# Patent

## Title
Method for directed self-assembly (DSA) of block copolymers

## Abstract

In directed self-assembly (DSA) of a block copolymer (BCP), a patterned sublayer on a substrate serves as a guiding chemical prepattern on which BCPs form more uniform and/or denser patterns. A layer of a blend of a BCP and functional homopolymers, referred to as inks, is deposited on the patterned sublayer and annealed to change the initial chemical prepattern to a 1:1-like chemical pattern that is more favorable to DSA. After annealing, the inks selectively distribute into blocks by DSA, and part of the inks graft on the substrate underneath the blocks. The BCP blend layer is then rinsed away, leaving the grafted inks. A second layer of BCP is then deposited and annealed as a second DSA step to form alternating lines of the BCP components. One of the BCP components is removed, leaving lines of the other BCP component as a mask for patterning the substrate.

## Background
RELATED APPLICATION
This application is a continuation of application Ser. No. 14/067,769 filed Oct. 30, 2013.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the directed self-assembly (DSA) of block copolymers (BCPs), and more particularly to the DSA of BCPs to make an etch mask for pattern transfer into a substrate.
2. Description of the Related Art
Directed self-assembly (DSA) of block copolymers (BCPs) has been proposed for making imprint templates. Imprint templates have application in making patterned-media magnetic recording disks and in semiconductor manufacturing, for example, for patterning parallel generally straight lines in MPU, DRAM and NAND flash devices. DSA of BCPs by use of a patterned sublayer for the BCP film is well-known. After the BCP components self-assemble on the patterned sublayer, one of the components is selectively removed, leaving the other component with the desired pattern, which can be used as an etch mask to transfer the pattern into an underlying substrate. The etched substrate can be used as an imprint template.
With the prior art method of DSA of BCP thin films, the quality of the self-assembled pattern depends upon a number of factors, including film thickness, the acceptable defect density, the density multiplication factor, the chemistry of the patterned sublayer and individual stripe width of the patterned sublayer. The design of a process to have a large process window for one parameter usually implies that the process window for the other parameters gets reduced or compromised. For example, thicker BCP films are desirable for pattern transfer into the template; however, a thicker film results in higher defect densities or in a lower tolerance for a high density multiplication factor. Conversely, if a higher density multiplication factor is desired, or if more tolerance is needed for the width of the stripes, the film thickness needs to be reduced considerably to avoid a large defect density.
What is needed is an improved method for DSA of BCPs that allows for a large process window.
SUMMARY OF THE INVENTION
Embodiments of the invention relate to a method to improve the process of DSA of BCPs, especially on chemical prepatterns. For a DSA process, a conventional lithography process is used to generate a patterned sublayer as a guiding chemical prepattern on which BCPs form more uniform and/or denser patterns. An additional DSA step uses a blend of BCP and functional homopolymers to change the initial chemical prepattern to a 1:1-like chemical pattern that is more favorable to DSA. The initial chemical prepattern can be generated by any conventional lithography process. Then a BCP blend with a small portion of functional homopolymers is deposited and annealed on the initial chemical prepattern. A functional homopolymer, referred to as an “ink”, is typically the same as one of the BCP blocks. After annealing, the BCP blend will be guided by the initial chemical prepattern. The inks will selectively distribute into blocks, and part of the inks will graft on the substrate underneath the blocks. The BCP blend layer is then rinsed away, leaving the grafted inks. When the grafted inks are selected to be longer than the brushes used in the initial chemical prepattern, they will form 1:1 chemical patterns in some regions that have the same geometry with BCP bulk morphology. Depending on the property of the initial chemical prepattern, the modified chemical pattern can be a fully 1:1, or a partially 1:1 chemical pattern. Either type of the modified chemical pattern will be acceptable for a second DSA of a BCP layer with a thickness of typically greater than L0. The resulting BCP layer will serve as mask for patterning other layers.
The method can significantly widen the process window of the DSA in comparison to prior art methods. The wider processing window can be first obtained by sacrificing one of the DSA parameters such as film thickness, e.g., to use a thin BCP blend layer with thickness substantially less than L0. The thin BCP film can form defect-free patterns that cannot be formed using a BCP film with a regular thickness equal to or greater than L0. The nearly perfect pattern will be “printed” on the initial chemical prepattern by the inks. Then a second DSA on the modified prepattern will be carried out without scarification on any parameter. Therefore, the method can reduce the requirements for the conventional lithography processes in terms of throughput, pattern pitch, pattern critical dimension, and pattern roughness. The method can also ease the strict restriction of the surface chemistry of the prepattern.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.



BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1A-1D are view illustrating the prior art method for making an imprint template using directed self-assembly (DSA) of block copolymers (BCPs).
FIGS. 2A-2G are illustrations of an embodiment of the invention, wherein FIGS. 2A, 2B, 2D, 2F and 2G are perspective views of the structure at various stages of the process, FIG. 2C is an expanded sectional view of a portion of FIG. 2B, and FIG. 2E is an expanded sectional view of a portion of FIG. 2D.
FIG. 3A is a perspective view showing a structure where L0=27 nm and LS=2L0 with a thickness of the additional BCP layer being about 30 nm.
FIG. 3B is a scanning electron microscopy (SEM) image of a top view of the structure of FIG. 3A but made according to the prior art method of DSA.
FIGS. 3C-3H are SEM images of a top view of structures made according to an embodiment of the method of this invention for values of W of 0.57L0, 0.69L0, 0.83L0, 0.93L0, 1.01L0 and 1.09L0, respectively.
FIG. 4A is a perspective view showing a structure where L0=27 nm, LS=5L0, W=2.5L0 and with a thickness of the additional BCP layer being about 30 nm.
FIG. 4B is a SEM image of a top view of the patterned and etched e-beam resist with cross-linked polystyrene XPS stripes underneath, showing the geometry of the chemical patterns with LS=135 nm and W≅67.5 nm.
FIG. 4C is a SEM image of a top view of the additional BCP layer with alternating parallel PS and PMMA lines.
FIG. 4D is a SEM image of chromium (Cr) lines formed by removal of the PMMA lines and dry lift-off using the remaining PS lines as a mask.
FIG. 5A is a SEM image of a top view of the patterned and etched e-beam resist with (XPS) stripes underneath, showing the geometry of the chemical patterns with LS=272.4 nm, W≅19 nm.
FIG. 5B is a SEM image of a top view of the additional BCP layer with a thickness of about 30 nm on chemical patterns with LS=266.1 nm.
FIG. 5C is a SEM image of a top view of the additional BCP layer with a thickness of about 30 nm on chemical patterns with LS=268.2 nm.
FIG. 5D is a SEM image of a top view of the additional BCP layer with a thickness of about 30 nm on chemical patterns with LS=270.0 nm.
FIG. 5E is a SEM image of a top view of the additional BCP layer with a thickness of about 30 nm on chemical patterns with LS=272.4 nm.
FIG. 5F is a SEM image of a top view of the additional BCP layer with a thickness of about 30 nm on chemical patterns with LS=274.2 nm.
FIG. 6A is a perspective view showing a structure where L0=27 nm, LS=nL0, W=0.6L0 and with a thickness of the additional BCP layer being about 2.08 L0.
FIG. 6B is a SEM image of a top view of the additional BCP layer with a thickness of about 56.25 nm on chemical patterns with LS=162 nm.
FIG. 6C is a SEM image of a top view of the additional BCP layer with a thickness of about 56.25 nm on chemical patterns with LS=189 nm.



DETAILED DESCRIPTION OF THE INVENTION
Self-assembling block copolymers (BCPs) have been proposed for creating periodic nanometer (nm) scale features. Self-assembling BCPs typically contain two or more different polymeric block components, for example components A and B, that are immiscible with one another. Under suitable conditions, the two or more immiscible polymeric block components separate into two or more different phases or microdomains on a nanometer scale and thereby form ordered patterns of isolated nano-sized structural units. There are many types of BCPs that can be used for forming the self-assembled periodic patterns. If one of the components A or B is selectively removable without having to remove the other, then an orderly arranged structural units of the un-removed component can be formed.
Specific examples of suitable BCPs that can be used for forming the self-assembled periodic patterns include, but are not limited to: poly(styrene-block-methyl methacrylate) (PS-b-PMMA), poly(ethylene oxide-block-isoprene) (PEO-b-PI), poly(ethylene oxide-block-butadiene) (PEO-b-PBD), poly(ethylene oxide-block-styrene) (PEO-b-PS), poly(ethylene oxide-block-methylmethacrylate) (PEO-b-PMMA), poly(ethyleneoxide-block-ethylethylene) (PEO-b-PEE), poly(styrene-block-vinylpyridine) (PS-b-PVP), poly(styrene-block-isoprene) (PS-b-PI), poly(styrene-block-butadiene) (PS-b-PBD), poly(styrene-block-ferrocenyldimethylsilane) (PS-b-PFS), poly(butadiene-block-vinylpyridine) (PBD-b-PVP), poly(isoprene-block-methyl methacrylate) (PI-b-PMMA), poly(styrene-block-lactic acid) (PS-b-PLA) and poly(styrene-block-dymethylsiloxane) (PS-b-PDMS).
The specific self-assembled periodic patterns formed by the BCP are determined by the molecular volume ratio between the first and second polymeric block components A and B. When the ratio of the molecular volume of the second polymeric block component B over the molecular volume of the first polymeric block component A is less than about 80:20 but greater than about 60:40, the BCP will form an ordered array of cylinders composed of the first polymeric block component A in a matrix composed of the second polymeric block component B. When the ratio of the molecular volume of the first polymeric block component A over the molecular volume of the second polymeric block component B is less than about 60:40 but is greater than about 40:60, the BCP will form alternating lamellae composed of the first and second polymeric block components A and B. When the ratio of B over A is greater than about 80:20 the BCP will form an ordered array of spheres in a matrix of the second component. For lamellar or cylinder forming BCPs, the orientation of the lamellae or the cylinders with respect to the substrate depends on the interfacial energies (wetting properties) of the block copolymer components at both the substrate interface and at the top interface. When one of the block components preferentially wets the substrate (or the top free interface) the block copolymers form layers parallel to the substrate. When the wetting properties at the interface are neutral to either block, then both block components can be in contact with the interface, facilitating the formation of block copolymer domains with perpendicular orientation. In practice, the wetting properties of the substrate are engineered by coating the substrate with “surface modification layers” that tune the wetting properties at the interface. Surface modification layers are usually made of polymer brushes or mats typically (but not necessarily) composed of a mixture of the constituent block materials of the BCP to be used.
The periodicity or natural pitch (L0) of the repeating structural units in the periodic pattern BCP components is determined by intrinsic polymeric properties such as the degree of polymerization N and the Flory-Huggins interaction parameter χ. L0 scales with the degree of polymerization N, which in turn correlates with the molecular weight M. Therefore, by adjusting the total molecular weight of the BCP, the natural pitch (L0) of the repeating structural units can be selected.
To form the self-assembled periodic patterns, the BCP is first dissolved in a suitable solvent system to form a BCP solution, which is then applied onto a surface to form a thin BCP layer, followed by annealing of the thin BCP layer, which causes phase separation between the different polymeric block components contained in the BCP. The solvent system used for dissolving the BCP and forming the BCP solution may comprise any suitable non-polar solvent, including, but not limited to: toluene, propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monomethyl ether (PGME), and acetone. The BCP solution can be applied to the substrate surface by any suitable techniques, including, but not limited to: spin casting, coating, spraying, ink coating, dip coating, etc. Preferably, the BCP solution is spin cast onto the substrate surface to form a thin BCP layer. After application of the thin BCP layer onto the substrate surface, the entire substrate is annealed to effectuate microphase segregation of the different block components contained by the BCP, thereby forming the periodic patterns with repeating structural units.
The BCP films in the above-described techniques self-assemble without any direction or guidance. This undirected self-assembly results in patterns with defects so it is not practical for applications that require long-range ordering, such as imprint templates for bit-patterned media or integrated circuits. However, directed self-assembly (DSA) of block copolymers (BCPs) has been proposed for making imprint templates for bit-patterned media or integrated circuits. DSA of BCPs by use of a patterned sublayer that acts as a chemical contrast pattern for the BCP film is well-known, as described for example in U.S. Pat. No. 7,976,715; U.S. Pat. No. 8,059,350; and U.S. Pat. No. 8,119,017. Pending application Ser. No. 13/627,492, filed Sep. 26, 2012 and assigned to the same assignee as this application, describes the use DSA of BCPs to make two submaster imprint templates, one with a pattern of generally radial lines, and the other with generally concentric rings, to make a master imprint template, which is then used to imprint patterned-media magnetic recording disks. Imprint templates made with DSA of BCPs have also been proposed for use in semiconductor manufacturing, for example, for patterning parallel generally straight lines in MPU, DRAM and NAND flash devices.
The prior art method for making an imprint template using DSA of BCPs will be described in general terms with FIGS. 1A-1D for an example where the template 50 will become an imprint template with protrusions 51 in a pattern of parallel bars. FIG. 1A is a side sectional view showing the template 50 with a patterned sublayer of generally parallel stripes 105 and intermediate substrate regions 106. Alternating A component polystyrene (PS) parallel lines 112 and B component (PMMA) parallel lines 115 are formed over the stripes 105 and substrate regions 106. The regions 106 can be exposed portions of the template 50 not covered by sublayer 105 or regions covered by a different sublayer. The sublayer has been patterned to direct the self-assembly of the BCP A and B components with a natural pitch of L0. In this example the stripes 105 have a width W of 1.5L0 and a stripe pitch LS of 2L0. In FIG. 1B, the portions of parallel lines 115, the B component (PMMA), are then selectively removed by a wet etch or a dry etch process. This leaves generally parallel lines 112 of the A component (PS) on the template 50. Then, a dry etch process is used to etch the template 50 to form recesses 52 using the parallel lines 112 as the etch mask. The material of parallel lines 112 and the remaining underlying sublayer 105 is then removed, leaving recesses 52 in template 50. This leaves the structure as shown in FIG. 1C, with a pattern of protrusions formed as parallel bars 51 and recesses formed as parallel bars 52. FIG. 1D is a side sectional view of the resulting imprint template.
With this prior art method of DSA of BCPs, the quality of the self-assembled pattern depends upon a number of factors, including the BCP film thickness, the acceptable defect density, the density multiplication factor “n” where LS=nL0, the chemistry of the prepatterns, and the width W of the individual stripes relative to L0. The design of a process to have a large process window for one parameter usually implies that the process window for the other parameters gets reduced or compromised. For example, thicker BCP films (about 1-2 times the L0) are desirable for pattern transfer into the template. However, a thicker BCP film results in higher defect densities or in a lower tolerance for a high density multiplication factor. Conversely, if a higher density multiplication factor is desired, or if more tolerance is needed for the width of the stripes, the BCP film thickness needs to be reduced considerably to avoid a large defect density. In general, it is known that when the chemical contrast patterns of the patterned sublayer are written at the same density as the resulting block copolymer pattern (i.e., LS=L0), the DSA results in the lowest defect densities with the largest tolerance for stripe pattern variation or for a wide range of film thicknesses. However, it can be difficult to form a chemical contrast pattern as the patterned sublayer with such a small stripe pitch. Thus it is desirable to have chemical contrast patterns with a high density multiplication factor (n greater than or equal to 2) that can still allow the use of thicker BCP films and result in low defect densities.
Embodiments of the method of this invention use intermediate steps between formation of the patterned sublayer and deposition of the BCP and thus replace the prior art method illustrated and described above with respect to FIG. 1A. The method is illustrated in FIGS. 2A-2F for an example where the BCP is poly(styrene-block-methyl methacrylate) (PS-b-PMMA) with L0=27 nm.
FIG. 2A is a perspective view of a substrate 200 with a patterned sublayer 205 that acts as a chemical contrast pattern. The substrate 200 may be formed of any suitable material, such as, but not limited to, single-crystal Si, amorphous Si, silica, fused quartz, silicon nitride, carbon, tantalum, molybdenum, chromium, alumina and sapphire.
In case of the DSA of perpendicularly oriented lamellae, a patterned sublayer 205 is typically a periodic pattern of generally parallel stripes 206 with a stripe pitch LS=nL0 and alternate stripes 207. One of the stripes (206) is typically preferentially wetted by one of the blocks, which is called guiding stripes. The chemistry of the alternate stripes (207) is tuned accordingly, depending on the chemistry of the guiding stripes 206. When stripes 206 and 207 have approximately the same height, they are typically referred to as chemical patterns. In the prior art, there are various types of chemical patterns specifically for PS-b-PMMA, which means different pairing of 206 and 207 stripes. For instance, the 206 and 207 stripes can be exposed silicon substrate and a PS-rich functionalized random copolymer PS-r-PMMA brush, respectively; exposed silicon substrate and a low molecular weight functionalized PS brush, respectively; exposed silicon substrate and a PS-rich crosslinked random copolymer PS-r-PMMA mat, respectively; crosslinked PMMA mat and a PS-rich functionalized random copolymer PS-r-PMMA brush, respectively; crosslinked PS mat and a PMMA-rich functionalized random copolymer PS-r-PMMA brush, respectively; e-beam resist HSQ (hydrogen silsesquioxane) and a PS-rich functionalized random copolymer PS-r-PMMA brush, respectively; a functionalized PS brush and a PMMA-rich crosslinked random copolymer PS-r-PMMA mat, respectively; or a functionalized PMMA brush and a PS-rich crosslinked random copolymer PS-r-PMMA mat, respectively.
A mat layer is a crosslinked polymer layer. The crosslinkable polymer may be spin-coated on the substrate to a thickness of 4-15 nm. The as-spun film is then annealed or treated by UV light for the cross-linking units to carry out the cross-linking. After cross-linking, the cross-linked polymer layer is typically referred as a mat layer. The film thickness is similar to that of the as-spun layer. A brush layer is a monolayer of a functional polymer grafted on the substrate. The functional polymer may be applied on the substrate to a thickness greater than 5 nm. The as-spun film is annealed for the functional groups to graft to the substrate surface. After annealing, any ungrafted brush material is rinsed away in a suitable solvent (e.g., toluene, PGMA, or NMP). The thickness of the brush layer is typically 1-15 nm, which is determined by the properties of the functional polymer such as chemistry, molecular weight, location of the functional group, etc. One of the main differences between these two is that a mat layer is denser than a brush layer and can prevent a further brush grafting on the underlying substrate surface. In embodiments of this invention, since the additive brush grafting on the original chemical patterns is required, at least part of the chemical patterns is a brush layer or bare substrate surface.
For DSA, additional steps are required to create a chemical contrast pattern. These steps may include e-beam lithography, photolithography or nanoimprint lithography and potentially a combination of polymer mats and brushes. Referring again to FIG. 2A, in the present example, a layer of cross-linked polystyrene XPS (205) with a thickness of 5-8 nm is first formed on a silicon substrate, followed by deposition of an e-beam resist layer on top. E-beam lithography is utilized to generate grating patterns with a stripe pitch LS=nL0. The resist pattern is then exposed to oxygen plasma etching so that the exposed portions of the XPS layer are etched away. Meanwhile, the width of the resulted XPS stripes (206) is also tuned by lateral etching. The remaining resist pattern is rinsed away in a suitable solvent (e.g., toluene, PGMA, or NMP). A functionalized random copolymer “PS-r-PMMA” with a hydroxyl (OH) group (e.g., PS-r-PMMA-OH) consisting of  ̃50% styrene is then spin-coated on the substrate and annealed. Since XPS stripes are dense, PS-r-PMMA-OH can only graft in the intermediate regions between XPS stripes (206). After rinsing away the ungrafted brush material in toluene or NMP, the remaining brush forms stripes 207 of grafted PS-r-PMMA-OH.
Next, in FIG. 2B and the expanded sectional view of FIG. 2C a solution of a BCP blended with homopolymers with functional groups is deposited, for example by spin-coating, as a thin film 210 onto the chemical pattern 205. Preferably the homopolymers with functional groups are the same as the polymers in the BCP and the functional groups are functional end groups. The preferred BCPs are PS-b-PMMA and polystyrene block poly 2-vinylpyridine (PS-b-P2VP). The preferred functional groups for the homopolymers are hydroxyl (OH) and amine (NH2). This layer is deposited to a thickness in the range of 10 to 20 nm, substantially less than L0. The functionalized homopolymers are sometimes called “inks” because they are added to the blend.
In the example of FIGS. 2A-2G, the BCP is PS-b-PMMA and the homopolymers are OH-terminated PS (item 220) and OH-terminated PMMA (item 225). The blend may be made up of 70-99% PS-b-PMMA and 30-1% inks. The ratio between PS-OH and PMMA-OH is typically chosen as the same as the ratio of PS block and PMMA block in the BCP. This type of blend of a BCP with the functionalized polymer inks 220, 225 has been used for molecular transfer printing. In that process the original self-assembled pattern that results after the blend is annealed is copied or transferred to a second top substrate. Molecular transfer printing is described in US20090260750 A1 and S. Ji, et al., “Molecular Transfer Printing Using Block Copolymers”, ACS Nano 2010, 4, (2) 599-609.
Next in FIG. 2D and the expanded sectional view of FIG. 2E the film 210 is annealed, for example by heating to 250° C. for at least 10 minutes. This results in a micro-phase separation (e.g., self-assembly into nanoscale domains) and allows the inks to be sequestered or phase separated into respective blocks of PS 240 and PMMA 245 within the BCP. Meanwhile, a fraction of the ink molecules will also react with the substrate regions 207, in this example the grafted PS-r-PMM-OH, thus becoming immobile and creating a pattern with the same geometry and feature size as the BCP pattern of PS 240 and PMMA 245. Since the thickness of BCP film 210 is substantially less than L0, the DSA process window is much wider. For example, defect-free patterns can be achieved with a density multiplication factor greater than 4, with lamellae pitch stretching or compressing for more than 5% for two times density multiplication, or with the width of XPS stripes greater than 0.7L0 for two times density multiplication. In comparison, if the film thickness is close to or greater than L0, defective patterns are expected at the same experimental conditions. However, the BCP film 210 is too thin to be useful for subsequent pattern transfer into substrate 200.
After the film 210 has been annealed, it is rinsed in a suitable solvent, for example in a solution of toluene or NMP, to remove the BCP and any functionalized polymers (inks) that are not bound to the substrate regions 207. This leaves the structure depicted in FIG. 2F with a patterned sublayer of stripes 206 of XPS mat material and intermediate regions 250 that contain the self-assembled pattern of bound PS-OH and PMMA-OH shown in the sectional view of FIG. 2E. After rinsing, region 250 will change to 1:1 chemical pattern. If the width of XPS stripes W≅0.5L0, the original 1:n chemical pattern is modified to 1:1. In cases of W≅1.5L0, 2.5L0, or other width greater than 0.5L0, the resulted chemical pattern will be a 1:1-like chemical pattern. Overall, the 1:1 chemical pattern in regions 250 will greatly increase the DSA process window in terms of film thickness, density multiplication factor, commensurability tolerance, etc.
Since 210 is too thin to be useful for pattern transfer, thus in FIG. 2G an additional layer 270 of BCP with a thickness greater than that of layer 210 is deposited, for example by spin-coating, over the patterned sublayer of stripes 206 and regions 250, and the additional layer of BCP is annealed by heating to about 250° C. for at least 2 minutes. This additional BCP layer 270 has a thickness preferably in the range of 25 to 200 nm. The underlying pattern of stripes 206 and regions 250 directs the BCP components to self-assemble into PS lines 260 and PMMA lines 265. The PS lines 260 and PMMA lines 265 self-assemble as lamellae perpendicular to the substrate.
If the BCP used in the solution deposited on the patterned sublayer in the step of FIGS. 2B and 2C was blended with the homopolymers of the BCP, e.g., if the BCP contains copolymers A and B and the homopolymers with functional groups are A and B, then this additional layer 270 of BCP is the same BCP. Thus in the example of FIG. 2G, the additional layer 270 of BCP is PS-b-PMMA. However, if the BCP used in the solution deposited on the patterned sublayer in the step of FIGS. 2B and 2C was blended with different homopolymers than are in the BCP, e.g., if the BCP contains copolymers A and B and the homopolymers with functional groups are polymers C and D, then this additional layer 270 is a BCP with polymers C and D.
After the structure shown in FIG. 2G is formed according to embodiments of the method of this invention, one of the BCP components can be removed, leaving the other component as an etch mask for transferring the pattern into substrate 200. These subsequent steps are known in the prior art, for example as shown and described above in FIGS. 1B-1D.
To determine the effect of ink length on the embodiments of the method of this invention, a set of experiments was performed with inks of varying polymer lengths. The PS-r-PMMA-OH, PS-OH, and PMMA-OH used in the experiments are all end-functionalized, thus the brush length is proportional to the molecular weight. The comparison between the length of PS-r-PMMA-OH and lengths of different inks can be tested by a water contact angle experiment. First, PS-r-PMMA-OH brush is formed on silicon substrates. On each of these substrates, one ink (either PS-OH, or PMMA-OH) with certain molecular weight was additionally grafted. Then the water contact angles of all samples were measured and compared. The contact angle of the original PS-r-PMMA-OH brush layer was 78.5°. If the substrate is additionally grafted by PS-OH with molecular weight of 1.2, 6, 10, or 17 kg/mol, the contact angle was 78.6°, 81.9°, 86.0°, or 89.4°, respectively. The contact angle of PS is  ̃89°, thus these results show that PS-OH with molecular weight of 17 kg/mol is much longer, PS-OH with molecular weight of 10 kg/mol is slightly longer, while PS-OH with molecular weight of 1.2 or 6 kg/mol is shorter than PS-r-PMMA-OH. The same experiments were carried for PMMA-OH with molecular weight of 9.5 or 30 kg/mol, and the contact angle changed to 73.4°, or 68.3° respectively. PMMA-OH with molecular weight of 30 kg/mol is much longer, while PMMA-OH with molecular weight of 9.5 kg/mol is slightly longer than PS-r-PMMA-OH brush, since the contact angle of PMMA is  ̃68°.
Another set of experiments was also performed to study the effect of the ink length. A series of identical chemical patterns with density multiplication factors of 2 through 9 were generated. The guiding stripes 206 are XPS lines with width of  ̃0.5L0. The stripes 207 are PS-r-PMMA-OH containing 50% styrene, which is the same brush used for the previous water contact angle experiments. Four samples of BCP or BCP-inks blend were used for layer 210 in FIG. 2B. Sample-A is pure PS-b-PMMA without inks; Sample-B is 90% PS-b-PMMA, 5% PS-OH with molecular weights of 6 kg/mol and 5% PMMA-OH with molecular weights of 6.5 kg/mol; Sample-C is 90% PS-b-PMMA, 5% PS-OH with molecular weights of 10 kg/mol and 5% PMMA-OH with molecular weights of 9.5 kg/mol; and Sample-D is 90% PS-b-PMMA, 5% PS-OH with molecular weights of 17 kg/mol and 5% PMMA-OH with molecular weights of 30 kg/mol. In all cases, the thickness of layer 210 is  ̃18 nm. After annealing, as shown in FIG. 2D, defect-free BCP pattern (layer 210) can be achieved on chemical patterns with density multiplication factor up to 6 or 7. Then the layer 210 is rinsed away in NMP, followed by deposition of 30 nm-thick pure BCP layer 270. The BCP layer 270 is then annealed. As a control, when no ink is used in layer 210, the maximum density multiplication factor is only about 3-4. When the inks are shorter than the PS-r-PMMA-OH, the 1:1 chemical pattern cannot form in regions 250, and the maximum density multiplication factor is about 3-4, similar to the control experiment. When the inks are longer than the PS-r-PMMA-OH, the longer molecules extend above the PS-r-PMMA-OH molecules to a height sufficient to allow for binding to the BCP components when the additional layer of BCP is deposited. The 1:1 chemical pattern can form in regions 250 and increase the DSA process window, e.g., the maximum density multiplication factor is 6-7, which is significantly better than the control experiment. Thus in the preferred embodiments the PS-OH and PMMA-OH molecules should be longer than the PS-r-PMMA-OH molecules. The length of a polymer brush highly depends on the chemistry, position of the functional group (at the end of the chain, on the side chain, on the main chain), and number of the functional groups. Typical molecular weight of a polymer brush used for DSA is  ̃1-100 kg/mol. In cases of using chemical prepatterns containing bare substrate surface, there is no requirement on the ink length.
DSA of BCP films with thickness greater than L0 on chemical patterns with wider range of W was demonstrated according to embodiments of the method of this invention. FIG. 3A is a perspective view of showing a structure where L0=27 nm and LS=2L0 with a thickness of the additional BCP layer being about 30 nm (equivalent to FIG. 2G). FIG. 3B is a scanning electron microscopy (SEM) image of a top view of this structure but made according to the prior art method of DSA, and clearly shows that the completed lines of PS and PMMA are not close to being parallel over any extended length. In contrast, FIGS. 3C-3H are SEM images of a top view of structures made according to embodiments of the method of this invention for values of W of 0.57L0, 0.69L0, 0.83L0, 0.93L0, 1.01L0 and 1.09L0, respectively. The inks used here were 5% PS-OH with molecular weights of 10 kg/mol and 5% PMMA-OH with molecular weights of 9.5 kg/mol. As shown, for FIGS. 3C-3F, all of the lines are nearly perfectly parallel without defects over an extended length.
Previous studies have shown that DSA works well when the width of the XPS guiding stripes W≅0.5L0 or 1.5L0, with 1 PS lamellae or 2 PS lamellae and 1 PMMA lamellae on top of the XPS guiding stripes, respectively. According to embodiments of the method of this invention, DSA works well also when W≅2.5L0, with 3 PS lamellae and 2 PMMA lamellae on top of the XPS guiding stripes. FIG. 4A is a perspective view showing a structure where L0=27 nm, LS=5L0, W≅2.5L0, n=5 and with a thickness of the additional BCP layer being about 30 nm. FIG. 4B is a SEM image of a top view of the oxygen plasma etched e-beam resist pattern with XPS stripes (206 in FIG. 2A) underneath. The resist pattern is then rinsed away and the intermediate regions between stripes 206 are then grafted with PS-r-PMMA-OH brush (stripes 207). FIG. 4C is a SEM image of a top view of the additional BCP layer (layer 270 in FIG. 2G) with alternating parallel PS lines and PMMA lines. FIG. 4D is a SEM image of chromium (Cr) lines formed by removal of the PMMA lines, deposition of a Cr layer, and dry lift-off of the Cr lines on top of the PS lines. FIGS. 4A-4D illustrate that embodiments of the method of the invention can be used to form generally parallel lines in a substrate for a relatively large stripe width (W=2.5L0) and a relatively large density multiplication factor (n=5). Based on these experimental results it is believed that W can have a width up to one-half the density multiplication factor times L0, i.e., W=(n/2)L0.
DSA with a high density multiplication factor was demonstrated according to embodiments of the method of this invention. A series of chemical patterns with density multiplication factors of 10 were generated. The chemical patterns have similar XPS guiding width W of 18-20 nm, identical PS-r-PMMA-OH brush as stripes 207, but various LS values of 266.1, 268.2, 270, 272.4, and 274.2 nm. FIG. 5A is a SEM image of a top view of the oxygen plasma etched e-beam resist pattern that was used to create the chemical pattern. FIGS. 5B-5F are top-down SEM images of the additional BCP layer (layer 270 in FIG. 2F) guided by chemical patterns with LS of 266.1, 268.2, 270, 272.4, and 274.2 nm, respectively. Nearly perfect BCP patterns are observed on chemical patterns with LS of 266.1, 268.2, 270, and 272.4 nm. When LS is 274.2 nm, a defective BCP pattern is observed.
Formation of a BCP layer with a thickness more than 2L0 on chemical patterns with high density multiplication factor was demonstrated according to embodiments of the method of this invention. FIG. 6A is a perspective view showing a structure where L0=27 nm, LS=nL0, W≅0.6L0 and with a thickness of the additional BCP layer being about 2.08L0. FIGS. 6B and 6C are top-down SEM images of the additional BCP layer (layer 270 in FIG. 2F) with a thickness of 56.25 nm guided by chemical patterns with Ls of 162 and 189 nm, respectively.
Thus the use of the thin film of bound self-assembled PS-OH and PMMA-OH as the patterned sublayer for the subsequent DSA of the additional BCP layer allows for a large design process window. A wide range of density multiplication factors and stripe widths can be used in the DSA to form nearly perfectly parallel lines of PS and PMMA with minimal defects.
While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.

## Claims

1. In a method for making a semiconductor device, a method for making a patterned mask on a substrate by use of directed self-assembly (DSA) of a block copolymer (BCP) having a natural pitch L0, the method comprising:
forming on the substrate a patterned sublayer with generally parallel and generally straight stripes having a stripe pitch Ls of approximately nL0, where n is an integer greater than or equal to 1;
depositing on the patterned sublayer a solution comprising a first BCP of polymers A and B without functional groups, a polymer C with a functional group and a polymer D with a functional group;
annealing said deposited solution to cause polymers C and D to self assemble and bind to the patterned sublayer;
removing the first BCP and unbound polymers, leaving the self-assembled bound polymers C and D;
depositing on the self-assembled bound polymers C and D a layer of a second BCP of polymers C and D without functional groups;
annealing said layer of second BCP to cause DSA of the second BCP into alternating lines of polymers C and D; and
removing the lines of one of the polymers of the second BCP, leaving the lines of the other of the polymers of the second BCP as a mask of generally parallel and generally straight lines with a line pitch of approximately L0.

2. The method of claim 1 wherein polymer A is identical to polymer C and polymer B is identical to polymer D, whereby the first BCP is identical to the second BCP.

3. The method of claim 1 wherein the functional groups are selected from OH and NH2.

4. The method of claim 1 wherein polymer C is polystyrene (PS), polymer D is poly(methyl methacrylate) (PMMA), and the second BCP is poly(styrene-block-methyl methacrylate) (PS-b-PMMA).

5. The method of claim 1 wherein n is greater than or equal to 2 and less than or equal to 10.

6. The method of claim 1 wherein the stripes have a width W greater than or equal to 0.5L0 and less than or equal to (n/2)L0.

7. The method of claim 1 wherein depositing the layer of second BCP comprises depositing the layer of second BCP to a thickness of at least 2L0.

8. The method of claim 1 further comprising, after removing the lines of one of the polymers of the second BCP, using the lines of the other of the polymers of the second BCP as an etch mask.

9. The method of claim 1 further comprising, after removing the lines of one of the polymers of the second BCP to leave the lines of the other of the polymers of the second BCP as a mask, depositing a layer of material over said mask and thereafter lifting off said mask, leaving generally parallel and generally straight lines of said deposited material.

10. The method of claim 1 wherein said alternating lines of first and second polymers form a pattern selected from parallel generally straight lines, generally radial lines, and generally concentric circular lines.

11. The method of claim 1 wherein the polymers C and D with functional groups are copolymers of a BCP.

12. The method of claim 1 wherein the stripes have a width W greater than or equal to 0.5L0 and less than or equal to (n/2)L0.

13. The method of claim 1 wherein n is greater than or equal to 2 and less than or equal to 10.

14. The method of claim 1 wherein said alternating lines of first and second polymers form a pattern selected from parallel generally straight lines, generally radial lines, and generally concentric circular lines.

15. A method of directed self-assembly (DSA) of a block copolymer (BCP) having a natural pitch L0 comprising:
providing a substrate;
forming on the substrate a patterned sublayer comprising generally parallel stripes of functionalized polymer brush material, the stripes having a stripe pitch Ls of approximately nL0, where n is an integer greater than or equal to 1;
depositing on the patterned sublayer a solution comprising a first BCP of polymers A and B without functional groups, a polymer C with a functional group and a polymer D with a functional group;
annealing said deposited solution to cause polymers C and D to self assemble and bind to the patterned sublayer;
removing the first BCP and unbound polymers, leaving the self-assembled bound polymers C and D;
depositing on the self-assembled bound polymers C and D a layer of a second BCP of polymers C and D without functional groups; and
annealing said layer of second BCP to cause DSA of the second BCP into alternating lines of polymers C and D, said alternating lines having a line pitch of approximately L0.

16. The method of claim 15 wherein the molecules of the polymer C with a functional group and the molecules of the polymer D with a functional group are longer than the molecules of the functionalized polymer brush material.

17. The method of 15 wherein the functionalized polymer brush material is a random copolymer brush material comprising polystyrene (PS) and poly(methyl methacrylate) (PMMA) having an OH end group (PS-r-PMMA-OH), wherein the polymer C with a functional group is PS having an OH end group (PS-OH), and wherein the polymer D with a functional group is PMMA having an OH end group (PMMA-OH).

18. The method of claim 17 wherein polymer A is identical to polymer C and polymer B is identical to polymer D, whereby the first BCP is identical to the second BCP.