U.S. patent application number 10/270950 was filed with the patent office on 2003-08-14 for method of forming a pattern of sub-micron broad features.
Invention is credited to Ketelaars, Wilhelmus Sebastianus Marcus Maria, Kroon, Mark, Van Delft, Falco Cornelius Marinus Jacobus Maria.
Application Number | 20030150737 10/270950 |
Document ID | / |
Family ID | 8181109 |
Filed Date | 2003-08-14 |
United States Patent
Application |
20030150737 |
Kind Code |
A1 |
Van Delft, Falco Cornelius Marinus
Jacobus Maria ; et al. |
August 14, 2003 |
Method of forming a pattern of sub-micron broad features
Abstract
A pattern of very fine features (18) can be produced by
illuminating an inorganic negative tone resist layer (16), provided
on an electroplating base layer (14), by a beam (EB), which is able
to cure the resist to a cured pattern according to the pattern to
be formed, removing the non-illuminated portions of the resist
layer and electroplating a layer (20) between the cured portions
(18) of the resist layer.
Inventors: |
Van Delft, Falco Cornelius Marinus
Jacobus Maria; (Eindhoven, NL) ; Ketelaars, Wilhelmus
Sebastianus Marcus Maria; (Eindhoven, NL) ; Kroon,
Mark; (Eindhoven, NL) |
Correspondence
Address: |
Michael E. Marion
c/o U.S. PHILIPS CORPORATION
Intellectual Property Department
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8181109 |
Appl. No.: |
10/270950 |
Filed: |
October 14, 2002 |
Current U.S.
Class: |
205/118 |
Current CPC
Class: |
G03F 7/11 20130101; G03F
7/0757 20130101; G03F 7/405 20130101; G03F 7/0005 20130101; G03F
7/40 20130101; H05K 3/108 20130101; G03F 7/001 20130101 |
Class at
Publication: |
205/118 |
International
Class: |
C25D 005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2001 |
EP |
01203998.8 |
Claims
1. A method of forming a pattern of sub-micron broad features in a
metal layer, which method includes the steps of: forming a resist
layer, comprising a negative tone resist material, on a substrate;
illuminating selected portions of the resist layer by a beam, which
is able to cure the resist to a cured pattern according to the
pattern to be formed, and removing non-illuminated portions of the
resist layer, which method is characterized in that the resist
material is an inorganic material and in that the additional steps
of: forming an electroplating base layer on the substrate before
applying the resist layer, and electroplating a layer between the
cured portions of the resist layer are carried out.
2. A method as claimed in claim 1, characterized in that siloxane
material is used as a resist material.
3. A method as claimed in claim 2, characterized in that hydrogen
silsesquioxane (HSQ) is used as a resist material.
4. A method as claimed in claim 1, 2, or 3, characterized in that a
double layer comprising a hydrogen silsesquioxane top layer and a
novolak bottom layer is used as a resist layer.
5. A method as claimed in claim 1, 2, 3 or 4, characterized in that
a layer of one of the materials: silver, nickel and permalloy is
electroplated between the illuminated portions of the resist
layer.
6. A method as claimed in claim 1, 2, 3, 4 or 5, characterized in
that a layer of one of the materials: silver, gold, copper,
aluminum and molybdenum is used as an electroplating base
layer.
7. A method as claimed in any one of the preceding claims,
characterized by the intermediate step of covering the substrate
surface, which is to be provided with the pattern of features, with
an insulating layer before the electroplating base layer is
applied.
8. A method as claimed in any one of the preceding claims,
characterized by the additional step of removing the illuminated
portions of the resist layer after finishing the electroplating
process.
9. A pattern of features manufactured by means of the method of any
one of claims 1 to 8, characterized in that the features have a
sub-micron width and are arranged at mutual distances which are
substantially larger than the feature widths.
10. A pattern of features manufactured by means of the method of
any one of claims 1 to 8, forming a lithographic mask, wherein the
pattern features constitute mask features which are transparent to
lithographic projection radiation and the pattern areas between the
features constitute mask areas which are non-transparent to
lithographic projection radiation.
11. A pattern of features manufactured by means of the method of
any one of claims 1 to 8, forming a grating structure, wherein the
pattern features constitute transparent grating strips and the
pattern areas between the features constitute non-transparent
intermediate strips.
12. A pattern of features manufactured by means of the method of
any one of claims 1 to 8, forming a structure of at least one
magnetic gap in a thin-film magnetic recording head.
Description
[0001] The invention relates to a method of forming a pattern of
sub-micron broad features in a metal layer, which method includes
the steps of:
[0002] forming a resist layer, comprising a negative tone resist
material, on a substrate;
[0003] illuminating selected portions of the resist layer by a
beam, which is able to cure the resist to a cured pattern according
to the pattern to be formed, and
[0004] removing non-illuminated portions of the resist layer.
[0005] The invention also relates to a pattern of features
manufactured by means of this method.
[0006] The pattern of features may be a grating structure, for
example an optical grating for use in an optical apparatus, or may
form part of an image sensor, which is used in a lithographic
projection apparatus. Such an apparatus is an essential tool in the
manufacture of integrated circuits (ICs) by means of masking,
material removing and implantation techniques. The projection
apparatus is used to successively image different mask patterns at
the same area of a semiconductor substrate, each mask pattern at a
different level of the substrate. This apparatus includes, in this
order, an illumination unit for supplying a projection beam, a mask
holder for accommodating a mask, a substrate holder for
accommodating a substrate and a projection system arranged between
the mask holder and the substrate holder. The mask is provided with
a mask pattern corresponding to the pattern of device features that
is to be formed in that substrate level that is to be configured by
the specific mask. The projection system images the mask pattern
into a resist layer coated on the substrate. This projection system
may be a system of lenses or a system of mirrors or a combination
of such systems. To control the performance of the projection
system and possibly of the illumination unit, the projection
apparatus comprises an image sensor. Such an image sensor is a
device composed of a radiation-sensitive element, for example an
array of photo diodes or a charge-coupled device (CCD), and a
light-shielding element comprising an array of radiation
transmission areas arranged in front of the radiation-sensitive
element. The image sensor may be arranged in or on the substrate
holder. For measuring the performance of the projection system, a
mask provided with a test pattern, for example a grating pattern,
is positioned in the mask holder and illuminated by the projection
beam. The test pattern is imaged, by means of the projection
system, on the image sensor. The lightshielding element has a
pattern of transmission areas corresponding to the test pattern.
The output signals of the image sensor are supplied to an
electronic processing circuit in which these signals are compared
with standard signals corresponding to the test pattern itself.
[0007] The size of the device features that can be imaged by the
lithographic apparatus in the resist layer depends on he resolving
power, or resolution, of the projection system of this apparatus.
This resolution is proportional to .lambda./ NA, wherein .lambda.
is the wavelength of the projection beam used in the apparatus and
NA is the numerical aperture of the projection system. To produce
devices, such as ICs, with a higher density, and hence higher
operating speeds, smaller device features have to be imaged so that
a projection system with a higher resolution should be used. To
control a lithographic projection apparatus with such a
high-resolution projection system, an image sensor with an
increased resolving power should be used. This means that the width
of the transparent openings in the radiation shield, for example
the width of the transparent strips of a grating, should be
considerably decreased.
[0008] Current lithographic projection apparatus employ ultraviolet
(UV) radiation having a wavelength of 365 nm, generated by mercury
lamps, or deep UV (DUV) radiation having a wavelength of 248 nm,
193 nm or 157 nm and generated by exciter lasersnm In principle, a
feature width as small as 100 mn can be imaged. with an apparatus
operating with a radiation of 157 nm. For future lithographic
projection apparatus, which should image device features having a
width smaller than 100 nm, it has been proposed to use extreme UV
(EUV) radiation, also called soft-X ray radiation, which has a
considerably smaller wavelength. EUV radiation is understood to
mean radiation with a wavelength from a few to some tens ofNMs and
preferably of the order of 13 nm. For an EUV image sensor, the
grating strips should be further decreased. A typical EUV image
sensor grating has strips in the form of grooves or ridges with a
width of 50-150 nm and a pitch, or grating period, of 2000 nm,
which strips are processed in a 50-100 nm thick metallic layer, for
example a nickel (In) or silver (Erg) layer. Such a layer is
commonly deposited by means of chemical vacuum deposition (CVD) on
any type of non-conductive substrate.
[0009] The most obvious technique to produce such a grating with
small grooves, or spaces in the metal layer, seems to be reactive
ion etching. However, for groove widths of 50-150 nm, this etching
technique does not provide the required quality in most transition
or alloy layers, because etching products are non-volatile. The
term transition refers to the fact that the grooves formed in these
layers do not show vertical walls between their bottom surface and
the upper surface of the layer, but a smooth transition between
these surfaces. Also the so-called "lift-off" method is not
suitable. This method may be used to produce a single isolated
groove having a small width, but when producing a series of such
grooves, like those of a grating, the grooves grow towards each
other, which results in a bad wall definition of the grooves. The
lift-off method is used to perform, when transferring the resist
pattern to the metal pattern, a contrast reversal, i.e. ridges in
the resist become slits in the metal. Grooves having widths of the
order of 50 nm can only be obtained in a reproducible manner by
writing corresponding strips in a negative tone resist by means of
an electron beam. A negative tone resist is understood to mean a
resist of which the illuminated portions remain after development
of the resist. The grating pattern formed in the resist layer is
the negative of the required grating pattern.
[0010] It is an object of the present invention to provide a method
which is very suitable for producing a pattern of very small
features, like a grating with very small grating grooves, which
method does not use a lift-off step and thus does not suffer from
the disadvantages inherent in such a step. The method is
characterized in that the resist material is an inorganic material
and in that the additional steps of:
[0011] forming an electroplating base layer on the substrate before
applying the resist layer, and
[0012] electroplating a layer between the cured portions of the
resist layer are carried out.
[0013] The grating structure is now formed by depositing the
supporting material between the grating strips and outside the area
of these strips, instead of transferring the resist structure to
the supporting material by means of etching techniques.
[0014] The method is preferably further characterized in that a
siloxane is used as a resist material.
[0015] Most preferably, the method is further characterized in that
hydrogen silsesquioxane (HSQ) is used as a resist material.
[0016] As described in the paper: "HSQ/ Novolak bilayer resist for
high aspect ratio nanoscale e-beam lithography " presented on Proc.
44.sup.th International Conference on Electron-, Ion- and
Photon-Beam Technology and Nanofabrication (EIPBN2000), Palm
Springs Calif. 2000 and published in Journal of Vacuum Science and
Technology B 18,6 (2000), 3419, hydrogen silsesquioxane (HSQ) is
sensitive to electrons and can be used as a negative tone resist
for a fine pattern writing electron beam. When manufacturing a
grating structure for EUV radiation, HSQ provides the great
advantage of being transparent to EUV radiation after it has been
cross-linked by the electron beam.
[0017] Other siloxane materials can also be used as negative tone
resist materials for the envisaged purpose.
[0018] If the absorption of a HSQ material is large and the
penetration depth for the writing beam small so that a thin layer
of HSQ would have to be used, the method is preferably further
characterized in that a double layer comprising a hydrogen
silsesquioxane top layer and a novolak bottom layer is used as a
resist layer.
[0019] Then a pattern of features having a larger depth can be
produced.
[0020] According to a further aspect of the invention, the method
is characterized in that a layer of one of the materials: silver,
nickel and permalloy is electroplated between the illuminated
portions of the resist layer.
[0021] These materials show the advantages of having a low
transmission for radiation, especially EUV radiation.
[0022] According to a still further aspect of the invention, the
method is characterized in that a layer of one of the materials:
silver, gold, aluminum, copper and molybdenum is used as an
electroplating base layer.
[0023] Because of their low electrical resistance, silver, gold,
aluminum and copper are excellent materials for an electroplating
base and allow several materials, having a higher resistance, to be
electroplated In addition to a low resistance, molybdenum has the
additional advantage of being transparent to EUV radiation.
[0024] To prevent that plating material from being deposited on the
substrate surface remote from the resist carrying-surface during
the electroplating process, the method is further characterized by
the intermediate step of covering the substrate surface, which is
to be provided with the pattern of features, with an insulating
layer before the electroplating base layer is applied.
[0025] As an alternative, instead of the resist-carrying surface,
said remote surface can be covered with an insulating layer.
[0026] As both a HSQ layer and a molybdenum layer are sufficiently
transparent to EUV radiation, a formed pattern structure comprising
the illuminated portions of the HSQ layer may be used as a pattern
of features for use in an EUV projection apparatus. If the pattern
of features is to be used with radiation other than EUV radiation,
other combinations of resist and electroplating base materials have
to be chosen. The presence of HSQ, or another resist material, in
the openings of the metal layer prevents contaminants from being
deposited in the openings, which is an important advantage in a
production environment for ICs or other devices. Another important
advantage of HSQ is that a pattern of features formed of this
material will not be ablated by EUV radiation, i.e. soft X-ray
radiation, during its use in an apparatus employing such a
radiation.
[0027] The pattern of features produced by means of the present
method may also be used outside the field of lithography and for
radiation other than EUV radiation. For such applications of the
pattern of features, the method may be characterized by the
additional step of removing the illuminated portions of the resist
layer after finishing the electroplating process.
[0028] The features of the structure thus obtained consist of fully
transparent openings in the electroplated, non-transparent, metal
layer.
[0029] The invention also relates to a pattern of features
manufactured by means of the method as described above. This
pattern is characterized in that the features have a submicron
width and are arranged at mutual distances which are substantially
larger than the feature widths.
[0030] The pattern of features may be implemented in several
applications. In a first application the pattern of features forms
a mask pattern of a lithographic mask, wherein the pattern of
features constitute mask features which are transparent to
lithographic projection radiation, and the pattern areas between
the features constitute mask areas which are nontransparent to
lithographic projection radiation.
[0031] This kind of mask pattern is especially suitable for an EUV
mask, but may also be used in a mask for a lithographic projection
apparatus which employs a different, short wavelength,
radiation.
[0032] In a second application, the pattern of features forms a
grating structure, wherein the pattern of features constitute
transparent grating strips and the pattern areas between the
features constitute non-transparent intermediate strips.
[0033] This kind of grating structure is especially suitable for
use in an EUV mask, or different wavelength, lithographic
projection apparatus, for example as an alignment mark or in an
image sensor for such an apparatus. The grating structure may also
be used outside the field of lithography; in general, in all
applications wherein gratings with small grating strips are
required.
[0034] In a third application, a feature in the form of a slit in a
magnetizable layer forms a magnetic gap in a thin-film magnetic
recording head.
[0035] These and other aspects of the invention are apparent from
and will be elucidated by way of non-limitative example, with
reference to the embodiments described hereinafter.
[0036] In the drawings:
[0037] FIG. 1 schematically shows an embodiment of a lithographic
projection apparatus, which comprises elements wherein the
invention may be implemented;
[0038] FIG. 2 shows a part of an embodiment of an EUV image
sensor;
[0039] FIGS. 3a-3d show the successive steps of the method;
[0040] FIGS. 4 and 5 show SEM photographs of a 160 nm wide ridge
produced by the method using different electron beam doses;
[0041] FIG. 6 shows a SEM photograph of a 40 nm wide ridge produced
by the method;
[0042] FIG. 7 shows a pattern of ridges comprising a HSQ/novolak
double resist layer;
[0043] FIG. 8 shows a known thin-film magnetic head, and
[0044] FIG. 9 shows such a head produced by the method of the
invention.
[0045] The main modules of the lithographic projection apparatus
schematically depicted in FIG. 1 are:
[0046] an illumination system LA/ IL for supplying a projection
beam PB of EUV radiation;
[0047] a mask table MT comprising, as is known in the art, a mask
holder (not shown) for holding a mask MA;
[0048] a substrate table WT comprising, as is known in the art, a
substrate holder (not shown) for holding a substrate W, e.g. a
resist coated silicon wafer, and
[0049] a projection system PL for imaging an illuminated portion of
the mask MA on a target portion, i.e. an IC area, or die, C
[0050] The projection system in an EUV projection apparatus is a
system of reflective elements.
[0051] The apparatus is also provided with a number of measuring
systems, one of which is an alignment measuring device for
determining mutual alignment, in an XY-plane of the mask MA and the
substrate W. Another measuring system is an interferometer system
IFw for measuring the X and Y-position and orientation of the
substrate holder, and thus of the substrate. Still another
measuring system is a focus-error detection system for determining
a deviation between the focus, or image, field of the projection
system PL and the surface of the resist layer on the substrate W.
These measuring systems form part of servosystems, which comprise
electronic signal processing and control circuits, by means of
which the position and orientation of the substrate and the focus
can be corrected with reference to the signals supplied by the
measuring systems. In FIG. 1, PW represents the actuator, or
positioning, means for the substrate table WT.
[0052] The mask MA for use with the lithographic projection
apparatus shown in FIG. 1 is a reflective mask. The apparatus may
be a stepping apparatus or a step-and-scanning apparatus, which are
both known in the art. In addition to a substrate a
step-and-scanning apparatus comprises, positioning means PW and a
substrate interferometer system IFw, also a mask positioning means
PM and a mask interferometer system IFm.
[0053] The exposure, or projection, beam PB supplied by the
illumination system LA/IL is a beam of EUV radiation with a
wavelength, for example, of the order of 13 nm. With such a beam,
very small device, or IC, features, of the order of 100 nm or
smaller can be imaged in the resist layer. The illumination system
supplying such a beam may comprise a plasma source LA, which may be
a discharge plasma source or a laser-produced plasma source, which
are both known in the art.
[0054] The illumination system comprises various optical components
to capture and guide source radiation and to shape this radiation
to a suitable projection beam PB, which illuminates the mask
pattern. The beam PB reflected by the mask passes through the
projection system PL, which focuses this beam in the resist layer
on top of the substrate to form an image of the mask pattern at the
position of a selected target, or IC area of the substrate.
[0055] As shown in the left section of FIG. 1, the mask MA
comprises, for example two, mask alignment marks M1, M2 outside the
area of the mask pattern C. Preferably, these alignment marks are
constituted by diffraction gratings. These marks are preferably
twodimensional, i.e. they include grating strips extending in the X
and Y-direction in FIG. 1. Substrate W comprises at least two wafer
alignment marks, two of which, P1 and P2 are shown in the
right-hand section of FIG. 1 The marks PI and P2 are positioned
outside the area of the substrate where images of the mask pattern
have to be formed. The mask and substrate alignment marks are used
to detect the degree of alignment of the substrate and the mask
during an alignment step, which precedes the step of exposure of
the substrate with the mask pattern. This detection can be
performed by imaging, by means of a dedicated alignment beam, a
mask alignment mark and a substrate wafer alignment mark onto each
other, or by imaging a mask alignment mark and a substrate
alignment mark onto a reference mark. The grating alignment marks
for use in an EUV projection apparatus should have grating strips
with a very small width. Such fine mask alignment marks are
difficult to produce with conventional techniques.
[0056] For monitoring the imaging performance of the projection
apparatus and for calibrating its measuring systems, the apparatus
comprises an image sensor, schematically represented by component
IS in FIG. 1. This image sensor may be integrated in the substrate
table WT. An early embodiment of an image sensor is described in
U.S. Pat. No. 4,540,277. This image sensor, which is used for
determining magnification of the projection system and/or for
calibration of the alignment system, comprises a glass plate coated
with a chromium layer. In this layer, light-transmitting zones
having a width of 1.5 .mu.m are etched, which zones correspond to
apertures in the mask. The mask is projected on the chromium layer
and the mutual alignment of the apertures and the corresponding
openings are determined by measuring the amount of light passing
through the openings by means of photodiodes arranged behind the
openings.
[0057] As EUV radiation is absorbed by glass, such an image sensor
cannot be used in an EUV lithographic apparatus. For such an
apparatus, the light-transmitting zones of the image sensor should
be openings to its radiation-sensitive elements. Moreover, these
openings should be much smaller than the light-transmitting zones
in the image sensor of U.S. Pat. No. 4,540,277. The structure of
openings for an EUV image sensor is typically a grating structure
with grating slits.
[0058] FIG. 2 shows a cross-section of a small part (only two
grating periods PE are shown) of an embodiment of such a grating
pattern. The grating slits SL have a right-angled cross-section.
The slits have a width WI of 50-150 nm and a depth d of 50-100 nm.
The grating period, or pitch, PE is of the order of 2000 nm. These
grooves are processed in a metal layer (ML), for example nickel
(Ni) or silver (Ag). The slit layer may be deposited on an
opto-electronic device OED, which comprises an EUV
radiation-sensitive detector DE, which converts the incident
radiation into an electric signal. The grating may be a
onedimensional or a two-dimensional grating, i.e. the grating slits
extend in one direction or in two, for example mutually
perpendicular, directions. These kinds of grating are used to
measure in one direction or in two directions, respectively. The
electronic circuitry for processing the detector signals may be
integrated in the opto-electronic device OED. Between the grating
and the OED, a radiation-converting layer CL may be interposed,
which converts the EUV radiation into a radiation for which the
detector, for example a photodiode, shows a better sensitivity.
[0059] According to the invention, a grating pattern like that
shown in Fig.2 and having the required quality can be obtained in a
relatively simple manner by performing the processing steps
illustrated in FIGS. 3a-3d. As shown in Fig.3a, a substrate 10, for
example a silicon substrate, or an OED (not shown) is coated with a
layer 14 of a conductive material, preferably molybdenum. This
layer is deposited by means of a sputter process. The layer 14 is
covered with a layer 16 of hydrogen silsesquioxane, which is a
negative tone resist sensitive to electron beam (E-beam) radiation,
for charged-particles radiation in general and also for
electromagnetic radiation with a wavelength smaller than 1576 nm.
When necessary, the resist may be submitted to a soft bake, for
example heated to 120-150.degree. for 2 minutes, which does not
change the essential characteristics of the resist. Next, as is
shown in FIG. 3b by the arrows EB, the resist layer 16 is
illuminated by an electron beam at those positions where
transparent strips are to be formed. A "writing " E-beam performs
this illumination, i.e. the E-beam is positioned at a point where a
grating strip has to start or to end and is scanned over a length
corresponding to the length of the strip to be formed. Instead of
an E-beam writing apparatus, an E-beam projection apparatus can be
used. Then the resist layer is illuminated by a broad beam via a
mask which contains a mask pattern corresponding to the pattern of
features to be formed. The electrons entering the HSQ layer cause a
cross-linking of the HSQ material. As a subsequent step, the HSQ
resist layer is developed and nonilluminated resist is removed. The
illuminated HSQ material, at the positions of the strips, remains
and this material forms a pattern of ridges 18 as shown in FIG. 3c.
Then a nontransparent layer 20 of plating material such as metal is
deposited by means of electroplating between these ridges and
outside the area of the ridges, as shown in FIG. 3d. An advantage
of the method is that no plating material is deposited on top of
the ridges.
[0060] The layer 20 may be a silver or nickel layer. If the pattern
of features is for use in an EUV apparatus, this layer preferably
comprises the alloy Ni.sub.0.78Fe.sub.0.22, known as Permalloy. The
attenuation length of this material for EUV radiation is 15 nm,
which means that after this beam has passed a 15 nm thick permalloy
layer, its intensity is reduced to 1/e (or 37%) of the original
intensity. For example, 100 nm thick molybdenum transmits 0,8% of
EUV radiation incident on it, so that such a layer is a
non-transparent layer for EUV radiation. The cross-linked HSQ
material of the ridges 18, which behaves like SiO.sub.2, has an
attenuation length of 98 nm and molybdenum has an attenuation
length of 162, nm so that these materials are to a high degree
transparent to EUV radiation, provided that their thickness is not
too large.
[0061] The pattern of ridges 18 obtained by the method of the
invention illustrated in FIGS. 3a-3d thus forms a pattern of
transparent slits in a non-transparent surface area. The advantage
of this method is that the ridges 18 and the molybdenum layer 14 do
not need to be removed to obtain a grating pattern which shows
sufficient contrast, i.e. a pattern having strips the transparency
of which differs sufficiently from that of their environment for
EUV radiation.
[0062] Such a grating pattern may be deposited on a detector or an
opto-electronic device to obtain an EUV image sensor. If the
sensitive layer of the detector is buried under an additional
layer, such layers should be removed beforehand so that the
detector is freed. For this application and for a grating structure
in general, the structure of ridges is periodic, which means that
the distance between all ridges is the same. The width WI of the
ridges, and thus of the grating slits, is determined by the width
of the electron beam EB and the dose of the beam, which is
expressed in .mu.C/cm.sup.2 wherein C stands for the unity of
charge: Coulomb. For patterns other than a grating pattern, for
example a mask pattern, the dose and width of the electron beam can
be varied to obtain a pattern having slits of different width, as
is shown in FIGS. 3c and 3d by the ridges 18a, 18b and 18c.
[0063] The method may also be performed with a resist material, an
electroplating base material and a plating material, which are
different from the materials mentioned above. The choice of the
materials is determined by the envisaged application of the pattern
of features and in particular by the wavelength of the radiation
used in such an application. The plating material should be
non-transparent to the radiation and both the resist material and
the electroplating base material should be transparent in case the
resist ridges remain present in the end product.
[0064] Instead of a one-dimensional pattern, a two-dimensional
pattern can also be produced. In the latter case, the electron beam
has to scan the HSQ layer in two, for example perpendicular,
directions at those positions of the HSQ layer where transparent
areas are to be formed.
[0065] FIG. 4 shows the principle of the electroplating process.
The substrate with the layer 14 and the pattern of ridges 18 is
brought into a holder 30 filled with an electrolyte 32, which
comprises the metal that is to be deposited on the plating base
layer 14 and between the ridges. The layer 14 is electrically
connected, for example by means of copper terminal block or clip
34, to the first pole of a current source 36. The holder comprises
an electrode 38, which is connected to the second pole of the
current source. When the source is switched on, electrons are
injected in the base layer 14 so that this layer becomes negatively
electrically charged. This layer begins to attract the metal ions
from the electrolyte 32. At the surface of layer 14, the ions are
de-charged and the neutral metal atoms precipitate on the surface
to form a metal layer 20 on the surface of the layer 14.
[0066] In order to obtain a stable metal layer of constant
thickness, it is necessary that the electrical resistance of the
plating base layer is smaller than that of material to be
deposited. If this is not the case, the metal ions will be first
deposited on the clip 34 and then on the already formed portion of
metal layer, instead of on the entire surface of the base layer 14.
An unstable metal layer of varying thickness will then be formed.
In general, materials having a low electrical resistance, i.e. a
high conductivity such as silver (Ag), aluminum (Al), gold (Au) and
copper (Cu) are suitable materials for the plating base layer 14.
These materials allow plating of a large number of metals having a
lower conductivity. If a pattern of slits or openings, which are
transparent to EUV radiation is to be produced, a layer of
molybdenum should be used as a plating base layer As this material
has a relatively high resistance of 5.20 .mu..OMEGA..cm, the choice
of materials which can be plated is limited. However, an alloy of
nickel (Ni) having a higher resistance of 6.84 .mu..OMEGA..cm, such
as Ni.sub.0.78Fe.sub.0.22 (permalloy) is a very suitable material
for this purpose.
[0067] To prevent metal from being deposited on the lower surface
of the substrate, this surface could be coated with an isolating
layer. For the same purpose, an isolating layer 12, shown in the
FIG. 3, is preferably applied to the upper surface of the
substrate. The layer 12 may be a silicon dioxide (SiO.sub.2) layer.
When the pattern on structure forms part of an EUV image sensor,
the isolating layer is preferably a silicon nitride
(Si.sub.3N.sub.4) layer. With such a layer, a good adhesion of the
plating base layer can be obtained and oxidation can be prevented.
The Si.sub.3N.sub.4 layer may form the top layer of the detector of
the EUV image sensor.
[0068] In an embodiment of the method, a molybdenum layer with a
thickness of 50 nm is used as a plating base layer and the
deposited Ni.sub.0.78Fe.sub.0.22 layer 20 has a thickness of 100
nm. Ridges having a width in the 100 nm range and in the sub-100 nm
range down to 50 nm or even less have been produced. The height of
the produced ridges may vary in a range of several tens to several
hundreds of nm.
[0069] By way of example, FIG. 5a shows a SEM (scanning electron
microscope) photograph of a central portion of an HSQ ridge
protruding from a Ni.sub.0.78Fe.sub.0.22 layer produced with an
electron beam dose of 700 .mu.C/cm.sup.2. FIG. 5b shows the central
portion of such a ridge produced with a dose of 500 .mu.C/cm.sup.2.
The design width of the ridge is 160 nm and the measured width is
about 160 nm. These Figures demonstrate the influence of the
electron beam dose on the width of the produced ridge. FIG. 6 shows
a SEM photograph, in perspective view, of two HSQ ridges protruding
from a Ni.sub.0.78Fe.sub.0.22 layer produced with an electron beam
dose of 1500 .mu.C/cm.sup.2. The design width of the ridges is 40
nm and the measured width is 50 mn.
[0070] The height of the HSQ ridges that can be produced is limited
by the electron forward scattering in HSQ, thus by the penetration
depth of electrons in this material. If ridges with a height of
more than 100 nm are required, a double layer resist comprising a
negative tone resist HSQ upper coat and a hard-baked novolak resist
as a lower coat can be used as resist layer, instead of a single
HSQ resist layer 16. FIG .7 shows a small part of an embodiment of
a pattern of ridges obtained by using such a double layer. These
ridges 18 are composed of a part 18a of novolak and a top part 18b
of cross-linked HSQ. The thickness of the top part may be of the
order of 100 nm and that of the lower part from 100 nm up to, for
example, 600 nm. For details about the HSQ/novolak double layer and
its use in E-beam lithography, reference is made to the
above-mentioned paper: "Hydrogen silses quioxane/novolak bilayer
resist for high aspect ratio nanoscale e-beam lithography". The
thickness of the ridges is determined by the required contrast in
the pattern of features, e.g. the grating, to be produced.
[0071] The method may also be used to produce a fine grating
pattern wherein the width of the transparent slits is of the order
of the width of the non-transparent areas between these slits. For
example, such a grating may have a duty cycle of 0.5, which means
that the width of the transparent slits is equal to that of the
non-transparent slits. Such a grating may be an optical grating,
i.e. a grating for visible and ultraviolet light. To obtain such a
grating, the ridges of HSQ of FIG. 3d are removed so that only the
metal layer 20 provided with slits remains. For a transmission
layer, the substrate onto which the grating is formed should be
transparent. For obtaining a reflective grating, the substrate
should be reflective.
[0072] An optical grating produced by the method of the invention
may be used in any optical apparatus where such a fine grating is
needed, for example for diffraction of a radiation beam, for beam
splitting, for colour separation of a white beam, etc. Such a
grating may also be used as an alignment mark in a lithographic
projection apparatus, for example an EUV projection apparatus such
as the mask alignment mark M1 or M2 in FIG. 1. As the transparent
slits which can be produced by the present method may be very
small, a very accurate alignment of the mask alignment mark, and
thus of the mask with respect to a reference mark becomes
possible.
[0073] The method can also be used to produce a pattern of fine
features, other than a grating, for use in an imaging system such
as an optical imaging system, an EUV imaging system and even an
X-ray imaging system. The pattern is, for example, an array of
annular transparent strips, or slits, having a varying width, which
strips together form a Fresnel lens.
[0074] The method can be further used for the production of
lithographic masks, for example, EUV masks. The method can be
applied for producing features of the mask pattern itself, for
example an IC pattern and/or for producing so-called assisting
features, for example scattering bars, which compensate for
proximity effects occurring when imaging fine mask patterns.
Currently, an electron beam writing apparatus is already used for
producing transparent masks for EUV and EUV lithography and
reflective masks for EUV lithography. The use of a HSQ resist and
electroplating allows production of a pattern of features, which
have well-defined perpendicular walls, also in materials hitherto
known as transition materials. Such a pattern cannot be obtained
with ion etching or lift-off techniques. Because the features are
in the form of ridges, they cannot fill with dust. In case the
ridges are HSQ ridges, they cannot be ablated by EUV radiation.
[0075] In general, the method can be used for accurately producing
sub-micron, and especially sub-100 nm wide slits, or transparent
strips in a metallic layer. Such a layer can be applied not only in
lithographic apparatus or, more generally, in optical apparatus,
but also in devices for other arts of techniques. An example of
such a device is a thin-film magnetic recording head. FIG. 9 is a
top view of an embodiment of such a magnetic head. For reasons of
comparison, a known thin-film magnetic head is shown in FIG. 8. The
known head comprises a yoke 40 of magnetically permeable, or
magnetizable, material, which yoke is provided with a gap 42 filled
with a non-magnetizable material 44. The gap has a length le and a
height he. A coil is wrapped around the yoke, of which coil only
one winding 46 is shown. The arrow 48 indicates the direction along
which the optical head and a track of the magnetic record carrier
(not shown) move relative to each other, in order to scan the
record carrier. This record carrier is read out by detecting the
variations in the magnetization of the head induced by the magnetic
domains on the carrier.
[0076] In the magnetic head of FIG. 9, the gap is replaced by a
ridge 52 of resist material such as HSQ, which ridge is embedded in
a layer of magnetically permeable material 51. Arrow 58 indicates
the track scanning direction. This head is produced by forming a
ridge of resist, preferably HSQ, on an electroplating base layer in
the way as described above and by electroplating the surroundings
of the ridge, with the exception of 57 which is reserved for the
coil windings, with a layer 51 of a magnetizable material. The
thickness of this layer may be equal to the ridge, which may be of
the order of 5 .mu.m. The ridge may have a width of, for example
100-200 nm.
* * * * *