Reduced mask count gate conductor definition

A combined wide-image and loop-cutter pattern is provided for both cutting and forming a wide-image section to a hard mask on a substrate formed by sidewall imaging techniques in a reduced number of photolithographic steps. A single mask is formed which provides a wide mask section while additionally providing a mask to protect the critical edges of an underlying hard mask during hard mask etching. After the hard mask is cut into sections, the protective portions of the follow-on mask are removed to expose the critical edges of the underlying hard mask while maintaining shapes necessary for defining wide-image sections. Thus, the hard mask cutting, hard mask critical edge protecting, and large area mask may be formed in a reduced number of steps.

FIELD OF THE INVENTION

The invention relates to semiconductor fabrication, and more particularly to fabricating a combination of small and large structures.

BACKGROUND DESCRIPTION

As the size of semiconductor devices has decreased, photolithographic techniques become unable to reliably create structures of the dimensions required. As photolithographic techniques have become unusable, other technologies have been developed to create the small structures required by the ever shrinking semiconductor devices. One example of a non-photolithographic imaging technique is sidewall image transfer (“SIT”).

SIT is able to produce structures substantially narrower than the minimum size achievable with photolithographic techniques, while maintaining excellent width control. However, SIT methods produce structures, usually hard masks, of generally closed-loop geometry. These loops have a single, well-controlled width (referred to henceforth as the “critical image width”). Conventional SIT applications thus require the use of two additional masks. One, called a “loop cutter mask”, is employed to segment the loops, and a second, called a wide-area mask, is employed to add shapes of any other, usually wider, dimensions.

Accordingly, subsequent processing is required to remove the unwanted portions of the closed-loop structures produced by SIT. While removing unwanted loop material with the loop-cutter mask, both edges of loop material which define portions of critical width images must be protected in order to maintain the excellent image size control afforded by SIT techniques. Both of these edges must subsequently be left exposed during etch of underlying films in order to accurately transfer their pattern into those underlying films.

Furthermore, not all structures being formed are to be of the same small dimensions typically produced by SIT methods. Wider shapes are often required, for example, to fabricate contact landing pads.

Combining other imaging processes with SIT methods to produce structures of varying dimensions, with some of those dimensions smaller than those achievable by photolithographic techniques, is typically required.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method of combining the wide-image mask and loop-cutter mask includes protecting a critical portion of a loop-geometry hard mask on a substrate with a first portion of a follow-on mask, and forming a wide-area pattern on the substrate proximate the hard mask with a second portion of the follow-on mask. The method also includes removing an exposed portion of the hard mask, and subsequently exposing the critical portion of the hard mask.

In another aspect of the invention, a method of combining a wide-image mask and a loop-cutter mask includes forming a follow-on mask in a loop-cutter pattern on a portion of a hard mask, wherein the follow-on mask comprises a wide-image section and narrow-image section, and removing a portion of the hard mask left exposed by the follow-on mask. The method also includes removing at least a portion of the narrow-image section of the follow-on mask. The method also includes employing remaining wide-image sections of the follow-on mask to pattern wide-area shapes under the hard mask and follow-on mask.

In another aspect of the invention, a combined wide-image and loop cutter mask includes a follow-on mask formed over a hard mask, and a wide-image section forming a first portion of the follow-on mask. A narrow-image section forms a second portion of the follow-on mask. The follow-on mask is configured to have the narrow-image section preferentially removed while the wide-image section remains substantially able to mask underlying films.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention relates to semiconductor fabrication, and more particularly to fabricating a combination of small and large structures. In embodiments of the invention, a reduction in the number of masks required to form a final image especially when combining SIT imaging processes with traditional masking techniques is possible. In one embodiment, the method of the invention generally includes, for example, forming a follow-on mask over a section of a sidewall image transfer (SIT) loop on a substrate for further imaging the underlying substrate. The follow-on mask enables cutting the loop into segments and adds larger features to the image formed by the SIT loop, without interfering with the accuracy for those portions of the final image formed by the SIT loop.

Referring toFIG. 1, a hard mask10image in the form of a SIT loop built using SIT-related techniques is shown. Although a hard mask loop10is shown, any imaging technique which builds a continuous, single-width loop, especially a single-width loop of very small and precise width, is suitable for use with embodiments of the invention. Typically, the SIT loop or hard mask10is on the order of a few tens of nanometers wide. However, embodiments of the invention are suitable for use with loops, or any hard mask image of virtually any size suitable for use in a semiconductor device. Also shown inFIG. 1are dashed lines indicating the target shape12of the final image.

The target shape12is the final shape to which the underlying substrate will be formed using the hard mask loop10and a follow-on mask. The target shape12includes a wide section14and a narrow section16. The wide section14of the target shape12overlaps the hard mask loop10; whereas, the narrow section16of the target shape12coincides with a portion of the hard mask loop10.

In accordance with the invention, after forming the hard mask loop10, it may be necessary to cut the loop into sections plus define wide pads for contact formation. The techniques of example embodiments, as described, follow loop definition by printing of a single follow-on mask. In general, the follow-on mask should protect 1) desired portions of the SIT-defined critical-dimension masking loop; and 2) less-critical, usually wider regions to be used, for example, for long-channel devices or contact pads. Portions of loop to be removed should be left uncovered. After etch/removal of unwanted loop material, removal of the part of the follow-on mask covering critical-dimension SIT-defined lines is desirable.

Referring toFIG. 2, a follow-on mask18is formed over a portion of the hard mask loop10. The follow-on mask18includes a wide section20, which corresponds to the wide section14of the target shape12. The follow-on mask18also includes a narrow section22which corresponds to the narrow section16of the target shape12. The follow-on mask18may be formed by any of the methods well know in the art suitable for forming a follow-on mask over a hard mask loop including, expanding a desired wide-feature design by an undercut/ashback amount, and adding tabs to cover desired portions of the hard mask loop10.

In other words, after forming a hard mask loop10using SIT, the process includes printing a single follow-on mask18. The patterns for mask18are created by expanding desired wide-poly designs by an undercut/ashback amount, and adding tabs to cover desired SIT-defined width regions. This produces a single gate-conductor follow-on mask which is a combined wide-image and loop-cutter pattern.

Referring toFIG. 3, after the follow-on mask18has been formed, the unwanted, uncovered regions of the hard mask loop loop10are etched away from the substrate5. The unwanted sections of the hard mask10may be removed using any of the suitable etching techniques well known in the art for removing the hard mask material. In other words, unwanted SIT-defined hard mask loop portions are removed/etched, without stripping resist.

Referring toFIG. 4, the follow-on mask18(seeFIG. 3) is etched in such a way as to reduce its length and width along its edges. Accordingly, in some instances, the narrow section22of the follow-on mask18will be almost completely etched away. The narrow section22of the follow-on mask18is at least etched away sufficiently to expose the underlying critical edges of the hard mask loop10. In other words, the process includes pulling back the follow-on mask18so that the narrow section22covering desired portions of the hard mask loop10are removed (at least the critical edges of the SIT line are uncovered).

In the example ofFIG. 4, the narrow section22of the follow-on mask18has been completely removed to expose the underlying remaining hard mask loop10section. Additionally, the wide section20of the follow-on mask18has its length and width reduced to produce an image pad24. The image pad24substantially conforms to the wide area14of the target shape12. Additionally, the remaining exposed portion of the hard mask loop10, formerly covered by the narrow section22of the follow-on mask18corresponds to the narrow section16of the target shape12.

As noted above, the etching of the follow-on mask18to form the image pad24may be performed by any of the methods well known in the art suitable for etching a follow-on mask on top of a hard mask loop10section and substrate5.

Referring toFIG. 5and comparing withFIG. 4, once the combination of the image pad24and the remaining hard mask loop10section are formed, the structure is used as a mask for etching the underlying substrate5. Accordingly, once the underlying substrate5has been etched, a structure of the underlying substrate5in the shape of the target shape12is formed where a portion of the final structure is shaped using the critical edges of the hard mask. In one embodiment, this includes etching the gate conductor stack using a combination of SIT-loop for the device-Lpoly section, plus resist pad for the wide section, and then stripping masking layers.

The final shape of the underlying substrate5may additionally include a stub26in addition to the target shape12. The stub26is a consequence of the pull-back process of the wide area20of the follow-on mask18exposing additional portions of hard mask loop10. In most applications, this stub26is an inconsequential artifact which has no effect on fabrication process or the circuit design. In other applications, the stub26may be a beneficial component to the final shape28.

Referring toFIG. 6, the size and overlap of the follow-mask18over the hard mask loop10is shown. The target image shape12is shown having a wide section14and a narrow section16. The narrow section16of the target shape12corresponds to the critical edges of the underlying hard mask loop10. The follow-on mask18includes a wide section20and a narrow section22. As indicated above, one purpose of the narrow section22of the follow-on mask18is to preserve the underlying section of hard mask10during the hard mask loop10cutting or etching process. Thus, the narrow section22of the follow-on mask18should extend beyond the edges of the hard mask loop10in order to protect those precision or critical edges.

The amount by which the follow-on mask18extends beyond the edges of the underlying hard mask loop10is the overlap32. The amount by which the follow-on masks18extends beyond the edges of the wide section14of target image12is the extended section30. Initially, the extended section30is generally larger than the overlap32. The amount by which the target image12is extended to form the follow-on mask18should preferably be sufficient so that despite follow-on mask18registration and image size errors, substantially all critical edges of the underlying hard mask10material remain covered during the loop etching process to avoid degrading the wanted portions such as the precision edges of the hard mask10. In other words, because preserving edges of the loop material maintains control of critical widths afforded by SIT processing, the amounts by which the original (designed) pattern of the follow-on mask is expanded (32,30) have to be sufficient so that, despite follow-on mask registration and image size errors, all critical edges of that loop material remain covered during loop-cutting in order to prevent degrading line width control.

Referring toFIG. 7, an illustration of the effects of a mis-registered follow-on mask18is shown. In this example, the follow-on mask44has been formed in mis-alignment and pull-back etched. As illustrated, for this embodiment the amount of pull-back34need not remove the entire width of narrow section22as long as critical edges of hard-mask loop section10are exposed despite some amount of mis-alignment.

In many cases, the follow-on mask masking pattern over critical-Leffregions will be completely removed. Since pull-back34is greater than the pattern expanded amount, a different, larger extended section30for pads than for device lines overlap32is needed.

Referring toFIG. 8, an example of image pad density criteria for situations where multiple image pads are densely packed is shown. The minimum image pad space36density is related to the minimum lithographic pad-pad space38of two follow-on masks,18and19, achievable with the method of the invention. Thus, the minimum image pad space36is related to the extended section30(FIG. 7). In other words, pad shapes on follow-on mask18should be sufficiently far apart to allow a minimum space to print between expanded adjacent pads.

Referring toFIGS. 9-13, another embodiment of a combined wide-image and loop-cutter pattern is shown. Referring first toFIG. 9, in this embodiment, a hard mask loop10is formed on a substrate5. An outline of the target shape12is shown by a dashed line. The target shape12has a wide section14and a narrow section16. Also shown is the follow-on mask46. Follow-on mask46has a narrow section48and a wide section50, corresponding to the wide and narrow sections14,16, respectively, of the target shape12.

The dimensions of the wide section50of the follow-on mask46approximately correspond to the dimensions of the wide section14of the target shape12. Accordingly, the narrow section48of the follow-on mask46extends or overlaps beyond the edges of the underlying hard mask loop10. The edges of the wide section50of the follow-on mask46also extend beyond the edges of the underlying hard mask loop10while still corresponding to the dimensions of the wide section14of the target shape12. Additionally, an edge of the wide section50may be configured to coincide with edge of the hard mask loop10. Thus, the wide section50of the follow-on mask46does not contain a section expanded beyond the underlying image shape12. In other words, this embodiment is similar to the previously discussed embodiment, except that while critical-width line portions of designs are expanded to make the narrow sections48of follow-on mask46, the follow-on mask pads50are not expanded.

Referring toFIG. 10, the undesired exposed portions of the hard mask loop10are etched from the surface of the substrate5. During this process, the follow-on mask46protects any hard mask loop10underlying the follow-on mask46. Thus, the original hard mask loop10is formed into hard mask loop10sections, where each section corresponds to a follow-on mask46.

Referring toFIG. 11, after removal of unwanted portions of hard mask loop10, the follow-on mask46has been pulled back to form a core pad52. The narrow section48of the follow-on mask46has been etched away to expose the underlying hard loop mask10, thus exposing the edges of the hard mask loop10. The follow-on mask46may be pulled back by any of the methods well known in the art suitable for etching the material forming the follow-on mask46, but which does not etch the hard mask loop10material. For example, a simple oxygen-based etch (ash) maybe used if the thickness of the follow-on mask46is sufficient.

Referring toFIG. 12, the core pad52is then re-grown along its edges to expand its length and width to form an image pad54. The image pad54is formed either over a section of hard mask loop10on top of the substrate5or over a core pad52employed for patterning shapes not attached directly to portions of the hard mask loop10. The core pad52is then grown back to approximately its original size using a side wall deposition and directional-etch process. The directional-etch portion of this process should have sufficient over etch to remove deposited material from the sidewalls of hard mask loop10, which thus must be thinner than the sidewall height of core pads52.

It should be noted that re-growing the core pad52to form the image pad54does not reform the thin section48of the original follow-on mask46because there is no structure remaining on which to re-grow the narrow section48. There is no structure remaining because pull-back completely removed narrow sections48of the follow-on mask46.

Referring toFIG. 13, after the image pad is grown, the combination (shown as reference numeral56) of the image pad54and the remaining section of the hard loop mask10is used as a mask to etch the final shape into the underlying substrate5. Once the substrate5has been etched, the image pad and section of hard mask loop10are removed. It should be noted that this method of combined wide-image and loop-cutter does not produce the stub26shown inFIG. 5. Additionally, the minimum pad-pad distance is decreased because the wide section50of the follow-on mask46does not need to extend beyond the edges of the target shape12.

The above method allows multiple initial follow-on masks46to be placed close to one another. However, image pads cannot be made with dimensions less than twice the pull-back distance, and thus either the minimum image pad size is limited or the pull-back distance is limited, both of which place size limitations on the target shape. Additionally, minimum pad-to-pad distance is decreased, but this method may be less effective for making pads with dimensions less than twice the pull-back/regrow distance plus enough to keep the desired structures in place.

Referring toFIGS. 14-16, another embodiment of a wide-image and loop-cutter combination pattern is shown. Beginning withFIG. 14, a substrate5has a hard mask loop10formed on its surface. Formed over the hard mask loop10is a follow-on mask58. The follow-on mask58includes a narrow region62and a wide region60. A target shape12is indicated by the dashed lines. The target shape12has a wide area14and a narrow section16.

The narrow section62of the follow-on mask58is formed at a reduced thickness as compared to the wide section60of the follow-on mask58. The narrow section62of the follow-on mask58is typically about 25% thinner than the wide section60of the follow-on mask58although other differences in thickness are also contemplated. The narrow section62of the follow-on mask58may be formed at a reduced thickness by any of the methods well known in the art for forming a thinner mask. For example, the reduced thickness of mask58in narrow section62is formed by operating the imaging tool so that narrow section62is of a dimension approaching the resolution limits of the tool.

The follow-on mask58will generally rely on degradation of resist profiles when smaller lines are printed. Additionally, it is well known to those of skill in the art that using photolithography to print “properly out of focus” narrow lines can maintain good nominal line width control, but degrade in as-developed resist height. Thus, the thinned narrow section62of the follow-on mask58may be printed concurrently with the wide section60of the follow-on mask58. Additionally, the narrow section62of the follow-on mask58overlaps, i.e., extends beyond the critical edges of the hard mask loop10.

After the follow-on mask58is formed, the unwanted exposed portion of the hard mask loop10is etched or otherwise removed. Thus, the unwanted portions of the hard mask loop10are removed while the critical portions of the hard mask loop10are protected and remain intact under the follow-on mask58. In other words, this embodiment uses a mask created from line and pad data very similar to that used for previously discussed embodiments. Also, critical SIT-defined regions have design expanded by an overlap amount so that critical edges of the hard mask loop10are protected despite overlay and image size variability.

Referring toFIG. 15, after the unwanted portions of the hard mask10are removed, the follow-on mask58is vertically etched. During the vertical etch process, the thin narrow section62(ofFIG. 14) of the follow-on hard mask58is preferentially removed, while the thick wide section60(ofFIG. 14) of the follow-on mask, while also being etched, will remain upon the completion of the vertical etch. Accordingly, the remaining structure includes an image pad64which corresponds to the original wide section60of the follow-on mask and an exposed section of the hard mask loop10.

Together, the image pad64and the remaining section of the hard mask10form a mask corresponding to the target shape12. Additionally, the critical edges of the section of the hard mask10are now exposed after having been protected during the hard mask etch step. In other words, vertically strip the follow-on mask18pattern so that the thinned areas are completely removed, but areas with full-height resist remain.

Referring toFIG. 16, the substrate5is etched using the combination of the image pad64and remaining section of the hard mask10as a mask to form the final shape66in the substrate5. The final shape66corresponds to the target shape12and is formed by a combination of portions of the original follow-on mask58(shown inFIG. 14) and exposed portions of the hard mask10.

In this embodiment, the minimum pad-to-pad distance is decreased over previously discussed embodiments. Also, it is possible to design pads with small dimensions as long as they are large enough so that they do not suffer resist thinning during follow-on mask18printing.

As should be now understood, in embodiments of the invention, methods include depositing on a section of the SIT loop an oversized follow-on mask of the final image, and etching the mask uniformly to leave a final large area image while removing those portions of the follow-on mask covering critical edges of the SIT loop. In general, those portions of the SIT loop having critical edges which will contribute to the final image are covered during the SIT loop cutting process. After SIT cutting, the edges are re-exposed for the final etch in order to take advantage of the high tolerances possible with SIT image forming methods.

While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modifications and in the spirit and scope of the appended claims.