Multicolor photolithography materials and methods

Abstract

The present invention relates to photoresist compositions comprising a base resin such as a monomer capable of radical polymerization upon photoinitiation, and photoinitiator molecules such as a diketone, and multicolor photolithography methods. Photoresist compositions comprise photoinitiator molecules that are exposed to a first radiation source, thereby exciting the photoinitiator molecules from a ground state to a pre-activated state. The pre-activated state molecules are then exposed to a second radiation source in selected locations, thereby deactivating the pre-activated state molecules in the selected locations. Any remaining pre-activated state molecules are exposed to a third radiation source, exciting such remaining pre-activated state photoinitiator molecules to an activated state. Polymerization of the base resin is then initiated.

Claims

1. A multicolor photolithography method, comprising the steps of: providing a substrate at least partially coated with a layer of a photoresist composition comprising a reactive monomer and photoinitiator (PI) molecules; exposing said PI molecules to a first radiation source, thereby exciting said PI molecules from a ground state to a pre-activated state; exposing said pre-activated state PI molecules to a second radiation source in selected locations, thereby deactivating said pre-activated state PI molecules in said selected locations; and exposing any of said pre-activated state PI molecules remaining to a third radiation source, thereby exciting said remaining pre-activated state PI molecules to an activated state, and initiating polymerization of said reactive monomer in said photoresist composition.

2. The method of claim 1, wherein said first radiation source is a pulsed laser.

3. The method of claim 1, wherein said second radiation source is a continuous-wave laser.

4. The method of claim 1, wherein said third radiation source is a pulsed laser.

5. The method of claim 1, wherein at least one of said first, second or third radiation source emits visible light.

6. The method of claim 1, wherein said photoinitiator molecules are a diketone having substituent groups R.sub.1 and R.sub.2, wherein at least one of R.sub.1 or R.sub.2 is selected from the group consisting of an alkyl substituent, an aryl substituent, a substituted aryl substituent, a heterocyclic aryl substituent, a cycloalkene substituent, and a heterocyclic cycloalkene substituent.

7. The method of claim 1, wherein said photoinitiator molecules are selected from the group consisting of biacetyl (C.sub.4H.sub.6O.sub.2), benzil (C.sub.6H.sub.5CO.sub.2), 2.2-pyridil (C.sub.12H.sub.8N.sub.2O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), and furil (C.sub.10H.sub.6O.sub.4).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing/photograph executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 illustrates the general structure of a vicinal diketone with substituents R.sub.1 and R.sub.2.

(3) FIG. 2, Plate A, is an SEM image of unpatterned, chopped deactivation of a photoresist containing biacetyl as the initiator. Plate B is an SEM image showing resolution enhancement by patterned deactivation of a photoresist containing biacetyl as the initiator.

(4) FIG. 3 illustrates a representative exemplary structure of biacetyl covalently bonded to a multiacrylic monomer.

(5) FIG. 4 illustrates a representative exemplary structure of biacetyl covalently bonded to two multiacrylic monomers.

(6) FIG. 5 illustrates a scheme for three-color photolithography using the disclosed class of molecules. The initiator is excited to S.sub.1 with color 1, then undergoes intersystem crossing (ISC) to the lowest triplet state, T.sub.1. T.sub.1 is a metastable and unreactive pre-activated state. The initiator is then deactivated by color 2, which causes absorption to a higher triplet state (T.sub.2) that undergoes reverse intersystem crossing (RISC) to a highly vibrationally excited level of S.sub.0. Molecules that have not been deactivated can be excited with color 3 to a reactive triplet state T.sub.3 (the activated state) to initiate polymerization.

(7) FIG. 6, Plate A, illustrates deactivation action spectrum from 720 nm to 940 nm for 1 wt % biacetyl in dipentaerythritol pentaacrylate excited by 2-photon absorption at 800 nm. Plate B illustrates transient absorption spectrum for biacetyl in carbon tetrachloride (e.g., see Singh et al. (1969), J. Phys. Chem. 73:2633-2643). The close correspondence between the peak positions indicates that deactivation occurs through excitation from T.sub.1 to T.sub.2 followed by reverse intersystem crossing.

(8) FIG. 7 illustrates three-dimensional, deactivation action spectrum showing that deactivation tracks the triplet absorption spectrum regardless of excitation wavelength.

(9) FIG. 8 illustrates resolution enhancement of line pairs in a 3-color photoresist with 1 wt % biacetyl as the photoinitiator in dipentaerythritol pentaacrylate with the implementation of deactivation in accordance with the present invention. Plate A shows the line pair without deactivation; Plate B shows the line pair with deactivation.

(10) FIG. 9 illustrates resolution enhancement of line pairs in a 3-color photoresist with 0.5 wt % benzil as the photoinitiator in 1:1 SR368/499 (tris(2-hydroxy ethyl) isocyanurate triacrylate/ethoxylated(6) trimethylolpropane triacrylate) with the implementation of deactivation in accordance with the present invention. Plate A shows the line pair without deactivation; Plate B shows the line pair with deactivation.

(11) FIG. 10, Plate A, is an SEM image of unpatterned, chopped deactivation of a photoresist (4 pitch lines; linewidth from about 150 nm to about 300 nm) containing benzil as the initiator. Plates B and C are SEM images showing resolution enhancement by patterned deactivation of a photoresist containing benzil as the initiator.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(12) The present invention is directed to photoresist compositions and photolithography methods preferably utilizing the disclosed compositions. The photoresist compositions comprise a class of molecules that act as three-color radical photoinitiators. The operating principle of the disclosed materials is to use a first radiation source that emits a first color or wavelength of light to excite the molecules to an unreactive, pre-activated state. A second radiation source emits a second color or wavelength of light which deactivates the molecules in selected spatial locations. Finally, a third radiation source emits a third color or wavelength of light takes any remaining pre-activated molecules to an activated state that leads to chemical reaction. Note that the first, second and third colors or wavelengths of light may be different and/or alternatively have differing emission characteristics (e.g., such as pulsed, continuous wave, etc.). Thus, in some embodiments, the first, second and/or third radiation sources emit radiation of similar or identical wavelength. In other embodiments, the radiation wavelength and/or emission characteristics of the first, second and/or third radiation sources differ.

(13) We disclose a general class of materials that undergoes this type of photochemistry: vicinal diketones, the general form of which is shown in FIG. 1. The two substituent groups R.sub.1 and R.sub.2 can be of any of a broad range, including alkyl groups such as ethyl, propyl, and isopropyl, and particularly alkyl groups for which there preferably will be no methylene or methane groups adjacent to the ketone, such as methyl or tert-butyl; aryl groups such as phenyl and naphthyl, and anthryl; substituted aryl groups such as tolyl, xylyl, chlorophenyl, and fluorophenyl; heterocyclic aryl groups such as pyridyl and pyrazyl; cycloalkenes such as cyclopentadiene; heterocyclic cycloalkenes, such as furyl, thiophenyl, pyrrolyl, imidazolyl, or thiozolyl; or more complex groups.

(14) In some embodiments, the photoresist composition includes a photoinitiator selected from the group consisting of biacetyl (C.sub.4H.sub.6O.sub.2), benzil (C.sub.6H.sub.5CO.sub.2), 2.2-pyridil (C.sub.12H.sub.8N.sub.2O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), and furil (C.sub.10H.sub.6O.sub.4).

(15) In one preferred embodiment of these materials, the photoinitiator is biacetyl, for which R.sub.1 and R.sub.2 are both methyl groups. In another preferred embodiment, the initiator is benzil (1,2-diphenylethane-1,2-dione; C.sub.6H.sub.5CO.sub.2), for which R.sub.1 and R.sub.2 are both phenyl groups. Preferably, the composition comprises the initiator mixed with a reactive acrylate or methacrylate monomer or mixture of monomers, with the photoinitiator at a few weight percent of the composition. Preferably, this monomer or monomer mixture is viscous and contains at least one component that has multiple acrylates or methacrylates in each molecule. Thus, a wide array of monomers and monomer mixtures are suitable for use in the disclosed compositions. Suitable monomers and monomer mixtures, and in particular acrylic and/or methacrylic monomer(s) capable of being photopolymerized using a radical photoinitiator, are available from Sartomer Americas (Exton, Pa.). In one preferred embodiment the monomer is dipentaerythritol pentaacrylate (C.sub.25H.sub.32O.sub.12). In another preferred embodiment the monomer consists of equal weight percentages of tris (2-hydroxyethyl) isocyanurate triacrylate (C.sub.21H.sub.27N.sub.3O.sub.9) and ethoxylated(6) trimethylolpropane triacrylate (C.sub.21H.sub.32O.sub.9).

(16) In the case of biacetyl, initial excitation to S.sub.1 may be accomplished using a two-photon transition driven by an ultrafast pulsed laser at a wavelength between about 720 nm and about 875 nm. Deactivation may then be accomplished using a continuous-wave laser tuned between about 720 nm and about 950 nm, though deactivation can be accomplished utilizing radiation having an even broader range of wavelength. As shown in FIG. 2A, the presence of the deactivation beam inhibits radical polymerization completely in a photoresist composed of 1 wt % of biacetyl in dipentaerythritol pentaacrylate. Any photoinitiator molecules that have not been deactivated are then excited to an activated state, e.g. using two-photon absorption from a second ultrafast pulsed laser that is tuned between about 720 nm and about 875 nm. Further, when the deactivation beam passes through an appropriate phase mask before being focused, it provides a substantial improvement in resolution (FIG. 2B).

(17) In another preferred embodiment, one of the R groups on the initiator comprises a reactive monomer (e.g., tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, neopentylglycol diacrylate, tricyclodecane dimethanol diacrylate, neopentylglycol dimethyl acrylate, polyethylene glycol diacrylate, polyethylene glycol dimethylacrylate, ethoxylated bisphenol A glycol diacrylate, ethoxylated bisphenol A glycol dimethyl acrylate, trimethylolpropane trimethacrylate, trimethyloipropane triacrylate, pentaerythritol triacrylate, ethoxylated trimethyloipropane triacrylate, glyceryl propoxy triacrylate, pentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, glycidyl acrylate, glycidylmethyl acrylate, or combinations thereof).

(18) In some embodiments, R.sub.1 is a methyl group and R.sub.2 is a group containing multiple acrylate groups. The chemical formula for such material is shown in FIG. 3, and exhibits the same deactivation behavior as compared to biacetyl in SR 399. However, the combined initiator/monomer material has reduced initiator diffusion, which contributes to finer resolution.

(19) In another preferred embodiment, both of the R groups on the initiator are also polyfunctional reactive monomers. For instance, R.sub.1 and R.sub.2 may be dipentaerythritol pentaacrylate groups, as shown in FIG. 4. Attaching two monomers to the functional initiator unit further inhibits diffusion and prevents initiators from interacting with one another.

(20) A potential mechanism for the disclosed three-color behavior is as follows. The first excitation takes the PI molecules to the first excited singlet state (S.sub.1), which then undergo intersystem crossing (ISC) to the lowest triplet state (T.sub.1, see FIG. 5). T.sub.1 is a metastable state that does not lead directly to reactivity, and so is called an unreactive pre-activated state. A second color or wavelength of light takes pre-activated PI molecules to the next triplet state, T.sub.2, which undergo RISC to a highly vibrationally excited level of the electronic ground state (S.sub.0), thereby deactivating the PI molecules in selected locations. Pre-activated PI molecules that have not been deactivated may then be excited further via exposure to a third color or wavelength of light to take them to an activated state, which allows chemistry to occur. The three-color nature of this process effectively circumvents the background buildup problem inherent in conventional two-color materials, and thus achieves substantially higher resolution. As shown in FIG. 6, the wavelength dependence of deactivation following two-photon excitation at 800 nm is in close agreement with the transient absorption spectrum of the T.sub.1 state of biacetyl, indicating that deactivation occurs through RISC.

(21) Expanded Range of Confirmed Initiators.

(22) A broader range of vicinal diketones has been tested for the ability to initiate and inhibit polymerization of radical photoresists with different beams of light (and/or with radiation having differing characteristics). In addition to biacetyl (C.sub.4H.sub.6O.sub.2), specific molecules that have been tested and proven to exhibit this behavior include: benzil (C.sub.6H.sub.5CO.sub.2), 2.2-pyridil (C.sub.12H.sub.8N.sub.2O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), -naphthil (C.sub.22H.sub.14O.sub.2), and furil (C.sub.10H.sub.6O.sub.4).

(23) Note that species in which the diketone is part of a closed ring system, such as camphorquinone, did not display this behavior.

(24) Further Evidence for the Mechanism of Initiation and Implications for Photoresists.

(25) As a further test of the mechanism of initiation, detailed transient absorption studies were performed on biacetyl in benzene. Excitation of biacetyl using radiation having a wavelength of about 355 nm was followed by intersystem crossing and relaxation to the lowest triplet state, giving a transient absorption spectrum akin to that shown in FIG. 6. The disappearance of the transient absorption arises from either relaxation of molecules (which is expected to be unimolecular, and therefore exponential), unimolecular reaction (which again should be exponential), or triplet-triplet annihilation (which should not be exponential). The major component of the decay kinetics is biexponential, indicating that triplet-triplet annihilation is the predominant mechanism of reactivity in solution.

(26) Referring to FIG. 7, a detailed study of the deactivation action spectrum as a function of excitation wavelength was conducted. As shown, there is a clear dependence on the excitation wavelength, but the deactivation spectrum resembles the triplet absorption spectrum. These data provide further evidence that deactivation occurs through reverse intersystem crossing following absorption out of the triplet state.

(27) Taken together, these data indicate that there are two paths to initiation of polymerization with these materials. The first path is the one outlined above, in which absorption from T.sub.1 to a sufficiently energetic triplet state leads to initiation via the generation of radicals. In the second path, triplet-triplet annihilation causes a molecule to become sufficiently excited to cause initiation via the generation of radicals. The bimolecular path is undesirable given it is uncontrolled in that it does not require an activation beam. However, in accordance with disclosed embodiments, photoresists containing diketone initiators minimize this effect by: (1) using sterically bulky diketones that are less likely to undergo triplet-triplet annihilation; (2) inhibiting diffusion of the diketones through steric bulk and/or attachment to monomers; (3) incorporation of unreactive triplet quenchers; or (4) and combination of (1), (2) and/or (3).

(28) Demonstration of Resolution Enhancement.

(29) The ultimate goal of enhancing resolution has been demonstrated by 3-color or wavelength materials and schemes disclosed herein. For example, such resolution enhancement was demonstrated using 1 wt % biacetyl as the photoinitiator in dipentaerythritol pentaacrylate. As shown in FIG. 8, Plate B, substantial resolution improvement was achieved when pre-activation and activation were accomplished with pulsed light at 800 nm and deactivation was accomplished with continuous-wave light at the same wavelength.

(30) Resolution enhancement was also demonstrated using 0.5 wt % benzil as the photoinitiator in 1:1 SR368/499 (tris(2-hydroxy ethyl) isocyanurate triacrylate/ethoxylated(6) trimethylolpropane triacrylate), with deactivation at different pitch (at maximum deactivation power of 100 mW after radial polarization). As shown in FIG. 9, Plate B, substantial resolution improvement was achieved when pre-activation and activation were accomplished with pulsed light at 800 nm and deactivation was accomplished with continuous-wave light also at 800 nm.

(31) As shown in FIG. 10, Plate A, the presence of the deactivation beam inhibits radical polymerization in a photoresist composed of 0.5 wt % of benzil in 1:1 tris(2-hydroxy ethyl) isocyanurate triacrylate/ethoxylated(6) trimethylolpropane triacrylate. PI molecules that have not been deactivated (using continuous-wave laser (800 nm; 200 nW)) are then excited to an activated state (e.g. using two-photon absorption from a pulsed laser (800 nm; ML power 12.2 mW)). Further, when the deactivation beam passes through an appropriate phase mask before being focused, it provides a substantial improvement in resolution, as shown in FIG. 10, Plates B and C.

(32) All identified publications and references are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference in its entirety. While the invention has been described in connection with exemplary embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the features hereinbefore set forth.