GB2501681A

GB2501681A - Nanoimprint lithographic methods

Info

Publication number: GB2501681A
Application number: GB1207464.7A
Authority: GB
Inventors: Felix Holzner; Armin W Knoll
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2012-04-30
Filing date: 2012-04-30
Publication date: 2013-11-06
Also published as: CN103376646A; DE102013206755B4; GB201207464D0; DE102013206755A1

Abstract

A nanoimprint lithographic method uses a mold 100 which comprises a topographic pattern 140 formed by structures 130 having nanometer scale dimensions. The nanoscale structures are heated above a decomposition temperature (Td) of a thermally decomposable polymer film 210 such as poly (phthalaldehyde) which is disposed on a substrate 220 facing the mold. The structures are brought into contact with the polymer film to thermally decompose portions which correspond to the pattern of the structures and the structures are removed from the polymer film. There may be a material layer 120 with low thermal conductivity to thermally insulate the structures from the body of the mold. The structures may be brought into contact with the substrate 220 to cool the structures and prevent further thermal decomposition of the polymer layer 210 to give a very clean pattern.

Description

NANOIMPRINT LITI-IOGRAPI-IIC METI-IODS

FIELD OF THE INVENTION

The invention relates in general to the field of nanoimprint lithographic methods, as well as related devices. ffi particular, it proposes new techniques for the fabrication of nanometer sized structures on a large scale.

BACKGROUND OF THE INVENTION

Nanoscale fabncation with high resolution and simultaneous high throughput is challenging.

Nanoimprint Lithography (or NIL) is a technology for patterning of nanoscale structures by means of molding or embossing. With NIL it is in principle possible to pattern very small features (e.g., shown down to 5 nrn features or less, limited only by the template) with a high throughput at low costs. This is why this technothgy is noted in the International Roadmap for Semiconductors (ITRS) as a potential technology for patterning of 32 nm and 22 nrn features. There exist various methods, which have all in common that they use a mold (or template) to transfer a pattern from a mold onto a substrate. NW finds applications for magnetic disc memory, biosensor microarrays, microfluidic channels and photonic crystal devices. However, NIL still faces technical problems. Some of them are illustrated through a bnef summary of two common NIL methods: hot embossing and Step and Flash Imprint Lithography.

In hot embossing NIL methods, a thermoplastic polymer film (e.g. PMMA) is deformed by the mold using high pressure and a temperature above the glass transition of the polymer.

Even so dot patterns as small as 6 nm has been patterned, this technique has some difficulties.

These are mainly the following: -the different theimal expansions of the polymer, the substrate and the mold lead to alignment problems; -incomplete template filling when patterning different features sizes; -low throughput because the process takes tong and needs temperature cycles; -degradation of the template due to the high pressures and temperatures; -difficulties to release small structures with high aspect ratios. Structures tend to rupture and stick to the master because of too high adhesion.

Step and Flash Imprint Lithography or S-FR. (trademark of Molecular Imprints. 11w.) was developed in 1999 and refers to a technique using low-viscosity monomer solutions and transparent templates (e.g. quartz). The liquid fills the structures in the mold without the need

I

of high pressures or temperatures. The monomer is then exposed to UV light through the transparent temp'ate. which causes it to photop&ymerize. Users have noted the following difficulties: -this technique is limited to fast photopolymenzable and low-viscosity matenals; -the mold must be fabricated from a transparent material; -the resist can shrink during curing; -small features with high aspect ratios may collapse; -the etch selectivity can be thw, hence a pattern transfer is difficult; -a release layer is needed to release the imprinted material from the template and to protect the template; -for features with different sizes an optimized droplet pattern of the resist must be printed with an ink-jet printer on the substrate.

Embodiments of the present invention aim at solving one or more of the above problems.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present invention is embodied as a nanoimprint lithographic method, comprising: providing: a mold comprising structures with dimensions on the nanoscale on a side of the mold, which structures form a topographic pattern; and a thermo-decomposable polymer film on a substrate, vis-à-vis said side of the mold, wherein the structures are heated above a decomposition temperature of the polymer film; bringing said structures into contact with the polymer film to thermally decompose portions thereof in correspondence with said structures; and removing the structures away from the polymer film.

In embodiments, the substrate is maintained at a temperature below a glass transition temperature of the polymer film, pnor to bnnging said structures into contact with the polymer film, and bringing said structures into contact with the polymer film preferably comprises approaching said structures toward the substrate to enable therma' contact therewith and cool the structures.

In variants, bringing said structures into contact with the polymer film is carried out to unzip and/or desorb molecules thereof. Preferably, the polymer film provided is such that bringing said structures into contact with the polymer film can be and is carried out to thermally decompose portions thereof according to an endothermic decomposition.

According to embodiments, the p&ymer film provided comprises a network of molecules cross-linked via intermolecular, non essentially covalent bonds, and bringing said structures into contact with the polymer film is carried out to clesorb molecules thereof.

Preferably, an average molecular mass of molecules in the polymer film provided is between Da and 2000 Da, more preferably in the range from 150 Da to 1000 Da, and said molecules are preferably cross-linked via hydrogen bonds.

According to other embodiments, the polymer film provided comprises a polymer material having polymer chains able to unzip upon thermal stimulation: bringing said structures into contact with the polymer film is carried out to unzip polymer chains of the polymer material in that case.

Preferably, the polymer film provided comprises polyKphthalaldehyde). The polymer film preferably has a glass transition temperature of 125°C ± 20°C and a thermal decomposition temperature of 150°C ± 30°C.

In embodiments, the mold provided comprises a body, preferably comprising crystalline silicon, and a material layer, the latter preferably comprising silicon oxide, between the body and the structures, said material layer having a thermal conductivity substantially lower than that of the body, preferably at least ten times lower, and more preferably at least fifty times lower.

Preferably, the respective linear thermal expansion coefficients of the material layer and the body differ by less than a factor of 5.

The material layer may have a thickness between 1 and 30 micrometers, preferably of 6 ± 4 micrometers.

In embodiments, one or more of the structures of the mold provided have a height, which, as measured in a direction perpendicu'ar to the polymer film, is arger than the average thickness of the polymer film, preferably larger than three times the average thickness of the polymer film.

According to embodiments, a temperature of the substrate is maintained below a glass transition temperature of the polymer film and the structures are heated at a temperature at which decomposition products of the polymer film are volatile, prior to bringing said structures into contact with the polymer film; bringing said structures into contact with the polymer film comprises approaching said structures onto the substrate to enaNe thermal contact therewith and cool the structure.

Preferably, a difference of temperature between the heated substrate and the heated structures prior to bringing said structures into contact with the polymer film is at least 100 °C, preferably at least 200 °C.

According to another aspect, the invention is embodied as a nanoimprint lithographic apparatus, comprising: a mold compnsing structures with dimensions on the nanoscale on a side of the mold, which structures form a topographic pattern; and a substrate, with a thermo-decomposable polymer film thereon, vis-à-vis said side of the mold, heating means configured to heat the structures above a decomposition temperature of the polymer film; and actuation means for bringing said structures into contact with the polymer film to thermally decompose portions thereof in correspondence with said structures, and removing the structures away from the polymer film.

In embodiments, the apparatus further comprises cooling means adapted for maintaining the substrate at a temperature below a glass transition temperature of the polymer film.

Devices, apparatuses and methods embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

-FIG. I schematically illustrates a vertical setup suitable for implementing steps of NIL methods according to embodiments; -FIG. 2. A -D illustrates steps of NIL methods, as involved in embodiments, together with approximate temperature profiles as they occur at various levels in components of the setup of FIG. 1; and -FIG. 3 is a flowchart depicting a sequence of steps as in embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The following description is structured as follows. First, general embodiments and high-level variants are described (sect. I). The next sections address specific embodiments (sect. 2 and 3).

1. General embodiments and high-level variants In reference to FIGS. 1 -3 a general aspect of the invention is first described, which concerns nanoimprint lithographic methods.

First, these methods use a mold 100, also called stamp or template. As seen in FIG. 1, the latter compnses a topographic pattern 140 on one side 101 thereof. This pattern is formed by imprinting structures 130 that have one or more dimensions on the nanoscale. These structures shall be heated to create a pattern on the imprint resist that reflects said topographic pattern 140. The structures 130 may accordingly be connected to heating means 104 allowing for heating the structures 130, and preferably to control the temperature at which structures are heated.

Second, on the other part 200, a polymer film 210 (the resist material) is placed on a substrate 220, facing the structures of the mold. This polymer film is thermo-decomposable, i.e., it may decompose into decomposition products upon suitable thermal stimulation. The structures are accordingly heated above a decomposition temperature Td of the polymer film, such that when bringing S20 said structures into contact with the polymer film, portions thereof that reflect said structures 130 and pattern 140 shall thermally decompose S30, typically into volatile decomposition products.

Finally, the structures can be removed S50 away from the polymer film. If necessary, pattern obtained thanks to the above thermal decomposition can be transfened to another layer, using any suitable transfer methods.

Like other NIL technologies, the above method has the potential of patterning high resolution features with high throughput. However, present techniques can be much faster, i.e., the underlying decomposition process works in the microsecond range (i.e., less than 10 microseconds); it is not the speed limiting factor. No high pressures are needed, whereas relatively low temperatures can be relied upon.

In addition, the above method does not present the issue of incomplete template filling when patterning different features sizes. There is also no need to adapt the resist material distribution to the pattern of template (as needed for S-FIL with ink-jet printing of the resist).

The resist material can be e.g. simply spin coated as a thin flat film. Furthermore, various template and resist combinations can be contemplated with present techniques. The process ensures that no adhesion occurs when retracting the stamp because the polymer material in contact with the template structures is evaporated. Therefore adhesion problems are avoided S like partial pattern transfer and conesponding template contamination.

In prefelTed embodiments, such as described in section 3 below, the substrate is maintained at a temperature b&ow a glass transition temperature Tg of the p&ymer film (and preferably at much lower temperatures), prior to bringing S20 said structures into contact with the polymer film. This limits the spread of material decomposed dunng the contact.

In addition, step 520 may possibly comprise approaching 540 said structures toward the substrate to enable thermal contact therewith and cool the structures.. As long as the mold structures 130 do not touch the substrate (or at least are sufficiently far to prevent substantial thermal exchanges with the substrate), the printing structures remain hot enough, i.e., above the decomposition temperature, allowing for decomposing the resist. Next, if the stamp is brought in firm contact with the substrate e.g.. exhibiting high thermally conduction, for example silicon, thermal exchanges occur and the printing structures 130 are cooled by the substrate 220 to a temperature below Tg. The temperature drop of the structures prevents further decomposition or melt induced flow of the polymer. whereby the process self-limits.

This allows for very clean patterns to be obtained.

Correspondingly with the above methods, apparatuses (10) suitable for implementing present methods comprise: a mold 100 with structures 130 and a substrate 220 with a thermo-decomposable polymer film 210 thereon, as described above. Such apparatuses further compnse suitable heating means 104 to heat the structures above the decomposition temperature Td of the polymer film, as well as actuation means 102, 240, 250 for enabling contact. In embodiments, apparatuses further comprise suited cooling means 230 for maintaining the substrate at a temperature below the glass transition temperature Tg of the polymer film, while urging the template against the resist 210.

Examples of resist materials best suited for the present methods are molecular glasses (or MG for short) or polyphthalaldehyde (hereafter PPA). Present inventors have realized that small amounts of these materials can be accurately removed when brought into contact with sharp NIL structures 130. So far, features with a half pitch down to 8 nm have successfully been imprinted with this method. Barely more material e.g.. PPA is removed than the shape of the structures 130 takes up. This means that the patterning resolution is only limited by the structure brought into contact with the thermo-decomposable materials.

More generally, the polymer film is preferably chosen such that molecules thereof can unzip andlor desorb upon thermal stimulation by the structures 130. Unzipping polymers allows for fast stimulation and decomposition. So does desorbing decomposition products of the polymer. Note that temperatures required to unzip an unzipping polymer suffice to evaporate the resulting decomposition products as the monomer units evaporate already at lower temperatures. Also, the polymer film is preferably designed to enable endothermic decomposition processes. Tt can be realized that such processes shall inherently prevent further heat spreading into the material and thus improve the imprint resolution.

Thus, beyond polyphthalaldehyde, other unzipping polymer may be used, i.e., materials that comprise polymer chains, which are able to unzip upon suitable thermal stimulation.

Accordingly, the film 210 can be thermally stimulated via structures 130 for triggering an unzipping reaction of polymer chains. Typically, a first degradation event triggers an unzipping effect, partial or total.

In the case of polyphthalaldehydes, an organocataytic approach to the polymerization is preferred, e.g., using dimeric I -lert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2A5,4A5-catenadi (phosphazene) (P2-/-Bu) phosphazene base as an anionic catalysts in presence of an alcoholic initiator. For example, a resulting polymer (comprising -200 monomer units equivalent to a molecular weight of 27 kDa) possess a low ceiling temperature and further facilitate the ability to create permanent patterns by selective therm&ysis. using heated structures. With such materials, deep patterns can be written with virtually no or small indentation force applied to the structure. This minimizes pattern distortion resulting from indenting or displacing the material. Furthermore, polymeric chains can be made of arbitrary length which offers substantial flexibility in tuning the material properties such as the glass temperature and solvent resistance. An additional advantage is that no tine-tuning of intermolecular forces is required, at variance with materials requiring stabilization from secondary structure such as hydrogen bonds. Particularly good imprints are obtained for a polymer film having a glass transition temperature of 125°C ± 20°C and a thermal decomposition temperature of 150°C ± 30°C.

In vanants to unzipping polymers, molecular glasses can be used, i.e., the matenal 210 comprises in that case molecules that are cross-linked via intermolecular (non essentially covalent) bonds. Such molecules can conveniently desorb when patterning the polymer material with heated structures 130. An average molecular mass of said molecules is preferably between 100 Da and 2000 Da, and more preferably in the range from 150 Da to 1000 Da, which offers enhanced desorbing properties. The film may be cross-linked via intermolecular bonds, such as van der Waals forces or 1-lydrogen bonds. When the structures 130, suitably heated, is urged against the surface of the film 210 and interacts therewith, the interaction is likely to desorb one or more molecules. The probe temperature and the time of exposure of the structures 130 to the surface can be adjusted to optimize desorption of molecules.

In all cases, it is advantageous to heat the structures 130 at a temperature at which decomposition products of the p&yrner film are volatile. Tn practice, a high difference of temperatures between the structures and the substrate is sought, e.g., more than 100 °C.

200 °C, possibly 300°C or even more.

Next, particularly preferred solutions make use of a mold compnsing a body 110, e.g., made of crystalline silicon, and a material layer 120, e.g., silicon oxide, as better seen in FIG. 1.

The layer 120 is between the body 110 and the structures 130. The material layer 120 has a thermal conductivity substantially tower than that of the body, e.g., ten times lower or more (preferably fifty or even hundred times tower).

In addition, the respective linear thermal expansion coefficients of the material layer 120 and the body 110 may ideally be as close as possible to each other. In practice, satisfactory results are obtained if they differ by less than a factor of 5.

On the other hand this material layer 120 is preferably made thin. i.e., thinner than the body layer 110, e.g.. between I and 30 micrometers. Very good results are achieved in practice with thicknesses of 6 ± 4 micrometers. In this respect, the following layer thicknesses are preferably contemplated: -body 110: between 100 and 800, preferably about 400 Rm; -buffer layer 120: between 1 and 30 Rm; -structures 130: larger than the polymer 210 thickness; -polymer film 210: between 20 nm and 1 m; and -substrate 220: between 100 and 800, preferably about 400 m.

Let us consider an example. The second layer 120 e.g., silicon oxide has a low thermal conductivity and a low thermal expansion. Because of the high thermal resistance of layer the temperature difference T1 -T2, see FIGS. 2.A -D will mainly drop at the level of this layer when the mold and the substrate come into contact (see FIGS. 2.B -C). This prevents heat spreading into the substrate and leaves the layer of thermo-decomposable material below the glass transition/melting temperature. The low thermal expansion reduces misalignment through the temperature drop. Misalignment is also reduced by using a layer that is e.g., only around 10 micrometers thick and hence lateral expansion is limited to layer 110.

Also, as implicitly touched above, it is advantageous to provide structures 130 with an aspect ratio, i.e., the height of the structures (as measured perpendicularly to the polymer film 210) should be larger than the average thickness of the polymer film. This height is preferably larger than two or three times the average polymer film thickness. Having such structures allows for leaving enough room for the thermo-decomposable material 2 I Oa to evaporate, as depicted in FIG. 2.B.

The above embodiments have been succinctly described in reference to the accompanying drawings and may accommodate a number of variants. In preferred embodiments, several combinations of the above features may be contemplated. Detailed examples are given in the next sections.

2. Specific embodiments of DRIL methods This section describes specific embodiments, hereafter denoted by "Dip and Run Imprint Lithography" or DRIL for short. As described above, use is made of a thin film of thermo-decomposable material brought onto a substrate. Materials such as molecular glasses (MG) or polyphthalaldehyde (UPA) can be used.

Only one single step to fabricate features on the wafer scale is needed. The template for DRIL may be fabricated using conventional nanofabrication techniques. It can be made of various materials, e.g., which are mechanical stable up to around 250°C. For example silicon is a suitable material.

The principle of DRIlL, can be explained in reference to FIG. I. The template 100 is heated and aligned parallel to the substrate side 200. Then the template is brought into contact with the polymer film 210 on the substrate 220. Wherever the thermo-decomposable material 210 on the substrate gets heated from the structures 130 on the hot template it will evaporate. The created gas molecules need space to evaporate. This space can be provided by open regions or cavities within the template, e.g., possibly arising from the structures 130 and patterns 140 they form. This process is improved when operated in vacuum, as more space is available for the decomposed gas.

The template can be moved either until a defined position that is controlled via piezo motors 102 or until the template touches the underlying substrate 220.

After reaching the target position the template is lifted again. The structures of the template are replicated in the layer of thermo-decomposable material and can be further processed or directly used for applications. The thermo-decomposable matenals mentioned above are very well suited for further processing, like for example reactive ion etching RIE).

In more detail, the decomposition reaction of the thermo-decomposable material occurs as soon as it reaches a distinct decomposition temperature Td. In the case of PPA as thermo-decomposable material the decomposition process of the polymer is endothernilc. This induces a self-limitation of the decomposition process, because the decomposition itself takes up heat. This lowers the temperature and therefore prevents spreading of heat. Also the following evaporation is endothermic. Therefore only the molecules that directly touch the heated structures 130 on the template decompose and evaporate. The gas flows out along the gap that is created through the hot structure. This gap with width d is preferably as small as possible in order to achieve high resolution. Ideally, only one layer of m6lecules is removed.

Vacuum will lower the heat conduction through this gap and will therefore help to keep the gap width d small.

A critical drawback of the prior art techniques as mentioned in the introduction is the fact that the (expensive) imprint master can be damaged or contaminated through the patterning process. On the contrary, in the present cases, the contamination of the thermo-decomposable resist shall not be problematic. Practically no residues of PPA stay on a silicon wafer, when the wafer with a thin film of PPA is heated to 215 °C or more. Sensitive surface analyses such as Tof-SfMS (Time-of-flight Secondary Ion Mass Spectrometry) were performed, which have confirmed this (not shown). Experimental results will be published in subsequent papers.

Note that in the device of FIG. 1, optional layers (between layers 210 and 220, or 220 and 230) might be used to adjust heat conduction. Similarly, other parameters may require (further) optimization, e.g., using trial-and-error.

3. Self-limiting thermal nanoimprint embodiments (cooled substrate) This section describes other specific embodiments, where the substrate is further maintained at a temperature below the glass transition temperature Tg of the polymer film, pnor to contact. In addition, a stamp/master is used, which is vertically structured into low and high thermal conductivity regions.

Thus, a heated master and a cooled substrate are used. As to be discussed in detafi below, the resulting structure allows for an optimal temperature distribution in the system during the imprint step: As long as the stamp does not touch the substrate, the printing features remain hot allowing for a dean decomposition of the resist. If the stamp is in firm contact with the substrate, the printing features get cooled by the substrate to a temperature below the glass transition temperature of the polymer.

The temperature drop occurs within the stamp, at the level of layer 120, see FIGS. 1 -2. This also occurs locally, if only one feature 130 touches the substrate 220. Therefore the setup of FIG. i can be used with high vertical forces enabling a good contact of master and substrate over the whole printing area. Since the bulk of the master stays at high temperature. thermal contraction is no issue.

FIG. 2.A illustrates the starting conditions with the mold iOU resting above the substrate 200 (as otherwise illustrated in FIG. I too). The mold is heated above the decomposition temperature Li of the thermo-decomposable material 210 and the substrate cooled below its g'ass transition temperature T5 (or at least below its melting temperature, if applicable).

FIG. 2.B: the mold approaches the substrate. When the heated structures touch the thermo-decomposable material 210 it evaporates into volatile decomposition products, e.g., monomers 210a. The temperature of the mold structures is reduced only slightly and still above Lu The film of thermo-decomposable material stays below Tg as theirnal conduction through the air/vacuum gap is very small. In addition, the film 210 is efficiently cooled through the substrate and further benefits from an endothermic decomposition and evaporation process.

FIG. 2.C: the mold touches the substrate. The structures 130 are cooled by the substrate 220 and shrink (see 130a) due to their high expansion coefficient. The temperature of the mold is only above Tg and Td in the layer 120 and above (layer 110, etc.), thus preventing further decomposition of the thermo-decomposable material 210. A clean patterned film 210b is finally obtained.

FIG. 2.D: the mold is removed from the substrate.

Concerning more specifically the thermo-decomposable material, the following points can be considered: 1) A material with a high glass transition/melting temperature is preferred so that no undesired deformation occurs; 2) The decomposition products are preferably volatile at these temperatures in order to allow immediate evaporation; 3) Endothermic decomposition processes prevent further heat spreading into the material thus enabling high resolution imprint; 4) A suitable material is polyphthalaldehyde, with a glass transition temperature of -125 ± °C and a decomposition temperature of 150 ± 30 °C; 5) The thermo-decomposable material 210 is coated onto the substrate 220. For example, this can be done using spin-coating.

Concerning now the mold features: 1) The mold components 110 -130 can be placed onto a heater 104 and a motor 102. as depicted in FIG. 1. The motor is designed to allow for accurate approach and removal of the mold to/from the film 210. The motor can be a piezo motor with very little lateral movement.

2) The mold is heated to temperatures above the decomposition temperature of the thermo-decomposable material, which in practice may typically be as high as 300 °C, or more; 3) The mold comprises layers with different heat conduction. Notably: -a first layer 110 (e.g. crystalline silicon) having high thermal conductivity. This guarantees a smooth temperature distribution in the mold; -a second layer 120 (e.g. silicon oxide Si02) having low thermal conductivity and low thermal expansion. As noted earlier, because of the high thermal resistance of this layer the temperature difference Ti -T2 will mainly drop at the level of layer 120 when the mold and the substrate come into contact (as seen in FIG. 2.C). This prevents heat spreading into the substrate and leaves the layer of thermo-decomposable material 210 below the glass transition / melting temperature. Misalignment induced by substrate cooling is reduced by using a layer that is «= 10 tm thick and hence the lateral expansion is dominated through the first layer; and.

-a third layer (e.g. crystalline silicon) having high thermal conductivity. This layer comprises the structures 130. The high thermal conductivity guarantees that also nanometer sized features get heated above the decomposition temperature of the thermo-decomposable material. Also, the high thermal conductivity reduces the temperature of the structures to the temperature of the substrate as soon as they come into contact. The structures l3Oa shrink as they cool down, which helps to prevents removal of too much material (see FIG. 2.C).

4) The structures 130 in the third layer can be arbitrarfly shaped. It does not matter if some structures touch the substrate (FIG. 2.C) while some do not as the second layer 120 prevents cooling more than the touching structures.

5) The structures advantageously have an aspect ratio (at least -2:1, typically -3:1) that is large enough to leave room for the evaporated thermo-decomposable material; 6) A suitable material for the fabrication of the mold are SOl (Silicon On Insolator) wafers.

These wafers are commercially available and comprise a Silicon wafer (1st layer) with a Silicon oxide layer (2nd ayer) and another Silicon layer (3rd layer) on top. Silicon has a very high thermal conductivity (k = 149 Wm'K'); its Unear thermal expansion coefficient is a = 2.6 j-6 K'. The corresponding values for silicon oxide are k = 1.4 Wm'K' and a = 8.4 io-K', which are lower enough to suit the present purpose.

Concerning now the substrate features: 1) The substrate 220 can be fixed onto a cooling unit 230 and an parallel alignment mechanism 240-250 (as depicted in FIG. 1); 2) The substrate can be fixed using a vacuum exhaust (not shown); 3) The substrate is preferably actively cooled (e.g., water cooling) below the glass transition temperature of the thermo-decomposable material; 4) Optimally, the substrate is thermally highly conductive, which improves the cooling.

Silicon is again suited; 5) For cooling, a Peltier-Element or water cooling can be used to cool the substrate to typically -5°C; 6) A possible mechanism for parallel alignment of the substrate and the m&d is the application of one ball-bearing 240 -250 underneath the substrate. When the mold starts to touch the substrate it gets tilted until three points of the mold touch the substrate. It is then perfectly parallel. This also can be done prior to the imprinting process with the unheated cold mold. The mold is then aligned parallel to the solid thermo-decomposable material layer.

The position of the substrate can be fixed, the mold can be lifted again and heated for a subsequent imprinting step.

Final remarks: 1) A high difference of temperatures between the mold and the substrate helps to prevent removal of too much material; 2) 11 the contact time of the substrate and the mold is reduced, the glass transition shifts to higher temperatures making the process more robust; 3) A very short contact time that corresponds to a frequency below the lateral resonance frequency helps to reduce distortions due to lateral vibrations; 4) Low atmospheric pressures or vacuum can help to reach improved resolution as heat conductance through the air is completely eliminated.

5) An alternative to the ball-bearing parallel alignment 240 -250 could be the use of piezo tilting motors and optical feedback; 6) The parallel alignment mechanism and the motor can be both situated at the substrate or at the mold.

WhUe the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or materia' to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In that respect, not all the components/steps depicted in the accompanying drawings need be involved, depending on the chosen embodiments. In addition, many other variants than explicitly touched above can be contemplated. For examp'e, other material layers could be involved, which are not shown in the apparatus of FIG. 1. be it for tuning thermal conduction.

It will also be apparent to the skilled person that features described in section 2 can be used in embodiments described in section 1 and reciprocally. More generally features described in section 1 can be combined in different manners.

REFERENCE LIST Mold

101 Mold Structure Side MoldBody 120 Low Thermal Conductivity Material Layer Mold Structures Mold Structure Pattern Receiver Body 210 Thermo-Decomposable Polymer Film 210 Polymer Resist Film 220 Substrate Td Decomposition Temperature Of The Polymer Film Tg Glass Transition Or Melting Temperature Of The Polymer Film

Claims

CLAIMS1. A nanoimprint lithographic method, comprising: providing (S 10): a mold (100) comprising structures (130) with dimensions on the nanoscale on a side (101) of the mold, which structures form a topographic pattern (140); and a thermo-decomposable polymer film (210) on a substrate (220), vis-à-vis said side of the mold, wherein the structures are heated above a decomposition temperature (Td) of the polymer film; bringing (S20) said structures into contact with the polymer film to thermally decompose (S30) portions thereof in colTespondence with said structures; and removing (S50) the structures away from the polymer film.
2. The nanoimprint lithographic method according to dairn I, wherein the substrate is further maintained at a temperature below a glass transition temperature (Tg) of the polymer film, prior to bringing (S20) said structures into contact with the polymer film, and wherein.preferably, bringing said structures into contact with the polymer film comprises approaching (S40) said structures toward the substrate (220) to enable thermal contact therewith and cool the structures.
3. The nanoimprint lithographic method according to claim I or 2, wherein bringing (S20) said structures into contact with the polymer film is carried out to unzip and/or desorb (S30) molecules thereof, and wherein, preferably, the polymer film provided is such that bringing (S20) said structures into contact with the polymer film can be and is carried out to thermally decompose portions thereof according to an endothermic decomposition.
4. The nanoimprint lithographic method according to claim 3, wherein the polymer film provided comprises a network of molecules cross-linked via intermolecular, non essentially covalent bonds, and wherein bringing (S20) said structures into contact with the polymer film is carried out to desorb (S30) rnolecues thereof.
5. The nanoimprint lithographic method according to claim 4, wherein an average molecular mass of molecules in the polymer film provided is between 100 Da and 2000 Da, preferably in the range from 150 Da to 1000 Da, and wherein said molecules are more preferably cross-linked via hydrogen bonds.
6. The nanoimprint lithographic method according to claim 3, wherein the polymer film provided comprises a polymer material having polymer chains able to unzip upon thermal stimulation, and wherein bringing (S20) said structures into contact with the polymer film is carried out to unzip (S30) polymer chains of the polymer material.
7. The nanoimprint lithographic method according to claim 6, wherein the polymer film provided comprises polyKphthalaldehyde). and wherein, preferably, the polymer film has a glass transition temperature of 125°C ± 20°C and a thermal decomposition temperature of 150°C ± 30°C.
8. The nanoimprint lithographic method according to any one of claims 1 to 7, wherein the mold comprises a body (110), preferably comprising crystalline silicon, and a material layer (120), preferably comprising silicon oxide, the material layer (120) between the body and the structures and having a thermal conductivity substantially lower than that of the body, preferably at east ten times thwer, and more preferably at least fifty times lower.
9. The nanoimprint lithographic method according to claim 8. the respective linear thermal expansion coefficients of the material layer (120) and the body (110) differ by less than a factor of 5.
10. The nanoimprint lithographic method according to claim 8 or 9. wherein the material layer (120) has a thickness between I and 30 micrometers, preferaNy of 6 ± 4 micrometers.
11. The nanoimprint lithographic method according to any one of claims Ito 11, wherein one or more of the structures (130) of the mold provided have a height, in a direction perpendicular to the polymer film, which is larger than the average thickness of the polymer film, preferably larger than three times the average thickness of the polymer film.
12. The nanoimprint lithographic method according to any one of claims I to 12, wherein a temperature of the substrate is maintained below a glass transition temperature (Tg) of the polymer film and wherein the structures are heated at a temperature at which decomposition products of the polymer film are volatile, prior to bringing (S20) said structures into contact with the polymer film, and wherein bnnging (S20) said structures into contact with the polymer film comprises approaching (S40) said structures onto the substrate to enable thermal contact therewith and cool the structure.
13. The nanoimprint Uthographic method according to daim 12, wherein a difference of temperature between the substrate (220) and the heated structures (130), prior to bringing (S20) said structures into contact with the polymer film, is at east 100 °C, preferably at least °C.
14. A nanoimprint lithographic apparatus (10), compnsing: a mold (100) compnsing structures (130) with dimensions on the nanoscale on a side (101) of the mold, which structures form a topographic pattern (140); and a substrate (220), with a thermo-decomposable polymer film (210) thereon, vis-à-vis said side C Ol) of the mold; heating means (104) configured to heat the structures above a decomposition temperature (Td) of the polymer film; and actuation means (1102, 240, 250) for bringing (S20) said structures into contact with the polymer film to thermally decompose portions thereof in correspondence with said structures, and removing (S50) the structures away from the polymer film.
15. The nanoimprint lithographic apparatus according to claim 14, further comprising cooling means (230) adapted for maintaining the substrate at a temperature below a glass transition temperature (Tg) of the polymer film.