Pattern forming method

According to one embodiment, a pattern forming method includes forming a physical guide that includes a first predetermined pattern in a first region on a lower layer film, and includes a second predetermined pattern and a dummy pattern in a second region on the lower layer film, forming a block polymer inside the physical guide, making the block polymer microphase-separated to form a pattern having a first polymer section and a second polymer section, removing the second polymer section to form a hole pattern, and processing the lower layer film after removal of the second polymer section, with the physical guide and the first polymer section used as a mask. Shapes of the hole patterns in the first and the second predetermined patterns are transferred to the lower layer film. A shape of the hole pattern in the dummy pattern is not transferred to the lower layer film.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims benefit of priority from the Japanese Patent Application No. 2012-159555, filed on Jul. 18, 2012, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a pattern forming method.

BACKGROUND

As a lithography technique in a semiconductor element manufacturing process, a double patterning technique by ArF-immersion exposure, EUV lithography, nanoimprinting and the like are known. A conventional lithography technique has held a variety of problems such as a cost increase and through-put deterioration, which have occurred with finer pattern processing.

Under such circumstances, application of directed self-assembly (DSA) to the lithography technique has been expected. Since DSA is generated by a voluntary behavior such as energy stabilization, a pattern with high dimensional accuracy can be formed. Especially, a technique of using microphase separation of a high-polymer block copolymer enables formation of periodic structures in a variety of shapes of several nm to several hundred nm by means of simple coating and an anneal process. The high-polymer block copolymer can be changed in shape to a spherical shape, a cylindrical shape, a lamella shape or the like in accordance with a composition ratio of blocks, and can be changed in size in accordance with a molecular weight, thereby forming a dot pattern, a hole or pillar pattern, line patterns or the like with a variety of dimensions.

Formation of a desired pattern in a broad range by use of DSA requires provision of a guide for controlling a generating location of a polymer phase formed by DSA. There are known as the guide a physical guide (grapho-epitaxy) that has a concavo-convex structure and forms a microphase separation pattern in its concave section, and a chemical guide (chemical-epitaxy) that is formed in a lower layer of the DSA material and controls, based on a difference in its surface energy, a forming location of the microphase separation pattern.

For example, a resist film is formed on a film to be processed, and a hole pattern to serve as the physical guide is formed in this resist film by photolithography. Coating of a block copolymer is applied so as to be embedded into the inside of the hole pattern, and then heated. This leads to microphase separation of the block copolymer into a first polymer section formed along a side wall of the hole pattern and a second polymer section formed in a midsection of the hole pattern. Subsequently, the second polymer section is selectively removed by irradiation with oxygen plasma, thereby to obtain a hole pattern reduced from the hole pattern formed in the resist film by photolithography. Then the film to be processed is processed using the resist film and the first polymer section as a mask.

However, when coating of the block copolymer is applied such that the block copolymer is appropriately embedded into the hole pattern, serving as the physical guide, in a region with a high pattern density of the hole pattern, the block copolymer is flooded from the hole pattern in a region with a low pattern density, and it has thus been not possible to obtain a desired microphase separation pattern. Conversely, when coating of the block copolymer is applied such that the block copolymer is appropriately embedded into the hole pattern in the region with a low pattern density, the block copolymer is not sufficiently embedded into the inside of the hole pattern in the region with a high pattern density, leading to formation of a microphase separation pattern with a small film thickness, and it has thus been not possible to obtain sufficient processing resistance.

DETAILED DESCRIPTION

According to one embodiment, a pattern forming method includes forming a physical guide that includes a first predetermined pattern in a first region on a lower layer film, and includes a second predetermined pattern and a dummy pattern in a second region on the lower layer film, forming a block polymer inside the physical guide, making the block polymer microphase-separated to form a pattern having a first polymer section and a second polymer section, removing the second polymer section to form a hole pattern, and processing the lower layer film after removal of the second polymer section, with the physical guide and the first polymer section used as a mask. Shapes of the hole patterns in the first predetermined pattern and the second predetermined pattern are transferred to the lower layer film. A shape of the hole pattern in the dummy pattern is not transferred to the lower layer film.

A pattern forming method according to the present embodiment will be described usingFIGS. 1 to 7.FIGS. 1,2and4to7are longitudinal sectional views, andFIG. 3is a top view.

First, as shown inFIG. 1, a hard mask102and a reflection preventive film103are sequentially formed on a film101to be processed. The film101to be processed is an oxide film having a film thickness of 300 nm, for example. The hard mask102is a carbon film having a film thickness of 100 nm and formed using CVD (chemical-vapor deposition), for example. Further, the reflection preventive film103is an oxide film having a film thickness of 15 nm and formed using CVD, for example.

Next, as shown inFIGS. 2A,2B and3, a resist film104having a film thickness of 120 nm is spin-coated onto the reflection preventive film103, which is then exposed to light with an exposure amount of 20 mJ/cm2by an ArF-immersion excimer laser and developed, thereby forming circular hole patterns105aand105bin the resist film104. A diameter of the hole pattern105ais 70 nm, for example, and a diameter of the hole pattern105bis 55 nm, for example. Ranges of values that can be taken as the diameters of the hole patterns105aand105bwill be described later.

The hole pattern105ahas a function to serve as a physical guide layer at the time of microphase separation of a block polymer that will be formed in a subsequent process. A microphase separation pattern formed inside the hole pattern105aincludes a pattern to be transferred to the film101to be processed.

Meanwhile, the hole pattern105bis a dummy pattern for adjusting a coverage ratio (or aperture ratio), and as shown inFIG. 3, it is formed in the vicinity of the hole pattern105ain a loose pattern region R2with a small number of hole patterns105a. Thereby, the coverage ratios (or aperture ratios) of a dense pattern region R1with a large number of hole patterns105aand the loose pattern region R2can be made at the same level.

It can be said here that the dense pattern region R1is a region with the resist film104having a lower coverage ratio (higher aperture ratio) than the loose pattern region R2in the case of not forming the dummy hole pattern105b. Further in the case of regarding the pattern that is transferred to the film101to be processed as a standard, it can be said that the dense pattern region R1is a region with a higher pattern density than the loose pattern region R2.

A cross section along a line A-A in the dense pattern region R1ofFIG. 3corresponds toFIG. 2A, and a cross section along a line B-B in the loose pattern region R2corresponds toFIG. 2B. Further,FIGS. 4A,5A,6A, and7A correspond to the cross section along the line A-A, andFIGS. 4B,5B,6B, and7B correspond to the cross section along the line B-B.

Next, as shown inFIGS. 4A and 4B, coating of a block polymer106is applied onto the resist film104. A random copolymer (PS-b-PMMA) of polyethylene (PS) and polymethyl methacrylate (PMMA), with a number average molecular weight (Mn) of the PS block/PMMA block being 4700/24000, is prepared and a solution of polyethylene glycol monoethyl ether acetate (PGMEA) containing the copolymer with a concentration of 1.0 wt % is spin-coated onto the resist film104at a revolution speed of 2000 rpm. Thereby, a block polymer106is embedded into the inside of the hole patterns105aand105b.

As shown inFIG. 4B, since the hole patterns105bas the dummy pattern for adjusting an aperture ratio are provided in the loose pattern region R2with a small number of hole patterns105a, it is possible to prevent the block polymer106from being flooded from the hole pattern105a.

Next, as shown inFIGS. 5A and 5B, the laminated body is placed on a hot plate and heated at 110° C. for 90 seconds, and further heated at 220° C. for 3 minutes in a nitrogen atmosphere. Thereby, the block polymer106is microphase-separated to form DSA phases109aand109bthat include first polymer sections107aand107bincluding first polymer block chains and second polymer sections108aand108bincluding second polymer block chains. For example, the first polymer sections107aand107bare formed (segregated) in side wall sections and bottom sections of the hole patterns105aand105b, and the second polymer sections108aand108bcontaining PMMA are formed in midsections of the hole patterns105aand105b. For example, a diameter of the second polymer section108ais 30 nm, and a diameter of the second polymer section108bis 20 nm.

Next, as shown inFIGS. 6A and 6B, the first polymer sections107aand107bare made to remain while the second polymer sections108aand108bare selectively removed by oxygen RIE (reactive ion etching), thereby forming hole patterns110aand110b. For example, the hole pattern110ahas a diameter of 30 nm and corresponds to one contracted from the hole pattern105aThe hole pattern110bhas a diameter of 20 nm, and corresponds to one contracted from the hole pattern105b.

Next, as shown inFIG. 7A,7B, the first polymer sections107aand107bhaving been made to remain and the resist film104are used as a mask, and the reflection preventive film103and the hard mask102are processed by RIE using fluorine gas. At this time, the hole pattern110ais transferred to the reflection preventive film103and the hard mask102, whereas the hole pattern110bis not transferred to the reflection preventive film103and the hard mask102.

The hole pattern110ahas a larger diameter than that of the hole pattern110b, and etching gas sufficiently reaches via the hole pattern110a, allowing removal of the first polymer section107ain a lower part of the hole pattern110a. For this reason, the hole pattern110ais transferred to the reflection preventive film103and the hard mask102.

On the other hand, the hole pattern110bhas a small diameter, and etching gas is not sufficiently distributed downward via the hole pattern110b. For this reason, the hole pattern110bis not transferred to the reflection preventive film103and the hard mask102.

Then the first polymer sections107aand107band the resist film104are removed, and the film101to be processed is processed using the hard mask102as a mask.

Next there will be described diameters of the hole pattern105aand the dummy hole pattern105bformed in the resist film104in the processes illustrated inFIGS. 2A,2B and3.

FIG. 8is a graph showing the relation between a size of the hole pattern formed in the resist film104and an aperture ratio of a pattern to be transferred to the hard mask102with respect to the hole pattern110a(110b) formed by selectively removing the second polymer section108a(108b). For example, when an aperture ratio of not higher than 40% is regarded as a state where “the pattern is not transferred”, it is found fromFIG. 8that the diameter of the dummy hole pattern105bshould be made smaller than 60 nm or larger than 76 nm.

When the diameter of the dummy hole pattern105bis smaller than 60 nm, as shown inFIGS. 6B and 7B, the first polymer section107bbelow the hole pattern110bis not sufficiently removed, and the pattern is hardly transferred to the hard mask102.

Further, when the diameter of the dummy hole pattern105bis larger than 76 nm, as shown inFIG. 9A, the second polymer section108bis formed with a small thickness in the midsection of the dummy hole pattern105b, and the first polymer section107bin the bottom section of the dummy hole pattern105bhas a large film thickness. As shown finFIG. 9B, even when such a second polymer section108bis selectively removed to form the hole pattern110b, the first polymer section107bbelow the hole pattern110bis not sufficiently removed in a subsequent RIE process using fluorine gas, and the pattern is hardly transferred to the hard mask102.

Accordingly, the diameter of the dummy hole pattern105bformed in the resist film104in the processes illustrated inFIGS. 2A,2B and3is preferably made smaller than 60 nm or larger than 76 nm. On the other hand, the diameter of the hole pattern105ais preferably made not larger than 60 nm and not smaller than 76 nm.

As thus described, according to the present embodiment, the dummy hole pattern105bfor adjusting an aperture ratio, which has such a size that the pattern is not transferred to the lower layer film, is formed in the loose pattern region R2with a small number of hole patterns105a, thereby allowing formation of an appropriate amount of block polymer106inside the hole pattern105ain the loose pattern region R2. This can lead to formation of the microphase separation pattern with desired film thickness and shape even when there is a difference in denseness (difference in aperture ratio) of the hole patterns105awhich are to serve as the physical guide depending on the regions.

Comparative Example

A pattern forming method according to a comparative example will be described usingFIGS. 10 to 14.FIGS. 10,11,13and14are longitudinal sectional views, andFIG. 12is a top view.

First, as shown inFIG. 10, a hard mask202and a reflection preventive film203are sequentially formed on a film201to be processed. For the film201to be processed, the hard mask202and the reflection preventive film203, similar materials to those for the film101to be processed, the hard mask102and the reflection preventive film103in the above first embodiment are respectively used.

Next, as shown inFIGS. 11A,11B and12, a resist film204is spin-coated onto the reflection preventive film203, which is then exposed to light and developed, to form a hole pattern205ain the resist film204. A diameter of the hole pattern205ais 70 nm, for example.

The hole pattern205ahas a function to serve as a physical guide layer at the time of microphase separation of a block polymer that will be formed in a subsequent process.

As shown inFIG. 12, a region R2is a loose pattern region with a small number of hole patterns205aas compared with the region R1. There is a difference in coverage ratio (or aperture ratio) between the dense pattern region R1with a large number of hole patterns205aand the loose pattern region R2.

It is to be noted that a cross section along a line C-C in the dense pattern region R1ofFIG. 12corresponds toFIG. 11A, and a cross section along a line D-D in the loose pattern region R2corresponds toFIG. 11B. Further,FIGS. 13A and 14Acorrespond to the cross section along the line C-C, andFIGS. 13B and 14Bcorrespond to the cross section along the line D-D.

Next, as shown inFIGS. 13A and 13B, coating of a block polymer206is applied onto the resist film204. A random copolymer (PS-b-PMMA) of polyethylene (PS) and polymethyl methacrylate (PMMA), with a number average molecular weight (Mn) of the PS block/PMMA block being 4700/24000, is prepared and a solution of polyethylene glycol monoethyl ether acetate (PGMEA) containing the copolymer with a concentration of 1.0 wt % is spin-coated onto the resist film204at a revolution speed of 2000 rpm.

As shown inFIG. 13A, when coating of the block polymer206is applied such that an appropriate amount of the block polymer206is embedded into the inside of the hole pattern205ain the dense pattern region R1, the block copolymer206is flooded from the hole pattern205ain the loose pattern region R2as shown inFIG. 13B.

When the block polymer206is microphase-separated by heating in a subsequent process, a desired microphase separation pattern cannot be obtained in the loose pattern region R2where the block copolymer206is flooded from the hole pattern205a.

On the contrary, when coating of the block polymer206is applied such that an appropriate amount of the block polymer206is embedded into the inside of the hole pattern205ain the loose pattern region R2as shown inFIG. 14B, the block copolymer206that is embedded into the inside of the hole pattern205ais small in amount in the dense pattern region R1as shown inFIG. 14A. When the block polymer206is microphase-separated by heating in a subsequent process, a microphase separation pattern formed in the dense pattern region R1has a small film thickness, and sufficient processing resistance cannot be obtained.

On the other hand, according to the foregoing present embodiment, the dummy hole pattern105bfor adjusting an aperture ratio, which has such a size that the pattern is not transferred to the lower layer film, is formed in the loose pattern region R2with a small number of hole patterns105a, whereby it is possible to prevent the block polymer106from being flooded from the hole pattern105ain the loose pattern region R2. Accordingly, an appropriate amount of block polymer106can be formed inside the hole pattern105aboth in the dense pattern region R1and the loose pattern region R2. Hence it is possible to form the microphase separation pattern with desired film thickness and shape even when there is a difference in denseness (difference in aperture ratio) of the hole patterns105a.

Although the dummy hole pattern105bhas been formed in the circular shape in the above embodiment, it may be formed in an elliptical shape as shown inFIG. 15A. Further, the dummy hole pattern105bmay be connected with the hole pattern105aas shown inFIG. 15Bso long as a desired microphase separation pattern is obtained.

Moreover, as shown inFIG. 16, the dummy hole pattern105bmay be formed as a terminal portion of a successive pattern formed by successive provision of a plurality of same patterns is regarded as the loose pattern region R2. At this time, a portion other than the terminal portion is regarded as the dense pattern region R1.

Further, the dummy hole pattern105bmay not pass through the resist film104. That is, a height of the dummy hole pattern105bmay be smaller than a film thickness of the resist film104.

Although the resist pattern to serve as the physical guide has been formed by ArF-immersion exposure in the above embodiment, it may be formed by a photolithography method such as EUV or nanoimprinting.

Further, not the resist pattern but the hard mask transferred with this pattern may be used as the physical guide. In this case, the film101to be processed is processed using the first polymer sections107aand107band the hard mask as the mask.

Further, although the first polymer sections107aand107bhave been formed in the side wall sections and the bottom sections of the hole patterns105aand105bin the above embodiment, the first polymer sections107aand107bmay be formed only in the side wall sections of the hole patterns105aand105b.

Moreover, although the case of forming the hole pattern has been described in the above embodiment, a line pattern may be formed. In this case, the physical guide has a rectangular shape, and a material to be microphase-separated in a lamella shape is used for the block polymer.