While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, the commonly used light exposure lithography is approaching the essential limit of resolution determined by the light source wavelength.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp has been widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit DRAM, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 1 G or more requiring a finer patterning technology (processing feature size 0.13 μm or less), a shorter wavelength light source is required, and in particular, photolithography using ArF excimer laser light (193 nm) is now under investigation.
On the other hand, it is known in the art that the bilayer resist method is advantageous in forming a high-aspect ratio pattern on a stepped substrate. In order that a bilayer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF laser exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are blocked with t-BOC groups, in combination with a photoacid generator (see JP-A 6-118651 and SPIE vol. 1925 (1993), p. 377). For ArF laser exposure, positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexylcarboxylic acid is substituted with an acid labile group were proposed (see JP-A 10-324748, JP-A 11-302382, and SPIE vol. 3333-07 (1998), p. 62). For F2 laser exposure, positive resist compositions comprising a silsesquioxane having hexafluoroisopropanol as a dissolvable group as a base were proposed (see JP-A 2002-55456). The above polymer bears in its backbone a polysilsesquioxane containing a ladder skeleton produced through polycondensation of a trialkoxysilane or trihalosilane.
Silicon-containing (meth)acrylate polymers were proposed as a resist base polymer having silicon pendants on side chains (see JP-A 9-110938, J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), pp. 435-446).
The undercoat layer of the bilayer resist process is formed of a hydrocarbon compound which can be etched with oxygen gas, and must have high etching resistance since it serves as a mask when the underlying substrate is to be etched. For oxygen gas etching, the undercoat layer must be formed solely of a silicon atom-free hydrocarbon. To improve the line-width controllability of the upper layer of silicon-containing resist and to minimize the sidewall corrugation and collapse of the pattern by standing waves, the undercoat layer must have the function of an antireflective film. Specifically, the reflectivity from the undercoat layer back into the resist film must be reduced to below 1%.
In the case of a subbing antireflective film used in the monolayer resist process, when a high reflection substrate such as polysilicon or aluminum underlies, a material having an optimum refractive index (n value) and absorption coefficient (k value) is designed to an appropriate film thickness, whereby the reflectivity from the substrate can be reduced to below 1%, achieving a significant antireflection effect. In an example wherein the wavelength is 193 nm and a resist has a refractive index of 1.7, if the subbing antireflective film has a refractive index (real part of complex refractive index) n of 1.5, an extinction coefficient (imaginary part of complex refractive index) k of 0.5, and a thickness of 42 nm, then the reflectivity becomes below 0.5% (see FIG. 1). However, if the substrate has steps, the antireflective film largely varies its thickness at the steps. The antireflection effect of the subbing film utilizes not only the absorption of light, but also the interference effect arising from a properly designed film thickness. The first base of 40 to 45 nm having the enhanced interference effect has an accordingly enhanced antireflection effect, but the reflectivity largely varies with a change of film thickness. JP-A 10-69072 discloses a high conformity antireflective film-forming material in which the molecular weight of a base resin is increased to minimize the variation of film thickness at steps. As the molecular weight of a base resin increases, there arise problems that more pinholes generate after spin coating, filtration becomes difficult, a viscosity change with the passage of time leads to a variation of film thickness, and crystals precipitate at the nozzle tip. The conformal behavior is developed only at relatively low steps.
In another method using a film thickness of at least the third base (i.e., of at least 170 nm) where the variation of reflectivity due to a film thickness variation is relatively small, the variation of reflectivity due to a film thickness variation is small and the reflectivity is kept below 1.5% as long as the k value is in a range of 0.2 to 0.3 and the film thickness is at least 170 nm. However, when the etching load that the overlying resist layer has to bear is considered, the approach of thickening the antireflective film encounters a limit, the limit of film thickening being of the order of the second base of up to 100 nm.
In the event the underlay below the antireflective film is a transparent film like an oxide or nitride film and steps exist below that transparent film, the thickness of the transparent film varies even if the surface of the transparent film is planarized as by chemical mechanical polishing (CMP). In this event, it is possible to make the thickness of the overlying antireflective film uniform. If the thickness of a transparent film underlying the antireflective film varies, the thickness of the minimum reflective film is shifted by the thickness of the transparent film. Even if the thickness of the antireflective film is set equal to the thickness of the minimum reflective film when the underlay is a reflective film, the reflectivity can be increased due to a variation of the thickness of the transparent film.
The materials of which the antireflective film is made are generally divided into inorganic and organic materials. A typical inorganic material is a SiON film. This has the advantages that it can be formed by CVD of a gas mixture of silane and ammonia, and the etching load on resist is light due to a high selective ratio of etching relative to resist, but the range of application is restricted because of difficulty of peeling. Because of a nitrogen atom-containing basic film, another drawback arises that it is susceptible to footing in the case of positive resist and an undercut profile in the case of negative resist.
The organic material has the advantages that spin coating is possible without a need for a special equipment as needed for CVD and sputtering, peeling is possible like resist, no footing occurs, the shape is obedient, and adhesion to resist is good. Thus a number of antireflective films based on organic materials have been proposed. For example, JP-B 7-69611 describes a composition comprising a condensate of a diphenylamine derivative with a formaldehyde-modified melamine resin, an alkali-soluble resin, and a light-absorbing agent; U.S. Pat. No. 5,294,680 describes the reaction product of a maleic anhydride copolymer with a diamine light-absorbing agent; JP-A 6-118631 describes a composition comprising a resin binder and a methylol melamine type heat crosslinking agent; JP-A 6-118656 describes a composition based on an acrylic resin having a carboxylic group, an epoxy group and a light-absorbing group within a common molecule; JP-A 8-87115 describes a composition comprising a methylol melamine and a benzophenone light-absorbing agent; and JP-A 8-179509 describes a polyvinyl alcohol resin with a low molecular weight light-absorbing agent added thereto. In all these patents, a light-absorbing agent is either added to a binder polymer or introduced into a polymer as substituent groups. However, since most light-absorbing agents have aromatic groups or double bonds, the addition of a light-absorbing agent undesirably increases dry etching resistance and rather reduces a selective ratio of dry etching relative to the resist. As the feature size becomes finer, the drive toward resist film slimming is accelerated. In the ArF exposure lithography of the next generation, acrylic or alicyclic polymers are used as the resist material, indicating that the etching resistance of the resist becomes poor. A further consideration is the problem that the thickness of antireflective film must be increased as mentioned above. Then, etching is an acute problem. There is a need for an antireflective film having a high selective ratio of etching relative to resist, that is, a high etching speed.
The function required as an antireflective film for the undercoat layer of the bilayer resist process differs from the function required in the monolayer resist process. Since the undercoat layer of the bilayer resist process serves as a mask when the substrate is etched, it must have a high etching resistance under the substrate etching conditions. As opposed to the antireflective film in the monolayer resist process required to have a high etching speed in order to mitigate the load to the monolayer resist, the inverse performance is required. To provide sufficient substrate etching resistance, the thickness of the undercoat layer must be equal to or greater than the monolayer resist, that is, as thick as 300 nm or greater. At a film thickness of at least 300 nm, the variation of reflectivity due to a film thickness change is substantially converged so that the antireflection effect due to phase difference control is no longer expected.
Now, the results of calculation of reflectivity at varying film thicknesses up to the maximum thickness of 500 nm are shown in FIGS. 2 and 3. Assume that the exposure wavelength is 193 nm, and the upper layer resist has an n value of 1.74 and a k value of 0.02. FIG. 2 shows substrate reflectivity when the k value of the undercoat layer is fixed at 0.3, the n value varies from 1.0 to 2.0 on the ordinate and the film thickness varies from 0 to 500 nm on the abscissa. In an imaginary case wherein the undercoat layer of the bilayer resist process has a thickness of 300 nm or greater, optimum values at which the reflectivity is reduced to or below 1% exist in the refractive index range of 1.6 to 1.9 which is approximate to or higher than that of the upper layer resist.
FIG. 3 shows substrate reflectivity when the n value of the undercoat layer is fixed at 1.5 and the k value varies from 0.1 to 0.8. In the k value range of 0.24 to 0.15, the reflectivity can be reduced to or below 1%. By contrast, the antireflective film in the form of a thin film of about 40 nm used in the monolayer resist process has an optimum k value in the range of 0.4 to 0.5, which differs from the optimum k value of the undercoat layer in the bilayer resist process having a thickness of 300 nm or greater. For the undercoat layer in the bilayer resist process, an undercoat film having a lower k value, that is, more transparent is necessary.
As the material for forming an undercoat layer for 193 nm exposure, copolymers of polyhydroxystyrene with acrylic compounds are under study as described in SPIE vol. 4345, p. 50 (2001). Polyhydroxystyrene has a very strong absorption at 193 nm and its k value is as high as around 0.6 by itself. By copolymerizing with an acrylic compound having a k value of almost 0, the k value is adjusted to around 0.25.
However, the resistance of an acrylic compound to substrate etching is weak as compared with polyhydroxystyrene, and a considerable proportion of the acrylic compound must be copolymerized in order to reduce the k value. As a result, the resistance to substrate etching is considerably reduced. The etching resistance is not only reflected by the etching speed, but also evidenced by the development of surface roughness after etching. Through copolymerization of acrylic compound, the surface roughness after etching is increased to a level of significance.
Naphthalene ring is one of rings that have a more transparency at 193 nm and a higher etching resistance than benzene ring. JP-A 2002-14474 proposes an undercoat layer having a naphthalene or anthracene ring. However, since naphthol-copolycondensed novolac resin and polyvinyl naphthalene resin have k values in the range of 0.3 to 0.4, the target transparency corresponding to a k value of 0.1 to 0.3 is not reached, with a further improvement in transparency being necessary. The naphthol-copolycondensed novolac resin and polyvinyl naphthalene resin have low n values at 193 nm, as evidenced by a value of 1.4 for the naphthol-copolycondensed novolac resin and a value of only 1.2 for the polyvinyl naphthalene resin as long as the inventors measured. JP-A 2001-40293 and JP-A 2002-214777 describe acenaphthylene polymers which have lower n values and higher k values at the wavelength of 193 nm than at 248 nm, both falling outside the target values. There is a need for an undercoat layer having a high n value, a low k value, transparency and high etching resistance.
JP-A 6-202317, JP-A 8-179502, JP-A 8-220750, JP-A 8-292565, and JP-A 9-15855 disclose i-line resists comprising a copolycondensed polymer of cresol and dicyclopentadiene as a base resin. Copolymerization with dicyclopentadiene was under study to develop higher transparency novolac resins. JP-A 10-282666 proposes a curable resin in the form of a resol-dicyclopentadiene copolycondensed polymer having pendant glycidyl groups. Further, JP-A 6-80760 and JP-A 7-5302 propose a resist composition comprising naphthol condensed with aldehyde.