U.S. patent application number 10/904766 was filed with the patent office on 2006-06-01 for differential interferometric light modulator and image display device.
Invention is credited to David M. Bloom.
Application Number | 20060114542 10/904766 |
Document ID | / |
Family ID | 36462676 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060114542 |
Kind Code |
A1 |
Bloom; David M. |
June 1, 2006 |
DIFFERENTIAL INTERFEROMETRIC LIGHT MODULATOR AND IMAGE DISPLAY
DEVICE
Abstract
A light modulator incorporates a polarization sensitive prism
and a novel MEMS ribbon device to impart a relative phase shift to
polarization components of an incident light beam. A linear array
of phase shifting elements in the MEMS device creates a linear
image which is scanned to form a two dimensional scene.
Alternatively the deflection of each cantilever in a linear array
of atomic force microscope cantilevers may be measured
simultaneously.
Inventors: |
Bloom; David M.; (Jackson,
WY) |
Correspondence
Address: |
MORRISON ULMAN
WOODSIDE IP GROUP 1900 EMBARCADERO ROAD SUITE 209
PALO ALTO
CA
94303-3327
US
|
Family ID: |
36462676 |
Appl. No.: |
10/904766 |
Filed: |
November 26, 2004 |
Current U.S.
Class: |
359/276 |
Current CPC
Class: |
G02B 26/06 20130101;
G02B 26/0841 20130101 |
Class at
Publication: |
359/276 |
International
Class: |
G02F 1/01 20060101
G02F001/01 |
Claims
1. An image display device comprising: a polarizing prism that
splits a light beam into different paths corresponding to
polarization components of the light; a microelectromechanical
device that imparts a relative phase shift to the light in the
paths prior to recombination of the light in the polarizing prism;
a scanning mirror for scanning the recombined beam; and, a
projection lens that magnifies the recombined beam.
2. A device as in claim 1 wherein the polarizing prism is a
Wollaston prism.
3. A device as in claim 1 wherein the polarizing prism is a Rochon
or Senarmont prism.
4. A device as in claim 1 wherein the microelectromechanical device
comprises: a thin reflective ribbon; end supports at each end of
the ribbon; an intermediate support located between the end
supports; and, a base.
5. A device as in claim 1 wherein the microelectromechanical device
comprises: a microcantilever such as an atomic force microscope
cantilever.
6. An atomic force microscope comprising: a polarizing prism that
splits a light beam into different paths corresponding to
polarization components of the light; and, an atomic force
microscope cantilever that imparts a relative phase shift to light
in the paths.
7. An image display device comprising: an interferometer that
splits a light beam into polarization components each of which
travels a separate path; and, a microelectromechanical phase
shifting surface device that imparts a relative phase shift to the
polarization component in each path.
8. A device as in claim 7 wherein the phase shifting surface device
is electrically controlled.
9. A device as in claim 7 wherein the phase shifting surface device
comprises a linear array of phase shifting elements.
10. A device as in claim 9 further comprising a scanning mirror and
a projection lens.
Description
TECHNICAL FIELD
[0001] The invention relates generally to visual display devices
and light modulators. In particular it relates to differential
interferometric light modulators containing polarizing prisms.
BACKGROUND
[0002] Display devices such as television sets and movie projectors
often incorporate a modulator for the purpose of distributing light
into a two-dimensional pattern or image. For example, the frames of
a movie reel modulate white light from a projector lamp into shapes
and colors that form an image on a movie screen. In modern displays
light modulators are used to turn on and off individual pixels in
an image in response to electronic signals that control the
modulator.
[0003] Texas Instruments introduced a microelectromechanical,
integrated circuit chip, light modulator called a digital mirror
device which includes millions of tiny mirrors on its surface. Each
mirror corresponds to a pixel in an image and electronic signals in
the chip cause the mirrors to move and reflect light in different
directions to form bright or dark pixels. See, for example, U.S.
Pat. No. 4,710,732 incorporated herein by reference. Stanford
University and Silicon Light Machines developed a
microelectromechanical chip called a grating light modulator in
which diffraction gratings can be turned on and off to diffract
light into bright or dark pixels. See, for example, U.S. Pat. No.
5,311,360 incorporated herein by reference.
[0004] Both of these reflective and diffractive light modulation
schemes for displays involve two-dimensional arrays of light
modulator elements. However, it is also possible to make a display
in which light is incident on a linear array of light emitters or
high speed light modulators. With appropriate magnifying optics and
scanning mirrors, a linear array can be made to appear
two-dimensional to an observer. Through the scanning action of a
vibrating mirror a single row of light modulators can be made to do
the work of as many rows of modulators as would be necessary to
provide a real two-dimensional display of the same resolution. See,
for example, U.S. Pat. No. 5,982,553 incorporated herein by
reference.
[0005] Many microelectromechanical light modulators are compatible
with digital imaging techniques. Digital information may be sent
electronically to the modulator. For example, gray scale images may
be achieved by turning pixels on only part time. A pixel that is
switched from bright to dark with a 50% duty cycle will appear to
an observer to have a constant intensity half way between bright
and dark. However, the pixel must be switched between bright or
dark states faster than the human eye's critical flicker frequency
of roughly 30 Hz or else it will appear to flicker. Therefore
two-dimensional digital light modulators for displays must switch
between states quickly to display a range of light levels between
bright and dark.
[0006] A one-dimensional digital light modulator array, scanned by
a vibrating mirror to make it appear two-dimensional, must
incorporate modulators with fast switching speeds. Each modulator
element must switch on and off quickly to provide the impression of
gray scale and this action must be repeated for each pixel in a
line within the scanning period of the mirror. Grating light
modulator devices in particular exhibit high switching speeds
because their mechanical elements move only very short distances.
The grating light modulator incorporates parallel ribbon structures
in which alternating ribbons are deflected electrostatically to
form diffraction gratings. The ribbons need only move a distance of
one quarter wavelength of light to switch a grating on or off. It
is also possible (and desirable in many instances) to operate one-
or two-dimensional light modulators in analog, rather than digital,
modes.
[0007] One limitation of the grating light modulator is that at
least two ribbons are required in order to form a diffractive
modulator element. Therefore each pixel requires at least two
ribbons each of which uses up valuable space on a chip. Another
limitation of grating light modulators is that they require
collimated light sources. Gudeman proposed an interferometric light
modulator based on a mechanical structure very similar to the
grating light modulator; see U.S. Pat. No. 6,466,354 incorporated
herein by reference. Gudeman's light modulator is a form of
Fabry-Perot interferometer based on a ribbon structure.
[0008] Microelectromechanical light modulators typified by the
Texas Instruments' digital mirror device and Stanford/Silicon Light
Machines grating light modulator devices mentioned above have
already enjoyed wide commercial success and have spawned other
related designs. See, for example, U.S. Pat. No. 6,724,515
incorporated herein by reference. However, they are not without
limitations and there is room for improvement.
[0009] The digital mirror device is comparatively slow and
therefore is usually supplied as a two-dimensional mirror array.
Usually two dimensional modulator arrays are more expensive to make
than one-dimensional arrays and require a sophisticated addressing
scheme for the mirrors. A two-dimensional array requires
defect-free manufacturing of N.times.N pixels over a large chip
area while a one-dimensional array with the same image resolution
requires only N working pixels on a chip in a single line.
[0010] Grating light modulator devices, while very fast, require
more than one ribbon structure per pixel as noted above. They are
also affected by limitations due to diffraction. A grating light
modulator has a reflective state or configuration and a diffractive
state. In the diffractive state incoming light is diffracted into
the +1 and -1 diffraction orders of an optical grating. However,
only about 80% of the light is collected in these two orders. Light
diffracted into higher orders is lost and overall light efficiency
suffers.
[0011] Grating-based devices use high numerical aperture optical
elements to collect diffracted light. It would be desirable for a
modulator to be able to use simpler, low numerical aperture optics.
Grating-based devices also have some difficulty achieving high
contrast in the dark state; i.e. displaying black areas in an
image. A light modulator that escaped as many of the limitations of
existing modulator designs as possible would be highly
desirable.
SUMMARY
[0012] An aspect of the invention provides a novel interferometric
light modulator combined with a scanner and projection optics to
form a visual display system. The light modulator incorporates a
polarization sensitive prism and a novel MEMS device to impart a
relative phase shift to polarization components of an incident
light beam. A linear array of phase shifting elements in the MEMS
device creates a linear image which is scanned to form a two
dimensional scene.
[0013] A further aspect of the invention provides a linear array of
atomic force microscope cantilevers in place of the MEMS device. In
this application, an aspect of the invention provides a device for
simultaneously measuring the deflection of each cantilever in a
linear array.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The drawings are heuristic for clarity. The foregoing and
other features, aspects and advantages of the invention will become
better understood with regard to the following descriptions,
appended claims and accompanying drawings in which:
[0015] FIGS. 1A and 1B show schematically the propagation of light
through Wollaston or Rochon prisms respectively.
[0016] FIG. 2A shows an interferometric light modulator and various
phase shifting surfaces.
[0017] FIG. 2B shows an image display system including the
modulator of FIG. 2A, a scanning mirror, projection lens, and
viewing screen.
[0018] FIG. 3 shows schematically a MEMS ribbon device.
[0019] FIG. 4 shows steps in a microfabrication process for a MEMS
ribbon device.
DETAILED DESCRIPTION
[0020] An aspect of the invention provides a novel light modulator,
a scanner and projection optics as a system for displaying images
to an observer. The system has several desirable features including
high contrast and speed.
[0021] According to an aspect of the invention a novel light
modulator incorporates a polarizing prism to split light beams into
components of orthogonal polarization. These polarization
components are made to travel unequal distances in the modulator
and are then recombined in the prism. When one polarization
component is phase shifted with respect to the other, the overall
polarization of the recombined beam is transformed. The
polarization of the recombined beam is then analyzed by a
polarizing beam splitter. Light intensity output from the
polarizing beam splitter depends on the polarization state of the
incident light beam which in turn depends on the relative phase
shift of the polarization components.
[0022] A phase shift is imparted to the orthogonal polarization
components in the modulator by focusing them on, and causing them
to reflect from, an engineered, uneven surface. This phase shift
surface has regions of slightly different displacement which cause
the light beams to travel slightly different distances upon
reflection. More specifically a novel microelectromechanical system
(MEMS) ribbon array device is provided that is used to modulate the
phase shift of light beams reflected from the surface of its
ribbons.
[0023] The MEMS ribbon array device has several advantages over
conventional devices including the property that a single ribbon in
the device represents a single pixel in a line image. Further,
non-active areas of the device do not cause artifacts in a
displayed image because they lie at the same surface height.
Features in the device at the same surface height do not cause
relative phase shifts in light that reflects from the device and
therefore do not appear at the output of the interferometer.
[0024] Furthermore, although aspects of the invention are described
below primarily in terms of their applicability to image display,
they may also be used profitably to perform other functions such as
parallel readout of cantilever position in a linear array of atomic
force microscope (AFM) cantilevers. A simple readout for an array
AFM would be of great benefit in advanced semiconductor
manufacturing since the AFM is useful for both surface
characterization and lithography. Alternatively, an array of
movable cantilevers could be used in place of the MEMS ribbon
device as phase shifting elements for a display.
[0025] For purposes of gaining a clearer understanding of certain
aspects of the invention it is useful to briefly review the
properties of birefringent optical prisms such as the Wollaston 100
and Rochon 150 prisms shown in FIG. 1A and FIG. 1B respectively. An
important characteristic of each of these polarizing prisms is that
they split an incoming light beam into orthogonally polarized
components that leave the prism in different directions.
[0026] In each of FIGS. 1A and 1B a linearly polarized light beam
102 or 152 enters a prism normal to one of the prism faces. The
components of the polarization in the plane of the figure and
perpendicular to the plane of the figure are shown by double-headed
arrows and bull's-eye symbols respectively.
[0027] In the Wollaston prism illustrated in FIG. 1A one of the
components of polarization of the light is oriented perpendicular
to the optic axis of the prism and is designated the ordinary or
"o" wave. The other component of polarization is oriented parallel
to the optic axis of the prism and is designated the extraordinary
or "e" wave. The Wollaston prism is comprised of two wedges 104 and
106 of a birefringent material such as quartz or calcite. The
wedges are the same size and shape but are cut so that their optic
axes are oriented perpendicular to one another when they are
assembled to form the prism.
[0028] In wedge 104 the optic axis is oriented 108 normal to the
plane of the figure while in wedge 106 the optic axis is oriented
110 in the plane of the figure and perpendicular to the direction
of propagation of light beam 102.
[0029] Inside the Wollaston prism o and e waves experience slightly
different refractive indices. In a quartz prism the difference in
refractive indices is about 0.6%. In a positive uniaxial crystal
such as quartz the o wave travels faster than the e wave because of
its slightly lower refractive index. In a negative uniaxial crystal
such as calcite the e wave experiences a lower refractive index and
travels faster than the o wave.
[0030] When the e and o waves reach the boundary between the two
halves of the Wollaston prism they are refracted. In addition the
orientation of the optic axis of the crystal is reversed so that
what was the o wave in the first half of the prism becomes the e
wave in the second half and vice versa. The net effect is that an
incoming wave is split into orthogonally polarized components 112
and 114 which exit the prism in different directions. The angle
between the two waves is a function of the wedge angle of the two
halves of the prism.
[0031] The Rochon prism, shown in FIG. 1B, is similar to a
Wollaston prism except that the incident light beam 152 enters the
first wedge 154 of the prism parallel to the optic axis 158 of the
crystal. When the beam passes the boundary between the halves of
the Rochon prism it enters second wedge 156 where the optic axis
160 is perpendicular to the direction of propagation and the light
is split into o and e components. The o wave 162 passes through the
entire prism with no angular deviation while the e wave 164 is
deflected away from the original direction of propagation. A
Senarmont prism (not shown) is very similar to a Rochon prism
except that the orientation of the optic axis in the second half of
the prism is such that the e wave passes straight through while the
o wave is deflected.
[0032] Wollaston, Rochon, Senarmont and other birefringent prisms
may be made from wedges of uniaxial birefringent crystals such as
quartz, calcite, tourmaline, sodium nitrate, or rutile (TiO.sub.2).
The wedges are sometimes cemented together with glycerine or castor
oil and sometimes not cemented at all if optical power handling
requirements are high.
[0033] FIG. 2A shows an interferometric light modulator according
to an aspect of the invention. The light modulator comprises a
light source, several lenses, polarization dependent optics, and a
reflective phase shifting surface. The polarization dependent
optics include a polarizing beam splitter, an optical 1/2-wave
plate, and a birefringent prism such as a Wollaston prism or a
Rochon prism. The phase shifting surface may take any one of
several forms such as a 1/4-wave step surface, a MEMS ribbon
device, or an AFM cantilever array.
[0034] The viewpoint of FIG. 2 is such that operation of the
modulator with only a single phase shifting surface is apparent.
However, as described below, an interferometric light modulator
according to an aspect of the invention may be operated with a
linear array of phase shifting surfaces simultaneously. For
example, the side view of MEMS ribbon device 224 corresponds to
side view 302 in FIG. 3 of linear array MEMS ribbon device 300.
Similarly, the side view of AFM cantilever 226 may represent a side
view of the first cantilever in a linear array of AFM
cantilevers.
[0035] Interferometric light modulator 200 illustrated
schematically in FIG. 2A is an important part of a display system
according to an aspect of the invention. Other parts of the display
system include scanning devices and projection optics.
[0036] In FIG. 2A, light is generated by light source 202 and
radiates through aperture 203. Light source 202 may be a lamp,
laser, or other light source. Aperture 203 may be a small hole or a
slit. Alternatively, aperture 203 may be inherent in the shape of
the light source itself. For example a laser beam may be focused to
a small spot or a lamp may have an elongated filament. Further, the
light source may incorporate beam shaping optics (not shown) for
the purpose of providing a circular, elongated slit-like, or other
transverse shape to the radiated light. Slits, line sources,
cylinder lenses, Powell lenses, and other condenser optics may all
be used for shaping the light source.
[0037] Light from light source 202 is incident on lens 204 which is
placed approximately one focal length away thereby collimating the
light. After passing through lens 204 light traverses several
optical elements before being focused by lens 212 onto a phase
shifting surface such as 1/4-wave step surface 222, MEMS ribbon
device 224, or AFM cantilever array 226.
[0038] The optical elements between lens 204 and lens 212 are a
polarizing beam splitter 206, an optical 1/2-wave plate 208, and a
birefringent prism such as Wollaston prism 210. Although these
elements are illustrated as separate components in the figure, they
could also be combined into a single optical element.
[0039] Optical 1/2-wave plate 208 is optional. It is included
because it is convenient to include it during optical prototyping
of the modulator. If it were not included, the Wollaston prism and
the phase shifting surface would have to be rotated 45 degrees
around the axis of lens 212. Using optical 1/2-wave plate 208 makes
it easier to align optical components and also easier to draw them
in schematic diagrams such as FIG. 2A.
[0040] It is also possible to build a modulator that operates in
exactly the same way as the one described in detail here by
replacing polarizing beam splitter 206 and birefringent Wollaston
prism 210 with a single Rochon or Senarmont prism. Such a modulator
would have its image plane (analogous to image plane 216) located
near, but off axis from light source 202. Lens 204 would
additionally serve the function of lens 214.
[0041] After light is reflected from the phase shifting surface it
passes back through lens 212, prism 210 and optical 1/2-wave plate
208. Modulated light is then directed by polarizing beam splitter
206 toward lens 214 and finally focused at image plane 216.
[0042] The optical elements of an interferometric light modulator
just described function to split an incoming light beam into two
polarization components that follow spatially distinct paths and
then recombine the beam after light in one of the paths has
undergone a phase shift with respect to light in the other path. To
understand how this happens, consider light from light source 202
that has been collimated by lens 204. Polarizing beam splitter 206
passes a linearly polarized component of light incident on it. The
linearly polarized light then passes through optical 1/2-wave plate
208 which is oriented so as to rotate the polarization by 45
degrees. In other words, the light now has in-phase polarization
components in the plane of the figure and perpendicular to it. This
light is split into two different propagation directions by
Wollaston prism 210. One polarization 218 is deflected upward in
the figure while the other 220 is deflected down. These two beams
218 and 220 are focused on a phase shifting surface such as
1/4-wave step surface 222.
[0043] The Wollaston prism is placed such that the point where the
incoming light beam is split into two is located half way between
and one focal length away from lenses 204 and 212. Because lens 212
performs a spatial Fourier transform, angular deviations between
light beams created in Wollaston prism 210 at one focal plane of
the lens lead to transverse displacements in focal spots at the
other focal plane. Therefore light beams 218 and 220 both arrive at
1/4-wave step surface 222 normal to the surface but displaced
laterally from the axis of lens 212.
[0044] Light beams 218 and 220 are reflected back on themselves at
1/4-wave step surface 222 and travel back through lens 212 before
being recombined in Wollaston prism 210. The combined beam passes
through optical 1/2-wave plate 208 for the second time and its
polarization is rotated back 45 degrees. Depending on the
polarization state at this point the beam then either passes
through polarizing beam splitter 206 back toward lens 204 and light
source 202 or is reflected within the polarizing beam splitter
toward lens 214 and image plane 216.
[0045] Whether or not the light is reflected toward lens 214 or
passed straight through toward lens 204 depends on its
polarization. The polarization of the light in turn is determined
by the phase relationship between its polarization components in
the plane of the figure and perpendicular to it. These are the
polarization components 218 and 220 into which the Wollaston prism
split the original incoming light beam. The phase relationship
between the polarization components depends on the path length each
component travels starting when the light beam is split in the
Wollaston prism until the components are recombined on their return
trip through the prism. Finally the path length difference depends
on the topography of the phase shifting surface on which light
beams 218 and 220 are focused by lens 212.
[0046] If the phase shifting surface is 1/4-wave step surface 222,
the path length difference for light beams 218 and 220 is 1/2
optical wavelength. The reason for this is that the step height on
1/4-wave step surface 222 is 1/4 optical wavelength. Therefore
light beam 218 travels 2.times.(1/4 optical wavelength) or 1/2
optical wavelength farther than light beam 220 upon reflection from
the phase shifting surface. When light beams 218 and 220 are
recombined at Wollaston prism 210 the polarization of the
recombined light beam is changed by 90 degrees because of the phase
delay between its polarization components.
[0047] The 90 degree change in the polarization of the light causes
the light to be reflected by polarizing beam splitter toward lens
214 rather than transmitted toward lens 204. Finally the light is
focused at image plane 216. Therefore a 1/4 optical wavelength
height difference in the surfaces that reflect orthogonally
polarized light beams 218 and 220 leads to maximum light output at
image plane 216. It may be readily verified that when a MEMS ribbon
device 224 (see also, FIG. 3) is used in place of 1/4-wave step
surface 222 the intensity of the light at image plane 216 is
proportional to sin.sup.2(kx) where x is the height difference of
the surfaces that reflect orthogonally polarized light beams 218
and 220, and k is two times pi divided by the wavelength of the
light. For example when voltage 320 is applied to a MEMS ribbon
306, 307 (as illustrated in FIG. 3) x=D.sub.1-D.sub.2.
[0048] The operating point of the interferometer may be adjusted by
translating Wollaston prism 210 perpendicular to the axis defined
by lenses 204 and 212 in such a way that light passes through
different amounts of prism material in the two wedges that form the
prism. As an example, this adjustment can be used to set the
interferometer to have minimum output when voltage source 320 in
FIG. 3 is set to zero.
[0049] In the interferometer, light beams 218 and 220 are
orthogonally polarized. However, it is not necessary, and it is
sometimes not desirable, for the two beams to be linearly
polarized. Linear, perpendicular polarizations would be undesirable
if, for example, a reflective phase shifting surface (e.g. 222,
224, 226) imparted a polarization dependent loss upon reflecting
incident light. Orthogonal, but not linear, polarizations could be
achieved by placing an optical 1/4-wave plate between Wollaston
prism 210 and lens 212, for example. Light beams 218 and 220 would
then be orthogonally, circularly polarized and would suffer the
same loss when incident on a reflective surface that imparts a
polarization dependent loss.
[0050] The linear array output that appears at image plane 216 in
the interferometer as shown in FIG. 2A is scanned and projected as
illustrated in FIG. 2B which shows a scanning mirror, projection
optics and a viewing screen. The scanning and projection system
permits the line of pixels output by the interferometer to appear
to an observer as a two dimensional image.
[0051] In FIG. 2B, the optical system enclosed within dotted box
250 is a perspective view of the optical system illustrated in FIG.
2A. In the perspective view 250 it is apparent that when a line
source 252 is used as the light source 202 and aperture 203 of FIG.
2A phase shifting device 272 may include a linear array of phase
shifting surfaces such as arrays of surfaces 222, 224, or 226.
[0052] In FIG. 2B lenses 254, 262 and 264 correspond to lenses 204,
212 and 214 respectively in FIG. 2A. Similarly polarizing beam
splitter 256, optical 1/2-wave plate 258, and Wollaston prism 260
correspond to polarizing beam splitter 206, optical 1/2-wave plate
208, and Wollaston prism 210. Finally image plane 266 corresponds
to image plane 216.
[0053] In FIG. 2B projection lens 280 magnifies the line image
formed at image plane 266. The magnified image 290 appears on
viewing screen 295. Scanning mirror 285 scans or sweeps line image
290 back and forth across viewing screen 295 quickly enough that an
observer sees a two dimension image on the screen. Reciprocating
rotation or vibration of mirror 285 leads to the side-to-side
movement of line image 290.
[0054] FIG. 3 shows top 300 and side 302 views of a MEMS ribbon
device according to an aspect of the invention. The device
incorporates a linear array of flexible, reflective ribbons. The
purpose of each ribbon is to form a phase shifting surface for
interferometric light modulator 200 described above.
[0055] Each ribbon in the MEMS ribbon device shown in FIG. 3 is
supported at its ends by supports 312 and 310 and also at a point
part way between the ends by support 308. Intermediate support 308
is intentionally located away from the midpoint between end
supports 312 and 310. In the figure, support 308 is located closer
to support 310 than support 312. The distance between support 312
and support 308 is represented by L.sub.1 while the distance
between support 308 and support 310 is represented by L.sub.2.
L.sub.1>L.sub.2. Supports 312, 308, and 310 are fixed to solid
base 304.
[0056] For purposes of discussion it is convenient to refer to each
ribbon in the array as being composed of two parts such as ribbon
segment 306 and ribbon segment 307 as shown in the figure. However,
segments 306 and 307 are understood to be part of the same ribbon.
Movement of segment 306 is isolated from movement of segment 307
because the ribbon is fixed to support 308.
[0057] The ribbons in the MEMS ribbon device illustrated in FIG. 3
are reflective to light. The ribbons are also electrically
conductive while supports 312, 308, and 310 are electrical
insulators. A ribbon in the device is deflected by electrostatic
force when a voltage is applied between it and the electrically
conductive base 304 as illustrated in side view 302.
[0058] When a voltage is applied between a ribbon and base 304, for
example, by voltage source 320, the ribbon bends toward the base.
However, ribbon segments 306 and 307 are deflected different
amounts. Ribbon segment 306 is deflected by distance D.sub.1 while
segment 307 is deflected by distance D.sub.2. Distance D.sub.1 is
greater than distance D.sub.2 because length L.sub.1 is longer than
length L.sub.2. It is important to note that a single voltage
applied to a single ribbon causes its segments 306 and 307 to
deflect different distances. The ribbon forms the phase shifting
surface for the interferometer.
[0059] Each ribbon in the MEMS ribbon device is actuated by its own
voltage signal. Therefore each ribbon represents a phase shifting
surface to be used in the interferometer of FIG. 2A. When the
linear array of ribbons is illuminated in an interferometer
incorporating an elongated or slit-shaped light source each ribbon
represents one pixel in a linear pixel array.
[0060] When several ribbons of the MEMS ribbon device are
illuminated by an elongated slit-shaped light beam some of the
light will fall between ribbons and be reflected back into the
interferometer by the base 304. However, since the base is flat it
does not impart a relative phase shift to light falling near ribbon
segment 306 as compared to light falling near ribbon segment 307.
Therefore one of the features of the interferometer is that the
support and base structures do not appear in the output. In other
words the interferometer has high common mode rejection which leads
to high contrast in the dark state. The interferometer also has
high rejection of common mode artifacts caused by vibration of the
apparatus for the same reason.
[0061] The MEMS ribbon device of FIG. 3 has been fabricated using
silicon microfabrication techniques. FIG. 4 illustrates a process
flow for the ribbon device. The ribbons are made of silicon
nitride. They are metallized with aluminum and the whole structure
is supported on a silicon wafer.
[0062] In FIG. 4, step A shows a silicon wafer 410 on which a layer
of silicon dioxide 420 has been grown and a sacrificial layer of
polysilicon 430 has been deposited. In step B, vias 435 have been
patterned in polysilicon layer 430 using standard lithographic
techniques. In step C, silicon nitride layer 440 has been
deposited. Silicon nitride layer 440 covers polysilicon layer 430
and also fills vias 435. In step D, silicon nitride layer 440 is
patterned using standard lithographic techniques. In step E,
sacrificial polysilicon layer 430 is removed by standard etch
techniques. Finally, in step F, aluminum metallization layer 450 is
deposited.
[0063] Comparing FIG. 4 to FIG. 3 it is apparent that base 304 is
realized by silicon wafer 410. Ribbon 306, 307 and its supports
308, 310, 312 are formed by silicon nitride layer 440. It will be
easily appreciated by those skilled in the art of microfabrication
that there are many possible variations on the basic process flow
presented in FIG. 4.
[0064] The interferometric display device described herein has
additional uses in atomic force microscopy applications. Briefly an
atomic force microscope comprises a small, flexible cantilever with
a sharp tip mounted on the free end. The cantilever is dragged over
a surface much like a stylus in a record player.
[0065] Various optical and electronic techniques have been
developed to measure the bending of the cantilever in an atomic
force microscope (AFM). The techniques are so sensitive that it is
possible to detect movement of the cantilever when the AFM tip is
displaced by as little as the size of a single atom. The most
commonly used technique is known as the optical lever and involves
illuminating the cantilever with a laser beam. The beam is
reflected and the intensity of the reflected light is detected with
a split photodiode several centimeters away from the cantilever. A
more recent interferometric technique for monitoring the deflection
of a single cantilever was described in U.S. Pat. No. 5,908,981
which is herein incorporated by reference.
[0066] Recently researchers have made AFMs with linear arrays of
cantilevers rather than just a single cantilever. However measuring
the deflection of each cantilever in an array is challenging. The
interferometer described above and illustrated in FIG. 2A may be
used to measure the deflection of each AFM cantilever in an array.
The cantilever array takes the place of the MEMS ribbon device as
the phase shifting surface in the interferometer. Specifically,
when cantilever 226 bends as it is traced over surface 228, light
beams incident on the surface of the cantilever, for example light
beams 218 and 220 are reflected after traveling slightly different
distances to the different segments of the cantilever. An entire
linear array of cantilevers may be monitored by placing a linear
array photodetector at image plane 216.
[0067] The separation of light beams 218 and 220 on a cantilever
may be chosen by selecting a convenient wedge angle for Wollaston
prism 210 and focal length for lens 212. If the focused spots of
light beams 218 and 220 are located close together or near the base
of the cantilever, then the deflection of the free end may be
deduced without ambiguity even if it is displaced more than
one-half of the wavelength of light used in the interferometer.
[0068] In fact the interferometric devices described herein are
applicable to any situation in which it is desirable to monitor a
single phase shifting surface element or an array of such elements.
And when the phase shifting surface itself is controlled, it may be
used to generate an image in concert with the interferometer,
scanning, and projection elements. Controllable phase shifting
surface arrays include, for example, the MEMS ribbon device
described herein and arrays of micro-cantilevers such as those used
in atomic force microscopy.
[0069] As one skilled in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, means, methods, or steps, presently existing or later
to be developed that perform substantially the same function or
achieve substantially the same result as the corresponding
embodiments described herein may be utilized according to the
present invention. Accordingly, the appended claims are intended to
include within their scope such processes, machines, manufacture,
means, methods, or steps.
[0070] While the invention has been described in connection with
what are presently considered to be the most practical and
preferred embodiments, it is to be understood that the invention is
not limited to the disclosed embodiments and alternatives as set
forth above, but on the contrary is intended to cover various
modifications and equivalent arrangements included within the scope
of the following claims.
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