WO2004099296A1

WO2004099296A1 - Nanoporous materials suitable for use in semiconductors

Info

Publication number: WO2004099296A1
Application number: PCT/NL2004/000242
Authority: WO
Inventors: Shahab Jahromi
Original assignee: Dsm Ip Assets B.V.
Priority date: 2003-05-05
Filing date: 2004-04-09
Publication date: 2004-11-18

Abstract

The composition according to the invention, comprises: (a) a porogen network system capable of forming a network comprising at least a functionalised porogen (i); (b) a resin; (c) a catalyst; and (d) an inert solvent. Surprisingly it has been shown that the composition according to the invention is suitable for use in a spin coating composition whilst not being susceptible to macroscopic phase separation. This allows the formation of a nanoporous material, having a low dielectric constant (k).

Description

NANOPOROUS MATERIALS SUITABLE FOR USE IN SEMICONDUCTORS

The present invention relates to nanoporous materials, which can for example be used as matrix for interlayer dielectrics, for example in semiconductors. The nanoporous materials according to the present invention can have a low dielectric constant (k), for example even lower than 2. The invention also relates to a composition on which the nanoporous materials according the invention can be based, a method of making the nanoporous materials according to the invention, and the products in which the nanoporous materials according to the invention can be used.

Continuing drive towards miniaturisation in chip manufacturing represents significant challenges to the chemical industry, for instance regarding developments of new type of interlayer dielectrics. Traditional dielectric interlayers have dielectric constants k of 3.9 to 4.2, are mostly based on silicon dioxide. These interlayers are commonly produced using chemical vapour deposition (CVD) methods.

One of the problems arising when electronics such as chips are made smaller is that the risk of cross talks between metal interconnect lines increases. Such cross talks result in increased signal delays. One of the methods to prevent such cross talks, is to replace the traditional material with new types of materials having significantly lower k values.

As a matrix, various types of resins, for example, organic polymers and inorganic resins, mostly polyorganosilicates, have been tested which are applied by spin-on technologies as opposed to chemical vapor deposition methods commonly used for application of conventional dielectric layers.

As the miniaturization in semiconductor manufacturing continues, there is a pressing need for insulating layers with ultra low k values (< 2). Inclusion of nanopores in the matrix system has been one of the methods for decreasing k values, since air has a very low dielectric constant (k = 1).

The common way of introducing nanopores in organosilicates, and thus decreasing dielectric constant (k) of the layer, is by use of organic porogens which are thermally decomposed at elevated temperatures, leaving nanopores. The more porogens are added, the larger the fraction of air in the resulting material after heating. In order to produce ultra low dielectric materials (k< 2), significant amount of porogens must be added to the spin-on formulations.

The problem is that porogens start to macroscopically phase separate with the resin at loadings typically exceeding 30-40 wt% prohibiting the formation of nanopores. Macroscopic phase separation is here and hereafter defined as a stage in which two different phases are present, of which the porogen phase comprises structures larger than the wavelength of visible light, i.e. higher than approximately 400 nm. As a result of the macroscopic phase separation large pores or even interconnected structures are formed, decreasing the mechanical properties of the interlayer once the porogen is burned out.

It is the object of the present invention to provide a composition not susceptible to macroscopic phase separation during spin coating whilst allowing formation of a nanoporous material.

The composition according to the invention comprises a porogen network system (a) capable of forming a network. The porogen network system comprises at least a functionalised porogen (i). The functionalised porogen is capable of forming a network either by itself or with another compound (ii). In case the porogen (i) is not capable of forming a network by itself, the presence of at least one other compound (ii) having functional groups capable of reacting with functional groups present on the porogen is required to make the porogen network system capable of forming a network.

Suitable porogens (i) for the composition according to the invention are either organic or inorganic compounds. Examples of inorganic porogens are compounds which are labile at higher temperatures. Preferably the used porogens are organic compounds. These organic compounds have to be decomposable, due to the fact that they have to be burned out the matrix structure during the production of the porous dielectric material. Suitable organic porogens are for example linear, branched and crosslinked polymers and copolymers, but also crosslinked polymeric nanoparticles with reactive surface functionality. The porogen may be a polymer comprised of aliphatic polycarbonates, polyester, polysulfones, polylactides, polylactones. The porogen may be a polymer comprised of monomer units as for example styrene, halogenated styrene, hydroxy-substituted styrene, lower alkyl- substituted styrene, acrylic acid, acrylamide, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, ethylene oxide, propylene oxide, and combinations of any of them. The porogen may be a homopolymer or a copolymer, for example based on the foregoing monomer units. Examples of suitable polymers include but are not limited to poly(methylmethacrylate), polystyrene, poly(α-methylstyrene) aliphatic polycarbonates, polyesters, polyesteramides, polysulfones, polylactides or polylactones. Branched polymers are suitable for the porogen, for example dendrimers, hyperbranched polymers, star shaped polymers. Preferably dendrimers, polyesteramides or star shaped poly (ε-caprolactone) are used.

Examples of suitable dendrimers to be used as porogen (i) in the present invention are for example described in WO93/14147 and WO95/02008 which dendrimers are incorporated by reference herein. In case of dendrimers used as porogens, preferably at least generation 3 dendrimers are used. More preferably generation 3 or 4 dendrimers are used.

Examples of suitable polyesteramides to be used as porogen (i) in the present invention are for example described in WO 99/16810, which describes linear or branched condensation polymers containing ester groups and at least one amide group in the backbone, having at least one hydroxy alkyl amide endgroup or modified hydroxyl amide endgroup, and having a weight average molecular mass of > 800 g/mol. The polyesteramides described in WO 99/16810 are hereby incorporated by reference.

A preferred polyesteramide is a branched polyesteramide having at least two groups according to formula II:

in which

R⁴ R⁶

Y = C— C— O— H , H, (C C₂₄)(cyclo)alkyl or (C_β-C₁₀) aryl, _R5 ,

B = (C₂-C₂₄), optionally substituted, aryl or (cyclo)alkyl aliphatic diradical, and R¹, R², R³, R⁴, R⁵ and R⁶ may, independently of one another, be the same or different, H, (C₆-C₁₀) aryl or (C C₈)(cyclo)alkyl radical. Also mixtures of porogens may be used.

The porogen has to be capable of forming a network either by itself or with another compound. Therefore the porogen has to be equipped with functional groups. The amount of functional groups is not limiting to the composition according to the invention, however, In all cases the average functionality of the porogen network system must be higher than 2.

The porogen should have a functionality of at least 2 per molecule. Introduction of functional groups to the porogen can be carried out using conventional methods, known to those skilled in the art. In a system wherein the porogen used is capable of reacting with itself, possible functional groups are for example oxazolines, nitriles, acryaltes, epoxy's, vinyl ethers.

In a system where the porogen is not capable of reacting with itself, possible functional groups are hydroxy or amine groups, preferably hydroxy groups. In one embodiment of the invention, wherein the porogen is not capable of reacting with itself, the porogen has at least 2 functional groups per molecule.

In case that the porogen is not capable of reacting with itself, compound (ii) is required to make the porogen network system of the present invention capable of forming a network. Such compound (ii) has to be capable of reacting with the functional groups of the porogen. Examples of suitable compounds (ii) are isocyanates, ketenes, cyano compounds, imino ethers, cardobiimides, aldehydes, ketons or the like. It is also possible that compound (ii) is a porogen containing another functionality than the other porogen. Preferably compound (ii) is an isocyanate. Examples of suitable isocyanates are trifunctional isocyanates or bifunctional isocyanates. Preferably, trifunctional isocyanates are used.

The functionalities of compounds (i) and (ii) should be high enough to enable the formation of the network. The person skilled in the art can determine the required functionality for that purpose easily. The functionality of compound (ii) should be at least 2. A preferred embodiments of the invention is a composition comprising a bifunctional porogen (i) and a trifunctional compound (ii)

The amount of compound (ii) used may be determined by the person skilled in the art. Preferably the ratio between the amount of functional groups of the porogen (i) and the amount of functional groups of the compound (ii) is at least 0.1 , more preferably at least 0,5, still more preferably at least 1.2 and most preferably about 1.

Preferably the ratio between the amount of functional groups of the porogen (i) and the amount of compatible functional groups of the compound (ii) is at most 10, more preferably at most 5 and still more preferably at most 1.2. Most preferably the functional groups of the porogen (i) and the functional groups of compound (ii) are present in stoichiometric amounts. The network reaction of the functional groups of the porogen network system may involve all types of reactions, either forming physical networks or chemical networks. Examples of possible reactions resulting in physical networks are hydrogen- bridges, VanderWaals bonding, ionic bonding or coordination bonding. Examples of reactions resulting in chemical networks are nucleophilic substitution, electrophilic substitution, free radical substitution, Diels Alder reaction.

Preferably the functional groups forming a network in the porogen network system are capable of reacting fast with each other, so that phase separation due to evaporation of solvent or due to the curing reaction of the resin does not take place any more, as the porogen is part of the netwerk already. It is believed that network formation of the porogen network system during the spin coating itself is most effective for preventing the macroscopic phase separation. Examples of fast combinations of functional groups are the combination of alcohols and isocyanates, or amines and isocayanates, free radical polymerisation of acrylates. Preferably, the functional groups in the porogen network system are combinations of alcohols and isocyanates.

An example of a suitable porogen network system for use in the formation of a nanoporous material is described by Jahromi et al (Macromolecules 2001 , 34, 1013-1027), which is incorporated herein by reference. Jahromi et al describes a network formed from functionalised dendrimers with isocyantes.

Any resin (b) can be used in the composition according to the invention as long as it is not susceptible to thermal degradation at the same temperature the porogen network will degrade. In general the resin should have, after curing, a glass transition temperature of at least about 400°C, more preferably at least about 440 °C, most preferably about 500 °C. Also mixtures of resins may be used.

Preferably the molecular weight (Mw) of the resin (b) is larger than 750. However, also high molecular weight resins are also suitable. The molecular weight (Mw) of the resin (b) is preferably lower than 100,000. Preferably, the resin is not capable of reacting with the porogen itself. More preferably the resin is not capable of reacting with any components of the porogen network system. This has the advantage that the efficiency of the porogen network system is not influenced by any side reactions.

Examples of suitable resins are silicon-containing polymers, such as organosilicates, polyarylenes, polyimides and polybenzocyclobutene (eg. Sumitomo Bakelite). Examples of suitable organosilicates are silsesquioxanes, alkoxy silanes, organic silicates, orthosilicates and organically modified silicates. Suitable silsesquioxanes are for example, hydrogen silsesquioxanes, alkyl silsesquioxanes, preferably lower alkyl silsesquioxanes, aryl or alkyl/aryl silsequioxanes, such as phenyl silsesquioxanes, and copolymers of silsesquioxanes with for example polyimides. Examples of suitable polyarylenes are polyphenylenes, poly(phenylquinoxalines) and poly(arylene ethers). An example of a commercially available polyarylene is SiLK™ (Dow Chem Inc.).

In a preferred embodiment of the invention organosilicates are used. Examples of suitable commercially available organosilicates are Zirkon Lk (Shiply), HOSP (Honeywell), Fox (Dow Corning), MezoELK (Chemicals affiliate Schumacher), polysilazane (AZ Electronics/Clariant), and polymethylphenylsiloxane resin, such as GR150F (Techneglas). Also mixtures of organosilicates may be used. Preferably hydrogen sisesquioxane (HSQ) or methyl silsesquioxane (MSQ) are used, due to their good dielectric properties. The amount of porogen network system (a) with respect to the amount of resin (b) determines the amount of pores in the matrix material to be produced. The more pores in the matrix material, the lower the dielectric constant (k). The composition according to the invention makes it possible that even an amount of 80 wt% porogen can be reached, without any sign of macroscopic phase separation. Preferably the composition according to the invention comprises at least an amount of 0.1 wt% porogen, more preferably at least an amount of 1 wt% porogen and most preferably an amount of 5 wt% porogen. Preferably the amount of porogen network system (a) is at most 80 wt% with respect to the total weight of the porogen network system (a) and the resin (b). Preferably the amount of porogen network system (a) is at most 70 wt %, more preferably at most 60 wt % and even more preferably at most 50 wt%.

In an embodiment of the invention, the composition according to the invention, also comprises a catalyst (c) in order to speed up the network formation of the porogen network system. Examples of suitable catalysts depend on the porogen network system used. The person skilled in the art can determine which catalyst is suitable for the porogen network system used. Also mixtures of catalysts may be used.

Examples of suitable catalysts for the composition according to the present invention are the present invention are acids and bases. Examples of suitable acid catalyst are Sb₂O₃, As₂O₃, dibutyltinlaurate LiX, BX₃, MgX₂, AIX₃, BiX₃, SnX₄, SbX₅, FeX₃, GeX₄, GaX₃, HgX₂, ZnX₂, AIX₃, TiX₄, MnX₂, ZrX₄, R₄NX, R₄PX, or HX where X is H, R, I, Br, Cl, F, acetylacetonate (acas), OR, O(O)CR or combinations of these and R is alkyl or aryl. Bronstedt acids such as H₂SO₄, HNO₃, HX, H₃PO₃, RH₂PO₂, RH₂PO₃, R[(CO)OH]_n, where n= 1-3, RSO₃H with R is alkyl or aryl, may also be used.

Examples of suitable bases are M(OH)_n, (RO)_nM (M= Alkali or earth alkali), NR_nH_4-nOH (R= alkyl with 1 to 20 carbon atoms or aryl, and n= 1-4), tertiary amines including triethylamine, tributylamine, triexylamine, trioctylamine, guanidine, cyclic amines such as diazobicyclo[2,2,2]octane (DABCO), dimethylaminopyridine (DMAP), and morfoline.

In case the functional groups in the porogen network system are alcohols and isocyanates, tin-based catalysts are preferred, for example dibutyltin acetate.

The amount of catalyst (c) used may be determined by the person skilled in the art. Generally the amount of catalyst is at least 0.01 wt % with respect to the porogen network system. Preferably the amount of catalyst is at least 0.1 wt%, more preferably at least 1 wt% and even more preferably at least 10 wt%. Generally the amount of catalyst is at most 100% with respect to the porogen network system. Additionally, the composition according to the invention may comprise an inert solvent (d). The solvent should be inert as to not react with any of the other components of the composition. The components of the composition should however dissolve in the solvent. The person skilled in the art can easily determine which type of solvent is most suitable for the specific components as used in the composition of the present invention. Suitable solvents include but are not limited to ketones, esters, ethers, alcohols, and/or hydrocarbons. Also mixtures of different solvents can be used.

The amount of solvent used for spin coating preferably may range between 50 and 98% (based on the total weight of the composition according to the invention). Preferably the solvent is chosen from the group including hydrocarbons, especially aromatic hydrocarbons; ethers, especially glycoethers and other equivalent ethers; and various esters, well-known to the person skilled in the art. Examples of suitable solvents are for example aromatic hydrocarbon compounds (for example the 'Solvesso' types), N-methylpyrolidone, xylene, dichlorobenzene, propylene glycol monomethylether, methylpropylene glycol acetate, butyl acetate, dibasic ester, isophoron, ethyl ethoxypropionate, ethylene-propylene glycol acetate and/or butyl glycol.

The present invention also relates to the use of a porogen network system (a) comprising at least a functionalised porogen for in the production of a porous material. Preferably the porogen network system is used during a spin coating process, or a similarly fast coating process, for example spray coating. Preferences for the porogen network system are as described above. Preferably the porogen network system (a) is used in combination with a catalyst (c). Preferences for the catalyst (c) are as described above. The composition according to the invention is suitable for use in a method for making porous material using a fast coating process. Examples of suitable fast coating processes are spin coating or spray coating. The coating of the substrate need not be performed at higher temperatures. Preferably, the coating takes place without additional heating, i.e. at room temperature. An example of a suitable method to produce porous material, but not intended to limit the scope of the invention, comprises the steps of:

(A) dissolving a porogen network system (a) with a resin (b) in an inert solvent (d) yielding a solution

(B) adding the catalyst (c) to the solution in an amount effective to start porogen network formation immediately followed by

(C) coating a substrate

(D) heating the coated substrate to a temperature Tc effective to cure the resin yielding a polymeric matrix having a glass transition temperature Tg; and

(E) heating the substrate to a temperature Td effective to degrade the porogen network without affecting the cured resin yielding a nanoporous material, wherein

Td is higher than Tc, but less than Tg. The porogen network system (a), the resin (b), the catalyst (c) and the inert solvent (d) being the same as already specified above, including the same preferences as to chemical consistence and amounts. Preferably the completion of porogen network formation takes place after the application of the composition to the substrate, so that the composition can flow to form the coating. The coating process preferably takes place at room temperature, thus the network formation preferably can take place at room temperature. As is known to the person skilled in the art room temperature depends on the circumstances. It is here and hereafter defined as approximately 20 °C +/- 5 °C. The nanoporous material can be used for several applications, for example in micro-electronics or anti-reflective coatings. For each of these applications there may be another optimal pore size. For anti-reflective coatings an average pore size of 100 nm can be sufficient, whilst for micro-electronics the average pore size needed is much smaller. Typically, but not restricted to, the pore size of the nanoporous material in micro-electronic applications is about 10% of the size of the individual metallic circuit lines or less. For example, in case the metallic circuit lines used are 130 nm, the averabe pore size preferably is about 13 nm or less. Pore sizes depend on the type of porogen used. The average pore sizes of can be for example at least about 5 nm. The pore size may be heterogeneously or homogeneously distributed. Preferably the pore size is more homogeneously distributed, since this may be of benefit to the material properties of the nanoporous material, as well as the dielectric constant.

The dielectric constant reached is influenced by the amount of air present in the nanoporous material. Nanoporous material prepared according to the invention can reach a dielectic constant of lower than 3, preferably lower than 2,5, and even more preferably lower than 2.

The substrate is generally an inert substrate, for example glass, silicon or ceramic. Suitable inert substrates also include epoxy composites, polyimides, phenolic polymers, high temperature polymers, and the like.

In case of micro-electronic applications, the substrate can optionally have integrated circuits disposed therein. The substrate may be provided with electrical conductor means such as input/output pins (I/O pins) for electrically connecting the packaging device to the circuit board. A plurality of electrically insulating and electrically conducting layers may be alternatively stacked up on the substrate. The layers are then generally formed on the substrate in a layer-by-layer process wherein each layer is formed in a separate process step.

For example, when forming an integrated circuit, first a metallic film may be deposited on the substrate, where after the metallic film is lithographically patterned to provide a pattern of individual metallic circuit lines on the substrate followed by the method of making a layer of a nanoporous material on the substrate as described above.

An example of micro-electronic applications are semi-conductors. The nanoporous material made from the composition according to the invention can be used in a semiconductor. For example a semiconductor may comprise a layer of the nanoporous material according to the invention.

It is to be understood that while the invention has been described in some specific embodiments thereof, that the foregoing description as well as the examples which follow are interned to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

The present invention is hereafter illustrated by the following example, not intended to limit the invention in any manner.

Sample preparation

Example I

Triisocyanate (Desmodur™ RFE, Bayer) was re-crystallised from dichloromethane to give white crystals having an isocyanate functionality of 3.

Polymethylphenylsiloxane resin (GR150F™, Techneglas, US). Dendritic wedges were synthesized according to the procedure described by Jahromi et. al., Macromolecules

2001 , 34, 1013.

The spin-on formulations were prepared by dissolving 3 g of GR150F in 100 ml dried dichlorobenzene (DCB) followed by adding 7 g of a stochiometric amount of dendritic diol and re-crystallised Desmodur™ RFE ([-NCO]/[-OH])=1). After mixing 100 mg of di-butyl tin acetate, the solutions were immediately spin-coated at

3000 rpm onto a glass substrate with a spincoater at ambient temperatures.

Samples were cured in an oven under nitrogen for 2 hours at 200°C and 2 hours at 530°C respectively, yielding coated substrates having a spin-coated film with a layer thickness below 2 μm.

Comparative Experiment A

A comparative example was prepared in the same manner as indicated above with the difference that no Desmodur RFE was added to the formulation.

Light microscopy

Light microscopy experiments were conducted after spin-coating on a Zeiss Axiophot microscope operating in phase contrast mode.

Photograph 1 shows the light microscopy image of the spin coated film of Example I. Photograph 2 shows the light microscopy image of the spin coated film of the comparative example.

First of all the coating of Comparative Experiment A appeared white milky indicative of macroscopic phase separation. This is also reflected in the light microscopy image of Comparative Experiment A showing a clear two-phase structure. In fact, investigations at lower loadings have indicated that the macroscopic phase separation already occurs above 10 wt% dendrimer in formulations without Desmodur RFE. In contrast, samples containing Desmodur RFE all appeared transparent even at a dendrimer loading of 70 wt%. This is also evident from the microscopy image of Example I, which shows no structure at least on microscopic scale.

Small angle X-ray scattering (SAXS) experiments

SAXS experiments were performed for Example I at ambient conditions with a modified Kratky setup, attached to a conventional sealed tube X-ray source (40kV and 50mA), which provides line-focused, Ni-filtered CuK_α-radiation

(0.154nm). The Kratky setup is equipped with an entrance slit of 40μm and features a sample-to-detector distance of 288mm. The scattering patterns from the samples and the background scattering were recorded with a position sensitive detector (MBRAUN 50M) for about 2-3 days. To correct for background signal, the signal recorded from a PE reference sample was used for intensity calibration, whereas sample transmissions were derived from its attenuation. For calibration of the scattering vector, q = 2πs - (4π/λ)*sm^' 3 , the position of the (attenuated) primary beam was recorded without beamstop. SAXS data processing including subtraction of the transmission- weighted background signal were performed with subroutines of the FFSAXS software, developed by Vonk. In this step, the overall background signal (including parasitic scattering from the SAXS camera and contributions from the glass substrate) was subtracted from the scattering pattern in order to minimize statistical errors at high s.

SAXS profiles were recorded from the coating of Example I after annealing at 530°C. The Guinier plot (figure 1) of the recorded data can be used to estimate the dimension of these structures. This reveals a radius of gyration (R_G) of ~9nm. The latter is related to the pore radius (R) via R - ^ ζ R_G and thus yields a dimension of R ~ 12nm.

Claims

1. Composition comprising at least:

(a) a porogen network system capable of forming a network comprising at least a functionalised porogen (i); and

(b) a resin.

2. Composition according to claim 1 , characterised in that the porogen network system (a) also comprises a compound (ii) capable of reacting with functional groups of the porogen (i).

3. Composition according to any one of claims 1-2, characterized in that the composition also comprises a catalyst (c).

4. Composition according to any one of claims 1-4, characterized in that the composition also comprises an inert solvent (d).

5. Method for producing porous material comprising the step of applying a composition according to any one of claims 1 to 4 in a spin-coating process to a substrate.

6. Method for producing porous material comprising the following steps: (A) dissolving a porogen network system (a) with a resin (c) in an inert solvent (d) yielding a solution (B) adding the catalyst (b) to the solution in an amount effective to start porogen network formation followed by

(C) spin-coating a substrate

(D) heating the spin-coated substrate to a temperature Tc effective to cure the resin yielding a polymeric matrix having a glass transition temperature Tg; and

(E) heating the substrate to a temperature Td effective to degrade the porogen network without affecting the cured resin yielding a nanoporous material, wherein Td is higher than Tc, but less than Tg.

7. Use of a porogen network system (a) capable of forming a network comprising at least a functionalised porogen (i) for the production of nanoporous material.

8. Nanoporous material produced by the method or use according to any one of claims 5 to 7 characterized in that it has a dielectric constant (k) lower than 2.

9. Use of porous material according to claim 8 in a semiconductor.

10. Semiconductor comprising a layer of the nanoporous material according to claim 9.