Source: http://www.google.com/patents/US6639676?dq=60/310,746
Timestamp: 2014-03-11 16:13:09
Document Index: 701756885

Matched Legal Cases: ['art 4000', 'art 4000', 'art 4000', 'art 4000', 'art 4000', 'art 4000', 'art 5000', 'art 5000']

Patent US6639676 - Method for determining rotational error portion of total misalignment error ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA method for determining rotational error portion of total misalignment error in a stepper. In one embodiment, the method comprises a series of steps in a stepper, starting with the step of receiving a wafer, having a first pattern and an error-free fine alignment target, in the stepper. In another step,...http://www.google.com/patents/US6639676?utm_source=gb-gplus-sharePatent US6639676 - Method for determining rotational error portion of total misalignment error in a stepperAdvanced Patent SearchPublication numberUS6639676 B1Publication typeGrantApplication numberUS 09/422,909Publication dateOct 28, 2003Filing dateOct 21, 1999Priority dateOct 21, 1999Fee statusPaidAlso published asEP1145081A1, US6950187, US20040138842, WO2001029618A1Publication number09422909, 422909, US 6639676 B1, US 6639676B1, US-B1-6639676, US6639676 B1, US6639676B1InventorsPierre LerouxOriginal AssigneeKoninklijke Philips Electronics N.V.Export CitationBiBTeX, EndNote, RefManPatent Citations (10), Classifications (9), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetMethod for determining rotational error portion of total misalignment error in a stepperUS 6639676 B1Abstract A method for determining rotational error portion of total misalignment error in a stepper. In one embodiment, the method comprises a series of steps in a stepper, starting with the step of receiving a wafer, having a first pattern and an error-free fine alignment target, in the stepper. In another step, the wafer is aligned in the stepper using the error-free fine alignment target. Then a second pattern is created on the wafer overlaying said first pattern. In another step, the rotational error portion of the total misalignment error is determined by measuring the circumferential misalignment between the first pattern and the second pattern.
I claim: 1. In a stepper, a method of determining a rotational error portion of a total misalignment error for a wafer, said method comprising the steps of:
a) receiving said wafer, said wafer having a first pattern, wherein said first pattern is located in an outer region of any shot and a fine alignment target formed therein, said fine alignment target located in a center region of a first shot; b) aligning said wafer in said stepper using said fine alignment target; c) creating a second pattern on said wafer, said second pattern overlaying said first pattern; and d) measuring a circumferential misalignment between said first pattern and said second pattern to obtain said rotational error portion of said total misalignment error. 2. The method recited in claim 1 wherein said fine alignment target in said wafer is approximately error-free.
3. The method recited in claim 1, further comprising the step of:
e) compensating said stepper for a translational misalignment portion of said total misalignment error for said wafer. 4. The method recited in claim 1 wherein said first pattern is an approximately error-free overlay.
5. The method recited in claim 4 wherein said second pattern is an overlay having rotational error, said second pattern corresponding in position to said first pattern.
6. The method recited in claim 1, further comprising the step of:
e) creating a layer of material on said wafer, said second pattern formed in said layer of material. 7. The method recited in claim 6 wherein said layer of material is a photo-resist material.
8. The method recited in claim 1 wherein step c) comprises the steps of:
c1) exposing an image of said second pattern, said image of said second pattern located in an outer region of a reticle; and c2) projecting said image of said second pattern through an outer region of a stepper lens onto said wafer. 9. The method recited in claim 1 wherein said wafer is a Preventative Maintenance (PM) wafer.
10. The method recited in claim 1 wherein said first pattern is a duplicate fine alignment target.
11. The method recited in claim 10 wherein said second pattern is a mating overlay for at least a portion of said duplicate fine alignment target.
12. The method recited in claim 1 wherein said first pattern is approximately error-free.
TECHNICAL FIELD The present claimed invention relates to the field of semiconductor wafer fabrication. More specifically, the present claimed invention relates to a method for determining the rotational portion of misalignment error in a stepper used to fabricate patterned layers on a wafer.
BACKGROUND ART Integrated circuits (ICs) are fabricated en masse on silicon wafers using well-known photolithography, etching, deposition, and polishing techniques. These techniques are used to define the size and shape of components and interconnects within a given layer of material built on a wafer. The IC is essentially built-up using a multitude of interconnecting layers, one formed on top of another. Because the layers interconnect, a need arises for ensuring that the patterns on adjacent layers of the wafer are accurately formed.
The conventional alignment reticle and conventional rotational error measurement process is corrupted by using an alignment target having magnification error, rotational error, and translational error. The conventional reticle includes an alignment target at an outer location of the reticle image, 132 of prior art FIG. 1B and 126b of prior art FIG. 1A, that is projected through an outer portion 128 b of the lens 128 of prior art FIG. 1A. Consequently, the alignment target created on the wafer suffers from magnification error, rotational error and translational error as well as reticle writing error. Furthermore, the conventional rotational error measurement process compares a full-field shot on each of two layers. However, a full-field shot includes errors other than rotational error. Hence, the rotational error measurement is confounded with other these other errors. Consequently, the rotational error measurement may not be accurate, and thus compromise yield of the wafer and performance of the IC formed on the wafer. Hence, a need arises for a more accurate reticle and for more accurate shots on a wafer, with which rotational error can be measured.
Confounding the rotational error also occurs by not separating out a translational portion of the misalignment error prior to forming images on a wafer for the rotational error measurement. The alignment of a wafer for a rotational error measurement process intrinsically includes a translational error. Conventionally, the translational error is not accounted for in a rotational error measurement. If this error is not compensated for, it will affect the results of the rotational error measurement. Thus, by using the rotational level to compensate for the translational portion of the alignment error, alignment accuracy can possibly be degraded, due to miscorrection.
Consequently, a need arises for compensating for the transitional error in the rotational error measurement.
DISCLOSURE OF THE INVENTION The present invention provides a method and an apparatus for ensuring accurate alignment of multiple layers formed on a wafer. More specifically, the present invention provides accurate rotational alignment of an image formed on a layer of a wafer from a reticle. The present invention accomplishes accurate rotational alignment by segregating other error-causing variables from the rotational misalignment error, so as to yield a true magnification error measurement. Additionally, the present invention provides a method for measuring magnification error using an alignment target that is approximately free of reticle writing error, magnification error, rotational error, and translational error. Furthermore, the present invention compensates for the transitional error in the stepper prior to the magnification error measurement. Thus, the present invention improves accuracy of the magnification error measurement, thereby satiating more stringent budget overlay requirements.
BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in, and form part of, this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. The drawings referred to in this description should be understood as not being drawn to scale except as specifically noted.
It should be borne in mind, however, that all of these terms are to be interpreted as referencing physical manipulations and quantities and are merely convenient labels and are to be interpreted further in view of terms commonly used in the art. Unless specifically stated otherwise as apparent from the following discussions, it is understood that throughout discussions of the present invention, terms such as or �receiving,� �aligning,� �creating,� �measuring,� �compensating,� �exposing,� �projecting,� �forming,� or the like, refer to the action and processes of fabricating material and patterns on a wafer.
Pattern box C3 in center portion 333 of alignment reticle 300 of FIG. 3A includes a first pattern, having a fine alignment target, and includes a second pattern, both of which are shown in subsequent figures. By locating the fine alignment target in the center portion 333 of alignment reticle 300, the present invention can more accurately locate to the fine alignment target during the translational misalignment measurement process. More specifically, the present invention eliminates other sources of error, such as. lens distortion, reticle writing error, and rotational misalignment from the fine alignment target by locating the fine alignment target in the center of the reticle.
Referring now to FIG. 3C, a second configuration of a pattern box in an alignment reticle is shown, in accordance with one embodiment of the present invention. In FIG. 3C, patterned box in center portion 333 of reticle 300 includes a first pattern 344 and a second pattern 346. In the present embodiment, first pattern.344 includes multiple large overlay boxes 345. Similarly, second pattern 346 of the present embodiment includes multiple small overlay boxes 347. Small overlay boxes 347 or large overlay boxes 345 can also be adapted for use as fine alignment targets for aligning the wafer in the stepper for a translational misalignment measurement. Thus, the fine alignment target has essentially no error from lens aberration, e.g. magnification error, from rotational misalignment, from reticle writing error, or from translational misalignment. Consequently, this embodiment isolates the true translational misalignment error between the reticle and the wafer because one of the overlay boxes actually is the alignment target.
Using fine alignment target for overlay box is described in greater detail in co-pending U.S. patent application Ser. No. 09/425,834 , concurrently filed herewith, entitled �Method for Determining Wafer Misalignment Using a Pattern on a Fine Alignment Target,� by Pierre Leroux, and assigned to the assignee of the present invention.
Referring now to FIG. 4B, one shot of alignment overlays in one layer of a wafer is shown, in accordance with one embodiment of the present invention. FIG. 4B shows the identity of the pattern boxes, e.g. pattern box A3 431, for a peripheral shot, e.g. shot 436. For purposes of clarity, pattern detail within each box, e.g. 431, and between boxes, is not shown in FIG. 4B. The shot shown is referred to as a �full-field� shot because the full field of a reticle, e.g. alignment reticle 300 of FIG. 3A, is projected to wafer in a single exposure of the reticle. Consequently, an outer portion of a reticle, e.g. pattern box A3 331 of FIG. 3A, is projected through an outer portion of a lens, e.g. region 208 b of lens 208 of FIG. 2, onto an outer portion of a shot, e.g. to form A3 431 of FIG. 4B, on a wafer. Similarly, an inner portion of a reticle, e.g. pattern box C3 333 of FIG. 3A, is projected through a center portion of a lens, e.g. region 208 a of lens 208 of FIG. 2, onto a center portion of a shot, e.g. to form A3 431 of FIG. 4B, on a wafer. An imaginary grid of dashed lines is shown to provide reference to the matrix grid of shots, as shown in FIG. 4A.
Referring now to FIG. 4C, another shot with alternative alignment overlays in one layer of a wafer is shown, in accordance with one embodiment of the present invention. FIG. 4C shows the identity of the pattern boxes, e.g. pattern box C3 433 a, for an internal shot, e.g. shot 434. For purposes of clarity, pattern detail within each box, e.g. pattern box C3 433 a, and between the pattern boxes, is not shown in FIG. 4C. The shot shown is referred to as a repeated pattern of a �bladed-down� shot. This term arises because only a small portion of the field of a reticle, e.g. alignment reticle 300 of FIG. 3A, is exposed to form each pattern box on the shot, e.g. shot 434. By repeating this process, the pattern shown is developed. That is, only a center portion of a reticle, e.g. pattern box C3 333 of FIG. 3A, is projected through an center portion of a lens, e.g. region 208 a of lens 208 of FIG. 2, onto a region of a shot on a wafer, e.g. to form pattern box C3 431 of FIG. 4B. To create a row of C3 pattern boxes, e.g. pattern boxes 442, the reticle or the wafer has to be indexed and the center portion of the reticle is re-projected through the center portion of the lens onto a different region of a shot on the wafer. Likewise, the reticle or wafer is indexed to form columns of pattern boxes. This process is repeated to fill the entire shot with C3 pattern boxes.
In step 4008 of the present embodiment, a full-field image of a reticle is exposed. Several embodiments implementing Step 4008 are shown in FIG. 2 and in FIGS. 3A through 3C. Specifically, step 4008 can be implemented by exposing a full field of reticle 206 shown in FIG. 2. Fullfield includes the full field of view of the reticle, e.g. including center portion 206 a and outer portion 206 b of reticle 206. Several embodiments of a reticle are shown in FIG. 3A through 3C. In the embodiment shown in FIG. 3A, the entire reticle 300, e.g. pattern boxes A1-E5, is exposed in step 4008. While the embodiments shown have a specific layout and geometry, the present invention is well-suited to using any reticle, including an alignment reticle or a product reticle. The purpose of exposing a full-field reticle is to form the full pattern of the reticle onto a wafer, as described in subsequent steps, for evaluation of alignment. In another embodiment, step 4008 is not used in the method to form a PM reticle. Following step 4008, flowchart 4000 proceeds to step 4010.
Step 4014 arises if additional full-field shots are not desired on the wafer, per step 4012. In step 4014 of the present embodiment, a fine alignment target image located in a center region of a reticle is exposed. Several embodiments implementing Step 4014 are shown in FIG. 2 and in FIGS. 3A through 3C. Specifically, step 4014 an be implemented by exposing a center region 206 a of reticle 206 as shown in FIG. 2. Several embodiments of a reticle are shown in FIGS. 3A through 3C. In the embodiment shown in FIG. 3A, center portion of reticle 300 includes pattern box C3 333. FIGS. 3B and 3C provide several embodiments of the fine alignment target located within pattern box C3 333. In one embodiment, the pattern box located in the center portion of the reticle includes a overlay boxes 334 and 336 in addition to a fine alignment target 338. In another embodiment, the large overlay boxes 345 and small overlay boxes 347 can be utilized as a fine alignment target as well as an overlay for misalignment measurement. The discussion presented for FIGS. 3B and 3C, hereinabove, provide additional information on the configuration of the fine alignment targets contained therein. While the embodiments shown have a specific layout and geometry, the present invention is well-suited to using any reticle, including an alignment reticle or a product reticle.
In step 4016 of the present embodiment, the fine alignment target is projected through a center region of a stepper lens onto a center region of a shot. Step 4016 is implemented, in one embodiment, in FIGS. 4A through 4C. Specifically, Step 4016 is implemented in FIG. 2, which shows the center portion 208 a of the stepper lens 208. Step 4016 is also implemented in FIG. 4A, where the image can be projected on a center region of a shot, not previously exposed. By using the center region of the stepper lens for this step, the present invention reduces any error arising from magnification error or rotation error for the first pattern that is created in the wafer. approximately error-free first pattern. This conclusion arises because magnification error is typically at a minimum at the center of the lens, where almost no magnification occurs. Similarly, circumferential, or rotational, offset 6 increases, approximately linearly, with the distance from the center, e.g. radius R, of a shot, for a given rotation error θ, e.g. δ=R*θ. Consequently, the minimum rotation error occurs at the center of the lens and reticle and shot. In one embodiment, the image formed from step 4016 will be used in a subsequent flowchart for determining translation error for the stepper that will be removed prior to determining a rotational error portion of the total misalignment error. This discussion will be discussed in that subsequent flowchart. Following step 4016, flowchart 4000 proceeds to step 4018.
In step 4024 of the present embodiment, the first pattern image from step 4022 is projected through a center region of a stepper lens onto an outer region of a shot. Step 4024 is implemented, in one embodiment, in FIG. 4A and 4C. In particular, one embodiment projects the first pattern onto an outer region of a shot, e.g. to form pattern box C3 443 of FIG. 4C. Outer location in this embodiment, is defined as any location outside of a center region, e.g. location 433 b, of a shot. The benefits of using the center part of the reticle in step 4022 and using the center part of the stepper lens in step 4024 is to provide an approximately error-free first pattern. This conclusion arises because magnification error is typically at a minimum at the center of the lens, where almost no magnification occurs. Similarly, circumferential, or rotational, offset δ increases, approximately linearly, with the distance from the center, e.g. radius R, of a shot, for a given rotation error θ, e.g. δ=R*θ. Consequently, the minimum rotation error occurs at the center of the lens and reticle and shot. Following step 4024, flowchart 4000 proceeds to step 4026.
In step 4026 of the present embodiment, an inquiry determines whether the first pattern image is to be projected onto,additional outer regions of a shot on the wafer. Step 4026 is implemented, in one embodiment, in FIGS. 4A through 4C. If it is desired to project the first pattern image onto additional outer regions of a shot on the wafer, then flowchart 4000 proceeds to step 4028. However, if it is not desired to project the first pattern image onto additional outer regions of a shot on the wafer, then flowchart 4000 proceeds to step 4030.
Step 4028 arises if additional shots are desired on the wafer, per step 4026. In step 4028 of the present embodiment, the reticle or the wafer is indexed to another region of a given shot location on the wafer. Step 4028 is implemented, in one embodiment, in FIG. 2 and FIGS. 4A and 4C. Specifically, FIG. 2 shows that stage 212 holding wafer 213 can be moved in any of multiple directions, e.g. X direction 212 a, as desired. Alternatively, reticle 206 can be indexed to another location if desired. Thus, it multiple projections, e.g. C3 433 a, and c-e, of a first pattern are desired on a shot, e.g. 434, as shown in FIG. 4C, then indexing can occur to repeat the projection of the given first pattern onto the shot of the wafer. In one embodiment, steps 4022 through 4028 are repeated until pattern boxes are formed in all regions of a shot, e.g. such as shot 434 shown in FIG. 4C. In one embodiment, the grid of pattern boxes formed by steps 4022 through 4028 correspond to a location of pattern boxes formed by a full-field exposure of a reticle, e.g. reticle 300 of FIG. 3A. In another embodiment, only a single C3 location, e.g. pattern box C3 433 a, is made on a shot. This latter embodiment provides sufficient information to obtain rotational error, or rotational misalignment. However, alternative embodiments provide additional benefits. Yet in another embodiment, four regions, e.g. for pattern boxes 433 a, 433 c-433 e, of a shot, e.g. shot 434, are chosen to receive the projection of a first pattern, as shown in FIG. 4C. The four outer regions chosen provide the greatest distance from the center of the lens and thus, in one embodiment, provide a worst case manifestation of rotational error for a shot. By using multiple projections of first pattern on a shot, the present embodiment can reduce noise in the measurement and fabrication operations by averaging the results of the alignment process on the multiple images. Following step 4028, flowchart 4000 returns to step 4022.
The set of pattern boxes in FIG. 5C can be from either type of shot on a wafer. That is, they can either be from an internal shot, e.g. shot 434 of FIG. 4C, or from a peripheral shot, e.g. shot 436 of FIG. 4B . Additionally, the set of pattern boxes 500 c can be from either a center region of a shot, e.g. C3 543 b on C3 433 b of set 540 b shown in FIG. 5B, or an outer region of a shot, e.g. A3 543 a over C3 433 a of set 540 a. Note that if set of pattern boxes 500 c are used as a center shot, to be used for alignment, then they also include a fine alignment target 506, in the present embodiment. Fine alignment target 506 is shown as an entity separate from alignment boxes 502 and 504. This embodiment, shown in FIG. 5C, is formed from a corresponding pattern in a reticle, e.g. image of fine alignment target 338 shown in FIG. 3B. However, in one embodiment, the alignment boxes 502 and 504 can also act as a fine alignment target. This latter embodiment is formed from a corresponding pattern in a reticle, as shown in FIG. 3C, and described therein.
In step 5008, of the present embodiment, a correction is made for the translational misalignment error between the reticle and the wafer in the stepper. The process of determining translation misalignment error is described in greater detail in co-pending US patent application Ser. No. 09/422,912, concurrently filed herewith, entitled �Method for Determining Translation Portion of Misalignment Error In a Stepper,� by Pierre Leroux, and assigned to the assignee of the present invention. This step is performed, in one embodiment, so that translational misalignment error will not affect, or be misinterpreted as, the rotational error. Consequently, the respective errors in wafer fabrication are segregated, isolated, and corrected with the appropriate control mechanisms.
In step 5012, of the present embodiment, a second pattern is exposed in an outer region of a reticle. In one embodiment, second pattern is a fullfield exposure of a reticle onto a shot of the wafer. Thus, for example, pattern boxes A1-A5, C1, B1-B5, C2, C4, C5, D1-D5, and E1-E5 are all exposed as outer regions of a reticle. In another embodiment, only a single pattern box can be exposed in an outer portion of a reticle. For example, only pattern box A3 of reticle 300 of FIG. 3A can be exposed as a second pattern. While the present embodiment shows specific size and shape of second pattern, the present invention is well-suited to using any size, shape, or location of second pattern that satisfies the steps of flowchart 5000. From another perspective, FIG. 2 shows how outer region 206 b of reticle 206 is exposed in stepper 200 a. In one embodiment, second pattern of step 5010 is an overlaying pattern, e.g. a large overlay box, located in each of the pattern boxes, e.g. pattern boxes A1-E5 of reticle 300 in FIG. 3A. Thus, for example, large overlay box 334 or 344 of FIGS. 3B and 3C respectively, can be used as second pattern shapes for each of the pattern boxes, e.g. pattern boxes A1-E5. In this manner, pattern boxes A1-E5 for the full-field shot will overlay a nearly error-free first pattern on the wafer. Following step 5012, flowchart 5000 proceeds to step 5014.
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