Microlithographic structures and method of fabrication

A method of formulating and fabricating a mask pattern and resulting mask for forming isolated or closely spaced contact holes in an integrated circuit. The mask has a transparent mask substrate and patterned regions of attenuating phase shift material and opaque, partially transmissive or transparent material arranged to reduce the effect of side lobes and improve depth of focus. The rims, frames and outrigger patterns for the attenuating phase shift material and opaque, partially transmissive or transparent material are determined according to calculations performed on a processor with imaging software for various optical conditions and exposed feature criteria.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to masks (or reticles) and other devices for use in microlithographic techniques, especially for use in forming contact holes in semiconductor products with improved depth of focus. The invention may be used to form isolated contact holes, arrays of contact holes and other structures. The present invention also relates to a method of determining a desired three-tone pattern for a microlithographic mask. The method, which may be performed on one or more programmed microprocessors, may involve the selection of one set of dimension data out of a plurality of dimension data sets, where the dimension data sets correspond to different mask patterns, and the selected set is the one that provides the greatest depth of focus.

2. Description of Related Art

A semiconductor device can be fabricated by photolithography, in which light is transmitted through a patterned mask (or reticle). The pattern on the mask is exposed on a layer of photoresist to form the desired feature or features in the semiconductor device. Examples of such features are isolated contact holes and contact holes formed in closely packed arrays. In certain circumstances, it is desirable to make contact holes with very small critical dimensions. The “critical dimension” is typically the diameter of the hole in the plane of the surface of the semiconductor device. Some devices require contact holes with critical dimensions that are less than the wavelength of the light that is used to expose the photoresist. A dimension that is less than the wavelength of the exposing light is referred to as a “sub-resolution” dimension.

A number of binary and phase-shifting masks have been proposed in the prior art. Such masks are shown, for example, in U.S. Pat. No. 6,114,071 (Chen et al.), U.S. Pat. No. 6,077,633 (Lin et al.), U.S. Pat. No. 6,022,644 (Lin et al.) and U.S. Pat. No. 5,707,765 (Chen). There is still a need in the art, however, for a three-tone mask (or other multi-tone mask) that can form holes and other structures with small critical dimensions and minimum side-lobing and with improved depth of focus. Depth of focus is especially important in connection with the formation of small structures in non-flat surfaces. Where the wafer is not flat, it may be necessary to image a pattern at different distances from the lithography system with essentially the same fidelity. In addition, it may be necessary to allow for wafer positioning in the system, wafer curvature, focal plane curvature, etc.

Moreover, there is a need in the art for an economical method of making three-tone masks (or other multi-tone masks) for use in the formation of small critical dimension features with minimal side-lobing and large depth of focus.

SUMMARY OF THE INVENTION

The present invention relates to a microlithographic mask for forming a sub-resolution feature in photoresist with improved depth of focus. As noted above, the term “sub-resolution” means that the critical dimension of the feature formed in the photoresist is less than the wavelength of the exposing light. According to one aspect of the invention, the mask has a three-tone structure, with a layer of transparent material, a layer of attenuating phase-shifting material overlying the transparent material, and a layer of light-obstructing material (i.e., opaque material and/or partially transmissive material) overlying the phase-shifting material. In a preferred embodiment of the invention, the layer of attenuating phase-shifting material is located between the transparent material and the light-obstructing material. The present invention should not be limited, however, to the specific features of the preferred embodiments shown and described in detail herein.

According to another aspect of the invention, opaque material and the attenuating phase-shifting material are patterned to form a square transparent opening, one or more partially transmissive assist features, which may include a rectangular frame, and an opaque frame and/or background. The opaque frame may be located at the edge of the opening (interposed between a partially transmissive assist feature and the transparent opening). Alternatively, opaque squares, triangles or other polygons can be placed at the corners (or inside) of a partially transmissive frame to control the exposure pattern of the light that is transmitted through the partially transmissive frame.

The transparent material may be quartz or another suitable material. The partially transmissive material causes a phase shift (e.g., 180° or an odd multiple thereof) relative to the light transmitted through the transparent material. The partially transmissive material also attenuates the phase-shifted light relative to the non-phase-shifted light. The transmissivity of the partially transmissive material relative to the transparent material may be in the range, for example, of from about 6% to 100%, more preferably from about 8% to about 24%. The partially transmissive material may be, for example, MoSi. The opaque material may be a metal such as chrome, and other suitable materials may be employed as desired.

The present invention may be used to form a variety of microlithographic features. The invention is especially well suited, however, for forming a contact hole that has a large aspect ratio of depth to width. The invention is also well suited to forming other structures where a large depth of focus is desirable, such as microlithographic features on substantially non-flat surfaces. According to one aspect of the invention, improved depth of focus is achieved by providing sub-resolution assist features that are patterned in the opaque material and/or the partially transmissive material.

The present invention also relates to masks for forming regular and asymmetric arrays of features, such as arrays of high aspect ratio contact holes. According to one aspect of the invention, elongated assist bars (of partially transmissive material and/or opaque material) are employed to interact with an array of transparent openings. According to another aspect of the invention, phase-shifting assist features are nested within transparent bars.

The present invention also relates to a method of forming elliptical holes and other structures with small critical dimensions and improved depth of focus. The holes may be isolated structures or they may be formed in a dense array.

The present invention also relates to a method of making a multi-tone microlithographic mask. The method, which may be performed at least in part on a digital microprocessor, includes the steps of: (1) providing sets of dimension data representative of multiple mask patterns; (2) for each set of dimension data, calculating feature dimension data as a function of optical conditions; and (3) for a desired optical condition, identifying the sets of dimension data that correlate to feature dimension data within desired limits. If desired, the method may also include the step of (4) selecting the one identified set of dimension data that achieves the smallest change in critical dimension between a zero defocus condition and a maximum considered defocus condition.

In a preferred embodiment of the invention, steps (1) and (2) are performed using a computer programmed with PERL/solid-c imaging software. Steps (3) and (4) may be performed using Visual BASIC/Excel software. As noted above, however, the present invention should not be limited to the specific features of the preferred embodiments.

The dimension data can include the widths of transparent openings and the corresponding dimensions of the opaque and partially transmissive assist features. There may be one set of such dimension data for each pattern under consideration. The limits considered in step (3) may include the critical dimension for the exposed feature, the allowable (or desirable) ellipticity, the absence of sidelobes, log-slope, etc. These limits operate to exclude patterns that do not form acceptable features at the desired operating conditions. Once a desired pattern is determined, the pattern is formed in layers of deposited partially transmissive and opaque materials to form the finished mask. As noted above, the two upper layers of the mask may be deposited on a layer of transparent quartz.

These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now to the drawings, where like reference numerals designate like elements, there is shown inFIG. 1a microlithographic mask30for forming an isolated contact hole (not shown) in photoresist (not shown). The mask30is formed of a transparent substrate32(FIG. 2), an attenuating phase shift layer34, and an opaque layer36. The attenuating phase shift layer34and the opaque layer36are patterned to define a square transparent opening38, a rectangular opaque frame40(FIG. 1), a partially transmissive outrigger frame42, and an opaque background44. In the illustrated embodiment, the inner edges of the opaque frame40define transparent opening38.

In operation, incident light46(FIG. 2) is transmitted through the opening38and the outrigger frame42. The incident light46may be generated by a suitable source (not shown) located above the mask30. The incident light46is prevented from passing through the opaque material40,44. The light46,48that is transmitted through the outrigger frame42is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent opening38. In addition, the outrigger frame42attenuates the phase-shifted light48relative to the non-phase-shifted light50. The attenuated phase-shifted light48interacts with the non-phase-shifted light50to form the contact hole in the photoresist.

In the illustrated embodiment, the opaque frame40causes the partially transmissive outrigger frame42to be effectively spaced apart from the edges52of the transparent opening38. That is, the opaque frame40is interposed between the transparent opening38and the partially transmissive frame42. By blocking the incident light46between the transparent opening38and the outrigger frame42, the contact hole can be formed in the shape of a cylinder (with minimal side lobing) with improved depth of focus. In a preferred embodiment, the cylindrical contact hole may be formed with a depth of focus of about 0.8 microns (μm) or greater. The depth of focus determines the length of the cylindrical hole that can be formed in the photoresist without unacceptable side-lobing. The depth of focus also characterizes the ability of the mask30to be used to form sub-resolution features in non-flat photoresist surfaces.

In the illustrated embodiment, the substrate32is formed of quartz, the attenuating phase shift layer34is formed of MoSi, and the opaque layer36is formed of chrome. The transmissivity of the attenuating phase shift layer34may be about 18% when the wavelength of the incident light is about 248 nanometers (nm). The transmissivity of the transparent quartz layer32may be essentially 100%. Other suitable materials may be employed in the mask30, and additional layers may be provided, if desired. Further, the width54of the transparent opening38is about 0.14 μm. The diameter of the hole (not shown) formed by the mask30is about 0.12 μm (less than the wavelength of the incident light46) in the exposed photoresist. The width56of the opaque frame40is about 0.125 μm, and the width58of the outrigger frame42is about 0.115 μm. The incident light46is propagated with a numerical aperture (NA) of about 0.63 and a sigma (σ) of about 0.35. The thickness of the three layers36,34,32may be 700 to 1000 Angstroms, 800 to 1200 Angstroms, and one-quarter inch, respectively. The present invention should not be limited, however, to the specific materials, dimensions and instrumentalities of the preferred embodiments shown and described in detail herein. As mentioned below, the scope of the invention should be defined by the appended claims.

Referring now toFIGS. 3 and 4, there is shown a mask70for forming an isolated cylindrical contact hole (not shown) in photoresist (not shown). The mask70includes a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. The attenuating phase shift layer34and the opaque layer36are patterned to define a square transparent opening72, rectangular, nested opaque frames74,76, rectangular, nested partially transmissive outrigger frames78,80, and an opaque background82.

In operation, incident light46is transmitted through the opening72and the outrigger frames78,80. The incident light46is prevented from passing through the opaque material74,76,82. The light46,48that is transmitted through the outrigger frames78,80is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent opening72. In addition, the phase-shifting layer34attenuates the phase-shifted light48relative to the non-phase-shifted light50. The attenuated phase-shifted light48interacts with the non-attenuated, non-phase-shifted light50to form the cylindrical contact hole in the underlying, exposed photoresist (not shown).

In the illustrated embodiment, the opaque frames74,76separate the partially transmissive outrigger frames78,80from the edges84of the transparent opening72(and from each other). By blocking the incident light46between the transparent opening72and the outrigger frames78,80, the mask70ofFIGS. 3 and 4is able to form a cylindrically-shaped hole with a critical dimension (CD) of 0.12 μm and with a depth of focus of 0.8 μm or more. As in the embodiment ofFIGS. 1 and 2, the attenuating phase shift layer34is formed of MoSi, and the opaque layer36is formed of chrome. The relative transmissivity of the phase shift layer34may be about 18%. The width86of the transparent opening72is about 0.16 μm. The width88of each opaque frame74,76is about 0.085 μm, and the width90of each outrigger frame78,80is about 0.11 μm. The mask70would be suitable for operation under the same optical and photoresist conditions described above in connection withFIGS. 1 and 2(i.e., where NA is about 0.63, and σ is about 0.35).

The structures shown inFIGS. 1–4are frame-based structures. In each illustrated embodiment there is opaque material abutting the contact. Polygons of attenuated phase shift material, particularly scattering bars, lay around the contact. The claimed invention should not be limited, however, to the embodiments shown and described in detail herein.

The structures shown inFIGS. 5–8are rim-based structures. In each embodiment there is attenuated phase shift material abutting the contact. Polygons, either opaque or transparent, or of another transmission, lay immersed inside the attenuated material or at either border, defining the form and size of the rim.

Referring now toFIGS. 5 and 6, there is shown a mask100for forming an isolated cylindrical contact hole with improved depth of focus. The mask100includes a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. The attenuating phase shift layer34and the opaque layer36are patterned to define a transparent opening102, a partially transmissive rim104, opaque corner squares106, and an opaque background108. In operation, incident light46is transmitted through the opening102and the rim104. The incident light46is prevented from passing through the opaque material106,108. The light46,48that is transmitted through the rim104is phase shifted (by 180° or an odd multiple thereof) and attenuated (to about 18%) relative to the light46,50that is transmitted through the opening102.

As in the embodiments ofFIGS. 1–4, the attenuated, phase-shifted light48interacts with the non-attenuated, non-phase-shifted light50to form the contact hole with the desired geometry. The four opaque corner squares106cover the partially transmissive layer34and thereby contribute to the formation of a cylindrically-shaped contact hole without sidelobes. In the illustrated embodiment, the corner squares106separate the partially transmissive rim104into four short bars. By blocking the incident light46at points between the transparent opening102and the opaque background108, the contact hole can be formed with minimal side-lobing and with improved depth of focus.

In the illustrated embodiment, the width110of the transparent opening102is about 0.21 μm, and the CD of the hole (not shown) formed by the mask100is about 0.12 μm. The width112of the partially transmissive rim104is about 0.24 μm, and the width114of the square corners106is about 0.19 μm. The mask100may be operated under the same optical and photoresist conditions as those described above in connection withFIGS. 1 and 2.

The mask130shown inFIGS. 7 and 8is similar to the mask100shown inFIGS. 5 and 6except that the opaque corners132in theFIGS. 7 and 8embodiment are triangular to provide improved side-lobing control.

The structures shown inFIGS. 9–12, like those shown inFIGS. 1–4, are frame-based structures. Referring now toFIGS. 9 and 10, there is shown yet another mask150for forming an isolated cylindrical contact hole with improved depth of focus. The mask150has a square transparent opening152, a partially transmissive short bar frame154, opaque corner squares156at the ends of the short bars of the frame154, and an opaque background158. In operation, incident light46is transmitted through the opening152and the frame154. The light46is prevented from passing through the opaque material156,158,160. The light46,48that is transmitted through the short bar frame154is phase-shifted (by 180° or an odd multiple thereof) and attenuated (to about 18%) relative to the light46,50that is transmitted through the opening152. The attenuated phase-shifted light48interacts with the non-phase-shifted light50to expose the photoresist such that the formed hole has the desired geometry.

The opaque frame160, like the frame40of theFIGS. 1 and 2embodiment, optically separates the partially transmissive frame154from the transparent opening152, to achieve improved depth of focus. The opaque corner squares156operate like the squares106ofFIGS. 5 and 6to spatially and selectively control the transmission of light through the frame154, to thereby make it easier to ensure that the contact hole has the desired cylindrical geometry, depth of focus and eliminate sidelobes.

In the illustrated embodiment, the width162of the transparent opening152is about 0.15 μm. The CD of the hole (not shown) formed by the mask150is about 0.12 μm. The width164of the partially transmissive frame154is about 0.15 μm, and the width166of the overlapping square corners156is likewise about 0.15 μm. The width168of the opaque frame160(i.e., the separation of the partially transmissive frame154from the transparent opening152in orthogonal directions) is about 0.10 μm. The mask150may be operated under the same optical and photoresist conditions as those described above in connection withFIGS. 1 and 2.

The mask180shown inFIGS. 11 and 12has nested opaque frames182,184and nested, short bar, partially transmissive frames186,188. In the illustrated embodiment, the width (or separation distance)190of each opaque frame182,184is about 0.12 μm, and the width192of each partially transmissive short bar186,188is about 0.175 μm. The width194of each overlapping opaque corner196is likewise about 0.175 μm. The width198of the square opening199is about 0.14 μm. Thus, the small dimensions of the features patterned on the mask180are all less than the wavelength of the incident light46.

Further, the layered structure32,34,36may be patterned to form a mask for forming an array of closely-spaced contact holes (or other features in the photoresist). The array may be regularly or asymmetrically configured. The contact holes may be cylindrical or elliptical, as discussed in more detail below.

FIGS. 13–25show frame-based structures for forming arrays of contact holes. Referring now toFIGS. 13–15, there is shown a mask200for forming a regular array of contact holes, where each contact hole has a cylindrical CD of 0.12 μm, for example. The mask200is formed of a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. The layers32,34,36may be formed of the same materials as described above in connection withFIGS. 1 and 2. The attenuating phase shift layer34and the opaque layer36are patterned to define square transparent openings202, rectangular opaque frames204surrounding the openings202, and partially transmissive/phase shifting outrigger bars206. The partially transmissive outrigger bars206are connected to each other to define a regular array of outrigger frames around each opaque frame204.

In operation, the mask200operates generally like the structure ofFIGS. 1 and 2, except in a closely packed array. Incident light46(FIG. 14) is transmitted through the openings202and the outrigger frames206, but is prevented from passing through the opaque material204. The light that is transmitted through the outrigger frames206is phase-shifted (by 180° or an odd multiple thereof) and attenuated (to about 18%) relative to the light46,50that is transmitted through the openings202. The attenuated phase-shifted light48(FIG. 14) and the non-phase-shifted light50interact with each other to form the desired densely packed array of contact holes in the photoresist. The opaque frames204operate to separate the partially transmissive material34,206from the square openings202to thereby provide the mask200with improved depth of focus (in the range of from about 0.4 to 0.8 μm or greater).

In the illustrated embodiment, the width208of the transparent openings202is about 0.15 μm. All of the small dimensions of the assist features204,206are sub-resolution. The separation width209of the rectangular frames204is about 0.15 μm. The widths210,212of the outrigger bars of the partially transmissive frame206are different from each other, and may be about 0.12 μm and about 0.14 μm in the respective orthogonal directions. As in the embodiments ofFIGS. 1–12, the incident light46is propagated with a NA of about 0.63 and an on-axis illumination σ of about 0.35. As noted, however, the present invention should not be limited to the specific materials, dimensions and instrumentalities of the preferred embodiments shown and described in detail herein. In all of the embodiments described herein, the NA may be as high as 0.70 or more, and the σ value may be in the range of from about 0.3 to 0.85, for example. The invention may be operated with on-axis and off-axis illumination systems.

The mask240shown inFIGS. 16–19is the same as the one shown inFIGS. 13–15, except that the partially transmissive outrigger frame for the mask240is formed of short bars242,244that are separated at their ends. Opaque squares246are formed between the ends of the short bars242,244to facilitate the design of the mask240. The opaque squares246spatially and selectively limit the amount of phase-shifted light48(FIG. 17) that is exposed on the photoresist (not shown) to thereby control the formation of the contact holes with the desired geometries and with improved depth of focus. In the embodiment ofFIGS. 16–19, the width248of each square portion244of the short bar frame is about 0.14 μm, and the width250of each elongated bar portion242of the short bar frame is about 0.12 μm. An advantage of the present invention is that the widths248,250of the various cooperating assist features244,242may be different from each other.

FIGS. 20–22show a mask260for forming an asymmetric array of cylindrical contact holes (not shown). The contact holes are aligned with transparent openings262patterned through the opaque layer36and the partially transmissive layer34. The opaque layer36is further patterned to expose bar-shaped regions264of the partially transmissive material34. The opaque material36that remains between the openings262and the partially transmissive bars264operate like opaque frames of the type shown inFIGS. 1 and 2. The dimensions of the asymmetric mask260may be as follows: The width266of the square openings262may be about 0.19 μm. The separation widths268,270of the opaque rectangular frames36may be about 0.21 and 0.19 μm, respectively. The width272of the elongated partially transmissive bars264may be about 0.15 μm.

In an alternative embodiment of the invention (not shown), the patterned layers of the mask260may be patterned in an inverse fashion such that the partially transmissive material34is contiguous with the transparent openings262, and bounded by longitudinal bars of opaque material36. In this alternative embodiment of the invention, the incident light46is transmitted (50) through the openings262without any phase-shifting or attenuation. The incident light46is blocked entirely by longitudinal, parallel bars of opaque material36, and the rest of the incident light46(48) is attenuated and phase-shifted by assist frames that surround the transparent openings262.

FIGS. 23–25show a mask300for forming an array of elliptical holes. The mask300of theFIGS. 23–25embodiment is like the mask260shown inFIGS. 20–22except that the transparent openings302are rectangular and not square. In theFIGS. 23–25embodiment, the width304and length306of the openings302may be about 0.15 and 0.23 μm, respectively. The separation widths308,310of the opaque rectangular frames36may be different from each other, and the length and width of the partially transmissive bars may be about 0.14 μm and 0.70 μm, respectively.

FIGS. 26–34show rim-based structures for forming arrays of contact holes. Referring now toFIGS. 26–28, there is shown a microlithographic mask500for forming a regular array of contact holes (not shown) in photoresist (not shown). The mask500is formed of a transparent substrate32(FIG. 27), an attenuating phase shift layer34, and an opaque layer36. The attenuating phase shift layer34and the opaque layer36are patterned to define rectangular transparent openings502, partially transmissive rims504, and an opaque background506.

In the structure shown inFIG. 26, the width and length of the opaque bars36between contacts502limit the size of the rims504. In an alternative embodiment of the invention, the bars surrounding the rims504may be non-opaque, but of another transmission. Whether the bars are opaque, light-obstructing, or transparent, they can still contribute to the function of defining the image.

In operation, incident light46(FIG. 27) is transmitted through the openings502and the rims504. The incident light46may be generated by a suitable source (not shown) located above the mask500. The incident light46is prevented from passing through the opaque material506. The light46,48that is transmitted through the rims504is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent openings502. In addition, the rims504attenuate the phase-shifted light48relative to the non-phase-shifted light50. The attenuated phase-shifted light48interacts with the non-phase-shifted light50to form the desired contact holes.

In the illustrated embodiment, the partially transmissive rims504cooperate with the square openings502to form the contact holes with minimal side lobing and improved depth of focus. In a preferred embodiment, the cylindrical contact hole may be formed with a depth of focus of about 0.8 μm or greater. As in the embodiments discussed above, the depth of focus determines the length of the cylindrical hole that can be formed in the photoresist without unacceptable side-lobing. The depth of focus also characterizes the ability of the mask500to be used to form sub-resolution features in non-flat photoresist surfaces.

In the illustrated embodiment, the substrate32is formed of quartz, the attenuating phase shift layer34is formed of MoSi, and the opaque layer36is formed of chrome. The transmissivity of the attenuating phase shift layer34may be about 18% when the wavelength of the incident light is about 248 nm. The transmissivity of the transparent quartz layer32may be essentially 100%. Other suitable materials may be employed in the mask500, and additional layers may be provided, if desired.

Further, the orthogonal dimensions508,510of each transparent opening502are about 0.22 μm and 0.20 μm, respectively. The diameter of each hole (not shown) formed by the mask500is about 0.12 μm (less than the wavelength of the incident light46) in the exposed photoresist. The widths512,514of the opaque bars506are about 0.10 μm and 0.06 μm, respectively. The incident light46is propagated with a numerical aperture (NA) of about 0.63 and a sigma (σ) of about 0.35. The thickness of the three layers36,34,32may be 700 to 1000 Angstroms, 800 to 1200 Angstroms, and one-quarter inch, respectively. The present invention should not be limited, however, to the specific materials, dimensions and instrumentalities of the preferred embodiments shown and described in detail herein.

FIGS. 29–31show another microlithographic mask550for forming a regular array of contact holes in photoresist (not shown). The mask550is formed of a transparent substrate32(FIG. 30), an attenuating phase shift layer34, and an opaque layer36. The three layers32,34,36may be formed of the same materials and with the same thicknesses as described above in connection with the mask500ofFIGS. 26–28. Alternatively, other suitable materials may be employed in the mask550, and additional layers may be provided, if desired.

In theFIGS. 29–31embodiment, the attenuating phase shift layer34and the opaque layer36are patterned to define square transparent openings552, partially transmissive rims554, and an opaque background formed of short bars556. In operation, incident light46is transmitted through the openings552and the rims554. The incident light46is prevented from passing through the opaque bars556. The light46,48that is transmitted through the rims554is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent openings552. In addition, the rims554attenuate the phase-shifted light48relative to the non-phase-shifted light50. The attenuated phase-shifted light48interacts with the non-phase-shifted light50to form the desired array of contact holes with minimal side lobing and improved depth of focus.

The width558of each transparent opening552is about 0.21 μm. The diameter of each hole (not shown) formed by the mask550is about 0.12 μm in the exposed photoresist. The width560of each short opaque bar556is about 0.10 μm. The incident light46may be propagated with a numerical aperture (NA) of about 0.63 and a sigma (σ) of about 0.35 inch. The present invention should not be limited, however, to the specific materials and dimensions shown and described in detail herein. The scope of the invention should be determined according to the appended claims.

FIG. 32shows another microlithographic mask600for forming a regular array of contact holes in photoresist. As shown inFIGS. 33 and 34, the illustrated mask600is formed of a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. The three layers32,34,36may be formed of the same materials and with the same thicknesses as described above in connection with the mask500ofFIGS. 26–28. The attenuating phase shift layer34and the opaque layer36are patterned to define rectangular transparent openings602, partially transmissive, asymmetric rims604, and an opaque background formed of long bars606. The parallel bars606are staggered with respect to the openings602. In operation, incident light46is transmitted through the openings602and the asymmetric rims604.

The incident light46is prevented from passing through the opaque bars606. The light46,48that is transmitted through the rims604is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent openings602. In addition, the rims604attenuate the phase-shifted light48relative to the non-phase-shifted light50. The attenuated phase-shifted light48interacts with the non-phase-shifted light50to form the desired array of closely packed contact holes with minimal side lobing and improved depth of focus.

In the illustrated embodiment, the orthogonal dimensions608,610of each transparent opening602are about 0.19 μm and 0.21 μm, respectively. The diameter of each hole (not shown) formed by the mask500is about 0.12 μm in the exposed photoresist, where the propagation characteristics of the incident light are as follows: NA=about 0.63; and σ=about 0.35. To define the desired asymmetric rims604, each long opaque bar may have a width610of about 0.15 μm and a length612of about 0.72 μm. The dimensions may be calculated by a suitable programmed computer as discussed in more detail below.

FIGS. 35–46show additional rim-based structures for forming isolated contacts. Thus,FIG. 35shows a microlithographic mask650for forming an isolated contact hole in photoresist. As shown inFIGS. 36 and 37, the mask650includes a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. The three layers32,34,36may be formed of the same materials and with the same thicknesses as described above in connection with the mask500ofFIGS. 26–28. The attenuating phase shift layer34and the opaque layer36are patterned to a square transparent opening652enclosed within concentric partially transmissive frames654,656,658. The partially transmissive assist features654–658are defined by a double set of long, orthogonal, opaque bars660,662, and an outer opaque background664.

In operation, incident light46is transmitted through the isolated opening652and the concentric rims654–658. The incident light46is prevented from passing through the opaque frames660,662. The light46,48that is transmitted through the rims654–658is phase-shifted (by 180° or an odd multiple thereof) relative to the light46,50that is transmitted through the transparent opening652. The rims654–658attenuate the phase-shifted light48relative to the non-attenuated light50. The light48,50interacts to form the desired contact hole with minimal side lobing and improved depth of focus.

In the illustrated embodiment, the width664of the opening652is about 0.20 μm to form a 0.12 μm diameter hole in the exposed photoresist, where the propagation characteristics of the incident light46are as follows: NA=about 0.63; and σ=about 0.35. The width668of each long bar frame660,662may be about 0.07 μm. If desired, the separation distances670between the opaque assist features may be about 0.1 μm.

FIGS. 38–40show a microlithographic mask700with a rim that includes double transparent long bars. The mask700may be used to form an isolated contact hole in photoresist. Like the masks discussed above, the mask700ofFIGS. 38–40is formed of a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. As shown inFIG. 38, the attenuating phase shift layer34is patterned to form a square transparent opening702(width704=about 0.22 μm) and concentric transparent frames706,708(width710=about 0.08 μm). The opening702is surrounded by the transparent frames706,708. The partially transmissive material34on opposite sides of the rims706,708forms phase-shifting assist features712,714,716. The assist features712–716(separation distance718=about 0.2 μm) may be concentrically, alternatingly nested with the transparent frames706,708. The outermost assist feature712may be surrounded by the opaque background36.

In operation, incident light46(NA=about 0.63; σ=about 0.35) is transmitted through the isolated opening702, the transparent frames706,708and the rim shaped assist features712–716. The light46,48that is transmitted through the concentric rims712–716is phase-shifted (by 180° or an odd multiple thereof) and attenuated relative to the light46,50that is transmitted through the transparent opening702and the frames706,708. Thus, the transmitted light48,50interacts to form the desired contact hole (not shown; diameter=about 0.12 μm) with minimal side lobing and improved depth of focus.

FIGS. 41–43show a microlithographic mask750, with a rim that includes double opaque short bars, for producing an isolated contact hole in photoresist (not shown; diameter=about 0.12 μm). Like the masks discussed above, the mask750is formed of a transparent substrate32, an attenuating phase shift layer34, and an opaque layer36. Throughout this specification, as noted above, like reference numerals designate like elements. Referring now toFIGS. 42 and 43, the attenuating phase shift layer34and the opaque layer36may be patterned to form a square transparent opening752(or “contact”; width754=about 0.2 μm) and concentric frames formed of short bars756,758(width760=about 0.08 μm) whose ends do not overlap each other.

The transparent opening752is surrounded by the orthogonally arranged short bars756,758. The opaque frames756,758define phase shifting assist features764,766,768in the partially transmissive layer34. The separation distance770between the opaque frames756,758may be about 0.1 μm, although changes may be made to the illustrated embodiments without departing from the scope of the present invention. The assist features764,766,768may be concentrically, alternatingly nested with the opaque frames756,758. The outermost assist feature764may be surrounded by an opaque background780. The opaque background780is formed by a non-patterned region of the opaque layer36.

In operation, incident light46(NA=about 0.63; σ=about 0.35) is transmitted through the isolated opening752and the assist features764,766,768. The light46,48that is transmitted through the assist features764,766,768is phase-shifted (by 180° or an odd multiple thereof) and attenuated relative to the light46,50that is transmitted through the transparent opening752. Thus, the transmitted light48,50interacts to form the desired contact hole with minimal side lobing and improved depth of focus.

FIG. 44shows a microlithographic mask800, with a rim that includes double transparent short bars, for producing an isolated contact hole in photoresist (not shown; diameter=about 0.12 μm). The mask800may be constructed generally like the mask750discussed above in connection withFIGS. 41–43except that the concentric frames are transparent (i.e., patterned through the partially transmissive material34) rather than partially transmissive (i.e., patterned only through the opaque layer36). Referring now toFIGS. 45 and 46, the mask800may have a square transparent opening802(contact width804=about 0.21 μm) and concentric frames formed of transparent short bars806,808(width810=about 0.08 μm) whose ends do not overlap each other.

The transparent opening802is surrounded by concentric assist features812,814separated by and nested within the transparent bars806,808. The illustrated separation distance816may be, for example, about 0.22 μm. The opaque background818is formed by a non-patterned region of the device800. In operation, incident light46(NA=about 0.63; σ=about 0.35) is transmitted through the isolated opening802, the clear outrigger frames806,808, and the assist features812,814. The attenuated and phase shifted light46,48interacts with the non-phase shifted light46,50to form the desired contact hole with minimal side lobing and improved depth of focus.

In connection withFIGS. 1–46, all of the dimensions provided for the illustrated masks are at 1×, i.e., at the final dimensions on the wafer or photoresist within which the contact hole(s) is(are) formed. The masks may be fabricated at a larger scale, for example, at 4× or 5×, such that when the geometry is imaged onto the wafer, the image is shrunk by the scale factor. In addition, satisfactory results may be obtained even through the dimensions are not exactly as specified in the detailed description. For example, each dimension may be in a tolerance range of 10% plus or minus the value specified herein. The invention should not be limited to the specified dimensions and ranges, however, except to the extent such values are recited in the claims.

Further, please note that, although the structures shown inFIGS. 35–46have two scattering bars per side, the invention may be employed with any number of bars per side, including one. The claimed invention should not be limited to the preferred embodiments shown and described in detail herein.

Referring now toFIG. 47, there is shown a flow chart for a method of making and/or designing a multi-tone microlithographic mask in accordance with one aspect of the present invention. The method may be arranged to identify or select the one mask pattern, out of numerous mask patterns of the types shown inFIGS. 1–46, that provides the greatest depth of focus under given optical and feature conditions. The given conditions may relate to the optical and operational parameters of the microlithography system that will be used, the type of photoresist within which the contact holes or other features will be formed, and the design criteria, limits or tolerances on the geometry of the features to be formed in the photoresist. The latter design limits may include the desired critical dimension (CD), lack of side-lobing, etc. In a preferred embodiment of the invention, the design criteria may also include the image log-slope. The term “image log-slope,” which is a well known term in the art, basically refers to the slope of the diffraction pattern. Higher slope provides an image with sharper edges, and therefore improved contrast.

In the illustrated method, sets of dimension data are input (Step400) into a programmed microprocessor or the like. The sets of dimension data are representative of the planar dimensions of various mask patterns. Each set of dimension data may include the width of the transparent opening(s), the width or separation distance of any opaque frame(s), the width, separation distance or other planar dimensions of any partially transmissive rims, the widths of corner squares (opaque or partially transmissive), the relevant dimensions of other assist features, etc. The dimensions that make up the dimension data may be varied within predetermined ranges. The process of varying the dimension data and inputting the sets of dimension data into the system (Step400) may be automated.

In a subsequent step (Step402), calculations are carried out for each set of dimension data. The calculations provide feature data, for each set of dimension data, as a function of optical conditions. The feature data may be, for example, representative of the CD, ellipticity, side-lobing, etc. of the features that would be produced by a mask having the dimensions of the respective dimension data set. The feature data is calculated as a function of optical conditions such as intensity range, NA, σ etc. In addition, the feature data is calculated for zero, intermediate and maximum defocus conditions. At the conclusion of Step402, there are multiple sets of feature data for each set of dimension data.

Subsequently, in Step404, for a desired optical condition, the sets of dimension data that result in feature data within acceptable limits (design criteria) are identified. The number of sets of dimension data output from Step404is less than the number of sets input in Step400. For example, for each optical condition (e.g., intensity or dose) considered in a range of optical condition values, the program investigates the calculated CD values as a function of dimension data, and discriminates according to the following design criteria: (1) CDs at zero, intermediate and maximum considered defocus conditions to be within tolerance of desired value; (2) ellipticity ((CDx−CDy)/(CDx+CDy)) to be within desired limits at zero defocus as well as on defocus. According to a preferred embodiment of the invention, the desired CD at a zero defocus condition may be about 0.12 μm, with a tolerance of about ±3%. The CD tolerance at an intermediate defocus condition (e.g., 0.40 μm) may be ±15%. The CD tolerance at a maximum considered defocus condition (e.g., ±0.6 um) may be, for example, ±30%.

Then, in Step406, the one set of dimension data, from among the sets selected in Step404, that achieves the smallest change in critical dimension (ΔCD) between a zero defocus condition and a maximum considered defocus condition is obtained by a sorting process. Note that the “intermediate defocus” is considered to provide a process latitude window within defocus values; however, to investigate depth of focus, a CD value is obtained at the maximum considered defocus condition.

Steps400and402may be performed on a microprocessor programmed with suitable imaging software, such as, for example, Solid C, Aerial Image version 5.5.11. The computer may be programmed in PERL script to define, write, and execute macros to analyze various surface dimensions like those mentioned above in connection withFIGS. 1–46.

Thus, the invention provides a method of forming a mask and the resulting structure to form contact holes in photoresist on semiconductor wafers. In an exemplary embodiment, a mask is formed with a transparent material mask substrate, attenuating phase shift material formed on the substrate, and opaque material regions formed on the attenuating phase shift material. The attenuating phase shift material and opaque (or partially transmissive) material are patterned to form a transparent hole. The dimensions of the patterns are determined using iterative methods and imaging software. An automated method is used to select the most desirable pattern for given conditions and design criteria. Certain dimensions, such as the size of the transparent opening and the size and spacing of the attenuating phase shift material and opaque material may be set as critical limits to reduce side lobes for a given illumination condition.

After a desired pattern is selected, then a mask, with the desired sub-resolution dimensions, is formed in the three-layer material32,34,36using electron beam lithography, ion milling, etc. Processes for forming the desired pattern in the three layer material32–36are described in U.S. Pat. No. 5,582,939 (Pierrat), for example. The entire disclosure of U.S. Pat. No. 5,582,939 is incorporated herein by reference.

The method described above may also be used to optimize the size of lines (as opposed to contacts) as well as the position and size of scattering bars. The method is applicable to the formation of lines and/or slots as well as for contacts.

Having thus described in detail certain exemplary embodiments of the invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the invention. Accordingly, the above description and accompanying drawings are only illustrative of exemplary embodiments which can achieve the features and advantages of the invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. The invention is only limited by the scope of the following claims.