Patent Publication Number: US-10784108-B2

Title: Method for forming a functionalised assembly guide

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of PCT/FR2017/052890, filed Oct. 20, 2017, which in turn claims priority to French Patent Application No. 1660231 filed Oct. 21, 2016, the entire contents of all applications are incorporated herein by reference in their entireties. 
     TECHNICAL FIELD 
     The present invention relates to a method for forming a functionalised assembly guide intended for the self-assembly of a block copolymer. The present invention also relates to a graphoepitaxy method using a functionalised assembly guide obtained by such a method. 
     PRIOR ART 
     The need for methods making it possible to produce patterns of nanometric dimensions has increased considerably over recent years, on account of the tendency towards miniaturisation of electronic components. 
     Among emerging lithography technologies, it is possible to cite techniques of directed self-assembly (DSA) of block copolymers. Block copolymers are polymers in which two repeating units, a monomer A and a monomer B, form chains bound together by a covalent bond. When the chains are given sufficient mobility, for example by heating these block copolymers, the chains of monomer A and the chains of monomer B have a tendency to separate into phases or blocks of polymer and to reorganise into specific conformations, which depend notably on the ratio between the monomer A and the monomer B. As a function of this ratio, it is possible to have spheres of A in a matrix of B, or cylinders of A in a matrix of B, or instead intercalated lamella of A and lamella of B. 
     Block copolymers thus have the property of forming polymer patterns that can be controlled thanks to the ratio of the monomers A and B. The periodicity of these patterns is directly linked to the molar mass of the block copolymers. Thus, by controlling this molar mass, it is possible to control the resolution of the polymer patterns formed by the block copolymers. 
     The properties of block copolymers are exploited within the scope of graphoepitaxy methods. Graphoepitaxy consists in forming primary patterns called “guide patterns” on the surface of a substrate, each guide pattern delimiting a cavity inside of which a block copolymer layer is deposited. The guide patterns make it possible to control the organisation of the copolymer blocks to form secondary patterns of higher resolution inside the cavities. Indeed, the mechanical strain exerted on the copolymer layer by the guide patterns and the chemical interactions of the copolymer with the sides of the guide patterns favour the separation of the phases. The guide patterns are conventionally formed by photolithography in a resin layer, and optionally, transferred into a hard mask. 
     It is often difficult to control the orientation of the secondary patterns relative to a guide pattern. Indeed, the orientation of the secondary patterns relative to the guide pattern depends on the interactions of the blocks of monomer with the surface of the guide pattern, with the substrate and with air. In the case of the PS-b-PMMA block copolymer, the interactions of the two phases—polystyrene (PS) and polymethyl methacrylate (PMMA)—with air are equivalent. 
     If the bottom and the side walls of the cavity have the same preferential affinity with one of the two phases of the block copolymer, for example PS, then this phase will extend over the sides of the guide pattern and over the substrate. In other words, the other phase of the copolymer, PMMA, will not extend over the whole height of the guide pattern on account of a residual layer of PS situated at the interface with the substrate. Yet this residual layer is detrimental for the transfer, into the substrate, of holes obtained by the selective etching of the PMMA phase. 
     The most advantageous configuration is to generate a cavity having a neutral bottom (equivalent interactions of the two blocks with the substrate) and side walls having a preferential affinity with one of the two polymer phases. In this case, the other polymer phase (that which will be etched selectively) extends up to the interface with the substrate. 
     A control of the surface affinities, which is selective between the bottom and the side walls of the cavity, is consequently necessary. 
     A conventional technique consists in grafting a neutralisation layer to the bottom of the cavity, whereas the walls of the cavity remain bare (the guide pattern having, by default, a preferential affinity for one of the phases). To do so, a random polymer is diluted, then deposited by spin coating inside the guide pattern. This technique does not allow the possibility of selecting the affinity of the side walls of the cavities, because this is dictated by the material of which the cavity is made. Furthermore, any treatment (deposition, plasma, etc.) aiming to modify the affinity of the walls of the cavity will also affect the bottom of the cavity. In addition, this technique gives good results for a single cavity or for several cavities of same dimensions regularly distributed on the substrate. However, from the moment that the density of the cavities varies within the assembly guide, the random polymer layer is also deposited on the side walls of the cavities in the least dense zones of the assembly guide. 
       FIGS. 1A and 1B  illustrate the steps of a method for forming a functionalised assembly guide  100 , as described in the document [“Soft Graphoexpitaxy of Block Copolymer Assembly with Disposable Photoresist”, Seong-Jun Jeong et al., Nano Letters, vol. 9, No. 6, 2300-2305, 2009]. At the step of  FIG. 1A , a substrate  101  is covered with a neutralisation layer  102  made of random copolymer, then a negative tone development resist resin layer  103 . The assembly guide  100  is formed by photolithography, by exposing a part only of the resin layer  103  to deep ultraviolet (DUV) then by developing the resin layer. During the development step represented by  FIG. 1B , the portions of the resin layer  103  not exposed to the ultraviolet are removed in order to form the cavities  104 . The neutralisation layer  102  constitutes the bottom of the cavities  104 . 
     This second technique resolves the problem linked to the variable density of cavities, but has two major drawbacks. The use of a resin to form the assembly guide does not always allow the possibility of selecting the affinity of the side walls of the cavities, because this is dictated by the chemistry of the resin. For example, for the block copolymer PS-b-PMMA, the negative tone development resist resin of the aforementioned document has an affinity for polystyrene. Any treatment (deposition, plasma, etc.) aiming to modify the affinity of the walls of the cavities will also affect the bottom of the cavities. Thus, control of the interactions with the block copolymer deposited in the cavities will be limited in this method of the prior art. Besides, the resin has poor thermal resistance which reduces the possibilities of annealing for the assembly of the block copolymer or implies dimensional variations of the assembly guide (the sides made of resin of the assembly guide are deformed on heating). 
     SUMMARY OF THE INVENTION 
     It therefore exists a need to provide a method for forming an assembly guide making it possible to select freely the chemical affinities of the bottom and the side walls of the cavities with regard to the block copolymer. 
     According to the invention, this need tends to be satisfied by providing the following steps:
         forming on the surface of a substrate a functionalisation layer made of a first material having a first chemical affinity with regard to the block copolymer;   forming on the functionalisation layer a first mask comprising at least one recess;   depositing on the functionalisation layer a second material having a second chemical affinity with regard to the block copolymer, in such a way as to fill said at least one recess of the first mask;   selectively etching the first mask relative to the first and second materials, thereby forming at least one guide pattern made of the second material arranged on the functionalisation layer.       

     The term “assembly guide” used above designates a structure making it possible to guide (or direct) the self-assembly of the block copolymer. This structure comprises the functionalisation layer formed of the first material and one or more guide patterns formed of the second material. 
     The guide patterns, comprised of the second material, thereby correspond to the complementary part of the mask formed on the functionalisation layer. Depending on their number and their geometry, fixed by the number and the shape of the recesses of the mask, the guide patterns delimit (at least in part) one or more cavities intended to be filled with a block copolymer. The bottom of each cavity is comprised of the functionalisation layer formed of the first material and consequently has the first chemical affinity. The side walls of the cavity are formed by the sides of the guide pattern made of second material and consequently have the second chemical affinity. 
     The chemical affinity of the side walls of the cavity is thus no longer dictated by the material of the mask, but by the second material deposited in the recess of the mask. This second material may be specially selected as a function of the desired affinity. Thus, the formation method according to the invention enables great freedom in the control of interactions with the block copolymer. This freedom with regard to the affinity of the walls is not achieved to the detriment of the affinity of the cavity bottom since this bottom is protected during the greater part of the method by the solid parts of the mask. 
     This method also offers the possibility, to form the assembly guide, of selecting a second material having good thermal resistance, that is to say capable of withstanding high temperatures without degrading or deforming. This makes it possible to resort, for the remainder of the graphoepitaxy method, to assembly or grafting annealings at higher temperature or for longer duration. 
     Thanks to this method, it is henceforth possible to form easily an assembly guide comprising at least one cavity of which the walls have a chemical affinity different from that of the bottom of the cavity, in other words a second affinity different from the first affinity. 
     Advantageously, the second material is deposited in such a way as to fill fully said at least one recess of the first mask and to form an extra thickness above the first mask. The method then comprises, before the step of etching the first mask, a step consisting in removing the extra thickness in order to uncover the first mask. 
     Thus, since the second material is deposited so as to extend beyond the mask (in which is printed the reverse of the assembly guide) then finally planed down to be able to remove the mask, the method according to the invention is insensitive to variations in the density of the recesses (i.e. the opening ratio) in the mask. 
     The first and second chemical affinities may be selected from the following possibilities: 
     preferential affinity for any one of the copolymer blocks; or 
     neutral, that is to say without preference for any one of the copolymer blocks (or equivalent for each of the copolymer blocks). 
     When a material or a surface, for example the bottom of the cavities, is designated “neutral” with regard to the block copolymer, this signifies that the interaction forces with the different copolymer blocks are equivalent. 
     According to a development of the method according to the invention, the first material is neutral relative to the blocks of the block copolymer and the second material has a preferential affinity for one of the blocks of the block copolymer. 
     Preferably, the first mask is etched by wet process. Thus, the chemical affinity of the functionalisation layer with regard to the block copolymer does not risk being modified. Conversely, in methods of the prior art, resort is often made to a step of etching or opening the mask by plasma, which alters the affinity of the functionalisation layer located under the mask. 
     According to a development, the formation method further comprises the following steps:
         forming on the functionalisation layer a second mask comprising at least one recess;   depositing a third material having a third chemical affinity with regard to the block copolymer, in such a way as to fill said at least one recess of the second mask; and   selectively etching the second mask relative to the first, second and third materials, thereby forming at least one pattern made of the third material arranged on the functionalisation layer.       

     The first and second masks are preferably formed successively in a same inorganic layer and etched simultaneously. Alternatively, the second mask is formed on the functionalisation layer after etching the first mask. 
     The method according to the invention may also have one or more of the characteristics below, considered individually or according to all technically possible combinations thereof:
         said at least one guide pattern delimits at least in part a cavity of depth comprised between 50 nm and 300 nm;   the extra thickness of the second material is removed by chemical mechanical planarization;   the extra thickness of the second material is removed by plasma etching;   the extra thickness of the second material has a flat surface and the second material is etched uniformly;   the functionalisation layer is formed of a polymer of which the surface energy corresponds to a neutrality regime of the block copolymer intended to occupy the cavities of the guide. It may notably be a random copolymer of same chemical nature (i.e. comprising the same monomers) as the block copolymer;   the second material is selected from a cross-linkable polymer (for example a styrene derivative or a methacrylate derivative), a silicon oxide, a silicon nitride, a titanium nitride, a metal such as gold, copper, platinum and palladium;   the first mask is a hard mask comprising an antireflective layer;   the first mask comprises a photosensitive resin layer;   the method further comprises the conformal deposition of a stop layer on the photosensitive resin layer; and   said at least one recess of the first mask and/or said at least one recess of the second mask open onto the functionalisation layer.       

     The invention also relates to a graphoepitaxy method comprising the formation of an assembly guide using the method described above, the assembly guide delimiting at least one cavity, and the deposition of a block copolymer inside the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Other characteristics and advantages of the invention will become clear from the description that is given thereof below, for indicative purposes and in no way limiting, with reference to the appended figures, among which: 
         FIGS. 1A and 1B  represent the steps of a method for forming a functionalised assembly guide according to the prior art; 
         FIG. 2  illustrates a functionalised assembly guide obtained by the formation method according to the invention; 
         FIGS. 3A to 3F  represent the steps S 1  to S 6  making it possible to manufacture the functionalised assembly guide of  FIG. 2 , according to a preferential embodiment of the method according to the invention; and 
         FIG. 4  represents a step S 2 ′ replacing steps S 2  and S 3  of  FIGS. 3B and 3C , according to an alternative embodiment of the method according to the invention; and 
         FIGS. 5A to 5E  represent optional steps S 7 -S 11  of the method according to the invention, following on from step S 5  of  FIG. 3E . 
     
    
    
     For greater clarity, identical or similar elements are marked by identical reference signs in all of the figures. 
     DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT 
     With reference to  FIG. 2 , the method according to the invention makes it possible to obtain on the surface of a substrate  200  a functionalised assembly guide  300  comprising one or more cavities  310 . Each cavity  310  is intended to be filled with a block copolymer in order to form therein at least one secondary pattern of high resolution, by directed self-assembly of the block copolymer. The bottom  311  of the cavities  310  is functionalised in such a way as to have a first chemical affinity with regard to the block copolymer, whereas the side walls  312  of the cavities are functionalised so as to have a second chemical affinity with regard to the block copolymer. The side walls  312  of the cavities extend, preferably, perpendicularly to the plane of the substrate  200 . 
     The cavities  310  of the assembly guide  300  may adopt different geometries. They may notably take the shape of a cylindrical well, a rectangular or elliptical well, a trench or any other shape suited to graphoepitaxy methods. The cavities  310  preferably have a depth comprised between 50 nm and 300 nm. 
       FIGS. 3A to 3F  represent the steps S 1  to S 6  making it possible to manufacture such a functionalised assembly guide  300 , according to a preferential embodiment of the method according to the invention. 
     The first step S 1 , illustrated by  FIG. 3A , consists in forming a functionalisation layer  210  on the surface of the substrate  200 . The functionalisation layer  210  is comprised of a first material, selected as a function of the chemical affinity that it is wished to give to the bottom of the cavities of the assembly guide. This first material is preferably a polymer material, graftable or cross-linkable, making it possible to control the surface energy of the substrate. The functionalisation layer  210  may also be formed of a self-assembled monolayer (SAM). Preferably, the functionalisation layer  210  has a substantially constant thickness, comprised between 2 nm and 15 nm. 
     Preferably, the functionalisation layer  210  is a neutralisation layer. In other words, the first material is neutral with regard to the block copolymer. This first material may be a polymer of which the surface energy corresponds to a neutrality regime of the block copolymer intended to occupy the cavities  310  of the guide  300 . It may notably be a random copolymer of same chemical nature (i.e. comprising the same monomers) as this block copolymer. 
     As an example, when the block copolymer is PS-b-PMMA of cylindrical morphology, the functionalisation layer  210  may be a layer of the random copolymer PS-r-PMMA, comprising 70% by weight of polystyrene (PS) and 30% by weight of polymethyl methacrylate (PMMA). When the block copolymer is PS-b-PMMA of lamellar morphology, the first functionalisation layer may be a PS-r-PMMA layer, comprising 50% by weight of PS and 50% by weight of PMMA. 
     Step S 1  of forming the functionalisation layer  210  preferably comprises an operation of depositing a layer formed of the first polymer material, for example by spin coating, an operation of fixing, also called grafting, the layer of the first polymer material onto the surface of the substrate  200 , and an operation of rinsing during which surplus polymer material (i.e. non-grafted material) is removed using a solvent. In an alternative embodiment, where the first polymer material is cross-linkable rather than graftable, the spin coating operation is followed by a thermal annealing in order to cross-link the first polymer material. 
     The substrate  200  comprises at least one active layer, in which it is provided to transfer the secondary patterns obtained by the assembly of the block copolymer, for example with the aim of forming contact holes. The substrate  200  is preferably comprised of a stack of several layers, comprising a base layer  201  made of semiconductor material, for example silicon, at least one active layer (for example made of oxide or nitride) and at least one organic layer (e.g. SOC, SOG, BARC, etc.). The number and the nature of the layers of the stack vary as a function of the envisaged application. 
     Steps S 2  and S 3 , illustrated by  FIGS. 3B and 3C , have the aim of forming a mask  220  on the functionalisation layer  210 . This mask  220  comprises one or more recesses, of which the shape and the dimensions correspond to the pattern(s) of the assembly guide  300  that it is wished to form. In other words, the solid parts of the mask  220  define the future cavities  310  of the assembly guide  300  (cf.  FIG. 2 ). In the example represented in  FIGS. 2 and 3 , it is wished to form an assembly guide  300  comprising two peripheral assembly patterns  320   a - 320   b  (in top view), that is to say of closed contour. The mask  220  then comprises two recesses  221   a - 221   b , also of peripheral shape (cf.  FIG. 3C ). Alternatively, the assembly patterns  320   a - 320   b  are lines that define between them a trench corresponding to the cavity  310 . 
     The mask  220  is, in this embodiment of the method, a hard mask formed by photolithography and etching. At step S 2  of  FIG. 3B , the functionalisation layer  210  is covered with at least one inorganic material layer  222 , then a photosensitive resin layer  223 . The inorganic material layer  222  is preferably an anti-reflective layer, for example a Silicon Anti-Reflective Coating (SiARC), in order to improve the quality of the photolithography. The photosensitive resin layer  223  is next exposed and developed, in order to reveal the future patterns of the assembly guide. At step S 3  ( FIG. 3C ), the patterns printed in the resin layer  223  are transferred into the underlying SiARC layer  222 , thereby forming the recesses  221   a - 221   b  of the mask  220 . For this transfer, a dry etching method, by plasma, may be employed. 
     The recesses  221   a - 221   b  of the mask  220  advantageously open onto the functionalisation layer  210 . This implies etching the SiARC layer  222  over its whole height during step S 3  of  FIG. 3C . In this way, the side walls of the cavities of the assembly guide will be entirely functionalised. 
     During the step of etching S 3  the recesses  221   a - 221   b  ( FIG. 3C ), the chemical affinity of the portions of the functionalisation layer  210  situated at the bottom of the recesses may have been modified by the plasma. This will not however have any incidence on the assembly of the block copolymer, because these portions will not constitute the bottom of the cavities  310  and thus will not be in contact with the block copolymer. As a reminder, the bottom of the future cavities is situated under the solid parts of the mask  220 . 
     Step S 4  of  FIG. 3D  consists in depositing a second material  230  on the functionalisation layer  210  and the mask  220 . This second material  230  is selected as a function of the chemical affinity that it is wished to give to the guide patterns  320   a - 320   b , of which the sides will form the side walls of the cavities  310  (cf.  FIG. 2 ). The second material  230  may be a cross-linkable polymer material, like the first material of the functionalisation layer  210 , a silicon oxide, a silicon nitride, a titanium nitride or a metal such as gold, copper, platinum and palladium. 
     Preferably, the second material  230  has a preferential affinity for one of the copolymer blocks. For example, in the case of the di-block copolymer PS-b-PMMA, the second material  230  may be PMMA-affine or PS-affine. Styrene derivatives and methacrylate derivatives are respectively examples of PS-affine and PMMA-affine material. Coupled to the “neutral” character of the functionalisation layer  210 , this preferential affinity with regard to one of the copolymer blocks makes it possible to orient the secondary patterns of the block copolymer (made of PS or PMMA, depending on the case) perpendicularly to the substrate  200 . 
     When the mask has a uniform opening ratio, it is possible to only fill partially the recesses  221   a - 221   b  of the mask  220 . The thickness of the block copolymer layer in the recesses  221   a - 221   b  will be substantially constant. 
     In the configuration illustrated in  FIG. 3D , where the mask has a non-uniform opening ratio (in this particular instance two recesses  221   b  wider than the two recesses  221   a ), the second material  230  is advantageously deposited in such a way as to fill entirely the recesses  221   a - 221   b  of the mask  200  and to form an extra thickness layer  231  above the mask. The deposition conditions are advantageously selected so as to obtain a uniform extra thickness layer  231 , that is to say of substantially constant thickness. To do so, the extra thickness  231  is preferably equal to around then times the depth of the recesses  221   a - 221   b , or even greater than ten times this depth. For example, for recesses of around 35 nm depth, it will be sought to obtain an extra thickness of the order of 300 nm. 
     The deposition of the second material  230  is advantageously carried out by spin coating. A solution containing a solvent and the second material (typically a polymer) is spread out over the substrate  200  (covered with the functionalisation layer  210  and the mask  220 ), by centrifugal force. This deposition mode is particularly suited to levelling out a surface topography, such as that generated by the recesses of the mask  220 , and thus to obtain a flat surface. Moreover, it may be carried out at ambient temperature. The risk of degrading the functionalisation layer  210  made of polymer material is then zero. 
     Other deposition modes may be envisaged according to the nature of the second material, for example PECVD (Plasma-Enhanced Chemical Vapour Deposition, notably in the case of oxides) and CVD (Chemical Vapour Deposition). The deposition preferably takes place at low temperature (&lt;300° C.) in order not to alter the functionalisation layer  210 . 
     Then, at step S 5  of  FIG. 3 , the extra thickness layer  231  formed of the second material  230  is removed in order to uncover the mask  220 . The removal of the second material  230  is carried out preferably down to the upper face of the mask  220 , in order to obtain patterns  320   a - 320   b  made of second material of same thickness as the mask  220 . In other words, the patterns  320   a - 320   b  of  FIG. 3E  are entirely contained in the recesses of the mask  220 . Thus, this step S 5  may be called “planarization step”, with reference to the flat surface formed by the patterns  320   a - 320   b  of the second material and the mask  220 . 
     In an embodiment of step S 5 , the extra thickness layer  231  made of second material is removed by chemical mechanical planarization (CMP). In this case, to obtain a flat surface at the end of step S 5 , it is not very important that the extra thickness layer  231  is initially of constant thickness. 
     In an alternative embodiment, the extra thickness layer  231  is removed by plasma etching. The etching chemistry is advantageously selected so as to obtain good etching selectivity relative to the SiARC of the mask  220 . This makes it possible to better control the removal of the second material while stopping selectively on the mask  220 . The mask  220  then constitutes an etching stop layer. As an example, when the extra thickness layer is an organic layer, an oxidising chemistry (based on O 2 , mixed with N 2  for example) or a reducing chemistry (based on H 2 , mixed with N 2  for example) may be used. 
     Since the thickness etched by the plasma is substantially the same at all points of the substrate, it is preferable to start from a uniform extra thickness layer  231  to end up with a planeness between the solid parts of the mask  220  and the patterns  320   a - 320   b  of the second material. An extra thickness of the order of—or even greater than—ten times the depth of the recesses will be selected for this. 
     It is also possible to combine chemical mechanical planarization (CMP) and plasma etching to remove the extra thickness layer  231 . It is moreover preferable to firstly carry out a CMP operation to roughly cut down the extra thickness layer  231 , then an operation of selective plasma etching, in order to finish the removal while stopping with precision on the mask  220 . 
     Thus, by making the second material  230  extend beyond the mask  220  at step S 4 , then by planing it down to the height of the mask  220  at step S 5 , it is guaranteed that all the patterns  320   a - 320   b  of the assembly guide will have the same thickness on the functionalisation layer  210 , whatever their number, their dimensions and their distribution on the functionalisation layer  210 . 
     Finally, at step S 6  ( FIG. 3F ), the mask is selectively etched relative to the first material of the functionalisation layer  210  and the second material, in order to release the patterns  320   a - 320   b  of the assembly guide  300 . This step may be qualified as inversion step, because the guide patterns  320   a - 320   b  are the negative of the patterns printed in the resin layer  223  at step S 2  of  FIG. 3B . 
     In order not to modify the chemical affinity of the portions of the functionalisation layer  210  which constitute the bottom  311  of the cavities  310 , the SiARC mask is advantageously etched by wet process, for example in a hydrofluoric acid bath. Indeed, other types of etching and notably plasma etching, although they may be envisaged, would risk modifying the chemical affinity of the bottom  311  of the cavities  310 . 
     A particular exemplary embodiment of steps S 1  to S 6  ( FIGS. 3A to 3F ) is given hereafter. The substrate  200  comprises, from bottom to top, a bulk silicon layer  201 , a silicon dioxide (SiO 2 ) layer  202  and a spin-on-carbon (SOC) layer  203 . 
     Firstly, at step S 1  ( FIG. 3A ), over around 30 nm thickness is deposited a solution of PS-r-PMMA diluted to 2% in weight concentration in propylene glycol methyl ether acetate (PGMEA). This deposition is followed by a grafting annealing at 230° C. for 10 min and rinsing with PGMEA, thereby forming a grafted PS-r-PMMA layer of around 8 nm thickness on the carbon layer  203 . This grafted PS-r-PMMA layer will serve as neutralisation layer  210 . 
     In S 2  ( FIG. 3B ), the PS-r-PMMA layer  210  is covered with a SiARC layer  222  of around 30 nm thickness, deposited by spin coating and annealed at 215° C. for 5 min, then with a negative tone development resist resin layer  223 . Then, line type patterns are printed in the resin layer, of around 100 nm thickness. 
     In S 3  ( FIG. 3C ), the patterns printed in the resin layer  223  are transferred by etching into the entire thickness of the SiARC layer  222 , in order to form the mask  220 . The transfer of the patterns may be obtained by means of a fluorocarbon plasma (CH x F y ), for example CH 2 F 2  (20 sccm)+CF 4  (40 sccm)+He (240 sccm) for 15 seconds. It may be carried out in a capacitive or inductive coupling reactor. 
     In S 4  ( FIG. 3D ), a cross-linkable polystyrene  230 , diluted to more than 5% in weight concentration in PGMEA, is deposited in the recesses of the SiARC mask  220 . The diluted polystyrene is deposited by spin coating at a speed of 1500 rpm for 30 seconds. The mask  220  is then covered with a polystyrene layer  231  of substantially constant thickness, of the order of 300 nm. 
     In S 5  ( FIG. 3E ), the polystyrene layer  231  is etched by an oxygen or hydrogen based plasma diluted or not with gases such as N 2 , Ar, etc. until the SiARC mask  220  is reached. The etching comes to an end by detection of end of attack of the polystyrene layer  231  (by in situ monitoring of the wavelength corresponding to the etching of this polymer). The substrate is next taken to a temperature of 180° C. for 100 min to cross-link the polystyrene, which confers on the material good thermal resistance. 
     Finally, in S 6  ( FIG. 3F ), the SiARC mask  220  is etched in a hydrofluoric acid bath of weight concentration equal to 0.1% for 30 seconds. This type of wet etching leaves intact the PS-r-PMMA of the neutralisation layer  210  and the cross-linked polystyrene. The functionalised assembly guide  300  is then formed. It comprises polystyrene patterns  320   a - 320   b  and a neutralisation layer  210 , on which lie the polystyrene patterns (cf.  FIG. 3F ) 
       FIG. 4  represents an alternative embodiment S 2 ′ of the steps making it possible to form the mask  220  on the functionalisation layer  210  (S 2  and S 3 ). 
     In this alternative embodiment, the mask  220  comprises a photosensitive resin layer  224 , rather than an inorganic layer (hard mask). Thus, at step S 2 ′, the resin layer  224  is deposited on the functionalisation layer  210 , insolated then developed in order to form the recesses  221   a - 221   b  of the mask  220 . 
     Using a resin layer rather than an inorganic layer to form the mask makes it possible to reach more easily greater heights of guide patterns. Moreover, this alternative embodiment is cheaper, because the formation of the mask includes one fewer step (no transfer of the patterns formed into the resin layer). 
     On the other hand, a mask  220  made of resin, that is to say made of polymer material, may make the planing down by plasma of step S 5  more difficult. Indeed, when the second material  230  deposited in the recesses of the mask is also a polymer material, there may exist a lack of chemical contrast, which does not make it possible to remove the second material  230  selectively relative to the material of the mask  220 . To solve this problem, the resin layer  224  is advantageously covered, after development, with a thin stop layer  225  of “liner” type, in other words deposited in a conformal manner on the resin layer  224  and the functionalisation layer  210 . This stop layer  225 , preferably made of oxide or nitride, protects the resin layer  224  from the plasma etching of the second material. The stop layer  225  has, preferably, a thickness comprised between 5 nm and 10 nm. 
     After step S 5  of planing down the second material  230  ( FIG. 3E ), the stop layer  225  covering the resin mask  220  is removed, for example in a hydrofluoric (HF) acid bath, then the resin mask  220  is removed, preferably by means of a solvent (step S 6 ;  FIG. 3F ). 
       FIGS. 5A to 5E  represent an alternative embodiment of the method for forming a functionalised assembly guide according to the invention. In this alternative, a first series of guide patterns  320  is produced thanks to the steps S 1  to S 5  described previously in relation with  FIGS. 3A to 3E . This first series of patterns is going to make it possible to create one (or more) first cavity or cavities  310  of which the walls have an affinity corresponding to the material of these patterns. At the stage of  FIG. 3E , rather than removing the mask  220  (step S 6 ,  FIG. 3F ), it is next possible to produce a second mask  250  by creating new openings  251  in the SiARC layer  222 , by lithography and etching (preferably in a manner similar to steps S 2  and S 3 ). These openings  251  are going to make it possible to create a second series of patterns  330  delimiting one (or more) second cavity or cavities  340  of which the affinity of the walls is different from the affinity of the walls of the first cavity  310 . 
     To do so, at step S 7  of  FIG. 5A , the patterns  320  of the first series and the SiARC layer  222  that surrounds them are covered with a second photosensitive resin layer  253 . This photosensitive resin layer  253  is next exposed and developed, in order to reveal the future patterns of the second series. 
     At step S 8  ( FIG. 5B ), the patterns printed in the second resin layer  253  are transferred into the underlying SiARC layer  222 , thereby forming the second mask  250  comprising the recesses  251 . For this transfer, a dry etching method, by plasma, may be employed. The recesses  251  of the second mask  250  open onto the functionalisation layer  210 . 
     Alternatively, it is possible from the structure of  FIG. 3F  (i.e. once the first mask  220  has been removed) to deposit a new inorganic material layer to produce the second mask  250 . This inorganic material layer is preferably an antireflective layer, for example a Silicon Anti-Reflective Coating (SiARC). The deposition of a new inorganic layer makes it possible to confer on the second mask  250  better properties, since unlike the first SiARC layer  222 , the second SiARC layer will not have undergone an aggressive step, such as CMP or plasma etching to remove the second excess material. 
     Step S 9  of  FIG. 5C  consists in depositing a third material  260  on the functionalisation layer  210  and the second mask  250 . This third material  260  is selected as a function of the chemical affinity that it is wished to give to the guide patterns  330  of the second series, of which the sides will form the side walls of the second cavity  340 . The third material  260  may be of same nature as the second material  230  but will advantageously have an affinity with regard to the block copolymer different to that of the second material. 
     For example, if the second material  230  has a preferential affinity for one of the copolymer blocks, the third material  260  may have a preferential affinity with regard to the other block. Coupled to the “neutral” character of the functionalisation layer  210 , these preferential affinities will make it possible to orient the secondary patterns of the block copolymer perpendicularly to the substrate  200  along the walls of the cavities  310  and  340 , but with a different location of the blocks between the two cavities. 
     In another example, the second material  230  has a preferential affinity for one of the copolymer blocks and the third material  260  has a neutral character with regard to the copolymer, like the functionalisation layer  210 . In the first cavity  310  will then be obtained secondary patterns oriented perpendicularly to the substrate and parallel to the walls of the cavity, and in the second cavity  340 , secondary patterns oriented perpendicularly to the substrate  200  and perpendicularly to the walls of the cavity. 
     As illustrated in  FIG. 5C , the third material  260  is advantageously deposited in such a way as to fill entirely the recesses  251  of the second mask  250  and to form an extra thickness layer  261  above the second mask  250 . 
     At step S 10  of  FIG. 5D , the extra thickness layer  261  of the third material is removed by CMP and/or plasma etching in order to uncover the second mask  250  and to obtain the patterns  330  comprised of the third material. When a single and same SiARC layer  222  has been used to form successively the masks  220  and  250 , the patterns  330  of the second series are of same thickness as the patterns  320  of the first series. 
     Finally, in S 11  ( FIG. 5E ), the first mask  220  (of which the recesses are occupied by the patterns  320 ) and the second mask  250  (of which the recesses are occupied by the patterns  330 ) are etched selectively relative to the first material of the functionalisation layer  210  and to the second and third materials, in order to release the patterns  320 - 330  of the assembly guide. The SiARC masks  220  and  250 , here arranged side by side, are advantageously etched by wet process, for example in a hydrofluoric acid bath. 
     The method that has just been described thus makes it possible to manufacture one (or more) assembly guide(s) comprising at least one cavity of which the bottom is functionalised with a first chemical affinity (functionalisation layer  210 ), whereas the side walls are functionalised with a second chemical affinity (second material  230 ), or even a third chemical affinity (third material  260 ). 
     The assembly guide may next be used in a method for directed self-assembly (DSA) of block copolymers, and more particularly in a graphoepitaxy method, in order to generate patterns of very high resolution and density. This graphoepitaxy method comprises a step of depositing a block copolymer in the cavity (or cavities) of the assembly guide and a step of assembly of the block copolymer, for example by annealing. 
     This block copolymer may notably be selected from the following:
         PS-b-PMMA: polystyrene-block-polymethyl methacrylate;   PS-b-PLA: polystyrene-block-polylactic acid;   PS-b-PEO: polystyrene-block-polyethylene oxide;   PS-b-PDMS: polystyrene-block-polydimethylsiloxane;   PS-b-PMMA-b-PEO: polystyrene-block-polymethyl methacrylate-block-polyethylene oxide;   PS-b-P2VP: polystyrene-block-poly(2vinylpyridine).       

     Of course, the method for forming an assembly guide according to the invention is not limited to the embodiments described with reference to  FIGS. 3 to 5  and many variants and modifications will become clear to those skilled in the art. In particular, the first material of the functionalisation layer, the second material and the third material could have other compositions than those described previously. Similarly, other block copolymers could be used. 
     The first chemical affinity (cavity bottom) is not necessary different from the second chemical affinity (lateral wall of the cavities). Indeed, other combinations between the first and second chemical affinities are possible. The assembly guide obtained thanks to the method according to the invention thereby makes it possible to obtain multiple configurations for the secondary patterns of the block copolymer. 
     Finally, thanks to the method according to the invention, it is possible to form an assembly guide comprising one (or more) cavity or cavities of which certain walls have the second chemical affinity and other walls have a third chemical affinity (or even a fourth affinity, etc.). To do so, it is possible to apply the method of  FIGS. 5A to 5E  to form a part of the guide patterns delimiting this cavity with the second material and the other part of the guide patterns delimiting this same cavity with the third material.