Patent Application
20050112476
The present invention relates to phase-shift masks and fabrication thereof. In particular, the present invention relates to phase-shift photolithographic masks comprising features created from cured sol-gel materials and fabrication process therefor using electron-beam. The present invention is useful, for example, in the fabrication of phase-shift photomasks and grayscale photomasks.
Photolithography is employed by, inter alia, semiconductor chip manufacturers to transfer integrated circuit patterns through a mask onto a silicon wafer. An exemplary traditional binary mask is a fused silica plate, with an opaque Cr film on it. Openings in the mask, corresponding to the IC features, allow light from an optical projection system (called a stepper because the exposure is a step and repeat process) to irradiate a photosensitive polymer (photoresist) layer coated on the silicon wafer. After resist development, or its selective removal, the silicon is now exposed to allow etching, metal deposition, ion implantation or other processing, followed by removal or “stripping” of the photoresist. To make a modern, complex microprocessor or memory chip requires as many as 20 iterations of this process with different but complementary (and critically aligned) masks (or mask set). One limitation of photolithography is that there is a minimum feature size that can be imaged on the wafer, determined by the optics of the stepper, the wavelength of the imaging light, and the particular process (e.g., contrast of the photoresist material). As the minimum feature size is reduced, speed and density in chips increase substantially as does the cost of the photolithography tool. Fortunately, a number of strategies have been developed to extend the usefulness of any optical lithography generation. One of these optical extensions is the phase-shift mask (PSM). It can enhance resolution beyond the wavelength-imposed diffraction limit. Since some fraction of the light used in lithography is coherent, phase-shift masks work by destructive optical interference to enhance imaging contrast.
The resolution of an image formed by a projection stepper in a photolithography system is defined by the following equation:
R=k1·(λ/NA) (1)
wherein R is resolution, k1 is a process-dependent constant, λ is the illumination wavelength, and NA=sin θ is numerical aperture of the projection lens. Depth of focus (DoF) is another important parameter of a photolithography process besides resolution R. Usually a large DoF is desired, because a larger DoF renders the process more tolerant to departure in wafer flatness and photoresist thickness uniformity. DoF is determined according to the following equation:
DoF=k2·(λ/NA2) (2)
where k2 is another process-dependent constant.
From the above equations (1), it can be seen that, in order to enhance resolution R, the following approaches may be employed: (i) using a shorter illumination wavelength λ; (ii) using a projection system having a larger numerical aperture NA; or (iii) lowering constant k1 by improving the process such as by using phase-shift mask or a higher contrast photoresist. However, from equation (2), it can be seen that when numerical aperture NA is increased, DoF is sacrificed.
Phase-shift masks can improve resolution without sacrificing DoF. Since optical interference does not depend critically upon a perfectly focused image, phase-shift masks can actually increase DoF in comparison to traditional Cr masks. Two types of phase-shift masks are commonly used in lithography: alternating aperture phase-shift masks (AAPSM) and embedded attenuating phase-shift mask (EAPSM).
The alternating aperture phase-shift mask is particularly well suited for printing closely spaced lines. Typically, it provides a 50% improvement in resolution compared to traditional binary Cr masks. In a conventional practical mask design, the silica substrate is etched to produce the 180° phase-shift masks, especially when the features to be printed are in complicated circuit patterns. An unwanted result is that the abrupt transition between 0° and 180° always prints as a dark line, and it can bridge or short-circuit isolated lines in some circuit designs. Although there are strategies to circumvent this, implementing them adds complexity to the mask design, especially for intricate circuits.
The other type of phase-shift mask is the embedded attenuating phase-shift mask (EAPSM). It is schematically illustrated in
Chromeless phase-shift mask has been developed recently in chromeless phase lithography (CPL). CPL uses chromeless features on the masks to define patterns that have nearly 100% transmission and are phase shifted by 180°.
In the production of all of the prior art phase-shift masks, very complex multi-step resist deposition, exposure, development and stripping are required.
In a phase-shift mask to be used in an ambient having a refractive index of about 1, such as in air or N2, in order to obtain a near 180° phase shift, the following requirement must be met:
d·(ns−1)≈λ/2 (3)
where d is the thickness of the phase-shifting film deposited on top of the substrate in an EAPSM, or the height of the phase-shifting steps in a chromeless phase-shift mask or an AAPSM, nS is the refractive index of the phase-shifting film in an EAPSM or the substrate in a chromeless phase-shift mask or AAPSM, and λ is the illumination wavelength.
The phase shifting approach offers great resolution improvement for a 25 nm gate length silicon-on-insulator (SOI) devices using a 248-nm stepper. This method has a deep subwavelength potential. SOI transistors with polysilicon gate lengths of 90, 25 and 9 nm have been demonstrated manufacturable by this approach using a 248-nm stepper. However, for the reasons mentioned above, this approach has so far suffered from impediments such as high mask cost, long turnaround time and difficult inspectability/repair.
Therefore, there remains a genuine need of a phase-shift mask and a production method thereof that overcome the drawbacks of the current phase-shift masks described above.
A first aspect of the present invention is a mask having a pattern P0 transferable onto an image-receiving substrate when subjected to illumination radiation in a lithographic process, comprising a substrate S0 bearing on a surface thereof patterned phase shift features P1 formed by a cured sol-gel material, wherein the sol-gel material has a refractive index n1 and a thickness d at the wavelength of the illumination radiation λ.
In a preferred embodiment, in the mask of the present invention, the refractive index n1 and thickness d of the phase-shift features P1 are chosen such that d·(n1−n0)≈λ/2, where n0 is the refractive index of the medium in which the mask is placed when being used, whereby a near 180° phase-shift is created relative to the medium in which the mask is used. Where the medium is air or N2, n0≈1. In one embodiment, pattern of the phase-shift features formed by the cured sol-gel is a grating having a pitch of less than 300 nm. In another embodiment, above the surface of the substrate S0 and/or the phase-shift features P1 of the mask of the present invention, there exist additional feature patterns P2 formed by materials opaque or attenuating to the illumination radiation used in the lithographic process. Such opaque material may be, for example, Cr or modified Cr. And the attenuating material may create 180° phase shift with respect to the medium in which the mask is placed during the lithographic process. The phase-shift features P1 and the additional features P2 may supplement and/or correct each other when the overall image pattern P0 is transferred to the image-receiving substrate in a lithographic process.
In an embodiment of the mask of the present invention, the substrate S0 is a plate having flat surfaces formed by fused silica, doped fused silica and/or low-expansion glass-ceramic materials used in typical photomasks for deep and vacuum UV lithography.
In a preferred embodiment, in the mask of the present invention, the sol-gel material forming the phase-shift features P1 is obtained from electron beam curing a sol-gel composition of at least one precursor material comprising at least one hydrolysable compound (I) having the following formula
Rm-M-Xn (I),
where M is a metal or metalloid having a valence of m+n, R independently is a non-hydrolysable group, X is a hydrolysable group, m is an integer from 0 to m+n−1, inclusive, and n is an integer from 1 to m+n, inclusive. Preferably, M is selected from the group consisting of Si, Ti, Al, Ge, Ta, B, Ga, Zr and Sb. Preferably, R is independently a C1-C12 group, optionally fluorinated. Preferably, X is independently selected from hydrogen, hydroxy, halogen (more preferably chlorine) and C1-C4 alkoxy groups. More preferably, n≧2 and all X groups are OR′ where R′ is a C1-C4 alkyl group. Preferably, the precursor material of the sol-gel composition comprises 3,3,3-trifluoropropyl trimethoxysilane. Preferably, the precursor material of the sol-gel composition comprises at least one compound (II) which is a compound (I) in which m=0. More preferably, the precursor material of the sol-gel composition comprises, in addition to at least one compound (II), at least one compound (III) which is a compound (I) in which R is independently a non-hydrolysable organic group and m≧1. More preferably, the precursor material of the sol-gel composition comprises, in addition to at least one compound (II), a compound (IV) which is a compound (I) in which R is independently a fluorinated alkyl or phenyl group, and m≧1. Where such a mixture of compounds (II) and (IV) is used, the molar ratio of compounds (II) to (IV) is preferred to be at least 50:50, more preferably at least 70:30.
A second aspect of the present invention is a process for making a phase-shift mask having a pattern P0 transferable onto an image-receiving substrate when subjected to illumination radiation in a lithographic process having a wavelength λ, said pattern P0 including a phase-shift pattern P1, comprising the following steps:
In a preferred embodiment of the process of the present invention, the cured parts P1 in step (c) has a thickness d and refractive index n1 such that d·(n1−n0)≈λ/2, where n0 is the refractive index of the medium in which the mask is placed at the lithographic wavelength λ when being used.
In a preferred embodiment of the process of the present invention, in step (b), the sol-gel film S1 is deposited on the surface of S0 by spin coating or dip coating, preferably by spin coating. Preferably, the film S1 has a uniform thickness, which leads to a uniform thickness of the electron beam cured phase-shift features P1.
In a preferred embodiment, step (e) is carried out, and this step comprises depositing a film of a material opaque or attenuating to the illumination radiation used in the lithographic process above the upper surface of S0 and/or P1, depositing a photoresist on top of the opaque/attenuating film, exposing the photoresist, developing the exposed photoresist, selectively etching the opaque/attenuating film, followed by stripping the remaining photoresist, whereby additional pattern features P2 of the opaque/attenuating material are formed. In a preferred embodiment, materials attenuating to the lithographic radiation is used to form at least part of the additional pattern features P2. The additional pattern features P2 comprising such materials are chosen to have a thickness d2 and a refractive index n2 at the lithographic wavelength λ, such that d·(n1−n0)≈λ/2, whereby the attenuating features cause 180° phase-shift of the lithographic radiation with respect to the medium in which the mask is used. In yet another preferred embodiment, materials opaque to the lithographic radiation is used to form at least part of the additional pattern features P2. Such opaque material can advantageously be a metal Cr layer, or a modified Cr layer, for example, a Cr layer with modified surface, such as an oxidized surface.
In a preferred embodiment of the process of the present invention, in step (a), the transparent substrate S0 is a plate having flat surfaces made of fused silica, doped fused silica or low expansion glass-ceramic materials.
In one embodiment of the process of the present invention, step (e) is performed after step (b), (c) and (d). In another embodiment of the process of the present invention, step (e) is performed before step (b).
In the process of the present invention, in the sol-gel composition, in the at least one hydrolysable compound (I), M is preferably selected from the group consisting of Si, Ti, Al, Ge, Ta, B, Ga, Zr and Sb. Preferably, R is independently a C1-C12 group, optionally fluorinated. Preferably, X is independently selected from hydrogen, hydroxy, halogen (preferably chlorine) and C1-C4 alkoxy groups. More preferably, n≧2 and all X groups are OR′ where R′ independently is a C1-C4 alkyl group. Preferably, the precursor material of the sol-gel composition comprises 3,3,3-trifluoropropyl trimethoxysilane. Preferably, the precursor material of the sol-gel composition comprises at least one compound (II) which is a compound (I) in which m=0. More preferably, the precursor material of the sol-gel composition comprises, in addition to at least one compound (II), at least one compound (III) which is a compound (I) in which R is independently a non-hydrolysable organic group and m≧1. More preferably, the precursor material of the sol-gel composition comprises, in addition to at least one compound (II), at least one compound (IV) which is a compound (I) in which R is independently a fluorinated alkyl or phenyl group, and m≧1. Where such a mixture of compounds (II) and (IV) is used, the molar ratio of compounds (II) to (IV) is preferred to be at least 50:50, more preferably at least 70:30.
The mask and method of the present invention can overcome the drawbacks of conventional phase-shift masks in terms of cost and turnaround time. The sol-gel film S1 is easy to be deposited with highly uniform thickness. The phase-shift features P1 having various complex patterns can be readily formed by electron beam curing and simple washing by using solvent. The thus cured sol-gel phase-shift pattern P1 can have a very uniform thickness and a very low surface roughness. In the mask of the present invention, very fine phase-shift features can be created by using electron beam cure. Compared to conventional phase-shift masks manufacture process, the process of the present invention uses less steps of CVD film deposition, resist coating, baking, developing and stripping and reactive ion etching.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.
In the accompanying drawings,
Sol-gel materials are well known. They have been used in the production of anti-reflective coatings for optical devices, such as photovoltaic systems. WO 02/17347 discloses sol-gel materials, electron beam curing of such sol-gel materials and use thereof in forming ultra-fine features in the micro or nanometer range in certain devices, the relevant portions thereof are relied upon and incorporated herein by reference in their entirety.
In the conventional processes for making phase-shift photomasks, described supra, the phase-shift features are created either by selective reactive ion etching of the photomask substrate, or by chemical vapor deposition of a phase-shift layer, followed by coating, baking, exposure, development of photoresist, selective reactive ion etching of the phase-shift layer, and stripping of the photoresist. As can be readily appreciated by those skilled in the art, these techniques are highly complex and usually very costly. Conditions in these processes must usually be strictly controlled in order to produce phase-shift masks with satisfactory quality and yield.
By using the technique of electron beam curing of sol-gel materials, the present inventors have found an unconventional, convenient, less costly way to produce phase-shift masks. In general terms, the process of the present invention for making a phase-shift mask having a pattern P0 transferable onto an image-receiving substrate when subjected to illumination radiation in a lithographic process having a wavelength λ, said pattern P0 including a phase-shift pattern P1, comprises the following steps:
The steps will be described in detail as follows.
In step (a), the transparent substrate S0 can be made of any material used for manufacturing conventional masks. The bottom line is the substrate S0 should be transparent to the lithographic wavelength λ of the lithographic process. Preferably, at the lithographic wavelength λ of the lithographic process, the substrate has a transmission of at least 70%, more preferably at least 75%, most preferably at least 80%. In traditional photomasks, the standard substrate material was soda lime glass. Later, white crown was introduced to reduce defects. And still later, borosilicate glass was introduced to reduce temperature effects on the mask. Currently, as the lithographic wavelength has gone shorter, fused silica has been introduced for further temperature effects and to give better transmission. For the purpose of example and illustration only, the substrate S0 in the present invention process can be made of borosilicate glass, fused silica, doped fused silica, low thermal expansion optical glass-ceramic materials, etc. For masks used in 248-nm and shorter wavelength photolithography, the substrate is advantageously made of fused silica or doped fused silica. Advantageously, the surfaces of the substrates S0 have a flatness that meets the requirement of optical distortion in mask manufacture. However, where necessary, the surfaces of substrate S0 may be engineered to any specific topography before the deposition of the photosensitive film in step (b) by using methods known in the art, such as dry etching and wet etching. Preferably, the thickness of the substrate S0 is sufficient to satisfy the requirement for gravitational sag and pattern placement accuracy. Preferably, the substrate S0 has a chemical durability that can withstand the mask producing environment, such as wet etching and dry etching.
Step (b) of the present invention mask-making process involves deposition of a film S1 of a sol-gel composition on a surface of the substrate S0. The sol-gel composition is typically an aqueous material system of a precursor material comprising at least one hydrolysable compound having the general formula (I) above. The element M in the compound is a network forming element, such as Si, Ti, Al, Ge, Ta, B, Ga, Zr or Sb. This compound (I) may contain, where m≧1, non-hydrolysable group R. Preferably, the hydrolysable group X is independently selected from hydrogen (preferably chlorine), hydrogen, hydroxy and OR′, where R′ is a C1-C4 alkyl. The group X in the compound, when in an aqueous medium, in the presence of acid or base catalyst where necessary, undergoes hydrolysis reaction through either an electrophillic reaction mechanism or a nucleophillic reaction mechanism to form compounds having hydroxies. Condensation reactions between the compounds may further occur to form a network bridged by the —O— linkages. Taking compound Si(OR′)4 for example, a simplified illustration of several possible reactions is provided as follows:
Hydrolysis:
Water Condensation:
Alcohol Condensation:
As can be understood by one skilled in the art, further hydrolysis and condensation reactions of compounds (i), (ii) and (iii) may take place via substantially the same mechanism. As a result, a complex network of condensed compounds can be formed. Thus the sol-gel composition as used in step (b) may comprise the hydrolysable compound (I), water, acid or base catalysts as typically used in sol-gel technology, alcohol and condensed products. The degree of condensation between the compounds in the composition is dependent on the chemical nature of the hydrolysable compound (I), nature and amount of catalyst used, nature and amount of optional components, such as alcohols, storage time and temperature, and other factors. Standard procedures of preparation of sol-gel compositions may be employed. For example, it can be prepared by mixing predetermined amounts of compound (I), acid or base catalyst and alcohol into deionized water, followed by aging at a given temperature for a given period of time before application to the photomask substrate. Where the at least one compound (I) comprises chlorine as the X group, it may not be necessary to add additional acid catalyst, insofar as the chlorine group hydrolyses to form HCl, which can act as the catalyst.
It is understood that, from the above reaction scheme, the more hydrolysable groups X the compound (I) has, the more OH can be generated on the molecule by hydrolysis, and a more highly condensed network can be formed. Thus it is desirable that a high proportion of compounds (I), where more than one type of compounds (I) is used, are those in which m=0 and all substituents to the element M are hydrolysable, such as tetraethoxysilane (TEOS, Si(OC2H5)4 and tetraisopropoxygermane (TIPG, Ge(OCH(CH3)2)4). For the purpose of convenience of description in the present application, those compounds (I) wherein m=0 are designated as compounds (II).
Compounds (III), which are compounds (I) in which m≧1 and R is a non-hydrolysable organic group, can impart flexibility to the network of the sol-gel material after cure if added. An example of such a compound (III) is methacryloxypropyl trimethoxysilane (MPTS). Sol-gel compositions comprising compounds (III) may also exhibit enhanced roughness, reduced brittleness, and relatively thicker films (up to 10 nanometers or more) may be cured without cracking using the electron-beam patterning and curing technique of the present invention. It is contemplated that in compound (III), at least one R group may be a functional group such as a photopolymerizable group, e.g., a (meth)acrylic group. Where desired, such functional groups can undergo further reactions, such as polymerization, under certain conditions, such as exposure to UV radiation. The addition of compounds (III) to the sol-gel composition can result in cured inorganic/organic hybrid sol-gel film.
In addition to compounds (II), the sol-gel composition as used in the process of the present invention may comprise compounds (IV) where m≧1 and R is independently a fluorinated alkyl or phenyl group. An example of compound (IV) is 3,3,3-trifluoropropyl trimethoxysilane. Sol-gel compositions comprising at least one compound (IV), defined supra, in effective amount, may also provide a highly non-wetting surface to the cured sol-gel film, which is sometimes desired in the production of the phase-shift mask of the present invention. Since the addition of compounds (IV) into the sol-gel composition may lead to a cured film having lower refractive index, such compounds (IV) may be added to tailor the refractive index of the phase-shift features of the mask after cure.
It is highly desired that the sol-gel composition, when applied to the surface of the substrate S0, is substantially homogeneous in composition.
The method of application of the sol-gel composition to the surface of the substrate S0 is not critical as long as a coating of uniform composition with desired uniform thickness of film S1 can be formed. As non-limiting examples of such coating method, mention can be made of spin-coating and dip-coating, with spin-coating being preferred. Spin-coating can result in a very thin film having very uniform thickness under 500 nm. The thickness of the coating can be easily adjusted.
After the sol-gel composition has been deposited to form a film in step (b) of the process of the present invention, the film may be prebaked to partially cure the sol-gel film S1 in order to stabilize the film on the substrate S0, and/or to eliminate tackiness at the surface of the substrate. However, a preheating step is not required, and may not be desirable for many sol-gel compositions.
In step (c) of the process of the present invention, a desired phase-shift pattern is formed in the sol-gel film S1 by selectively curing predetermined areas of the sol-gel film S1 that correspond to a desired pattern by directing an electron beam at the selected areas of the film. In other words, a desired pattern is written directly into the sol-gel film using an electron beam. An apparatus for patterning and curing a sol-gel film by writing the desired pattern directly into the sol-gel film using an electron beam can be provided by converting a scanning electron microscope (SEM) (for example, JEOL 840) for use as an electron beam patterning tool using a commercially available SEM conversion package. The commercially available package includes pattern generation software and hardware that directs the electron beam to write patterns that are created by the user in a computer aided design (CAD) software package. A system of this type is capable of writing generally any type of pattern that can be drawn using the CAD software.
Although the mechanism by which curing of the sol-gel film is achieved using an electron beam is perhaps not yet fully understood, it is believed that the electron beam induces localized heating of the sol-gel film, and that this localized heating causes localized curing or condensation of the sol-gel film in those areas of the film bombarded with or exposed to the electron beam by way of the hydrolysis and condensation reactions schematically illustrated above. FTIR studies have shown that e-beam promoted the condensation of silanol groups and localized formation of glass network. It was also observed that in inorganic/organic hybrid sol-gels, e-beam did not cause changes to the organic moieties. The cured areas of the sol-gel film form a condensed three-dimensional network comprised of metal or metalloid atoms linked together through the oxygen atom. The resulting cured product has chemical and physical properties that are very similar, and in some cases almost identical to conventional glasses.
A great advantage of the electronic beam writing process is that ultra-fine phase-shift features, in a nanoscopic dimension, can be created by the electron beam scanning with highly precise control of position and dimension, as demonstrated by the examples of WO 02/17347. Fully cured areas of the film S1 have a highly uniform thickness that can be controlled by controlling the thickness of the film S1 when coating the sol-gel composition to the surface of substrate S0. The cured film can be obtained with a very low surface roughness. In addition, stress-free cured film can be easily obtained by electron beam cure of sol-gel materials. This is beneficial for both the mechanical and optical properties of the phase-shift features of the mask.
For many phase-shift masks, it is desired that the phase-shift features provide a phase-shift amount of near 180°. This can be done, in the process of the present invention, by controlling the thickness d and the refractive index n1 of the cured sol-gel phase-shift patterns by means described supra, such that they satisfy the equation d·(n1−n0)≈λ/2, where λ is the wavelength of the lithographic radiation and n0 is the refractive index of the medium in which the mask is placed when being used. Where the mask is used in air or N2, n0≈1. If the mask is used in a high-refractive-index medium, such as oil, one skilled in the art can adjust d and n1 accordingly. It is desirable that a strictly 180° phase shift can be obtained. However, in practice, this is difficult, if not impossible, to achieve. Thus as used herein, a 180° or near 180° phase shift amount, is meant to be a phase shift amount of 180±10°, preferably 180±5°, more preferably 180±3°. Where other amounts of phase-shift is desired, such as near 90° or 270°, it can be realized by adjusting the feature thickness and refractive index accordingly. For example, to realize phase shift amounts of near 90° or 270°, the thickness d and refractive index n1 should satisfy the equation d·(n1−n0)≈λ/4 and d·(n1−n1)≈3λ/2, respectively.
After the desired pattern P1 has been written into the sol-gel film S1 by electron beam scanning, in step (d) of the process of the present invention, the film is contacted with a solvent to dissolve areas of the film that have not been cured. This allows the uncured areas of the film to be easily removed, leaving behind the desired phase-shift pattern of cured sol-gel material. A suitable solvent that may be used to remove the areas of the sol-gel film that have not cured is toluene. In order to improve the wettability and effectiveness of the solvent, the solvent may include a small amount of surfactant. The cured sol-gel pattern P1 should be transmissive to the illumination radiation used in the lithographic process. Preferably, at the lithographic wavelength λ of the lithographic process, the sol-gel pattern has a transmission of at least 70%, more preferably at least 75%, most preferably at least 80%.
Preferably, the cured sol-gel phase-shift features P1 has a chemical durability that can withstand the chemical environment of the downstream steps in the process of forming the mask of the present invention, such as the dry etching and/or wet etching steps where necessary. If this is the case, the additional features P2 can be formed directly by depositing the relevant materials on the upper surface of S0 and the cured sol-gel pattern P2. In case the cured sol-gel phase-shift features P1 is not robust enough to withstand the environment, it is contemplated that a very thin protective layer resistant to the environment, such as a silica layer, may be formed on the upper surface of the cured sol-gel pattern P1 and optionally on the exposed surface of the substrate S0 before the formation of additional patterns features P2 by techniques such as chemical vapor deposition. Of course, the protective layer should be transmissive to the lithographic radiation. As long as the thickness of the protective layer can prevent undesired etch of the cured sol-gel phase-shift pattern P1, the thinner the protective layer is, the better. In addition, the protective layer should preferably have an even thickness and a low surface roughness in order not to create optical distortion.
The substrate S0 bearing the cured sol-gel phase-shift features P1 may be used directly as a phase-shift mask. In such a phase-shift mask, the overall image pattern P0 is equivalent to the phase-shift feature pattern P1. Such a simple phase-shift mask can be a simple one- or two-dimensional grating of the phase-shift features. More detailed description of this type of mask will be given infra.
However, in most phase-shift masks, additional features P2 are formed to combine with the sol-gel phase-shift features P1 to form the overall image pattern P0. Such additional features can be formed in the step (e) of the process of the present invention. Such additional features P2 can be formed by materials opaque or attenuating to the lithographic radiation. For example and for the purpose of illustration only, opaque Cr and/or modified Cr features used on conventional photomasks can be formed. As mentioned above, an intermediate protective layer, particularly an etch stopping layer, such as a silica layer, may be formed between the surface of the cured sol-gel features P1 before the deposition of the opaque or attenuating layer where necessary, as long as it meets the requirements described above, where the sol-gel features P1 cannot resist the down-stream photomask forming environment. The formation of such additional opaque or attenuating features P2 can be effected using methods known in the art, for example, by steps such as depositing a film of the opaque or attenuating material on the surface of the substrate S0 and/or the phase-shift features P1, depositing a photoresist on top of the opaque/attenuating film, exposing the photoresist, developing the exposed photoresist, selectively etching the opaque/attenuating film, followed by stripping the remaining photoresist. Deposition of the film of the opaque or attenuating material can be effected via conventional methods such as sputtering, ion plating, and the like. The opaque or attenuating film as deposited may be further modified to obtain a differing etching rate, reflectivity, etc. For example, where part of the additional features P2 is formed by Cr, the Cr film as deposited may be modified in accordance with U.S. Pat. Nos. 4,530,891 and 4,463,407, the relevant portion of which are incorporated herein by reference, before the lithographic steps are performed on it.
In one embodiment of the process of the present invention, attenuating materials are used to form the additional features P2. Understandably, the attenuating materials causes phase-shift to the radiation passing through it relative to the medium in which the mask is used, if the refractive indexes of the attenuating material and the medium are not identical. It is desirable in certain phase-shift masks that features formed from these materials create a near 180° phase shift as well.
It is to be noted that in the process of the present invention, step (e) may be carried out after steps (b), (c) and (d). In this case, the phase-shift features P1 are formed first on the surface of the substrate S0. Subsequently the additional features P2 are formed on the remaining area of the surface S0 and/or on top of the features P2. Alternatively, the step (e) may be carried out prior to steps (b), (c) and (d). In this embodiment, the additional features P2 are formed first on a surface of the substrate S0, and the phase-shift features P1 formed from sol-gel material are formed thereafter on the remaining area of the surface of the substrate S0 and/or on top of the features P2 according to the methods described supra. In this embodiment, pre-formed photomask substrates, such as those bearing Cr or modified Cr films may be used. The combination of P1 and P2 forms the overall feature P0 of the photomask.
The production of the
Various electronic design automation tools are known for preparing the patterns used in conventional and phase-shift masks. In addition, OPC tools alter those patterns to account for the realities of the exposure systems. It is also known that the pattern of apertures on the phase-shift mask need not correspond closely to the ultimate circuit pattern, at least not when a conventional block-out mask is employed for a second exposure on the resist film in concert with a first exposure made using a an alternating-aperture PSM. Such second exposures erase anomalies due to phase-conflicts. All these tools and strategies developed for conventional masks, phase-shifting or not, can be adapted for use in the production and use of the mask of the present invention.
A specific example of the mask of the present invention involves a grating pattern. The pattern is a 180° phase shifting 1-D or 2-D grating system created by scanning the sol-gel film S1 with electron beam. The grating pitch can be lower than 300 nm, and may be as short as 100 nm. These low pitch gratings can be used for creating very dense sub-wavelength features. Such mask can be used in conjunction with trim mask and/or chrome binary masks via multiple exposure to create desired image patterns on an image-receiving substrate, such as a wafer. The trim mask can be a phase-shifting trim mask produced using the method of the present invention, or a conventional chrome trim mask. Advantageously, an additional feature P2 formed by chrome or other weak phase shifting materials is formed to supplement the gratings.
Other various features and advantages of the phase-shift photomask of the present invention can be readily appreciated by reference to the above description of the process for making the same.
The following non-limiting examples further illustrate the present invention.
In these examples, electron-beam patterning of sol-gel films was accomplished by using a custom built electron-beam lithography system. The system was based on a JEOL 840 scanning electron microscope that was converted to write patterns using a commercially available SEM conversion package. The package included pattern generation software and hardware that is used to direct the electron beam to write patterns that are created by the user in a computer aided design (CAD) software package.
In the examples illustrated in
Referring particularly to
Further background information relevant to sol-gel compositions, the formation of sol-gel films, methods for patterning, curing, and etching of such sol-gels, and the fabrication of waveguides and other structures using sol-gels are contained in U.S. patent application Ser. No. 09/319,800 (filed Dec. 12, 1997) and Ser. No. 09/494,073 (filed Jan. 28, 2000), as well as the U.S. patent application Ser. No. 09/641,358 of Hancock, Ukrainczuk, and Sewalt filed on Aug. 17, 2000 entitled Direct Writing of Waveguides in Sol-Gels, which are each incorporated herein by reference as though fully set forth.
It will be apparent to those skilled in the art that various modifications and alterations can be made to the present invention without departing from the scope and spirit of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
The present invention claims priority of U.S. Provisional Patent Application Ser. No. 60/524,465, filed on Nov. 24, 2003, entitled “Phase-Shift Mask and Fabrication Thereof,” the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60524465 | Nov 2003 | US |
