U.S. patent application number 11/560900 was filed with the patent office on 2007-04-19 for multipass multiphoton absorption method and apparatus.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Todd A. Ballen, Robert J. DeVoe, Patrick R. Fleming, Jeffrey M. Florezak, Catherine A. Leatherdale.
Application Number | 20070087284 11/560900 |
Document ID | / |
Family ID | 22788012 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070087284 |
Kind Code |
A1 |
Fleming; Patrick R. ; et
al. |
April 19, 2007 |
MULTIPASS MULTIPHOTON ABSORPTION METHOD AND APPARATUS
Abstract
A method of increasing the efficiency of a multiphoton
absorption process and apparatus. The method includes: providing a
photoreactive composition; providing a source of sufficient light
for simultaneous absorption of at least two photons; exposing the
photoreactive composition to at least one transit of light from the
light source; and directing at least a portion of the first transit
of the light back into the photoreactive composition using at least
one optical element, wherein a plurality of photons not absorbed in
at least one transit are used to expose the photoreactive
composition in a subsequent transit.
Inventors: |
Fleming; Patrick R.; (Lake
Elmo, MN) ; DeVoe; Robert J.; (Oakdale, MN) ;
Leatherdale; Catherine A.; (St. Paul, MN) ; Ballen;
Todd A.; (St. Paul, MN) ; Florezak; Jeffrey M.;
(Austin, TX) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
22788012 |
Appl. No.: |
11/560900 |
Filed: |
November 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10297968 |
Dec 12, 2002 |
7166409 |
|
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PCT/US01/19125 |
Jun 14, 2001 |
|
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11560900 |
Nov 17, 2006 |
|
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60211704 |
Jun 15, 2000 |
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10297968 |
Dec 12, 2002 |
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Current U.S.
Class: |
430/269 ; 359/3;
430/1; 430/2; 430/270.11 |
Current CPC
Class: |
B33Y 30/00 20141201;
G03F 7/2053 20130101; G03F 7/038 20130101 |
Class at
Publication: |
430/269 ;
430/001; 430/002; 430/270.11; 359/003 |
International
Class: |
G03C 5/00 20060101
G03C005/00; G03H 1/04 20060101 G03H001/04 |
Claims
1.-29. (canceled)
30. An apparatus for multiphoton absorption, comprising: a
photoreactive composition; a light source providing sufficient
light for simultaneous absorption of at least two photons by the
photoreactive composition; a plurality of optical elements, wherein
the photoreactive compositions is located between at least two of
the plurality of optical elements, wherein at least one optical
element of the plurality of the optical elements is capable of
selectively reflecting the light through the photoreactive
composition without focusing, and at least one optical element of
the plurality of optical elements is capable of selectively
focusing the light at a focal point within the photoreactive
composition.
31.-33. (canceled)
34. The apparatus of claim 30 wherein the light source provides
infrared or visible light sufficient for simultaneous absorption of
at least two photons by the photoreactive composition.
35. The apparatus of claim 30 wherein the photoreactive composition
comprises a reactive species.
36. The apparatus of claim 35 wherein the reactive species is a
curable species.
37. The apparatus of claim 30 wherein the photoreactive composition
comprises a multiphoton photosensitizer.
38. The apparatus of claim 30 wherein the photoreactive composition
comprises an electron donor compound.
39. The apparatus of claim 30 wherein the photoreactive composition
comprises a photoinitiator.
40. The apparatus of claim 30 wherein the photoreactive composition
comprises about 5% to about 99.79% by weight of the least one
reactive species, about 0.01% to about 10% by weight of the at
least one multiphoton photosensitizer, up to about 10% by weight of
the at least one electron donor compound, and about 0.1% to about
10% by weight of the at least one photoinitiator, based upon the
total weight of solids.
41. The apparatus of claim 30 wherein the light source is capable
of pulse irradiating.
42. The apparatus of claim 41 wherein the light source comprises a
pulsed laser.
43. The apparatus of claim 42 wherein the pulsed laser comprises a
near infrared pulsed laser having a pulse length less than about 10
nanoseconds.
44. An apparatus for multiphoton absorption, comprising: a
photoreactive composition; a light source providing infrared or
visible light sufficient for simultaneous absorption of at least
two photons by the photoreactive composition; a plurality of
optical elements, wherein the photoreactive composition is located
between at least two of the plurality of optical elements, wherein
at least one optical element of the plurality of optical elements
is capable of selectively reflecting the light through the
photoreactive composition without focusing, and at least one
optical element of the plurality of optical elements is capable of
selectively focusing the light at a focal point within the
photoreactive composition for increasing the efficiency of the
multiphoton absorption process relative to same process that uses
only one transit of light.
45. An apparatus for multiphoton absorption, comprising: a
photoreactive composition; a light source providing infrared or
visible light sufficient for simultaneous absorption of at least
two photons by the photoreactive composition; a plurality of
optical elements, wherein the photoreactive composition is located
between at least two of the plurality of optical elements, wherein
at least two optical elements of the plurality of optical elements
are capable of selectively focusing the light at one or more focal
points within the photoreactive composition for increasing the
efficiency of the multiphoton absorption process relative to same
process that uses only one transit of light, and further wherein
each of the plurality of optical elements is aligned so the light
encounters each of the plurality of optical elements in
sequence.
46. The apparatus of claim 45 wherein at least one optical element
of the plurality of optical elements is capable of selectively
reflecting the light through the photoreactive composition without
focusing.
Description
STATEMENT OF PRIORITY
[0001] This application claims the priority of U.S. Provisional
Application No. 60/211,704 filed Jun. 15, 2000, the contents of
which are hereby incorporated by reference.
FIELD
[0002] This invention relates to a method of enhancing the
efficiency of a light source (e.g., a short pulse laser) used in a
multiphoton absorption (e.g., curing) process, the method
comprising passing the light through a photoreactive composition a
plurality of times.
BACKGROUND
[0003] Molecular two-photon absorption was predicted by
Goppert-Mayer in 1931. Upon the invention of pulsed ruby lasers in
1960, experimental observation of two-photon absorption became a
reality. Subsequently, two-photon excitation has found application
in biology and optical data storage, as well as in other
fields.
[0004] There are two key differences between two-photon-induced
photoprocesses and single-photon induced processes. Whereas
single-photon absorption scales linearly with the intensity of the
incident light, two-photon absorption scales quadratically.
Higher-order absorptions scale with a related higher power of
incident intensity. As a result, it is possible to perform
multiphoton processes with three-dimensional spatial resolution.
Also, because multiphoton processes involve the simultaneous
absorption of two or more photons, the absorbing chromophore is
excited with a number of photons whose total energy equals the
energy of an excited state of a multiphoton photosensitizer, even
though each photon individually has insufficient energy to excite
the chromophore. Because the exciting light is not attenuated by
single-photon absorption within a curable matrix or material, it is
possible to selectively excite molecules at a greater depth within
a material than would be possible via single-photon excitation by
use of a beam that is focused to that depth in the material. These
two phenomena also apply, for example, to excitation within tissue
or other biological materials.
[0005] Major benefits have been achieved by applying multiphoton
absorption to the areas of photocuring and microfabrication. For
example, in multiphoton lithography or stereolithography, the
nonlinear scaling of multiphoton absorption with intensity has
provided the ability to write features having a size that is less
than the diffraction limit of the light utilized, as well as the
ability to write features in three dimensions (which is also of
interest for holography). Such work has been limited, however, to
slow writing speeds and high laser powers. Thus, there is a need
for methods of improving the throughput and efficiency of
multiphoton absorption systems.
SUMMARY
[0006] The present invention provides a method of increasing the
efficiency of a multiphoton absorption process. The method
includes: providing a photoreactive composition; providing a source
of sufficient light for simultaneous absorption of at least two
photons by the photoreactive composition; exposing (preferably,
pulse irradiating, for example, using a near infrared pulsed laser
having a pulse length of less than about 10 nanoseconds) the
photoreactive composition to at least a first transit of light from
the light source; and directing at least a portion of the first
transit of the light back into the photoreactive composition using
at least one optical element, wherein a plurality of photons not
absorbed in the first transit are used to expose the photoreactive
composition in a subsequent transit.
[0007] It is an advantage of the invention that more efficient use
of high power laser light in a multiphoton absorption process can
be obtained by passing the laser light through the photoreactive
composition a plurality of times. This can be accomplished by use
of an appropriate optical element, such as a focusing mirror, a
waveguide, or a cube corner reflective element, for example.
[0008] Preferably, directing at least a portion of the first
transit of the light back into the photoreactive composition
includes directing at least a portion of the first transit of the
light back into the photoreactive composition at the same location
exposed to the first transit of light. Alternatively, directing at
least a portion of the first transit of the light back into the
photoreactive composition includes directing at least a portion of
the first transit of the light back into the photoreactive
composition at a location different from that exposed to the first
transit of light.
[0009] The present invention also provides a method of increasing
the efficiency of a multiphoton absorption process that includes:
providing a photoreactive composition; providing a source of
sufficient light for simultaneous absorption of at least two
photons by the photoreactive composition; focusing the light at a
first focal point within the photoreactive composition, wherein a
first portion of light is absorbed by the photoreactive composition
and a second portion of light transits the photoreactive
composition; and focusing the second portion of light at a second
focal point within the photoreactive composition.
[0010] Preferably, focusing the second portion of light at a second
focal point further includes reflecting the second portion of light
through the photoreactive composition. Alternatively, focusing the
second portion of light includes focusing the second portion of
light at a plurality of focal points. Preferably, reflecting the
second portion of light includes reflecting multiple transits of
the second portion of light through the photoreactive composition
without focusing.
[0011] Preferably, reflecting multiple transits of the second
portion of light includes selectively directing the second portion
of light between a plurality of optical elements, wherein at least
one optical element of the plurality of optical elements is capable
of selectively reflecting the light through the photoreactive
composition without focusing, and at least one optical element of
the plurality of optical elements is capable of selectively
focusing the light at a focal point within the photoreactive
composition.
[0012] If desired, reflecting the second portion of light through
the photoreactive composition and focusing the second portion of
light is repeated one or more times to create a plurality of focal
points. Furthermore, if desired, reflecting the second portion of
light involves reflecting multiple transits of the second portion
of light through the photoreactive composition without
focusing.
[0013] Preferably, the photoreactive composition includes a curable
species that is cured proximate the first focal point and proximate
the second focal point. If desired, the first focal point and the
second focal point are at the same location within the
photoreactive composition.
[0014] The present invention also provides a method of increasing
the efficiency of a multiphoton absorption process. The method
includes: providing a photoreactive composition disposed on a
reflective substrate; providing a source of sufficient light for
simultaneous absorption of at least two photons by the
photoreactive composition; exposing the photoreactive composition
to the light from the light source at a first focal point; and
reflecting the light back into the photoreactive composition by the
reflective substrate. Preferably, the method further includes
directing the light to an optical element for reflecting the light
back into the photoreactive composition at a second focal point.
More preferably, in this method, reflecting the light by the
reflective substrate and reflecting the light by an optical element
are repeated one or more times to create a plurality of focal
points.
[0015] A photoreactive composition of the present invention
includes a reactive species, which is preferably a curable species,
such as monomers, oligomers, reactive polymers, and mixtures
thereof, although non-curable species are also possible. Preferred
examples of a curable species include addition-polymerizable
monomers and oligomers, addition-crosslinkable polymers,
cationically-polymerizable monomers and oligomers,
cationically-crosslinkable polymers, and mixtures thereof.
[0016] Preferably, the photoreactive composition also includes a
multiphoton photosensitizer. A photoreactive composition may or may
not include an electron donor compound. A photoreactive composition
can optionally include a photoinitiator.
[0017] A preferred photoreactive composition includes about 5% to
about 99.79% by weight of the at least one reactive species, about
0.01% to about 10% by weight of the at least one multiphoton
photosensitizer, up to about 10% by weight of the at least one
electron donor compound, and about 0.1% to about 10% by weight of
the at least one photoinitiator, based upon the total weight of
solids.
[0018] The present invention also provides an apparatus for
multiphoton absorption. The apparatus includes: a photoreactive
composition; a light source providing sufficient light for
simultaneous absorption of at least two photons by the
photoreactive composition; a plurality of optical elements, wherein
the photoreactive composition is located between at least two of
the plurality of optical elements, wherein at least one optical
element of the plurality of optical elements is capable of
selectively reflecting the light through the photoreactive
composition without focusing, and at least one optical element of
the plurality of optical elements is capable of selectively
focusing the light at a focal point within the photoreactive
composition.
[0019] The optical element is preferably one or more of concave
spherical mirrors, concave aspheric mirrors, planar mirrors,
digital micromirror devices, polarizers, lenses, retroreflectors,
gratings, phase masks, holograms, diffusers, Pockels cells,
wave-guides, wave plates, birefringent liquid crystals, prisms, and
combinations thereof.
[0020] The light source preferably includes a pulsed laser.
Preferably, the wavelength of the light is about 300 nm to about
1500 nm, more preferably, about 600 nm to about 1100 nm, and most
preferably, about 750 nm to about 850 nm.
Definitions
[0021] As used herein:
[0022] "multiphoton absorption" means simultaneous absorption of
two or more photons to reach a reactive, electronic excited state
that is energetically inaccessible by the absorption of a single
photon of the same energy;
[0023] "simultaneous" means two events that occur within the period
of 10.sup.-14 second or less;
[0024] "electronic excited state" means an electronic state of a
molecule that is higher in energy than the molecule's electronic
ground state, that is accessible via absorption of light, and that
has a lifetime greater than 10.sup.-13 second;
[0025] "react" means to effect curing (polymerization and/or
crosslinking) as well as to effect depolymerization or other
reactions;
[0026] "optical system" means a system for controlling light, the
system including at least one element chosen from refractive
optical elements such as lenses, reflective optical elements such
as mirrors, and diffractive optical elements such as gratings;
optical elements shall also include diffusers, wave-guides, and
other elements known in the optical arts;
[0027] "exposure system" means an optical system plus a light
source;
[0028] "sufficient light" means light of sufficient intensity and
appropriate wavelength to effect multiphoton absorption;
[0029] "photosensitizer" means a molecule that lowers the energy
required to activate a photoinitiator by absorbing light of lower
energy than is required by the photoinitiator for activation and
interacting with the photoinitiator to produce a photoinitiating
species therefrom;
[0030] "photochemically effective amounts" (of the components of
the photoinitiator system) means amounts sufficient to enable the
reactive species to undergo at least partial reaction under the
selected exposure conditions (as evidenced, for example, by a
change in density, viscosity, color, pH, refractive index, or other
physical or chemical property);
[0031] "transit" means passing light completely through a volume of
a photoreactive composition; and
[0032] "focus" or "focusing" means bringing collimated light to a
point or forming an image of an object.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 illustrates a multipass multiphoton absorption
apparatus of one embodiment of the present invention.
[0034] FIG. 2 illustrates an alternative embodiment of a multipass
multiphoton absorption apparatus.
[0035] FIGS. 3a-3e illustrate alternative embodiments of a
multipass multiphoton apparatus extended to any number (N) of
passes through a sample.
[0036] FIG. 4 illustrates an embodiment of a multipass multiphoton
apparatus whereby active focal length control of the focusing
mirrors is depicted.
[0037] FIG. 5 illustrates a multipass multiphoton absorption
apparatus used in Example 2.
[0038] FIG. 6 illustrates a multipass multiphoton absorption
apparatus used in Example 5.
[0039] FIG. 7 is a graph of the power dependence of threshold
writing speed.
[0040] FIG. 8a is an optical micrograph of filled cubes.
[0041] FIG. 8b is an optical micrograph of open cubes.
[0042] FIG. 9a is an optical micrograph of scanned lines (mirror
unblocked).
[0043] FIG. 9b is an optical micrograph of scanned lines (mirror
blocked).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0044] A preferred system for multiphoton absorption can include an
exposure system that includes a light source and an appropriate
optical element, and a photoreactive composition that includes at
least one reactive species, at least one multiphoton
photosensitizer, optionally at least one electron donor compound,
and optionally at least one photoinitiator for the photoreactive
composition. The photoinitiator is typically optional except when
the reactive species is a cationic resin.
[0045] In practice, the method of the present invention can be used
to prepare complex, three-dimensional objects by exposure of a
photoreactive composition to a light source of sufficient energy to
cause a photoreactive composition to react (e.g., cure).
Preferably, unreacted material is separated from the desired object
by, e.g., washing with a solvent or other art-known means.
[0046] Unlike conventional photocuring, very little of the incident
light is absorbed and used when a photoreactive composition is
exposed. While curing systems are not limited by the amount of
light available, it is probable that they will be sometime in the
near future. By passing the light (e.g., laser) through the sample
more than once, one can make much more efficient use of the light.
Since short pulse photons require considerable resources to
generate, and there is currently no way to scale up existing
sources, multipassing may be important to make two photon
polymerization practical. Light that is directed back into the
photoreactive composition can be directed to the same focus spot of
interest for the previous transit, or it may be directed to a
separate focus spot.
[0047] FIG. 1, illustrates a multipass multiphoton absorption
apparatus 10 of one embodiment of the present invention where light
is directed back into the photoreactive composition at the same
focus spot of interest as the previous transit. The apparatus 10
includes at least one first optical element 20, at least one second
optical element 30, and a photoreactive composition 12. The
photoreactive composition 12, which is described in greater detail
below, is preferably of a highly uniform thickness to prevent
imaging aberrations and so that the full working distance of the
imaging system can be utilized.
[0048] As shown in FIG. 1, light 40 approaches the at least one
first optical element 20 generally parallel to an axis 50. The at
least one first optical element 20 focuses the light 40 to a focal
point 14 within the photoreactive composition 12. The at least one
first optical element 20 includes a converging lens 22 with its
center 22a lying on axis 50. Although depicted as a converging lens
22, the at least one first optical element 20 may include any
suitable optical device known in the art capable of focusing light
to a focal point. As is well known in the art, converging lens 22
will focus parallel light to a focal point at a distance from the
center of the lens 22a equal to a focal length f Here, the distance
from the center 22a of the converging lens 22 to the focal point 14
is equal to the focal length f for the lens 22. After being focused
at focal point 14, light 40 begins to diverge as it leaves the
photoreactive composition 12.
[0049] The divergent beam 40 is collected by an at least one second
optical element 30 located on the opposite side of the
photoreactive composition 12 from the at least one first optical
element 20. The at least one second optical element 30 includes a
spherical focusing mirror 32 having a first reflective surface 34.
Although depicted in FIG. 1 as a spherical focusing mirror, the at
least one second optical element 30 may be any suitable optical
device known in the art, e.g., an aspheric mirror, or a combination
of elements that produce the same effect, e.g., a collimating lens
and a planar mirror, etc. After collecting the divergent light 40,
the spherical focusing mirror 32 reflects the light 40 back toward
to photoreactive composition 12, focusing the light 40 back to
focal point 14. As is well known in the art, reflective spherical
focusing elements such as spherical focusing mirror 32 have a focal
length f that is equal to one-half the radius of curvature of the
mirror times the index of refraction n for the medium between the
mirror and its focus. Here, spherical focusing mirror 32 has a
focal length equal to the distance from a vertex 36 of the mirror
to focal point 14 along axis 50, such that the mirror 32 projects
the image of focal point 14 back to precisely the same location in
the photoreactive composition 12.
[0050] Because the at least one second optical element 30 refocuses
the light 40 at the focal point 14, the dose of light received by
the photoreactive composition 12 is effectively doubled. The
photoreactive composition 12 is moved relative to the optical
elements 20 and 30 to react (e.g., cure) the composition in an
arbitrary pattern. This allows for an increase in manufacturing
speed since the photoreactive composition may now be moved at least
twice as fast as a single transit geometry and still receive the
same amount of absorbed energy.
[0051] FIG. 2 illustrates another embodiment of the multipass
multiphoton absorption apparatus 10 of FIG. 1. In FIG. 2, the at
least one second optical element 30 is tilted at an angle .theta.,
which is formed by the axis 50 and an axis 38 that intersects the
vertex 36 and a second focal point 16. The tilt in the at least one
second optical element 30 is such that the light 40 is refocused at
a second focal point 16. This also allows a second gain in
efficiency since the same laser beam 40 is being used to react
(e.g., cure) multiple locations (i.e., first focal point 14 and
second focal point 16) in the photoreactive composition 12. In both
cases, the at least one second optical element 30 should have a
numerical aperture greater than or equal to that utilized by the at
least one first optical element 20 to maximize light gathering.
[0052] In theory at least, the principles described in FIG. 2 may
be extended to any number (N) of passes through the sample as long
as sufficient light intensity is retained. FIGS. 3a-3e show some
possible embodiments of such a system.
[0053] FIG. 3a shows a plurality of first optical elements 130, a
plurality of second optical elements 140, a photoreactive
composition 112, and a light beam 150 focused by optical element
132. The plurality of first optical elements 130 includes spherical
focusing mirrors 130a-130e, and the plurality of second optical
elements 140 includes spherical focusing mirrors 140a-140e. Optical
element 132 includes converging lens 134. The photoreactive
composition 112 is positioned such that it is in between the
plurality of first optical elements 130 and the plurality of second
optical elements 140. As depicted, the distance between the
plurality of first optical elements 130 and the plurality of second
optical elements 140 is equal to the radius of curvature (R) of
each of the spherical focusing mirrors 130a-130e and 140a-
140e.
[0054] As illustrated in FIG. 3a, the light beam 150 is sent
through the converging lens 134, which focuses the beam 150 at a
first focal point 114a in the photoreactive composition 112. The
first focal point 114a is at a distance f that is the focal length
of converging lens 134. The diverging light leaving the
photoreactive composition 112 is collected and reflected by
spherical focusing mirror 140a. Because the first focal point 114a
is at a distance f that is approximately equal to the focal length
of the spherical focusing mirror 140a, light from the first focal
point 114a will be reflected by mirror 140a as parallel, or
recollimated beam 154a, which will pass through the photoreactive
composition 112 without focusing at a focal point and thus no
reaction with the photoreactive composition 112 (e.g., curing)
takes place. This can be accomplished by setting the intensity of
the beam of pulsed light 150 such that negligible 2-photon
absorption occurs as the large diameter, collimated beams 154a-e
pass through the photoreactive composition.
[0055] When the recollimated light beam 154a reaches the spherical
focusing mirror 130a, it is reflected as light beam 152b, which is
focused to a second focal point 114b, thus reacting with (e.g.,
curing) the photoreactive composition 112 at focal point 114b. The
spot size of focal point 114b will depend on the numerical aperture
of the plurality of first and second optical elements 130 and 140,
respectively.
[0056] As can be seen in FIG. 3a, the multipass multiphoton
absorption apparatus 100 can include any number of first and second
optical elements that continue to collect and reflect either
focused or unfocused light through the photoreactive composition
112.
[0057] To produce a three-dimensional pattern, the photoreactive
composition 112 is scanned in the xyz planes, holding the positions
of both banks of the plurality of first and second optical elements
130 and 140 fixed. Alternatively, the arrays of the plurality of
first and second optical elements 130 and 140 can be moved as a
group, keeping the distance between them constant (e.g., the radius
of curvature R) and the photoreactive composition held fixed. This
system is suitable for reproducing three-dimensional patterns at
multiple locations.
[0058] An alternative system also suitable for reproducing a
three-dimensional pattern at multiple locations can be made by
placing the banks of optical elements (e.g., mirrors) 130 and 140 a
distance equal to two times the radius of curvature R apart. In
FIG. 3b, the multipass multiphoton absorption apparatus 100 of FIG.
3a is depicted in an alternative embodiment where the plurality of
first optical elements 130 is located a distance from the plurality
of second optical elements 140 of twice the radius of curvature R
of spherical focusing mirrors 130a-130b and 140a-140b. By placing
the mirrors at a distance of 2R, the image of the first focal point
114a will be reproduced 1:1 (i.e., without magnification) each time
the light beams 152b and 154a-154b pass through the photoreactive
composition 112 at focal points 114b-144d. The photoreactive
composition 112 is then scanned in the xyz planes to produce the
pattern.
[0059] To produce an arbitrary pattern that is stitched together
from multiple imaging spots it may be preferable to incorporate
active control of focusing into each of the mirrors. One way to
accomplish this is to place a flat mirror (whose reflectivity may
be turned on and off) in front of each curved mirror. FIG. 3c is an
illustration of this technique.
[0060] The embodiment depicted in FIG. 3c is similar to the
multipass multiphoton absorption apparatus 100 depicted in FIG. 3a.
In FIG. 3c, a multipass multiphoton absorption apparatus 200 is
configured to produce four writing spots (i.e., focal points 214a,
214b, 214c, and 214d) and two regions where absorption (resulting
in a reaction such as curing) doesn't take place. The apparatus 200
includes a plurality of first optical elements 230, a plurality of
second optical elements 240, and a photoreactive composition 212.
As in FIG. 3a, the plurality of first optical elements 230 are
placed at a distance from the plurality of second optical elements
240 equal to the radius of curvature R of the spherical focusing
mirrors 230a-230e and 240a-240e.
[0061] What differentiates the multiphoton absorption apparatus 200
of FIG. 3c from the embodiment depicted in FIG. 3a is the placement
of flat mirrors 232a, 232b, 242a, and 242b in front of spherical
focusing mirrors 230b, 230c, 240c, and 240d, respectively. In this
embodiment, focusing lens 232 initially focuses light beam 250 at
focal point 214a, which is then reflected by spherical focusing
mirror 240a. After passing through the photoreactive composition
212, light beam 254a is reflected by spherical focusing mirror 230a
back through focal point 214b along 252b to spherical focusing
mirror 240b, and back through the photoreactive composition 212 to
optical element 232a, which, as pictured in FIG. 3c, is a flat
mirror placed in front of spherical focusing mirror 230b. The flat
mirror 232a is any suitable flat mirror known in the art. The flat
mirror 232a may be of the type that can be turned on and off by an
electrical field. When the flat mirror is "on," the light beam 254b
is directed to the next writing location without focusing. When the
flat mirror is "off" the underlying spherical focusing mirror 230b
focuses the light into the photoreactive composition 212. Depending
on the speed of actuation of the mirrors, the configuration of flat
and curved mirrors could be changed during processing or preset at
the beginning of the photoimaging process.
[0062] An example of a configuration using the planar mirror 232a
and spherical focusing mirror 230b of FIG. 3c is illustrated as an
alternative embodiment in FIG. 3d. Here, a planar mirror system 300
includes a planar mirror 310 and a concave mirror 370. The planar
mirror 310 has controllable reflectivity (which can be used with
polarized light). When the planar mirror system 300 is in its "on"
state, light beam 350 passes through polarizer 320 along path 350a
and enters a birefringent liquid crystal layer 330. When in the
"on" state, the birefringent liquid crystal layer 330 rotates the
polarization state of the light so that it is perpendicular to the
transmission axis of an underlying reflective polarizer 340.
Because the polarization of light beam 350 is now perpendicular to
the underlying reflective polarizer 340, the beam 350 will not pass
through polarizer 340. Instead, beam 350 is reflected back along
path 350b and through polarizer 320.
[0063] When the planar mirror system 300 is in its "off" state,
light beam 360 passes through polarizer 320 along path 360a and
into the liquid crystal layer 330 where the liquid crystal layer
330 rotates the polarization state of the beam 360 so that it is
parallel to the transmission axis of the reflective polarizer 340.
The beam 360 passes through the reflective polarizer 340 and
proceeds to the concave mirror 370, where it is focused back
through the reflective polarizer 340 and the liquid crystal layer
330 along path 360b to form a focal point within the photoreactive
composition (see, e.g., photoreactive composition 212 of FIG.
3c).
[0064] A similar action can be achieved by using total internal
reflection (TIR) and frustrated TIR at the rear interface of the
mirror to achieve the "on" and "off" states of the mirror. The TIR
effect could be frustrated through the use of a piezoelectric
driven membrane (or other means) positioned near the rear interface
of the mirror. When the membrane (or other material) is brought
near to the dielectric mirror, the evanescent wave present at the
interface couples into the membrane and reduces the reflectivity of
the mirror.
[0065] Another possibility is to physically move the mirrors to
select (in a binary manner) particular locations to write during
multiple passes through the photoreactive composition. An
embodiment of this is shown in FIG. 3e, where stationary spherical
focusing mirrors and movable planar mirrors are interlaced in a
linear array configuration. In FIG. 3e, a multipass multiphoton
absorption apparatus 400 includes a plurality of first optical
elements 420, a plurality of second optical elements 430, and a
photoreactive composition 412 located between the plurality of
first optical elements 420 and the plurality of second optical
elements 430. The plurality of first optical elements 420 includes
spherical focusing mirrors 422a, 422b, 422c, and 422d, which
alternate with planar mirrors 424a, 424b, and 424c. The plurality
of second optical elements 430 includes spherical focusing mirrors
432a, 432b, 432c, and 432d, and planar mirrors 434a, 434b, 434c,
and 434d. The planar mirrors can be moved to steer a collimated
light beam either to a concave mirror, which then focuses the beam
into the photoreactive composition 412, or to the next planar
mirror, effectively skipping one of the focal points.
[0066] For example, planar mirror 434a reflects light beam 440 to
spherical focusing mirror 422a, and planar mirror 434d reflects the
light beam 440 to spherical focusing mirror 422d. Alternatively,
planar mirror 434a can be tilted to reflect beam 440 to planar
mirror 424a, which in turn reflects the beam 440 to planar mirror
434b, by-passing spherical focusing mirror 422a. The mirror
arrangement shown in FIG. 3e is configured to write one spot at a
first focal point 414a, leave two regions of unreacted (e.g.,
uncured) material, and to write another spot at a second focal
point 414b. Micro-electromechanical systems such as the digital
micromirror device arrays produced by Texas Instruments may be
useful in constructing this configuration.
[0067] As the beam undergoes multiple passes through the
multiphoton reactive (i.e., photoreactive) composition of the
present invention, its intensity will be slowly reduced by losses
from a variety of sources, e.g., scattering from material
imperfections, and diffractive losses from small mirrors and
lenses. This leads to a lower dose of light in the last writing
spots than in the first passes. This defect may be overcome by
placing a mirror or retroreflector at the end of the array that
directs the beam precisely back along the path it just traveled.
This will reduce the variation in dose. Once the loss per pass is
known precisely, the reflector can be positioned so as to make the
dose per pass as constant as possible. Dispersion compensating
elements may also be added to recompress the light pulse.
[0068] Also of concern are imaging aberrations that can be
propagated through the system. Like spherical lenses, spherical
mirrors are prone to off-axis aberrations and are best used for
beams striking the mirror close to normal. Aspherical surfaces may
be preferable in off-axis focusing situations such as shown in
FIGS. 3a-3e.
[0069] In a multiple pass system such as those described in FIGS.
3a-3e, considerable attention is given to both the reflectance of
the mirrors and the pulse dispersion as these may contribute more
to the loss of useful laser intensity than the material
absorption.
[0070] The substrate for the photoreactive composition is
preferably transparent at the laser wavelength and should also
preferably exhibit minimal dispersion; however, expanding the
system capabilities to include active focal length control of the
focusing mirrors allows the use of a reflective substrate.
[0071] FIG. 4 depicts an illustrative embodiment of the present
invention whereby active focal length control of the focusing
mirrors is depicted. As shown in FIG. 4, active focal length
control is necessary to compensate for the difference in
propagation distance before and after the focal point. This focal
length control can be achieved by either adjusting the shape of the
focusing mirror or by introducing a variable optical delay into
portions of the beam path.
[0072] FIG. 4 depicts a multipass multiphoton absorption apparatus
500 that includes a plurality of optical elements 520, a light beam
530, a photoreactive composition 512, and a reflective substrate
540. The plurality of optical elements 520 includes spherical
focusing mirrors 520a, 520b, and 520c.
[0073] As shown, beam 530 is reflected and focused by spherical
focusing mirror 520 a to a first focal point 514 within the
photoreactive substrate 512. After focusing at first focal point
514, the beam 530 is reflected by the reflective substrate 540
toward the spherical focusing mirror 520b, where it is reflected
back toward the substrate and refocused at a second focal point
516. As is depicted by FIG. 4, the spherical focusing mirror 520b
is more curved than mirror 520a and thus has a shorter radius of
curvature. This shorter radius of curvature causes the beam 530 to
be refocused within the photoreactive composition 512 at second
focal point 516 that is closer to the surface of the photoreactive
composition 512 than first focal point 514.
[0074] After focusing at second reflective point 516, the beam 530
is again reflected by the reflective substrate 540 and directed
toward spherical focusing mirror 520c. As depicted, mirror 520c has
a slightly longer radius of curvature than mirrors 520a or 520b.
This longer radius causes the beam 530 to be reflected and then
refocused at a third focal point 518 that is closer to the
reflective substrate 540 than either the first focal point 514 or
second focal point 516. By changing the radius of curvature of the
plurality of optical elements 520, varying patterns can be formed
within the photoreactive composition 512.
[0075] Photoreactive compositions useful in the present invention
include at least one reactive species, at least one multiphoton
photosensitizer, optionally at least one electron donor compound,
and optionally at least one photoinitiator for the photoreactive
composition.
Reactive Species
[0076] Reactive species suitable for use in the photoreactive
compositions include both curable and non-curable species. Curable
species are generally preferred and include, for example,
addition-polymerizable monomers and oligomers and
addition-crosslinkable polymers (such as free-radically
polymerizable or crosslinkable ethylenically-unsaturated species
including, for example, acrylates, methacrylates, and certain vinyl
compounds such as styrenes), as well as cationically-polymerizable
monomers and oligomers and cationically-crosslinkable polymers
(including, for example, epoxies, vinyl ethers, cyanate esters,
etc.), and the like, and mixtures thereof.
[0077] Suitable ethylenically-unsaturated species are described,
for example, by Palazzotto et al. in U.S. Pat. No. 5,545,676 at
column 1, line 65, through column 2, line 26, and include mono-,
di-, and poly-acrylates and methacrylates (for example, methyl
acrylate, methyl methacrylate, ethyl acrylate, isopropyl
methacrylate, n-hexyl acrylate, stearyl acrylate, allyl acrylate,
glycerol diacrylate, glycerol triacrylate, ethyleneglycol
diacrylate, diethyleneglycol diacrylate, triethyleneglycol
dimethacrylate, 1,3-propanediol diacrylate, 1,3-propanediol
dimethacrylate, trimethylolpropane triacrylate, 1,2,4-butanetriol
trimethacrylate, 1,4-cyclohexanediol diacrylate, pentaerythritol
triacrylate, pentaerythritol tetraacrylate, pentaerythritol
tetramethacrylate, sorbitol hexacrylate,
bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane,
bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane,
trishydroxyethyl-isocyanurate trimethacrylate, the bis-acrylates
and bis-methacrylates of polyethylene glycols of molecular weight
about 200-500, copolymerizable mixtures of acrylated monomers such
as those of U.S. Pat. No. 4,652,274, and acrylated oligomers such
as those of U.S. Pat. No. 4, 642,126); unsaturated amides (for
example, methylene bis-acrylamide, methylene bis-methacrylamide,
1,6-hexamethylene bis-acrylamide, diethylene triamine
tris-acrylamide and beta-methacrylaminoethyl methacrylate); vinyl
compounds (for example, styrene, diallyl phthalate, divinyl
succinate, divinyl adipate, and divinyl phthalate); and the like;
and mixtures thereof. Suitable reactive polymers include polymers
with pendant (meth)acrylate groups, for example, having from 1 to
about 50 (meth)acrylate groups per polymer chain. Examples of such
polymers include aromatic acid (meth)acrylate half ester resins
such as Sarbox.TM. resins available from Sartomer (for example,
Sarbox.TM. 400, 401, 402, 404, and 405). Other useful reactive
polymers curable by free radical chemistry include those polymers
that have a hydrocarbyl backbone and pendant peptide groups with
free-radically polymerizable functionality attached thereto, such
as those described in U.S. Pat. No. 5,235,015 (Ali et al.).
Mixtures of two or more monomers, oligomers, and/or reactive
polymers can be used if desired. Preferred
ethylenically-unsaturated species include acrylates, aromatic acid
(meth)acrylate half ester resins, and polymers that have a
hydrocarbyl backbone and pendant peptide groups with free-radically
polymerizable functionality attached thereto.
[0078] Suitable cationically-reactive species are described, for
example, by Oxman et al. in U.S. Pat. Nos. 5,998,495 and 6,025,406
and include epoxy resins. Such materials, broadly called epoxides,
include monomeric epoxy compounds and epoxides of the polymeric
type and can be aliphatic, alicyclic, aromatic, or heterocyclic.
These materials generally have, on the average, at least 1
polymerizable epoxy group per molecule (preferably, at least about
1.5 and, more preferably, at least about 2). The polymeric epoxides
include linear polymers having terminal epoxy groups (for example,
a diglycidyl ether of a polyoxyalkylene glycol), polymers having
skeletal oxirane units (for example, polybutadiene polyepoxide),
and polymers having pendant epoxy groups (for example, a glycidyl
methacrylate polymer or copolymer). The epoxides can be pure
compounds or can be mixtures of compounds containing one, two, or
more epoxy groups per molecule. These epoxy-containing materials
can vary greatly in the nature of their backbone and substituent
groups. For example, the backbone can be of any type and
substituent groups thereon can be any group that does not
substantially interfere with cationic cure at room temperature.
Illustrative of permissible substituent groups include halogens,
ester groups, ethers, sulfonate groups, siloxane groups, nitro
groups, phosphate groups, and the like. The molecular weight of the
epoxy-containing materials can vary from about 58 to about 100,000
or more.
[0079] Useful epoxy-containing materials include those which
contain cyclohexene oxide groups such as
epoxycyclohexanecarboxylates, typified by
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate,
3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexane
carboxylate, and bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate. A
more detailed list of useful epoxides of this nature is set forth
in U.S. Pat. No. 3,117,099.
[0080] Other epoxy-containing materials that are useful include
glycidyl ether monomers of the formula ##STR1## where R' is alkyl
or aryl and n is an integer of 1 to 6. Examples are glycidyl ethers
of polyhydric phenols obtained by reacting a polyhydric phenol with
an excess of a chlorohydrin such as epichlorohydrin (for example,
the diglycidyl ether of 2,2-bis-(2,3-epoxypropoxyphenol)-propane).
Additional examples of epoxides of this type are described in U.S.
Pat. No. 3,018,262, and in Handbook of Epoxy Resins, Lee and
Neville, McGraw-Hill Book Co., New York (1967).
[0081] Numerous commercially available epoxy resins can also be
utilized. In particular, epoxides that are readily available
include octadecylene oxide, epichlorohydrin, styrene oxide, vinyl
cyclohexene oxide, glycidol, glycidylmethacrylate, diglycidyl
ethers of Bisphenol A (for example, those available under the trade
designations Epon.TM. 828, Epon.TM. 825, Epon.TM. 1004, and
Epon.TM. 1010 from Resolution Performance Products, formerly Shell
Chemical Co., as well as DER.TM.-331, DER.TM.-332, and DER.TM.-334
from Dow Chemical Co.), vinylcyclohexene dioxide (for example,
ERL-4206 from Union Carbide Corp.),
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexene carboxylate (for
example, ERL-4221 or Cyracure.TM. UVR 6110 or UVR 6105 from Union
Carbide Corp.),
3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methyl-cyclohexene
carboxylate (for example, ERL-4201 from Union Carbide Corp.),
bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate (for example,
ERL-4289 from Union Carbide Corp.), bis(2,3-epoxycyclopentyl) ether
(for example, ERL-0400 from Union Carbide Corp.), aliphatic epoxy
modified from polypropylene glycol (for example, ERL-4050 and
ERL-4052 from Union Carbide Corp.), dipentene dioxide (for example,
ERL-4269 from Union Carbide Corp.), epoxidized polybutadiene (for
example, Oxiron.TM. 2001 from FMC Corp.), silicone resin containing
epoxy functionality, flame retardant epoxy resins (for example,
DER.TM.-580, a brominated bisphenol type epoxy resin available from
Dow Chemical Co.), 1,4-butanediol diglycidyl ether of
phenolformaldehyde novolak (for example, DEN.TM.-431 and
DEN.TM.-438 from Dow Chemical Co.), resorcinol diglycidyl ether
(for example, Kopoxite.TM. from Koppers Company, Inc.),
bis(3,4-epoxycyclohexyl)adipate (for example, ERL-4299 or UVR-6128,
from Union Carbide Corp.),
2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)
cyclohexane-meta-dioxane (for example, ERL-4234 from Union Carbide
Corp.), vinylcyclohexene monoxide 1,2-epoxyhexadecane (for example,
UVR-6216 from Union Carbide Corp.), alkyl glycidyl ethers such as
alkyl C.sub.8-C.sub.10 glycidyl ether (for example, Heloxy.TM.
Modifier 7 from Resolution Performance Products), alkyl
C.sub.12-C.sub.14 glycidyl ether (for example, Heloxy.TM. Modifier
8 from Resolution Performance Products), butyl glycidyl ether (for
example, Heloxy.TM. Modifier 61 from Resolution Performance
Products), cresyl glycidyl ether (for example, Heloxy.TM. Modifier
62 from Resolution Performance Products), p-tert-butylphenyl
glycidyl ether (for example, Heloxy.TM. Modifier 65 from Resolution
Performance Products), polyfunctional glycidyl ethers such as
diglycidyl ether of 1,4-butanediol (for example, Heloxy.TM.
Modifier 67 from Resolution Performance Products), diglycidyl ether
of neopentyl glycol (for example, Heloxy.TM. Modifier 68 from
Resolution Performance Products), diglycidyl ether of
cyclohexanedimethanol (for example, Heloxy.TM. Modifier 107 from
Resolution Performance Products), trimethylol ethane triglycidyl
ether (for example, Heloxy.TM. Modifier 44 from Resolution
Performance Products), trimethylol propane triglycidyl ether (for
example, Heloxy.TM. Modifier 48 from Resolution Performance
Products), polyglycidyl ether of an aliphatic polyol (for example,
Heloxy.TM. Modifier 84 from Resolution Performance Products),
polyglycol diepoxide (for example, Heloxy.TM. Modifier 32 from
Resolution Performance Products), bisphenol F epoxides (for
example, Epon .TM.-1138 or GY-281 from Ciba-Geigy Corp.), and
9,9-bis[4-(2,3-epoxypropoxy)-phenyl]fluorenone (for example,
Epon.TM. 1079 from Resolution Performance Products).
[0082] Other useful epoxy resins comprise copolymers of acrylic
acid esters of glycidol (such as glycidylacrylate and
glycidylmethacrylate) with one or more copolymerizable vinyl
compounds. Examples of such copolymers are 1:1
styrene-glycidylmethacrylate, 1:1
methylmethacrylate-glycidylacrylate, and a 62.5:24:13.5
methylmethacrylate-ethyl acrylate-glycidylmethacrylate. Other
useful epoxy resins are well known and contain such epoxides as
epichlorohydrins, alkylene oxides (for example, propylene oxide),
styrene oxide, alkenyl oxides (for example, butadiene oxide), and
glycidyl esters (for example, ethyl glycidate).
[0083] Useful epoxy-functional polymers include epoxy-functional
silicones such as those described in U.S. Pat. No. 4,279,717
(Eckberg), which are commercially available from the General
Electric Company. These are polydimethylsiloxanes in which 1-20
mole % of the silicon atoms have been substituted with epoxyalkyl
groups (preferably, epoxy cyclohexylethyl, as described in U.S.
Pat. No. 5,753,346 (Kessel)).
[0084] Blends of various epoxy-containing materials can also be
utilized. Such blends can comprise two or more weight average
molecular weight distributions of epoxy-containing compounds (such
as low molecular weight (below 200), intermediate molecular weight
(about 200 to 10,000), and higher molecular weight (above about
10,000)). Alternatively or additionally, the epoxy resin can
contain a blend of epoxy-containing materials having different
chemical natures (such as aliphatic and aromatic) or
functionalities (such as polar and non-polar). Other
cationically-reactive polymers (such as vinyl ethers and the like)
can additionally be incorporated, if desired.
[0085] Preferred epoxies include aromatic glycidyl epoxies (such as
the Epon.TM. resins available from Resolution Performance Products)
and cycloaliphatic epoxies (such as ERL-4221 and ERL-4299 available
from Union Carbide).
[0086] Suitable cationally-reactive species also include vinyl
ether monomers, oligomers, and reactive polymers (for example,
methyl vinyl ether, ethyl vinyl ether, tert-butyl vinyl ether,
isobutyl vinyl ether, triethyleneglycol divinyl ether
(Rapi-Cure.TM. DVE-3, available from International Specialty
Products, Wayne, N.J.), trimethylolpropane trivinyl ether (TMPTVE,
available from BASF Corp., Mount Olive, N.J.), and the Vectomer.TM.
divinyl ether resins from Allied Signal (for example, Vectomer.TM.
2010, Vectomer.TM. 2020, Vectomer.TM. 4010, and Vectomer.TM. 4020
and their equivalents available from other manufacturers)), and
mixtures thereof. Blends (in any proportion) of one or more vinyl
ether resins and/or one or more epoxy resins can also be utilized.
Polyhydroxy-functional materials (such as those described, for
example, in U.S. Pat. No. 5,856,373 (Kaisaki et al.)) can also be
utilized in combination with epoxy- and/or vinyl ether-functional
materials.
[0087] Non-curable species include, for example, reactive polymers
whose solubility can be increased upon acid- or radical-induced
reaction. Such reactive polymers include, for example, aqueous
insoluble polymers bearing ester groups that can be converted by
photogenerated acid to aqueous soluble acid groups (for example,
poly(4-tert-butoxycarbonyloxystyrene). Non-curable species also
include the chemically-amplified photoresists described by R. D.
Allen, G. M. Wallraff, W. D. Hinsberg, and L. L. Simpson in "High
Performance Acrylic Polymers for Chemically Amplified Photoresist
Applications," J. Vac. Sci. Technol. B, 9, 3357 (1991). The
chemically-amplified photoresist concept is now widely used for
microchip manufacturing, especially with sub-0.5 micron (or even
sub-0.2 micron) features. In such photoresist systems, catalytic
species (typically hydrogen ions) can be generated by irradiation,
which induces a cascade of chemical reactions. This cascade occurs
when hydrogen ions initiate reactions that generate more hydrogen
ions or other acidic species, thereby amplifying reaction rate.
Examples of typical acid-catalyzed chemically-amplified photoresist
systems include deprotection (for example,
t-butoxycarbonyloxystyrene resists as described in U.S. Pat. No.
4,491,628, tetrahydropyran (THP) methacrylate-based materials,
THP-phenolic materials such as those described in U.S. Pat. No.
3,779,778, t-butyl methacrylate-based materials such as those
described by R. D Allen et al. in Proc. SPIE, 2438, 474 (1995), and
the like); depolymerization (for example, polyphthalaldehyde-based
materials); and rearrangement (for example, materials based on the
pinacol rearrangements).
[0088] Useful non-curable species also include leuco dyes, which
tend to be colorless until they are oxidized by acid generated by
the multiphoton photoinitiator system, and which, once oxidized,
exhibit a visible color. (Oxidized dyes are colored by virtue of
their absorbance of light in the visible portion of the
electromagnetic spectrum (approximately 400-700 nm).) Leuco dyes
useful in the present invention are those that are reactive or
oxidizable under moderate oxidizing conditions and yet that are not
so reactive as to oxidize under common environmental conditions.
There are many such chemical classes of leuco dyes known to the
imaging chemist.
[0089] Leuco dyes useful as reactive species in the present
invention include acrylated leuco azine, phenoxazine, and
phenothiazine, which can, in part, be represented by the structural
formula: ##STR2## wherein X is selected from O, S, and
--N--R.sup.11, with S being preferred; R.sup.1 and R.sup.2 are
independently selected from H and alkyl groups of 1 to about 4
carbon atoms; R.sup.3, R.sup.4, R.sup.6, and R.sup.7 are
independently selected from H and alkyl groups of 1 to about 4
carbon atoms, preferably methyl; R.sup.5 is selected from alkyl
groups of 1 to about 16 carbon atoms, alkoxy groups of 1 to about
16 carbon atoms, and aryl groups of up to about 16 carbon atoms;
R.sup.8 is selected from --N(R.sup.1)(R.sup.2), H, alkyl groups of
1 to about 4 carbon atoms, wherein R.sup.1 and R.sup.2 are
independently selected and defined as above; R.sup.9 and R.sup.10
are independently selected from H and alkyl groups of 1 to about 4
carbon atoms; and R.sup.11 is selected from alkyl groups of 1 to
about 4 carbon atoms and aryl groups of up to about 11 carbon atoms
(preferably, phenyl groups). The following compounds are examples
of this type of leuco dye: ##STR3##
[0090] Other useful leuco dyes include, but are not limited to,
Leuco Crystal Violet
(4,4',4''-methylidynetris-(N,N-dimethylaniline)), Leuco Malachite
Green (p,p'-benzylidenebis-(N,N-dimethylaniline)), Leuco Atacryl
Orange-LGM
[0091] (Color Index Basic Orange 21, Comp. No. 48035 (a Fischer's
Base Type Compound)) Having the Structure ##STR4## Leuco Atacryl
Brilliant Red-4G (Color Index Basic Red 14) Having the Structure
##STR5## Leuco Atacryl Yellow-R (Color Index Basic Yellow 11, Comp.
No. 48055) Having the Structure ##STR6## Leuco Ethyl Violet
(4,4',4''-methylidynetris-(N,N-diethylaniline), Leuco Victoria
Blu-BGO (Color Index Basic Blue 728a, Comp. No. 44040;
4,4'-methylidynebis-(N,N,-dimethylaniline)-4-(N-ethyl-1-napthalamine)),
and LeucoAtlantic Fuchsine Crude
(4,4',4''-methylidynetris-aniline).
[0092] The leuco dye(s) can generally be present at levels of at
least about 0.01% by weight of the total weight of a light
sensitive layer (preferably, at least about 0.3% by weight; more
preferably, at least about 1% by weight; most preferably, at least
about 2% to 10% or more by weight). Other materials such as
binders, plasticizers, stabilizers, surfactants, antistatic agents,
coating aids, lubricants, fillers, and the like can also be present
in the light sensitive layer. One of skill in the art can readily
determine the desirable amount of additives. For example, the
amount of filler is chosen such that there is no undesirable
scatter at the writing wavelength.
[0093] If desired, mixtures of different types of reactive species
can be utilized in the photoreactive compositions. For example,
mixtures of free-radically-reactive species and
cationically-reactive species, mixtures of curable species and
non-curable species, and so forth, are also useful.
Photoinitiator System
[0094] (1) Multiphoton Photosensitizers
[0095] Multiphoton photosensitizers suitable for use in the
multiphoton photoinitiator system of the photoreactive compositions
are those that are capable of simultaneously absorbing at least two
photons when exposed to sufficient light. Preferably, they have a
two-photon absorption cross-section greater than that of
fluorescein (that is, greater than that of
3',6'-dihydroxyspiro[isobenzofuran-1(3H), 9'-[9H]xanthen]3-one).
Generally, the cross-section can be greater than about
50.times.10.sup.-50 cm.sup.4 sec/photon, as measured by the method
described by C. Xu and W. W. Webb in J. Opt. Soc. Am. B, 13, 481
(1996) (which is referenced by Marder and Perry et al. in
International Publication No. WO 98/21521 at page 85, lines
18-22).
[0096] This method involves the comparison (under identical
excitation intensity and photosensitizer concentration conditions)
of the two-photon fluorescence intensity of the photosensitizer
with that of a reference compound. The reference compound can be
selected to match as closely as possible the spectral range covered
by the photosensitizer absorption and fluorescence. In one possible
experimental set-up, an excitation beam can be split into two arms,
with 50% of the excitation intensity going to the photosensitizer
and 50% to the reference compound. The relative fluorescence
intensity of the photosensitizer with respect to the reference
compound can then be measured using two photomultiplier tubes or
other calibrated detector. Finally, the fluorescence quantum
efficiency of both compounds can be measured under one-photon
excitation.
[0097] Methods of determining fluorescence and phosphorescence
quantum yields are well-known in the art. Typically, the area under
the fluorescence (or phosphorescence) spectrum of a compound of
interest is compared with the area under the fluorescence (or
phosphorescence) spectrum of a standard luminescent compound having
a known fluorescence (or phosphorescence) quantum yield, and
appropriate corrections are made (which take into account, for
example, the optical density of the composition at the excitation
wavelength, the geometry of the fluorescence detection apparatus,
the differences in the emission wavelengths, and the response of
the detector to different wavelengths). Standard methods are
described, for example, by I. B. Berlman in Handbook of
Fluorescence Spectra of Aromatic Molecules, Second Edition, pages
24-27, Academic Press, New York (1971); by J. N. Demas and G. A.
Crosby in J. Phys. Chem., 75, 991-1024 (1971); and by J. V. Morris,
M. A. Mahoney, and J. R. Huber in J. Phys. Chem., 80, 969-974
(1976).
[0098] Assuming that the emitting state is the same under one- and
two-photon excitation (a common assumption), the two-photon
absorption cross-section of the photosensitizer (.delta..sub.sam),
is equal to .delta..sub.refK
(I.sub.sam/I.sub.ref)(.phi..sub.sam/.phi..sub.ref), wherein
.delta..sub.ref is the two-photon absorption cross-section of the
reference compound, I.sub.sam is the fluorescence intensity of the
photosensitizer, I.sub.ref is the fluorescence intensity of the
reference compound, .phi..sub.sam is the fluorescence quantum
efficiency of the photosensitizer, .phi..sub.ref is the
fluorescence quantum efficiency of the reference compound, and K is
a correction factor to account for slight differences in the
optical path and response of the two detectors. K can be determined
by measuring the response with the same photosensitizer in both the
sample and reference arms. To ensure a valid measurement, the clear
quadratic dependence of the two-photon fluorescence intensity on
excitation power can be confirmed, and relatively low
concentrations of both the photosensitizer and the reference
compound can be utilized (to avoid fluorescence reabsorption and
photosensitizer aggregration effects).
[0099] When the photosensitizer is not fluorescent, the yield of
electronic excited states can to be measured and compared with a
known standard. In addition to the above-described method of
determining fluorescence yield, various methods of measuring
excited state yield are known (including, for example, transient
absorbance, phosphorescence yield, photoproduct formation or
disappearance of photosensitizer (from photoreaction), and the
like).
[0100] Preferably, the two-photon absorption cross-section of the
photosensitizer is greater than about 1.5 times that of fluorescein
(or, alternatively, greater than about 75.times.10.sup.-50 cm.sup.4
sec/photon, as measured by the above method); more preferably,
greater than about twice that of fluorescein (or, alternatively,
greater than about 100.times.10.sup.-50 cm.sup.4 sec/photon); most
preferably, greater than about three times that of fluorescein (or,
alternatively, greater than about 150.times.10.sup.-50 cm.sup.4
sec/photon); and optimally, greater than about four times that of
fluorescein (or, alternatively, greater than about
200.times.10.sup.-50 cm.sup.4 sec/photon).
[0101] Preferably, the photosensitizer is soluble in the reactive
species (if the reactive species is liquid) or is compatible with
the reactive species and with any binders (as described below) that
are included in the composition. Most preferably, the
photosensitizer is also capable of sensitizing
2-methyl-4,6-bis(trichloromethyl)-s-triazine under continuous
irradiation in a wavelength range that overlaps the single photon
absorption spectrum of the photosensitizer (single photon
absorption conditions), using the test procedure described in U.S.
Pat. No. 3,729,313. Using currently available materials, that test
can be carried out as follows:
[0102] A standard test solution can be prepared having the
following composition: 5.0 parts of a 5% (weight by volume)
solution in methanol of 45,000-55,000 molecular weight, 9.0-13.0%
hydroxyl content polyvinyl butyral (Butvar.TM. B76, Monsanto); 0.3
parts trimethylolpropane trimethacrylate; and 0.03 parts
2-methyl-4,6-bis(trichloromethyl)-s-triazine (see Bull. Chem. Soc.
Japan, 42, 2924-2930 (1969)).
[0103] To this solution can be added 0.01 parts of the compound to
be tested as a photosensitizer. The resulting solution can then be
knife-coated onto a 0.05 mm clear polyester film using a knife
orifice of 0.05 mm, and the coating can be air dried for about 30
minutes. A 0.05 mm clear polyester cover film can be carefully
placed over the dried but soft and tacky coating with minimum
entrapment of air. The resulting sandwich construction can then be
exposed for three minutes to 161,000 Lux of incident light from a
tungsten light source providing light in both the visible and
ultraviolet range (FCH.TM. 650 watt quartz-iodine lamp, General
Electric). Exposure can be made through a stencil so as to provide
exposed and unexposed areas in the construction. After exposure the
cover film can be removed, and the coating can be treated with a
finely divided colored powder, such as a color toner powder of the
type conventionally used in xerography. If the tested compound is a
photosensitizer, the trimethylolpropane trimethacrylate monomer
will be polymerized in the light-exposed areas by the
light-generated free radicals from the
2-methyl-4,6-bis(trichloromethyl)-s-triazine. Since the polymerized
areas will be essentially tack-free, the colored powder will
selectively adhere essentially only to the tacky, unexposed areas
of the coating, providing a visual image corresponding to that in
the stencil.
[0104] Preferably, a photosensitizer can also be selected based in
part upon shelf stability considerations. Accordingly, selection of
a particular photosensitizer can depend to some extent upon the
particular reactive species utilized (as well as upon the choices
of electron donor compound and/or photoinitiator).
[0105] Particularly preferred multiphoton photosensitizers include
those exhibiting large multiphoton absorption cross-sections, such
as Rhodamine B (that is,
N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethyletha-
naminium chloride, and the hexafluoroantimonate salt of Rhodamine
B) and the four classes of photosensitizers described, for example,
by Marder and Perry et al. in International Patent Publication Nos.
WO 98/21521 and WO 99/53242. The four classes can be described as
follows: (a) molecules in which two donors are connected to a
conjugated .pi. (pi)-electron bridge; (b) molecules in which two
donors are connected to a conjugated .pi. (pi)-electron bridge
which is substituted with one or more electron accepting groups;
(c) molecules in which two acceptors are connected to a conjugated
.pi. (pi)-electron bridge; and (d) molecules in which two acceptors
are connected to a conjugated .pi. (pi)-electron bridge which is
substituted with one or more electron donating groups (where
"bridge" means a molecular fragment that connects two or more
chemical groups, "donor" means an atom or group of atoms with a low
ionization potential that can be bonded to a conjugated .pi.
(pi)-electron bridge, and "acceptor" means an atom or group of
atoms with a high electron affinity that can be bonded to a
conjugated .pi. (pi)-electron bridge).
[0106] Representative examples of such photosensitizers include:
##STR7## ##STR8## ##STR9## ##STR10##
[0107] The four above-described classes of photosensitizers can be
prepared by reacting aldehydes with ylides under standard Wittig
conditions or by using the McMurray reaction, as detailed in
International Patent Publication No. WO 98/21521.
[0108] Other compounds are described by Reinhardt et al. (for
example, in U.S. Pat. Nos. 6,100,405, 5,859,251, and 5,770,737) as
having large multiphoton absorption cross-sections, although these
cross-sections were determined by a method other than that
described above. Representative examples of such compounds include:
##STR11## ##STR12##
[0109] Other compounds which may be useful as photosensitizers in
the present invention include but are not limited to fluorescein,
p-bis(o-methylstyryl)benzene, eosin, rose Bengal, erythrosin,
Coumarin 307 (Eastman Kodak), Cascade Blue hydrazide trisodium
salt, Lucifer Yellow CH ammonium
salt, 4,4-difluoro- 1,3,5,7,8-pentamethyl-4-bora-3.alpha.,
4.alpha.-diazaindacene-2,6-disulfonic acid disodium salt,
1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate,
Indo-1 pentapotassium salt
[0110] (Molecular Probes), 5-dimethylaminonaphthalene-1-sulfonyl
hydrazine, 4',6-diamidino-2-phenylindole dihydrochloride,
5,7-diiodo-3-butoxy-6-fluorone, 9-fluorenone-2-carboxylic acid, and
compounds having the following structures: ##STR13## ##STR14##
[0111] (2) Electron Donor Compounds
[0112] Electron donor compounds useful in the multiphoton
photoinitiator system of the photoreactive compositions are those
compounds (other than the photosensitizer itself) that are capable
of donating an electron to an electronic excited state of the
photosensitizer. The electron donor compounds preferably have an
oxidation potential that is greater than zero and less than or
equal to that of p-dimethoxybenzene vs. a standard saturated
calomel electrode. Preferably, the oxidation potential is between
about 0.3 and 1 volt vs. a standard saturated calomel electrode
(SCE).
[0113] The electron donor compound is also preferably soluble in
the reactive species and is selected based in part upon shelf
stability considerations (as described above). Suitable donors are
generally capable of increasing the speed of reaction (e.g., cure)
or the image density of a photoreactive composition upon exposure
to light of the desired wavelength.
[0114] When working with cationically-reactive species, those
skilled in the art will recognize that the electron donor compound,
if of significant basicity, can adversely affect the cationic
reaction. (See, for example, the discussion in U.S. Pat. No.
6,025,406 (Oxman et al.) at column 7, line 62, through column 8,
line 49.)
[0115] In general, electron donor compounds suitable for use with
particular photosensitizers and photoinitiators can be selected by
comparing the oxidation and reduction potentials of the three
components (as described, for example, in U.S. Pat. No. 4,859,572
(Farid et al.)). Such potentials can be measured experimentally
(for example, by the methods described by R. J. Cox, Photographic
Sensitivity, Chapter 15, Academic Press (1973)) or can be obtained
from references such as N. L. Weinburg, Ed., Technique of
Electroorganic Synthesis Part II Techniques of Chemistry, Vol. V
(1975), and C. K. Mann and K. K. Barnes, Electrochemical Reactions
in Nonaqueous Systems (1970). The potentials reflect relative
energy relationships and can be used in the following manner to
guide electron donor compound selection.
[0116] When the photosensitizer is in an electronic excited state,
an electron in the highest occupied molecular orbital (HOMO) of the
photosensitizer has been lifted to a higher energy level (namely,
the lowest unoccupied molecular orbital (LUMO) of the
photosensitizer), and a vacancy is left behind in the molecular
orbital it initially occupied. The photoinitiator can accept the
electron from the higher energy orbital, and the electron donor
compound can donate an electron to fill the vacancy in the
originally occupied orbital, provided that certain relative energy
relationships are satisfied.
[0117] If the reduction potential of the photoinitiator is less
negative (or more positive) than that of the photosensitizer, an
electron in the higher energy orbital of the photosensitizer is
readily transferred from the photosensitizer to the lowest
unoccupied molecular orbital (LUMO) of the photoinitiator, since
this represents an exothermic process. Even if the process is
instead slightly endothermic (that is, even if the reduction
potential of the photosensitizer is up to 0.1 volt more negative
than that of the photoinitiator) ambient thermal activation can
readily overcome such a small barrier.
[0118] In an analogous manner, if the oxidation potential of the
electron donor compound is less positive (or more negative) than
that of the photosensitizer, an electron moving from the HOMO of
the electron donor compound to the orbital vacancy in the
photosensitizer is moving from a higher to a lower potential, which
again represents an exothermic process. Even if the process is
slightly endothermic (that is, even if the oxidation potential of
the photosensitizer is up to 0.1 volt more positive than that of
the electron donor compound), ambient thermal activation can
readily overcome such a small barrier.
[0119] Slightly endothermic reactions in which the reduction
potential of the photosensitizer is up to 0.1 volt more negative
than that of the photoinitiator, or the oxidation potential of the
photosensitizer is up to 0.1 volt more positive than that of the
electron donor compound, occur in every instance, regardless of
whether the photoinitiator or the electron donor compound first
reacts with the photosensitizer in its excited state. When the
photoinitiator or the electron donor compound is reacting with the
photosensitizer in its excited state, it is preferred that the
reaction be exothermic or only slightly endothermic. When the
photoinitiator or the electron donor compound is reacting with the
photosensitizer ion radical, exothermic reactions are still
preferred, but still more endothermic reactions can be expected in
many instances to occur. Thus, the reduction potential of the
photosensitizer can be up to 0.2 volt (or more) more negative than
that of a second-to-react photoinitiator, or the oxidation
potential of the photosensitizer can be up to 0.2 volt (or more)
more positive than that of a second-to-react electron donor
compound.
[0120] Suitable electron donor compounds include, for example,
those described by D. F. Eaton in Advances in Photochemistry,
edited by B. Voman et al., Volume 13, pp. 427-488, John Wiley and
Sons, New York (1986); by Oxman et al. in U.S. Pat. No. 6,025,406
at column 7, lines 42-61; and by Palazzotto et al. in U.S. Pat. No.
5, 545,676 at column 4, line 14 through column 5, line 18. Such
electron donor compounds include amines (including triethanolamine,
hydrazine, 1,4-diazabicyclo[2.2.2]octane, triphenylamine (and its
triphenylphosphine and triphenylarsine analogs), aminoaldehydes,
and aminosilanes), amides (including phosphoramides), ethers
(including thioethers), ureas (including thioureas), sulfinic acids
and their salts, salts of ferrocyanide, ascorbic acid and its
salts, dithiocarbamic acid and its salts, salts of xanthates, salts
of ethylene diamine tetraacetic acid, salts of
(alkyl).sub.n(aryl).sub.mborates (n+m=4) (tetraalkylammonium salts
preferred), various organometallic compounds such as SnR.sub.4
compounds (where each R is independently chosen from among alkyl,
aralkyl (particularly, benzyl), aryl, and alkaryl groups) (for
example, such compounds as n-C.sub.3H.sub.7Sn(CH.sub.3).sub.3,
(allyl)Sn(CH.sub.3).sub.3, and (benzyl)Sn(n-C.sub.3H.sub.7).sub.3),
ferrocene, and the like, and mixtures thereof. The electron donor
compound can be unsubstituted or can be substituted with one or
more non-interfering substituents. Particularly preferred electron
donor compounds contain an electron donor atom (such as a nitrogen,
oxygen, phosphorus, or sulfur atom) and an abstractable hydrogen
atom bonded to a carbon or silicon atom alpha to the electron donor
atom.
[0121] Preferred amine electron donor compounds include alkyl-,
aryl-, alkaryl- and aralkyl-amines (for example, methylamine,
ethylamine, propylamine, butylamine, triethanolamine, amylamine,
hexylamine, 2,4-dimethylaniline, 2,3-dimethylaniline, o-, m- and
p-toluidine, benzylamine, aminopyridine,
N,N'-dimethylethylenediamine, N,N'-diethylethylenediamine,
N,N'-dibenzylethylenediamine, N,N'-diethyl-1,3-propanediamine,
N,N'-diethyl-2-butene-1,4-diamine, N,N'-dimethyl-1,6-hexanediamine,
piperazine, 4,4'-trimethylenedipiperidine,
4,4'-ethylenedipiperidine, p-N,N-dimethyl-aminophenethanol and
p-N-dimethylaminobenzonitrile); aminoaldehydes (for example,
p-N,N-dimethylaminobenzaldehyde, p-N,N-diethylaminobenzaldehyde,
9-julolidine carboxaldehyde, and 4-morpholinobenzaldehyde); and
aminosilanes (for example, trimethylsilylmorpholine,
trimethylsilylpiperidine, bis(dimethylamino)diphenylsilane,
tris(dimethylamino)methylsilane, N,N-diethylaminotrimethylsilane,
tris(dimethylamino)phenylsilane, tris(methylsilyl)amine,
tris(dimethylsilyl)amine, bis(dimethylsilyl)amine,
N,N-bis(dimethylsilyl)aniline, N-phenyl-N-dimethylsilylaniline, and
N,N-dimethyl-N-dimethylsilylamine); and mixtures thereof. Tertiary
aromatic alkylamines, particularly those having at least one
electron-withdrawing group on the aromatic ring, have been found to
provide especially good shelf stability. Good shelf stability has
also been obtained using amines that are solids at room
temperature. Good photographic speed has been obtained using amines
that contain one or more julolidinyl moieties.
[0122] Preferred amide electron donor compounds include
N,N-dimethylacetamide, N,N-diethylacetamide,
N-methyl-N-phenylacetamide, hexamethylphosphoramide,
hexaethylphosphoramide, hexapropylphosphoramide,
trimorpholinophosphine oxide, tripiperidinophosphine oxide, and
mixtures thereof
[0123] Preferred alkylarylborate salts include [0124]
Ar.sub.3B.sup.-(n-C.sub.4H.sub.9)N.sup.+(C.sub.2H.sub.5).sub.4
[0125] Ar.sub.3B.sup.-(n-C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.4
[0126] Ar.sub.3B.sup.-(n-C.sub.4H.sub.9)N.sup.30
(n-C.sub.4H.sub.9).sub.4 [0127]
Ar.sub.3B.sup.-(n-C.sub.4H.sub.9)Li.sup.+ [0128]
Ar.sub.3B.sup.---(n-C.sub.4H.sub.9)N (C.sub.6H.sub.13).sub.4 [0129]
Ar.sub.3B.sup.---(C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.3(CH.sub.2).sub.2C-
O.sub.2(CH.sub.2).sub.2CH.sub.3 [0130]
Ar.sub.3B.sup.---(C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.3(CH.sub.2).sub.2O-
CO(CH.sub.2).sub.2CH.sub.3 [0131]
Ar.sub.3B.sup.---(sec-C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.3(CH.sub.2).su-
b.2CO.sub.2(CH.sub.2).sub.2CH.sub.3 [0132]
Ar.sub.3B.sup.---(sec-C.sub.4H.sub.9)N.sup.+(C.sub.6H.sub.13).sub.4
[0133]
Ar.sub.3B.sup.---(C.sub.4H.sub.9)N.sup.+(C.sub.8H.sub.17).sub.4
[0134] Ar.sub.3B.sup.---(C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.4
[0135]
(p-CH.sub.3O--C.sub.6H.sub.4).sub.3B.sup.-(n-C.sub.4H.sub.9)N.sup.+(n-C.s-
ub.4H.sub.9).sub.4 [0136]
Ar.sub.3B.sup.---(C.sub.4H.sub.9)N.sup.+(CH.sub.3).sub.3(CH.sub.2).sub.2O-
H [0137] ArB.sup.---(n-C.sub.4H.sub.9).sub.3N.sup.+(CH.sub.3).sub.4
[0138] ArB.sup.-(C.sub.2H.sub.5).sub.3N.sup.+(CH.sub.3).sub.4
[0139]
Ar.sub.2B.sup.-(n-C.sub.4H.sub.9).sub.2N.sup.+(CH.sub.3).sub.4
[0140] Ar.sub.3B.sup.-(C.sub.4H.sub.9)N.sup.+(C.sub.4H.sub.9
).sub.4 [0141] Ar.sub.4B.sup.-N.sup.+(C.sub.4H.sub.9).sub.4 [0142]
ArB.sup.-(CH.sub.3).sub.3N.sup.+(CH.sub.3).sub.4 [0143]
(n-C.sub.4H.sub.9).sub.4B.sup.-N.sup.+(CH.sub.3).sub.4 [0144]
Ar.sub.3B-(C.sub.4H.sub.9)P.sup.+(C.sub.4H.sub.9).sub.4 (where Ar
is phenyl, naphthyl, substituted (preferably, fluoro-substituted)
phenyl, substituted naphthyl, and like groups having greater
numbers of fused aromatic rings), as well as tetramethylammonium
n-butyltriphenylborate and tetrabutylammonium
n-hexyl-tris(3-fluorophenyl)borate (available as CGI 437 and CGI
746 from Ciba Specialty Chemicals Corporation), and mixtures
thereof.
[0145] Suitable ether electron donor compounds include b
4,4'-dimethoxybiphenyl, 1,2,4-trimethoxybenzene,
1,2,4,5-tetramethoxybenzene, and the like, and mixtures thereof.
Suitable urea electron donor compounds include N,N'-dimethylurea,
N,N-dimethylurea, N,N'-diphenylurea, tetramethylthiourea,
tetraethylthiourea, tetra-n-butylthiourea, N,N-di-n-butylthiourea,
N,N'-di-n-butylthiourea, N,N-diphenylthiourea,
N,N'-diphenyl-N,N'-diethylthiourea, and the like, and mixtures
thereof.
[0146] Preferred electron donor compounds for free radical-induced
reactions include amines that contain one or more julolidinyl
moieties, alkylarylborate salts, and salts of aromatic sulfinic
acids. However, for such reactions, the electron donor compound can
also be omitted, if desired (for example, to improve the shelf
stability of the photoreactive composition or to modify resolution,
contrast, and reciprocity). Preferred electron donor compounds for
acid-induced reactions include 4-dimethylaminobenzoic acid, ethyl
4-dimethylaminobenzoate, 3-dimethylaminobenzoic acid,
4-dimethylaminobenzoin, 4-dimethylaminobenzaldehyde,
4-dimethylaminobenzonitrile, 4-dimethylaminophenethyl alcohol, and
1,2,4-trimethoxybenzene.
[0147] (3) Photoinitiators
[0148] Suitable photoinitiators for the reactive species of the
photoreactive compositions are those that are capable of being
photosensitized by accepting an electron from an electronic excited
state of the multiphoton photosensitizer, resulting in the
formation of at least one free radical and/or acid. Such
photoinitiators include iodonium salts (for example, diaryliodonium
salts), chloromethylated triazines (for example,
2-methyl-4,6-bis(trichloromethyl)-s-triazine,
2,4,6-tris(trichloromethyl)-s-triazine, and
2-aryl-4,6-bis(trichloromethyl)-s-triazine), diazonium salts (for
example, phenyldiazonium salts optionally substituted with groups
such as alkyl, alkoxy, halo, or nitro), sulfonium salts (for
example, triarylsulfonium salts optionally substituted with alkyl
or alkoxy groups, and optionally having 2,2' oxy groups bridging
adjacent aryl moieties), azinium salts (for example, an
N-alkoxypyridinium salt), and triarylimidazolyl dimers (preferably,
2,4,5-triphenylimidazolyl dimers such as
2,2',4,4',5,5'-tetraphenyl-1,1'-biimidazole, optionally substituted
with groups such as alkyl, alkoxy, or halo), and the like, and
mixtures thereof.
[0149] The photoinitiator is preferably soluble in the reactive
species and is preferably shelf-stable (that is, does not
spontaneously promote reaction of the reactive species when
dissolved therein in the presence of the photosensitizer and the
electron donor compound). Accordingly, selection of a particular
photoinitiator can depend to some extent upon the particular
reactive species, photosensitizer, and electron donor compound
chosen, as described above. Preferred photoinitiators are those
that exhibit large multiphoton adsorption cross-sections, as
described, e.g., by Marder, Perry et al., in PCT Patent
Applications WO 98/21521 and WO 995/3242, and by Goodman et al., in
PCT Patent Application WO 99/54784.
[0150] Suitable iodonium salts include those described by
Palazzotto et al. in U.S. Pat. No. 5,545,676 at column 2, lines 28
through 46. Suitable iodonium salts are also described in U.S. Pat.
Nos. 3,729,313, 3,741,769, 3,808,006, 4,250,053 and 4,394,403. The
iodonium salt can be a simple salt (for example, containing an
anion such as Cl.sup.-, Br.sup.-, I.sup.- or
C.sub.4H.sub.5SO.sub.3.sup.-) or a metal complex salt (for example,
containing SbF.sub.6.sup.-, PF.sub.6.sup.-, BF.sub.4.sup.-,
tetrakis(perfluorophenyl)borate, SbF.sub.5 OH.sup.31 or
AsF.sub.6.sup.-). Mixtures of iodonium salts can be used if
desired.
[0151] Examples of useful aromatic iodonium complex salt
photoinitiators include diphenyliodonium tetrafluoroborate;
di(4-methylphenyl)iodonium tetrafluoroborate;
phenyl-4-methylphenyliodonium tetrafluoroborate;
di(4-heptylphenyl)iodonium tetrafluoroborate;
di(3-nitrophenyl)iodonium hexafluorophosphate;
di(4-chlorophenyl)iodonium hexafluorophosphate;
di(naphthyl)iodonium tetrafluoroborate;
di(4-trifluoromethylphenyl)iodonium tetrafluoroborate;
diphenyliodonium hexafluorophosphate; di(4-methylphenyl)iodonium
hexafluorophosphate; diphenyliodonium hexafluoroarsenate;
di(4-phenoxyphenyl)iodonium tetrafluoroborate;
phenyl-2-thienyliodonium hexafluorophosphate;
3,5-dimethylpyrazolyl-4-phenyliodonium hexafluorophosphate;
diphenyliodonium hexafluoroantimonate; 2,2'-diphenyliodonium
tetrafluoroborate; di(2,4-dichlorophenyl)iodonium
hexafluorophosphate; di(4-bromophenyl)iodonium hexafluorophosphate;
di(4-methoxyphenyl)iodonium hexafluorophosphate;
di(3-carboxyphenyl)iodonium hexafluorophosphate;
di(3-methoxycarbonylphenyl)iodonium hexafluorophosphate;
di(3-methoxysulfonylphenyl)iodonium hexafluorophosphate;
di(4-acetamidophenyl)iodonium hexafluorophosphate;
di(2-benzothienyl)iodonium hexafluorophosphate; and
diphenyliodonium hexafluoroantimonate; and the like; and mixtures
thereof. Aromatic iodonium complex salts can be prepared by
metathesis of corresponding aromatic iodonium simple salts (such
as, for example, diphenyliodonium bisulfate) in accordance with the
teachings of Beringer et al., J. Am. Chem. Soc 81, 342 (1959).
[0152] Preferred iodonium salts include diphenyliodonium salts
(such as diphenyliodonium chloride, diphenyliodonium
hexafluorophosphate, and diphenyliodonium tetrafluoroborate),
diaryliodonium hexafluoroantimonate (for example, SarCat.TM. SR
1012 available from Sartomer Company), and mixtures thereof.
[0153] Useful chloromethylated triazines include those described in
U.S. Pat. No. 3,779,778 (Smith et al.) at column 8, lines 45-50,
which include 2,4-bis(trichloromethyl)-6-methyl-s-triazine,
2,4,6-tris(trichloromethyl)-s-triazine, and the more preferred
chromophore-substituted vinylhalomethyl-s-triazines disclosed in
U.S. Pat. Nos. 3,987,037 and 3,954,475 (Bonham et al.).
[0154] Useful diazonium salts include those described in U.S. Pat.
No. 4,394,433 (Gatzke), which comprise a light sensitive aromatic
moiety (for example, pyrrolidine, morpholine, aniline, and diphenyl
amine) with an external diazonium group (--N.sup.m.sup.+.dbd.N) and
an anion (for example, chloride, tri-isopropyl naphthalene
sulfonate, tetrafluoroborate, and the
bis(perfluoroalkylsulfonyl)methides) associated therewith. Examples
of useful diazonium cations include 1-diazo-4-anilinobenzene,
N-(4-diazo-2,4-dimethoxy phenyl)pyrrolidine,
1-diazo-2,4-diethoxy-4-morpholino benzene, 1-diazo-4-benzoyl
amino-2,5-diethoxy benzene, 4-diazo-2,5-dibutoxy phenyl morpholino,
4-diazo-1-dimethyl aniline, 1-diazo-N,N-dimethylaniline,
1-diazo-4-N-methyl-N-hydroxyethyl aniline, and the like.
[0155] Useful sulfonium salts include those described in U.S. Pat.
No. 4,250,053 (Smith) at column 1, line 66, through column 4, line
2, which can be represented by the formulas: ##STR15## wherein
R.sub.1, R.sub.2, and R.sub.3 are each independently selected from
aromatic groups having from about 4 to about 20 carbon atoms (for
example, substituted or unsubstituted phenyl, naphthyl, thienyl,
and furanyl, where substitution can be with such groups as alkoxy,
alkylthio, arylthio, halogen, and so forth) and alkyl groups having
from 1 to about 20 carbon atoms. As used here, the term "alkyl"
includes substituted alkyl (for example, substituted with such
groups as halogen, hydroxy, alkoxy, or aryl). At least one of
R.sub.1, R.sub.2, and R.sub.3 is aromatic, and, preferably, each is
independently aromatic. Z is selected from the group consisting of
a covalent bond, oxygen, sulfur, --S(.dbd.O)--, --C(.dbd.O)--,
--(O.dbd.)S(.dbd.O)--, and --N(R)--, where R is aryl (of about 6 to
about 20 carbons, such as phenyl), acyl (of about 2 to about 20
carbons, such as acetyl, benzoyl, and so forth), a carbon-to-carbon
bond, or --(R.sub.4--)C(--R.sub.5)--, where R.sub.4 and R.sub.5 are
independently selected from the group consisting of hydrogen, alkyl
groups having from 1 to about 4 carbon atoms, and alkenyl groups
having from about 2 to about 4 carbon atoms, and X.sup.- is as
described below.
[0156] Suitable anions, X.sup.-, for the sulfonium salts (and for
any of the other types of photoinitiators) include a variety of
anion types such as, for example, imide, methide, boron-centered,
phosphorous-centered, antimony-centered, arsenic-centered, and
aluminum-centered anions.
[0157] Illustrative, but not limiting, examples of suitable imide
and methide anions include (C.sub.2F.sub.5SO.sub.2).sub.2N.sup.-,
(C.sub.4F.sub.9SO.sub.2).sub.2N.sup.-,
(C.sub.8F.sub.17SO.sub.2).sub.3C.sup.-,
(CF.sub.3SO.sub.2).sub.3C.sup.-, (CF.sub.3SO.sub.2).sub.2N.sup.-,
(C.sub.4F.sub.9SO.sub.2).sub.3 C.sup.-,
(CF.sub.3SO.sub.2).sub.2(C.sub.4F.sub.9SO.sub.2)C.sup.-,
(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2)N.sup.-,
((CF.sub.3).sub.2NC.sub.2F.sub.4SO.sub.2).sub.2N.sup.-,
(CF.sub.3).sub.2NC.sub.2F.sub.4SO.sub.2C.sup.-(SO.sub.2CF.sub.3).sub.2,
(3,5-bis(CF.sub.3)C.sub.6H.sub.3)SO.sub.2N.sup.-SO.sub.2CF.sub.3,
C.sub.6H.sub.5SO.sub.2C.sup.-(SO.sub.2CF.sub.3).sub.2,
C.sub.6H.sub.5SO.sub.2N.sup.-SO.sub.2CF.sub.3, and the like.
Preferred anions of this type include those represented by the
formula (R.sub.fSO.sub.2).sub.3C.sup.-, wherein R.sub.f is a
perfluoroalkyl radical having from 1 to about 4 carbon atoms.
[0158] Illustrative, but not limiting, examples of suitable
boron-centered anions include F.sub.4B.sup.-,
(3,5-bis(CF.sub.3)C.sub.6H.sub.3).sub.4B.sup.-,
(C.sub.6F.sub.5).sub.4B.sup.-,
(p-CF.sub.3C.sub.6H.sub.4).sub.4B.sup.-,
(m-CF.sub.3C.sub.6H.sub.4).sub.4B.sup.-,
(p-FC.sub.6H.sub.4).sub.4B.sup.-,
(C.sub.6F.sub.5).sub.3(CH.sub.3)B.sup.-,
(C.sub.6F.sub.5).sub.3(n-C.sub.4H.sub.9)B.sup.-,
(p-CH.sub.3C.sub.6H.sub.4).sub.3(C.sub.6F.sub.5)B.sup.-,
(C.sub.6F.sub.5).sub.3FB.sup.-,
(C.sub.6H.sub.5).sub.3(C.sub.6F.sub.5)B.sup.-,
(CH.sub.3).sub.2(p-CF.sub.3C.sub.6H.sub.4).sub.2B.sup.-,
(C.sub.6F.sub.5).sub.3(n-C.sub.18H.sub.37O)B.sup.-, and the like.
Preferred boron-centered anions generally contain 3 or more
halogen-substituted aromatic hydrocarbon radicals attached to
boron, with fluorine being the most preferred halogen.
Illustrative, but not limiting, examples of the preferred anions
include (3,5-bis(CF.sub.3)C.sub.6H.sub.3).sub.4B.sup.-,
(C.sub.6F.sub.5).sub.4B.sup.-,
(C.sub.6F.sub.5).sub.3(n-C.sub.4H.sub.9)B.sup.-,
(C.sub.6F.sub.5).sub.3FB.sup.-, and
(C.sub.6F.sub.5).sub.3(CH.sub.3)B.sup.-.
[0159] Suitable anions containing other metal or metalloid centers
include, for example,
(3,5-bis(CF.sub.3)C.sub.6H.sub.3).sub.4Al.sup.-,
(C.sub.6F.sub.5).sub.4Al.sup.-,
(C.sub.6F.sub.5).sub.2F.sub.4P.sup.-,
(C.sub.6F.sub.5)F.sub.5P.sup.-, F.sub.6P.sup.-,
(C.sub.6F.sub.5)F.sub.5Sb.sup.-, F.sub.6Sb.sup.-,
(HO)F.sub.5Sb.sup.-, and F.sub.6 As.sup.-. The foregoing lists are
not intended to be exhaustive, as other useful boron-centered
nonnucleophilic salts, as well as other useful anions containing
other metals or metalloids, will be readily apparent (from the
foregoing general formulas) to those skilled in the art.
[0160] Preferably, the anion, X.sup.-, is selected from
tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate,
hexafluoroantimonate, and hydroxypentafluoroantimonate (for
example, for use with cationically-reactive species such as epoxy
resins).
[0161] Examples of suitable sulfonium salt photoinitiators include:
[0162] triphenylsulfonium tetrafluoroborate [0163]
methyldiphenylsulfonium tetrafluoroborate [0164]
dimethylphenylsulfonium hexafluorophosphate [0165]
triphenylsulfonium hexafluorophosphate [0166] triphenylsulfonium
hexafluoroantimonate [0167] diphenylnaphthylsulfonium
hexafluoroarsenate [0168] tritolysulfonium hexafluorophosphate
[0169] anisyldiphenylsulfonium hexafluoroantimonate [0170]
4-butoxyphenyldiphenylsulfonium tetrafluoroborate [0171]
4-chlorophenyldiphenylsulfonium hexafluorophosphate [0172]
tri(4-phenoxyphenyl)sulfonium hexafluorophosphate [0173]
di(4-ethoxyphenyl)methylsulfonium hexafluoroarsenate [0174]
4-acetonylphenyldiphenylsulfonium tetrafluoroborate [0175]
4-thiomethoxyphenyldiphenylsulfonium hexafluorophosphate [0176]
di(methoxysulfonylphenyl)methylsulfonium hexafluoroantimonate
[0177] di(nitrophenyl)phenylsulfonium hexafluoroantimonate [0178]
di(carbomethoxyphenyl)methylsulfonium hexafluorophosphate [0179]
4-acetamidophenyldiphenylsulfonium tetrafluoroborate [0180]
dimethylnaphthylsulfonium hexafluorophosphate [0181]
trifluoromethyldiphenylsulfonium tetrafluoroborate [0182]
p-(phenylthiophenyl)diphenylsulfonium hexafluoroantimonate [0183]
10-methylphenoxathiinium hexafluorophosphate [0184]
5-methylthianthrenium hexafluorophosphate [0185]
10-phenyl-9,9-dimethylthioxanthenium hexafluorophosphate [0186]
10-phenyl-9-oxothioxanthenium tetrafluoroborate [0187]
5-methyl-10-oxothianthrenium tetrafluoroborate [0188]
5-methyl-10,10-dioxothianthrenium hexafluorophosphate
[0189] Preferred sulfonium salts include triaryl-substituted salts
such as triarylsulfonium hexafluoroantimonate (for example,
SarCat.TM. SR1010 available from Sartomer Company),
triarylsulfonium hexafluorophosphate (for example, SarCat.TM. SR
1011 available from Sartomer Company), and triarylsulfonium
hexafluoroantimonate (for example, SarCat.TM. K185 available from
Sartomer Company).
[0190] Useful azinium salts include those described in U.S. Pat.
No. 4,859,572 (Farid et al.) at column 8, line 51, through column
9, line 46, which include an azinium moiety, such as a pyridinium,
diazinium, or triazinium moiety. The azinium moiety can include one
or more aromatic rings, typically carbocyclic aromatic rings (for
example, quinolinium, isoquinolinium, benzodiazinium, and
naphthodiazonium moieties), fused with an azinium ring. A
quaternizing substituent of a nitrogen atom in the azinium ring can
be released as a free radical upon electron transfer from the
electronic excited state of the photosensitizer to the azinium
photoinitiator. In one preferred form, the quaternizing substituent
is an oxy substituent. The oxy substituent, --O-T, which
quaternizes a ring nitrogen atom of the azinium moiety can be
selected from among a variety of synthetically convenient oxy
substituents. The moiety T can, for example, be an alkyl radical,
such as methyl, ethyl, butyl, and so forth. The alkyl radical can
be substituted. For example, aralkyl (for example, benzyl and
phenethyl) and sulfoalkyl (for example, sulfomethyl) radicals can
be useful. In another form, T can be an acyl radical, such as an
--OC(O)-T.sup.1 radical, where T.sup.1 can be any of the various
alkyl and aralkyl radicals described above. In addition, T.sup.1
can be an aryl radical, such as phenyl or naphthyl. The aryl
radical can in turn be substituted. For example, T.sup.1 can be a
tolyl or xylyl radical. T typically contains from 1 to about 18
carbon atoms, with alkyl moieties in each instance above preferably
being lower alkyl moieties and aryl moieties in each instance
preferably containing about 6 to about 10 carbon atoms. Highest
activity levels have been realized when the oxy substituent, --O-T,
contains 1 or 2 carbon atoms. The azinium nuclei need include no
substituent other than the quaternizing substituent. However, the
presence of other substituents is not detrimental to the activity
of these photoinitiators.
[0191] Useful triarylimidazolyl dimers include those described in
U.S. Pat. No. 4,963,471 (Trout et al.) at column 8, lines 18-28.
These dimers include, for example,
2-(o-chlorophenyl)-4,5-bis(m-methoxyphenyl)-1,1'-biimidazole;
2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenyl-1,1'-biimidazole;
and
2,5-bis(o-chlorophenyl)-4-[3,4-dimethoxyphenyl]-1,1'-biimidazole.
[0192] Preferred photoinitiators include iodonium salts (more
preferably, aryliodonium salts), chloromethylated triazines,
triarylimidazolyl dimers (more preferably,
2,4,5-triphenylimidazolyl dimers), sulfonium salts, and diazonium
salts. More preferred are aryliodonium salts, chloromethylated
triazines, and the 2,4,5-triphenylimidazolyl dimers (with
aryliodonium salts and the triazines being most preferred).
Preparation of Photoreactive Composition
[0193] The reactive species, multiphoton photosensitizers, electron
donor compounds, and photoinitiators can be prepared by the methods
described above or by other methods known in the art, and many are
commercially available. These four components can be combined under
"safe light" conditions using any order and manner of combination
(optionally, with stirring or agitation), although it is sometimes
preferable (from a shelf life and thermal stability standpoint) to
add the photoinitiator last (and after any heating step that is
optionally used to facilitate dissolution of other components).
Solvent can be used, if desired, provided that the solvent is
chosen so as to not react appreciably with the components of the
composition. Suitable solvents include, for example, acetone,
dichloromethane, and acetonitrile. The reactive species itself can
also sometimes serve as a solvent for the other components.
[0194] The components of the photoinitiator system are present in
photochemically effective amounts (as defined above). Generally,
the composition contains at least about 5%, preferably at least
about 10%, and more preferably, at least about 20%, by weight of
one or more reactive species. Generally, the composition contains
up to about 99.79%, preferably up to about 95%, and more preferably
up to about 80%, by weight of one or more reactive species.
Generally, the composition contains at least about 0.01%,
preferably at least about 0.1%, more preferably, at least about
0.2%, by weight of one or more photosensitizers. Generally, the
composition contains up to about 10 %, preferably up to about 5%,
and more preferably up to about 2%, by weight of one or more
photosensitizers. Preferably, the composition contains at least
about 0.1% by weight of one or more electron donors. Preferably,
the composition contains up to about 10%, and preferably up to
about 5%, by weight of one or more electron donors. Preferably, the
composition contains at least about 0.1% by weight of one or more
photoinitiators. Preferably, the composition contains up to about
10%, and preferably up to about 5%, by weight of one or more
photoinitiators. When the reactive species is a leuco dye, the
composition generally can contain at least about 0.01%, preferably
at least about 0.3%, more preferably at least about 1%, and most
preferably at least about 2%, by weight of one or more leuco dyes.
When the reactive species is a leuco dye, the composition generally
can contain up to about 10% by weight of one or more leuco dyes.
These percentages are based on the total weight of solids, i.e.,
the total weight of components other than solvent.
[0195] A wide variety of adjuvants can be included in the
photoreactive compositions, depending upon the desired end use.
Suitable adjuvants include solvents, diluents, resins, binders,
plasticizers, pigments, dyes, inorganic or organic reinforcing or
extending fillers (at preferred amounts of about 10% to 90% by
weight based on the total weight of the composition), thixotropic
agents, indicators, inhibitors, stabilizers, ultraviolet absorbers,
medicaments (for example, leachable fluorides), and the like. The
amounts and types of such adjuvants and their manner of addition to
the compositions will be familiar to those skilled in the art.
[0196] It is within the scope of this invention to include
nonreactive polymeric binders in the compositions in order, for
example, to control viscosity and to provide film-forming
properties. Such polymeric binders can generally be chosen to be
compatible with the reactive species. For example, polymeric
binders that are soluble in the same solvent that is used for the
reactive species, and that are free of functional groups that can
adversely affect the course of reaction of the reactive species,
can be utilized. Binders can be of a molecular weight suitable to
achieve desired film-forming properties and solution rheology (for
example, molecular weights between about 5,000 and 1,000,000
daltons; preferably between about 10,000 and 500,000 daltons; more
preferably, between about 15,000 and 250,000 daltons). Suitable
polymeric binders include, for example, polystyrene, poly(methyl
methacrylate), poly(styrene)-co-(acrylonitrile), cellulose acetate
butyrate, and the like.
[0197] Prior to exposure, the resulting photoreactive compositions
can be coated on a substrate, if desired, by any of a variety of
coating methods known to those skilled in the art (including, for
example, knife coating and spin coating). The substrate can be
chosen from a wide variety of films, sheets, and other surfaces,
depending upon the particular application and the method of
exposure to be utilized. Preferred substrates are generally
sufficiently flat to enable the preparation of a layer of
photoreactive composition having a uniform thickness. For
applications where coating is less desirable, the photoreactive
compositions can alternatively be exposed in bulk form.
Exposure System and Its Use
[0198] Useful exposure systems include at least one light source
(usually a pulsed laser) and at least one optical element. Suitable
light sources include, for example, femtosecond near-infrared
titanium sapphire oscillators (for example, a Coherent Mira Optima
900-F) pumped by an argon ion laser (for example, a Coherent
Innova). This laser, operating at 76 MHz, has a pulse width of less
than 200 femtoseconds, is tunable between 700 and 980 nm, and has
average power up to 1.4 Watts. However, in practice, any light
source that provides sufficient intensity (to effect multiphoton
absorption) at a wavelength appropriate for the photosensitizer
(used in the photoreactive composition) can be utilized. Such
wavelengths can generally be in the range of about 300 nm to about
1500 nm; preferably, from about 600 nm to about 1100 nm; more
preferably, from about 750 nm to about 850 nm. Peak intensities can
generally range from at least about 10.sup.6 W/cm.sup.2. The upper
limit on the pulse fluence (energy per pulse per unit area) is
generally dictated by the ablation threshold of the photoreactive
composition. For example, Q-switched Nd:YAG lasers (for example, a
Spectra-Physics Quanta-Ray PRO), visible wavelength dye lasers (for
example, a Spectra-Physics Sirah pumped by a Spectra-Physics
Quanta-Ray PRO), and Q-switched diode pumped lasers (for example, a
Spectra-Physics FCbar.TM.) can also be utilized. Preferred light
sources are near infrared-pulsed lasers having a pulse length less
than about 10 nanoseconds (more preferably, less than about 1
nanosecond; most preferably, less than about 10 picoseconds). Other
pulse lengths can be used as long as the peak intensity and fluence
criteria given above are met.
[0199] Optical elements useful in carrying out the method of the
invention include refractive optical elements (for example, lenses
and prisms), reflective optical elements (for example,
retroreflectors or focusing mirrors), diffractive optical elements
(for example, gratings, phase masks, and holograms), diffusers,
Pockels cells, wave-guides, wave plates, birefringent liquid
crystals, and the like. Such optical elements are useful for
focusing, beam delivery, beam/mode shaping, pulse shaping, and
pulse timing. Generally, combinations of optical elements can be
utilized, and other appropriate combinations will be recognized by
those skilled in the art. It is often desirable to use optics with
large numerical aperture to provide highly-focused light. However,
any combination of optical elements that provides a desired
intensity profile (and spatial placement thereof) can be utilized.
For example, the exposure system can include a scanning confocal
microscope (BioRad MRC600) equipped with a 0.75 NA objective (Zeiss
20X Fluar).
[0200] Generally, exposure of the photoreactive composition can be
carried out using a light source (as described above) along with an
optical system as a means for controlling the three-dimensional
spatial distribution of light intensity within the composition. For
example, the light from a pulsed laser can be passed through a
focusing lens in a manner such that the focal point is within the
volume of the composition. The focal point can be scanned or
translated in a three-dimensional pattern that corresponds to a
desired shape, thereby creating the desired shape. The exposed or
illuminated volume of the composition can be scanned either by
moving the composition itself or by moving the light source (for
example, moving a laser beam using galvo-mirrors).
[0201] If the light induces, for example, a reaction of the
reactive species that produces a material having solubility
characteristics different from those of the reactive species, the
resulting image can optionally be developed by removing either the
exposed or the unexposed regions through use of an appropriate
solvent, for example, or by other art-known means. Complex,
three-dimensional objects can be prepared in this manner.
[0202] Exposure times generally depend upon the type of exposure
system used to cause image formation (and its accompanying
variables such as numerical aperture, geometry of light intensity
spatial distribution, the peak light intensity during the laser
pulse (higher intensity and shorter pulse duration roughly
correspond to peak light intensity)), as well as upon the nature of
the composition exposed (and its concentrations of photosensitizer,
photoinitiator, and electron donor compound). Generally, higher
peak light intensity in the regions of focus allows shorter
exposure times, everything else being equal. Linear imaging or
"writing" speeds generally can be about 5 to 100,000 microns/second
using a laser pulse duration of about 10.sup.-8 to 10.sup.-15
seconds (preferably, about 10.sup.-11 to 10.sup.-14 second) and
about 10.sup.2 to 10.sup.9 pulses per second (preferably, about
10.sup.3 to 10.sup.8 pulses per second).
EXAMPLES
[0203] Objects and advantages of this invention are further
illustrated by the following examples, but the particular materials
and amounts thereof recited in these examples, as well as other
conditions and details, should not be construed to unduly limit
this invention. These examples discuss the use of optical elements
to recycle unabsorbed light used in a multiphoton absorption
process back into the focal region to increase the conversion
efficiency of the incident light during, e.g., a
photopolymerization process.
[0204] Unless otherwise noted, chemicals used in the examples were
commercially available from Aldrich Chemical Co., Milwaukee, Wis.
Tris(2-hydroxyethylene)isocyanurate triacrylate, 2-phenoxyethyl
acrylate, and trimethylolpropane triacrylate were commercially
available from Sartomer Co., West Chester, Pa. under the tradenames
SR-368, SR339, and SR-351 respectively. SR-9008 is an alkoxylated
trifunctional acrylate commercially available from Sartomer Co.,
West Chester, Pa.
[0205] The aromatic iodonium complex salts such as diphenyliodonium
hexafluorophosphate may be prepared by metathesis of corresponding
aromatic iodonium simple salts (such as, for example,
diphenyliodonium bisulfate) in accordance with the teachings of
Beringer et al., J. Am. Chem. Soc., 81, 342 (1959). Thus, for
example, the complex salt diphenyliodonium tetrafluoroborate was
prepared by the addition at 60.degree. C. of an aqueous solution
containing 29.2 grams (g) silver fluoroborate, 2 g fluoroboric
acid, and 0.5 g phosphorous acid in about 30 milliliters (mL) of
water to a solution of 44 g (139 millimoles) of diphenyliodonium
chloride. The silver halide that precipitated was filtered off and
the filtrate concentrated to yield diphenyliodonium fluoroborate,
which may be purified by recrystallization.
[0206] The two-photon sensitizing dye,
bis-[4-(diphenylamino)styryl]-1,4-(dimethoxy)benzene was prepared
as follows: (1) Reaction of
1,4-bis-bromomethyl-2,5-dimethoxybenzene with triethyl phosphite
(Horner Eamons reagent): 1,4-bis-bromomethyl-2,5-dimethoxybenzene
was prepared according to the literature procedure (Syper et. al.,
Tetrahedron, 1983, 39, 781-792).
1,4-bis-bromomethyl-2,5-dimethoxybenzene (253 g, 0.78 mol) was
placed into a 1000-mL round bottom flask. Triethyl phosphite (300
g, 2.10 mol) was added. The reaction was heated to vigorous reflux
with stirring for 48 hours under nitrogen atmosphere. The reaction
mixture was cooled and the excess P(OEt).sub.3 was removed under
vacuum using a Kugelrohr apparatus. The desired product was not
actually distilled, but the Kugelrohr was used to remove the excess
P(OEt).sub.3 by distilling it away from the product. Upon heating
to 100.degree. C. at 0.1 millimeter (mm) Hg, a clear oil resulted.
Upon cooling the desired product solidified. The product was
suitable for use directly in the next step, and .sup.1H NMR was
consistent with the proposed structure. Recrystallization from
toluene yielded colorless needles and resulted in a purer product,
but this was not necessary for subsequent steps in most cases.
[0207] (2) Synthesis of
Bis-[4-(diphenylamino)stryl]-1,4-(dimethoxy)benzene: A 1000-mL
round bottom flask was fitted with a calibrated dropping funnel and
a magnetic stirrer. The flask was charged with the product from the
above synthesis (a Homer Eamons reagent) (19.8 g, 45.2 mol), and it
was also charged with N,N-diphenylamino-p-benzaldehyde (Fluka, 25
g, 91.5 mmol). The flask was flushed with nitrogen and sealed with
septa. Anhydrous tetrahydrofuran (750 mL) was cannulated into the
flask and all solids dissolved. The dropping funnel was charged
with KOtBu (potassium t-butoxide) (125 mL, 1.0 M in THF). The
solution in the flask was stirred and the KOtBu solution was added
to the contents of the flask over the course of 30 minutes. The
solution was then left to stir at ambient temperature overnight.
The reaction was then quenched by the addition of H.sub.2O (500
mL). The reaction continued to stir and after about 30 minutes a
highly fluorescent yellow solid had formed in the flask. The solid
was isolated by filtration and air-dried. It was then
recrystallized from toluene (450 mL). The desired product was
obtained as fluorescent needles (24.7 g, 81% yield). .sup.1H NMR
was consistent with the proposed structure.
Example 1
[0208] This example describes the use of an array of curved
reflective mirrors to recycle unabsorbed light used for reacting
one or more monomers with two-photon initiators. The light source
consists of a Nd:YAG microchip laser operating at a wavelength of
1.06 micrometer (.mu.m), pulse width of 800 picoseconds (ps), pulse
repetition rate of 12 kilohertz (kHz), and an average output power
of 35 milliwatts (mW). The optical train consists of low dispersion
turning mirrors and an optical attenuator to vary the optical power
to the sample. After the attenuator, a beam expansion system
enlarges the light from the laser to fill an imaging lens
consisting of a 40.times.-microscope objective having a numerical
aperture of 0.65. The focal length of this objective lens is 4.39
mm. With this optical system, the lens produces focal spots having
a radius of approximately 2 .mu.m. For these test substrates,
reaction (e.g., photocuring) is accomplished by positioning the
focal spot of the objective lens to coincide with the
substrate/polymer interface. The test structures consist of
continuously scanned lines that are 5 centimeters (cm) long with
0.5 cm spacing between lines, with all lines lying in the same
plane. Each line is the result of a single pass of the energy from
the laser.
[0209] The example substrates have two faces or sides. One face
consists of an array of microreflectors having spherical focusing
surfaces directing and focusing light at or near the second face of
the substrate. The actual microreflectors in the microreflector
array come from aluminizing glass or polymeric substrates that have
spherical microlens arrays formed on one face of the substrate. The
microreflector array is an aluminized microlens array with
spherical focusing elements. The individual spherical lenslets,
once aluminized, behave as spherical focusing microreflectors. For
a spherical focusing element, the focal length is f=(1/2n)r, where
n is the index of refraction for the medium between the reflector
and its focus and r is the radius of curvature. In this example,
the value of the radius of curvature yields a focal length equal to
the thickness of the substrate. For the comparative analysis, half
of the array is vacuum coated with aluminum and half is not.
[0210] A thin layer of a photoreactive composition having a 1%
loading of a two-photon initiator covers the opposite face of the
substrate. The photoreactive composition consists of 40% by weight
tris(2-hydroxyethylene) isocyanurate triacrylate, 59% by weight
methyl methacrylate (MMA), and 1% by weight two-photon absorber
such as, for example,
1,4-bis(dodecyl)-2,5-bis(2,5-dimethoxy-4-(2,2-dicyanoethenyl)sty-
ryl)benzene, dissolved to 40% concentration in a dioxane solvent.
This layer of photoreactive composition on the planar side of the
substrate is approximately 100 .mu.m thick.
[0211] Exposure of the photoreactive composition covered
microreflector substrate occurs by continuously moving the sample
beneath the highly focused light from the microscope objective.
Lines are scanned across the mircroreflector substrate from the
metallized to the unmetallized sections at a constant speed. A
second comparative sample of the same photoreactive material coats
a planar substrate and serves to establish the writing threshold.
The writing threshold for this sample provides the baseline power
level for the microreflector substrate. Writing powers bracketing
the threshold expose the two-photon initiator loaded photoreactive
composition covering the microreflector substrate.
[0212] Developing the reacted polymeric coating using a dioxane
solvent removes the unreacted regions from the substrate, revealing
photoreacted lines. The individual photoreacted lines have
thicknesses of approximately 20 .mu.m and widths of approximately
15 .mu.m. Moreover, the photoreacted polymeric lines exhibit good
adhesion to the polymeric substrate. At the writing threshold,
photoreacted lines appear on both metallized and unmetallized
portions of the microreflector substrate. Below the writing
threshold, photoreacted lines appear only on the metallized portion
of the microreflector substrate.
Example 2
[0213] As depicted in FIG. 5, a multipass multiphoton absorption
apparatus 600 includes a test substrate 610 consisting of a
polymeric film with two faces: a first face 612 and a second face
614. The first face 612 is planar while the second face 614 has a
microreplicated array of retroreflective corner cubes 614a-614e.
These retroreflective corner cube elements have edge lengths of 20
.mu.m. On the planar face 612, a two-photon initiator loaded
photoreactive composition 620 coats the surface 612. On the corner
cube face 614, one half of the corner cubes are as fabricated and
one half are filled with an acrylate polymer to effectively
eliminate the retroreflective properties of the corner cubes.
[0214] The 2-photon writing light source (not shown) consists of a
Nd:YAG microchip laser operating at a wavelength of 1.06 .mu.m,
pulse width of 800 ps, pulse repetition rate of 12 kHz, and an
average output power of 35 mW. The optical train consists of
turning mirrors and an optical attenuator to vary the optical power
to the sample. After the attenuator, a beam expansion system
enlarges the light from the laser to fill an imaging lens 630
consisting of a 40.times.-microscope objective having a numerical
aperture of 0.65. The focal length of this objective lens 630 is
4.39 mm. With this optical system, the lens 630 produces focal
points having a radius of approximately 2 .mu.m. For these test
substrates, reaction (e.g., photocuring) occurs by positioning the
focal point of the objective lens 630 to coincide with the
substrate/photoreactive composition interface. The test structures
consist of continuously scanned lines that are 5 cm long with 0.5
cm spacing between lines, with all lines lying in the same plane.
Each line is the result of a single pass of the energy from the
laser.
[0215] A thin layer of a photoreactive composition 620 having a 1%
loading of a two-photon initiator covers the opposite face 612 of
the substrate 610. The photoreactive composition 620 consists of
40% by weight tris(2-hydroxyethylene) isocyanurate triacrylate, 59%
by weight methyl methacrylate (MMA), and 1% by weight two-photon
absorber dissolved to 40% concentration in a dioxane solvent. This
layer of photoreactive composition 620 on the planar side 612 of
the substrate 610 is approximately 100 .mu.m thick.
[0216] Exposure of the photoreactive composition covered
retroreflective substrate 610 occurs by continuously moving the
sample beneath the highly focused light from the microscope
objective. Lines are scanned at constant speed across the substrate
610 from the retroreflective to the nonretroreflective sections. A
second comparative sample of the same photoreactive composition
coats a planar substrate and serves to establish the writing
threshold. The writing threshold for this sample provides the
baseline power level for the retroreflective substrate. Writing
powers bracketing the threshold expose the two-photon initiator
loaded photoreactive composition covering the retroreflective
substrate.
[0217] Developing the reacted coating in a dioxane solvent removes
the unreacted regions from the sample substrate, revealing
photoreacted lines. The individual photoreacted lines have
thicknesses of approximately 20 .mu.m and widths of approximately
15 .mu.m. Moreover, the photoreacted polymeric lines have good
adhesion to the polymeric substrate. At the writing threshold,
photoreacted lines appear on both retroreflective regions and
nonretroreflective portions of the substrate 610. Below the writing
threshold, photoreacted lines appear only on the retroreflective
portion of the sample substrate.
Example 3
[0218] This example describes the use of a reflective substrate to
recycle unabsorbed light used for reacting one or monomers with
two-photon initiators. The light source was a diode pumped
Ti:sapphire laser (Spectra-Physics, Mountain View, Calif.)
operating at a wavelength of 800 nanometers (nm), pulse width 100
femptoseconds (fs), pulse repetition rate of 80 megahertz (MHz),
beam diameter of approximately 2 millimeters (mm), and an average
output power of 860 milliwatts (mW). The optical train consisted of
low dispersion turning mirrors, an optical attenuator to vary the
optical power, and a 10.times.-microscope objective (0.25 NA) to
focus the light into the sample. With this optical system, the lens
produced focal spots having a diameter of approximately 8 .mu.m and
a depth of focus of approximately 120 .mu.m. For these test
substrates, reaction was accomplished by positioning the focal spot
of the objective lens to coincide with the substrate/polymer
interface. The test structures consisted of continuously scanned
lines that were 0.5 to 5 cm long with 0.0625 cm spacing between the
lines, with all lines lying in the same plane. Each line was the
result of a single pass of energy from the laser. The average power
delivered to the sample, measured where the beam exits the
microscope objective using a calibrated photodiode (photodiode head
PD 300-3, commercially available from Ophir Optronics Inc.,
Danvers, Mass.), was 20 mW in this example.
[0219] The sample substrates were 4-inch silicon wafer with a
native oxide, where one half were vacuum coated with 1500 .ANG. of
aluminum to form a mirror-like coating. To promote polymer
adhesion, a 2% solution of trimethoxysilylpropylmethacrylate in
aqueous ethanol (pH of approximately 4.5) was spun coated on the
substrates, which were then baked in a 130.degree. C. oven for 10
minutes (min). A thin layer of the photoreactive composition (see
Table 1), 40% by weight of solids in dioxane (available from
Mallinckrodt Baker, Phillipsburg, N.J.), was then spun coated over
the entire wafer and baked in an 80.degree. C. oven to remove the
solvent. The final layer of photoreactive coating was approximately
20 .mu.m thick. TABLE-US-00001 TABLE 1 Photoreactive composition
for Example 3 Ingredient Weight % Poly(styrene-co-acrylonitrile)
26.55 (MW approximately 165,000 g/mol) SR-368 (Sartomer Co., West
Chester, PA) 35.40 SR-9008 (Sartomer Co., West Chester, PA) 35.40
Diphenyliodonium hexafluorophosphate 1.77
Bis-[4-(diphenylamino)stryl]-1,4- 0.88 (dimethoxy)benzene
[0220] Exposure of the photoreactive composition covered, partially
aluminized wafer occurred by continuously moving the sample beneath
the highly focussed light from the microscope objective. Lines were
scanned across the substrate from the metallized to the
unmetallized regions at constant speed. With each successive pass
of the laser, the stage velocity was increased by a factor 2 to
cover a range from 77 to 27520 micrometers per second (.mu.m/s).
The dose array was repeated on different areas of the substate with
different average laser powers in order to bracket the threshold
dose. Developing the reacted polymeric coating using N,N-dimethyl
formamide removed the unreacted regions from the substrate,
revealing photoreacted lines. The individual photoreacted lines had
widths that decreased from 24 .mu.m to 8 .mu.m as the speed was
increased. Moreover, the photoreacted lines exhibited good adhesion
to both the metallized and unmetallized parts of the silicon
substrate. The writing threshold, for a given average laser power,
is defined as the highest stage speed at which a photoreacted line
is visible when examined using an optical microscope after
development. The results on silicon and on aluminum are shown in
FIG. 7. The increase in the slope of the power dependence indicates
that the writing speed on the metallized region was at least twice
that of the bare silicon. Such a result is consistent with the
forward and reflected pulses acting independently so that the
photoreactive composition on the aluminum receives twice the energy
or dose of the photoreactive composition on the silicon.
Example 4
[0221] A test substrate consisted of a polymeric film with two
faces. One face was planar while the second face had a
microreplicated array of retroreflective corner cubes prepared as
described in U.S. Pat. No. 5,138,488, Comparative Example A. The
thickness of the test substrate was approximately 380 .mu.m. On the
planar face, a two-photon initiator loaded, photoreactive
composition was coating the surface. The photoreactive composition
consisted of reactive monomers in a thermoplastic matrix. The
refractive index and density of the photoreactive composition was
increased in the illuminated areas as a result of polymerization
and subsequent monomer diffusion into the illuminated area. After
the desired structures were created, the entire film was blanket
exposed using a one-photon source to permanently fix the image. On
the corner cube face, one half of the corner cubes were as
fabricated and one half were filled with an acrylate polymer to
effectively eliminate the retroreflective properties of the corner
cubes. The polymeric film was mounted on a glass substrate and
lightly taped at the edges.
[0222] The light source was a diode pumped Ti:sapphire laser
(Spectra-Physics) operating at a wavelength of 800 nm, pulse width
100 fs, pulse repetition rate of 80 MHz, beam diameter of
approximately 2 mm, and an average output power of 860 mW. The
optical train consisted of low dispersion turning mirrors, an
optical attenuator to vary the optical power, and a
5.times.-microscope objective with 39 mm effective focal length and
numerical aperture of 0.09 to focus the light into the sample. With
this optical system, the objective produced focal spots having a
diameter of approximately 18 .mu.m and a depth of focus of
approximately 650 .mu.m. The long depth of focus was necessary so
that both the forward and retroreflected spots were in focus in the
photoreactive composition (see FIG. 5). The average power delivered
to the sample, measured where the beam exits the microscope
objective using a calibrated photodiode (photodiode head PD 300-3W,
commercially available from Ophir Optronics Inc., Danvers, Mass.),
was 120 mW in this example.
[0223] Reaction was accomplished by positioning the focal spot of
the objective lens to coincide with the corner cube/glass substrate
interface. The test structures consisted of continuously scanned
lines that are 0.5 to 5 cm long with 0.0625 cm spacing between the
lines, with all lines lying in the same plane. Each line was the
result of a single pass of energy from the laser. TABLE-US-00002
TABLE 2 Photoreactive composition for Example 4 Ingredient Weight %
Cellulose acetate butyrate CAB-531-1 (Eastman 50.96 Chemicals,
Kingsport, TN) Phenoxyethyl acrylate SR-339 (Sartomer Co., West
39.51 Chester, PA) 2-(1-Naphthoxy)ethyl acrylate* 5.64 SR-351
(Sartomer Co., West Chester, PA) 0.94
Bis-[4-(diphenylamino)stryl]-1,4-(dimethoxy)benzene 0.98
Diphenyliodonium hexafluorophosphate SR1012 (Sartomer 1.96 Co.,
West Chester, PA) 2-(1-Naphthoxy)ethyl acrylate was made as
described in U.S. patent application Ser. No. 09/746613, filed on
Dec. 21, 2000.
[0224] The photoreactive composition described in Table 2 was
dissolved in 1,2-dichloroethane to 33% solids and spun coated on to
the planar side of the retroreflective substrate. The solvent was
removed by baking in an 80.degree. C. oven for 10 minutes. The
final layer of photoreactive coating was approximately 20 .mu.m
thick. Exposure of the photoreactive composition covered,
retroreflective substrate occurred by continuously moving the
sample beneath the highly focused light from the microscope
objective. Lines were scanned across the substrate at constant
speed. With each successive pass of the laser, the stage velocity
was increased by a factor 2 to cover a range from 77 to 27520
micrometers per second (.mu.m/s). The dose array was repeated on
different areas of the substrate with different average laser
powers in order to bracket the threshold dose. Following imagewise
exposure, the photoreactive composition was blanket exposed using a
bank of three Philips TLD 15W-03 bulbs for 30 minutes. A second
comparative sample of the same photoreactive composition coated a
planar substrate and served to establish the writing threshold.
[0225] FIGS. 8a and 8b shows optical micrographs of the scanned
lines. An index matching oil was placed beneath the corner cubes to
reduce the reflection from the corner cubes. Retroreflected spots
appear along side the line created by the first pass of the focused
beam. The spots do not appear where the retroreflective properties
were eliminated by filling with acrylate monomer. It is interesting
to note that retroreflected spots continued to appear at faster
writing speeds, even when the line created by the initial pass
through the photoreactive composition was no longer discernable.
This may be due to imperfect positioning of the focal point or
self-focusing of the light in the photoreactive composition.
Example 5
[0226] As depicted in FIG. 6, a multipass multiphoton absorption
apparatus 700 utilizing a transparent substrate 710 and the
transmitted light was recycled and used to perform another
exposure. The light source 720 was a diode pumped Ti:sapphire laser
(Spectra-Physics) operating at a wavelength of 800 nm, pulse width
100 fs, pulse repetition rate of 80 MHz, beam diameter of
approximately 2 mm, and an average output power of 860 W. The
optical train, shown in FIG. 6, consisted of low dispersion turning
mirrors 730 and 732, an optical attenuator 740 to vary the optical
power, a shutter 745, and a 10.times.-microscope objective 750
(0.25 NA) to focus the light into the sample 710. The objective 750
produced focal points having a diameter of approximately 8 .mu.m
and a depth of focus of approximately 120 .mu.m. The transmitted
light was collected and re-focused on to the sample 710 by a 25-mm
diameter, aluminum coated spherical focusing mirror 760
(commercially available from Newport Corp., Irvine, Calif.) with
10-cm focal length, that was mounted on a 3-axis translation stage
and gimbal mirror mount, 20 cm from the photoreactive composition
coated substrate 710. A zero order quarter wave plate 770
(commercially available from CVI Laser Corp.) was placed between
the sample 710 and the focusing mirror 760 to rotate the
polarization of the reflected light by ninety degrees. Combined
with the linear polarizing cube 780, these optics formed an
efficient optical isolator to prevent the reflected light from
returning to the laser cavity 720.
[0227] The test substrate 710 consisted of a photoreactive
composition 712 coated on to one side of an optically transparent
microscope slide. To promote polymer adhesion, a 2% solution of
trimethoxysilylpropylmethacrylate in aqueous ethanol (pH of
approximately 4.5) was spun coat on the substrates, which were then
baked in an 130.degree. C. oven for 10 min. A thin layer of the
photoreactive composition (see Table 3), 40% by weight of solids in
dioxane, was then spun coated over the entire wafer and baked in an
80.degree. C. oven to remove the solvent. The final layer of
photoreactive coating was approximately 20 .mu.m thick.
TABLE-US-00003 TABLE 3 Photoreactive composition for Example 5
Ingredient Weight % Polymethylmethacrylate (MW approximately 300,00
g/mol) 26.55 SR-368 (Sartomer Company, West Chester, PA) 35.40
SR-9008 (Sartomer Company, West Chester, PA) 35.40 Diphenyliodonium
hexafluorophosphate 1.77
Bis-[4-(diphenylamino)stryl]-1,4-(dimethoxy)benzene 0.88
[0228] Reaction was accomplished by positioning a focal point 714
of the objective lens 750 to coincide with the substrate polymer
interface 716. The image of the focal point 714 produced by the
focusing mirror 760 was also positioned at this interface 716. Test
structures consisted of continuously scanned lines that were 0.5 cm
long with 0.125 cm spacing between the lines, all with lines lying
in the same plane. Each line was the result of a single pass of
energy from the laser 720. Exposure of the photoreactive
composition covered slides 710 occurred by continuously moving the
sample beneath the highly focused light from the microscope
objective 750 and the focusing mirrors 760. Lines were scanned
across the substrate 710 at constant speed. With each successive
pass of the laser, the stage velocity was increased by a factor 2
to cover a range from 77 to 27520 .mu.m/s. The dose array was
repeated on different areas of the substrate with different average
laser powers in order to bracket the threshold dose. Developing the
exposed photoreactive composition 712 using N,N-dimethyl formamide
removed the unreacted regions from the substrate 710, revealing
photoreacted lines. The individual photoreacted lines had widths
that decreased from 24 .mu.m to 8 .mu.m as the speed was increased.
Moreover, the photoreacted lines exhibited good adhesion to the
glass substrate 710. The writing threshold, for a given average
laser power, was defined as the highest stage speed at which a
photoreacted line was visible when examined using an optical
microscope after development.
[0229] In one example, the reflected spot 714 from the focusing
mirror 760 was positioned slightly offset from the region exposed
on the first pass through the sample. In this case, two parallel
lines appeared wherever the sample had been scanned. See, FIG. 9a.
When the reflected beam from the focusing mirror was blocked, only
a single line appeared (see FIG. 9b). In a second example, the
reflected spot 714 was directed back on to the first focus region
to increase the exposure. The sample was then scanned both with the
mirror 760 blocked and when it is covered by a non-focusing,
scattering substrate. Table 4 shows the threshold speed using an
average power of approximately 53 mW, both with the mirror blocked
and when the transmitted and reflected focal points were
overlapped. Sample IV of Table 4 demonstrates that when the mirror
760 was unblocked but the focal points were not overlapped, there
was no enhancement of the threshold writing speed. TABLE-US-00004
TABLE 4 Threshold speed with and without the reflected spot
Threshold speed Threshold speed (.mu.m/s) (.mu.m/s) Sample Mirror
Blocked Mirror Unblocked Comments I 310 620 Overlapped II 215 430
Overlapped III -- 525 Overlapped IV 215 215 Not overlapped
[0230] The complete disclosures of the patents, patent documents,
and publications cited herein are incorporated by reference in
their entirety as if each were individually incorporated. Various
modifications and alterations to this invention will become
apparent to those skilled in the art without departing from the
scope and spirit of this invention. It should be understood that
this invention is not intended to be unduly limited by the
illustrative embodiments and examples set forth herein and that
such examples and embodiments are presented by way of example only
with the scope of the invention intended to be limited only by the
claims set forth herein as follows.
* * * * *