A package structure and method of packaging an interferometric
modulator with an integrated desiccant. An interferometric modulator
is formed on a transparent substrate. A backplane is joined to the
transparent substrate to form a package structure and to encapsulate
the interferometric modulator. A desiccant integrated into the backplane
or the transparent substrate is provided to absorb moisture within
1. A display device, comprising: a transparent substrate; an interferometric
modulator configured to modulate light transmitted through the transparent
substrate; and a backplane cover disposed on the modulator and sealing
the modulator within a package between said transparent substrate
and the backplane cover, wherein the backplane cover has an integrated
desiccant configured to absorb moisture within the package.
2. The display device of claim 1 wherein the backplane cover is
formed of a material comprising a desiccant.
3. The display device of claim 2 wherein the material comprises
a polymer and one of a molecular sieve or silica gel.
4. The display device of claim 2 wherein the material comprises
a desiccant, a channeling agent, and a polymer.
5. The display device of claim 2 wherein the material can be molded
6. The display device of claim 2 wherein the desiccant is applied
to a surface of the backplane cover as a spray coat.
7. The display device of claim 2 wherein the desiccant is applied
to recessed areas formed in a surface of the backplane.
8. The display device of claim 1 wherein the desiccant is a zeolite.
9. The display device of claim 1 wherein the desiccant is calcium
10. The display device of claim 1 wherein the transparent substrate
has an integrated desiccant configured to absorb moisture within
11. A method of manufacturing a display device, comprising: providing
a transparent substrate; forming an interferometric modulator on
the transparent substrate; joining a backplane to the transparent
substrate to form a package, wherein the package encapsulates the
interferometric modulator; and providing within the package a desiccant
contained within a membrane.
12. The method of claim 11 wherein the membrane is adhered to
13. The method of claim 11 wherein the membrane is adhered to
the transparent substrate.
14. The method of claim 11 wherein the membrane is formed of polyethylene.
15. The method of claim 11 wherein the desiccant is selected from
a group consisting of zeolites, calcium sulfate, calcium oxide,
silica gel, molecular sieves, surface adsorbents, bulk adsorbents,
chemical reactants, and indicating silica gel.
16. A display device, comprising: a package comprising a transparent
substrate, a backplane, and a seal applied between the backplane
and the transparent substrate; an electronic display configured
to modulate light transmitted through the transparent substrate,
wherein the electronic display is formed on the transparent substrate
and positioned between the transparent substrate and the backplane;
and a desiccant contained within a pouch adhered inside the package,
wherein the desiccant is configured to absorb moisture within the
17. The display device of claim 16 wherein the desiccant is adhered
to an interior surface of the backplane.
18. The display device of claim 16 wherein the desiccant is adhered
to the transparent substrate.
19. The display device of claim 16 wherein the pouch is formed
20. A display device, comprising: a transmitting means for transmitting
light therethrough; a modulating means configured to modulating
light transmitted through the transmitting means, wherein the modulating
means comprises an interferometric modulator; an encapsulating means
for sealing the modulating means within a package between the transmitting
means and the encapsulating means; and a moisture absorbing means
integrated into either the transmitting means or the encapsulating
means, the moisture absorbing means configured to absorb moisture
within the package.
21. The display device of claim 20 wherein the moisture absorbing
means is selected from a group consisting of zeolites, calcium sulfate,
calcium oxide, silica gel, molecular sieves, surface adsorbents,
bulk adsorbents, chemical reactants, and indicating silica gel.
22. The display device of claim 20 wherein the encapsulating means
is formed of a material comprising the moisture absorbing means.
23. The display device of claim 20 wherein at least a portion
of the encapsulating means is formed of a material comprising the
moisture absorbing means.
24. A display device, comprising: a transparent substrate; an electronic
display configured to modulate light transmitted through the transparent
substrate, wherein the electronic display is formed on the transparent
substrate; a backplane joined to the transparent substrate to form
a package, wherein the electronic display is positioned between
the backplane and the transparent substrate; and a desiccant within
at least one pouch in the package, the desiccant being configured
to absorb moisture within the package.
25. The display device of claim 24 wherein the at least one pouch
is adhered to the backplane.
26. The display device of claim 24 wherein the at least one pouch
is adhered to the transparent substrate.
27. The display device of claim 24 wherein the desiccant is contained
in two pouches.
28. The display device of claim 27 wherein each of the two pouches
contains a different type of desiccant.
29. A method of manufacturing a backplane, comprising: providing
a backplane having a recessed area on a surface, wherein the recessed
area is created by applying a mask to the surface of the backplane
and etching an area of the backplane exposed by the mask; and applying
desiccant to the recessed area on the interior surface while the
mask is on the surface.
30. The method of claim 29 wherein etching is sandblasting.
31. The method of claim 29 wherein etching is wet etching.
32. The method of Claim, 29 further comprising: providing a transparent
substrate having an interferometric modulator formed thereon; and
joining the backplane to the transparent substrate to form a package
after applying the desiccant, wherein the surface is an interior
surface of the backplane.
 This application claims priority to U.S. Provisional Application
No. 60/613300 filed Sep. 27 2004 the contents of which are hereby
incorporated by reference in their entirety.
 1. Field
 The field of the invention relates to microelectromechanical
systems (MEMS) and the packaging of such systems. More specifically,
the field of the invention relates to interferometric modulators
and methods of fabricating such modulators with a desiccant material.
 2. Description of the Related Technology
 Microelectromechanical systems (MEMS) include micro mechanical
elements, actuators, and electronics. Micromechanical elements may
be created using deposition, etching, and or other micromachining
processes that etch away parts of substrates and/or deposited material
layers or that add layers to form electrical and electromechanical
 One type of MEMS device is called an interferometric modulator.
An interferometric modulator may comprise a pair of conductive plates,
one or both of which may be transparent and/or reflective in whole
or part and capable of relative motion upon application of an appropriate
electrical signal. One plate may comprise a stationary layer deposited
on a substrate, the other plate may comprise a metallic membrane
separated from the stationary layer by an air gap. Such devices
have a wide range of applications, and it would be beneficial in
the art to utilize and/or modify the characteristics of these types
of devices so that their features can be exploited in improving
existing products and creating new products that have not yet been
 The system, method, and devices of the invention each have
several aspects, no single one of which is solely responsible for
its desirable attributes. Without limiting the scope of this invention,
its more prominent features will now be discussed briefly. After
considering this discussion, and particularly after reading the
section entitled "Detailed Description of Certain Embodiments"
one will understand how the features of this invention provide advantages
over other display devices.
 An embodiment provides a display device comprising a transparent
substrate, an interferometric modulator configured to modulate light
transmitted through the transparent substrate, and a backplane cover
disposed on the modulator and sealing the modulator within a package
between said transparent substrate and the backplane cover, wherein
the backplane cover has an integrated desiccant configured to absorb
moisture within the package.
 In accordance with another embodiment, a method of manufacturing
a display device is provided. According to this method, a transparent
substrate is provided and an interferometric modulator is formed
on the transparent substrate. A backplane is then joined to the
transparent substrate to form a package to encapsulate the interferometric
modulator. A desiccant integrated within the package is also provided.
 According to another embodiment, a display device is provided,
comprising a package, an electronic display, and a desiccant. The
package comprises a transparent substrate, a backplane, and a seal
applied between the backplane and the transparent substrate. The
electronic display is configured to modulate light transmitted through
the transparent substrate, and is formed on the transparent substrate
and positioned between the transparent substrate and the backplane.
The desiccant integrated into the package, and is configured to
absorb moisture within the package.
 According to yet another embodiment, a display device is
provided. The display device includes a transmitting means for transmitting
light therethrough, a modulating means configured to modulating
light transmitted through the transmitting means, an encapsulating
means for sealing the modulating means within a package between
the transmitting means and the encapsulating means, and a moisture
absorbing means for absorbing moisture within a package. The modulating
means comprises an interferometric modulator, and the moisture absorbing
means is integrated into either the transmitting means or the encapsulating
 According to another embodiment, a method of manufacturing
a display device is provided. A transparent substrate is provided
and an interferometric modulator is formed on the transparent substrate.
A backplane having recessed areas on an interior surface is provided.
Desiccant is applied to the recessed areas on the interior surface,
and the backplane is joined to the transparent substrate to form
a package after applying the desiccant.
BRIEF DESCRIPTION OF THE DRAWINGS
 These and other aspects of the invention will be readily
apparent from the following description and from the appended drawings
(not to scale), which are meant to illustrate and not to limit the
invention, and wherein:
 FIG. 1 is an isometric view depicting a portion of one embodiment
of an interferometric modulator display in which a movable reflective
layer of a first interferometric modulator is in a released position
and a movable reflective layer of a second interferometric modulator
is in an actuated position.
 FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device incorporating a 3.times.3 interferometric
 FIG. 3 is a diagram of movable mirror position versus applied
voltage for one exemplary embodiment of an interferometric modulator
of FIG. 1.
 FIG. 4 is an illustration of a set of row and column voltages
that may be used to drive an interferometric modulator display.
 FIGS. 5A and 5B illustrate one exemplary timing diagram
for row and column signals that may be used to write a frame of
display data to the 3.times.3 interferometric modulator display
of FIG. 2.
 FIG. 6A is a cross section of the device of FIG. 1.
 FIG. 6B is a cross section of an alternative embodiment
of an interferometric modulator.
 FIG. 6C is a cross section of another alternative embodiment
of an interferometric modulator.
 FIG. 7 schematically illustrates a front view of one embodiment
of a wireless telephone handset having an electronic display.
 FIG. 8 schematically illustrates a perspective view of one
embodiment of an electronic display.
 FIG. 9A schematically illustrates a cross-sectional view
of one embodiment of an electronic display taken across the line
10-10 from FIG. 8.
 FIG. 9B schematically illustrates a cross-sectional view
of a first alternate embodiment of an electronic display taken across
the line 10-10 from FIG. 8.
 FIG. 10 schematically illustrates a second alternate embodiment
of an electronic display taken across the line 10-10 from FIG. 8.
 FIG. 11 schematically illustrates a third alternate embodiment
of an electronic display taken across the line 10-10 from FIG. 8.
 FIG. 12 schematically illustrates a fourth alternate embodiment
of an electronic display taken across the line 10-10 from FIG. 8
showing desiccant material integrated into the display backplane.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The following detailed description is directed to certain
specific embodiments of the invention. However, the invention can
be embodied in a multitude of different ways. In this description,
reference is made to the drawings wherein like parts are designated
with like numerals throughout. As will be apparent from the following
description, the invention may be implemented in any device that
is configured to display an image, whether in motion (e.g., video)
or stationary (e.g., still image), and whether textual or pictorial.
More particularly, it is contemplated that the invention may be
implemented in or associated with a variety of electronic devices
such as, but not limited to, mobile telephones, wireless devices,
personal data assistants (PDAs), hand-held or portable computers,
GPS receivers/navigators, cameras, MP3 players, camcorders, game
consoles, wrist watches, clocks, calculators, television monitors,
flat panel displays, computer monitors, auto displays (e.g., odometer
display, etc.), cockpit controls and/or displays, display of camera
views (e.g., display of a rear view camera in a vehicle), electronic
photographs, electronic billboards or signs, projectors, architectural
structures, packaging, and aesthetic structures (e.g., display of
images on a piece of jewelry). MEMS devices of similar structure
to those described herein can also be used in non-display applications
such as in electronic switching devices.
 One interferometric modulator display embodiment comprising
an interferometric MEMS display element is illustrated in FIG. 1.
In these devices, the pixels are in either a bright or dark state.
In the bright ("on" or "open") state, the display
element reflects a large portion of incident visible light to a
user. When in the dark ("off" or "closed") state,
the display element reflects little incident visible light to the
user. Depending on the embodiment, the light reflectance properties
of the "on" and "off" states may be reversed.
MEMS pixels can be configured to reflect predominantly at selected
colors, allowing for a color display in addition to black and white.
 FIG. 1 is an isometric view depicting two adjacent pixels
in a series of pixels of a visual display, wherein each pixel comprises
a MEMS interferometric modulator. In some embodiments, an interferometric
modulator display comprises a row/column array of these interferometric
modulators. Each interferometric modulator includes a pair of reflective
layers positioned at a variable and controllable distance from each
other to form a resonant optical cavity with at least one variable
dimension. In one embodiment, one of the reflective layers may be
moved between two positions. In the first position, referred to
herein as the released state, the movable layer is positioned at
a relatively large distance from a fixed partially reflective layer.
In the second position, the movable layer is positioned more closely
adjacent to the partially reflective layer. Incident light that
reflects from the two layers interferes constructively or destructively
depending on the position of the movable reflective layer, producing
either an overall reflective or non-reflective state for each pixel.
 The depicted portion of the pixel array in FIG. 1 includes
two adjacent interferometric modulators 12a and 12b. In the interferometric
modulator 12a on the left, a movable and highly reflective layer
14a is illustrated in a released position at a predetermined distance
from a fixed partially reflective layer 16a. In the interferometric
modulator 12b on the right, the movable highly reflective layer
14b is illustrated in an actuated position adjacent to the fixed
partially reflective layer 16b.
 The fixed layers 16a, 16b are electrically conductive, partially
transparent and partially reflective, and may be fabricated, for
example, by depositing one or more layers each of chromium and indium-tin-oxide
onto a transparent substrate 20. The layers are patterned into parallel
strips, and may form row electrodes in a display device as described
further below. The movable layers 14a, 14b may be formed as a series
of parallel strips of a deposited metal layer or layers (orthogonal
to the row electrodes 16a, 16b) deposited on top of posts 18 and
an intervening sacrificial material deposited between the posts
18. When the sacrificial material is etched away, the deformable
metal layers are separated from the fixed metal layers by a defined
air gap 19. A highly conductive and reflective material such as
aluminum may be used for the deformable layers, and these strips
may form column electrodes in a display device.
 With no applied voltage, the cavity 19 remains between the
layers 14a, 16a and the deformable layer is in a mechanically relaxed
state as illustrated by the pixel 12a in FIG. 1. However, when a
potential difference is applied to a selected row and column, the
capacitor formed at the intersection of the row and column electrodes
at the corresponding pixel becomes charged, and electrostatic forces
pull the electrodes together. If the voltage is high enough, the
movable layer is deformed and is forced against the fixed layer
(a dielectric material which is not illustrated in this Figure may
be deposited on the fixed layer to prevent shorting and control
the separation distance) as illustrated by the pixel 12b on the
right in FIG. 1. The behavior is the same regardless of the polarity
of the applied potential difference. In this way, row/column actuation
that can control the reflective vs. non-reflective pixel states
is analogous in many ways to that used in conventional LCD and other
 FIGS. 2 through 5 illustrate one exemplary process and system
for using an array of interferometric modulators in a display application.
FIG. 2 is a system block diagram illustrating one embodiment of
an electronic device that may incorporate aspects of the invention.
In the exemplary embodiment, the electronic device includes a processor
21 which may be any general purpose single-or multi-chip microprocessor
such as an ARM, Pentium.RTM., Pentium II.RTM., Pentium III.RTM.,
Pentium IV.RTM., Pentium.RTM. Pro, an 8051 a MIPS.RTM., a Power
PC.RTM., an ALPHA.RTM., or any special purpose microprocessor such
as a digital signal processor, microcontroller, or a programmable
gate array. As is conventional in the art, the processor 21 may
be configured to execute one or more software modules. In addition
to executing an operating system, the processor may be configured
to execute one or more software applications, including a web browser,
a telephone application, an email program, or any other software
 In one embodiment, the processor 21 is also configured to
communicate with an array controller 22. In one embodiment, the
array controller 22 includes a row driver circuit 24 and a column
driver circuit 26 that provide signals to a pixel array 30. The
cross section of the array illustrated in FIG. 1 is shown by the
lines 1-1 in FIG. 2. For MEMS interferometric modulators, the row/column
actuation protocol may take advantage of a hysteresis property of
these devices illustrated in FIG. 3. It may require, for example,
a 10 volt potential difference to cause a movable layer to deform
from the released state to the actuated state. However, when the
voltage is reduced from that value, the movable layer maintains
its state as the voltage drops back below 10 volts. In the exemplary
embodiment of FIG. 3 the movable layer does not release completely
until the voltage drops below 2 volts. There is thus a range of
voltage, about 3 to 7 V in the example illustrated in FIG. 3 where
there exists a window of applied voltage within which the device
is stable in either the released or actuated state. This is referred
to herein as the "hysteresis window" or "stability
window." For a display array having the hysteresis characteristics
of FIG. 3 the row/column actuation protocol can be designed such
that during row strobing, pixels in the strobed row that are to
be actuated are exposed to a voltage difference of about 10 volts,
and pixels that are to be released are exposed to a voltage difference
of close to zero volts. After the strobe, the pixels are exposed
to a steady state voltage difference of about 5 volts such that
they remain in whatever state the row strobe put them in. After
being written, each pixel sees a potential difference within the
"stability window" of 3-7 volts in this example. This
feature makes the pixel design illustrated in FIG. 1 stable under
the same applied voltage conditions in either an actuated or released
pre-existing state. Since each pixel of the interferometric modulator,
whether in the actuated or released state, is essentially a capacitor
formed by the fixed and moving reflective layers, this stable state
can be held at a voltage within the hysteresis window with almost
no power dissipation. Essentially no current flows into the pixel
if the applied potential is fixed.
 In typical applications, a display frame may be created
by asserting the set of column electrodes in accordance with the
desired set of actuated pixels in the first row. A row pulse is
then applied to the row 1 electrode, actuating the pixels corresponding
to the asserted column lines. The asserted set of column electrodes
is then changed to correspond to the desired set of actuated pixels
in the second row. A pulse is then applied to the row 2 electrode,
actuating the appropriate pixels in row 2 in accordance with the
asserted column electrodes. The row 1 pixels are unaffected by the
row 2 pulse, and remain in the state they were set to during the
row 1 pulse. This may be repeated for the entire series of rows
in a sequential fashion to produce the frame. Generally, the frames
are refreshed and/or updated with new display data by continually
repeating this process at some desired number of frames per second.
A wide variety of protocols for driving row and column electrodes
of pixel arrays to produce display frames are also well known and
may be used in conjunction with the present invention.
 FIGS. 4 and 5 illustrate one possible actuation protocol
for creating a display frame on the 3.times.3 array of FIG. 2. FIG.
4 illustrates a possible set of column and row voltage levels that
may be used for pixels exhibiting the hysteresis curves of FIG.
3. In the FIG. 4 embodiment, actuating a pixel involves setting
the appropriate column to -V.sub.bias, and the appropriate row to
+.DELTA.V, which may correspond to -5 volts and +5 volts respectively
Releasing the pixel is accomplished by setting the appropriate column
to +V.sub.bias, and the appropriate row to the same +.DELTA.V, producing
a zero volt potential difference across the pixel. In those rows
where the row voltage is held at zero volts, the pixels are stable
in whatever state they were originally in, regardless of whether
the column is at +V.sub.bias, or -V.sub.bis.
 FIG. 5B is a timing diagram showing a series of row and
column signals applied to the 3.times.3 array of FIG. 2 which will
result in the display arrangement illustrated in FIG. 5A, where
actuated pixels are non-reflective. Prior to writing the frame illustrated
in FIG. 5A, the pixels can be in any state, and in this example,
all the rows are at 0 volts, and all the columns are at +5 volts.
With these applied voltages, all pixels are stable in their existing
actuated or released states.
 In the FIG. 5A frame, pixels (11), (12), (22), (32)
and (33) are actuated. To accomplish this, during a "line
time" for row 1 columns 1 and 2 are set to -5 volts, and column
3 is set to +5 volts. This does not change the state of any pixels,
because all the pixels remain in the 3-7 volt stability window.
Row 1 is then strobed with a pulse that goes from 0 up to 5 volts,
and back to zero. This actuates the (11) and (12) pixels and releases
the (13) pixel. No other pixels in the array are affected. To set
row 2 as desired, column 2 is set to -5 volts, and columns 1 and
3 are set to +5 volts. The same strobe applied to row 2 will then
actuate pixel (22) and release pixels (21) and (23). Again, no
other pixels of the array are affected. Row 3 is similarly set by
setting columns 2 and to -5 volts, and column 1 to +5 volts. The
row 3 strobe sets the row 3 pixels as shown in FIG 5A. After writing
the frame, the row potentials are zero, and the column potentials
can remain at either +5 or -5 volts, and the display is then stable
in the arrangement of FIG 5A. It will be appreciated that the same
procedure can be employed for arrays of dozens or hundreds of rows
and columns. It will also be appreciated that the timing, sequence,
and levels of voltages used to perform row and column actuation
can be varied widely within the general principles outlined above,
and the above example is exemplary only, and any actuation voltage
method can be used with the present invention.
 The details of the structure of interferometric modulators
that operate in accordance with the principles set forth above may
vary widely. For example, FIGS. 6A-6C illustrate three different
embodiments of the moving mirror structure. FIG. 6A is a cross section
of the embodiment of FIG. 1 where a strip of metal material 14
is deposited on orthogonally extending supports 18. In FIG. 6B,
the moveable reflective material 14 is attached to supports at the
corners only, on tethers 32. In FIG. 6C, the moveable reflective
material 14 is suspended from a deformable layer 34. This embodiment
has benefits because the structural design and materials used for
the reflective material 14 can be optimized with respect to the
optical properties, and the structural design and materials used
for the deformable layer 34 can be optimized with respect to desired
mechanical properties. The production of various types of interferometric
devices is described in a variety of published documents, including,
for example, U.S. Published Application 2004/0051929. A wide variety
of well known techniques may be used to produce the above described
structures involving a series of material deposition, patterning,
and etching steps.
 FIG. 7 illustrates a wireless telephone handset 100 having
an electronic display 200. In this illustration, the electronic
display 200 displays the telephone number "555-1212".
It will be understood that the electronic display 200 may display
other information, including, but not limited to, other text and
images, either moving or static.
 The electronic display 200 can be any type of display, including,
but not limited to, light emitting diode (LED), organic light emitting
diode (OLED), or an interferometric modulator (IMOD) direct view
electronic display. Embodiments of the invention relate to the manufacturing
and packaging of these types of electronic displays with a desiccant.
The packages and packaging methods described herein may be used
for packaging a variety of electronic displays, including, but not
limited to, the interferometric modulators described above.
 FIG. 8 shows a perspective view of the display 200 from
FIG. 7. As shown in FIG. 8 the display 200 of this embodiment has
a transparent or semi-transparent front surface 250 a seal 280
and a backplane 300. As will be explained below, within the display
200 and between the transparent front surface 250 and backplane
300 are the electronics for the particular display technology. For
example, within the display may be the electronics for an LED, OLED
or IMOD display device. It should be realized that each of these
display types has a different degree of sensitivity to moisture.
Thus, it is advantageous to provide a means for reducing the amount
of moisture that may come in contact with the display device.
 Packaging techniques for a MEMS device will be described
in more detail below. A schematic of a basic package structure for
a MEMS device, such as an interferometric modulator array, is illustrated
in FIG. 9A. As shown in FIG. 9A, a basic package structure 200 includes
a substrate 250 and a backplane cover or "cap" 300 wherein
an interferometric modulator array 400 is formed on the substrate
250. This backplane or cap 300 may also be referred to as a "backplate."
It will be understood that the terms "display," "package
structure," and "package" may be used interchangeably,
as used herein.
 According to the embodiment shown in FIG. 9A, the substrate
250 and the backplane 300 are joined by a seal 280 to form the package
structure 200 such that the interferometric modulator array 400
is encapsulated by the substrate 250 backplane 300 and the seal
280. The interferometric modulator provides a The substrate provides
a means for transmitting transmitting light therethrough. The backplane
provides an encapsulating means for sealing the interferometric
modulator within a package between the transparent substrate and
 As shown in FIG. 9A, there is a cavity 350 between the backplane
300 and the substrate 250. The moving parts of a MEMS device, such
as the movable mirrors 14a, 14b of an interferometric modulator
array described above, preferably have a protected space in which
to move. As illustrated in FIG. 9A, the cavity 350 can be provided
by the use of a backplane 300 that has a recessed cavity. Using
a recessed cavity 350 allows the seal 280 to be relatively thin,
and thus less subject to transmission of water vapor.
 The seal 280 is provided to join the substrate 250 and the
backplane 300 to form the package structure 200. The seal 280 may
be a non-hermetic seal, such as a conventional epoxy-based adhesive.
In other embodiments, the seal 280 may be a polyisobutylene (sometimes
called butyl rubber, and other times PEB), o-rings, polyurethane,
thin film metal weld, liquid spin-on glass, solder, polymers, or
plastics, among other types of seals that may have a range of permeability
of water vapor of about 0.2-4.7 g mm/m.sup.2kPa day. In still other
embodiments, the seal 280 may be a hermetic seal.
 The substrate 250 may be a semi-transparent or transparent
substance capable of having thin film, MEMS devices built upon it.
Such transparent substances include, but are not limited to, glass,
plastic, and transparent polymers. Images are displayed through
the substrate 250 which serves as an imaging surface. The interferometric
modulator array 400 may comprise membrane modulators or modulators
of the separable type. Examples of such devices are described in
U.S. Pat. No. 5835255 to Miles, which is hereby incorporated by
reference in its entirety. The skilled artisan will appreciate that
the backplane 300 may be formed of any suitable material, such as
glass, metal, foil, polymer, plastic, ceramic, or semiconductor
materials (e.g., silicon).
 Generally, it is desirable to minimize the permeation of
water vapor into the package structure and thus control the environment
inside the package structure 200 and hermetically seal it to ensure
that the environment remains constant. An example of a hermetic
sealing process is disclosed in U.S. Pat. No. 6589625. When the
humidity within the package structure 200 exceeds a level beyond
which surface tension from the moisture becomes higher than the
restoration force of a movable element (e.g., the movable mirrors
14a, 14b described above) in the interferometric modulator 400
the movable element may become permanently stuck to the surface.
 A desiccant may be used to control moisture resident within
the package structure 200. The package structure 200 preferably
includes an integrated desiccant (e.g., desiccant integrated into
the backplane material or transparent substrate material, desiccant
contained within a pouch integrated with the backplane, or desiccant
that is deposited or otherwise incorporated into the backplane during
fabrication of the backplane) configured to reduce moisture within
the cavity 350. In the embodiment shown in FIG. 9A, a desiccant
pouch 480 is positioned between the interferometric modulator array
400 and the backplane 300. The desiccant may also be applied to
or otherwise integrated with the backplane in the recessed area(s)
during fabrication of the backplane, as described in more detail
 Desiccants may be used for packages that have either hermetic
or non-hermetic seals. In packages having a hermetic seal, desiccants
are typically used to control moisture resident within the interior
of the package. In packages having a non-hermetic seal, a desiccant
may be used to control moisture moving into the package from the
environment. The skilled artisan will appreciate that a desiccant
may not be necessary for a hermetically sealed package, but may
be desirable to control moisture resident within the package or
to capture outgassed or residual water from epoxy or other outgassed
materials or materials from surfaces inside the package.
 According to the embodiments described herein, the desiccant
preferably is configured to absorb water molecules that permeate
the display package structure once it has been manufactured as well
as after sealing. As can be appreciated, the desiccant maintains
a low humidity environment within the package structure and prevents
water vapor from adversely affecting the operation of the display
electronics (e.g., interferometric modulator). This maintenance
of a low humidity environment will be explained more completely
with reference to FIGS. 9-12 below.
 As illustrated in FIG. 9A, also sealed within the display
200 is a desiccant pouch 480. In these embodiments, the desiccant
pouch 480 is formed within the cavity 350 and attached to the backplane
300. The desiccant pouch 480 includes a desiccant material 500
and a membrane cover 550. The desiccant pouch 480 may be used within
displays that have either hermetic or non-hermetic sealants. In
displays having a hermetic seal, the desiccant pouch 480 can be
used to control moisture resident within the interior of the package.
In displays having a non-hermetic seal, the desiccant pouch 480
may be used to control moisture moving into the package from the
 The membrane 550 of the pouch 480 preferably is made from
a compound that is strong enough to contain the desiccant material
500 but also allows water vapor to pass through the membrane 550
and contact the desiccant material 500. An example of such a material
is Tyvek.RTM. (DuPont Corporation) or polyethylene, preferably with
a low moisture vapor transmission rate (MVTR). The MVTR of the membrane
550 depends upon the type and thickness of the materials used and
the external environmental conditions. It should be realized that,
in some embodiments, the membrane 550 can adhere directly to the
backplane 300 310 or be sealed to the backplane 300 310 with
an adhesive. Suitable adhesives include, but are not limited to,
adhesives in a PSA (pressure sensitive adhesive) thin-film patch
and dispensed adhesives, preferably epoxies, thermal or UV, with
low outgassing specifications, such as those compliant with NASA
specifications. Table 1 below provides the MVTR for a number of
membrane materials suitable for the membrane 550. By knowing the
MVTR (in grams of water per square foot per day), the total surface
area of the membrane 550 (membrane surface area) and the rate of
water permeation into the package 200 210 through the perimeter
seal, the required MVTR of the membrane 550 can be calculated to
ensure that the desiccant can absorb at a sufficient rate to keep
the interior of the package 200 210 dry enough for proper operation.
TABLE-US-00001 TABLE 1 MVTR* Material gm/m.sup.2-day gm/ft.sup.2-day
Aluminum Foil Wrapping 0.025 mm 0.5 0.05 Aluminum Foil Wrapping
0.009 mm 1.0 0.09 Cellulose Films (`Cellophane`) 1.5 0.14 400's
MXXT Grade (Polyvinylidene Chloride Coated) Polyvinylidene/Polyvinyl
Chloride Films (`Saran`) 0.005 cm (0.002 in) Polyvinylidene/Polyvinyl
Chloride Films 3.0 0.28 (`Saran`) 0.0013 cm (0.0005 in) Polyethylene
Films (`Polythene`) 4.0 0.37 0.0125 cm (0.005 in) Waxed Paper (45.5
kg (100 lb) per DC Ream) Cellulose Films (`Cellophane`) 7.5 0.70
300's MSAT Grade (Cellulose Nitrate Coated) Glassine Lacquered 9.0
0.84 (16 kg (35 lb) per DC Ream) Polyethylene Film (`Polythene`)
10.0 0.93 0.005 cm (0.002) in) Polyethylene Film (`Polythene`) 20.0
1.86 0.0025 cm (0.001 in) Polyethylene Coated Kraft 30.0 2.79 (9
kg (20 lb) per DC Ream) *Determined at 100.degree. F. and 90% relative
 Generally, any substance that can trap moisture while not
interfering with the optical properties of the interferometric modulator
array may be used as the desiccant material 500. Preferably, the
desiccant does not interfere with the optical properties of the
interferometric modulators 400. Suitable desiccant materials 500
include, but are not limited to, zeolites, calcium sulfate, calcium
oxide, silica gel, molecular sieves, surface adsorbents, bulk adsorbents,
and chemical reactants. Other desiccant materials include indicating
silica gel, which is silica gel with some of its granules coated
with cobalt chloride. The silica changes color as it becomes saturated
with water. Calcium oxide is a material that relatively slowly absorbs
 It will be understood that, in certain embodiments, the
desiccant material 500 may be inserted into the cavity 350 of a
package structure 210 without a pouch 480 or membrane cover 550
as shown in FIG. 9B. The backplane 310 in the embodiment shown in
FIG. 9B does not have as deep a recessed cavity as the backplane
300 of the embodiment shown in FIG. 9A. The skilled artisan will
appreciate that the desiccant material 500 may be inserted into
the cavity 350 without a pouch 480 or membrane cover 550 in a
package having a backplane with a recessed cavity, such as the one
shown in FIG. 9A.
 The desiccant may be in different forms, shapes, and sizes.
In addition to being in solid or gel form, the desiccant material
500 may alternatively be in powder form. These powders may be inserted
directly into the pouch 480 or directly into the package without
a pouch 480 or they may be mixed with an adhesive for application.
In an alternative embodiment, the desiccant may be formed into different
shapes, such as cylinders or sheets, before being applied inside
the package. It should be realized that the desiccant pouch 480
may take any form, and can be of any thickness that provides the
proper desiccating function for the display 200 210.
 The skilled artisan will understand that the desiccant material
500 can be applied and integrated with the package in different
ways. In one embodiment, the desiccant material 500 is deposited
as part of the interferometric modulator array 400. In another embodiment,
the desiccant material 500 is applied inside the package as a spray
or a dip coat.
 In another embodiment, the desiccant material 500 may be
printed or sprayed onto a surface of the interior of the package,
such as the backplane after it has been sandblasted or etched using
standard photolithographic techniques. A mask is preferably first
applied to the backplane prior to etching, preferably using standard
photolithographic techniques, in order to form recessed pockets
or windows in the backplane, allowing the package to be thinner
with a thinner perimeter seal, preferably having a thickness of
about 15 microns. The skilled artisan will also appreciate that
a thinned perimeter seal allows lower water vapor flux into the
package and the package/device would therefore have a longer lifetime.
It will be understood that etching techniques, such as sandblasting
and wet etching, are preferred. The skilled artisan will understand
that, alternatively, a stencil may be used instead of a photolithographic
mask. After the pockets or windows have been created, the desiccant
material 500 is applied (e.g., sprayed or brushed on) in the recessed
pockets or windows. It will be understood that the mask is preferably
not removed until the desiccant material 500 has been applied to
the recessed pockets or windows so that there is little danger of
applying the desiccant material 500 to the non-recessed areas of
the backplane. A thin foil may be applied over the desiccant material
to protect the desiccant material 500 if the backplane is manufactured
and transported prior to assembly with other parts of the package.
The desiccant material 500 may be activated after the package is
 Typically, in packages containing desiccants, the lifetime
expectation of the device may depend on the lifetime of the desiccant.
When the desiccant is fully consumed, the interferometric modulator
400 may fail to operate as sufficient moisture enters the cavity
350 and causes damage to the interferometric modulator 400. The
theoretical maximum lifetime of the display device is determined
by the water vapor flux into the cavity 350 as well as the amount
and type of desiccant material.
 The theoretical lifetime of the device may be calculated
with the following equations: lifetime = desiccant_capacity .times.
( g ) water_vapor .times. _flux .times. ( g .times. / .times. area
.times. / .times. day ) * perimeter_seal .times. _area water .times.
.times. vapor .times. .times. flux = - P .times. d p d t
 where P is the water vapor permeation coefficient for the
perimeter seal 280 and dp/dt is the water vapor pressure gradient
across the width of the seal 280.
 In the embodiment of a display having a hermetic seal, the
lifetime of the device is not as dependent on the desiccant capacity,
or the geometry of the seal. In display devices wherein the seal
280 is not hermetic, the lifetime of the device is more dependent
on the capacity of the desiccant to absorb and retain moisture.
 Another embodiment of a display 580 is illustrated in FIG.
10. As shown, two desiccant pouches 650 700 are formed within the
interior cavity 350. The two desiccant pouches 650 700 function
to remove moisture from within the cavity 350. It should be realized
that, in this embodiment, the desiccant material 500 used to fill
the pouches 650 700 can be the same or different in the two pouches
650 700. For example, one pouch may be filled with a desiccant
that binds water molecules very quickly, but wears out in a relatively
short period of time. An example of such a desiccant is zeolite.
The other pouch may be filled with a desiccant that absorbs water
molecules more slowly, but lasts longer. One example of such a compound
is calcium oxide. Of course, embodiments of the invention are not
limited to a particular number of integrated desiccant pouches,
or a particular desiccant used within each pouch. The display device
may have 1 2 3 4 5 6 or more desiccant pouches inside without
departing from the spirit of the invention.
 Yet another embodiment of a display 780 is illustrated in
FIG. 1. In this embodiment, the desiccant pouches 850 950 are integrated
with and adhered to the transparent or semi-transparent substrate
250 instead of to the backplane 300. Preferably, the pouches 850
950 do not contact or interfere with the display electronics. It
should be realized that this embodiment is not limited to having
desiccant pouches adhered only to the substrate 250. In other embodiments,
both the substrate 250 and the backplane 300 have integrated desiccant
 FIG. 12 shows yet another embodiment of a display 1000 wherein
the desiccant material 500 is integrated into the material that
forms the backplane 1050. Such material can be made by incorporating
the desiccant 500 into the plastic that forms the backplane 1050.
Preferably, the desiccant is incorporated into the backplane 1050
on the internal side of the backplane 1050 as shown in FIG. 12.
Examples of such material include 2AP (Sud-Chemie), which combines
precise amounts of a desiccant, such as molecular sieve or silica
gel, with a polymer. Because the desiccant material 500 is incorporated
into the backplane 1050 itself, there is no need to add desiccant
material 500 in a separate step during the packaging process. In
addition, 2AP can be customized to control the moisture adsorption
 Another material suitable for a backplane 1050 is made by
Capitol Specialty Plastics Inc. (Auburn, Ala.). This material combines
a desiccant 500 with a channeling agent into a polymer that can
be molded or extruded into many shapes. Almost any type of polymer
can be used with the desiccant 500. This type of desiccant plastic
allows the entire backplane 1050 to act as a moisture absorber.
Other materials suitable for such a backplane 1050 include, but
are not limited to, material delivered with foil protection, which
can be chemically or plasma etched off, such as amorphous silicon,
chrome, and similar materials.
 Generally, the packaging process to produce the display
may be accomplished in a vacuum, pressure between a vacuum up to
and including ambient pressure, or pressure higher than ambient
pressure. The packaging process may also be accomplished in an environment
of varied and controlled high or low pressure during the sealing
process. There may be advantages to packaging the display in a completely
dry environment, but it is not necessary. Similarly, the packaging
environment may be of an inert gas at ambient conditions, or the
cavity 350 may be created to contain an inert gas, such as nitrogen,
at ambient conditions. Packaging at ambient conditions allows for
a lower cost process and more potential for versatility in equipment
choice because the device may be transported through ambient conditions
without affecting the operation of the device.
 While the above detailed description has shown, described,
and pointed out novel features of the invention as applied to various
embodiments, it will be understood that various omissions, substitutions,
and changes in the form and details of the device or process illustrated
may be made by those skilled in the art without departing from the
spirit of the invention. As will be recognized, the present invention
may be embodied within a form that does not provide all of the features
and benefits set forth herein, as some features may be used or practiced
separately from others.