FR3101354A1 - Neutral underlayer for block copolymer and polymer stack comprising such an underlayer covered with a film of block copolymer - Google Patents

Abstract

The invention relates to a homopolymer or (co-) polymer sublayer intended to be deposited on the surface of a substrate and covered with a block copolymer, said sublayer being characterized in that it exhibits an aliphatic chemical structure whose carbon chain comprises a number N of carbon atoms such that N ≥ 6 and N <12. Such a sub-layer has a surface energy that is neutral with respect to each of the blocks of the copolymer to blocks and makes it possible to avoid or delay the dewetting of a block copolymer deposited on its surface. Figure to be published with the abstract: figure 1

Description

Neutral sublayer for block copolymer and polymer stack comprising such a sublayer covered with a film of block copolymer

The invention relates to the field of microelectronics, and more particularly to applications of nano-lithography by directed self-assembly, also called DSA (from the English acronym "Directed Self-Assembly").

The invention relates more particularly to a neutral underlayer for a block copolymer as well as a polymer stack comprising such an underlayer deposited on the surface of a substrate and covered by a film of block copolymer intended to serve as a nano-lithography.

Since the 1960s, block copolymers have been a very vast field of research for the development of new materials. It is possible to modulate and control their properties by the chemical nature of the blocks and their architecture for the intended application. For specific macromolecular parameters (M _n , I _p , f , χ, N), block copolymers are able to self-assemble and form structures whose characteristic dimensions (10 -100 nm) are today an issue. major in the field of microelectronics and micro-electro-mechanical systems (MEMS).

In the particular context of applications in the field of nano-lithography by directed self-assembly, or DSA (English acronym for "Directed Self-Assembly"), block copolymers, capable of nano-structuring at an assembly temperature, are used as nano-lithography masks. Block copolymers, once nano-structured, make it possible to obtain cylinders, lamellae, spheres or even gyroids which respectively form patterns, for the creation of nano-lithography masks, with a periodicity of less than 20 nm, which difficult to achieve with conventional lithography techniques.

Applications in microelectronics, of DSA type technology, are potentially limited by the dewetting conditions of the block copolymers used. Indeed, in order to be able to present sufficient mobility during their nano-structuring at the assembly temperature, the block copolymers are deposited on a solid substrate in liquid or viscous form. However, the fact of depositing a liquid/viscous polymer layer on a solid surface can lead to the appearance of dewetting phenomena, in particular when said polymer layer is brought to a temperature higher than its glass transition temperature. If a block copolymer film dewets from its surface, either before or after its self-organization at the assembly temperature, then it becomes unusable as a nano-lithography mask. Indeed, the dewetting leads to the obtaining of different block copolymer film thicknesses and an inhomogeneity of the patterns in terms of dimensions and shape factor. In such a case, the patterns transferred by dry etching into the underlying substrate are then not homogeneous in terms of dimensions and form factor. Consequently, it appears essential that the block copolymer film can remain perfectly flat throughout the manufacturing process of a nano-lithography mask and throughout the subsequent etching process.

On the other hand, obtaining a self-assembled block copolymer with a periodicity of less than 10 nm is made possible thanks to the use of block copolymers whose blocks have a high incompatibility, i.e. say with a high Flory-Huggins interaction parameter, χ. This high parameter results in a difference in physico-chemical properties between the blocks and in particular in surface energy. In the case of the lamellar phase in particular, this large difference in surface energy favors the orientation of the domains parallel to the surface of the substrate. However, in order to be able to serve as a nano-lithography mask, such a block copolymer must have nano-domains oriented perpendicularly to the lower and upper interfaces of the block copolymer, in order to then be able to selectively remove one of the nano-domains from the block copolymer , create a porous film with the residual nano-domain(s) and transfer, by etching, the patterns thus created into the underlying substrate. However, this condition of perpendicularity of the nano-domains is met only if each of the lower (substrate / block copolymer) and upper (block copolymer / ambient atmosphere) interfaces is "neutral" with respect to each of the blocks of said copolymer, i.e. that is to say that there is no preponderant affinity of the interface considered for at least one of the blocks constituting the block copolymer. A neutralization of the energies at the lower (copolymer/substrate) and upper (copolymer/ambient air) interfaces is therefore necessary to obtain an orientation of the nano-domains perpendicular to the surface of the substrate, over the entire thickness of the film. This is why a lot of research has focused on the development of neutralizing overcoats and undercoats.

With this in mind, the possibilities for controlling the affinity of the so-called “lower” interface, located between the substrate and the block copolymer, are well known and mastered today. By way of example, among the possible solutions, if the intrinsic chemistry of the monomers constituting the block copolymer allows it, a random copolymer comprising a judiciously chosen ratio of the same monomers as those of the block copolymer can be grafted onto the substrate, allowing thus to balance the initial affinity of the substrate for the block copolymer. This is, for example, the classic method of choice, used for a system comprising a block copolymer such as PS- b -PMMA and described in the article by Mansky et al, Science , 1997 , 275, 1458).

Regarding the control of the so-called "upper" interface of the system, that is to say the interface between the block copolymer and the surrounding atmosphere, one solution consists in depositing a coating material, called "top coat" , on the block copolymer film. Such a coating is intended to neutralize the upper interface of the block copolymer, in order to compensate for a large difference in surface energy of the blocks of the block copolymer leading to a film oriented parallel to the substrate, even if the lower interface of the film was previously neutralized. This approach of neutralizing the upper interface by a top coat coating presents its own problems, such as for example the non-miscibility of the top coat layer with the block copolymer, the solubilization of the top coat layer coat in a solvent which does not dissolve the underlying block copolymer, the difficulties in synthesizing the top coat material, etc. In addition, at this upper interface, dewetting phenomena appear which can be of the liquid/solid type or liquid/liquid depending on the configuration of the deposited top coat material. Thus, while at the lower interface the dewetting phenomena appearing are of the solid/liquid type, at the upper interface they may be different, in particular of the liquid/liquid type if the top coat layer is still in liquid form. or viscous when deposited.

Although the neutrality of the lower interface with respect to the block copolymer in such a polymer stack is well mastered today, the prior art only describes very few methods making it possible to limit, or even eliminate, the dewetting of polymer films in a polymer stack. Only the document L. Xue, Y. Han / Progress in Materials Science 57 (2012) 947–979, entitled "Inhibition of dewetting of thin polymer films", presents the state of the art in the field of polymeric stacks. It describes in particular that the two principles making it possible to inhibit the dewetting of a film of polymer deposited on the surface of a substrate consist, on the one hand, in modifying the interfacial tension between the film of polymer and the substrate and on the other hand, to reduce the mobility of the chains of the polymer. However, the methods described in this document do not apply to the dewetting of block copolymer films, which must necessarily be deposited in a liquid or viscous state in order to be able to nano-structure at the assembly temperature, and even less at the dewetting of block copolymer films requiring perfect control of the interfacial energy to obtain a perpendicular organization of the nano-domains, within the framework of a DSA-type process, for applications in microelectronics.

In addition, there are two major techniques for controlling and guiding the orientation of the blocks of a block copolymer on a substrate: graphoepitaxy and/or chemo-epitaxy. Graphoepitaxy uses a topological constraint to force the block copolymer to organize itself in a predefined space commensurate with the periodicity of the block copolymer. For this, graphoepitaxy consists in forming primary patterns, called guides, on the surface of the substrate. These guides, of any chemical affinity with respect to the blocks of the block copolymer, delimit zones inside which a layer of block copolymer is deposited. The guides allow to control the organization of the blocks of the block copolymer to form secondary patterns of higher resolution, inside these zones. Conventionally, the guides are formed by photolithography. Chemo-epitaxy uses a contrast of chemical affinities between a pre-drawn pattern on the substrate and the different blocks of the block copolymer. Thus, a pattern exhibiting a strong affinity for only one of the blocks of the block copolymer is pre-drawn on the surface of the underlying substrate, in order to allow the perpendicular orientation of the blocks of the block copolymer, while the rest of the surface has no particular affinity for the blocks of the block copolymer. For this, a layer is deposited on the surface of the substrate comprising, on the one hand, neutral zones (consisting for example of grafted random copolymer), having no particular affinity with the blocks of the block copolymer to be deposited and on the other hand, affine zones (consisting for example of homopolymer grafted with one of the blocks of the block copolymer to be deposited and serving as an anchoring point for this block of the block copolymer). The homopolymer serving as an anchor point can be made with a width slightly greater than that of the block with which it has a preferential affinity and allows, in this case, a "pseudo-equitable" distribution of the blocks of the block copolymer at the substrate surface. Such a layer is called "pseudo-neutral" because it allows an equitable or "pseudo-equitable" distribution of the blocks of the block copolymer on the surface of the substrate, so that the layer does not present, as a whole, affinity preferentially with one of the blocks of the block copolymer. Consequently, such a chemo-epitaxial layer at the surface of the substrate is considered to be neutral with respect to the block copolymer.

In view of the foregoing, the applicant sought a solution to avoid the dewetting of block copolymer films, or at least to delay it as much as possible, in order to have a greater range of assembly temperatures accessible for the self-organization of the block copolymer and to maximize the chances of obtaining assemblies with fewer defects, when they are guided with methods such as chemo-epitaxy and/or graphoepitaxy for example.

[Technical problem]

The object of the invention is therefore to remedy the drawbacks of the prior art. In particular, the object of the invention is to propose a sub-layer of homopolymer or of (co-)polymer intended to be deposited on the surface of a substrate and covered with a block copolymer, said sub-layer having a neutral or pseudo-neutral surface energy with respect to each of the blocks of the block copolymer and making it possible to avoid, or at least to delay as much as possible, any phenomenon of dewetting of the film of block copolymer so as to obtain block copolymer film as flat as possible.

The invention further relates to a polymer stack comprising a substrate on the surface of which is deposited an underlayer of homopolymer or (co-)polymer, itself covered by a film of block copolymer intended to serve as a nano-lithography, said sub-layer exhibiting a neutral or pseudo-neutral affinity with respect to each of the blocks of the block copolymer and making it possible to avoid, or at least to delay as much as possible, any phenomenon of dewetting of the film of block copolymer so as to obtain the flattest possible block copolymer film.

[Brief description of the invention]

Surprisingly, the Applicant has discovered that a homopolymer or (co-)polymer sub-layer intended to be deposited and grafted onto the surface of a substrate and to be covered with a block copolymer, said sub-layer layer being characterized in that it has an aliphatic chemical structure whose carbon chain comprises a number N of carbon atoms such that N ≥ 6 and N < 12, makes it possible to respond very effectively both in terms of neutrality of said underlayer with respect to the block copolymer, and also in terms of limiting the dewetting of the block copolymer.

This is due to the fact that this type of underlayer promotes a multiplication of weak interactions with the block copolymer, said interactions being of the Van der Walls type and not very selective for each of the blocks of the block copolymer. Such a sub-layer makes it possible both to obtain a perpendicular assembly of the nano-domains of the block copolymer and to effectively limit the dewetting of the block copolymer from this sub-layer via the establishment of a multitude of these weak interactions. with the chains of the block copolymer.

According to other optional characteristics of the underlay:
- it comprises one or more hydroxy group(s) at the end of the polymer chain, allowing it to be grafted onto the surface of the substrate;
- it is obtained by cationic, anionic, controlled or uncontrolled radical polymerization, or by ring opening, from acrylic and/or (meth)acrylic monomers;
- it further comprises a nitroxy function allowing its grafting onto the surface of the substrate, when it is obtained by controlled radical polymerization;
- it has a chemical structure of the acrylate or methacrylate type based on co-monomers chosen from acrylic monomers such as acrylic acid or its salts, alkyl, cycloalkyl or aryl acrylates such as methyl, ethyl, butyl, tert-butyl of 2-ethyl-hexyl, hexyl, 1-methyl-heptyl or phenyl, hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate, acrylates of alkyl ether such as 2-methoxyethyl acrylate, alkoxy- or aryloxy-polyalkylene glycol acrylates such as methoxypolyethylene glycol acrylates, ethoxypolyethylene glycol acrylates, methoxypolypropylene glycol acrylates, methoxy-polyethylene glycol-polypropylene glycol acrylates or their mixtures, aminoalkyl acrylates such as 2-(dimethylamino)ethyl acrylate (ADAME), fluorinated acrylates, silylated acrylates, phosphorus-containing acrylates such as alkylene glycol phosphate acrylates, glycidyl acrylates, dicyclopentenyloxyethyl, methacrylic monomers such as methacrylic acid or its salts, alkyl, cycloalkyl, alkenyl or aryl methacrylates such as methyl methacrylate (MAM), lauryl, cyclohexyl, allyl, phenyl or naphthyl, hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate, alkyl ether methacrylates such as 2-ethoxyethyl methacrylate, alkoxy- or aryloxy- polyalkylene glycol such as methoxypolyethylene glycol methacrylates, ethoxypolyethylene glycol methacrylates, methoxypolypropylene glycol methacrylates, methoxy-polyethylene glycol-polypropylene glycol methacrylates or mixtures thereof, aminoalkyl methacrylates such as 2-(dimethylamino)ethyl methacrylate (MADAME) , fluorinated methacrylates such as 2,2,2-trifluoroethyl methacrylate, silylated methacrylates such as 3-methacryloylpropyltrimethylsilane, phosphorus methacrylates such as alkylene glycol phosphate methacrylates, hydroxy-ethylimidazolidone methacrylate, methacrylate hydroxy-ethylimidazolidinone, 2-(2-oxo-1-imidazolidinyl)ethyl methacrylate, glycidyl methacrylates, or dicyclopentenyloxyethyl, at least one compound of said underlayer comprising between 6 and 12 carbon atoms;
- It consists of poly (2-ethyl-hexyl methacrylate) or poly (methyl-heptyl methacrylate) up to at least 50% by mass. ;
- it is grafted onto the substrate by heating to a temperature less than or equal to 280°C, preferably less than or equal to 250°C, and more preferably less than or equal to 200°C, for a time less than or equal to 60 minutes, preferably less than or equal to 15 minutes, and more preferably less than or equal to 2 minutes;
- it is neutral with respect to each of the blocks of the block copolymer.

The invention also relates to a polymer stack comprising a substrate on the surface of which is deposited an underlayer of homopolymer or (co-)polymer, itself covered by a film of block copolymer intended to serve as a mask of nano -lithography, said stack being characterized in that the underlayer of homopolymer or (co-)polymer conforms to that which has just been described above.

Other advantages and characteristics of the invention will appear on reading the following description given by way of illustrative and non-limiting example, with reference to the appended Figures which represent:
FIG. 1, a graph comprising two curves representing the percentage of sub-layer surface covered by a block copolymer as a function of the temperature at which said block copolymer is nanostructured, said curves being obtained for two energy sub-layers of equivalent surface, one of the two sub-layers being in accordance with the invention and the other being a comparative sub-layer;
FIG. 2, a photo of a nanostructured lamellar block copolymer, seen from above, said block copolymer having been deposited on an underlayer in accordance with the invention;
FIG. 3, a photo of a nanostructured lamellar block copolymer, seen from above, said block copolymer having been deposited on a comparative underlayer.

[Description of the invention]

By “ polymers ” is meant either a copolymer (of random, gradient, block, alternating type), or a homopolymer.

The term " monomer " as used refers to a molecule which can undergo polymerization.

The term “ polymerization ” as used relates to the process of transforming a monomer or a mixture of monomers into a polymer of predefined architecture (block, gradient, random, etc.).

The term " copolymer " means a polymer combining several different monomer units.

The term “ random copolymer ” means a copolymer in which the distribution of the monomer units along the chain follows a statistical law, for example of the Bernoullian type (Markov zero order) or first or second order Markovian. When the repeating units are randomly distributed along the chain, the polymers were formed by a Bernoulli process and are called random copolymers. The term random copolymer is often used, even when the statistical process that prevailed during the synthesis of the copolymer is not known.

The term “ gradient copolymer ” means a copolymer in which the distribution of the monomer units varies progressively along the chains.

The term " alternating copolymer " means a copolymer comprising at least two monomer entities which are distributed alternately along the chains.

The term " block copolymer " means a polymer comprising one or more uninterrupted sequences of each of the distinct polymer species, the polymer sequences being chemically different from one or more of the other(s) and being bonded together by a bond chemical (covalent, ionic, hydrogen bond, or coordination). These polymer sequences are also called polymer blocks. These blocks have a phase segregation parameter (Flory-Huggins interaction parameter) such that, if the degree of polymerization of each block is greater than a critical value, they are not miscible with each other and separate into nano- areas.

The term " miscibility " above means the ability of two or more compounds to mix completely to form a homogeneous or "pseudo-homogeneous" phase, i.e. without apparent crystalline or quasi-crystalline symmetry. short or long distance. The miscible nature of a mixture can be determined when the sum of the glass transition temperatures (Tg) of the mixture is strictly lower than the sum of the Tgs of the compounds taken in isolation.

In the description, we speak both of “self-assembly ” and of “self-organization ” or even of “ nano-structuring ” to describe the well-known phenomenon of phase separation of block copolymers, at an assembly temperature also called annealing temperature.

The term “ porous film ” designates a block copolymer film in which one or more nano-domains have been removed, leaving holes whose shapes correspond to the shapes of the nano-domains having been removed and which can be spherical, cylindrical, lamellar or helical.

By “ neutral ” or “ pseudo-neutral ” surface is meant a surface which, as a whole, does not exhibit any preferential affinity with one of the blocks of a block copolymer. It thus allows an equitable or “pseudo-equitable” distribution of the blocks of the block copolymer on the surface. Neutralizing the surface of a substrate makes it possible to obtain such a “ neutral ” or “ pseudo-neutral ” surface.

By “ non-neutral ” surface is meant a surface which, as a whole, has a preferential affinity with one of the blocks of a block copolymer. It allows an orientation of the nano-domains of the block copolymer in a parallel or non-perpendicular way.

We define the surface energy (denoted γx) of a given material “ x ”, as being the excess energy at the surface of the material compared to that of the material taken as a whole. When the material is in liquid form, its surface energy is equivalent to its surface tension.

When talking about the surface energies or more precisely the interfacial tensions of a material and of a block of a given block copolymer, these are compared to a given temperature, and more particularly to a temperature allowing the self-organization of the block copolymer.

The term " lower interface " of a block copolymer means the interface in contact with an underlying layer or substrate on which/which said block copolymer is deposited. It is noted that this lower interface is neutralized by a conventional technique, that is to say that it does not exhibit, as a whole, any preferential affinity with one of the blocks of the block copolymer.

The term “ upper interface ” or “upper surface” of a block copolymer means the interface in contact with an upper layer, called top coat and denoted TC, applied to the surface of said block copolymer. It is noted that the upper layer of top coat TC, like the underlying layer, preferably has no preferential affinity with one of the blocks of the block copolymer so that the nano-domains of the block copolymer can orient themselves perpendicular to the interfaces at the time of joint annealing.

“ Solvent orthogonal to a (co)polymer ” is understood to mean a solvent that is not capable of attacking or dissolving said (co)polymer.

The term " liquid polymer " or " viscous polymer " means a polymer exhibiting, at a temperature above the glass transition temperature, due to its rubbery state, an increased capacity for deformation due to the possibility given to its molecular chains of move freely. The hydrodynamic phenomena at the origin of dewetting appear as long as the material is not in a solid state, that is to say undeformable due to the negligible mobility of its molecular chains.

The term " discontinuous film " means a film whose thickness is not constant due to the shrinkage of one or more zones, leaving holes to appear.

By “ pattern ” in a nano-lithography mask, is meant an area of a film comprising a succession of alternately recessed and protruding shapes, said area having a desired geometric shape, and the recessed and protruding shapes that can be lamellae, cylinders, spheres or gyroids.

One way of delaying the dewetting of a block copolymer in a final stack of the top coat (TC)/block copolymer (BCP)/neutral underlayer (NL) type is to consider only the half-system corresponding to the lower block copolymer (BCP)/neutral underlayer (NL)/substrate interface.

The applicant therefore took an interest in this half-system and developed a polymer stack comprising a substrate, a sub-layer of homopolymer or (co-)polymer deposited on the surface of the substrate and covered with a block copolymer intended forming a nano-lithography mask.

In such a half-system, it is not enough for the (co-)polymer underlayer to prevent, reduce or slow down the phenomenon of dewetting of the block copolymer at the surface of the underlayer, but it is also necessary that it is neutral with respect to the block copolymer. The underlayer must be neutral with respect to the block copolymer, i.e. it must have a suitable surface energy to allow the block copolymer to generate nano-domains oriented perpendicular to the surface. interface at the time of its nano-structuring at the assembly temperature, whether the block copolymer is alone or covered with a top coat to neutralize the upper interface.

The substrate is preferably solid and of any nature (oxide, metal, semiconductor, polymer, etc.) depending on the applications for which it is intended. Preferably, but not exhaustively, the constituent material of the substrate can be chosen from: silicon Si, a silicon oxide Si _x O _y , silicon nitride Si ₃ N ₄ , an aluminum oxide Al _x O _y , a titanium nitroxide Ti _x O _y N _z , a hafnium oxide Hf _x O _y , metals, or even a polymeric material such as polydimethylsiloxane PDMS, polyimide, polycarbonate for example. The constituent material of the substrate can also be covered with a stack of organic layers of the SOC type (English acronym for "Spin-On-Carbon") or BARC (English acronym for "bottom anti-reflectant coating") and SiARC (acronym English for "Silicon Anti-Refelctive Coating") or SOG (English acronym for "Spin-on-Glass"), in order to allow successive etching steps and transfer throughout the thickness of the underlying substrate, the pattern generated in the block copolymer film after removal of at least one of its nano-domains.

To allow the guidance of the nano-domains of the block copolymer at the time of its nano-structuring, the substrate may or may not include patterns, said patterns being pre-drawn by a step or a sequence of lithography steps of any kind. and being intended to guide the organization of the BCP block copolymer by the so-called chemo-epitaxy or grapho-epitaxy technique, or even a combination of these two techniques, to obtain a neutralized or pseudo-neutralized surface.

A sub-layer of (co-)polymer, neutral with respect to the block copolymer, is then deposited on the surface of the substrate. Advantageously, the applicant has discovered that the use of sub-layers favoring a multiplication of weak interactions, of the Van der Waals type, with the block copolymer, makes it possible to respond very effectively both in terms of neutrality of said sub-layer with respect to the block copolymer, and also in terms of limiting dewetting.

Thus, a sub-layer having an aliphatic chemical structure, branched or not, whose carbon chain comprises a number N of carbon atoms such that N ≥ 6 and N < 12, makes it possible, on the one hand, to obtain a sub -neutral layer vis-à-vis the block copolymer and an orientation of the nano-domains of the block copolymer perpendicular to the interface and on the other hand, to effectively limit the dewetting of the block copolymer from the surface of this sub- layer due to the establishment of these weak interactions, of the Van der Walls type, and not very selective for each of the blocks.

Regarding the synthesis of this (co-)polymer sub-layer, neutral with respect to the block copolymer, it can be synthesized by any appropriate polymerization technique, such as for example anionic polymerization, cationic polymerization, controlled or uncontrolled radical polymerization, ring-opening polymerization.

When the polymerization process is carried out by a controlled radical route, any controlled radical polymerization technique may be used, whether NMP (“Nitroxide Mediated Polymerization”), RAFT (“Reversible Addition and Fragmentation Transfer”), ATRP (“ Atom Transfer Radical Polymerization”), INIFERTER (“Initiator-Transfer-Termination”), RITP (“Reverse Iodine Transfer Polymerization”), ITP (“Iodine Transfer Polymerization). Preferably, the polymerization process by a controlled radical route will be carried out by NMP.

More particularly the nitroxides derived from alkoxyamines derived from the stable free radical (1) are preferred.

in which the RL radical has a molar mass greater than 15.0342 g/mole. The RL radical can be a halogen atom such as chlorine, bromine or iodine, a linear, branched or cyclic, saturated or unsaturated hydrocarbon group such as an alkyl or phenyl radical, or an ester group -COOR or an alkoxyl group -OR, or a phosphonate group -PO(OR)2, provided that it has a molar mass greater than 15.0342. The monovalent RL radical is said to be in the β position with respect to the nitrogen atom of the nitroxide radical. The remaining valences of carbon atom and nitrogen atom in formula (1) can be bonded to various radicals such as hydrogen atom, hydrocarbon radical such as alkyl, aryl or aryl radical -alkyl, comprising from 1 to 10 carbon atoms. It is not excluded that the carbon atom and the nitrogen atom in the formula (1) are linked together via a bivalent radical, so as to form a ring. Preferably, however, the remaining valences of the carbon atom and the nitrogen atom of formula (1) are bonded to monovalent radicals. Preferably, the RL radical has a molar mass greater than 30 g/mole. The radical RL can for example have a molar mass of between 40 and 450 g/mole. By way of example, the radical RL may be a radical comprising a phosphoryl group, said radical RL possibly being represented by the formula:

in which R3 and R4, which may be identical or different, may be chosen from alkyl, cycloalkyl, alkoxyl, aryloxyl, aryl, aralkyloxy, perfluoroalkyl, aralkyl radicals, and may comprise from 1 to 20 carbon atoms. R3 and/or R4 can also be a halogen atom such as a chlorine or bromine or fluorine or iodine atom. The RL radical can also comprise at least one aromatic ring as for the phenyl radical or the naphthyl radical, the latter possibly being substituted, for example by an alkyl radical comprising from 1 to 4 carbon atoms.

More particularly alkoxyamines derived from the following stable radicals are preferred:

- N-tertiobutyl-1-phenyl-2 methyl propyl nitroxide,

- N-tertiobutyl-1-(2-naphthyl)-2-methyl propyl nitroxide,

- N-tertiobutyl-1-diethylphosphono-2,2-dimethyl propyl nitroxide,

- N-tertiobutyl-1-dibenzylphosphono-2,2-dimethyl propyl nitroxide,

- N-phenyl-1-diethyl phosphono-2,2-dimethyl propyl nitroxide,

- N-phenyl-1-diethyl phosphono-1-methyl ethyl nitroxide,

- N-(1-phenyl 2-methyl propyl)-1-diethylphosphono-1-methyl ethyl nitroxide,

- 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy,

- 2,4,6-tri-tert-butylphenoxy.

Preferably, the alkoxyamines derived from N-tert-butyl-1-diethylphosphono-2,2-dimethyl-propyl nitroxide will be used.

When the polymerization process is carried out by an anionic pathway, any anionic polymerization mechanism may be considered, whether it be liganded anionic polymerization or else anionic polymerization by ring opening. Preferably, in the context of such an anionic polymerization, an anionic polymerization process will be used in an apolar solvent, and preferably toluene, as described in patent EP0749987, and involving a micro-mixer.

The (co-)monomers constituting the (co-)polymer sub-layer, synthesized by any of the aforementioned polymerization routes, are for example chosen from acrylic monomers such as acrylic acid or its salts, alkyl, cycloalkyl or aryl acrylates such as methyl, ethyl, butyl, tert-butyl, 2-ethyl-hexyl, hexyl, 1-methyl-heptyl or phenyl acrylate, hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate, alkyl ether acrylates such as 2-methoxyethyl acrylate, alkoxy- or aryloxy-polyalkylene glycol acrylates such as methoxypolyethylene glycol acrylates, ethoxypolyethylene glycol, methoxypolypropylene glycol acrylates, methoxy-polyethylene glycol-polypropylene glycol acrylates or mixtures thereof, aminoalkyl acrylates such as 2-(dimethylamino)ethyl acrylate (ADAME), fluorinated acrylates, silylated acrylates, phosphorus compounds such as alkylene glycol phosphate acrylates, glycidyl or dicyclopentenyloxyethyl acrylates, methacrylic monomers such as methacrylic acid or its salts, alkyl, cycloalkyl, alkenyl or aryl methacrylates such as methyl (MAM), lauryl, cyclohexyl, allyl, phenyl or naphthyl methacrylate, hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate, etheralkyl methacrylates such as 2-ethoxyethyl methacrylate, alkoxy- or aryloxy-polyalkylene glycol methacrylates such as methoxypolyethylene glycol methacrylates, ethoxypolyethylene glycol methacrylates, methoxypolypropylene glycol methacrylates, methoxy-polyethylene glycol-polypropylene glycol methacrylates or mixtures thereof, methacrylates aminoalkyl such as 2-(dimethylamino)ethyl methacrylate (MADAME), fluorinated methacrylates such as 2,2,2-trifluoroethyl methacrylate, silylated methacrylates such as 3-methacryloylpropyltrimethylsilane, phosphorus methacrylates such as alkylene glycol phosphate methacrylates, hydroxy-ethylimidazolidone methacrylate, hydroxy-ethylimidazolidinone methacrylate, 2-(2-oxo-1-imidazolidinyl)ethyl methacrylate, glycidyl methacrylate, or dicyclopentenyloxyethyl methacrylate, at least a constituent compound of said sub-layer comprising a number of carbon atoms greater than or equal to 6 and less than 12.

The constituent polymer of the underlayer, of (meth)acrylate architecture, comprising a number N of carbon atoms greater than or equal to 6 and less than 12, is deposited on the surface of the substrate according to techniques known from the skilled in the art, such as the technique known as "spin coating", "Doctor Blade", "knife system", "slot die system" for example. The layer thus deposited may have a high thickness, for example of the order of 60 nm.

Preferably, the undercoat is obtained on an acrylate or methacrylate architecture, graftable onto the surface of the substrate via a hydroxy group at the end of the chain and/or a nitroxy function originating from an alkoxyamine used for the polymerization reaction, when this Polymerization is carried out by a controlled radical route and more particularly by the NMP (“Nitroxide Mediated Polymerization”) route. The ester function of the acrylate comprises an aliphatic carbon chain in which the number N of carbon atoms is greater than or equal to 6 and less than 12.

The polymer sub-layer thus deposited, in the form of a film on the surface of the substrate, is then subjected to a treatment with a view to allowing the polymer to be grafted onto the surface of the substrate. This treatment can be done in different ways depending on the polymer and the chemical functions it contains. Thus, the treatment allowing the grafting of the constituent polymer of the underlayer on the surface of the substrate, can be chosen from at least one of the following treatments: a heat treatment, also called annealing, a treatment by organic oxidation-reduction or inorganic, electrochemical treatment, photochemical treatment, shear treatment or ionizing radiation treatment. Preferably, the grafting is carried out by heat treatment at a temperature less than or equal to 280°C, preferably less than or equal to 250°C and more preferably less than or equal to 200°C, for a time less than or equal to 60 minutes, preferably less than or equal to 15 minutes and more preferably less than or equal to 2 minutes .

Rinsing in a solvent, such as propylene glycol monomethyl ether acetate (PGMEA) for example, then makes it possible to eliminate the excess non-grafted polymer chains. Then the substrate is dried, for example under a stream of nitrogen. The residual thickness of the underlayer is then measured, by ellipsometry for example. The grafted sublayer has a thickness of the order of 5 nm, i.e. of the order of the thickness of a monolayer of molecules.

The sub-layer of (co-)polymer thus fixed on the surface of the substrate makes it possible to control its surface energy with respect to a subsequently deposited block copolymer, so as to obtain a particular orientation of the nano-domains of the block copolymer with respect to the surface. More particularly, the surface energy of the sub-layer thus deposited is neutral or pseudo-neutral with respect to each of the blocks of the block copolymer so that, during its self-organization at the assembly temperature , the nano-domains of the block copolymer are oriented perpendicular to the underlayer/block copolymer interface.

The block copolymer is then deposited by any deposition technique known to those skilled in the art cited above and then subjected to a heat treatment to allow its nano-structuring into nano-domains oriented perpendicular to the surface.

With regard to this BCP block copolymer to be nano-structured, it comprises "n" blocks, n being any whole number greater than or equal to 2. The BCP block copolymer is more particularly defined by the following general formula:

where A, B, C, D,…, Z, are as many “i”… “j” blocks representing either pure chemical entities, i.e. each block is a set of monomers of identical chemical natures , polymerized together, or a set of co-monomers copolymerized together, in the form, in whole or in part, of block or random or random or gradient or alternating copolymer.

Each of the “i”… “j” blocks of the BCP block copolymer to be nano-structured can therefore potentially be written in the form: i= a _i - co -b _i - co -…- co -z _i , with i ≠…≠j, in whole or in part.

The volume fraction of each entity a _i …z _i can range from 1 to 99%, in monomer units, in each of the i…j blocks of the BCP block copolymer.

The volume fraction of each of the i…j blocks can range from 5 to 95% of the BCP block copolymer.

The volume fraction is defined as being the volume of an entity compared to that of a block, or the volume of a block compared to that of the block copolymer.

The volume fraction of each entity of a block of a copolymer, or of each block of a block copolymer, is measured as described below. Within a copolymer in which at least one of the entities, or one of the blocks if it is a block copolymer, comprises several co-monomers, it is possible to measure, by NMR, the proton, the molar fraction of each monomer in the whole copolymer, and then working up to the mass fraction using the molar mass of each monomer unit. To obtain the mass fractions of each entity of a block, or each block of a copolymer, it suffices to add the mass fractions of the comonomers constituting the entity or the block. The volume fraction of each entity or block can then be determined from the mass fraction of each entity or block and the density of the polymer forming the entity or block. However, it is not always possible to obtain the density of polymers whose monomers are co-polymerized. In this case, the volume fraction of an entity or a block is determined from its mass fraction and the density of the majority compound by mass of the entity or block.

The molecular weight of the BCP block copolymer can range from 1000 to 500000 g.mol ⁻¹ .

The BCP block copolymer can have any type of architecture: linear, star (tri- or multi-arm), grafted, dendritic, comb.

Each of the blocks i, … j of a block copolymer, has a surface energy denoted γ _i … γ _j , which is specific to it and which depends on its chemical constituents, i.e. on the chemical nature monomers or co-monomers which compose it. Similarly, each of the constituent materials of a substrate has its own surface energy value.

Each of the blocks i, … j of the block copolymer also has a Flory-Huggins type interaction parameter, denoted: χ_ix, when interacting with a given material "x", which can be a gas, a liquid, a solid surface, or another polymer phase for example, and an inter-facial energy denoted "γ_ix», with γ_ix= γ_I-(γ_xcosθ_ix), where θ_ixis the contact angle between materials i and x. The interaction parameter between two blocks i and j of the block copolymer is therefore denoted χ_ij.

There is a relation linking γ_Iand Hildebrand's solubility parameter δ_Iof a given material i, as described in the document Jia & al.,Journal of Macromolecular Science, B,2011, 50, 1042. In fact, the Flory Huggins interaction parameter between two given materials i and x is indirectly related to the surface energies γ_I and γ_x specific to the materials, we can therefore either speak in terms of surface energies, or in terms of interaction parameters to describe the physical phenomenon appearing at the interface of the materials.

When we speak of the surface energies of a material and those of a given BCP block copolymer, we imply that we are comparing the surface energies at a given temperature, and this temperature is that (or is at least part of the temperature range) allowing the self-organization of the BCP.

The block copolymer film is deposited on the surface of the underlayer by a conventional technique such as, for example, spin coating.

The block copolymer is necessarily deposited in a liquid/viscous state, so that it can nano-structure at the assembly temperature, during a subsequent annealing step.

Preferably, but not limiting for the invention, the block copolymer used comprises at least one block where a heteroatom such as silicon, germanium, titanium, hafnium, zirconium, aluminum, is present. in all or part of the (co)monomer constituting said block.

Preferably, but not limiting for the invention, the block copolymer used is said to be "high-χ" (has a high Flory-Huggins parameter), that is to say it must have a parameter more important than that of the so-called "PS-b-PMMA" system at the considered assembly temperature, as defined by Y. Zhao, E. Sivaniah and T. Hashimoto, Macromolecules, 2008, 41 (24), pp 9948–9951 (Determination of the Flory-Huggins parameter between styrene ("S") and MMA ("M"): χSM = 0.0282 + (4.46/T)).

The following example illustrates the scope of the invention in a non-limiting manner.

Example :

In this example, a block copolymer film of the PDMSB- b- PS (poly(dimethylsilacyclobutan) -block -polystyrene) type, with a lamellar morphology and a period of the order of 18 nm, is deposited on a substrate on which different sub-layers are grafted, in order to compare the dewetting of the block copolymer according to the sub-layers used. The block copolymer is deposited over a thickness of the order of 30 nm.

Advantageously, the different sub-layers compared have a similar or equivalent surface energy. The measurement of the surface energy of these sub-layers is carried out according to the OWRK method (from the English acronym Owens-Wendt-Rabel-Kaelble" (https://www.kruss-scientific.com/services/education- theory/glossary/owens-wendt-rabel-and-kaelble-owrk-method/)or static resting drop method which consists in measuring the contact angle of a drop of liquid on a solid substrate by means of a goniometer.

The sub-layers of interest are obtained on an acrylate or methacrylate architecture, and can be grafted onto the substrate via a hydroxy group at the end of the chain and/or a nitroxy function originating from an alkoxyamine used for the polymerization reaction, when this polymerization is carried out by a controlled radical route. The ester function of the (meth)acrylate comprises an aliphatic carbon chain in which the number N of carbon atoms is greater than or equal to 6 and less than 12.

In a particular example, the underlayer studied is more particularly a homopolymer of poly(2-ethyl-hexyl methacrylate), denoted "PMali" below. Such a sub-layer has a surface energy of the order of 29 mN/m, measured on a Drop Shape Analyzer 100 instrument from Krüss Scientific (https://www.kruss-scientific.com/products/drop-shape/ dsa100/drop-shape-analyzer-dsa100/).

By way of comparison, the sub-layer in accordance with the invention is compared with a range of sub-layers where the Van der Walls interactions with the block copolymer which covers them are limited and where the surface energy is adjusted via the introduction of fluorinated groups into the polymer chain. Such an underlayer is, for example, in the form of random copolymers of poly(trifluoroethyl methacrylate-co-n-butyl acrylate) (PMATRIFE- co -ABu), with a mass composition in each of the comonomers equal to 50/50. Such a sub-layer of PMATRIFE- co -ABU has a surface energy of the order of 28.5 mN/m.

Consequently, taking into account the experimental error on the measurement, the two compared subshells present an equivalent surface energy.

In order to be able to compare the dewetting of the block copolymer film of PDMSB- b -PS deposited on a substrate on which the various sub-layers are grafted, the rate of recovery of the block copolymer on the surface of the sub-layer under -jacente when it is annealed at a given temperature in a range of temperatures studied, the two sub-layers having an equivalent surface energy.

The results obtained are reported on the graph of Figure 1 which includes two curves each representing the dewetting properties of the PDMS- b -PS block copolymer on the two sub-layers subjected to comparison of PMali and PMATRIFE- co -Abu, equivalent surface energies.

It follows from the curves in FIG. 1 that the PMali sub-layer, in accordance with the invention, has a wider range of annealing temperatures than the PMATRIFE- co -Abu sub-layer. Indeed, while the PDMSB- b -PS block copolymer deposited on a PMATRIFE- co -Abu sub-layer begins to dewet when the annealing temperature, allowing nano-structuring of the block copolymer, reaches 110°C , the same block copolymer deposited on a sub-layer of PMali only begins to dewet when the annealing temperature, allowing nano-structuring of the block copolymer, reaches 150°C.

The multiplication of Van der Walls type interactions between the underlayer and the block copolymer therefore makes it possible to stabilize the system with respect to dewetting at higher temperatures.

Conversely, in the case of the PMATRIFE- co -Abu sub-layer, weak Van der Walls type interactions are not very present due to the fact that the Abu chain contains only 4 carbon atoms and only the fluorinated groups introduced into the MATRIFE chain of the copolymer make it possible to adjust the surface energy of the sub-layer to find a zone of neutrality. In this case, significant dewetting is observed when the assembly temperature of the block copolymer exceeds a temperature of 110°C. At equivalent surface energy, the PMATRIFE- co -Abu copolymer does not exhibit enough Van der Walls type interactions with the block copolymer to be able to maintain the block copolymer and for the latter to remain flat, without dewetting its surface.

The photos in Figures 2 and 3 represent respectively the nanostructured PDMSB- b -PS block copolymer, seen from above, when it is deposited on the PMali sub-layer in accordance with the invention and on the comparative PMATRIFE sub-layer. -co -Abu. These photos were taken by scanning electron microscopy, after having deposited a layer of TC top coat material, of appropriate surface energy, on the block copolymer in order to neutralize its upper interface, then having nanostructured the block copolymer at an assembly temperature so that its nano-domains orient perpendicular to the interfaces and finally, having removed the top coat material. The PDMSB- b -PS block copolymer deposited on the PMali sublayer was nanostructured at an assembly temperature of 240°C for 5 minutes (Figure 2), while the PDMSB -b -PS block copolymer PS deposited that the PMATRIFE- co -Abu sublayer was nanostructured at a temperature of 110°C for 5 minutes (Figure 3). These two photos highlight a perpendicular orientation of the nano-domains of the block copolymer whatever the underlayer used, thus demonstrating the neutrality of the two types of underlayers for the block copolymer while they have properties of different dewetting.

The sub-layer of PMali therefore makes it possible to avoid, or at least to delay, dewetting of the block copolymer film of PDMSB- b -PS, which remains flat throughout the process, so that the block copolymer has a homogeneous thickness and nano-domains whose dimensions and shape factor are homogeneous, so that it is possible to use it as a nano-lithography mask.

Claims (9)

Underlayer of homopolymer or (co-)polymer intended to be deposited and grafted onto the surface of a substrate and to be covered with a block copolymer, the said underlayer being characterized in that it has a chemical structure aliphatic whose carbon chain comprises a number N of carbon atoms such that N ≥ 6 and N < 12. Undercoat according to Claim 1, characterized in that it comprises one or more hydroxy group(s) at the end of the polymer chain, allowing it to be grafted onto the surface of the substrate. Undercoat according to claim 1 or 2, characterized in that it is obtained by cationic, anionic, controlled or uncontrolled radical polymerization, or by ring opening, from acrylic and/or (meth)acrylic monomers. Undercoat according to one of Claims 1 to 3, characterized in that it also comprises a nitroxy function allowing it to be grafted onto the surface of the substrate, when it is obtained by controlled radical polymerization. Undercoat according to one of Claims 1 or 4, characterized in that it has a chemical structure of the acrylate or methacrylate type based on co-monomers chosen from acrylic monomers such as acrylic acid or its salts, alkyl, cycloalkyl or aryl acrylates such as methyl, ethyl, butyl, tert-butyl, 2-ethyl-hexyl, hexyl, 1-methyl-heptyl or phenyl acrylate, hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate, alkyl ether acrylates such as 2-methoxyethyl acrylate, alkoxy- or aryloxy-polyalkylene glycol acrylates such as methoxypolyethylene glycol acrylates, ethoxypolyethylene glycol, methoxypolypropylene glycol acrylates, methoxy-polyethylene glycol-polypropylene glycol acrylates or mixtures thereof, aminoalkyl acrylates such as 2-(dimethylamino)ethyl acrylate (ADAME), fluorinated acrylates, silylated acrylates, phosphorus compounds such as alkylene glycol phosphate acrylates, glycidyl or dicyclopentenyloxyethyl acrylates, methacrylic monomers such as methacrylic acid or its salts, alkyl, cycloalkyl, alkenyl or aryl methacrylates such as methyl (MAM), lauryl, cyclohexyl, allyl, phenyl or naphthyl methacrylate, hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate, etheralkyl methacrylates such as 2-ethoxyethyl methacrylate, alkoxy- or aryloxy-polyalkylene glycol methacrylates such as methoxypolyethylene glycol methacrylates, ethoxypolyethylene glycol methacrylates, methoxypolypropylene glycol methacrylates, methoxy-polyethylene glycol-polypropylene glycol methacrylates or mixtures thereof, methacrylates aminoalkyl such as 2-(dimethylamino)ethyl methacrylate (MADAME), fluorinated methacrylates such as 2,2,2-trifluoroethyl methacrylate, silylated methacrylates such as 3-methacryloylpropyltrimethylsilane, phosphorus methacrylates such as alkylene glycol phosphate methacrylates, hydroxy-ethylimidazolidone methacrylate, hydroxy-ethylimidazolidinone methacrylate, 2-(2-oxo-1-imidazolidinyl)ethyl methacrylate, glycidyl methacrylate, or dicyclopentenyloxyethyl methacrylate, at least a compound of said underlayer comprising between 6 and 12 carbon atoms. Undercoat according to one of Claims 1 to 5, characterized in that it consists of poly(2-ethyl-hexyl methacrylate) or poly(1-methyl-heptyl methacrylate) present in the copolymer chain at height of at least 50% by mass. Undercoat according to one of Claims 1 to 6, characterized in that it is grafted onto the substrate by heating to a temperature less than or equal to 280°C, preferably less than or equal to 250°C, and in such a way more preferably less than or equal to 200°C, for a time less than or equal to 60 minutes, preferably less than or equal to 15 minutes, and more preferably less than or equal to 2 minutes. Undercoat according to one of Claims 1 to 7, characterized in that it is neutral with respect to each of the blocks of the block copolymer. Polymer stack comprising a substrate on the surface of which is deposited a sub-layer of (co-)polymer, itself covered by a film of block copolymer intended to serve as a nano-lithography mask, said stack being characterized in that the (co-)polymer underlayer is in accordance with one of claims 1 to 8.