U.S. patent application number 10/169816 was filed with the patent office on 2003-07-31 for device and method for carrying out spatially directed detection of an electroencephalogram.
Invention is credited to Berkes, Sebastian, Henning, Gunter, Husar, Peter, Schellhorn, Klaus, Schlegelmilch, Falk.
Application Number | 20030144599 10/169816 |
Document ID | / |
Family ID | 7664365 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030144599 |
Kind Code |
A1 |
Husar, Peter ; et
al. |
July 31, 2003 |
Device and method for carrying out spatially directed detection of
an electroencephalogram
Abstract
A first layer of photoresist material, sensitive to radiation of
a first wavelength, has a second layer of photoresist material,
sensitive to radiation of a second wavelength, deposited thereon. A
pattern of radiation of the second wavelength is then formed on the
second layer of photoresist material which is subsequently treated
with a solvent, in which the first layer of photoresist material is
insoluble, to develop a first periodic profile. The first layer of
photoresist material is then exposed to radiation of the first
wavelength through the first periodic profile and treated to
develop a second periodic profile. By directing the radiation of
the second wavelength through the first periodic profile a an angle
to normal to the first photoresist material a blaze profile may be
obtained.
Inventors: |
Husar, Peter; (Ilmenau,
DE) ; Henning, Gunter; (Ilmenau, DE) ;
Schellhorn, Klaus; (Ilmenau, DE) ; Berkes,
Sebastian; (Ilmenau, DE) ; Schlegelmilch, Falk;
(Ilmenau, DE) |
Correspondence
Address: |
Douglas J Christensen
Patterson Thuente Skaar & Christensen
4800 IDS Center
80 South Eighth Street
Minneapolis
MN
55402
US
|
Family ID: |
7664365 |
Appl. No.: |
10/169816 |
Filed: |
November 12, 2002 |
PCT Filed: |
November 21, 2001 |
PCT NO: |
PCT/EP01/13476 |
Current U.S.
Class: |
600/544 |
Current CPC
Class: |
A61B 5/369 20210101;
A61B 5/291 20210101 |
Class at
Publication: |
600/544 |
International
Class: |
A61B 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 22, 2000 |
DE |
100 58 128.5 |
Claims
1. A process for making a blazed periodic profile in a photoresist
layer comprising the steps of; taking a first layer of photoresist
material which will react to radiation of a first wavelength and
which has deposited on it a second layer of photoresist material
which will react to radiation of a second wavelength wherein the
first photoresist material is substantially insoluble in the second
solvent and wherein the layer of second photoresist material is
substantially opaque to the radiation of the first wavelength;
exposing the layer of second photoresist material to a pattern of
radiation of the second wavelength; treating the layer of second
photoresist material with the second solvent so as to develop a
first periodic profile; exposing the layer of first photoresist
material to radiation of the first wavelength through the first
periodic profile; treating the layer of first photoresist material
with the first solvent so as to develop a second periodic profile,
wherein the step of exposing the layer of first photoresist
material to radiation of the first wavelength through the first
periodic profile is performed with the radiation being directed
toward the first layer at an angle away from the normal to the
first photo-resist material.
2. A process for making a periodic profile in a photoresist layer
as claimed in any of the preceding claims and further comprising
the initial step of depositing the layer of second photoresist
material on the layer of first photoresist material.
3. A process for making a periodic profile in a photoresist layer
as claimed in claim 2 and further comprising the initial step of
depositing a layer of first photoresist material on a
substrate.
4 A process for making a periodic profile in a photoresist layer as
claimed in any of the preceding claims wherein the layer of second
photoresist material is a positive photoresist material.
5. A liquid crystal device comprising a photoresist layer having a
blazed periodic profile, the blazed periodic profile being
configured to impart alignment to liquid crystal molecules of the
liquid crystal device, wherein the blazed periodic profile is
formed using the process as claimed in any one of the preceding
claims.
Description
[0001] This invention relates to a process for making periodic
profiles, in particular a to a method of fabricating grating
structures, especially blazed grating structures in photoresist
material.
[0002] Grating structures, which are structures having a periodic
profile in one particular cross section, can be fabricated using a
variety of techniques. A convenient and well known method of
producing a grating structure is to expose a layer of photo-resist
material to a pattern of light of a periodically varying intensity.
For a positive photoresist material, any area which is exposed to
light can be removed by a development process whilst for a negative
photoresist material any area which is not exposed to light can be
removed by a development process. Photoresist materials are
generally only photosensitive to radiation in a certain range.
[0003] One method of producing light of a periodically varying
intensity is to form an interference pattern. The exposure of a
photoresist layer to such an interference pattern, which can be
produced by any number of interferometric techniques that are well
known to a person skilled in the art, allows a range of symmetric
grating profiles to be produced. However, by the nature of this
technique, it is difficult to obtain a high degree of grating
asymmetry uniformly over a large area. Obtaining short pitch
gratings with large groove depths is also difficult using this
technique. To obtain grating profiles other than sinusoidal
requires over-exposure of the layer, and the level of background
radiation associated with the interference pattern may then expose
the whole photoresist layer.
[0004] A greater range of grating profiles may be obtained using
hard contact photolithography, in which a chrome mask is placed in
intimate contact (within 0.2 .mu.m) with the photo-resist coated
substrate. This approach allows asymmetric grating profiles to be
attained but, because of the requirement for the mask to be in
intimate contact with the substrate, it is difficult to realise
asynmmetric gratings over a large area. Additionally, the minimum
pitch of grating which can be produced using this technique is
limited by the resolution of chrome mask fabrication. The highest
resolution chrome mask which can be readily fabricated is of
approximately 0.8 .mu.m pitch, and such masks are generally
fragile, therefore having a short useful lifetime, and may produce
unwanted diffraction effects.
[0005] It is also possible to fabricate grating structures using
ruling techniques, but these tend to be prohibitively
expensive.
[0006] It is an object of the present invention to provide an
inexpensive method of making gratings that mitigates some of the
disadvantages, as described above, that are associated with
conventional methods of grating fabrication.
[0007] According to this invention, a process for making a periodic
profile in a photoresist layer comprises the steps of taking a
first layer of photoresist material which will react to radiation
of a first wavelength and which has deposited on it a second layer
of photoresist material which will react to radiation of a second
wavelength and wherein the first photoresist material is
substantially insoluble in the second solvent and wherein the layer
of second photoresist material is substantially opaque to the
radiation of the first wavelength, exposing the layer of second
photoresist material to a pattern of radiation of the second
wavelength, treating the layer of second photoresist material with
the second solvent so as to develop a first periodic profile,
exposing the layer of first photoresist material to radiation of
the first wavelength through the first periodic profile and
treating the layer of first photoresist material with the first
solvent so as to develop a second periodic profile.
[0008] The use of dual layers of photoresist material in the
process offers various advantages, including the ability to
fabricate short pitch gratings which, because of the proximity of
the mask formed in the second photoresist material, can be formed
deep into the first photoresist material with a very high
resolution. This mitigates some of the disadvantages associated
with the chrome mask method described above.
[0009] Additional advantages of the dual layer photoresist process
also include the masked areas of the layer of first photoresist
material having minimal exposure to radiation of the first
wavelength when the layer of first photoresist material is exposed
to radiation of the first wavelength through the first periodic
profile. This mitigates some of the disadvantage associated with
the known technique of exposing a single layer of photoresist to an
interference pattern.
[0010] A preferred embodiment of this invention provides a process
for making a periodic profile in a photoresist layer wherein
exposing the layer of first photoresist material to radiation of
the first wavelength through the first periodic profile is
performed with the radiation being directed toward the first layer
at an angle away from the normal to the first photoresist material.
This allows blazed grating structures to be fabricated. A
particular advantage of this embodiment is that it allows deep,
short pitch blazed gratings to be made.
[0011] Additionally, the process for making a periodic profile in a
photoresist layer described above may further comprise the initial
step of depositing a layer of second photoresist on the layer of
first photoresist and possibly depositing a layer of first
photoresist on a substrate.
[0012] In a preferred embodiment, the process for making a periodic
profile in a photoresist layer, as described above, makes use of
PMGI as the first photoresist material and Shipley's S1805 resist
as the second photoresist material.
[0013] A person skilled in the art would immediately recognise that
the gratings made using the process described above could be used,
amongst other things, as diffraction gratings or to impart
alignment to liquid crystal molecules as part of a liquid crystal
device (for example the Zenithal Bistable Device of WO 97/14990 or
the Azimuthal Bistable device of GB Application 0001955.4).
[0014] This invention will now be described, by way of example
only, with reference to FIGS. 1 to 3, wherein;
[0015] FIG. 1 illustrates the principle behind the dual photoresist
technique for blazed grating fabrication;
[0016] FIG. 2 is a cross-sectional SEM image of a deep groove
symmetric grating fabricated using the dual photoresist technique;
and
[0017] FIG. 3 is a cross-sectional SEM image of an asymmetric
grating fabricated using the dual photoresist technique.
[0018] An general embodiment of the present invention will now be
described with reference to FIG. 1.
[0019] Referring to FIGS. 1(a) and 1(b), a substantially flat
substrate (2) is spin-coated with a layer of PMGI photoresist
material (4). A layer of Shipley's S1805 photo-resist (6) is then
spin coated on top of the layer of PMGI material (4).
[0020] An interference pattern (8) is formed in the vicinity of the
layer of Shipley's S1805 photo-resist material (6), using laser
light of 457.8 nm wavelength. The Shipley's S1805 is sensitive to
the 457.8 nm radiation, but the PMGI is insensitive to light of
such a wavelength. After the Shipley's S1805 layer has been exposed
for an appropriate time the grating is developed using the
recommended Shipley process. The layer of PMGI is insoluble in the
solvent that is used to develop the Shipley's S1805 layer.
[0021] Referring to FIG. 1(d), after developing the Shipley's S1805
material a grating structure (10) is formed in what was the
substantially flat layer of Shipley's S1805 material (6).
[0022] The PMGI is sensitive to radiation of 254 nm in wavelength,
whilst the Shipley's S1805 layer is opaque at this wavelength and
will act as a mask. Referring to FIG. 1(d), exposing the PMGI layer
(4) to a collimated light source of 254 nm (12) through the mask
formed by the Shipley's S1805 layer (10) results in a periodic
variation of the intensity of light to which the PMGI layer (4) is
exposed. The PMGI can then be developed, whereupon a grating
structure (14) is formed in the previously substantially flat PMGI
layer.
[0023] The angle at which the substantially collimated light is
incident on the PMGI layer, (4), through the mask formed by the
Shipley's S1805 layer (10), can be varied away from normal
incidence thereby producing a blazed grating. The resolution of the
mask formed by the Shipley's S1805 layer (10) is limited only by
the resolution of the interference pattern, therefore short pitch
grating structures (down to around 200 nm) are readily attainable.
The close proximity of the mask formed by the Shipley's S1805 layer
(10) and the underlying PMGI layer (4) will also allow relatively
deep grating grooves, for a given pitch, to be attained.
[0024] Any two photo-resist materials may be used in the method
described above, provided that:
[0025] i) the first layer (i.e. the layer in contact with the
substrate) is insoluble in the solvents used to process the second
(i.e. upper most) layer;
[0026] ii) the first layer is insensitive to light used to expose
the second layer; and
[0027] iii) that the second layer is opaque to light of the
wavelength used to expose the first layer.
[0028] The layer of second photoresist material is ideally a
positive material, although a negative material would be effective
to a lesser extent, whilst the layer of first photoresist material
may be either a positive or negative material.
EXAMPLE 1
[0029] The technique described with reference to FIG. 1 above has
been used to fabricate a grating structure possessing deep grooves.
The precise fabrication process used will now be described with
reference to FIG. 2.
[0030] A glass substrate was cleaned and neat PMGI
(poly(dimethyl-glutarim- ide) supplied by the Microlithography
corporation) was deposited on it. The substrate was spun at 3000
rpm for 30 seconds. The solvent was removed by placing on a
hotplate at 90.degree. C. for 3 minutes. The substrate was then
oven baked on a hotplate at 250.degree. C. for 15 minutes which
results in 1.1 .mu.m thick film.
[0031] When cool, a layer of Shipley photoresist S1805 was spun (at
3000 rpm) onto the substrate which resulted in a 550 nm film of
photoresist. The substrate was prebaked on hotplate at 100.degree.
C. for 10 minutes.
[0032] The substrate was exposed in interferometer, for 900
seconds, to radiation of wavelength .lambda.=457.9 nm using a 300
mW expanded laser beam.
[0033] The substrate was developed in Shipley MF319 developer for
45 seconds to obtain a truncated profile in photoresist.
[0034] The substrate was then exposed at normal incidence, and for
400 seconds, to light of .lambda.=254 nm. The substrate was
developed in MF319 for 10 seconds to obtain symmetric grating
profile shown in FIG. 2.
EXAMPLE 2
[0035] The technique described with reference to FIG. 1 above has
been used to fabricate a blazed grating structure. The precise
fabrication process used will now be described with reference to
FIG. 3.
[0036] A glass substrate was cleaned and neat PMGI
(poly(dimethyl-glutarim- ide) supplied by the Microlithography
corporation) was deposited on it. The substrate was spun at 3000
rpm for 30 seconds. The solvent was removed by placing on a
hotplate at 90.degree. C. for 3 minutes. The substrate was then
oven baked on a hotplate at 250.degree. C. for 15 minutes which
results in 1.1 .mu.m thick film.
[0037] A layer of S1805 photoresist diluted 2:1 (resist to
thinners) in thinner PGMEA was spun on to the substrate, providing
a 303 nm layer of S1805 resist. The substrate was then prebaked at
100.degree. C for 10 minutes.
[0038] The substrate was exposed in interferometer, for 600
seconds, to radiation of wavelength .lambda.=457.9 nm using a 300
mW expanded laser beam.
[0039] The substrate was developed in Shipley MF319 developer for
25 seconds to obtain a truncated profile in photoresist.
[0040] The substrate was then exposed, for 400 seconds, to light of
.lambda.=254 nm incident at an angle of 18.degree. away from the
substrate normal, The substrate was developed in MF319 for 10
seconds to obtain symmetric grating profile shown in FIG. 2.
[0041] The reason for using thinner resist for the 18.degree.
exposure was that self shadowing eventually becomes a problem for
large exposure angles for example gratings exposed at 60.degree.
failed even under these conditions, nevertheless, because the
photoresist acts as a conformal mask, we obtain a high degree of
asymmetry even from a modest asymmetry exposure angle. A
photoresist layer 303 nm thick preserved good contrast in the PMGI
whilst being as thin as possible.
* * * * *