Contact hole printing by packing and unpacking

A method is provided for the creation of contact holes. The invention provides two masks. The first mask, referred to as the packed mask, comprises the desired contact holes, which are part of the creation of a semiconductor device. To the packed mask are added padding holes in order to increase the hole density of the packed mask. An insulation layer is formed to protect the first layer of material. The second mask, referred to an the unpacking mask, comprises openings at the same locations as the locations of the padding holes of the first mask, the openings provided in the second mask have slightly larger dimensions than the padding holes of the first mask. A first exposure is made using the packed mask, a second exposure of the same surface area is made using the unpacking mask. The unpacking mask is used to selectively cover the padding contact holes, resulting in the final image. Two types of unpacking masks can be used, a first type having unpacking holes that surround the desired hole pattern, a second type having unpacking holes that align with the desired hole pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of improved contact hole creation for ultra-small contact holes.

2. Description of the Prior Art

Integrated semiconductor devices comprise electronic circuits of sub-micron and deep sub-micron dimensions and are typically created in or on the surface of a silicon substrate. The creation of semiconductor devices is supported by sophisticated and interacting, mutually supporting disciplines.

Device features are transposed from a mask onto a semiconductor surface using photolithographic imaging processes. Since this exposure depends on the transfer of photo energy from a source to a target surface, it is to be expected that, for target features that are created in very close proximity to each other, the transfer of photo energy interacts for very closely spaced device features, most commonly interconnect lines or contact holes having sub-micron spacing between adjacent lines.

A common measure for the occurrence of such interactions are applications where the Critical Dimensions (CD) of the layout of the Integrated Circuit (IC) approach the resolution limit of the lithography equipment. For such applications, proximity effects begin to influence the manner in which mask images are transferred to target surfaces. This interaction imposes limitations on the proximity of adjacent device features; these limitations are referred to as Critical Dimensions (CD) of a design and device layout.

The invention addresses problems and limitations that are encountered when creating openings on a semiconductor surface such as the surface of a layer of photoresist or the surface of a semiconductor substrate.

U.S. Pat. No. 6,238,850 (Bula et al.) shows a photo method for contact holes using 2 masks/exposures.

SUMMARY OF THE INVENTION

A principal objective of the invention is to provide a method of creating contact holes of sub-micron dimensions whereby the provided method is not limited to holes of one hole-diameter to hole-distance ratio.

Another objective of the invention is to provide a method of creating contact holes of sub-micron dimensions whereby the range of the ratio of hole-diameter to hole-distance is narrowed to an optimum range of off axis illumination.

Another objective of the invention is to provide a method of creating contact holes using a packing and unpacking procedure.

Another objective of the invention is to provide improved Depth Of Focus (DOF) during the creation of contact holes.

Yet another objective of the invention is to improve a Mask Error Factor (MEF) associated with the creation of contact holes.

Thus, the present invention relates to a method of creating closely spaced contact holes, comprising the steps of: (1) providing a substrate, said substrate having been provided with a first layer of material for creation of a pattern of contact holes; (2) exposing the surface of said first layer of material with a first mask, said first mask comprising a first and a second pattern of contact holes; (3) creating openings in said first layer of material in accordance with said first and second pattern of contact holes; (4) forming an insulation layer over the first layer of material, wherein (i) a difference in polarity is used to form the protection of the first layer; (ii) a water solution which includes a water soluble resin, a thermal acid generator and a cross-linker is coated over the first layer of material; (iii) a thermal process is applied, wherein the water soluble resin undergoes a cross-linking reaction under catalysis by acid to form the insulation layer, followed by; (iv) a water-rinse step to remove unreacted components; (5) depositing a second layer of material over the surface of said protected first layer of material, including said openings created in said first layer of material in accordance with said first and second pattern of holes; (6) exposing the surface of said second layer of material with a mask selected from the group consisting of: (i) a second mask comprising a third pattern of holes, said third pattern of holes being aligned with said second pattern of holes, said third pattern of holes being a transparent pattern of holes with a surrounding opaque background surface; (ii) a third mask comprising a third pattern of holes, said third pattern of holes being aligned with said second pattern of holes, said third pattern of holes being an opaque pattern of holes with a surrounding transparent background surface; (iii) a fourth mask comprising a fourth pattern of holes, said fourth pattern of holes being aligned with said first pattern of holes, said fourth pattern of holes being a transparent pattern of holes with a surrounding opaque background surface; (iv) a fifth mask comprising a fourth pattern of holes, said fourth pattern of holes being aligned with said first pattern of holes, said fourth pattern of holes being an opaque pattern of holes with a surrounding transparent background surface; and (7) creating openings or covering in said second layer of material in accordance with said third or fourth pattern of holes, holes of said third or fourth pattern of holes having a diameter being larger than a diameter of holes of said first and second pattern of holes by a measurable amount.

In accordance with the objectives of the invention a new method is provided for the creation of contact holes. The invention provides two masks. The first mask, referred to as the packed mask, comprises the desired contact holes, which are part of the creation of a semiconductor device; in the present invention, an insulation coating is applied over the patterns from the printing of the first mask. To the packed mask are added padding holes in order to increase and condense the hole density of the packed mask. The second mask, referred to as the unpacking mask, comprises openings at the same locations as the locations of the padding holes of the first mask, the openings provided in the second mask have slightly larger dimensions than the padding holes of the first mask. An alternative approach for the second mask includes openings at the same location of the desired holes of the first mask, but with larger dimensions. A first exposure is made using the packed mask, a second exposure of the same surface area is made using the unpacking mask. The unpacking mask is used to selectively cover the padding contact holes or to open the desired contact holes, resulting in the final image. Two types of unpacking masks can be used, a first type having unpacking holes that surround the desired hole pattern, a second type having unpacking holes that align with the desired hole pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention addresses the problems that are typically caused in imaging contact holes. In past practices, these problems have been addressed by off-axis illumination in the form of annular, quadruple of dipole configurations. This method however is highly dependent on the density of the created images and can be optimized for only one density of images at the time. The invention provides a method of creating contact holes that improves Depth Of Focus (DOF) and a Mask Error Factor (MEF) for the creation of the contact holes. The invention purposely packs the holes up in order to reduce the spread of pitches and to take advantage of the improvement of off-axis illumination to closely packed and equally pitched features. Proximity effects have to be corrected but are effectively irrelevant to the invention.

In applying conventional micro-photolithography with a reduction of the feature size below 0.4 lambda/NA, where lambda is the actinic wavelength and NA is the numerical aperture of the imaging lens, the depth of focus (DOF) is on the order of 0.21 lambda/NHA, where NHA is the numerical half aperture. To create 0.1 μm contact holes, using a wavelength of 193 nm and a NA of 0.65, the feature size is 0.34 lambda/NA and the DOF is about 0.32 μm. In addition, the Mask Error Factor (MEF), which is defined as the ratio between the incremental change of the image dimension and the incremental change of the object dimension on the mask, is on the order of 4.0. Both indicated values of DOF and MEF are unacceptable for the creation of contact holes for sub-micron and deep-sub-micron semiconductor devices. The invention provides a method for the improvement of both DOF and MEF for such contact holes.

Methods have been provided to improve the DOF of closely packed contact holes with off-axis illumination. This off-axis illumination can be applied in annular, quadruple or bipolar configurations, but the location of these configurations can be optimized only for a specific packing density. For example, only the image of holes with a hole-diameter to hole-separation ratio of 1:1 is optimized. After this optimization has been implemented for the 1:1 ratio, holes with a ratio of 1:2 benefit to a lesser extent than the 1:1 ratio holes. This decrease in benefit is even more pronounced for holes that are further separated, such as isolated holes. Alternately, holes may be optimized for a hole-diameter to hole-separation ratio of 1:2, with worsened results for holes that are packed closer and holes that are packed less tightly than the optimized holes.

The invention addresses the above stated concerns by providing two masks. The first mask, referred to as the packed mask, comprises the desired contact holes, which are part of the creation of a semiconductor device. An insulation layer is then coated over the first resist image. This insulation layer is preferably a water solution which includes a water soluble resin, a thermal acid generator and a cross-linker. After the solution is coated, high temperature baking may be applied.

During the thermal process, the water soluble resin undergoes the cross-linking reaction under catalysis by acid to form the insulation layer. The acid is generated by heat from the thermal acid generator in the water solution, or by exposure of the first resist. In a preferred embodiment, the cross-linking reaction forms an insulation layer on the surface of the first resist image. A water-rinse step follows to remove unreacted components.

In a further preferred embodiment, a compatible second layer resist is coated on the delineated insulation layer and first resist image. Note that either positive or negative resist may be used for the second layer. When a negative resist is chosen, a dark field unpacking mask based on padded holes exposes the second layer resist at the padded holes to make them insoluble in the resist developer, resulting in coverage of the padded holes. Alternatively, a light-field unpacking mask, based on the desired holes, exposes the negative second layer resist in all areas, except where the desired holes are located, and results in coverage of the background.

Note that when a positive resist is chosen for the second layer, a light-field unpacking mask based on the padding holes results in coverage of the padded holes, and a dark-field unpacking mask based on the desired holes results in coverage of the background.

To the packed mask are added padding holes in order to increase the hole density of the packed mask and to reduce the range of hole pitches. The second mask, referred to as the unpacking mask, comprises openings at the same locations as the locations of the padding holes of the first mask; the openings provided in the second mask have slightly larger dimensions than the padding holes of the first mask. A first exposure is made using the packed mask, a second exposure of the same surface area is made using the unpacking mask. The unpacking mask is used to selectively cover the padding contact holes, resulting in the final image. Two types of unpacking masks can be used, a first type having unpacking holes that surround the desired hole pattern, a second type having unpacking holes that align with the desired hole pattern.

To formalize and further expand the above stated procedures of the invention, these procedures can be stated as follows:the invention provides a first mask, images of the desired holes and of padding holes are provided in the first maskthe invention provides a second mask, unpacking images are provided in the second mask, the unpacking images align with the padding images; the images that are provided in the second mask are slightly larger than the corresponding images of the padding holes of the first maskthe first and the second mask can be implemented by making the mask background surface (which surrounds the therein created images) opaque, the therein created images being transparentthe first and the second mask can be implemented by making the mask background surface (which surrounds the therein created images) transparent, the therein created images being opaquethe application of the first and the second mask can be further extended, using a first layer of photoresist, in which a pattern of holes is created using the two masks of the invention, and an insulation coating is applied above the first layer; this pattern of holes can then further be extended by the deposition of a second layer of photoresist of the surface of the protected patterned first layer of photoresist, the second layer of photoresist is in turn patterned and developedpositive and negative photoresist can be usedspecial arrangements can be made relating to spacing the holes that are created, andspecial arrangements can be made relating to providing extra holes surrounding the holes that are created, the diameter of these extra holes can be adjusted with respect to the diameter of the holes that are created.

The invention will now be further explained usingFIGS. 1athrough7c. For these explanations, the following must be kept in mind. Photolithography is a common approach wherein patterned layers are usually formed by spinning on a layer of photoresist, projecting light through a photomask with the desired pattern onto the photoresist to expose the photoresist to the pattern, developing the photoresist, washing off the undeveloped photoresist, and plasma etching to clean out the areas where the photoresist has been washed away. The exposed resist may be rendered soluble (positive working) and form the pattern, or insoluble (negative working) and be washed away.

FIG. 1athrough1cshow the application of the packed and the unpacking mask where the unpacking features provided in the unpacking mask align with the padding holes in the packed mask. unpacking features and referring specifically toFIGS. 1athrough1c, there is shown:10,FIG. 1a, a top view of the packed mask12,FIG. 1b, a top view of the unpacking mask14,FIG. 1c, a top view of the final image16, a top view of padding holes contained in the packed mask1017, a top view of desired holes contained in the packed mask1018, a top view of the unpacking features contained in the unpacking mask12; it must be noted that the unpacking features18in unpacking mask12are in the same relative positions (align with) within the mask12as the padding holes16in the packed mask10; it must further be noted that the unpacking features18are slightly larger dimensions than padding holes16, and19, a top view of the final images of the desired holes.

FIGS. 2athrough2cshow the application of the packed and the unpacking mask where the unpacking features provided in the unpacking mask align with the desired holes in the packed mask. Referring specifically toFIGS. 2athrough2c, there is shown:20,FIG. 2a, a top view of the packed mask22,FIG. 2b, a top view of the unpacking mask24,FIG. 2c, a top view of the final image26, a top view of padding holes contained in the packed mask2027, a top view of desired holes contained in the packed mask2028, a top view of the unpacking features contained in the unpacking mask22; it must be noted that the unpacking features28in unpacking mask22are in the same relative positions (align with) within the mask28as the desired holes27in the packed mask20; it must further be noted that the unpacking features28are slightly larger dimensions than the desired holes27, and29, a top view of the final images of the desired holes.

FIGS. 3a–eshow cross sections of implementations of the masks of the invention.

Shown in cross section inFIGS. 3athrough3eare:30,FIG. 3a, a semiconductor surface over which a pattern of holes (desired holes) is to be created; this semiconductor surface typically is the surface of a monocrystalline silicon substrate32, a first layer of photoresist, coated over surface3033, a padded hole created through the first layer32of photoresist34, a desired hole created through the first layer32of photoresist31,FIG. 3b, an insulation layer, formed over the first layer32of photoresist36,FIG. 3c, a second layer of photoresist, coated over surface of the insulation layer31and the patterned first layer32of photoresist, including openings33and34created in the first layer32of photoresist37,FIG. 3d, a first cross section that shows unpacking results, resulting in filling padded hole33, and38,FIG. 3e, a second cross section that shows unpacking results, resulting in filling padded hole33and in filling the background.

Five cross sections are shown inFIGS. 3athrough3ewith the processing steps that are required to create these cross sections. The sequences ofFIGS. 3a,3b,3cand3dand ofFIGS. 3a,3b,3cand3erelate to the masks as shown inFIGS. 1athrough1cas follows:

FIG. 3ashows a cross section after exposure and development of the first layer32of photoresist with a mask comprising desired contact holes and dummy padding contact holes, that is mask10ofFIG. 1a.FIG. 3bshows a cross section of forming an insulation layer31to protect the patterns of the first layer32of photoresist.FIG. 3cshows a cross section of coating the second layer of photoresist36, without any impact on the first layer of photoresist32due to the protection by the insulation layer31.

FIG. 3dshows a cross section after exposure and development of the second layer36of photoresist with a mask12,FIG. 1b, comprising unpacking features, with the unpacking features being opaque and the mask background being transparent, not exposing the padded holes (since the unpacking features align with the padded holes), using positive photoresist for layer36so that the exposed surface of the layer36is soluble and is washed away after exposure, the layer36of photoresist remains in place overlying the padded hole, with the remaining layer36of photoresist being slightly larger than the padded hole33.

FIG. 3dfurther shows a cross section after exposure and development of the second layer36of photoresist with a mask12,FIG. 1b, comprising unpacking features, with the unpacking features being transparent and the mask background being opaque, exposing the padded holes (since the unpacking features align with the padded holes), using negative photoresist for layer36so that the exposed surface of the layer36is insoluble, the layer36of photoresist remains in place overlying the padded hole, with the remaining layer36of photoresist being slightly larger than the padded hole33.

FIG. 3eshows a cross section after exposure and development of the second layer36of photoresist with a mask22,FIG. 2b, comprising unpacking features, with the unpacking features being transparent and the mask background being opaque, exposing the desired holes (since the unpacking features align with the desired holes), using positive photoresist for layer36so that the exposed surface of the layer36is soluble and is washed away after exposure, the layer36of photoresist remains in place overlying the desired hole, with the remaining layer36of photoresist being slightly larger than the desired hole34.

FIG. 3efurther shows a cross section after exposure and development of the second layer36of photoresist with a mask22,FIG. 2b, comprising unpacking features with the unpacking features being opaque and the mask background being transparent, exposing the desired holes (since the unpacking features align with the desired holes), using negative photoresist for layer36so that the exposed surface of the layer36is insoluble, the layer36of photoresist remains in place overlying the desired hole, with the remaining layer36of photoresist being slightly larger than the desired hole34.

FIGS. 4athrough4cshow a sequence of mask layouts that follows the process of creation from an original layout to packed mask layout, as follows:70,FIG. 4a, the desired hole pattern provided on the surface of a mask72,FIG. 4b, a pattern of padded holes that is added to the desired hole pattern7074,FIG. 4c, a mask of the combined desired holes and the padding holes76, the desired holes provided in the mask7077, the padding hole provided in the mask7271, one of the desired holes that has been selected as an example, four padding holes73have been added to the desired hole7175, the distance between the desired hole71and the four surrounding padding holes73; it is noted that the pattern that is created by the four padding holes73is applied to each of the desired holes76, this is shown in the view7279, an example of combined padding holes; by comparing the view72with the view74, it is clear that added padding holes that overlap or that essentially overlap are combined into one padding hole, such as for instance padding holes81and83.
Further explanations that relate to the highlighted drawings are provided next in order to further explain the invention.FIGS. 3athrough3eshow the two-resist coating process to perform PAU. The first layer of photoresist is exposed with the packed mask and developed as usual. A compatible second layer of photoresist is coated on the delineated image in the first layer of photoresist image without affecting it. For some second layers of photoresist, in order to preserve the first photoresist image, an insulation layer is formed. When a negative resist is chosen, a dark-field unpacking mask, which is based on the padded holes (mask12,FIG. 1b, with unpacking features18being transparent and the background of the mask that surrounds the unpacking features18being opaque), exposes the second layer of photoresist at the padded holes to make the padded holes insoluble in the resist developer, resulting in coverage of the padded holes (see alsoFIG. 5afor additional details). Alternately, a light-field unpacking mask, which is based on the desired holes (mask22,FIG. 2b, with unpacking features28being opaque and the background of the mask that surrounds the unpacking features28being transparent), exposes the negative second layer of photoresist in all areas except where the desired holes are located and results in coverage of the background (see alsoFIG. 8dfor additional details). When a positive photoresist is chosen for the second layer of photoresist, a light-field unpacking mask, which is based on the padding holes (mask12,FIG. 1b, with unpacking features18being opaque and the background of the mask that surrounds the unpacking features18being transparent), results in coverage of the padded holes (see alsoFIG. 8bfor additional details) while a dark-field unpacking mask (mask22,FIG. 2b, with unpacking features28being transparent and the background of the mask that surrounds the unpacking features28being opaque), which is based on the desired holes, results in coverage of the background (see alsoFIG. 5cfor additional details).

One method to add the padding holes is shown inFIGS. 4athrough4c. Four padding holes73, which are similar in size with the desired hole71, are placed at the four sides of71as shown inFIG. 4a. They are separated from71by a distance75.FIG. 4ais a plot of the desired holes with the exception of the four padding holes73for illustration purpose. The process of adding four holes to one desired hole continues until all desired holes are treated as shown inFIG. 4b. Notice that the padding holes may overlap each other irregularity as in cluster77or they may overlap a desired hole as in cluster. Cluster77is actually constituted from the padding holes to desired holes71,87and89. For cluster77, a rectangle enclosing all three overlapping padding holes is drawn and replaces the three padding holes as shown inFIG. 4c. For cluster85, the padding holes are simply omitted.

The invention, in view of the number of independent parameters that can be selected in the creation of the desired openings, involves a somewhat complex thought process that can perhaps best be focused by a set of drawings, for this reason drawingsFIGS. 5athrough5dare provided.

In viewing these drawings, the following must be realized:the pattern that is created in the first layer of photoresist, seeFIG. 3a, is a base pattern that applies to all of these drawingsan insulation layer is formed to protect the base pattern, see layer31,FIG. 3b.a second layer of photoresist is deposited over the base pattern, see layer36,FIG. 3c; this second layer of photoresist is patterned, creating an opening in this layer of photoresist that in all cases and by definition must align with the desired opening, that is opening34,FIG. 3a.

Using mask12,FIG. 1b, where the unpacking features do not align with the desired holes but align with the padding holes, the second layer of photoresist will only be removed from the desired holes under one of two conditions:1. the second layer of photoresist is a negative layer of photoresist and the unpacking mask exposes (exposure91) the padding holes, that is the unpacking features of the unpacking mask are transparent and the surrounding background surface of the unpacking mask is opaque; seeFIG. 5awhere layer36is a negative (second) layer of photoresist, mask90has an opening that aligns with the padding holes and does not align with the desired hole34.2. the second layer of photoresist is a positive layer of photoresist and the unpacking mask does not expose the padding holes, that is the unpacking features of the unpacking mask are opaque and the surrounding background surface of the unpacking mask is transparent; seeFIG. 5bwhere layer36is a positive (second) layer of photoresist, mask95is opaque in a surface area of the mask95that aligns with the padding holes and does not align with the desired hole34.

The above two conditions can be summarized as follows, using the same sequence of conditions as followed above:1. a packed mask is used for creating closely spaced contact holes, creating a first pattern of contact holes (holes34,FIG. 5a) and a second pattern of contact holes (holes33,FIG. 5a). A mask90,FIG. 5a, is used comprising a pattern of contact holes, the pattern of holes in the mask90is aligned with the second pattern of holes33,FIG. 5a, the pattern of holes in mask90comprising a transparent pattern of holes with a surrounding opaque background surface2. a packed mask is used for creating closely spaced contact holes, comprising a first pattern of contact holes (holes34,FIG. 5b) and a second pattern of contact holes (holes33,FIG. 5b). A mask95,FIG. 5b, is used comprising a pattern of holes, said pattern of holes in mask95being aligned with said second pattern of holes33, said pattern of holes in mask95being an opaque pattern of holes with a surrounding transparent background surface.

Using mask22,FIG. 2b, where the unpacking features align with the desired holes and do not align with the padding holes, the second layer of photoresist will only be removed from the desired holes under one of two conditions:1. the second layer of photoresist is a positive layer of photoresist and the unpacking mask exposes the desired holes, that is the unpacking features of the unpacking mask are transparent and the surrounding background surface of the unpacking mask is opaque; seeFIG. 5cwhere layer36is a positive (second) layer of photoresist, mask93has an opening that aligns with the desired hole34.2. the second layer of photoresist is a negative layer of photoresist and the unpacking does not expose the desired holes, that is the unpacking features of the unpacking mask are opaque and the surrounding background surface of the unpacking mask is transparent; seeFIG. 5dwhere layer36is a negative (second) layer of photoresist, mask92is transparent in a surface area of the mask93that aligns with the desired hole34.

The above two conditions can be summarized as follows, using the same sequence of conditions as followed above:1. a packing and unpacking mask is used for creating closely spaced contact holes, comprising a first pattern of contact holes34,FIG. 5c, and a second pattern of contact holes33,FIG. 5c. A mask93,FIG. 5c, is used comprising a pattern of holes, the pattern of holes in mask93being aligned with the first pattern of holes34, the pattern of holes in mask93being a transparent pattern of holes with a surrounding opaque background surface.2. a packing and unpacking mask is used for creating closely spaced contact holes, comprising a first pattern of contact holes34,FIG. 5d, and a second pattern of contact holes33,FIG. 5d. A mask92,FIG. 5d, is used comprising a pattern of holes, said pattern of holes in mask92being aligned with said first pattern of holes34,FIG. 5d, said pattern of holes in mask92being an opaque pattern of holes with a surrounding transparent background surface.