Title: Integrated spacer technology for LCOS light modulators
Abstract: An LCOS display, including specially manufactured spacers, and a process for making the display, are disclosed. The spacers ensure a uniform cell gap along the entire display. The spacers occupy a region between pixels, such that they do not interfere with light modulation and are not visible during magnification. The spacers are manufactured using known deposition, lithography and etching techniques, and are made from widely available materials. The process results in a high yield of high-quality LCOS displays.
Patent Number: 6,940,577 Issued on 09/06/2005 to Kozhukh
| Inventors:
|
Kozhukh; Michael (Palto Alto, CA)
|
| Assignee:
|
Intel Corporation (Santa Clara, CA)
|
| Appl. No.:
|
867405 |
| Filed:
|
June 14, 2004 |
| Current U.S. Class: |
349/156 |
| Intern'l Class: |
G02F 001/13 |
| Field of Search: |
349/156
|
References Cited [Referenced By]
U.S. Patent Documents
| 6154267 | Nov., 2000 | Kondo et al.
| |
| 6184959 | Feb., 2001 | Izumi.
| |
| 6236445 | May., 2001 | Foschaar et al.
| |
| 6642987 | Nov., 2003 | Chung et al.
| |
| 6750938 | Jun., 2004 | Koyama.
| |
| 6774955 | Aug., 2004 | Chung et al.
| |
| 6788380 | Sep., 2004 | Melnik et al.
| |
| 2001/0040665 | Nov., 2001 | Ahn.
| |
| 2002/0060771 | May., 2002 | Sumino et al.
| |
| 2002/0149720 | Oct., 2002 | Janssen et al.
| |
| 2002/0149734 | Oct., 2002 | Melnik et al.
| |
| 2003/0071959 | Apr., 2003 | Koyama.
| |
Other References
Voltolina, Guido, "Liquid Crystal Meets Silicon in Projection Displays", Semiconductor
International, Dec. 1, 2002.
|
Primary Examiner: Dudek; James A.
Attorney, Agent or Firm: Boone P.C.; Carrie A.
Parent Case Text
This application is a divisional of application Ser. No. 10/369,074, filed in
Feb. 18, 2003.
Claims
1. A method, comprising:
depositing a material on a patterned metal film layer affixed to a substrate,
the patterned metal film layer comprising micro-mirror and no-micro-mirror regions;
chemically and mechanically polishing the material to a predetermined thickness;
etching away a first portion of the material, such that a second portion of the
material remains deposited upon the no-micro-mirror region of the patterned metal
film; and
coating the patterned metal film and the second portion with an anti-reflective
film.
2. The method of claim 1, further comprising:
seating a transparent plate upon the second portion, wherein the transparent
plate is equidistant from the substrate at all points of the substrate.
3. The method of claim 2, further comprising:
adding a liquid crystal material to a space between the patterned metal film
and the transparent plate.
4. The method of claim 1, further comprising:
grinding a groove into the deposited material, the groove being of a predetermined
depth, wherein the groove indicates how much of the material is to be chemically
and mechanically polished.
5. The method of claim 1, coating the patterned metal film and the second portion
with an anti-reflective film further comprising:
depositing a layer of silicon dioxide to the patterned metal film and the second
portion; and
depositing a layer of silicon nitride to the patterned metal film and the second
portion.
6. The method of claim 1, wherein the material comprises silicon nitride and
the second portion is cross-shaped.
7. The method of claim 1, coating the patterned metal film and the second portion
with an anti-reflective film further comprising:
coating the patterned metal film and the second portion with silicon dioxide.
8. A method, comprising:
depositing a layer of silicon nitride on a patterned metal film, the patterned
metal film comprising a plurality of micro-mirrors arranged in pixel regions, the
patterned metal film further comprising no-pixel regions;
polishing the silicon nitride layer to a uniform thickness on the patterned metal
film, wherein the polishing causes a sub-layer of a predetermined thickness to
be removed from the patterned metal film;
generating a plurality of spacer patterns over the silicon nitride layer, wherein
the spacer patterns are disposed over the no-pixel regions;
removing the silicon nitride layer from the patterned metal film, except where
the silicon nitride layer is disposed beneath the plurality of spacer patterns,
such that the remaining silicon nitride forms a plurality of spacers; and
depositing an anti-reflective coating on the plurality of spacers and the exposed
patterned metal film.
9. The method of claim 8, further comprising:
depositing a silicon dioxide coating on the plurality of spacers and the exposed
patterned metal film.
10. The method of claim 8, polishing the silver nitride layer further comprising
chemically and mechanically polishing the silver nitride layer.
11. The method of claim 8, generating a plurality of spacer patterns over the
silicon nitride layer further comprising disposing the plurality of spacer patterns
upon the silicon nitride layer using lithography and etching techniques.
12. The method of claim 8, further comprising:
grinding a groove In the silicon nitride layer before polishing, the groove having
the predetermined thickness within the layer.
13. A method, comprising:
depositing a material upon a patterned metal film, the patterned metal film being
disposed upon a substrate, the patterned metal film including a pixel region and
a no-pixel region, a plurality of micro-mirrors being disposed in the pixel region,
wherein the material is disposed over both the pixel region and the no-pixel region
of the substrate;
removing the material from the pixel region of the patterned metal film such
that a remaining material is disposed over the no-pixel region, wherein the remaining
material has a uniform predetermined thickness all along the substrate;
disposing a pattern along the no-pixel region;
sculpting the material away from the patterned metal film, except where the pattern
is disposed, such that the patterned metal film in the pixel region is exposed;
depositing a silicon dioxide coating on the remaining material and the exposed
patterned metal film; and
depositing a silicon nitride film on the silicon dioxide deposit.
14. The method of claim 13, further comprising:
chemically and mechanically polishing the deposited material, until the material
has the predetermined uniform thickness.
15. The method of claim 14, further comprising:
marking the predetermined uniform thickness of the deposited material before
chemically and mechanically polishing the deposited material.
Description
FIELD OF THE INVENTION
This invention relates to optical projection systems and, more particularly,
to liquid crystal on silicon (LCOS) display systems.
BACKGROUND OF THE INVENTION
Optical projection systems such as televisions and computer monitors use
cathode ray tubes (CRTs) as displays. A liquid crystal on silicon, or LCOS, light
modulator, is an alternative display component that has some advantages over CRTs.
In particular, LCOS light modulators are flat, thus occupying less space, and use
less power than CRTs.
LCOS displays consist of layered components. A surface layer of glass or transparent
plastic substrate is disposed over a middle layer of liquid crystal material, which
is further supported by an underlying layer of silicon substrate, known as a back
plane. The glass or transparent plastic layers are manufactured with transparent
electrodes on the surface adjacent to the liquid crystal material. A patterned
metal layer is further disposed upon the back plane, comprising hundreds or thousands
of reflecting mirrors, known as micro-mirrors.
The LCOS display thus comprises a first glass or plastic layer, an electrode
layer, a liquid crystal layer, a second electrode layer, a patterned metal film
layer, and a second glass or plastic layer. A pixel of the LCOS display includes
a single micro-mirror, along with its associated layers. The LCOS display is thus
an array of pixels, including hundreds or thousands of individual pixels.
LCOS displays use polarized lenses to control the transmission of light, known
as light modulation, to individual pixels within the pixel array. LCOS displays
are thus sometimes referred to as light modulators. The electrodes embedded in
the glass or plastic layers allow an electric field to be applied across the liquid
crystal material. Molecules in the liquid crystal medium exhibit polarization alignment
when subjected to the electric field. The liquid crystal thus acts as a dynamic
polarizing medium.
One of the glass or plastic layers of the LCOS display is coated to respond as
a static polarizing filter. Together the two polarizing filters modulate the light
received by individual pixels of the LCOS display. When the pixel is turned "off,"
the light attempting to transmit through the pixel is blocked by the polarizing
filters. When the pixel is turned "on," the light travels through the polarizing
filters unimpeded, reflecting off the micro-mirror.
Due to minute manufacturing inconsistencies or defects, the materials that make
up the LCOS light modulator do not have a precise thickness at the microscopic
level. These differences in material thickness cause differences in the thickness
of the cell gap, or space between the two glass or plastic layers, in which liquid
crystal material occupies the cell gap.
To more precisely control cell gap spacing, many manufacturers spray minute glass
or plastic beads over the patterned metal film. The glass or plastic beads, or
spacers, are meant to ensure that the cell gap is uniform all along the pixel array,
thus improving image quality. However, the glass or plastic beads, which share
space with the liquid crystal material in the cell gap, may interfere with light
traveling through the liquid crystal. Further, when the pixel is turned "on," and
the image is magnified, the interference caused by the beads is also enlarged,
often appearing as a defect in the display. While the defect may not be apparent
on an optical projection system from a distance, it is more likely visible at close
proximity to the discriminating eye.
Because of these issues, some LCOS manufacturers are making spacerless displays,
with unsatisfying results. Generally, the quality of spacerless displays is poor.
Other manufacturers control the cell gap spacing by etching spacers into the overlying
glass or plastic plate. This etching technique, while more precisely controlling
spacing between the pixels of the display, may continue to adversely affect image
quality, as the spacers block the transmission of light through the LCOS media.
Thus, there is a need for an LCOS display in which spacers precisely control
the cell gap, but which do not interfere with light transmission through each pixel
of the display.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an LCOS display with spacers;
FIG. 2 is a side view of the LCOS display, including a couple of pixels;
FIG. 3 is a perspective view of the LCOS display of FIG. 1, featuring various dimensions;
FIG. 4 is a flow diagram of operations performed in manufacturing the LCOS display
of FIG. 1; and
FIG. 5 is a diagram, including top and side views, of operations performed in
manufacturing the LCOS display of FIG. 1.
DETAILED DESCRIPTION
In accordance with some embodiments described herein, an LCOS display is disclosed,
including novel spacers superimposed upon a patterned metal film. The patterned
metal film is an array of micro-mirrors disposed upon a silicon back plane or substrate.
The spacers ensure that a precise distance between the patterned metal film and
an overlying transparent layer is maintained, which improves the image quality
of the LCOS display.
The spacers are depicted as cross-shaped, but may assume any shape. Each spacer
may lie within a gap between four adjoining pixels of the patterned metal film.
The spacers may be oriented so as to be outside of the space in which the transmitted
light of the pixel travels. By occupying the nonfunctional space of the LCOS display,
the spacers avoid interfering with the light traveling through the display.
A process for making the LCOS display, including the spacers, is disclosed. Spacers
of a uniform height along the entire LCOS display are generated during its manufacture,
thus creating a uniform cell gap, or gap between the micro-mirrors and the overlying
transparent layer. The cell gap is then injected with the liquid crystal material,
which is activated as a polarizing filter by transistors disposed beneath the micro-mirrors.
The use of the spacers may result in a high-quality projected image from the LCOS
display and a higher yield of LCOS displays during manufacture.
In the following detailed description, reference is made to the accompanying
drawings,
which show by way of illustration specific embodiments in which the invention may
be practiced. However, it is to be understood that other embodiments will become
apparent to those of ordinary skill in the art upon reading this disclosure. The
following detailed description is, therefore, not to be construed in a limiting
sense, as the scope of the present invention is defined by the claims.
FIG. 1 is a perspective view of an LCOS display
100, including two spacers
20, according to some embodiments. The spacers
20 are affixed or
otherwise attached to a patterned metal film
14, which is superimposed upon
a silicon substrate or back plane
30. The patterned metal film
14
is patterned or etched into multiple distinct micro-mirrors
10, each of
which reflects light during light modulation.
The LCOS display
100 further includes a transparent layer or plate
40,
comprising glass or plastic material, disposed on top of the spacers
20.
Between the patterned metal film
14 and the transparent layer
40
is a layer of liquid crystal
50. For example, the liquid crystal material
50 may be injected between the layers after the spacers
20 are formed.
By design, not all the ambient, or unpolarized, light reaches the micro-mirrors
10 of the LCOS display
100. Using the principles of polarization,
the light is filtered to control which pixels receive light. Unpolarized light,
which consists of light waves oriented in all directions, can be converted to polarized
light, or light waves oriented in a particular direction, using a polarizing filter
or lens.
A polarizing lens blocks light waves that are oriented orthogonal to the lens,
for example, but transmits light oriented parallel to the lens. Such a polarized
lens neither wholly blocks nor wholly transmits the unpolarized light. Two polarized
lenses, however, may be able to wholly block or wholly transmit unpolarized light,
such as by adjusting the relative positions of the lenses.
Accordingly, in the LCOS display
100, the transparent layer
40
includes electrodes (not shown) which, in conjunction with electrodes disposed
within the substrate
30 (not shown), produce a polarizing effect on the
liquid crystal
50 when an electric signal is sent to the LCOS display
100.
Circuitry, such as transistors, is embedded within the substrate
30 (not
shown), to produce the electric signal. The electrodes may be made of aluminum
oxide, tin oxide, or aluminum-tin oxide, as examples. The transparent layer
40
operates as a second polarizing filter.
The LCOS display
100 is a pixel array, typically comprising hundreds or
thousands or more of individual pixels. As used herein, a pixel
12 consists
of the portion of the LCOS display
100 that includes a single micro-mirror
10, including the substrate
30 beneath the micro-mirror as well as
the liquid crystal
50 and transparent layer
40 above the micro-mirror.
The pixel
12 does not include any of the space
32 between the micro-mirrors
10, which is essentially a no-micro-mirror region. This no-micro-mirror
region is known herein as a no-pixel region
32. In the side view of FIG.
2, two pixels
12 of the LCOS display
100 are depicted, separated
by a single spacer
20.
During the manufacture of the LCOS display
100, the spacers
20
are built upon the silicon substrate
30 using techniques familiar to those
of skill in the art of integrated circuit fabrication. The spacers
20 are
generated prior to adding the liquid crystal and surface layers to the LCOS display.
Ultimately, the spacers
20 determine the uniformity of the liquid crystal
material and, thus, the quality of light modulation achieved by the LCOS display
100.
Between each pixel
12 are spaces, known as the no-pixel region
32,
some portion of which may be occupied by the spacers
20. The no-pixel region
32 is also the part of the patterned metal film
14 in which the reflective
surface (micro-mirror) has been etched away. Since the micro-mirrors
10
are generally square in shape, the no-pixel region
32 forms a waffle-like
pattern across the silicon substrate
30. The spacers
20 preferably
occupy at least some, but preferably not all, of the no-pixel region
32
between the micro-mirrors
10.
In FIG. 3, a second perspective view of the LCOS display
100 features
spacer
dimensions, as well as distance between spacers
20, according to some embodiments.
The height
22, the length
24, the width
26, the thickness
16, the spacer distance
28, and the no-pixel region width
18
are included. The height
22 of the spacer
20 is dependent upon the
properties of the liquid crystal material
50 used in the LCOS display
100.
In some embodiments, the height
22 of the cross-shaped spacer
20
is between 1 and 5 microns.
The width
18 is the width of the no-pixel region
32. The no-pixel
region
32 is the space where the patterned metal film
14 has been
etched away from the substrate
30. Preferably, the thickness
16 of
each spacer
20 is less than the no-pixel region width
18, so as to
avoid the likelihood that the spacers
20 produce visible defects during
light modulation. In some embodiments, the thickness
16 of the spacer
20
is between 0.2 and 0.5 microns, making the no-pixel region
32 at least slightly
thicker than these dimensions.
FIG. 3 also shows the length
24 and the width
26 of the spacer
20. In some embodiments, the length
24 and the width
26 of
the spacer
20 are identical, such that the spacer
20 comprises a
uniform cross within a square region. Alternatively, the length
24 can be
longer than the width
26, or vice-versa, such that the spacer
20
comprises an elongated cross within a rectangular region. The spacers
20
can also be diamond shaped, circular, oval, or other nonconforming shape, as long
as the shape of the spacers does not extend beyond the no-pixel region
32.
The spacer distance
28, indicating the distance between spacers
20,
is also variable. Preferably, the spacers
20 are sufficient in number along
the LCOS display
100 to ensure that the overlying glass layer
40
is equidistant from the underlying substrate
30 at all points along the
LCOS display
100. A spacer
20 may be positioned between each group
of four pixels where one corner of each pixel is joined at a single point, known
herein as a pixel group
56 (see FIG.
3). Preferably, however, a minimum
number of spacers
20 are used to ensure that the layer
40 is equidistant
from the substrate
30 at all locations. In some embodiments, the spacer
distance
28 is 1.5 to 2.0 millimeters. Spacer distance can also be measured
in the relation to the pixels
12. For example, in FIG. 3, spacers are positioned
along every fourth pixel group
56.
The spacers
20 are preferably confined to within the no-pixel region
32.
This arrangement reduces the possibility of interference with the transmitted light,
which would otherwise create unwanted dark spots and image defects to the LCOS
display
100.
FIGS. 4 and 5 depict manufacturing steps for producing the LCOS display
100,
including the spacers
20, according to some embodiments. The steps of FIG.
4 (blocks
200-
214) are correlated with the top (
200A-
214A)
and side (
200B-
214B) views of FIG. 5, using like reference numerals.
A pixel group
56 of the LCOS display
100 is depicted in FIG.
5.
Prior to commencement of the process (
200A and
200B), the patterned
metal film is shown, etched into four micro-mirrors
10 and affixed to the
substrate
30. For example, the four micro-mirrors
10 are equally
spaced apart and adjacent to the no-pixel region
32 upon which the spacers
20 are to be generated.
Upon commencement, silicon nitride (Si
3N
4)
38 is
deposited on the patterned metal film
14 (block
202). The material
from which the spacers
20 will be made, silicon nitride
38 is commonly
used in the manufacture of integrated circuits. In the top view (
202A) of
FIG. 5, the micro-mirrors
10 are no longer visible, having been covered
by the silver nitride material. In the side view (
202B), the silicon nitride
is visible as a block of material, from which the spacers
20 will be formed.
The height of the silicon nitride deposition is d+r, where d is the desired height
of the spacers
20 and r is the thickness of an anti-reflective film or coating,
to be deposited on the silicon nitride
38 later in the process (blocks
212
and
214).
Once the silicon nitride
38 is deposited on the patterned metal film
14, grooves
98 are ground into the surface of the silicon nitride
(block
204). The top view (
204A) and the side view (
204B)
of FIG. 5 depict the groove
98 as a channel along the silicon nitride
38.
The depth of the groove
98 is limited such that the height of the silicon
nitride deposit
38 remains at least of height d, or greater, even at the
lowest point of the groove
98.
Next, the silver nitride surface is chemically and mechanically polished until
the silver nitride
38 is of at least height d at all points (block
206).
Most likely, the chemical and mechanical polishing is an automatic process performed
by a machine. The bottom of the groove
98 provides a physical guide for
the machine during the polishing process, indicating the desired minimum height
of the silver nitride
38 after polishing. The top view of the pixel group
(
206A) shows only silver nitrate
38 while the side view (
206B)
shows the silver nitrate
38 deposited over the micro-mirrors
10.
Once the polishing is complete, a pattern
48 for the spacer
20
is generated, using standard lithography and dry etching techniques (block
208).
The pattern
48, a uniform cross shape, is visible in the top view (
208A)
but not in the side view (
208B) of FIG.
5. Alternatively, the pattern
48 may assume a distinct shape upon the no-pixel region
32. Once
the pattern
48 is etched, lithography and dry etching techniques are again
used to carve or sculpt through the silver nitride
38, using the pattern
48 as a guide, to form the spacer
20 (block
210). Preferably,
care is taken not to damage the patterned metal film
14 as the silver nitride
38 is removed. In the top view (
210A) of FIG. 5, the spacer
20,
made entirely from the silver nitride deposit, is visible on top of the patterned
metal film. Notice that the micro-mirrors
10 are again visible.
Subsequently, a first anti-reflective film or coating
78 is
deposited on the pixel group
56, including both the newly formed spacer
20 and the patterned metal film
14 (block
212). Optionally,
a second anti-reflective film or coating
88 may be deposited on the pixel
group
56 (block
214). In some embodiments, a 75-Angstrom deposit
of silicon dioxide (SiO
2) is used as the first layer of anti-reflective
coating and a 75-Angstrom deposit of silicon nitride (Si
3N
4)
is used as the second layer, for a total anti-reflective coating of 15 microns.
The side view (
214B) of FIG. 5 shows the two layers
78 and
88
of the anti-reflective coating. Recall that, when the original deposit of silicon
nitride
38 was made, the thickness was d+r, where d is the desired thickness
of the spacers
20 and r is the thickness of the anti-reflective material.
In some embodiments, the desired thickness of the anti-reflective material is 15
microns, which is achieved using two materials with a thickness of 75 angstroms each.
This additional material adds to the thickness of the spacer
20 itself.
The actual desired height of the spacer
20 depends, in some part, upon the
properties of the liquid crystal
30 used in the LCOS display
100.
The desired spacer height is usually 1 to 5 microns, with the actual desired height
depending on the properties of the liquid crystal material used in the light modulator.
Once the process described in FIGS. 4 and 5 is complete, the transparent layer
40 (see FIG. 1) is placed upon the spacers
20. The liquid crystal
material
30 is injected into the cell gap, e.g., the space between the transparent
layer
40 and the patterned metal film
14.
Because the spacers
20 are substantially uniform in height, the transparent
plate
40 is substantially parallel with the patterned metal film
14.
Further, the transparent plate
40 is equidistant from the substrate
30
at all points along the LCOS display
100. Advantageously, cell gap spacing
is substantially uniform. The uniformity of cell gap spacing results in a higher
quality LCOS display.
The benefits of the LCOS display
100 are particularly noticeable when
the display is magnified. One application of LCOS displays is the development of
small display footprints (on the order of an inch or less along its diagonal),
which are then magnified substantially twenty-five times magnification is not uncommon).
The resulting display is competitive in size with current CRT and thin film transistor
(TFT) displays.
Due to the small size of the LCOS display, defects in the display may not be
evident. However, once the display is magnified, the defects can be most disruptive.
On the LCOS display
100, visual defects are not evident, either before or
after magnification, because the spacers
20 remain in the no-pixel region
32 of the substrate
30. In some embodiments, this results in a better
quality display.
Further, the LCOS display
100 can be manufactured using known techniques
and materials. The deposition, lithography, and etching techniques employed in
building the spacers
20 are well known in the industry. The materials used
are likewise familiar and widely available. In some embodiments, LCOS displays
manufactured using the methods described in FIGS. 4 and 5 produce a high yield,
relative to the prior art.
While the invention has been described with respect to a limited number of
embodiments, those skilled in the art will appreciate numerous modifications and
variations therefrom. It is intended that the appended claims cover all such modifications
and variations as fall within the true spirit and scope of the invention.
*
