Optical: systems and elements – Holographic system or element – Having optical element between object and recording medium
Reexamination Certificate
1999-05-14
2001-02-06
Henry, Jon (Department: 2872)
Optical: systems and elements
Holographic system or element
Having optical element between object and recording medium
C359S034000, C359S035000
Reexamination Certificate
active
06185019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high throughput holographic lithography system for generating interferometric patterns suitable for selectively exposing a photosensitive material and, more particularly, to an easily reconfigurable lithographic patterning tool employing a fiber optic beam delivery system and a prism optically coupled to the photosensitive material.
2. Description of the Related Art
Holographic or interferometric lithography has been proven in laboratory environments to be feasible for generating patterns of light suitable for exposing photosensitive materials in the manufacture of devices having sub-micron features. Holographic lithography exploits the mutual coherence of multiple optical beams derived from a single light source such as a laser. The laser beams are made to overlap in some region of space and interfere to produce patterns of light and dark areas that repeat on a scale proportional to the wavelength of the light source. The interferometric patterns of light are recorded in a photosensitive medium, such as photoresist, properly positioned within the region of space. Conventional contact or projection photo masks are not required; thus, holographic lithography is known as a “maskless” lithography technique. Additionally, by exploiting inherent photoresist and etching process non-linearities, a variety of surface relief structures can be generated with no change in the optical configuration.
There are a number of applications that would benefit from the sub-micron sized structures that can be produced using interferometric patterns generated from holographic lithography techniques. For example, holographic lithography can be used to produce improved distributed feedback (DFB) gratings which are employed in the telecommunications field. More specifically, the telecommunications market is experiencing rapid growth and prosperity due to advances in fiber-optic technology and the advent of wavelength division multiplexing (WDM). WDM techniques allow the efficient combination of multi-channel (multi-carrier frequency), high-bit rate signals onto a single optical fiber. Solid state laser sources operating about wavelength bands centered at 1310 nanometers (nm) and 1550 nm are employed to transmit digital information at rates as high as 2.5 gigabits per second. These solid state lasers emit light in a multi-longitudinal mode wherein numerous narrow band wavelengths are clustered about the center wavelength. Interference between these wavelengths limits the number of channels that can be transmitted along an optical fiber.
Distributed feedback solid state lasers incorporate DFB gratings within the lasing medium to act as filters to limit the lasing output to a single narrow-banded mode. This narrow-band operation is required for long haul and high-speed telecommunications. Such gratings are typically produced via a phase mask technique or by e-beam lithography. Both techniques suffer from practical limitations such as small field size, stitching errors, short mask lifetimes, low throughput, and high cost. Thus, a practical, low cost, reliable, easily reconfigured production tool is needed to more efficiently produce DFB gratings.
Holographic, or interferometric lithography has been used experimentally to produce DFB gratings with the advantages of large area, high throughput, no stitching errors and no photomasks. The technology enables the realization of a non-contact, non-scanned, maskless pattern generator. No intermediate photomasks are required. The principal advantages of holographic lithography include: sub-half-micron resolution; nearly unlimited field size; a lensless configuration; the capability to form patterns on arbitrary surfaces; a cost effective mechanism for obtaining high yield and throughput; and compatibility with all current semiconductor, photoresist and mask production technologies.
The line arrays, or gratings required for distributed feedback are derived by recording a classical two-beam interference pattern.
FIG. 1
depicts a typical laboratory system
10
for exposing a photosensitive material with a two-beam interference pattern. A laser source
12
, such as a single frequency argon-ion laser, produces a laser beam that is split into two substantially equal beams by a beam splitter
14
. The two beams are incident on respective turning mirrors
16
and
18
which direct the two beams toward respective spatial filters
20
and
22
. Each of spatial filters
20
and
22
causes its respective laser beam to diverge, such that two divergent illuminating beams
24
and
26
are respectively projected from the two spatial filters. The points from which the illuminating beams are emitted from the spatial filters lie in a point source plane
28
, and the spatial filters are oriented such that the illuminating beams overlap at some distance from the point source plane to produce a patternable volume
30
. A wafer or panel
32
coated with a photosensitive material such as a photoresist is placed in the patternable volume
30
with the surface of the photosensitive material lying in a recording plane
34
that is substantially parallel to the point source plane
28
. Typically, the distance between the recording plane and the point source plane is on the order of at least one meter. Exposure timing and duration can be controlled with an electronic shutter/timer
36
lying in the optical path of the laser beam (e.g., between the laser source
12
and beam splitter
14
).
As shown in
FIG. 2
a
, wafer
32
includes at least a substrate
40
coated with a photoresist layer
42
. Photoresist layer
42
is subjected to two-beam holographic exposure by illuminating beams
24
and
26
. Specifically, beams
24
and
26
create an interference pattern in the recording plane
34
in the form of a grating of parallel lines of high light intensity alternating with parallel lines of low light intensity. The light intensity varies sinusoidally in the recording plane
43
in the direction perpendicular to the orientation of the lines of the grating pattern.
The two expanded beams
24
and
26
overlap in the recording plane
34
to form the grating pattern and selectively expose the photoresist layer
42
. This exposure creates a latent image in the photoresist consisting of parallel lines in an array. The sinusoidal intensity distribution of the interference pattern has a modulation contrast of four to one and a peak-to-peak distance, or pitch, given by the relation:
&Lgr;=&lgr;
o
(2
n
i
sin &thgr;
i
) (1)
where &lgr;
o
is the vacuum wavelength, &thgr;
i
is the incident beam angle measured from the surface normal, and n
i
is the index of refraction of the incident medium.
After a suitable non-linear photoresist development process, a grating in the photoresist is produced which can then be employed as a mask during a subsequent etch process which replicates the grating in the distributed feedback material. Specifically, as shown in
FIG. 2
b
, the developed photoresist layer
42
′ is a grating corresponding to the illumination grating pattern, with parallel lines of developed photoresist separated by linear regions where the photoresist has been removed. An etching process, such as reactive ion etching (RIE), can then employ the photoresist grating as a mask to reproduce the grating pattern in the substrate
40
(see
FIG. 2
c
).
A graphical representation of the intensity distribution and the resulting calculated photoresist profile predicted from the measured response characteristics of a commercially available resist are respectively shown in
FIGS. 3
a
and
3
b
. The nature of the photoresist grating mask that is recorded using two-beam interferometric lithography can be clearly seen in the scanning electron micrographs (SEM) of
FIGS. 4
a
(0.8 &mgr;m pitch, 1.2 &mgr;m depth) and
4
b
(resist on Si, 460 nm pitch, ~200 nm CD) showing edge profile cross-sections of grating masks. Lines in the photoresist slightly less than 0.4 micron in width are located on 0.8 a
Hobbs Douglas S.
Kelsey Adam F.
MacLeod Bruce D.
Henry Jon
Optical Switch Corporation
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