US 20060081557 A1
In a substantially planar circuit, the conductors are separated by an inorganic material with a dielectric constant of less than about 3.0. The dielectric layers are formed in a process that includes defining trenches and/or vias for the conductors by imprinting an initially planar layer of a radiation curable composition. The imprinting die is preferably UV transparent such that the composition is UV cured while the imprint die is in place. The curable composition includes an organic modified silicate compound and a second decomposable organic component, the latter forming nanometer scale pores as the organic compounds are subsequently decomposed to provide a polysilicate matrix. The pores reduce the effective dielectric constant from that of otherwise dense silicon dioxide.
8. A composition of matter that comprises:
a) a UV curable organic modified silicate, and
b) a decomposable organic compound,
c) wherein a viscosity of said compound is less than about 200,000 cps.
9. A composition of matter according to
10. A composition of matter according to
11. A composition of matter according to
12. A composition of matter that comprises:
a) a UV curable organic modified silicate,
b) a decomposable organic compound,
c) a fluorosurfactant, and
d) a solvent for said UV curable organic silicate compound, said decomposable organic compound and said fluorosurfactant.
13. A composition of matter according to
14. A composition of matter according to
15. A composition of matter according to
16. A composition of matter according to
17. A composition of matter according to
18. A composition of matter according to
The present invention relates to a method or material of fabricating integrated circuits, and in particular to a method of forming an integrated circuit on a substrate having a low dielectric constant.
There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices, e.g., memory and logic chips, thereby increasing their performance and reducing their cost. In order to accomplish this goal, it is also desirable to reduce the minimum feature size on the chip, e.g., circuit line width, and also to decrease the dielectric constant of the interposed dielectric material to enable closer spacing of circuit lines without an increase in crosstalk and capacitive coupling. Further, there is a desire to reduce the dielectric constant of the dielectric materials such as utilized in the back end of the line (BEOL) portion of integrated circuit devices, which contain input/output circuitry, to reduce the requisite drive current and power consumption for the device.
The most commonly used dielectric material in integrated circuits is silicon dioxide, which has a dielectric constant of about 4.0. Silicon dioxide is readily grown or formed on the surface of a planar silicon wafer that is used to form the majority of the current semiconductor devices. Silicon dioxide has the requisite mechanical and thermal properties to withstand processing operations and thermal cycling associated with semiconductor manufacturing. However, it is desired that dielectric materials for future integrated circuit devices exhibit a lower dielectric constant (e.g., <3.0) than exhibited by current silicon dioxide. As inorganic materials have an inherent limitation to dielectric constants of lower than about three, several types of alternative materials have been developed to achieve lower dielectric constants. A number of these alternative materials are organic polymers, which if at least partially fluorinated can have a dielectric constant less than about three. However, the development of appropriate organic polymers, as well as their depositions and patterning methods poses significant challenges. The selection or choice of an organic material is frequently limited by the need for higher temperature steps in other aspects of the process, such as metallization or semiconductor fabrication. Another type of alternative material is an inorganic material with dispersed micro voids or pores to achieve a lower effective dielectric constant. Efforts to develop such materials are generally described in J. H. Golden, C. J. Hawker and P. S. Ho, “Designing Porous low-K Dielectrics”, Semiconductor International, May 2001, which is incorporated herein by reference. Further, U.S. Pat. No. 5,895,263, to Carter, et al. and issued on Apr. 20, 1999, which is incorporated herein by reference, teaches a process for forming an integrated circuit device comprising (i) a substrate; (ii) metallic circuit lines positioned on the substrate and (iii) a dielectric material positioned on the circuit lines. The dielectric material comprising porous organic modified polysilica.
Although porous inorganic materials can inherently withstand higher processing temperatures, like other dielectric materials, additional challenges arise due to the complexity of the patterning processes. Lithographic techniques are often employed in device micro fabrication. Traditionally photolithography has been used to define or remove a portion of the dielectric material after it deposited on the substrate. See S. Wolf et al., Silicon Processing for the VLSI Era, Volume 1—Process Technology, (1986), pp. 407-413. Using microcircuit fabrication as an example, photo resist materials are applied to a dielectric material after deposition on a planar substrate. Next, the resist layer is selectively exposed to a form of radiation. An exposure tool and mask are often used to affect the desired selective exposure. Patterns in the resist are formed when the dielectric layer undergoes a subsequent “developing” step. The areas of resist remaining after development protect the dielectric and substrate regions that they cover. Locations from which resist has been removed can be subjected to a variety of additive (e.g., lift-off) or subtractive (e.g., etching) processes that transfer the pattern onto the substrate surface. However, photolithography has inherent size limitations that demand the use of shorter wavelength sources and more sophisticated optics to reduce the line width and feature sizes in the micro circuitry.
Thus in the process of U.S. Pat. No. 5,895,263 the low-K dielectric layer must be first formed, and then patterned prior to deposition of the conductor material. The plurality of required processing steps inherently increases the processing time, resulting in higher costs as well as generally reduced product yield.
Further, as it is desirable to decrease the size of circuit features, that is line width and spacing between conductors, the alternative inorganic materials must be capable of deposition with pore sizes that are a fraction of the size of these features.
It is therefore a first object of the present invention to provide an improved method of fabricating an integrated circuit device comprising a low dielectric constant material between conductive lines and/or vias.
It is another object of the present invention to provide a process to deposit a patterned low dielectric constant inorganic material on a planar substrate in a minimum number of process steps.
It is a further object of the invention to provide a robust, repeatable process for depositing a patterned low dielectric constant inorganic material.
Other objects and advantages will be apparent from the following disclosure.
In the present invention, the aforementioned objects are achieved by deploying imprint lithography to mold a relief image corresponding to microcircuit features on a substantially planar substrate. The imprint molding process deploys a polymerizable resin composition that is subsequently converted to a porous low dielectric constant inorganic material.
The method of forming a relief image involves at least the steps of covering a substantially planar substrate with a polymerizable fluid composition; then contacting the polymerizable fluid composition with a mold having a relief structure formed therein such that the polymerizable fluid composition substantially fills the relief structure in the mold; subjecting the polymerizable fluid composition to conditions to polymerize the fluid composition and form a solidified polymeric material there from on the substrate; separating the mold from the solid polymeric material such that a replica of the relief structure in the mold is formed in the solidified polymeric material. The polymerizable composition is preferably a UV curable organic modified silicate that compromises a decomposable organic component known as a porogen. Pores remain as the organic porogen decomposes during the subsequent processing that converts the polymerized organic modified silicate to an inorganic material.
As the UV curing is preferably conducted through a mold that is UV transparent, another object of achieving a robust process for imprinting includes using a UV curable polymerizable fluid that includes an organic modified silicate, a decomposable organic compound, and a fluorosurfactant to improve the release of the cured composition from the imprint-molding tool.
Other objects of the invention are achieved by using a process that includes the steps of providing a composition that includes a UV curable organic modified silicate, a decomposable organic compound and a solvent; then spin coating the composition on a substrate, removing the solvent, imprinting a circuit pattern in the remaining composition, UV curing the remaining composition, heating the composition to condense the organic modified silicate and decompose the decomposable polymer to form a porous patterned dielectric layer, and depositing metal conductors within the patterns formed in the porous dielectric material.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.
Methods of imprinting to form a relief pattern are taught in U.S. Pat. No. 6,334,960, to Wilson, et al., which issued on Jan. 1, 2002, the disclosure of which is incorporated herein by reference.
In the instant invention, the polymerizable material is a modified silicate having organic functional groups that upon exposure to actinic radiation cross-link or react to form a non-fluid material replicating the shape of the mold. The cured or polymerized organic modified silicate is subsequently, after removal of the mold, converted to an inorganic silicate upon thermal decomposition of the organic functional groups therein. The polymerizable material also contains one or more components, which upon decomposition form pores or voids in the inorganic silicate. As shown in further detail in
The mold used in the methods of the invention may be formed from various conventional materials, such as, but not limited to, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, and combinations of the above. Preferably, the material is selected such that the mold is UV transparent, which allows the polymerizable fluid composition covered by the mold to be exposed to an external radiation source. Thus, quartz molds are most preferred. To facilitate release of the mold from the solid polymeric material, the mold may be treated with a surface modifying agent. Surface modifying agents that may be employed include those that are known in the art. An example of a surface modifying agent is a fluorocarbon silylating agent. These surface modifying agents or release materials may be applied, for example, from plasma sources, a Chemical Vapor Deposition method (CVD) such as analogs of the Parylene deposition process, or a treatment involving deposition from a solution.
The methods of the invention will now be described in detail to the accompanying drawing in which a preferred embodiment of the invention is shown.
Alternatively, the polymerizable fluid can be first deposited as a substantially uniform fluid layer on substrate 10 employing, for example, spin-coating techniques. Thereafter mold 30 is brought to the same proximity as shown in
Further, to the extent that it is otherwise preferable to use different cross-linkable organic polysilicates having higher molecular weight than the preferred oligomers, as described below, the mixture may contain a solvent as an inert diluent. The solvent may be selected to dissolve a particular pore forming material as well as a fluorosurfactant (described in the more preferred embodiments below) or simply to lower the viscosity to a level low enough for spin coating on a planar substrate. After spin coating, the solvent is removed by vacuum or thermal evaporation, for example at about 100° C. for about 1 min. The then solvent free, planarized fluid can be directly imprinted by contacting the mold thereto.
Suitable substrates for the device of the present invention comprise silicon, silicon dioxide, glass, silicon nitride, ceramics, aluminum, copper and gallium arsenide. Other suitable substrates will be known to those skilled in the art. In a multilayer integrated circuit device, an underlying layer of insulated, planarized circuit lines can also function as a substrate.
Referring now to
The selection of a method of initiating the polymerization of the fluid composition is known to one skilled in the art, and typically depends on the specific application that is desired. Generally speaking, organic modified polysilica is an oligomeric or polymeric compound comprising silicon, carbon, oxygen and hydrogen atoms. The polymerizable (or crosslinkable) materials that may be used in the methods of the invention may include various silicon-containing materials that are often present themselves in the form of polymers or oligomers. Suitable organic polysilica include (i) silsesquioxanes (ii) partially condensed alkoxysilanes (e.g., partially condensed by controlled hydrolysis tetraethoxysilane having a number average molecular weight of about 500 to 20,000); (iii) organically modified silicates having the composition RSiO3 and R2SiO2 wherein R is an organic substituent and (iv) partially condensed orthosilicates having the composition SiOR4. Silsesquioxanes are polymeric silicate materials of the type RSiO1.5 where R is an organic constituent. The silicon-containing materials preferably contains the element silicon in an amount greater than about 10 percent based on the weight of the polymerizable fluid composition, and more preferably greater than about 20 weight percent.
The silicon-containing polymerizable material also includes one or more pendant functional from a variety that includes, as non-limiting examples, epoxy groups, ketone groups, acetyl groups, vinyl groups, acrylate groups, methacrylate groups, and combinations of the above. Although not wishing to be bound by any theory, it is believed that suitable polymerizable fluid compositions may react according to a variety of reaction mechanisms such as, but not limited to, acid catalysis, free radical polymerization, cationic polymerization, or 2+2 photocycloaddition, and the like.
The most preferable forms of organic polysilica are of relatively low molecular weight, but predominantly have two or more pendent and reactive functional group per molecule. Such organically modified silicates are available under the trade name “ORMOCER” type resins are available from Micro Resist Technology GmbH (Berlin, Germany). Typically, these materials are formed through the controlled hydrolysis and condensation of organically modified silanes, particularly alkyltrialkoxysilanes, such as the mixture of molecules 710, 720, 730 and 740 as is illustrated in
Upon the condensation reaction 700, the aforementioned trialkoxysilane reactants form various types of cross-linked networks with one or more reactive functional groups. Thus, upon the initial condensation reaction —OX groups are eliminated such that a Si—O— bonded network is formed having the generic structure illustrated as 750. The silicate portion of the network 750 is illustrated schematically as an oval for the other species formed in condensation reaction 700. Depending on the exact composition and ratios of the initial reactants, polycondensation reaction 700 produces a variety of species having one of more methacrylate, acrylate, vinyl, epoxide, and the like pendent function groups capable of cross-linking with each other either thermally or on exposure to actinic radiation with a suitable photo initiator and/or catalyst. When R or Z is alternatively a porogen, designated P2 or P3 wherein the above the condensation reaction bonds the porogen pendent groups to the Si—O— bonded network 750, as 755. P3 is intended to encompass structures and molecules having an additional pendent methacrylate, acrylate, vinyl, epoxide, and the like pendent function groups capable of cross-linking. P2 and P3 thus can be oligomeric or polymeric, to vary or optimize the pore size and distribution. For example, trimethoxysilyl norbornene (TMSNB) and triethoxysilyl norbornene (TESNB) polymers (Promerus, Brecksville, Ohio) have been used as such chemically bonded porogens as described by Padovani, et al in “Chemically Bonded Porogens in Methylsilsesquioxane, I. Structure and Bonding, Journal of the Electrochemical Society, 149 (12) F161-F170 (2002).), which is incorporated herein by references. Alternatively, P1 or P2 can be poly (caprolactone) or other polyols of various molecular weights with polyhydroxyl terminated or branched hydroxyl terminated species preferred to minimize the viscosity of the polymerizable fluid.
Thus, reaction 700 result in, among others, species 741, which has a Si—O— bonded network 750 with an epoxide pendent group, whereas other products include species 744 has a Si—O— bonded network 750 with a methacrylate pendent group. In contrast, as an alternative species 742 has Si—O— bonded network 750 with both an epoxide and a methacrylate pendent group, while species 743 has Si—O— bonded network 750 with two methacrylate pendent groups. Another product of reaction 700 is species 755 has Si—O— bonded network 750 with an epoxide, methacrylate and pore forming pendent group, P2. In species 760 the Si—O— bonded network 750 has pendent epoxide and methacrylate groups as well as the pore forming pendent group, P3, with P3 having the third pendent reactive group, that is methacrylate, bonded or pendent from it.
Preferably, the subsequent polymerization step 815, wherein the fluid is exposed to actinic radiation with the mold in place, results in the solid cross-linked resin 880. Thus, if the mixture contains epoxide groups it is preferable to include a photo initiator that creates an acid such that the complete curing of a cross-linked network can be accomplished in a single step, such that the mold can be rapidly removed and used to imprint other devices or portions of a substrate.
Alternatively, depending on the photo initiator, the subsequent cross-linking reaction 810 may initially occur via the methacrylate groups. This may be preferable if one wishes to increase the viscosity or provide a partially cross-linked the organic silicate precursors prior to a final thermal cure process 820, which would cross-link any remaining epoxy groups, also forming a solid material having a three dimension cross-linked network 880. When epoxide group are present after the initial exposure to actinic radiation, the curing can be accomplished in multiple steps, using what is termed a soft bake at between 80 to 120° C. for 5 min. or less, followed by a higher temperature cure at between about 120 to 240° C., for up to about 3 hrs.
The final step to decompose the organic modified silicate, to form porous silicate 70 in
Referring back to
Another feature of the present invention is forming the dielectric material, which is positioned over the circuit lines and/or between the circuit lines and on the substrate. In multilevel integrated circuit devices, the dielectric material is often planarized to function as a substrate for lithographic formation of the next layer of circuit lines. The dielectric material comprises porous organic polysilicate.
In the more preferred embodiments, the polymerizable composition also includes a fluorosurfactant to improve the release properties and performance life of the imprint mold or tool. A presently preferred fluorosurfactant a non-ionic polymeric fluorochemical surfactant sold under the trade name NOVEC FC-4432 by 3M Performance Materials Division (St. Paul, Minn.) Fluorosurfactant. An alternative fluorosurfacants include ZONYL FSO-100, available from DuPont Corporation (Wilmington, Del.)
In a preferred polymerizable fluid composition, percentage or fraction decomposable polymer (porogen) to Si— is selected to produce a pore volume from about 10 to 40 volume %, and more preferably 20 to 30%, depending on the desired dielectric constant and the ultimate mechanical strength and durability required of the dielectric layer, it being understood that even for nanoscale pores, increasing the total porosity decreases the strength and durability. The porogen component preferably comprises from about 10 to 50 weight percent of the composition. Additionally it is preferable if the organic modified silicate comprises at least about 10 weight percent silicon. More preferably, the organic modified silicate has a molecular weight of less than about 50,000. Under such conditions, the polymerizable fluid composition preferably has viscosity of less than about 200,000 cps.
As a theoretical example of a preferred composition for the polymerizable fluid of the instant invention, 79.5 g ORMOCER b59 UV curable organic modified silicate, 20 g TONE 0301 as the porogen and 0.5 g FC4432 of fluorosurfactant are mixed together. As ORMOCER b59 is available from the manufacturer premixed with the appropriate photo initiator the above composition can be used for imprint molding as described above when exposed to UV radiation of a wavelength that includes 365 nm. “TONE” 0310 is a poly(caprolactone) polyol (CAS Reg. No. 37625-56-2) having a relatively low-melting point and is tri-functional (3 —OH groups per molecule) with a number average molecular weight of about 900, and a hydroxyl number (mg KOH/g) of 187.0, being available from the Dow Chemical Company (Midland, Mich.). Other polycaprolactones deemed suitable without undue experimentation include CAPA 3031, which is available from Solvay Caprolactones (Warrington, Cheshire, United Kingdom).
It is expected that the inventive process is susceptible to achieving the smallest pore sizes, as the presence of the micro relief of the mold prior to the pore generation process minimizes the tendency for the nucleation and growth of larger pores.
It should be appreciated that one skilled in the art may select the substrate, mold, polymerizable fluid composition, surface modifying agent, as well as any other materials such that the method of the invention optimally functions according to the specific needs of the end user.
While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.