion polygon of the second set (intersecting the offending polygon of the first set) is reassigned to be of the first set.
At step 2520, change the assignment of a chain of all other polygons that are overlapping or touching the offending polygon of step 2510 to belong to the second set. For example, if a second red polygon overlaps the first new blue polygon, that second red polygon should be changed to a second new blue polygon. If a third red polygon overlaps either the first new blue polygon or the second new blue polygon, then the third red polygon should be changed to a third new blue polygon and so forth until all polygons of different colors overlapping each other at unintended locations (i.e., not the locations of the desired contact holes) have been changed to have the same color.
At step 2530, change the assignment of a chain of all other polygons that are overlapping or touching the companion polygon of step 2510 to belong to the first set. For example, if a second blue polygon overlaps the companion new red polygon, that second blue polygon should be changed to a second new red polygon. If a third blue polygon overlaps either the companion new red polygon or the second new red polygon, then the third blue polygon should be changed to a third new red polygon and so forth until all polygons of different colors overlapping each other at unintended locations (i.e., not the locations of the desired contact holes) have been changed to have the same color.
B. Adjust the Shapes of the Polygons
Another technique to resolve phase conflict is to change the shapes of one or more offending (or nearby) polygons. For example, smaller polygons may be used to replace one or both offending polygons overlapping at unintended locations.
C. Re-Classify (or Regroup) Contact Holes
Another technique to resolve phase conflict is to re-classify or regroup contact holes to be printed by the offending polygons. For example, contact holes of one group may be assigned to another group to enable, for example, a redesign of the size/shape of the offending polygons.
D. Add Redundant Contact Holes
Another technique to resolve phase conflict is to add one or more redundant (or dummy) holes to enable redesign or removal of one or more offending polygons.
E. Print Single Contact Holes
Another technique to resolve phase conflict is to print one or more single contact holes to eliminate the need of one or more offending polygons.
IX. An Exemplary Operating Environment
The embodiments described herein may be implemented in an operating environment comprising software installed on a computer, in hardware, or in a combination of software and hardware.
The software and/or hardware would typically include some type of computer-readable media which can store data and logic instructions that are accessible by the computer or the processing logic within the hardware. Such media might include, without limitation, hard disks, floppy disks, flash memory cards, digital video disks, random access memories (RAMs), read only memories (ROMs), and the like.
X. Conclusion
The foregoing examples illustrate certain exemplary embodiments from which other embodiments, variations, and modifications will be apparent to those skilled in the art. The inventions should therefore not be limited to the particular embodiments discussed above, but rather are defined by the claims. Furthermore, some of the claims may include alphanumeric identifiers to distinguish the elements thereof. Such identifiers are merely provided for convenience in reading, and should not necessarily be construed as requiring or implying a particular order of steps, or a particular sequential relationship among the claim elements.
This application claims priority to provisional patent applications filed on Oct. 24, 2003, bearing application Ser. No. 60/514,227 and Nov. 5, 2003, bearing application Ser. No. 60/517,741.
Number | Date | Country | |
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60514227 | Oct 2003 | US | |
60517741 | Nov 2003 | US |