U.S. patent application number 10/880047 was filed with the patent office on 2005-12-29 for method of making grayscale mask for grayscale doe production by using an absorber layer.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Liu, Xinbing.
Application Number | 20050287445 10/880047 |
Document ID | / |
Family ID | 35482516 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050287445 |
Kind Code |
A1 |
Liu, Xinbing |
December 29, 2005 |
METHOD OF MAKING GRAYSCALE MASK FOR GRAYSCALE DOE PRODUCTION BY
USING AN ABSORBER LAYER
Abstract
The present invention relates to manufacturing grayscale masks
that are used for mass-producing grayscale DOEs. More specifically,
the present invention provides a method whereby a grayscale mask is
fabricated by using an absorber layer and a photoresist with a
laser writer. The method of the present invention includes the
steps of providing a substrate with a known layer of absorber and a
layer of photoresist, exposing the photoresist to a grayscale
pattern from a laser writer, developing the photoresist into
variable thickness, and transferring the surface relief pattern
from the photoresist layer onto the absorber layer by etching.
Inventors: |
Liu, Xinbing; (Acton,
MA) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
Osaka
JP
|
Family ID: |
35482516 |
Appl. No.: |
10/880047 |
Filed: |
June 29, 2004 |
Current U.S.
Class: |
430/5 |
Current CPC
Class: |
G03F 1/54 20130101; G03F
1/50 20130101 |
Class at
Publication: |
430/005 |
International
Class: |
G03F 001/00 |
Claims
1. A method of manufacturing a grayscale mask, comprising:
providing a substrate having a layer of absorber that includes a
carrier material embedded with a light absorbing material and
having a layer of photoresist; exposing the layer of photoresist to
a grayscale pattern from a laser writer; developing the photoresist
layer into a surface relief pattern of variable thickness;
transferring the surface relief pattern from the photoresist layer
onto the absorber layer by etching; and adjusting a concentration
of the light absorbing material embedded in the carrier material in
order to vary a maximum absorption of the absorber layer.
2. The method of claim 1, wherein providing the substrate includes
providing a layer of absorber atop which the layer of photoresist
is overlaid.
3. The method of claim 1, wherein providing the substrate includes
providing a fused silica (SiO.sub.2), which is a polished and
transparent glass substrate.
4. The method of claim 1, wherein providing the substrate includes
providing a substrate having a layer of absorber including
semiconductor material.
5. The method of claim 1, wherein providing the substrate includes
providing a substrate having a layer of absorber including
silicon.
6. The method of claim 1, wherein providing the substrate includes
providing a substrate having a layer of absorber including glass
embedded with absorbing quantum dots.
7. (canceled)
8. The method of claim 1, wherein providing the substrate includes
providing a carrier material embedded with a light absorbing
material that is a dye.
9. The method of claim 1, wherein providing the substrate includes
providing a carrier material embedded with a light absorbing
material that is a pigment.
10. The method of claim 1, wherein providing the substrate includes
providing a carrier material embedded with a light absorbing
material that is nanocrystals (quantum dots).
11. The method of claim 1, wherein providing the substrate includes
providing a carrier material that is a polymer material in a
solvent.
12. The method of claim 1, wherein providing the substrate includes
providing a carrier material that is the photoresist layer.
13. The method of claim 1, wherein providing the substrate includes
providing a carrier material in which absorbing materials are
embedded with various concentrations, thus varying the amount of
light absorbed.
14. The method of claim 1, wherein providing the substrate includes
providing a substrate having absorber spin-coated to a desired
thickness on the substrate.
15. The method of claim 1, wherein providing the substrate includes
determining a target absorption based on a target optical density
and a thickness of the layer of absorber.
16. The method of claim 15, wherein determining the target
absorption includes determining the target optical density based on
a dynamic range of grayscale levels of the grayscale pattern.
17. The method of claim 15, wherein determining the target
absorption includes determining the thickness of the layer of
absorber based on a feature size of a product to be manufactured by
use of the mask, and based on a depth of focus of an imaging system
capable of using the mask to manufacture the product.
18. The method of claim 15, further comprising dispersing the
absorber material in the carrier material to achieve the target
absorption by attaining an absorption material concentration as a
function of the thickness of the layer of absorber and absorption
characteristics of the absorber material.
19. The method of claim 1, wherein providing the substrate includes
providing a substrate having the layer of absorber, wherein the
layer of absorber has a thickness in a range of 0.1-3.0 micrometers
(.mu.m), such that light may transmit variably through the mask,
depending on its thickness, due to exponential attenuation of light
with thickness in an absorbing material.
20. The method of claim 1, wherein exposing the layer of
photoresist to the grayscale pattern from the laser writer includes
exposing the layer of photoresist to a focused laser beam from a
laser writer in a grayscale manner which, by varying light
intensity, creates an exposed photoresist and grayscale
pattern.
21. The method of claim 1, wherein exposing the layer of
photoresist to the grayscale pattern from the laser writer includes
exposing a layer of positive photoresist to the greyscale pattern
from the laser writer.
22. The method of claim 1, wherein exposing the layer of
photoresist to the grayscale pattern from the laser writer includes
exposing a layer of negative photoresist to the greyscale pattern
from the laser writer.
23. The method of claim 1, wherein exposing the layer of
photoresist to the grayscale pattern from the laser writer includes
exposing the layer of photoresist to the grayscale pattern from an
electron-beam (e-beam) writer, wherein the layer of photoresist is
an e-beam resist.
24. The method of claim 1, wherein developing the photoresist layer
into a surface relief pattern of variable thickness includes
causing exposed photoresist to exfoliate, thereby producing the
surface relief pattern.
25. The method of claim 1, wherein transferring the surface relief
pattern from the photoresist layer onto the absorber layer by
etching includes using an ion-beam milling process, wherein
ion-beam milling is performed uniformly over the photoresist layer,
such that chemically developed photoresist and, subsequently,
absorber layer uniformly mills.
26. The method of claim 1, wherein transferring the surface relief
pattern from the photoresist layer onto the absorber layer by
etching includes using a reactive ion etching (RIE) process.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to manufacturing grayscale
masks that are used for mass-producing grayscale diffractive
optical lenses (DOEs). More specifically, the present invention
provides a method whereby a grayscale mask is fabricated by
exposing photoresist to a laser writer.
BACKGROUND OF THE INVENTION
[0002] There is an ever-increasing demand for more sophisticated,
smaller, and less expensive consumer electronic devices in today's
high-tech marketplace. Consumer products, such as the optical disc
reader and writer (e.g., compact disc (CD), digital versatile disc
(DVD), and blu-ray disc (BD) players and writers), are selling in
record numbers. As a result, new and innovative technologies for
these optical disc readers and writers keep emerging from
developers. One area of design in the optical disc readers and
writers is the optical disc pick up.
[0003] The optical pickup uses DOEs and lasers to optically read or
write to an optical disc (e.g., CD, DVD, or BD). Unlike
conventional optical components that utilize refraction and/or
reflection, the DOE enables parallel processing by optically
diffracting and directly controlling the optical phases. Therefore,
a wide range of applications, including, for example, multi-spot
beam splitters or shapers, can be expected as a result of this
preferred benefit. Conventionally, the DOEs in optical pickups are
manufactured to be binary, i.e., the DOEs only have two phase
levels. Binary DOEs are easier to manufacture because they are
compatible with the standard semiconductor fabrication processes,
which are well developed. However, binary DOEs generally suffer
lower diffraction efficiencies. Additionally, binary DOEs produces
symmetric diffraction orders (e.g., a -1 order has the same
intensity as a +1 order), whereas for many DOE
applications--optical pickups included--asymmetric diffraction
orders are desired. These drawbacks result in an inefficient
optical pickup. Therefore, there exists a need to provide a more
efficient DOE which does not employ binary beam splitting for
optical pickups.
[0004] The grayscale DOE, a DOE that has more that two phase
levels, is a type of DOE that can be more efficient and produce
asymmetric diffraction orders. However, grayscale DOEs have been
difficult and expensive to fabricate and, therefore, inhibit
manufacturers from using them in optical pickups. Therefore, there
exists a need to provide a means of inexpensively manufacturing
grayscale DOEs.
[0005] Currently, there are three major methods of manufacturing
grayscale masks which are used to fabricate grayscale DOEs.
[0006] First, a method of creating grayscale masks for grayscale
DOE fabrication is described in U.S. Pat. No. 5,310,623, entitled,
"Method for fabricating microlenses." The '623 patent details a
half-tone method of manufacturing grayscale masks. However,
half-tone manufacturing techniques employ a series of binary
pixels, which collectively vary the transmission of light
approximating a grayscale mask which may be used to create
grayscale DOEs. Because this is another binary approach, the
resolution is limited. Therefore, there exists a need to provide a
means of fabricating grayscale masks with greater resolution than
that of half-tone grayscale masks.
[0007] Second, a method of creating grayscale masks for grayscale
DOE fabrication is described in U.S. Pat. No. 5,078,771, entitled,
"Method of making high energy beam sensitive glasses." The '771
patent details a method whereby a high-energy beam sensitive-glass
(HEBS-glass) illuminates the mask with varying intensities, and
thereby creates a grayscale mask. However, the HEBS-glass
transmission often changes during the exposure times, which results
in non-identical grayscale masks. Therefore, there exists a need to
provide a means of identically manufacturing a number of grayscale
masks.
[0008] Third, a method of making a grayscale mask for manufacturing
grayscale DOEs is described in U.S. Pat. No. 6,638,667, entitled,
"Fabricating optical elements using a photoresist formed using of a
gray level mask." The '667 patent details a method whereby
grayscale patterns are created by varying the thickness of a light
absorber layer. The varying thickness in the absorber layer is
created by using a series of binary masks. However, employing a
series of binary masks is a cumbersome and costly means of
manufacturing a grayscale mask. Also, a series of binary masks
create only an approximation of a true grayscale mask. Therefore,
there exists a need to provide a means of creating true grayscale
masks in an efficient manner without employing binary masks.
[0009] The '667 patent further discloses use of a nickel alloy
called "inconel" as the absorber layer. However, because metals
have very high light attenuation, the metal layer must be very thin
(.about.0.1 .mu.m or less) to allow adequate light to transmit
through the layer. In practice it is difficult to control the
thickness of such layers due to the small overall thickness
required--any small variation in thickness can have a large
variation in transmitted light. In U.S. Pat. No. 6,613,498,
assigned to Mems Optical, the use of SiO for the absorber layer is
described. However, an absorber material, such as SiO, has fixed
absorption coefficient at a given wavelength. It is difficult to
obtain desired total absorption and the desired thickness since the
absorption coefficient is fixed--to obtain a desired maximum
absorption one must use a fixed thickness. Therefore, there exists
a need to provide a means of absorber layer with adjustable levels
of absorption. Also, applying the metal layer or SiO absorber by
evaporation can be a difficult process. Therefore, there exists a
need to provide a means of applying the absorber layer in an easy
and convenient way.
[0010] It is therefore an object of the invention to provide a
means of creating a more efficient DOE for optical pickups which do
not employ binary beam splitting.
[0011] It is another object of the invention to provide a means of
inexpensively manufacturing grayscale DOEs.
[0012] It is yet another object of the invention to provide a means
of creating true grayscale masks in an efficient manner without
employing binary masks.
[0013] It is yet another object of the invention to provide a means
of fabricating grayscale masks with greater resolution than that of
half-tone grayscale masks.
[0014] It is yet another object of the invention to provide a means
of identically manufacturing a number of grayscale masks.
[0015] It is yet another object of the invention to provide a means
of easily creating a light-absorbing layer for making a grayscale
mask whose absorption and thickness are adjustable to satisfy a
broad range of specifications.
SUMMARY OF THE INVENTION
[0016] The present invention relates to manufacturing grayscale
masks that are used for mass-producing grayscale DOEs. More
specifically, the present invention provides a method whereby a
grayscale mask is fabricated by using an absorber layer and a
photoresist with a laser writer. The method of the present
invention includes the steps of providing a substrate with a known
layer of absorber and a layer of photoresist, exposing the
photoresist to a grayscale pattern from a laser writer, developing
the photoresist into variable thickness, and transferring the
surface relief pattern from the photoresist layer onto the absorber
layer by etching.
[0017] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention will become more fully understood from
the detailed description and the accompanying drawings,
wherein:
[0019] FIG. 1A illustrates a grayscale DOE fabrication process
using contact imaging, including a grayscale mask consisting of a
substrate and a surface relief pattern that has variable light
transmission, an incident beam, a plurality of transmitted
sub-beams, photoresist, and another substrate, in a grayscale DOE
fabrication process;
[0020] FIG. 1B illustrates grayscale photoresist after light
exposure and chemical development, plasma etching beams, and
substrate, in a grayscale DOE fabrication process;
[0021] FIG. 1C illustrates a grayscale DOE created by grayscale DOE
fabrication process in a grayscale DOE fabrication process;
[0022] FIG. 2A illustrates a first structure, including a
substrate, a photoresist, and an absorber;
[0023] FIG. 2B illustrates a second structure, including a
substrate, a photoresist, an absorber, a laser beam, a grayscale
pattern, and an exposed photoresist;
[0024] FIG. 2C illustrates a third structure, including a
substrate, an absorber, an ion-beam milling, surface relief
pattern, and an ion-beam-milled photoresist;
[0025] FIG. 2D illustrates a grayscale mask, including a substrate
and a translated surface relief pattern; and
[0026] FIG. 3 illustrates a flow chart method of fabricating a
grayscale mask.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The following description of the preferred embodiment(s) is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0028] The present invention relates to manufacturing grayscale
masks used for mass-producing grayscale diffractive optical lenses
(DOEs). More specifically, the present invention provides a method
whereby a grayscale mask is fabricated by exposing photoresist with
a laser writer. The present invention also provides a means of
creating a light-absorbing layer for making a grayscale mask whose
absorption and thickness are adjustable.
[0029] FIGS. 1A, 1B, and 1C illustrate an example grayscale DOE
fabrication process 100 where a grayscale mask 120 is employed.
[0030] FIG. 1A illustrates a grayscale DOE fabrication process 100
using contact imaging, including a grayscale mask 120 including of
a substrate 110 and a surface relief pattern 130 that has variable
light transmission, an incident beam 140, a plurality of
transmitted sub-beams 150, photoresist 160, substrate 115 in
grayscale DOE fabrication process 100.
[0031] FIG. 1B illustrates grayscale photoresist after light
exposure and chemical development, plasma etching beams 180, and
substrate 115, in grayscale DOE fabrication process 100.
[0032] FIG. 1C illustrates a grayscale DOE 170 created by grayscale
DOE fabrication process 100 in grayscale DOE fabrication process
100.
[0033] In operation, grayscale mask 120 is used with beam 140 to
pattern photoresist 160. Since grayscale mask 120 has variable
transmission from point to point depending on the thickness of
surface relief pattern 130 the patterned photoresist 160 after
light exposure and chemical development (not shown) becomes
grayscale photoresist 165. Plasma etching beam 180 is conducted on
grayscale photoresist 165 transferring the grayscale photoresist
165 into substrate 115 creating grayscale DOE 170.
[0034] FIG. 2A illustrates first structure 200, including a
substrate 110, a photoresist 215 and an absorber 220.
[0035] FIG. 2B illustrates second structure 225, including
substrate 110, photoresist 215, absorber 220, a focusing lens 225,
a laser beam 227, a focused laser beam 230, a grayscale pattern 235
and an exposed photoresist 240.
[0036] FIG. 2C illustrates third structure 245, including substrate
110, absorber 220, ion-beam milling 250, surface relief pattern 255
and chemically developed photoresist 260.
[0037] FIG. 2D illustrates grayscale mask 120, including substrate
110 and translated surface relief pattern 130.
[0038] FIGS. 2A, 2B, 2C, and 2D are described in more detail in
reference to Method 300.
[0039] FIG. 3 illustrates a flow chart method 300 of fabricating
grayscale mask 120.
[0040] Step 310: Providing Substrate with Layer of Known Absorber
and Layer of Photoresist
[0041] In this step, substrate 110 for grayscale mask 120 is
provided with a layer of absorber 220 atop which photoresist 215 is
overlaid to form first structure 200, as illustrated in FIG. 2A. In
one example, substrate 110 may be a fused silica (SiO.sub.2), which
is a polished and transparent glass substrate manufactured by
Corning Inc. Example absorber layers may be semiconductor materials
such as silicon, glass imbedded with absorbing quantum dots, or a
carrier material imbedded with a light absorbing material, such as
dyes, pigments, or nanocrystals (quantum dots). The absorber layer
may be thin, e.g., 0.1-3.0 micrometers (.mu.m), such that light may
transmit variably through the mask, depending on its thickness, due
to the exponential attenuation of light with thickness in an
absorbing material. It is envisioned, however, that thicker
absorber layers may be used for applications requiring less
resolution and/or larger features. Any photoresist which is
compatible with ion-beam milling 150 or reactive ion etch (RIE) may
be used. Method 300 proceeds to step 320.
[0042] The use of absorber 220 embedded within carrier materials is
particularly advantageous; examples of absorber include dyes,
pigments, and/or nanocrystals. By changing the concentration of the
absorbing material in the carrier material, the maximum absorption
of absorber 220 is changed. In one example, the carrier material is
a polymer material in a solvent, thus in a liquid form initially.
The carrier itself may or may not be light-absorbing. One example
of a carrier is the photoresist itself. Examples of absorbing
materials include dyes, such as UV388 made by ColorChem
International Corp; pigments, such as Neolor made by ColorChem; and
nanocrystals, such as CdSe or CdTe nanocrystals (quantum dots),
particularly EviDots manufactured by Evident Technologies.
Nanocrystals are engineered to different sizes that exhibit
different light absorption and fluorescence properties. In one
example these absorbing materials are embedded in the carrier
material with various concentrations, thus varying the amount of
light it absorbs. In one example, the carrier embedded with
absorber is then spin-coated to the desired thickness on substrate
110.
[0043] A sub-method within the step of providing the substrate
having the layer of absorber is a sub-method of producing the layer
of absorber. The sub-method includes determining a target
absorption of the layer of absorber based on a target optical
density and a thickness of the layer of absorber. The target
optical density is determined based on a dynamic range (i.e.,
number of steps) of grayscale levels of the grayscale pattern. The
thickness of the layer of absorber is determined based on a feature
size of a product to be manufactured by use of the mask, and based
on a depth of focus of an imaging system capable of using the mask
to manufacture the product. Maximum and minimum thicknesses may be
determined to define a thickness range. Then a target thickness may
be selected within the range for one or more imaging systems that
can use the mask to manufacture the product. It is envisioned that
any imaging system that can be designed or adapted to use the mask
is capable of using the mask to manufacture the product. Once the
target absorption of the absorber layer is determined, absorber
material is dispersed in the carrier material to achieve the target
absorption by attaining an absorption material concentration as a
function of the thickness of the layer of absorber and absorption
characteristics of the absorber material.
[0044] Step 320: Exposing Photoresist with Grayscale Pattern from
Laser Writer
[0045] In this step, photoresist 215 is exposed to focused laser
beam 230 from a laser writer (not shown) in a grayscale manner
which, by varying light intensity, creates exposed photoresist 240
and grayscale pattern 235 to form second structure 225 as
illustrated in FIG. 2B. In one example, the laser writer may be
LW2003 from Microtech. LW2003 uses a helium-cadmium (HeCD) laser
with a wavelength (.lambda.) equal to 442 nm. The photoresist 240
may be either a positive or negative photoresist.
[0046] In an alternate embodiment, an electron-beam (e-beam) writer
may be substituted for the laser writer, in this case the
photoresist 215 is replaced with a suitable e-beam resist. An
example e-beam writer is a Leica SB350 DW. Method 300 proceeds to
step 330.
[0047] Step 330: Developing Photoresist into Variable Thickness
[0048] In this step, the exposed photoresist 240 is developed into
a surface relief pattern with varying thickness, depending on the
exposure. During the developing step, the exposed photoresist 240
exfoliates, which produces surface relief pattern 255, as
illustrated in FIG. 2C. If a positive photoresist is used, the
surface relief pattern 255 becomes thinner with more light
exposure. Conversely, if a negative photoresist is used it becomes
thicker with more light. Method 300 proceeds to step 340.
[0049] Step 340: Transferring Translated Surface Relief Pattern
from Photoresist Layer onto Absorber Layer by Etching
[0050] In this step, the surface relief pattern 255 is translated
onto absorber 220 by an etching process such as ion-beam milling
250, which forms translated surface relief pattern 130, as
illustrated in FIGS. 2C and 2D. Ion-beam milling 250 is done
uniformly over the photoresist 215, such that chemically developed
photoresist 260 and, subsequently, absorber 220 uniformly mills.
The resultant grayscale mask 120 with translated surface relief
pattern 130 in the absorber layer emerges. Note: the dynamic range
of grayscale mask 120 is dependent on the maximum thickness
difference in translated surface relief pattern 130 in the absorber
layer.
[0051] In an alternate embodiment, RIE may be substituted for
ion-beam milling 250. Method 300 ends.
[0052] Thus, a means of manufacturing a more efficient DOE for
optical pickups which do not employ binary beam splitting is
provided. Second, a means of inexpensively manufacturing grayscale
DOEs is provided. Third, a means of creating true grayscale masks
in an efficient manner without employing binary masks is provided.
Fourth, a means of fabricating grayscale masks with greater
resolution than that of half-tone grayscale masks is provided.
Finally, a means of identically manufacturing a number grayscale
masks is provided.
[0053] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *