|Title of Invention||
"METHOD FOR TRANSFERRING A PATTERN FROM A TEMPLATE TO A SUBSTRATE AT A CONSTANT TEMPERATURE"
|Abstract||Method for transferring a pattern from a template (10) having a structured surface (11) to a substrate (12) carrying a surface layer (14) of a material devised to 5 solidify upon exposure to radiation, comprising: arranging said template and substrate mutually parallel in an imprint apparatus, with said structured surface facing said surface layer, heating the template and the substrate to a temperature Tp by means of a heater device (20); and while maintaining said temperature Tp performing the steps of: pressing the template towards the substrate for imprinting said pattern into said layer; exposing said layer to radiation (19) for solidifying the layer, and - postbaking the layer.|
|Full Text||METHOD FOR IMPRINT LITHOGRAPHY AT CONSTANT TEMPERATURE
Field of the invention
The invention relates to a method for imprint lithography of structures on a
micro or nanometre scale. In particular, the invention relates to constant temperature
imprint lithography at constant temperature for improved accuracy.
The trend in microelectronics, as well as in micromechanics, is towards ever
smaller dimensions. Some of the most interesting techniques for fabrication of
micro and submicro structures include different types of lithography.
Photolithography typically involves the steps of coating a substrate with a
photoresist material to form a resist layer on a surface of the substrate. The resist
layer is then exposed to radiation at selective portions, preferably by using a mask
Subsequent developing steps remove portions of the resist, thereby forming a
pattern in the resist corresponding to the mask. The removal of resist portions
exposes the substrate surface, which may be processed by e.g. etching, doping, or
metallization. Fur fine scale replication, photolithography is limited by diffraction,
which is dependent on the wavelength of the radiation used. For fabrication of
structures on a scale of less than 50 nm, such a short wavelength is needed that the
material requirements on the optical systems will be major.
An alternative technique is imprint technology. In an imprint lithography
process, a substrate to be patterned is covered by a mouldable layer. A pattern to be
transferred to the substrate is predefined in three dimensions on a stamp or template.
The template is brought into contact with the mouldable layer, and the layer is
softened, preferably by heating. The template is then pressed into the softened layer,
thereby making an imprint of the template pattern in the mouldable layer. The layer
is cooled down until it hardens to a satisfactory degree followed by detachment and
removal of the template. Subsequent etching may be employed to replicate the
template pattern in the substrate. The steps of heating and cooling the combined
template and substrate can bring about movement in the engaging surfaces due to
heat expansion. The larger the area to be imprinted, the larger the actual expansion
and contraction, which can make the imprint process more difficult for larger
A different form of imprint technology, generally known as step and flash
imprint lithography has been proposed by Willson et al. in US patent 6,334,960, and
also by Mancini et al in US patent 6,387,787. Similar to the imprint technique
briefly described above, this technique involves a template having a structured
surface defining a pattern to be transferred to a substrate. The substrate is covered
by a layer of polymerisable fluid, a pre-polymer, into which layer the template is
pressed such that the fluid fills recesses in the pattern structure. The template is
made from a material which is transparent to a radiation wavelength range which is
usable for polymerising the polymerisable fluid, typically UV light By applying
radiation to the fluid through the template, the fluid is solidified. The template is
subsequently removed, after which the pattern thereof is replicated in the solid
polymer material layer made from the polymerised fluid. Further processing
transfers the structure in the solid polymer material layer to the substrate.
WO 02/067055 to Board of Regents, the University of Texas System,
discloses a system for applying step and flash imprint lithography. Among other
things, this document relates to production-scale implementation of a step and flash
apparatus, also called a stepper. The template used in such an apparatus has a rigid
body of transparent material, typically quartz. The template is supported in the
stepper by flexure members, which allow the template to pivot about both X and Y
axes, which are mutually perpendicular in a plane parallel to the substrate surface to
be imprinted. This mechanism also involves a piezo actuator for controlling
parallelism and the gap between the template and the substrate. Such a system is,
however, not capable of handling large area substrate surfaces in a single imprint
step. A step and flash system offered on the market is the IMPRIO 100, provided by
Molecular Imprints, Inc, 1807-C West Braker Lane, Austin, TX 78758, U.S.A. This
system has a template image area of 25x25 mrn, and a street width of 0.1 mm.
Although this system is capable of handling substrate wafers of up to 8 inches, the
mprint process has to be repeated by lifting the template, moving it sideways, and
lowering it to Hie substrate again, by means of an X-Y translation stage.
Furthermore., for each such step, renewed alignment as well as new dispensation of
polymerisable fluid has to be performed. This technique is therefore very timeconsuming,
and less man optimum for large scale production. Furthermore, besides
problems of repeated alignment errors, and high accuracy demands on the
translation stage, this technique suffers from the drawback that continuous
structures which are larger than said template size cannot be produced. All in all,
this means the productions costs may be too high to make mis technique interesting
for large scale production of fine structure devices.
Another drawback with the state of the art technology for UV-assisted
imprint, is that in many cases it is desirable to use non-transparent templates. Nickel
is typically used as a template material, for its excellent material properties.
However, a nickel template is of course not transparent, wherefore UV radiation has
' to be supplied through the substrate. In such a case, a substrate of e.g. glass or
quartz, or a suitable plastic material may be used. Furthermore, using different
materials in the template and the substrate generally means that they have different
coefficients of thermal expansion. This, in turn, may cause problems during stops of
heating and cooling, limiting the accuracy of the process.
Accordingly, an object of the present invention is to provide methods and
means for improving fabrication of structures comprising three-dimensional
features on a micro or nanometre scale. Aspects of this object involve providing an
improved method for transferring a pattern to a substrate with improved accuracy, a
method involving a simplified production process, and a method making it possible
to imprint large continuous structures on substrates having widths of more than one
inch, and even for 8 inch diameters, 12 inch diameters, and larger.
According to the invention, this object is fulfilled by a method for
transferring a pattern from a template having a structured surface to a substrate
carrying a surface layer of a material devised to solidify upon exposure to radiation,
- arranging said template and substrate mutually parallel in an imprint apparatus,
with said structured surface feeing said surface layer;
- heating the template and me substrate to a temperature Tp by means of a heater
while maintaining said temperature Tp, performing the steps of:
- pressing the template towards the substrate for imprinting said pattern into said
- exposing said layer to radiation for solidifying the layer, and
- postbaking the layer.
In one embodiment, said material is a crosslinkable thermoplastic polymer
having a glass temperature Tg, and wherein Tp exceeds Tg.
In one embodiment, said material is a UV-crosslinkable thermoplastic
polymer having a glass temperature Tg, wherein temperature Tp exceeds
temperature Tg, and wherein said radiation is UV radiation.
In one embodiment, said material is photo chemically amplified.
la one embodiment, the method comprises:
- applying said surface layer on the substrate by spin-coating said material, prior to
me step of arranging said template and substrate mutually parallel.
In one embodiment, said material is a UV-curable thermoplastic prepolymer,
and wherein said radiation is UV radiation.
In one embodiment; the method comprises:
- arranging the template and the substrate sandwiched between a stop member and a
first side of a flexible membrane, and wherein
- the pressing of the template towards the substrate involves applying an
overpressure to a medium present on a second side of the membrane, for obtaining.
In one embodiment, said medium comprises a gas.
In one embodiment, said medium comprises air.
In one embodiment, said medium comprises a liquid.
In one embodiment, said medium comprises a gel.
In one embodiment, the method comprises:
- emitting radiation to said layer through said template, which template is
transparent to a wavelength range of a radiation usable for solidifying said material;
- heating said substrate by direct contact with said heater device.
In one embodiment, the method comprises:
- emitting radiation to said layer through said substrate, which substrate is
transparent to a wavelength range of a radiation usable for solidifying said material;
- heating said template by direct contact with said heater device.
In one embodiment, the method comprises:
- emitting radiation to said layer through said membrane, which membrane is
transparent to a wavelength range of a radiation usable for solidifying said material
In one embodiment, the method comprises:
- emitting radiation to said layer through said membrane, and through a transparent
wall opposing said membrane, defining a back wall for a cavity for said medium,
which back wall and membrane are transparent to a wavelength range of a radiation
usable for solidifying said material.
In one embodiment, the step of exposing said layer comprises:
- emitting radiation from a radiation source within a wavelength range of 100-500
Tn one embodiment, the method comprises:
- emitting pulsating radiation with a pulse duration in the range of 0.5-10 us and a
pulse rate in the range of 1-10 pulses per second.
In one embodiment, the method comprises:
- clamping said substrate and template together prior to arranging said template and
substrate between said stop member and said flexible membrane.
In one embodiment, the method comprises:
- applying a vacuum between said template and said substrate in order to extract air
inclusions from said surface layer prior to exposing said layer to radiation.
In one embodiment, said structured surface includes protrusions defining a
pattern, which protrusions are non-transparent to said radiation, whereby the step of
exposing said layer to radiation involves solidifying said layer at portions between
In one embodiment, said protrusions include a layer of a non-transparent
In one embodiment, a layer of a non-transparent material is applied as an
outermost layer on said protrusions.
In one embodiment, the temperature Tp is within the range of 50-250°C.
Brief description of the drawings
The invention will be described in greater detail below with reference to the
accompanying drawings, on which:
Figs 1 -3 schematically illustrate the main process steps for transferring a
pattern from a template to a substrate, wherein radiation is applied through a
transparent template to solidify a polymerisable fluid on the substrate surface;
Figs 4-6 schematically illustrate corresponding process steps for transferring
a pattern from a template to a substrate, wherein radiation is applied through a
transparent substrate to solidify a polymerisable fluid on the substrate surface;
Fig. 7 schematically illustrates an embodiment of an apparatus according to
the invention, for performing the process as generally described in Figs 1-3 or 4-6;
Fig. 8 schematically illustrates the apparatus of Fig. 7, when loaded with a
template and a substrate at an initial step of the process;
Fig. 9 illustrates the apparatus of Figs 7 and 8, at an active process step of
transferring a pattern from the template to the substrate;
Figs 10-12 illustrates an alternative embodiment of an imprint process
according to the present invention; and
Figs 13-14 illustrate test results of a 2.5" substrate imprinted with a single
imprint step according to the invention, with AFM pictures taken close to the centre
and close to the edge of the substrate, respectively.
Detailed description of preferred embodiments
The present invention relates, in general, to a method of transferring a pattern
from a template to a substrate, by creating a relief image of a structure on a surface
of the template on a surface of the substrate. The surface of the template and the
surface of the substrate are in this process arranged generally parallel to each other,
and the transfer of the pattern is obtained by pressing the structured template surface
into a formable layer disposed on the substrate surface. The formable layer is
treated to solidify, such that its shape is forced to resemble the template surface.
The template can thereafter be removed from the substrate and its layer, said layer
now being an inverted topographical replica of the template. In order to permanent
the transferred pattern in the substrate, further processing may be required.
Typically, wet or dry etching is performed to selectively etch the surface of the
substrate under the solidified layer, whereby the pattern in the solidified layer is
transferred to the substrate surface. This much is state of the art, and is well
described in prior art documents, such as the aforementioned US Patent No.
Figs 1 -3 schematically present the basic process steps of the actual pattern
transfer steps, or imprint steps, of an embodiment of the invention.
In Fig. 1 a template 10 is illustrated, the template having a structured surface
11, in which three-dimensional protrusions and recesses are formed with a feature
size in height and width within a range of 1 run to several urn, and potentially both
smaller and larger. The thickness of template 10 is typically between 10 and 1000
jim. A substrate 12 has a surface 17 which is arranged substantially parallel to
template surface 11, with an intermediate spacing between the surfaces at the initial
stage shown in Fig. 1. The substrate 12 comprises a substrate base 13, to which the
pattern of template surface 11 is to be transferred. Though not shown, the substrate
may also include a support layer below the substrate base 13. In a process where the
pattern of template 10 is to be transferred to substrate 12 directly through an imprint
in a polymer material, said material may be applied as a surface layer 14 directly
onto the substrate base surface 17. In alternative embodiments, indicated by the
dashed line, a transfer layer 15 is also employed, of e.g. a second polymer material.
Examples of such transfer layers, and how they are used in the subsequent process
of transferring the imprinted pattern to the substrate base 13, are also described in
US 6,334,960. In an embodiment including a transfer layer 15, substrate surface 17
denotes the upper or outer surface of the transfer layer 15, which in turn is arranged
on the substrate base surface 18.
Substrate 12 is positioned on a heater device 20. Heater device 20 preferably
comprises a heater body 21 of metal, e.g. aluminium. A heater element 22 is
connected to or included in heater body 21, for transferring thermal energy to heater
body 21. In one embodiment, heater element 22 is an electrical immersion heater
inserted in a socket in heater body 21. In another embodiment, an electrical heating
coil is provided inside heater body 21, or attached to a lower surface of heater body
21. In yet another embodiment, heating element 22 is a formed channel in heater
body 21, for passing a heating fluid through said channel. Heater element 22 is
further provided with connectors 23 for connection to an external energy source
(not shown). In the case of electrical heating, connectors 23 are preferably galvanic
contacts for connection to a current source. For an embodiment with formed
channels for passing a heating fluid, said connectors 23 are preferably conduits for
attachment to a heated fluid source. The heating fluid may e.g. be water, or an oil.
Yet another option is to employ an IR radiation heater as a heater element 22,
devised to emit infrared radiation onto heater body 21. Furthermore, a temperature
controller is included in heater device 20 (not shown), comprising means for heating
heater element 22 to a selected temperature and maintaining that temperature within
a certain temperature tolerance. Different types of temperature controllers a well
known within the art, and are therefore not discussed in further detail.
Heater body 21 is preferably a piece of cast metal, such as aluminium,
stainless steel, or other metal. Furthermore, a body 21 of a certain mass and
thickness is preferably used such that an even distribution of heat at an upper side of
heater device 20 is achieved, which upper side is connected to substrate 12 for
transferring heat from body 21 through substrate 12 to heat layer 14. For an imprint
process used to imprint 2.5" substrates, a heater body 21 of at least 2.5" diameter,
and preferably 3" or more, is used, with a thickness of at least 1 cm, preferably at
least 2 or 3 cm. For an imprint process used to imprint 6" substrates, a heater body
21 of at least 6" diameter, and preferably 7" or more, is used, with a thickness of at
least 2 cm, preferably at least 3 or 4 cm. Heater device 20 is preferably capable of
heating heater body 21 to a temperature of up to 200-300°C, though lower
temperatures will be sufficient for most processes.
For me purpose of providing controlled cooling of layer 14, heater device 20
may further be provided with a cooling element 24 connected to or included in
heater body 21, for transferring thermal energy from heater body 21. In a preferred
embodiment, cooling element 24 comprises a formed channel or channels in heater
body 21, for passing a cooling fluid through said channel or channels. Cooling
element 24 is further provided with connectors 25 for connection to an external
cooling source (not shown). Preferably, said connectors 25 are conduits for
attachment to a cooling fluid source. Said cooling fluid is preferably water, but may
alternatively be an oil, e.g. an insulating oil.
A preferred embodiment of the invention makes use of a radiationcrosslinkable
thermoplastic polymer solution material for layer 14, which preferably
is spin-coatable. These polymer solutions may also be photo chemically amplified.
An examples of such a material is mr-L6000.1 XP from Micro Resist Technology,
which is UV-crosslinkable. Other examples of such radiation-crosslinkable
materials are negative photoresist materials like Shipley ma-N 1400, SC100, and
MicroChem SU-8. A material which is spin-coatable is advantageous, since it
allows complete and accurate coating of en entire substrate.
Another embodiment makes use of a liquid or near liquid pre-polymer
material for layer 14, which is polymerisable by means of radiation. Examples of
> available and usable polymerisable materials for layer 14 comprise NIP-K17, NDPK22,
andMP-K28 from ZEN Photonics, 104-11 Moonj i-Dong, Yusong-Gu,
Daejeon 305-308, South Korea. NIP-K17 has a main component of acrylate, and
has a viscosity at 25°C of about 9.63 cps. NEP-K22 also has a main component of
acrylate, and a viscosity at 25°C of about 5.85 cps. These substances are devised to
cure under exposure to ultraviolet radiation above 12 mW/cm2 for 2 minutes.
Another example of an available and usable polymerisable material for layer
14 is Ormocore from Micro Resist Technology GmbH, Koepenicker Strasse 325,
Haus 211, D-12555 Berlin, Germany. This substance has a composition of
inorganic-organic hybrid polymer, unsaturated, with a 1-3% photopolymerisation
initiator. The viscosity of 3-8 mPas at 25°C is fairly high, and the fluid may be
cured under exposure of radiation with 500 mJ/cm2 at a wavelength of 365 nm.
Other usable materials are mentioned in US 6,334,960.
Common for all these materials, and any other material usable for carrying
out the invention, is that they have the capability to solidify when exposed to
radiation, particularly UV radiation, e.g. by cross-linking of polymer solution
materials or curing of pre-polymers. Herein, such materials used for layer 14 are
commonly called radiation polymerisable.
The thickness of layer 14 when deposited on the substrate surface is typically
10 nm-10 um, depending on application area. The polymerisable material is
preferably applied in liquid form onto substrate 12, preferably by spin coating, or
optionally by roller coating, dip coating or similar. One advantage with the present
invention compared to the prior art step and flash method, typically when using a
cross-linkable polymer material, is that the polymer material may be spin coated on
the entire substrate, which is an advantageous and fast process offering excellent
layer evenness. Cross-linkable materials, such as those mentioned, are typically
solid in normal room temperature, and a substrate which has been pre-coated at an
elevated temperature may therefore conveniently be used. The step and flash
method, on the other hand, has to use repeated dispensation by dripping on repeated
surface portions, since that method is incapable of handling large surfaces in single
steps. This makes both the step and flash process and the machine for carrying out
such a process complex and hard to control.
A preferred embodiment of the method according to the invention will now
be described with reference to Figs 1-3. According to the invention, the process
steps of imprinting; solidifying the imprint layer material by radiation, and
postbaMng the material, are performed at a constant temperature.
The arrows of Fig. 1 illustrate that the template surface 11 is pressed into
surface 16 of the polymerisable material layer 14. At this step, heater device 20 is
preferably used to control the temperature of layer 14, for obtaining a suitable
viscosity in the material of layer 14. For a crosslinkable material of layer 14, heater
device 20 is therefore controlled to heat layer 14 to a temperature Tp exceeding the
glass temperature Tg of the material of layer 14. In this context, Tp stands for
process temperature, indicating that it is one temperature level common for the
process steps of imprint; exposure, and postbaking. The level of constant
temperature Tp is of course dependent on the type of material chosen for layer 14,
since it must exceed the glass transition temperature Tg for the case of a crosslinkable
material and also be suitable for postbaking the radiation-cured material of
the layer. For radiation-crosshnkable materials Tp typically ranges within 50-250°C.
For the example of mr-L6000.1 XP, successful tests have been performed with a
constant temperature throughout imprint, exposure and postbake of 100-120°C. For
embodiments using radiation-curable pre-polymers, such materials are typically
liquid or near liquid in room temperature, and therefore need little or no heating to
become soft enough for imprinting. However, also these materials must generally
go through post-baking for complete hardening after exposure, prior to separation
from the template. The process temperature Tp is therefore set to a suitable postbaking
temperature level already in the imprint step beginning at the step of Fig. 1.
Fig. 2 illustrates how the structures of template surface 11 has made an
imprint in the material layer 14, which is in fluid or at least soft form, at which the
fluid has been forced to fill the recesses in template surface 11. In the illustrated
embodiment; the highest protrusions in template surface 11 do not penetrate all the
way down to substrate surface 17. This may be beneficial for protecting the
substrate surface 17, and particularly the template surface 11, from damage.
However, in alternative embodiments, such as one including a transfer layer,
imprint may be performed all the way down to transfer layer surfacel7. In the
embodiment illustrated in Figs 1-3, the template is made from a material which is
transparent to radiation 19 of a predetermined wavelength or wavelength range,
which is usable for solidifying a selected polymerisable material. Such materials
may e.g. be quartz or various forms of polymers, dependent on the radiation
wavelength. Since the template generally is extremely thin, typically less than a
millimetre, also glass templates may be used even if a UV-sensitive material is used
in layer 14, since there will be very little absorption in the template material.
Radiation 19 is typically applied when template 10 has been pressed into layer 14
with a suitable alignment between template 10 and substrate 12. When exposed to
this radiation 19, solidification of the polymerisable material is initiated, for
solidification to a solid body 14* taking the shape determined by the templatelO.
During the step of exposing layer 14 to radiation, heater 20 is controlled to maintain
the temperature of layer 14 at temperature Tp.
After exposure to radiation, a postbaking step is performed, to completely
harden the material of layer 14Mh this step, heater device 20 is used to provide
heat to layer 14*, for baking layer 14' to a hardened body before separation of
template 10 and substrate 12. Furthermore, postbaking is performed by maintaining
the aforementioned temperature Tp. This way, template 10 and material layer 14,
14 will maintain the same temperature from the beginning of solidification of
material 14 by exposure to radiation, to finalized postbaking, and optionally also
through separation of template 10 and substrate 12. This way, accuracy limitations
due to differences in thermal expansion in any of the materials used for the substrate
and the template are eliminated.
The template 10 is e.g. removed by a peeling and pulling process. The
formed and solidified polymer layer 14* remains on the substrate 12. The various
different ways of further processing of the substrate and its layer 14* will not be
dealt with here in any detail, since the invention as such is neither related to such
further processing, nor is it dependent on how such further processing is achieved
Generally speaking, further processing for transferring the pattern of template 10 to
the substrate base 13 may e.g. include etching or plating followed by a lift-off step.
Figs 4-6 schematically present the basic process steps of the actual pattern
transfer steps, or imprint steps, of an alternative embodiment of the invention. The
only real difference from the embodiment of Figs 1-3 is that in this embodiment the
radiation 19 is applied through substrate 12 instead of through template 10, while
the same reference marks have been used. Furthermore, heater device 20 is instead
connected to template 10, for heating layer 14 through template 10. In an
embodiment such as the one depicted in Figs 4-6 an opaque template may be used,
which has certain advantages. For one thing, this makes it possible to use nickel
templates, which are suitable for imprint Heater device 20 of Figs 4-6 otherwise
comprises me same features as the heater device of Figs 1-3, wherefore the same
reference markings have been used. No further explanation of the features of Figs 4-
6 will therefore be made.
Fig. 7 schematically illustrates a preferred embodiment of an apparatus
according to the present invention, also usable for carrying out an embodiment of
the method according to the present invention. It should be noted that this drawing
is purely schematic, for me purpose of clarifying the different features thereof In
particular, dimensions of the different features are not on a common scale.
The apparatus 100 comprises a first main part 101 and a second main part
102. In the illustrated preferred embodiment these main parts are arranged with the
first main part 101 on top of second main part, with an adjustable spacing 103
between said main parts. When making a surface imprint by a process as illustrated
in Figs 1 -6, it may be of great importance mat the template and the substrate are
properly aligned in the lateral direction, typically called the X-Y plane. This is
particularly important if the imprint is to be made on top of or adjacent to a
previously existing pattern in tho substrate. However, the specific problems of
alignment, and different ways of overcoming them, are not addressed herein, but
may of course be combined with the present invention when needed.
The first, upper, main part 101 has a downwards facing surface 104, and the
second, lower, main part 102 has an upwards facing surface 105. Upwards facing
surface 105 is, or has a portion that is, substantially flat, and which is placed on or
forms part of a plate 106 which acts as a support structure for a template or a
substrate to be used in an imprint process, as will be more thoroughly described in
conjunction with Figs 8 and 9. A heater body 21 is placed on plate 106, or forms
part of plate 106. Heater body 21 forms part of a heater device 20, and includes a
heating element 22 and preferably also a cooling element 24, as shown in Figs 1-6.
Heating element 22 is connected through connectors 23 to a energy source 26, e.g.
an electrical power supply with current control means. Furthermore, cooling
element 24 is connected through connectors 25 to a cooling source 27, e.g. a cooling
fluid reservoir and pump, with control means for controlling flow and temperature
of the cooling fluid.
Means for adjusting spacing 103 are, in the illustrated embodiment, provided
by a piston member 107 attached at its outer end to plate 106. Piston member 107 is
displaceably linked to a cylinder member 108, which preferably is held in fixed
relation to first main part 101. As is indicated by the arrow in the drawing, the
means for adjusting spacing 103 are devised to displace second main part 102 closer
to or farther from first main part 101, by means of a movement substantially
perpendicular to the substantially flat surface 105, i.e. in the Z direction.
Displacement may be achieved manually, but is preferably assisted by employing
either a hydraulic or pneumatic arrangement The illustrated embodiment may be
varied in a number of ways in this respect, for instance by instead attaching plate
106 to a cylinder member about a fixed piston member. It should further be noted
that the displacement of second main part 102 is mainly employed for loading and
unloading the apparatus 100 with a template and a substrate, and for arranging the
apparatus in an initial operation position. The movement of second main part 102 is,
however, preferably not included in the actual imprint process as such in the
illustrated embodiment, as will be described.
First main part 101 comprises a peripheral seal member 108, which encircles
surface 104. Preferably, seal member 108 is an endless seal such as an o-ring, but
may alternatively be composed of several interconnected seal members which
together form a continuous seal 108. Seal member 108 is disposed in a recess 109
outwardly of surface 104, and is preferably detachable from said recess. The
apparatus further comprises a radiation source 110, in the illustrated embodiment
disposed in the first main part 101 behind surface 104. Radiation source 110 is
connectable to a radiation source driver 111, which preferably comprises or is
connected to a power source (not shown). Radiation source driver 111 may be
included in the apparatus 100, or be an external connectable member. A surface
portion 112 of surface 104, disposed adjacent to radiation source 110, is formed in a
material which is transparent to radiation of a certain wavelength or wavelength
range of radiation source 110. This way, radiation emitted from radiation source
110 is transmitted towards spacing 103 between first main part 101 and second
mam part 102, through said surface portion 112. Surface portion 112, acting as a
window, may be formed in available fused silica, quartz, or sapphire.
In operation, apparatus 100 is further provided with a flexible membrane
113, which is substantially flat and engages seal member 108. In a preferred
embodiment, seal member 113 is a separate member from seal member 108, and is
only engaged with seal member 108 by applying a counter pressure from surface
105 of plate 106, as will be explained. However, in an alternative embodiment,
membrane 113 is attached to seal member 108, e.g. by means of a cement, or by
being an integral part of seal member 108. Furthermore, in such an alternative
embodiment, membrane 113 may be firmly attached to main part 101, whereas seal
108 is disposed outwardly of membrane 113. For an embodiment such as the one
illustrated, also membrane 113 is formed in a material which is transparent to
radiation of a certain wavelength or wavelength range of radiation source 110. This
way, radiation emitted from radiation source 110 is transmitted into spacing 103
through said cavity 115 and its boundary walls 104 and 113. Examples of usable
materials for membrane 113, for the embodiment of Figs 7-9, include
polycarbonate, polypropylene, polyethylene, PDMS and PEEK. The thickness of
membrane 113 may typically be 10-500 um.
A conduit 114 is formed in first main part 101 for allowing a fluid medium,
either a gas, a liquid or a gel, to pass to a space defined by surface 104, seal member
108 and membrane 113, which space acts as a cavity 115 for said fluid medium.
Conduit 114 is connectable to a pressure source 116, such as a pump, which may be
an external or a built in part of apparatus 100. Pressure source 116 is devised to
apply an adjustable pressure, in particular an overpressure, to a fluid medium
contained in said cavity 115. An embodiment such as the one illustrated is suitable
for use with a gaseous pressure medium. Preferably, said medium is selected from
the group containing air, nitrogen, and argon. If instead a liquid medium is used, it
is preferred to have the membrane attached to seal member 108. Such a liquid may
be a hydraulic oil. As mentioned, another possibility is to use a gel for said medium.
Fig. 8 illustrates the apparatus embodiment of Fig. 7, when being loaded with
a substrate and a template for a lithographic process. For better understanding of
this drawing, reference is also made to Figs 1-3. Second main part 102 has been
displaced downwards from first main part 101, for opening up spacing 103. As
indicated in Figs 1-6, either the template or the substrate are transparent to radiation
of a certain wavelength or wavelength range of radiation source 110. The illustrated
embodiment of Fig. 8 shows an apparatus loaded with a transparent template 10 on
top of a substrate 12. Substrate 12 is placed with a backside thereof on surface 105
of heater body 21, placed on or in the second main part 102. Thereby, substrate 12
has its substrate surface 17 with the layer 14 of a polymerisable material, e.g. a
UV-crosslinkable polymer solution, facing upwards. For the sake of simplicity, all
features of heater device 20, as seen in Figs 1-6 are not shown in Fig. 8. Template
10 is placed on or adjacent to substrate 12, with its structured surface 11 facing
substrate 12. Means for aligning template 10 with substrate 12 may be provided, but
are not illustrated in this schematic drawing. Membrane 113 is then placed on top of
template 10. For an embodiment where membrane 113 is attached to the first main
part; the step of actually placing membrane 113 on the template is, of course,
dispensed with. In Fig. 8 template 10, substrate 12 and membrane 113 are shown
completely separated for the sake of clarity only, whereas in a real situation they
would be stacked on surface 105.
Fig. 9 illustrates an operative position of apparatus 100. Second main part
102 has been raised to a position where membrane 113 is clamped between seal
member 108 and surface 105. In reality, both template 10 and substrate 12 are very
thin, typically only parts of a millimetre, and the actual bending of membrane 113
as illustrated is minimal. Still, surface 105 may optionally be devised with a raised
peripheral portion at the point where it contacts seal member 108 through
membrane 113, for compensating for the combined thickness of template 10 and
Once main parts 101 and 102 are engaged to clamp membrane 113, cavity
115 is sealed. Pressure source 116 is then devised to apply an overpressure to a fluid
medium in cavity 115, which may be a gas, a liquid or a gel. The pressure in cavity
115 is transferred by membrane 113 to template 10, which is pressed towards
substrate 12 for imprinting the template pattern in layer 14, cf Fig. 2. For a prepolymer
material of layer 14 having sufficient viscosity at typically room
temperature, typically between 20 and 25°C, imprint may be made directly.
However, crosslinkable polymer solutions typically need pre-heating to overcome
its glass transition temperature Tg, which may be about 60°C. An example of such a
polymer is the afore mr-L6000.1 XP. When using such polymers, the apparatus 100,
having combined radiation and heating capabilities, is particularly useful. However,
for bom these types of materials a post-baking step is generally needed to harden
the radiation-solidified layer 14'. As previously mentioned, an aspect of the
invention is therefore to apply a raised temperature Tp to the material of layer 14,
which is higher than Tg for the case of a crosslinkable material, and also suitable for
postbaking of the radiation-exposed material. Heater device 20 is activated to heat
layer 14 through substrate 12, by means of heater body 21, until Tp has been
reached. The actual value of Tp is naturally dependent on the material chosen for
layer 14. For the example of mr-L6000.1 XP, a temperature Tp within the range of
50-150°C may be used, dependent on the molecular weight distribution in the
material. The pressure of the medium in cavity 115 is then increased to 5-500 bar,
advantageously to 5-200 bar, and preferably to 20-100 bar. Template 10 and
substrate 12 are thereby pressed together with a corresponding pressure. Thanks to
flexible membrane 113, an absolutely even distribution of force is obtained over the
whole of the contact surface between the substrate and the template. The template
and the substrate are thereby made to arrange themselves absolutely parallel in
relation to one another and, the influence of any irregularities in the surface of the
substrate or template being eliminated.
When template 10 and substrate 12 have been brought together by means of
the applied fluid medium pressure, radiation source is triggered to emit radiation 19.
The radiation is transmitted through surface portion 112, which acts as a window,
through cavity 115, membrane 113, and template 10. The radiation is partly or
completely absorbed in layer 14, the material of which thereby is solidified by
crosslinking or curing in the perfectly parallel arrangement between template 10 and
substrate 12, provided by the pressure and membrane assisted compression.
Radiation exposure time is dependent on the type and amount of material in layer
14, the radiation wavelength combined with the type of material, and of the
radiation power. The feature of solidifying such a polymerisable material is well
known as such, and the relevant combinations of the mentioned parameters are
likewise known to the skilled person. Once the fluid has solidified to form a layer
14*, further exposure has no major effect However, after exposure the material of
layer 14' is allowed to post bake, or hard bake, at the predetermined constant
temperature Tp for a certain time period of e.g. 1-10 minutes. For the example of
mr-L6000.1 XP, postbaking is typically performed for 1-10 minutes, preferably
about 3 minutes, at the common process temperature Tp of 100-120°C.
With the apparatus 100 according to the present invention, post-baking is
performed in the imprint machine 100, which means that it is not necessary to bring
the substrate out of the apparatus and into a separate oven. This saves one process
step, which makes both time and cost savings possible in the imprint process. By
performing the post-baking step while the template 10 is still held at a constant
temperature Tp, and potentially also with the selected pressure towards substrate 10,
and, higher accuracy in the resulting structure pattern in layer 14 is also achieved,
which makes it possible to produce finer structures. Following compression,
exposure and post-baking, the pressure in cavity 115 is reduced and the two main
parts 101 and 102 are separated from one another. In one embodiment, cooling
element 24 of heater device 20 may be used to cool down the substrate 12 after
separation of the main parts. After this, the substrate is separated from the template
and subjected to further treatment according to what is previously known for
Figs 8 and 9 illustrate a process similar to that of Figs 1-3. Again, it should
be noted that with a transparent substrate 12, template 10 may instead be placed on
surface 105 of heater body 21, with substrate on top of template 10, as shown in
Figs 10-12 illustrates an alternative method of using apparatus 100, in
accordance with an embodiment of the invention. The same reference markings are
used for like features as in Figs 1-3. However, in the process of Figs 10-12, a
transparent template 200 is used, preferably made from glass or quartz. Template
200 has a structured surface facing substrate 12, with projecting pattern-defining
protrusions 201 which are non-transparent Preferably, this is achieved by including
a layer of an opaque material in the protrusions. The illustrated preferred
embodiment includes opaque layers 202 covering the outer end surfaces of
protrusions 201. Preferably, layers 202 are metal layers. In one embodiment,
template 200 is manufactured by means of first applying a metal mask 202 on
selected areas of the template surface, where after an etching process is used for
defining grooves between the masked portions. Instead of removing the mask after
the etching step, the mask 202 is kept on the template to define the non-transparent
outer end surfaces of the template protrusions 201. By manufacturing template 200
by means of this process, it is also ensured that a near completely even common
plane for the outer end surfaces of protrusions 201 is achieved, since the template
manufacturing process starts from a flat template body with a plane surface. It
should be noted that dimensions illustrated in Figs 1-12 are exaggerated for the sake
of easy understanding. For instance, layers 202 may be only a few atomic
In Fig. 10, template 200 is pressed into layer 214 on substrate 12, preferably
by using an apparatus as described with reference to Figs 7-9. The material of layer
214 is in this case e.g. a UV-curable pre-polymer or a-UV-crosslinfcable negative
resist; which may be of any known type. Heater device 20 is controlled to raise the
temperature of substrate 12 to a suitable process temperature Tp. For the case of a
crosslinkable material, heater device 20 is set to pre-heat layer 214 trough substrate
12 in order for the material of layer 214 to overcome the glass transition
temperature Tg and reach the elevated temperature Tp An even pressure is achieved
over the entire engaging surfaces of template 200 and substrate 12, thanks to the
imprint technique using a membrane and gas pressure as described above.
Preferably, the template 200 is pressed into layer 214 such that the outer ends of
protrusions 201 come extremely close to substrate layer 17, preferably only a few
In Fig. 11, at which template 200 has been fully pressed into layer 214,
radiation 19 is applied through template 200, towards substrate 12. Radiation which
hits layers 202 is stopped and reflected, and does not reach layer portions 214
positioned there under. Radiation which fells in between protrusions 201, however,
will hit layer 214 and start a curing or solidification process in layer portions 214",
while maintaining layer 214 at a temperature Tp. Preferably, a post-baking process
at the same temperature Tp is then performed using heater device 20 for completing
the solidification process.
In the step illustrated in Fig. 12, template 200 is separated and removed from
template 12, leaving layer 214 as imprinted. In this shape, substrate 12 is exposed to
a negative resist developer fluid The exact type of fluid may be of any known kind,
although the skilled person realises that developer type has to be selected dependent
on the resist polymer used. The developer will only remove portions 214' which
were not exposed to radiation, and which remain only as very thin layers at the
bottom of the recesses in the polymer layer formed by protrusions 201. Compared
to prior art processes, where an ashing or etching process has to be applied to
remove the remaining polymer portions 214' in the recesses, which are then also
solid, this process is considerably easier and faster. Furthermore, ashing or etching
of the patterned polymer layer 14 will remove material from all parts of layer 214,
both portions 214* and 214*', whereas the proposed method only takes away the
portions 214' which were not exposed to radiation.
One embodiment of the system according to the invention further comprises
mechanical clamping means, for clamping together substrate 12 and template 10.
This is particularly preferred in an embodiment with an external alignment system
for aligning substrate and template prior to pattern transfer, where the aligned stack
comprising the template and the substrate has to be transferred into the imprint
apparatus. The system may also contain means for applying a vacuum between
template and substrate in order to extract air inclusions fbom the polymerisable layer
of the stacked sandwich prior to hardening of the polymerisable material through
In a preferred embodiment, the template surface 11 is preferably treated with
an anti-adhesion layer to prevent the solidified polymer layer 14' from sticking to it
after me imprint process. An example of such an anti-adhesion layer comprises a
fliwrine-contaming group, as presented in WO 03/005124 and invented by one of
the inventors of the instant invention. The contents of WO 03/005124 are also
hereby incorporated by reference.
A first mode of the invention, with a transparent template, which has been
successfully tested by the inventors, involves a substrate 12 of silicon covered by a
layer 14 of NIP-K17 with a thickness of 1 urn. A template of glass or fused
silica/quartz, with a thickness of 600 pm, has been used.
A second mode of the invention, with a transparent substrate, which has been
successfully tested by the inventors, involves a substrate 12 of glass or fused
silica/quartz covered by a layer 14 of NEP-K17 with a thickness of 1 urn. A template
of e.g. nickel or silicon has been used, with a thickness of about 600 urn, though
any other suitable non-transparent material can be used.
After compression by means of membrane 113 with a pressure of 5-100 bar
for about 30 seconds, radiation source 110 is turned on. Radiation source 110 is
typically devised to emit at least in the ultraviolet region below 400 nm. In a
preferred embodiment an air-cooled xenon lamp with an emission spectrum
ranging from 200-1000 nm is employed as the radiation source 110. The preferred
xenon type radiation source 110 provides a radiation of 1-10 W/cm2, and is devised
to flash 1-5 jis pulses, with a pulse rate of 1-5 pulses per second. A window 112 of
quartz is formed in surface 104 for passing through radiation. Exposure time is
preferably between 1-30 seconds, for polymerising fluid layer 14 into a solid layer
14', but may be up to 2 minutes.
Tests with mr-L6000.1 XP have been performed with about 1.8 W/ cm2
integrated from 200-1000 nm, with 1 minute exposure time. It should, in this
context, be noted that the radiation used need not be restricted to a wavelength
range within which the polymer applied in layer 14 solidifies, radiation outside that
range may of course also be emitted from the radiation source used. After
successful exposure and subsequent postbaking at a constant process temperature,
second main part 102 is lowered to a position similar to that of Fig. 8, following
which template 10 and substrate 12 are removed from the apparatus for separation
and further processing of the substrate.
The present invention brings about a novel imprint method, which combines
UV and thermal NIL, allowing the complete imprint sequence into UVcrosslinkable
thermoplastic polymers to be performed at a constant temperature.
Thereby, the method according to the invention overcomes problems related to
different thermal expansions in template and substrate materials. As a result, it is
possible to perform high accuracy large area imprints using different template and
substrate materials. Furthermore, the method allows the use of spin-coatable UVcrosslinkable
polymers with a homogeneous thickness distribution on wafer scale,
which is difficult to accomplish with dispensing low viscosity UV-curable prepolymers.
The general process scheme includes three main steps; a thermal imprint
sequence, followed by a UV post exposure, and a hard bake to entirely cure the
polymer. In a preferred embodiment, a photo chemically amplified polymer is used,
such as for example mr-L6000.1 XP.
The three steps are combined and performed at a constant temperature,
giving the following process scheme. Template and substrate are heated to a
temperature Tp, which is above Tg for the case of a crosslinkable material.
Preferably, this is performed by placing the template in contact with the substrate in
a sandwich arrangement, and then heating either the template or the substrate by
means of a heater device. This way, both the template and the substrate, and in
particular the layer on the substrate which is to be imprinted, are heated to a
common temperature by heat conduction. The template-substrate sandwich is then
exposed to a high pressure in order to imprint the template pattern into the polymer
layer. After a certain time, typically 30-60 seconds, a UV flood exposure is started
in order to initiate the curing of the polymer. Before releasing the pressure the
temperature is kept constant at Tp to hard bake the polymer until it is entirely cured.
Different template and substrate materials can easily be used to produce high
accuracy large area imprints. The fact that cheap reproducible nickel templates can
be used makes large area imprinting significantly more cost effective and easy to
perform showing a great potential for this method.
The inventors have successfully imprinted the complete area of 2.5" glass
substrates using a Blu-ray nickel template, with a line width of about 140 nm. The
' imprint quality shows no tendency of degradation, due to thermal effects, when
moving towards the edge of the substrate. This is clearly visualized in Figs 13 and
14, which show AFM (Atomic Force Microscope) pictures of results obtained.
Fig. 13 illustrates an AFM picture 137 of an area close the centre of a Bluray
imprint on a 2.5" glass substrate. To the left in Fig. 13, an AFM depth analysis
result on the area of 137 is shown, which is measured along the horizontal line in
picture 137. Selected points along that line are indicated by references 131-136,
shown bom in picture 137 and in the depth analysis diagram. As can be seen from
the latter, the grooves formed in the imprint process according to the invention are
deep and smooth.
Furthermore, Fig. 14 illustrates a corresponding picture 147 of the same
imprinted substrate, of an area located a few millimetres inside of the substrate
edge. Similar to Fig. 13, selected points along the horizontal line in picture 147 are
indicated by references 141-146, shown both in picture 147 and in the depth
analysis diagram to the left of picture 147. Also in Fig. 14, it can be seen that the
grooves formed in the imprint process according to the invention are deep and
smooth also close to the edge of the 2.5" glass substrate, and show no tendency to
unevenness or distortion due to thermal expansion.
By a single process, including three main process steps, performed in one
and the same machine without having to extract the substrate from the machine in
between those main steps, imprint with excellent quality is possible on large
substrate surfaces. Since the entire substrate surface is imprinted in one step, the
polymer layer 14 may be spin-coated onto the substrate, and continuous structures
can be created over the entire substrate surface. None of this is possible with the so
called step and flash method. The disclosed apparatus and method is therefore
particularly advantageous for large area imprint, and has as such huge benefits over
the step and flash method. By using membrane-transferred fluid pressure, the
present invention can be used for one step imprint of substrates of 8 inch, 12 inch,
and even larger discs. Even full fiat panel displays with sizes of about 400x600 mm
and larger can be patterned with a single imprint and exposure step with the present
The present invention therefore provides a technique which may for the first
time make radiation-assisted polymerisation imprint attractive to large scale
production. The invention is usable for forming patterns in a substrate for
production of e.g. printed wire boards or circuit boards, electronic circuits,
miniaturised mechanical or electromechanical structures, magnetic and optical
storage media etc. The embodiments described herein relate to radiation exposure of
UV-crosslinkable polymers or UV-curable pre-polymers, in combination with a
hearer. However, from the object of providing a solution which overcomes the
problem caused by thermal expansion due to having different template and substrate
materials, the skilled person would realise that the present invention could equally
well be implemented for a method including radiation in other wavelength ranges,
to which the resist material used in the imprint layer on the substrate is responsive
by solidifying during exposure. Furthermore, while the invention is particularly
advantageous for imprint processes including a template and a substrate of different
materials, a technical effect is obtained also when the same material is used in the
template and the substrate, in that the substrate need not be removed from the
imprint machine during postbaking, and the facilitated control of using a constant
By the term constant temperature is meant substantially constant, meaning
that even though a temperature controller is set to maintain a certain temperature,
the actual temperature obtained will inevitably fluctuate to a certain extent The
stability of the constant temperature is mainly dependent on the accuracy of the
temperature controller, and inertia of the entire setup. Furthermore, it is understood
that even though the method according to the invention is usable for imprinting
extremely fine structures down to single nanometres, a slight temperature variation
will not have a major effect as long as the template is not too large. Assuming that
the structures at the periphery of the template has a width x, and a reasonable spatial
tolerance is a fraction of that width, such as y=x/10, then y becomes the parameter
setting the temperature tolerance. In fact, it can easily be calculated which effect
differences in thennal expansion will have, by applying the respective coefficients
of thermal expansion for the materials of the template and substrate, the size,
typically the radius, of the template, and the spatial tolerance parameter y. From
such a calculation, a suitable temperature tolerance for the temperature controller
can be calculated and applied to the machine for performing the process.
The invention is defined by the appended claims.
1. A Method for transferring a pattern from a template having a structured surface to
a substrate carrying a surface layer of a material devised to solidify upon exposure
to radiation, the method comprising:
-arranging said template and substrate mutually parallel in an imprint apparatus,
with said structured surface facing said surface layer;
-heating the template and the substrate to a temperature Tp suitable for postbaking
the surface layer, by means of a heater device; and
while maintaining a constant temperature Tp, performing the steps of:
-pressing the template towards the substrate for imprinting said pattern into said
-exposing said layer to radiation for solidifying the layer, and
-postbaking the layer.
2. The method as claimed in claim 1, wherein said material is a crosslinkable thermoplastic polymer having a glass temperature Tg, and wherein Tp exceeds Tg.
3. The method as claimed in claim 1, wherein said material is a UV- crosslinkable thermoplastic polymer having a glass temperature Tg, wherein temperature Tp exceeds temperature Tg, and wherein said radiation is UV radiation.
4. The method as claimed in claim 2 or 3, wherein said material is photo-chemically amplified.
5. The method as claimed in claim 1, wherein
- said surface layer is applied on the substrate by spin-coating said material, prior to the step of arranging said template and substrate mutually parallel.
6. The method as claimed in claim 1, wherein said material is a UV-curable
thermoplastic pre-polymer, and wherein said radiation is UV radiation.
7. The method as claimed in claim 1, wherein,
-the template and the substrate are sandwiched between a stop member and a first side of a flexible membrane, and wherein
-the pressing of the template towards the substrate is done by applying an overpressure to a medium present on a second side of the membrane.
8. The method as claimed in claim 7, wherein said medium is a gas.
9. The method as claimed in claim 7, wherein said medium is air.
10. The method as claimed in claim 7, wherein said medium is a liquid.
11. The method as claimed in claim 7, wherein said medium is a gel.
12. The method as claimed in claim 1, wherein
-radiation is emitted to said layer through said template, which template is transparent to a wavelength range of a radiation usable for solidifying said material; and
- said substrate is heated by direct contact with said heater device.
13. The method as claimed in claim 1, wherein
-radiation is emitted to said layer through said substrate, which substrate is transparent to a wavelength range of a radiation usable for solidifying said material; and
- said template is heated by direct contact with said heater device.
14. The method as claimed in claim 7, wherein
-radiation is emitted to said layer through said membrane, which membrane is transparent to a wavelength range of a radiation usable for solidifying said material.
15. The method as claimed in claim 7, wherein
- radiation is emitted to said layer through said membrane, and through a transparent wall opposing said membrane, defining a back wall for a cavity for said medium, which back wall and membrane are transparent to a wavelength range of a radiation usable for solidifying said material.
16. The method as claimed in claim 1, wherein the step of exposing said layer is done
-emitting radiation from a radiation source within a wavelength range of 100-500 nm.
17. The method as claimed in claim 16, wherein
-pulsating radiation is emitted with a pulse duration in the range of 0.5-10 us and a pulse rate in the range of 1 -10 pulses per second.
18. The method as claimed in claim 7, wherein
-said substrate and template are clamped together prior to arranging said template and substrate between said stop member and said flexible membrane.
19. The method as claimed in claim 1, wherein
-a vacuum is applied between said template and said substrate in order to extract air inclusions from said surface layer prior to exposing said layer to radiation.
20. The method as claimed in claim 1, wherein said structured surface consists
protrusions defining a pattern, which protrusions are non-transparent to said
radiation, whereby the step of exposing said layer to radiation involves solidifying
said layer at portions between said protrusions.
21. The method as claimed in claim 20, wherein said protrusions have a layer of a non-transparent material.
22. The method as claimed in claim 20, wherein a layer of a non-transparent material is applied as an outermost layer on said protrusions.
23. The method as claimed in claim 1, wherein the temperature Tp is within the range of 50-250° C.
|Indian Patent Application Number||5315/DELNP/2006|
|PG Journal Number||38/2011|
|Date of Filing||14-Sep-2006|
|Name of Patentee||OBDUCAT AB|
|Applicant Address||P.O. BOX 580,S-201 25 MALMO, SWEDEN|
|PCT International Classification Number||G03F 7/00|
|PCT International Application Number||PCT/EP2004/053106|
|PCT International Filing date||2004-11-25|